Femtosecond Technology for Technical and Medical Applications

Topics in Applied Physics Volume 96

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Topics in Applied Physics Topics in Applied Physics is a well-established series of review books, each of which presents a comprehensive survey of a selected topic within the broad area of applied physics. Edited and written by leading research scientists in the field concerned, each volume contains review contributions covering the various aspects of the topic. Together these provide an overview of the state of the art in the respective field, extending from an introduction to the subject right up to the frontiers of contemporary research. Topics in Applied Physics is addressed to all scientists at universities and in industry who wish to obtain an overview and to keep abreast of advances in applied physics. The series also provides easy but comprehensive access to the fields for newcomers starting research. Contributions are specially commissioned. The Managing Editors are open to any suggestions for topics coming from the community of applied physicists no matter what the field and encourage prospective editors to approach them with ideas.

Managing Editors Dr. Claus E. Ascheron

Dr. Hans J. Koelsch

Springer-Verlag GmbH Tiergartenstr. 17 69121 Heidelberg Germany Email: [email protected]

Springer-Verlag New York, LLC 233, Spring Street New York, NY 10013 USA Email: [email protected]

Assistant Editor Dr. Werner Skolaut Springer-Verlag GmbH Tiergartenstr. 17 69121 Heidelberg Germany Email: [email protected]

Friedrich Dausinger Friedemann Lichtner Holger Lubatschowski (Eds.)

Femtosecond Technology for Technical and Medical Applications With 224 Figures

123

Professor Friedrich Dausinger

Dipl.-Ing. Friedemann Lichtner

Universität Stuttgart Institut für Stahlwerkzeuge Pfaffenwaldring 43 70569 Stuttgart, Germany [email protected]

Forschungsgesellschaft für Stahlwerkzeuge mbH Nobelstr. 15 70569 Stuttgart, Germany [email protected]

Dr. Holger Lubatschowski Laserzentrum Hannover e.V. Hollerithalle 8 30419 Hannover, Germany [email protected]

Library of Congress Control Number: 2004107208

Physics and Astronomy Classification Scheme (PACS): 42.55.Xi, 42.65.Re, 42.62.Be, 42.62.Cf, 42.62.Eh, 87.50.Hj, 07.85.Fv

ISSN print edition: 0303-4216 ISSN electronic edition: 1437-0859 ISBN 3-540-20114-9 Springer Berlin Heidelberg New York This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer. Violations are liable for prosecution under the German Copyright Law. Springer is a part of Springer Science+Business springeronline.com © Springer-Verlag Berlin Heidelberg 2004 Printed in Germany The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Typesetting: DA-TEX · Gerd Blumenstein · www.da-tex.de ockler GbR, Leipzig Production: LE-TEX Jelonek, Schmidt & V¨ Cover design: design & production GmbH, Heidelberg Printed on acid-free paper

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Preface

Record numbers are always fascinating. Femto, a millionth part of nano, is going much further to short scales than nano. Is femtotechnology now topping nanotechnology? Certainly not: If researchers and funding organizations are talking about nano, they mean nanometer, whereas femto stands for femtosecond. Both notions, however, have in common that they denominate research fields of actual interest. It is our hope that this volume contributes to making femtosecond technology (FST), which is now called femtonic, as popular as nanotechnology. With the beginning of the new century a German research and development program FST (http://www.fgsw.de/fst/) started with the intention of exploiting the potential of femtosecond technology. In a preliminary competition five research consortia had been selected: PRIMUS, FESMET, MUSKL, FLIM and SAFEST. Some time later, the project cluster GEPULAM was associated. In total, 24 companies and 21 research institutes were involved as full members. Additionally, a number of institutions in Eastern Europe and Israel contributed as subcontractors. The program was funded by the Federal Ministry of Education and Research (BMBF) with almost 30 million Euros. An equivalent amount was spent by industrial partners. The VDI-Technologiezentrum in D¨ usseldorf acted as the project agency. The investigations concentrated on the following fields: • • • • •

Micromachining of technical materials (microstructuring and drilling) Medical therapy (ophtalmology, dentistry, neurology and ear surgery) Metrology X-ray sources Laser safety

Lasers, systems and technologies required in these potential fields of applications were investigated and developed. The program aimed at industrial success and, therefore, was dominated by industrial partners. The more fundamental research was done in university institutes and research centers. At the end of a four-year long funding period remarkable success has been achieved: • Ultrafast laser sources with increased performance and reliability at reduced costs are now available.

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• An in-depth knowledge of basic interactions of ultrashort pulses with technical and biological matter allows development of ultraprecise processes with still sufficient productivity. • A new surgical method to correct ametropia using nonlinear optical effects instead of knives is on the eve of clinical qualification. • ... The completion of this list of successes would go far beyond the scope of a preface. A comprehensive description of all the interesting results requires a book, such as the one presented here. The remarkable progress described here would not have been achieved in such a short time without the engagement of the young researchers doing the project work and the funding by the German Federal Government. The editors and authors of this volume wish to thank both of these. Stuttgart and Hannover, June 2004

Friedrich Dausinger Friedemann Lichtner Holger Lubatschowski

Contents

Introduction to Femtosecond Technology Friedrich Dausinger, Stefan Nolte . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 4

High-Power Ultrashort Pulse Lasers Principles of Ultrashort Pulse Generation Jochen Kleinbauer, Ralf Knappe, Richard Wallenstein . . . . . . . . . . . . . . . 1 Q-Switching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Mode Locking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Active Mode Locking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Passive Mode Locking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ultrafast Nd:YVO4 and Yb:YAG Bulk Lasers and Amplifiers Jochen Kleinbauer, Ralf Knappe, Richard Wallenstein . . . . . . . . . . . . . . . 1 Diode-Pumped Ultrashort Pulse Laser Oscillators . . . . . . . . . . . . . . . 1.1 SESAM Mode Locked Picosecond Nd:YVO4 Oscillators . . . . . . 1.2 PSM Mode Locked Picosecond Nd:YVO4 and Femtosecond Yb:YAG Oscillators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Nd:YVO4 Regenerative Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Additional Amplification in a Power Amplifier . . . . . . . . . . . . . . 2.2 Nonlinear Frequency Conversion in the Visible and the UV . . . 2.3 Scaling to Higher Repetition Rates . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ultrashort Pulse Fiber Lasers and Amplifiers Andreas T¨ unnermann, Jens Limpert, Stefan Nolte . . . . . . . . . . . . . . . . . . 1 The Low Numerical Aperture Large Mode Area Concept . . . . . . . . . 2 Fiber-Based Chirped-Pulse Amplification . . . . . . . . . . . . . . . . . . . . . . . 3 High Average Power Femtosecond Fiber CPA System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 High Pulse Energy Ultrafast Fiber CPA System . . . . . . . . . . . . . . . . . 4.1 The Consequences of SRS and SPM in a Fiber CPA System . . 4.2 Energy-Scaling Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . .

9 9 10 11 13 15 17 20 20 22 24 27 29 31 31 35 38 40 41 44 44 47

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5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 Ultrashort Pulse Thin-Disk Lasers and Amplifiers Daniel M¨ uller, Adolf Giesen, R¨ udiger Paschotta, Ursula Keller . . . . . . . 1 Ultrafast Thin-Disk Oscillators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Ultrashort Thin-Disk Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Geometrical Multipass Amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Resonator Multipass (or Regenerative) Amplifier . . . . . . . . . . . . 2.3 Pump Phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Multipass Pulse Amplification . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5 Regenerative Amplifier Resonator Design . . . . . . . . . . . . . . . . . . 2.6 Pockels Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Ultrafast Thin-Disk Laser Scaling Limits . . . . . . . . . . . . . . . . . . . . . . . 4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

55 58 62 62 63 65 66 68 69 69 70 70

Ultrashort Interaction with Materials Interaction with Atmosphere Detlef Breitling, Sergei Klimentov, Friedrich Dausinger . . . . . . . . . . . . . . 1 Optical Breakdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Conical Emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Shock Waves and Material Vapor Flow . . . . . . . . . . . . . . . . . . . . . . . . . 4 Residual Ablated Matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Interaction with Biological Tissue Holger Lubatschowski, Alexander Heisterkamp . . . . . . . . . . . . . . . . . . . . . 1 Nonlinear Propagation of Ultrashort Laser Pulses in Transparent Media . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Self-Focusing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Self-Phase Modulation (SPM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Group-Velocity Dispersion (GVD) . . . . . . . . . . . . . . . . . . . . . . . . 1.4 Continuum (White Light) Generation . . . . . . . . . . . . . . . . . . . . . 2 Plasma Ignition: Multiphoton Ionization vs. Avalanche Ionization . 2.1 Multiphoton Ionization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Avalanche Ionization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Time History for the Density of Free Electrons . . . . . . . . . . . . . 3 Mechanical and Chemical Side Effects . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

75 75 78 82 86 88 91 92 92 93 93 94 95 95 96 96 98 102

Interaction with Metals Andreas Ruf, Friedrich Dausinger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

Contents

IX

Technical Applications Surface Structuring Michael Weikert, Friedrich Dausinger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Influence of Basic Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Influence of Beam Movement on Geometry of Ablated Zone . . 1.3 Process Speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Examples of Possible Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Tribological Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Structures for Printing and Embossing . . . . . . . . . . . . . . . . . . . . 3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

117 117 117 121 125 128 128 128 129 129

Drilling of Metals Detlef Breitling, Christian F¨ ohl, Friedrich Dausinger, Taras Kononenko, Vitali Konov . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Basic Understanding of Short-Pulsed Laser Drilling . . . . . . . . . . . . . . 1.1 Energy Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Heat Conduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Plasma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 Impeded Material Expulsion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Drilling Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Helical Drilling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Drilling with Beam Inclination . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Means to Increase Productivity and Processing Quality . . . . . . . . . . 3.1 Pulse Repetition Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Polarization Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Vacuum Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

131 131 133 134 134 136 140 140 142 146 146 147 150 151

Cutting of Diamond Michael Weikert, Friedrich Dausinger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Influence of Process Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Pulse Duration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Polarization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Process Gases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Repetition Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

155 156 157 157 160 162 163 164

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Medical Therapy Dental Applications Paul Weigl, Anton Kasenbacher, Kristian Werelius . . . . . . . . . . . . . . . . . . 1 Caries Therapy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Femtosecond-Laser-Based Caries Removal . . . . . . . . . . . . . . . . . . 1.1.1 Laser–Tissue Interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1.2 Threefold Caries Selectivity . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Retentive Patterns at Dentin Surfaces Facing Filling Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Intraoral Application of a Femtosecond Laser Beam . . . . . . . . . 2 Manufacturing of All-Ceramic Restorations . . . . . . . . . . . . . . . . . . . . . 2.1 Coherence Radar (KoRad) for Surface Measurement . . . . . . . . 2.2 Femtosecond-Laser-Based Shaping of All Ceramic Restorations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Material Properties After Femtosecond-Laser Processing . . . . . 2.3.1 Effects of Pulse Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2 Effect of Pulse Duration . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.3 Innovative Strategies of an Efficient 3D Shaping of Ceramics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.4 Ablation Strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.5 Online Depth Measurement . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Innovative Femtosecond-Laser-Based Dental CAD/CAM System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

167 167 167 168 171 172 173 174 175 176 177 177 179 180 180 181 182 184

Ophthalmic Applications Holger Lubatschowski, Alexander Heisterkamp . . . . . . . . . . . . . . . . . . . . . 1 Refractive Surgery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Photorefractive Keratectomy (PRK) . . . . . . . . . . . . . . . . . . . . . . 1.2 Laser in situ Keratomileusis (LASIK) . . . . . . . . . . . . . . . . . . . . . 2 Femtosecond LASIK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Laser System and Beam Delivery . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Morphological Studies and Nonlinear Side Effects . . . . . . . . . . . 3 Keratoplasty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Presbyopia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

187 188 188 189 190 191 192 197 198 201

Neurosurgical Applications Marcus G¨ otz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Ablation of Brain Tissue with Ultrashort Laser Pulses . . . . . . . . . . . 2 Potential Applications of Ultrashort Laser Pulses in Neurosurgery . 2.1 Movement Disorders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Requirements for Neurosurgical Instruments . . . . . . . . . . . . . . . . . . . .

203 203 204 204 205

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3.1 Stereotactic or Navigated Surgery . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Ultrashort Laser-Beam Delivery . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Operation Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Use of Femtosecond Technology in Otosurgery Burkard Schwab, Dietrich Hagner, J¨ org Bornemann, Ralf Heermann . . 1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Determination of the Ablation Threshold . . . . . . . . . . . . . . . . . . 3.2 Determination of Ablation Rates . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Model-Based Measurement of Pressure and Temperature . . . . . 3.4 Comparative Investigation into Heat Accumulation Using Femtosecond Pulses, a Free-Running Er:YAG Laser and a CO2 Laser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Interpretation of Measurement Results . . . . . . . . . . . . . . . . . . . . 3.6 Optimization of Ablation Rates in Relation to the Scan Algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7 Spectral Analysis of the Laser-Generated Plasma . . . . . . . . . . . 3.8 Pressure Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.9 Histological Evaluation of the Irradiated Soft Tissue . . . . . . . . 4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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205 207 207 210 211 211 212 212 212 213 214

215 216 217 218 220 221 224 225

Subcellular Photodisruption Alexander Heisterkamp, Holger Lubatschowski . . . . . . . . . . . . . . . . . . . . . 227 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231 Generation of X-Rays by Intense Femtosecond Lasers Generation of X-Rays by Intense Femtosecond Lasers Heinrich Schwoerer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Laser Light and X-Rays: An Introduction . . . . . . . . . . . . . . . . . . . . . . 2 From Laser Light to X-Rays: Basic Physics . . . . . . . . . . . . . . . . . . . . . 2.1 Multiphoton and Field Ionization . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Collisional Heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Collective Absorption Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Relativistic Electron Acceleration . . . . . . . . . . . . . . . . . . . . . . . . . 3 High-Intensity Femtosecond Lasers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Laser-Generated X-Ray Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Plasma Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Ultrashort Kα -Pulses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Hard Bremsstrahlung . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Transmutation of Isotopes from the Nuclear Fuel Cycle . . . . . .

235 235 236 236 238 238 240 241 242 243 245 247 248

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Contents

4.5 Production of Short-Lived β + -Active Isotopes . . . . . . . . . . . . . . 250 5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251 Metrological Applications Metrological Applications Ralf Menzel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Advantages of Femtosecond Pulses for Metrology Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Investigated Measuring Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Coherence Radar (KoRad) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Optical Coherence Tomography (OCT) . . . . . . . . . . . . . . . . . . . . 3.3 Femtosecond Radar (FemRad) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Realized Broadband Light Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Short-Pulse Laser with Microstructured Fiber . . . . . . . . . . . . . . 4.2 Gain-Switched Ti:sapphire Laser . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Broadband Operation of Diode Lasers . . . . . . . . . . . . . . . . . . . . . 5 Measurement Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Coherence Radar (KoRad) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Femtosecond Radar (FemRad) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Optical Coherence Tomography (OCT) . . . . . . . . . . . . . . . . . . . . 6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

257 257 258 259 259 263 265 267 267 270 273 275 275 278 279 280 281

Safety Aspects in Femtosecond Technology Primary Hazards and Reliability of Protective Materials Andreas Hertwig, Sven Martin, J¨ org Kr¨ uger, Christian Spielmann, Martin Lenner, Wolfgang Kautek . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Optical Filter Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Linear Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Nonlinear Behavior: Induced Transmission . . . . . . . . . . . . . . . . . 3.3 Surface Damage on Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Guards and Curtains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Introduction: Strategies of Guard Design . . . . . . . . . . . . . . . . . . 4.2 Surface-Damage Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Failure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

287 287 288 289 289 290 296 302 302 303 304 306 306

Contents

Secondary Hazards: Particle and X-Ray Emission Jens Bunte, Stephan Barcikowski, Thomas Puester, Tomas Burmester, Martin Brose, Thomas Ludwig . . . . . . . . . . . . . . . . . . 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Particle Emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 X-Ray Emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

XIII

309 309 310 311 315 317 318

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321

Introduction to Femtosecond Technology Friedrich Dausinger1 and Stefan Nolte2 1

2

Institut f¨ ur Strahlwerkzeuge, Universit¨ at Stuttgart, Pfaffenwaldring 43, 70569 Stuttgart, Germany [email protected] Institut f¨ ur Angewandte Physik, Friedrich-Schiller-Universit¨ at Jena, Max-Wien-Platz 1, 07743 Jena, Germany

Abstract. Femtosecond technology, with its ultrashort light pulses, is an innovative laser technology that can be used for multiple applications, e.g., in industrial manufacturing, information and communication technologies, environmental technology and life sciences (medicine, biology, chemistry). This volume concentrates on the use of ultrashort pulses as a tool for ultraprecise material removal in manufacturing and medical therapy, as well as a tool for metrology and for X-ray production.

The most striking feature of the new technology is the extreme shortness of the laser pulses ranging from about 10 fs (10−14 s) to 10 ps (10−11 s). To give an example: Within 100 fs, light travels only as far as a fraction of the diameter of a human hair. For comparison: Within one second, light can circle the earth about 7.5 times. Another predominant feature of ultrashort pulses is their extremely high intensity. During the pulse a power level of hundreds of Gigawatts is achieved, that is as much as all the power plants of Germany deliver together. At a pulse width of 100 fs and a focal area of 100 µm2 a pulse energy of 1 mJ yields an intensity value of 1016 W/cm2 , for example.

Fig. 1. The shortness of a femtosecond is illustrated by the distance light travels during the pulse duration. During 100 fs light crosses only half of the diameter of a human hair. During 1 s, on the other hand, light travels a distance corresponding to 7.5 times the circumference of the earth F. Dausinger, F. Lichtner, H. Lubatschowski (Eds.): Femtosecond Technology for Technical and Medical Applications, Topics Appl. Phys. 96, 1–6 (2004) c Springer-Verlag Berlin Heidelberg 2004


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Friedrich Dausinger and Stefan Nolte

What is common to the very different applications of femtosecond technology, like drilling of fuel-injection nozzles, correction of ametropia by cutting inside the cornea, profilometry with coherent radar and production of X-rays? What determines its limit towards other technologies? Why does it reach into the picosecond timescale? How is the intensity related to the pulse duration? The answers to these questions come from two completely different aspects: • The generation of ultrashort pulses and • The interaction of ultrashort pulses with matter Laser pulses are conventionally produced either by pulsing of the pump source, which leads to long pulses with duration longer than 0.1 ms, or by Q-switching, which allows minimum pulse lengths of several nanoseconds (short pulses). A further shortening to the picosecond and femtosecond range is enabled by a completely different method, the mode-locking technique (see the Chapter “High-Power Ultrashort Pulse Lasers”). By superposition of modes with slightly different wavelengths coupled together with appropriate active or passive optical devices a strong temporal concentration of energy is achieved, leading to a proportional increase of peak power. From the lasers used in femtosecond technology very different beam parameters are required depending on their application. It is obvious that, for example, for eye surgery much lower energy density is needed than for drilling of hard steel. All FST lasers have in common, however, that mode locking is used to generate the required shortness of pulses. High intensity, on the other hand, is a natural consequence of the modelocking technique that changes energy delivery by extreme temporal concentration, automatically increasing energy per time, i.e., power. Extreme intensities enable multiphoton effects allowing, e.g., materials treatment inside of transparent materials like glass or the human eye cornea and, on the other hand, efficient generation of X-rays. The more modes that can be locked together the shorter the pulse duration can be. For this reason, laser-active materials with broad gain-bandwidth delivering a wide spectrum of wavelengths are needed. This is, on the other hand, a prerequisite for white-light interferometry used in metrology. As a consequence, ultrashort laser pulses are interesting tools for nondestructive measuring techniques, as well, see the Chapter “Metrological Applications”. Besides metrology, all other applications described in this book make use of precise ablation of matter, be it of a biological or technical nature ranging from soft and transparent ones, like in the eye, to hard and opaque ones like in teeth or wear-resistant technical components. In all cases, the interaction of light with matter is primarily an energy transfer to the electrons contained in it. A multitude of collisions is needed to transfer the absorbed energy from the heated electrons to the heavy particles (atoms, ions) the matter is built of. This energy-transfer process takes a long time, typically more than 10 ps, compared to the pulse duration of mode-locked lasers, see the Chapter “Ultrashort Interaction with Materials”. The consequence of this

Introduction to Femtosecond Technology

3

is that the material remains essentially cold during ultrashort pulses with a maximum duration of about 10 ps. This enables ultrahigh precision and minimized heat load. All techniques making use of this new possibility are included in femtosecond technology. The upper limit of pulse duration still allowing this effect of cold interaction is material dependent, the value of 10 ps is agreed on as being typical. Laser materials processing has been an intensive research topic within the past few decades. As a consequence, the laser is nowadays routinely used in many industrial processes like cutting, hardening, and welding. When the highest precision is required, however, the laser has not yet become a universal tool. In general, one needs a laser with matched properties for a certain microstructuring application. Marking applications, for example, are typically addressed by Nd:YAG lasers, while excimer lasers are used for the micromachining of ceramics and polymers. In addition, there are several applications, e.g., in the field of precise microstructuring of metallic materials, which are limited by thermal or mechanical damage, when lasers with pulse durations in the range of nanoseconds to microseconds are used. These limitations have stimulated widespread research activities in order to minimize collateral damage and thermal diffusion out of the irradiated area by using ultrashort laser pulses (see, e.g., [1, 2, 3, 4, 5, 6, 7, 8, 9]). A spectacular demonstration of the advantages of ultrashort pulse laser ablation is the cutting of explosives [10]. When using pulses with a duration of 600 ps the explosives were ignited due to the thermal load. In contrast, the irradiation with femtosecond pulses results in clean cuts and no chemical-reaction products were observable. This indicates that thermal transfer and shock waves are substantially smaller than is necessary for ignition. Examples like this have spurred interest not only from industry, but also from biology and medicine. Several applications have been tested and the specific advantages of using ultrashort laser pulses for the precise micromachining have been worked out [11]. To summarize the results of this research: ultrashort pulse lasers with pulse durations of a few picoseconds or below can be used for the precise micromachining of a wide variety of different materials like metals, semiconductors, dielectrics, polymers, etc. Even the processing of transparent media is possible due to efficient nonlinear absorption associated with the high intensities achievable using ultrashort pulses. By appropriate choice of the processing parameters the mechanical and thermal modification of the surrounding areas can be minimized and postprocessing can be avoided. Even the smart modification of material properties like the refractive index inside the volume of glasses [12, 13, 14, 15] and crystals [16] has become possible that allows the direct writing of buried optical waveguides for applications in integrated optics. The extensive research using ultrashort laser pulses is associated with improvements in the laser systems. Research topics have been limited and

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Friedrich Dausinger and Stefan Nolte

rather esoteric when dye-laser-based ultrashort pulse laser systems had to be used since these systems were too complex, hard to operate and offered only moderate output powers and pulse energies. This changed in the early 1990s, when the technique of Kerr-lens mode locking was developed [17] and Ti:sapphire emerged as a reliable gain medium with a very wide emission bandwidth. Starting from this date laser sources with pulse widths of less then 20 fs have become commonplace in many research laboratories. The scaling to high pulse energies has become possible by the development of the so-called chirped-pulse amplification (CPA) technique [18]. A further step to more compact and efficient systems has been achieved by the development of gain media that can be directly diode pumped, such as Cr- and Yb-based materials. However, the output power is still limited by thermo-optical effects, which in turn limits process throughput. As a consequence, only a few niche applications, like the repair of photolithographic masks [19], have been realized until now. For the majority of real-world applications the laser output power has to be increased and more ruggedized laser systems are required in order to improve processing speed, yield, reliability and to reduce costs. This volume will present several approaches to overcome the limitations related to beam sources (see the Chapter “High-Power Ultrashort Pulse Lasers”). The Chapter “Ultrashort Interactions with Materials” summarizes the latest state of knowledge about interactions of ultrashort pulses with matter. In the next Chapter “Technical Applications” possible technical applications of femtonic lasers are described. The Chapter “Medical Therapy” is devoted to the use of ultrashort pulses in medical therapy. The following Chapter “Generation of X-Rays by Intense Femtosecond Lasers” deals with the generation of X-rays and the Chapter “Metrological Applications” with metrological applications. Last, but not least, safety aspects that have to be considered when using ultrashort pulses are treated in the Chapter “Safety Aspects in Femtosecond Technology”.

References [1] D. Du, X. Liu, G. Korn, J. Squier, G. Mourou: Laser-induced breakdown by impact ionization in SiO2 with pulse widths from 7 ns to 150 fs, Appl. Phys. Lett. 64, 3071–3073 (1994) 3 [2] S. Preuss, A. Demchuk, M. Stuke: Sub-picosecond UV laser ablation of metals, Appl. Phys. A-Mater. 61, 33–37 (1995) 3 [3] J. Kr¨ uger, W. Kautek: Femtosecond-pulse laser processing of metallic and semiconducting thin films, in Laser-Induced Thin Film Processing, Proc. SPIE 2403 (1995) pp. 436–447 3 [4] B. C. Stuart, M. D. Feit, S. Herman, A. M. Rubenchik, B. W. Shore, M. D. Perry: Optical ablation by high-power short-pulse lasers, J. Opt. Soc. Am. B 13, 459–468 (1996) 3

Introduction to Femtosecond Technology

5

[5] B. N. Chichkov, C. Momma, S. Nolte, F. von Alvensleben, A. T¨ unnermann: Femtosecond, picosecond and nanosecond laser ablation of solids, Appl. Phys. A 63, 109–115 (1996) 3 [6] C. Momma, B. N. Chichkov, S. Nolte, F. von Alvensleben, A. T¨ unnermann, H. Welling, B. Wellegehausen: Short-pulse laser ablation of solid targets, Opt. Commun. 129, 134–142 (1996) 3 [7] H. Varel, D. Ashkenasi, A. Rosenfeld, M. W¨ ahmer, E. E. B. Campbell: Micromachining of quartz with ultrashort laser pulses, Appl. Phys. A-Mater. 65, 367–373 (1997) 3 [8] S. Nolte, C. Momma, H. Jacobs, A. T¨ unnermann, B. N. Chichkov, B. Wellegehausen, H. Welling: Ablation of metals by ultrashort laser pulses, J. Opt. Soc. Am. B 14, 2716–2722 (1997) 3 [9] S. Nolte: Micromachining, in M. E. Fermann, A. Galvanauskas, G. Sucha (Eds.): Ultrafast Lasers: Technology and Applications (Decker, New York 2002) 3 [10] M. D. Perry, B. C. Stuart, P. S. Banks, M. D. Feit, V. Yanovsky, A. M. Rubenchik: Ultrashort-pulse laser machining of dielectric materials, J. Appl. Phys. 85, 6803–6810 (1999) 3 [11] M. E. Fermann, A. Galvanauskas, G. Sucha: Ultrafast Lasers: Technology and Applications (Decker, New York 2002) 3 [12] K. M. Davis, K. Miura, N. Sugimoto, K. Hirao: Writing waveguides in glass with a femtosecond laser, Opt. Lett. 21, 1729–1731 (1996) 3 [13] K. Miura, J. Qiu, H. Inouye, T. Mitsuyu, K. Hirao: Photowritten optical waveguides in various glasses with ultrashort pulse laser, Appl. Phys. Lett. 71, 3329–3331 (1997) 3 [14] C. B. Schaffer, A. O. Jamison, J. F. Garcia, E. Mazur: Structural changes induced in transparent materials with ultrashort laser pulses, in M. E. Fermann, A. Galvanauskas, G. Sucha (Eds.): Ultrafast Lasers: Technology and Applications (Decker, New York 2002) 3 [15] S. Nolte, M. Will, J. Burghoff, A. T¨ unnermann: Femtosecond waveguide writing: A new avenue to three-dimensional integrated optics, Appl. Phys. AMater. 77, 109–111 (2003) 3 [16] T. Gorelik, M. Will, S. Nolte, A. T¨ unnermann, U. Glatzel: Transmission electron microscopy studies of femtosecond laser induced modifications in quartz, Appl. Phys. A-Mater. 76, 309–311 (2003) 3 [17] D. E. Spence, P. N. Kean, W. Sibbett: 60-fsec pulse generation from a selfmode-locked Ti:sapphire laser, Opt. Lett. 16, 42–44 (1991) 4 [18] D. Strickland, G. Mourou: Compression of amplified chirped optical pulses, Opt. Commun. 56, 219–221 (1985) 4 [19] R. Haight, D. Hayden, P. Longo, T. Neary, A. Wagner: MARS: Femtosecond laser mask advanced repair system in manufacturing, J. Vac. Sci. & Tech. B 17, 3137–3143 (1999) 4

Index

femtosecond technology, 2 FST, 2 intensity, 1 multiphoton effect, 2 nonlinear absorption, 3

transparent material, 2 transparent media, 3 ultrashort, 1 waveguide, 3

Principles of Ultrashort Pulse Generation Jochen Kleinbauer, Ralf Knappe, and Richard Wallenstein Fachbereich Physik, Universit¨ at Kaiserslautern, Erwin-Schr¨ odinger-Straße 46, 67663 Kaiserslautern, Germany [email protected] Abstract. This Chapter gives an overview on various techniques employed today for ultrashort pulse generation. As an introduction to the concept of mode locking, it is briefly explained how phase locking the axial modes of a laser cavity can lead to the generation of short pulses. Both active mode locking, where an external signal modulates the radiation inside the cavity, and passive mode locking, where an intensity-dependent loss in combination with the laser radiation itself provides the necessary modulation, are explained. Since passive mode locking allows for the generation of the shortest pulses, three different experimental schemes are introduced: Passive mode locking employing an intracavity Kerr-lens (KLM), additive pulse mode locking (APM) and mode locking using a semiconductor saturable absorber.

The most important and commonly applied techniques for the generation of short laser pulses are Q-switching and mode locking. The following Chapter will give a brief overview on the basic principles of these two methods. For a deeper understanding, [1, 2, 3, 4] are recommended for further reading.

1

Q-Switching

In the case of Q-switching, a modulator is placed inside a laser resonator, in order to control the cavity losses. While the laser is pumped and energy is stored inside the active medium, the modulator is set to produce high losses, to prevent the system from lasing. In this way, the population inversion inside the active medium reaches a level far above that of normal laser action. When the maximum population inversion has been obtained, the losses are switched to a minimum and the complete energy stored inside the active medium is released in a single laser pulse of high intensity [1, 4]. The duration of the generated pulse is thereby much shorter than the actual rise time of the modulator, but always longer than the cavity roundtrip time. Typically, the pulse duration obtainable with Q-switched solid-state lasers is in the range of several nanoseconds. Due to the statistical nature of the pulse-generating effects inside the laser, each ejected pulse can be considered independent of the one before. Every pulse evolutes from random noise fluctuations, which in turn leads to a high amount of temporal and spectral jittering of the emitted radiation. F. Dausinger, F. Lichtner, H. Lubatschowski (Eds.): Femtosecond Technology for Technical and Medical Applications, Topics Appl. Phys. 96, 9–17 (2004) c Springer-Verlag Berlin Heidelberg 2004


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Jochen Kleinbauer et al. 1.2

Amplitude [ a. U. ]

1.0

1.2 Gain profile Axial cavity modes

0.8

dn = c/2L

tp = 1/nrep = 2L/c 1.0 0.8

0.6

0.6 Dn

Dtp

0.4

0.4

0.2

0.2

0.0

0.0 n0 Frequency

10

20

30

40

50

Time [ a. U. ]

Fig. 1. Left: Axial laser cavity modes with spacing δν = c/2L below the gain profile. Right: Temporal development of the superposition of seven axial modes of the same intensity and with constant phase difference ∆Φ

2

Mode Locking

To obtain pulses with pulse durations in the range of picoseconds down to several femtoseconds, other techniques have to be applied. Consider a laser cavity with many axial modes of distance δν = c/2L, where L is the cavity length and c is the speed of light. All the modes inside the gain-bandwidth of the laser material can oscillate (see Fig. 1, left), given that they reach the threshold. The emitted electrical field is a superposition of the contribution of each mode [2]: E(t) =

n


Ak exp {i [2π(ν0 + kδν)t + Φk ]}

(1)

k=−n

Here, ν0 denotes the center frequency (i.e., the frequency of the central axial mode), Ak and Φk refer to the amplitude and the phase of mode k, respectively. Generally, as each mode is subject to random fluctuations, its phase Φk is not correlated to that of the adjacent modes. Both the electrical field E(t) and the intensity I(t) are statistically varying functions of time, incoherent and without regular temporal structure [3]. If it can be arranged that all the modes oscillate in phase, i.e., in the case of Φk − Φk−1 = ∆Φ = const., then the resulting electrical field becomes a temporally well-defined periodic function of time. When considering 2n + 1 oscillating axial modes with constant phase difference ∆Φ and the same amplitude A0 , (1) yields

Principles of Ultrashort Pulse Generation

E(t) = A0

  sin (2n + 1) ∆ωt+∆Φ  ∆ωt+∆Φ2 eiω0 t , where ∆ω = 2πδν . sin 2

11

(2)

Figure 1 (right) shows the resulting function in the time domain and in the case of seven coupled modes (n = 3). It can be described by a train of short pulses, temporally spaced by the cavity roundtrip time τp = 1/δν = 2L/c. In this way, the superposition of the electrical field of all the phase-locked axial cavity modes can be explained by a single optical pulse, circulating inside the cavity. After each roundtrip, i.e., after the time τp , a certain fraction of the circulating pulse is ejected from the cavity and a periodic signal with repetition rate νrep = 1/τp (see Fig. 1, right) can be detected. It can be seen from (2) that the pulse duration ∆τp is getting shorter when the number of locked modes increases. From the same equation, ∆τp can be estimated as τp 1 1 ∆τp = = . (3) 2n + 1 νrep 2n + 1 Assuming that all axial modes inside a rectangular gain profile of the laser medium can be coupled together, the pulse duration can finally be approximated by 1 , (4) ∆τp
∆ν where ∆ν is the linewidth of the laser transition, i.e., the width of the gain profile. Therefore, a laser medium with a large gain-bandwidth is required for the generation of ultrashort laser pulses. Figure 2 illustrates the different modes of operation of a laser [5]. In singleaxial mode operation, the laser can generate either continuous-wave (CW) or pulsed output, in the case when Q-switching is employed. Accordingly, phase locking of the axial modes can either lead to CW mode locking or Q-switched mode locking, where the train of mode-locked ultrashort pulses is additionally modulated by a superposed Q-switched process. For the generation of ultrashort pulses of high stability, the laser is typically operated in the regime of CW mode locking. However, depending on the actual method of mode locking applied, the tendency for Q-switched mode locking cannot be eliminated completely. The different techniques for mode locking are classified in two main categories, active and passive mode locking. Active mode locking generally refers to the case where the radiation in the laser cavity is actively modulated by an external signal with a repetition rate matched to the cavity roundtrip time. In the second case, the radiation itself in combination with an intracavity nonlinear device provides the necessary modulation (see Fig. 3). 2.1

Active Mode Locking

The technique of active mode locking involves a periodic change of the loss or the gain that the radiation inside the laser cavity experiences. This is

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Jochen Kleinbauer et al. 1.2

1.2

Q-switched

Intensity [ a. U. ]

Continuous-wave (cw) 1.0

1.0

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0 Time

1.2

Time 1.2

Intensity [ a. U. ]

cw mode-locked

Q-switched mode-locked

1.0

1.0

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0 Time

Time

Fig. 2. Intensity–time characteristics of the different modes of operation of a laser: continuous-wave (CW), Q-switched, CW mode locked and Q-switched mode locked

A m p lifie r

G a in - o r lo s s m o d u la tio n

A c tiv e m o d e -lo c k in g

A m p lifie r

In te n s ity d e p e n d e n t lo s s

P a s s iv e m o d e -lo c k in g

Fig. 3. Schematic setup of active (top) and passive (bottom) mode locking

generally done by either placing a modulator inside the cavity, which periodically incorporates high or low losses (loss modulation), or by using a pulsed pump laser to periodically switch on and off the gain (gain modulation or synchronous pumping). In both cases, the modulator generates sidebands on the cavity modes νk , spaced at the modulation frequency ∆Ω. When the modulation frequency is chosen to be equal to the axial-mode separation δν, i.e., if ∆Ω = δν, energy is transferred between adjacent modes and they all oscillate in phase. Thus, the

Principles of Ultrashort Pulse Generation

13

modulation frequency has to correspond to the inverse of the cavity roundtrip time ∆Ω = δν =

1 c . = τp 2L

(5)

This condition can readily be explained in the time domain: Radiation that is attenuated in the modulator at a moment of high losses will again get attenuated in the next roundtrip, since the modulation function is periodic with the roundtrip time. Correspondingly, radiation passing the modulator in the moment of low losses will always pass it at low losses and get efficiently amplified in the gain medium at the expense of the radiation experiencing higher losses. After several roundtrips, one pulse evolves that circulates in the cavity and always passes the modulator at the moment of minimal losses [4]. Since the modulation function has no sharp edge, the wings of the pulse will be attenuated more strongly than the central part of the pulse, resulting in a temporal narrowing and a corresponding spectral broadening of the pulse in each roundtrip. The gain profile of the laser medium is also a peaked function and for this reason leads to a stronger amplification of the central part of the spectrum compared to the wings. This effect will therefore narrow the spectrum and correspondingly temporally broaden the pulses. Temporal compression and the opposing spectral compression, or more generally mode locking and dispersion, will counterbalance each other after the evolution of the pulse and finally lead to a steady state [3]. The basic disadvantage of active mode locking is the relative weakness of the pulse-compression mechanism, which is only capable of generating picosecond pulses. The external modulation stays the same during the evolution of the pulse, leading to a strong compression of the cavity radiation in the beginning, but to a weaker effect as the pulse shortens. 2.2

Passive Mode Locking

Passive mode locking is often also referred to as self-mode locking, owing to the fact that the cavity internal radiation itself, in conjunction with some intracavity nonlinear element, provides the necessary amplitude modulation. Essentially, the circulating pulses induce their own modulation function, providing a much stronger compression mechanism than in the case of active mode locking. For this reason, passive mode locking usually generates shorter pulses than active mode locking. Passive mode locking generally implies the insertion of a saturable absorption inside the cavity, which, in the most general case, is an element that generates high losses at low intensities and low losses at high intensities. Depending on the timescale that the saturated absorption needs to recover, two general cases are distinguished between: If the recovery time is much shorter than the pulse duration, the transmission follows the intensity profile of the pulse almost immediately. This leads to pulse compression, since the wings of

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the pulse experience stronger absorption than the central part. The absorber is thus said to be fast. If the recovery time is longer, the absorber is called slow. In this case, mode locking is achieved either by the combined interaction of saturable absorption and saturable gain (to form the falling edge of the pulse), or by the interaction of a slow and a fast component of the recovery function, e.g., of a semiconductor saturable absorber [6]. Stable mode locking with pulse durations shorter than the actual recovery time of the absorber can be achieved in this way. The most exploited physical effect for the construction of a virtually instantaneous saturable absorber is the optical Kerr effect, i.e., the nonlinear intensity dependence of the refractive index n(I) = n0 + n2 I .

(6)

• In the time domain, the nonlinear refractive index leads to self-phase modulation, the generation of new spectral components and the introduction of a frequency chirp, i.e., the change of the instantaneous frequency with time. This effect is used for additive pulse mode locking (APM). • In the spatial domain, the Kerr effect leads to self-focusing, which is utilized for Kerr-lens mode locking (KLM). Additive pulse mode locking (APM) is usually accomplished by coupling an additional cavity, which contains an optical fiber, to the main resonator. A pulse circulating in the main cavity is partially injected into the nonlinear cavity, where self-phase modulation in the optical fiber applies an intensitydependent phase profile. After the pulses in the main and in the slave cavity have finished one roundtrip in their respective cavities, they interfere at the common mirror. Provided that the optical lengths of the cavities are matched, only the central part of the pulses superpose in phase, whereas the wings are out of phase and interfere destructively. Thus, the nonlinear cavity can be considered an instantaneous saturable absorber, providing pulse compression by an intensity-dependent loss [7, 8, 9, 10, 11]. The most important practical disadvantage is the requirement for interferometric stabilization of the cavities, usually accomplished by electronic feedback loops. Recently, a variation of APM called phase self-adjusting mode locking (PSM) was subject to intensive investigations. As a result, the requirement for active length stabilization of the nonlinear cavity is no longer true [12]. The principal setup and results of this mode locking scheme will be given later in this chapter. Just like the temporal profile of a laser pulse is used to conduct APM, the spatial profile of the beam is utilized for a mode locking scheme called Kerr-lens mode locking (KLM). The intensity-dependent loss is provided by the fact that the high intensity in the center and the low intensity in the wings of the beam are causing self-focusing in a nonlinear medium, i.e., the generation of an intensity-dependent intracavity lens. If the resonator is set up in such a way that the cavity losses are lower in the presence of the lens

Principles of Ultrashort Pulse Generation

15

(e.g., by the insertion of an aperture), mode-locked operation of the laser is favored to continuous-wave operation. The Kerr medium can either be the crystal itself, or an additional intracavity element, such as a piece of glass with a high nonlinear refractive index. Additionally, the resonator can include a real aperture (which is said to be hard ), or a soft aperture, which means the confined region of the pumped volume in the laser crystal. Thus, in the simplest case, the laser crystal both provides the Kerr lens and the aperture, and no additional intracavity elements are required [13, 14, 15, 16]. As opposed to APM, the Kerr lens represents an intracavity saturable absorber, and so no control of the cavity length is required. On the other hand, for the Kerr lens to provide a maximum change of the beam size at the position of the aperture, the resonator of a KLM laser is usually operated close to the end of the stability region, making it sensitive to environmental changes. Moreover, the beam has to be focused very tightly into the Kerr medium for the Kerr lens to form, which limits the output power of the system due to possible optical damage of the medium. Additionally, KLM usually is not self-starting, since the fluctuations of the intracavity radiation are generally not strong enough to form the Kerr lens, and an additional mechanism is required to initiate the pulse formation. When these are getting sufficiently short, KLM is induced and can further compress the pulses to a minimum. Another important and fairly common mode locking scheme is based on the saturable absorption in semiconductors. In these devices, multiple quantum-well (MQW) structures provide a resonant nonlinear absorption at the laser wavelength. With increasing intensity, the absorption bleaches and the structure becomes transmissive. Most common designs are implemented by placing the MQW inside the spacer layer of a Fabry–P´erot etalon, formed by a highly reflective Bragg mirror on one side and a (partially) reflective layer grown on top. This is done to easily adjust key parameters of the absorber device and to overcome the inherent low saturation fluence and the damage threshold of the semiconductor material [17]. The interferometer is set for antiresonance by adjusting the thickness of the spacer layer. Therefore, a device like this is often referred to as an antiresonant Fabry–P´erot saturable absorber mirror (A-FPSA). The whole device can simply be used to replace one of the cavity mirrors, and therefore is often also called a semiconductor saturable absorber mirror (SESAM) [5, 18, 19, 20, 21].

References [1] A. E. Siegman: Lasers (Univ. Science Books, Mill Valley 1986) 9 [2] F. K. Kneub¨ uhl, M. W. Sigrist: Laser (Teubner, 0Stuttgart 1995) 9, 10 [3] P. M. W. French: The generation of ultrashort laser pulses, Rep. Prog. Phys. 58, 169–267 (1995) 9, 10, 13 [4] W. Koechner: Solid-State Laser Engineering (Springer, Berlin, Heidelberg 1999) 9, 13

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[5] U. Keller, K. J. Weingarten, F. X. K¨ artner, D. Kopf, B. Braun, I. D. Jung, R. Fluck, C. H¨ onninger, N. Matuschek, J. Aus der Au: Semiconductor saturable absorber mirrors (SESAMs) for femtosecond to nanosecond pulse generation in solid-state lasers, IEEE J. Sel. Top. Quant. 2, 435–453 (1996) 11, 15 [6] F. X. K¨ artner, J. Aus der Au, U. Keller: Mode-locking with slow and fast saturable absorbers – what’s the difference?, IEEE J. Sel. Top. Quant. 4, 159– 168 (1998) 14 [7] K. J. Blow, D. Wood: Mode-locked lasers with nonlinear external cavities, J. Opt. Soc. Am. B 5, 629–632 (1988) 14 [8] J. Mark, L. Y. Liu, K. L. Hall, H. A. Haus, E. P. Ippen: Femtosecond pulse generation in a laser with a nonlinear external resonator, Opt. Lett. 14, 48–50 (1989) 14 [9] E. P. Ippen, H. A. Haus, L. Y. Liu: Additive pulse mode locking, J. Opt. Soc. Am. B 6, 1736–1745 (1989) 14 [10] J. Goodberlet, J. Wang, J. G. Fujimoto, P. A. Schulz: Femtosecond passively mode-locked Ti:Al2 O3 laser with a nonlinear external cavity, Opt. Lett. 14, 1125–1127 (1989) 14 [11] H. A. Haus, J. G. Fujimoto, E. P. Ippen: Structures for additive pulse mode locking, J. Opt. Soc. Am. B 8, 2068–2076 (1991) 14 [12] B. Henrich: Neuartige Lasersysteme zur Erzeugung ultrakurzer Lichtimpulse hoher mittlerer Leistung, Dissertation, Universit¨ at Kaiserslautern (2000) 14 [13] D. E. Spence, P. N. Kean, W. Sibbett: 60-fsec pulse generation from a selfmode-locked Ti:sapphire laser, Opt. Lett. 16, 42–44 (1991) 15 [14] G. P. A. Malcolm, A. I. Ferguson: Self-mode locking of a diode-pumped Nd:YLF laser, Opt. Lett. 16, 1967–1969 (1991) 15 [15] H. A. Haus, J. G. Fujimoto, E. P. Ippen: Analytic theory of additive pulse and Kerr lens mode locking, IEEE J. Quantum Elect. 28, 2086 (1992) 15 [16] T. Brabec, C. Spielmann, P. F. Curley, F. Krausz: Kerr lens mode locking, Opt. Lett. 17, 1292–1294 (1992) 15 [17] U. Keller: Ultrafast all-solid-state laser technology, Appl. Phys. B-Lasers O. 58, 347–363 (1994) 15 [18] I. D. Jung, F. X. K¨ artner, N. Matuschek, D. H. Sutter, F. Mourier-Genoud, Z. Shi, V. Scheuer, M. Tilsch, T. Tschudi, U. Keller: Semiconductor saturable absorber mirrors supporting sub-10-fs pulses, Appl. Phys. B-Lasers O. 65, 137–150 (1997) 15 [19] R. Fluck, I. D. Jung, G. Zhang, F. X. K¨ artner, U. Keller: Broadband saturable absorber for 10-fs pulse generation, Opt. Lett. 21, 743–745 (1996) 15 [20] L. R. Brovelli, U. Keller, T. H. Chiu: Design and operation of antiresonant Fabry–Perot saturable absorbers for mode-locked solid-state lasers, J. Opt. Soc. Am. B 12, 311–322 (1995) 15 [21] S. Tsuda, W. H. Knox, S. T. Cundiff, W. Y. Jan, J. E. Cunningham: Modelocking ultrafast solid-state lasers with saturable Bragg reflectors, IEEE J. Sel. Top. Quant. 2, 454–464 (1996) 15

Index

active mode locking, 11 additive pulse mode locking (APM), 14 fast saturable absorber, 14 gain modulation, 12 Kerr effect, 14 Kerr-lens mode locking (KLM), 14 loss modulation, 12 mode locking, 10 passive mode locking, 13

phase self-adjusting mode locking (PSM), 14 Q-switching, 9 saturable absorber, 14, 15 self-mode locking, 13 self-phase modulation (SPM), 14 SESAM, 15 slow saturable absorber, 14 synchronous pumping, 12 ultrashort pulse generation, 9

Ultrashort Pulse Lasers and Amplifiers Based on Nd:YVO4 and Yb:YAG Bulk Crystals Jochen Kleinbauer, Ralf Knappe, and Richard Wallenstein Fachbereich Physik, Universit¨ at Kaiserslautern, Erwin-Schr¨ odinger-Straße 46, 67663 Kaiserslautern, Germany [email protected] Abstract. This Chapter focuses on the generation of ultrashort pulses and their amplification to high energies in Nd:YVO4 and Yb:YAG bulk crystals. Particularly with regard to applications such as high-precision micromachining, we briefly describe why these materials can be a promising alternative for current systems mostly based on Ti:sapphire, and how the actual setup of a laser would benefit from the possible simplifications. Following this, recent progress in the development of highpower diode-pumped picosecond Nd:YVO4 and femtosecond Yb:YAG oscillators is briefly outlined. Mode locking of the lasers presented here is achieved with either semiconductor saturable absorber mirrors (SESAMs) or by employing phase selfadjusted mode locking (PSM). The setup and the characterization of a Nd:YVO4 regenerative amplifier is subsequently explained in detail and experimental results are presented. The output of the amplifier is also frequency converted to the visible spectral range by second-harmonic generation (SHG) in LBO and to the UV by fourth-harmonic generation (FHG) in BBO. For higher average output powers in the IR and to demonstrate the potential of a further power scaling, an additional single-pass amplifier is applied to the system.

The last few years have seen a rapid development in the field of high-power ultrashort lasers and amplifiers, both with regard to systems based on the bulk material, as well as advanced concepts such as fiber lasers and the thindisk scheme. Much progress has been made to achieve the output powers and the pulse energies needed for numerous high-precision applications, as outlined in a later part of the book. Particularly with regard to the simplicity of the laser systems and their suitability for applications in an industrial environment, promising results with systems based on bulk crystals have been obtained over the last couple of years, and this Chapter will give an overview on these recent developments. The vast majority of industrial lasers for micromachining today is still based on bulk laser materials, mostly Ti:sapphire, which has become the standard material for femtosecond pulse generation and amplification. For this reason, some general remarks on the properties of this laser material and its application for the generation and amplification of ultrashort laser pulses will be given here. Compared to other classical active materials supporting femtosecond pulse generation, such as laser dyes, Ti:sapphire exhibits far superior thermal and laser properties. It offers an upper-state lifetime of F. Dausinger, F. Lichtner, H. Lubatschowski (Eds.): Femtosecond Technology for Technical and Medical Applications, Topics Appl. Phys. 96, 17–34 (2004) c Springer-Verlag Berlin Heidelberg 2004


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about 3 µs, broad absorption and emission linewidths and a relatively high gain cross section, which make it an ideal candidate for a tunable femtosecond oscillator and amplifier system [1]. Probably the most notable disadvantage of Ti:sapphire is the principal inability to pump it with standard high-power laser diodes. Because of its absorption bands, Ti:sapphire has to be pumped in the visible spectral range (between about 500 nm to 550 nm), which is commonly done by argon-ion lasers or diode-pumped, frequency-doubled solid-state lasers, such as Nd:YAG. Additionally, the generation of femtosecond pulses with high energy requires chirped-pulse amplification (CPA), to ensure that no damage occurs inside the amplifier due to the high peak power of the femtosecond pulses. CPA will be explained extensively in a later section, as it also applies for the generation of high-energy pulses in fiber amplifier systems. Basically, the ultrashort pulses from an oscillator have to be temporally stretched to the nanosecond regime, (regeneratively) amplified and afterwards shortened again in a grating compressor, typically to about 100 fs to 200 fs [2, 3]. Pulses as short as 20 fs at a repetition rate of 1 kHz directly from a Ti:sapphire regenerative amplifier have been demonstrated some time ago [4] and are now commercially available [5]. Significantly increased repetition rates and output powers were realized only recently with a system providing 60 fs pulses at 20 kHz with an average power of 6.5 W [6]. However, the inability to pump these lasers with high-power diodes, combined with the complexity of chirped-pulse amplification, results in an expensive and inefficient system with very limited average power that is also susceptible to mechanical misalignment. Maybe even more important, employing such systems for applications such as micromachining requires extended processing times, since the repetition rate is typically limited to below 5 kHz. To overcome most of these deficiencies, many research activities in the last few years have concentrated on the investigation of alternative laser materials for high-power ultrashort pulse generation. Yb-doped materials such as Yb:YAG and Yb:KGW are promising candidates for this purpose, since unlike Ti:sapphire they can be pumped directly with high-power diode lasers, leading to a significantly reduced complexity of the overall system. Compared to standard neodymium-doped materials (such as Nd:YAG or Nd:YLF), Yb:YAG exhibits a high efficiency, caused by both the small quantum defect (i.e., the energy difference between the pump and the laser photons) and the absence of other loss mechanisms, such as excited-state absorption and upconversion. Additionally, it provides a large gain bandwidth of more than 5 nm, which allows for the generation of ultrashort pulses as short as 350 fs. The thermal conductivity is very high, which, in conjunction with a fracture limit of almost 240 MPa (about four times that of Nd:YVO4 ), makes it particularly suitable for high-power femtosecond oscillators. As a consequence of all the properties described above, Yb:YAG allows the advantages of more elaborate schemes, such as the thin-disk design or the fiber laser, to be taken.

Ultrashort Pulse Lasers and Amplifiers Based on Nd:YVO4

19

H ig h p o w e r d io d e la s e r s

F r e q u e n c y - d o u b le d N d :Y A G la s e r

fs T i:S a p p h ir e O s c illa to r

p s N d :Y V O O s c illa to r 4

H ig h p o w e r d io d e la s e r s

fs , n J p u ls e s

H ig h p o w e r d io d e la s e r s

P u ls e - s tr e tc h e r

p s , n J p u ls e s

n s , n J p u ls e s

F r e q u e n c y - d o u b le d N d :Y A G la s e r

T i:S a p p h ir e R G A

N d :Y V O R G A 4

H ig h p o w e r d io d e la s e r s

n s , µ J p u ls e s

P u ls e - c o m p r e s s o r

p s , m J p u ls e s

fs , µ J p u ls e s

O u tp u t (» 2 W , < 1 5 0 fs , < 5 k H z )

O u tp u t (» 1 0 W , 1 0 p s , < 1 0 0 k H z )

Fig. 1. Principal experimental setup of a regenerative amplifier system based on Ti:sapphire (left) and Nd:YVO4 (right). Most notably, the Nd:YVO4 system can be pumped directly with diodes and does not require CPA. The same holds for a similar setup incorporating an Yb:YAG thin-disk laser

Due to its even larger gain bandwidth, Yb:KGW on the other hand is especially interesting for the amplification of ultrashort laser pulses. It allows for the generation of femtosecond pulses directly from the amplifier, and has only recently been employed successfully in a commercial high-power regenerative amplifier system, which delivers 500 fs pulses with more than 4 W of average power at a repetition rate of 7 kHz [7]. An attractive alternative to Ti:sapphire for a certain range of applications that allow picosecond pulses are systems based on Nd:YVO4 , which has probably been one of the most investigated and analyzed laser materials in the last couple of years. Compared to other neodymium-doped materials, it shows the advantage of a very high gain in conjunction with a linewidth of about 1 nm, which allows for most simple, stable setups and pulse durations in the range of about 5 ps to 10 ps. The pulse duration of several picoseconds is a limit of the material, but, following recent investigations, which will be described in detail in a later chapter, represents an optimum for the machining of metals [8, 9]. Moreover, the choice of Nd:YVO4 as the laser medium for a regenerative amplifier system greatly simplifies the experimental setup. The principal difference between a system based on Ti:sapphire and one based on Nd:YVO4 is illustrated in Fig. 1. It can be seen immediately, that the complexity of

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the system can be reduced significantly when switching to Nd:YVO4 , since, contrary to Ti:sapphire, it can be be pumped directly with high-power diode lasers. Maybe even more important, the system is working in the picosecond regime and therefore does not require chirped-pulse amplification to avoid optical damage of the amplifier’s components. This leads to further simplification and enhanced mechanical stability of the system, as the pulse-stretching and -compression units become redundant.

1

Diode-Pumped Ultrashort Pulse Laser Oscillators

In the following section, current progress in the development of end-pumped ultrafast high-power Nd:YVO4 and Yb:YAG oscillators will be discussed. These lasers are of special interest as seed oscillators for regenerative amplification of the pulse energy to the millijoule level, e.g. for applications such as micromachining. Furthermore, they can be power amplified and nonlinear frequency converted to the visible spectral range for display and projection applications. 1.1

SESAM Mode Locked Picosecond Nd:YVO4 Oscillators

One of the most reliable and well-proven systems for the generation of picosecond laser pulses is a diode-pumped Nd:YVO4 laser, mode locked with a semiconductor saturable absorber mirror (SESAM). The principal setup for such a system is shown in Fig. 2. A Nd:YVO4 crystal with typical dimensions of 4 mm × 4 mm × 4 mm, doped with 0.5–1.0 at. % Nd, is pumped by a single, fiber-coupled diode laser at a wavelength of 809 nm, providing about 10 W of total pump power. To broaden the spectrum and consequently shorten the pulse duration, the crystal is placed very close to one of the cavity mirrors to exploit spatial hole burning [11, 12]. The SESAM is set at one end of the cavity and in front of a curved mirror, which allows adjustment of the spot size on the absorber. An output coupler with typical transmissions of about 8%–15% at the laser wavelength of 1064 nm forms the other end of the cavity. O u tp u t c o u p le r

c u rv e d m ir r o r s fib e r - c o u p le d d io d e la s e r

N d :Y V O c ry s ta l 4

S E S A M

Fig. 2. Schematic setup of a diode-pumped Nd:YVO4 oscillator, passively mode locked with a SESAM [10]

Ultrashort Pulse Lasers and Amplifiers Based on Nd:YVO4

21

Systems similar to this nowadays readily provide average output powers exceeding 4 W in a diffraction-limited beam. The duration of the generated pulses can be as short as 6 ps with a repetition rate of about 80 MHz. The simple setup and the small number of optical elements needed allows rugged systems to be built with a high degree of reproducibility and excellent stability. They can be employed as seed oscillators for regenerative amplification, which is, in fact, one of the main applications for this system. Since the first realization of a semiconductor device as a saturable absorber, substantial progress has been made in the development of novel, sophisticated SESAM designs with higher damage threshold and optimized properties. The power scalability of the laser concept introduced above was also demonstrated recently. In a very simple setup, a Nd:YVO4 crystal was end-pumped from one side by a fiber-coupled diode laser with a total pump power of about 35 W. Crystal dimensions, doping concentration, pump wavelength and spot size were adapted in order to keep the stress below the fracture threshold of the crystal. Furthermore, the spot size on the saturable absorber was adjusted to compensate for the higher intracavity power of the system. With the modifications and adjustments described above, the setup generated a maximum output power of 16 W at a repetition rate of 80 MHz. Figure 3 shows the autocorrelation trace and the optical spectrum of the system. Assuming a sech2 pulse shape, a pulse duration of 11.6 ps was determined. The total spectral width was 41 GHz, measured using a scanning Fabry–P´erot interferometer. 1.2 Dn = 41 GHz

Dt = 11.6 ps 1.0

Intensity [ n. U. ]

0.8 0.6 0.4 0.2 0.0 -80 -60 -40 -20

0

20

Delay [ ps ]

40

60

80 -150 -100 -50 0 50 100 Relative Frequency [ GHz ]

150

Fig. 3. Left: Autocorrelation trace of the high-power Nd:YVO4 oscillator, showing a pulsewidth of 11.6 ps, assuming a sech2 pulse shape. Right: Corresponding optical spectrum, measured with a scanning Fabry–P´erot interferometer and giving a spectral width of 41 GHz

22

Jochen Kleinbauer et al. O u tp u t c o u p le r

O u tp u t L a s e r c ry s ta l P u m p lig h t

P u m p lig h t

O u tp u t c o u p le r

O p tic a l F ib e r

H R

Fig. 4. Schematic setup of a phase self-adjusting mode locked (PSM) diode-pumped ultrashort pulse laser

Similar results with output powers of about 20 W, albeit with pulse durations above 20 ps, have been reported earlier and can be found in the literature [13, 14]. 1.2 PSM Mode Locked Picosecond Nd:YVO4 and Femtosecond Yb:YAG Oscillators In the last section, low- and high-power Nd:YVO4 lasers mode locked with semiconductor saturable absorbers have been introduced. This mode locking scheme seems to be very attractive, both because of the simple setup, allowing for compact, reliable systems, and because of the high degree of stability these lasers provide. However, mode locking with a semiconductor saturable absorber can also introduce problems, since the mode locked system becomes susceptible to Q-switched mode locking. This is especially true for laser materials with low gain or with a long lifetime of the upper laser level. As a consequence, laser resonators with long cavity roundtrip times have to be employed to prevent the respective system from Q-switching [15, 16]. This makes the systems vulnerable to mechanical and environmental changes and perturbations. One way around this problem is to employ a different mode locking scheme that does not exhibit this limitation. A promising candidate, which fulfills this requirement, is additive pulse mode locking (APM). APM, on the other hand, relies on the interferometric length stabilization of the coupled cavities, as shown in a previous section. However, this has turned out not to be true for phase self-adjusting mode locking (PSM), a new technique and a variation of APM, that has recently been developed [17]. The basic setup of a system employing PSM is shown in Fig. 4. The main cavity contains the end-pumped gain medium, curved mirrors to compensate for the thermal lens induced inside the crystal and two output couplers. A second cavity of the same length and including an optical fiber is set up behind one of the output couplers of the main cavity. Stable mode locking is observed when the optical lengths of the two cavities are properly matched. As opposed to APM, there is no requirement for active length stabilization of the two coupled cavities and to maintain CW mode locking. Additionally,

Ultrashort Pulse Lasers and Amplifiers Based on Nd:YVO4

23

1.2 Dl = 2.07 nm

Dt = 578 fs 1.0

Intensity [ n. U. ]

0.8 0.6 0.4 0.2 0.0 -3

-2

-1

0

Delay [ ps ]

1

2

1028

1030 1032 Wavelength [ nm ]

1034

Fig. 5. Left: Autocorrelation trace of the high-power Yb:YAG oscillator, showing a pulsewidth of 578 fs, assuming a sech2 pulse shape. Right: Corresponding optical spectrum, giving a total width of 2.07 nm

there is no upper limit for the repetition rate and no increased tendency for Q-switched mode locking could be observed for shorter cavities. Nd:YVO4 lasers mode locked with PSM have been studied intensively in the last couple of years and are now commercially available [18]. With a total pump power of about 50 W, these systems generate a typical output power of 18 W with a pulse duration of about 9 ps. With a repetition-rate of 160 MHz, a compact and efficient system is available for the generation of high-power picosecond radiation. The same mode locking scheme has also been applied to a different gain medium, Yb:YAG. With its low gain (compared to Nd:YVO4 ) and its long upper-state lifetime of about 1 µs, mode locked lasers based on Yb:YAG are generally prone to Q-switching, requiring long cavities when using semiconductor saturable absorbers. On applying PSM to an Yb:YAG oscillator containing two laser crystals pumped by fiber-coupled diode lasers, it was possible to generate a maximum output power of 7.2 W at a repetiton rate of 83 MHz. To compensate for the dispersion of the laser crystal, Gires–Tournois interferometer mirrors were included in the cavity, resulting in a pulse duration of 520 fs. Stable mode locking was maintained without any active synchronization of the cavity lengths [19]. The main advantage of using the PSM mode locking scheme is the scalability of the output power and the repetition rate. The potential of this technique can be seen in the example of a high-power Yb:YAG laser with a repetition rate of as high as 124 MHz. With a single fiber-coupled diode, the crystal was end-pumped to a spot-size of 600 µm. In two successive passes through the gain medium, 84 W of total pump power were absorbed, leading to a maximum mode locked output power of 21.2 W in a diffraction-limited beam. Figure 5 shows the autocorrelation trace and the optical spectrum of

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the system. Assuming a sech2 pulse shape, a pulse duration of 578 fs was measured. The optical spectrum has a width of about 2.1 nm, indicating almost Fourier-limited pulses with a time–bandwidth product of 0.34 [20].

2

Nd:YVO4 Regenerative Amplifiers

It has been shown in the introduction to this chapter that powerful sources of ultrafast infrared radiation are required for a number of applications, like nonlinear processes (e.g., the generation of higher harmonics) or micromachining. An established method for the generation of optical pulses with megawatts of peak power and pulse durations in the picosecond and femtosecond regime is regenerative amplification. The basic setup and principle of operation of a regenerative amplifier system will be described in detail in a later chapter. In principle, a mode locked oscillator provides a continuous train of picosecond or femtosecond pulses at high repetition rates. The pulse energy usually is in the range of several nJ and is intended to be amplified to the microjoule or millijoule regime. A pulse-picker unit, basically a very fast optical switch, is cutting out single pulses from the megahertz pulse train of the oscillator, thus lowering the repetition rate of the signal to that of the pulse picker. Until very recently, this repetition rate was limited to about 5 kHz by the availability of fast and efficient electro-optical switches, i.e., Pockels cells. However, ongoing investigations and substantial research of new techniques have led to the development of new Pockels cell drivers, which now allow for repetition rates well above 50 kHz at voltages to drive standard BBO Pockels cells. A typical switching sequence of a regenerative amplifier is illustrated in Fig. 6. A single optical pulse from the pulse picker is injected into the amplifier’s cavity through a thin-film polariser (TFP). The voltage of the Pockels cell initially is switched off, so two passes of the quarterwave plate cause the polarisation to turn by 90 degrees. When the voltage is switched on, the Pockels cell compensates the effect of the quarterwave plate, thus trapping the pulse inside the amplifier for a specified number of roundtrips through the gain medium. When the pulse energy is saturated, the voltage on the Pockels cell is switched off again, the pulse is ejected out of the amplifier and the gain regenerates until the next pulse is injected. Figure 7 shows the experimental setup of a regenerative amplifier system, which has been realized for applications in micromachining and material processing [21]. The setup basically consists of three main parts: A mode locked seed oscillator, a pulse picker and the regenerative amplifier. The seed oscillator is a diode-pumped Nd:YVO4 laser, CW mode locked by a semiconductor saturable absorber mirror (SESAM). It is pumped with a power of 10 W, provided by a single fiber-coupled diode, and generates a maximum output power of 4.2 W at a repetition rate of 82 MHz.

Ultrashort Pulse Lasers and Amplifiers Based on Nd:YVO4 P u ls e - T r a in fr o m th e P u ls - P ic k e r

H R 0 °

H R 0 °

l / 4 G a in

T F P

G a in

P C

R e g e n e r a tiv e A m p lifie r = H R -R e s o n a to r + G a in + fa s t o p tic a l s w itc h

P C

N o v o lta g e a p p lie d : A s in g le p u ls e is in je c te d in to th e a m p lifie r

V = 0

G a in

l /4 - V o lta g e a p p lie d : T h e tr a p p e d p u ls e is m a k in g a n u m b e r o f r o u n d tr ip s .

P C V = l /4

G a in

25

V o lta g e is s w itc h e d o ff: T h e a m p lifie d p u ls e is e je c te d a fte r th e n e x t r o u n d tr ip .

P C V = 0

Fig. 6. Typical switching sequence of a regenerative amplifier k H z - P u ls e T r a in

P u ls e P ic k e r

M H z - P u ls e T r a in

M o d e lo c k e d S e e d - O s c illa to r

T F P

O u tp u t F a ra d a y R o ta to r H W P R e g e n e r a tiv e A m p lifie r

P u m p lig h t T F P N d :Y V O

Q W P B B O P o c k e ls - C e ll

4

C ry s ta l

P u m p lig h t

Fig. 7. Experimental setup of the Nd:YVO4 regenerative amplifier system

The subsequent pulse picker comprises a BBO Pockels cell with a clear aperture of 3 mm and a polariser and reduces the repetition rate to a maximum of 20 kHz. A combination of a thin-film polariser (TFP), a Faraday rotator and a half-wave plate (HWP) separates the input from the amplified output beam. The kilohertz pulse train from the pulse picker is then injected into the amplifier through a polariser. The regenerative amplifier consists of a cavity of high-reflective mirrors, containing the gain medium and a BBO Pockels cell with a clear aperture of

Jochen Kleinbauer et al.

11

0.65

10

0.60

9

0.55

12

14

16

18

Pulse-Energy [ mJ ]

Output-Power [ W ]

26

20

Repetition-Rate [ kHz ]

Fig. 8. Output power (filled squares) and pulse energy (open squares) of the regenerative amplifier system

4 mm for fast optical switching. The laser crystal is a 4 mm × 4 mm × 12 mm Nd:YVO4 , end pumped from both sides with a total pump power of 42 W by fiber-coupled diode lasers. The time required for switching the Pockels cell to the quarterwave state and to trap the seed pulse inside the amplifier is about 22 ns. This time required a total optical length of the amplifier of about 3.3 m. Figure 8 shows the average output power and the pulse energy of the generated radiation against the repetition rate. It was varied between 12 kHz and 20 kHz, which was the upper limit for the high-voltage Pockels cell drivers that were used. At this repetition rate, a maximum output power of 10.8 W and a pulse energy exceeding 0.5 mJ were obtained, which corresponds to a maximum peak power of about 53 MW. A typical oscilloscope trace of the output signal, measured with a fast photodiode, is displayed in the left part of Fig. 9. It shows a clean signal without noticeable prepulses or postpulses, which were to be expected 22 ns before and after the main pulse when the cavity roundtrip time is shorter than the rise and fall time of the electro-optic switch. The center of Fig. 9 shows the cavity roundtrip signal, i.e., the signal measured behind one of the cavity’s mirrors. The signal shows the build-up of the amplified pulse during each roundtrip inside the cavity. After only seven roundtrips (i.e., fourteen passes through the gain medium), the amplified pulse is ejected. The small postpulses after each roundtrip are due to an echo from the diagnostic electronics and do not originate from the detected signal. In the right part of the figure, 50 output pulses of the system were measured in a sequence with high resolution. It can be seen that the amplifier is showing an excellent pulse-to-pulse stability with typical standard deviations well below 5%. During the process of regenerative amplification, the pulse duration typically broadens due to gain narrowing. This effect can be seen by measuring

Ultrashort Pulse Lasers and Amplifiers Based on Nd:YVO4

27

Fig. 9. Typical oscilloscope trace of the amplifier output (left), cavity roundtrip signal (middle) and a sequence of 50 output pulses (right) measured with a fast photo diode

the pulse duration of the seed and the output pulses, as well as their optical spectra. Figure 10 shows autocorrelation traces and optical spectra of the seed oscillator and the regenerative amplifier. The pulse duration has been measured by intensity autocorrelation, where a certain pulse shape has to be assumed to determine the actual pulse duration, since there is no way of knowing the phase of the pulse at a given time. When assuming a sech2 pulse shape, the measured pulse duration of the input pulses is 6.6 ps, whereas the pulse duration of the amplified pulses is about 10.2 ps. The temporal broadening observed here is due to the aforementioned effect of gain narrowing. At the same time, the optical spectrum of the amplified pulses is also broadened from about 90 GHz to 150 GHz. This spectral broadening and also the distinct modulation of the spectrum of the amplified pulses are a consequence of self-phase modulation (SPM) due to the nonlinear refractive index and the high peak intensities in both the Pockels cell and the laser crystal. A common problem of most Ti:sapphire regenerative amplifier systems is the spatial quality of the amplified beam. The transversal beam profile relies sensitively on the exact alignment of the optical gratings in the pulsecompression unit. Slight misalignments and temperature drifts induce spatial distortions and result in a poor beam quality. The concept of the Nd:YVO4 amplifier system presented here does not imply any such restrictions, since chirped-pulse amplification (CPA) and the usage of gratings is not required at picosecond pulse durations. Accordingly, the spatial beam quality of this system is close to the diffraction limit with a measured value of M2 below 1.2. 2.1

Additional Amplification in a Power Amplifier

For various applications, the average output power and pulse energy generated by the regenerative amplifier system might not be sufficient. The output

28

Jochen Kleinbauer et al. 1.2 Dn = 89 GHz

Dt = 6.6 ps 1.0

Intensity [ n. u. ]

0.8 0.6 0.4 0.2 0.0 -20

-10

0 10 Delay [ ps ]

20

-100

-50 0 50 100 Relative Frequency [ GHz ]

1.2 Dt = 10.2 ps

Dn = 150 GHz

1.0

Intensity [ n. u. ]

0.8 0.6 0.4 0.2 0.0 -40

-20

0 Delay [ ps ]

20

40 -200

-100

0

100

Relative Frequency [ GHz ]

Fig. 10. Top: Autocorrelation trace (left) and optical spectrum (right) of the seed pulses. Bottom: Autocorrelation trace (left) and optical spectrum (right) of the amplified pulses

power of the system is limited by both the maximum pump power possible without fracture of the laser crystal and by the damage threshold of intracavity optical elements, especially the Pockels cell. One possible solution to overcome this limitation and to generate even higher pulse energies is the setup of an additional power amplifier behind the regenerative amplifier. Figure 11 shows the basic setup of a simple additional single-pass amplifier. It consists of another 4 mm × 4 mm × 12 mm Nd:YVO4 crystal, endpumped from both sides by fiber-coupled diode lasers, providing 50 W of total pump power. The output beam of the regenerative amplifier is focused into the laser crystal to match the pump spot size of the laser diodes.

Ultrashort Pulse Lasers and Amplifiers Based on Nd:YVO4

F ro m r e g e n e r a tiv e a m p lifie r

S in g le - P a s s A m p lifie r

P u m p lig h t

L e n s

29

N d :Y V O 4

C ry s ta l

L e n s O u tp u t

P u m p lig h t

Fig. 11. Experimental setup of a simple single-pass power amplifier to increase the infrared output power of the regenerative amplifier system

24 nrep = 20 kHz Ppump = 50 W

Output-Power [ W ]

20 16 12 8 4 0

0

2

4

6

8

Input-Power [ W ]

10

Fig. 12. Output power of the single-pass power amplifier against input power from the regenerative amplifier. At a repetition rate of 12 20 kHz, a maximum output power of 20.3 W was obtained

At a repetition rate of 20 kHz and a maximum input power of 10.4 W, the single-pass amplifier generated an output power of 20.3 W. This corresponds to a pulse energy of more than 1 mJ. The output power of the amplifier in dependence of the input power provided by the regenerative amplifier is shown in Fig. 12. 2.2

Nonlinear Frequency Conversion in the Visible and the UV

A variety of applications for high-energy ultrashort laser pulses require radiation at shorter wavelengths than the fundamental of the Nd- or Yb-doped laser materials of about 1 µm. One way to generate high-energy picosecond pulses in the visible and the UV spectral range is the method of nonlinear frequency conversion, e.g., frequency doubling. Figure 13 shows a straightforward setup for frequency conversion of the output of a Nd:YVO4 regenerative amplifier system operating at a wavelength of 1064 nm. To generate the second harmonic, an LBO crystal with a length of 5 mm is placed in the output beam of the regenerative amplifier. The crystal was AR coated for both the fundamental and the second harmonic and critically phase matched at room temperature (φ = 11.3◦ , θ = 90◦ ). The second har-

30

Jochen Kleinbauer et al. H R 5 3 2 / H T 2 6 6

H R 1 0 6 4 / H T 5 3 2

F ro m r e g e n e r a tiv e a m p lifie r 1 0 6 4 n m

2 6 6 n m (U V )

B B O

L B O

1 0 6 4 n m (IR )

5 3 2 n m (V IS )

Fig. 13. Experimental setup for the generation of radiation at the second and the fourth harmonic of 1064 nm

SHG Pulse Energy @ 532 nm [ µJ ]

300 250

nrep = 20 kHz LBO, 5 mm

200 150 100 50 0 0.0

0.1

0.2

0.3

0.4

0.5

Fundamental Pulse Energy @ 1064 nm [ mJ ]

0.6

Fig. 14. Pulse energy of the second-harmonic green radiation at 532 nm versus the input fundamental pulse energy

monic at 532 nm is separated from the remaining fundamental power using a dichroic mirror. Similarly, the fourth harmonic in the UV was generated by frequency doubling the second-harmonic radiation in a BBO crystal with a length of 3 mm. This crystal was also AR coated and critically phase matched at room temperature (θ = 47◦ ). The fourth harmonic at 266 nm was then separated from the remaining green radiation with another beamsplitter. Figure 14 shows the output pulse energy of the 532 nm second harmonic in dependence of the energy of the fundamental 1064 nm laser pulses. The maximum obtained output pulse energy at a repetition rate of 20 kHz is 270 µJ, corresponding to an output power of 5.4 W. The conversion efficiency obtained is about 50%, which is in good agreement with an estimation based on the Boyd–Kleinman model, which predicted a maximum of about 60%. This also indicates the good spatial and temporal quality of the generated laser pulses. In the UV, a maximum output pulse energy of 75 µJ was generated at 266 nm, corresponding to an average output power of 1.5 W. A maximum conversion efficiency of about 28% has been achieved in this process.

Ultrashort Pulse Lasers and Amplifiers Based on Nd:YVO4

31

600 14

12

400

10

300

200

Pulse-Energy [ mJ ]

Output-Power [ W ]

500

8 0

20

40

60

80

100

100

Repetition-Rate [ kHz ]

Fig. 15. Output power (filled squares) and pulse energy (open squares) of the 100 kHz regenerative amplifier system

2.3

Scaling to Higher Repetition Rates

During the intensive investigations of recent years in the field of material processing with lasers, it has become obvious, that not only higher average powers but also higher repetition rates are desirable in order to minimize processing times. This is especially true when considering the cost effectiveness in industrial applications. Novel electro-optical modulators (Pockels cells) are now offering faster rise times, higher repetition rates and alternative crystal materials with significantly lower switching voltages (e.g. RTP). Their availability made it possible to further reduce the complexity and minimize the setup, due to the relaxed constraints regarding cavity lengths. A maximum repetition rate of up to 100 kHz has recently been demonstrated with a high-power Nd:YVO4 regenerative laser amplifier system [22]. Figure 15 shows the average output power and the pulse energy of the system as a function of the repetition rate. The highest pulse energy achieved is about 0.5 mJ at a repetition rate of 15 kHz, while a maximum output power of 14 W is generated at 100 kHz. The measured width of the amplified pulses is 13 ps.

References [1] P. M. W. French: The generation of ultrashort laser pulses, Rep. Prog. Phys. 58, 169–267 (1995) 18 [2] J. D. Kafka, M. L. Watts, J. J. Pierterse: Picosecond and femtosecond pulse generation in a regeneratively modelocked Ti:sapphire laser, IEEE J. Quantum Elect. 28, 2151 (1992) 18

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[3] S. Takeuchi, T. Kobayashi: Highly efficient Ti:sapphire regenerative amplifier, Opt. Commun. 109, 518 (1994) 18 [4] S. Sartania, Z. Cheng, M. Lenzner, G. Tempea, C. Spielmann, F. Krausz, F. Ferencz: Generation of 0.1-TW 5-fs optical pulses at a 1-kHz repetition rate, Opt. Lett. 22, 1562–1564 (1997) 18 [5] Femtolasers Produktions GmbH, Fernkorngasse 10, 1100 Vienna, Austria: Model Femtopower Compact Pro URL http://www.femtolasers.com 18 [6] N. Zhavoronkov, G. Korn: Regenerative amplification of femtosecond laser pulses in Ti:sapphire at multikilohertz repetition rates, Opt. Lett. 29, 198–200 (2004) 18 [7] Spectra Physics, 1335 Terra Bella Avenue, Mountain View, CA 94039, USA: Model Eclipse URL http://www.spectra-physics.com 19 [8] F. Dausinger, H. H¨ ugel, V. Konov: Micromachining with ultrashort laser pulses, from basic understanding to technical application, in (Int. Conf. on Advanced Laser Technologies 2002) pp. 15–20 19 [9] F. Dausinger: Machining of metals with ultrashort laser pulses: Fundamental aspects and their consequences, in Europhys. Conf. (Abstracts) 27E (2003) 19 [10] Spectra-Physics Lasers, United States Patent 5,812,308: Mode locked laser and amplifier (1998) 20 [11] C. J. Flood, D. R. Walker, H. M. van Driel: Effect of spatial hole burning in a mode-locked diode end-pumped Nd:YAG laser, Opt. Lett. 20, 58–60 (1995) 20 [12] B. Braun, K. J. Weingarten, F. X. K¨ artner, U. Keller: Continuous-wave modelocked solid-state lasers with enhanced spatial hole burning, Appl. Phys. BLasers O. 61, 429–437 (1995) 20 [13] D. Burns, M. Hetterich, A. I. Ferguson, E. Bente, M. D. Dawson, J. I. Davies, S. W. Bland: High-average-power (> 20W) Nd:YVO4 lasers mode locked by strain-compensated saturable Bragg reflectors, J. Opt. Soc. Am. B 17, 919–926 (2000) 22 [14] Y. F. Chen, S. W. Tsai, Y. P. Lan, S. C. Wang, K. F. Huang: Diode-end-pumped passively mode-locked high-power Nd:YVO4 laser with a relaxed saturable Bragg reflector, Opt. Lett. 26, 199–2001 (2001) 22 [15] F. Brunner, T. S¨ udmeyer, E. Innerhofer, F. Morier-Genoud, R. Paschotta, V. E. Kisel, V. G. Shcherbitsky, N. V. Kuleshov, J. Gao, K. Contag, A. Giesen, U. Keller: 240-fs pulses with 22-W average power from a mode-locked thin-disk Yb:KY(WO4 )2 laser, Opt. Lett. 27, 1162–1164 (2002) 22 [16] E. Innerhofer, T. S¨ udmeyer, F. Brunner, R. H¨ aring, A. Aschwanden, R. Paschotta, C. H¨ onninger, M. Kumkar, U. Keller: 60-W average power in 810-fs pulses from a thin-disk Yb:YAG laser, Opt. Lett. 28, 367–369 (2003) 22 [17] B. Henrich: Neuartige Lasersysteme zur Erzeugung ultrakurzer Lichtimpulse hoher mittlerer Leistung, Dissertation, Universit¨ at Kaiserslautern (2000) 22 [18] Lumera Laser GmbH, Opelstraße 10, D-67661 Kaiserslautern: Model UPL-20 URL http://www.lumera-laser.de 23 [19] S. Reuter, J. Kleinbauer, R. Knappe, R. Walkenstein, B. Henrich: High average power 80 MHz repetition rate additive pulse mode-locked femtosecond Yb:YAG laser, in OSA Tech. Dig. Ser. CWA 42 (Opt. Soc. Am. 2002) p. 343 23

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[20] M. Weitz, S. Reuter, R. Knappe, B. Henrich, R. Wallenstein: Passive modelocked 21 W femtosecond Yb:YAG laser with 124 MHz repetition-rate, in OSA Tech. Dig. Ser. (Opt. Soc. Am. 2004) 24 [21] J. Kleinbauer, R. Reuter, R. Knappe, R. Wallenstein: High power, high repetition-rate picosecond Nd:YVO4 regenerative amplifier, in Tech. Dig. CA3-2-TUE (Conf. Lasers and Electrooptics 2003) 24 [22] R. Knappe, B. Henrich, T. Herrmann, A. Nebel: High power Nd:YVO4 regenerative laser amplifier with 100 kHz repetition-rate, in Tech. Dig. CP23-THU (CLEO 2003) postdeadline paper 31

Index

additive pulse mode locking (APM), 22 amplifier, 17 BBO Pockels cell, 25 bulk crystal, 17 chirped-pulse amplification, 18, 27 CPA, 18, 27 electro-optical modulator, 31 fiber laser, 17, 18 frequency conversion, 29 frequency doubling, 29 gain narrowing, 26, 27 intensity autocorrelation, 27

phase self-adjusting mode locking (PSM), 22 Pockels cell, 24, 31 pulse picker, 24, 25 regenerative amplifier, 24, 25 RTP, 31 seed oscillator, 20 self-phase modulation (SPM), 27 SESAM, 20, 21 spatial hole burning, 20 thin-disk, 17, 18 Ti:sapphire, 17–19, 27 Ti:sapphire regenerative amplifier, 18

material processing, 24, 31 micromachining, 17, 24

ultrashort pulse laser, 17 ultrashort pulse laser oscillator, 20

Nd:YVO4 , 17, 19–21, 24 Nd:YVO4 oscillator, 20, 22 neodymium-doped material, 19 nonlinear frequency conversion, 29

Yb-doped material, 18 Yb:KGW, 18, 19 Yb:YAG, 17, 18, 23 Yb:YAG oscillator, 22

Ultrashort Pulse Fiber Lasers and Amplifiers Andreas T¨ unnermann, Jens Limpert, and Stefan Nolte Institut f¨ ur Angewandte Physik, Friedrich-Schiller-Universit¨ at Jena, Max-Wien-Platz 1, 07743 Jena, Germany [email protected] Abstract. Reliable turn-key high-power ultrafast laser sources are required for a variety of applications in science, industry, and medicine. Fiber-based laser systems have the potential to fulfill this requirement. In this Chapter the possibilities of rare-earth doped fibers for the amplification of ultrashort pulses are discussed. Novel concepts to overcome limitations due to nonlinear effects are presented.

Micromachining with ultrafast lasers has grown into an intensive research topic during the past ten years. Various applications have been explored and several advantages of this technology have been demonstrated, however, for the majority of real-world applications the achieved process throughput is still too low. Thus, one key element for a successful implementation of ultrafast laser technology in an industrial environment is a significant power increase of the laser sources. Fiber-based laser systems are appropriate candidates for power scaling since they are, in general, immune against any thermo-optical problems due to their geometry. The excellent heat dissipation is due to the large ratio of surface-to-active volume. In addition, the beam quality of the guided mode is determined by the fiber-core design and is therefore power independent. Moreover, due to the confinement of both the laser and pump radiation, a high intensity is maintained over the entire fiber length. As a consequence the product of pump-light intensity and interaction length with the laser radiation in the gain medium, which determines the gain, can be orders of magnitude higher in fibers [1] than in other bulk solid-state lasers. This leads to a highly efficient operation of fiber-laser systems, with very high gain and low pump threshold values. Additionally, the complete integration of the laser process leads to the inherent compactness and long-term stability of fiber lasers, because no components are necessary in a long free-space cavity. Ytterbium-doped fiber-laser systems are especially interesting for highpower ultrashort pulse generation and amplification. The fundamental requirement for broad-bandwidth short-pulse amplification, a broad emission spectrum, is fulfilled. In ytterbium-doped glass fibers the amplification bandwidth is approximately 40 nm [2, 3], which supports pulses of durations as low as 30 fs. Due to the low quantum defect level of less than 10% Yb-doped fibers can provide optical-to-optical efficiencies well above 80% [4] and have an inherent low thermal load. Moreover, excited-state absorption of pump F. Dausinger, F. Lichtner, H. Lubatschowski (Eds.): Femtosecond Technology for Technical and Medical Applications, Topics Appl. Phys. 96, 35–54 (2004) c Springer-Verlag Berlin Heidelberg 2004


36 pump light

Andreas T¨ unnermann et al. active core

inner cladding

refractive index profile

laser radiation

outer cladding

n

Fig. 1. The double-clad fiber concept

or signal radiation, or concentration quenching by ion–ion energy-transfer processes do not occur with ytterbium, because only two-level manifolds are relevant for all optical wavelengths. To generate very high powers the double-clad fiber design, invented in 1988 by Snitzer et al. [5], has to be used. Here, the active doped core is surrounded by a second waveguide, which is highly multimode, shown in Fig. 1. In this second waveguide, also called the inner cladding or pump core, low-brightness high-power diode lasers can be launched. This pump light is gradually absorbed over the entire fiber length and is converted into highbrightness high-power laser radiation. Thus, the double-clad concept can be used as a highly efficient brightness improvement using the laser process in rare-earth ions. One drawback of the double-clad fiber concept is a reduced pump-light absorption due to inner cladding modes that have no overlap with the doped core. This can be overcome by breaking the cylindrical symmetry of the inner cladding. Geometries such as D-shaped or rectangular pump cores prevent the propagation of these undesired intensity distributions of the pump radiation by permanent mode mixing. Alternatively, the absorption of pump radiation can be enhanced in symmetrical fibers using the technique of periodic bending of the fiber, e.g. in a kidney-shaped arrangement [6]. Taking all this into account rare-earth-doped fibers are superior to other solid-state laser concepts in a variety of performance categories. This has become obvious following several recent demonstrations of kilowatt-class continuous-wave fiber-laser systems with excellent beam quality [7]. However, the attributes that make fiber-based laser systems so attractive – the confinement of the laser radiation and the long interaction length – constitute the principal limitations of high peak power pulsed fiber-laser systems, the nonlinear effects. Nonlinearity can lead to severe pulse distortions and even to damage of the fiber. Since SiO2 is a symmetric molecule, contributions of the second-order susceptibility vanish for silica glass fibers. Thus, the lowest-order nonlinear effect in optical fibers originates from the third-order susceptibility χ(3)

Ultrashort Pulse Fiber Lasers and Amplifiers

37

and is responsible for an intensity-dependent refractive index in the form n ˜ = n + n2I. Consequently, an optical field propagating through a fiber experiences a self-induced phase shift, a phenomenon referred to as self-phase modulation (SPM). A second important class of nonlinear effects results from stimulated inelastic scattering processes, whereby the radiation transfers a part of its energy to the glass host in the form of excited vibrational modes. Two phenomena known as stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS) fall into this category. Both manifest themselves as a significant power-loss mechanism in fiber-based laser systems. Since the spectra of ultrashort laser pulses are significantly broader than the Brillouin gain bandwidth, the Brillouin gain is significantly reduced. As a consequence, the effect of stimulated Brillouin scattering (SBS) can be neglected in ultrashort pulse fiber systems. In general, the nonlinearity coefficients in glass fibers are intrinsically small. Both the nonlinear index coefficient n2 and the gain coefficients of SRS (and SBS) are at least two orders of magnitude smaller than in other common nonlinear media [8]. Nevertheless, due to the large product of intensity and interaction length inside the fiber core nonlinearity can be observed at very low power levels that basically limits the performance of rare-earth-doped fiber systems in ultrashort pulse operation. To describe the basic effects during the propagation of an optical pulse in a single-mode fiber the nonlinear Schr¨odinger equation (NLSE) can be used. In a frame of reference (the so-called retarded time frame T ) this equation can be written as
∞   in−1 ∂ n α ∂A 2 + βn (1) A + A = iγ |A| A , n ∂z n! ∂T 2 n=2 where βn is the n-th derivative of the mode-propagation constant at ω = ω0 , α is the gain or loss coefficient and the pulse amplitude A(z, t) is assumed to be normalized such that |A|2 represents the optical power. The nonlinearity coefficient γ is defined by n2 ω 0 γ= , (2) c Aeff with Aeff as the effective mode-field area. Phenomena that are included in (1) are dispersion, loss and self-phase modulation as the lowest-order nonlinear effect. In many nonlinear media spontaneous Raman scattering converts a small fraction of the input power to another optical beam at a frequency downshifted by an amount determined by the vibrational modes of the medium. This process is called the Raman effect [9] and can be described quantum mechanically as an inelastic scattering process. An incident photon (pump wave) is annihilated to create a photon at a downshifted frequency, the Stokes wave, and a phonon with the appropriate energy and momentum. Also, a frequencyupshifted photon, the so-called anti-Stokes wave, can be created if a phonon

38

Andreas T¨ unnermann et al.

with the corresponding energy and momentum is available. The evolution of the peak powers of the pump (PP ) and Stokes (PS ) wave can be described by the following coupled differential equations ωP ∂PP = − gR PS PP + gP PP , ∂z ωS ∂PS = gR PS PP , ∂z

(3) (4)

where ωP and ωS are the frequencies of the pump and Stokes field, respectively, and gR is the peak Raman gain. The numerical integration of these equations with arbitrarily chosen parameters indicates that after an exponential growth of the laser power a rapid energy transfer to the Stokes wave occurs at a certain threshold value. Due to the mentioned nonlinear distortions, the generation of high peak powers in pulsed fiber-laser systems is very challenging. Novel fiber designs and experimental strategies have to be applied to scale the output parameters.

1 The Low Numerical Aperture Large Mode Area Concept Basically, as discussed above, the nonlinear effects are proportional to the fiber length and the intensity in the fiber core, and therefore inversely proportional to the mode-field area of the guided radiation in the fiber. Thus, an enlargement of the mode-field diameter and a reduction of fiber length help to reduce disturbing nonlinear effects. Using special techniques and fiber designs the mode-field area of fiber devices in a single-transverse mode has been significantly increased in the past years. One approach is to decrease the numerical aperture relative to a standard telecommunication value of about 0.16, which allows an increase in the core size, while single-mode operation is maintained [10]. However, a further reduction in the numerical aperture below a certain limit is not tolerable in terms of bending losses. Other fiber designs are based on modified index profiles, which increase the single-mode area by using an outer ring structure. Such large mode area fibers are demonstrated with core diameters up to several tens of micrometer and diffraction-limited output [11]. Preferential gain to the fundamental mode is created by an optimally overlapping rare-earth dopant distribution [12]. This concept can be extended to gain- and loss-managed multimode fibers to discriminate higher-order modes [13]. Stable fundamental mode propagation over more than 20 m at 1.5 µm wavelength is obtained in a conventional step-index core double-clad fiber with a core diameter of 45 µm with a numerical aperture of 0.13 [14]. Further discrimination of higher-order modes is achieved by a careful optimization of the seed-launching conditions and incorporated tapered sections [15] inside the fiber-laser or amplifier. Using

Ultrashort Pulse Fiber Lasers and Amplifiers

39

Fig. 2. Calculated bending losses in a 30 µm LMA fiber (NA = 0.06) for the first four transverse modes subject to the bending radius

these techniques the single-mode operation of a 50 µm core fiber amplifier at 1.06 µm is reported [16]. In this Chapter the concept of low numerical aperture large mode area (LMA) fibers is pursued for power scaling of rare-earth-doped fiber lasers and amplifiers. Such large mode area fibers possess core diameters in the range of 30 µm and a reduced numerical aperture of about 0.06. Using such LMA fibers the power density in the fiber core can be reduced by one order of magnitude compared to conventional single-mode fibers. Furthermore, the smaller ratio of active core area to pump core area of the double-clad fiber leads to an improved pump-light absorption and therefore to a reduction in the fiber length necessary for efficient amplification. Both effects allow for power scaling of ultrashort pulse fiber-based lasers and amplifiers. However, such large mode area fibers can guide several higher-order transverse modes. In order to achieve a stable fundamental-mode operation bending losses can be exploited [17]. Figure 2 shows the calculated bending losses, based on equations published in [18], of a 30 µm LMA fiber (NA = 0.06) for the fundamental and three higher-order modes as a function of the bending radius. This calculation reveals a significant discrimination of high-order transverse modes. As an example, at a bending radius of 50 mm the induced bending loss for the LP01 mode is 0.01 dB/m, while it already amounts to 52 dB/m for the first higher-order mode LP11 . This difference of approximately 5 orders of magnitude enforces fundamental-mode operation. Owing to the described nonlinear effects the direct amplification of femtosecond pulses seems to be a questionable approach. Nevertheless, the nonlinearity during the propagation of optical pulses in a high-power fiber ampli-

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fier can even be used to overcome this limitation. The combined interaction of normal dispersion, gain and nonlinearity (self-phase modulation) can create linearly chirped parabolic pulses, which resist optical wave breaking [19, 20]. The linear chirp can be removed using a grating compressor, resulting in high-power femtosecond pulses. Pursuing this approach of direct amplification of femtosecond pulses in fibers 10.2 W average power of 80 fs pulses has been demonstrated [21]. To further scale the average power or pulse energy the peak intensity in the fiber core has to be reduced. This can be done in the spatial and temporal domains, leading to an enlargement of the mode-field diameter of the fiber core or the sufficient stretching in the time domain, the so-called chirped-pulse amplification technique [22].

2

Fiber-Based Chirped-Pulse Amplification

Fiber-based chirped-pulse amplification systems are an outstanding candidate for practical turnkey femtosecond laser systems with high pulse energies and high average powers. The high single-pass gain of a rare-earth-doped fiber makes the application of regenerative or multipass amplification schemes unnecessary. Therefore, the use of Pockels cells, which are applied for pulse inlet and outlet in a regenerative amplifier and restrict the repetition rate to the kilohertz range, can be waived. In fiber-based CPA the upper limit of repetition rate is given by the repetition rate of the oscillator and the lower limit by the build-up time of amplified spontaneous emission (ASE) in the high-gain medium. The ASE build-up time in ytterbium-doped fibers is in the range of 0.4 ms [23], thus, repetition rates down to a few kilohertz are feasible in continuous-wave pumped systems. Over the last decade the pulse energy and average power of fiber-based CPA has been increased by orders of magnitude. Starting at 1.5 µm (the wavelength region of erbium-doped fibers) pulse energies in the microjoule range and milliwatt average powers are generated, which constitutes approximately the limit achievable with single-mode fibers [24]. The introduction of cladding pumped fibers led to an increase of average output power into the watt level [25]. The increase in mode size of the fiber enabled the extraction of 10 µJ [26] up to 100 µJ [27] from erbium-ytterbium-codoped fiber CPA systems. Ytterbium-doped fibers are the more suitable gain medium for high-energy high-power short-pulse generation. This is primarily due to the broader amplification bandwidth, the higher optical pumping efficiencies, and the larger saturation fluence. Up to 100 µJ pulse energy of 220 fs pulses at kHz repetition rate and up to 5.5 W average power at 1 MHz are demonstrated using an ytterbium-doped fiber CPA system employing a 25 µm core fiber in the final stage of the system [28]. Recently, average powers well above 100 W at repetition rates of 75 MHz have been demonstrated from an Yb-doped fiber CPA system [29] as described in the following section.

Ultrashort Pulse Fiber Lasers and Amplifiers

Nd:glass oscillator

Yb-doped preamplifier

fiber stretcher

41

Transmission grating compressor

Yb-doped poweramplifier OI

OI LD

LD Output

Fig. 3. Schematic setup of the high average power fiber CPA system. OI: optical isolator, LD: laser diode

3 High Average Power Femtosecond Fiber CPA System For the amplification of ultrashort laser pulses to high average output powers pulses of a passively mode-locked, diode-pumped solid-state laser system are first stretched in a fiber stretcher, then amplified in ytterbium-doped fibers and finally compressed in a transmission grating compressor, as shown in Fig. 3. The femtosecond seed source is a passively mode-locked (SESAM) Nd:glass laser system producing up to 150 mW average power of 144 fs pulses at 75 MHz repetition rate and 1060 nm center wavelength [30]. These pulses are coupled into a 300 m long step-index single-mode fiber, which is applied as a dispersive delay line. Along with the stretching in the time domain the transmitted pulses exhibit a power-dependent spectral broadening due to self-phase modulation. The maximum acceptable spectral width is determined by the gain characteristics of the amplifier chain and the size of the applied diffraction gratings in the compressor stage. In our case the optimum has been a transmitted average power of 15 mW, where the pulses are stretched to 250 ps and possess a spectral width of 23 nm. The preamplifier is constructed using a 15 m long double-clad fiber, where the 10 µm core (NA = 0.08) is doped with 1000 ppm (mol) Yb2 O3 . The preamplifier boosts the few milliwatts out of the stretcher fiber up to 2 W average power. The spectral width and pulse duration are slightly reduced to 20 nm and 215 ps, respectively, due to the restricted transmission range of the optical isolator in front of the preamplifier and the gain distribution of the ytterbium-doped fiber. The power amplifier consists of a 13.5 m length large mode area fiber with a 28.5 µm diameter, 0.06 NA step-index ytterbium-doped core, surrounded by a 400 µm D-shaped inner cladding with NA = 0.38. The ytterbium doping concentration is 700 ppm (mol) Yb2 O3 . The core parameters lead to a calculated mode-field diameter of the fundamental mode of 23 µm, resulting in an effective mode-field area of 415 µm2 . Both fiber ends are polished at an angle of 8◦ in order to suppress seeding of amplified spontaneous emission. As pump source a pigtailed diode-laser emitting up to 250 W at 976 nm is

42

Andreas T¨ unnermann et al. 140

Slope efficiency ~80%

Output power [W]

120 100 80 60 40 20 0 0

20

40

60

80

100

120

140

160

180

Launched pump power [W]

Fig. 4. Output characteristics of the ytterbium-doped fiber power amplifier

employed. To avoid interaction between the two high-power fiber amplifiers a second optical isolator is applied. Figure 4 shows the output characteristics of the power amplifier. When seeding the power amplifier with approximately 1.5 W, we were able to generate average output powers up to 140 W with a slope efficiency of 80%, with respect to the launched pump power. The pulse energy is as high as 1.86 µJ. The LMA fiber has a normalized frequency parameter of approximately 5 and can therefore support 4 transverse modes. However, as previously discussed, introducing bending losses by coiling the fiber discriminates higherorder transverse modes. Thus, only the fundamental mode is guided and amplified. We measured an M 2 -value of 1.1 at the highest output power, implying a diffraction-limited beam quality. Furthermore, the coiling provides sufficient stress birefringence to stabilize the polarization. The degree of polarization is characterized as ∼ 50% (power ratio of 1 to 3 behind a polarization beam splitter). The emitted spectral width and pulse duration of the power amplifier at the highest power level is characterized to be 20 nm and 224 ps, respectively, resulting in a peak power of 8.3 kW. The stretched and amplified pulses are recompressed using a diffractiongrating compressor based on transmission gratings in fused silica [31]. The gratings have a pitch of 800 nm (1250 lines per millimeter) and are designed to have a maximum diffraction efficiency at 1060 nm under Littrow conditions. The grating period is chosen as a trade-off to work with a moderate grating separation to compensate the imposed chirp and avoid a large amount of residual higher-order dispersion. The fabrication of the gratings with a size of 10 × 60 mm2 is done by electron-beam lithography. The following etching process (reactive ion etching and reactive ion beam etching) results in a binary grating with the targeted parameters (groove depth = 1.54 µm, duty cycle = 0.45). A scanning electron microscope image of the transmis-

Ultrashort Pulse Fiber Lasers and Amplifiers

43

Fig. 5. Scanning-electron microscope image of the transmission grating in fused silica

Fig. 6. Measured autocorrelation trace of the high-power femtosecond pulses

sion grating is shown in Fig. 5, and reveals that the intended binary profile is achieved with good quality. The diffraction efficiency of a single grating for TE-polarized light is measured as 95%. Besides their high efficiency these transmission gratings possess a damage threshold only a factor of 2 below unstructured fused silica, which is significantly higher than conventional goldcoated diffraction gratings. This is a prerequisite for the compression of such high-power ultrafast laser pulses. Figure 6 shows the measured autocorrelation trace at 140 W output power of the fiber amplifier. Best compression is achieved at a grating separation of

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0.61 m. The gratings are used under a diffraction angle of 44.5◦ (3◦ off Littrow mounting). The autocorrelation width is determined as 620 fs, corresponding to 400 fs pulse duration (FWHM) by assuming a sech2 pulse shape. The wings in the autocorrelation trace are due to uncompensated higherorder phase contributions of the fiber-stretcher grating compressor setup. No influence of nonlinear pulse distortions was visible. Therefore, the quality of the recompressed pulses can be simply increased by revising the dispersion management of the fiber CPA system. The overall compressor throughput efficiency at the highest power level is ∼ 54% resulting in 76 W average power of femtosecond laser pulses. At a repetition rate of 75 MHz this corresponds to a pulse energy of about 1 µJ and a peak power of 2.5 MW. As demonstrated, the fiber CPA system supports a bandwidth of about 20 nm. Therefore, the amplification of even sub-100 fs pulses should be possible. Moreover, the recent demonstration of 500 W output of a continuouswave fiber laser with nearly diffraction-limited beam quality [7] demonstrates the achievable power range.

4

High Pulse Energy Ultrafast Fiber CPA System

When not only high average output powers but also high pulse energies are targeted several other aspects have to be taken into account. Thus, the basics of a high-energy femtosecond fiber-based chirped-pulse amplification system are discussed in this section. The main focus is on the discussion of limitations due to parasitic nonlinear effects, which already become dominant at the microjoule pulse energy level in single-mode fiber amplifiers and strategies are presented to overcome these restrictions. In general, a high-energy fiber CPA system also consists of a femtosecond oscillator, a stretcher, an amplifier chain and a compressor. Additionally, pulse selectors, such as acousto-optical modulators, are employed to reduce the repetition rate and therefore to enhance the pulse energy. A schematic setup of a high pulse energy ultrafast fiber CPA system is shown in Fig. 7. 4.1

The Consequences of SRS and SPM in a Fiber CPA System

The impact of self-phase modulation in CPA systems has already been discussed for bulk amplifiers [32]. It has been shown that even moderate selfphase modulation can significantly distort the recompressed pulse after amplification, thereby the peak power is reduced and the pulse contrast is degraded. When using a fiber as gain medium these effects are much more pronounced due to the tight confinement of the pulses over a long interaction length. To illustrate the consequence of SPM a fiber CPA system is simulated with typical parameters by solving the NLSE (1) using the split-step Fourier

Ultrashort Pulse Fiber Lasers and Amplifiers Femtosecond oscillator

Yb-doped SM-preamplifier

Pigtailed diode laser

Yb-doped SM-preamplifier

45

Pigtailed diode laser

AOM I

Pigtailed diode laser

AOM II

grating stretcher

grating compressor

Output

Yb-doped SM or LMA amplifier

Fig. 7. Principle setup of the fiber CPA system; AOM: acousto-optical modulator

Fig. 8. Impact of SPM in a fiber CPA system on the pulse quality of the recompressed pulses

method [33] for a 20 m long single-mode fiber. The pulses are amplified to the kilowatt peak power level and the autocorrelation traces of the compressed pulses are shown in Fig. 8. This reveals a significant reduction of pulse quality in comparison to a transform-limited sech2 pulse and the evolution of a pedestal with increasing pulse energy due to the nonlinear phase contribution of SPM. This behavior is experimentally confirmed by using a 20 m long singlemode fiber (MFD = 11 µm) as the final amplifier in the fiber CPA system

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Fig. 9. Experimentally measured autocorrelation traces of the CPA system based on single-mode fibers

shown in Fig. 7. Figure 9 presents the autocorrelation traces, at optimum compression using the grating compressor, for different peak powers out of the final stage of the fiber CPA system. At lower peak powers the measured autocorrelation is pedestal free and the autocorrelation width of 470 fs corresponds to a pulse duration of 300 fs, assuming a sech2 pulse shape. With increasing pulse energy the pulse quality is reduced due to the imposed nonlinear phase of SPM in the fiber amplifier. The measured emitted spectrum reveals severe changes in the emission spectrum with increasing peak power, as shown in Fig. 10. At lower pulse energies only the signal at 1060 nm is observable, with a good ASE suppression of approximately 30 dB. At peak powers higher than 2.3 kW a signal at about 1120 nm increases rapidly, indicating that the threshold of stimulated Raman scattering is reached. At a peak power of 3.4 kW even the frequency upshifted anti-Stokes wave at about 1010 nm becomes detectable. At 4.6 kW the pulse breaks up, the most intense central part of the spectrum, and therefore of the pulse, is shifted into the Stokes wave. There are basically two approaches to avoid these restrictions due to nonlinearity, a reduction of the intensity by temporal or spatial scaling. Temporal scaling means the application of a stretcher-compressor unit with higher stretching factors. Spatial scaling means the use of fibers with reduced nonlinearity, i.e. an increased mode-field diameter and a reduced fiber length. A very promising fiber technology to achieve this is the so-called air-clad large mode area microstructured fiber. Air clad means that the inner cladding is surrounded by a net of silica bridges, which are substantially narrower

Ultrashort Pulse Fiber Lasers and Amplifiers Anti-Stokes

-10

47

Stokes

Pulse

4.6 kW 3.4 kW

-20

Intensity [dB]

2.3 kW -30

1.2 kW 0.4 kW

-40

-50

-60 1000

1020

1040

1060

1080

1100

1120

1140

Wavelength [nm]

Fig. 10. Emission spectrum of the final fiber amplifier as a function of pulse energy

than the wavelength of the guided radiation. This leads to a high numerical aperture of the inner cladding of up to 0.8 [34]. As a consequence the diameter of the pump core can be significantly reduced with remaining brightness acceptation of pump radiation. Due to the reduced ratio of pump core area to active core area, the pump-light absorption is improved and shorter fiber lengths are possible. Figure 11 shows the scanning electron microscope image of such a high-NA double-clad ytterbium-doped large mode area fiber. This special fiber has a pump-light absorption of approximately 10 dB/m at 976 nm. Thus, fiber lengths between 1 m and 2 m are sufficient for an efficient operation. A further advantage of such an air-clad fiber is that no radiation has direct contact with the coating material, which makes these fibers suitable for high-power operation. Up to 80 W of output power in a 2.3 m long ytterbium-doped large mode area air-clad fiber laser has been reported [35], recently, even 260 W has been demonstrated in a similar fiber structure. Using a 1.3 m length of this air-clad large mode area (MFD = 21 µm) microstructured fiber as the power amplifier in the described CPA system an average power of 12 W at a repetition rate of 114 kHz could be achieved. This corresponds to a pulse energy of 105 µJ without any limitations due to nonlinearity. The pulses are recompressible to subpicosecond pulse duration with a compression efficiency of ∼ 50%. 4.2

Energy-Scaling Considerations

In this section a summary of energy-scaling limitations, regarding the nonlinearity, damage and extractable energy, of a fiber CPA system is given. These considerations are based on a ytterbium-doped fiber with a mode-field diameter of 35 µm and a stretched pulse duration of 1 ns. Both the funda-

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Fig. 11. SEM image of an ytterbium-doped LMA double-clad microstructure fiber

mental mode propagation with a MFD of 35 µm and the 1 ns pulse duration are technologically achievable with current large mode area single-mode photonic crystal fibers [36] and grating stretcher-compressor systems. All limits are summarized in Fig. 13. The extractable energy of an amplifier can be estimated using the following equation [37] Eextract = εsat Aeff ln(G0 ) ,

(5)

where εsat is the saturation fluence, Aeff the effective mode area and G0 the small-signal gain. Here, a further advantage of ytterbium compared to other rare-earth ions becomes important. Ytterbium possesses a huge saturation fluence of about 35 J/cm2. Thus, (5) implies that a few hundred microjoule can be extracted from single-mode fibers, while using large mode area or multimode fibers, the millijoule range can be reached [38]. For the calculation of the extractable energy of the considered fiber with 35 µm MFD a small signal gain of 50 dB is assumed. Therefore, the extractable energy yield is ∼ 5 mJ. For high-energy fiber-laser systems, issues associated with surface damage have to be considered. The surface damage fluence threshold of fused silica, which is significantly lower than the bulk damage fluence threshold, at a wavelength of 1060 nm is given by [39] 22 × (∆τ )

0.4

J/cm2 ,

(6)

where ∆τ is the pulse duration in nanoseconds. Therefore, the damage threshold for the given MFD of 35 µm is only ∼ 280 µJ, which is more than an order of magnitude less than the extractable energy. This problem can be solved by special treatment of the fiber end. One solution is to splice a coreless end cap

Ultrashort Pulse Fiber Lasers and Amplifiers

49

Expanding mode

Coreless endcap

Splice

Fiber core

Fig. 12. Coreless fiber end cap to avoid facet damage

on the output side of the fiber amplifier, as shown in Fig. 12. The expansion of the beam reduces the fluence and avoids fiber-facet damage. Assuming an expansion of the mode to a diameter of 200 µm the damage threshold is increased to ∼ 8.5 mJ and is, therefore, well above the extractable energy. In multimode fibers self-focusing, a consequence of the intensity dependence of the refractive index (Kerr effect), sets the fundamental limit of achievable pulse peak power. Since the mode is more intense at the center than at the wings the mode tends to converge. Consequently, the mode will be focused to a small filament and the medium will usually break down via multiphoton and avalanche ionization. Self-focusing is characterized by a critical power for the propagating mode at which it starts to collapse. This critical power, which is identical for multimode fibers and bulk media [40], is given by [41, 42] Pcr = a

λ20 , 8πnn2

(7)

where a is a correction factor, assumed to have a value of 4 [43], n is the refractive index and n2 the nonlinear index coefficient of the medium. In fused silica this critical power is approximately 3.7 MW for a wavelength of λ0 ≈ 1 µm, corresponding to a pulse energy of 3.7 mJ at a stretched pulse duration of 1 ns. The limits due to nonlinear effects are based on numerical simulations as described above. For all fiber lengths a gain of 30 dB is assumed, which is a realistic value for sufficient ASE suppression. The resulting limits are summarized in Fig. 13. These simulations reveal that self-phase modulation limits the performance of a fiber-based CPA system. Using a fiber length of about 2 m it is possible to reach the 200 µJ level without significant nonlinear phase distortion due to SPM.

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Fig. 13. Summary of restricting effects in a fiber CPA system subject to the fiber length, assuming a MFD of 35 µm and a stretched pulse duration of 1 ns

5

Conclusion

The fiber amplifier is a power-scalable solid-state laser concept due to outstanding thermo-optical properties, providing robustness, compactness and long-term stability. Diode-pumped ytterbium-doped fiber laser systems are especially suitable for high-power ultrashort pulse amplification because of several unique properties, such as the broad gain bandwidth and the high optical-to-optical efficiencies. However, due to the tight confinement of the laser radiation over considerably long interaction lengths, nonlinear effects are enhanced, which constitutes the main performance limitation of fiberlaser systems. The application of the fiber-based chirped-pulse amplification technique, where sufficient pulse stretching in the time domain reduces the pulse peak power and therefore the nonlinearity in the gain fiber, allows the amplification of ultrashort laser pulses. Additionally, innovative fiber designs such as large mode area fibers can be employed, which reduce the intensity of the guided laser radiation in the core by approximately one order of magnitude. Because of this spatial scaling in combination with refined experimental concepts a significant power scaling of ultrafast fiber amplifiers can be achieved. Future developments using novel advanced fiber designs are discussed, which allow a further significant power and energy improvement. Presented energy-scaling considerations lead to the conclusion that pulse energies in the few hundred microjoule range with pulse durations in the femtosecond range can be efficiently produced using an ytterbium-doped fiber CPA system. Even pulse energies up to the mJ range can be approached using improvements in fiber design with larger mode-field diameters. These high-energy levels in

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51

combination with the presented high average powers make a fiber-based CPA system attractive for a variety of industrial and scientific applications.

References [1] D. Hanna: Confined solid-state structures (fibers and waveguides) compared to bulk gain lasers, in OSA Tech. Dig. Ser. JWA1 (Opt. Soc. Am., Baltimore 1995) tutorial 35 [2] L. Goldberg, J. Koplow, R. P. Moeller, D. A. V. Kliner: High-power superfluorescent source with a side-pumped Yb-doped double-cladding fiber, Opt. Lett. 23, 1037–1039 (1998) 35 [3] R. Paschotta, J. Nilsson, A. C. Tropper, D. Hanna: Ytterbium-doped fiber amplifiers, IEEE J. Quantum Elect. 33, 1049–1056 (1997) 35 [4] L. Goldberg, J. P. Koplow, D. A. V. Kliner: Highly efficient 4 W Yb-doped fiber amplifier pump by a broad stripe laser diode, Opt. Lett. 24, 673–675 (1999) 35 [5] E. Snitzer, H. Po, F. Hakimi, R. Tumminelli, B. C. McCollum: Double-clad offset core Nd fiber laser, in Optical Fiber Sensors, OSA Tech. Dig. Ser. 2 (Opt. Soc. Am., Washington 1988) postdeadline paper PD5 36 [6] H. Zellmer, A. T¨ unnermann, H. Welling, V. Reichel: Double-clad fiber laser with 30 W output power, TOPS 16, 137 (1997) 36 [7] J. Limpert, A. Liem, H. Zellmer, A. T¨ unnermann: 500 W continuous-wave fiber laser with excellent beam quality, Electron. Lett. 39, 645–647 (2003) 36, 44 [8] R. R. Alfano: The Supercontinuum Laser Source (Springer, Berlin, Heidelberg 1989) 37 [9] C. V. Raman: A new radiation, Indian J. Phys. 2, 387 (1928) 37 [10] D. Taverner, D. J. Richardson, L. Dong, J. E. Caplen, K. Williams, R. V. Penty: 158-µJ pulses from a single-transverse-mode, large-mode-area erbium-doped fiber amplifier, Opt. Lett. 22, 378 (1997) 38 [11] J. A. Alvarez-Chavez, H. L. Offerhaus, J. Nilsson, P. W. Turner, W. A. Clarkson, D. J. Richardson: High-energy, high-power ytterbium-doped Q-switched fiber laser, Opt. Lett. 25, 37–39 (2000) 38 [12] J. M. Sousa, O. G. Okhotnikov: Multimode Er-doped fiber for single-transversemode amplification, Appl. Phys. Lett. 74, 1528 (1999) 38 [13] J. Limpert, H. Zellmer, A. T¨ unnermann, T. Pertsch, F. Lederer: Suppression of higher order modes in a multimode fiber amplifier using efficient gain-lossmanagement (GLM), in (Advanced Solid State Lasers 2002) paper MB20 38 [14] M. E. Fermann: Single-mode excitation of multimode fibers with ultrashort pulses, Opt. Lett. 23, 52–54 (1998) 38 [15] J. A. Alvarez-Chavez, A. B. Grudinin, J. Nilsson, P. W. Turner, W. A. Clarkson: Mode selection in high power cladding pumped fiber lasers with tapered sections, in OSA Tech. Dig. Ser. (Opt. Soc. Am. 1999) p. 247 38 [16] A. Galvanauskas: Mode-scalable fiber-based chirped pulse amplification systems, IEEE J. Sel. Top. Quant. 7, 504–517 (2001) 39 [17] J. P. Koplow, L. Goldberg, R. P. Moeller, D. A. V. Kliner: Single-mode operation of a coiled multimode fiber amplifier, Opt. Lett. 25, 442–444 (2000) 39 [18] J. Sakai, T. Kimura: Bending loss of propagation modes in arbitrary-index profile optical fibers, Appl. Opt. 17, 1499 (1978) 39

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Index

amplified spontaneous emission (ASE), 40 bending loss, 39 chirped-pulse amplification, 40 coreless end cap, 48

parabolic pulse, 40 photonic crystal fiber, 48 Raman effect, 37

microstructured fiber, 46

saturation fluence, 48 self-focusing, 49 self-phase modulation (SPM), 37, 44 stimulated Brillouin scattering (SBS), 37 stimulated Raman scattering (SRS), 37, 46 stress birefringence, 42

nonlinear Schr¨ odinger equation (NLSE), 37

transmission grating, 42

double-clad fiber, 36 extractable energy, 48 large mode area fiber, 38, 39

Ultrashort Pulse Thin-Disk Lasers and Amplifiers Daniel M¨ uller1 , Adolf Giesen1 , R¨ udiger Paschotta2, and Ursula Keller2 1

2

Institut f¨ ur Strahlwerkzeuge, Universit¨ at Stuttgart, Pfaffenwaldring 43, 70569 Stuttgart, Germany [email protected] Physics Department / Institute of Quantum Electronics, Swiss Federal Institute of Technology (ETH), Wolfgang-Pauli-Str. 16, ETH Z¨ urich H¨ onggerberg–HPT, 8093 Z¨ urich, Switzerland

Abstract. This Chapter deals with the generation and the amplification of ultrashort pulses in mode-locked oscillators and multipass amplifiers based on the thin-disk laser design. The first section covers soliton mode locking of a thin-disk laser employing semiconductor saturable absorber mirrors (SESAMs). Advantages over Kerr-lens mode locking and the prevention of Q-switching instabilities are also addressed. Results with Yb:YAG and Yb:KYW as laser materials are presented along with possible applications. The second section treats pulse amplification in both geometrical multipass systems and regenerative amplifiers. The processes of pumping and amplification are explained with the help of a simple model. Important issues for the design of the resonator and the Pockels cell are also part of this section. The third section discusses power scaling of the thin-disk laser by increasing the mode area. Scaling limits like amplified spontaneous emission and thermally induced phase distortions are considered.

A thin disk has a substantially higher surface to volume ratio than a cylindrical rod. Applying this thin-disk geometry to the laser crystal in a solidstate laser considerately improves the removal of waste heat generated by the pumping process and would therefore allow the operation of new laser materials like Yb:YAG. This simple, but important insight led to the invention of the thin-disk laser in the early 1990s [1]. The thin-disk laser crystal is antireflection coated for both the pump and the laser wavelength at the front side and high-reflection coated for these wavelengths at the rear side, which is mounted on a heat sink. A circular area in the center of the thin disk is optically excited (pumped). The energy thereby deposed in the pump spot area of the thin disk can be extracted by a laser beam passing through the pumped area of the thin disk. This beam is reflected on the high-reflective coating under normal incidence or a small angle and passing the thin disk a second time. So the excited thin disk may act as an active, amplifying mirror in a laser resonator or as a laser amplifier as shown in Fig. 1. F. Dausinger, F. Lichtner, H. Lubatschowski (Eds.): Femtosecond Technology for Technical and Medical Applications, Topics Appl. Phys. 96, 55–73 (2004) c Springer-Verlag Berlin Heidelberg 2004


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la s e r b e a m

fib e r

p u m p r a d ia tio n ( c o llim a te d )

p a r a b o lic

d is k m o u n te d o n h e a t s in k

Fig. 1. Schematic principle of the thin-disk laser concept

Several advantages, but also some difficulties, arise from this geometry with a crystal thickness of 100 µm to 200 µm and a pump spot diameter of 1 mm to 5 mm (disk diameter 8 mm to 15 mm): Cooling of the thin disk is not only very effective, which permits to be applied high pump power densities without a drastic temperature raise in the thin disk, but the resulting heat flux is also very homogenous and mainly perpendicular to the surface of the disk with only small tangential components. This results in smaller phase and polarisation distortion of a reflected laser beam compared to conventional designs, improving the efficiency in both CW lasers and short-pulse amplifiers. Another consequence of the small thickness of the disk is that the density of laser ions (per area) Ndot ddisk is lower than in other laser designs. This reduces the threshold of the pump power density in three-level systems (a positive effect), but also the possible small-signal gain and the pump-radiation absorption per double pass (a negative effect). To achieve a sufficient pump-radiation absorption (> 80%) despite the small disk thickness, the pump radiation must either propagate parallel to the surfaces of the thin disk, guided by total internal reflection (side pumping), or pass the thin-disk multiple times through its front side (multipass pumping). Both schemes have been proposed and implemented, but multipass pumping is more commonly used. Figure 2 shows such a pump optic configuration, that images the nonabsorbed fraction of the pump radiation multiple times onto the disk by means of a parabolic mirror. The pump radiation is first collimated by a lens to a beam parallel to the optical axis of the parabolic mirror that focuses the off-axis incident pump radiation to its focal plane where the thin disk is located. The nonabsorbed pump radiation hits the parabolic mirror a second time after a double pass through the thin disk and it is collimated again. By means of a pair of mirrors the collimated beam is redirected on another segment of the parabolic mirror, which focuses it once more onto the thin disk and the transmitted fraction is collimated again. This scheme is repeated until most of the incident pump power is absorbed in up to 16 double passes, depending on the configuration.

Ultrashort Pulse Thin-Disk Lasers and Amplifiers

8

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5

4 1

2 3 6 7

Fig. 2. Pump optics for the thin-disk laser allowing 8 double passes of the pump radiation trough the thin disk

Multipass pumping of a small number of laser ions (per unit area) in combination with effective cooling is particularly suited for quasi-three-level laser materials like Yb:YAG. The effective pump power density, calculated as the sum of the superimposed pump beam passes, is higher then the simple incident pump power density of the laser diodes. With this high effective pump power density, the material can be excited to an inversion considerably above the transparency threshold, thus allowing efficient operation. Despite the high inversion, the absorption of the pump radiation remains good because of the multipass pumping concept. The effective cooling is important because the thermal population of the lower laser level strongly increases with increasing crystal temperature, raising the laser threshold. The comparatively small double-pass gain in the thin-disk design is not problematic by itself, but obtaining a good gain-to-loss ratio, which determines the efficiency of a laser both in CW and pulsed operation, is more difficult than in other laser designs with higher gain. In consequence, the optical components used in the thin-disk laser should be of superior quality. A second effect of the small gain is that saturation of a thin-disk amplifier and thus efficient energy extraction requires either a high initial pulse energy density (comparable to or higher than the saturation energy density) or extensive multipass amplification. A smaller gain is also connected to a smaller extractable energy for a given beam cross section compared to other laser designs. This may be disadvantageous in some cases, because high pulse energies will require large beam cross sections. However, to avoid critical optical damage, it is often desirable to keep the pulse energy density low, which is assured automatically when the density of the extractable energy is small. A small pulse energy density can also be achieved with a high area density of

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laser ions by keeping the inversion low or by extracting only a small fraction of the stored energy, but in both ways only at the cost of efficiency. To summarize, a well-balanced combination of crystal thickness, doping concentration and pump power density must be chosen for efficient operation. Increasing the output power or energy of the thin-disk laser without disturbing this equilibrium leads to the power-scaling law of the thin-disk laser: The active area is enlarged by increasing the pump spot diameter on the thin disk or by using more than one thin disk in the laser system, while keeping the pump power density constant. A more extended general presentation of the thin-disk laser is found in [2]. Details on the analytical and numerical simulation of the thin-disk laser are given in [3], whereas appropriate resonator designs and pump optic configurations are discussed in [4].

1

Ultrafast Thin-Disk Oscillators

In the beginning of the 1990s, continuously operating diode-pumped solidstate lasers had begun to penetrate new regimes of output power and brightness. However, for mode-locked lasers it took longer until similarly high average powers were reached. The first step was the development of diode-pumped solid-state lasers that generate a high power in a diffraction-limited beam, because this is not only a valuable feature for applications but also a prerequisite for mode locking. The thin-disk laser design appeared particularly suitable because it minimizes the effects of thermal lensing and thus can generate quite high power in a diffraction-limited beam. Furthermore, so far it has worked best with Yb:YAG as the gain medium, which has a much higher amplification bandwidth than Nd:YAG, the most common gain medium for other high-power lasers. This high bandwidth results in the potential to generate significantly shorter pulses. The second major challenge was to find an appropriate technique for mode locking. Passive rather than active mode locking appeared to be preferable due to its simplicity and the potential for shorter pulses, but it was necessary to find a technique that is suitable for very high powers. Although the use of semiconductor saturable absorber mirrors (SESAMs) [5, 6] had been very successful at lower power levels, initially there were serious concerns about the ability of these devices to operate at high-power levels without damage. After all, SESAM damage had been observed even at moderate power levels, and the search for much more damage-resistant types of SESAMs appeared to be a great challenge. However, it turned out that SESAMs could be used in high-power lasers without significantly changing the materials and/or designs. The key point was to optimize the overall laser design. First, to avoid nonthermal damage (e.g., involving multiphoton ionization), one has to limit the intensity on the SESAM. Even at high-power levels, this is easily accomplished by increasing the mode area on the SESAM in proportion to the intracavity pulse energy. Secondly, thermal damage (overheating) has to be

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avoided by minimizing the dissipated power and by maximizing the mode area. The former is achieved by using a SESAM with relatively low modulation depth (i.e., a thin absorber layer), while a large mode area can be chosen by using a design with relatively low saturation fluence, which needs lower pulse fluences to achieve the saturation. First applied for mode locking with side-pumped Nd:YAG laser heads, this led to the demonstration of a passively mode-locked 10 W laser [7] and later to a 27 W version [8]. It became clear that the SESAM, when properly used, would not be the limiting factor for high mode-locked powers. To some researchers, the technique of Kerr-lens mode locking (KLM [9]) appeared to be more suitable for high powers. The reasoning was that softaperture KLM does not introduce an optical element that has to dissipate power, so that thermal effects can be avoided even at high powers. However, KLM requires operation near the stability limit of the laser resonator, particularly for longer pulses where the peak powers are moderate and the Kerr-lensing effect is weak. So far, a mode-locked thin-disk laser based on KLM has not been demonstrated. Another challenge for passive mode locking is to suppress the tendency for Q-switching instability [10], which are caused by the saturable absorber. This kind of instability is caused by the fact that a saturable absorber typically “rewards” a higher pulse energy with lower loss, so that any deviations from the steady-state pulse energy tend to grow. However, gain saturation will sooner or later pull the pulse energy back towards the steady state. If this happens fast enough, Q-switching instabilities are suppressed; otherwise, the pulse energy may undergo undamped oscillations (Q-switched mode locking). The difficulty with many high-power lasers is that the poor beam quality of the pump diodes and/or thermal effects make it necessary to design the laser head for a large mode area in the gain medium, which reduces the stabilizing effect of gain saturation. A thin-disk laser head is not bad in this respect, as it allows operation of the gain medium with a fairly high intensity, but the low laser cross sections of Yb:YAG lead to a relatively strong Q-switching tendency. This has to be kept under control by using a relatively long laser cavity, which slows down the growth of fluctuations without weakening the effect of gain saturation. An additional stabilizing effect comes from the use of soliton pulses [10] that also help to generate shorter pulses. Apart from these measures, a rather small modulation depth of the SESAM (below 1%) is necessary. Finally, it is important to note that we always have counterpropagating laser beams in the gain medium of a thin-disk laser. This leads to spatial hole burning, which distorts the shape of the gain spectrum and can consequently cause various kinds of instabilities [11]. All these can be suppressed by using a SESAM with a high enough modulation depth, provided that the modulation depth is still small enough to avoid Q-switching instabilities. By reducing the tendency for the latter with other available means as mentioned above,

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a parameter window of sufficient width with stable operation can be found. A remarkable feature is that stable mode locking can normally be achieved only in a certain range of pulse durations, while in most other mode-locked lasers there is only a strict lower limit to the pulse duration. The first passively mode-locked thin-disk laser was demonstrated in the year 1999 [12]. This laser was based on Yb:YAG as the gain medium, was passively mode locked with a SESAM, and made use of soliton-like pulses in the negative-dispersion regime (with negative dispersion from a pair of silica prisms). It generated 16 W of average output power in 730 fs pulses. At that time, this was about an order of magnitude more power than had been achieved with other lasers in the subpicosecond domain (without using amplifiers). Note that particularly in the subpicosecond domain it is advantageous to generate the high power directly with a laser, because broadband gain media for amplifiers in this regime tend to generate a low gain, so that multipass arrangements are required. This complication does not arise with fiber amplifiers, which can provide a high gain in a high bandwidth with good efficiency. However, these amplifiers still increase the complexity of the setup and also raise issues concerning pulse quality in terms of spectral width, polarization, and possible satellite pulses or pedestals. For comparison, the pulses emitted by mode-locked thin-disk lasers are usually close to transform-limited soliton pulses with a clean temporal and spectral shape. This is important for many applications, e.g. those involving frequency conversion. Since the first demonstration of a mode-locked thin-disk laser, research has developed in two directions: higher powers and shorter pulses. The most successful route towards higher powers so far was to use a similar thin-disk Yb:YAG laser head with a higher pump power of up to 370 W. Due to the reduced thickness of the disk (≈ 100 µm), stress-induced abberations were minimized, and diffraction-limited operation in the continuous-wave regime was achieved at up to 100 W of output power. With accordingly increased mode areas, insertion of the SESAM into the cavity did not raise problems. However, a new challenge arose from thermal and other effects in other intracavity elements. In particular, a Gires–Tournois interferometer (GTI) could no longer be used because the intracavity power of about 700 W led to strong thermal distortions, making it too difficult to achieve diffraction-limited operation. Finally, negative dispersion was introduced through dispersive dielectric mirrors. These exhibited only moderate thermal effects, but appeared to introduce an additional Kerr-type nonlinearity, which made it necessary to introduce substantially more negative dispersion than originally anticipated. (Compared to standard highly reflecting mirrors, the electric field penetrates further into these structures, causing a higher nonlinearity.) In this way, 60 W of average output power have been obtained in 820 fs pulses [13]. Figure 3 shows the cavity setup. Further substantial improvements of the output power (possibly above 100 W) may be achieved with improved dielectric mirrors and/or other optimizations.

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Fig. 3. Cavity of passively mode-locked thin-disk laser with 60 W average power in 820 fs pulses

In order to obtain shorter pulses, a gain medium with broader bandwidth is required. Although a large number of ytterbium-doped gain media with very broad amplification bandwidth have recently been developed, most of them do not appear to be suitable for passively mode-locked high-power lasers due to their moderate thermal conductivity and their typically very low laser cross sections, which make the tendency for Q-switching instabilities too strong. As a notable exception, Yb-doped tungstate crystals such as Yb:KGW and Yb:KYW [14] have peak laser cross sections similar to those of Yb:YAG and a thermal conductivity that is reasonably high – lower than for Yb:YAG, but still high enough due to the very small quantum defect level for pumping around 981 nm. The latter is possible because this transition has sufficient bandwidth (a few nanometers) for diode pumping, while, e.g., Yb:YAG has to be pumped around 940 nm with a correspondingly larger quantum defect level. This made it possible to generate 240 fs pulses with 22 W of average power from a thin-disk Yb:KYW laser [15]. Higher powers – comparable to those from Yb:YAG lasers – should be possible, but require further advances of crystal quality and the procedure for mounting the crystals on the heat sink. Note that one might expect that even significantly shorter pulses could be generated with Yb:KGW or Yb:KYW, as the total bandwidth is a few tens of nanometers. However, the relatively strong curvature of the gain spectrum near the peak gain (for polarization in the a-direction in a b-cut crystal, which gives the highest gain) makes this difficult. Shorter pulses may be generated by using a c-cut crystal, which can be operated with polarization in the b-direction where the gain spectrum is much smoother. However, it appears to be more difficult to fabricate high-quality c-cut crystals, and the gain for b-polarization is somewhat lower. We finally note that due to their exceptional performance in the subpicosecond domain, passively mode-locked lasers have enabled a number of interesting experiments in nonlinear optics. By using a novel large mode area single-mode fiber, pumped with up to 38 W of incident average power from

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a thin-disk Yb:YAG laser, it was possible to demonstrate efficient nonlinear pulse compression leading to 33 fs pulses with 18 W of average power [16] and 12 MW of peak power. With tight focusing, these pulses could be used to generate intensities of the order of 1014 W/cm2 , which are expected to be sufficient for experiments in high-intensity physics, e.g. high-harmonic generation [17] and laser-plasma-generated X-rays [18]. Nonlinear wavelength conversion at very high average power levels is another interesting domain for mode-locked thin-disk lasers. In the simplest case, frequency doubling has been demonstrated with 58% efficiency, using a critically phase-matched LBO crystal [12]. The advantage of critical phase matching is that the nonlinear crystal can be operated at room temperature, so that a temperature-stabilized crystal oven is not required. For comparison, a picosecond laser with similar average power would deliver too low a peak power for efficient critically phase-matched frequency doubling, and a noncritically phase-matched crystal kept in an oven would be required. For other wavelengths, synchronously pumped parametric oscillators and optical generators can be used. Particularly with periodically poled LiNbO3 and LiTaO3 crystals, the high peak powers from passively mode-locked thindisk lasers allow a very high parametric gain to be generated. This has enabled a number of interesting devices, such as a fiber-feedback parametric oscillator [19, 20] which has a compact setup and an unusually large tolerance both to intracavity losses and to a mismatch of the OPO cavity length, apart from generating very high powers in femtosecond or picosecond pulses. Also, it became possible to pump an optical parametric generator (i.e. a device that does not need another cavity) at the full laser repetition rate [20]. Compared to earlier parametric generators, pumped with sophisticated amplified laser systems, this approach allows much higher pulse repetition rates and average powers to be obtained, while having a simpler setup.

2

Ultrashort Thin-Disk Amplifiers

With a typical double-pass gain of 20% (Yb:YAG), thin-disk amplifiers must, in general, employ a kind of multipass amplification scheme to reach the required total gain. So far, two approaches have been investigated: geometrical multipass amplification and amplification in an optically switched resonator (regenerative amplifier). 2.1

Geometrical Multipass Amplifier

One realization of the first approach uses angular multiplexing in a concept similar to the pump optics described earlier. The laser beam hits the thin disk from different angles and is reflected in the opposite direction after each double pass. All double-pass segments form the same angle with the central axis of the thin disk, thereby lying on a cone with the disk in its apex.

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fo ld in g u n its in p u t o u tp u t

le n s th in d is k

Fig. 4. Optical path in a geometrical multipass amplifier with one lens and two folding units consisting of plane mirrors (The lens can be substituted by a parabolic mirror to reduce losses and avoid lens errors and the the folding units can be modified such that the foci are more elongated from the mirror coatings)

A lens with its focal point in the center of the thin disk refracts all laser beams so that they propagate parallel to the central axis on the surface of a cylinder. These individual double-pass segments are connected by two folding units, each consisting of a pair of plane mirrors, to form a single path for the laser beam as depicted in Fig. 4. This geometrical multipass scheme requires no active components prone to optical damage (e.g. a Pockels cell), has a practical total gain in the order of 102 and is therefore best used as a high-power booster amplifier. It is a recent development that has been studied so far only with nanosecond pulses [21], but would also work with (stretched and unstretched) picosecond pulses. 2.2

Resonator Multipass (or Regenerative) Amplifier

The latter approach is more common and was first transferred to the thin-disk laser concept at the end of the 1990s [22]. Figure 5 gives a schematic overview of a thin-disk regenerative amplifier system. The seed pulses are generated in a mode-locked oscillator, which can also be based on the thin-disk concept. A pulse-picking system decreases the high repetition rate of the oscillator to a suitable repetition rate for the regenerative amplifier. The pulse-picking system uses a Pockels cell, that can change the polarization of the transmitted seed pulses depending on a high voltage applied to the Pockels-cell crystal. Only the selected pulses pass the thin-film polarizer (TFP).

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m o d e lo c k e d o s c illa to r o p tic a l is o la to r

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P o c k e ls c e ll

b e a m d ia g n o s tic

h a lf- w a v e p la te p o la r iz e r

o u tp u t

p u ls e p ic k e r

in p u t p o la r iz e r

F a ra d a y ro ta to r

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p o la r iz e r m ir r o r

Fig. 5. A short-pulse laser system, consisting of a mode-locked seed laser, an electrooptically modulated pulse picker, a thin-disk amplifier resonator with Pockels cell to inject and eject the pulses and an optical isolator separation unit to redirect the amplified pulses

Before the seed pulses are injected into the resonator, their beam parameters are adapted to the amplifier resonator by a mode-matching optic. They pass directly through the separation unit, because the polarization change of the half-wave plate and the Faraday rotator compensate in this direction. When no voltage is applied to the Pockels cell inside the resonator, the polarization of the injected seed pulse changes from horizontal to vertical during its double pass through the quarterwave plate and it is reflected at the TFP of the resonator. While the pulse is travelling in the other part of the resonator, the quarterwave voltage is applied to the Pockels cell thus compensating the effect of the quarterwave plate, which traps the pulse in the resonator. The pulse circulates in the resonator until the Pockels-cell voltage is switched off again. In the next double pass through the quarterwave plate, the polarization of the pulse is turned back to a horizontal state and the pulse is transmitted through the TFP of the resonator. When travelling in this direction through the separation unit, the polarization rotation accumulates to 90◦ and the amplified pulse is reflected at the TFP to the output of the amplifier system. The advantage of regenerative amplification is the high total gain of 106 to 109 and the spatial filtering of the laser beam in the resonator, whereby this approach is ideal to build a one-stage amplifier for quite high average power levels. So far, average output powers of 10 W with a pulse width of 4.5 ps [23] and 0.8 W with subpicosecond pulses [24] have been published.

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Pump Phase

Although the energy stored in the thin disk is extracted in a comparatively short time interval of ≈ 1 µs during the amplification phase, it can be transferred to the thin disk continuously. CW pumping is adequate, as long as the fluorescence lifetime of the upper laser level (τfluo = 0.95 ms for Yb:YAG) is longer than the cycle time of the amplifier (i.e. f rep > 1 kHz). For typical repetition rates of 1 kHz to 100 kHz, the cycle time is 10 µs to 1000 µs and therefore much longer than the amplification phase. One can therefore distinguish a pump phase having almost the length of the cycle time and the amplification phase, where only a negligible amount of pump energy is deposited in the thin disk. In a simple model, the pumped thin disk is characterized by the density of the extractable energy Hextr . This parameter, assumed to be spatially uniform, corresponds to the energy that could be extracted in the absence of any losses from a certain area of the pumped region. The double-pass gain gdp is proportional to the density of the extractable energy: gdp = 2

Hextr . Hsat

(1)

Here Hsat is the saturation energy density. The extractable energy density decreases due to fluorescence with a time constant τfluo (the fluorescence lifetime of the upper laser level) and is increased by optical pumping with an absorbed pump power density Epump, abs , Hextr (t) + |Hextr, unpumped| dHextr (t) =− + Epump, abs ηq . dt τfluo

(2)

The quantum efficiency ηq corresponds to the energy ratio of laser photons and pump-radiation photons. Hextr, unpumped is the (negative) extractable energy density of the unpumped thin disk. The disk is transparent when Epump, abs = |Hextr, unpumped|/τfluo ηq . In a first approximation, the extractable energy density tends exponentially to an equilibrium state where dHextr (t)/dt = 0. The temporal evolution is not exactly exponential, because Hextr, unpumped and Epump,abs depend on the temperature and the actual inversion of the thin disk. Furthermore, the equilibrium is not attained, because the pump phase of the amplifier for typical repetition rates is shorter than the fluorescence lifetime. The efficiency of the pumping process ηpump , defined as the ratio of pump energy to the increase of the extractable energy stored in the thin disk during the pump phase, depends on many factors: the employed pumping diodes, the pump optics configuration, the thin disk itself, but also on the mode of operation of the amplifier, namely the repetition rate and the (average) level of the extractable energy density. A small density of extractable energy is disadvantageous in three-level systems like Yb:YAG, because a large fraction

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of the pump radiation is then spent to reach the transparency of the thin disk. Very high extractable energy density levels will also reduce the pump efficiency, because the high inversion implies large fluorescence losses. Additionally, the absorption of the pump radiation decreases with the inversion and the temperature in the thin disk increases with high pump power densities, whereby both effects reduce the efficiency. Moderate extractable energy densities hence give the best pump efficiency. It must also be considered that a high double-pass gain, which is proportional to the extractable energy density, has a positive effect on the energy extraction efficiency, as can be seen in the next section. 2.4

Multipass Pulse Amplification

The amplification phase in a solid-state multipass amplifier has been analyzed in [25]. This model can also be adopted to the thin-disk laser, but it has to be taken into account that the laser pulse always passes the thin disk twice. An analytical solution can only be found if the superimposition of the pulse with itself in the thin disk is neglected. Fortunately, the deviation caused by this approximation is insignificant for typical laser parameters. Assuming, furthermore, a complete gain recovery, the extractable energy density Hextr (k + 1) and the pulse energy density Hpulse (k + 1) after one double pass is given by the following recurrence relations: ⎛ ⎞ Hextr (k + 1) 1 ⎜ = ln ⎝ Hsat 2



1 −2

Hextr (k) Hsat



−2

Hpulse (k) Hsat

e 1− 1−e 

H (k) H (k) 1 Hpulse (k + 1) 2 pulse 2 extr Hsat Hsat . = ln 1 + e −1 e Hsat 2

⎟ ⎠,

(3)

(4)

These equations can be iterated to calculate the respective energy densities after a certain number of double passes. The resonator round triplosses lr of a regenerative amplifier are taken into account by multiplication of the pulse energy density with (1 − lr ) after each resonator round trip, consisting of ndp double passes, depending on the resonator design. Losses in a geometrical multipass amplifier are considered analogously. A numerical example shown in Fig. 6 illustrates the three parts of the amplification phase. During the first part, the pulse energy increases exponentially over several orders of magnitude without a significant decrease of the extractable energy stored in the disk. The high gain of this first part is an important feature of a regenerative amplifier. When using a geometrical multipass amplifier, one will typically avoid this part of the amplification phase, because it is difficult to realize this large number of double passes. In the second part, the gain is saturated and the energy is transferred rapidly from the disk to the pulse. The pulse energy rises, until the roundtrip gain equals the roundtrip losses. At this point the

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Fig. 6. Simulation of a regenerative amplifier with two double passes per resonator roundtrip. The pulse energy density (pulse), the density of the extractable energy (thin disk) and the sum of both energy densities (total) is shown after each round trip. Identical data is depicted on a linear (lower graph) and a logarithmic scale (upper graph)

pulse should be extracted from the resonator or leave the geometrical multipass amplifier, respectively, to get the maximum pulse energy. In the third part, which is of course avoided in practical applications, the pulse energy decreases exponentially while the extractable energy tends to a small constant value. The extraction efficiency ηextr of the pulse-amplification process, defined as the ratio of the increase in pulse energy density to the decrease of the extractable energy density depends mainly on the (initial) resonator roundtrip gain to loss ratio ρg/l , ρg/l = 2ndp Hextr (0)/lr , ηextr =

− ln [1 − ∆Hextr /Hextr (0)] ∆Hpulse . = 1 − ρg/l −1 ∆Hextr ∆Hextr /Hextr (0)

(5) (6)

In the case of small energy extraction per pulse at high repetition rates (∆Hextr  Hextr (0)), the deviation of the efficiency from 100% is given by the reciprocal of the initial gain to loss ratio. In the case of a larger energy extraction, the efficiency is further reduced by the last factor in (6), accounting

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for the reduction of the initial gain to loss ratio ρg/l during the amplification phase. 2.5

Regenerative Amplifier Resonator Design

The resonator design is directly infuenced by the power-scaling law of the thin-disk laser. The beam diameter of the desired resonator mode on the thin disk must correspond to the diameter of the pumped area, which means that the resonator must be adapted to the average output power of the system. The advantage of this concept is that not only the pump power density but also the pulse energy density on the thin disk is kept constant. This implies that high average power systems require just the same damage threshold and threshold for nonlinear interactions for the thin disk as low-power systems (with identical repetition rates) to safely reach higher average powers [24]. Obviously, the beam diameter must likewise be expanded on the other delicate optical components, namely the Pockels cell and the thin-film polarizers. In addition to spatially stretching the pulse by enlarging the mode diameter, temporal pulse stretching can further reduce the pulse power density. The use of this chirped-pulse amplification (CPA) is recommended when working with very short pulses and thin-disk materials with a low damage threshold. The resonator has a minimal length because the roundtrip time must be long enough to allow the Pockels cell to switch between two states while the pulse travels in the resonator. Typical switching times are 10 ns (plus timing jitter), requiring a length greater than 1.5 m. There is no maximal length for the resonator if a pulse picker is used to prevent trapping of two or more pulses, but the resonator should of course be kept small and stable. Reasonable resonator lengths lie therefore in the range of 2 m to 3 m. The laser head containing the thin disk can in principle replace any of the resonator mirrors. It should not be used as an end mirror in the resonator (ndp = 1), but always as a folding mirror (ndp = 2). This doubles the roundtrip gain and thereby improves the gain to loss ratio ρg/l . Resonators containing two thin disks (ndp = 4) as folding mirrors or a mirror configuration guiding the beam twice under different angles over the disk (ndp = 4) further improve the gain-to-loss ratio, but also raise the complexity of the system. Beyond these issues and the need to ensure the correct beam diameters on the optical elements in a static case [26], the resonator should be “dynamically stable” and of “Type II” in the sense of Magni [27]. Dynamically stable means that a small variation of the dioptric power of the thin disk, which changes with pump power and energy extraction, has, to a first approximation, no influence on the beam diameter on the thin disk, on the TFP and in the Pockels cell. “Type II” implies that a larger variation of the dioptric power, either negative or positive, will never reduce the mode diameters and thus increase the energy densities in the optical elements.

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Pockels Cell

The Pockels cell is a key component of the regenerative amplifier. It consists essentially of a crystal changing its index of refraction in one axis when a high voltage is applied across the crystal. The applied voltage can be changed within nanoseconds by means of a high-voltage switch, allowing of control the polarization of a train of laser pulses passing the crystal. Though Pockels cells have been wellknown for many years, constructing a suitable Pockels cell for high-power high-repetition-rate amplifiers is still a challenging task. Currently, beta barium borate (BBO) is probably the best choice as crystal material, because it combines a high damage threshold, a good transparency, a very small thermal lens and sufficiently low thermally induced depolarization loss. Unfortunately, the material is hygroscopic, requiring special protective coatings or housings and has only a moderately high electro-optical coefficient. Typical crystals have physical dimensions of 6 mm×6 mm×20 mm and need a quarterwave voltage of 7 kV to 8 kV at a wavelength of 1 µm. Suitable high-voltage switches for repetition rates of up to 50 kHz are actually becoming available.

3

Ultrafast Thin-Disk Laser Scaling Limits

Most scaling limits are of a technological nature, but some are fundamental limitations inherent to the design. One of them is the amplified spontaneous emission (ASE), i.e. fluorescence photons propagating in the plane of the thin disk that are amplified along their way, reducing the inversion. This effect becomes more important with increasing pump spot diameter and limits the applicable pump power for a 200 µm thick Yb:YAG disk to approx. 11 kW [3] and the maximal extractable energy to 2.5 J. These values decrease quadratically with the crystal thickness. Reducing the thickness to 20 µm, which would be favorable in CW lasers (with 100 at.% doping) to reduce phase distortions, would also decrease the extractable energy to 25 mJ. Pulse amplifiers require therefore a careful compromise in crystal thickness between low phase distortions and high extractable energy. One approach to overcome the ASE problem is the segmentation of the thin disk to limit the propagation of ASE to smaller subapertures [28]. Additionally, it is possible to combine two or more thin disks in one amplifier or oscillator to multiply the average power. Despite the mainly axial heat flow in the thin disk, a laser pulse still experiences a certain phase distortion in a double pass through the disk. The origin of the distortion are the temperature dependency of the index of refraction of Yb:YAG, the expansion of the thin disk itself and the deformation of the heat sink. The parabolic component of this distortion is the pump-powerdependent “thermal” lens, counteracted with the concept of the dynamically stable resonator. As the acceptable variation range of the thermal lens is in-

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versely proportional to the mode area, it will be very important to reduce the effect of the thermal lens according to the increase of the mode area. The higher-order components of the distortion are associated with diffraction loss, which increases with the diameter of the pump spot and currently limits CW fundamental mode operation to about 100 W. This limitation is, however, merely a technological problem, which may be solved by an improved heat-sink design, a better contacting technique or with the help of passive/active wavefront correctors. Thermally induced phase distortion can also arise from other optical elements in the resonator, like the Pockels cell, the mirrors or the SESAM. They can also be reduced by a proper choice of the materials, an adapted cooling technique, a careful resonator design and, if required, the use of wavefront correctors. A last important, but also technological limitation is, that the apertures of certain optical components, especially the Pockels cell, may not be sufficient. The maximum aperture of commercially available high-quality BBO crystals currently is 8 mm. Besides the difficulty to grow larger crystals, the construction of appropriate high repetition rate high-voltage switches is also problematic, as the quarterwave voltage is proportional to the crystal aperture.

4

Summary

Ultrafast lasers are an emerging technology with promising applications in science and industry. However, real-world applications have been excluded so far because of the sophisticated setups of available laser sources and the limited productivity. At present, different approaches are under investigation to overcome these limitations. As demonstrated here, high-power ultrafast thin disk and rare-earth-doped fiber lasers systems are promising candidates to fill this gap. Output powers in the 100 W range are accessible with pulse durations around or below 1 ps. High repetition rates will allow the processing speed to be increased significantly making ultrafast laser sources an interesting solution for many high-precision machining tasks in industry and medicine.

References [1] A. Giesen, H. H¨ ugel, A. Voss, K. Wittig, U. Brauch, H. Opower: Scalable concept for diode-pumped high power solid-state lasers, Appl. Phys. B-Lasers O. 58, 365–372 (1994) 55 [2] A. T¨ unnermann, H. Zellmer, W. Sch¨ one, A. Giesen, K. Contag: New concepts for diode-pumped solid-state lasers, in R. Diehl (Ed.): High-Power Diode Lasers, Top. Appl. Phys. 78 (2000) pp. 369–408 58 [3] C. Contag: Modellierung und numerische Auslegung des Yb:YAG-Scheibenlasers (Utz, M¨ unchen 2002) in German 58, 69

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[4] S. Erhard: Pumpoptiken f¨ ur den Scheibenlaser (Utz, M¨ unchen 2002) in German 58 [5] U. Keller, D. A. B. Miller, G. D. Boyd, T. H. Chiu, J. F. Ferguson, M. T. Asom: Solid-state low-loss intracavity saturable absorber for Nd:YLF lasers: an antiresonant semiconductor Fabry–Perot saturable absorber, Opt. Lett. 17, 505– 507 (1992) 58 [6] U. Keller, K. J. Weingarten, F. X. K¨ artner, D. Kopf, B. Braun, I. D. Jung, R. Fluck, C. H¨ onninger, N. Matuschek, J. Aus der Au: Semiconductor saturable absorber mirrors (SESAMs) for femtosecond to nanosecond pulse generation in solid-state lasers, IEEE J. Sel. Top. Quant. 2, 435–453 (1996) 58 [7] G. J. Sp¨ uhler, R. Paschotta, U. Keller, M. Moser, M. J. P. Dymott, D. Kopf, J. Meier, K. J. Weingarten, J. D. Kmetec, J. Alexander, G. Truong: Diodepumped passively mode-locked Nd:YAG laser with 10 W average power in diffraction-limited beam, Opt. Lett. 24, 528–530 (1999) 59 [8] G. J. Sp¨ uhler, T. S¨ udmeyer, R. Paschotta, M. Moser, K. J. Weingarten, U. Keller: Passively mode-locked high-power Nd:YAG lasers with multiple laser heads, Appl. Phys. B-Lasers O. 71, 19–25 (2000) 59 [9] D. E. Spence, P. N. Kean, W. Sibbett: 60-fsec pulse generation from a selfmode-locked Ti:sapphire laser, Opt. Lett. 16, 42–44 (1991) 59 [10] C. H¨ onninger, R. Paschotta, F. Morier-Genoud, M. Moser, U. Keller: Qswitching stability limits of CW passive mode locking, J. Opt. Soc. Am. B 16, 46–56 (1999) 59 [11] R. Paschotta, J. Aus der Au, G. J. Sp¨ uhler, S. Erhard, A. Giesen, U. Keller: Passive mode locking of thin disk lasers: effects of spatial hole burning, Appl. Phys. B-Lasers O. 72, 267–278 (2001) 59 [12] J. Aus der Au, G. J. Sp¨ uhler, T. S¨ udmeyer, R. Paschotta, R. H¨ ovel, M. Moser, S. Erhard, M. Karszewski, A. Giesen, U. Keller: 16.2 W average power from a diode-pumped femtosecond Yb:YAG thin disk laser, Opt. Lett. 25, 859–861 (2000) 60, 62 [13] E. Innerhofer, T. S¨ udmeyer, F. Brunner, R. H¨ aring, A. Aschwanden, R. Paschotta, C. H¨ onninger, M. Kumkar, U. Keller: 60-W average power in 810-fs pulses from a thindisk Yb:YAG laser, Opt. Lett. 28, 367–369 (2003) 60 [14] N. V. Kuleshov, A. A. Lagatsky, V. G. Shcherbitsky, V. P. Mikhailov, E. Heumann, T. Jensen, A. Diening, G. Huber: CW laser performance of Yb and Er,Yb doped tungstates, Appl. Phys. B-Lasers O. B 64, 409–413 (1997) 61 [15] F. Brunner, T. S¨ udmeyer, E. Innerhofer, F. Morier-Genoud, R. Paschotta, V. E. Kisel, V. G. Shcherbitsky, N. V. Kuleshov, J. Gao, K. Contag, A. Giesen, U. Keller: 240-fs pulses with 22-W average power from a mode-locked thindisk Yb:KY(WO4 )2 laser, Opt. Lett. 27, 1162–1164 (2002) 61 [16] T. S¨ udmeyer, F. Brunner, E. Innerhofer, R. Paschotta, K. Furusawa, J. C. Baggett, T. M. Monro, D. J. Richardson, U. Keller: Nonlinear femtosecond pulse compression at high average power levels using a large mode area holey fiber, Opt. Lett. 28, 1951–1953 (2003) 62 [17] M. Ferray, A. L’Huillier, X. F. Li, L. A. Lompr´e, G. Mainfray, C. Manus: Multiple-harmonic conversion of 1064 nm radiation in rare gases, J. Phys. BAt. Mol. Opt. 21, L31–L35 (1988) 62 [18] M. M. Murnane, H. C. Kapteyn, M. D. Rosen, R. W. Falcone: Ultrafast X-ray pulses from laser-produced plasmas, Science 251, 531–536 (1991) 62

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[19] T. S¨ udmeyer, J. Aus der Au, R. Paschotta, U. Keller, P. G. R. Smith, G. W. Ross, D. C. Hanna: Femtosecond fiber-feedback OPO, Opt. Lett. 26, 304–306 (2001) 62 [20] T. S¨ udmeyer, J. Aus der Au, R. Paschotta, U. Keller, P. G. R. Smith, G. W. Ross, D. C. Hanna: Novel ultrafast parametric systems: high repetition rate single-pass OPG and fiber-feedback OPO, J. Phys. D Appl. Phys. 34, 2433– 2439 (2001) 62 [21] D. M¨ uller, S. Erhard, O. Ronsin, A. Giesen: Thin disk multi-pass amplifier, TOPS 83, 278–284 (2003) 63 [22] C. H¨ onniger, I. Johannson, M. Moser, A. Giesen, U. Keller: Diode-pumped thin disk Yb:YAG regenerative amplifer, Appl. Phys. B-Lasers O. 65, 423–426 (1997) 63 [23] D. M¨ uller, S. Erhard, A. Giesen: High power thin disk regenerative amplifier, TOPS 50, 319 (2000) 64 [24] A. Beyertt, D. M¨ uller, D. Nickel, A. Giesen: Femtosecond thin disk Yb:KYW regenerative amplifer without CP, in OSA Tech. Dig. Ser. (Opt. Soc. Am., Washington 2003) p. 372 64, 68 [25] W. H. Lowdermilk, J. E. Murray: The multipass amplifier: Theory and numerical analysis, J. Appl. Phys. 51, 2436–2444 (1980) 66 [26] A. E. Siegman: Lasers (Univ. Science Books, Mill Valley 1986) 68 [27] V. Magni: Multielement stable resonators containing a variable lens, J. Opt. Soc. Am. A 4, 1962–1969 (1987) 68 [28] L. E. Zapata, R. J. Beach, S. A. Payne: High power Yb:YAG/YAG composite thin-disk laser, in Tech. Dig. (CLEO 2001) 69

Index

amplified spontaneous emission (ASE), 69

Pockels cell, 64, 69 pump optic, 56

density of the extractable energy, 65

regenerative amplifier, 63

extraction efficiency, 67

SESAM, 58

geometrical multipass amplifier, 63 Kerr-lens mode locking (KLM), 59

“thermal” lens, 69 thin-disk laser, 55

mode locking, 58 multipass amplifier, 63

Yb:KYW, 61 Yb:YAG, 60

Interaction with Atmosphere Detlef Breitling1 , Sergei Klimentov2 , and Friedrich Dausinger1 1

2

Institut f¨ ur Strahlwerkzeuge (IFSW), Universit¨ at Stuttgart, Pfaffenwaldring 43, 70569 Stuttgart, Germany [email protected] General Physics Institute (GPI) of Russian Academy of Sciences, Vavilov St. 38, 119991 Moscow, Russia

Abstract. Contrary to longer-pulsed laser irradiation, ultrashort laser pulses in the femto- and low picosecond-pulse duration domains are expected to be too short to interact directly with the material vapor they produce during ablation. Nonetheless, the very high intensities reached by focused ultrashort pulses cause a number of interaction phenomena of the pulses with the ambient atmosphere. The various effects are discussed with respect to their relevance to femtosecond laser machining. Optical breakdown, the threshold of which is easily reached with ultrashort pulses, can cause absorption losses despite the short irradiation times. When nonlinear interaction with the atmospheric gas takes place additionally at even higher intensities, the phenomenon of conical emission leads to beam-profile disruption and increased beam divergence. During ablation shock waves are created by the rapid vapor flow. Their visualization provides insight into individual ablation processes and allows the effects of laser parameter variations to be judged. In the case of multi-pulse machining, residual ablated matter left behind by previous pulses is accumulated in the atmosphere and can interact with subsequent pulses.

1

Optical Breakdown

In gaseous media optical breakdown occurs when the electric-field strength of the optical field is sufficient to accelerate free electrons enough to enable further impact ionization and a subsequent avalanche of free-electron generation. Generally, the process depends on the presence of a few so-called “lucky” energetic electrons among the free-electron distribution [1]. For ultrashort laser pulses, however, especially in the femtosecond pulse domain, high power densities are easily obtained that enable multiphoton absorption to generate a much higher number of starting electrons for the impact ionization avalanche [2]. Thus optical breakdown in the femtosecond regime can start more efficiently and is, furthermore, less subject to statistical variations compared to longer laser pulses [3]. When an ultrashort pulsed laser beam is focused under ambient atmospheric conditions, each pulse creates a small, short-lived plasma cloud visible to the observer through its optical emission. The onset of this plasma emission with increasing power density may be regarded as the breakdown −1/2 dependence on pulse duration, which is depicted threshold. It exhibits a τH F. Dausinger, F. Lichtner, H. Lubatschowski (Eds.): Femtosecond Technology for Technical and Medical Applications, Topics Appl. Phys. 96, 75–91 (2004) c Springer-Verlag Berlin Heidelberg 2004


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in Fig. 1. Consequently, in terms of energy density the threshold increases 1/2 with pulse duration following a τH trend. The generation of a laser-induced plasma for optical breakdown in gases is associated with energy loss for the laser pulse. In Fig. 2 the total energy transmission through the breakdown spark is shown as a function of pulse duration. It is interesting to note that despite the lower energy density threshold for breakdown in the femtosecond domain and the subsequently stronger spark emission, the actual energy loss is near to zero at about 100 fs. This could be explained by the multiphoton ionizationseeded electron avalanche occurring only late during these short pulses, which causes only a small frac-

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Fig. 3. Shadowgraphs of the gas-dynamic shock waves generated by laser-induced breakdown sparks in air atmosphere for different pulse durations. The laser beam is incident from the right-hand side and the nominal, unperturbed focal plane is indicated by arrows. The frames are recorded approximately 35 ns after the laser pulse (λ = 800 nm, Q = 500 µJ, df = 15 µm, H = 280 J/cm2 , pair = 950 hPa) [6]

tion of the pulse tail to be subject to strong absorption in the breakdown plasma [5]. With increasing pulse duration, the absorption in the plasma increases considerably, see Fig. 2. The breakdown strength can also be judged by means of shadowgraphy on nansosecond timescales, which allows the gas-dynamic shock fronts that have been caused by the plasma spark initiated by gas breakdown to be visualized. The frames in Fig. 3 have been recorded for several pulse durations in the femtosecond and picosecond time domains. They display the extent of the shock waves about 35 ns after the breakdown-generating laser pulse. Being caused by a breakdown filament of a certain length, the shock fronts show cylindrical symmetry with little or no variation of the shock waves’ lengths along the laser-beam axis during their temporal expansion. At comparable energy-density levels the breakdown is considerably more violent for femtosecond pulses, leaving a long plasma streak of up to 2 mm length in the atmosphere. Especially for these short pulse durations, the breakdown occurs almost entirely in front of the nominal focal plane. The theory of gas-dynamic blast-wave expansion [7] enables the energy content of the breakdown shock waves in air to be determined, Fig. 4. According to this analysis the energy loss in the breakdown can amount to as much as 15% of the entire pulse energy for 120 fs pulses or to 3% and 2%, respectively, for 500 fs and 1 ps pulses. These results seem to be in clear disagreement with the measurements of energy-transmission losses in the breakdown spark depicted in Fig. 2. As the mechanisms underlying the occurrence of optical breakdown with ultrashort pulses are still not fully understood, the discrepancy cannot be explained to date.

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Position w. respect to focus zRel in µm Fig. 4. Line energy deposition along the laser beam in air breakdown sparks for 120 fs, 500 fs, and 1 ps pulse duration vs. position along the spark. Using a time series of shadowgraphs comparable to those in Fig. 3, Sedov–Taylor analysis of the radial expansion of the gas-dynamic shock waves at each position zrel provides the distribution of deposited energy per unit length q(zrel ) according to [7]: r(t) = Λ0 (q/ρ0 )1/4 t1/2 , where ρ = 1.293 kg/m3 is the density of the surrounding air atmosphere and Λ0 a dimensionless constant amounting to 1.00 for cylindrical symmetry. Integration of q(zrel ) along the spark length leads to the net energy contents denoted in the graph (λ = 800 nm, Q = 500 µJ, df = 15 µm, H = 280 J/cm2 , pair = 950 hPa) [8]

2

Conical Emission

For focused ultrashort pulsed laser radiation optical breakdown is associated with a number of nonlinear phenomena. Some of them can contribute to a heavy widening and distortion of the beam profile in the far field behind the breakdown spark as depicted in Fig. 5. The effect is known as conical emission (CE) [9, 10]. The colorful ring patterns indicate that broadband nonlinear wavelength conversion is taking place in the optical-breakdown region, while the blue central spot results from fluorescence on the paper screen illuminated by third-harmonic radiation. As the left and middle frames indicate, the patterns are strongly dependent on laser power density in the focus. The spectral distribution along a cross section of the high-fluence pattern in the right frame of Fig. 5 reveals the complex substructure of the beam profiles and confirms the notion that radiation components at shorter wavelengths occur primarily, although not exclusively, towards the outer regions of the ring structure. It is most striking that no spectral contributions are found for long wavelengths beyond the original laser line at about 805 nm on the spectrometer scale. Spatially integrated spectra are shown in Fig. 6 for different pulse dura-

Interaction with Atmosphere 260 J/cm2

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Fig. 5. Left and middle: far-field beam profiles captured by a commercial digital photo camera on a white paper screen about 620 mm beyond a dielectric breakdown spark in the laser focus in air atmosphere for medium and high fluence levels. Horizontally polarized laser operating at 800 nm wavelength, τH = 125 fs and focused to a spot diameter of df = 18 µm, M 2 = 2. The dashed circle corresponds to the size of an unperturbed beam profile for focusing conditions with an effective F -number of 9. The original color of the profile patterns is blue within the circle and the rings display shades of red, starting with a long-wavelength red for the innermost ring towards orange-red for the outermost portion of the large ring structure. Right: spectral content of the spatial distribution along a slit aperture positioned off-center in the beam profile at high energy density [11]

tions and energy densities. At low fluences the spectra only contain the laser line of the Ti:sapphire laser and shorter contributions appear successively with increasing fluence levels. The threshold energy density for wavelengthconversion processes is considerably higher for longer laser pulses, but the spectra also reveal that conical emission is mainly, but not solely governed by intensity. At comparable power densities the spectra for 105 fs pulses exhibit slightly wider spectral ranges than the corresponding spectra at 1.1 ps, leading in turn to a noticeably weaker beam deformation at longer pulses. Experimental ablation results show that even for a focus position at the target surface, the ablation zones can be significantly enlarged when conical emission occurs for femtosecond laser pulses at high energy densities in the focal spot. This, however, suggests that the nonlinear interaction leading to conical emission must be taking place at a considerable distance in front of the actual focal plane. To verify this notion, beam cross sections along the propagation direction have been imaged directly onto the entrance slit of a spectrometer and the corresponding spectra have been analyzed in terms of their spectrally converted fractions, Fig. 7. Normalized with respect to the peak intensity at the original laser line, the distributions reveal indeed that the highest conversion efficiency occurs several hundred micrometers in front of the focus. Taking into account a total far-field divergence angle of as much as 18◦ (Fig. 5), the size of the ablation zone in the focal plane can be determined from the location of the most intensive interaction (in this case about 600 µm in front of the focal plane). Ablation craters of 190 µm diameter must be expected and have indeed been found under corresponding experimental

Detlef Breitling et al. 1.1 ps

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Fig. 6. Spectra of far-field patterns as displayed in Fig. 5 for 105 fs (left) and 1.1 ps (right) pulse duration and different energy levels. A collimated far-field beam profile was projected onto the entrance slit of a spectrometer and optimized for maximum spectral content. The relative height of the various observed peaks depends strongly on the position of the slit relative to the profile [11]

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1.2 mm 0.6 mm 0.3 mm In focus 650

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Fig. 7. Normalized spectra at different beam cross sections along the propagation direction for 150 fs pulses in ambient air atmosphere. The region of highest wavelengthconversion efficiency does not coincide with the focal plane but is located several hundred micrometers before (λ = 800 nm, Q = 660 µJ, df = 18 µm, H = 260 J/cm2 , pair = 950 hPa) [4]

conditions, Fig. 8. Similarly, these findings are in good agreement with a location of the breakdown shock wave far in front of the focal plane as depicted in Fig. 3. For a more quantitative analysis of the amount of redistributed radiation generated by conical emission, a small oblique disk has been inserted into the far-field beam profile behind the breakdown region such that it blocks most of the unperturbed laser beam when no nonlinear interaction takes place. Thus it is possible to measure the amount of radiation that is transferred into larger divergence angles due to conical emission by a simple energy measurement behind the disk, Fig. 9. The decrease of atmospheric pressure or energy density as well as the use of helium atmosphere can reduce the amount of widely scattered radiation considerably. In Fig. 10 the dependence of conical emission strength on energy density is depicted in more detail. The graph,

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Fig. 8. Longitudinal section of a blind hole drilled in stainless steel by 800 125 fs pulses (λ = 800 nm, Q = 640 µJ, df = 18 µm, H = 250 J/cm2 , pair = 950 hPa) [4]

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furthermore, indicates that the amount of scattered radiation can also be reduced significantly by the use of longer pulse durations. The strong dependence on the atmospheric gas type and pressure confirms nonlinear interaction with the ambient gas as the origin of conical emission. Indeed, early theoretical descriptions of the phenomenon have proposed a number of phenomena as explanations. However, models based on the third-order optical nonlinearity of the medium, such as self-phase modulation (SPM) and four-wave mixing (FWM), or even SPM in combination with FWM and Raman processes have each failed to account for the host of observations that are associated with conical emission, especially concerning the spatial properties of the phenomenon [9]. More successful was the approach to describe the colorful ring structures by means of a boundary refraction

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Detlef Breitling et al. 60

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Fig. 10. Conical-emission radiation component TCE passing around an oblique disk aperture on the beam axis. Variation of the focus energy density for 130 fs and 1.5 ps pulses in air and helium atmospheres at 950 hPa (λ = 800 nm, df = 18 µm) [4]

at the surface of self-trapped filaments. The nonlinear polarization propaˇ gating along this surface will invoke light emission analogous to Cerenkov ˇ radiation, the so-called light-induced Cerenkov emission [9]. The filament itself is frequently explained by the balance between nonlinear self-focusing by SPM (based on the third-order nonlinear optical Kerr effect) and defocusing caused by the negative refractivity contribution due to free electrons generated via multiphoton ionization (MPI) in the strong optical field [10, 12]. An alternative explanation to this self-channeling model is based on self-focusing because of the optical Kerr effect as well. Due to the nonlinear response of the Kerr effect, successive time slices of the ultrashort pulse with different intensities are focused at variable distances along the laser-beam axis [13]. The time-integrated detection of the moving foci only results in an apparent confined filament. Refined by defocusing due to free electrons generated via MPI [14] or by absorption and refraction at the MPI-generated electrons and by group-velocity dispersion [15], the moving-focus model is also claimed to reproduce the entire emission characteristics observed for conical emission. However, despite the scientific dispute about the interpretation of their respective roles in the model, it is generally agreed that both self-phase modulation and multiphoton ionization must be regarded as fundamental mechanisms responsible for conical emission.

3

Shock Waves and Material Vapor Flow

The strong interaction of the intense ultrashort-pulsed laser radiation with the atmosphere leads to the expectation that the presence of material-vapor plumes during ablation will deteriorate the beam further, especially when ablated material accumulates over multiple pulses at high repetition rates. Indeed, these plasma-related effects have been found to exhibit a significant impact on laser processes and might even impose a limit for efficient highquality machining at high repetition rates [4, 11]. The interaction of material

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vapor and the laser radiation has been studied by means of resonance absorption photography. The interaction zone above the target surface is visualized by shadowgraphy but with an illumination tuned to a wavelength at which certain species within the plume absorb resonantly. Aluminum has proven to be an excellent sample material since its neutral atoms in their ground states can absorb resonantly at wavelengths of 394.4 nm and 396.15 nm [16, 17], thereby providing sufficient sensitivity and contrast in the back-illuminated frames for the visualization of the ablated vapor [18]. Resonance-absorption shadowgraphs of the laser-induced ablation plume about 100 ns after the ablation of aluminum by a single laser pulse are depicted in Fig. 11 for different pulse durations. The frames show the expansion of both the hemispherical shock waves in air (narrow black-and-white lines) that precede the plume of actual ablated aluminum vapor (dark region inside the plume). Along the beam axis of the processing laser, a second shock front can be distinguished that is caused by the expansion of the plasma filament. This plasma, in turn, is created by the gas breakdown initiated in the laser-beam focal region by the ultrashort laser pulses similarly to the pure breakdown shock waves shown in Fig. 3. While the main shock wave exhibits spherical symmetry and expands radially outward from the ablation zone, the gas-breakdown wave above shows a cylindrical symmetry, which is especially prominent for the subpicosecond laser pulses. The air-breakdown shock wave being far more pronounced for shorter pulses is due to the much stronger radiation–atmosphere interaction that causes the breakdown. The vapor flow further reveals a much more regular mushroom-cloud pattern for longer pulses. In the shorter femtosecond regime, the turbulence is too strong to allow similar patterns to develop. The wavefront disturbance that is caused by the nonlinear radiation–gas interaction before the development of the actual air breakdown is believed to be responsible for this [19]. The disturbance increases for shorter pulses because of the longer propagation length during which nonlinear effects can occur. As shown by Fig. 5 and Fig. 8, this nonlinear interaction can distort the beam profile to a considerable extent, causing irregular ablation conditions at the target, which, in turn, lead to turbulent vapor flow. For pulsed ablation with multiple pulses, the breakdown strength can be dramatically increased when subsequent pulses hit vapor that was left behind above the target by previous pulses [11]. In Fig. 12 frames of the ablation plumes are displayed that have been captured for the last in a series of 500 fs pulses with 1 kHz repetition rate. Both hemispherical and cylindrical shock waves and the material-vapor cloud each show a distinct behavior with larger pulse numbers. While the hemispherical shock wave is only little affected in its extent and symmetry, the vapor distribution quickly loses its symmetry and becomes more and more turbulent towards higher number of pulses. Furthermore, the outline of the cloud becomes less defined and the vapor mixes strongly with the atmospheric gas inside the shock fronts. Most striking

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5 ps

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500 fs

250 fs

120 fs

500 µm

Target

Fig. 11. Resonance-absorption shadowgraphs displaying the morphology for singlepulse ablation plumes on an aluminum target for different pulse durations. The frames have been recorded approximately 100 ns after the processing-laser pulse. Laser parameters: processing wavelength λ = 800 nm, pulse energy Q = 500 µJ, beam-propagation ratio M 2 = 1.5. Focusing conditions: effective F -number 9, focus diameter df = 15 µm, energy density H = 280 J/cm2 ; focus on target surface [19]

2nd pulse

5th pulse

10th pulse

20th pulse

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100th pulse

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Fig. 12. Material-vapor distribution and shock waves of ablation plumes during high repetition rate percussion drilling of an aluminum target in air visualized by resonance-absorption photography. The plume generated for the last of a series of pulses is shown at a temporal delay of approximately 50 ns with respect to that pulse. Processing-laser parameters: λ = 800 nm, τH = 500 fs, Q = 500 µJ, df = 18 µm, H = 200 J/cm2 , focus on target surface [11]

is the change of the breakdown shock wave in the focal region with multipulse ablation. There is no direct correlation with the pulse number. Apparently, already the second pulse might ignite a plume that is entirely different from the breakdown-spark shock wave of a single pulse and the shock wave for the 5th pulse shows a strong enhancement of the breakdown feature in a region of 1 mm and more above the target surface. On the other hand, the effect is not seen, for example, in the frames for the 20th and 50th pulses. The breakdown behavior for multiple pulses being obviously of considerable statistical nature renders the effect of potential material accumulation difficult to judge – especially when short subpicosecond laser pulses are concerned. For picosecond pulse durations many pulses do not experience strong disruption in the interaction zone even at higher pulse numbers. They therefore allow the effect of material accumulation to be studied in more detail.

Interaction with Atmosphere 1 Hz

1000 Hz

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100 Hz

100th pulse

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Fig. 13. Influence of repetition rate and pulse number for 5 ps pulses analyzed by resonance-absorption shadowgraphy. Frames recorded approximately 100 ns after the last processing-laser pulse of the pulse train. Aluminum target. Processinglaser parameters: λ = 800 nm, Q = 500 µJ, df = 15 µm, H = 280 J/cm2 , focus on target surface [19]

In Fig. 13 ablation plumes during drilling for different repetition rates (columns) and pulse numbers (rows) are presented. A corresponding plume of a single pulse is shown to the left. Comparing the 2nd, 10th, and 100th pulses at 1 kHz repetition rate (last column), the shock fronts reveal a transfer of energy content away from the hemispherical wave towards the breakdown for higher pulse numbers creating a larger shock-front feature at the tip of the plume. In terms of the material-vapor fractions increasingly irregular distributions are found for higher pulse numbers. At first, vapor also penetrates the shock wave initially created by the air breakdown (2nd pulse), then the mushroom cloud becomes partially disrupted (10th pulse). Finally, clear flow patterns are no longer discernible and the vapor more or less fills the plume’s entire central section along the laser-beam axis (100th pulse). However, experiments at a single repetition rate do not clarify the nature of the irregular, turbulent vapor distributions at higher pulse numbers: whether they are indeed due to an accumulation of ablated material in the atmosphere or merely an issue of a change in the ablation characteristics – and thus of the initial vapor flow – due to the increasing crater depth and the burr around it. To this end Fig. 13 also compares the variation of pulse number for several repetition rates. Most striking is the fact that despite the change of three orders of magnitude there is very little change of the plume morphology and vapor clouds for a fixed pulse number. To a certain extent, disorder and turbulent flow within the vapor fraction increase slightly with higher repetition rates, but they hardly exceed the fluctuations encountered among

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several equivalent experiments. The dominant effect is thus the change of ablation geometry in deeper craters. However, the enlargement of the ablation plumes’ breakdown component cannot be easily explained by the changing surface morphology of the target. Accumulation of ablated vapor and particles during multiple pulses seem to be responsible for this particular effect. In consequence, the ablated matter must be very long living so that it can present a similar obstacle to a subsequent laser pulse even after a 1 s delay as it is for short millisecond delay times (compare first and fourth columns in Fig. 13).

4

Residual Ablated Matter

For delay times exceeding about 10 µs to 15 µs, the vapor is too thinned out by the expansion or even by a certain degree of condensation to generate enough contrast in the back-illuminated resonance-absorption shadowgraphs. To a certain extent, experiments employing various pulse numbers as in Fig. 13 can provide useful information about the potential whereabouts of ablated matter when a subsequent pulse reaches the interaction zone. Nonetheless, in order to directly visualize ablated material on timescales reaching up to the millisecond domain, Mie-scattering photography can be applied. In this case scattered UV radiation enables vapor and condensed particles to be detected for a wide range of temperatures and densities of the plume [20, 21]. In Fig. 14 a series of Mie-scattering photographs is displayed for plumes generated by single-pulse ablation with 500 fs pulses. Until about 100 µs, the plume remains relatively compact without considerable inner structure. As it gradually expands, it also moves away from the target, leaving a relatively vapor-free zone directly above the workpiece that stretches to about 800 µm after 100 µs. On a 100 µs timescale the plume breaks up, sometimes forming interesting flow patterns such as the eddy in the frame recorded after 1 ms. While the extent and morphology for the compact plumes at short delay times are fairly reproducible, these complex flow patterns exhibit a very large variation even under identical experimental conditions. In general, however, the zone above the target remains clear of vapor for times exceeding 100 µs. Thus the region of highest laser intensity, where the gas breakdown occurs initially for a following pulse, is only moderately affected by ablated material of the first pulse. This explains why most subsequent pulses in Fig. 13 do not show dramatically increased breakdown phenomena [19]. With a considerable amount of ablated material remaining in the atmosphere above the target for periods of several milliseconds, it is of interest to also apply the Mie-scattering method to pulse bursts and to visualize directly the extent of material accumulation during multiple pulses. A series of frames for femtosecond-pulse ablation is displayed in Fig. 15. For comparison a corresponding image for the 50th pulse is also shown for 2 ps pulse duration. Similar to frames with long time delays in Fig. 14, the scattering photographs reveal the formation of clouds with quite intriguing vapor distributions and

Interaction with Atmosphere 20 µs

100 µs

1 ms

2 ms

Laser

5 µs

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2 mm Target 500 µs

Fig. 14. Vapor-cloud expansion in air on microsecond and millisecond timescales obtained for single-pulse ablation of an aluminum target with Mie-scattering photography. On the frames’ lower edges the target is indicated by a gray bar. Laser parameters: λ = 800 nm, τH = 500 fs, Q = 500 µJ, df = 15 µm, H = 280 J/cm2 , focus on target surface [19]

flow patterns. Despite the reproducibility of certain patterns being very low, ablated matter does indeed accumulate strongly with increasing pulse number. Nonetheless, the cloud does not become more dense towards higher pulse numbers but rather increases in size as more and more vapor accumulates. Hence in most cases, the gas-breakdown behavior – in terms of increasing breakdown strength and probability – does not change dramatically even for higher burst-pulse numbers. Finally, it is interesting to note that for femtosecond pulses the plumes in Fig. 15 are mainly composed of fine vapor, creating a nebula-like appearance of the clouds. For picosecond pulses, on the contrary, the plume is dominated by small, brightly scattering centers whose intensity is much higher than that of the fine vapor that is still present in between. These bright spots must be attributed to the presence of larger particles or clusters of particles that can scatter radiation far more efficiently [19].

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τH = 110 fs

τH = 2 ps

Laser

Single pulse

2nd pulse

5th pulse

50th pulse

50th pulse

2 mm Target

Fig. 15. Accumulation of ablated material visualized by Mie-scattering photography. The frames have been recorded approximately 500 µs after the last of a burst of pulses of the processing laser. The gray bar on the lower edge indicates the target with a lighter section denoting the position of laser processing. Aluminum target. Laser parameters: λ = 800 nm, fP = 1 kHz, Q = 500 µJ, df = 15 µm, H = 280 J/cm2 , focus on target surface [19]

References [1] M. v. Allmen: Laser-Beam Interaction with Materials (Springer, Berlin, Heidelberg 1987) 75 [2] A. C. Tien, S. Backus, H. Kapteyn, M. Murnane, G. Mourou: Short-pulse laser damage in transparent materials as a function of pulse duration, Phys. Rev. Lett. 88, 3883–3886 (1999) 75 [3] D. Du, X. Liu, G. Korn, J. Squier, G. Mourou: Laser-induced breakdown by impact ionization in SiO2 with pulse widths from 7 ns to 150 fs, Appl. Phys. Lett. 64, 3071–3073 (1994) 75 [4] S. M. Klimentov, T. V. Kononenko, P. A. Pivovarov, S. V. Garnov, V. I. Konov, D. Breitling, F. Dausinger: Effect of nonlinear scattering of radiation in air on material ablation by femtosecond laser pulses, in F. H. Dausinger, V. I. Konov, V. Y. Baranov, V. Y. Panchenko (Eds.): Proc. SPIE 5121 (Intl. Soc. for Opt. Eng., Bellingham 2003) pp. 77–86 76, 80, 81, 82 [5] B. C. Stuart, M. D. Feit, S. Herman, A. M. Rubenchik, B. W. Shore, M. D. Perry: Nanosecond-to-femtosecond laser-induced breakdown in dielectrics, Phys. Rev. B 53, 1749–1761 (1996) 77 [6] D. Breitling, A. Ruf, F. Dausinger: Fundamental aspects in machining of metals with short and ultrashort laser pulses, in P. R. Herman, J. Fieret, A. Pique, T. Okada, F. G. Bachmann, W. Hoving, K. Washio, X. Xu, J. J. Dubowski, D. B. Geogehan, F. Tr¨ ager (Eds.): Proc. SPIE 5339 (Intl. Soc. for Opt. Eng., Bellingham 2004) pp. 49–63 77 [7] L. I. Sedov: Similarity and Dimensional Methods in Mechanics (Cleaver–Hume, London 1959) 77, 78 [8] D. Breitling: Ablation plume dynamics and consequences for materials processing, in P. Berger, F. Dausinger, C. F¨ ohl (Eds.): Proc. 5th Intl. Workshop on Fundamentals of Ablation with Short Pulsed Solid State Lasers 2004 (Hirschegg, Austria) (Forschungsgesellschaft f¨ ur Strahlwerkzeuge FGSW, Stuttgart 2004) 78 [9] I. Golub: Optical characteristics of supercontinuum generation, Opt. Lett. 15, 305–307 (1990) 78, 81, 82

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[10] E. T. J. Nibbering, P. F. Curley, G. Grillon, B. S. Prade, M. A. Franco, F. Salin, A. Mysyrowicz: Conical emission from self-guided femtosecond pulses in air, Opt. Lett. 21, 62–64 (1996) 78, 82 [11] D. Breitling, A. Ruf, P. W. Berger, F. H. Dausinger, S. M. Klimentov, P. A. Pivovarov, T. V. Kononenko, V. I. Konov: Plasma effects during ablation and drilling using pulsed solid state lasers, in F. H. Dausinger, V. I. Konov, V. Y. Baranov, V. Panchenko (Eds.): Proc. SPIE 5121 (Intl. Soc. for Opt. Eng., Bellingham 2003) pp. 24–33 79, 80, 82, 83, 84 [12] H. R. Lange, G. Grillon, J. F. Ripoche, M. A. Franco, B. Lamouroux, B. S. Prade, A. Mysyrowicz, E. T. J. Nibbering, A. Chiron: Anomalous long-range propagation of femtosecond laser pulses through air: moving focus or pulse self-guiding?, Opt. Lett. 23, 120–122 (1998) 82 [13] A. Brodeur, C. Y. Chien, F. A. Ilkov, S. L. Chin, O. G. Kosareva, V. P. Kandidov: Moving focus in the propagation of ultrashort laser pulses in air, Opt. Lett. 22, 304–306 (1997) 82 [14] O. G. Kosareva, V. P. Kandidov, A. Brodeur, C. Y. Chien, S. L. Chin: Conical emission from laser–plasma interactions in the filamentation of powerful ultrashort laser pulses in air, Opt. Lett. 22, 1332–1334 (1997) 82 [15] M. Mlejnek, E. M. Wright, J. V. Moloney: Dynamic spatial replenishment of femtosecond pulses propagating in air, Opt. Lett. 23, 382–384 (1998) 82 [16] R. W. Dreyfus, R. Kelly, R. E. Walkup: Laser-induced fluorescence studies of excimer laser ablation of Al2 O3 , Appl. Phys. Lett. 49, 1478–1480 (1986) 83 [17] H. R. Griem: Plasma Spectroscopy (McGraw–Hill, New York 1964) p. 405 83 [18] R. M. Gilgenbach, P. L. G. Ventzeck: Dynamics of excimer laser-ablated aluminum neutral atom plume measured by dye laser resonance absorption photography, Appl. Phys. Lett. 58, 1597–1599 (1991) 83 [19] D. Breitling, K. P. M¨ uller, A. Ruf, P. Berger, F. Dausinger: Material-vapor dynamics during ablation with ultrashort pulses, in I. Miyamoto, A. Ostendorf, K. Sugioka, H. Helvajian (Eds.): Proc. SPIE 5063 (Intl. Soc. for Opt. Eng. 2003) pp. 81–86 83, 84, 85, 86, 87, 88 [20] D. B. Geoheagan, A. A. Puretzky, D. J. Rader: Gas-phase nanoparticle formation and transport during pulsed laser deposition of Y1 Ba2 Cu3 O7−d , Appl. Phys. Lett. 74, 3788–3790 (1999) 86 [21] A. A. Puretzky, H. Schittenhelm, X. Fan, M. J. Lance, L. F. Allard, Jr., D. B. Geoheagan: Investigations of single-wall carbon nanotube growth by timerestricted laser vaporization, Phys. Rev. B 65, 245425 (2002) 86

Index

ablated material, 85 ablation plume, 83, 84 absorption, 77 accumulated vapor, 87 accumulation, 85 air atmosphere, 77–82 atmosphere, 75, 80, 81 atmosphere pressure, 80, 81 avalanche ionization, 75, 76 beam divergence, 79, 80 beam profile, 78, 83 breakdown, 75, 77, 78, 83, 86, 87 breakdown by accumulated vapor, 87 breakdown filament, 77 breakdown shock wave, 83, 84 breakdown threshold, 75, 76

interaction with atmosphere, 75 ionization, 75 ˇ light-induced Cerenkov emission, 82 location of breakdown, 77, 80 location of nonlinear interaction, 79 Mie-scattering photography, 86–88 multiphoton absorption, 75 multiphoton ionization, 76, 82 nonlinear phenomena, 78, 81 optical breakdown, 75–77 optical Kerr effect, 82

conical emission, 78, 80 cylindrical shock wave, 83 cylindrical symmetry, 77, 78

percussion drilling, 84 plasma, 76, 83 plasma absorption, 77 plasma emission, 75, 76 pulse duration, 77

dependence on pulse duration, 75 distortion, 83 distortion of the beam profile, 78

radiation–atmosphere interaction, 83 repetition rate, 85 resonance absorption, 83

energy content, 77, 78 energy loss, 76, 77

self-defocusing, 82 self-focusing, 82 self-phase modulation (SPM), 81 shadowgraphy, 77, 83 shock wave, 77, 82 shock wave energy content, 85 shock wave symmetry, 83 symmetry, 77

filament, 77, 82, 83 focal plane, 77, 79, 80 four-wave mixing (FWM), 81 gas breakdown, 77 helium atmosphere, 80, 82 hemispherical shock wave, 83 impact ionization, 75

vapor, 82, 83 vapor accumulation, 84, 86, 88 vapor cloud, 83, 87 vapor cloud expansion, 87

Index vapor expansion, 86 vapor flow, 83, 85–87 vapor particle, 86, 87

vapor plume, 83 wavelength conversion, 78–80

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Interaction with Biological Tissue Holger Lubatschowski and Alexander Heisterkamp Laserzentrum Hannover e.V., Hollerithallee 8, 30419 Hannover, Germany [email protected] Abstract. In low-absorbing and low-scattering material, like biological tissue, optical breakdown can occur not only at the surface of the tissue but also inside the bulk. Here, different mechanical phenomena like cavitation and bubble development take place. In this chapter the basic interaction processes of ultrashort laser pulses with biological tissue like nonlinear effects during propagation through transparent media as well as plasma formation and mechanical side effects are discussed.

Biological tissue has a relatively low linear absorption coefficient especially in the mid-infrared region, where most femtosecond lasers emit (Fig. 1). As a consequence, plasma formation has a different origin and time history as is known from metals, where the direct linear absorption plays a major role for generating the first free electrons of the plasma. In low-absorbing and lowscattering material thus, optical breakdown can occur not only at the surface of the tissue but also inside the bulk. Here, different mechanical phenomena like cavitation and bubble development take place that cannot be observed at a free surface.

1 10 10 10 10 10 10 10

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Fig. 1. Absorption of different chromophores of biological tissue. In the infrared region, where most of the femtosecond lasers operate the average penetration depth 1/a is in the range of some millimeter (Data from [1]) F. Dausinger, F. Lichtner, H. Lubatschowski (Eds.): Femtosecond Technology for Technical and Medical Applications, Topics Appl. Phys. 96, 91–105 (2004) c Springer-Verlag Berlin Heidelberg 2004


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To get a better understanding of the dynamics of laser–tissue interaction with ultrashort pulses, water is used as a model substance, where most of the relevant effects can be studied in detail. At least, it has been shown that many parameters like the threshold for optical breakdown or stress-wave formation in water is similar to that in soft biological tissue [2, 3, 4, 5]. Therefore, in this Chapter the basic interaction processes of ultrashort laser pulses with biological tissue are mostly explained on pure water.

1 Nonlinear Propagation of Ultrashort Laser Pulses in Transparent Media When ultrashort laser pulses are focused inside transparent materials, extremely high field intensities lead to nonlinear interaction with the material. The most prominent effects are described in this section. 1.1

Self-Focusing

At high field strengths or light intensities, respectively, the polarization P of a transparent medium does not follow the oscillating field strength E linearly anylonger. Rather, susceptibilities χ of higher order become important: P (r, t) = 0 χ1 E(r, t) + 0 χ2 E(r, t)E(r, t) + 0 χ3 E(r, t)E(r, t)E(r, t) + · · · . (1) As a consequence, the index of refraction
n0 = 1 + χ1

(2)

has to be replaced by an index of refraction n(I) that depends on the intensity in isotropic media I: n(I) = n0 + n2 I + · · · .

(3)

Generally, the absolute value of n2 is positive [6]. For example, in water n2 = 1.8 × 10−16 cm2 /W [7], and for TiSa n2 = 3.2 × 10−16 cm2 /W [8]. Thus, the spatial beam profile of a high-intensity laser beam leads to a spatial variation in the index of refraction of the medium, in which the laser is propagating. In the case of a Gaussian beam profile, the central part along the beam axis has a higher index of refraction than its wings. As a result, the light will be focused as is known from graded-index devices. The divergence of the laser beam acts directly opposed to the focusing effect. At a certain threshold Pcr , both effects are exactly balanced [9]: Pcr =

cλ2 . 32π 2 n2

(4)

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When the laser power exceeds Pcr , self-focusing predominates and the beam collapses [9]. Numerical calculations yield a theoretical value of: Pcollaps ≥ 3.77Pcr .

(5)

Typical numbers for the Pcr are in the range of some megawatt. 1.2

Self-Phase Modulation (SPM)

The temporal development I(t) of a laser pulse can be described by a Gauss function: I(t) = I0 e−2(t/τ ) , 2

(6)

where T is the FWHM of the laser pulse. After propagation of a distance L within a medium, the phase φL of the electromagnetic wave yields:   L [n0 + n2 I(t)] φL = ω . (7) c The actual frequency ω(t) of the wave train can be written as the temporal derivation of the phase: ω(t) = −

n2 ωL dφ =ω− ∂t I(t) . dt c

(8)

At the beginning of the laser pulse, where its intensity increases, the phase front will be delayed due to the positive temporal derivation. The wavelength will be shifted to larger wavelengths. On the other hand, at the latter part of the pulse, where the derivation of the intensity is negative, the pulse will be shifted to shorter wavelength. Consequently, the spectral profile of the laser pulse becomes broader. 1.3

Group-Velocity Dispersion (GVD)

A typical 100 fs pulse has a spectral width of approximately 8 nm. Normal dispersion (dn2 / dλ2 > 0) of common media leads to an additional broadening of the ultrashort pulses. After passing the transparent material, having a positive group-velocity dispersion (GVD), the longer part of the spectrum propagates ahead of the pulse and the shorter wavelengths are delayed. This again results in a positive “chirp” of the pulse. In contrast to self-phase modulation, GVD leads to a spatial separation of each fraction of the frequency spectrum. In terms of application of ultrashort pulses, long passages through optically dense materials such as glass have to be minimized in order to avoid GVD. As an example, a 100 fs pulse elongates in conventional optical material √ by a factor of 2 after traveling a distance of 1 cm [10].

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On the other hand, using elaborate optics, it is possible to impress an adequate “pre-chirp” onto the laser pulse, in such a way that due to the GVD of the optics, the pulse has its minimum pulse width within its focal point. 1.4

Continuum (White Light) Generation

Focusing ultrashort laser pulses into a Kerr medium can lead to the generation of a broad continuum of radiation with a spectral width of several tens of nanometer (Fig. 2). The largest part of the spectrum usually is represented by the anti-Stokes component. The origin of this radiation is still discussed controversially in the literature. Most often an interplay between self-phase modulation and self-focusing is assumed [11]. The anti-Stokes part of the spectrum is believed to be generated inside the free-electron gas by SPM and the smaller Stokes part is attributed to SPM within the Kerr medium. Subsequently, this radiation is amplified by stimulated Raman processes [12] and, moreover, broadened by four-wave mixing. According to experimental investigations, this broad spectrum has the same coherence capacity as the generating-laser pulse [13].

Spectrum a.u.

Laser pulse τ=150fs (FWHM), λ=780nm Spectrum after passing a water cell

700

750

800

850

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λ / nm

Fig. 2. Spectral broadening of a 150 fs laser pulse (6 µJ) after passing a 15 mm water cell

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2 Plasma Ignition: Multiphoton Ionization vs. Avalanche Ionization In order to achive a sufficient energy deposition in low-absorbing material, the laser intensity has to exceed a certain threshold, which leads to nonlinear absorption. Nonlinear absorption is the basic mechanism for depositing energy into transparent media. The process is followed by a so-called optical breakdown, with the generation of a free-electron density ρ exceeding 1021 cm−3 [14, 15, 16]. The intensity threshold for nonlinear absorption of ultrashort pulses with subsequent plasma formation is 1011 W/cm2 in water [17] and 1 × 1013 W/cm2 to 5 × 1013 W/cm2 in different types of glass [18]. In contrast to high-absorbing material, where the interaction process takes place on a surface, nonlinear absorption enables processing material three-dimensionally inside a bulk. The mechanism of nonlinear absorption can be explained by two different processes: multiphoton ionization (MPI) and avalanche ionization (AI). 2.1

Multiphoton Ionization

Due to simultaneous absorption of several photons by an atom or a molecule at the same time, multiphoton ionization can occur if the total of each single photon energy exceeds the ionization energy. In the case of water the ionization energy is Eion = 12.6 eV [19]. However, in the liquid phase this threshold decreases to Eion = 6.5 eV. Here the electrons are only lifted to the exciton band where they are called quasi-free (Fig. 3). The probability for ionization is proporional to I k , where I is the laser intensity and k the minimum number of photons that has to be absorbed simultaneously related by khν > Eion .

(9)

The required number for ionizing a water molecule thus is k = 4, assuming a wavelength of 780 nm and the probability for ionization is proportional to I 4 .

Eion 12.6 eV

Eexc hωL hω L

hωL

E0

hωL

6.5 eV

Fig. 3. Due to simultaneously absorption of several photons, multiphoton ionization can occur if the total of each single photon energy exceeds the ionization energy. In the case of water the ionization energy is Eion = 12.6 eV. However, in the liquid phase this threshold decreases to Eion = 6.5 eV. Here the electrons are only lifted to the exciton band where they are called quasi-free

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hωL

e−

hωL

hωL

hω L

e−

e−

2.2

Fig. 4. Free electrons can be accelerated by absorbing photons (inverse bremsstrahlung). If their kinetic energy is high enough, these electrons are able to ionize other atoms by collision. In order to guarantee impulse conservation, it is necessary to have a third particle in direct environment

Avalanche Ionization

Already existing free electrons, so-called lucky electrons or seed electrons, can be accelerated by absorbing photons (inverse bremsstrahlung). If their kinetic energy is high enough, these electrons are able to ionize other atoms by collision. In order to ensure impulse conservation, it is necessary to have a third particle in direct environment (Fig. 4). The seed electrons may already exist in the medium or they are generated by MPI or linear absorption. As a result of absorbing inverse bremsstrahlung by one electron, two free electrons exist after the ionization process. Therefore, this process is called avalanche or cascade ionization, because, after a certain time, an avalanche process of ionization develops. The time that is necessary for such an ionization process results from the rate of collisions of the electrons with another particle. For water the time tcoll for one collision is approximately 1 fs [17, 20]. This results in a time for ionization tion = 4 fs if, for example, k = 4 photons are needed for one ionization process. 2.3

Time History for the Density of Free Electrons

The temporal behavior of the density ρ of free electrons that are generated by MPI or AI can be described by a rate equation [21]:   dρ(t) = ηMPI (I, t) + ηAI (I, t)ρ(t) − gρ(t) + ηrec ρ2 (t) . dt

(10)

Here, ηMPI stands for the rate of generating free electrons by multiphoton ionization and ηMPI ρ the rate of generating free electrons by avalanche ionization. The rate of generating free electrons by multiphoton ionization is independent of the already-existing free electrons, whereas avalanche ionization becomes stronger, the more free electrons that already exist. The rate of generating free electrons by avalanche ionization therefore is proportional to the density of free electrons ρ inside the medium. Losses of free electrons occur by diffusion (gρ) and by recombination (ηrec ρ2 ). The probability for diffusion is proportional to the number of free

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(b)

Fig. 5. Temporal development of the free-electron density, calculated as a function of time for 6 ns (a) and 100 fs pulses, respectively (b). The laser intensity was chosen close to the threshold for optical breakdown, where the number density of free electrons is assumed to be in the order of 1021 cm−3 . In order to have a better comparison between the different pulse durations, the time t is normalized with respect to the laser-pulse duration τL . The contribution of multiphoton ionization in comparison to the total free-electron density is plotted as a dashed line [16]

electrons inside the focal volume. On the other hand, the probability for recombination is proportional to ρ2 because this process needs both one electron and one ionized atom. If one is interested only in the electron density in the timescale, the term (gρ) can be neglected, because the losses of free electrons by diffusion are very low during that time. In Fig. 5, the temporal development of the free-electron density is calculated as a function of time for 6 ns and 100 fs pulses, respectively, by solving (10) [16]. The laser intensity was chosen close to the threshold for optical breakdown. As a threshold for plasma ignition, the number density of free electrons is assumed to be of the order of 1021 cm−3 , which is in good agreement with other theoretical contributions [14,15,22]. In order to have a better comparison between the different pulse durations, the time t is normalized with respect to the laser-pulse duration τL . In both graphs, the contribution of multiphoton ionization in comparison to the total free-electron density is plotted as a dashed line. For longer pulses (Fig. 5a) the irradiance has to build up on a certain level to provide seed electrons to start with AI. Once an ionization cascade has started, the electron density increases by 9 orders of magnitude within a very short time. For femtosecond pulses, a much higher intensity is necessary for plasma formation compared to ns pulses. Here the generation of free electrons by MPI is preferred because of its stronger irradiance dependence on the intensity (I k ), in contrast to the avalanche-ionization rate, which is linear to the intensity (Fig. 5b). At the end of the laser pulse, 99% of the free electrons

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are generated by AI. Only for pulse durations below 40 fs, does MPI become dominant [20]. As a result of the dominating multiphoton ionization with shorter pulses there is no need for the existence of seed electrons to start avalanche ionization. Plasma formation induced by fs laser pulses therefore is a rather deterministic process compared to ns optical breakdown, where the plasma formation depends on the existence of seed electrons that are distributed statistically near the threshold. This results in a much sharper threshold for optical breakdown for femtosecond pulses compared to nanosecond pulses. The upper limit for the electron density in laser-induced plasmas is determined by the plasma frequency. For plasma frequencies above the laser frequency, the radiation will be reflected by the plasma and no further energy deposition is possible [23].

3

Mechanical and Chemical Side Effects

The fast expansion of the laser-generated plasma induces a cavitation bubble, which can have a serious mechanical damage potential [24, 25]. To study the temporal behavior of the cavitation process and the formation of gas bubbles, flash photography is a common technique (Fig. 6). Here, the laser beam is focused into a water-filled cuvette and the breakdown region can be illuminated perpendicularly to the beam either by flash lamp or by a part of the original laser pulse, which is guided over a variable delay line. With a CCD camera the shadow of the cavitation as well as laser-induced variations in the index of refraction of the water can be observed. At nanosecond timescales after the optical breakdown, and at pulse energies of the order of some hundred microjoule, the generation of fan-like filaments originating from the laser focus became visible (Fig. 7). Their length

Fig. 6. Experimental setup for the observation of laser-induced cavitation by flash photography. The interaction process can be illuminated either by the delayed femtosecond pulse (0 ns to 10 ns) or by a triggered flash lamp (> 50 ns)

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varied with pulse energy, reaching up to 1 mm at 600 µJ. Due to the liquid nature of water, the filaments vanished after some microseconds. On replacing the water by a solid medium like gelatin or PMMA, the streaks are permanent and can be analyzed in more detail. In Fig. 7 on the left, structures, generated in gelatin 10 min after laser treatment are shown. The laser pulse energy was 500 µJ and 100 pulses were applied. The diameter of each filament was determined to be 3 µm. The initial shape of the filament structure appears to be statistical. However, the contrast of each filament increases on a consecutive application of more laser pulses without generating any additional filaments. Similar structures can be found in PMMA (Fig. 8, right). At pulse energies of 300 µJ various locations of cracks, generated by optical breakdown can be observed. As an origin for the filaments, self-focusing probably plays a major role. Due to inhomogeneities in the beam profile (“hot spots”), scattered locations with higher intensity occur, which lead to filamentation by self-focusing [26]. The nature of the filaments might be a change in the structure of the medium by local melting and solidification [27] or due to the interaction of the free electrons, which are generated by MPI. Different authors use these effects to write waveguides inside a glass by applying several tens of pulses on the same spot, each pulse being below the threshold of optical breakdown [18, 28, 29]. A typical increase of the index of refraction is in the range of 0.03 to 0.035. The authors assign the change in index of refraction to a local increase of the density. In water, the effect might be caused by a local increase of the temperature. If one roughly estimates the volume of all filaments to be in the range of 3 × 10−7 cm3 (10 filaments with 1 mm in length and 3 µm in diameter) and 50% of the laser pulse energy is converted into heat, the temperature would increase by 38 K. This would cause a change in the index of refraction of 0.005. On the timescale of several microsecond, cavitation can be observed (Figure 9). Due to the intensity distribution of the focused laser beam, a cylindrical shape of the bubble develops. Because of the surface tension, the bubble collapses faster at smaller radii of curvature, which turns the bubble into

Fig. 7. Filamentation of the laser beam in water. The picture was taken 5 ns after the laser pulse. The focused beam entered from the right (arrow )

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Fig. 8. Filamentation of the laser beam in gelatin 10 min after laser treatment (left). The laser pulse energy was 500 µJ and 100 pulses were applied. Similar structures can be found in PMMA (right). At pulse energies of 300 µJ various locations of cracks, generated by optical breakdown, can be observed

Fig. 9. Sequence of laser-induced cavitation bubbles 50 ns to 10 µs after optical breakdown at a pulse energy of 5 µJ and a pulse duration of 175 fs. The laser beam entered from the right

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a sphere during collapse. After 3 µs, the bubble has blown up again in a second cycle. Now its shape is like a small disc, perpendicular to the original elongation of the cylinder. After 7.5 µs the bubble has finally collapsed to a size of approximately 6 µm in diameter and begins to rise to the surface. It has to be pointed out that the cavitation bubble does not collapse completely, as would be expected if it contained only a small vapor pressure. The residual bubbles maintain their size and rise to the surface of the water. Gas-chromatographic analyses have shown that the gas inside the bubble contains hydrogen, oxygen and, if the optical breakdown takes place in corneal tissue, nitrogen, CO as well as CH4 [3]. The origin for these gas particles can be attributed to photodissociation during plasma ignition. Starting from the maximum radius of the cavitation bubble and the time for one oscillating period, it is possible to estimate the mechanical energy Ecav of the bubble [30] Ecav =

4 3 π(p0 − pv )Rmax . 3

(11)

Here Rmax is the maximum radius of a spherical bubble, pv stands for the vapor pressure at ambient temperature and p0 is the vapor pressure at atmospheric pressure. For nonspherical bubbles a sphere with the same volume can be assumed for the calculation [31]. The term p0 − pv can be calculated from Rayleigh’s formula [32]: Rmax =

T  0.915

4ρ0 p0 −pv

.

(12)

Here ρ0 is the density of the water and T is the time for collapsing. For a 5 µJ laser pulse (T = 100 fs) a maximum radius (spherical equivalent) of 36 µm can be observed, which yields a mechanical energy of Ecav = 0.23 nJ. The conversion rate of laser energy to mechanical energy is thus approximately 0.011%, assuming that 48% of the laser energy is transmitted through the focal region and does not interact with the material [33]. Besides cavitation, pressure transients are generated due to the rapid plasma expansion. The amplitude of these acoustic transients can be measured by piezoelectric PVDF films [3,34]. According to spherical expansion of the pressure wave their amplitude decreases with 1/r. For microjoule pulses the amplitudes reache values in the range of some bar at a distance of some 100 µm from the focal region (Fig. 10). Near the laser focus, calculations by Noak [21] show amplitudes of some kbar. Therefore a strong dissipation of acoustic energy into heat can be assumed for the first few micrometer.

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20

pressure amplitude [bar]

15

220 fs, 10 µJ

10

250 fs, 4 µJ 5

0 200

400

600

800

1000

1200

distance to focal point [µm]

Fig. 10. Measured pressure amplitudes as a function of the distance to the laser focus at pulses energies of 4 µJ and 10 µJ. The fitting curves follow a 1/r function

References [1] J. L. Boulnois: Photophysical processes in recent medical laser developments: a review, Laser Med. Sci. 1, 47–66 (1986) 91 [2] F. Docchio, C. A. Sachhi, J. Marshall: Experimental investigation of optical breakdown thresholds in ocular media under single pulse irradiation with different pulse durations, Lasers Ophthalmol. 1, 83–93 (1986) 92 [3] G. Maatz, A. Heisterkamp, H. Lubatschowski, S. Barcikowski, C. Fallnich, H. Welling, W. Ertmer: Chemical and physical side effects at applications of ultrashort laser pulses for intrastromal refractive surgery, J. Opt. A-Pure Appl. Op. 2, 59–64 (2000) 92, 101 [4] A. Heisterkamp, T. Ripken, T. Mamom, W. Drommer, H. Welling, W. Ertmer, H. Lubatschowski: Nonlinear side effects of fs-pulses inside corneal tissue during photodisruption, Appl. Phys. B-Lasers O. 74, 1–7 (2002) 92 [5] A. Heisterkamp, T. Ripken, U. Oberheide, O. Kermani, T. Mamom, W. Drommer, W. Ertmer, H. Lubatschowski: Applications of ultrafast lasers in ophthalmology, in Proc. SPIE 5142 (2003) pp. 146–153 92 [6] I. N. Duling: Compact Sources of Ultrashort Pulses (Cambridge Univ. Press, Cambridge 1995) 92 [7] E. T. J. Nibbering, M. A. Franco, B. S. Prade, G. Grillon, C. LeBlanc, A. Mysyrowicz: Measurement of the nonlinear refractive index of transparent materials by spectral analysis after nonlinear propagation, Opt. Commun. 119, 479–484 (1993) 92 [8] C. L. Blanc: Principes et r´ealisation d’une source laser t´erawatt femtoseconde bas´ee sur le saphir dop´e au titane, Dissertation, L’ecole de Polytechnique, Paris (1993) 92 [9] J. H. Marburger: Self-focusing: Theory, Prog. Quantum Electron. 4, 35–110 (2000) 92, 93 [10] A. E. Siegmann: Lasers (Univ. Science Books, Mill Valley 1986) 93 [11] Y. R. Shen: Principles of Nonlinear Optics (Wiley, New York 1984) 94

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[12] W. L. Smith, P. Liu, N. Bloembergen: Superbroadening in H2 O and D2 O by self-focused picosecond pulses from YAlGG:Nd laser, Phys. Rev. Lett. 151, 2396–2403 (1977) 94 [13] S. L. Chin, S. Petit, F. Borne, K. Miyazaki: The white light supercontinuum is indeed an ultrafast white laser, Jpn. J. Appl. Phys. 38, 126–128 (1999) 94 [14] A. Vogel: Optical Breakdown in Water and Ocular Media and its Use for Intraocular Photodisruption (Shaker, Aachen 2001) 95, 97 [15] A. Vogel, J. Noack, K. Nahen, D. Theisen, S. Busch, U. Parlitz, D. X. Hammer, G. D. Nojin, B. A. Rockwell, R. Birngruber: Energy balance of optical breakdown in water at nanosecond to femtosecond time scales, Appl. Phys. B-Lasers O. 68, 271–280 (1999) 95, 97 [16] J. Noack, A. Vogel: Laser-induced plasma formation in water at nanosecond to femtosecond time scales: Calculation of thresholds, absorption coefficients and energy density, IEEE J. Quantum Elect. 35, 1156–1167 (1999) 95, 97 [17] A. Vogel, J. Noack, K. Nahen, D. Theisen, R. Birngruber, D. X. Hammer, G. D. Noojin, B. A. Rockwell: Laser-induced breakdown in the eye at pulse durations from 80 ns to 100 fs, in Proc. SPIE 3255 (1998) pp. 34–47 95, 96 [18] C. Schaffer: Interaction of Femtosecond Laser Pulses with Transparent Materials, Ph.D. thesis, Harvard University (2000) 95, 99 [19] F. Williams, S. P. Varma, S. Hillenius: Liquid water as a lone pair amorphous semiconductor, J. Comp. Phys. 64, 1549 (1976) 95 [20] Q. Feng, J. L. Maloney, A. C. Newell, E. M. Wright, K. Cook, P. K. Kennedy, D. X. Hammer, B. A. Rockwell, C. R. Thomson: Theory and simulation on the treshold water breakdown induced by focused ultrashort laser pulses, IEEE J. Quantum Elect. QE-33, 127–137 (1997) 96, 98 [21] J. Noack: Optischer Durchbruch in Wasser mit Laserpulsen zwischen 100 ns und 100 fs, Dissertation, Universit¨ at L¨ ubeck (1998) 96, 101 [22] B. C. Stuart, M. D. Feit, S. Hermann, A. M. Rubenchik, B. W. Shore, M. D. Perry: Nanosecond to femtosecond laser-induced breakdown in dielectrics, Phys. Rev. B 53, 1749–1761 (1996) 97 [23] N. Bloembergen: Laser-induced electric breakdown in solids, IEEE J. Quantum Elect. 10, 375–386 (1974) 98 [24] A. Vogel, S. Busch, M. Asiyo-Vogel: Time-resolved measurements of shockwave emission and cavitation-bubble generation in intraocular laser surgery with ps- and ns-pulses and related tissue effects, in Proc. SPIE 1877 (1993) pp. 312–323 98 [25] A. Vogel, S. Busch, U. Parlitz: Shock wave emission and cavitation bubble generation by picosecond and nanosecond optical breakdown in water, J. Acoust. Soc. Am. 100, 148–165 (1996) 98 [26] K. Yamada, W. Watanabe, T. Toma, K. Itoh: In situ observation of photoinduced refractive-index changes in filaments formed in glasses by femtosecond laser pulses, Opt. Lett. 26, 19–21 (2001) 99 [27] A. C. K. Solokowski-Tinten, J. Bialkowski, D. von der Linde: Transient states of matter during short pulse laser ablation, Phys. Rev. Lett. 81, 224–227 (1998) 99 [28] K. Hirao, K. Miura: Writing waveguides and gratings in silica and related materials by a femtosecond laser, J. Non-Cryst. Solids 239, 91 (1998) 99 [29] S. Nolte, B. Chichkov, H.Welling, Y.Shani, K. Lieberman, H. Terkel: Nanostructuring with spatially localized femtosecond laser pulses, Optics Letters 24, 914–916 (1999) 99

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[30] R. H. Cole: Underwater Explosions (Princeton Univ. Press, Princeton 1948) 101 [31] A. Vogel, J. Noack, K. Nahen, D. Theisen, S. Busch, U. Parlitz, D. X. Hammer, G. D. Noojin, B. A. Rockwell, R. Birngruber: Energy balance of optical breakdown at nanosecond to femtosecond time scales, in Proc. SPIE 3255 (1998) pp. 34–43 101 [32] R. T. Knapp, J. W. Daily, F. G. Hammit: Cavitation (McGraw–Hill, New York 1971) 101 [33] A. Vogel: Optical Breakdown in Water and Ocular Media and its Use for intraocular Photodisruption, Habilitationsthesis, Universit¨ at L¨ ubeck (2000) 101 [34] A. Heisterkamp, T. Mamom, O. Kermani, W. Drommer, H. Welling, W. Ertmer, H. Lubatschowski: Intrastromal refractive surgery with ultrashort laser pulses in living animals, in Proc. SPIE 4611 (2002) pp. 136–142 101

Index

avalanche ionization, 96 cavitation, 98

nonlinear absorption, 95

electron, 96

photodissociation, 101 plasma formation, 95 pressure, 101

filament, 99 filamentation, 99 free, 96 free electron, 96 free-electron, 95

self-focusing, 92, 93, 99 self-phase modulation (SPM), 93 streak, 99

group-velocity dispersion, 93 multiphoton ionization, 95

transients, 101 white light, 94

Interaction with Metals Andreas Ruf1 and Friedrich Dausinger2 1

2

DaimlerChrysler AG, Forschungszentrum Ulm, RBP/MJ, P. O. Box 2360, 89013 Ulm, Germany Institut f¨ ur Strahlwerkzeuge (IFSW), Universit¨ at Stuttgart, Pfaffenwaldring 43, 70569 Stuttgart, Germany [email protected]

Abstract. In contrast to dielectric materials, metals are characterized by a high density of quasi-free electrons causing strong absorption of laser light from the beginning of the ultrashort laser pulse without the need of nonlinear absorption effects like multiphoton absorption. In a femtosecond timescale the energy transfer to the electrons can therefore be regarded as instantaneous. Their thermalization and movement is fast enough to occur during a typical femtosecond pulse duration. The transfer of energy from the electronic system to the lattice, however, needs relaxation times of the order of some picoseconds. The heating process of the lattice is for subpicosecond pulses no longer determined by the pulse duration but by the material-dependent relaxation time. This circumstance limits the reduction of heat load and explains recast occurring even when using ultrashort pulses.

Metals differ from dielectrics and semiconductors by a high density of quasifree electrons (“electron gas”) that causes their comparably high values of electrical and thermal conductivity and determines their optical properties, i.e. the interaction with light. At the high intensity levels that can be achieved with ultrashort laser pulses multiphoton absorption followed by impact ionization produces such an electron gas even in dielectrics and semiconductors, making them “metallic”. In this respect, most of the findings presented below for metals apply more or less for other solid materials, as well. There remains one important difference, however: The change in electronic properties, among them the most important one in this context, the absorption coefficient, happens only locally where the intensity exceeds the threshold for multiphoton absorption. This allows a strong local concentration of the interaction in the vicinity of the focus enabling materials treatment inside transparent media. When electromagnetic waves interact with metals, they do so primarily with the electronic system. With intense laser pulses the electrons are heated nearly instantaneously. For material treatment the crucial question then is, what follows? At the extreme intensity of ultrashort pulses the transferred energy could be high enough to allow electrons to leave the solid, leaving behind positively charged ions that repel each other so strongly that their binding is broken up and the material decomposes. Such an ablation mechanism called Coulomb F. Dausinger, F. Lichtner, H. Lubatschowski (Eds.): Femtosecond Technology for Technical and Medical Applications, Topics Appl. Phys. 96, 105–114 (2004) c Springer-Verlag Berlin Heidelberg 2004


Normalized yield

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Al+ ion yield

2

Thermal peak

Coulomb explosion

1

0

1

2

3

Velocity in 10 4 m/s

4

0 0.1

1

10

1000

100

Time delay in ps Fig. 1. Aluminum ion yield in time-of-flight mass spectroscopy for pump-probe measurements on Al2 O3 targets in dependence of time delay between two subthreshold 100 fs pulses of equal intensity. Slow ions with a velocity of 1.2 × 104 m/s (circles) have a maximum yield during thermal melting of the target between 20 ps to 30 ps. The fast ions (2 × 104 m/s, squares) show a double-peak structure. The first peak is caused by Coulomb explosion and the second comprises thermal ions in the high-velocity tail of the distribution. Inset: velocity distribution for delay times of 12 ps (circles) and 0.6 ps (squares) [1] 1.2

Si+ from a-SiO2

Fast ion yield

Normalized yield

0.8 0.4

Slow ion yield

0

Si+ from silicon (fast yield)

0.8 0.4 0

Au+ from gold 0.1

1

10

100

1000

Time delay in ps Fig. 2. Ion yield in time-of-flight mass spectroscopy for pump-probe measurements as in Fig. 1 for dielectric (a-SiO2 , upper graph) and semiconductor/metallic targets (Si and Au, lower graph). For Si+ ions from SiO2 the fast ion yield rate has a strong contribution on femtosecond timescales (Coulomb explosion) whereas in silicon and gold almost no ions are found until melting of the surface occurs corresponding to the respective electron–phonon relaxation times [1]

explosion can indeed be observed in dielectric materials, see Fig. 1, but not in metals. In metals, only a retarded thermal emission of particles was found, Fig. 2.

Av. ablation rate per Lpulseuls] in µm

Interaction with Metals

107

0.3

Copper Thermal penetration zone

0.2

0.1

0 0.1

Optical penetration zone 1

10 2

H density H in J/cm² Energiedichte[mJ/cm] Energy

Fig. 3. Average ablation rate of copper in vacuum versus energy density. 150 fs pulses of a Ti:sapphire laser operating at 780 nm wavelength [2, 3]. The two ablation regimes have been interpreted as the optical-penetration regime at low fluence levels, where ablation is dominated by the optical penetration depth, and as the thermal-penetration regime for higher energy density levels with electronic heat conduction leading to strongly increasing ablation rates [2]

The laser-heated hot electrons transfer energy to colder particles by collisions. The energy transfer to other electrons leading to a thermalization of the local electronic system takes place on a timescale of the order of 100 fs. Even during that time energy can already be transported out of the directly laser-heated volume by electronic heat conduction. This can lead to an increased interaction depth and explain the experimental findings shown in Fig. 3. According to the authors, at low energy density the ablation process is dominated by the optical penetration depth, at higher energy density by the thermal penetration depth [2]. At about 100 fs after the start of radiation, the lattice of the metal is still cold. It is heated indirectly by collisions with the hot electrons. Due to the low mass and consequently low heat capacity of the electrons a multitude of collisions and, therefore, a long time is needed to raise the lattice temperature. The timescale for energy transfer from the electrons to the lattice is characterized by the relaxation time τep . In metals with strong electron–phonon coupling, such as Fe, the relaxation time is near to 0.5 ps, in those with weak coupling like Al or Cu it is one or two orders of magnitude larger [4, 5]. Only after a time interval several times the relaxation time, do electrons and lattice achieve the same temperature. Before that, electrons and lattice have to be characterized by different temperatures, the classical thermodynamical description has to be replaced by a two-temperature model. In such a model, individual heat-conduction equations are solved both for the electronic temperature Te and for that of the lattice Ti [2, 5, 6, 7, 8, 9, 10]. Both equations

Surface temperature TS in K

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8000

Lattice Gitter Electrons Elektronen Classical 1T klassisch

Al

6000

1-temp. model

τHH = 1 ps

4000

τH = 1 ns

Tb

2000 0 -14 10

Tm -13

10

-12

-11

-10

10 10 10 Time t in s

-9

10

-8

10

Fig. 4. Temporal evolution of the surface temperature of aluminum when absorbing a laser pulse of 1 ps (AH = 10 mJ/cm2 ) or 1 ns (AH = 100 mJ/cm2 ) duration. For ultrashort pulses the electronic temperature exceeds the lattice temperature considerably in the beginning, but after about 100 ps the difference becomes negligible [5, 11]

are coupled by a coupling term proportional to the temperature difference: dTe (x, t) = −∇j th,e (x, t) − G(Te − Ti ) + q(x, ˙ t) , dt dTi (x, t) = −∇j th,i (x, t) − G(Te − Ti ) . Ci (Ti ) dt

Ce (Te )

(1) (2)

Figure 4 compares the evolution of the electronic and lattice temperatures for two pulse durations. One of them, 1 ns, is much longer than the relaxation time. In that case the classical one-temperature model is quite sufficient for describing both temperatures, which are nearly identical. For the ultrashort pulse duration of 1 ps a strong difference between the temperatures can be observed up to 100 ps, i.e. 20 times the relaxation time. During the pulse duration only a strong heating of the electronic system is observed, a significant increase of the lattice temperature is delayed by the relaxation time of 5 ps. This leads to an energy distribution as shown in Fig. 5. During a pulse of 1 ps duration energy is stored nearly exclusively in the electrons. After transfer to the lattice vaporization starts taking away a major share of the energy. A considerable amount of energy remains in the lattice, however, causing an unwanted thermal load leading to reduced accuracy of the machining process, generally. The two-temperature model calculation thus shows that the expectation of cold machining when using ultrashort pulses can not be fulfilled completely. The matter remains essentially cold during an ultrashort pulse only, but reaches high temperature values leading to thermal ablation after the pulse. The difference between short (τH  τep ) and ultrashort interaction (τH < τep )

Interaction with Metals

Energy fraction in %

100 80

109

Al τH = 1 ps A H = 1 J/cm

2

Vapor 60 40 20 0 -15 10

Electrons -14

10

-13

10

-12

10

Lattice -11

10

-10

10

-9

10

-8

10

Time t in s Fig. 5. Development of the energy fractions stored in the electronic and lattice systems as well as in the vapor during absorption of a laser pulse of 1 ps duration (AH = 10 mJ/cm2 ). Due to electron–phonon relaxation, evaporation sets in only after several picosecond and continues up to nanosecond [5, 11]

is therefore, mainly, that in the case of short (nanosecond) pulses the thermalinteraction time is determined by the pulse duration, whereas at ultrashort pulses it is determined by a material property, the relaxation time. This means, on the other hand, that by shortening the pulse duration to the ultrashort regime, a thermal load can be minimized but not completely avoided, a fact that is clearly illustrated by Fig. 7 [5]. This figure shows how the thermal response of an Al sample depends on the duration of the laser pulse. It can be seen that the characteristic time spans for evaporation and melt saturate below 100 ps. Even at ultrashort pulses of 1 ps or shorter, evaporation lasts more than 1 ns and melt is present for several tens of nanosecond. Figure 6 additionally reveals that even with ultrashort pulses a melt depth of nearly one micrometer has to be taken into account. It has to be mentioned, however, that for other materials such as iron considerably lower values can be expected because of a shorter relaxation time, a lower thermal diffusivity, and a higher melting temperature. The amount of melt can be reduced by minimization of the pulse energy to a value slightly above the ablation threshold, see Fig. 7a, however, at the expense of a strongly reduced ablation rate, see Fig. 7b. In the example shown in the figure, the effect on the ablation rate is nearly one order of magnitude larger than that on the melt production. Heavy evaporation caused by intense laser irradiation produces high pressure acting on the melt layer like a piston and accelerating it. The accelerating forces are active at least as long as vaporization continues, the movement ends ultimately upon solidification. See Fig. 6 for calculated values of these time

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Andreas Ruf and Friedrich Dausinger

Time t in ns

1000

←τep

τH→ tion ca i th f i ep lid d o t l S e x. m Ma ion rat o ap Ev

100

10

1 -12 10

-11

10

-10

10

-9

10

-8

10

-7

10

Pulse duration τH in s Fig. 6. Characteristic times for evaporation, reaching the maximum melt-film thickness, and for solidification as a function of pulse duration (AH = 1 J/cm2 ). For pulse durations τH far beyond the electron–phonon relaxation time τep , the process is governed predominantly by τH . For shorter pulses a material-dependent saturation is noticeable [5, 11]

spans. Figure 8a shows melt-velocity values and Fig. 8b shows maximum melt-travel distances calculated with a so-called piston model. It turns out that with ultrashort pulses melt can be transported only small distances of less than one micrometer per pulse. With decreasing energy density, the travel distance decreases strongly reducing the possibility to accumulate recast to a larger layer thickness. In conclusion, three mechanisms have been mentioned above that affect the accuracy of a machining process: 1. Thermal penetration by electronic heat conduction (see Fig. 4) 2. Melt produced by retarded energy transfer to lattice (Fig. 6 and Fig. 7) and 3. Increased recast thickness caused by melt transport and accumulation The effect of all three mechanisms can be minimized by reducing energy density, which on the other hand, leads to a dramatic drop of process velocity. In a given application one has to choose between highest precision at the expense of a very low productivity or a relatively productive process with a tolerable lack of precision. The Chapter “Technical Applications” will show that this choice depends on the process, e.g. whether surface structuring or drilling is attempted.

Interaction with Metals

111

Max. melt depth dm,max in µm

3 Al 1 J/cm

2

2 0.5 J/cm

2

1 0.2 J/cm A H = 0.1 J/cm 0 -12 10

-11

-10

10

10

-9

10

2

2

-8

10

-7

10

Pulse duration τH in s (a)

Ablation per pulse ∆z in nm

100 1 J/cm

Al

2

0.5 J/cm

2

0.2 J/cm

2

10

2

A H = 0.1 J/cm 1 -12 10

-11

10

-10

10

-9

10

-8

10

-7

10

Pulse duration τH in s (b) Fig. 7. Influence of pulse duration on the maximum melt-layer thickness (a) and on ablation depth (b) during ablation of aluminum. A reduction of pulse duration or energy density can minimize the melt depth but due to saturation effects for ultrashort pulses, melt-free processing is not achievable. On the other hand, lower fluences drastically reduce the ablation rate, which cannot be compensated by the use of shorter pulses due to a saturation of the ablation rate towards shorter pulses [5, 11]

Melt flow velocity u 'm in m/s

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Andreas Ruf and Friedrich Dausinger

8 7 6 5

Al

dh = 100 µm

1 J / cm

∆h = 25 µm

2

4 3

0.5 J / cm

2

2 1

A H = 0.2 J / cm

0 -12 10

-11

10

2

-10

10

-9

-8

-7

10 10 Pulse duration τH in s

10

Melt flow distance lm in µm

(a)

0.6 0.5 0.4

Al

1 J / cm

dh = 100 µm

2

∆h = 25 µm

0.3 0.2 0.1 0.0 -12 10

2

A H = 0.5 J / cm -11

10

-10

10

-9

10

-8

10

-7

10

Pulse duration τH in s (b) Fig. 8. Maximum melt flow velocity versus pulse duration (a) and total melt-travel distance (b) for ablation of aluminum calculated using a modified piston model [5]. Due to the transient nature of the melt acceleration, only small maximum velocity values can be reached and consequently, the travel distances remain too low to allow melt to be transported from the crater ground to the rim during a single pulse

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113

References [1] R. Stoian, A. Rosenfeld, D. Ashkenasi, I. V. Hertel, N. M. Bulgakova, E. E. B. Campbell: Surface charging and impulsive ion ejection during ultrashort pulsed laser ablation, Phys. Rev. Lett. 88, 97603–1–97603–4 (2002) 106 [2] S. Nolte, C. Momma, H. Jacobs, A. T¨ unnermann, B. N. Chichkov, B. Wellegehausen, H. Welling: Ablation of metals by ultrashort laser pulses, J. Opt. Soc. Am. B 14, 2716–2722 (1997) 107 [3] C. Momma, S. Nolte, B. N. Chichkov, F. v. Alvensleben, A. T¨ unnermann: Precise laser ablation with ultrashort pulses, Appl. Surf. Sci. 109/110, 15–19 (1997) 107 [4] B. H¨ uttner, G. Rohr: On the theory of ps and sub-ps laser pulse interaction with metals I. surface temperature, Appl. Surf. Sci. 103, 269–274 (1996) 107 [5] A. Ruf: Modellierung des Perkussionsbohrens von Metallen mit kurz und -ultrakurz gepulsten Lasern, Laser in der Materialbearbeitung – Forschungsberichte des IFSW (Utz, Munich 2004) 107, 108, 109, 110, 111, 112 [6] S. I. Anisimov, A. M. Bonch-Bruevich, M. A. Elyashevich, Y. A. Imas, N. A. P. G. S. Romanov: Effect of powerful light fluxes on metals, Sov. Phys.– Tech. Phys. 11, 945–952 (1967) 107 [7] S. I. Anisimov: Vaporization of metals absorbing laser radiation, Sov. Phys. JETP 27, 182–183 (1968) 107 [8] B. N. Chichkov, C. Momma, S. Nolte, F. v. Alvensleben, A. T¨ unnermann: Femtosecond, picosecond and nanosecond laser ablation of solids, Appl. Phys. A-Mater. 63, 109–115 (1996) 107 [9] C. K¨ orner: Theoretische Untersuchungen zur Wechselwirkung von ultrakurzen Laserpulsen mit Metallen, Dissertation, University of Erlangen-N¨ urnberg (1997) 107 [10] A. P. Kanavin, I. V. Smetanin, V. A. Isakov, Y. V. Afanasiev, B. N. Chichkov, B. Wellegehausen, S. Nolte, C. Momma, A. T¨ unnermann: Heat transport in metals irradiated by ultrashort laser pulses, Phys. Rev. B 57, 14698–14703 (1998) 107 [11] D. Breitling, A. Ruf, F. Dausinger: Fundamental aspects in machining of metals with short and ultrashort laser pulses, in P. R. Herman, J. Fieret, A. Pique, T. Okada, F. G. Baumann, W. Hoving, K. Klashio, X. Xu, J. J. Dubowski, D. B. Geohegan, F. Tr¨ ager (Eds.): Proc. SPIE 5339 (Int. Soc. for Opt. Eng., Bellingham 2004) pp. 49–63 108, 109, 110, 111

Index

ablation rate, 109

metal, 105

Coulomb explosion, 105

piston model, 110

dielectric, 105 dielectric material, 105

recast, 105, 110 relaxation time, 105, 107, 109

electron–phonon, 107 evaporation, 109

solidification, 109

heat load, 105

two-temperature model, 107

melt, 109

vaporization, 108

Surface Structuring Michael Weikert and Friedrich Dausinger Institut f¨ ur Strahlwerkzeuge (IFSW), Universit¨ at Stuttgart, Pfaffenwaldring 43, 70569 Stuttgart, Germany [email protected] Abstract. Several industrial applications use nanosecond laser pulses for microstructuring of surfaces on macroscopic workpieces. The quality of the resulting structures, however, is limited due to the formation of recast that has to be removed by additional postprocessing. Experimental results presented here show that it is possible to avoid melt formation by shortening the pulse duration into the picosecond regime if low energy density values are used. The resulting ablation rates are not satisfactory in most applications but can be increased by using laser systems with a higher repetition rate.

In recent years industry has shown a growing interest in microstructuring of surfaces to improve the characteristics of macroscopic workpieces. Microcavities on the surface of parts moving against each other can, for example, improve their tribological behavior. The first known industrial application of this technique is the so-called laser honing of cylinder walls in combustion engines. For this application, the cylinder wall is structured with microcavities near the top dead center of the piston movement, decreasing the tendency of the oil film to break down during the short standstill before the piston starts to move in the other direction. The outcome is a significant reduction of oil consumption and particle emission [1]. Nowadays, Q-switched lasers are used, generally. However, the use of nanosecond laser pulses limits the quality of the structures due to the formation of recast during the ablation process. The reduction of the pulse duration is a promising technology to avoid the formation of recast and consequently increase quality and precision of the ablated structures [2].

1 1.1

Experimental Results Influence of Basic Parameters

During the ablation process material is ejected partially by evaporation and partially by melt ejection. Usually some of the molten material solidifies at the walls forming recast layers and burr. As mentioned in the Chapter “Interaction with Metals” it should be possible to reduce the proportions of the melt volume by reducing the pulse duration. The experimental results depicted in Fig. 1 confirm these expectations, showing that it is possible to F. Dausinger, F. Lichtner, H. Lubatschowski (Eds.): Femtosecond Technology for Technical and Medical Applications, Topics Appl. Phys. 96, 117–130 (2004) c Springer-Verlag Berlin Heidelberg 2004


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Fig. 1. Burr height at the border of shallow structures in steel. The number of scans was selected to obtain an average ablation depth of 40 µm. Energy density: 50 J/cm2 , material: steel Ck 101

reduce burr height by using shorter laser pulses. Process parameters were optimized seperatly for each regime of pulse duration to obtain maximum quality at a constant pulse energy density of 50 J/cm2. Nevertheless, it is clear that even with femtosecond pulses it was not possible to avoid burr completely [3]. These observations confirm numerical calculations presented in the Chapter “Interaction with Metals” showing that completely melt-free ablation of metal may not be possible even with femtosecond pulses. But the calculations also show that at small energy density levels the thickness of the melt layer as well as the transport velocity of the melt (causing burr formation) is reduced. Experiments indicate that ablation without noticable recast is possible with pulse durations of 10 ps and lower if very low energy densities are used. Figure 2 depicts grooves in steel [4]. The groove at the left was machined with a comparatively high energy density of 175 J/cm2. A layer of molten material with a thickness of several micrometers and distinct burr are recognizable. The wavy structure on the bottom of the groove indicates the flow of the molten material. The groove on the right is produced with an energy density slightly above the ablation threshold and shows no signs of molten material. The surface of the groove is covered by ripple structures frequently observed at near-threshold ablation with ultrashort pulses. At higher energy densities and longer pulses these structures are covered by a layer of recast material. Apparently recast-free structuring with femtosecond laser pulses is only possible at energy-density levels slightly above the ablation threshold. In Fig. 3 the ablation behavior of cast iron at different energy densities above the ablation threshold is shown. At 2 J/cm2 the groove is narrow since only in the central part of the beam profile is the energy density above the ablation

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Fig. 2. Grooves in steel ablated by 2 ps pulses with mean pulse energy densities of 175 J/cm2 (left) and 4 J/cm2 (right). Due to the asymmetrical energy-density profile of the laser beam, the cross section of the groove is slightly asymmetrical

threshold. The optimum energy density is around 4 J/cm2 , as groove width and quality are optimal. At an average energy density of 12 J/cm2 the energy density in the central part of the beam profile is already too high, as structures from molten material are appearing in the center of the groove. The edges are still free from irregularities since there the energy density is still below the critical value. Due to the very low energy densities used for recast-free structuring, the process speed is very low. In Fig. 4 the ablation depth of grooves is depicted depending on the scanning feedrate. For an ablation depth of 10 µm to 25 µm a scanning feedrate of several millimeter per minute is required. The ablation depth depends largely on the pulse overlap. Figure 5 sketches the basic geometry of the ablation front. The angle of the ablation front is dependent on the ablation depth t and the length of the ramp on the ablation front determined by the laser-beam diameter df . The ablation depth t is dependent on the number of pulses that hit the increment of ablation. A common approach to increase ablation depth is the reduction of the scanning feedrate in order to increase the pulse overlap. This approach is only successful in a certain range of scanning speed. For high scanning speed the pulse overlap is too low to produce continuous grooves, the single pulses appear as a row of overlapping craters. At a very high pulse overlap, each

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Fig. 3. Grooves in steel ablated by 120 fs pulses with mean pulse energy densities of 2 J/cm2 (upper left), 4 J/cm2 (upper right), 12 J/cm2 (lower left) and 20 J/cm2 (lower right). Structured in one scan with a pulse overlap of 99%

pulse hits a substantial portion of the cavity that was already ablated by preceding pulses. This influences the coupling of energy at the ablation front, leading to irregularities in the ablation process. Additionally grooves with a high aspect ratio are usually filled with ablated material that could not be expulsed. The chemical composition of the base material and the redeposited material in the groove is shown in Fig. 6. Compared to its original state, the affected material in the groove shows a distinct peak corresponding to oxygen accumulation. Evidently the pressure inside the groove is not sufficient for the ejection of the molten material, which can then react with ambient oxygen during the cooling process after the laser pulse. The width of the groove is influenced by the spot diameter on the workpiece surface. A simple approach to alter the spot diameter is to change the position of the focus with respect to the workpiece surface. As depicted in Fig. 7, ablation behavior is different depending on the focus being positioned above the workpiece or below. As described in the Chapter “Interaction with

Surface Structuring

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13 J/cm² 25 J/cm² 40 J/cm²

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20

0 0

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Fig. 5. Geometry of the ablation front

Atmosphere”, the laser beam is deformed in the air breakdown, resulting in a more divergent beam with irregular distribution of energy density and wavelength. If the focus is positioned above the workpiece surface, these irregularities influence the ablated structure to an extent increasing with distance. This effect can be avoided if the focus is positioned below the surface. 1.2

Influence of Beam Movement on Geometry of Ablated Zone

For several applications in tribology and in printing technology, dimpleshaped structures are required. In nanosecond applications usually each dimple is ablated by a single laser pulse. Depending on the repetition rate, high process speeds are possible if the dimples are ablated on the fly. Due to the volume of the dimples, a high energy density is necessary to ablate a dimple with a single pulse. At very low energy densities that are used for recast free structuring with ultrashort laser pulses, the ablation depth for each single pulse is in the nanometer range. Since the required ablation depth for known

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Fig. 6. Chemical composition of the material in the groove

Fig. 7. Influence of focal position in comparison to workpiece surface. Laser: wavelength: 800 nm, pulse duration: 120 fs, energy density: 118 J/cm2 , material: cast iron

applications is in the range of 10 µm to 50 µm, it is obvious that more than a single pulse is necessary to ablate recast-free dimples with the intended volume. In Fig. 8 several strategies for the structuring of dimples with more than one laser pulse are illustrated. The simplest technique is to use several laser pulses on the same position, comparable to percussion drilling. The dimple depicted on the laser-scanning microscope picture in Fig. 9 was ablated with 100 pulses and shows no signs of recast or burr.

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Fig. 8. Ablation strategy for dimples

Fig. 9. Dimple in copper. Laser: wavelength: 800 nm, pulse duration: 2 ps, repetition rate: 1 kHz, pulse energy: 0.04 mJ, energy density: 25 J/cm2 , 100 pulses per dimple, dimple depth: 20 µm

The beam profile has a great influence on the shape of the dimple if the percussion technique is used. In Fig. 10 a comparison between two different laser systems is shown. Corresponding to the beam profile of the laser system, which was not circular at the time of the experiment, the dimple on the left has an oval shape. The dimple on the right was ablated with an almost rotationally symmetrical beam profile resulting in an approximately circular dimple. In printing technology sometimes dimples with an approximately square shape are desired. The dimples depicted in Fig. 11 were structured by filling with parallel lines. The shape of the dimple is to much less extent dependent on the beam profile. Obviously, it is possible to obtain recast-free structures in copper even with higher pulse energy than in steel.

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Fig. 10. Dimples produced with different laser beams. Energy density: 5 J/cm2 , pulse duration: 2 ps (left), 5 ps (right), 100 pulses per dimple, dimple depth: 20 µm, material: copper

Fig. 11. Dimples in copper produced with scanning (parallel lines). Laser: wavelength: 800 nm, pulse duration: 120 fs, repetition rate: 1 kHz, strategy: filled by parallel lines

Surface Structuring

1.3

125

Process Speed

Since for ablation without recast the energy density is limited, a remaining approach to increase the ablation rate is the use of a laser system with higher repetition rate. In Fig. 12 the ablation rate for a repetition rate between 1 kHz and 8 kHz is depicted. The solid line is the calculated ablation rate for the assumption of a linear behavior. It can be seen that the ablation rate increases approximately proportional to the repetition rate. In Fig. 13 two grooves in cast iron are depicted. The groove in the left picture was structured with a repetition rate of 1 kHz, the groove in the right picture with a repetition rate of 8 kHz at a scanning feedrate increased by the same factor, assuring the same overlap. The shape and quality of the grooves are almost identical. Figure 14 shows the ablation rate for a variation of repetition rate from 10 kHz up to 200 kHz. Some scatter of the values is obvious, which is due to the fact that the laser system had to be adjusted carefully when the repetition rate was changed. Therefore constant beam parameters could not be guaranteed. Taking this into account, the proportionality of the ablation rate to the repetition rate is quite good. For several values the ablation rate is even slightly higher than expected. In Fig. 15 two grooves are depicted referring to the lowest and the highest repetition rate used. The shape and size of the grooves are similar. To compare different laser systems with different parameters, the crosssectional areas for different grooves structured with different laser systems are shown in Fig. 16. As a reference, the cross section of a typical groove produced by laser honing with nanosecond laser pulses is shown as a dotted

Fig. 12. Influence of repetition rate on ablation rate for structuring of grooves. Laser wavelength: 1030 nm, pulse duration: 5 ps, energy density: 5 J/cm2 , 1 scan, material: cast iron

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Fig. 13. Influence of repetition rate on grooves in cast iron, repetition rate: left: 1 kHz, right: 8 kHz. Laser: wavelength: 1030 nm, pulse duration: 5 ps, energy density: 5 J/cm2 , 1 scan

Fig. 14. Ablation rate at high repetition rates. Laser wavelength: 800 nm, pulse duration: 200 fs, energy density: 5 J/cm2 , 1 scan, material: steel

line. To assure a comparability of the results, all grooves are structured with the same energy density of 5 J/cm2 . To reach the ablation rate and the feedrate of the industrial process, a laser source with a repetition rate of several hundred kilohertz and a pulse energy of more than 20 µJ on the workpiece would be necessary.

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Fig. 15. Grooves in steel, repetition rate: left: 10 kHz, right: 200 kHz. Laser: wavelength: 800 nm, pulse duration: 200 fs, energy density: 5 J/cm2 , feedrate per pulse: 0.025 µm, 1 scan, material: steel. The slight asymmetrical tilt of the groove is based on the fact that the laser beam did not hit the target perfectly perpendicular 1000000

1 kHz, 2 ps 8 kHz, 5 ps 20 kHz, 10 ps 200 kHz, 200 fs 25 kHz, 150 ns

cross section in µm²

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Fig. 16. Comparison of cross-sectional area for different laser systems. Pulse energy density: 5 J/cm2 , 1 scan, material: cast iron

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Examples of Possible Applications Tribological Structures

In Fig. 17 a cast-iron ring with laser-structured grooves is shown. The machining time for such rings with different groove arrangements was between 2 h and 40 h. Such rings are used for so-called pin-on-ring experiments. In tribometrical examinations, a chromium ring is pressed against the structured surface of the rotating ring. The forces measured give an idea of the tribological coefficients of the surface. It could be shown that laser structuring of surfaces could improve the tribological behavior over a large range. 2.2

Structures for Printing and Embossing

Today, printing and embossing tools are usually structured by mechanical or etching technologies. Lasers are already used to structure soft materials like laquer or rubber, but quality and economical issues prevented the use of lasers for metals like steel or copper. Femto- or picosecond lasers have the potential to fill this gap. In printing technology, dimple structures are used. As mentioned before, it is possible to produce dimples without recast if an energy density of several joule per square meter is selected. At this energydensity level a number of pulses is necessary to produce the required dimple depth of 5 µm to 20 µm. As a demonstrator a picture was transferred to the software of a scanning head and structured in galvanic copper to simulate a printing tool. In Fig. 18 a section of this structure can be seen. Due to the asymmetrical energy-density profile of the laser beam, the cross section of the groove is slightly asymmetrical.

Fig. 17. Cast-iron ring with laser-structured surface for tribological experiments. Laser: wavelength: 800 nm, pulse duration: 2 ps, repetition rate: 1 kHz, pulse energy: 10 µJ, energy density: 1 J/cm2 , 150 scans, effective feedrate: 4 mm/min, material: cast iron, machining time: 6.5 h

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Fig. 18. Printing structure in copper. Laser wavelength: 800 nm, pulse duration: 2 ps, pulse energy: 10 µJ, energy density: 1 J/cm2 , 100 pulses per dimple, dimple depth: 20 µm, resolution: 200 dpi, material: copper

3

Summary

Reduction of the pulse duration is a way to reduce the formation of burr and layers of molten material. Structures without noticeable amounts of recast can be produced with pulse durations between 120 fs and 10 ps if an energydensity level slightly above the ablation threshold is selected. The very low ablation rate could be compensated to some extent by using laser systems with a high repetition rate.

References [1] T. Abeln, U. Klink: Laserstrukturieren zur Verbesserung der tribologischen Eigenschaften von Oberfl¨ achen, in Proc. of Stuttgarter Lasertage Stuttgart (Forschungsgesellschaft f¨ ur Strahlwerkzeuge 2001) p. 61 117 [2] C. Momma, S. Nolte, B. N. Chirchov, F. von Alvensleben, A. T¨ unnermann: Precise laser ablation with ultrashort pulses, Appl. Surf. Sci. 109/110, 15 (1997) 117 [3] J. Radtke, C. F¨ ohl, M. Weikert, F. Dausinger: Bohren und Mikrostrukturieren mit Ultrakurzpulslasern, in Proc. of Stuttgarter Lasertage Stuttgart (Forschungsgesellschaft f¨ ur Strahlwerkzeuge 2001) p. 56 118 [4] M. Weikert, C. F¨ ohl, F. Dausinger: Surface structuring of metals with ultrashort laser pulses, in Proc. SPIE 4830 (2002) pp. 501–505 118

Index

ablation rate, 117, 125 application, 117, 121, 128 burr, 117

precision, 117 printing, 128 printing technology, 121, 123 pulse duration, 117

embossing, 128

quality, 117

melt, 117 microstructuring, 117 nanosecond, 117 overlap, 119

recast, 117 repetition rate, 117, 125 tribological structure, 128 tribology, 117, 121

Drilling of Metals Detlef Breitling1 , Christian F¨ ohl2 , Friedrich Dausinger1 , Taras Kononenko3, 3 and Vitali Konov 1

2

3

Institut f¨ ur Strahlwerkzeuge (IFSW), Universit¨ at Stuttgart, Pfaffenwaldring 43, 70569 Stuttgart, Germany [email protected] Forschungsgesellschaft f¨ ur Strahlwerkzeuge (FGSW), Nobelstraße 15, 70569 Stuttgart, Germany General Physics Institute (GPI) of Russian Academy of Sciences, Vavilov St. 38, 119991 Moscow, Russia

Abstract. The use of ultrashort laser pulses for drilling of metals is highly promising with respect to reduced melt production and recast formation, as well as minimized heat-affected zones. It enables a machining precision far superior to that achieved by longer pulses, the full potential of which, however, can frequently only be reached using low laser fluence levels at the expense of processing speed. Hence, ablation rates are presented versus drilling depth and energy density and discussed in view of an empirical model of laser drilling with respect to their limiting mechanisms. On this basis, processing windows for ideal laser parameters are concluded. With helical drilling being by far the most successful drilling technique using laser pulses in the nano-, pico-, or femtosecond-pulse duration domains, the method is discussed in detail and various technical means are presented for its improvement towards a successful implementation in an industrial environment.

1

Basic Understanding of Short-Pulsed Laser Drilling

Drilling in the short pulse range that spans from approximately 100 ps to 100 ns has been investigated intensively [1, 2, 3, 4, 5]. Irrespective of the chemical, optical or thermophysical properties of a large variety of materials including diamond, ceramics, plastics and steel, the evolution of the drilling process shows a quite general behavior that is illustrated in Fig. 1 and Fig. 2. At first sight it becomes obvious that the achieved drill depth does not increase linearly with the number of applied pulses. The highest drilling rate (increase in depth per pulse) of the order of micrometer per pulse is observed at the beginning of the penetration (phase I). Then the drilling rate drops rapidly to a level that is lower by one to two orders of magnitude. Figure 2a shows, for instance, that in drilling of steel with 300 ps pulses the drilling rate has decreased to about 10 nanometers per pulse. The end of the drop, which is by definition the end of phase II, strongly depends on energy density. In phase III the depth increases linearly with the number of pulses. Depending on material and energy density, this phase may span over a relatively large depth, see Fig. 1. After the end of phase III the drilling rate falls again F. Dausinger, F. Lichtner, H. Lubatschowski (Eds.): Femtosecond Technology for Technical and Medical Applications, Topics Appl. Phys. 96, 131–156 (2004) c Springer-Verlag Berlin Heidelberg 2004


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0 .1

II

IV

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2 0 0

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Fig. 1. Ablation rate versus hole depth for drilling in CVD-diamond with 220 ps and 9 ns laser pulses. Nd:YAP laser operating at λ = 539 nm (220 ps) or λ = 1078 nm (9 ns) wavelength. Focus diameter df = 40 µm, focus energy density H = 10 J/cm2 [1, 6]

IV

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Fig. 2. Average ablation rate for drilling in steel with 300 ps laser pulses: (a) Development with hole depth for three energy densities and (b) dependence on energy density for two material thicknesses (λ = 1078 nm, df = 40 µm) [4]

quite rapidly, leading to a more or less sudden stop of the penetration process. As Fig. 2a shows, the depth where this stop is observed, which is at the same time the maximum thickness that can be drilled through, increases with energy density. Furthermore, it becomes obvious that in order to maintain a high-level drilling rate a certain minimum energy density is needed that strongly increases with material thickness, see Fig. 2b. When drilling with femtosecond pulses, a similar behavior is observed, see Fig. 3. The energy density needed to drill through 500 µm thick steel is much higher than the one for 30 µm irrespective of the pulse duration being 125 fs or 1 ps. Depending on the energy density the average drilling rate drops by one to two orders of magnitude when the thickness of the sample is increased to 500 µm, see Fig. 4. How can these obviously general phenomena be explained? The so-called Hirschegg model developed in a Russian–German collaboration [1] gives four possible reasons for the decrease of the drilling rate that are illustrated in Fig. 5 and will be discussed in the following:

Av. ablation rate per pulse in µm

Drilling of Metals 1

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133

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1

0.1 970 hPa

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0.01

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200

300

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Fig. 5. Schematic illustration of possible mechanisms that might be the cause for the strong drop of ablation rate during drilling

1.1

Energy Coupling

The formation of a capillary obviously changes the geometry illuminated by the laser beam. The increase of surface area can locally lead to a considerable reduction of intensity. In particular, the energy density acting on the side wall, given by the normal component En of the laser flux, is decreased by this projection of the radiation onto the actual hole shape [8]. This certainly leads to a reduction of the ablation rate in these areas.

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Ablation per pulse in µm

1

analytical 1D analytical with slope analytical with curvature numerical

0.1

Aluminum 7

-2

E0 = 7 × 10 W cm τH = 50 ns w = 20 µm

0.01 0

200

400

600

Number of pulses

1.2

800

1000

Fig. 6. Calculated ablation rates as a function of pulse number for drilling of aluminum. Comparison of a numerical model and various analytical approximations (see text) [8]

Heat Conduction

A one-dimensional description of heat conduction is only applicable as long as the lateral variations of absorbed energy or surface topology are negligible compared to the heat-penetration depth. Particularly at the strongly curved surface areas near the tip or the hole entrance of deep and narrow drillings this approximation does not hold, in general. Here, the material will heat slower or faster than on flat surfaces due to a more efficient heat diffusion or accumulation. In these regions three-dimensional heat conduction should be used for theoretical modeling. The influence of the above geometrical effects (neglecting questions regarding the beam propagation within the hole such as the influence of multiple reflections) are demonstrated by model calculations [8, 9]. In Fig. 6 several different stages of analytical approximations for the depth dependence of ablation rate are compared with a numerical result. While the purely onedimensional description is, of course, independent of hole depth, the combination of one-dimensional heat conduction with projected intensity shows a slight decrease of drilling velocity with depth. Nevertheless, such calculated evaporation rates are still much higher than the numerical results. Only when the local surface curvature is also taken into account does the analytical method yield values similar to the numerical simulation. The calculated drop of ablation rate within several hundred laser pulses is larger than one order of magnitude. Obviously the decrease of surface temperature at the tip due to three-dimensional heat diffusion has a considerable influence on the drilling velocities for such small holes. 1.3

Plasma

At high intensity a laser beam is able to ionize the material it interacts with and thus to produce plasma. Depending on the ionization energy of the material different intensity levels are needed: The formation of an atmospheric

Drilling of Metals

135

Purely atmospheric plasma (gas breakdown) df

dB Material-vapor plasma Particle ignited atmospheric plasma

Fig. 7. Schematic illustration of the types of plasma that can occur during drilling with ultrashort laser pulses [10]

plasma (“breakdown”, see the Chapter “Interaction with Atmosphere”) generally requires two orders of magnitude higher intensity than, for instance, metal plasma. The absolute threshold additionally depends on wavelength, interaction time, and on surrounding conditions such as the number of start electrons. Figure 7 illustrates the different locations where we can expect the different types of plasma. An atmospheric plasma is ignited near the location of highest intensity that is in the laser beam focus. The presence of a workpiece is not necessary but facilitates the initiation. The material-vapor plasma, on the other hand, can only be produced where material vapor is present, which is the case close to the surface that is heated by the laser beam. The maximum distance from this surface is given by the product of expansion velocity times pulse duration. For a reasonable velocity of 104 m/s the vapor is able to expand only 10 nm during one picosecond. The third type of plasma shown in Fig. 7 at some distance from the bottom of the drilled capillary is again an atmospheric one, the ignition of which is facilitated by particles that are still present in the capillary after the previous laser pulse. This particle-ignited plasma requires less laser intensity than the purely atmospheric one and can be produced far away from the bottom of the capillary. A plasma is able to absorb laser energy strongly and thereby affect the linear drilling rate by reducing the intensity reaching the bottom of the capillary. The absorbed energy is not necessarily lost for the drilling process, however, because a large share of it is transmitted to the capillary walls by radiative and convective heat transport if the plasma is located in the capillary. This means that in addition to the laser beam that acts as a primary tool in the direction of its propagation, a secondary tool, the plasma, is acting radially to widen the capillary and to smoothen its wall.

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The formation of atmospheric plasmas can be influenced by the composition of the atmosphere. Gases of high ionization potential like helium increase the threshold intensity and weaken plasma effects. The most efficient way of suppression is to remove particles that can be ionized by decreasing the pressure. 1.4

Impeded Material Expulsion

The transport of gaseous or liquid material out of the capillary is enforced by pressure gradients inside the capillary. It can be expected that the pressure distribution is strongly influenced by the geometry of the ablated surface. In comparison to the beginning of a drilling process when material flows away from a still nearly planar surface, the outflow from a narrow capillary certainly is impeded. Additionally, plasma formation inside the capillary at locations distant from the bottom will strongly increase the pressure locally and can even cause pressure gradients enforcing material flow in a direction towards the bottom. Which of the above mentioned mechanisms is effective in reducing the ablation rate in a given drilling process will depend on the process parameters. At intensity values below the threshold for plasma formation only geometrical effects are expected. In that case the entrance of the capillary corresponds approximately to the focus diameter. The formation of plasma widens the capillary and thus reduces geometric effects. The strength of the secondary drilling tool plasma can be controlled by the laser parameters wavelength, intensity and pulse duration. Intensive investigations [11, 12] made clear that at a pulse duration of 300 ps two of the above mechanisms are predominant. As long as the ignition of a particle-ignited atmosphere plasma is avoided by using a subthreshold intensity or by applying vacuum, geometrical effects reduce the drilling rate in phase II. When such a plasma is ignited it strongly influences the drilling process by eroding the walls of the capillary and by reducing the drilling rate and suppressing the outflow of vapor. The plasma plume may, due to the high pressure involved, be regarded as a plug. At the beginning of phase II, the plasma plume may freely expand outside the capillary. As a consequence it does not affect the outflow and the observed reduction of the drilling rate is caused by geometrical effects related to the formation of a capillary. At a pulse duration near 100 fs, a strong reduction of drilling rate with increasing thickness is observed, as well, see Fig. 3 and Fig. 4. At low energy density atmospheric pressure has no significant influence. From this one can conclude that the reduction is caused by geometrical effects. At higher energy density, on the other hand, the strong influence of pressure indicates that atmospheric plasma is involved. What we observe at this ultrashort timescale is mainly the result of a deformation of the laser beam (see the Chapter “Interaction with Atmosphere”) changing the intensity profile and enlarging the angle of divergence without absorbing a considerable amount of energy,

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Fig. 8. Schematic illustration of the geometry of energy scattering and beam-profile deformation due to conical emission. The power density in the focused beam reaches a level sufficient to initiate nonlinear interaction with the atmospheric gas already in front of the nominal focus, thereby causing broadband radiation to be emitted within a cone that is considerably larger than the original beam divergence. The central blue spot results from fluorescence on the paper screen initiated by thirdharmonic radiation [13]

see Fig. 8 and Fig. 9. The enlargement of the beam profile that starts before the focal plane already, see Fig. 8, causes an increase of the hole diameter as shown in Fig. 10. In contrast to what is observed at longer pulse duration where the eroding effect of plasma inside the capillary smooths the wall surface, a pronounced ripple structure is encountered with femtosecond pulses, generally, the origin of which could be found in the structure of the deformed beam. Figure 11 shows how the linear ablation rate and the hole diameter depend on the energy density when a steel sample is drilled with femtosecond pulses. Below about 10 J/cm2 the ablation rate increases with energy density, while the diameter remains nearly constant. A strong widening of the hole diameter is observed at higher energy density values, at the same time the linear ablation rate saturates. As discussed in the Chapter “Interaction with Metals”, high accuracy, or in this case small hole diameters can be attained at low energy density at the expense of a very small drilling rate. If instead of normal pressure the drilling is performed in a mild vacuum, both the strong widening and the saturation of the ablation rate are avoided, allowing accurate drilling with higher productivity. Figure 12 compares the volume ablation rate instead of the linear one used so far. In this case no significant difference between normal and reduced pressure can be detected. From this observation one can conclude that the scattering effect of the plasma is approximately energy conserving, which confirms the transmission result shown in Fig. 9. With increasing pulse duration absorption in the plasma grows, see Fig. 9, the situation of drilling with long







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Fig. 9. Transmission measurements of breakdown sparks in air atmosphere (970 hPa) for various pulse durations. Scattered-energy fraction TS passing around an opaque disk centered in the diverging beam beyond the focus (open symbols) and total transmitted energy fraction Ttotal for high fluence (filled circles). TS = QQT −T0 , where QT and Q are the transmitted and incident pulse energies and T0 is the “geometrical transmittance” of the disk for an unperturbed beam (Ti:Al2 O3 laser. λ = 800 nm, df = 18 µm) [14]

Fig. 10. Blind holes in 500 µm thick steel percussion drilled in air atmosphere (970 hPa) by a Ti:Al3 O3 laser with the focus positioned on the front surface (λ = 800 nm, τH = 125 fs, fp = 1 kHz, Q = 640 µJ, df = 18 µm, H = 250 J/cm2 ). Number of pulses (left to right) 500, 800, 1400, 2500, 4000, 8000 [14, 15]

picosecond pulses (see above) is gradually approached. At the same time the scattering effect of the atmospheric plasma decreases and even falls below the detection level at a moderate energy density level of 65 J/cm2. From this, one may deduce an optimal pulse duration allowing scattering effects at the short side and thermal effects on the long side to be avoided, as is illustrated in Fig. 13. As has been shown in the Chapter “Interaction with Metals” in the case of aluminum, thermal effects start to increase with pulse duration at about 10 ps. For other metals with shorter relaxation time τep , such as

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Fig. 12. Volume ablation rate versus energy density for drilling in air atmosphere (970 hPa) and vacuum (< 1 hPa). Steel, 500 µm thickness, 125 fs pulses at 800 nm wavelength. A tenfold increase in laser fluence leads to an approximately hundred times higher volume drilling velocity, i.e. a tenfold increase in efficiency per unit pulse energy [16]

iron, the optimum is expected at shorter pulse duration. In general, pulses between 1 ps and 10 ps are regarded as optimal for the treatment of metals. It is important to note that the above conclusion holds for metals only, because for them nonlinear absorption can be dispensed due to their high linear absorption. In transparent media, however, shortening the pulse duration and thus increasing the intensity enables a minimization of the optical penetration, thus allowing precision to be increased and damage to be avoided, see the Chapter “Cutting of Diamond” and the Chapter “Dental Applications”.

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Fig. 13. Schematic illustration of the achievable the precision level in drilling of metals versus laser-pulse duration. For long pulses in the nanosecond regime, the accuracy is limited by melt and burr formation, for ultrashort pulses in the femtosecond domain, nonlinear wavefront disruption impairs the beam profile and, in consequence, the ablation precision [17]

2

Drilling Techniques

The fundamental discussion above shows the principal limitations of the drilling process: On the one hand, high energy density is required to enable penetration of a given material thickness in the first place and secondly to achieve a high level of process efficiency. This, on the other hand, strongly increases melt production and beam deformation due to nonlinear effects, both effects causing unacceptable quality losses. As illustrated in Fig. 13, a proper choice of the pulse duration is one step on the way out of this dilemma. The technical means presented in the following enable further improvements. 2.1

Helical Drilling

So far the basic dependences of laser drilling at ultrashort pulses have been discussed using the example of the percussion drilling process in which the full penetration is reached with a multitude of pulses. As Fig. 14 illustrates, recast is an issue even at ultrashort pulse duration if this technique is used. As explained in the Chapter “Interaction with Metals”, melt formation can indeed not be avoided entirely even for ultrashort pulses. However, the resulting recast layers are, to a large extent, a result of insufficient material expulsion and thus can be reduced drastically when more sophisticated techniques for drilling are used. These involve a relative movement of the workpiece and the laser spot with respect to each other, Fig. 15. Among these, helical drilling has already turned out to be beneficial for achieving highest accuracy levels

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Fig. 14. Entrance view of a bore in steel percussion drilled with ultrashort pulses showing strong melt burr formation at the hole edges. Parameters: target thickness 1 mm, τH = 500 fs, λ = 775 nm, H = 390 J/cm2 [19, 20] S in g le p u ls e

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with pulse durations in the nanosecond regime [3, 18]. In this technique the beam is moved in a circular motion relative to the target, such that the ablation front proceeds into the material on a helical path. Since the laser-spot diameter is smaller than the final bore, material expulsion becomes easier and thus melt films do not accumulate to thick layers or can be removed by successive passes of the beam. Furthermore, helical drilling has proven to be beneficial also in terms of increased shape accuracy, since for a beam diameter smaller than the bore hole in combination with a rotational beam movement, the hole shape will depend less on the actual – and generally imperfect – beam profile but rather on the superimposed beam motion. The relative motion of beam and workpiece, which is fundamental to the technique of helical drilling, can be accomplished in several ways. One approach is to move the workpiece in a circular fashion, either by a controlled movement of crossed precision-positioning axes in the x–y-plane of the motion, or – and even more naturally – by rotating the entire workpiece around

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Fig. 16. Entrance (left) and exit view (right) of a hole produced in steel of 0.5 mm thickness by means of helical drilling through a rotation of the workpiece plate [20]. Experimental picosecond laser [21] with τH = 5 ps pulse duration and λ = 1030 nm wavelength, Q = 600 µJ, fP = 10 kHz

an axis that is slightly displaced from the laser-beam axis in order to impose a helical drilling radius. While this idea seems simple enough and has indeed proved the full potential of helical drilling for small flat sample plates (Fig. 16), the method quickly finds its limitations when large workpieces of complex geometries are to be accurately handled and positioned and moved precisely with several hundred revolutions per minute. Hence, the opposite approach, the movement of the beam with respect to the target by an optical deflection of the laser beam is considered to be more promising. Frequently, galvanometric optical scanners are used for this purpose. In their most common configuration the laser beam is deflected by a system of two small, lightweight mirrors, each of which can be quickly adjusted around a rotational axis in order to position the laser beam along the perpendicular axis in the plane of focus (Fig. 17). Since the laser beam is deflected at variable angles and therefore travels different distances to the target, a specially designed lens system, a so-called f-theta objective, ensures that the focus is always moved within a plane on the workpiece surface. Scanner systems provide an extremely flexible tool for beam movement, the lightweight mirrors allow for very high dynamics of the beam motion such that structures of virtually any shape can be machined (Fig. 18). If, similarly to the process of helical drilling, several passes are needed to cut a complex shape through thicker materials, the method is referred to as multipass micromachining. 2.2

Drilling with Beam Inclination

Frequently, circular symmetry of the bore is sufficient, but additionally negative conicity may be desired in order to provide exit diameters of the drilled hole that are larger than the entrance diameter (Fig. 19). This geometry is, for example, required for automotive fuel-injection nozzles. In order to produce these geometries by means of a laser, it is required not only to move the

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Fig. 18. Entrance (left) and exit view (right) of a spinneret nozzle design with sixfold symmetry [22]. Produced by laser machining with a prototype picosecond laser system [23]: τH = 10 ps, λ = 1064 nm, Q = 200 µJ, fP = 20 kHz

laser focus in a circular fashion but to additionally impose a defined inclination angle on the beam that is to be rotated simultaneously. A special optical system has been designed [24] to facilitate the decoupled adjustment of these parameters by means of an arrangement of optical wedges (Fig. 20). The first pair of wedges is oriented directly opposed to each other and the first wedge can be adjusted in terms of its distance along the optic axis in relation to all the other wedges. This provides a beam displacement parallel to the optic axis before the focusing lens. It is transformed by the lens into an inclination angle at the target. The second pair of wedges (with a considerably smaller

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Fig. 19. Example of conical holes drilled by nanosecond-laser pulses in a AlN-ceramics plate of 640 µm thickness (left) and in 1 mm thick steel (right). In both cases the exit diameter is twice as large as the entrance [24]

wedge angle) is mounted at fixed distances with respect to each other and the fourth wedge is able to be rotated relative to the other wedges. Depending on its rotational orientation, the deflection angle introduced by wedge 3 is not compensated fully by wedge 4. It is therefore converted to a displacement of the focal spot in the target plane. Since the entire optical system rotates around the optic axis, a circular beam movement as well as an inclination of the beam toward the outside of the hole is provided. If the wedge angles and distances are carefully chosen, the parameters of trepanning radius and inclination angle can be adjusted virtually independently [24]. Helical drilling with the above-mentioned trepanning optic is not only beneficial when special hole geometries are required that cannot be achieved by standard drilling techniques. On introducing a laser-beam inclination, it furthermore enables processing times to be reduced even when cylindrical cross sections are sufficient. Figure 21 shows the achieved ratios of the exit and entrance diameters of bore holes versus the drilling time for helical drilling with various pulse durations and for two beam-inclination angles. The initial breakthrough is reached after only a couple of seconds (starting point of the curves), followed by a widening of the exit diameter (increase of diameter ratio). The widening and cleaning phase is characteristic of helical drilling, it is generally necessary to obtain both the desired geometry as well as highest precision. During the widening, a growing fraction of the laser radiation passes through the bore unhindered by the hole walls and is thus entirely lost to the process, especially when the hole is close to its final shape. But since the widening frequently requires the major portion of the total processing time, sometimes up to 90%, this part of the process should also offer the highest potential for an increase in efficiency. To this end, the

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Fig. 20. Schematic of the optical principle for a trepanning optic that provides independent adjustment of the trepanning radius rh as well as of the inclination angle γ by means of two pairs of optical wedges that are oriented in opposite directions and can be translated with respect to each other (1st and 2nd wedge) or rotated relative to each other at a fixed, minimal distance (3rd and 4th wedge). The entire optical arrangement rotates around the optic axis to facilitate the helical drilling motion [19]

Fig. 21. Influence of drilling time on cylindricity – the ratio of exit (dout ) and entrance (din ) diameters – for drilling of 500 µm thick steel plates with ultrashort laser pulses at various pulse durations (λ = 780 nm, Q = 900 µJ, fP = 1 kHz). Helical drilling with trepanning optic and beam inclination angles of 0◦ (top) and 4◦ (bottom) [20]

trepanning optic is indeed beneficial. As Fig. 21 demonstrates, the drilling time that is necessary to reach hole entrances and exits of equal diameters can be cut down by about 50% when a beam inclination of 4◦ is introduced (bottom) compared to ordinary helical drilling with perpendicular incidence

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(top). Moreover, in this case, the use of the shortest femtosecond pulses does not even enable entrance and exit diameters of similar sizes to be reached even for long drilling times unless a beam inclination is used. It can be concluded that due to the angular deflection of the beam, a larger fraction of the radiation takes part in the hole-widening process that can thus be shortened considerably.

3 Means to Increase Productivity and Processing Quality 3.1

Pulse Repetition Rate

While the helical drilling technique enables bore holes with good quality to be produced even with high laser-pulse energies and thus allows use of the superior ablation rates at high fluence levels to be made, compare Fig. 12, with ultrashort pulses the processing times are still far from being feasible in industrial environments. At present, commercially available ultrashortpulsed laser systems with sufficient pulse energy for efficient drilling offer repetition rates of the order of one or a few kilohertz at best. Thus, with the exception of accumulated ablated material vapor and particle clouds, whose lifetimes can range far into the millisecond domain but that do not disturb the majority of subsequent laser pulses, compare with the Chapter “Interaction with Atmosphere”, the time spans between two laser pulses exceeds most typical time spans of the interactions of the ablation process by several orders of magnitude. It can hence be expected, at least for metallic workpieces, that while each pulse may find a target that is changed in geometry by the preceding pulses, the target does not actually retain much further “processing history”. Processing at higher repetition rates should therefore lead to an increase in drilling speed, that is, for all practical aspects, proportional to the repetition rate. Figure 22 displays the drilling time needed to produce holes of equivalent geometries at various repetition rates. Since, in this case, the experimental laser system has been equipped with an external acousto-optical pulse picker and energy modulator, which allows both pulse energies and repetition rates to be selected, while the entire laser system – oscillator and regenerative amplifier – can operate at its optimized setpoint, the comparison of the processing times is possible without reservations. Indeed, compared to the dashed line for the expected drilling time corresponding to a constant total number of laser pulses, the drilling time in Fig. 22 shows a nearly reciprocal dependence on repetition rate. While this result has been obtained for percussion drilling, where the total processing time for through drilling can be easily determined, a similar behavior is also observed for helical drilling. It is especially important to note that the quality of the drilled holes remains unchanged irrespective of repetition rate, as Fig. 23 proves for entrance views

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Fig. 22. Influence of the repetition rate on drilling time for percussion drilling of 1 mm thick steel plates by an experimental picosecond laser system: τH = 5 ps, λ = 1030 nm, H = 330 J/cm2 [22]

Fig. 23. Front view of holes revealing no effect of repetition rate on hole quality. Helical drilling in 1 mm thick steel by an experimental picosecond laser system: τH = 5 ps, λ = 1030 nm, H = 330 J/cm2 , 235 µm diameter, 410 000 (1 kHz) and 520 000 pulses (4 kHz) [22]

of two holes drilled at one and 4 kHz repetition rates and with comparable total pulse numbers. 3.2

Polarization Control

To date, all available ultrashort-pulsed laser systems for materials processing emit linearly polarized laser radiation due to the necessity of fast optical switches for the selective amplification of seed-laser pulses. The orientation of the polarization has proven to be of considerable importance whenever the processing involves craters, through holes, or cuts with high aspect ratios. A phenomenon known already from materials processing with laser pulses in the longer picosecond and in the nanosecond time domains, is that sidewall

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ablation in deep structures shows different efficiency depending on the direction of polarization. Thus, one often finds bending of drilling capillaries in a direction perpendicular to the polarization [6], which is given by the orientation of the electric-field vector in the radiation field, or irregular hole exits with larger diameters perpendicular to the plane of polarization [1, 2]. These effects have been explained with the higher reflection for s-polarized light (defined with respect to the plane of incidence given by the light propagation direction and the direction normal to the surface) at grazing incidence. It can lead to a higher fraction of multiply reflected light in the capillary and thus to more efficient ablation at the crater ground or hole exit [20,25]. For ultrashortpulsed laser machining similar observations have been reported [9, 20, 25, 26]. The pronounced influence of polarization on the hole exits is depicted in Fig. 24a. They not only show elongated shapes but exhibit actual corners. Holes drilled in steel by femtosecond laser pulses have revealed an additional polarization-dependent effect. As shown in Fig. 24b, ripple formation occurs at the side walls of the bore near the entrance surface. However, these ripples do not appear uniformly around the entire hole. In some areas, ripples with a pitch of several micrometers stretch all the way along the entire length of the hole wall, i.e. to a depth easily exceeding several hundred micrometers. In other parts of the wall, they only propagate some 10 µm to 20 µm from the front surface of the target. An analysis of the polarization reveals that the long ripples form in regions where the polarization is perpendicular to the plane of incidence, i.e. parallel to the hole wall, while the short ripples occur for parallel polarization (Fig. 24b). It is believed that both types of ripples have the same origin, a characteristic surface structure forming immediately after the beginning of drilling [20], which might be caused by beam-profile disturbances introduced by conical emission as described in the Chapter “Interaction with Atmosphere”. In order to achieve smooth hole walls to satisfy even the highest-quality demands, a means to suppress the long ripple formation must be regarded as beneficial. In many cases, the use of circular polarization has proved to be able to inhibit the disadvantages of linear polarization. However, where machining quality is directly affected by polarization, circular polarization often acts similarly to a mere mixture of two mutually perpendicular, linear orientations. In this case, a tendency towards ripple formation remains for circular polarization. Sufficient suppression of ripple generation can only be achieved by means of polarization control, where the plane of polarization for linearly polarized light is rotated in synchronization with the laser spot during helical drilling. A suitable technique has been devised to be used in combination with the described trepanning optic. It is schematically shown in Fig. 25. Figure 26 shows the potential of this method of polarization control. In Fig. 26a an exit view of a hole drilled in steel by helical drilling is depicted when a fixed, vertical linear polarization is employed. The considerable de-

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Fig. 24. Influence of the direction of polarization on the shape of hole exits (a) and ripple formation on the wall surface near the entrance (b) for holes drilled through 0.5 mm thick steel in air atmosphere by means of helical drilling with femtosecond laser pulses: TH = 120 fs, λ = 775 nm, df = 18 µm, H = 310 J/cm2 , fP = 1 kHz [20, 22]

Fig. 25. Schematic drawing of the principle of polarization control by a trepanning optic. A pair of quarterwave plates, one of which is fixed with respect to the laser source, while the other is mounted to the trepanning optic and thus rotates with the same frequency ω, ensures that linear polarization is generated that rotates in synchronization with the focal-spot movement [20]

formation of the hole can be reduced noticeably by the polarization rotation in combination with a trepanning optic as described in Fig. 25. However, some irregularity remains, which can be explained by the fact that despite the polarization being always oriented perpendicular to the wall, the beam intensity profile does not rotate. Thus it faces the hole wall with different parts during one revolution. When it is not perfectly circularly symmetric, as in the case of the measured profiles mounted inside Fig. 26b, it can cause the hole to be ablated anisotropically in different sections of the wall. Therefore, it must be concluded that beam-profile circular symmetry must be regarded as a very stringent requirement for future laser-system development. A perfectly symmetric beam cross section is not always available. In this case, a rotation of the beam profile itself might be necessary such that it

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Fig. 26. Improvement of hole geometry using polarization control during helical drilling. Exit views of holes drilled (a) without polarization control and (b) with linear polarization rotating synchronously with the laser-spot movement. (c) View of exit of a hole drilled in the ideal case for additional control of the beam profile with respect to the workpiece, e.g. by a rotation of the workpiece itself (d) [20]

always faces the hole wall with the same side. In general, this requires the use of an additional image rotator to be introduced into the laser beam. Figure 26c displays the remarkable improvement that this method implies, although in this case, both beam cross section and polarization rotation have been simulated by a rotating workpiece as depicted in Fig. 26d. 3.3

Vacuum Processing

As shown in Sect. 1 and in Chapter “Interaction with Atmosphere”, the deformation of the laser beam due to conical emission leading to an unwanted irregular widening as in Fig. 10 and the strong decrease of the depth-ablation rate (Fig. 11) can be suppressed under vacuum conditions. The implementation of a vacuum chamber into a processing station would require time-consuming and therefore expensive loading and unloading processes of the chamber and is, furthermore, relatively inflexible for practical purposes. Moreover, experiments have shown that, in general, the positive aspects of drilling in vacuum can already be achieved in a relatively moderate low-pressure atmosphere of approximately 100 hPa. This allowed a special vacuum nozzle to be developed, which is based on the principle of an aerodynamic window, Fig. 27. A supersonic gas jet created by the supply nozzle generates a freely propagating flow in the window section. The carefully designed nozzle geometry allows this jet to travel along a curved path towards the diffusor, thereby sucking gas out of the cavity volume. In other applications, these windows have been shown to be able to provide a cavity pressure down to 70 hPa to 90 hPa while atmospheric conditions prevail on the gas jet’s opposite side [27, 28]. The nozzle offers high flexibility for the geometry and handling of the workpiece. Enabling, furthermore, a fast variation of the pressure even during the drilling process, the nozzle allows development of novel laser processes that combine the advantages of fast drilling at reduced pressure with the smoothing effect of the laser-induced plasma during the hole-widening and cleaning phase, after the material is pierced through [29].

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Fig. 27. Concept of a vacuum nozzle based on the principle of an aerodynamic window. A supersonic gas jet provided by the supply nozzle is enabled by the special nozzle geometry to travel on a curved path towards the diffusor. In the freepropagation section it is able to suck gas from the cavity volume and thus provides a moderate vacuum of pcav ≤ 100 hPa in the laser–target interaction zone. Thus laser drilling can be performed in a subatmospheric pressure ambient with high efficiency without the need for a fully enclosed and bulky vacuum chamber [22, 30]

References [1] F. Dausinger, T. Abeln, D. Breitling, J. Radtke, V. Konov, S. Garnov, S. Klimentov, T. Kononenko, O. Tsarkova: Bohren keramischer Werkstoffe mit Kurzpuls-Festk¨ orperlasern, LaserOpto 31 (3), 78–85 (1999) 131, 132, 148 [2] S. M. Klimentov, S. V. Garnov, T. V. Kononenko, V. I. Konov, P. A. Pivovarov, F. Dausinger: High rate deep channel ablative formation by picosecond– nanosecond combined laser pulses, Appl. Phys. A-Mater. 69 Suppl., 633–636 (1999) 131, 148 [3] T. Abeln, J. Radtke, F. Dausinger: High precision drilling with short-pulsed solid-state lasers, in P. Christensen, P. Herman, R. Patel (Eds.): LIA 88 (Laser Institute of America, Orlando 2000) pp. 195–203 131, 141 [4] T. Kononenko, S. M. Klimentov, V. I. Konov, P. A. Pivovarov, S. V. Garnov, F. Dausinger, D. Breitling: Propagation of short-pulsed laser radiation and stages of ablative deep-channel formation, in M. C. Gower, H. Helvajian, K. Sugioka, J. J. Dubowski (Eds.): Proc. SPIE 4274 (Intl. Soc. for Opt. Eng. 2001) pp. 248–257 131, 132, 133

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[5] J. Radke: Herstellung von Pr¨ azisionsdurchbr¨ uchen mittels repetierender Laserbearbeitung am Beispiel keramischer Werkstoffe, Laser in der Materialbearbeitung – Forschungsberichte des IFSW (Utz, Munich 2003) Dissertation Univ. Stuttgart 131 [6] T. V. Kononenko, V. Konov, S. Garnov, S. Klimentov, F. Dausinger: Dynamics of deep short pulse laser drilling: ablative stages and light propagation, Laser Phys. 11, 343–351 (2001) 132, 148 [7] V. I. Konov, S. V. Garnov, S. M. Klimentov, T. V. Kononenko, P. A. Pivovarov, F. Dausinger, D. Breitling: Role of gas pressure in the process of high intensity ultra-short laser pulse drilling with special regard to plasmas, in P. Berger, F. Dausinger, C. F¨ ohl (Eds.): Proc. 3rd Intl. Workshop on Fundamentals of Ablation with Short Pulsed Solid State Lasers 2002 (Hirschegg, Austria) (Forschungsgesellschaft f¨ ur Strahlwerkzeuge, Stuttgart 2002) 133 [8] A. Ruf, P. Berger, F. Dausinger, H. H¨ ugel: Analytical investigations on geometrical influences on laser drilling, J. Phys. D Appl. Phys. 34, 2918–2925 (2001) 133, 134 [9] A. Ruf, D. Breitling, C. F¨ ohl, J. Radtke, F. Dausinger, H. H¨ ugel, T. Kononenko, S. Klimentov, S. Garnov, V. Konov, J. Suzuki: Modeling and experimental analysis of hole formation in laser deep drilling with short and ultra-short pulses, in Wissenschaftliche Gesellschaft Lasertechnik (WLT) e. V. (Ed.): Proc. First Intl. WLT-Conference on Lasers in Manufacturing (ATFachverlag, Stuttgart 2001) pp. 214–226 134, 148 [10] S. M. Klimentov, P. A. Pivovarov, S. V. Garnov, T. V. Kononenko, V. I. Konov, D. Breitling, F. Dausinger: Ablation rate enhancement by combination of picosecond and nanosecond pulse trains: effect of polarization, in P. Berger, F. Dausinger, C. F¨ ohl (Eds.): Proc. Intl. Workshop on Fundamentals of Ablation with Short Pulsed Solid State Lasers 2001 (Hirschegg, Austria) (Forschungsgesellschaft f¨ ur Strahlwerkzeuge, Stuttgart 2001) 135 [11] T. V. Kononenko, S. M. Klimentov, S. V. Garnov, V. I. Konov, D. Breitling, C. F¨ ohl, A. Ruf, J. Radtke, F. Dausinger: Hole formation process in laser deep drilling with short and ultrashort pulses, in I. Miyamoto, Y. F. Lu, K. Sugioka, J. J. Dubowski (Eds.): Proc. SPIE 4426 (Intl. Soc. for Opt. Eng. 2002) pp. 108– 112 136 [12] T. V. Kononenko: Spectroscopic study of plasmas produced by subnanosecond pulses, in P. Berger, F. Dausinger, C. F¨ ohl (Eds.): Proc. 4th Intl. Workshop on Fundamentals of Ablation with Short Pulsed Solid State Lasers 2003 (Hirschegg, Austria) (Forschungsgesellschaft f¨ ur Strahlwerkzeuge, Stuttgart 2003) 136 [13] S. M. Klimentov, P. A. Pivovarov, V. I. Konov, D. Breitling, F. Dausinger: Spectral and energy characteristics of conical emission in gases, in P. Berger, F. Dausinger, C. F¨ ohl (Eds.): Proc. 4th Intl. Workshop on Fundamentals of Ablation with Short Pulsed Solid State Lasers 2003 (Hirschegg, Austria) (Forschungsgesellschaft f¨ ur Strahlwerkzeuge, Stuttgart 2003) 137 [14] D. Breitling, A. Ruf, P. W. Berger, F. H. Dausinger, S. M. Klimentov, P. A. Pivovarov, T. V. Kononenko, V. I. Konov: Plasma effects during ablation and drilling using pulsed solid state lasers, in F. H. Dausinger, V. I. Konov, V. Y. Baranov, V. Y. Panchenko (Eds.): Proc. SPIE 5121 (Proc. Laser Processing of Advanced Materials and Laser Microtechnologies (Conf. on Lasers, Applications, and Technologies LAT 2002, Moscow) 2003) pp. 24–33 138

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[15] M. Klimentov, T. V. Kononenko, P. A. Pivovarov, S. V. Garnov, V. I. Konov, D. Breitling, F. Dausinger: Effect of nonlinear scattering of radiation in air on material ablation by femtosecond laser pulses, in F. H. Dausinger, V. I. Konov, V. Y. Baranov, V. Y. Panchenko (Eds.): Proc. SPIE 5121 (Proc. Laser Processing of Advanced Materials and Laser Microtechnologies (Conf. on Lasers, Applications, and Technologies LAT 2002, Moscow) 2003) pp. 77–86 138 [16] F. Dausinger: Femtosecond technology for precision manufacturing: Fundamental and technical aspects, in I. Miyamoto, K. Kobayashi, K. Sugioka, R. Poprawe, H. Helvajian (Eds.): Proc. SPIE 4830 (Third Intl. Symposium on Laser Precision Microfabrication LPM 2002 (Osaka, Japan) 2003) pp. 471–478 139 [17] F. Dausinger: Machining of metals with ultrashort laser pulses – fundamental aspects and their consequences, in G. Huber (Ed.): Adv. Prog. Conf. on Lasers and Electro-Optics/Europe and European Quantum Electronics Conf. CLEO/Europe 2003 – EQEC (European Physical Society, Mulhouse 2003) p. 31 140 [18] F. Dausinger: Precise drilling with short pulsed lasers, in X. Chen, T. Fujioka, A. Matsunawa (Eds.): Proc. SPIE 3888 (Intl. Soc. for Opt. Eng. 2000) pp. 180– 187 141 [19] C. F¨ ohl, D. Breitling, K. Jasper, J. Radtke, F. Dausinger: Precision drilling of metals and ceramics with short and ultrashort pulsed solid state lasers, in I. Miyamoto, Y. F. Lu, K. Sugioka, J. J. Dubowski (Eds.): Proc. SPIE 4426 (Second Intl. Symposium on Laser Precision Microfabrication LPM 2001 (Singapore) 2002) pp. 104–107 141, 145 [20] C. F¨ ohl, D. Breitling, F. Dausinger: Influences on hole quality in high precision drilling of steel with ultra-short pulsed laser systems, in E. Beyer, R. Patel, A. Ostendorf (Eds.): LIA 94 (Proc. ICALEO 2002) 141, 142, 145, 148, 149, 150 [21] D. M¨ uller, S. Erhard, A. Giesen: High power thin disk Yb:YAG regenerative amplifier, in C. Marshall (Ed.): TOPS 50 (Opt. Soc. Am., Washington 2001) pp. 319–324 142 [22] C. F¨ ohl, F. Dausinger: Influences on hole quality in high precision drilling of steel with ultra-short pulsed laser systems, in I. Miyamoto, A. Ostendorf, K. Sugioka, H. Helvajian (Eds.): Proc. SPIE 5063 (Intl. Soc. for Opt. Eng. 2003) pp. 346–351 143, 147, 149, 151 [23] J. Kleinbauer, S. Reuter, R. Knappe, R.Wallenstein: High power, high repetition-rate picosecond Nd:YVO4 regenerative amplifier, in G. Huber (Ed.): Adv. Prog. Conf. on Lasers and Electro-Optics/Europe and European Quantum Electronics Conf. CLEO/Europe 2003 – EQEC 2003, Munich, Germany (European Physical Society, Mulhouse 2003) pp. 40 (CA3–2–TUE) 143 [24] K. Jasper: Neue Konzepte in der Laserstrahlformung und -f¨ uhrung f¨ ur die Mikrotechnik, Laser in der Materialbearbeitung – Forschungsberichte des IFSW (Utz, Munich 2003) Dissertation Univ. Suttgart 143, 144 [25] S. Nolte, C. Momma, G. Kamlage, A. Ostendorf, C. Fallnich, F. von Alvensleben, H. Welling: Polarization effects in ultrashort-pulse laser drilling, Appl. Phys. A-Mater. 68 Suppl., 563–567 (1999) 148 [26] D. Breitling, S. M. Klimentov, T. V. Kononenko: Drilling progress visualization – hole formation and drilling rate analysis during high-aspect ratio drilling using ultra-short pulsed solid-state lasers, in P. Berger, F. Dausinger, C. F¨ ohl (Eds.): Proc. Intl. Workshop on Fundamentals of Ablation with Short

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[27]

[28] [29]

[30]

Detlef Breitling et al. Pulsed Solid State Lasers 2001 (Hirschegg, Austria) (Forschungsgesellschaft f¨ ur Strahlwerkzeuge, Stuttgart 2001) 148 W. Krepulat, P. Berger, H. H¨ ugel: Investigation of a low pressure free vortex aerodynamic window for industrial lasers, in W. L. Bohn, H. H¨ ugel (Eds.): Proc. SPIE 2502 (Proc. Tenth Intl. Symposium on Gas Flow and Chemical Lasers GCL’94 1995) pp. 559–564 150 W. Krepulat: Aerodynamische Fenster f¨ ur industrielle Hochleistungslaser, Laser in der Materialbearbeitung – Forschungsberichte des IFSW (Teubner 1996) Dissertation Univ. Stuttgart 150 J. Radtke, T. Abeln, M. Weikert, F. Dausinger: High precision micro cutting of ceramics with nanosecond lasers, in H. H¨ ugel, A. Matsunawa, J. Mazumder, P. Christensen (Eds.): LIA 90 (Laser Institute of America, Orlando 1999) pp. 27–35 150 C. F¨ ohl, D. Breitling, F. Dausinger: Precise drilling of steel with ultrashort pulsed solid state lasers, in F. H. Dausinger, V. I. Konov, V. Y. Baranov, V. Y. Panchenko (Eds.): Proc. SPIE 5121 (Proc. Laser Processing of Advanced Materials and Laser Microtechnologies (Conf. on Lasers, Applications, and Technologies LAT 2002, Moscow) 2003) pp. 271–279 151

Index

ablation rate, 132, 133, 136 ablation rate average, 132, 133, 139 ablation rate depth dependence, 134 ablation rate linear, 137 ablation rate reduction, 133, 136 ablation rate volume, 137, 139 absorption linear, 139 absorption nonlinear, 139 aerodynamic window, 150, 151 atmosphere air, 138, 139, 149 atmosphere helium, 136 atmosphere low pressure, 150 atmosphere pressure, 136, 137, 150, 151 beam profile, 141, 149, 150 beam profile distortion, 137, 148 breakdown dependence on pulse duration, 138 capillary, 136, 148 conical emission beam divergence, 137 conical emission beam-profile distortion, 137 drilling conical holes, 142, 144 drilling efficiency, 140, 144, 146, 151 drilling energy coupling, 133 drilling geometry, 133, 134, 136, 142, 144 drilling heat conduction, 134 drilling hole diameter, 137, 139, 141, 142, 144 drilling in vacuum, 137, 139, 150, 151 drilling inclination angle, 143–145 drilling of aluminum, 134 drilling of metals, 131, 140 drilling of steel, 131–133, 138, 139, 141, 142, 144, 147, 149

drilling drilling drilling drilling drilling drilling drilling drilling drilling

percussion, 140 precision, 140, 144 quality, 140, 146 rate, see ablation rate, 136 rate linear, 135 techniques, 140 time, 144–147 trepanning, 141 trepanning radius, 144, 145

energy coupling, 133 f-theta objective, 142 heat conduction, 134 heat transport, 135 heat-penetration depth, 134 helical drilling, 140–142, 144–150 Hirschegg model, 132 laser drilling, see drilling laser machining, 143 material expulsion, 136, 140 melt, 140, 141 micromachining, 142 modeling of drilling, 134 percussion drilling, 138, 140, 141, 147 plasma, 134, 150 plasma absorption, 137 plasma atmospheric, 134, 136, 138 plasma metal, 135 plasma particle-ignited, 135, 136 plasma threshold, 135, 136 plasma vapor plasma, 135 polarization, 147, 149, 150 polarization control, 148–150 polarization in laser machining, 148

156

Index

pulse duration, 140 recast, 140 repetition rate, 146, 147 ripple, 137, 148, 149 scanner, 142, 143 steel, 133

thermal effects, 138 thermal effects dependence on pulse duration, 138 trepanning optic, 144, 145, 148

vacuum nozzle, 150, 151

Cutting of Diamond Michael Weikert and Friedrich Dausinger Institut f¨ ur Strahlwerkzeuge (IFSW), Universit¨ at Stuttgart, Pfaffenwaldring 43, 70569 Stuttgart, Germany [email protected] Abstract. For a number of years, diamond has been used as a material for cutting tools. Since conventional machining of diamond is difficult and expensive, laser cutting is considered a promising technology for future production of diamond-cutting tools. Experiments with nanosecond-lasers showed a tendency to crack formation in the material, especially in monocrystalline diamond. With femtosecond laser pulses it is possible to avoid the cracks. The roughness of the laser cut surface is better than the surface produced by EDM, but inferior to a polished one. The cutting speed with available laser systems is, however, low compared to conventional techniques.

The edge of cutting and machining tools has to be harder than the material that is machined. Traditionally, hard metals are used, but in the past diamond became more and more popular due to its hardness and with its reduced wear compared to hard metal. Usually, polycrystalline diamond (PCD), consisting of diamond crystallites bonded together in a matrix material, is used. If the matrix material is selected to be electrically conducting, PCD can be machined economically by EDM techniques. However, in recent years monocrystalline diamond (MCD) became popular since its service life is even longer than with PCD. But the hardness of MCD makes it difficult to machine. Since diamond is harder than all other materials in existence, it can only be machined mechanically by grinding with diamond. The grinding material is worn off itself, making it a complex and expensive process. Since MCD is electrically nonconductive, it is impossible to use EDM techniques. CVD (chemical vapor deposition) diamond is an alternative to expensive MCD. CVD diamond consists of grains with monocrystalline phases. In comparison to PCD, CVD diamond has no matrix material between the crystallites. Therefore the mechanical properties of CVD diamond are very similar to MCD. Laser cutting would have the potential to replace or complement conventional techniques in the economical production of diamond tools. Today, state-of-the-art laser systems in industrial use have pulse durations in the nanosecond range or longer. Earlier experiments have shown that diamond is prone to produce cracks during the laser processing. Additionally, quality and precision of laser-shaped surfaces do not meet the requirements for the use as cutting tools without further postprocessing. The use of ultrashort laser pulses is considered to be a promising way to prevent cracks and to increase quality and precision of the surface in order to reduce or avoid further postprocessing. F. Dausinger, F. Lichtner, H. Lubatschowski (Eds.): Femtosecond Technology for Technical and Medical Applications, Topics Appl. Phys. 96, 155–165 (2004) c Springer-Verlag Berlin Heidelberg 2004


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Fig. 1. Typical morphology of the cut surface. Laser: wavelength: 800 nm, pulse duration: 2 ps, pulse energy: 0.75 mJ, energy density: 78 J/cm2 , material: CVD, thickness: 500 µm

1

Experimental Results

In order to examine the potential of laser cutting, diamond samples were cut perpendicular to the sample surface in straight lines. In typical cutting or machining tools, the edge is usually shaped in an angle different from 90◦ , while the shape is not always a straight line. The typical morphology of a lasershaped surface is shown in Fig. 1. With optimised process parameters, the first 200 µm to 300 µm of cutting depth show a comparably smooth surface. Usually it could be observed that from this depth onwards, the cut becomes irregular. Depending on the process parameters, these irregularities could be minimized, but it was not possible to avoid them completely. It could be observed that the quality of the cut surface increased if the pulse energy is decreased. On the other hand, a high energy is necessary to cut through the material, as it was observed that the ablation rate decreases with depth. Comparable to laser drilling, where the process stops in a certain depth if the energy is not high enough, it becomes very time consuming or even impossible to cut through the material if the pulse energy is too low. Therefore, a balance between quality and the ability to cut through the material has to be maintained. As shown before, the surface of the cut becomes irregular at a certain material depth. In the case of helical drilling (see the Chapter “Drilling of Metals”) the process is usually continued for some time after the first breakthrough, in order to remove material in some kind of cleaning process. This technique was adapted to the laser cutting of diamond to explore if it would be possible to clean the surface after the cut. As depicted in Fig. 2 it is possible to remove some of the large irregularities at the exit of the cut. However,

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Fig. 2. Polishing of the cut surface by continued processing. Laser: wavelength: 800 nm, pulse duration: 2 ps, pulse energy: 0.75 mJ, energy density: 78 J/cm2 , material: CVD, thickness: 500 µm

it is only possible to remove material, not to fill depressions in the surface. Therefore, it is only to some extent possible to improve the quality of the laser cut with continued processing. Beside the surface, the edge is important for the performance of a cutting tool. In Fig. 3 the edge quality of laser-shaped diamond is compared with conventionally machined samples. The quality of the laser cutting lies between the quality of the samples produced by EDM or grinding, respectively. Except for a slight curvature, the edge of laser-shaped PCD is almost comparable to the edge produced by grinding, for MCD the edge is even better. In Fig. 4 the laser-shaped surface of MCD is compared with the polished surface. The laser cut surface is very smooth but does not reach the quality of the polished one. However, as is clear in the left picture, the surface of the laser cut begins to become irregular at a depth of about 300 µm.

2 2.1

Influence of Process Parameters Pulse Duration

As observed earlier, diamond is prone to cracks when machined with nanosecond-laser pulses. The use of pulse durations in the femtosecond range reduces the tendency of the material to form cracks. A comparison between laser cutting with different pulse durations and conventional techniques is depicted in Fig. 5. The surface roughnessies of samples machined with different pulse durations at different energy densities were measured with a mechanical sensor approximately 200 µm from the upper edge and are compared with the roughnesses of samples machined by polishing and EDM. For the majority of parameters, the roughness of the laser-cut surface is better than the surface produced by EDM. However, the quality of the pol-

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Fig. 3. Comparison of conventional technologies with laser cutting. Laser wavelength: 800 nm, pulse duration: 120 fs, pulse energy: 0.65 mJ, material: thickness: 1.5 mm

ished surface could not be achieved. A distinct influence of the pulse duration on the surface roughness is not discernible. To explore the influence of pulse duration on the formaton of cracks, Fig. 6 shows samples of MCD that were laser shaped with different pulse durations. As is clear in the left picture of Fig. 6, it is possible to shape MCD with femtosecond laser pulses without the formation of any cracks. Beginning at a pulse duration of about 1 ps more and more cracks appear, causing parts of the edge to break away. At a pulse duration of 5.2 ps almost the whole edge was broken away, as seen in the right picture of Fig. 6. It could be observed that the breaking planes are parallel corresponding to the crystal-lattice orientation of the diamond. In cutting of PCD less significant differences between femto- and picosecond pulses are visible as depicted in Fig. 7. With femtosecond pulses the quality of the edge is better than with picosecond pulses, but contrary to MCD even at 5.2 ps, no cracks were detected.

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Fig. 4. Comparison of polished and laser-cut surface. Laser wavelength: 800 nm, pulse duration: 130 fs, pulse energy: 0.6 mJ, energy density: 62 J/cm2 , feedrate: 0.6 mm/min, material: MCD, thickness: 1.5 mm

10 MCD, 10 J/cm² mean rougness Rz in µm

PCD, 10 J/cm² MCD, 78 J/cm² PCD, 78 J/cm² 5 PCD, EDM

PCD, polished 0 0

2500 pulse duration in fs

5000

Fig. 5. Mean roughness of the shaped surface. Laser wavelength: 800 nm, feedrate: 0.6 mm/min, 1 scan

The same experiments were also carried out with CVD, the behavior was to a large degree similar to PCD. It is even possible to shape PCD with nanosecond pulses as depicted in Fig. 8, without cracking the material. Due to the comparatively high pulse energy, the cutting speed was higher than with ultrashort laser pulses. Cracks were not detected. It was even possible to cut through the thick hard metal carrier of the diamond sample. The quality of the cut is inferior to the one achieved with ultrashort laser pulses. Cutting of MCD was not possible with nanosecond pulses. The samples broke into several pieces during the process.

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Fig. 6. Influence of pulse duration on cutting of diamond. Laser wavelength: 800 nm, energy density: 78 J/cm2 , feedrate: 600 mm/min, number of scans: left: 6000, right: 4600, material: MCD, thickness: 1.5 mm

Fig. 7. Influence of pulse duration on cutting of diamond. Laser wavelength: 800 nm, energy density: 78 J/cm2 , feedrate: 600 mm/min, number of scans: left: 6000, right: 4600, material: PCD on hard metal, thickness: 1.5 mm

2.2

Polarization

Due to their design, femtosecond lasers are usually linearly polarised. It is well known that in laser cutting the orientation of the polarization with respect

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Fig. 8. Laser cutting with nanosecond pulses. Laser wavelength: 1064 nm, pulse duration: 15 ns, repetition rate: 2 kHz, pulse energy: 4.5 mJ, feedrate per pulse: 60 mm/min, material: PCD on hard metal, thickness: 1.5 mm

Fig. 9. Influence of polarization. Laser wavelength: 800 nm, pulse duration: 130 fs, pulse energy: 0.6 mJ, energy density: 62 J/cm2 , polarization: left: parallel to cutting direction, 192 scans, right: perpendicular to cutting direction, 107 scans, material: CVD, thickness: 500 µm

to the feeding direction has a great influence on the quality. This effect is also noticeable in the cutting of diamond, as depicted in Fig. 9. With a polarization parallel to the cutting direction the roughness of the cut surface is very high below a certain depth. If the polarization is perpendicular to the cutting direction, the cut surface is significantly smoother.

Michael Weikert and Friedrich Dausinger

ablation depth in µm

600

600

nitrogen helium ambient air argon

400

ablation depth in µm

162

200

nitrogen helium ambient air argon

400 200 0

0 0

2

4

6

effective feedrate in mm/min

8

0

2

4

6

8

effective feedrate in mm/min

Fig. 10. Cutting speed with different process gases. Laser wavelength: 800 nm, pulse duration: 120 fs, pulse energy: 675 µJ, left: 1 scan, right: 100 scans, material: PCD on hard metal, thickness: 500 µm

Fig. 11. Cutting with different process gases. Laser wavelength: 800 nm, pulse duration: 120 fs, pulse energy: 675 µJ, 100 scans, left: ambient atmosphere, right: helium, material: PCD on hard metal, thickness: 1.5 mm

2.3

Process Gases

To examine the influence of process gases on the cutting of diamond, experiments with different ones were conducted. The gases were blown into the interaction zone under pressure by a flexible nozzle. In Fig. 10 the ablation depth after 1 scan or 100 scans, respectively, is depicted. For the experiments shown in the right diagram, the feedrate was selected as 100 times higher than in the left diagram in order to achieve a comparable effective feedrate. After 100 scans a slight advantage for the use of helium is visible. This is due to the fact that the use of helium helps to suppress the formation of conical emission as described in the Chapter “Interaction with Atmosphere”. With 1 scan the advantage of helium is not visible. In Fig. 11 the cut surface for a cut in ambient atmosphere and with helium, respectively, is shown. On the diamond there are practically no differences visible. The surface of the cut hard metal is smoother when using helium. The use of helium

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Fig. 12. Process speed for cutting of diamond, Laser wavelength: 800 nm, repetition rate: 1 kHz, pulse duration: 2 ps

shows slight advantages over the process in ambient atmosphere, however, the economic efficiency is questionable due to the comparatively high price of helium. 2.4

Repetition Rate

The speed for laser cutting of diamond is strongly dependent on the thickness of the material. With the available laser systems, cutting speed is low, as depicted in Fig. 12. For a material thickness of 200 µm a cutting speed of the order of 1 mm/s could be achieved. This speed is 1 to 2 orders of magnitude lower compared to conventional techniques. For a given pulse energy density the only possibility to increase the speed significantly is the use of a laser source providing a higher repetition rate with a comparable pulse energy. To explore the influence of the repetition rate on the process speed, experiments were conducted with a laser source providing a repetition rate of 8 kHz at a pulse energy of up to 350 µJ. The repetition rate could be decreased in whole numbers by outcoupling only certain pulses. Therefore it was guaranteed that the pulse parameters were kept unchanged. For the results shown in Fig. 13, the scan speed was selected according to the repetition rate in order to keep the pulse overlap constant. For the diagram, the ablation depth per scan was calculated from the number of scans that were necessary to cut the whole material thickness. The ablation per pulse increases slightly with increasing repetition rate. For the higher repetition rate, the cooling time between two scans is shorter due to the higher scanning feedrate. Probably the shorter cooling time has an influence on the ablation rate.

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ablation per scan in µm

6

4 2

0 0

2

4

6

repetition rate in kHz

2.5

8

10

Fig. 13. Influence of repetition rate on cutting speed. Laser wavelength: 1030 nm, pulse duration: 5 ps, pulse energy: 350 µJ, material: CVD, thickness: 500 µm

Summary

Laser cutting with ultrashort laser pulses is a viable technique to cut all kinds of diamond. Edge quality and surface roughness of the laser cutting exceeds the quality of EDM technology but is inferior to the quality reached by grinding and polishing. For cutting of MCD the pulse duration should be below 1 ps, whereas PCD and CVD can be machined even by short ns pulses. Typical of the laser process is the fact that the cut surface is very smooth for the upper few hundred micrometers but it becomes rougher on the rest of the surface. This effect can be minimized by parameter optimization, but can not be avoided completely. Compared to conventional techniques like grinding and EDM, the process speed for the laser process is dependent on the material thickness. With current laser systems it is not possible to reach the process speed of the other techniques. The orientation of the polarization with respect to the feeding direction is important. The influence of process gasses is only minor, only the use of helium shows a slight advantage over the use of ambient atmosphere.

Index

crack, 155, 157, 159 cutting, 155 cutting tool, 155

polarization, 160 precision, 155 pulse duration, 157

diamond, 155

quality, 155–157

gas, 162

repetition rate, 163 roughnessies, 157

nanosecond, 155

ultrashort, 155

Dental Applications Paul Weigl1 , Anton Kasenbacher2, and Kristian Werelius1 1

2

Poliklinik f¨ ur Zahn¨ arztliche Prothetik, Goethe-Universit¨ at Frankfurt/Main, Theodor-Stern-Kai 7, Haus 29, 60590 Frankfurt/Main [email protected] Zahnarztpraxis Dr. Kasenbacher, Obere Hammerstr. 5, 83278 Traunstein

Abstract. In dentistry the specific interactions between ultrashort laser pulses and enamel/dentin of a tooth [1] or ceramic restoration materials [2] are advantageous due to minimal collateral damage. The femtosecond laser beam can be used as a tool to remove either decayed enamel/dentin – also named caries – or as a tool to remove ceramic from a bulk until the shape of an all-ceramic restoration is left. Therefore a completely new caries therapy and an innovative way to manufacture all ceramic dental restorations are possible if femtosecond laser devices emitting pulses with a high energy and repetition rate are available. Although the latter has not become a commercial possibility yet, basic research has to be done to evaluate the interactions between lasers and materials depending on various laser parameters. This strategy enables a specific development of dental femtosecond laser sources based on optimized laser parameters. Additionally the high demand of an intraorally application of a femtosecond laser beam can be met by developing a dental handpiece.

1

Caries Therapy

Caries – the most frequent cause for dental surgery – is still mainly treated with conventional mechanical drills, although lasers have meanwhile been successfully applied to various clinical disciplines. Due to the unavailability of ultrashort laser pulses in recent decades, only continuous-wave radiation or pulse durations longer than thermal diffusion processes had been applied with the result of severe thermal damage and pain. 1.1

Femtosecond-Laser-Based Caries Removal

The first lasers to irradiate teeth were pulsed ruby lasers [3, 4]. However, due to their relatively long pulse durations of up to a few hundred microseconds, these lasers induced severe thermal side effects inside the tooth substance. Similar findings were reported just a few years later using a CO2 laser [5]. Meanwhile, several experiments have been conducted using alternative laser systems, such as Er:YAG lasers [6, 7, 8], Ho:YAG lasers [9], excimer lasers [10, 11], and frequency-doubled Alexandrite lasers [12, 13]. All of these lasers still induce severe thermal effects or do not supply sufficient ablation rates (ablated volume per time) to compete with the mechanical drill [9, 14, 15]. F. Dausinger, F. Lichtner, H. Lubatschowski (Eds.): Femtosecond Technology for Technical and Medical Applications, Topics Appl. Phys. 96, 167–187 (2004) c Springer-Verlag Berlin Heidelberg 2004


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Even the newest generation of superpulsed CO2 laser [16] needs water for cooling, is wavelength dependent and is far from a direct ionization of the material, so that heat- and/or shock-affected zones are principally inevitable. In 1993, Niemz proposed the application of ultrashort laser pulses to overcome these detrimental side effects [9]. Using a picosecond Nd:YLF laser, ablation qualities superior to any other laser systems had been achieved [17, 18, 19]. Although, at the early stage of experiments, uncertainty predominated concerning potential shock-wave effects, it has meanwhile been verified by independent tests that mechanical impacts are negligible [20, 21, 22]. Recent progress in femtosecond laser technology gives promise for a breakthrough in ultrashort laser dentistry soon [23]. Taken together, lasers have not currently succeeded in replacing the dental drill in tooth hard tissue ablations due to slow material removal rates and unacceptable collateral damage. It should be kept in mind that the traditional drill is cheaper, more universal and faster than the laser, but it has an irritating sound, is unselective and not minimally invasive, transmits uncomfortable vibrations, creates heat and shock affecte zones despite cooling by air–water spray and thus pain, with the need for local anesthesia. 1.1.1

Laser–Tissue Interactions

In extracted human third molars, cavities are generated with a CPA (chirpedpulse amplification) Ti:sapphire femtosecond laser (Concerto, Thales Laser). The cavities are ablated by scanning the focused laser pulses (ϕfocal spot = 100 µm) with a computer-controlled x–y galvanometer scanner over the tooth surface at a scan velocity of 200 mm/s. The SEM in Fig. 1a–c shows the result created at a wavelength of 780 nm, a pulse duration of 700 fs, a pulse energy of 100 µJ and a pulse repetition rate of 5 kHz. The extremely precise tetragonal cavity (Fig. 1a) is located within healthy dentin. It has a lateral dimension of 2 mm × 2 mm and a depth of approximately 1.4 mm. The two white rectangles are enlarged (Fig. 1b–c). The roughness of the cavity bottom (Fig. 1b) is of the order of 5 µm to 10 µm and thus facilitates the direct adhesion of most filling materials without any etching gel. The cavity margin is sharp and free of chippings, the cavity wall (Fig. 1c) is extremely steep and clean, the cavity bottom shows no smear layer and open dentinal tubuli, taken together these are ideal conditions for a perfect filling. A second cavity has been created within healthy and carious dentin with the carious lesion being located in the top left corner at the tip of the white arrow (Fig. 2a). Obviously, carious substance is removed more efficiently than healthy dentin. The enlargement (Fig. 2b) shows a very clean surface after laser exposure, thus proving that all carious substance has been completely removed in that area. Again, the cavity wall is extremely steep and precise. A third cavity has been located within carious enamel at the most typical location of a carious lesion that is on top of the chewing surfaces (Fig. 3a).

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Fig. 1. SEM of a cavity 2 mm × 2 mm × 1.4 mm (a), its bottom (b), and one of its side walls (c) (target: healthy dentin, processed by λ = 780 nm, t = 700 fs, E = 100 µJ, repetition rate is 5 kHz)

Even here, an excellent quality is achieved as demonstrated in the enlargement (Fig. 3b). Carious enamel is removed without breaking apart. Small microcracking proved to be superficial only, indicating that this artefact was caused by the drying process during the SEM preparation. When comparing femtosecond laser dentistry with previous dental laser applications, the quality achievable with femtosecond laser pulses is very impressive. This quality is primarily due to the single fact that femtosecond laser interaction with biological tissues is a direct multiphoton ionization of bound and free electrons, which leads to pure plasma-induced ablation [20] of the material. Focusing of femtosecond laser pulses to spot sizes of several micrometers leads to such high intensities that these free electrons are generated at pulse energies in the range of a few microjoules and in a very thin layer of material of less than 1 µm, resulting in a well-defined “optical

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Fig. 2. (a): SEM of a cavity with a carious lesion (white arrow ) and its bottom (target: healthy dentin with a carious lesion at the tip of the white arrow). (b): Enlargement of (a) shows a caries-free surface

Fig. 3. (a) SEM of a cavity (target: carious enamel at an occlusal fissure). (b) Enlargement of (a) shows a caries-free surface at the corner of the cavity

breakdown”, called “ablation threshold”, which is independent of beam size and repetition rate for a fixed pulse duration [24]. This ultrafast and minimally invasive laser–tissue interaction is why the use of femtosecond laser pulses for tooth hard tissue ablation minimizes mechanical and thermal effects that precise structuring is possible without collateral injuries and most probably without activation of damage-sensing neurons, called nociceptors.

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Fig. 4. Ablation rates of carious dentin, healthy dentin, and healthy enamel, respectively

Therefore, the excavation of caries is expected to be at least significantly pain reduced, maybe completely pain free. As a result, the femtosecond laser ablation is significantly superior to laser material processing with pulses longer than 1 ps [25] and to the conventional tooth preparation by mechanical drills. 1.1.2

Threefold Caries Selectivity

The ablation rates of carious dentin, healthy dentin, and healthy enamel are summarized in a single graph (Fig. 4) and extrapolated at a constant average power of 5 W. The data indicates that smaller pulse energies (at higher repetition rates) induce higher ablation rates than higher pulse energies (at lower repetition rates). Furthermore, a target-dependent pulse energy exists at which the ablation rate is optimized. At a pulse energy of 30 µJ the ablation rate is almost halved when crossing from carious to healthy dentin, turning the femtosecond laser itself into a caries-selective tool. We also derive that – at a given average power of 5 W – more than 20 mm3 of carious dentin can be ablated per minute. Thus, even regarding efficiency, femtosecond lasers can compete with conventional mechanical drills. Caries can be detected during the laser treatment by LIBS (laser-induced breakdown spectroscopy) analyzing the laser-induced microplasma sparks at the surface of the tooth. Spectra obtained from healthy and carious dentin significantly differ in the spectral intensities of corresponding calcium transitions. Carious teeth provide far less spectral intensity than healthy teeth.

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Summarizing, a threefold caries selectivity in the treatment of dental decay with femtosecond laser pulses is possible. A first means of caries selectivity has been evaluated when comparing the ablation efficiencies of healthy and carious tooth substances. A second means of caries selectivity is derived from the fact that the pulse energy at the threshold of ablation is significantly lower when exposing carious surfaces. Finally, a third means of caries selectivity becomes evident during optical spectroscopy of the laser-induced plasma spark at the surface of the tooth. Spectra obtained from carious dentin provide far less spectral intensity of corresponding calcium transitions than spectra obtained from healthy dentin. This decrease in spectral intensity is due to the demineralization process associated with caries. Femtosecond laser pulses may even be regarded as a superior tool because of their three means of caries selectivity, which may be combined during treatment to achieve best results. For the first time ever, dentists will have an objective, reproducible and minimally invasive concept for the treatment of carious lesions. Due to its threefold caries selectivity, this novel technology eliminates any overexcavation and lowers the chances of an artificial pulp exposure. By reducing the ablation of sufficiently mineralized “healthy” tooth substance to a minimum, the need for expensive dental crowns and bridges is likely to significantly decrease in the future. 1.2 Retentive Patterns at Dentin Surfaces Facing Filling Materials Adhesion of restorative materials to the hard components of the tooth structure has been a goal pursued by numerous researchers ever since Buonocore [26] established the foundation for adhesive dentistry. The adhesion of filling materials like tooth-colored resins, filled with small ceramic particles creating best aesthetic outcomes of a tooth filling, is well established between resin and enamel. The bonding can be achieved easily by surface enlargement conditioning the enamel with phosphoric acid. The micropatterns increase the mechanical bond between enamel and resin. While bonding to enamel is a reliable technique, bonding to dentin represents a greater challenge [27]: dentin is an intrinsically wet organic tissue penetrated by a tubular labyrinth containing the odontoblastic process, which communicates with the pulp [15]. The manufacturers of so-called adhesive systems recommend the application of their adhesive material on moist dentin. The main reason is that the spatial alteration that occurs upon drying demineralised dentin may prevent the monomers from penetrating the labyrinth of nanochannels formed by dissolution of hydroxyapatite crystals between collagen fibrils [15]. Another characteristic of dentin is the presence of a smear layer that forms on the dentin surface after instrumentation; it occludes the tubules and decreases dentin permeability by 86% [28] and is composed of hydroxyapatite and altered collagen with an external surface formed by gel-like denatured collagen. In particular, fatigue of the interface dentin – smear layer – monomer (resin)

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Fig. 5. Shaped dentin pins for micromechanic retention of acrylic filling materials and cements, created by a precise femtosecond laser tissue ablation without signs of collateral damage

leads to a compromised long-term durability of restorations if they are retained only at dentin without additional mechanical retention patterns [27]. An improvement of retention can be achieved by creating patterns working as a micromechanical bond to the dental restoration. This kind of pattern is achieved by femtosecond laser application running under a defined scanning mode of the beam. Figure 5 shows a dentin surface with femtosecond lasershaped pins by SEM. Note the regularly spaced pins of dentin serving as a micromechanical pattern for the monomer component of tooth-colored dental-filling materials. Additionally, there is no sign of any collateral damage of the dentin [21, 29]. This micromechanical pattern will obviously prevent the fatigue of the bond due to the mainly chemically based bond mechanism of current dentin adhesive systems [27]. 1.3

Intraoral Application of a Femtosecond Laser Beam

One of the highest challenges of a femtosecond laser-based caries therapy is the integrating of the laser beam into a dental handpiece. It has to accomplish the demands of easy handling by the dentists and the miniaturization of scanning components, refracting the beam at a defined scanning mode. A first prototype was developed in close cooperation with Straßl et al. [22] and the company W&H Dentalwerk B¨ urmoos GmbH (www.wh.com). The first principle of proof was shown in an in vitro setup (Fig. 6). Currently the main drawback is the miniaturized galvanometer scanner integrated into the dental handpiece. Further development has to be done either to improve the stability of this scanner or to eliminate the problem by a substitution with a normal-sized scanner placed outside of the handpiece.

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Fig. 6. Dental handpiece for femtosecond laser pulses

2

Manufacturing of All-Ceramic Restorations

If a large amount of tooth structure is lost or a whole tooth is lost, the first choice of therapy is to manufacture crowns (see Fig. 9b,d) and bridges fixed to the decayed teeth or to the adjacent teeth of the missing tooth. The dentist has to grind either the restored, filled tooth or the adjacent teeth to a conical shaped tooth stump (see Fig. 9a,c), bordered by a chamfer line to achieve a well-fitting crown or bridge. An impression enables a replica of gypsum – so-called master cast – of this prepared tooth. The dental technician uses this master cast to manufacture an individual crown, bridge or a ceramic filling (inlay). This hand-made restoration lacks a standardized quality and generates high costs. In detail, the sometimes complicated manual production steps cause the resulting high financial and temporal expenditure, poor fitting accuracy, leading to insufficient closeness and durability of the restoration, the limitation to castable materials, and unavoidable material errors connected with this production method. Nevertheless, manufacturing all ceramic restorations by hand is limited, because the kinds of shaping methods of dental ceramics results in low material strengths. All the above disadvantages can be met by an industrial manufacturing based on a CAD/CAM process chain. The CAD/CAM systems currently available can be divided into three independent components (Fig. 7): 1. The sensor for three-dimensional evaluation and digitalisation of the surface of the prepared tooth and the surrounding structures 2. The CAD system for the computer-supported design of the denture 3. The CAM system for producing the modelled denture with the help of a numerically controlled milling machine For example, if a crown is needed, the stump and the surrounding teeth (see Fig. 9a,c) have to be digitized in order to determine the boundaries for the crown (Fig. 7). The data gained by digitization is used to design the crown by a CAD program. The subsequent CAM process computes the needed milling paths and converts the data into a machine-readable format. This data can then be used to control many kinds of manufacturing machines, e.g., milling machines.

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Fig. 7. Dental CAD/CAM system consisting of three parts: (a) optical measurement of tooth stump; (b) CAD construction of the restoration; (c) NC-Milling machine manufacturing the restoration

2.1

Coherence Radar (KoRad) for Surface Measurement

For the mechanized automated production of dental restorations, a data set on the exact dimensions of the crown or bridge to be produced must be provided. One region that has especially strict requirements on dimensional tolerances is the contact surface between the tooth stump (see Fig. 9c) ground by the dentist, and the tooth replacement (see Fig. 9d), which is to be set on the tooth stump and that has to be produced. Therefore, a very precise measurement of the ground tooth stump is imperative. In addition, it is also important to measure the adjacent teeth and their distance to the ground tooth to get borders for defining the outer surface of the tooth replacement, e.g., a crown. Typically, a cast model of the stump is measured. Conventionally, this is performed either by contact, that is, by means of mechanical scanning, or by means of an optical, three-dimensional measurement of the tooth by stripe projection, that is, a triangulation method (Fig. 7a). The principles of a coherence radar (KoRad) are based on a white-light interferometric measuring technique and described in the Chapter “Metrological Applications”. The cast model is measured with a coherent-radar device (see the Chapter “Metrological Applications”, Fig. 25, right). The dental KoRad device was developed in close cooperation with Sch¨ ussler and the company Polytec GmbH (www.polytec.com). The cast model and the KoRad are aligned relative to each other such that the direction of the measurement beam and the KoRad device essentially coincides with the insertion or placement direction of the dental restoration (see Fig. 9c). This is based on the knowledge that a tooth that has been ground for tooth replacement has no undercuts when viewed in the direction in which the tooth replacement is inserted or set. Therefore, if the prepared tooth is measured from this direction, absolutely no undercuts have to be taken into account, so that one measurement is sufficient to represent the tooth surface of interest. Further measurement data from other spatial directions can be eliminated.

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(a)

(b)

(c)

Fig. 8. (a) Measuring data set of a tooth stump created by the KoRad. (b) Measuring data set after detection of the preparation line (red curve) finishing the crown margin. (c) Surface reconstruction of the 3D point cloud

In this way, the amount of data generated during the measurement becomes smaller, and various pictures from different spatial directions also do not have to be assembled to obtain a three-dimensional picture of the sample. The KoRad technique can measure even quite steep surfaces (Fig. 8a–c). 2.2 Femtosecond-Laser-Based Shaping of All Ceramic Restorations Forming techniques for small, tooth-like objects with complex sculptured surfaces, depend on the dental materials used. Nevertheless, the main challenge for the CAM system is the three-dimensional shaping of the highly complex, free-form surfaces of dental restorations (Fig. 9d). In particular, all-ceramic restorations simulate the optical properties up to a point, where only a thorough examination reveals the artificial tooth. The increasing demand for a strong, chemically stable, biocompatible and aesthetic ceramic can be satisfied to a high degree by dense sintered oxide ceramics. One biocompatible oxide ceramic, yttria-tetragonal zirconia polycrystal (Y-TZP) has the potential of being the most suitable material for all-ceramic dental restorations due to its superior mechanical, biological and chemical properties. Hot isostatic-pressed (HIP) yttria-tetragonal zirconia polycrystal (Y-TZP) ceramic consists of fine particles of ZrO2 and Y2 O3 , which when sintered, form a stable tetragonal structure at room temperature. The mechanical strength results from a transformation toughening mechanism [30]. The transformation from tetragonal t to the monoclinic m phase occurs when stress is exerted, e.g., by grinding, impact or fracture. A relatively large volume expansion (3% to 5%) is associated with the t → m phase transformation, which leads to the development of internal stresses opposing further opening of the crack and therefore acting to increase the resistance of the material to crack propagation [31].

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Fig. 9. A ground tooth stump of the left first lower molar fixed on an all-ceramic crown, which free-form occlusal surface is handmade by a dental technician. The fine structures (fissures) can not be milled conventionally

2.3

Material Properties After Femtosecond-Laser Processing

Grinding HIP Y-TZP is costly and causes possible surface damage, decreasing the survival rate of the dental restoration. Grinding Y-TZP leads to a significant loss in flexural strength. Y-TZP sintered material shows a flexural strength of σ = 900 MPa to 1000 MPa and a Weibull parameter m = 10.7 to 14.9. These values are reduced after wet or dry grinding down to σ = 543 MPa to 751 MPa with a Weibull parameter m = 4.5 to 6.2 resulting in a loss between 25% and nearly 50% [30, 32]. Processing ZrO2 using a conventional laser causes microcracking [33]. Using ultrashort laser pulses is an alternative possibility in processing HIP Y-TZP. Due to the nature of ultrashort laser pulses little or no collateral damage is expected [34]. 2.3.1

Effects of Pulse Energy

Investigations on material damage were carried out on a femtosecond laser system, generating 140 fs at 780 nm and up to 1.5 W at 5 kHz pulse repetition

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rate. The laser beam was focused to 40 µm in diameter, providing a maximum laser intensity of 1.7 × 1014 W/cm2 (maximum laser fluence of 23.9 J/cm2). The laser processing was performed with different pulse energy and spot separation on bars of HIP Y-TZP. The bars were tested in 6 groups, the reference was group no. 6 (Table 1). Table 1. Process parameters for the ceramic bars Group

Pulse energy (µJ) Spot separation (µm)

1 2 3 4 5 6

200 144 56 200 85

20 20 20 26.6 11.4

All processing was done in air. Each bar was cracked in a four-point bending test technique [35]. The results were evaluated using the analysis technique described in ENV 843-5 [36]. In addition to the bending tests, a sample bar of each group was verified by X-ray diffractometry (XRD) using CuKα radiation to detect the transformed monoclinic zirconia on the treated surface. The results from the bending tests are listed in Table 2. The Weibull parameter m indicates the reliability of the tested ceramic [37]. The very low value of the Weibull parameter m for the reference group and the significant higher values for the processed HIP Y-TZP results from very high bending strength values up to 2145 MPa found in the reference group. The high value of the Weibull parameter m for group 4 results from overall lower values in measured bending strength. Table 2. Mean flexural strength σmean and the Weibull parametres σu and m Group

σmean (MPa) σu (MPa) m

1 2 3 4 5 6

1354 1408 1088 1103 1240 1495

1434 1492 1181 1147 1344 1613

8.9 8.5 5.5 13.7 5.6 5.3

The mean flexural strength σmean decreases from 1495 MPa down to a minimum of 1108 MPa, the Weibull parameter σu from 1645 MPa to a minimum of 1147 MPa. Nevertheless, the values obtained are, even after laser processing, still a factor two higher than the Y-TZP material, which was ground.

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Fig. 10. Relationship between pulse duration and ablation rate

The XRD patterns show traces of monoclinic zirconia and indicate induced stress. The highest amount of monoclinic zirconia on the surface was found in group 2. All other examined bars showed far less monoclinic zirconia. 2.3.2

Effect of Pulse Duration

Another femtosecond laser system generated pulses between 100 fs and 5.2 ps at 800 nm and an average power of 1.0 W at 1 kHz pulse repetition rate to evaluate the effects of pulse duration. The laser beam was focused to a focal point of 18 µm in diameter. The laser beam was scanned over a 2 mm × 2 mm with a dual-axis galvanometer scanner and formed a square crater in the HIP Y-TZP. The scanning speed was chosen so that consecutive laser pulses were separated by 9 µm, thus overlapping 50%. The crater depth was studied at 5.3 ps, 2.02 ps, 1.2 ps, 700 fs, 400 fs and 100 fs with pulse energies of 50 µJ, 100 µJ, 300 µJ and 500 µJ resulting in a laser fluence of 20 J/cm2, 39 J/cm2 , 118 J/cm2 and 196 J/cm2. The results show a dependence between pulse duration and ablation rate (Fig. 10). The crater depth as an indicator for the ablation rate is increased for shorter pulses up to 400 fs. The maximum for higher laser fluences is reached at around 400 fs. The shortest pulse duration evaluated, 100 fs, shows a decrease in ablation rate for the two higher laser fluences, whereas the lowest laser fluences remains about constant over the whole range of pulse durations. A slight increase in ablation rate towards lower pulse durations occurs for the series of 39 J/cm2, where the maximum ablation rate was found for 100 fs. Enviromental scanning electron microscope (ESEM) images of the crater floor at 118 J/cm2 reveal areas of molten material. This is only observed for the longer pulse durations of 5.3 ps and 2.02 ps (Fig. 11a and b).

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Fig. 11. (a) Floor of crater with molten material of ZrO2 (5.3 ps, 118 J/cm2 ). (b) Floor of crater, no melting occurred (100 fs, 118 J/cm2 )

2.3.3

Innovative Strategies of an Efficient 3D Shaping of Ceramics

Basically there are two ways to manufacture high-strength all-ceramic dental restorations using Y-TZP: shaping the pressed ceramic powder prior to sintering with a conventional milling machine or grinding the already sintered ceramic blank. Milling pressed ceramic powder is easily accomplished. Clinical experience shows, however, that this results in a lower accuracy due to shrinking during the sintering process. Grinding sintered high-strength ceramics, such as HIP Y-TZP, requires rigid grinding machines to achieve the needed accuracy of the dental restoration. Long grinding times and a severe tool wear cause high costs for the restoration. In addition, during the grinding process, small surface fractures are produced leading to a reduced strength and increased fatigue of the ceramic [30]. In addition, the three-dimensional shaping of the free-form surfaces of dental restorations is highly complex (see Fig. 9). However, the fine spot of a femtosecond laser focus can form even miniaturized occlusal surfaces of a crown (Fig. 12), which is not possible by milling machines due to the limited diameter reduction of milling tools. 2.3.4

Ablation Strategy

Commercially available femtosecond laser systems have several degrees of freedom, e.g., pulse duration, pulse energy and wavelength. Average power above 2 W with repetition rates more than 10 kHz and pulse peak power in the range of terawatts are possible. Still, this is not enough to machine a crown out of standardized ceramic blanks with dimensions of approximately 10 mm × 12 mm × 16 mm (Fig. 13a). Creating a dental restoration, e.g., a crown, by complete ablation would take more than 30 h of machining time using a state-of-the-art femtosecond laser system. To overcome this obstacle and reduce the machining time drastically, a special ablation strategy is needed. One way is to cut out volumes of bulk material, which would otherwise have to be fully ablated (Fig. 13b–d).

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Fig. 12. Miniaturized occlusal surfaces shaped three-dimensionally by a femtosecond laser beam. Compare the diameter of a conventional grinding tool working on a NC-milling machine

(a)

(b)

(c)

(d)

Fig. 13. (a) A crown within a ceramic blank. The crown is to be cut out of the blank. (b) Cutting away bulk material. (c) The advanced process of cutting away bulk material. (d) At this stage no more material is cut away. The remaining material has to be fully ablated. This is the most time-consuming step

During the CAM process, the pathway for the laser beam is calculated in such a way that volumes are created and will separate during the cutting with the laser beam. Calculations for such a model [38], using the standardized ceramic blank and a typical molar, show that 84% of the volume that has to be removed can be separated through this strategy. The remaining 16% must be fully ablated. The total machining time can thus be reduced to 6 h, 80% of which is needed for the ablation of the remaining material. 2.3.5

Online Depth Measurement

While the theoretical gain in machining speed is more than a factor of 5, a further concern has to be taken into account. Cutting through a material is no problem, but to stop cutting at a defined depth is difficult but essential in

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Fig. 14. The left sketch shows cuts that stopped before reaching the defined depth. The right sketch shows cuts possibly damaging the crown

this ablation strategy. A small uncertainty in cutting depth is not a problem with the first bulk cuts near the surface of the blank. The closer to the crown the removal takes place, the more critical it is to keep the defined depth and not to cut too deep into the blank, otherwise the crown would be damaged (Fig. 14). Thus it is very important to control the ablation depth. Regarding deep cuts into bulk material, it is difficult to measure the depth with conventional optical triangulation methods. Optical scanning methods or measurements with structured light require an offline measurement, thus increasing the processing time. So far two methods for measuring the depth online during processing have been developed: Lausten and Balling [39] describe a method of time-gated imaging of the backscattered radiation from the ablation region. Originally designed to determine the depth profile along a chosen axis, the method is capable of determining the depth at a fixed point resulting in a higher sampling speed and therefore higher temporal resolution. The spatial resolution depends on the laser-pulse duration: x=

cT , 2

(1)

where c is the speed of light, T the laser-pulse duration and x the spatial resolution [39]. With a laser-pulse duration of 100 fs, a spatial resolution of 15 µm is achieved. Klinger and H¨ ausler [40] present a sensor based on measurements of the evolving plasma during processing. Although this sensor is not described for use with ultrashort laser pulses, the principle should work with ultrashort laser pulses as well. The method is based on imaging the process spot through a double slit onto a CCD line array. By defocusing the CCD line array, the two projections from the plasma vary in distance depending on the variation in distance of the plasma. Through correlation of the two projections on the CCD line array, the distance to the plasma can be calculated. A measurement uncertainty between 4 µm and 20 µm is reported, depending on the removal rate. 2.4 Innovative Femtosecond-Laser-Based Dental CAD/CAM System The use of a femtosecond laser system running a dental CAD/CAM system offers new possibilities either in measuring of complex three-dimensional sur-

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Fig. 15. Femtosecond laser-based dental CAD/CAM consisting of three parts: (a) optical measurement by a coherence radar; (b) CAD construction of the restoration; (c) 3D micromachining of dental ceramics with a femtosecond laser beam

faces or in machining dental restorations with minimal collateral damage to the bulk material. This indicates the possibility of machining dental restorations with high quality because the accuracy of the measuring data is high and the damage of the processed ceramic is low. Applying the described ablation strategy considerably reduces the machining time. The time needed is, however, still too high and a further reduction in time is necessary to compete with conventional machining methods. The ongoing research into more powerful femtosecond lasers indicates that the goal to use femtosecond laser pulses to create dental restorations can be achieved (Fig. 15). Acknowledgements The authors would like to thank the Laser Zentrum Hannover e. V. for their support and to acknowledge the financial support by the Federal Ministry of Education and Research (BMBF) for funding several projects under files 13N7781, 13N7852, 13N7790, 13N7788 and 13N8047. Parts of the laser–tissue interaction work have been supported by Prof. Dr. M. H. Niemz (MABEL, Mannheim, Germany) and Dipl.-Phys. J. Serbin (LZH, Hannover, Germany). We would like to thank Prof. Dr. F. Dausinger and Dipl.-Ing. M. Weikert at the Institut f¨ ur Strahlenwerkzeuge, University of Stuttgart for their great help with the studies on the effects of pulse durations and Dr. R. Oberacker at the Institut f¨ ur Keramik im Maschinenbau, University of Karlsruhe for the great help in the work on the material damage with ultrashort laser pulses.

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References [1] M. H. Niemz: Ultrashort laser pulses in dentistry – advantages and limitations, in Proc. SPIE 3255 (1998) pp. 84–91 167 [2] K. Werelius, P. Weigl: Precise machining of ceramic dental prothesis with ultrashort laser pulses using online depth profiling, in (Proc. 1st Pacific Int. Conf. Application of Lasers and Optics 2004) 167 [3] L. Goldman, P. Hornby, R. Mayer, B. Goldman: Impact of the laser on dental caries, Nature 203, 417 (1964) 167 [4] R. H. Stern, R. F. Sognnaes: Laser beam effect on dental hard tissues, J. Dent. Res. 43, 873 (1964) 167 [5] R. H. Stern, J. Vahl, R. Sognnaes: Lased enamel: ultrastructural observations of pulsed carbon dioxide laser effects, J. Dent. Res. 51, 455–460 (1972) 167 [6] R. Hibst, U. Keller: Experimental studies of the application of the Er:YAG laser on dental hard substances: I. Measurement of the ablation rate, Lasers Surg. Med. 9, 338–344 (1989) 167 [7] R. Hibst, U. Keller: Experimental studies of the application of the Er:YAG laser on dental hard substances: II. Light microscopic and SEM investigations, Lasers Surg. Med. 9, 345–351 (1989) 167 [8] T. Kayano, S. Ochiai, K. Kiyono, H. Yamamoto, S. Nakajima, T. Mochizuki: Effects of Er:YAG laser irradiation on human extracted teeth, J. Stomat. Soc. Jap. 56, 381–392 (1989) 167 [9] M. H. Niemz, L. Eisenmann, T. Pioch: Vergleich von drei Lasersystemen zur Abtragung von Zahnschmelz, Schweiz. Monatsschr. Zahnmed. 103, 1252–1256 (1993) in German 167, 168 [10] M. Frentzen, H. J. Koort, O. Kermani, M. U. Dardenne: Bearbeitung von Zahnhartgeweben mit einem Excimer-Laser, Dtsch. Zahnaerztl. Z. 44, 431–435 (1989) in German 167 [11] T. Liesenhoff, T. Bende, H. Lenz, T. Seiler: Abtragen von Zahnhartsubstanzen mit Excimer-Laserstrahlen, Dtsch. Zahnaerztl. Z. 44, 426–430 (1989) 167 [12] E. Steiger, N. Maurer, G. Geisel: The frequency-doubled alexandrite laser: an alternative dental device, in Proc. SPIE 1880 (1993) pp. 149–152 167 [13] P. Rechmann, T. Hennig, U. von den Hoff, R. Kaufmann: Caries selective ablation: wavelength 377 nm versus 2.9 µm, in Proc. SPIE 1880 (1993) pp. 235–239 167 [14] M. Frentzen, C. Winkelstraeter, H. van Benthem, H. J. Koort: Bearbeitung der Schmelzoberflaechen mit gepulster Laserstrahlung, Dtsch. Zahnaerztl. Z. 49, 166–168 (1994) 167 [15] K. M. Hargreaves: Seltzer and Bender’s dental pulp, Quintessence (2002) 167, 172 [16] O. R. Keller, F. E. Weber, K. W. Gratz, M. M. Baltensperger, G. K. Eyrich: Laser-induced temperature changes in dentine, J. Clin. Laser. Med. Surg. 21, 375–381 (2003) 168 [17] M. H. Niemz: Investigation and spectral analysis of the plasma-induced ablation mechanism of dental hydroxyapatite, Appl. Phys. B-Lasers O. 58, 273–281 (1994) 168 [18] M. Niemz: Cavity preparation with the Nd:YLF picosecond laser, J. Dent. Res. 74, 1194–1199 (1995) 168

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[19] L. Willms, A. Herschel, M. H. Niemz, T. Pioch: Preparation of dental hard tissue with picosecond laser pulses, Laser Med. Sci. 11, 45–51 (1996) 168 [20] M. H. Niemz: Laser-Tissue Interactions – Fundamentals and Applications, 3rd ed. (Springer, Berlin, Heidelberg 2003) 168, 169 [21] J. Serbin, T. Bauer, C. Fallnich, A. Kasenbacher, W. H. Arnold: Femtosecond lasers as novel tool in dental surgery, Appl. Surf. Sci. 197/198, 737–740 (2002) 168, 173 [22] A. K. M. Straßl, E. Wintner: Ultrashort laser pulses in dentistry, J. Oral Laser Appl. 2, 213–222 (2002) 168, 173 [23] A. Beyertt, D. M¨ uller, D. Nickel, A. Giesen: Femtosecond thin disk Yb:KYW regenerative amplifier without CPA, in Proc. ASSP (Opt. Soc. Am. 2003) pp. 407–413 168 [24] A. Vogel, V. Venugopalan: Mechanisms of pulsed laser ablation of biological tissues, Chem. Rev. 103, 577–644 (2003) 170 [25] B. M. Kim, M. D. Feit, A. M. Rubenchik, E. J. Joslin, P. M. Celliers, J. Eichler, L. B. D. Silva: Influence of pulse duration on ultrashort laser pulse ablation of biological tissues, J. Biomed. Opt. 6, 332–338 (2001) 171 [26] M. G. Buonocore: A simple method of increasing the adhesion of acrylic filling materials to enamel surfaces, J. Dent. Res. 34, 846–851 (1955) 172 [27] J. Perdigao, M. Lopes: Dentin bonding – questions for the new millennium, J. Adhesive Dentistry 1, 191–209 (1999) 172, 173 [28] D. H. Pashley, M. J. Livingstone, J. D. Greenhill: Regional resistances to fluid flow in human dentin in vitro, Arch. Oral Biol. 23, 807–810 (1978) 172 [29] C. Momma, S. Nolte, H. Welling, A. Kasenbacher, M. H. Niemz: Ablation von Zahnhartsubstanz mit ps- und fs-Laserpulsen, in Laser in Medicine (Springer, Berlin, Heidelberg 1997) 173 [30] T. Kosmaˇc, C. Oblak, P. Jevnikar, N. Funduk, L. Marion: The effect of surface and sandblasting on flexural strengh and reliability of Y-TZP zirconia ceramic, Dent. Mater 15, 426 (1999) 176, 177, 180 [31] K. Werelius, P. Weigl, H. Lubatschowski: Processing hip-zirconia with ultrashort laser pulses, in Proc. SPIE (SPIE Proceedings of LPM 2003) 176 [32] R. G. Luthardt, M. Holzh¨ uter, O. Sandkuhl, V. Herold, J. D. Schnapp, E. Kuhlisch, M. Walter: Reliability and properties of ground Y-TZP-zirconia ceramics, J. Dent. Res. 81, 487 (2002) 177 [33] G. Lu, E. Siores, B. Wang: An emperical equation for crack formation in the laser cutting of ceramic plates, J. Mat. Proc. Technol. 88, 154–158 (1999) 177 [34] E. G. Gamaly, A. V. Rode, B. Luther-Davies, V. T. Tikhonchuk: Ablation of solids by femtosecond lasers: Ablation mechanism and ablation thresholds for metals and dielectrics, Phys. Plasmas 9, 949 (2002) 177 [35] ENV 843/1: Advanced Technical Ceramics – Mechanical Properties of Monolithic Ceramics At Room Temperature – Part 1: Flexural Strength Tests (Beuth, Berlin 1995) 178 [36] ENV 843/5: Advanced Technical Ceramics – Mechanical Properties of Monolithic Ceramics At Room Temperature – Part 5: Statistical Analysis (Beuth, Berlin 1997) 178 [37] D. Munz, T. Fett: Ceramics (Springer, Berlin, Heidelberg 2001) 178 [38] T. Weber: Steuerungsalgorithmus zur Laserbearbeitung von Zahnersatz, Diplomarbeit, Technical University of Darmstadt (2002) 181

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[39] R. Lausten, P. Balling: On-the-fly depth profiling during ablation with ultrashort laser pulses: A tool for accurate micromachining and laser surgery, Appl. Phys. Lett. 79, 884–886 (2001) 182 [40] P. Klinger, G. H¨ ausler: On-line ablation measurement for laser material processing and its applications, in Proc. SPIE 4900 (2002) pp. 195–201 182

Index

3D shaping of ceramics, 180 ablation strategy, 180 adhesive, 172 adhesive dentistry, 172 all ceramic restoration, 174 CAD/CAM, 174 caries, 167 caries selectivity, 172 caries therapy, 167 caries-selective tool, 171 cavity, 168 coherence radar (KoRad), 175 dental CAD/CAM system, 182 dental ceramic, 174 dental handpiece, 173 dentin, 168 depth measurement, 181

manufacturing, 174 microcracking, 169 micromachining, 174 milling, 174 minimally invasive, 170 oxide ceramic, 176 pain, 167 pulse duration, 179 retentive pattern, 172 surface measurement, 175 thermal damage, 167 Weibull, 178 X-ray diffractometry, 178

filling material, 172 free-form surfaces, 176

Y-TZP, 176

grinding, 177

zirconium, 176

Ophthalmic Applications Holger Lubatschowski and Alexander Heisterkamp Laserzentrum Hannover e. V., Hollerithallee 8, 30419 Hannover, Germany [email protected] Abstract. Femtosecond photodisruption opens new pathways in refractive surgery due to its precise interaction mechanism with biological tissue. The quality of tissue processing allows correction of refractive errors and preparing donor and recipient tissue for keratoplasty with a much higher flexibility of what is known from the use of mechanical knives. In this chapter, the potential of ultrashort laser pulses is shown in the field of refractive surgery, keratoplasty and the treatment of presbyopia.

In ophthalmic surgery photodisruption by means of short laser pulses in the ns and ps regime is a well-known interaction process since being introduced by Krasnov in 1973 [1]. Today, so-called capsulotomy, performed with a Q-switched Nd:YAG laser, has been well established as a standard surgical technique for treating secondary cataracts [2]. During this procedure, a troublesome membrane, which has grown on the artificial lens after cataract surgery, is cut by means of photodisruption without opening the patient’s eye. First attempts at refractive corneal surgery were made to reshape the corneal curvature by means of photodisruption with picosecond pulses [3,4,5] This approach failed because of the strong mechanical side effects, especially the considerable bubble formation inside the corneal stroma. The size of the laser-induced bubbles as well as the extent of the shock-wave formation depends on the laser pulse energy that has been applied (Table 1). A decrease of pulse energy can be achieved by shortening the laser pulse duration because a certain threshold intensity has to be exceeded for optical breakdown. Recently, turnkey laser systems have become available emitting laser pulses in the fs scale. Focusing the beam to a spot size of some micrometer in diameter, the threshold intensity is reached at pulse energies of only 1 µJ [6] or, when focusing with extremely high numerical aperture even in the nanojoule regime [7]. As a consequence, the secondary mechanical effects are reduced dramatically. Thus, femtosecond photodisruption offers the possibility to perform a refractive surgical operation by preparing an intrastromal lenticule, analogous to the so-called LASIK procedure (laser in situ keratomileusis), without using a mechanical knife, as is necessary in conventional LASIK.

F. Dausinger, F. Lichtner, H. Lubatschowski (Eds.): Femtosecond Technology for Technical and Medical Applications, Topics Appl. Phys. 96, 187–203 (2004) c Springer-Verlag Berlin Heidelberg 2004


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Table 1. Typical laser parameters and tissue effects of photodisruption in the nanosecond, picosecond and femtosecond regime ns Intensity (1012 W/cm2 ) Fluence (J/cm2 ) Pulse energy (µJ) Amplitude of the acoustic transient (bar)† Diameter of the caviation bubble (µm) *

own investigation

1

†

ps 0.05

[8]

fs 0.5 to

1

[8]

5

to 10

∗

, [8] , [9, 10] , [8]

100

[9]

2

to 10

[9, 10]

1

to

3

∗

100

to 10 000

[8]

1

to

[8]

0.5 to

3

∗

100

to

500

[11]

10

to 100

[11]

1

to

5

∗

1000

to

2000

[12] 200

to 500

[12]

< 30

(*)

10

to

5

1 mm distance

Refractive Surgery

Refractive laser surgery, in terms of reshaping the surface of the human cornea with an UV-excimer laser was proposed in 1983 by Trockel and Srinivasan [13]. In the early 1990s the first clinically controlled studies were performed and today it is almost a routine surgical procedure, shared by two different methods: PRK, and LASIK. 1.1

Photorefractive Keratectomy (PRK)

The principle of the so-called photorefractive keratectomy (PRK) is shown in Fig. 1. For myopic eyes, where the refractive power of the cornea and the lens is too strong in relation to the length of the eyeball, the corneal surface will be flattened by ablation of corneal tissue in the center, in order to reduce its refractive power. On the other hand, the cornea of a hyperopic eye, where the refractive power is too low, the surface will be steepened by removing most of the tissue at the edge of the cornea. By shaping the corneal surface to a certain degree on different axis, refractive errors like astigmatism can be corrected. According to the amount of the refractive error, some 10 µm or as much as 100 µm of the superficial layer of the cornea will be removed. Before the laser treatment starts, a 70 µm thick superficial cell layer, the so-called epithelium of the cornea (Fig. 2) is removed mechanically. The epithelium is a thin multilayer of cells with a rapid turnover. The cell layer heals up within 24 h to 48 h after removal, in contrast to the underlying corneal stroma, which shows almost no regression after ablation. The postoperative recovery of the epithelium in combination with the tear film has a smoothing effect. Thus small irregularities on the submicrometer scale, which will occur during laser processing of the tissue, will cause no problems for the visual acuity of the patient’s eye.

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Fig. 1. Principle of photorefractive keratectomy (PRK). On myopic eyes (left), where the refractive power of the cornea and the lens is too strong in relation to the length of the eyeball, the corneal surface will be flattened by ablation of corneal tissue in the center, in order to reduce its refractive power. On the other hand, at the cornea of a hyperopic eye (right), where the refractive power is too low, the surface will be steepened by removing most of the tissue at the edge of the cornea

Fig. 2. Cross section of the human cornea. The thickness of the cornea is about 500 µm

1.2

Laser in situ Keratomileusis (LASIK)

At low and medium myopia (up to −6 dpt.), PRK is a safe and established procedure. At higher corrections of the refractive error, however, complications like scar formation and regression occur. This is in the first instance caused by a stronger induction of wound healing, induced by the removal of a larger amount of tissue [14]. These difficulties can be avoided by a procedure that was proposed by Pallikaris et al. in the early 1990s [15, 16]. In so-called “laser in situ keratomileusis” (LASIK), the inner part of the cornea not the superficial layer of the corneal stroma will be processed with the ArF excimer laser (Fig. 3). For this, a layer with a thickness of approximately 160 µm is separated partially by a knife, the so-called microkeratome, and opened. Afterwards, corneal tissue is ablated by the laser, as is necessary

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Fig. 3. Principle course of laser in situ keratomileusis (LASIK). A layer with a thickness of approximately 160 µm is separated partially by a knife and opened. Afterwards corneal tissue is ablated by the laser, as is necessary to correct the refractive error. Finally, the top lamella is replaced. This flap joins to the new surface and fits without any suture due to adhesion forces

to correct the refractive error. Finally, the top lamella is replaced. This flap joins to the new surface and fits without any suture due to adhesion forces. By means of LASIK, myopic corrections well over −6 dpt. can be performed and also hyperopia and astigmatism of more than 4 dpt. can be corrected. Although LASIK is an accepted procedure to correct refractive errors of the eye, there is still some room for improvement, especially in that part of the procedure where the flap is created by the microkeratome. One of the main problems is to assure a standardized flap geometry. Variations in thickness of the flap within up to 30% are one of the most frequent complications in refractive laser surgery [17, 18].

2

Femtosecond LASIK

One way to overcome the problems of creating a flap with a microkeratome is by using ultrashort laser pulses. The principle of this procedure is shown in Fig. 4. In a first step, a lamellar intrastromal cut is performed by scanning the laser in a spiral pattern. This procedure is analogous to the mechanical lamellar cut of a microkeratome (diamond knife) in the conventional LASIK procedure. But not only can the troublesome lamellar LASIK cut be done with the femtosecond laser. In a second step, another cut prepares a stromal lenticule

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Fig. 4. Principle of femtosecond laser keratomileusis: In a first step, a lamellar intrastromal cut is performed by scanning the laser in a spiral pattern. In a second step, another cut prepares a lenticule with the desired shape, depending on the refractive error of the treated eye. In the third step, the anterior corneal flap is opened, and the prepared lenticule can be extracted. Finally, the flap will be repositioned on the cornea. The surface of the cornea follows the missing volume of the lenticule leading to a change in refractive power

with the desired shape, depending on the refractive error of the treated eye. In the third step, the anterior corneal flap is opened, and the prepared lenticule can be extracted. Finally, the flap will be repositioned on the cornea. The surface of the cornea follows the missing volume of the lenticule, thereby leading to a change in refractive power. Although the mechanical and thermal damage of the surrounding tissue were already shown to be small [19, 20], several other side effects may take place, due to the nonlinear character of the photodisruption. 2.1

Laser System and Beam Delivery

A typical femtosecond laser system that delivers sufficient pulse energy for material or tissue processing usually consists of an oscillator-amplifier arrangement. One of the most compact femtosecond oscillators, which was also used in the experiments demonstrated here, are erbium fiber laser oscillators [21]. The oscillator pulses have a typical repetition rate of 60 MHz to 80 MHz and a pulse duration of 100 fs. In the setup used in this study, the pulses were frequency-doubled in chirped periodically poled lithium niobate, resulting in 2 mW output power at a central wavelength of 780 nm. Subsequently, these pulses were amplified by means of chirped-pulse amplification (CPA) in a Ti:sapphire regenerative amplifier [22]. The complete system al-

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Fig. 5. Experimental setup for the beam-delivery system. The laser light is guided to a scanner system and a focusing lens with a 75 mm operating distance. The eyes are fixed below a suction ring with a contact glass plate on its top. The glass plate has a fixed position relative to the focal plane of the laser beam

lows variable repetition rates of up to 3 kHz at output powers of 300 mW. The minimum achievable pulse duration of the system was approximately 110 fs. Longer pulse durations were achieved by a detuning of the compressor system and not fully recompressing the output pulse, leading to a defined chirp and a longer pulse duration. The amplified laser light is guided to a computer-controlled, two-axis scanner system and a focusing lens with a 75 mm operating distance (Fig. 5). The spot size of the laser focus was estimated to be smaller than 4 µm in diameter. The eyes are fixated by a suction ring with a contact glass plate on its top. The anterior segment of the cornea is slightly flattened by the glass plate. The glass plate has a fixed position relative to the focal plane of the laser beam. With this setup the focus of the laser beam can be well defined, with submicrometer precision inside the corneal stroma. 2.2

Morphological Studies and Nonlinear Side Effects

Histological analysis demonstrates the smooth and precise character of femtosecond tissue processing by ultrashort laser pulses. In Fig. 6 the light microscopic cross section of a porcine cornea shows the cutting line of 160 fs laser pulses. In this experiment, the corneal flap remains closed in order to analyze the cutting characteristics without mechanical influence due to any movements of the flap. The pulse energy of each laser pulse was 0.8 µJ. The effective spot separation was set to 3 µm, whereas the spot diameter was esti-

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Fig. 6. Histological section (HE staining) of a porcine cornea, irradiated with a series of 160 fs laser pulses with a pulse energy of 0.8 µJ. The focal spot had a diameter ≥ 4 µm. The spot separation during the scan was 3 µm. The cutting line is indicated by the black arrows. On the left, the laser focus was moved upwards into the epithelium. Although optical breakdown takes place inside the corneal stroma and epithelium, no denudation or bursting of the tissue could be observed

mated to be 4 µm. The mechanical as well as the thermal side effects caused by laser interaction are negligibly small. The zone of visible thermal alteration of the adjacent tissue is below 1 µm. The SEM micrographs in Fig. 7 illustrate the precision of the corneal cut in comparison with a conventional cut of a mechanical knife (Nidek Microkeratome). At higher magnification, the laser-induced cut seems to be slightly smoother than the cut performed with the diamond knife, provided that the histological preparation technique has no significant influence on the morphology of the samples. One step further in magnification, the corneal fibrils can be seen on the laser-generated cutting surface (Fig. 8). Another indication for the absence of thermal stress during laser–tissue interaction. Otherwise, the corneal fibrils would be agglomerated. The macroscopic smoothness and precision of corneal-tissue processing strongly depends on the pulse energy and on the scan algorithm. Each optical breakdown inside the corneal stroma is accompanied by the production of a small amount of H2 , O2 , CO and CH4 gas via photodissociation resulting in a small bubble remaining in the corneal stroma for some seconds [23, 24]. At higher pulse energies and at a small spot separation these microbubbles join to larger bubbles and affect the beam path of the following laser pulses substantially. As a consequence, the intended cut is highly irregular (Fig. 9). To overcome this problem, the intrastromal cuts were produced by scanning the laser focus in two different spiral patterns, each with a spot separation of about 8 µm. After performing the first pattern the second was scanned by placing each laser pulse between two of the first spiral. At that time, the gas of the bubbles created during the first scan has dissolved in the liquid of the

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Fig. 7. SEM micrographs of a prepared intrastromal lenticule with femtosecond laser pulses (top) in comparison with a conventional cut of a mechanical diamond knife (Nidek Microkeratome, bottom). In the upper micrograph, the same laser parameters were used as in Fig. 6

Fig. 8. At higher magnification, the corneal fibrils can be seen on the laser-generated cutting surface. Another indication for the absence of thermal stress during laser–tissue interaction. Otherwise, the corneal fibrils would be agglomerated

surrounding tissue. Hence, the effective spot separation of this scan algorithm was in the range of 4 µm (Fig. 10). Another side effect can be observed, when using small numerical apertures of the focusing lens. Caused by the high intensities within the Rayleigh

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Fig. 9. Histological section (HE staining) of a porcine cornea, irradiated with a series of 300 fs laser pulses with a pulse energy of 4 µJ. At these higher pulse energies the laser-induced microbubbles join to larger bubbles and affect the beam path of the following laser pulses. As a consequence, the desired cut is highly irregular

Fig. 10. Each optical breakdown inside the corneal stroma is accompanied by the production of a small amount of gas, forming a microbubble. The microbubbles can fuse to larger bubbles and affect the beam path of the following laser pulses substantially (left). To overcome this problem, the laser focus is moved in two different patterns, each with a spot separation twice as long as the desired spot separation. After performing the first scanning line the second is scanned by placing each laser pulse between the foci of the first line (right). At that time, the gas of the bubbles created during the first scan has dissolved in the liquid of the surrounding tissue

range of the laser beam, streaks were created inside the corneal tissue that image each single laser pulse (Fig. 11). The strength of the streaks depends on the laser intensity. At a constant pulse energy of 2 µJ almost no streaks

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Fig. 11. Histological section (HE staining) of a porcine cornea, irradiated with 160 fs laser pulses at a pulse energy of 2 µJ. Caused by the high intensities within the Rayleigh range of the laser beam, streaks were created inside the corneal tissue that image each single laser pulse. The corneal flap was opened and closed for better demonstration of the cutting line

could be observed at 930 fs, whereas at 160 fs the strongest streaks occurred. In the same way the intensity of the streaks increased with decreasing numerical aperture. In TEM the streaks can be seen as a dark staining, crossing the picture in the vertical direction (Fig. 12). The diameters of the streaks were estimated to be in the range of 200 nm to 500 nm, which is below the diffraction limit of the focused laser beam. The distance between two single streaks is equal to the separation of the laser pulses, which is 3.5 µm. It is important to note that the streak appearance is obviously caused by accumulation of electron-opaque material (the contrast medium was uranyl acetate and lead citrate) and not by a change in the structure of the collagen fibrils as would occur when thermal stress is exposed to the collagen. Thus, it is more likely that the high photon density leads to multiphoton absorption and consequently induces UV damage by photodissociation. This assumption competes with the hypothesis, in which the authors assume that due to the high intensity a significiant number of free electrons is produced [25] which do not reach plasma density. However, the free electrons might have induced radical reactions that damage the collagen and lead to the streak formation. With respect to recently performed in vivo experiments, these tissue alterations seemed to be permanent. The streaks could be found even 7 days and 14 days after treatment of the animals [24]. Nevertheless, the cornea of these animals was clear and no haze during observation of the eyes by a slit lamp could be detected. As a conclusion, intrastromal refractive surgery, in principle, is feasible with ultrashort laser pulses, taking advantage of the fact that no mechanical keratome has to be used with all its chances for complications. The first clinical trials [26,27], which only used the fs laser as a microkeratome, already

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Fig. 12. In TEM the streaks can be seen as a dark staining, crossing the picture in the vertical direction. The diameters of the streaks were estimated to be in the range of 200 nm to 500 nm, which is below the diffraction limit of the focused laser beam (3 µJ, 180 fs). The distance between two single streaks is equal to the separation of the laser pulses, which is 3.5 µm. The left micrograph shows two streaks within the focusing plane of the cornea. On the left streak, optical breakdown occurred, which is indicated by the bubble. On the right streak, obviously no optical breakdown took place. The right micrograph shows the collagen fibrils around an optical breakdown. In the upper third of the image the fibrils come perpendicular to the image plane and appear as small dots (50 nm). In the lower part, the fibrils run parallel to the image plane from left to right. Obviously, the structure of the fibrils is not changed significantly, as would happen when thermal stress is applied to the collagen

demonstrated that the thickness of the created flaps showed significantly lower standard deviation than conventionally created flaps. Expecting small, compact and low-priced laser sources, femtosecond photodisruption has the potential to be an attractive tool for intrastromal refractive surgery.

3

Keratoplasty

In fields of keratoplasty (transplantation of corneal tissue), parts of the cornea are cut out, to be implanted into an acceptor eye. In this procedure, similar cuts as for the flap in femtosecond LASIK are performed. As in femtosecond LASIK, the cutting geometry is no longer limited to round-shaped forms, as are done with conventional mechanical trepans (Fig. 13). Using a similar setup as in refractive surgery, nearly any shape is possible and the amount of tissue that has to be transplanted is determined predominantly by the pathological tissue that has to be removed. If the tissue alteration is mainly on the anterior part of the cornea, even lamellar cuts can be performed, leaving the posterior part of the receiver cornea intact (Fig. 14).

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Fig. 13. Perforating keratoplasty of a pig cornea, 6 mm in diameter, a triangle was integrated for a better replacement of the donor cornea (2 o’clock position)

Fig. 14. Lamellar keratoplasty. If the tissue alteration on the cornea is mainly on the anterior part, lamellar cuts can be performed with the femtosecond laser, leaving the posterior part of the receiver cornea intact

4

Presbyopia

Presbyopia starts at the age around 45 and leads usually within 15 years to a complete loss of the accommodation of the human eye. In consequence almost every middle-age person needs glasses or contact lenses to see a sharp image at low distances. After several studies on the reason for presbyopia it is today accepted that a loss of flexibility of the lens is responsible for this. To regain the elasticity, it might be possible to create small cuts inside the lens and therefore crack some hard-tissue structures of collagen as well as reduce the density of the tissue [28]. A surgery is desirable that keeps the eye unopened and cuts inside the lens without an influence on the surrounding tissue. For near-infrared light both the cornea and the lens are transparent. So it is possible to focus ultrashort Ti:sapphire laser pulses inside the lens and create a series of microplasmas. While scanning the focal point of the laser beam inside the lens it is possible to place one spot next to another and disrupt tissue in a two- or three-dimensional pattern.

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Fig. 15. Schematic diagram of the cutting patterns inside the lens; left: annulus pattern; right: sagittal pattern

Fig. 16. Sagittal cut in ex vivo pig eye performed with 2.4 µJ and 5 µm spot separation. On the left only the big bubbles are visible that merged together from more then one optical breakdown. On the right, at higher magnification also the microbubbles, created by each individual breakdown, are visible

In a first study ex vivo pig and in vivo New Zealand albino rabbits were treated with this method. The scanner unit was very similar to the setup used in femtosecond LASIK studies. Basically, two different patterns inside lens tissue were performed. First, an annular pattern (Fig. 15, left) with up to four rings was scanned. The cutting lines were fixed in one plane, perpendicular to the beam axis. Secondly, a sagittal pattern (Fig. 15, right) was written inside the lens, while leaving the central region untreated. Figure 16 shows the image of a treated pig eye. Here, the inner, untreated zone has a diameter of 2 mm and the outer diameter of the pattern is about 6 mm. At the magnification on the left only the big bubbles are visible that merged together from more than one optical breakdown. On the right, at higher magnification also the microbubbles, created by each individual breakdown are visible. These microbubbles vanish after a couple of minutes. In Fig. 17 four rings each 500 µm in width are shown, which were scanned into a living rabbit eye. While cutting the sagittal patterns in the rabbit eye in vivo, a centered fixation of the rabbit eye below the scanner unit was very difficult. Therefore, the sagittal cuts were not always complete in diameter concerning the shielding by the iris. Nevertheless, the creation of microcuts was possible in all eyes.

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Fig. 17. 4 annulus cuts in in vivo rabbit eyes with 1 µJ and 10 µm spot separation; the right side is shaded due to difficulties in centering and is thus shielded by the iris

Fig. 18. Ex vivo annulus cuts in rabbit eyes with 2 µJ and 4 µJ pulse energy and 5 µm, 7 µm and 9 µm spot separation. The higher the pulse energy and the closer the spot separation, the more larger merged bubbles are visible

Photodisruption inside the lens tissue leads to a small gas-filled bubble that remains in the breakthrough region for some minutes after their gaseous filling dissolve into the surrounding lens tissue. It could be shown that the amount of larger bubbles scaled with the pulse energy and the spot separa-

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tion (Fig. 18). For higher energies many more and bigger bubbles could be observed. With respect to this, the spot separation has to be increased to get a smooth cut. In conclusion, cutting inside the lens is possible without opening the eyes. No haze and no damage of the eye and especially the lens related to the laser radiation could be observed during this first study. Because of the sharp focusing a pulse energy in the range of 1 µJ and below seems to be sufficient to create cuts with widths of a few micrometer. Due to this, ultrashort laser pulses seem to be a promising tool for a possible treatment of presbyopia.

References [1] M. M. Krasnov: Laserpuncture of anterior chamber angle in glaucoma, Am. J. Ophthalmol. 75, 674–678 (1973) 187 [2] D. Aron-Rosa, J. J. Aron, M. Griesemann, R. Thyzel: Use of the Nd:YAG laser to open the posterior capsule after lens implant surgery: A preliminary report, Am. Intra-Ocular Implant. Soc. J. 6, 352 (1980) 187 [3] M. Remmel, C. M. Dardenne, J. F. Bille: Intrastromal tissue removal using an infrared picosecond Nd:YLF ophthalmic laser operating at 1053 nm, Lasers Opthalmol. 4, 169–173 (1992) 187 [4] M. H. Niemz, E. G. Klancnik, J. F. Bille: Plasma-mediated ablation of corneal tissue at 1053 nm using a Nd:YLF oscillator/regenerative amplifier laser, Laser Surg. Med. 11, 426–431 (1991) 187 [5] M. H. Niemz, T. P. Hoppeler, T. Juhasz, J. F. Bille: Intrastromal ablations for refractive corneal surgery using picosecond infrared laser pulses, Lasers Light Ophthalmol. 5, 149–155 (1993) 187 [6] A. Heisterkamp, T. Ripken, E. L¨ utkefels, W. Drommer, H. Lubatschowski, W. Welling, W. Ertmer: Intrastromal cutting effects in rabbit cornea using femtosecond laser pulses, in Proc. SPIE 4161 (2000) pp. 52–60 187 [7] K. K¨ onig, I. Riemann, W. Fritzsche: Nanodissection of human chromosomes with near-infrared femtosecond laser pulses, Opt. Lett. 26, 819–821 (2001) 187 [8] Vogel, et al.: in Proc. SPIE 3255 (1998) p. 34 188 [9] Loesel, et al.: in Proc. SPIE 2923 (1997) p. 118 188 [10] Niemz, et al.: Lasers Light Ophtalmol. 5, 149 (1993) 188 [11] Vogel, et al.: J. Acoust. Soc. Am. 100, 148 (1996) 188 [12] Vogel, et al.: in Proc. SPIE 1877 (1993) p. 312 188 [13] S. L. Trokel, R. Srinivasan, B. Braren: Excimer laser surgery of the cornea, Am. J. Ophthalmol. 96, 710–715 (1983) 188 [14] C. P. Lohmann, E. Hoffmann, U. Reischl: Epidermaler Wachstumsfaktor (EGF) in der Tr¨ anenfl¨ ussigkeit bei der Excimerlaser PRK. Verantwortlich fur postoperative Refraktion und “Haze”?, Der Ophthalmol. 95, 80–87 (1998) in German 189 [15] I. G. Pallikaris, M. E. Papatzanaki, E. Z. Stathi, O. Frenschock, A. Georgiadis: Laser in situ keratomileusis, Lasers Surg. Med. 10, 463–468 (1990) 189 [16] I. G. Pallikaris, D. S. Siganos: Excimer laser in situ keratomileusis and photorefractive keratectomy for correction of high myopia, J. Refract. Corneal. Surg. 10, 498–510 (1994) 189

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[17] M. C. Knorz, B. Jendritza, P. Hugger, A. Liermann: Komplikationen der Laserin-situ-Keratomileusis (LASIK), Der Ophthalmol. 96, 503–508 (1999) in German 190 [18] H. Petersen, T. Seiler: Laser-in-situ-keratomileusis (LASIK), Der Ophthalmol. 96, 240–247 (1999) 190 [19] H. Lubatschowski, G. Maatz, A. Heisterkamp, U. Hetzel, W. Drommer, H. Welling, W. Ertmer: Application of ultrashort laser pulses for intrastromal refractive surgery, Graefes Arch. Clin. Exp. Ophthalmol. 238, 33–39 (2000) 191 [20] A. Heisterkamp, M. Thanongsak, O. Kermani, W. Drommer, H. Welling, W. Ertmer, H. Lubatschowski: Intrastromal refractive surgery with ultrashort laser pulses – in vivo study on rabbit eyes, Graefes Arch. Clin. Exp. Ophthalmol. 241, 511–517 (2003) 191 [21] A. Heisterkamp, G. Maatz, T. Ripken, E. L¨ utkefels, W. Drommer, H. Lubatschowski, W. Welling, W. Ertmer: Intrastromal refractive surgery by ultrashort laser pulses: side effects and mechanisms, in Proc. SPIE 3908 (2000) pp. 146–156 191 [22] G. Morou: The ultrahigh-peak-power laser: present and future, Appl. Phys. B-Lasers O. 65, 205–211 (1997) 191 [23] G. Maatz, A. Heisterkamp, H. Lubatschowski, S. Barcikowski, C. Fallnich, H. Welling, W. Ertmer: Chemical and physical side effects at applications of ultrashort laser pulses for intrastromal refractive surgery, J. Opt. A-Pure Appl. Op. 2, 59–64 (2000) 193 [24] A. Heisterkamp, G. Maatz, U. Hetzel, W. Drommer, H. Lubatschowski, W. Welling, W. Ertmer: Optimierung der Laserparameter f¨ ur die intrastromale Schnittf¨ uhrung mittels ultrakurzer Laserpulse, Der Ophthalmol. 98, 623–628 (2001) in German 193, 196 [25] J. Noack, A. Vogel: Laser-induced plasma formation in water at nanosecond to femtosecond time scales: Calculation of thresholds, absorption coefficients, and energy density, IEEE J. Quantum Elect. 35, 1156–1167 (1999) 196 [26] L. T. Nordan, S. G. Slade, R. N. Baker, C. Suarez, T. Juhasz, R. Kurtz: Femtosecond laser flap creation for laser in situ keratomileusis: six month follow-up of initial US clinical series, J. Refract. Surg. 19, 8–14 (2003) 196 [27] I. Ratkay-Traub, I. E. Feruncz, T. Juhasz, R. Kurtzs, R. R. Krueger: First clinical results with the femtosecond neodymium-glass laser refractive surgery, J. Refract. Surg. 19, 94–103 (2003) 196 [28] Krueger, et al.: Experimental increase in accomadative potential after Nd:YAG laser photodisruption of paired cadaver lenses, Ophthalmol. 108, 2122–2129 (2001) 198

Index

bubble, 187, 193 cornea, 188 flap, 190 keratoplasty, 197 laser in situ keratomileusis (LASIK), 187

photodisruption, 187 Presbyopia, 198 PRK, 188 refractive surgery, 187 side effect, 192 streak, 196

Neurosurgical Applications Marcus G¨ otz MRC Systems GmbH – Medizintechnische Systeme, Hans-Bunte-Str. 10, 69123 Heidelberg, Germany [email protected] Abstract. The high precision of femtosecond laser ablation makes it an interesting tool for neurosurgical applications. The resection of arbitrary-shaped volumes of brain tissue can be of interest for the treatment of movement disorders. Fundamental studies of bovine brain-tissue ablation have shown precise cutting effects of femtosecond laser pulses with no thermal or structural side effects. If a large number of laser pulses is applied with an appropriate strategy, the quality of the laser cut is comparable to a mechanical cut with a scalpel. A selection of important requirements for the application of these laser pulses with a neurosurgical instrument are presented with respect to navigation, beam delivery, and operation monitoring. Only the resection capability of femtosecond lasers is discussed in this Chapter. Other applications, like 2-photon fluorescent microscopy or optical coherence tomography (OCT) with femtosecond lasers, which might support neurosurgical interventions, are not included.

1 Ablation of Brain Tissue with Ultrashort Laser Pulses The interaction of femtosecond laser pulses with brain tissue follows the same mechanism as described in the previous Chapter for ophthalmic applications. Ablation of bovine brain has shown precise cutting effects without thermal or structural changes to adjacent tissue [1]. In addition, the ablation with femtosecond laser pulses was found to be more efficient than the ablation with picosecond laser pulses: Laser pulses from a Ti:sapphire laser with 140 fs duration showed a two times higher efficiency than the longer 30 ps from a Nd:YLF laser with identical pulse energy [1]. The same fundamental study has shown that the threshold fluence needed for initiating the ablation process is lower for the shorter femtosecond laser pulses than for picosecond or nanosecond laser pulses. This result is in accordance with the results shown in the previous section with respect to ophthalmic applications. The ablation threshold of bovine tissue was at 1.5 J/cm2 for 100 fs and at 20 J/cm2 at pulse widths of 35 ps [2]. Therefore, lower pulse energies can be used for the removal of neural tissue if femtosecond laser pulses are applied. As discussed with reference to ophthalmic applications, the decrease of the applied pulse energy leads to smaller sizes of cavitation bubbles as well as less extensive shock-wave formation. In contrast to the transparent cornea the F. Dausinger, F. Lichtner, H. Lubatschowski (Eds.): Femtosecond Technology for Technical and Medical Applications, Topics Appl. Phys. 96, 203–211 (2004) c Springer-Verlag Berlin Heidelberg 2004


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brain tissue absorbs infrared laser radiation to a high extent and the ablation takes place on the tissue surface. However, since the laser pulses are applied through a minimally invasive instrument, ablation within a rinsing liquid has to be considered. Therefore, the reduction of cavitation bubbles is important. The histological quality of the femtosecond laser cut was comparable to a mechanical cut with a scalpel [1].

2 Potential Applications of Ultrashort Laser Pulses in Neurosurgery In the following section we will describe potential applications of femtosecond lasers, which make use of the special characteristics of these light sources. It has to be pointed out that the presented applications have not been introduced into clinical practice up to now. 2.1

Movement Disorders

Neurology knows a number of indications where the lesion of a specific area in the brain can reduce or eliminate the symptoms. Probably the most prominent indication of these is Parkinson’s disease. Patients suffering from Parkinson’s disease show a triad of leading symptoms: 1. akinesia or hypokinesia, i.e. a slow-down of all movements, a ducked posture and a shuffling walk as well as a monotonous speech and other concomitants, 2. rigor, i.e. stiffness, and 3. tremor. Medicinal therapy can help these patients for several years. As soon as the medicinal therapy loses its efficacy or the side effects get too strong, a surgical treatment is indicated. In this case, a functional or structural lesion of clearly circumscribed areas in the brain takes place. The area to be lesioned or destroyed is called the target volume. In the case of Parkinson’s disease the target volume is a tailor-shaped area with a range of about 3 mm in all dimensions. There are, of course, other diseases of the brain that result in a movement disorder, like hereditary monosymptomatic tremor and MS tremor. However, the treatment concept is comparable and there is no need to give more details in this publication. Figure 1 shows examples of size and location of different target volumes. Today, there are mainly two surgical methods for lesioning. A structural lesion is achieved by means of coagulation. A functional lesion is established by means of deep-brain stimulation [3, 4, 5]. However, these methods have therapeutic or economic restrictions, which makes the development of alternative treatment concepts very attractive.

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Fig. 1. The target volume to be lesioned is a circumscribed area deep in the brain. In this sagittal view of a patient’s brain, two examples of deep-seated target volumes are marked. Stn: Nucleus subthalamicus, VIM: Nucleus ventralis intermedia. Another important target volume, the Globus pallidus internus (GPi) is not shown in this image

3

Requirements for Neurosurgical Instruments

As mentioned in the previous chapters, femtosecond lasers have a number of advantages compared to other surgical lasers. The main advantage is the precision of the laser resection and the absence of thermal side effects. However, in order to transfer the precision of the laser into the precision of a complete surgical system, a dedicated application system is required. The application system must have one main feature: The positioning of the laser focus in the target volume. In more detail, the laser focus has to be scanned to any point of the target volume, since laser ablation only takes place in the focus of the laser beam. Therefore, an instrument is required that transfers and focuses the laser beam into the brain and scans the focus spot over the target volume. Another important feature of the application system is that it is minimally invasive. It must satisfy the ergonomic and hygienic requirements of neurosurgery. The following sections will present concepts that allow for the application of a femtosecond laser through a thin hollow tube, which is inserted directly into the target volume. A current setup of an instrument works with an outer tube diameter of less than 3 mm (see Fig. 2). 3.1

Stereotactic or Navigated Surgery

The success of neurosurgery strongly depends on the growing use of imagebased methods, which arose since the introduction of 3-dimensional computer

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Fig. 2. Minimally invasive neurosurgical instrument attached to a stereotactic device. An articulated mirror arm, flexible shafts and electronic cables are connected to the instrument’s body

tomography. Magnetic resonance and positron emission tomography are just two more examples of imaging techniques that provide the neurosurgeon with detailed anatomic (and functional) information of the patient’s brain. There are a number of methods that enable the navigation of surgical instruments based on these images. In stereotactic neurosurgery and neuronavigation, a direct connection of a coordinate system to the images of the individual patient is established. The patient’s skull is fixed and markers are attached to the patient or the fixation system before the imaging procedure. These markers are recognised in the images and therefore a correlation of the real-world coordinate system to the images can be obtained. The neurosurgeon determines the target volume in the images of the patient and defines an approach through the brain with minimal damage to vital structures. In stereotactic neurosurgery, this is done by marking a target point and a trepanation point in different slices of CT or MR images. These points describe the trajectory through the brain. A head ring serves as the fixation system (see Fig. 2). Since the orientation of the images to the head ring is known and a targeting system can be attached to the head ring in a well-known manner, the introduction of a neurosurgical instrument by means of that targeting system can be calculated before the operation. In neuronavigation, registered markers are placed on the instrument, thus the orientation of the instrument can be slotted into the anatomic images obtained before or during the intervention. In this way, the neurosurgeon can introduce the instrument on a visualised pathway. Both methods can be used

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as a basis for the application of femtosecond lasers by means of a minimally invasive instrument. 3.2

Ultrashort Laser-Beam Delivery

One disadvantage of femtosecond lasers is the high damaging potential leading to the fact that it is not possible to transmit a laser beam with required pulse energies and appropriate beam quality through a flexible fiber. Since such a fiber is ruled out as an application system, a straightforward concept for transferring the femtosecond laser beam into the brain is the use of a hollow tube. Figure 3 shows an arrangement of optical components that are attached to further tubes integrated into such a hollow tube. The lens at the tip of the inner tube focuses the laser beam. The beam is deflected by means of a mirror at the tip of the middle tube. The mirror can be rotated and moved up and down, thereby moving the laser focus along a cylindrical shape. In order to change the working distance of the instrument, i.e. the position of the focal spot relative to the instrument’s axis, the distance between lens and mirror can be changed. Further details of a realised instrument based on this concept can be found in [6]. The space between the tubes can be used for the in and outflow of rinsing liquid [7]. A computer-controlled movement of the optical components enables an accurate correlation between the resection and the target volume. Besides the accurate scanning of the laser focus inside the brain, a connection between the laser source and the surgical instrument is required. Since the transmission of the femtosecond laser pulses through a flexible fiber is not possible without affecting the beam quality, articulated mirror arms must be used to transfer the laser beam into the neurosurgical instrument. Depending on the actual approach to the target volume, the neurosurgical instrument must be moved in space. Hence, an accurate alignment of the femtosecond laser beam with the optical channel of the instrument must be maintained for all orientations. Due to the small dimensions of the optical channel, this can be crucial. If the tolerances of the articulated mirror arm alone do not fulfill the alignment requirements, an active beam control must be used. Figure 4 shows an example of such an active beam alignment. Two position-sensitive detectors are directly connected to the surgical instrument. Since the orientation of the detectors to the optical channel of the instrument is known, the deviation of the laser-beam axis from its target position can be measured. A pair of steerable mirrors is then used to adjust the laser beam onto the target position. 3.3

Operation Monitoring

The known ablation efficiency of single laser pulses can be used to plan the movement of the laser focus over the target volume. However, due to biological

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(a)

(c)

laser probe

up and down movement of focusing lens

up and down movement of deflection mirror

(b)

laser opening

rotation

resection primitive

ablation geometry

Fig. 3. Arrangement of optical components capable of moving the laser focus in three dimensions. (a) movable deflection mirror and focusing lens in hollow tube, (b) cylindrical ablation geometry obtained by standard movement of optical components, (c) ablation geometry, if the mirror is moved over an angular segment

variations the actual effect can differ from pulse to pulse. Furthermore, brain shift has to be considered, which means movements of the brain due to liquor outflow and changing space requirements. Differing cleanliness of the rinsing liquid also leads to the fact that the ablation efficiency will fluctuate with time. Therefore, after a period of time without further measures, the laser focus would not hit the current surface of the operation cavity any longer. In comparison to a detailed precalculation of the focus movement, a direct observation of the ablation progress is preferable. The plasma spark, which is an intrinsic characteristic of plasma-mediated ablation, can support this task. The plasma-spark intensity can be measured by a photodiode. It is a sensitive criterion for the ablation efficiency and can be used to determine the position of the laser focus relative to the current tissue surface. In Fig. 5 three situations are shown to illustrate the method.

Neurosurgical Applications detector 1

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detector 2

M2 R2

R1

M1

Fig. 4. Active beam alignment comprising two position-sensitive detectors and two steerable mirrors M1 and M2. Two reflectors R1 and R2 deflect a small portion of the laser power to the detectors

instrument

operation cavity focusing lens target volume

3 2 1 r

Fig. 5. The position of the laser focus relative to the tissue surface influences the intensity of the plasma spark. See text for details

If the laser focus is located deep inside the tissue, the fluence on the tissue surface is below the ablation threshold. No ablation and also no plasma spark will occur. If the laser focus is located in the rinsing liquid in front of the tissue, the plasma-spark intensity is low due to the higher ablation threshold of clean saline compared to brain tissue. Only if the laser focus hits the tissue surface within a specific depth range, is a high signal obtained. This information can be used to visualise the ablation efficiency and to readjust the laser focus onto the tissue surface during the operation. In this way, the laser resection can be controlled to obtain the precision, which makes the femtosecond laser an attractive tool for neurosurgery.

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References [1] F. H. L¨ osel, J. P. Fischer, M. H. G¨ otz, C. Horvath, T. Juhasz, F. Noack, N. Suhm, J. F. Bille: Non-thermal ablation of neural tissue with femtosecond pulses, Appl. Phys. Lett. 66, 121–128 (1998) 203, 204 [2] F. H. L¨ osel, M. H. Niemz, J. F. Bille, T. Juhasz: Laser-induced optical breakdown on hard and soft tissues and its dependence on the pulse duration: Experiment and model, IEEE J. Quantum Elect. 32 (1996) 203 [3] J. Volkmann, V. Sturm, P. Weiss: Bilateral high frequency stimulation of the internal globus pallidus in advanced Parkinson’s disease, Ann. Neurol. 44, 953– 961 (1998) 204 [4] P. Krack, P. Pollak, P. Limousin: Subthalamic nucleus or internal pallidum stimulation in young onset Parkinson’s disease, Brain 121, 451–457 (1998) 204 [5] A. L. Benabid, P. Pollak, D. Gao, et al.: Chronic electrical stimulation of the ventralis intermedius nucleus of the thalamus as a treatment of movement disorders, J. Neurosurg. 84, 203–214 (1996) 204 [6] M. H. G¨ otz, S. K. Fischer, A. Velten, J. F. Bille, V. Sturm: Computer-guided laser probe for ablation of brain tumours with ulrashort laser pulses, Phys. Med. Biol. 44, N119–N127 (1999) 207 [7] J. Wahrburg, K. U. Schmidt, M. H. G¨ otz, K. Kappings, S. G¨ olz: Concept of a novel laser probe for minimal invasive applications in neurosurgery, J. Mechatron. 6, 479–489 (1996) 207

Index

ablation threshold of bovine tissue, 203 active beam control, 207 brain tissue, 203 laser-beam delivery, 207 neuronavigation, 206 neurosurgery, 204

neurosurgical instrument, 205 Parkinson’s disease, 204 plasma-spark intensity, 208 removal of neural tissue, 203 stereotactic neurosurgery, 206

The Use of Femtosecond Technology in Otosurgery Burkard Schwab, Dietrich Hagner, J¨ org Bornemann, and Ralf Heermann Medizinische Hochschule Hannover, Klinik f¨ ur Hals-Nasen-Ohren-Heilkunde, Konstanty-Gutschow-Str. 8, 30623 Hannover, Germany [email protected]

1

Summary

Introduction: Laser applications within the tympanic cavity area are fully accepted. Commonly used systems are CO2 , argon-, KTP and erbium devices. The disadvantages are heat development and/or acoustic load of the inner ear. A new laser with ultrashort pulses was examined concerning its ablation characteristics and tested for possible applications in the tympanic cavity. Material and methods: Investigations on human ossicles and bovine compacta were executed in order to accurately determine the ablation parameters. This included measurements of the dependency of the threshold energy on the pulse duration and the determination of the ablation ratio using different pulse energy levels. On the basis of histological slices the thermal damages of the bone were examined. Additionally, the processed samples were analyzed with an optical microscope and with a scanning electron microscope in order to evaluate the quality of the perforations. Results: The measurements showed that the threshold energy has a lower level than the threshold energy of the conventional laser systems. At a pulse duration of 170 fs the smallest fluence with which an erosion can be achieved, is below 1 J/cm2 . With increasing pulse duration the necessary threshold energy also rises. Due to the low energy level necessary for ablation and the extremely short pulse duration, less thermal damage is induced to the surrounding bone tissue as compared to conventional laser systems. The analysis with a scanning electron microscope demonstrates the extreme precision of this laser system. The achieved accuracy of the incisions and drillings ranges in the micrometer area. Discussion: The femtosecond laser represents a further possibility of processing tympanic cavity structures efficiently and in a contact-free procedure. Due to its high precision and the reduced side effects an advantage in the handling of bony structures is to be expected in relation to other laser systems. Apart from the perforation of the stapes footplate, in particular the handling and modelling of the incus, a further field of applications includes enhanced coupling, e.g. for implantable hearing aids. F. Dausinger, F. Lichtner, H. Lubatschowski (Eds.): Femtosecond Technology for Technical and Medical Applications, Topics Appl. Phys. 96, 211–226 (2004) c Springer-Verlag Berlin Heidelberg 2004


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Introduction

Surgical procedures on the inner and middle ear require techniques that maximize precision while minimizing strain on the highly sensitive anatomic structures present. Mechanical stresses, which may, for example, be coupled into the sound-processing system owing to pressure exerted by surgical instruments, can cause irreversible damage to the inner ear. The use of clinically established laser systems has thus far offered therapeutic improvements in only a few applications, because the thermic and/or photoacoustic effects that occur may lead to intolerable stresses on functionally important structures. In surgery to improve hearing, femtosecond technology promises significantly higher precision and reduced side effects in the ablation of both soft and hard tissue. This opens up prospects for procedures such as the nondamaging treatment of ossicles, either where they are affected by adhesions or in order to couple them to electromechanical hearing devices in the middle-ear region. It is conceivable that application of femtosecond technology in the inner-ear region may allow surgery that enhances the coupling of stimulating electrodes to the auditory nerve. First, the key variables were investigated by using tissue samples to study laser–tissue interactions; appropriate laser parameters were established. Follow-up animal trials will hopefully corroborate these findings. In addition, functional studies were conducted using the experimental setup; these will be consulted in order to describe the risk of damage when applying femtosecond pulses directly to the middle and inner ear.

3 3.1

Results Determination of the Ablation Threshold

First, ablation thresholds were determined at various pulse durations in porcine compact bone. As porcine compact bone behaves very similarly to human auditory ossicles when irradiated with ultrashort pulses, investigations using ossicles were dispensed with (the provision and preparation of ossicles would also have been far more work intensive). The threshold energy was measured at different pulse durations between 130 fs and 1 ps and plotted against pulse duration (Fig. 1). In order to determine threshold energy the samples were treated using different pulse energies at a given pulse duration; they were then analyzed under a light microscope. The criterion taken to indicate that the threshold had been reached was the presence of changes to osseous structure that were just visible under microscopic magnification. In order to avoid problems with the focusing plane, sanded-down bones were used for these studies. It is clearly apparent that threshold energy is highly dependent on pulse duration; this also applies to the subpicosecond range. Extending the pulse duration increases the energy required for ablation.

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Fig. 1. Threshold energy in porcine compact bone as a function of pulse duration when using a 200 mm Achromat with a spot size of 0.0024 mm2

Fig. 2. Dependence of threshold fluence on pulse duration in porcine compact bone

Figure 2 shows this contingency as a log–log relation; the linear dependency between pulse duration and threshold fluence is striking. Bone material stored in NaCl was used for these measurements. The use of slightly dried bone has the effect of altering the threshold fluence. 3.2

Determination of Ablation Rates

The ablation rates for different pulse energies were determined at a pulse duration of 180 fs. The 200 mm Achromat, with a spot size of 0.0024 mm2, was again used for these measurements. For this purpose a defined number of pulses was applied to the bone: between 200 pulses (at high pulse energy)

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Fig. 3. Dependence of ablation depth on pulse energy at a pulse duration of 180 fs and a spot size of 0.0024 mm2

and 800 pulses (at low pulse energy) were used at a repetition rate of 10 Hz. The depth of the cavities produced in this way was then measured under a light microscope. If the cavities are too shallow, the imprecision of the microscope (which is only accurate to around 10 µm) results in an excessively high percentage error, whereas if they are too deep the cavity floor is very difficult to discern. The optimal ideal cavity depth for analysis is therefore 10 µm to 30 µm (see also [1]). For this reason the number of pulses was varied at different pulse energies. After measuring the cavities the ablation depth per pulse was calculated. Figure 3 shows that the ablation depth over the range of energies studied increases linearly with increasing energy. The error of the individual measurement points was below 10%; for the sake of clarity these additional data are not included in Fig. 3. 3.3

Model-Based Measurement of Pressure and Temperature

If femtosecond technology is to be suitable for use in otosurgery, it is crucial to determine laser parameters relevant to ablation and to compensate for any undesirable effects. Owing to the spatially confined anatomical conditions and the close proximity to sensitive nerve and sensory cells, it is vitally important to minimize the impact of thermal and photoacoustic effects in order to ensure operational safety. In terms of protecting the filigree structures of the inner and middle ear, two relevant factors are the total heat input into the system and the expansion of the thermally damaged zone directly into the marginal area of the zone

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Fig. 4. Plexiglas model of the inner ear with a target volume of 2.3 ml. For comparative purposes heat was introduced in the case of a femtosecond laser, a free-running Er:YAG laser and a CO2 laser in pulsedmode operation

of interaction between hard and soft tissue. Furthermore, it must be ensured that photoacoustic stress waves do not lead to the direct formation of cracks in tissue or to pressure transients being coupled into the inner ear, which may cause damage to the hair cells. Accordingly, the following aspects were studied: 1. Heat accumulation in a defined target volume when heat is introduced in the form of femtosecond pulses 2. The formation of a thermal damage zone in the area surrounding the ablation site in hard and soft tissue 3. The formation of pressure transients in fluid media 4. Photoacoustic effects in hard and soft tissue 3.4 Comparative Investigation into Heat Accumulation Using Femtosecond Pulses, a Free-Running Er:YAG Laser and a CO2 Laser For modeling purposes the cochlea was regarded as a fluid-filled target space (endolymph) embedded in a solid body with decelerated heat transfer (bone). The thermal adjustment of this open system when subjected to laser bombardment depends on laser-induced heat input and heat conduction to the environment; convection and radiation are relatively insignificant. For experimental purposes the temperature changes in the cochlea were simulated using a drilled PMMA body filled with 0.3 ml of water. The rise in temperature of the target space upon laser bombardment was registered with the aid of a temperature sensor (compensated NTC) (Fig. 4). The laser parameters used are given in Table 1. The temperature increase recorded over the duration of laser exposure is influenced by the mixing process within the measuring chamber and the emission of heat to the surroundings. The temperature reaches its peak when the input energy flow and the outward heat flux are equal. By means of double exponential adaptation, a mathematical model was devised that allows the temperature change to be accurately predicted. The peak value so determined (labelled “maximum temperature” in the curves in

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Table 1. Laser parameters Laser type

CO2 Er:YAG Femtosecond laser (100 fs)

nm

Focus diameter µm

Mean output power mW

Pulse energy mJ

Repetition rate Hz

10 640

300

2940

200

780

80

50 100 33 57 64 30 60 120

12.5 25 9 15 21 0.010 0.020 0.040

4 4 3.7 3.7 3 3000 3000 3000

λ

Fig. 5. Temperature change in H2 O with the Er:YAG laser (33 mW at 3.7 Hz; 57 mW at 3.7 Hz, 63 mW at 3 Hz)

Fig. 5, Fig. 6, and Fig. 7) allows a direct comparison between the heat input for the various lasers and settings. 3.5

Interpretation of Measurement Results

Figure 8 shows the extrapolated steady-state temperature increase as a function of input laser power for the laser types investigated. The gradient of the “averaging” line drawn between the points yields the heat input ratio for the various laser types (Fig. 8). From this it can be inferred that, for a given radiated power output, the fs laser releases only half the amount of heat emitted by the Er:YAG laser, so it appears that thermal damage to the surrounding tissue can be virtually ruled out.

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Fig. 6. Temperature change in H2 O with the CO2 laser, pulsed, repetition rate 4 Hz

Fig. 7. Temperature change in H2 O with the femtosecond laser (100 fs, 3 kHz)

3.6 Optimization of Ablation Rates in Relation to the Scan Algorithm If the scan algorithm is altered by varying the operational speed and line spacing, it can be seen that the ablation rate declines as both speed and spacing increase (see Fig. 9). A scanning electron micrograph (see Fig. 10) reveals the characteristics of the cavity floor at varied line spacing. The waffle-

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Fig. 8. Temperature increases using different laser systems

Fig. 9. Dependence of ablation rate on operational speed and line spacing (LS)

like structure can clearly be seen in the left-hand image at a line spacing of 80 µm. This “waffle effect” is far less marked with a closer spacing of 40 µm. This “waffle effect” disappears completely when line spacing is further reduced to 20 µm (see Fig. 11). 3.7

Spectral Analysis of the Laser-Generated Plasma

The ablated tissue can be monitored online by analyzing the plasma induced by laser ablation. An FFT analysis of the plasma of both ablated bone

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Fig. 10. SE micrographs of a cavity floor at line spacing of 80 µm (left) and 40 µm (right)

Fig. 11. Images depicting two floors of laser-generated cavities (upper images, 140-fold magnification) at higher magnification (lower images, left: 710-fold, right: 2800-fold). Line spacing 20 µm in all cases

and NaCl (as an approximate substitute for the perilymph leaking out behind the footplate) reveals marked differences in the analyzed spectrum (see Fig. 12). This method enables the ablated tissue to be directly monitored, which will be especially useful in terms of developing intelligent guidance for the laser system.

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Fig. 12. Spectral analysis of the laser-generated plasma for bone (left) and NaCl (right)

Fig. 13. Schematic representation of an inner-ear model to determine pressure as an in vitro parameter

3.8

Pressure Changes

The pressure load on the inner ear was determined in further model-based investigations (see Fig. 13 and Fig. 14). The femtosecond laser was operated at a repetition rate of 10 Hz. The pressure sensor, made of PVDF film and with a sensitive surface area of 1 mm2 , has a response time of 5 ns. Figure 14 shows the linear increase of pressure in relation to input energy. If the pressures generated by the femtosecond laser are compared with the pressure values that result upon application of the Er:YAG laser, which is currently used in middle-ear surgery, it is apparent that the pressure values are considerably reduced at energy input levels that are also substantially lower (see Fig. 15 and Fig. 16); both the energy values and the pressure pulse are one order of magnitude lower when the fs laser is used. Owing to the much lower energies required for tissue ablation when using

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Fig. 14. Linear increase in pressure in relation to pulse energy

Fig. 15. Peak pressure in relation to pulse energy using the Er:YAG laser, at varying focal distance from the measuring element

the femtosecond laser (also in the area of the threshold), the resulting pressure load on the inner ear can be reduced, thus avoiding potential damage. 3.9

Histological Evaluation of the Irradiated Soft Tissue

Histological examination of different soft-tissue types reveals virtually no thermal damage (such as a coagulation zone) in the area of laser penetration. The laser parameters used are: • Pulse energy: 130 µJ • Repetition rate: 3000 Hz • Mean output power: 400 mW

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Fig. 16. Peak pressure in relation to pulse energy using the femtosecond laser, at varying femtosecond pulse length

Fig. 17. Laser penetration into fatty tissue (HE staining)

Figure 17 shows laser penetration into the fatty tissue at the boundary layer between fat and muscle. Figure 18 shows laser penetration through muscle tissue at the laser parameters stated above. Here, too, virtually no thermal damage is discernible; all that can be detected is a damage zone only a single cell layer (approximately 5 µm) in thickness. The outcome is somewhat different following laser bombardment of nerve structures (here: infraorbital nerve of pig, see Fig. 19) at the following laser parameters:

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Fig. 18. Laser penetration through muscle tissue (HE staining)

Fig. 19. Laser penetration into nerve tissue (infraorbital nerve of pig, HE staining). Cracks running longitudinal to the nerve bundle are clearly visible in the vicinity of the laser penetration site (circle)

• Pulse energy: 33 µJ • Repetition rate: 3000 Hz • Mean output power: 100 mW Dissociations or disruptions can be seen running longitudinally at a distance of up to 1000 µm from the laser impact site; these may be attributable to a photoacoustic effect. Remarkably, evidence was obtained that the laser

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does indeed have a certain potential to damage sensitive structures; an effect that, in any case, necessitates further studies, especially given that the confined spaces in the inner-ear region are in the immediate vicinity of nerve structures (e.g. the first neuron of the cochlear nerve).

4

Discussion

Laser types currently used in middle-ear surgery are either based on the principle of continuous-wave technology or use relatively long pulse durations. In the latter case, however, the ablation process is dependent on the thermal and optical properties (such as thermal diffusion and absorption coefficient) of the material to be treated. The two parameters of wavelength and pulse duration therefore represent limiting factors. Owing to the low absorption of laser energy in water or bone within the visible green range [2], the KTP (Nd:YAG) laser (λ = 532 nm) or the argon laser (λ = 514 nm) appear to be suitable for middle-ear and stapes surgery, respectively, because the expected generation of heat does not occur. There is, however, the risk that deeper-lying structures will be damaged owing to poor absorption in water (or perilymph). Lasers with infrared wavelengths, such as the CO2 laser (λ = 10.6 µm) or the Er:YAG laser (λ = 2.94 nm), achieve a high absorption rate in bone and surrounding tissue [3, 4] although this may be accompanied by the generation of relatively large amounts of heat [5]. By contrast, when ultrashort pulses are applied the ablation process is virtually independent of both the material properties and the wavelength used, and the mechanical and thermal side effects are far less severe [6], since a large portion of the input energy is carried away with the ablated tissue. The total input energy used for ablation is considerably lower than with conventional laser systems. Whereas the Er:YAG laser requires total energy of 0.35 J to 0.75 J for footplate perforation, and the CO2 laser requires energy of 0.2 J to 1.8 J (depending on pulse duration) [7, 8], the energy values are of the order of millijoules when ultrashort pulses are applied. This leads in turn to a substantial reduction in undesirable tissue reactions. The technique of multiphoton ablation allows precise spatial definition of the ablation process, as it is entirely restricted to the focus area. The risk that direct laser bombardment will cause damage to deeper-lying structures (such as the utricle and saccule) following footplate perforation is thereby significantly reduced. Possible acoustic stresses, especially pressure-related phenomena, are currently the subject of further studies. Although this ablation is “cold”, thus minimizing side effects, this aspect does have a potential drawback: as temperatures do not rise during the ablation process, the coagulation effect – which is sometimes actually beneficial – does not occur. As a result, even minor bleeding, particularly from the soft-tissue sheath encasing the middle-ear structures, restricts the view of the operator and makes the surgical procedure more difficult. Here, the sur-

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geon must resort to conventional means of hemostasis (use of vasoconstrictive substances). A possible alternative would be to alter the laser’s operational parameters, allowing localized hemostasis. Our investigations demonstrated that the application of the femtosecond laser in middle-ear surgery presents a new and promising addition to the range of ultrashort wavelength lasers used for this purpose. The concept of femtosecond laser application combines the advantages of the various laser types while dispensing with their respective disadvantages. Further studies are currently in preparation; predominantly using animal models, these are aimed at reviewing the effectiveness and safety of femtosecond application. Given the growing interest in this new technology, and assuming that further advances are made, it can be expected that the system will undergo miniaturization, since clinical use is still prevented by both the size of the laser setup and the as-yet-unfilled requirements for beam-guidance systems.

References [1] S. Nolte: Mikromaterialbearbeitung mit ultrakurzen Laserpulsen (Cuvillier, G¨ ottingen 1999) in German 214 [2] G. M. Hale, M. R. Querry: Optical constants of water in the 200 nm to 200 mm wavelength region, Appl. Opt. 12, 555–563 (1973) 224 [3] S. G. Lesinski, A. Palmer: Lasers for otosclerosis: CO2 vs. argon and KTP-532, Laryngoscope 99, 1–8 (1989) 224 [4] C. W. Robertson, D. Williams: Lambert absorption coefficients of water in the infrared, J. Opt. Soc. Am. 61, 1316–1320 (1971) 224 [5] B. J. Wong, J. Neev, M. J. van Gemert: Surface temperature distributions in carbon dioxide, argon, and KTP (Nd:YAG) laser ablated otic capsule and calvarial bone, Am. J. Otol. 18, 766–772 (1997) 224 [6] J. Neev, J. S. Nelson, M. Critelli, J. L. McCullough, E. Cheung, W. A. Carrasco, A. M. Rubenchik, L. B. D. Silva, M. D. Perry, B. C. Stuart: Ablation of human nail by pulsed lasers, Laser Surg. Med. 21, 186–192 (1997) 224 [7] S. Jovanovic, U. Sch¨ onfeld, V. Prapavat, A. Berghaus, R. Fischer, H. Scherer, G. M¨ uller: Die Bearbeitung der Steigb¨ ugelfussplatte mit verschiedenen Lasersystemen. Teil 1: Kontinuierlich strahlende Laser, HNO 43, 149–158 (1995) 224 [8] S. Jovanovic, U. Sch¨ onfeld, V. Prapavat, A. Berghaus, R. Fischer, H. Scherer, G. M¨ uller: Die Bearbeitung der Steigb¨ ugelfussplatte mit verschiedenen Lasersystemen. Teil 2: Gepulste Laser, HNO 44, 6–13 (1996) 224

Index

bone tissue, 211

middle ear, 212

electromechanical hearing devices, 212

ossicles, 212

implantable hearing aid, 211

tympanic, 211

Subcellular Photodisruption Alexander Heisterkamp and Holger Lubatschowski Laserzentrum Hannover e. V., Hollerithallee 8, 30419 Hannover, Germany [email protected] Abstract. Using high numerical focusing and pulse energies close above threshold for optical breakdown, disruptive effects can be generated in a range below the diffraction limit. This effect can be used to cut tissue with a precision at the cellular level. This kind of cell surgery offers a new field of experimental investigations in cell biology.

Laser micromanipulation can be used in various applications at the cellular level. For example, single organelles, cytoskeletal filaments, chromosomes, flagella, mitochondria can be cut or altered in their functionality. Moreover, it is possible to perforate cell membranes by a laser beam, in order to induce cell fusion [1] or enable the transfer of foreign DNA into the cell, so-called optoporation or, in the case of a single cell, optoinjection. The high resolution for such a process at the cellular level was first realised by the use of UV-laser radiation [2]. By tightly focusing with high numerical microscope-objectives, holes of below a micrometer in diameter were drilled into the cell wall in order to achieve a gene transfection into living cells [3]. These laser microbeams were subsequently applied in the various mentioned fields [2, 4]. Combined with optical traps, laser tweezers, or laser pressure catapulting fully contactfree manipulation of cells was possible for the first time [1]. Figure 1 shows the application of such a laser microbeam for cutting out and analyzing a selected single Hep3B cell, which was fixed in paraffin.

Fig. 1. Laser microdissection and subsequent catapulting of Hep3B cells for subsequent p53 gene analysis by a UV-laser in the nanosecond regime [1] F. Dausinger, F. Lichtner, H. Lubatschowski (Eds.): Femtosecond Technology for Technical and Medical Applications, Topics Appl. Phys. 96, 227–233 (2004) c Springer-Verlag Berlin Heidelberg 2004


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Fig. 2. Excitation volume of fluorescence microscopy (a) in contrast to multiphoton microscopy (b)

However, the application of UV-laser beams has several disadvantages, resulting in photodamage caused by the high photon energy and high absorption of this radiation in biological samples. As a consequence, the survival rate of optoinjection by UV-laser beams, for example at 337 nm by a nitrogen laser, is as low as only a few percent [2, 3]. Additionally, the laser radiation is not able to cut structures inside the cell without possibility harming the cell membrane. Thus, the low penetration depth is another disadvantage of the UV-laser application. Similarly, these problems are prominent in fluorescence microscopy, (Fig. 2a). A considerable amount of the radiation is absorbed in front of and behind the focus spot, which leads to a lower axial resolution or damage and unwanted side effects outside the focus. In contrast, near-infrared laser radiation has a very high penetration depth in biological samples, due to the so-called optical window. If ultrashort laser pulses (100 fs to 200 fs) with wavelengths in the near infrared are focused very tightly by microscope objectives (0.9 NA or higher), precision at the cellular level can be achieved by nonlinear absorption. This absorption is very well confined to the focus region, due to the intensity dependence of the nonlinear effects (Fig. 2b). The applied pulse energies are in the range of only 0.5 nJ to 4 nJ. However, the first application of femtosecond pulses in the field of singlecell manipulation was for illumination purposes in multiphoton microscopy [5]. To achieve a higher resolution during the excitation of a sample and to reduce out of focus photobleaching or damage, a tightly focused ultrashort laser beam is used to induce the excitation by simultaneous absorption of photons. Thereby, only in a small volume at the focus region are the intensities high enough to reach the threshold for multiphoton processes (Fig. 2b). The axial resolution is several times higher when compared to conventional illumination and comparable to a conventional confocal laser scanning microscope [6]. Besides, the femtosecond pulses can be used to illuminate a variety of fluorophores simultaneously, ranging from the blue, with 461 nm (DAPI), to the red part of the spectra, 695 nm (Cy 5.5) [7].

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Fig. 3. Human chromosomes treated by 100 fs laser pulses at 800 nm, the linear cuts have a minimum diameter of 110 nm, the holes have diameters in the range of some hundred nanometers [8]

Fig. 4. Cut in a capillary endothelial cell by a 2 nJ pulse at a pulse duration of 100 fs at 800 nm. The mitochondrion in the square is cut by 1 s illumination by the ultrashort laser, see detail on the right. After treatment the cell remains alive [9]

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However, too high pulse intensities can be harmful to the cells and cause damage by the creation of considerable numbers of free electrons [10]. These free electrons can lead to oxidative reactions, which can cause cell damage or cell death [11]. K¨ onig et al. investigated the threshold for multiphotoninduced cell damage and found illumination intensities in the range of some megawatt per square centimeter up to several terawatt per square centimeter. At these intensities two different damage mechanisms are prominent: One, at relatively long timescales and moderate intensities of megawatt per square centimeter, leads to the mentioned photo-oxidation processes. Although the density of free electrons of 1012 cm−3 is considerably lower, possible bond breaking of DNA strands can be observed [12]. K¨ onig et al. found a P/t2 dependence of the damage threshold, consequently, photodamage is more pronounced at short pulse duration. The second damaging effect takes place at even higher light intensities, in the range of terawatt per square centimeter. At these intensities, an optical breakdown is created at the focus region, comparable to the loose focusing conditions mentioned in the Chapter “Surface Structuring” on the cutting of diamond and the material is ablated or fragmented. Similar to the interaction area in the fields of multiphoton microscopy, the breakdown site can be quite small and lead to very precise effects, even below the diffraction limit of the laser beam. This kind of cell surgery was first performed by K¨ onig et al., by raising the pulse energy of the multiphoton illumination source up to the nanojoule level. As can be seen in the atomic force microscopy image in Fig. 3, the processing of single chromosomes becomes possible, offering resolutions of about 100 nm. The image shows several cuts through human chromosomes, having a minimum cutting size of 110 nm. In the lower part of Fig. 3 several chromosomes with a hole drilled into them with diameters in the 100 nm range are shown. The minimum volume of ablated material was approximately 0.008 µm3 at an exposure time of 1.3 ms, which represents roughly 10 000 laser pulses. At similar pulse durations but at kHz repetition rates the group of Eric Mazur showed the application of ultrashort laser pulses to dissect single mitochondria in living cells (Fig. 4) [8, 9]. The picture was taken by a fluorescence microscope, through which the ultrashort laser beam was focused onto the targeted cell. The epithelial cell remained alive after the processing by the laser. In the fields of optoinjection or gene transfection, the ultrashort laser pulses offer very high precision in drilling holes into the cell membrane, combined with survival rates of 100% irrespective of cell type [13]. Although many authors believe that the optical breakdown is a plasmamediated effect [14], some questions remain. As a main point, the shown cuts and holes are achieved by a highly overlapped and repetitive application of thousands of laser pulses, although only a very small volume is ablated. Similarly to the processing of waveguides or other structures in solid transparent

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materials, a kind of cumulative effect may play an important role in the ablation process at the cellular level. Moreover, it was up to now not possible to measure any kind of mechanical or pressure effect near the focus while ablating tissue at these high focusing conditions. Investigations of the mechanical damage of the processed cells could be determined by AFM microscopy, the shock zone, i.e. the pressure-affected area at the cellular level, for a 2 nJ 100 fs pulse was shown to be below 1 µm [9]. As was already mentioned, the plasmamediated ablation at loose focusing always occurs with a mechanical effect inside the tissue and the following creating of a gas and vapor-filled bubble, which is completely missing or has not yet been observed at high numerical processing. The effect of the high density of free electrons near the focal area is also not clear yet. Fs radiation alos altered tissue around the focus, which appeared as darkened areas under light and electron microscopy [15]. These streaks are probably due to filamentation of some parts of the beam, creating higher electron densities, which might lead to chemical reaction near the focus. In particular, these high electron densities can possibly induce chemical changes inside the cells and consequently lead to cell death [14]. The application of femtosecond lasers in cell surgery and micromanipulation is a growing field of interest and many applications are feasible. However, the underlying mechanisms of femtosecond tissue interaction at the cellular level are still to be investigated and remain an exciting field of research.

References [1] K. Sch¨ utze, H. P¨ osl, G. Lahr: Laser micromanipulation systems as universal tools in cellular and molecular biology, Cell. Mol. Biol. 44, 735–746 (1998) 227 [2] M. W. Berns, J. Aist, J. Edwards, K. Strahs, J. Girton, P. McNeill, J. B. Rattner, M. Kitzes, M. Wilson-Hammer, L. H. Liaw, A. Siemens, M. Koonce, S. Peterson, S. Brenner, J. Burt, R. Walter, P. J. Bryant, D. van Dyk, J. Coulombe, T. Cahill, G. S. Berns: Laser microsurgery in cell and developmental biology, Scinece 213, 505–513 (1981) 227, 228 [3] Tsukakoshi: A novel method of DNA transfection by laser microbeam cell surgery, Appl. Phys. B-Lasers O. 35, 135–140 (1984) 227, 228 [4] K. O. Greulich: Micromanipulation by Light in Biology and Medicine (Birkh¨ auser, Basel 1999) 227 [5] W. Denk, J. H. Stricker, W. W. Webb: Two-photon laser scanning fluorescence microscopy, Nature 248, 73–76 (1990) 228 [6] W. Denk, D. W. Piston, W. W. Webb: Two-photon molecular excitation in laser-scanning microscopy, in Handbook of Biological Confocal Microscopy (Plenum, New York 1995) pp. 445–458 228 [7] K. K¨ onig: Laser tweezers and multiphoton microscopes in life sciences, Histochem. Cell. Biol. 114, 79–92 (2000) 228 [8] K. K¨ onig, I. Riemann, W. Fritzsche: Nanodissection of human chromosomes with near-infrared femtosecond laser pulses, Opt. Lett. 26 (2001) 229, 230

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[9] N. Shen: Photodisruption in Biological Tissues Using Femtosecond Lasers, Ph.D. thesis, Harvard University (2003) 229, 230, 231 [10] A. Vogel, J. Noack: in Proc. SPIE 4260 (2001) numerical simulations of Optical Breakdown for Cellular Surgery at Nanosecond to Femtosecond Time Scales 230 [11] V. Shafirovich, A. Dourandin, N. Luneva, C. Singh, F. Kirigin, N. Geacintov: Multiphoton near-infrared femtosecond laser pulse induce DNA damage with and without the photosensitizer proflavine, Photobiol. 69, 265–274 (1999) 230 [12] U. K. Tirlapur, K. K¨ onig: Femtosecond near-infrared laser pulse induced strand breaks in mammalian cells, Cell. Mol. Biol. 47, OL131–134 (2001) 230 [13] U. K. Tirlapur, K. K¨ onig: Targeted transfection by femtosecond laser, Nature 418, 290–291 (2002) 230 [14] A. Vogel, J. Noack, G. H¨ uttmann, G. Paltauf: Femtosecond-laser-produced low density plasmas in transparent biological media: A tool for the creation of chemical, thermal and thermomechanical effects below the optical breakdown threshold, in Proc. SPIE 4633A (2002) 230, 231 [15] A. Heisterkamp, T. Ripken, T. Mamom, W. Drommer, H. Welling, W. Ertmer, H. Lubatschowski: Nonlinear side effects of fs pulses inside corneal tissue during photodisruption, Appl. Phys. B-Lasers O. 74, 1–7 (2002) 231

Index

cell, 229, 230 cell membrane, 228 cell surgery, 227, 230 chromosomes, 229, 230 confocal laser scanning microscope, 228 DNA, 227 DNA strands, 230 fluorescence microscope, 230 fluorescence microscopy, 228 gene transfection, 230 laser microbeam, 227 laser pressure catapulting, 227

laser tweezer, 227 microdissection, 227 microscope, 228 mitochondria, 230 mitochondrion, 229 multiphoton microscopy, 228 nonlinear absorption, 228 optical breakdown, 227 optoinjection, 228, 230 photobleaching, 228 subcellular photodisruption, 227

Generation of X-Rays by Intense Femtosecond Lasers Heinrich Schwoerer Friedrich-Schiller-Universit¨ at Jena, Institut f¨ ur Optik und Quantenelektronik, Max-Wien-Platz 1, 07743 Jena, Germany [email protected] Abstract. Nowadays the most intense lasers, working in the near-infrared wavelength range, are able to generate incoherent X-ray radiation from the soft X-ray region up to hard γ-rays. The X-ray radiation is not emitted by the laser itself but by a hot plasma that is produced by the interaction of the laser light with matter. Laser-produced X-rays all have in common an ultrashort duration, a small source size and an extremely high intensity. These properties qualify them for a variety of scientific and technological applications. This Chapter intends to discuss the mechanisms responsible for the generation of X-rays with femtosecond lasers and will highlight some of their most interesting applications.

1

Laser Light and X-Rays: An Introduction

How can visible laser light generate X-rays? The energy of visible and nearinfrared laser photons is between about one and two electronvolt, whereas X-ray photons have energies of thousands or millions of electronvolt. What are the mechanisms responsible for this tremendous upconversion of photon energy? How efficiently can this be accomplished and what kind of laser sources are required? What are the properties of laser-generated X-rays that make them unique? In what aspects and applications are they superior to conventional X-ray sources, like the tube at the dentist’s surgery? We will try to answer these questions in this Chapter, starting with a discussion of the basic physics of laser-generated X-rays in Sect. 2. Since the X-ray photon energy of laser-generated X-rays is mainly determined by the intensity of the laser light and since this intensity is high compared to the intensity that conventional femtosecond laser systems deliver, we will dicuss the current frontier high-intensity laser systems and we will learn, on this occasion, that these lasers are surprisingly simple (Sect. 3). We will then classify laser-generated X-rays on the basis of their generation mechanism that will also reflect a classification in photon energies. We start with laser-generated extreme ultraviolet radiation and its potential applications in lithography (Sect. 4.1), then cover X-ray line radiation in the keV range (Sect. 4.2) and end with the most recent development in laser generation of high-energy radiation, which is bremsstrahlung generation with photon energies in the range of giant nuclear resonances (Sect. 4.3). Thereby we will restrict ourselves to F. Dausinger, F. Lichtner, H. Lubatschowski (Eds.): Femtosecond Technology for Technical and Medical Applications, Topics Appl. Phys. 96, 235–254 (2004) c Springer-Verlag Berlin Heidelberg 2004


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incoherent X-ray sources and exclude X-ray lasers as well as high-harmonic generation from the discussion, since the applications of the latter will be purely scientific for a long time to come. In a well-intended contrast to other chapters of this book we will not treat femtosecond technology that is close to industrial application and commercial availability, but we try to lead the reader through modern aspects of applied femtosecond science, like ultrashort wavelength lithography, dose reduction in radiology, movies of molecular motion and transmutation of radioactive nuclei. Some of this is closer to reality, some is closer to the imagination, but only the combination of both can generate new ideas and new solutions of current scientific questions, and will finally evolve into smarter technology and better products.

2

From Laser Light to X-Rays: Basic Physics

The conversion of visible photons into X-ray photons requires mediation by electrons. Electrons can acquire enough energy within an ultrashort laser pulse to generate hard X-ray radiation. However, due to the high degree of upconversion the description of the laser light as consisting of photons with energies of a few electronvolt (eV) is replaced by the description in terms of amplitudes of the electric and magnetic field of the light wave. X-rays, in general, arise either by ionization of atoms or by inelastic scattering of fast charged particles, e.g., electrons by nuclei. The first process yields X-ray line radiation from the recombination and de-excitation of the ionized atoms, the latter results in continuous bremsstrahlung, see Fig. 1 and Fig. 2. Line emission is usually categorized into plasma radiation, generated by recombination in outer shells of multiply ionized atoms and cold line emission from inner shell holes of atoms, typically in the solid phase. Plasma radiation has energies in the 10 eV to 100 eV range (corresponding to the wavelength range ∼ 100 nm to 10 nm), whereas inner-shell radiation has photon energies of 1 keV to 100 keV (∼ 1 nm to 10 pm). Bremsstrahlung photons are the most energetic photons with energies up to many tens of megaelectronvolt. The generation of either of these X-rays first needs an electron of at least the energy of the X-ray photon. For this simple reason we will summarize in the following section the fundamental acceleration mechanisms of electrons in strong light fields as they are dominant at increasing light intensity, see Fig. 3 and [1]. 2.1

Multiphoton and Field Ionization

The first step in the electron acceleration is its ionization from the atom. At light intensities around 1010 W/cm2 atoms start to be ionized by multiphoton processes. For outer-shell electrons typically two to four laser photons are

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Fig. 1. Generation processes and applications of femtosecond-laser-produced X-rays. In the focus of an intense laser pulse a hot plasma is produced by ionizing and accelerating electrons. X-rays arise in the plasma from recombination in the outer shells of ionized atoms (left), in solid targets from inner-shell excitation (middle) and from inelastic scattering on nuclei (Bremsstrahlung). These laser-produced X-rays can be applied in lithography, in radiology and in isotopetransmutation technology

Fig. 2. Ionization mechanisms (left) and origins of X-rays (right). The thick line symbolizes the Coulomb potential of electrons in an atom. Left: Bound electrons can be ionized by multiple absorption of photons (multiphoton ionization, MPI), by tunneling through the barrier, if it is lowered by the external electric field of the laser light (tunneling ionization, TI) or by field ionization if the potential is reduced below their energy level during a half-cycle of the light wave (field ionization, FI). Right: Plasma radiation arises from de-excitation in outer shells of ionized atoms, K-shell radiation from recombination of K-shell holes and bremsstrahlung from inelastic scattering of electrons on the atom

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required for ionization. In the absence of an external field the probability of direct (“geminate”) recombination is very high. At slightly higher intensities, however, the electric field of the light wave is strong enough to lower the potential energy asymmetrically during one half-cycle of the laser pulse. Now the electrons can tunnel through the (new) barrier (see Fig. 2). At even higher intensities the external electric field completely overcomes the binding field of the electron in the atom. This process is referred to as field ionization and dominates the laser–matter interaction above 1014 W/cm2 [2]. 2.2

Collisional Heating

Since these mechanisms take place within the rising edge of even stronger laser pulses, we can now deal with the acceleration of free electrons in the strong electromagnetic wave. The oscillation or quiver energy of electrons in the electromagnetic field of frequency ω is given by e2 E02 , (1) 4mω 2
where E0 = 2I/c0 is the amplitude of the electric field, I its intensity, 0 the dielectric constant, m the electron mass and c the speed of light. Wq linearly scales with the light intensity and amounts to 5 eV at 1014 W/cm2 or 5 keV at 1017 W/cm2 at a wavelength of 1 µm. These electrons are again fast enough to ionize further electrons by collisions. These are again accelerated. Depending on the density of the target so many electrons are produced that they cannot be treated as free particles any longer but must be treated as a plasma. In consequence, the electrons undergo collisions and thermalize to an equilibrium state, see, e.g., [2]. This energy transfer from the laser field into the plasma is called collisional heating or inverse bremsstrahlung (photon energy to electron energy instead of vice versa). It is the dominating process up to intensities of 1016 W/cm2 . In laser plasma or laser–matter experiments with nanosecond laser pulses, the plasma is heated to an equilibrium state with a temperature that can be estimated by Wq =

 Te ≈

I ne

2/3

m1/3 ≈ 3 × 107 kB



I(W/cm2 ) ne

2/3 eV ,

(2)

see, for example, [3]. The collisional regime is the regime of conventional femtosecond laser–matter interaction that is covered in the other contributions in this book. 2.3

Collective Absorption Mechanisms

At intensities above 1017 W/cm2 a new class of mechanisms, the collective processes, begin to dominate the laser–plasma interaction. If the density of

E U V lith o g ra p h y

p la s m a th re s h o ld

p la s m a ra d ia tio n 1 0 e V - 1 k e V E U V

m u ltip h o to n io n iz a tio n

m e d ic a l im a g in g

1 0

c o llis io n a l h e a tin g fie ld io n iz a tio n

1 2

m ic ro s c o p ic d y n a m ic s

239

Is o to p e tra n s m u ta tio n

B re m s s tra h lu n g (M e V )

K a ra d ia tio n (5 - 3 0 ) k e V

g

re s o n a n c e a b s o rp tio n p o n d e ro m o tiv e a c c e le ra tio n re la tiv is tic e le c tro n s p o n d e ro m o tiv e s e lf fo c u s in g re la tiv is tic s e lf fo c u s in g

K e rr s e lf fo c u s in g

o p tic s

p h y s ic s

x -ra y s

a p p lic a tio n s

Generation of X-Rays by Intense Femtosecond Lasers

1 4 1 6 lo g ( L a s e r in te n s ity

/ W /c m

1 8 2 )

2 0

2 2

Fig. 3. Summary of laser–atom and laser–plasma interaction, X-ray generation and applications of laser-produced X-rays as they take place at light intensities between 1010 W/cm2 and 1020 W/cm2

the plasma is high enough and the Debye shielding length is small enough, plasma waves can develop. Both conditions are easily accessible in intense laser-produced plasmas. The electric field of the electromagnetic wave displaces the electrons of the plasma transversely with aspect to the propagation direction of the laser light. The plasma reacts on this push through Coulomb repulsion of the electrons. A density modulation evolves that propagates through the plasma oscillating with the eigenfrequency

(3) ωP = ne e2 /0 me = 5.7 × 104 ne (cm−3 ) Hz . The plasma frequency ωP is only determined by the plasma density ne . If now the laser frequency ωL is in resonance with ωP , which is the case if ne = ncr = 0 me ωL2 /e2 with ncr the critical density, the energy transfer from the light into the plasma wave can be very efficient. Since ωL is usually fixed, the plasma density must be chosen correctly to fulfil the resonance condition. This requirement sounds more difficult than it is in real laser– plasma experiments, since the temporally leading edge of the strong laser pulse generates a plasma on the surface of the target that rapidly expands into the volume in front of the target. The density profile of this preplasma decreases exponentially and at some point the electron density is just the critical density ncr [2]. The optimum geometry for this so-called resonance absorption is an incident angle of 45◦ and p-polarised light: The laser pulse penetrates the underdense plasma up to ncr , deposits most of its energy there by exciting a plasma wave and is reflected back out. The plasma wave

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penetrates into the overdense regime and is damped due to the off-resonance condition for ne > ncr . In total, the plasma is heated very efficiently. 2.4

Relativistic Electron Acceleration

At an intensity of IL = 3 × 1018 W/cm2 and a laser wavelength of λ = 800 nm the quiver energy Wq of an electron in the light field reaches its rest mass of 511 keV/c2 . Since now the electron velocity approaches the speed of light, the interaction with an electromagnetic wave enters a completely new regime: the magnetic force of the wave becomes as large as the electric force. This induces a forward acceleration of electrons due to the v/c × B term in the Lorentz force, which is superimposed on the transverse oscillation due to the transverse electric force. Electrons are driven along the direction of laser propagation with an energy of the order of the quiver energy and even higher at ultrarelativistic intensities. An ultrashort and focused laser pulse is constrained in all spatial dimensions, longitudinal by its duration, transversal through the beam waist. Since the electromagnetic forces are stronger in the center of the pulse than on its wings, electrons do not oscillate back to the center once they have left it. The potential behind this effect is called the ponderomotive potential of the laser pulse. The ponderomotive force is proportional and parallel to the gradient of the light intensity. Since the pulse propagates through the plasma with a velocity close to the vacuum speed of light the main effect of the ponderomotive force is a forward acceleration of the electrons. The energy distribution of these hot electrons can be described as Maxwellian with a temperature Te given by [4]   1/2 Iλ2 −1 . (4) kB Te  0.511 1 + 1.37 × 1018 W · cm−2 · µm2 In particular, at the highest intensities achievable with current state-of-theart tabletop laser systems of 1021 W/cm2 , the electron temperature reaches the giant resonance energies of most nuclei and thus opens up a revolutionary new aspect of laser physics, which is the induction of nuclear reactions by lasers [5]. We will come to this at the end of this Chapter. To come back to X-rays: the photon energy of X-rays generated by these electrons follows the electron energy by less than an order of magnitude, through all the range from hundreds of electronvolt up to tens of megaelectronvolt [6]. Furthermore, since the laser source is ultrashort in time and ultrasmall in size the X-ray duration is again ultrashort and the source size is small [7, 8, 9]. These two properties make laser-generated X-rays superior to conventional X-ray sources in many aspects – as we will see in Sect. 4. But before we discuss applications of femtosecond-laser-generated X-ray sources we will briefly describe the laser systems currently used in these experiments.

Generation of X-Rays by Intense Femtosecond Lasers

3

241

High-Intensity Femtosecond Lasers

Looking at Fig. 3 we realize that light intensities of at least 1016 W/cm2 are necessary for the generation of inner-shell X-rays and 1019 W/cm2 for laserproduced megaelectronvolt bremsstrahlung. These enormous light intensities can only be reached by compressing a reasonable amount of energy into an ultrashort and perfectly focused laser pulse. Typical state-of-the-art laser systems deliver 5 mJ within 50 fs into a spot of ∼ 10 µm diameter for inner-shell radiation or 1 J within 100 fs into 5 µm2 for hard bremsstrahlung. Lasers with the first characteristics will become commercially available in the near future even though they still require well-experienced users. Lasers of the second type are individually developed and they still remain highly sophisticated items [10]. The technological challenge of femtosecond X-ray lasers is the large pulse energy rather than the short duration, which can easily be generated with simple fiber lasers or Kerr-lens mode-locked systems. The laser active medium has to have a huge storage capacity within a large bandwidth to support ultrashort pulses containing sufficient energy. In particular, this requires largeaperture laser media with high optical quality, high damage threshold and small nonlinear coefficients to avoid self-phase modulation of the laser pulse or optical damage during the amplification process. Furthermore, the absorption wavelength should match with the wavelength of existing high-power pump lasers. The most frequently used material fulfilling these demands is titanium-doped sapphire that can be pumped by frequency-doubled Nd:YAG or Nd:YLF lasers and that is available as single crystals up to a few centimeters in diameter and many centimeters in length. Recently, also rare-earthdoped glasses, pumped by high-power diode lasers, have proven to generate ultrashort pulses with Joules of energy. For thermal reasons and for pump-laser restrictions the repetition rate of these laser systems is typically limited to several kHz for the low-energy lasers and to tens of Hz for the high-energy systems. Within the FST program the University of M¨ unster, Germany, developed a laser system that delivers 5 mJ within 50 fs at 1 kHz repetition rate. It can be focused to intensities slightly above 1017 W/cm2 and was proven to be able to generate Kα -radiation of light metals as nickel, titanium or copper (4.5 keV to 8.9 keV) [11]. A similar system was set up at the Max-Born-Institut in Berlin, again within the FST program, which is used for the generation of gallium Kα -radiation at 9.2 keV. Both systems rely on chirped-pulse amplification. They consist of an ultrashort pulse oscillator, a stretcher-compressor combination, a regenerative amplifier and one multipass amplifier stage. Figure 4 displays the schematic setup of a laser systems that is able to deliver intensities of more than 1020 W/cm2 . It basically starts with the abovedescribed system, increases the beam diameter to several square centimeter, adds one more amplification stage and reduces the repetition rate to 10 Hz. It finally generates laser pulses with 1 Joule of energy within 70 fs, which can

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s tre tc h e r N d :Y V O

1 5 0 p s

5 W

c w

4

fs - o s c illa to r

1 0 n J 4 5 fs 8 0 M H z

p u ls e p ic k e r 1 0 H z

is o la to r

N d :Y A G 5 0 0 m J , 1 0 H z

2 2

c o m p re s s o r

P Z

T iS a

p r e p u ls e

P Z l /2

2 m J N d :Y A G 5 0 0 m J , 1 0 H z

1 0 20 W /c m o n 5 m m

f/2 v a c u u m

r e g e n e r a tiv e a m p lifie r N d :Y A G 4 0 m J , 1 0 H z

ta rg e t

E = 0 ,9 J , t = 7 5 fs , 1 0 H z l = 8 0 0 n m , D l = 1 6 n m O = 7 0 m m

l /2

4 - p a s s - a m p lifie r T iS a

1 ,2 J

p o la r iz e r

3 0 0 m J

T iS a

3 - p a s s - a m p lifie r

N d :Y A G 5 J , 1 0 H z

Fig. 4. General setup of a high-intensity laser system for the generation of hard bremsstrahlung. It relies on the chirped-pulse amplification technique. It consists of a Kerr-lens mode-locked oscillator, a stretcher to reduce the intensity during amplification, three amplification stages and a vacuum compressor. The first amplification stage is either a regenerative amplifier or a multipass design. The amplifiers are usually pumped by frequency-doubled Nd:YAG lasers at repetition rates of a few kilohertz or 10 Hz, depending on the final output energy. The beam size is enlarged in between the amplifier stages for a maximum energy extraction and for the prevention of optical damage of the components. The low-energy kilohertz systems typically require only two amplifiers

be focused down to 5 µm2 . These systems, one of which is operated at the Jena University, have emerged from years of sophisticated laser development and optimization. It is still large in volume, the described system is still a three-tabletop laser. But even though they are optimized in all conceivable details, they are finally still based on well-known regenerative or multipass amplification technologies. Due to the experience gained with these high-end lasers and due to the emergence of powerful and reliable diode-based pump lasers, real tabletop and turnkey lasers producing 1020 W/cm2 and more will be developed within the next few years.

4

Laser-Generated X-Ray Radiation

With the knowledge on electron acceleration in intense light fields and the different mechanisms of X-ray generation we can now discuss applications of this new technology. The applications are listed by increasing photon energy, starting in the extreme ultraviolet (EUV) with an application in lithography, followed by Kα -radiation in the multi-keV range for medical and scientific purposes and ending with hard bremsstrahlung for the generation of radioactive nuclei and the transmutation of isotopes from the nuclear-fuel cycle.

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Fig. 5. General setup for laser-generated EUV generation from a mass-limited target. The short pulse laser is focused, for example, on liquid droplets of a few micrometer in diameter. The produced plasma emits preferentially radiation around 13 nm, which is collected by a large aperture, reflective condenser optic. The light is then used to image a mask onto the wafer. Since EUV radiation is strongly absorbed in matter all optics have to be reflective, in contrast to present technology, and the whole stepper has to be evacuated

4.1

Plasma Radiation

One of the difficult decisions in the development of the next generation of short-wavelength lithography is the choice of the light source. Laser-generated EUV radiation is one of two proposals together with discharge plasmas. The technical requirements for a light source for the use in a lithography stepper system are manifold, see, for example, [12]: The wavelength has to be 13 nm, because only there can high-reflective optics be produced. The source volume has to be small and the angular distribution of the collected light has to be homogeneous, in order to support the very precise imaging of a mask onto the wafer. The shot-to-shot stability must be excellent and the repetition rate should be in the multikilohertz range, in order to allow fast and controlled processing. The average 13 nm output power should lie in the kilowatt range, even though that number has changed several times (upwards) in recent years. The source should not produce matter, like, for example, fast particles, or radiation that degenerate the nearby expensive condenser optics. And finally the whole system has to be service free for a long period of time and it has to be profitable, which means that the wall plug efficiency has to be as high as possible. On the other hand, the light has not to be coherent as in the present lithography technology using excimer lasers. Laser-based EUV sources almost fulfil these requirements. Figure 5 shows a schematic of a laser EUV source: It consists of a short-pulse laser system and a fast regenerating target. The target atoms must have a strong and narrowband ionic transition at 13 nm, like, for example, the 4d → 2p transition

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Fig. 6. Optimization of conversion efficiency from laser light into EUV radiation using short-pulse lasers and mass-limited targets. Left: Conversion efficiency versus laser-pulse duration: picosecond pulses are favorable over nanosecond and femtosecond pulses [13]. Right: Enhancement of conversion efficiency through introduction of a laser prepulse. The prepulse generates an expanding plasma. The main laser pulse heats the plasma, if optimum absorption conditions are achieved. Enhancement factors of more than 10 are obtained [14]

of oxygen5+ , transitions in highly ionized xenon or zinc. In the case of oxygen the target can be just water in the shape of small droplets or a jet. In the case of xenon a liquid jet is discussed, zinc can be used in a solution again as droplets or as a jet. The problem of degradation of nearby optics by fast particles can be diminished, if the whole target is evaporated by the laser pulse. For this reason mass-limited targets, such as droplets, are preferred to extended solids. Using these mass-limited targets the conversion efficiency from laser light into EUV radiation strongly depends on the duration of the laser pulse for simple plasma-physics arguments: Light is efficiently absorbed at plasma densities close to the critical density of the used laser wavelength (e.g., ncr = 1.1 × 1021 cm−3 at 1 µm). At higher densities most of the light is reflected as from a metal surface, at lower densities the light propagates through the matter basically unattenuated. Long laser pulses in the ns range generate a plasma with their leading part that expands rapidly below the critical density and out of the focal area. The trailing edge of the pulse is not absorbed in the thin matter and therefore the conversion efficiency is small. On the other hand, short pulses in the fs range are mainly reflected at the high density of the sample because there is no time left for expansion to ncr . Somewhere in between, around tens of ps, the optimum compromise between expansion and pulse duration can be found [13]. From the discussion, it follows that a pair of ultrashort pulses is the ultimate solution to maximize the conversion efficiency: The first pulse generates the plasma, the second pulse heats the plasma just when it has expanded to the critical density [14]. This concept is independent of the atom used, it simply follows from plasma properties. Only the specific energy has to be

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Fig. 7. Left: Spectrum of laser-generated titanium Kα - and Kβ -radiation. Right: Intensity dependence of the Kα yield: The maximum yield is achieved at a laser intensity corresponding to electron temperatures slightly higher than the K-shell ionization energy of 5 keV (for titanium). The slow rise of the yield at high intensities is a relativistic effect [15]

adapted to the substance, since it has to be heated to the desired ionic state. Energy-conversion efficiencies from laser light into 13 nm in-band EUV radiation of up to 2% were achieved by using ultrashort laser pulses. The emitted EUV radiation was proven to be perfectly isotropic, the source size can be smaller than 100 µm in diameter, the repetition rate can, in principle, be hundreds of kilohertz by using a fast droplet or a jet source and multiplexing the laser systems, which might even solve the output-power requirement. This example of femtosecond-laser-generated plasma radiation shows one potential application of this new technology. The small source size of the radiation and the ultrashort and therefore optimum energy-transfer process make femtosecond-laser sources superior to long-pulse laser sources in the context of imaging technology. 4.2

Ultrashort Kα -Pulses

Femtosecond-laser-generated inner-shell radiation, in particular Kα -radiation, has the property of being ultrashort. A K-shell hole has a lifetime between one and a few tens of femtoseconds depending on the element. This opens up the possibility to use these ultrashort and monochromatic X-rays in timeresolved spectroscopy of dynamic processes in solids or molecules. If elements with atomic numbers around Z ≈ 25 are used, e.g. titanium Z = 22, E(Kα ) = 4.5 keV or copper Z = 29, E(Kα ) = 8 keV, the wavelength of the radiation perfectly fits interatomic distances and therefore allows for efficient Bragg refraction. The generation of Kα -radiation as it depends on the laser intensity and the electron temperature was thoroughly investigated by Eder et al. and Ewald et al. [9,15]. Figure 7 shows the variation of the Kα yield with the laser

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intensity on a thin titanium target. The yield rises with intensity or equivalent with electron temperature to a maximum around 3 × 1017 W/cm2 , reaching a maximum photon yield of 1012 photons per 200 mJ laser shot. The yield drops to a minimum around 2 × 1018 W/cm2 and then starts rising again for even higher intensities. The explanation can be found in the K-shell ionization cross section for electrons: The cross section is peaked at electron energies of a few times the K-shell ionization energy. 3 × 1017 W/cm2 corresponds to a ponderomotive electron temperature equivalent to 50 keV. The yield drops at higher energies because the interaction time with the atom decreases with higher electron velocity. Above 1018 W/cm2 , however, where the electrons become relativistic, the electric field of the electron is Lorentz-contracted and therefore affects the atom stronger. This leads to the logarithmic increase of the yield at relativistic laser intensities. A detailed analysis of the Kα yield versus laser intensity confirms this argumentation, see [15]. Recent measurements of Pretzler et al. [8] reveal a small source size of the Kα radiation in the range of the laser focal area at relativistic electron energies, which is reasonable since megaelectronvolt electrons are not deflected substantially within a thin foil target. The generation of ultrashort K-shell radiation by compact kilohertz laser systems was successfully demonstrated by several groups. Within the FST-program, Hagedorn et al. [11], for example, developed a source of titanium up to zinc Kα -radiation with kilohertz repetition rate. The temporal duration of the Kα pulses cannot be measured analogous to nonlinear optics in the visible wavelength regime, because nonlinear coefficients for two-photon processes in the X-ray regime are too small. On the other hand, a classical pump-probe experiment with a visible ultrashort pump pulse and an X-ray probe pulse can be performed. The visible laser pulse is focused onto the surface of a semiconductor crystal and induces, for example, an ultrafast melting process. The X-ray probe pulse is Bragg scattered at the semiconductor lattice. The anisotropic Bragg reflection is monitored as a function of the relative delay between visible and X-ray pulses. The signal drops if the X-ray pulse probes the surface after being pumped by the visible pulse. Analysis of the experiment reveals an X-ray duration between 200 fs and 500 fs for 100 fs laser pulses [16]. The mentioned ultrafast melting process, the so-called nonthermal melting, is also of interest beyond methodology. For the first time nuclear motion can be directly monitored in real time in contrast to visible femtosecond spectroscopy, which indirectly detects nuclear dynamics by probing electronic wave functions. In general, the vis-pump–x-probe technique can be applied to the observation of a variety of dynamical processes like shock waves, phonons and phase transitions in the solid phase. Due to the intrinsic temporal and spatial resolution on the molecular scale, it will serve as a powerful tool to understand dynamical properties of fundamental chemical processes.

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tim e g a te d d e te c to r

s c a tte rin g m e d iu m b a llis tic

u ltra s h o rt x -ra y p o in t s o u rc e

s c a tt e re d

a b s o rb in g m a tte r tim e in te g ra te d s ig n a l

b a llis tic s ig n a l

s c a tte re d s ig n a l

Fig. 8. Time-gated X-ray imaging for contrast enhancement. Photons from the laser-generated, point-like and ultrashort X-ray source propagate through a scattering medium with an absorber inside. The ballistic photons produce a sharp shadow of the absorber on the detector. Scattered photons wash out this image. By time gating the detector the ballistic photons can be discriminated, see, e.g., [17]

To summarize, the exceptional properties of femtosecond-laser-generated Kα -radiation: 1012 photons at 4.5 keV and a spectral width of dλ/λ = 0.001 can be generated within less than half a picosecond out of a volume of 10 µm3 . Femtosecond inner-shell X-rays can also be used in medical applications, by far the most extensive X-ray application. Due to the ultrashort duration of the X-ray pulses a fast detector can distinguish between ballistic and scattered photons [17]: The ballistic photons, which flew the straight way from the source through the sample arrive first, the scattered or even multiply scattered photons had to propagate a longer distance and therefore enter the detector at later times, see Fig. 8. By discriminating between the first and the delayed photons, i.e., closing the gate after the arrival of the ballistic photons, the contrast of a shadow image, for example, of a bone fracture, is enhanced. Or vice versa: images of the same contrast can be produced by reduced X-ray doses, which is particularly important for tumor prevention. This effect was demonstrated by the group of Svanberg in a variety of experiments including tomographic imaging of targets within thick scattering samples. The method gains further importance by the small source size compared to conventional X-ray tubes, which leads to higher imaging quality [18]. 4.3

Hard Bremsstrahlung

In Sect. 2 we have learned that the electron quiver energy in a strong electromagnetic wave can exceed its rest mass of 511 keV/c2 . The velocity of these hot electrons approaches the speed of light and acceleration as well as radiation processes become relativistic. Beyond the onset of relativistic self-focusing (above the critical power of Pcr = 17(ne /ncr ) GW) the electron temperature further increases to many tens of megaelectronvolt. Due to

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1 0

2 0

W /c m 2

b re m s s tra h lu n g c o n v e rte r T a , A u

T h , U , I ...

la s e r re la tiv is tic c h a n n e l

e le c tro n s (E > 1 0 M e V )

g (E > 1 0 M e V )

Fig. 9. General scheme of a laser-induced nuclear-reaction experiment. An intense laser pulse is focused into a gas jet of about 1 bar pressure. It collapses due to relativistic self-focusing into a long, tiny filament. Electrons are accelerated along the filament in the forward direction to energies up to many tens of megaelectronvolt. A primary target serves as a bremsstrahlung converter. Photon temperatures at and above the giant dipole resonances of nuclei are easily obtained. Nuclei in a secondary target, placed in the pure bremsstrahlung field can be excited

conservation of relativistic momentum the electron beam collapses to a narrow filament with low divergence. The energy of the electrons and of the bremsstrahlung that is produced when these electrons hit a target with high atomic number, is exactly in the range of the giant resonances of nuclei. Nuclei can be excited to emit neutrons, an alpha particle or even to fission into fragments (see Fig. 1). The first successful laser-induced nuclear experiments were done in 1999 with the two largest lasers in the world, at the Rutherford Appleton Laboratory RAL in the UK and at the Lawrence Livermore National Laboratory LLNL in the US [19, 20]. Both installations are factory-size systems and almost infinitely far from real-world applications. Through the work of two groups in France and in Germany it was shown within the last two years that many common photonuclear reactions can be triggered with simple tabletop terawatt laser systems [21, 22, 23]. In addition, the reaction yields per time of tabletop lasers exceed the yields of the RAL and LLNL laser experiments by far, mainly because the intensities are almost the same despite the energy being much lower, and the repetition rate of the tabletop lasers is about 10 000 times higher. This scenario sounds rather fantastic at first sight: Visible lasers make alchemy. Let us have a look at two examples: the transmutation of longlived isotopes from the nuclear-fuel cycle and the production of short-lived β + -active isotopes for radiological diagnostics. 4.4

Transmutation of Isotopes from the Nuclear Fuel Cycle

The hazardous waste from nuclear-energy production consists of long-lived higher actinides like plutonium, americium and curium, which are produced

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(C ,n )

X e 1 2 8 s ta b le

I 1 2 7

I 1 2 8

s ta b le

2 5 m in

> -

> +

I 1 2 9 1 .5 7 1 0

>

7

a

-

T e 1 2 8 s ta b le

Fig. 10. Laser transmutation of iodine-129. Right: 129 I is excited by a laser-generated photon to emit a neutron. The produced 128 I decays within 25 min by β-decay into 128 Xe and 128 Te, which are both stable and harmless The inset shows the build up of the 443 keV line that follows the 25 min lifetime of 128 I. Left: The decay of 128 I into 128 Xe is accompanied by a 443 keV γ-line, which is used to monitor the yield of laser transmutation [23]

by neutron capture after the fission of uranium as well as of long-lived fission products with mass numbers around 90 and 135. The most problematic of the latter are 99 Tc, 129 I and 135 Cs. Both types of waste isotopes can be transmuted by photoexcitation, the actinides by inducing fission, the fission products by (γ, xn)-reactions, where x is a small number of emitted neutrons. In both cases photon energies between 8 MeV and 25 MeV are required, which can easily be generated by femtosecond terawatt lasers [21]. A typical experimental scheme for laser-induced isotope transmutation is sketched in Fig. 9. The laser pulse is focused onto a gas jet, collapses due to ponderomotive and relativistic self-focusing to a long channel and accelerates electrons in the forward direction to many tens of megaelectronvolt. In a first target of high atomic number, e.g., tantalum or gold, these electrons are stopped and efficiently generate bremsstrahlung with photon energies of again tens of megaelectronvolt. This radiation now interacts with the radioactive sample. In the case of 129 I with a half-life of 15.7 Mio years for example, a (γ, n)-reaction was induced with laser-produced 12 MeV photons, leading to 128 I, which decays into the stable and harmless 128 Xe and 128 Te with a half-life of only 25 min. The successful transmutation is monitored after the irradiation by detecting characteristic emissions from the decay of a short lived product, in this case the 443 keV γ-emission of 128 I, see Fig. 10 [23]. Beyond the proof of principle this experiment delivered the former unknown photo-cross section value of the 129 I(γ, n)128 I reaction. This result opens up a fundamental new technology to study photonuclear reactions that will allow detailed investigations of transmutation scenarios of isotopes from the nuclear-fuel cycle. On the other hand, the method is still far from a pos-

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2 0

W /c m 2

p rim a ry ta rg e t

se c o n d a ry ta rg e t

(p ,n ) (d ,n )

n

(g ,2 n ) g

la s e r p la s m a

(g ,n ) (e ,n ) (g ,p n ) (g ,p )

e

(n ,g )

p

(g ,f)

Fig. 11. Intense laser–matter interaction generates photons, neutrons, electrons and small ions like protons or deuterons, each of them with energies up to many tens of megaelectronvolt. These projectiles can induce a variety of nuclear reactions as sketched in the symbolized nuclear chart (number of neutrons increase from left to right, number of protons bottom-up). The first symbol in the brackets represents the projectile onto the nucleus, the second the emitted particle, e.g. (γ, n) describes a photoinduced emission of a neutron, the isotope moves one step to the left

itive-energy balance in the sense that less energy is needed to transmute a nucleus than is set free during the burning process in the reactor. But overviewing the progress of laser-plasma physics in the last decade and the prospects of high-intensity laser development, in particular the tremendous gain in efficiency as soon as diode lasers will be used for amplifier pumping, there is some hope that at some time in the future lasers can play a role in solving a huge problem: the handling of long-lived nuclear waste. 4.5

Production of Short-Lived β + -Active Isotopes

The second application of laser-induced nuclear physics is the generation of short-lived β + -active isotopes like 11 C, 15 O or 18 F, which are used in positron emission tomography. In this example, the energy balance of the process is much less important, since the amount of matter to be generated is small and the benefit will justify the means. These isotopes are attached to pharmaceutics that are injected into the human body and quickly become enriched in the tissue of interest like bone or thyroid tumors. The positron emitted during the β + -decay annihilates to two counterpropagating 511 keV photons, which can be monitored from outside the body. A tomographic imaging technique allows for exact localization of the emitter and therefore of the tumor itself. In a similar scenario, short-lived isotopes are used for localized treatment, that is destructive irradiation, of tumors. In this case again, mostly energetic β- and α-radiators are used that are produced just before injection into the body. Again, since the energy of the emissions of relativistic laser plasmas lie in the range of the giant dipole resonances of most nuclei, all these radioactive isotopes, in principle, can be produced by powerful femtosecond lasers. Experiments by Nemoto et al. [24] and the Jena group proved that activities of

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many kilobequerel can already be achieved, which is only two to three orders of magnitude below the current radiology need. It is within the foreseeable future that femtosecond lasers can replace the nuclear reactors and large particle accelerators that are still required to supply radiology with short-lived isotopes [25]. This will not only reduce logistic efforts and costs, but a small turnkey laser system in the basement of the hospital will allow for the use of even shorter-lived radioactive nuclei because the time-consuming transport from the reactor site to the hospital is avoided. Particularly for diagnostic purposes, shorter lifetimes reduce the total dose applied to the patient for an equal quality of the diagnostic itself.

5

Summary

We have described in this Chapter how X-ray radiation with remarkable properties can be generated by focusing intense femtosecond laser pulses onto matter. Different mechanisms govern the interaction of light in the region of interest of laser intensities between 1015 W/cm2 and 1021 W/cm2 , starting with collisional plasma heating through collective phenomena such as resonance absorption up to relativistic effects at the highest intensities. We have introduced applications of laser-generated X-ray radiation in lithography, in radiology, in basic science and in isotope transmutation. All the discussed technologies utilize one or more of the unique qualities of laser-generated hard X-ray radiation, which are the ultrashort duration, the small source size and the high intensity. The rapid evolution of femtosecond laser technology and of the understanding of laser–matter interaction make us confident that new, hitherto undiscovered applications will be opened up in technology and in science.

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[7] T. Feurer, A. Morak, I. Uschmann, C. Ziener, H. Schwoerer, C. Reich, P. Gibbon, E. F¨ orster, R. Sauerbrey, K. Ortner, C. Becker: Femtosecond silicon Kα pulses from laser produced plasmas, Phys. Rev. E. 65, 016412 (2001) 240 [8] G. Pretzler, F. Brandl, J. Stein, E. Fill, J. Kuba: High-intensity regime of X-ray generation from relativistic laser plasmas, Appl. Phys. Lett. 82, 3623 (2003) 240, 246 [9] D. C. Eder, G. Pretzler, E. Fill, K. Eidmann, A. Saemann: Spatial characteristics of Kα radiation from weakly relativistic laser plasmas, Appl. Phys. B 70, 211–217 (1999) 240, 245 [10] M. Pittman, S. Ferre, J. P. Rousseau, L. Notebaert, J. P. Chambaret, G. Cheriaux: Design and characterization of a near-diffraction limited femtosecond 100TW 10Hz high-intensity laser system, Appl. Phys. B 74, 529–535 (2002) 241 [11] M. Hagedorn, J. Kutzner, G. Tsimilis, H. Zacharias: High-repetition-rate hard X-ray generation with sub-millijoule femtosecond laser pulses, Appl. Phys. B 77, 49–57 (2003) 241, 246 [12] U. Stamm, H. Schwoerer, R. Lebert: Strahlungsquellen f¨ ur die EUVLithographie, Physik J. 1, 33–39 (2002) 243 [13] S. D¨ usterer, H. Schwoerer, W. Ziegler, C. Ziener, R. Sauerbrey: Optimization of EUV radiation yield from laser produced-plasma, Appl. Phys. B. 73, 693– 698 (2001) 244 [14] S. D¨ usterer, H. Schwoerer, W. Ziegler, D. Salzmann, R. Sauerbrey: Effects of a prepulse on a laser-induced EUV radiation conversion efficiency, Appl. Phys. B 76, 17–21 (2003) 244 [15] F. Ewald, H. Schwoerer, R. Sauerbrey: Kα -radiation from relativistic laser produced plasmas, Europhys. Lett. 60, 710–716 (2002) 245, 246 [16] T. Feurer, A. Morak, I. Uschmann, C. Ziener, H. Schwoerer, E. F¨orster, R. Sauerbrey: An incoherent sub-picosecond X-ray source for time-resolved X-ray-diffraction experiments, Appl. Phys. B 72, 15–20 (2001) 246 [17] M. Gr¨ atz, L. Kiernan, K. Herrlin: Time-gated imaging in planar and tomographic X-ray imaging, Med. Phys. 26, 438 (1999) 247 [18] A. Sj¨ orgen, M. Harbst, C. G. Wahlstr¨ om, S. Svanberg, C. Olson: Highrepetition-rate, hard X-ray radiation from a laser produced plasma: photon yield and application considerations, Rev. Sci. Instrum. 74, 2300 (2003) 247 [19] K. W. D. Ledingham, I. Spencer, T. McCanny, R. P. Singhal, M. I. K. Santala, E. Clark, I. Watts, F. N. Beg, M. Zepf, K. Krushelnick, M. Tatarakis, A. D. Dangor, P. A. Norreys, R. Allott, D. Neely, R. J. Clark, A. C. Machacek, J. S. Wark, A. J. Cresswell, D. C. W. Sanderson, J. Magill: Photonuclear physics when a multiterawatt laser pulse interacts with solid targets, Phys. Rev. Lett. 84, 899–902 (2000) 248 [20] T. E. Cowan, A. W. Hunt, T. W. Phillips, S. C. Wilks, M. D. Perry, C. Brown, W. Fountain, S. Hatchett, J. Johnson, M. H. Key, T. Parnell, D. M. Pennington, R. A. Snavely, Y. Takahashi: Photonuclear fission from high energy electrons from ultraintense laser-solid interactions, Phys. Rev. Lett. 84, 903–906 (2000) 248 [21] G. Malka, M. M. Aleonard, J. F. Chemin, G. Claverie, M. R. Harston, J. N. Scheurer, V. Tikhonchuk, S. Fritzler, V. Malka, P. Balcou, G. Grillon, S. Moustakis, L. Notebard, E. Levebre, N. Cochet: Relativistic electron generation ininteractions of a 30 TW laser pulse with a thin foil target, Phys. Rev. E. 66, 066402 (2002) 248, 249

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[22] H. Schwoerer, F. Ewald, R. Sauerbrey, J. Galy, J. Magill, V. Rondinella, R. Schenkel, T. Butz: Fission of actinides using a tabletop laser, Europhys. Lett. 61, 47–52 (2003) 248 [23] J. Magill, H. Schwoerer, F. Ewald, J. Galy, R. Schenkel, R. Sauerbrey: Laser transmutation of iodine-129, Appl. Phys. B 77, 387–390 (2003) 248, 249 [24] K. Nemoto, A. Maksimchuk, S. Banerjee, K. Flippo, G. Mourou, D. Umstadter, Y. Y. Bychenkov: Laser-triggered ion acceleration and table top isotope production, Appl. Phys. Lett. 78, 595–597 (2001) 250 [25] S. Fritzler, V. Malka, G. Grillon, J. Rousseau, F. Burgy, E. Levebre, E. d’Humieres, P. McKenna, K. Ledingham: Proton beams generated with high-intensity lasers: Applications to medical isotope production, Appl. Phys. Lett. 83, 3039–3041 (2003) 251

Index

extreme ultraviolet (EUV), 242, 243

transmutation, 248

hard bremsstrahlung, 247 high-intensity femtosecond laser, 241

ultrashort Kα -puls, 245

ionization, 236

X-ray, 235

Metrological Applications Ralf Menzel Lehrstuhl f¨ ur Photonik, Physik, Universit¨ at Potsdam, Am Neuen Palais 10, 14469 Potsdam, Germany [email protected] Abstract. Using short laser pulses in metrological applications offers the possibility of very short observation times in combination with high accuracies. The high intensities allow for nonlinear techniques and very short pulses result in broad spectra as is necessary for white-light interferometric measurements. With respect to the demands of commercial devices the costs of the light sources had to be sufficiently low and thus new concepts had to be investigated. Therefore broadband laser pulses were generated on the one hand by gain switching of Ti:sapphire lasers. In addition, nonlinear absorbers were used for further broadening. 30 nm to 90 nm bandwidths are realized with these techniques. On the other hand, microstructured fibers are used in combination with femtosecond and picosecond pulses. The optimized compact picosecond source provided a total bandwidth of 900 nm with 2.6 W average output power in the IR and very good beam quality. These new light sources are evaluated in comparision to new CW-light sources based on diode-laser structures in different metrology techniques. In particular, the coherence radar (KoRad) technique, optical coherence tomography (OCT) and femtosecond radar (FemRad) are investigated. In addition, several prototypes of lasers and measuring devices are presented.

1

Introduction

Higher precision and faster processes in manufacturing, as, for example, realized with femtosecond lasers, generate new demands in metrology. Typically, improved accuracy and shorter measuring times are needed. Optical methods have fundamental advantages in this regard. In addition, large working distances are possible, optically. Therefore it was investigated how existing optical methods can be improved by applying new light sources of short pulse duration in the femtosecond or picosecond range. This was compared to the results using other newly developed light sources with longer pulses or even CW operation under the realistic measurement conditions. Furthermore, a new measuring method, which becomes possible by using very short laser pulses in a radar technique, was developed. Advantages and disadvantages of femtosecond technology in these metrology applications should be identified. In recent years many new other measurement techniques based on femtosecond pulses were developed, such as, for example, spectroscopic or imaging methods, and even used in commercial applications. These are, for example, new types of nonlinear microscopy [1, 2, 3], of imaging using new Raman F. Dausinger, F. Lichtner, H. Lubatschowski (Eds.): Femtosecond Technology for Technical and Medical Applications, Topics Appl. Phys. 96, 257–285 (2004) c Springer-Verlag Berlin Heidelberg 2004


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techniques [4], and all kinds of new types of nonlinear spectroscopy as described, for example, in [5, 6]. But the focus here is the measurement of the three-dimensional shape of more or less solid surfaces.

2 Advantages of Femtosecond Pulses for Metrology Applications Using light in metrology applications has the advantage of a totally nondestructive method without any contact with the sample to be measured. Several methods were developed to fulfill the needs of the different metrology applications [7, 8, 9, 10]. For example, triangulation [11, 12] allows for measurements of three-dimensional surfaces with accuracies in the range of some micrometers with CW light sources. Large machinery parts can be investigated in this way. Using femtosecond pulses introduces completely new possibilities based on three advantages following from the specific properties of this light. Femtosecond pulses are characterized by: • Short pulse durations (5 fs to 1 ps) • High peak powers (100 kW to > 10 GW) • Broad pulse spectra (10 nm to > 300 nm) Short pulses can be realized with pulse durations of about 100 fs or longer, routinely, in comparatively simple apparatuses. With special setups it is possible to realize very short pulses of 10 fs to 30 fs, and in a few experiments pulse durations even smaller than 5 fs were demonstrated [13,14]. These short pulses allow for the observation of processes occurring with very high speed. It is, for example, possible to observe molecular processes in real time as has been demonstrated, for example, with the vibrational motion of molecular systems [15]. For metrology applications it follows that samples moving with the speed of sound can be observed, in principle, with accuracies of much less than 1 nm, which is usually not possible for other reasons. Femtosecond metrology has, in principle, the advantage of being faster than any mechanical process. The availability of high peak powers of the laser pulses corresponding to high electric field strengths in the material allows for new nonlinear measuring techniques as demonstrated, for example, in two-photon microscopy [16] or other nonlinear measurements [5]. Peak powers of a few hundred kilowatt can easily be realized even with comparably simple oscillators. With amplifiers, system peak powers above 10 GW can be reached, and with special devices even very high peak powers up to terawatt are possible. In measurement applications nonlinear processes can be activated to improve the measurement technique in different ways. Large areas can be investigated in parallel. Thus, femtosecond laser pulses opened the door to many new nonlinear measurement techniques demanding very high peak powers of the used laser pulses in combination with low average powers and low prices of the whole device.

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With respect to the metrology topic described here, mainly the broad spectra of the femtosecond pulses were of interest. These broad spectra have advantages for many reasons. For example, the problem of speckles in imaging can be solved by using broadband light. Routinely, spectral bandwidths of 10 nm can easily be obtained from femtosecond lasers. With special devices, 50 nm bandwidths are possible, and in a few measurements bandwidths of more than 600 nm are obtained from the oscillator itself [17]. Using other nonlinear techniques, as will be described below, it is possible to broaden the spectra of short pulses to very large values. As shown here a spectral width of up to 900 nm from a single pulse was realized by nonlinear broadening mechanisms. Usually, not all these parameters are necessary in one measurement technique, and the effort to establish the femtosecond laser device has to be compared to other methods to realize the specifications for the measurement. Thus, in this investigation the possibilities of realizing light sources with certain parameters comparable to those of femtosecond laser pulses are investigated and measurement results are given for the different light sources.

3

Investigated Measuring Techniques

With respect to the framework of the project structure three methods for metrology applications will be described: 1. The so-called coherence radar technique (KoRad) was developed as an interferometric measurement technique to get the highest resolution of surfaces even for noncooperative samples. 2. Optical coherence tomography (OCT) is useful to get information about the three-dimensional shape of surfaces and also information about the structure of a material below the surface, which will be described shortly. 3. A new technique is the so-called femtosecond radar technique (FemRad) that allows for the three-dimensional measurement of the surface with just one laser pulse. 3.1

Coherence Radar (KoRad)

The technique of coherence radar (as well as the optical coherence tomography, OCT) is a white-light interferometric measuring technique. KoRad was developed by the group of Prof. Haeusler, Erlangen, to get high-precision measurements of the three-dimensional shape of rough surfaces [18]. In best cases the resolution of the 3D shape of the surfaces is about 10 nm. Usually, resolutions in the range between 15 nm and 100 nm are sufficient for the applications [19]. The scheme of a coherence radar setup is shown in Fig. 1. It is based on a Michelson interferometer where one of the mirrors is substituted by the object to be measured, and the reference mirror is replaced by a rough surface. Both the object and the reference surface are illuminated by the

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Fig. 1. Scheme of the coherence radar setup (KoRad)

same light source via a beam splitter (BS). For observation of the interference pattern a CCD camera is used. The surface of the object is imaged on it. If the reference is moved and thus the length of the reference arm is varied a correlogram structure can be measured by each pixel of the CCD camera as shown in Fig. 1 (as a function of the position of the surface of the object belonging to that pixel). Each pixel will show the absolute maximum of the correlogram structure as a function of the shape of the object in correlation to the position of the rough surface. The position of the absolute maximum of the correlogram per pixel allows for the reconstruction of the 3D surface of the whole observed object. A detailed description of this measuring method can be found in [18, 19, 20]. Without discussing details, it is obvious that the lateral resolution is dependent on the lateral resolution of the imaging process itself. But the evaluation of the correlogram and especially the safe determination of the absolute maximum of the oscillating structure demands a sufficiently narrow correlogram. The width of the correlogram is, on the other hand, a function of the coherence length of the laser, and thus it follows that the resolution is better for shorter coherence lengths of the laser [21]. A more detailed discussion will follow below. As an example of a measurement result, in Fig. 2 the metrology of the back side of a German Euro coin is shown. The picture-like graph shows the 3D profile of this coin coded in grayscale and measured with the coherence radar as shown in Fig. 1. The two lines across the coin graph indicate the dissections of the diagrams below and on the right-hand side of this picture. It can be seen that the details of the coin are very nicely reproduced. In this case the whole setup was aligned for micrometer depth resolution. Because this measurement technique is a scanning technology, the scanning steps have

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Fig. 2. Example of measuring a Euro coin

to be chosen small enough in comparison with the required resolution of the measurement. On the other hand small scanning steps mean long measuring times, and for practical applications a compromise between scanning speed and resolution usually needs to be made. As mentioned before for realizing a narrow correlogram a short coherence length of the light pulse is necessary, which in turn demands a large spectral bandwidth. The interference signal on the detector is a function of the frequency of the used light, and thus, the spectral intensity of the superposition of the two electric fields with the field strengths A1 and A2 from the two interferometer arms can be described by: 2

I(ω, t) = 0 c0 |A1 (ω, t) + A2 (ω, t)|
 2 2 = 0 c0 |A1 (ω, t)| + |A2 (ω, t)| + Re [A1 (ω, t)A∗2 (ω, t)] ,

(1)

with c0 as the speed of light in vacuum (3 × 108 m/s) and 0 as the permittivity of vacuum (8.85 × 10−12 F/m) [21]. At each pixel of the CCD camera this intensity will be integrated as an average signal over the whole spectrum and over the integration time of the CCD that is usually much larger than the pulse duration. For a simple analysis the spectral sensitivity curve of the CCD camera will be approximated by a constant sensitivity. Furthermore, it is assumed that the intensities from both interferometer arms are equal in magnitude, which can be experimentally realized in the setup by using neutral-density filters. With these assumptions the electric field strength can be written as:   2∆l = A(ω, t)ei[ωt+Φ(ω)] . (2) A1 (ω, t) = A2 ω, t + c

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For the obtained signal per pixel, it follows that:  ∞ ∞  ∞ ∞ 2∆l 2 2 ¯ S=2 |A(ω, t)| dω dt + Re |A(ω, t)| e−iω c dω dt . −∞

0

−∞

0

(3) In this formula ∆l describes the detuning of the object point compared to the exact alignment of the Michelson interferometer for this pixel. This formula shows that the first term is independent of the position of the reference plane and the second term describes the modulation of the signal resulting in the already shown correlogram of Fig. 1. If, furthermore, a Gaussian shape of the temporal and spectral profiles of the used laser pulses are assumed, and ∆ω and ∆t describe the full width half-maximum (FWHM) widths of these distributions the first term can be explicitly determined as:        2 2 ω − ω0 t 2 ln 2 exp −4 ln 2 . (4) |A(ω, t)| = I0 exp −4 ∆ω ∆t This spectral intensity can be used in the above written equation for the signals. It follows that:       ∞ 2 t 2∆lω0 ∆l2 ∆ω 2 ¯ S ≈ 2 + 2 cos I0 e−( ∆t ) ln 2 dt . (5) exp − 2 c c ln 2 −∞ The spectral frequency bandwidth can be expressed by the width of the wavelength distribution with the following formula under the assumption that the spectral bandwidth is small enough: ∆ω c c c∆λ c∆λ = − = ≈ 2 . 2π λ λ + ∆λ λ (λ + ∆λ) λ

(6)

For illustration, the influence of the bandwidth is demonstrated in Fig. 3 for a laser pulse with a center wavelength of 800 nm. This correlogram shows the observed pattern for a bandwidth of 8 nm and of 30 nm as a function of the detuning ∆l. From the signal equation it can also be seen that the amplitude of the correlogram is the larger, the more intensity is available. Thus, for measurements of dark objects or other noncooperative structures that, for example, reflect the light out of the interferometer, it is necessary to have enough light intensity. From this brief analysis, it follows that the KoRad technique will work the better the higher the light intensity of the source. But the main preposition is a sufficiently large spectral width to allow for safe determination of the absolute maximum of the obtained correlograms. In practice, bandwidths in the range of 10 nm to 30 nm seem to be optimal for these purposes.

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Fig. 3. Correlogram as a function of the laser bandwidth

3.2

Optical Coherence Tomography (OCT)

Using a Michelson interferometer with a broadband light source also allows for another white-light interference technique: optical coherence tomography [22, 23, 24, 25]. A special technique that avoids the mechanical depth scan is “spectral radar” [26], which is explained in Fig. 4 and Fig. 5. Because this technique is frequently used in medical applications, the whole construction is usually built by a fiber interferometer [27] as shown in Fig. 4.

Fig. 4. Scheme of optical coherence tomography according to the “spectral radar” principle

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The beamsplitter provides the illumination of the object that is, for example, a skin tissue, and a reference mirror in the lower right part of the figure. The scattered light of object and reference is then recombined at the beamsplitter, and this signal is measured by a CCD detector behind the spectrometer. The main idea of this measuring principle is sketched in Fig. 5. The single object layers will cause different interference patterns for several colors of a spectrum as shown in the upper right part of the figure. The different

Fig. 5. Principle of “spectral radar”

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wavelengths are detected on different pixels of the CCD detector behind the spectrometer, and the associated correlograms are available there. If this result is Fourier transformed, the amplitudes will be different for the single layers of the object as a function of position z. In more detail, the observed intensities are a function of the frequency: u0 (t) = 12 U0 SΓ (t) cos [(Ω0 − ω) t + Φ0 − φ]

− 12 U0 SΓ (t) cos [(Ω0 + ω) t + Φ0 + φ] = U0 SΓ (t) sin (Ω0 t + Φ0 ) sin (ωt + φ) ,

(7)

with the amplitude-modulated signal of object u0 , the amplitude of signal U0 , the amplitude of object S, the Gamma function+Γ , the frequency of object Ω0 , the frequency of signal ω, the time t, and the phase of object Φ0 . The phase of signal φ can be transformed to the scattering amplitudes as given by: s (t) =

U0 UR SΓ (t) sin(Φ0 − ΦR ) sin(ωt + φ) , 2

(8)

with the modified signal s , the amplitude of reference UR , and the frequency of the reference ΦR . Thus, this measuring principle allows for the detection of the structure of slightly transparent materials. As can be seen from the scattering formula, the resolution in depth is a function of the bandwidth of the used laser light. The larger the bandwidth the more accurate are the results of the measurements. With about 30 nm bandwidth resolutions in the range of several micrometers can be obtained. If the bandwidth is increased above 100 nm, the resolution can be smaller than 1 µm in depth. This would be especially interesting for the investigation of skin tissues and other medical applications as, for example, the retina in the eye. On the other hand, the signal is again strongly dependent on the available intensity of the light source. For optical coherence tomography applications the bandwidth should be as large as possible, and the intensity of the light source has to be sufficient. Furthermore, it is important to avoid any distortions in the Fourier analysis, and thus the spectra of the light sources should be as smooth as possible. More detailed information can be found, among others, in [28, 29, 30, 31]. 3.3

Femtosecond Radar (FemRad)

A newly developed metrology principle is the femtosecond-radar method (FemRad) [32]. This measuring principle is essentially based on short pulse durations of the used laser light. The scheme is shown in Fig. 6. In this scheme the object is illuminated by a very short femtosecond laser pulse. The light scattered at the object is imaged on a CCD camera 1 that would usually show the photographic picture of the object. The

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Fig. 6. Scheme of femtosecond-radar setup (FemRad)

FemRad measuring principle is based on the fact that the scattered light shows different delays as a function of the z-position of the scattering area of the object. Thus the scattered light will show different delays at the different pixels. The delays are as large as twice the surface distance from the beamsplitter. In the FemRad scheme this delay is encoded as an intensity distribution via a special converter in the beam. The scattered light coming from larger distances will show less intensity on the CCD camera. As a result of this simple measuring principle, the obtained intensity distribution on the CCD camera shows directly the grayscale-coded three-dimensional shape of the investigated object. But because the object itself may show some variation of the scattered intensity, a reference beam is needed. In this reference beam the scattered light is imaged directly on a second CCD camera 2 that obtains the photographic picture of the investigated object. With a simple calculation of the signals of the two CCD cameras the object reflectivity can be calibrated with the accuracy of the CCD camera dynamic. Thus, as a result only the pure delay information is available, and thus the 3D information of the object can be determined with just one laser pulse. If laser pulses with about 100 fs pulse duration are applied, the resolution of the method can easily be in the range of a few micrometers. With deconvolution techniques it should be possible to obtain resolutions much less than 1 µm. Using longer or shorter laser pulses would decrease or increase the resolution of this method. As a result, the measuring technique can be optimized for different applications by chosing a suitable laser-pulse duration and different converters. The advantage of this measuring technique is that the whole 3D shape of the surface is available from just one single laser shot, and the measuring time results from the laser-pulse duration, only. This new measuring technique can provide very stable determination of the 3D shape of light-scattering rough surfaces.

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267

Realized Broadband Light Sources

As described above, the main properties of the light sources for coherence radar (KoRad) and optical coherence tomography (OCT) are sufficiently large spectral bandwidths for short coherence length and sufficient average output power for good signal-to-noise ratio. In the case of the femtosecond radar method appropriate short femtosecond laser pulses are needed. For the practical application also the size, reliability, robustness, and price of the light source are important considerations. So far, a commercial laser (Tsunami, regenerative amplifier, optical parametric amplifier; all Spectra Physics) has been used as the femtosecond light source. This laser system allowed short pulses with pulse durations tunable between 100 fs and 2 ps in the wavelength range between 300 nm and 3 µm. The maximum pulse energy was in the region of roughly 1 mJ, and the repetition rate was 1 kHz. The spectral bandwidth of the amplified pulses was in the region of 10 nm. These pulses were used in the femtosecond radar system. For the white-light interferometric applications the femto- and picosecond pulses were further broadened by using the nonlinear effects in a microstructured fiber as described next. This was also investigated with another picosecond laser that is, compared to the femtosecond system, a much smaller and simpler and thus much cheaper device. For comparison broadband laser sources based on the gain switching of Ti:sapphire laser material were also developed. These light sources were synchronized to the video frequency of the CCD cameras, and they were operated at 25 Hz and up to 500 Hz. As a very low cost light source diode lasers were prepared for emitting broadband spectra of up to 30 nm spectral bandwidth. 4.1

Short-Pulse Laser with Microstructured Fiber

Bandwidth-limited laser pulses with 100 fs or 10 ps pulse duration, respectively, show a spectral width of 5.3 nm and 0.05 nm at 1 µm wavelength. This is usually too small for the white-light interferometric applications. Thus, for this type of laser, the spectral broadening using microstructured fibers was applied. A picture of these microstructured fibers as used in the experiments is shown in Fig. 7.

Fig. 7. Cross section of one of the used microstructured fibers

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By the design of the fiber structure the dispersion of broadband light inside the fiber can be optimized for very long interaction lengths of the nonlinear processes. These fibers are sometimes also called photonic crystal fibers. The efficiency of the nonlinear processes can be increased by orders of magnitude. The broadening effect in the comparable conventional fiber showed much narrower spectral bandwidth, whereas with photonic fibers, as described below, a maximum spectral bandwidth of 900 nm was obtained. As a consequence of this enhancement effect simple picosecond-laser oscillators can be applied for pumping the nonlinear processes, and no expensive amplification of the pulses is needed. The “threshold” of the nonlinear processes in these fibers can be as low as 100 W peak power. Therefore, besides the commercial femtosecond laser system also a simple passively mode-locked Nd:YVO4 laser (Jenoptik Laser, Optik, Systeme GmbH, Germany) was applied. This laser provided an output power of up to 5 W with a pulse duration of 10 ps and a repetition rate of 85 MHz at a wavelength of 1064 nm. The peak power of this laser was roughly 5.5 kW. The laser beam was of diffraction-limited quality (M 2 ≤ 1.05) and linearly polarised. The laser light was coupled into the microstructured fiber using an aspheric microscope objective with a NA of 0.5. A similar lens was used for collimation behind the fiber. The light was then detected by a calibrated optical spectrum analyzer (ANDO AQ6315A).

Fig. 8. Compact multiwatt white-light source

The experiments were done with two different fibers called MF1 and MF2 with lengths of 1 m and 5 m. The core diameter of fiber 1 was 3.5 µm, and the core diameter of fiber 2 was 5 µm, respectively. The zero-dispersion wavelengths of the two fibers were 976 nm and 1065 nm. The obtained emission spectra of both fibers are shown in Fig. 9. 0 MF1 MF2

Output [dBm]

-10 -20 -30 -40 -50 -60 600

800

1000

1200

1400

Wavelength [nm]

1600

Fig. 9. Spectra of microstructured fibers MF1 and MF2

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Fig. 10. Spectra of MF1 for different pump wavelengths

In both fibers a very strong broadening of the light could be obtained. In the case of MF1 the spectrum ranged from 550 nm to 1800 nm, and the average output power was 2.3 W. MF2 delivered a 900 nm broad spectrum with an average output power of 2.4 W. The structure of the two spectra is different because of different zero-dispersion wavelengths. As is known, the spectral broadening is generated by self-phase modulation (SPM), fourwave mixing (FWM), stimulated Raman scattering (SRS), and soliton effects [6, 33, 34, 35, 36]. The phase matching of the different components is strongly controlled by the relation of these wavelengths compared to the zero-dispersion wavelength. Thus, the tuning of the zero-dispersion wavelength to the laser wavelength resulted in a comparably flat spectral shape as is necessary for the white-light interferometric applications. More details about these investigations are described in [37, 38, 39]. In Fig. 10, the obtained spectra using the commercial femtosecond laser source with the parameters 1.3 ps pulse duration, 0.1 µJ pulse energy, and 1.0 kHz repetition rate are plotted. As can be seen from the figure, it is possible to shift especially the short-wavelength edge of the spectra to smaller values by varying other laser parameters. This may be useful for applications where visible light is needed. Summarizing these investigations for realizing a broadband light source for white-light interferometric applications, such as KoRad and OCT, the application of a simple and inexpensive picosecond laser provides enough spectral bandwidth and average output power. These specifications are realized in combination with the very compact design, as can be seen from Fig. 11, which shows the commercially available laser containing the picosecond pump source and the microstructured fiber in a box with a size of 500 mm × 250 mm × 100 mm. It turned out that the well-adapted fiber parameters with respect to the used laser pulses are the key element for getting

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high efficiency in combination with extremely large broadening. This type of system operating with repetition rates of 120 MHz may be useful for all kinds of applications in the white-light interferometric field. It may also find applications in spectroscopy and other measuring techniques such as nonlinear microscopy and others.

Fig. 11. Commercially available picosecond laser with microstructured fiber (Jenoptik) with an average output power of 2.6 W, a repetition rate of 120 MHz, and a broad output spectrum ranging from 700 nm to 1600 nm

4.2

Gain-Switched Ti:sapphire Laser

Another method to realize broadband laser emission is the gain switching of an oscillator using an active material that allows broadband operation [21]. Therefore the active material should be pumped with nanosecond pulses. Besides Cr:YAG as the active material, which will be described elsewhere, Ti:sapphire was used for this purpose. The setup of a gain-switched Ti:sapphire laser is shown in Fig. 12.

TEM00 Nd-laser

λ/2 λ/ pol. KTP

0.53 µm

1.06 µm

L

Ti:sapphire

HR @0.8 µm AR @0.53 µm

R @0.8 µm

Fig. 12. Experimental setup of the gain-switched Ti:sapphire laser

The pump-laser pulses were generated with a Nd:YAG laser that was passively Q-switched. The light of this laser was frequency doubled in a KTP crystal, and the remaining fundamental was taken out using a beamsplitter. The Nd:YAG laser was designed for pulse-burst operation with five identical pulses within the pump-pulse duration of 140 µs and a repetition rate of

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1.0 0.8 0.6 0.4 0.2 0.0

760

16.6 15 14.3 13.7 12.7 12.5 11.2 10.4 9.6 8.9 7.9 7.2 5.7 5.01 3.84 2.53 770

780

790

800

Pump energy [m J]

Intensity [a.u.]

25 Hz for the bursts. The total energy of the five pulses was 55 mJ. The half-wave plate and the polariser were used to tune the output pulse energy of that system for a detailed investigation of the Ti:sapphire laser operation. The conversion efficiency was 35%. The Ti:sapphire laser crystal was pumped with a beam radius of roughly 250 µm. The high-reflecting mirror of this laser was transparent to the pump wavelength of the frequency-doubled Nd:YAG laser. The Ti:sapphire laser operated almost diffraction limited. The spectra of the emitted laser radiation as a function of the pump energy are shown in Fig. 13.

810

W avelength [nm ] Fig. 13. Spectra of the Ti:sapphire laser for different pump energies

As can be seen, the peak wavelength is shifted slightly to longer wavelengths using higher pump energies, and the spectral bandwidth is slightly increased from about 20 nm to 27 nm in this case. In the optimized case the observed spectral bandwidth was 31 nm for this type of laser, as can be seen in Fig. 14. More details about this laser are desribed in [21]. Because the spectral bandwidth of the emission spectrum of the gainswitched laser is strongly dependent on the initial emission, the spectrum can be broadened even further by using a saturable absorber in the laser cavity. The scheme of the setup is shown in Fig. 15. In this case the saturable absorber is placed between two lenses for adjusting the spot size of the laser beam inside the absorber cell to the optimal value. Different dyes were investigated as the saturable absorber. It is important for the selection of a dye that the absorption spectrum is almost the inverse of the emission spectrum of a free-running Ti:sapphire laser [see Fig. 16]. Thus the emission spectrum can be broadened by the absorption. Secondly, the dye should be bleachable by the available light intensities inside

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Ti:sapphire laser (R = 90 % )

Intensity [a. u.]

1.2

Calculated laser spectrum

1.0 0.8 0.6

∆ λ = 31.3 nm

0.4 0.2 0.0 750 760 770 780 790 800 810 820 830 840 850

W avelength [nm] Fig. 14. Spectrum of the Ti:sapphire laser, comparison of theory and experiment

Ti:sapphire

absorber

R = 80%

0.54 µm HR @0.8 µm AR @0.54 µm

f = 75 mm

wTi:sapphire= 200 µm

f = 100 mm

wabsorber= 80 µm

Fig. 15. Scheme of gain-switched laser with saturable absorber to realize further spectral broadening of the laser emission

Free running Ti:sapphire laser 1.0

Absorber DDTTCI

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0 720

740

760

780

800

820

840

Transmission

Intensity [a. u.]

1.0

0.0 860

W avelength [nm]

Fig. 16. Spectrum of saturable absorber and Ti:sapphire laser

Intensity [a.u.]

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3 2 1 0

79 72 70

Transmission [%]

83

66 700

725

750

775

800

825

850

875

900

Wavelength [nm]

Fig. 17. Spectra of gain-switched Ti:sapphire laser with saturable absorber

the cavity [6]. By changing the spot size the optimal point of operation for the dye can be chosen. In the best case the dye is bleached during the laser operation, and consequently it does not cause losses for the power emission. In Fig. 17 the obtained spectra of the Ti:sapphire laser as a function of the initial dye transmission are plotted. It can be seen that very low transmissions lead to a shift of the laser emission to the red edge and a strong narrowing. If the dye concentration is decreased the laser emits a broadened spectrum around the initial peak wavelength. In the best cases emission spectra with almost 100 nm bandwidth could be obtained. In this case some structure occurred in the spectrum. Finally, several lasers based on the gain switching of Ti:sapphire were developed and are now commercially available. The pictures and the specifications of these lasers are shown in Fig. 18. 4.3

Broadband Operation of Diode Lasers

As alternative light sources gain-guided broad stripe diode lasers were prepared for broadband operation. It was possible to operate these laser diodes with spectral bandwidths of up to 30 nm and average output powers of 220 mW. With a specially developed collimation optic the beam profile of this light source could be modified to a homogenous spot. This was sufficient for the KoRad metrology applications. Applying the collimator the average output power decreased to values of about 93 mW. The emission spectra of such laser diodes are shown in Fig. 19.

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Fig. 18. Commercially available gain-switched Ti:sapphire lasers (IB Laser, Berlin)

1.0

Intensity [a.u.]

0.8 0.6 0.4 0.2 0.0 740 760 780 800 820

880 900 920 940 960 980

Wavelength [nm] Fig. 19. Spectra of two broadband diode lasers

These laser diodes could be packaged with the collimation optics in a cylindrical body of about 50 mm diameter and about 160 mm length, as shown in Fig. 20.

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Fig. 20. Broadband diode lasers assembled with collimation optics

The useful diameter of the collimated light was in this case 40 mm. The collimation optics could be tuned in such a way that the light is also slightly divergent for illuminating objects larger than 40 mm in size. The brightness improvement of these light sources compared to superluminescent diodes was roughly a factor of 8, and they are much cheaper.

5

Measurement Examples

The usefulness of the described light sources in metrology applications will be shown in different examples for the different measuring methods. These measurements were done mostly in collaboration with the partners of the Universities of Erlangen, Frankfurt, and Vienna, as well as with the companies Polytec, Bosch, Rolls Royce Germany, and WiSenT. 5.1

Coherence Radar (KoRad)

One of the most developed devices is built for the 3D characterization of tooth structures for the further preparation of replacements. In Fig. 21 in the upper part the gray-coded surface shape of a prepared part of three teeth is shown. The middle one is already prepared from the doctor for the further construction of the replacement in this case. The line in the upper part of this graph gives the position of a cut that is shown in the lower part of the figure. As can be seen by using the KoRad technique it is possible to measure even quite steep surfaces. It turned out that up to 1◦ compared with the direct measuring direction it was possible to determine the surface structure with sufficient accuracy in the range below 10 µm. These measurements are possible with the commercial product as is shown in Fig. 25 on the right-hand side. In the next example the result of a measurement of a turbine fan from an aircraft engine is shown. On the left-hand side on Fig. 22, the photograph of this turbine is presented.

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© by Polytec, WiSEnT

Fig. 21. Result of a measurement of a prepared tooth structure using the KoRad method as realized in the TopDent device as shown in Fig. 26 (right)

6 cm Schnitt 1 2 mm

1

2

3

Schnitt 2 2 mm

© by Rolls Royce

Photo

0.2 mm Schnitt 3 1 mm Schnitt 2a

© by Uni Erlangen, Rolls Royce

Coherence radar measurement Fig. 22. Photograph and KoRad measurement of a turbine blade

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On the right-hand side of the same figure the result of a preliminary measurement of this turbine surface is given. Different lines show the planes for which the diagrams are given on the right-hand side or below the graycoded graph. As can be seen from this figure the diagram shows quite well the surface structure of this part. These diagrams can be used for quality control and production-process optimization of these parts. Further optimization of the measuring process and the following mathematical evaluation of the data led to the results for KoRad measurements as shown in Fig. 23.

Fig. 23. 3D reconstruction of a turbine blade

In Fig. 24 the potential of the KoRad measurements using high-brightness laser sources with sufficient spectral bandwidth is shown. In this case the field of measurement is 100 mm × 100 nm and the realized accuracy is in the range of a few micrometers. Therefore, even the distortions from the mechanical production of this part can be seen, easily. Another example is given in Fig. 25. In Fig. 26 three commercially available measuring devices that are based on the coherence radar method are shown. They are designed for different purposes regarding field of measurement, speed of measurement, and accuracy. On the left-hand side the Top Map (Polytec) that is useful for areas of 3 cm × 4 cm and accuracies of 10 nm is shown. This is a compact device that allows for routine investigations of industrial parts. In the middle, the sensor S18 (3D Shape) is shown. It is useful for object diameters of 1.8 cm and provides a lateral resolution of 30 µm. On the right-hand side of Fig. 26 a measuring device especially developed for the routine measurement of tooth structures is shown. This device is called TopDent (Polytec).

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Fig. 24. KoRad measurement of a pump body (Haeusler, Erlangen)

Fig. 25. Windscreen wiper and the KoRad measurement of the spring steel (Bosch)

5.2

Femtosecond Radar (FemRad)

As a measuring example using the FemRad technique in Fig. 27 the result for a stopper is shown. On the left-hand side of this graph the photograph is given. On the right-hand side the 3D surface as measured with a single-shot technique of the FemRad is shown. This preliminary result was realized with a nonoptimized setup. As described above, using deconvolution techniques it should be possible to reach resolutions even below 1 µm with this technique. Because of the high speed of this measurement technique the real-time observation of very fast moving parts is possible. The limitations of this technique are mostly given by the detection system. With modern cameras repetition rates of the measurement for different samples up to a few 100 Hz would be possible. Thus, this tech-

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Fig. 26. Commercially available measuring devices using the coherence radar technique (KoRad): left: TopMap (Polytec), middle: KORAD (3D-Shape GmbH, Erlangen), right: TopDent (Polytec)

Fig. 27. Photograph and 3-dimensional surface of the measured stopper using the femtosecond-radar setup (FemRad)

nique may be useful, for example, for real-time quality control in production lines. Another possible application is, as mentioned above, the observation of very fast moving machinery parts that otherwise is difficult to obtain. 5.3

Optical Coherence Tomography (OCT)

The following example should demonstrate the possibilities using a very broadband picosecond light source as described above in OCT. In Fig. 28 such an OCT measurement of the retina of a monkey is shown. As can be seen from this the resolution is higher than in conventional measurements. In this case the application of the short pulse light source in combination with the microstructured fiber technique to increase the band-

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Fig. 28. OCT-measurement of the retina of a monkey (Macaca Fascicularis). c by Departement of Medical Physics Picture dimension: 1 mm × 0.25 mm;
(W. Drexler), University Vienna

width of the still diffraction-limited light enables this type of measurement in a very simple way. Because of the compactness and reliability of this type of light source, this technique may improve the commercial application.

6

Summary

Using short light pulses in metrology applications for the investigation of the 3-dimensional surface shapes of samples allows for substantial improvements of these measurement techniques on the one hand, and the development of completely new measurement techniques on the other. In the investigated coherence radar measurement technique (KoRad) the field of measurement could be improved substantially by using the new high-brightness sources with a spectral bandwidth of 20 nm to 100 nm. In optical coherence tomography (OCT) based on the extremely large bandwidth using the nonlinear processes in microstructured fibers, the accuracy of a measurement can be increased. The femtosecond radar technique (FemRad) is a new technique that allows for the measurement of three-dimensional surfaces with just one femtosecond or picosecond laser shot. The spectral broadening processes as they are applied in the microstructured fiber for OCT measurement or the femtosecond radar technique are essentially dependent on the availability of very short laser pulses in the femto- to picosecond range. In the coherence radar measurements (KoRad) the spectral bandwidth and the brightness of the light sources are the more important parameters, and even quasi-CW or CW light sources are useful in that case. In all these applications the availability of comparatively cheap, small and reliable light sources is the precondition for the final commercial success of these new measurement techniques. Femto- and picosecond lasers that allow new techniques with improved accuracy, speed of measurement, and reliability are very much dependent on the development of cheaper and smaller short-pulse light sources. Therefore, the commercial success of this type of

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metrology applications that are based on short-pulse lasers depend very much on future developments of new lasers for these applications. Acknowledgements The described work was partially funded by the BMBF under the guidance of the VDI and thus Mr Sellhorst is especially acknowledged for his friendly and constructive support. The basic idea of the KoRad was initiated by Prof. Dr. Haeusler (University Erlangen-N¨ urnberg), who helped with his coworkers R. Gross, B. Wiesner, C. Richter to develop this measuring technique to the given stage. The measurement devices were built by Polytec GmbH with software support from WiSenT GmbH and very strong support from Dr Weigl (University Frankfurt/Main), who initiated the tooth project, and his coworkers Axel Bauer, Roland Felber, and Kristian Werelius. Furthermore, helpful and financial support from Rolls Royce Germany (Dahlewitz) and Bosch GmbH (Stuttgart) are acknowledged. The laser developments were based on close collaborations and financial support from the Jenoptik AG (Jena) and the IB-Laser (Berlin). For helping to organize this project and for their successful scientific work I thank my coworkers Michael Seefeldt, Ingo Brandenburg, Dr Axel Heuer, Dr Dieter Lorenz, Dr Volker Raab, and Dr Christian Spitz.

References [1] K. Bahlmann, S. Jakobs, S. W. Hell: 4pi-confocal microscopy of live cells, Ultramicrosc. 87, 155–164 (2001) 257 [2] V. Westphal, L. Kastrup, S. W. Hell: Lateral resolution of 28 nm (λ/25) in far-field fluorescence microscopy, Appl. Phys. B-Lasers O. 77, 377–380 (2003) 257 [3] A. Diaspro, M. Robello: Two-photon excitation of fluorescence for threedimensional optical imaging of biological structures, J. Photochem. Photobiol. B: Biol 55, 1–8 (2000) 257 [4] J. X. Cheng, Y. K. Jia, G. Zheng, X. S. Xie: Laser-scanning coherent anti-Stokes Raman scattering microscopy and applications to cell biology, Biophys. J. 83, 502–509 (2002) 258 [5] R. Menzel: Transient state laser spectrometer for observations of excited state absorptions (ESA), fractional bleaching (FB) and photophysical hole burning (PPHB), in Laser Jahrbuch 1990 (Vulkan, Essen 1990) pp. 223–229 258 [6] R. Menzel: Photonics, Linear and Nonlinear Interactions of Laser Light and Matter (Springer, Berlin, Heidelberg 2001) 258, 269, 273 [7] K. B. Atkinson: Close Range Photogrammetry and Machine Vision (Whittles 1996) 258 [8] H. J. Tiziani, H. M. Uhde: Three-dimensional image lensing with chromatic confocal microscopy, Appl. Opt. 33, 1838–1843 (1994) 258 [9] M. Idesawa, T. Yatagai, T. Soma: Scanning Moir´e method and automatic measurement of 3D-shapes, Appl. Opt. 16, 2152 (1977) 258

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[10] H. M. Shang, Y. Y. Hung, W. D. Luo, F. Chen: Surface profiling using shearography, Opt. Eng. 39, 23–31 (2000) 258 [11] L. S. Wang, D. L. Lee, M. Y. Nie, Z. W. Zheng: A study of the precision factors of large-scale object surface profile laser scanning measurement, J. Proc. Technol. Mater. 129, 584–587 (2002) 258 [12] G. Wang, B. Zheng, X. Li, Z. Houkes, P. P. L. Regtien: Modelling and calibration of the laser beam-scanning triangulation measurement system, Robot. Auton. Syst. pp. 267–277 (2002) 258 [13] M. Spanner, M. Y. Ivanov: Optimal generation of single-dispersion precompensated 1-fs-pulses by molecular phase modulation, Opt. Lett. 28, 576–578 (2003) 258 [14] M. Nisoli, S. D. Silvestri, O. Svelto, R. Szipcs, K. Ferencz, C. Spielmann, S. Sartania, F. Krausz: Compression of high-energy laser pulses below 5 fs, Opt. Lett. 22, 522–524 (1997) 258 [15] T. S. Rose, M. J. Rosker, A. H. Zewail: Femtosecond real-time observation of wave packet oscillations (resonance) in dissociation reactions, J. Comp. Phys. 88, 6672–6673 (1988) 258 [16] R. A. Farrer, M. J. R. Previte, C. E. Olson, L. A. Geyser, J. T. Fourkas, P. T. C. So: Single-molecule detection using a two-photon fluorescence microscope with fast-scanning capabilities and polarization sensitivity, Opt. Lett. 24, 1832 (1999) 258 [17] R. Ell, U. Morgner, F. X. K¨ artner, J. G. Fujimoto, E. P. Ippen, V. Scheuer, G. Angelow, T. Tschudi, M. J. Lederer, A. Boiko, B. Luther-Davies: Generation of 5-fs-pulses and octave-spanning spectra directly from a Ti:sapphire laser, Opt. Lett. 26, 373–375 (2001) 259 [18] T. Dresel, G. H¨ ausler, H. Venzke: Three-dimensional sensing of rough surfaces by coherence radar, Appl. Opt. 31, 919–925 (1992) 259, 260 [19] P. Ettl: Studien zur hochgenauen Objektvermessung mit dem Koh¨ arenzradar, Diplomarbeit, Universit¨ at Erlangen-N¨ urnberg (1995), in German 259, 260 [20] B. Golubovic, G. E. Bouma, G. J. Tearney, J. G. Fujimoto: Optical frequencydomain reflectometry using rapid wavelength tuning of a Cr4+ :forsterite laser, Opt. Lett. 22, 1704 (1997) 260 [21] D. Lorenz: Verst¨ arkungsgeschaltete Cr:YAG und Ti:Saphir Laser mit großer spektraler Bandbreite und guter Strahlqualit¨ at, Dissertation, Universit¨ at Potsdam (1999), in German 260, 261, 270, 271 [22] U. Morgner, W. Drexler, F. X. K¨ artner, X. D. Li, C. Pitris, E. P. Ippen, J. G. Fujimoto: Spectroscopic optical coherence tomography, Opt. Lett. 25, 111–113 (2000) 263 [23] B. E. Bouma, G. J. Tearney: Power-efficient nonreciprocal interferometer and linear-scanning fiber–optic catheter for optical coherence tomography, Opt. Lett. 24, 531–533 (1999) 263 [24] W. Drexler, U. Morgner, F. X. K¨ artner, C. Pitris, S. A. Boppart, X. D. Li, E. P. Ippen, J. G. Fujimoto: In vivo ultrahigh-resolution optical coherence tomography, Opt. Lett. 24, 1221–1223 (1999) 263 [25] G. M¨ uller (Ed.): Optical Tomography, Proc. SPIE (1994) 263 [26] G. H¨ ausler, M. Bail, J. Herrmann, R. Ringler, M. Lindner: Optical coherence tomography with the spectral radar, in Proc. SPIE 2925 (1996) pp. 298–303 263

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[27] A. R. Lazar: Eignung der fl¨ achenhaften Detektion zur Erweiterung der optischen Koh¨ arenztomographie an stark streuenden dermalen Strukturen, Dissertation, Universit¨ at Ulm (1999) 263 [28] J. G. Fujimoto, W. Drexler, U. Morgner, F. K¨ artner, E. Ippen: Optical coherence tomography: High resolution imaging using echoes of light, Opt. Phot. News 11, 25–31 (2000) 265 [29] I. Hartl, X. D. Li, C. Chudoba, R. K. Ghanta, T. H. Ko, J. G. Fujimoto, J. K. Ranka, R. S. Windsor: Ultrahigh-resolution optical coherence tomography using continuum generation in an air-silica microstructure optical fiber, Opt. Lett. 26, 608–610 (2001) 265 [30] B. Povazay, K. Bizheva, A. Unterhuber, B. Hermann, H. Sattmann, A. F. Fercher, W. Drexler, A. Apolonski, W. J. Wadsworth, J. C. Knight, P. S. J. Russell, M. Vetterlein, E. Scherzer: Submicrometer axial resolution optical coherence tomography, Opt. Lett. 27, 1800 (2002) 265 [31] M. Lindner, P. Andretzky, F. Kiesewetter, G. H¨ ausler: Spectral radar: Optical coherence tomography in the Fourier domain, in B. E. Bouma, G. J. Tearney, M. Dekker (Eds.): Handbook of Optical Coherence Tomography (New York 2002) pp. 335–357 265 [32] patent pending 265 [33] J. K. Ranka, R. S. Windeler, A. J. Stentz: Visible continuum generation in airsilica microstructure optical fibers with anomalous dispersion at 800 nm, Opt. Lett. 25, 25–27 (2000) 269 [34] S. Coen, A. H. L. Chau, R. Leonhardt, J. D. Harvey, J. C. Knight, W. J. Wadsworth, P. S. J. Russel: White-light supercontinuum generation with 60ps pump pulses in a photonic crystal fiber, Opt. Lett. 26, 1356–1358 (2001) 269 [35] J. Herrmann, U. Griebner, N. Zhavoronkov, A. Husakou, D. Nickel, J. C. Knight, W. J. Wadsworth, P. S. J. Russell, G. Korn: Experimental evidence for supercontinuum generation by fission of higher-order solitons in photonic fibers, Phys. Rev. Lett. 88, 173901–1–4 (2002) 269 [36] A. V. Husakou, J. Herrmann: Supercontinuum generation, four-wave mixing, and fission of higher-order solitons in photonic-crystal fibers, J. Opt. Soc. Am. B 19, 2171–2182 (2002) 269 [37] M. Seefeldt, A. Heuer, R. Menzel: Compact white-light source with an average output power of 24 W and 900 nm spectral bandwidth, Opt. Commun. 216, 199–202 (2003) 269 [38] W. J. Wadsworth, A. Ortigosa-Blanch, J. C. Knight, T. A. Birks, T. P. M. Man, P. S. J. Russell: Supercontinuum generation in photonic crystal fibers and optical fiber tapers: A novel light source, J. Opt. Soc. Am. B 19, 2148–2155 (2002) 269 [39] G. Genty, M. Lehtonen, H. Ludvigsen, J. Broeng, M. Kaivola: Spectral broadening of femtosecond pulses into continuum radiation in microstructured fibers, Opt. Express 10, 1083–1098 (2002) 269

Index

3D 3D 3D 3D 3D

information, 266 profile, 260 shape, 266 surface, 260, 278 surface shape, 280

absorption spectrum, 271 accuracy, 258, 275 bandwidth, 259, 265, 273 bandwidth-limited laser pulse, 267 brightness, 280 broad spectrum, 259 broadband diode lasers, 274, 275 broadband light, 259 broadband light source, 263, 267, 269 broadband operation, 270, 273 broadband operation of diode lasers, 273 broadened spectrum, 273 broadening, 269 camera, 278 coherence length of the laser, 260 coherence radar (KoRad), 259, 262, 267, 269, 273, 275–277 coherence radar (KoRad) measurement technique, 280 collimated light, 275 collimation optics, 273, 275 commercially available gain-switched Ti:sapphire laser, 274 converter, 266 correlogram, 260, 262 correlogram structure, 260 Cr:YAG, 270 deconvolution technique, 278

depth resolution, 260 diffraction-limited, 268, 271 dispersion, 268 dye, 271, 273 dye concentration, 273 dye transmission, 273 efficiency of the nonlinear process, 268 electric field strength, 261 fast moving machinery part, 279 femtosecond laser pulse, 265 femtosecond pulse, 258 femtosecond radar (FemRad), 259, 265, 267, 278, 280 femtosecond radar (FemRad) measuring principle, 266 fiber interferometer, 263 four-wave mixing (FWM), 269 Fourier analysis, 265 Fourier transform, 265 frequency bandwidth, 262 gain-guided broad stripe diode laser, 273 gain-switched laser, 271 gain-switched Ti:sapphire laser, 270, 273 high-brightness laser source, 277 interaction length, 268 KTP, 270 large spectral bandwidth, 267 lateral resolution, 260 Macaca Fascicularis, 280 mathematical evaluation, 277

Index measurement example, 275 medical application, 263 metrology, 260 metrology application, 275 Michelson interferometer, 259, 262, 263 microstructured fiber, 267, 279, 280 molecular process, 258 Nd:YAG laser, 270 nonlinear effect, 267 nonlinear measuring technique, 258 nonlinear microscopy, 257 nonlinear process, 268 nonlinear spectroscopy, 258 object reflectivity, 266 optical coherence tomography (OCT), 259, 263, 267, 269, 279, 280 optical spectrum analyzer, 268 peak power, 258 phase matching, 269 photographic picture, 266 photonic crystal fiber, 268 picosecond laser, 267, 269 prepared tooth structure, 276 production-process optimization, 277 pulse duration, 258 pulse-burst operation, 270 Q-switch, 270 real-time observation, 278 real-time quality control, 279 reference beam, 266 repetition rate, 278 resolution, 259, 265, 266, 279 retina, 265 retina of a monkey, 279, 280 rough surface, 259

285

saturable absorber, 271–273 scanning speed, 261 scanning steps, 260 self-phase modulation (SPM), 269 short coherence length, 267 short pulse, 258 signal-to-noise ratio, 267 single-shot technique, 278 skin tissue, 264, 265 soliton effect, 269 speckle, 259 spectral bandwidth, 262, 268, 269, 271, 273, 280 spectral broadening, 267, 269, 272 spectral intensity, 262 “spectral radar”, 263 spectrum of the Ti:sapphire laser, 272, 273 steep surface, 275 stimulated Raman scattering (SRS), 269 superluminescent diodes, 275 Ti:sapphire, 270 Ti:sapphire laser, 271 tooth structures, 275 TopDent, 276 transparent material, 265 triangulation, 258 turbine blade, 276, 277 turbine fan, 275 turbine surface, 277 white-light interference technique, 263 white-light interferometric, 267, 269 white-light interferometric application, 267 zero-dispersion wavelength, 268, 269

Primary Hazards and Reliability of Protective Materials Andreas Hertwig1 , Sven Martin1 , J¨ org Kr¨ uger1, Christian Spielmann2,3 , 2 1 Martin Lenner , and Wolfgang Kautek 1

2

3

Laboratory for Thin Film Technology, Federal Institute for Materials Research and Testing, Unter den Eichen 87, 12205 Berlin, Germany [email protected] Photonics Institute, Vienna University of Technology, Gusshausstr. 27–29/387, 1040 Vienna, Austria Department of Physics, University of W¨ urzburg, Am Hubland, 97074 W¨ urzburg, Germany

Abstract. Femtosecond laser pulses are close to industrial use. The advantages of ultrashort laser pulses for micromachining applications especially in the case of dielectric and biological samples down to pulse durations of 5 fs have been established. The current international standards of laser safety are primarily concerned with CW and pulsed lasers down to the nanosecond range. Therefore, human tissue and laser-protection equipment was investigated with respect to its resistance and protection performance for femtosecond laser illumination down to 30 fs. This included filter glasses for laser protection eyewear, polymer and textile materials used in curtains and guards. Bulk absorber filters can provide enough protection against laser radiation, give a sufficiently broad absorption spectrum. Materials are damaged more easily by femtosecond laser radiation. The need for sufficient spectral broadness as well as the different damage thresholds have to be included in international laser safety standards. This work should trigger the development of novel eye-protection devices that are lighter and ergonomically more acceptable than present commercial models.

1

Introduction

Ultrashort laser pulses have many advantages for the use in laser materials processing [1, 2, 3]. The most important are high precision and a very small heat-affected zone. Recently, even higher machining precision could be demonstrated when the pulse duration was below 50 fs down to less than 5 fs [4, 5, 6, 7]. With lower costs and greater simplicity, industrial and medical application of ultrashort sources comes into reach. This development brings up the issue of laser safety. The current international standards of laser safety (mainly IEC207, IEC11254, IEC60825) are primarily concerned with CW lasers and pulsed lasers down to the nanosecond range. The shortest pulse length respected in an international standard so far is 100 fs [8, 9, 10]. Therefore, the whole field of laser safety with pulsed laser sources has to be re-evaluated and extended into the femtosecond field [11, 12, 13, 14, 15]. This F. Dausinger, F. Lichtner, H. Lubatschowski (Eds.): Femtosecond Technology for Technical and Medical Applications, Topics Appl. Phys. 96, 287–308 (2004) c Springer-Verlag Berlin Heidelberg 2004


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includes all laser-protection materials, namely filter glasses, used in laser-protection eyewear, metal parts, polymer and textile materials used in curtains and guards, etc. Another very important field is the impact of femtosecond pulses on human tissue, mainly cornea and retina, but also all other skin surfaces [16]. There is an urgent need for exact numbers on the maximum acceptable dose of femtosecond pulses for human skin and eye tissue, on the protection levels of the common materials and their damage resistance.

2

Experimental

For all experiments, similar laser systems were used at Vienna University of Technology, W¨ urzburg University and at the Federal Institute for Materials Research and Testing, Berlin. The output pulses have durations of 20 fs to 30 fs at 800 nm center wavelength and an energy of up to 1 mJ at a repetition rate of 1 kHz. By controlling the trigger of the Pockels cell we were able to select a single or a train of a well-defined number of pulses without influencing other laser parameters. Additionally, the repetition rate could be varied. The pulse energy could be adjusted using a rotatable half-wave plate inserted before the compressor, which acts as a polarisation analyzer. At the facilities of the Federal Institute for Materials Research and Testing, pulse energies were measured with a 3Sigma Energy meter with a J25LP photodiode head of Molectron and with a 407A Power Meter of Spectra Physics. For online monitoring the pulse energy as well as the transmission of the samples, Si photodiodes were used together with a boxcar integrator/ amplifier of Stanford Research Systems. For some materials, damage detection was supported by acoustical measurements [17, 18]. In this case a Sony ECM-360 microphone was placed at a distance of ∼ 3 cm and at an angle of 45◦ from the sample surface. The damage was detected by recording the microphone signal with a Tektronix TDS 3032 storage oscilloscope. The amplitude of the first positive-going peak of the signal served as the identifier for the ablation strength. Transmission spectra were measured with a Perkin Elmer Lambda 900 spectrometer (Federal Institute for Materials Research and Testing). All post-mortem analysis and damage geometry/morphology measurements were done with a Nikon Eclipse 200 optical microscope and corresponding measurement software. The mode of operation (Nomarski polarimetry, dark-field microscopy) was chosen appropriately for maximum contrast. SEM (Scanning Electron Microscopy) pictures were taken with a Hitachi S-4100 (Federal Institute for Materials Research and Testing). At Vienna University of Technology and at W¨ urzburg University the compressed pulses were launched into a 1 m long hollow fiber with an inner diameter of 250 µm for an additional spatial filtering. It has been demonstrated that a hollow fiber efficiently suppresses high-order spatial modes resulting in an output beam with M 2 < 1.2. Filling the fiber with Ne also allowed the

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input spectrum (60 nm FWHM) to be broadened up to 150 nm. In this case the pulse could be further shortened to less than 10 fs using a chirped mirror compressor. The details of the laser system are described elsewhere [19]. The hollow fiber was evacuated for most of our experiments, acting only as spatial filter. The pulse duration was adjusted by varying the dispersion in the compressor by adding heavy flint-glass blocks. The pulse durations have been measured with a background-free autocorrelator. Only the 10 fs pulses have been characterized with a fringe-resolved autocorrelator. For pulses longer than 25 fs the pulse energy was controlled by a half-wave plate and a polariser. For the very short pulses this method is not applicable due to the large bandwidth. In a limited dynamic range we controlled the energy by inserting pellicle beamsplitters at different angles of incidence. By choosing the number and angle the energy can be reduced with negligible modifications of the pulse duration and beam direction. After setting the pulse parameters the pulses have been focused by spherical mirrors. The incident reflected and transmitted energy for each laser pulse was measured with fast diodes. The diode signals have been recorded with a sampling oscilloscope read out by a PC. Prior to the measurement the diodes were calibrated with a pyrolectric head connected to an energy meter (Molectron EPM-1000). The head (J3-09) was suitable to measure the energy of single pulses in a range between 0.4 µJ and 2 mJ. The sample thickness of 0.5 mm was chosen to have comparable energy levels for the transmitted and reflected pulses allowing the diodes to be calibrated with the same pyroelectric head, avoiding calibration uncertainties by using different detectors. The laser spot size in the focal range has been measured by imaging the spot onto a CCD camera. The CCD camera was not only used to measure the spot size, it allowed us also to measure the beam profile of the transmitted beam and, after slight modifications, also of the reflected beam.

3

Optical Filter Materials

Optical filter materials are the key component in laser safety goggles. They must show a sufficient extinction at the laser wavelengths (specified by the protection level) as well as sufficient visible light transmission (VLT). This leads to the use of mostly inorganic, and, more recently, organic glasses for these devices. Furthermore, dichroic mirrors and interference filters have been introduced recently by the manufacturers. The linear transmission spectrum, nonlinear effects (laser-induced transmission) as well as the damage behavior are of key importance when designing protective devices. 3.1

Linear Behavior

When looking at the spectral broadness of femtosecond laser pulses, it becomes clear that single-line absorption, as sufficient for narrowband CW lasers

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Fig. 1. Linear absorption spectra of Laservision BG36 and BG18 eye-protection filters. Filter thickness 5.3 mm each, as used in commercial laser-protection eyewear

provides insufficient protection. Therefore, current materials have first to be critically reviewed for absorption bandwidth. So far, two materials specified for protection at 800 nm have been investigated for the use with 30 fs pulses, namely the filters BG36 and BG16 of Schott. In Fig. 1, the linear absorption spectra of BG36 and BG18 are depicted. Clearly, BG18 shows a much broader absorption around 800 nm. When irradiated with 30 fs pulses of 40 nm spectral FWHM, the BG36 system shows considerable transmission, can be seen in Fig. 2. Here, the gray area gives the laser spectral intensity, the black line the transmission spectrum of the BG36 filter and the black area the transmitted part of the laser spectrum. While the majority of the laser radiation is absorbed in the filter a small quantity at the sides of the spectrum is transmitted. This amounts to 5% of the total integral pulse energy. Therefore the BG36 system – while acceptable for a narrow laser line at 800 nm – is insufficient for the broadband femtosecond laser radiation. In recent times, dielectric reflective filters are used in protective eyewear. The filter type T23 consists of two reflective layers on glass and an additional absorption filter block, which absorbs the near-infrared part. Figure 3 shows the transmission spectrum. The main component is a broadband high-reflective mirror around 800 nm. As dielectric mirrors provide an excellent contrast ratio they show a high-protection level as well as high VLT and can be very thin and light. 3.2

Nonlinear Behavior: Induced Transmission

With the described setup we measured the incident, transmitted and reflected energy as a function of the incident fluence, the pulse duration, and the num-

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Fig. 2. Transmission of parts of the laser light by BG36. The gray area is the laser emission spectrum. The black line is the transmission spectrum of the BG36 filter. The black area is the resulting transmitted light. The total energy of this amounts to ∼ 5% of the total laser energy

Fig. 3. Transmission spectrum of T23

ber of shots. To check the energy calibration of the diodes we measured the linear transmission. The input energy was reduced to a few microjoule and the sample was placed out of the focus to make the fluence as low as possible. For the estimated fluence of 1 mJ/cm2 we could safely rule out any nonlinear effects. The measured linear transmission for the 25 fs pulses (bandwidth < 60 nm at 800 nm) agrees well with the value calculated from the transmission data at 800 nm. The same transmission has also been estimated for the stretched pulses providing evidence for the lack of nonlinear effects. Only for the 10 fs pulses having a larger bandwidth (> 100 nm at 800 nm) we measured a more than 50% higher linear transmission. The higher transmis-

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Fig. 4. Measured spectra for the pulses having a duration of 25 fs and 10 fs and the transmission curve for a 0.5 mm thick BG18 sample

sion can be well explained in terms of the broader spectrum. A substantial amount of the energy is located in a spectral region where the absorption of the filter is reduced. The corresponding spectra and the spectral transmission curve are shown in Fig. 4. The transmission data have been taken from the Schott catalogue. From the data shown in Fig. 4 we calculated the integrated transmission for the spectrally broadened pulses, which was in reasonable agreement with the measured data. From these measurements a potential problem for the design and specification of laser-protective eyewear becomes evident: the large bandwidth. For the 10 fs pulses we have measured only a 50% higher transmission. Having a spectrum that is not only broadened, but also slightly shifted, the transmission could be easily up to an order of magnitude higher than the specified value. Therefore it is worth considering whether to specify not only the optical density at the center wavelength but also the maximum bandwidth for safe operation. In the first series we measured the single-shot behavior, i.e., we moved the sample after each shot to ensure a fresh surface for each shot. The transmission was measured for fluences ranging roughly from one order below to an order above the damage fluence. The measured transmission as a function of fluence is displayed in Fig. 5. The pulse duration was set to 10 fs, 25 fs, 175 fs and 1200 fs, respectively. For all pulse durations we found a qualitatively similar behavior. For the discussion of the obtained results we have therefore divided the whole fluence range into four subranges: 1. The linear range, the transmission is independent of the fluence, 2. the damage range, the transmission is reduced with increasing fluence, 3. the saturation range, where the transmission increases with increasing fluence around 1 ps, and 4. the so-called revival range, characterized by an increasing transmission for fluences well above the damage fluence. In the first range, the linear range, we measured a transmission that was the same as that obtained for the linear absorption measurement. In a wide

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Fig. 5. Measured single-shot transmission as a function of fluence and pulse duration for a 0.5 mm thick sample of BG 18. The total fluence range can be divided into four subranges: 1. linear range, 2. fluence above damage fluence and below saturation fluence, 3. above saturation fluence and below damage fluence, and 4. fluence much larger than the damage fluence

range the transmission is independent of pulse duration and fluence. Only for the 10 fs pulses we did measure an increased transmission, which is caused by the broader spectrum as described in the previous paragraph. The results suggest that in a wide fluence and pulse width range the filter operates properly. The second range is the so-called damage range. After reaching a welldefined fluence the transmission starts to drop. The magnitude of the fluence at which the decrease starts is in reasonable agreement with measured damage thresholds. It should be noted that determining the damage threshold as described above is based on the measurement of the ablated volume versus the fluence. In spite of this difference we obtained the same damage threshold with the transmission measurements. Therefore both methods are equally well suited for estimating the damage threshold. The reduction of the transmission is caused by several reasons: laser energy is nonlinearly absorbed by the glass causing a lattice heating and subsequently melting and irreversible damage. For femtosecond laser pulses the damage threshold is very well defined. Therefore damage at the threshold occurs only at the central part of the beam with a Gaussian intensity distribution. The well-localized, and at the beginning very small damaged area explains the moderate drop of the transmission at the threshold. Increasing the fluence well above the damage threshold is accompanied by a substantial reduction of the transmitted energy. A larger damaged area, and an increased reflectivity for the trailing part of the pulse caused by the plasma generated by the leading edge of

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the pulse are mainly responsible for the observed behavior. Measurements of the reflected energy and time–space resolved measurements of the absorbed and transmitted energy confirmed the described mechanism. The third range is the so-called saturation regime. For pulses longer than 1 ps we have observed an increased transmission prior to the onset of damage. Our measurements and previous publications predicted a pulse-duration-dependent damage threshold fluence for dielectric materials. We measured an increase of the damage threshold of about 1.2 J/cm2 to 3 J/cm2 for pulse durations of 10 fs and 1.2 ps, respectively. The higher damage threshold for picosecond pulses allows deposition of more energy into the sample prior to damage. Increasing the amount of deposited energy makes it feasible to observe a saturation of the absorption. Using a model considering the temporal and spatial distribution of the pulses we calculated the transmission for various saturation fluences. The best agreement between the measurement and our calculation was found by assuming a saturation fluence of about 1.5 J/cm2. The observed increase in the transmission is moderate, and limited by the onset of damage. Our measurements suggest that there is no intensity-dependent saturation of the absorption and an energy-dependent saturation of the absorption can only be observed for longer pulses. Summing up, the characterized filter maintains its protective properties for pulses below 1 ps up to fluences nearly one order of magnitude above the damage threshold. The energy-dependent saturation of the absorption is a much more severe limitation for longer pulses due to the increased damage threshold. However, despite the optimum filter characteristic up to fluences one order of magnitude above the damage threshold we measured an increase of the transmission for very high fluences. To explain the observed behavior in this fourth region we measured the transmitted spectra. For the highest fluences we observed a substantial blueshift of the transmitted spectrum. The frequency components at the short-wavelength range were generated by selfphase modulation in the sample and suffered a lower linear absorption by the filter. Frequency broadening by self-phase modulation can only be observed for very high intensities. Due to the linear absorption the peak intensity is reduced substantially during propagation through the sample, i.e., the effective sample length is very short. The short effective length is sufficient for a substantial broadening only for very high intensities . The increased transmission due to spectral broadening is well observable for the 25 fs pulses, sets in for the 175 fs pulses at the highest fluence and is absent for the 1.2 ps pulses. This pulse-length dependence provides further evidence for the proposed mechanism. All the measurements described so far are single-shot measurements, which are well suited to study the underlying physical mechanisms. However, for real-world applications, the multishot transmission is of much more interest. The corresponding safety regulations require that laser-safety eyewear must maintain their protective properties for 10 s. To test the material we repeated

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Fig. 6. Measured transmission of a BG18 sample as a function of fluence for a pulse duration of 25 fs and an increasing number of laser shots on the same spot

Fig. 7. Measured transmission of a BG18 sample as a function of fluence for a pulse duration of 1.2 ps and an increasing number of laser shots on the same spot

the above-described measurement with more than 1 shot on the same spot. After applying a well-defined number N of pulses we measured the transmission of the last pulse of the series. The results obtained for pulse durations of 25 fs and 1.2 ps are shown in Fig. 6 and Fig. 7, respectively. First, we did not observe any severe degradation in the multishot regime. For both pulse durations we were able to confirm the results of previous measurements predicting a decreasing damage threshold with increasing number of shots. For the shorter pulses the damage threshold decreased from 1.5 J/cm2 to 1 J/cm2 by increasing N from 1 to 10 000, respectively. The

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minimum observed transmission scales with the number of shots. The lowest measured transmission was about 0.1% after applying 10 000 pulses with a fluence of 10 J/cm2 at the same spot. Independent measurements with a CCD camera have shown that the amount of radiation scattered in all directions increased with the number of shots on the same spot. The strong scattering is mainly responsible for the reduction of the transmission. For pulses as long as 1.2 ps we have observed the same behavior. The only difference is that the reduction of the damage threshold with increasing number of shots, results in damage before saturation could take place. The minimum observed transmittance is about a factor of three lower than for the 25 fs pulses. The further reduction is attributed to an enhanced scattering of the damaged spot. It is well known that the morphology of the damaged spot depends on the pulse duration, and for longer pulse durations the surface is strongly modified. We have measured the transmission of pulses from 10 fs to 1 ps through filters used for laser-protective eyewear. For all pulse durations the “linear optical density” is nearly maintained for fluences up to the damage threshold. Above the damage threshold the nonlinear absorption reduces the amount of transmitted energy, enhancing the protective properties. Only for fluences far above the damage threshold we observed an increased transmission due to self-phase modulation induced spectral broadening. Our measurements suggest that the filter material used for protective eyewear keeps its protective properties for subpicosecond pulses without any degradation. 3.3

Surface Damage on Filters

An important indicator of filter failure is the occurrence of surface damage. Surface damage can have several effects on a glass filter, such as an increase of transmittance and a lower resistance to thermal and mechanical stress. Therefore, a damaged filter should always be replaced, even if it still shows sufficient absorption. For the normal case (irradiation at a highly absorbed wavelength), the occurrence of damage in the bulk is insignificant. Thus surface damage (ablation) is also an indicator for damage in general. It is especially important to examine ablation thresholds when using shorter pulses, because these thresholds are generally shown to be lower for shorter pulse durations [20]. This is usually explained in terms of an avalanche ionization picture [21] where a sufficient charge density is necessary to damage a dielectric material. The threshold fluence is decreasing with the pulse duration in the subpicosecond range [5]. With respect to this, it becomes necessary to re-evaluate all damage thresholds for very short laser pulses. In the case of a Gaussian beam profile, damage thresholds can be obtained easily from damage evaluation at different pulse energies [22]
 F0 2 2 . (1) D = 2w ln Fth

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Fig. 8. Determination of the beam radius and ablation threshold in a BG18 sample with 100 pulses of 30 fs each. The upper panel shows the squared crater diameter versus pulse energy. The lower shows the same data in terms of fluence. The intersection point with the x-axis is the ablation threshold fluence

Here, Fth denotes the ablation threshold, F0 the maximum fluence of the beam, w the Gaussian beam radius (1/e2 value) and D the observed ablation crater diameter. According to this, the ablation threshold can be obtained together with the beam radius when plotting the squared crater diameter versus the pulse energy, which yields the beam radius as the slope. The beam radius is also obtained when only the integral pulse energies are known, so the fluence can be calculated with only energy and crater-size measurements. The advantage of this method – later denoted as the “D2 method” – is that no additional measurement of the beam radius is necessary, both parameters, F0 and w are obtained from one single fit. This procedure is not yet accepted in the international standards, namely ISO11254-2, which requests a probabilistic evaluation after measuring the beam profile very carefully. However, the D2 method is fully equivalent in its results to the standard method, as has been shown elsewhere [23]. In Fig. 8, this process is demonstrated on a sample of the BG18 filter. The main component of BG18 is an ion-doped phosphate glass matrix [24]. Regarding the damage morphology there are different fluence regimes. Figure 9 shows a comparison of these. At very high fluences, deep holes are created and the debris is still hot enough to heat the surrounding filter surface above the softening point. At low fluences, mainly surface ablation with very high lateral ablation is observed. The damage morphology shows the well-known ripple pattern in the ablation crater, the origin of which is not yet clarified [25].

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(a)

(b)

Fig. 9. Damage morphology of BG18 at 30 fs pulse duration: (a) low-fluence regime (1 J/cm2 , 100-on-1) at 100 Hz repetition rate, (b) high-fluence regime (5.5 J/cm2 , 1000-on-1) at 1000 Hz repetition rate

As mentioned above, the dependence of the ablation threshold on the pulse duration is of prime importance when evaluating eye-protection equipment for femtosecond lasers. As can be seen in Fig. 10, the threshold rises by a factor of 2 to 4 when going from 30 fs to 350 fs. This complies with the theory of avalanche photoionization as mentioned above. With shorter pulses, the probability of high-order multiphoton processes rises, resulting in production of more seed electrons as needed for ablation. The effect scales oppositely to the pulse number and is strongest for single-pulse experiments. This behavior is not fully understood. One explanation gives rise to thermal effects when regarding multipulse experiments. As thermal accumulation and predamage come into play, the avalanche-ionization mechanism has less influence on the threshold. The threshold depending on the pulse number shows a corresponding behavior (Fig. 11): Longer pulses lead to a strong dependence of the threshold on the pulse number, which means that for long pulses the incubation is much stronger than for short pulse lengths. This becomes even more evident by the fact that in our experiments we observed little or no dependence on the repetition rate, as Table 1 shows. Table 1. Ablation threshold depending on the repetition rate of BG18. Number of pulses: 1000, pulse duration: 30 fs Repetition rate Hz

Ablation threshold fluence J/cm2

1 10 100 1000

0.97 0.96 0.97 0.94

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Fig. 10. Dependence of the ablation threshold on the pulse duration. Measured on BG18 with 1000 Hz repetition rate, pulse number as given

We can conclude that the ablation with very short pulses is less sensitive to the “history” of the irradiated site, i.e., shows less incubation effects. Comparison with other colored and uncolored glass matrices [12] leads to the following conclusions: • Dopants show surprisingly low influence on the surface-damage behavior of dielectrics in this pulse-duration range. This holds for the absolute value of the thresholds as well as for the dependence on pulse number and duration. • The threshold is also independent of the absorbance of the glass. These findings strongly indicate that the ablation process is based on multiphoton processes and is mostly nonthermal in nature. The key material property for the ablation threshold with femtosecond pulses seems to be the bandgap of the matrix material. A very interesting finding in this context is a – normally unexpected – dependence of the ablation threshold on the laser spot size. This effect has already been observed in various materials [26, 27] and explained with different theoretical models. In Fig. 12 this effect is displayed for BG18. Clearly, the area-normalized threshold becomes smaller when the beam size is bigger. The thermal-ablation model according to [27] is insufficient for describing this behavior as it diverges to +∞ for small spot sizes. Therefore we focus on a defect-site model [26] that is more qualitative in nature but explains the findings much more exactly. It is based on the following assumptions: • There are randomly distributed defect sites on the sample surface with a significantly smaller damage threshold than the surface itself. • There is a certain probability P of hitting a defect site, depending on the site density (expressed in terms of a mean site distance d0 , which is a material property) and on the beam radius.

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Fig. 11. Ablation threshold over pulse number at different pulse durations. Repetition rate: 1000 Hz

Fig. 12. Ablation threshold versus beam radius. Points: Measured threshold of BG18 at 1000 Hz repetition rate, 30 fs, 1000 pulses. Solid line: defect-site model according to [26]

In this theory the size-dependent threshold is Fth (w0 ) = P (w0 )Fd + (1 − P (w0 )) Fi ,

(2)

where Fd and Fi are the thresholds of the defect sites and of the surface

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(intrinsic threshold), P (w0 ) is the probability of hitting a defect site: 
2  π 2 w0 P (w0 ) = 1 − exp − . 32 d0

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(3)

This model fits very accurately to the measured thresholds, as can be seen in Fig. 12. The corresponding parameters are Fd = 0.1 J/cm2, Fi = 0.9 J/cm2 , d0 = 605 µm. These values are reasonable, but the method does not provide information about the nature of the defects or the mechanism of ablation. Furthermore, one should note that this is a multipulse phenomenon, which occurs at very high pulse numbers. The dependence of this effect on pulse numbers, especially the implications on single-pulse measurements, are yet to be assessed. The implications of these findings on laser-safety equipment are small as absorption filters are normally thick enough to provide protection for a certain time (service life). However, as surface-damage thresholds are to be specified (IEC 60825-1) this effect should not be neglected. For future normative texts it may be necessary to require very specific beam sizes when measuring surface-ablation thresholds. While thick absorption filters provide some protection even when damaged they are heavy and inconvenient for the operator. This is also a security issue because inconveniencies while using security equipment always increase the chance of insecure behavior. Therefore, many efforts are currently being made to develop thinner – lighter – filters based on dielectric reflectors. While providing extremely low transmission at very specific wavelengths and potentially a high VLT, they incorporate some dangers: • They have an angle-dependent transmission spectrum: The reflectivity must be broad enough spectrally to ensure sufficient protection at all possible incident angles. • They have no security reserve: Once the reflective surface is damaged by the laser or by mechanical influences, the protection is made void. We have addressed the latter issue experimentally by measuring the transmission of the laser beam by surface-coated mirrors during surface-ablation measurements. Figure 13 shows the result of this at two different fluences, both only slightly above the ablation threshold (0.15 J/cm2 for 1000 pulses at 1000 Hz repetition rate and 30 fs pulse duration). As expected, the filter still provides protection for ∼ 50 pulses and ∼ 120 pulses, respectively, but then undergoes a sudden failure. The transmission reaches a maximum when the mirror layer is completely destroyed and decreases afterwards due to dispersion losses, as the laser also ablates the bulk-substrate material. This behavior comes from the lower ablation threshold of the thin mirror layer compared to the substrate (∼ 0.5 J/cm2). Obviously, this property of a mirrored filter holds an inherent danger. Within only a few milliseconds, the

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Fig. 13. Transmission of a ISS008 mirror (used in the T23 filter) on float glass during an ablation experiment at 1000 Hz, 30 fs with 1000 pulses, respectively. Fluences as given in the graph; both fluences were slightly above the ablation threshold for 1000 pulses (0.15 J/cm2 )

protection is annihilated, even at fluences near the surface-ablation threshold, while bulk-absorption filters provide some protection even when ablated (if not destroyed suddenly by thermal stress). Therefore, a surface-mirrored design is not sufficient for eye protection. Absorption filters, though they may be bulky and heavy, are a failsafe design for protection against femtosecond-laser radiation. Their surface-damage thresholds are generally smaller with femtosecond pulses than in the nanosecond regime, but they still show very high service lives. With the standard commercial femtosecond equipment (∼ 1 mJ, sub-100 fs, 1000 Hz) it is not possible to drill a hole through a BG18 filter. For future devices, it will be necessary to sensibly combine the advantages of bulk filters and mirror filters.

4 4.1

Guards and Curtains Introduction: Strategies of Guard Design

For practical use of lasers, especially in the industrial field, it is of key importance to have reliable laser guards. There are many guard designs possible, such as hard metal plates, partly transparent polymer foils, or flexible curtains of textile fabric. The key feature is always the resistance to the laser radiation, i.e., a service life as long as possible when irradiated. Further important properties are: • Low reflectivity. The material must absorb or disperse the incident light. • Fire protection. The material must not be flammable.

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Fig. 14. Design of the laser-protection curtains

• Mechanical and acoustical properties, antistatic and electrical isolator properties, ease of use and assembly, etc. As the use of polymer foils in the area of laser protection is developed at this time, we will focus on textile compound laser-protection curtains (Laservision). The curtains consist of alternating layers of a black polymer felt material and polymer-stabilized aluminum foil. This stack is enclosed in a textile fabric. Two different curtains have been investigated, one thinner with 4 inner layers (2 aluminum, 2 felt) and a simple polymer-mixture textile outside, one thicker with 6 inner layers and a rough metal-coated fabric outside (Fig. 14). The protective effect of these materials comes from all components together. The combination of felt and metal foils causes a high radiation resistance. If the laser beam accidentally penetrates the outer textile layer, it is “caught” by the inner layers by being reflected by the metal and absorbed by the felt. 4.2

Surface-Damage Behavior

All materials have been investigated for their surface-damage behavior with the same method as the filter materials. The special surface morphology of textile materials causes some difficulty when detecting the damage crater size. Figure 15 shows this. While the crater is still more or less clearly detectable on an Al surface (Fig. 15a), this becomes more and more difficult in a textile mesh (Fig. 15b) or a felt sample (Fig. 15c). Therefore, we have incorporated the acoustic measurement as a very simple method of detecting the surface damage as described above. Figure 16 shows how the threshold is determined

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(a)

(b)

(c)

Fig. 15. Damage craters on single layers of the laser-protection curtains. (a) Aluminum, (b) fine-meshed textile, (c) felt

Fig. 16. Determination of the ablation-threshold fluence with an acoustic measurement on the fine textile mesh with 30 fs pulses at 1000 Hz

by plotting the microphone signal against the fluence. Unfortunately, this method is only practicable for pulse numbers of 10 or less, the most accurate results coming from single-pulse measurements. At least these measurements allow a comparison of the different components (see Table 2). The thresholds are very similar. None of these components could serve as a laser-protection device on its own, as they are thin, easily destroyed, mechanically unstable and often (in the case of the textile materials) do not provide a close shield. Only all components together offer enough protection. 4.3

Failure

From these findings arises the question of failure of the overall system. Apart from mechanical damages, this can only happen when the laser drills through

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Table 2. Ablation thresholds of different laser-curtain components Material

Number of pulses

Fth (J/cm2 )

Polyethylene on Al Rough fabric, metal coated Fine fabric Aluminum Felt

1000-on-1 1000-on-1 10-on-1 1000-on-1 10-on-1

0.19 0.3 0.3 0.2 0.3

the entire width of the curtain. This can easily be measured by detecting the transmitted laser energy behind the curtain with a power meter. As the damage hole size is of the same order of magnitude as the beam the situation is that of a beam diffracted at a self-created pinhole. For these experiments, the sample is placed very near to the focus of the beam to ensure sufficient energy for complete destruction of the curtain and also to ensure little variation of the beam size over the sample thickness. There are several standardized times that the service life of laser guards must exceed (according to IEC60825-4). These are 10 s for operation under constant observation by an operator, 100 s for operation under frequent observation (job-to-job operation), 30 000 s for unobserved operation. The currently commercially available 30 fs system offers not enough long-time stability for day-long operation, so we have focused on 10 s and 100 s (10 000 and 100 000 pulses). We have measured the transmitted energy at the end of these time periods. This gives a fluence threshold at which complete failure of the curtain occurs (interleaf threshold Fil ). In Fig. 17, this is displayed for one of the examined curtains. Surprisingly, the interleaf threshold does not depend on the irradiation time. Only the transmitted energy becomes larger at longer irradiation times, which can be attributed to a broader damage channel through the sample. Furthermore, Fil varies very little with the repetition rate of the laser. Fil is a very valuable parameter for the resistance of a guard system to constant laser irradiation. As is seen in Table 3, thicker curtains do not necessarily give longer protection, once the failure fluence is reached. When going to thick curtains, this fluence only becomes higher. Table 3. Interleaf thresholds for the different curtain types Curtain type

Irradiation time s

Fil J/cm2

6 layers

10 100 10 100

1.05 1.04 2.1 2.0

8 layers

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Fig. 17. Determination of the interleaf threshold at different irradiation times with 30 fs pulses at 1000 Hz. Sample: thin curtain (6 layers). Irradiation time as given

5

Summary

The necessity of evaluating potential dangers of femtosecond lasers is evident. With our measurements in the context of the SAFEST project we have examined some important laser-protection materials. We have shown that bulk absorber filters can provide enough protection against laser radiation, given a sufficiently broad absorption spectrum. Generally, the materials are damaged on the surface more easily by femtosecond laser radiation, but this does not mean that they do not provide protection. The need for spectral broadness as well as the different damage thresholds should be included in future versions of the international standards texts concerned with laser safety. Furthermore, the use of the D2 method that gives a very simple access to ablation thresholds should be standardized as a material-testing procedure. Our ongoing work on new filter materials aims at the development of new eye-protection devices that are thinner and lighter than the currently used ones. This incorporates use of new materials, especially optical polymers and of combinations of mirror layers and absorber filters.

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Index

absorption bandwidth, 290 acoustic measurement, 288, 303 avalanche ionization, 298 avalanche photoionization, 298

laser safety, 287 laser spot size, 299

damage threshold, 287 defect-site, 299 dielectric mirror, 290

nonlinear behavior, 290

eye-protection devices, 287 filter glass, 287 Gaussian beam radius, 297 human tissue, 287 induced transmission, 290

multiphoton processes, 299

primary hazard, 287 protective eyewear, 290 protective material, 287 reliability, 287 saturation, 294 textile material, 287 transmission spectrum, 290

Secondary Hazards: Particle and X-Ray Emission Jens Bunte1 , Stephan Barcikowski1, Thomas Puester1 , Tomas Burmester1 , Martin Brose2 , and Thomas Ludwig2 1 2

Laser Zentrum Hannover e. V., Hollerithallee 8, 30419 Hannover, Germany Berufsgenossenschaft Feinmechanik und Elektrotechnik, [email protected] Gustav-Heinemann-Ufer 130, 50968 K¨ oln, Germany

Abstract. Apart from primary hazards caused by the laser beam, ultrashort pulse lasers also pose secondary nonbeam hazards by gaseous and particulate laser-generated air contaminants or ionizing radiation. Though the emission rates for femtosecond-laser applications are remarkably lower than for conventional laser technologies (such as cutting, welding or cladding), the high respirability of particles can pose health risks, especially if carcinogenic, toxic materials (such as Ni or Cu), or, in medical applications infectious tissues, are processed. For human tissue a clear shift of the mean aerodynamic diameter of the aerosols to smaller diameters compared to conventional lasers is observed. This raises new questions in the field of ultrafine particles. Suitable capture systems near to the processing zone or personal protective equipment (PPE) such as respiratory masks are required to avoid possible health risks. Due to the extremely high pulse intensities, X-rays can be generated if laser radiation interacts with matter. In cases of material processing, X-rays are unintentionally directly generated. The emission rate decisively depends on the laser parameters and the physical properties of the target material. The dose rate the employees are exposed to results from the emission rate and the design of the process zone (open or enclosed) or their distance from the target. Depending on the laser process parameters, investigations reveal that legal TLV for exposure to ionising radiation can be exceeded. This obliges a risk assessment to be carried out for X-rays on the basis of which adequate safety measures have to be applied.

1

Introduction

Apart from primary hazards caused by the laser beam, a laser installation can also pose secondary hazards. These nonbeam hazards are either related to the laser device (system-component-related hazards such as electricity, laser gases, optics) or to the application. Application-related hazards arise by the application process itself; such as the generation of fumes, gases or secondary radiation. Concerning femtosecond laser applications, two application-related hazards are relevant from their risk potential: • Gaseous and particulate laser-generated air contaminants (LGACs), which are generated by the ablation process and released to the environment F. Dausinger, F. Lichtner, H. Lubatschowski (Eds.): Femtosecond Technology for Technical and Medical Applications, Topics Appl. Phys. 96, 309–321 (2004) c Springer-Verlag Berlin Heidelberg 2004


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• Ionizing radiation, which is produced when the hot electrons of the plasma re-enter the target material and generate X-rays Nonbeam hazards, in particular laser-generated air contaminants (LGACs) released during laser material processing, are well characterized up to now and the decomposition products that emerge during macroprocessing have been found to be hazardous [1, 2, 3, 4, 5, 6, 7]. Though the emission rates for femtosecond-laser applications are expected to be remarkable lower than for conventional laser technologies (such as cutting, welding or cladding), the high respirability of the particles poses health risks, especially if carcinogenic, toxic materials (such as Ni or Cu) or infectious tissues are processed. Hence, emission characterization for femtosecond laser material processing is necessary to assure a safe use of the laser and environmentally compatible handling of the waste gas. Since femtosecond-laser applications cover industrial and medical fields, investigations have been carried out on technical materials and human tissue. Hazards from ionizing radiation are a new risk potential related to lasers, raised with the use of femtosecond-laser systems and up to now that is much less studied than other hazards. Investigations on ionizing radiation have been aimed at unintentional generation of X-rays, as they can arise during material processing in ambient air (e.g. drilling of injection nozzles). Devices that are specially designed to generate ionizing radiation (for material analysis) such as laser-driven X-ray tubes [8, 9] are not the subject of investigation, since in most cases, these systems are enclosed to meet the safety requirements defined in legal guidelines (Germany: Strahlenschutzverordnung, R¨ ontgenverordnung [10, 11]).

2

Experimental

Investigations on the characterization of LGACs have been performed using a Ti:sapphire fs laser system (Thales Bright, see Table 1) The target materials were processed using a scanner optic (Scanlab GmbH; Typ Scangine 14). The feed rate was adjusted to a pulse-to-pulse distance of 15 µm at a beam diameter of 50 µm. Due to natural inhomogenities of the tissue material, samples from different body parts (back of the hand, groin, chest, leg, torso) and from different patients were taken. Immediately after the removal from the patient, the tissue samples were ice-cooled and stretched onto cork. The PM (particulate matter) was captured using a semiopen process chamber. The aerosols were sucked out of the chamber and guided to the online measuring unit. The characterization of the particle-size distribution was carried out by an automatic 12-level low-pressure cascade impactor (Dekati, Inc.; Typ ELPI), which allows postanalysis of the particle morphology by scanning electron microscopy (SEM). To assess the hazards caused by ionizing radiation, the dose or the dose rate, respectively, and the spectrum of the X-rays are to be determined.

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The dose was measured with thermoluminescence dosimeter chips and sticks (TLDs). Two TLD chips (size: 3.2 mm × 3.2 mm × 0.05 mm) and sticks (size: 1 mm × 1 mm × 6 mm) are combined into a 4-detector array. Several detector arrays were placed at different distances from the target. The detector array is shrink-wrapped in a plastic film, to avoid exposure to light that would cause errors. The TLD measuring principle is advantageous compared to electronic measuring systems, since even short-pulse radiation can be recorded without inaccuracies. The TLDs are calibrated using an average energy of 17 keV. The spectrum of the ionizing radiation was measured with an Amptek X-ray detector (XR-100CR), using a thermoelectrically cooled Si-PIN photodiode. The detector size is 2.4 mm × 2.8 mm with a silicon layer thickness of 300 µm. The measuring range is limited with regard to the lower level by the eryllium window (approx. 4 keV at 50% transmission rate, window thickness: 0.5 mm) and to the upper level by the active depth of the silicon detector (approx. 11 keV at 50% detection efficiency, active detector depth: 300 µm). The energy resolution at 5.9 keV (55 Fe) is 220 eV FWHM with 12 µs shaping time. Investigations on the characterization of ionizing radiation have been carried out with several femtosecond laser systems (Table 1). The spot size was adjusted in each case to 50 µm. Table 1. Specification of the Ti:sapphire laser systems used for the characterization of secondary hazards Laser

Pulse duration fs

Center wavelength nm

Repetition rate kHz

Thales bright Femto compactPro

150 30

780 796

5 1

3

Particle Emissions

Laser manufacturers and users are obliged to identify and evaluate the actual risks in the plant on the basis of risk assessments. This includes occupational safety (workplace) as well as air quality (environment) as a consequence of the use of femtosecond-laser machines for material processing [12]. If the LGACs are released to the workplace, regulations defining the requirements on the workplace air by limiting the concentration of harmful substances (threshold limit values TLVs) have to be considered. The declaration 26 of the article 137 formulates directives that are specifically related to safety and health [13]. Occupational safety is regulated by law for chemicals (98/24/EG [14]) and carcinogenic compounds (97/42/EG [15]), which includes harmful aerosols as well. The national implementation in Germany, for example, is the regulation for the execution of the federal emission control law (BImSchG [16]) and the

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Fig. 1. Comparison of the particle-size distribution generated during femtosecondlaser ablation of polypropylene and human dermal tissue

regulation of hazardous substances list of the employer’s liability insurance association of the Institute for Industrial Safety. Also in the debate on public health, particulate matter components are becoming more important in policy terms. Particle deposition in the respiratory tract depends upon the dimensions of the particles [17]. Inhaling fine particles, particularly those under 10 µm in diameter (PM10 ), can increase the frequency and severity of lung problems and even trigger premature death. Thus, in describing the particle loading of the air, information on the distribution of particle size has to be given in addition to the mass concentration [18]. This shows the neccessity to evaluate the hazards by LGACs for femtosecond-laser technology. Due to the high energy density of the femtosecond-laser radiation, the material or human tissue is removed by either photoablation and/or photodisruption processes [19]. Generally, the particles generated by femtosecond-laser processing have small aerodynamic diameters, are airborne and highly respirable. When processing polymers or tissue the particulate emissions contain toxic hydrocarbon components and – for tissue – can even have a virolent potential [7]. Investigations on polymers – polypropylene (PP) – generates particles with a mean aerodynamic diameter of 0.3 µm (Fig. 1), which is comparable to those of conventional laser processing [7]. However, it was found that the particle emissions of human dermal tissue show a significantly smaller median diameter compared to those of polymer materials. Detailed investigations on processing human tissue reveal that with increasing laser fluence (0.01 J/cm2 to J/cm2 the median diameter decreases

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Fig. 2. Particle morphology and particle-size distribution generated during femtosecond-laser ablation at different laser fluences (wavelength: 780 nm; pulse duration: 150 fs; repetition rate: 5000 s−1 ; spot diameter: 50 µm)

slightly lying in a range of 0.045 µm to 0.035 µm (Fig. 2). On varying the pulse duration in the range of 150 fs to 400 fs or the pulse rate no significant changes in the particle-size distribution could be observed. Due to the high yield of particulate matter of a size < 50 nm, providing sufficient effective respirator mask devices is essential [19]. The morphology of the PM shows an inhomogeneous distribution of different structures over all investigated particle-size fractions (see Fig. 2 REM images). The various forms of spherical as well as fibrous particles show a significant tendency to agglomerate and a strong adhesive character. Due to the sticky character of the particles, strict requirements concerning air-cleaning systems as well as respiratory masks have to be met [20]. Particle emissions generated by femtosecond-laser treatment of human dermal tissue show a median diameter that is considerably smaller than particles generated with conventional laser sources such as CO2 or Nd:YAG laser (Fig. 3). Due to the toxic or infectious potential of PM the operator and medical personnel have to be efficiently protected from inhaling particle emissions. Since the ELPI measurement system (0.03 µm to 3 µm) used only provides a measuring range down to 0.03 µm, the existence of ultrafine particles less then 30 nm is most likely. The fraction precipitation rates of the respiratory masks or filters have to be designed to meet these requirements.

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Fig. 3. Comparison of the particle-size distribution generated during femtosecondlaser ablation of human dermal tissue with the PM generated during Nd:YAG and CO2 laser ablation

The fine particles have a negligible settling velocity in air, so that efficient capture near the source of emission is necessary to avoid contamination of the workplace [21, 22]. Even if the emission source (the femtosecond-laser material processing machine) is not subject to US or EU standard in detail, the “lowest achievable emission rate” should be applied [23, 24]. To reduce the impact on the environment, air-cleaning systems have to be used. The technical demands on air-cleaning systems for LGACs released during processing of organic materials are determined by the characteristics of the LGACs and the levels of the emission rates. As shown above, the LGACs released during femtosecond-laser ablation are characterised by fine and ultrafine particles and different levels of emission rates. Based upon available information, the maximum achievable control technology (MACT) emission limitation and control technology should achieve the maximum degree of reduction in emission of hazardous air pollutants (HAP) that can be achieved by utilising those control technologies. Numerous filter systems such as activated-carbon adsorption [20], thermal combustion [7], catalytic combustion [7] and biofiltration systems [2,25] have been characterised and qualified for various conventional laser material processing applications [25,26,27]. The surface filter or deep-bed filter is the MACT for the filtration of LGACs of femtosecond ablation. Deep-bed filtration has the capability to separate particles down to < 0.1 mm, but the filter material has to be disposed of after saturation, increasing operational costs. Surface filtration allows the separation of a high particle concentration and automatic cleaning of the filter media, but is often more expensive (investment costs). The application of these MACT as off-gas techniques is recommended to guarantee

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best employee protection and occupational safety and health. But supporting documentation including identification of the selected control technology to meet the recommended MACT emission limitation apply to the owner or operator of the source of hazardous air pollutants for which the administrator has failed to promulgate an emission standard [28, 29, 30]. As shown above, process byproducts of femtosecond-laser ablation are found to be hazardous due to the small median diameter of the particulate matter. But the overall mass rates are comparatively low, so that TLVs for harmful substances in the workplace air will not be exceeded, if an emission capture module is applied. An exhaust volume flow of 20 m3 /min to 100 m3 /min is proved to assure sufficient effectively. As a result, safety precautions regarding nonbeam hazards will contribute marginally to the operating costs of a femtosecond-laser system. Even if in future, higher repetition rates and pulse power result in increased emission mass rates, waste-gas filtration systems for LGACs are already available as MACT. Further studies are to be carried out on the nonbeam hazards of femtosecond-laser ablation of tissue or materials that may generate infectious particles.

4

X-Ray Emissions

Due to the extremely high pulse intensities, ultrashort-pulsed lasers can produce a plasma and hot electrons when interacting with matter. When these electrons re-enter the solid material, X-rays are generated. Generally, X-rays are characterized by the polychromatic continuum (bremsstrahlung) and the characteristic line radiation. The ratio of Bremsstrahlung and line radiation depends on the physical properties of the material being processed and the process parameters. To assess the hazards of these unintentionally generated X-rays for people (laser operators) one has to consider the workplace and the properties of ionizing radiation. Many femtosecond micromachining applications are carried out in an open process zone in ambient air. The typical distance of the operator to the target can be assumed to be 0.5 m. The propagation of X-rays through air is determined by two factors, which together have to be considered (Fig. 4): • The transmission of air for X-rays of a certain photon energy at ambient pressure • The intensity decrease with distance from the X-ray source; if one assumes the X-ray source to be a point source, the intensity will decrease with distance according to the distance square law (1/r2 ) Investigations have been carried out on ablating copper target plates that were moved by an x–y handling system. Results reveal that there is

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Open Process Zone Transmission of Air

Intensity

1.0 0.9

0.0030

Normalized Signal: I s/I0

Transmission

0.8 0.7 0.6

Path: 1 cm Path: 5 cm Path: 10 cm Path: 25 cm Path: 50 cm

0.5 0.4 0.3 0.2

Area = 16 A0

Area = 4 A0 Area = 1 A0 Point source r 2r

0.0035

0.0025 0.0020

4r

0.0015 0.0010 0.0005

0.1 0.0000

0.0 0

2.5

5

7.5

10

12.5

15

17.5

0

20

5

10

15

20

25

30

35

40

45

50

55

Distance from the Target [cm]

Photon Energy [keV]

Intensity decreases by 1/r²

©LZH

Fig. 4. Criteria for propagation of the ionizing radiation Dose rate (TLD-signals) Laser Laser Parameter Parameter Pulse Pulse Duration: Duration: Center Center Wavelength: Wavelength: Repetition Repetition Rate: Rate: Pulse Energy: Pulse Energy: Spot Size: Spot Size: Peak Peak Pulse Pulse Intensity: Intensity: Target Target Material: Material:

Dose rate [µSv/h]

50

40

30

20

30 30 fs fs 796 796 nm nm 11 kHz kHz 500 µJ 500 µJ 50 50 µm µm 8.5E+14 8.5E+14 W/cm² W/cm² Copper Copper

10

0 0

10

20

30

40

50

60

Distance to the target [cm] ©LZH

Fig. 5. Results of X-ray measurements

a threshold area, and X-ray generation starts. For laser-pulse intensities below 1 × 1014 W/cm2 , the TLD signals are below the detection limit of 2.7 µSv, which corresponds to a dose rate less than 1.8 µSv/h. Analysis of the X-ray spectrum shows a bremsstrahlung spectrum with a small FWHM at low photon energies. With the femto compactpro laser, pulse intensities of 8.5 × 1014 W/cm2 could be achieved. TLD signals confirm that the dose rate decreases with increasing distance (Fig. 5). For a distance of 0.5 m, a dose rate of 2 µSv/h is calculated. The X-ray spectrum is dominated by a line spectrum of Kα at 8.05 keV.

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The potential risk of X-rays for people is affected by a number of parameters. Apart from the material to be processed and its physical properties (electron shell/passes) the laser parameters such as pulse intensity, repetition rate, and spot size (at constant intensity) are found to largely determine the emission rate and thus the dose rate for exposed persons. The design of the process zone – absorption properties of wall materials – and the distance of the operator from the emission source are workplace-related parameters, decisively determining the dose rate. Pursuant to the German laws and legal regulations for ionizing radiation – acc. R¨ontgenverordnung [11] – a femtosecond-laser installation for material processing has to be categorized as an “external interferring radiation emitter”. Depending on the emission rate it is subject to legal authorization, or subject to notification, if the dose rate at a distance of 0.1 m to the touchable surface is less than 1 µSv/h. According to the Strahlenschutzverordnung/R¨ ontgenverordnung [10,11] – the operators of the femtosecond-laser system are classified into the category of nonoccupationally exposed persons (to ionizing radiation). For this category of persons, the following TLV has been defined: • For the effective dose: 1 µSv/a • For the dose for skin: 50 µSv/a Comparing the results of the above-mentioned investigations with these thresholds, it can be concluded that with the tabletop systems used and the realized laser-parameter settings, at a typical distance of the laser operator to the X-ray source of about half a meter no acute danger will arise. Proceeding from a dose rate of 2 µSv/h measured in a typical distance of 0.5 m, • 500 h of exposure will cause an exceedance of the TLV for the effective dose • 25 000 h will cause an exceedance of the TLV for the dose for skin However, the first investigation on laser systems providing higher pulse intensities show an increased emission rate and dose rate, giving potential for considerably exceeding the TLVs. These data have to be verified and supplemented by further measurements. To ensure a safe use of femtosecond-laser technology, a risk assessment must include the evaluation of hazards by ionizing radiation. On the basis of the risk analysis a safety concept has to be drawn up and – if necessary – adequate safety measures such as shielding have to be applied.

5

Summary

As can be concluded, the assessment of secondary hazards for femtosecondlaser applications is important with regard to safety of machinery and workplace safety. Both the manufacturer of a femtosecond-laser material process-

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ing machine and the user have to be aware of the hazards and should be able to apply qualified safety measures. Investigations reveal that the femtosecond-laser technology poses hazards already known from material processing with conventional lasers, such as hazards by the generation of particulate and gaseous emissions. However, the detailed characteristics of the particles generated by femtosecond-laser processing of human tissue show a remarkable shift of the mean aerodynamic diameter of the aerosols to smaller diameters. This raises new questions in the field of ultrafine particles. Since the particles are highly respirable, hazards occur especially if materials are processed that release fumes containing carcinogenic or toxic substances. In medical applications the ablation of infectious tissue causes high risks, if the particles are inhaled. Therefore, suitable capture systems near to the processing zone or personal protective equipment (PPE) such as respiratory masks are required to avoid possible health risks. On the other hand, the application of ultrashort-pulsed lasers causes new hazards, which have to be considered. Due to the extremely high pulse intensities, X-rays can be generated if laser radiation interacts with matter. In cases of material processing the X-rays are unintentionally directly generated. The emission rate decisively depends on the laser parameters and the physical properties of the target material. The dose rate the employees are exposed to results from the emission rate and the design of the process zone or their distance from the target. Depending on the laser process parameters, legal TLV for exposure to ionizing radiation can be exceeded. With increasing output power of the femtosecond-laser systems marked exceedances of legal TLVs are to be expected. This obliges all concerned to carry out a risk assessment for X-rays on the basis of which adequate safety measures have to be applied. The investigations reveal that for femtosecond-laser material processing suitable safety concepts for nonbeam hazards must be drawn up on the basis of technical standards for safety of machinery and risk analysis [31, 32]. The risk analysis has to be addressed to the manifold applications of femtosecond laser technology on the laboratory scale, in industry and in medical facilities.

References [1] S. Barcikowski, T. Burmester, M. Goede, T. P¨ uster: The fume hazard in laser cutting and how to deal with it, in Proc. AILU Workshop “Processing plastics with lasers” (AILU, Coventry 2000) 310 [2] S. Barcikowski, M. Goede, H. Haferkamp: Waste gas treatment of particular and volatile emissions from laser material processing of polymers - comparison of multifunctional filtration concepts, in (Proc. USC-TRG Conference on Biofiltration 2000) pp. 115–122 310, 314 [3] S. Barcikowski, M. Goede, H. Haferkamp: Incorporating environmental aspects in quality control of laser material processing, in (LaserOpto 2001) pp. 68–71 310

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[4] J. S. Wittbecker: Gefahrstoffermittlung bei der CO: 2-Laserstrahlbearbeitung, Dissertation, Univ. Hannover (1993), in German 310 [5] A. Hampe: Filtration von Emissionen bei der Laserstrahlbearbeitung (VDI, D¨ usseldorf 1997) 310 [6] K. D. Engel: Partikel- und gasf¨ ormige Emissionen bei der Materialbearbeitung mit gepulsten Nd: YAG-Lasern (VDI, D¨ usseldorf 1995) 310 [7] M. Goede: Enstehung und Minderung der Schadstoffemissionen bei der Laserstarhlbearbeitung von Polymerwerkstoffen, VDI Fortschrittberichte, Reihe 5 587 (2000) in German 310, 312, 314 [8] A. Egbert, B. Mader, B. Tkachenko, C. Fallnich, B. N. Chichkov: Highrepetition rate femtosecond laser-driven hard-X-ray source, Appl. Phys. Lett. 81 (2002) 310 [9] A. Rousse, C. Rischel, J. C. Gauthier: Colloquium: Femtosecond X-ray crystallography, Rev. Mod. Phys. 73 (2001) 310 [10] H. M. Veith: Strahlenschutzverordnung – Neufassung 2001 (Bundesanzeiger, K¨ oln 2001) in German 310, 317 [11] E. Witt: R¨ ontgenverordnung, 5th ed. (Carl Heymanns Verlag, K¨ oln 2002) in German 310, 317 [12] Council Directive 96/62/EC of 27 September 1996 on ambient air quality assessment and management. Official Journal L 296, 21/11/1996 P. 0055-0063 311 [13] Declaration (No.26) of the article 137 (ex-article 118) of the contract for the establishment of the European community 311 [14] Council Directive 98/24/EC of 7 April 1998 on the protection of the health and safety of workers from the risks related to chemical agents at work (fourteenth individual Directive within the meaning of Article 16(1) of Directive 89/391/EEC) 311 [15] Council Directive 97/42/EC of 27 June 1997 amending for the first time Directive 90/394/EEC on the protection of workers from the risks related to exposure to carcinogens at work (Sixth individual Directive within the meaning of Article 16 (1) of Directive 89/391/EEC). Official Journal L 179, 08/07/1997 P. 0004 - 0006 CONSLEG - 90L0394 - 08/07/1997 - 20 P. 311 [16] N.N.: Gesetz zum Schutz vor sch¨ adlichen Umwelteinwirkungen durch Luftverunreinigungen, Ger¨ ausche, Ersch¨ utterungen und a ¨hnliche Vorg¨ ange (Bundes-Immissionsschutzgesetz – BImSchG) in der Fassung der Bekanntmachung vom 26. September 2002 (BGBl. I S. 3830), einschließlich der ¨ Anderung vom 21.8.2002 (BGBl. I S. 3322 (3341)) 311 [17] N.N.: Sulfur oxides and suspended particulate matter. Environmental Health Criteria No. 8. World Health Organization, Geneva 1979 312 [18] ISO 1995 Air Quality – Particle size fraction definitions for health-related sampling. International Standard ISO (7708) International Organization for Standardization, Geneva 312 [19] T. Meier: Bewertung von Abbrandprodukten bei der medizinischen Laseranwendung, in VDI-Sonderband Laser in der Materialbearbeitung (VDITechnologiezentrum 1996) in German 312, 313 [20] H. Haferkamp, T. Burmester, K. Schulz, M. Goede, J. Bunte: Effizientes und wirtschaftliches Abluftreinigungsverfahren f¨ ur die thermische Polymerwerkstoffbearbeitung, WLB Wasser, Luft und Boden 7/8, 69–72 (2001) in German 313, 314

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[21] H. Haferkamp, F. von Alvensleben, D. Seebaum, M. Goede, T. P¨ uster: Air contaminants generated during laser processing of organic materials and protective measures, in (Int. Laser Safety Conf. 1997) 314 [22] H. Haferkamp, M. Goede, T. P¨ uster, et al.: The fume hazards in laser cutting and how to deal with it, in Processing plastics with lasers (AILU workshop 2000) 314 [23] Council Directive 1999/30/EC of 22 April 1999 relating to limit values for sulphur dioxide, nitrogen dioxide and oxides of nitrogen, particulate matter and lead in ambient air. Official Journal L 163, 29/06/1999 P. 0041 - 0060 314 [24] Environmental Data, Germany (2003) 314 [25] H. Haferkamp, J. Bunte, S. Barcikowski, R. Sattari, D. Hesse, D. M¨ uller: Geruchsminderung Aerosol enthaltender Abluft - Erfolgreicher Einsatz der Biofiltration f¨ ur das Laserschneiden von Holzwerkstoffen, WLB Wasser, Luft und Boden 7/8, 54–58 (2002) 314 [26] F. von Alvensleben, S. Barcikowski, E. Vitzthum, M. Goede, H. Haferkamp: Abl¨ ufte aus der laserbearbeitung richtig filtrieren, Laser Mag. (1998) in German 314 [27] S. Barcikowski, M. Goede, H. Haferkamp: Bioremediation of micron and submicron particular emissions exhausted from laser material processing of polymers, in (Biotechnology 2000 2000) p. 354 314 [28] 40 CFR Part 63: Part II Environmental Protection Agency. Federal Register: March 23, 2001 (Volume 66, Number 57) Proposed Rules, Page 16317-1(6360) Revised as of July 1, (2000) From the Federal Register Online via GPO Access: DOCID:fr23mr01-36 315 [29] CAA90: Clean Air Act 1990, Sec. 112 and Title 42 - The public health and welfare. Chapter 85 – Air pollution prevention and control. Subchapter 1 – Programs and activities. Part A – Air quality emission limitations. Hazardous air pollutants. From the U.S. Code Online via GPO Access: CITE: 42USC(7412) (Laws in effect as of January 27, 1998) 315 [30] 29CFR1910: Occupational safety and health standards. Title 29 – Labor chapter 17 – Occupational safety and health administration, department of labor (continued) Part (1910) 29 CFR 1910, Page 7–543. 315 [31] ISO/TR 12100-1:1992, Safety Of Machinery – Part 1: Basic Concepts, General Principles of Design - Basic Terminology, Methodology (1992) 318 [32] DIN EN:1997-01, Safety of machinery – Principles for risk assessment 318

Index

ablation, 314 aerosol, 310, 311 contamination, 314 dose, 310, 317 filter, 314 infectious particle, 315 ionizing radiation, 310 laser-generated air contaminant (LGAC), 309

particle, 312, 314 particle emission, 311 photoablation, 312 photodisruption, 312 pollutant, 314 primary hazard, 309 propagation of X-rays, 315 safety, 311 secondary hazard, 309 tissue, 310 X-ray, 310, 315

Index

3D 3D 3D 3D 3D 3D

information, 266 profile, 260 shape, 266 shaping of ceramics, 180 surface, 260, 278 surface shape, 280

ablated material, 85 ablation, 314 ablation plume, 83, 84 ablation rate, 109, 117, 125, 132, 133, 136 ablation rate average, 132, 133, 139 ablation rate depth dependence, 134 ablation rate linear, 137 ablation rate reduction, 133, 136 ablation rate volume, 137, 139 ablation strategy, 180 ablation threshold of bovine tissue, 203 absorption, 77 absorption bandwidth, 290 absorption linear, 139 absorption nonlinear, 139 absorption spectrum, 271 accumulated vapor, 87 accumulation, 85 accuracy, 258, 275 acoustic measurement, 288, 303 active beam control, 207 active mode locking, 11 additive pulse mode locking (APM), 14, 22 adhesive, 172 adhesive dentistry, 172 aerodynamic window, 150, 151 aerosol, 310, 311 air atmosphere, 77–82 all ceramic restoration, 174

amplified spontaneous emission (ASE), 40, 69 amplifier, 17 application, 117, 121, 128 atmosphere, 75, 80, 81 atmosphere air, 138, 139, 149 atmosphere helium, 136 atmosphere low pressure, 150 atmosphere pressure, 80, 81, 136, 137, 150, 151 avalanche ionization, 75, 76, 96, 298 avalanche photoionization, 298 bandwidth, 259, 265, 273 bandwidth-limited laser pulse, 267 BBO Pockels cell, 25 beam divergence, 79, 80 beam profile, 78, 83, 141, 149, 150 beam profile distortion, 137, 148 bending loss, 39 BMBF, V bone tissue, 211 brain tissue, 203 breakdown, 75, 77, 78, 83, 86, 87 breakdown by accumulated vapor, 87 breakdown dependence on pulse duration, 138 breakdown filament, 77 breakdown shock wave, 83, 84 breakdown threshold, 75, 76 brightness, 280 broad spectrum, 259 broadband diode lasers, 274, 275 broadband light, 259 broadband light source, 263, 267, 269 broadband operation, 270, 273 broadband operation of diode lasers, 273

322

Index

broadened spectrum, 273 broadening, 269 bubble, 187, 193 bulk crystal, 17 burr, 117 CAD/CAM, 174 camera, 278 capillary, 136, 148 caries, 167 caries selectivity, 172 caries therapy, 167 caries-selective tool, 171 cavitation, 98 cavity, 168 cell, 229, 230 cell membrane, 228 cell surgery, 227, 230 chirped-pulse amplification, 18, 27, 40 chromosomes, 229, 230 coherence length of the laser, 260 coherence radar (KoRad), 175, 259, 262, 267, 269, 273, 275–277 coherence radar (KoRad) measurement technique, 280 collimated light, 275 collimation optics, 273, 275 commercially available gain-switched Ti:sapphire laser, 274 confocal laser scanning microscope, 228 conical emission, 78, 80 conical emission beam divergence, 137 conical emission beam-profile distortion, 137 contamination, 314 converter, 266 coreless end cap, 48 cornea, 188 correlogram, 260, 262 correlogram structure, 260 Coulomb explosion, 105 CPA, 18, 27 Cr:YAG, 270 crack, 155, 157, 159 cutting, 155 cutting tool, 155 cylindrical shock wave, 83 cylindrical symmetry, 77, 78

damage threshold, 287 deconvolution technique, 278 defect-site, 299 density of the extractable energy, 65 dental CAD/CAM system, 182 dental ceramic, 174 dental handpiece, 173 dentin, 168 dependence on pulse duration, 75 depth measurement, 181 depth resolution, 260 diamond, 155 dielectric, 105 dielectric material, 105 dielectric mirror, 290 diffraction-limited, 268, 271 dispersion, 268 distortion, 83 distortion of the beam profile, 78 DNA, 227 DNA strands, 230 dose, 310, 317 double-clad fiber, 36 drilling conical holes, 142, 144 drilling efficiency, 140, 144, 146, 151 drilling energy coupling, 133 drilling geometry, 133, 134, 136, 142, 144 drilling heat conduction, 134 drilling hole diameter, 137, 139, 141, 142, 144 drilling in vacuum, 137, 139, 150, 151 drilling inclination angle, 143–145 drilling of aluminum, 134 drilling of metals, 131, 140 drilling of steel, 131–133, 138, 139, 141, 142, 144, 147, 149 drilling percussion, 140 drilling precision, 140, 144 drilling quality, 140, 146 drilling rate, see ablation rate, 136 drilling rate linear, 135 drilling techniques, 140 drilling time, 144–147 drilling trepanning, 141 drilling trepanning radius, 144, 145 dye, 271, 273 dye concentration, 273

Index

323

dye transmission, 273

FST, 2

efficiency of the nonlinear process, 268 electric field strength, 261 electro-optical modulator, 31 electromechanical hearing devices, 212 electron, 96 electron–phonon, 107 embossing, 128 energy content, 77, 78 energy coupling, 133 energy loss, 76, 77 evaporation, 109 extractable energy, 48 extraction efficiency, 67 extreme ultraviolet (EUV), 242, 243 eye-protection devices, 287

gain modulation, 12 gain narrowing, 26, 27 gain-guided broad stripe diode laser, 273 gain-switched laser, 271 gain-switched Ti:sapphire laser, 270, 273 gas, 162 gas breakdown, 77 Gaussian beam radius, 297 gene transfection, 230 geometrical multipass amplifier, 63 grinding, 177 group-velocity dispersion, 93

f-theta objective, 142 fast moving machinery part, 279 fast saturable absorber, 14 femtosecond laser pulse, 265 femtosecond pulse, 258 femtosecond radar (FemRad), 259, 265, 267, 278, 280 femtosecond radar (FemRad) measuring principle, 266 femtosecond technology, 2 fiber interferometer, 263 fiber laser, 17, 18 filament, 77, 82, 83, 99 filamentation, 99 filling material, 172 filter, 314 filter glass, 287 flap, 190 fluorescence microscope, 230 fluorescence microscopy, 228 focal plane, 77, 79, 80 four-wave mixing (FWM), 81, 269 Fourier analysis, 265 Fourier transform, 265 free, 96 free electron, 96 free-electron, 95 free-form surfaces, 176 frequency bandwidth, 262 frequency conversion, 29 frequency doubling, 29

hard bremsstrahlung, 247 heat conduction, 134 heat load, 105 heat transport, 135 heat-penetration depth, 134 helical drilling, 140–142, 144–150 helium atmosphere, 80, 82 hemispherical shock wave, 83 high-brightness laser source, 277 high-intensity femtosecond laser, 241 Hirschegg model, 132 human tissue, 287 impact ionization, 75 implantable hearing aid, 211 induced transmission, 290 infectious particle, 315 intensity, 1 intensity autocorrelation, 27 interaction length, 268 interaction with atmosphere, 75 ionization, 75, 236 ionizing radiation, 310 keratoplasty, 197 Kerr effect, 14 Kerr-lens mode locking (KLM), 14, 59 KTP, 270 large mode area fiber, 38, 39 large spectral bandwidth, 267 laser drilling, see drilling

324

Index

laser in situ keratomileusis (LASIK), 187 laser machining, 143 laser microbeam, 227 laser pressure catapulting, 227 laser safety, 287 laser spot size, 299 laser tweezer, 227 laser-beam delivery, 207 laser-generated air contaminant (LGAC), 309 lateral resolution, 260 ˇ light-induced Cerenkov emission, 82 location of breakdown, 77, 80 location of nonlinear interaction, 79 loss modulation, 12 Macaca Fascicularis, 280 manufacturing, 174 material expulsion, 136, 140 material processing, 24, 31 mathematical evaluation, 277 measurement example, 275 medical application, 263 melt, 109, 117, 140, 141 metal, 105 metrology, 260 metrology application, 275 Michelson interferometer, 259, 262, 263 microcracking, 169 microdissection, 227 micromachining, 17, 24, 142, 174 microscope, 228 microstructured fiber, 46, 267, 279, 280 microstructuring, 117 middle ear, 212 Mie-scattering photography, 86–88 milling, 174 minimally invasive, 170 mitochondria, 230 mitochondrion, 229 mode locking, 10, 58 modeling of drilling, 134 molecular process, 258 multipass amplifier, 63 multiphoton absorption, 75 multiphoton effect, 2 multiphoton ionization, 76, 82, 95 multiphoton microscopy, 228

multiphoton processes, 299 nanosecond, 117, 155 Nd:YAG laser, 270 Nd:YVO4 , 17, 19–21, 24 Nd:YVO4 oscillator, 20, 22 neodymium-doped material, 19 neuronavigation, 206 neurosurgery, 204 neurosurgical instrument, 205 nonlinear absorption, 3, 95, 228 nonlinear behavior, 290 nonlinear effect, 267 nonlinear frequency conversion, 29 nonlinear measuring technique, 258 nonlinear microscopy, 257 nonlinear phenomena, 78, 81 nonlinear process, 268 nonlinear Schr¨ odinger equation (NLSE), 37 nonlinear spectroscopy, 258 object reflectivity, 266 optical breakdown, 75–77, 227 optical coherence tomography (OCT), 259, 263, 267, 269, 279, 280 optical Kerr effect, 82 optical spectrum analyzer, 268 optoinjection, 228, 230 ossicles, 212 overlap, 119 oxide ceramic, 176 pain, 167 parabolic pulse, 40 Parkinson’s disease, 204 particle, 312, 314 particle emission, 311 passive mode locking, 13 peak power, 258 percussion drilling, 84, 138, 140, 141, 147 phase matching, 269 phase self-adjusting mode locking (PSM), 14, 22 photoablation, 312 photobleaching, 228 photodisruption, 187, 312 photodissociation, 101

Index photographic picture, 266 photonic crystal fiber, 48, 268 picosecond laser, 267, 269 piston model, 110 plasma, 76, 83, 134, 150 plasma absorption, 77, 137 plasma atmospheric, 134, 136, 138 plasma emission, 75, 76 plasma formation, 95 plasma metal, 135 plasma particle-ignited, 135, 136 plasma threshold, 135, 136 plasma vapor plasma, 135 plasma-spark intensity, 208 Pockels cell, 24, 31, 64, 69 polarization, 147, 149, 150, 160 polarization control, 148–150 polarization in laser machining, 148 pollutant, 314 precision, 117, 155 prepared tooth structure, 276 Presbyopia, 198 pressure, 101 primary hazard, 287, 309 printing, 128 printing technology, 121, 123 PRK, 188 production-process optimization, 277 propagation of X-rays, 315 protective eyewear, 290 protective material, 287 pulse duration, 77, 117, 140, 157, 179, 258 pulse picker, 24, 25 pulse-burst operation, 270 pump optic, 56 Q-switching, 9 Q-switch, 270 quality, 117, 155–157 radiation–atmosphere interaction, 83 Raman effect, 37 real-time observation, 278 real-time quality control, 279 recast, 105, 110, 117, 140 reference beam, 266 refractive surgery, 187 regenerative amplifier, 24, 25, 63

325

relaxation time, 105, 107, 109 reliability, 287 removal of neural tissue, 203 repetition rate, 85, 117, 125, 146, 147, 163, 278 resolution, 259, 265, 266, 279 resonance absorption, 83 retentive pattern, 172 retina, 265 retina of a monkey, 279, 280 ripple, 137, 148, 149 rough surface, 259 roughnessies, 157 RTP, 31 safety, 311 saturable absorber, 14, 15, 271–273 saturation, 294 saturation fluence, 48 scanner, 142, 143 scanning speed, 261 scanning steps, 260 secondary hazard, 309 seed oscillator, 20 self-defocusing, 82 self-focusing, 49, 82, 92, 93, 99 self-mode locking, 13 self-phase modulation (SPM), 14, 27, 37, 44, 81, 93, 269 SESAM, 15, 20, 21, 58 shadowgraphy, 77, 83 shock wave, 77, 82 shock wave energy content, 85 shock wave symmetry, 83 short coherence length, 267 short pulse, 258 side effect, 192 signal-to-noise ratio, 267 single-shot technique, 278 skin tissue, 264, 265 slow saturable absorber, 14 solidification, 109 soliton effect, 269 spatial hole burning, 20 speckle, 259 spectral bandwidth, 262, 268, 269, 271, 273, 280 spectral broadening, 267, 269, 272 spectral intensity, 262

326

Index

“spectral radar”, 263 spectrum of the Ti:sapphire laser, 272, 273 steel, 133 steep surface, 275 stereotactic neurosurgery, 206 stimulated Brillouin scattering (SBS), 37 stimulated Raman scattering (SRS), 37, 46, 269 streak, 99, 196 stress birefringence, 42 subcellular photodisruption, 227 superluminescent diodes, 275 surface measurement, 175 symmetry, 77 synchronous pumping, 12 textile material, 287 thermal damage, 167 thermal effects, 138 thermal effects dependence on pulse duration, 138 “thermal” lens, 69 thin-disk, 17, 18 thin-disk laser, 55 Ti:sapphire, 17–19, 27, 270 Ti:sapphire laser, 271 Ti:sapphire regenerative amplifier, 18 tissue, 310 tooth structures, 275 TopDent, 276 transients, 101 transmission grating, 42 transmission spectrum, 290 transmutation, 248 transparent material, 2, 265 transparent media, 3 trepanning optic, 144, 145, 148 triangulation, 258 tribological structure, 128 tribology, 117, 121 turbine blade, 276, 277

turbine fan, 275 turbine surface, 277 two-temperature model, 107 tympanic, 211 ultrashort, 1, 155 ultrashort Kα -puls, 245 ultrashort pulse generation, 9 ultrashort pulse laser, 17 ultrashort pulse laser oscillator, 20 vacuum nozzle, 150, 151 vapor, 82, 83 vapor accumulation, 84, 86, 88 vapor cloud, 83, 87 vapor cloud expansion, 87 vapor expansion, 86 vapor flow, 83, 85–87 vapor particle, 86, 87 vapor plume, 83 vaporization, 108 VDI, V waveguide, 3 wavelength conversion, 78–80 Weibull, 178 white light, 94 white-light interference technique, 263 white-light interferometric, 267, 269 white-light interferometric application, 267 X-ray, 235, 310, 315 X-ray diffractometry, 178 Y-TZP, 176 Yb-doped material, 18 Yb:KGW, 18, 19 Yb:KYW, 61 Yb:YAG, 17, 18, 23, 60 Yb:YAG oscillator, 22 zero-dispersion wavelength, 268, 269 zirconium, 176