Photonic Glasses

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Fuxi Gan • Lei Xu editors

PHOTONIC GlflSS€S

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Fuxi Gan .. Lei Xu Fudan University, China

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Published by World Scientific Publishing Co. Pte. Ltd. 5 Toh Tuck Link, Singapore 596224 USA office: 27 Warren Street, Suite 401-402, Hackensack, NJ 07601 UK office: 57 Shelton Street, Covent Garden, London WC2H 9HE

British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library.

PHOTONIC GLASSES Copyright © 2006 by World Scientific Publishing Co. Pte. Ltd. All rights reserved. This book, or parts thereof, may not be reproduced in any form or by any means, electronic or mechanical, including photocopying, recording or any information storage and retrieval system now known or to be invented, without written permission from the Publisher.

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ISBN 981-256-820-4

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Preface

Photonics is a systematic science dealing with the photon generation and detection, as well as stimulated emission, photon frequency conversion and polarization change. The invention of laser in the middle of the last century was a great event for the development of photonics, in which photons are performed as the information and energy carriers. The photonics has been popularized recently, and indicates its rapidly escalating importance in the future. The exploration of new photonic materials, including photonic glasses, is a key issue for advance photonic devices. Inorganic glasses have been used as optical materials for a long time due to their isotropy, hence to make the large size and high optical homogeneity more easily, and high transparency over a wide spectral range from ultraviolet to infrared, as well as linear functional properties. Therefore, inorganic glasses still play essential role in photonics used as transmitting and linear functional media for photonics. Since the emergence of lasers, transition element doped glasses have become one of the most important laser materials. The laser glasses, glass fibers and waveguides are still the main groups of solid state laser materials. The nonlinear optical effects of inorganic glasses are always happened with intense electromagnetic field, where as the glasses can perform the nonlinear optical functional roles. The interaction of ultra-short laser pulse with glass media produces a series of new optical phenomena, which can be applied in fabrication of new photonic devices. Inorganic glasses have also been developed from three dimensional (bulk materials) to low dimensional (thin films and fibers), which play a significant role

V

VI

Preface

in photonic devices for optical data storage, optical communication, as well as optical processing and display. Authors of this book have been actively engaged in this field and made notable contributions to the above mentioned subjects and published many papers in both domestic and international journals. An attempt was made in this monograph to summarize the research results in photonic glasses, which have been achieved by the authors' research groups in Shanghai Institute of Optics and Fine Mechanics and Fudan University mainly in recent 15 years, and to compile all experimental information from journals and proceedings in a book. It is not quite enough to select those chapters in this book for reflecting all aspects of photonic glasses due to imbalance for our research achievements. It might be possible to ignore some important subjects or contents due to the limit of the authors' knowledge and incomplete access to published materials. We hope that our readers, especially the photonics experts, will give us their valuable suggestions in order to make correction and complements for us in the next edition. The authors of this book are grateful to the China National Natural Science foundation, Chinese Academy of Science and Chinese Ministry of Education for their continuous support to the research projects. Acknowledgements are made to the journals from which some figures have been reproduced. The editors wish to extend the gratitude to our associates who contributed to compile this book, and particularly thank Prof. Shouyun Tian, Ms. Bo Ma, Ms. Hongxia Zhao, for their assistance in preparing the manuscript.

Fuxi Gan Lei Xu Fudan University, Shanghai December, 2005

Contents

Preface

v

List of Contributors

xi

1. From Optical Glass to Photonic Glass 1.1 Introduction 1.2 Physical Fundamentals 1.3 Optical Glasses 1.4 Photonic Glasses 2. Structure and Properties of Amorphous Thin Films for Optical Data Storage 2.1 Amorphous Rare Earth-transition Metal (RE-TM) Alloy Thin Films 2.2 Amorphous Metallic and Chalcogenide Thin Films 2.3 Nonlinear Optical Amorphous Alloy Thin Films 3. New Developments in Optics and Spectroscopy of Rare Earth Ions Doped Glasses 3.1 Laser Spectroscopy of Nd 3+ and Yb 3+ High Doped Glasses 3.2 Nonlinear Luminescence of Rare-Earth (RE) Ions in Glasses 3.3 Super-luminescence of RE-doped Glass Fibers 4. Third-Order Optical Nonlinear Properties of Glasses 4.1 Measurement of Third-Order Optical Nonlinear Susceptibility of Glass 4.2 Optical Nonlinearity of Dielectric Glasses 4.3 Optical Nonlinearity of Organic-Inorganic Hybrid Glasses 4.4 Optical Nonlinearity of Nano-composite Glasses 4.5 Optical Limiting Effects

vii

1 1 2 6 18 39 40 53 64

77 77 94 110 117 118 123 131 137 146

viii

Contents

5. Second-Order Optical Nonlinear Properties of Glasses 5.1 Introduction 5.2 Second-Order Optical Nonlinearity in Silica Glasses 5.3 Second-Order Optical Nonlinearity in High Refractive Index Glasses 5.4 Applications

153 153 167

6. Glass Fibers for Optical Amplification 6.1 Brief Introduction of Optical Fiber Amplifier 6.2 Er3+-Doped Phosphate Glass Fiber Amplifiers

191 191 193

7. Glass Fibers for High Power Lasers 7.1 Introduction of Optical Fibers 7.2 Fabrication and Materials 7.3 High Power Lasers Based on Rare-Earth Ions Doped Fibers 7.4 High Power Pulsed Fiber Lasers 7.5 Recent Development and Applications of Fiber Lasers

227 227 234 239 246 254

8. Hybrid Organic-Inorganic Solid-State Dye Laser Glasses 8.1 Organic Dyes and Liquid Dye Lasers 8.2 Hybrid Solid-State Dye Laser Glasses and Preparation Techniques 8.3 Photostabilities and Photodegradation Mechanisms of Hybrid Solid-State Dye Laser Glasses 8.4 Hybrid Solid Dye Laser Glass Based on Energy Transfer Between Laser Dyes 8.5 Solid-State Dye Lasers and Parameter Optimization 8.6 DFB Laser Based on Sol-Gel Derived Organic-Inorganic Hybrid Thin Film Waveguides 8.7 Summary and Future Prospects

261 262

9. Optical Glass Waveguides 9.1 Principles of Optical Waveguides 9.2 Glass Waveguides Fabrication and Optical Properties 9.3 Organic/inorganic Hybrid Glass Waveguide Materials 9.4 Functional Glass Waveguide Devices

299 300 302 310 315

10. Glass Photosensitivity and Fiber Gratings 10.1 Glass Photosensitivity 10.2 Principles of Fiber Gratings 10.3 Fiber Grating Fabrications 10.4 Fiber Grating Applications

172 186

264 273 281 285 290 292

339 340 355 361 363

Contents

ix

11. Glass Fibers for Photonic Crystals 11.1 Light Guidance in PCF 11.2 Fabrication 11.3 Properties of PCFs and Device Applications 11.4 Non-Silica Glasses for PCFs

375 377 383 386 400

12. Functional Microstructures in Glass Induced by a Femtosecond Laser 12.1 Introduction 12.2 Micro-Structural Changes Induced by Femtosecond Lasers 12.3 Valence State Manipulation of Active Ions 12.4 Precipitation of Functional Crystals 12.5 Novel Phenomena Induced by Femtosecond Lasers

405 405 407 417 424 435

Index

445

List of Contributors

Chapters 1-4 Fuxi Gan Shanghai Institutes of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800; Department of Optical Science and Engineering, Fudan University, Shanghai 200433 Chapter 5 Liying Liu Department of Optical Science and Engineering, Fudan University, Shanghai 200433 Chapter 6 Shibin Jiang NPPhotonics, Arizona, USA Chapter 7 Qihong Lou Laboratory of Advanced Laser Technology, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800 Chapter 8 Guodong Qian, Yu Yang Department of Materials Science, Zhejiang University, Hangzhou 310027 Chapter 9 Lei Xu Department of Optics Science and Engineering, Fudan University, Shanghai 200433 Chapter 10 Yigang Li, Guanghui Chen and Lei Xu Department of Optical Science and Engineering, Fudan University, Shanghai 200433 Chapter 11 Danping Chen Photon Craft Project Laboratory, Shanghai Institutes of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800 Chapter 12 Jianrong Qiu Department of Materials Science, Zhejiang University, Hangzhou 310027; Photon Craft Project Laboratory, Shanghai Institutes of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800 xi

Chapter 1

From Optical Glass to Photonic Glass

1.1 Introduction Optics is dealing with the propagation of electromagnetic wave and interaction of substance with electromagnetic radiation in light frequency. The former is geometric Optics and the later is molecular Optics. Optical glasses, which used for optics purposes are mainly light transmission media developed in 1880s by R&D works of Abbe and Schott in Germany. Manufacture of optical glass possessed mysterious and secret characters for a long time. After one century scientific research and development, the design of physical properties and chemical composition, as well as manufacturing technology have been put on modern scientific bases.'1"31 Optical glasses have also been developed from three dimensional (bulk materials) to low dimensional (thin films and fibers), which play an important role in modern optical and optoelectronic engineering. Photonics is dealing with the photon generation and detection, as well as stimulated emission, photon frequency conversion and polarization change. The invention of laser in the middle of last century was a great event for development of photonics, while photonics becomes more and more important in information science and engineering. The first definition of "Photonics" in 9th International Symposium on High Speed Photograph in 1970 was as following: "Photonics is a systematic science concerning the photon as the information carrier". Since photonpropagation speed (~1012 cm/s) is much higher than electron propagation speed (109 cm/s), therefore, photonic devices have very short time 1


2

response and super-high information capacity for single channel. As 21st century is the multi-media era and the tera-information era, the information capacity in Tb (1012 bits), or information density in Tb/cm2, information stream for transportation, storage, display and calculation in Tb/s, super-high frequency processing (modulation, switching, crossexchanging, coding and decoding) in T Hz (ps time response) are expected. The exploration of new photonic material, including photonic glasses, is a key issue for advance photonic devices. Here, photonic glasses with optical glasses perform different functional roles, such as laser generation, frequency conversion, light modulation and switching etc. In marketplace optics have already played significant roles in optical instruments, ophthalmic and medical industries, as well as optical engineering in space and nuclear industries. The photonics has only been popularized recently, in many ways an indication of its rapidly escalating importance in optoelectronic industries, such as optical communication, optical data storage and data processing, flat panel displays and many others. The development of photonics as well as related or overlapping fields with optics and optoelectronics, still needs to explore new materials, in which glassy or amorphous materials are the important ones. 1.2 Physical Fundamentals Optical properties of glasses are much concerning with molecular optics: light refraction, absorption, reflection and scattering based on electromagnetic radiation theory.t4] In the classic electromagnetic radiation theory the induced electrodipole moment P by electromagnetic field is P = P(1)+/>(2)+P(3) , where t* =aE, F^^l/Ijfi-E-E, I*3)=(\l6)rE-E-E.

(1.1)

l)

In which a, fi, y is the second rank, the third rank and the fourth rank polarization tensor respectively. E is the electrical field vector. In the weak electromagnetic field (classic optics) only P*1} should be considered. Therefore, the optical effect is linear. Optical properties with

3

1.2 Physical Fundamentals

classical optical glasses are linear ones and based on two main optical effects: optical dispersion and scattering. The optical functions of photonic glasses are always happened with intense electromagnetic field, where by the nonlinear optical effects are induced. In the intense electromagnetic field, the P®\ P (3) should be important, therefore the optical properties of photonic glasses are mostly nonlinear ones. 1.2.1 Light scattering The spontaneous light scattering of optical glass involves molecular vibration. Polarization tensor i* 1 ' is expanded into Taylor series. Pa> = a 0 c o s c y + (dak /dQk )Qk cosco0tcos(cot + 8k),

(1.2)

where Qk is the normal coordinate of vibration frequency cok, Sk is the phase factor. The first term is Rayleigh scattering, and second one is Raman scattering, we have studied them in optical glasses.[4] During intense coherent light (laser) irradiation it is accompanied by coherent light scattering, resulting in stimulated light scattering, e.g. stimulated Raman scattering and stimulated Brillouin scattering by P^ action. Differing from spontaneous light scattering, the incident photons are scattered by excited phonons rather than thermo-vibration phonons. The scattering light is also coherent, since the coherent light scattering is nonlinear one, we have to pay a great attention in photonic glasses. 1.2.2 Optical dispersion Optical dispersion can be described by classical electrodynamics and quantum mechanics. Absorption, reflection and refraction can be expressed by optical dispersion equation 2/,

2N ,

4nNe2 TTP m

fk{cok2-co2) 2

2 2

„ „N 2

2

^(a>k -co ) +co rk

where x is the absorption coefficient. If the light absorption can be neglected, % «1

4


•1=1+

x m

k

fk(k2-a>2)2+co2rk2

(1.4)

If the light interaction is not in the resonant region ()»71 4xHe2 ^ fk n -1 = 1 +3- ' k COk -co m

(1.5)

where 71 is the damping coefficient,^ is the oscillator strength, co* is the eigen frequency. Fig. 1.1 shows the optical dispersion curve in the resonant region and non-resonant regions, it can be divided into optical normal dispersion at non-resonant region and abnormal dispersion at resonant region. It is difficult to measure the refractive index of bulk glass at abnormal region, but for thin films it can be done by optical polarized ellipsometric method. In the normal dispersion region the refractive index of glass at definite light wavelength only depends on oscillator strength and eigen frequency.

y*-^ y ^

co

(b)

Fig. 1.1 Optical dispersion graphs near the absorption band (a) and with three absorption bands (b).

5

1.2 Physical Fundamentals

1.2.3 Optical nonlinearity In the intense electromagnetic field the dispersion curve generates a Stokes displacement and shifts toward the longer wavelength as the intensity of the field increasing. As shown in Fig. 1.2, the increment of refractive index 8n can be expressed as Sn=n2\E\\

(1.6)

where n2 is the nonlinear refractive index, which can be expressed as: n2 = {2x/n0 ) r = {2nN/n0 ) # ,

(1.7)

where y is the third order and fourth rank tensor, 6 is the nonlinear polarizability, i.e. the electric field induced change of polarizability (da/dE), a is first order and second rank polarization tensor as mentioned before.

Fig. 1.2 Stokes shift of dispersion curve.

The third order optical nonlinearity is consisted of real part Re ^3\ nonlinear refraction and imaginary ^3\ nonlinear absorption resulted from saturated absorption and multi-photon absorption.

6


^3>=[(Re^3)) +(lm^»)J •

(1-8)

Optical nonlinear refractive index n2 is related with ^ 3 ) n2 =\6xm

ln2ce,

(1.9)

where c is the light velocity and s is the dielectric susceptibility. The third order optical nonlinearity ^ 3 ) or nonlinear refractive index «2 is presented in all glassy materials. In non-resonant region the third order optical nonlinearity is caused by electronic shell (cloud) distortion, which is called electronic polarization, and nuclear core displacement, where the electronic part is dominant. We have deduced the method for calculating the nonlinear refractive index of different glasses by eigen absorption wavelength or refractive index of glass at three wavelengths,[4] based on the hypotheses of Stokes displacement of eigen absorption of glass mentioned above. The third order optical nonlinearity of glasses is described in detail in Chapter 4 of this book. As the glass is an isotropic substance, there is no second order optical nonlinear effect in glass, the second order and third rank polarization tensor p should be zero. But recent research results confirm that in intense electromagnetic field if electric charge separation could be performed, the second order optical nonlinear effects, such as light second harmonic generation etc. can be observed. By photo-voltaic model, X(2)=X0KEdc,

(1.10)

where ^ 2 ) and ^ are the second and third order optical susceptibility respectively, Eic is the electric field resulted from charge separation. In Chapter 5 of this book the second order optical nonlinearty of glasses is introduced furthermore. 1.3 Optical Glasses Modern Optical glasses are composed of following branches, which still play an important role in modern optics, optoelectronics and photonics.

7

1.3 Optical Glasses

1.3.1 Transparent optical glasses The main optical characteristics of transparent optical glasses are optical dispersion and spectral transmission. The optical dispersion characteristics of optical glasses can be expressed by «d~ua diagram, here nd is the refractive index at d spectral line and «d is called Abbe value (ud=nd-l/ nF-nc) , «F and nc are the refractive index at F and C spectral lines respectively, the n?-nc is so called mean dispersion. Different sorts of optical glasses are located in the different positions in the «d~fd diagram. The UV-transparent optical glasses are high purity oxide and fluoride glasses, and the IR-transparent glasses are heavy metal oxide and non-oxide glasses.[5"71 Fig. 1.3 shows the optical dispersion characteristics of oxide and fluoride glasses, the oxide glasses possess higher «d and Od value than that of fluoride glasses. The optical dispersion characteristics of different optical glasses in near infrared region are shown in Fig. 1.4, here °2 = n2 ~V n\ ~n3, n\, «2. «3 represent the refractive index at 1 urn, 2 um and 3um wavelength respectively. The chalcogenide glasses are located at the upper right, the oxide glasses at lower right and the fluoride glasses at lower left respectively.141 1.70

>oy i

1.65 1.60 1.55 &

1.50

/

1.45

^"W>^

II

1.40 1.35

a

1.30 1.25 0 "BeFi 110 105 100 95 90 85 80 75 70 65 60 55 50

v„ v

Fig. 1.3 H-0.30 * 0 . 2 5 400

600 800 wavelength(nm)

Fig. 2.2 Influence of bilayer period on Kerr rotation spectra of PrGd/FeCo MLFs.

350

550 Wavelength(nm)

750

Fig. 2.3 Spectra of figure of merit F for the three films NdGd/FeCo, PrGd/FeCo and TbFeCo. O modulated multilayer Pr35Gd65( 1 nm)/Fe89Con( 1 nm). • modulated multi-layer Nd36Gd64 (1 nm)/Fe89Con( 1 nm). • Single layer TbFeCo.

2.1.2.2 RE-TM thinfilmsfor magnetic super-resolution For high density M-0 data storage the magnetic alloy induced superresolution (MSR) technology is applied, which includes center aperture detection (CAD), magnetic amplifying magneto-optical system (MAMMOS) and domain wall displacements (DWD). MSR technology can make us to read the magnetic domain, which is smaller than laser spot.[7] Fig. 2.4 shows the schematic diagram of magnetic superresolution with central aperture detection (CAD-MSR) mechanism. The magnetic layer configurations are exchange-coupled double-layer (ECDL) or exchange-coupled multi-layer (ECML) consisted of the readout layer, and recording layer or the intermediate (switching) layer. In addition, the function of readout layer is to replicate magnetic domain of the recording layer by magnetic coupling or the switching layer by exchange catena force. The readout layer with RE-rich possesses high magnetization and

44

Structure and Properties ofAmorphous Thin Films for Optical Data Storage

low coercivity to make magnetic coupling easy and the recording layer with TM-rich possesses high coercivity to guarantee the perpendicular recording. Tables 2.1, 2.2 listed the materials composition and magnetic properties of ECDL and ECML for MSR.[8]

(b) Fig. 2.4 Schematic diagram of CAD-MSR (a) ECDL (b) ECML. Table 2.1 Magneto-optical materials and properties of ECDLforMRS Layer Readout Recording

Layer

Material

r Q (°c)

-*comp V W

Nd8Gd27(Fe75Co25)7o TbI9(Fe85Co,5)8i

306 250

130

-

A

J*V~ « H

j !

T =300 "C. 1 hr

-

• I1 11

F e EXAFS

;,. *-J.,i., •

*.*±*.*+*.i'....„

E in-plane

Ell

0.0

Fe EXAFS: Tb0MFeOM

-

0.4 0.2

•

E out-of-plane

EX 0

1 2 3 4 5 6 7 Radial Coordinate (A)

8

Fig. 2.10 Fourier-transformed Fe XAFS data for a Tbo.26Feo.74 alloy film at room temperature (a) and heat treated at 300 °C for 1 hr.

We use the magnetic effect in scanning tunnel microscope (STM) to investigate the structure of amorphous RE-TM thin films. The tunnel current of STM can be modulated by the external magnetic field. The transverse of longitudinal magnetic field is applied between the tip and the sample, the tunnel current is correlated with the strength and the direction of the external field. The experimental arrangement of transverse magnetic effect in STM and the tunnel current modulation

52

Structure and Properties of Amorphous Thin Films for Optical Data Storage

with the alternating transverse magnetic field are shown in Fig. 2.11 and Fig. 2.12, respectively.p2]

Voltage source

-t,

Fig. 2.11 Experimental arrangement of transverse magnetic effect in STM.

The STM images of sputtered amorphous TbFeCo thin film are illustrated in Fig. 2.13. By our experimental results, sputtered amorphous. TbFeCo thin films are not real amorphous, they have cluster structure (10~30 nm). In the cluster there exist nano-scale fine crystallites in 2.2-2.8 nm size. The atomic lattice spacing of nano-crystallites is about 0.2 nm. These nano-crystallite, orienting randomly (amorphous structure in atomic scale) may be contributed to the origin of perpendicular magnetic anisotropy.[23] to

ac tunnel current 0

|

> |-20 3 "3

-40

(a)

\

« — J J k

-u—

100

200

Magnetic &M (10~*T) (a)

ac magnetic field (b)

Fig. 2.12 Z voltage of piezotube depending on transverse magnetic field (a) and alternating tunnel current signal generated by an alternating magnetic field (b).

2.2 Amorphous Metallic and Chalcogenide Thin Films

(a)

53

(b)

(c) Fig. 2.13 STM image of sputtered amorphous TbFeCo thin film, (a) Cluster microstracture image with 109.5nmxl09.5nm scan area, (b) nanocrystallite image with 10.7nmx 10.7nm scan area, and (c) with 5.9nm> \n , 4In/2 and 4I3/2 steady-state fluorescence spectra at room temperature is measured for the phosphate glass with 2.2wt% of Nd3+ by exciting the sample with an Ar laser (Fig. 3.2). The emission spectrum consists of three large and non-symmetric bands centered near 897, 1054 and 1325 nm, and they are in-homogeneously broadened due to site-to-site variation in the local ligand field.

3.1 Laser Spectroscopy ofNds+ and Yb3+ High Doped Glasses

79

Table 3.1 Judd-Ofelt calculation results a2 a, Q6 Transition 4F3/2 —• 4 I9/2 4 In/2 4 I3/2

3.9 4.6 5.3 Branching ratio, /? 0.431 0.478 0.09

0 30 £ 0.25 s a

1o.20 !> SO

§ 0.15 | 0.10 : a. O 0.05

ft

ML Jl

300

wl

400

, .

l/w

500 600 700 800 Wavelength >7nm

900 1000

Fig. 3.1 Absorption spectrum of the neodymium-doped phosphate glass (with 2.2wt% Nd 2 0 3 ). 1UU

.

80 jS

e

».M)

Jl

.

u

-8 •^40 £• xn §20 ts

.

•

A ' A \ \

i i 1

V

0 -20

800

A.

i

1000 1200 Wavelength Wnm

1400

1600

Fig. 3.2 Room-temperature fluorescence spectrum of Nd3+ in phosphate glass (with 2.2wt% Nd 2 0 3 ).

80 New Developments in Optics and Spectroscopy of Rare Earth Ions Doped Glasses

Table 3.2 lists the emission properties of Nd +-doped phosphate glasses,A,p is the emission peak wavelength, AA,eff is the effective emission line-width, a p is the stimulated emission cross-section, n is the refractive index, xexP is the measured lifetime. Table 3.2 Room-temperature emission properties of Nd3+ (2.2wt% of Nd 2 0 3 ) in phosphate glass A,p/nm 4

4

F3/2-* I,i/2 1054

AA.eff/nm 25

o-p/l(r20cm2 4.0

n 1.52

Texp fa 330

The 4F3/2 —• 4In/2 emission cross-section in Nd3+-doped phosphate glasses is much higher than those obtained in other glass systems.[8'9'10] The larger cross section is mainly due to the narrower effective emission line-width. In phosphate glasses, there is a systematic decrease in the effective line-width with decreasing charge and increasing size of the glass-modifying cations. Therefore the introduction of K 2 0 and BaO modifiers has the effect of reducing the emission bandwidth. 3.1.1.2 Non-radiative transition It is well known that the emission efficiency Cne) is determined by the ratio of radiative transition probability Pr and non-radiative transition probability Pnon, it can be expressed by the ratio of calculated lifetime rc (1/P r ) and measured lifetime Tm (1/P„0„) rie=PrIPnon=*J*c-

(3-1)

The non-radiative transition probability is mainly composed of nonradiative transition by the interaction between active ions itself and with un-active ions, as well as with glass host. The last one is glass host phonon assisted non-radiative transition probability, which is determined by glass phonon energy and the energy gap between upper and lower levels of the transition (9040 cm"1, 4 F 3/2 —%m for Nd3+). Owing to the high vibration frequencies of the phosphate hosts (1121cm"1), this process is more pronounced in phosphate glass than in other oxide glasses. It can be measured by luminescence decay change. The decays of the 4F3/2 —>• 4In/2 transition are measured with a narrow band tunable

3.1 Laser Spectroscopy ofNd3* and Yb3+ High Doped Glasses

81

dye laser exciting the sample at the 4I9/2 -» 4G5/2 absorption band (575 nm). The measured fluorescence lifetimes at room temperature of seven Nd-doped phosphate glass samples with different Nd3+ concentrations are displayed in Fig. 3.3. It can be observed that the lifetime x decreases linearly with Nd3+ ions increasing. The rate of concentration quenching in phosphate glass is lower than that in silicate glass, because the interactions of Nd-Nd ions are weaker subjected to the chain structure of phosphate glass. 500 r

q t-

s a

•

400 •

, 300 •

1

200 •

S §

100 •

u

E

0

Nd ions concentration n/1010cm~3 Fig. 3.3 Lifetime of the 4F3/2 state as a function of the neodymium ion.

The energy transfer processes in case of higher Nd3+ concentrations are of cross-relaxation between Nd3+ ions, the non-radiative transition probability is proportional to Nd3+ ion volume concentration square.[11] The interaction of Nd3+ ions with OH" anion is a particular case for phosphate glasses. Due to the hydrophilicity the content of OH" groups in phosphate glasses is considerably higher than that in silicate, borate and fluoro-phosphate glasses. By dehydration treatment during the glass melting the hydroxyl group absorption of tested glass samples is low, less than 1.0 cm"1 at 3000 cm"1 wavelength. The phonon assisted energy transfer from the 4F3/2 level to OH group plays an important role in phosphate glasses and reduces the 4F3/2 state lifetime. 3.1.1.3 Site-dependent effects Taking advantage of the tunability and narrow bandwidth of the Ti:


sapphire ring laser as an excitation source for the 4I9/2 -» 4F3/2 transition, the emission spectra of the 4F3/2 —> 4In/2 transition at different excitation wavelengths along the 4I9/2 -» 4F3/2 absorption band of the Nd3+-doped phosphate glasses can be obtained. Fig. 3.4 shows the steady-state emission spectra obtained at different excitation wavelengths measured at 77K. None of the laser-excited fluorescence spectra show the extreme line narrowing in spite of the good resolution of narrow bandwidth of Ti: sapphire ring laser. Instead, the line-widths of the 4F3/2 —» 4lm transition are much broader than the homogeneous widths. This is attributed to a residual inhomogeneous broadening, which occurs for non-resonant fluorescence, because the excitation energy of ions in physically distinct sites may accidentally coincide while the location of other levels remains different.[12] However, the characteristics of site-selection of spectra are still exhibited. As it can be observed, the spectrum slightly narrows and red-shift occurs as the excitation wavelength increases, and the shape of the emission band changes as the excitation wavelength varies. 5

§4 .o

Is & 1 2 'e a. o

877mn

—

875iun 873nm870ran-

1000

1020

1040

1060

1080

1100

Wavelength X/nm Fig. 3.4 Steady-state emission spectra of the 4 F 3/2 -> 4 I n / 2 transition in phosphate glass with 2.2wt% of Nd 2 0 3 for different excitation wavelengths. Measurements were performed at 77K.

The fluorescence decay for the 4 F 3/2 -> 4In/2 emission is also measured as a function of the excitation wavelength along the lm -> G5/2 absorption band at 6K by using a dye laser. The variations in the time x with excitation wavelength are shown in Fig. 3.5, which displays that longer decay time occurs at shorter excitation wavelengths and vice versa, shorter decay time occurs at longer excitation wavelengths.

3.1 Laser Spectroscopy ofNd3+ and Yb3+ High Doped Glasses

83

567 568 569 570 571 572 573 574 575 576 Wavelength A/nm Fig. 3.5 Lifetime t of the 4 F 3/2 state as a function of excitation wavelength for the phosphate glass with 2.2wt% of Nd 2 0 3 . Lifetime were obtained at 6K and collecting the fluorescence at the emission peak of the F3/2 -* I11/2 transition.

These results suggest that the Nd3+ ions in phosphate glasses have a different nearest neighbour coordination. At the shorter wavelength excitations we should observe the emissions from Nd3+ ions with higher field strength of ligand Dq, whereas at the longer wavelength excitations the emission from lower field strength of ligand Dq sites becomes dominant. According to non-radiative transition theory,[13] the ions with larger Dq values have a larger energy gap between the excited state and ground state, higher radiative transition probability and so possess shorter emission wavelength, lower non-radiative decay rates and larger fluorescence lifetime. The ions with smaller Dq values have reverse properties. Therefore, the peaks of emission wavelengths shift to high energy and longer lifetime with excitation energy increasing. The distribution of lifetime as a function of excitation energy shows us a picture of the type of local coordination around Nd ions. However, mpared to other Nd3+-doped glasses system,1141 the smaller site-to-site variation in the fluorescence lifetime indicates that the phosphate glass has smaller site-to-site variations in local field and a comparatively low degree of inhomogeneous broadening, so it has a relatively higher emission cross-section.


3.1.2 Laser spectroscopy ofYb

high doped glasses

Yb3+ ions possess wide absorption band (Av~18 nm), it is easy to couple InGaAs diode laser pumping without strict temperature control. The laser wavelength is close to the absorption band, and the quantum efficiency should be high (>90%), thus the heat effect is low. The emission lifetime is high, this allows more energy to be stored at high power pumping. There are only two manifolds in the Yb3+ energy level scheme, it is commonly believed that excited absorption and non-radiative energy transfer between electronic energy levels should not occur. Yb3+-doped glasses have been considered as a promising high power laser glasses and have attracted much attentions. The spectroscopic properties and laser parameters of Yb3+ ions in different glasses have been studied before,[15~20] the laser selective excitation and time resolved spectra of high doping Yb3+ phosphate glasses are reported recently.[21] The summary of the these research results is presented in this section. 3.1.2.1 Absorption and emission spectra The absorption and emission spectra of Yb3+ doped phosphate (PNK), telluorogermanate (GTN), niobosilicate (SN), borate (BL), fluorophosphates (FP) and fluoride (FL) glasses are given in Fig. 3.6 and Fig. 3.7. The absorption and emission bands of Yb3+ in different glass hosts are listed in Table 3.3. Table 3.3 Absorption and emission bands of Ytterbium doped phosphate, borate, silicate and tellurate glasses, fluorophosphate and fluoride Glass Phosphate Borate Silicate Tellurate Fluorophossphate Fluoride

Absorption band (nm)

h

h.

^3

942 940 939 932 930 930

I I I 956 950 /

971 973 974 975 970 970

Emission band (nm) X2 X3 A-i 1006 972 984 974 988 1010 994 1017 975 995 1018 975 / 1005 970 970 / 1000

3.1 Laser Spectroscopy ofNd3+ and Yb High Doped Glasses

900

850

950 1000 Wavctength/nm

1050

900

1100

85

950 1000 1050 Wavelength/nm

1100

- Absorption ' Emission

= 20 V ~ IK

•£»«

g 1" c 12

it

1050

1100

(b) 3+

Fig. 3.7 Absorption and emission cross-section of Yb -doped glasses (a) FP; (b) FL.


3.1.2.2 Absorption of high Yb + doped phosphate glasses at low temperature Due to low concentration quenching effect the high Yb3+ ions doped phosphate glasses are noted as good laser media for thin slab and fiber lasers. The molar composition of studied Yb3+ doped phosphate glass is (60-65) P 2 0 5 : (4-8) B 2 0 3 : (5-10) A1203: (10-15)K2O: (5-10) BaO: (0-2) La 2 0 3 : (0-2)Nb2O5: (4-6)Yb203. Fig. 3.8 shows the absorption spectra of Yb3+ in phosphate glass at different temperature (8K-300K). Because lattice vibrations are weakly coupled to the 4f electrons of the rare earths, it is the usual case that only zero-phonon lines are observed in rare earth spectra. There are four distinct peaks at low temperature, which correspond to the lowest sub-level of 2F7/2 to 2F5/2 transitions. The width of these lines remains essentially constant, which indicates that these lines show strong inhomogeneous broadening. The position of absorption peak of 2F7/2 (1) — 2F5/2 (1) transition does not shift, while the other peaks shift to shorter wavelength with the temperature increasing. The positions of the peaks at different temperatures are listed in Table 3.4. Table 3.4 The peak-position of the four absorption bands at different temperature

8K 80K 150K 200K 300K

First peak/nm

Second peak/nm Third peak/nm

918.2 918.0 917.6 Dispersion Dispersion

933.0 932.6 931.9 930.8 930.7

959.0 958.1 957.2 956.7 Dispersion

Fourth peak/nm 975.2 975.2 975.2 975.2 975.2

According to crystal-field theory, the maximum of allowed splitting number of J=5/2 level is three; therefore, the absorption spectrum should be resolved into three broad bands. However, the absorption spectrum consists of four bands. This is the fourth weaker peak besides the other three stronger peaks. Apparently, a small portion of Yb3+ ions is found in the sites distinctly different from those occupied by Yb3+ ions, which give rise to the principal spectra. The energy level diagram of Yb3+ in phosphate glass can be determined by absorption spectra at low temperature, as well as emission spectra, and is shown in Fig. 3.9.


87

I06I6 10460 10269 2

: 800

900

1000

—

Wavelength/hm

_,_:: j

3+

F.„

1

570 372 150

7/2

Fig. 3.8 Absorption spectra of Yb in phosphate glass at different temperature. Fig. 3.9 Energy level structures of ytterbium ion in phosphate glass.

3.1.2.3 Spectroscopic parameters Due to overlap of emission and absorption bands of Yb3+, the emission cross-section measurement error is mostly attributed to the radiation trapping effect. We have already proposed the determination of emission cross-sections of Yb3+ in glasses by the reciprocity method Okm(A)

= Ohbs(A)

exp

An

(Ki-hcV) kT

(3.2)

where oab(A,) and aem(k) are the absorption and emission cross-sections at wavelength A. respectively. Zu Zu represent the partition functions for lower and upper levels. At the high temperature limit, the ratio of ZXIZ^ simply becomes the degeneracy weighing of the two states; Ez\ is the zero-line energy, which is defined as the energy separation between the lowest components of Stark split levels of the upper and lower states, h is the Planck constant, c is the light velocity, and Tis the temperature (K). The spectroscopic parameters can be calculated by FuchbauerLadenburger equations.[19] Table 3.5 gives spectroscopic properties such as peak absorption cross-section (op) and emission cross-section, which are determined from the absorption and emission spectra shown in Figs. 3.6, 3.7. The spectroscopic properties of the Yb-doped laser glasses


developed recently are listed in Table 3.6 to compare them with above glasses. The ADY, LY and PN are phosphate laser glasses which were reported to have high emission cross-sections and were recently developed by HOYA Corporation/231 QX is phosphate laser glass which was reported to have high heat capacity and the largest laser output power so far and was developed by KIGRE Company in USA,[24] and FP is fluorophosphates laser glass which was reported to have femtosecond ultra-fast pulse laser output and was developed by JENA University and the BONN-MAX Institute.[25] Table 3.5 Spectroscopic properties of Yb3+-doped PNK, GTN, SN, BL, FP glasses. Glass PNK GTN SN BL FP FL

Xp/nm 972.0 975.0 974.2 974.0 980.0 975.0

0D/pm2 0.68 1.45 1.52 0.98 0.62 0.65

0-e.r/pm2 1.09 2.29 1.92 1.93 0.68 0.65

tf/ms 2.0 0.90 1.00 0.90 1.50 1.77

Wan 1019 1030 1026 1016 1005 998

tf-Oen,/

2.07 2.06 1.92 1.74 1.02 1.15

Table 3.6 Spectroscopic properties of Yb3+-doped ADY, LY, PN, QX glasses. Glass ADY LY PN QX

XJnm 971.0 970.5 973.0 970.5

oypm 2 0.60 0.55 1.00 0.5

aem/pm2 1.03 0.80 1.35 0.70

T(/ms 1.58 1.68 1.09 2.00

A.em/nm 0.020 1.028 1.019 1.010

TfOem/ms-pm2 1.63 1.35 1.48 1.40

From Tables 3.5, 3.6 it is shown that PNK has higher emission cross section and longer fluorescence lifetime than ADY, LY and FP, and has a much higher emission cross section than QX, and has the same lifetime as QX. Although the emission cross-section of PNK is lower than that of PN, its lifetime is much longer than that of PN. In addition, in all glasses, PNK has the largest if, that is, the highest extraction efficiency. GTN has the highest emission cross section and a shorter lifetime and its is nearly equal to that of PNK; SN and BL have a nearly equal emission cross section, but the former has a longer fluorescence lifetime, and thus larger C e m *T,[ 1 9 l


89

3.1.2.4 Laser selective excited fluorescence line narrowing (FLN) spectra Fluorescence line narrowing (FLN) spectroscopy is used for studying the local environment around the Yb3+ ions. The FLN spectra were measured at 77K under Ti: sapphire laser excitation and the 2F5/2-2F7/2 fluorescence spectra were recorded after excitation at each absorption peak according to the absorption spectra. Fig. 3.10 shows the fluorescence spectra with the excitation wavelengths corresponding to three absorption peaks (918 nm, 933 nm and 958 nm) at 77 K. The fluorescence spectrum excited by 975 nm absorption peak is not shown, because the 975 nm wavelength overlaps with the fluorescence spectra, the positions of the emission peaks are identical for excitation wavelength at 933 nm and 958 nm, and located at 975 nm, 1002 nm, 1013 nm and 1026 nm corresponding to the transitions from the lowest sub-level of 2F5/2 to the components of the 2 F7/2 level, according to energy level diagram shown in Fig. 3.9. In the case of 918 nm excitation wavelength, there is a redshift at the shortest wavelength emission band, while the other three emission bands have no obvious shift. This behavior indicates that the Yb3+ ions possibly occupy two groups of sites. The absorption peak of 918 nm probably arises from another type of site.[21] 4 3 3

•5 2 ^. in

c * •a c

1 i

0 1000

1050

1100

1150

Wavelength /nm Fig. 3.10 Fluorescence spectra of Yb3+ in phosphate glass at 77K with excitation wavelength at 918nm, 933nm and 958nm, respectively.


Additionally, we measured the fluorescence spectra at 77K with the excitation wavelength shifted inside the inhomogeneous absorption band. Fig. 3.11 and 3.12 show the site-selective fluorescence spectra with the variation of excitation wavelength inside 933 nm and 958 nm absorption lines respectively. It can be seen that similar fluorescence spectra occur with continuously changing the excitation wavelength within the absorption bands 933 nm and 958 nm respectively. It indicates that the site structure of Yb3+ ions in phosphate glass is uniform. We can still find some spectacular things, the shortest emission wavelength (2F5/2(1)2 F7/2 (1)) is sharper than the other three emission band and its location is much more sensitive to the excitation wavelength. It shifted systematically to longer wavelength as the excitation wavelength decreasing, which revealed the sited dependent behaviours of Yb3+ ions in glass attributed to distortion of the local environment of Yb3+ ions. However, comparing to the other rare earth ions, as Nd3+ ions shown above in Fig. 3.4, the variation of the fluorescence wavelength with the excitation frequency in Yb3+-doped glasses is smaller. 8 6 -5 4 ' B

2

0 950

1000

1050

1100

1150

Emission Wavelength /nm Fig. 3.11 Fluorescence of Yb3+ in phosphate glass at 77K with changing wavelength within the absorption line at 933nm.


91

990 1020 1050 1080 E m i s s o n W a v e l e n g t h inm Fig. 3.12 Fluorescence of Yb3+ in phosphate glass at 77K with changing wavelength within the absorption line at 958nm.

3.1.2.5 Yb ions concentration effect on luminescence Table 3.7 shows the Yb3+ ion concentration effect on measured luminescence lifetime. It can be seen that the concentration quenching is very weak in Yb3+ doped phosphate and fluorophosphates glasses, it is pronounced in borate glasses. The Yb3+ ion concentration effect on luminescence in different glass hosts is similar to that of Nd +-doped glasses.1111 The optimum concentration of Yb3+ ions in borate, niobosilicate, tellurate, phosphate, fluorophosphates and fluoride glasses appears in the range of (3-5)xl0 20 , (5-7) xlO20, (3-8) xlO20, (5-11) xlO20, (7-15) xio 20 and >5xl020/cm3 respectively. The concentration quenching is attributed to the energy transfer between Yb3+ ions and cooperative upconversion. [21'26] Table 3.7 Concentration effect of Yb 2 0 3 on measured lifetime in different glasses Glass system 60TeO210La2O3-30ZnO 50P2O3-10Nb2O5-20K2O-20BaO 40SiO2-20Nb2O5-20BaO20SrO 53ZrF4-17BaF2-4LaF3-3 A1F3-10PbF2-13LiF 50B2O3-10La2O3-40ZnO 15Ba(P03)2-75 (35A1F3-15YF3-50MF2)

xm (ms) Tm= Tm= Tm= Tm= Tm= *m=

Refe-

Yb 2 0 3 %wt 3 0.81 1.60 1.0 2.4 1.15 1.6

5 0.71 2.0 0.9 2.5 1.10 1.8

7 0.60 1.90 0.7 / 0.76 1.8

9 0.64 1.85 0.64

11 0.60 1.72 0.60

/

/

0.39 0.3 2.0 2.0

rence 17 17 17 26 27 16


Due to more homogeneous site structure of Yb ions in phosphate glass, the cross-relaxation loss between Yb -Yb ions should be small; therefore, the concentration quenching effect should be weak. It can be confirmed by experiment of concentration dependence of luminescence lifetime of Yb3+-doped phosphate glass. As shown in Fig. 3.13 a constant up to 5mol% of Yb 2 0 3 can be kept, which is higher than Nd3+-doped phosphate glass in comparison of Fig. 3.13(a) with Fig. 3.13(b).

Yb phosphate

•

*

«

a

•

2

Nd ioiis concertration/xl(J!0/cm (a)

4

6

8

10

12

14

16

Ybîons concentration/ xlO^/cm3 (b)

Fig. 3.13 Lifetime as a function of the RE ion concentration, in phosphate glass of (a) Nd3+ ion (b) Yb3+ ion.

3.1.2.6 Laser performance parameters The laser performance parameters of Yb3+-doped laser materials are characterized by /?min, which is defined as the minimum fraction of Yb3+ ions that must be excited to balance the gain exactly with the ground state absorption at laser wavelength, and 7sat which is the pumping saturation intensity, that is to acquire, as the ground state depletion may be accomplished by available laser diodes, which can pump all Yb3+ ions into the excited state. Im\a, which is defined as the minimum pumping intensity, is a measure of the case of pumping the laser material to overcome the threshold power in the absence of other losses. With smaller /?min, 7sat and 7m;n values, the better laser performance can be obtained.


93

The main parameters describing laser performance of laser glasses include minimum pumping intensity (/min), saturating pumping intensity (7sat) and minimum fraction of excitations G#min). The relationship between Imin, Isat and /tan, is as follows: min _ Pmin=

A'min

(3.3)

sat '

Oabs(Ao)

L Z\ = > a

Intens o

/•

:

550 nm /

o 5 •o c a o r, c u

530 nm slope 1 .85

A

jlope

1.89

0

ta

i

, '

.

i

,

5

,

,

, . i

10

30

Excitation Intensity (a.u.) (1)

1 Excitation

5 10 Intensity

30 (a.u.)

(2)

Fig. 3.19 The dependence of up-conversion emission intensity against pumping intensity of Nd3+ and Er3+ ions ZBLAN glasses: (l)Nd3+, (2) Er3+.

3.2 Nonlinear Luminescence of Rare-Earth (RE) Ions in Glasses

ioa 1

too 2

ioa 1

ion 2

ion I

iee 2

ton 1

99

i&n 2

d)

\

V

a b c

|S

ftn Q

Sft

:s (in/! ion 2 (2)

Fig. 3.20 Energy levels and excitation processes of Er3+ (1) and Nd3+ (2) in ZBLAN glass.

3.2.3 Cooperative luminescence The energy level scheme for cooperative luminescence is shown in Fig. 1.13(c). Yb3+ doped glass is a good example to demonstrate cooperative luminescence. With 4f-13 electronic configuration the energy level diagram of Yb3+ is very simple, in the range of visible to near IR, there are only two energy levels (ground state 2F7/2 and excited state 2F5/2). The luminescence owing to the transition 2F7/2—>-2F5/2 is in the vicinity of 1 um and the excitation wavelength is around 900 nm. When the excitation light intensity increases, it is easy to observe the green luminescence with a peak wavelength at 480 nm.13'1 Fig 3.21 gives the visible fluorescence of Yb3+ doped sample, which is excited by a Ti: sapphire laser at wavelength of 919.6 nm. The fluorescence wavelength range extends approximately from 460 nm to 490 nm, with a peak wavelength


at 480 nm. It can be found that there is a wavelength correspondence between cooperative and one photon fluorescence of Yb3+, which is X cooP=V2, where A,C00p is the wavelength of cooperative emission and X; is the wavelength of the of 2F5/2—>2F7/2 transition in Yb3+. In view of cooperative theory, a photon is produced by an ion pair resulted from the interaction of two Yb3+ ions. Therefore, we have the relationship hvcoop= hv! +hvj, where I and j denote the Stark levels of 2F5/2 and 2F7/2 respectively, in terms of conservation of energy. From this, in the case of a low broadening approximation the above wavelength relationship can be deduced easily in consideration of the approximation of hvi +hvj. 100

r

is

50 -

460

470

480 490 500 Wavelength (nm)

Fig. 3.21 Cooperative luminescence of Yb3+ ions in ZBLAN glass. Excitation wavelength: 919.6nm.

Rate equations can give a relationship between excitation intensity and cooperative luminescence intensity. These are as follows: rl,

2

dt = o<J>Mn ,^

at

'12"1 n "0 , C„n, n

^ 1 2 ' ' l "0

'

(3.7)

(3.8)

T0

iV = n0 + nx + 2n2.

(3.9)

In the case of continuous-wave excitation, we find a solution for n2 as


101

Cn{zxo^NfN

(3.10)

(l + r,o)3 In the low pumping saturation approximation, that is r1oO»/.

(3.11)

We reduce equation (3.10) to /i 2 *r 2 C 1 2 (T,oa>JV) 2 ^,

(3-12) +

where Cu is the cooperative possibility; N is the Y b molar concentration: a is the absorption cross section for pumping light; ii is the experimental lifetime of the 2F5/2 level; Tm (3F2) -> Tm (3H4) (ii) Tm: 3H4 - • 3H6 ^ H o : 5I7 -> 5F4 (iii)Tm: 3H4 -» 3 F 4 =>Tm: 3H6 -» 3 F 4 and Tm: 3 F 4 =>Ho: (5I7). Clearly, populations at 3F5 and 5I4 of holmium ions are moved to 3Hi of thulium ions by energy transfers between Tm and Ho ions. In addition, for sample 4 the avalanche 'threshold' is about 3 mW, while for sample 2 it is only about a fraction of a milliwatt. It also indicates that the losses of population looping increase owing to the decrease of populations of 3F5 and 5I4. As the slope efficiency of up-conversion process changes to 2.6, it can be considered as a three-photon process. % D

F4

5

F5

0= - ^ \

:3F23F3

J

3

F4

_3H*

I8. 3+

Ho

Tm3+

Fig. 3.30 Energy transfers between Tm 3 + ions and Ho 3 + ions.

3.2.5.3 Energy transfer in Nd3+/Yb3+/Tb3+ co-doped glass Up-conversion luminescence of a ZrF4-based fluoride glass co-doped with Nd , Yb and Tb was examined under 800 nm excitation by Qiu et al.[3S] Fig. 3.31 shows the up-conversion luminescence excited by 800 nm laser light of Nd/Yb/ Tb co-doped glass and normal fluorescence excited by 350 nm light of Tb3+ doped glass. The up-conversion luminescence around 490, 545, 580 and 624 nm, which originate from the 5D4 level of Tb3+ ions, were observed. The schematic diagram of up-


109

conversion mechanism, as shown in Fig. 3.32 has been proposed. The conclusions are as follows: (1) only Nd3+ is excited by 800 nm light, (2) the phonon relaxation, (3) the energy of Nd3+ transfers to Yb3+ and (4) the Tb3+: 5D4 level is populated by the co-operative energy transfer from two Yb3+ ions. •T

-

-r 1 ' i—r—f—r— | . Ujwwnversion luminescence (X „= 800nm) Normal fluorescence ( X ^^SOnm)

• •?">

-J\

'

£

-

t

JFf

A jp-

_ .

380 400 420 440 Wavelength (nm)

Jp*

T P~

.... i . . . . i . , . . J iW./ 400

450

500

-"•

U \600A .650

550

Wavelength (nm)

Fig. 3.31 Up-conversion luminescence spectrum of 50ZrF4-29BaF2-lNdF3-10YbF2-10TbF3 glass under 800nm excitation (solid line) and normal fluorescence spectrum of 60ZrF4-30BaF2-5TbF3- 5La F3 glass under 325nm excitation (broken line). xlO'

I

'D,

" *D„ S G 4

24

e JO E?

A

3

D4\

•• "

" 4<WO,n

16

4>

a W

2

= 1

4f7

^

I8

\ -l-m

m

s-

Nd3"*

4

JJl

- 7I;,IM).l.2l • 'F, •If, •7F,

*i,

'

H»*IV

% V

ii

Yb)*

2f

"

Tb"

• 1=6

2

'

Yb-1*

Nd-1*

Fig. 3.32 Schematic diagram of up-conversion mechanism under 800nm excitation in 50ZrF 4 -29BaF 2 lNdF 3 10YbF 2 10TbF 3 (ZBNdYbTb) glass.


For development of visible solid-state lasers by frequency upconversion, the most practical approaches are using RE co-doped nonoxide glass fibers and pumped by near IR laser diodes. The possible upconversion laser designs are summarized in Chapter 1 (1.4.1.6). 3.3 Super-luminescence of RE-doped Glass Fibers Super-fluorescence is an amplified spontaneous emission (ASE) under extremely strong excitation. In this process the spectral width of the emission is narrowed, but the line shape still keep smooth and a large output can be achieved. For getting the high gain the RE-doped fluoride glass fibers are always used. 3.3.1 Super-luminescence in RE-doped silica glass fibers To produce the super-luminescence in glass fibers the experiment is arranged as follows. A laser beam was coupled into the fiber core by an achromatic microscopic objective (the magnifying power 10x, and the N.A. 0.3). An Ar-ion laser and CW tunable Ti: sapphire laser was used as pumping source. The output signal light from the fiber was dispersed by a GDM-1000 monochromater and detected by a near-infrared photomultiplier tube GDB-411. There is no resonant cavity in the experiment for double pass experiment, only a reflective mirror put at single pass output side.[39'40] The schematic diagram of measuring arrangement is shown in Fig. 3.33.

(a) ' Power meter

Ar Laser

Recorder

PMT

GDM-1000 Monochromator

Fig. 3.33 The experimental configuration for fiber super-fluorescence.

3.3 Super-luminescence ofRE-doped Glass Fibers

111

The fiber we used in this experiment was a heavily doped one. Its length was 3m. The fiber core was 5um in diameter with NA=0.15. Super-fluorescence characteristics were investigated in single-pass and double-pass configurations. The changes of the spectral profile with increasing pumping power of a Nd3+-doped silica glass fiber is shown in Fig. 3.34. No obvious threshold effect was observed in this process. When the pumping was weak, the peak of the spectrum was at 1080 nm with spectral width 28 nm, but the intensity at 1088 nm increased faster with increasing pump power than that at 1080 nm, and soon it became the most intense. In this process, the spectra narrowing are very obvious. The narrowest spectral width we have obtained in this experiment is 3 nm when about 40 mW of pump power was absorbed. No further narrowing has been observed when the pumping power increased after this. The maximum outputs we achieved were 0.32 mW and 6 mW in the single-pass and double-pass configurations respectively.'41'421

1080 1130 W a v e l e n g t h (ran)

Fig. 3.34 Fluorescence spectrum narrowed with the increase of pump power of a Nd3+doped silica glass fiber. 1, 5mW; 2, lOmW; 3, 15mW; 4, 20mW; 5, 40mW.

The Er +-doped silica glass fiber used in this experiment is 4m long and with a core diameter of 6(am. The doping concentration is 1000 PPM. Fig. 3.35 shows the fluorescence spectrum and super-fluorescence spectra of an Er-doped silica fiber at different pump levels. The


fluorescence spectrum was measured in a short fiber and it has a peak at 1530 nm and a shoulder peak at 1550 nm. Curves 2-5 in Fig. 3.35 are the super-fluorescence spectra of the fiber of 4m measured at different WP, which exhibit the change of spectral profile with pump intensity. This change was for similar reasons to those indicated in the last section. The maximum output is 0.28 mW in the single-pass configuration while WP=290 mW. In the double-pass configuration, 3.2 mW output was achieved with Wp=240 mW. The corresponding conversion efficiencies were 0.31% and 4.1%. Although these values will increase with the pump power, they are too small compared with the ideal values 33.8%and 16.9% in the case that the quantum efficiency is unity. We regard this as the effect of ESA, a sizeable fraction of the absorbed pump power is absorbed from the excited state and not contributed to signal photon emission.[41] 3.3.2 Evolution of super-luminescence in Nd3+-dopoed fluoride glass fibers Nd3+-doped ZBLAN glass fibers with a core diameter of 5-1Oum were prepared. The inactive loss at emission wavelength is about 20-30 dB/km. An evolving process of the spectral shapes from fluorescence to laser emission of the ZBLAN fiber pumped by the Ar-ion laser is shown in Fig. 3.36. As a comparison, Fig. 3.36(a) gives a fluorescence spectrum of a bulk glass sample. Its peak is situated at 9544 cm"1, and the line width (FWHM) is about 60 cm"1 (-176 nm). Fig. 3.36(b) is the fluorescence spectrum of the fiber at 7.5 mW of injected pump power; the fluorescence peak locates at 9549 cm"1, and the line-width becomes 127 cm"1, which is a little narrower than that of the bulk sample. When pump power is doubled, the line-width narrows to 114 cm"1, and another peak at 9499 cm"1 appears (Fig. 3.36(c)). At 30 mW, there are three well resolved peaks located at 9515, 9480 and 9450 cm"1, respectively (Fig. 3.36 (d)), and the line-width drops to 108 cm"1 (-120 nm). But the peak at 9549 cm"1 disappears. So with increasing the pump intensity, the linewidth narrows progressively. At the same time not only the splitting of

3.3 Super-luminescence ofRE-doped Glass Fibers

113

fluorescence but also the change of these peak locations has been observed.[43]

9800

1500 1620 1540 1560 1530 1600 Wavelength (ran)

Fig. 3.35 Fluorescence spectrum (1) and Super-fluorescence spectra (2-5) of Erdoped silica fiber at different pump power: 2, 40mW; 3, 85mW;4, 120mW; 5, 180mW.

9500 9200 lfavenumber(cm"')

Fig. 3.36 The developing process from fluorescence to laser emission pumped by 514.5nm of Ar-ion laser, a: bulk sample, b: 7.5mW, c: 15mW d: 30mW, e: lasing close to the threshold.

Fig. 3.37 shows the development of fluorescence spectra of doublepass pumped by Ti: sapphire laser, in which there is a clear saturation

100T3

T025 1050 1075 Wavelength(nm)

1100

Fig. 3.37 Typical spectra of fluorescence for double-pass at different injected pump power levels pumped by 800nm,l: the line-width 11.6nm (injected pump power 7.0mw), 2: the line-width 8.6nm (injected pump power 9.2mw), 3. the line-width 8.2nm (injected pump power ll.Omw), 4. the line-width 7.7nm (injected pump power 14.4mw), 5. the line-width 7.5nm (injected pump power 20.0mw).


range. In this regime, the line-width maintains about 7.5 nm although the pump power increases. The line-widths of output spectra change progressively. For the single-pass, the line-width decreases from 16.2 nm (injected pump power 7 mW) to 7.6 nm (24 mW). For the double-pass, it decreases from 11.6 nm (7 mW) to 7.5 nm. No clear saturation, however, is recorded for single-pass due to the limit of the pump power. From the change of line-widths, it can be seen that there is a super-fluorescence phenomenon.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

J Murray, Proceedings of SPIE.3492 (1998)1-10. J Campbell and T Suratwala, J. Non-Cryst. Solids, 263/264 (2000) 318-345. W.F.Krupke, IEEE J.Select. Topics Quantum Electron., 6 (2000) 1287-1298. Fuxi Gan, Optical Spectroscopy of laser Materials, In: Laser Materials, World Scientific, (1995)1-102. Yanli Mao, Zhanrong Sun, Xiuli Jiang, Peizhen Deng and Fuxi Gan, Chinese Physics, 11 (2002) 613-618. B R Judd, Phys. Rev. 127 (1962) 750. G S Ofelt, J. Chem. Phys. 37 (1983) 511. M Ajroud, M Haouari, Ouada Ben, H Maaref, A H Brenier, and C Garapon, J.Phys.: Condens. Matter 12 (2000) 3181. V Mehta, G Aka, A L Dawar and A Mansingh, Opt. Mater. 12 (1999) 53. G A Kumar, P R Biju, C Venugopal and N V Unnikrishnan, J. Non-Cryst. Solids 111 (1997)47. Fuxi Gan, Optical and spectroscopic properties of transition elements in glass. In Optical and spectroscopic properties of glass. Springer Verlag. 1992,176-200. M J Weber, J A Paisner, S S Sussman, W M Yen, L A Riseberg and C Brecher, J.Lumin. 12/13 (1976) 729. F Auzel, Luminescence of Inorganic Solids, New York: Plenum, (1978) 67. C Brecher, L A Risberg and M J Weber, Phys. Rev. B18 (1978) 5799. Hongbin Yin, Peizhen Deng, Fuxi Gan, J.Noncryst. Solids. 210 (1997) 243-248. Hongbing Yin, Peizhen Deng and Fuxi Gan, J.Noncryst.Solids. 210 (1997) 243-248. Peizhen Deng, Chun Jiang, Hongbin Yin and Fuxi Gan, Proc. SPIE. 3416 (1998) 99103. Hongbin Yin, Peizhen Deng, Fuxi Gan, Junzhou Zhang, Proceedings of 17th Intern. Congress on Glass, Beijing. 17 (1995) 240-243.

References

115

19. Chun Jiang, Peizhen Deng, Guosong Huang, Fuxi Gan, Science in China, 42 (1999) 616. 20. Chun Jiang, Fuxi Gan, Peizhen Deng, Materials letters, 41 (1999) 209. 21. Peizhen Deng, Yanli Mao and Fuxi Gan, Proceedings of 7,h Intern. Otto Schott Colloquium, C.Russel, G. Volksch, eds. Glastech. Ber. Glass Sci. Techn. 75 (2000) 203-209. 22. Hongbing Yin, Peizhen Deng, Junzhou Zhang, Fuxi Gan, Materails Lett. 30 (1997) 29-33. 23. Xuelu Zou, H. Toratani, Phys. Rev. B, 52 (1995) 15889. 24. R.Koch, U.Griebner, H.Schonnagel, etal, Optical Communication, 134 (1997) 175. 25. V.Petrov, U.Griebner, D.Ehrt, et al, Optics Letters, 22 (1997) 19. 26. Junjie Zhang, Long Zhang, Changhong Qi, Fengying Lin, Hafeng Hu, Chinese J. of lasers, A26 (1999) 739-744(in Chinese). 27. Chung Jiang, Fuxi Gan, Junzhou Zhang, Peizhen Deng and Guosong Huang, J. solid state chemistry 144 (1999) 449-453. 28. Fuxi Gan, Jie Wang, Yihong Chen, J.Noncryst. Solids, 213&214 (1997) 261-265. 29. Fuxi Gan and Yihong Chen, Opt. Mater.l (1993) 45-52. 30. R.Piramidowicz, P.Witonski, M.Klimczak, M.Malinowski, Opt .Mater, 28 (2006) 152-156. 31. Fuxi Gan, Jie Wang, Yihong Chen, Pure Appl. Opt. 5 (1996) 855-862. 32. Yihong Chen, Jie Wang, Fuxi Gan, Proc XVII International Congress on Glass,! (1995)15-21. 33. Y. Chen and F. Auzel, Electron. Lett. 30 (1995) 1602. 34. Y.H.Chen, G.K.Liu, J.V.Beitz, Fuxi Gan, Jie Wang, 10th International Sympoium on Non-oxide Glasses, June 19-22, Corning, NY (1996). 35. Marie-France Joubert, Opt. Mater. 11 (1999) 181-203. 36. Liyan Zhang, Jianghu Yang, Lili Hu et.al. Chin. Phys. Lett. 20 (2003) 1344-1346. 37. P. Lapota, S. Taccheo, S. Longhi et. al., Optical Mater. 11 (1999) 269-288. 38. J.Qiu, M Shojiya, R. Kanno, Y.Kawamoto, Opt. Mater. 13 (1999) 319-325. 39. Fuxi Gan, Chemical Express, 6 (1992) 933-948. 40. Yihong Chen, Ruihua Cheng and Fuxi Gan, Chinese Science Bulletin 67 (1992) 556559. 41. Fuxi Gan and Yihong Chen, Pure Appl. Opt. 2 (1993) 359-365 42. Yihong Chen, Ruihua Cheng and Fuxi Gan, Chinese Journal of Infrared and Millimeter Waves, 11 (1992) 415-419. 43. Jie Wang, Fuxi Gan, Changhong Qi, J.Noncryst. Solids 184 (1995) 235-239.

Chapter 4

Third-Order Optical Nonlinear Properties of Glasses

The nonlinear optical effects of glass have recently become a focus of increasing scientific and technological interest. All glassy materials possess third-order optical nonlinearity, which is remained a subject of considerable theoretical and experimental efforts stimulated by the need of materials understanding in applications. In the past ten years of development of high power solid state laser devices the self-focusing and laser induced damage were the serious obstacles for application of laser glasses, a great progress has been achieved in development of new laser glasses with low figures of third-order optical nonlinear susceptibility. The optical wavefront distortion in optical glass fibers should be prevented for long haul optical fiber communication, some new approaches to diminish or compensate the nonlinear optical effects in glasses fibers have been explored. Recently on the contrary the strong third-order nonlinear optical effects in glasses have come into a hot spot of many researchers because of their potential applications in photonic devices, such as switching, modulation, optical signal processing etc. It is worth to understand deeply the mechanism and glass composition relationship of the third-order optical nonlinearity of glasses. In the previous monograph we have introduced the third-order nonlinear optical effects, such as laser induced self-focusing and damage, and optical blooming etc. in glasses, much concerning on dielectric bulk glasses.[I] Therefore, we put more emphases in this chapter on the thirdorder optical nonlinearity of dielectric glass, thin films, semiconductor and metallic particles doped composite glasses, and organic and inorganic hybrid glasses. 117

118


4.1 Measurement of Third-Order Optical Nonlinear Susceptibility of Glass According to nonlinear optical effects several methods have been developed for measuring the third-order optical nonlinear susceptibility of different bulk glasses and glass thin films. In optical and laser glasses the third-order optical nonlinearity is always expressed as nonlinear refractive index n2(E), that is the refractive index change (
Exp. Value of n2 (IP"13 esu) 6.4+1.1 4.1+0.4 1.3+0.4 1.9+0.4 1.6+0.5 1.6+0.5 1.3+0.3 1.2

127

4.2 Optical Nonlinearity of Dielectric Glass

4.2.2 Optical nonlinearity of chalcogenide glasses and glass thin films Chalcogenides, included sulfides, selenides and tellurides, can form amorphous state in a wide range of composition. Due to anions, such as S2", Se2" and Te2", with lone electron pairs the third-order optical susceptibility x

t

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Fig. 4.17 Measured z-scan of the Pr4 VOPc-doped ormosil (a) and octa-PdPc-doped gel. (b)

4.4 Optical Nonlinearity ofNano-composite Glasses

137

4.4 Optical Nonlinearity ofNano-composite Glasses Nano-composite glass means the nano-particles dispersed in inorganic glass matrix. There are two kinds of nano-particles doped glasses: the semiconductor dots doped glasses with quantum confinement effects and the metallic dots doped glasses with dielectric confinement effects. A comprehensive review of nonlinear optical properties of semiconductordoped and metal-doped glasses has been reviewed before.[331 Here we present the new progress in this field. There are emerged new groups of nano-composite glasses. They are semiconductor micro-crystallites with high %(3) value, such as CdTe, PbSe and Bi2S3, and fullerenes (C6o) nano-particles and semiconductor oxide, such as ZnO, Sn0 2 , Fe 2 0 3 etc, nano-particles doped composite glasses, which were developed by us recently, the experimental results are shown in this section. 4.4.1 Semiconductor micro-crystallites doped nano-composite glasses The optical properties of semiconductor micro-crystallites are very different from that of the bulk one. Due to the quantum confinement effect the conduction and valence bands of bulk materials split into a series of discrete energy levels, and the surface effects also become increasingly important.[34] The quantum size effects can be classified into two categories depending on the ratio of micro-crystallite radius y to Bohr exciton radius y^. If the radius of the micro-crystallites is close to or smaller than Bohr radius of exciton (y<jB), it belongs to electron and hole individual confinement, which is called the strong confinement, and can be observed in chalcogenide micro-crystallites, such as CdS, CdSe etc. If the radius of the micro-crystallites is larger than the Bohr radius of exciton (y>yB), all the excitons take place to confinement, which is called weak quantum size confinement. The halide semiconductor, such as CuCl, CuBr, belongs to this category. The third-order optical nonlinearity of semiconductor microcrystallite doped glasses can be divided into two cases: wavelength of optical nonlinear effects in the resonant region and the non-resonant region in dependence of the ratio of incident light wavelength ^ n to eigen

138


absorption. In resonant region the optical nonhnearity is originated from enharmonic electronic structure of different electronic energy levels, the optical nonlinear effect is large, but the response time is slow and the signal absorption loss is high, sometimes it is called active optical nonlinear process. In non-resonant region the third-order optical nonhnearity is induced by the distortion of electronic cloud (or shell), the nonlinear effect is smaller than that of the former, but the response time is quick and the signal loss is low, it can be used as passive optical nonlinear process. 4.4.1.1 Chalcogenide semiconductor micro-crystallites Since Jain and Lind (1983) firstly reported on the high nonlinear optical properties of CdSSe-doped silicate glasses,[35] there have been many papers on the optical nonhnearity of semiconductor microcrystallines in glass.[36"38] The approaches for increasing optical nonhnearity of semiconductor nano-composite materials are as follows: increasing particle concentration, working at near resonant wavelength and adding dielectric confinement effect besides quantum size confinement effect. (l)For increasing particle concentration the sol-gel method has been adopted. As an example, under near resonant condition the ^ 3 ) of 2% CdS doped glass was estimated to be 1.5xl0"12esu (390 nm). With increasing the doping concentration up to 8%wt of CdS by sol-gel process the ^ 3 ) value rises to 7xl0"10 esu at room temperature measured by DFWM method.[39] (2)Semiconductor micro-crystallites with higher optical nonhnearity, such as CdTe, PbeSe and Bi2S3 doped in glasses have been developed. The CdTe micro-crystallites doped glasses were prepared by laser evaporation method.[40] The third-order nonlinear susceptibility ^ and decay time v of the thick glass films (thickness is around 1-1.5um) were measured by DFWM method. The ^ value depends on the light wavelength, as shown in Fig. 4.18. It can be seen that near the eigen absorption the ^ value increases rapidly. The x value was estimated to be shorter than 10 ps.

139

4.4 Optical Nonlinearity ofNano-composite Glasses

Recently the PbSe micro-crystallites doped glasses have been studies in detail.[41] Te host glass composition is borosilicate (Na20-PbO-ZnOB 2 0 3 -Si0 2 ). The absorption spectra of glass samples with different heat treatments are shown in Fig. 4.19. The spectra do not show well resolved band structures but only the exciton peaks or so-called exciton shoulders at a wavelength less than the absorption band edge. The exciton shoulder and the absorption band edge shift to a higher-energy region (blue shift)

700 Wavelength (ran)

Fig. 4.18 x values measured on CdTe micro-crystallites doped glass sample, plotted as a function of wavelength. Linear absorption spectrum is also presented.

1300 1700 Wavelenglh (nm)

Fig. 4.19 Absorption spectra of samples 1,2,3,4 and 5.

as the average size of the PbSe micro-crystallite decreases. The blue shift can be attributed to the quantum size effect of the carriers (electron and hole) and the exciton shoulder is tentatively assigned to a transition from the highest hole sub-band to the lowest electron sub-band (Is-Is transition). The energies at the exciton shoulders are summarized in Table 4.6. Table 4.6 Heat treatment of samples and the experimental results at room temperature Sample

Heat treatment temperature(°C)

1 2 3 4 5

460 480 500 520 540

Average radius (nm) 1.6+0. 3 2.510.3 4.010.4 6.510.3 7.3+0.4

Band gap (eV)

%(3)(esu) ( + 20%)

0.730 0.654 0.591 0.565 0.552

1.46xl0"9 1.15xl0~9 1.30xl0"9 LlOxlO" 9 0.75x10"9

140


The optical nonlinearities of PbSe micro-crystallites doped glass were measured by the single beam Z-scan technique. The irradiation source is a mode-locked Nd-doped yttrium aluminium garnet laser operating at 1 Hz with 50ps pulse width and 1.06um wavelength at room temperature. Fig. 4.20(a) gives the Z-scan curve with a collecting aperture for sample 1. The normalized far-field transmittance for a small aperture behaved as a function of sample position relative to the focal plane of the lens shows a decreasing and then increasing intensity while moving through the plane, which indicates a positive intensity-dependent

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refractive index y. All the samples displayed similar behaviours. Fig. 4.20(b) gives the Z-scan curve without the collecting aperture for sample 1, which shows that the normalized transmittance has a peak at the focus (z=0). This curve indicates that there is a saturable absorption in the sample 1. The estimated values of x(3> for different samples are shown in Table 4.6. As shown in Fig. 4.19, the exciton shoulder was clearly observed, which indicates the presence of exciton effect in the present system, which would enhance the third-order optical nonlinearity. Thus, the larger optical nonlinearity may arise from the exciton resonance effect.

4.4 Optical NonHnearity ofNano-composite Glasses

141

The response time of optical nonHnearity for sample 1 was also measured by pump-probe time-resolved Z scan (Fig. 4.21), which indicates that the response time is much faster than the 5 Ops pulse duration. These observations are of the results of electronic third-order nonHnearity for the samples of PbSe micro-crystallites doped in glass. The larger third-order susceptibility %0) (about 10"9 esu) and shorter response time (less than 50ps) for such material were estimated. These experimental results suggest that PbSe micro-crystallites doped in glass constitute a promising material for applications in photonics devices. 8c 1.05 to

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Fig. 4.21 Normalized probe transmittance versus probe delay for sample 1. A pulse duration of 50ps full width at half-maximum is calculated from the fit indicated by the solid curve.

(3)Optical nonHnearity can be enhanced by dielectric confinement effect, which is s surface polarization effect induced by trapped state and atomic vacancy defect. This effect depends on the dielectric constant ratio (ei/e2) of micro-particle and surrounding. The influence of surface coating and heat treatment on the third-order optical nonHnearity of Bi2S3 nano-particles with benzene sulfonic acid sodium salt (DBS) doped ormosil has been studied.[42] Fig. 4.22 shows the absorption spectra of Bi2S3 doped ormosils (sample 1) dried at nitrogen atmosphere and heated at 150°C for 2 hrs. The obvious red-shift can be observed, it demonstrates the dielectric confinement effect, because the Bi2S3 particles are coated with organic DBS doped layer during heat treatment. Fig. 4.23 shows the z-scan curves of sample I and sample II. It can be seen that the self-focusing effect of both two samples and only exists the nonlinear refraction, while the nonlinear absorption is very small,

142


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^08 and x(3) represent the secondorder nonlinear susceptibility and third-order nonlinear susceptibility, respectively. However, the second-order nonlinear susceptibility %(2) is zero for conventional homogeneous glass because of macroscopic inversion symmetry. The centrosymmetry of glass can be broken via various poling techniques. Different methods of poling have been successfully used to induce permanent second-order optical nonlinearity in glasses.181 The methods include thermal poling,[1] corona poling,[2'9] electron beam implantation,'3' proton implantation,[10] UV poling[11'12] and photoinduced poling.[13] Among these methods, photo-induced poling was the first one used to induce second-order optical nonlinearity in glasses,17'14] and thermal poling is the most extensively used method to induce the nonlinearity in glass, because of its potentiality to induce stable and efficient nonlinearity in different kinds of glasses. 5.1.1.1 Photo-induced poling (optical poling) Photo-induced poling, or so-called optical poling, was first reported to induce second-order optical nonlinearity in Ge0 2 -Si0 2 glass fiber by

5.1 Introduction

155

Sasaki and Ohmori in 1981.[14] In 1986, Osterberg and Margulis discovered that an efficient SHG could be generated in a Ge-doped silica fiber of about lm in length by irradiation with 1.06um Nd:YAG laser light for a long time. The second harmonic conversion efficiency was as high as 5% after 12 hours irradiation.[7] A year later, Stolen and Tom showed that, when infrared fundamental light was introduced into a fiber together with its second harmonic light, the time to generate SHG was shortened to 5 min.[13] They believed that the incident fundamental and its second harmonic light beams caused a periodic dc electric field to build up in the glass and to permit periodic phase matching of SHG. Since that time, second-order optical nonlinearities were demonstrated using various kinds of glass fibers. Later, efficient SHG was also demonstrated to generate in bulk Ge-doped silica fiber preforms by optical poling.[15] Several other bulk glass systems, including commercial silicate and semiconductor-doped glasses, have been shown to exhibit the same effect.1161 Though optical poling has been used to induce second-order optical nonlinearity both in fiber and in bulk glass, the experiments on optical fibers gave best results. Since the induced nonlinearity is relatively small (%(2) ~ 10"4 pm/V), long interaction length is needed for high efficiency quasi-phase-matching SHG resulted from periodic electric field. 5.1.1.2 Thermal poling Thermal poling offers a simple and reproducible way to induce permanent second-order optical nonlinearity in glass. It is also called parallel-plate poling sometimes in order to distinguish with corona poling. The experimental setup is shown in Fig. 5.1. Basically the process of thermal poling of a glass is, heating the glass to high temperature of typically ~300°C while, at the same time, a strong external electric field ~107 V/m is applied across the sample. After a sufficient duration (about 30 minutes), the sample is cooled down to room temperature and the applied field is subsequently removed.tl] Thermal poling was first demonstrated by R. A. Myers et al. in 1991 to induce large second-order optical nonlinearity in bulk fused silica.[1] The induced nonlinearity, ~1 pm/V, which was evaluated from SHG

156

Second-Order Optical Nonlinear Properties of Glasses

measurement, is of similar magnitude to that from quartz. Since the nonlinearity was considerably large, the method has been applied widely to other oxide and chalcogenide glasses[17] and it can also be used for optical fiber poling.[18]

Power supply Silica glass plate 20mm in diameter 1.2mm in thickness

Fig. 5.1 Experimental setup for thermal poling of bulk silica glass. After Ref. [19].

5.1.1.3 Corona poling Corona poling is actually another method of thermal poling. It differs from parallel plate poling in the way that the electric field is applied. Instead of using plate electrode in thermal poling, a tungsten needle is placed about 1 cm above the grounded planar electrode and a voltage of ~5 kV is applied to the needle. Poling is carried out at temperature -200°C. The experimental setup[9] is shown in Fig. 5.2. Corona poling has been a technique commonly employed to orient the organic dye molecules in polymer films, which are generally called poled polymer. •HV

, Tungsten needle , Glass fUm Planar Electrode

Substrate

Fig. 5.2 Electrode configuration for corona poling. After Ref. [9].

5.1 Introduction

The first corona poling was et al.[2] With this method, they susceptibility x(2) ~ 1 pm/V in Corning7059 films on Pyrex glass

157

applied to glass films by A. Okata demonstrated second-order nonlinear radio-frequency sputtering deposited substrates.

5.1.1.4 Electron beam implantation Electron beam implantation was used to induce second-order optical nonlinearity in glass with charge implantation by exposure to a lowenergy electron beam. The first successful use of electron implantation was reported by Kanzansky et al. in lead silicate glass. The result showed X(2) ~0.7 pm/V.[3] In the experiment, a scanning electron microscope was used for electron-beam irradiation of the sample for 1 min, the beam currents used were in the range 0.3-10 nA, and the beam voltage ranged between 5 and 40 kV. The electron-beam spot size was 0.5 urn. The SH efficiency was found to increase with both the electron energy and the electron-beam current.[3] The injection of electrons into dielectrics can lead to the formation of a space-charge electrostatic field directed perpendicular to the surface of the sample. An advantage of electron beam implantation method is high resolution, which may be promising for fabricating optical waveguide or periodic structure for quasi-phase-matched SHG. In addition, proton implantation into silica glass has also been performed with minimum dosage of 0.25-0.5 mC and induced a x(2) of the order of 1.0 pm/V.[10] 5.1.1.5 UVpoling The method of UV poling was first realized by Fujiwara et al. in 1995.[11] In their pioneer work, Fujiwara et al. discovered that a particularly enhanced electro-optic effect could be induced in a Ge-doped silica fiber if the fiber was irradiated by nanosecond UV laser pulses at 193 nm in the presence of an applied electric field of >800 kV/cm.[11] The poling electric-field strength is at least one-order of magnitude higher than early reported works. Later, Fujiwara et al. reported the induction of a secondorder nonlinear coefficient d33 as high as 3.4 pm/V (i.e. x(2) = 6.8 pm/V)

158


in highly Ge-doped (15.7 mol%) fused silica glass. for UV-poling is shown in Fig. 5.3.

The configuration

Fig. 5.3 Configuration for UV-poling in a Ge-doped silica glass. After Ref. [20].

Besides UV-poling, other auxiliary methods have also been used to enlarge second-order optical nonlinearity in silica glass, y ray and X ray irradiation[19] have been demonstrated earlier, irradiation of silica glass with infrared femtosecond laser also gave good results.[21] As thermal poling technique was used most frequently to induce nonlinearity in glass, the rest of this chapter will mainly emphasis on this technique. 5.1.2 Measurements of second-order optical nonlinearity in glasses Second harmonic generation measurement or the so-called Maker-fringe measurement^1 is the most commonly used technique to deduce the second-order nonlinear susceptibilities ^ of the poled glasses. Besides SHG measurement, interferometric measurement of the linear electrooptic (EO) effect is also widely used for probing second-order optical nonlinearity of glasses or fibers.[22] 5.1.2.1 Maker-fringe measurements The Maker fringe method has been widely used to determine the intensity and effective thickness of second-order optical nonlinearity of bulk materials.'1' 23] In the SHG experiment, as shown in Fig. 5.4, fundamental laser beam is incident onto the glass sample at different

159

5.1 Introduction

angle of incidence; the generated SHG signal is measured, which is proportional to the product (x(2)L)2, in which L is the material thickness contributed to the nonlinearity, i.e., the nonlinear region. The fundamental light used is usually the 1.064 um laser beam from a Nd:YAG laser operating at 10 Hz at an intensity of about 10 MW/cm2. By comparing the second harmonic signal of the glass sample with the signal generated by the same fundamental laser beam on a known nonlinear material, e.g. quartz, the second-order nonlinear susceptibilities "(2) of the glasses can be deduced. Ml

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To generate the Maker-fringe pattern, sample is rotated around an axis perpendicular to the laser beam. The resulting Maker-fringe pattern is the result of a phase mismatch Ak between the fundamental and the second harmonic waves: Ak =

{A,nlX){nacos0J-n2a)cos02o)'),

(5.2)

where na and n2m are the refractive index at fundamental and second harmonic wavelength, and 0m' and Q2m' are the angles of refraction inside the glass for the fundamental and the second harmonic waves, respectively, X is the wavelength of the fundamental beam. To get a clear fringe in Maker-fringe measurement, the thickness of second-order nonlinear region must be larger than the coherent length lc=n/Ak. In this bulk case, from the interval of the fringe, the thickness of nonlinear region L could be deduced. If only a thin layer of second-order optical nonlinearity is induced in the glass, i.e., a surface case with

160


thickness less than the coherent length of silica glass (~24um), angular dependence of SHG measurement gives an envelope. Both results have been observed in poled silica glasses as shown in Fig. 5.5.[1'23] In most case, the later one was observed.

-60 -40 -20 0 20 40 Angle of Incidence / degre«

0 20 40 Angle of Incidence (degrees)

60

Fig. 5.5 Two examples of Maker-fringe patterns for fused silica: a clear fringe (left)[23) and an envelope (right)1'1.

The induced second-order optical nonlinearity in glass, both in bulk and surface case, is usually not homogenous over the nonlinear region. The second harmonic signal is thus described theoretically by integrating the second-order nonlinear susceptibility profile of sample x(2) (z) along z: I la, = A0)\ txi2)(z)exp[jAk(0)z]dz2,

(5.3)

where x(2)(z) is the second-order nonlinear susceptibility profile, z is the distance into the sample normal to its surface, 9 is angle of incidence, Ak is phase mismatch, A(9) is a function related to Fresnel function. In-order to obtain nonlinear profile and thickness of nonlinear region, especially in the surface case, other methods must be used together with SHG measurement, which will be shown in section 5.1.3. 5.1.2.2 EO measurements The linear electro-optic effect is also proportional to the second-order nonlinear susceptibility and offers a further potential for applications in modulators and optical switches. EO measurement gives the electro-optic

5.1 Introduction

161

coefficient y of the poled glasses, which is related directly to secondorder nonlinear susceptibility/^ via y=2rf2>/n4.[22] 5.1.3 Characterizations of the depth profile of the poled region Determining the second-order optical nonlinearity profile of poled glass is important, as the knowledge of this profile is essential to understand the physical origin of the nonlinearity, and it might also assist in developing methods to improve the strength and the uniformity of the nonlinear region. In thermal poling process, a dc electric field is recorded in the sample, which arises from a space-charge distribution in the depletion layer near the anodic surface.[1] Since the second-order optical nonlinearity is created near this layer, several studies have also been carried out to characterize this electric field and charge distribution. Different methods have been used to characterize the nonlinear profile or electric field distribution in poled glass. Apart from Maker fringe analysis, chemical etching of the poled region followed by either optical interferometric measurement^51 or atomic force microscope imaging1261 was used, as well as laser induced pressure pulse method[27] and optical second harmonic microscopy.[4] 5.1.3.1 Chemical etching method Chemical etching with hydrofluoric acid (HF) was first used by Myers et al.[1] to obtain the thickness of poled region. Later it was found that the built-in electric field distribution can be obtained by measuring the HF etching rate, and the variance in the etching rate has been suggested to be an indication of the onset of the frozen-in electric field.[25] By etching samples transverse to the poling direction together with AFM imaging, T. G. Alley and S. R. J. Brueck gave spatially resolved visualization of the space charge region directly, as showed in Fig. 5.6.[26] Interferometric measurement in real time could be used to deduce the HF removed thickness of silica with submicrometer precision.[25] It was used to determine the depth of poling region as a function of poling voltage, an average electric field Edc of 5,3x108 V/m was thus obtained.[28]

162


Unlike SHG giving direct evidence of second-order optical nonlinearity profile of poled glass, HF etching alone can only give information of electric field or charge distribution in glass. An improved method to fully reconstruct the nonlinear profile with an accuracy of ~50 nm was subsequently proposed. It consists in HF etching the anodic surface of a poled sample, while the SHG intensity is recorded. The total nonlinear distribution is obtained after a step-by-step reconstruction.1291

Fig. 5.6 AFM image of a poled silica glass etched transversely to the poling direction, which reveals two ridges for regions of slower etch rates near anode. After Ref. [26],

The drawback of HF etching method is that it cannot distinguish changes in the etching rate from changes in the electric field and charge densities, which are both present within the nonlinear region. It also destroys the nonlinear layer during etching, therefore the measured nonlinear profile may not represent the original profile well. 5.1.3.2 Laser induced pressure pulse method Laser induced pressure pulse (LIPP) method was used to determine the space-charge distribution in poled glass. During LIPP measurements, a pressure pulse is produced by the impact of a short duration high power laser pulse on an absorbing layer deposited on the sample surface. This pressure pulse propagates inside the sample and gives rise to an electric

5.1 Introduction

163

current. The measurement of the current allows one to infer the sign and amplitude of the charges and their location.[27] Using LIPP method, Kazansky et al. have been able to accurately localize the electric charges responsible for the nonlinearity with a spatial resolution of ~3 um.[27] They have found that in a thermally poled silica glass, a 10 um thick space charged region consists of a positive layer at anodic surface and a negative one deeper in the poled sample. Later, they used LIPP method to study the difference of charge distribution in silica glasses poled in air and under vacuum.130' 5.1.3.3 Improved Maker fringe method When only a thin layer (L< 4) of second-order optical nonlinearity is induced in glasses, it is usually impossible to get second-order nonlinear susceptibility x(2) and the depth of the nonlinear region L separately from a simple Maker-fringe measurement, nor the nonlinear profile could be retrieved uniquely. Several improved SHG methods are issued, such as using prism,[31] glass hemispheres,132' or cylinder[33] to overcome the problem of total internal reflection at the rear glass-air interface, making L longer than lc at large angles. Another variant of Maker-fringe technique employed two fundamental beams at an angle to produce a non-collinear SH beam, which has much smaller coherent length (~2um in silica).[34] In both cases, clear fringe instead of envelope might be obtained, which was used for deducing the depth of nonlinear region. Later, Ozcan et al. showed that the Fourier Transform phase o$x(2)(z) could be retrieved by measuring the Maker-fringe generated by the interference of two samples.[35] This phase was used to get the unique recovery of^(z), which agreed well with results of Ref. [27]. 5.1.3.4 Second harmonic microscope Second harmonic imaging of poled silica glass was first performed by Kazansky et al.[4] The extent and distribution of nonlinear region have been investigated by SHG measurement when the incident laser beam was scanned transversely to the sample's poling direction. The result showed that SH spot was localized 12 um below the anodic surface, and

164


the spread of the depth was ~ 7 urn. Later, SH microscopy has been used for investigating the nonlinear profile in thermally poled silica glass,[36] planar waveguides[37] and optical fibers[38] with sub-micron spatial resolution. Compared with other methods, the SH microscopy can provide direct visualization of the induced nonlinear profile no matter what mechanism is responsible. 5.1.4 Mechanism Many investigations have been devoted to the determination of the microscopic mechanism underlying the formation of second-order optical nonlinearity in silica glass.[8] Cations migration is usually assumed, and the occurrence of poling-induced charge (mainly Na+) migration has been reported.126'39_42] In most case, mobile alkali ions, such as Na+, Li+, or K+, migrate towards the cathode, leaving a depletion region of several micrometers thick at the anode. The negative charge centers are motionless in the silica matrix as compared to the positive charges. Due to the charge separation, a high permanent built-in electric field Edc as high as ~4xl0 8 V/m is frozen along the electric field direction (z axis) within the depleted region.[43] The rectification of third-order non-linear susceptibility x(3) by this built-in electric field induces an effective y2^ susceptibility as[1'13]: Xijk

=

^Xijkz^dc •

(5-4)

On the other hand, alignment of hyperpolarizable entities (such as bond, dipole, defect, or nano-crystallite) leads to the creation of a macroscopic %(2) susceptibility through relation'1'441:

where /? is the hyperpolarizability, p is the permanent dipole moment, N is the concentration of entities, k is Boltzmann's constant, and T is the temperature. Though dipole orientation model could not be excluded completely, a lot of experimental results supported the frozen-in electric field model.

5.1 Introduction

165

5.1.4.1 Charge migration model Charge migration models that predicts the electric field distribution have been given in order to understand the mechanism for x(2) creation.'44"461 The contribution of the built-in electric field to the x(2> susceptibility can be deduced from an understanding of how the charge concentration evolves with time and space during the poling process.[8] The evolution of the local charge concentration with time can be described by a set of partial derivative equations that involve diffusion, conduction and charge trapping and de-trapping. An analytical expression for the thickness w of the poled region' 40 ' 41 ' 45] has been obtained for the case of one mobile positive charge carrier: w=l—(V0-VT), Veco

(5.6)

where V0 is the applied potential, c0 is the initial charge concentration and e the elementary charge of electron. VT is a threshold voltage[45] that depends on the chemical glass composition, the poling temperature, the attraction energy between the mobile cation and the fixed anion (The VT was measured to be 900 V in Infrasil sample[45]). So the thickness of the cation depletion region depends mainly on the initial concentration of cations and the applied voltage. Neglect charge injection, values of w deduced from Eq. (5.6) are in good agreement with experimental ,„ [1,45-46]

ones. The time dependence of thermal poling and the values of the nonlinear thickness obtained in experiment[43] indicate that the poling process takes place in two stages: the rapid formation, within tens of seconds, of negatively charged region depleted with cations (Na+) followed by a slower process which is responsible for charge separation within the depletion region.141'47] To explain the experimental results, two charge carrier model was proposed and solved.[46] It has proposed that during thermal poling in air, If1" or H 3 0 + are driven by the high electric field at the anode into the region depleted by Na+, thus neutralizing the negative charges left behind by Na+. At the same time, the region depleted with Na+ increases and this ion exchange process results in the forward movement of the negative depleted region.[41] When using a

166


non-blocking electrode, for increasing time, charge injection can no longer be neglected. In this case, the nonlinear thickness increases, thus leading to a decrease of the electric field, hence of /(2)- The poling stops when the electric field near the anode drops to values corresponding to which H+ /H30+ ions cannot be driven into the sample and when the total voltage drops across the depletion region, so that the movement of Na+ in the bulk stops. Fig. 5.7 shows one of the charge distributions in poled silica glass by LIPP measurements.[30]

Fig. 5.7 Charge distribution in the poled silica glass revealed by LIPP measurement. After Ref. [30].

5.1.4.2 Third-order nonlinear susceptibility modification In frozen-in electric field model, the second-order optical nonlinearity is shown to proportional to /3). %(i) varies from glass to glass, and is typically in the range (l-3)xl0" 22 m2/V2 in silica glass. Its value is generally assumed to be constant and unmodified after poling. In face, in some experiment, %(3) was shown to be the same for different samples poled with different time.[43] However, there are reports of third-order nonlinear susceptibility modification inside the nonlinear region after poling. Kashyap has reported that the x(3) susceptibility can be modified by a factor of 1.9 after poling in a Ge-doped silica waveguide.'481 The experiment in Ge-doped silica glass also showed an approximately 15 times enhancement of x(V due to the formation of crystallites after UVpoling.[12]

5.2 Second-Order Optical Nonlinearity in Silica Glasses

167

5.1.4.3 Structure characterizations Some studies have been performed in order to reveal the microscopic structure change in glass after poling. Here a few examples are given. Cabrillo et al. published neutron diffraction and inelastic neutron scattering results on poled silica glass.[49] They concluded that large microscopic alterations were found in silica glass after thermal poling. These resulted in the breakdown of isotropy which involved at least next nearest Si neighbors. The effectively evidenced an elongation of the shortest Si-Si distance from 3.06 to 3.23 A in this direction was attributed to the opening of Si-O-Si bond angles in the same direction. In Ref. [50], using X-ray absorption near-edge structure (XANES) and FTIR spectroscopy, Nazabal et al. have shown that Si-O-Si bridging bonds have been broken in the non-linear region during poling. At the same time, a decrease of hydroxyl Si-OH bonds associated with an increase of nonbridging oxygen (Si-CT) defect center was interpreted by a proton conduction during poling.[41] 5.2 Second-Order Optical Nonlinearity in Silica Glasses Second-order optical nonlinearity induced in silica is widely studied since the first report by Myers in 1991. Charge migration models are issued trying to elucidate the mechanism of second-order optical nonlinearity in silica glasses. Here we will give some results on silica glasses, which mainly come from our own work. Other results on silicate glasses are also briefly reviewed. 5.2.1 The influence of poling condition on the optical nonlinearity Poling condition must be optimized to obtain large second-order optical nonlinearity in poled glasses. We have studied the influence of poling condition on the second-order optical nonlinearity induced in silica glass.[51] The sample used in the experiments was commercial JGS] fused silica. During poling process, the sample was heated to the temperature of 150-450 °C, and a high voltage varied from 1.0 to 8.0 KV was applied across the sample plates for 0.5 to 6 hours.

168


The SHG signal was obtained in poled silica glass by the Makerfringe method. In our experiment, a clear fringe pattern instead of an envelope was observed for the poled silica glasses under all poling conditions. Fig. 5.8(a) shows the dependence of SHG intensity on the poling temperature after the samples were poled with 7.5 KV for 1 hour. From 150°C onwards, the SH signal could be observed and the signal reached a maximum at 250°C and then quickly reduced to zero. The SHG intensity dependence on the poling voltage is shown in Fig. 5.8(b). When the voltage increases, the SHG intensity increases monotonically. The SHG intensity dependence on the poling time is given in Fig. 5.8(c). The samples were poled at 250°C and 7.5kV. This figure shows the SHG intensity increase as the poling time increases and approaches saturation after 3 hours. These experimental results revealed that there existed a optimum poling condition for generating a large second-order optical nonlinearity in silica glass.

Polna Temperature 2-Si02 film. Preheating the film in vacuum prior to UV-poling was found to further increase the second-order optical nonlinearity of the film. The value of second-order nonlinear susceptibility was obtained to be as large as 12.5 pm/V, which is the largest ever reported for poled glasses.[53] 5.2.4 Soft glasses The method for poling soft glass is a little different from that of silica

172


glass. A step-like increase in the poling voltage must be used to limit the current flow during poling. The second-order optical nonlinearity has been induced in soft glass,[54] but it decays spontaneously after poling. It is also found possible to fabricate waveguides in soft glass by thermal poling, exploiting the change in refractive index caused by ion depletion.155 ] 5.3 Second-Order Optical Nonlinearity in High Refractive Index Glasses The frozen-field model given in section 5.1.4 predicts that x(2) should increase proportionally to /3\ for a given value of Edc. Enhancement of X(2) in proportion to x(3) is therefore expected in thermally poled glasses with higher /3) than silica. High refractive index glasses are potentially interesting for this purpose since they exhibit intrinsically high x(3)- Here we will give some of experimental results on high refractive index glasses. 5.3.1 Lead borate glass 5.3.1.1 Non-uniform bulk second-order optical nonlinearity distribution In most of the reported experiments on second-order optical nonlinearity in fused silica, the nonlinearity was regarded as coming from a thin layer near the anode surface.11'39] However, according to Le Calvez et al., the nonlinearity was also induced in the cathode area.[56] Furthermore, some groups reported clear Maker fringe observations on poled fused silica and other glasses.15'52] Although it was concluded that the second-order optical nonlinearity of these poled glasses may come from a bulk effect, the interference of the second harmonic signals generated from the two surfaces of a sample might result in a fringe pattern as well. In our paper, [57] a detailed Maker fringe investigation was reported on thermally poled PbO/B203 bulk glass. Our experimental results showed that the Maker fringes come from a bulk second-order optical nonlinearity. In particular, we showed that this bulk effect is strongly influenced by the negative charge layer frozen in the anode area. A theoretical model based on a

5.3 Second-Order Optical Nonlinearity in High Refractive Index Glasses

173

non-uniform second-order nonlinear coefficient distribution was proposed and the experimental results are explained. We used 0.43PbO:B2O3 glasses in the experiment. The samples were made through a standard melt-quenching method. The sample was thermally poled with a voltage of 5.5 KV at temperature of 400°C for 30 min. •

783 urn 634 fim —

7 M t*m calculated

100 90 80 70-

~. «o: 4 80^ -«' 40-

f\ki% A A R * tl

o

J *>: 20-

.^SL^Z'

to0-

,

^\~_

vvvv^^^

^J$Jv n.r-*^^^^ 20

fa)

i \ A A «,

40

WO

80

60

Incident angle (degree) » largest mmmna {cathode grind) B maximum (calhodr frind) O If rgesi Minimum (ftMdtgriad) a rruiimum (anode grind) -•—tartest minimum i > i - i • i 100 MO 300 400 5 M 600

depth Er3+(4Ii3/2-4F9/2)]. After the pump is switched off, the rate equations can be rewritten as d

-¥ = -^~N-{t^FNENY2

^

=dt

^ =d t

^ r»

+

-KcNE2NYi = ^ , ( 6 , 6 )

KFNENY-CNE, F

£,

Y2

E2,

6.2 Er3^-Doped Phosphate Glass Fiber Amplifiers

201

where r° =1/ A§ and zY is the measured lifetime. Values of 5.2 x 1021 and 6.8 x 1021 cm2/ion were used for the absorption cross section and the stimulated-emission cross section at 980 nm, respectively; a value of 2.2 ms was used for the Yb3+ lifetime (rr) in the samples without Er3+ doping. To retrieve KF, the following procedure was adopted. For low pump intensities the population in the level 4IB/2 is small, and both the cooperative upconversion and the cumulative energy-transfer effects for the Er3+ ions are negligible. Moreover, during the 250 \xs after the pump was switched off, the population of level 4Ii3/2 could be considered constant because of the long lifetime of the Er3+ ions in the metastable level. Therefore the spontaneous-emission decays of Yb3+ ions were measured and fitted between 20 and 250us. Assuming NE constant, the energy-transfer coefficients were calculated from the forward energytransfer rate (WF = KFNE ) and the Er3+ ground-state population at low pump intensity. The cooperative upconversion of Er3+ ions of an Yb 3+ Er3+-codoped system can be found by fitting luminescence decay of Er3+ ions at 1.5 um with Eq. (6.3) at higher pump intensity. However, NE2(0) needs to be solved numerically, and the fitting process is somewhat iterative. The cooperative-upconversion coefficient and lifetime were obtained as a function of Er3+ concentration, shown in Fig. 6.3.

2

2.5

3

3.5

4

4.5

Erbium Doping Concentration (10 20 ions/cm3)

Fig. 6.3 Cooperative-upconversion coefficient and lifetime as a function of Er3+ concentrations.

202

Glass Fibers for Optical Amplification

Experimental decay curves of the weak excitation were fitted very well with single exponential curves. The lifetimes are very close for different Er3+ concentrations, indicating that the concentration quenching is negligible. Cooperative upconversion coefficients were fitted with Eq. (6.3) at the highest pump intensity. The cooperative upconversion coefficients increase with higher Er3+ concentrations, as expected. This shows increasing interactions between the excited ions owing to reduced ion separation. The cooperative upconversion coefficient for an Er3+ concentration of 4 x 1020 ions/cm3 is -1.1 x 10"18 cm3/s, which is smaller than those in other hosts with lower Er3+ concentrations.137"401 Experimental data for lifetime and cooperative upconversion have shown that these Er3+-doped glasses are excellent candidates for gain media. In the Yb3+-Er3+ codoped system the energy-transfer process from Yb3+ to the metastable level of Er3+ ions is not instantaneous owing to the finite lifetime of level 4In/2. However, the residual pumping effect from the energy transfer of Yb3+ ions is negligible after 1 ms. Therefore, based on the steady-state population for t= 0, the luminescence decays were fitted from 1 to 30 ms for the cooperative upconversion coefficients to minimize the error from the residual pumping effect of the energy transfer process. The energy transfer and cooperative upconversion coefficients are shown in Fig. 6.4.

1

2

3

4

5

Yb3* Doping Concentration (1020 ins/cm3)

Fig. 6.4 Energy-transfer coefficient and cooperative upconversion coefficient as a function of Yb3+ concentrations.

6.2 Er3*-Doped Phosphate Glass Fiber Amplifiers

203

The cooperative-upconversion coefficient of sample Erl is also indicated for comparison. Owing to the stronger Yb3+-Er3+ interactions, the energy-transfer coefficient was found to increase with Yb3+ concentration, as expected. The energy-transfer coefficient for the Yb + concentration of 6 x 1020 ions/cm3 (YE3) was found to be 1.1 x 10"16 cm3/s, which is a reasonable value when compared with other results.'41' The cooperative upconversion coefficient that results from Er3+-Er3+ interaction was almost constant, as expected. This also indicates that the residual pumping effect was indeed negligible in our fitting procedure. Table 6.2 summarizes the cooperative upconversion and energy transfer coefficients. Phosphate glass has one of the lowest cooperative upconversion coefficients. A commonly used silica glass co-doped with Ge, Al, and P shows a much higher cooperative upconversion coefficient of 2 x 10"16cm3/s. It has been suggested that the relatively large amount of nonbridging oxygens in phosphate glasses contribute to a more homogeneous Er3+ distribution, which in turn leads to a smaller cooperative upconversion effect compared with that in other host materials. Because of the low Yb3+ concentration, the energy transfer coefficients obtained are smaller than those from other work. Table 6.2 Summary of the Cooperative-Upconversion and Energy-Transfer Coefficients. Er3+ Yb3+ Concentration Concentration (lO-'W/s) (10-"W/s) (1020 ions/cm3) (1020 ions/cm3) 2-4

2-6

0.8-1.1

-1

0

3.8

-1.2

0

4

0.4-5

0

0.7-4

0

0.4

Ref.

Phosphate

19

Soda-lime silicate

37

Er-implanted A1203

38

0.3-1

Soda-lime silicate

39

0.5-2

Alumino-silicate

39

Ge/Al/P-doped fused silica

40

Phosphate

41

-0.01

0

200

2

10

1.2

1.1

GlassHost

5

Measured lifetimes of Yb3+ ions and effective-transfer rates (1/x-r- l/xY°) from Yb3+ to Er3+ as a function of pump power. The energy transfer efficiency is calculated for low pump power by

204


ri = l-f.

(6.7)

The energy-transfer efficiency mainly depends on the ratio of the backward-transfer rate and the multiphonon relaxation rate of the Er3+ I11/2 level. Compared with silicate and germanate glasses, phosphate glasses have high efficiencies because of the small values of this ratio.[42] In tellurate glasses, low observed transfer rates indicate that a low transfer efficiency and/or a presence of back transfer from Er3+ to Yb3+ because the slow decay of the 4ln/2 level cannot provide an efficient sink for the excitation transferred from Yb3+.[43] The Ce3+ incorporated with the Yb3+-Er3+ system was studied to minimize the back-energy transfer by reduction of the lifetime of level 4In/2.[44'45] An energy-transfer efficiency of 45% was reported under weak excitation at 977 nm in borosilicate glasses with an Yb3+-Er3+ concentration ratio of 5 (Er3+, 8 x 1019 ions/cm3; Yb3+, 4 x 1020 ions/cm3). The low transfer efficiency of 45% is because of a low Yb3+-Er3+ ratio in borosilicate glass.[46] In phosphate glasses, energy-transfer efficiencies higher than 95% were measured for low pump power. The high transfer efficiency is not only because the back energy transfer is negligible, but also because both Yb3+ and Er3+ concentrations are high to cause high excitation diffusion among Yb3+ and Er3+ ions. Moreover, Er3+ concentration in phosphate glass is 2 x 10 ions/cm , which is higher than other glasses (-A-A-A-A.-A_a_

-20

Q,

f

-15

Output signal power (dBm)

Fig. 6.13 Internal gain vs. output signal power for different fiber lengths at both 1535 and 1550 nm at pump power of 224 mW.

Saturation behaviors for both 1535 and 1550 nm signals are shown. The 980 nm pump power used was 224 mW. The output 1535 nm signal powers at 3 dB compression are measured to be 5.6, 8.1, and 10.9 dBm for 5.55-, 4.4-, and 3.6-cm-long fiber, respectively. Although all three

6.2 Er +-Doped Phosphate Glass Fiber Amplifiers

221

1550 nm saturation curves have not reached 3 dB compression in this measurement, it can be seen that the saturation output powers are higher for the signal at 1550 nm because of its lower cross section.[16] 6.2.4.3 Multi-mode pumped fiber amplifiers Fig. 6.14 shows the layout of an amplifier in a reflective geometry by using a circulator. Pump light from a multimode 50-um broad-area laser diode is coupled into the first cladding of a passive double-clad fiber (DCF) through a proprietary coupler.[59] The DCF is then fusion-spliced to an 8-cm erbium-doped phosphate fiber (EDPF). Signal is launched from port 1 of the circulator and directed to EDPF through port 2. A dielectric mirror is coated on the tip of the DCF fusion-spliced to the other side of the EDPF and is used to highly reflect both the pump and the signal. Most of the residual pump is immediately dumped when it reaches the SMF and further reduced to -60dBm level by the built-in isolator in the circulator en route to the output of the amplifier. The inset in Fig. 6.14 shows a cross-sectional view of the EDPF. As shown in the figure, the core, the 1st and the 2nd cladding are all axially symmetric. The numerical apertures of the core and the 1st cladding were calculated to be 0.145 at 1550 nm and 0.24 at 980 nm, respectively. bUFJh^ Cross sectional view _Core — 1st cladding

Output Fig. 6.14 Schematics of the multimode pumped double pass amplifier. SMF: Corning single-mode fiber SMF-28; LD: laser diode; C: proprietary pump coupler; DCF: doubleclad fiber; EDPF: erbium-doped phosphate fiber; DM: dielectric mirror.

222


Amplifier performance was evaluated experimentally using fibers with a variety of lengths. Excluding the circulator and the dielectric mirror, the single-pass insertion loss with this EDPF was measured at 1310 nm to be 3 dB. Almost 1/3 of this insertion loss was from the propagation loss of the EDPF. Fig. 6.15 illustrates the measured and simulated amplifier gain spectra from the 8-cm-long EDPF at signal input powers of-30 dBm, -10 dBm and 0 dBm with a 1W pump power from the broad-area laser diode. A peak gain of 41 dB was achieved at 1535 nm with -30 dBm input signal. At the -30 dBm input signal level, the noise figures at 1530 nm, 1535 nm, 1550 nm and 1565 nm were measured to be 6.3 dB, 6.1 dB, 5.3 dB, and 4.8 dB, respectively. Fig. 6.15 shows that greater than 15dBm output power could be delivered over the whole C-band when the input power was 0 dBm. The output power increased to 17.5 dBm when the pump power increased to 1.5 W. The saturation output power (3 dB compression) at 1535 nm, 1550nm and 1565 nm are 11 dBm, 12.5 dBm and 14 dBm, respectively for the 8-cm-long fiber excited with 1W pump power. The output saturation power is 2 dB higher when the fiber is excited with 1.5 W pump power. •

1525

Pin=-30 dBm, exp. Pin=-30dBm, simulation

1535

1545 1555 1565 Wavelength (nm) Fig. 6.15 Measured and simulated gain spectra with various input signal powers.

The simulated gain performance agrees closely with the experimental results as indicated in Fig. 6.15. According to the modeling results, the core in the 8-cm EDPF absorbed approximately 15% of the multi-mode pump. The model also showed that such high absorption in such a short fiber was caused by the high pump absorption coefficient of the core.

6.2 Er * -Doped Phosphate Glass Fiber Amplifiers

223

Furthermore, the model indicated that an absorption efficiency of 30% is achievable with optimized EDPF design in terms of doping concentrations and fiber geometry. Fig. 6.16 illustrates the net gain and noise figure versus the pump power at 1550nm for different fiber lengths. The input signal power was -30 dBm. Fig. 6.16(a) indicates that the optimum fiber length is around 8 cm. Fig. 6.16(b) shows the gain saturation at 1530 nm, 1535 nm, 1550 nm, and 1565 nm for the 8-cm-long fiber excited with 1 W pump power. 45 23 40 —«G—L=7 cm — 0 » L = 8 cm *L=9 cm X L=7cm NF L=8cm NF A L=9cm NF

•

QHHQ H • 400

700

1000

1300

'1 "I

•

35

O1530nm • 1535im. A1550nm • 1565 nm

130 •a 25 k> ^ 0A O A O* 75%

DCYDF-650 6000 ppm 25~50um 0.07-0.15 D 650um 600um 0.46 >75%

7.3 High Power Lasers Based on Rare-Earth Ions Doped Fibers 7.3.1 Principle Rare-earth doped silica fiber has been proven to be an excellent solution for all-optical network and has been extensively and reliably employed in the telecommunication area for many years. Silica fibers are also attractive for high power amplifiers and lasers. Silica based glasses have high damage thresholds. The surface-to-volume ratio of an optical fiber is high so that heat dissipation is straightforward, and the gain medium is incorporated in a waveguide so it is possible to maintain single mode

240

Glass Fibers for High Power Lasers

wave propagation. Fiber manufacturing technology allows producing long fibers, and fiber laser cavities can be tailored to provide the beam quality for almost all applications. Recently there have been significant developments in the area of cladding pumped double-clad doped-fiber for very high power fiber lasers and amplifiers. In this approach the fiber core is heavily doped with active ions (e.g. Yb, Er, Nd, Tm) and the silica-clad fiber is coated with low refractive index material; pump light is launched into the cladding of the optical fiber (typically in the range 125 [im to 650 urn diameter). The silica inner cladding and the low index coating give rise to a high numerical-aperture multimode waveguide for the pump light, allowing high-power multi-mode semiconductor diode lasers be used. This offers advantages both in terms of cost-per-Watt, and of the availability of much high-power high-brightness pump lasers. In operations, the pump light propagates in the undoped cladding of the optical fibre and is absorbed by the active ions in the core. The pump power into the inner clad can be expressed as P = 4BD2NA\

(7.16)

in which D2 and NA are the area and numerical aperture of the inner cladding respectively, B is the brightness of the pump source. To increase the maximum pump power coupled into the fiber, the inner cladding is designed with a high NA and a large area (typically in the range of 350 to 650 urn). From Eq. (7.16), in order to realize high-power output for a certain double-clad fiber, the most import thing is to have a high brightness pump source. The pump absorption efficiency changes with the inner cladding shape. The inner cladding shape is normally non-circular to let more pump power enter the fiber core. If the inner cladding is designed to be circular, then only few pump modes cross the doped core. In this case, the pump efficiency is low. However, the core can be offset from the center, or the inner cladding can be designed to have no-circular shape to improve the absorption efficiency. To optimize the pump absorption, various shapes of the inner cladding have been proposed. The most common shapes are described in Fig. 7.4. In which the inner cladding are circular (a), offset (b), square(c), rectangular (d), hexagonal (e), flower shape (f), D shape (g), "unstable cavity" shape (h). The absorption

7.3 High Power Lasers Based on Rare-Earth Ions Doped Fibers

241

characteristics are simulated for double-clad fibers with (a), (b), (g) and (h) inner cladding shapes by the 2-D ray tracing method.[5]

(c)

(d)

(e)

(f)

Fig. 7.4 Typical double clad fibers.

The pump light absorption efficiency is 10%, 50%, 80% and 98%, correspondingly. The results are shown in Fig. 7.5.

40

60

Times of reflection N Fig. 7.5 Variation of absorption efficiency versus different inner shape fibers.

A fundamental limit to power scaling of double-clad fiber lasers is the damage of fiber core end facet in building high-power fiber lasers. In the multimode regime (large core), it is relatively easy to get a high output power, but single-mode output is a great challenge due to the optical damage. Also, nonlinear optical processes in the fiber degrade the laser performance. Several important nonlinear processes that limit the output power and energy are stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS), and self-phase modulation. Their influences are determined by the pulse duration, spectral linewidth, and fiber length.


242

The simplest solution to overcome these problems is to increase the size of the core diameter. As shown in Fig. 7.6, a large dc0re correspond to larger V. when V>2.4, the beam quality is poor due to the propagation of higher order modes. Recently, many scientists pay attention to find a way to make use of the multimode power capabilities but remaining the single mode beam quality. 60

..y=8.o 20

... V=4.0 V=2.405 — i —

0.14

0.06

0.16

COTeNAj

Fig. 7.6 Doped core diameter Vs. Core NA (The wavelength is near l.Oum region). The size of dcore for single mode fiber is less than 20um with an NA,.,,,.,, 0.06.

7.3.2 Laser diodes and beam shaping The choice of pump source is very important in the overall performance of a high-power fiber laser. It influences the reliability and cooling methods, as well as the efficiency. Historically bulk solid state lasers have been widely used for high power applications. They can be easily pumped with lamps. But for the high-power fiber lasers, laser diodes are the best choice for the pumping. Fortunately, the developments of highpower pump lasers and low-loss rare-earth doped fibers make high power fiber lasers possible. The difficulty in high-brightness delivery of high-power diode laser beams stems from the geometries and structures of these devices. Thus, a high-power LDA has a broad-area light-emitting aperture of about 1 cm


243

x 1 |im. With such a configuration, some commercially available high power LDAs can provide 40 to 50 W of power. Much higher power can be achieved by layering high-power LDA bars in stacks. The raw output beam from an LDA is highly divergent and suffers from two asymmetries—astigmatism and an elliptical beam profile. The divergence angles are different in two axes, the so-called "fast axis" and "slow axis." Typically the fast axis divergence is about 40° full-width half-maximum (FWHM) while that of the slow-axis is about 10° FWHM. Sometimes the divergence can be as high as 50° x 15°. To improve the beam quality and brightness, sophisticated beam rearrangement mechanisms are normally used. Two typical examples are the step mirror approach used by the Fraunhofer Institute for Laser Technology (Aachen, Germany) and the two-reflector approach invented by researchers in the University of Southampton (Southampton, England). Both methods have been used commercially to provide fibercoupled laser-diode devices. A new efficient approach is developed by using groups of thin prisms for beam shaping at Shanghai Institute of Optics and Fine Mechanics (China patent No. ZL 03115584.7 by Qihong LOU). 7.3.3 End-pumping fiber laser Coupling the pump beam into the inner cladding of double-clad fiber through its ends by so-called end-pumping scheme is the simplest and most efficient way to pump double-clad fibers with high-power pump sources. In end-pumping configuration, a large inner cladding is required in order to accommodate the large pump beam of the high-power laser diode source. The laser diode pump source is either coupled to the double-clad fiber with bulk optics or fiber optics.[6] The bulk optics approach, in which the pump light is launched into the end of the fiber, is often used in laboratory applications. A major drawback is that one or both of the fiber ends are obstructed by the bulk optics used to launch the pump light. In addition, this approach lacks scalability (one fiber has only two ends) and is difficult to implement in a compact and rugged manner. However, most double-clad fibers made to date use a low-index


244

polymer as the outer clad material to achieve the desired high numerical aperture (NA 0.3-0.45). These polymers have much poorer thermal stability than glass. Under high power pump, the polymer near the innerouter clad interface can easily burn or gradually degrade. Because of any faulty steps in beam shaping and assembling, the high-power collimated pumping beam is not so good for pumping the double-clad fiber directly. A spatial filter can be used to improve the beam quality of the highpower pump light. In order to inject the pump light into the inner cladding with high coupling efficiency, a special aspheric lens is designed and fabricated. Benefiting from the diffraction limited performance of the aspheric lens, as well as the optical spatial filter, the focus spot and the cone angle of the pump light match well with the corresponding parameters of the double clad fiber. And the pump light can be safely and efficiently coupled into the inner-cladding. The experimental setup is shown in Fig. 7.7.

a

Yb-doped fiber

Pump source

Dichroic mirror Spatial filter

Collimation lens

Fig. 7.7 Experimental setup of fiber laser with one-end-pumping using spatial filter.

The output power is a function of fiber length when the injected pump power is fixed. Optimum fiber length of 20-m was achieved theoretically as shown in Fig. 7.8. The experiment results of 6-, 21-, and 52-m fibers are presented in Fig. 7.9. The experimental results are in agreement with the calculated ones. For one end pumping, the maximum laser output power of 20-m DCF is more than 200W at 1.1 um with a slope efficiency more than 69%. With two-end scheme, the output power was more than 440 Watts.[7S1


245

20 26 Fiber length L (m)

Fig. 7.8 Output power as a function of fiber length. 50

FiberLengh Slope Efficiency - • - 22m 67.1% - A - 53m 53.7% -A- 72m 52.6% - a - 6m 45.9%

40

30

a. 3 20-

/ & *

talK&. 20

30

40

50

Launched pump power (W) Fig. 7.9 Output power of different fiber lengths.

The fiber optics approach, or so-called fused fiber bundle (pump combiner), in which several multi-mode(MM) fibers are bundled together, fused and drawn into a taper, fusion spliced to a double-clad fiber, and then recoated with a low index polymer; pump light is launched into the double-clad fiber from individual diode lasers that are coupled to the MM fibers. Optionally, the fiber bundle can include a single-mode (SM) fiber that is used to couple signal light into or out of

Glass Fibers for High Povxr Lasers

246

the core of the double-clad fiber. This method is stable and robust and can provide high coupling efficiency (ultimately limited by the efficiency of fiber-coupling of the pump diodes). The approach allows unidirectional pumping and is scalable. The shape and size of the fiber bundle and of the SM pigtail must be matched to the double fiber being pumped (see Fig. 7.10, from IPG catalog).

\^vA

%

&JJ)

or ; J>& &3*^ : A

;.'*&/*****" ««*' Jii* *LD

""* ^ ^

Fig. 7.10 Fiber laser with fused fiber bundles pumping schematic.

7.4 High Power Pulsed Fiber Lasers 7.4.1 Introduction Recent improvements in the modified chemical vapor deposition (MCVD) process and diode-pumped lasers offer the possibility of incorporating a large variety of rare-earth ions in low-loss silica glass fibers. Interests in Er3+ ions were stimulated by the fact that its peak laser transition near 1.5 um falls into the low-loss window of silica fibers. Erbium doped fiber amplifiers (EDFA) are developed rapidly and used widely in broad-band optical communication system. In recent years, high power pulsed fiber lasers have caused particular interest as a reliable, efficient and compact, low cost source for a variety of applications in industry. There are several methods to get pulsed operation of fiber laser, one is the Q-switch fiber laser with lower power output, another one is a master oscillator power amplifier (MOPA) system. MOPA system is used frequently comparing with CW fiber

7.4 High Power Pulsed Fiber Lasers

247

lasers, pulse fiber laser need higher peak power intensity at the fiber surface, also how to reduce amplified spontaneous emission(ASE) signal is important for MOPA system. In this section, we will discuss these problems in detail. In the past few years, Yb-doped fiber amplifiers have made great strides in the optical power amplification field. Performances have been improved firstly thanks to new pumping technologies: end pumping has been progressively replaced by side pumping, making it possible to leave the double clad fiber ends free for fiber splicing. Moreover, the use of large emitting area laser diodes makes it possible to launch very high pump power, typically from a few to several tens of watts. The inner clad shapes of the fibers lead to different absorption. High level saturated signal output power can thus be reached, but with reduced efficiency. This is mainly due to the fact that the shape of the cross section of the fiber and/or the transverse distribution of rare-earth ions are not optimized, leading to an inefficient absorption of the pump power. In 2000, A Q-switched, 5W average output power amplifier using 3+ Yb -doped double clad fiber was reported by J.Alvarez-Chavez.[9] Further research was carried out by J. Limpert et al. at Jena, Germany. Single-frequency (MOPA) emits up to 20W. Picosecond pulse duration fiber amplifier is capable of generating 51.2 W. Nanosecond fiber amplifier can even produce up to 100 W average power at 50 KHz repetition rate, corresponding to pulse energy of 2 mJ. [1012] We launched the research of high power and high energy Yb-doped double clad fiber and amplifier at Shanghai Institute of Optics and Fine Mechanics, CAS. We used the MOPA system and homemade large mode area (LMA) double clad fibers, realize 133.8 W average power of amplified radiation at the wavelength of 1064 nm and a repetition rate of 100 KHz, limited only by the available pump power. Peak power of 300 KW at 20 kHz with the pulse duration of 15 ns is obtained. Compared to the previous results with similar arrangement, it is the highest single-fiber output average power to our knowledge.[7] 7.4.2 Transient response of Yb-doped fiber Amplifier Quasi-three-level systems such as erbium and ytterbium-doped glass


248

fibers take part in commonly used technologies in many applications such as telecommunication devices and high-average-power systems. Because of the importance of these devices, much theoretical research has been undertaken to understand and optimize. Two main areas have been prospected in order to characterize the behavior of the fiber amplifiers. On one hand, fully numerical models allow us to predict accurately the gain and the ASE spectra, but these approaches usually take a lot of computing time and are not very suitable for a better understanding of the fiber device properties. On the other hand, several analytical or semi-analytical models have been developed. The two-level rate equations neglecting of ASE can be given as : 9N /

=

) dt

(RP°+W^Ni-(Rl*+W-

+

^

N

i

'

Nl+N2=p, ^

-

(7-17>

(7.18)

^

= ±?;-rp[N2*pe-N]apa],

(7 .19)

dP± 1 8P± ~^ + ^- = ±P;Ts[N2ase-NlaJ, oz vg at

(7.20)

The transition rates can be described as follows:

R

/T

T~"

= pa'e

p

p

T^

w =^L±P

A= y

Here, Ni and N2 are the population of upper and lower status of laser. p is total Yb3+ density. apa!e is the pump absorption and emission cross section respectively, &sa,e is the signal absorption and emission cross section. Aeff is the effective core area of the YDFA and Ts and Tp give a measure of the overlap of the optical modes with the Yb distribution. x2i is the fluorescence lifetime of the metastable level of the two-level system. Pp and Ps are pump and signal power respectively. vp and vs are pump frequency and signal frequency, vg is the group velocity, h is Planck constant.


249

We suppose nt = Nt j p (i = 1, 2), the above equations can be written as: ^

= (Rpa + WJn, ~(Rpe + Wse + Ae)n2,

«i+«2=l.

(7.21) (7.22)

And we can obtain: ^

+

Zn2=V,

(7.23)

at t = Rp„+Wsa+Rpe+Wse + A2l , t} = Rpa+W„. The general solution is n2 = ce'1'" + TJ/% , c = n\- n/4 . So the characteristic time constant t0=\/Z = T/l + q + p, q = Pp/P?, P = PjPrThe initial conditions can be calculated under steady-state conditions, the pump is constant for pumping scheme, the Gaussian signal power is 0.3 W and the duration is 1 (is, the pulse repetition rate is 20 KHz, the length of fiber is 10 meters, P psat =2xl0" 3 W, P ssat =1.5xl0" 2 W. We use finite-difference method mentioned above to integrate numerically with the parameters below: Xp=9\5nm,

As =\064nm , opa =2.5x1 (T2W ,

crpe = 3 xl(T26 m2, z2l=0Mms, <jsa=3x\0-26m2, ase=5.5xl0-26m2,

p = 1.9xl025w3, ^#=5xl0""w2,

T p = 0.0012, T s =0.82, P = 1000, ^ = 100. The calculated result is shown in Fig. 7.11. The peak of pulse shift to the rising edge, which is due to the long population inversion recovery time of Yb ions. The experimental result is shown in Fig. 7.12, which is in agreement with the calculated results. The phenomenon demonstrates that the low repetition rate pulse has obvious transient gain. The gain saturation and recovery time depend on pump rate.


250

Fig. 7.11 The transient changes of Gauss pulse as functions of time in YDFA.

^ 3

0.020

i?

0.015

d

0

1

2

3

4

Time( urn) Fig. 7.12 Amplified pulsed profile from pulse fiber laser.

7.4.3 High power Yb-doped fiber amplifier We have done some research on double-clad fiber lasers and amplifiers. A 133 W fiber laser and amplifier was fabricated using double-clad fiber made by the Fiberhome Technology, Inc, Wuhan, China.[1319] Increasing the size of core appears is one of the main directions of the


251

technological advancement towards high pulse energy and power, and also short length fibers are preferred as the Raman scattering can be reduced and stimulated Raman scattering threshold increased. The double-clad large-mode-area Yb-doped fibers used in the experiment were designed by independent technology and fabricated by the standard MCVD method. The fiber has a 43 urn diameter Yb-doped core with a NA of 0.08, centered in the preform and 650/600 um D-shaped inner cladding with a NA of 0.37. The doping Yb3+concentration is evaluated to be -6500 ppm. The setup of MOPA system is shown in Fig. 7.13. A Q-switched laser is applied as the seed source. The laser delivers average powers up to 1 W with repetition rate between 20 and 100 KHz. A Faraday isolator was used to protect the seed laser from back-reflections. The length of double-clad fiber is 4 m. The large cladding size ensures that more than 90% of the launched pump light is absorbed in the fiber. Mi*'

Seed source

LD

~l

isolator

\

Ettâ^ DCF

M^

Fig. 7.13 Experiment setup of MOPA system.

The fiber was coiled to a 10-cm diameter cylindrical mandrel in air, without any special cooling device. Both fiber ends were polished at angles of 5-10° to suppresses ASE arising from the Fresnel reflections. The fiber amplifier was pumped by a laser diode which was water-cooled and the operating temperature was from 18 to 22 °C. The central wavelength of the diode was about 975nm. Two lenses, which have short focal length, were used to couple the pump light into the inner cladding with a coupling efficiency of -90%. A dichroic mirror with AR for pump light and HR for amplified light was placed by an angle of 45° to separate the pump and amplified light. Two reflective mirrors Ml and M2 were used to decrease the length of the total system. An aspheric lens

252


was used to couple the seed light into the active core with high efficiency. The output characteristics are shown in Fig. 7.14.

50

100 150 Launched pump power, W

200

250

Fig. 7.14 Output power against launched pump power.

At a repetition rate of 100 KHz, we were able to produce an average output power up to 133.8 W at the maximum diode driven current. The slope efficiency with respect to the launched pump power was 56 %, and the output power increased linearly with the launched pump power. Compared to the previous results with similar arrangement, it is the highest output average power to our knowledge. We have not found any facet damage at the maximum power. So more output power can be realized if we increase the pump power. Due to transient gain of the MOP A system, the amplified pulse duration is reduced from 30 ns to 15 ns at the repetition rate of 20 KHz, corresponding to a peak power of 300 KW. The pulse duration decreases with the decrease of repetition rate. Fig. 7.15 shows the emission spectrum at the maximum output power, against the seed source spectrum and ASE on the logarithmic scale at a repetition rate of 100 kHz. When we pumped the fiber without injection the seed source, the output spectrum was ASE spectrum and centered at 1040nm with 3 dB bandwidth of 20 nm, but when the seed source was coupled into the core, the output peak spectrum was shifted to 1064 nm due to mode competition. The ASE spectrum was reduced effectively and no stimulated Raman scattering occurs. Coiling the fiber in a diameter less than 10 cm suppressed higher order transversal mode


253

through bending losses, and only lower order modes are amplified. The M value is characterized to be 3.2. This value can be improved with a smaller diameter of the cylindrical mandrel.

M

1000

'

1

1020

1

1

1

h

1040 1060 Wavelength, nm

"n

1

1080

1

1

1100

Fig. 7.15 Emitted spectrum of seed source, amplifier output and ASE.

The modeling and experiments presented here are not the last word on any of these subjects. There is still a great deal of work to be done to scale fiber amplifier system to high power. Additional work must be done to improve beam quality. The variation in amplifier performance with temperature is unlikely to be a major difficulty. However, as amplifiers are scaled to KW levels, eliminating waste heat becomes a concern. Short fibers, while reducing SBS, make heat removal more difficult. A very short fiber will have some of the same thermal problems as a rod laser. In addition, elevated core temperatures may cause diffusion of the core dopants and changes in the guiding properties. A great challenge that has not received much attention is the need for high power, low-loss isolators. Low-loss fiber pig-tailed isolators can only operate at the hundred mW regime. High-power bulk isolators require free-space propagation. It also introduces a place subject to misalignment, negating the greatest advantage of fiber systems.

254


7.5 Recent Development and Applications of Fiber Lasers 7.5.1 Recent development Fiber lasers are technological revolution in the fields of solid state lasers. There has been a rapid, large increase in the power produced by fiber laser systems.[20"231 Recently, the level of CW power available in single double-clad fiber has been increased from hundreds of Watts to more than 1 KW.[24"261 This breakthrough has been driven by two factors, one is the progress in development of high brightness semiconductor diode pumping sources; the second is by using large mode area ( LMA) fibers. The remarkable rises in CW fiber laser output power that has been reported by several research groups over the past few years are illustrated in Fig. 1.9 of Chapter 1. This figure shows only the results achieved in a single double-clad fiber. An array of combined fiber laser with an output close to 10 KW has been reported. The achievement of higher laser powers is limited by three effects: first, optical damage of the fiber material by high power laser, second, limitation by the usable pump powers with higher brightness, and third, the effects of nonlinear interaction in the fiber core.[27] The development of the optical fiber has traditionally been driven by the need of the long distance optical communication networks. During 1970's, the trend was to move from large multimode to small singlemode core, the modal dispersion is much smaller in single mode core compared to multimode fiber core. However, the need to obtain high CW power or high average pulse fiber lasers is pushing the double clad fibers to the opposite direction: increasing the fiber core dimensions. For standard telecommunication fiber, the core diameter is usually less than 10 um, a typical high power LMA double cladding fiber core diameter is around 30 um. The ten times larger core area allows ten times more output power. The pumping power with high brightness is another key point in developing the high power fiber lasers. Recently, the high power diode laser module with total output of more than 750 Watts and centre wavelength of 975-980 nm is available. The output can be transmitted by a fiber with core diameter of 600 um. By using two end pumping configuration, 1 kW laser output can be obtained. On the other hand,

7.5 Recent Development and Applications of Fiber Lasers

255

several fiber pigtail diode pumps can be coupled into a double cladding fiber by using a fiber combiner. This combiner consists of several relatively large diameter pump fibers fused together with signal-carrying fiber, which contains a core matching that of a LMA fiber. The fused pump fibers are tapered at the end to match the dimensions of the double cladding fiber, being cleaved and then spliced to a rare-earth doped LMA fiber. With this diode -combining scheme, several low power diodes can be used instead of a single high power one. The maximum power of this kind of combining system is around 200 Watts. Since the magnitude of laser - signal distortion induced by nonlinear effects (such as stimulated Raman scattering, SRS) is inversely proportional to the fiber mode area, the LMA fiber has a mode area more than ten times larger than the standard one. For standard single-mode fiber at 1.064 um typically supports a fundamental mode with diameter ~ 6.2 um, and corresponding mode area of 30 um2, when LMA fiber is used with core diameter of 30 um, the mode field diameter is ~ 24 um, corresponding mode area of 452 um2. The SRS threshold for CW fiber laser is typically in the 100 W range and that of LMA is in KW range. For pulsed fiber laser, the SRS threshold for standard fiber is only ~ 20 KW peak power, and for LMA (30 um core), it can increased to ~ 300 KW. Although the LMA fibers have advantages described above to increase the total output power from a single double cladding fiber laser with higher SRS threshold, the beam quality of the LMA fiber laser is decreased. Because LMA fibers with core sizes larger than 10 um support more than one transverse mode, it is necessary to employ some specific techniques to achieve single-mode operation of LMA fiber laser. For example, in a coiled fiber, higher order mode can be filtered to offset the effects of intermodal scattering, Practically by using short length high quality LMA fiber, it has proved possible to get a nearly diffractionlimited beam output. In contrast to all other optically pumped solid state lasers, fiber lasers use guided-mode propagation to create a new kind of laser structure with very long resonator lengths. The laser cavity is extremely simple without need for complex alignment. What are the limiting factors for scaling the power of the fiber laser in future? It is likely that within the next few

256


years, research and development efforts will continue to extend output power with a single fiber into the several KW range, since current approach to produce LMA fibers are closed to their limits in terms of the mode area that can be achieved, to design and develop new, large mode area fiber would be highly advantageous. One new idea is hollow core rare-earth doped photonic crystal fiber (PCF), which can reduce the nonlinearity of the fibers, the other one is short centimeter-size fiber lasers with large core areas. Fiber geometry offers another promising avenue of power scaling, several fibers can easily be packaged together, optical combination of the outputs of these fibers could lead to higher power than what individual one can provide. A variety of techniques for incoherent and coherent combination of fiber lasers is now being studied and developed. Much higher power can be obtained in future. 7.5.2 Applications of fiber lasers Fiber lasers have become the "hotbed of solid state laser development" with advance in powerful output, better ways to generate short pulses at high average power and higher efficiency. According the annual market review and forecast, the LASER FOCUS WORLD starts a report on fiber laser sales only a few years ago, in 2004 total sales of fiber lasers reached $ 84 million, it is 14% over 2003, additional growth of 47% is expected in 2005. The reasons for this strong growth are new technology and ideas, including improvements in fiber design and fabrication, steady improvements in pumping diodes source. In various applications, fiber laser can be used in the fields of materials processing, medical applications, instrument and basic research, in which, the main field is the material processing. It includes welding, cutting, drilling, marking and semiconductor or microelectronics manufacturing. The requirements for the flexible tool are as follows[28]: 1. Good beam quality: compare with other lasers, fiber laser has very nice beam quality with M2 less than 1.5 with core diameter < 10 |j,m, for large core diameter, more than 600 W with M2 < 1.2 was demonstrated recently.

7.5 Recent Development and Applications of Fiber Lasers

2.

257

Flexibility in spot geometry: the fiber laser is delivery by the double cladding itself, no additional transmission fiber is needed. 3. Flexibility in time: CW and pulse output are available from fiber laser, usually, a master oscillator power amplifier scheme is used to generate pulse output. Laser processing technology is widely used in car manufacture, aerospace industry and many other fields. It is the tendency of material processing in the future and has most advanced advantages comparing with other processing method. Three dimensional laser processing makes it possible to realize the processing of three dimensional workpiece by high intensity laser beam along the complicated special curve for welding, cutting and other treatment. The fiber laser is held by robot arm, normally, five or six degree of freedom to reach any position of the workpiece. In this system, the focus characteristics of the fiber laser beam keep constant at different processed position during the processing. Laser micro-machining is another application field for fiber lasers, as microelectronic circuits continue to shrink, the use of laser in circuit production continues to grow, based on the fiber laser can be focused to very small point, it can be used for laser trimming of thick- and thin-film resistor on ceramic and silicon substrates and laser repair of redundant memory device, the fiber laser rapidly becomes a viable tool in a wide variety of micro-machining applications. Typical lamp pumped laser for trimming applications required high energy input power supplies ( for example 3 KW ), large cooling water system, periodic lamp and water filter replacement, it also need frequently output power monitoring and adjustment. In comparison, a LD pumped fiber laser requires only a few hundred watts of input power and provides constant, stable laser energy for thousands of hours. With improved quality of nonlinear crystals, such as BBO and LBO, the process in the laser micromachining can get smaller focal point by using second harmonic or third harmonic generation in the green or UV regions. Better results can be obtained with much cleaner cut and less thermal effected zone. Laser marking is one of the most important applications for laser material processing especially in China. Fiber laser can be used in laser marking system with several advantages: good beam quality with almost

258


diffraction limited beam spread angle, compact size with flexible fiber output, high plug efficiency (more than 20 %) with low operating cost and air-cooling system. Fig. 7.16 shows the fiber laser marking system developed by Shanghai Institute of Optics and Fine mechanics.

Fig. 7.16 Laser marking system developed by SIOM.

The system can make marks in the region of 100mm x 100mm with scanning speed of 30 m/sec. The fiber laser used in this system is a 10 Watts pulsed fiber with repetition rate between 20 KHz to 100 KHz, the laser beam spread angle is 2 mrad.

References 1. M.Born and E. Wolf, "Principle of optics," (Cambridge University Press, Cambridge; New York) 1999. 2. S. Chuang, "Physics of optoelectronic device," (John Wiley & Sons, Inc., New York) 1995. 3. Kao, K..C. and Hockham, G.A., "Dielectric-fibre Surface Waveguides for Optical Frequencies", Proc. I.E.E.E 113 (1966) 1151.

References

259

4. S.Millar and I.Kaminow, Optical fiber telecommunications-II, New York: Academic, (1988). 5. J.Zhou, Q.H.Lou, Z.J.Wang, J.X.Dong, Y.R.Wei, SPIE, 4914 (2002) 141-145. 6. V.Dominic, S.MacCormack, R.Waarts et al., Electronics Letters, 35 (1999) 11581160. 7. Qihong Lou, Jun Zhou, Jianqiang Zhu, CLEO-PR 2005, Tokyo, Japan, Post Deadline paper. No. 00989 (2005). 8. D.Xue, Q.Lou, J.Zhou, L.Kong, J.Li, S.Li, Chinese optics Letters, 3 (2005) 345-347. 9. J.A.Alvarez-Chavez , H.L.Offerhaus, Opt.Lett, 25 (2000) 37. 10. S.Hofer, A.Liem, Opt.Lett, 26(2001) 1326-1329. 11. J.Limpert, A.Liem, Opt.Lett, 26 (2001) 1849-1851. 12. J.Limpert, S.Hofer, Appl.Phys.B75 (2002) 477-479. 13. Jun Zhou, Qi-Hong Lou, Lingfeng Kong, Chin.Phys.Lett, 21 (2004) 1083-1085. 14. Lingfeng Kong, Qihong Lou, Jun Zhou, Zhonglin Wu, Chinese Optics Letters, 2 (2004) 98-99. 15. Dong xue, Qihong Lou, Jun Zhou, High power laser and particle beams, 17 (2005) 665-668. 16. Lingfeng Kong, Qihong Lou, Jun Zhou, Dong Xue, Optics & Laser Technology, 37 (2005) 597-600. 17. Qihong Lou, Jianqiang Zhu, Jun Zhou, Chinese J. of lasers, A32 (2005) 552-555. 18. Lingfeng Kong, Qihong Lou, Jun Zhou, Actaphotonica sinica, 33 (2004) 1286-1289. 19. Lingfeng Kong, Qihong Lou, Jun Zhou, Chinese J. of lasers, Ail (2004) 93-95. 20. Zhonglin Wu, Qihong Lou, Jingxing Dong, Yunrong Wei, SPIE, 5623 (2004) 137140. 21. Zhonglin Wu, Qihong Lou, Jingxing Dong, Yunrong Wei, Chinese J. of lasers, A 32 (2005) 953-955. 22. Qihong Lou, Jun Zhou, Jianqiang ZhuChinese J. of lasers, A 31 (2004) 1029 . 23. Qihong Lou, Jun Zhou, Jianqiang Zhu, Chinese J. of lasers, A 32 (2005) 20-21. 24. Lou Qihong, Zhou Jun, Zhu Jianqiang, Wang Zhijiang, CLEO-Pacific-Rim, Japan, (2005). 25. Lou Qihong, Sino-German Workshop on Advances in Diodes and diode Pumped Lasers, Beijing (2005). 26. Lou Qihong, Zhou Jun, Zhu Jianqiang, Wang Zhijiang, The 2ed China-Korea Workshop on Optical Technologies and 3rd Joint Steering Committee Meeting of OSTCC, Sanya, (2005). 27. A.Galvanauskas High Power Fiber lasers, Optics & Photonics News, July (2004) 4247. 28. Y. Sun and E. J. Swenson , Laser Micro-machining in the microelectronics industry. SPIE 4915 (2002) 17-22.

Chapter 8

Hybrid Organic-Inorganic Solid-State Dye Laser Glasses

Since the born of the first laser in 1960 with ruby as active medium, several kinds of laser materials have been developed. They include crystals, gas, semiconductors and dyes. Crystal media are boasted for their high strength, however, the "thermal lens" effect resulted from their poor optical homogeneity deteriorates the laser performance. The strength and lifetime of gas as laser materials are limited. The lifetime and strength of semiconductors are both high and the laser configuration can be very compact, but the tuning range is only several nanometers. Laser dyes can be used in solid or liquid, their concentration can be readily controlled and the laser wavelength can be tuned continuously over a very wide spectrum range, producing narrow linewidth laser pulses without substantial energy loss. Also, organic dyes are very cheap. At present, dye lasers are still the major tunable laser sources. Because of the above characteristics, dye lasers have found extremely wide applications in industry, military, environment monitoring, medical diagnoses and therapy, laboratory and optical communication. In scientific research, tunable dye laser is especially useful in spectroscopy, biology, holography, photochemistry, isotope separation, non-linear optics and integrated optics, etc. Dye lasers in early stage mainly use liquid solution as active media. Since mid 1980s, intensive efforts have been devoted to the research on all solid state dye laser media and devices. In this chapter, a brief review is given on the history, recent progress and developing trends in this field, especially for hybrid organic-inorganic solid state dye laser glasses.

261

262

Hybrid Organic-Inorganic Solid-State Dye Laser Glasses

8.1 Organic Dyes and Liquid Dye Lasers 8.1.1 Structure Characters and spectra ranges of Organic laser dyes Dyes are the substances containing conjugated double bonds. Generally, the absorption and fluorescence band of a dye molecule is often very wide, covering more than tens of nanometers.[1] The output wavelength covers a wide spectrum range and is tunable. The spectra ranges of some organic laser dyes are shown in Fig. 8.1.[2]

360

400

440

480

520

560 600 640 680 WAVELENGTH (nm)

720

760

840

920

Fig. 8.1 Wavelength ranges of different laser dyes.

8.1.2 Synthesis and selection of new laser dyes From approximately 500 organic dyes identified to exhibit laser action, only a few show efficient laser output,[3] such as coumarin, rhodamine, pyrromethene and perylene family dyes. The majority of current laser dyes are in the visible region. In recent years, the most prominent progress is the synthesis of two kinds of efficient dyes, perylene family dyes and pyrromethene family dyes.[4"U] The former has extremely high photostability,[12"1316] and the latter has high quantum yields.[1415]

8.1 Organic Dyes and Liquid Dye Lasers

263

Pyrromethene family dyes have extremely high fluorescence quantum yields (near 1) whilst very low triplet yields.[17] Up to now, the most efficient dye laser output was obtained from pyrromethene dyes.[1215'18] The transition mechanisms and spectra characteristics of these dyes in various hosts have also been investigated^17'19"201 M. D. Rahn et al. summarized that the recent progress in the performance of solid-state dye lasers was brought by the newly synthesized pyrromethene dyest21] The molecular structure of pyrromethene dyes is shown in Table 8.1. The photostability of pyrromethene family dyes was investigated and the order of photostability of several kinds of pyrromethene dyes can be listed as follows: P567d, d2E(x,y)/d2x2+(k2n2-j32)E(x,y) 0<x3 and NaNOs) over melting temperature (300-350 °C, depending on the composition), put cleaned glass samples into the melted salt for several minutes. Na+ in the glass drifts to the solution, while Ag+ in the melted salt moves into the glass. Ion exchange occurs in a 10 um surface region, generating a gradient refractive index distribution with its maximum on the surface. The refractive index change can be well described by equations :[8] n(x) = n0+An- erfc(x/d)

,

An = n(0)-n0,

(9.7)

where n0 is the refractive index before ion exchange, d is the diffusion depth, De is the diffusion coefficient, erfc(x) is the error function. Table 9.1 Ion exchange parameters for Schott glass IOG-1. Surface An

8wt%AgN03+92wt%KN03 335 °C, 7 minutes

0.034

Diffusion depth Diffusion coefficient d(um) D(u,m2/min) 6.32 1.33

Table 9.2 Ion exchange parameters for BK7 glass. Surface An

5wt%AgN03+95wt%KN03, 360 °C 180 minutes

0.101

Diffusion depth Diffusion d(u.m) coefficient D(um2/min) 8.3

0.0958

9.2 Glass Waveguides Fabrication and Optical Properties

305

Table 9.3 Ion-exchange parameters for soda lime glass. Surface An

2xl0_3mol%AgNO3+ 9.9mol%NaN03 330 °C, 60 minutes

0.045

Diffusion depth Diffusion d(um) coefficient D(um2/min) 5.94

0.15

The asymmetric refractive index distribution can be modified by a second step ion-exchange, either decrease the surface refractive index through reverse Ag+-Na+ exchange, or push the maximum Ag+ distribution into glass by applying external electric field. Tables 9.1-9.3 give the ion exchange parameters and the final refractive index change, as well as the refractive index profile parameters for 3 kinds of frequently used glasses. Schott glass IOG-1 is a phosphate glass doped with Er 2 0 3 (2.31 wt%) and Yb 2 0 3 (3.57 wt%). BK7 and soda lime glasses are popular silicate glasses. All the three kind of glasses are commercially available. 9.2.5 Sol-gel Sol-gel technique was first realized as a chemical method to generate Si0 2 structure through hydrolysis and condensation of precursors (like tetraethoxysilane, TEOS) in acidic environment. After exploration for over 100 years, sol-gel technique has been developed to be a key wet chemical method to produce inorganic materials or organic/inorganic hybrid nanocomposites.[9] Sol refers to nano size colloids suspended in liquids, when these nano-size colloids grow up and connect to each other, the suspension can no longer flow freely, the product is called Gel. Since 1980, researches started to realize that sol-gel technique could be a possible and convenient way to produce thin films with excellent optical quality. A new division of sol gel was developed very quickly and soon formed an important community called Sol-gel Optics.[10] Conventional sol-gel reactions are used to fabricate multi-component oxide films with precisely controlled refractive index. Active elements, like rare-earth elements can be doped in liquid phase with pretty high concentration, thus active waveguides are formed. The most difficulty in

306

Optical Glass Waveguides

this approach is, when depositing thin films on substrate (usually silicon wafer is used as substrate), deposited film thickness should be below 100 nm to avoid cracking, crackingfrequentlyhappens in films prepared by sol-gel technique. To overcome cracking, multi-cycle deposition process is necessary, For each cycle, film thickness is controlled to be less than 100 nm. After coating, the film must be annealed at high temperature (1100-1300 °C), then the next coating cycle starts. A normal waveguide structure consists three layers, the total structure thickness is around 4050 um. Therefore, 400-1000 cycles is needed. Syms et al. developed a spin-coating and rapid thermal annealing (SC-RTA) technique to repeatedly coat thin films on silicon wafer.[u] This technique requires that after each coating, the sample should be treated at 1300°C for a few minutes in a oven that use high power Xe lamps to rapidly heat the samples. We modified the SC-RTA process by a fully computercontrolled multi-layer deposition-annealing apparatus, the apparatus has been successfully used to prepare thick sol-gelfilms.Fig. 9.3 is the AFM image of a 10-um thick sol gel film that we fabricated following the SC-RTA procedure. The film composition was 10%P2O5, 5%A1205, 0.5%Er2O5, 0.5%Yb2O5. The surface roughness is less than 2 nm.

Fig. 9.3 AFM image of a 10 pxa thick sol gel film.

SC-RTA has turned out to be one of the best way to produce erbium doped multi-component sol gel glass films and waveguides. Refractive index of the material can be easily controlled by slightly adjust the ratios of the components. Table 9.4 shows the refractive index of several compositions.

9.2 Glass Waveguides Fabrication and Optical Properties

307

Table 9.4 Refractive indices of erbium doped multi-component sol gel glass. Composition

Refractive index @ 632.8 nm

10%P2O5, 5%A1205, 0.5%Er2O5, 0.5%Yb2O5 10%P2O5 5%P205, 5%B205

1.4668 1.4676

5%P205, 10%B2O3

1.4647 1.4614

9.2.6 Fabrication techniques for non-oxide glass waveguides 9.2.6.1 Fluoride glass waveguides Fluoride glasses were received attention because of their optical transmission in the infrared, theoretical low losses and potential as hosts for active rare-earth ions.[12] In 1990, these glasses emerged as an outstanding class of materials for fiber laser sources and optical fiber amplifiers. Lucas and his group described an ion exchange procedure to generate planar and channel waveguides in fluoride glasses.[13] The process is based on the idea that the F" anion in the glass matrix can be exchanged with its equivalent in charge and size, namely OH" and OD". The basic reaction occurring at the surface of the glass is MFn+H20^

MFn_x (OH)x + xHF

(9 g)

The anionic exchange process takes place at temperatures below, or very close to glass transition temperature Tg. The replacement of F" ions by OH+ or OD" ions produces an increase of the surface refractive index in an order of 0.01 by controlling the exchange time and temperature. R.Almeida el al reported vapor-phase film deposition of rare-earth doped fluorozirconate glasses.tl4] Zirconium fluoride based glassy films include ZrF4-PbF2, ZrF4-PbF2-NdF3 systems. As the vapor pressure of ZrF4 and PbF2 are much higher than those of NdF3, it is necessary to find adequate evaporation boats, as far as the material type and its electric resistance. The deposition was operated at a pressure near 2xl0"5 mbar. Single crystal silicon wafers and microscope glass slides were used as substrates. 3 minutes of deposition yielded a film thickness of about

308


3 micron. The propagation loss of the planar waveguides thus fabricated is around 4 dB/cm at He-Ne laser wavelength (632.8 nm). B.Boulard and his group explored the possibility of fabricating ternary fluoride glass films PZG (PbF2-ZnF2-GaF2) by physical vapor deposition/151 From a thermodynamic point of view, a multi-component glass can be seen as a eutectic mixture. The non-congruent vaporization of a binary compound causes a composition displacement, and crystallization phenomena occur in the melt on solidification. To prevent such crystallization of the melt, which stops the vaporization, they looked for a starting glass composition which would allow the melt to remain in a glassy domain during the entire evaporation process. This can be accomplished by diluting the PZG glass into another flux glass which is fully compatible with PZG and has very low vapor pressure and good stability towards crystallization. One kind of flux material (BaF2InF3-MnF2) has a melting temperature of 591 °C, which is obviously higher than PZG glass (547 °C).[16] 9.2.6.2 Chalcogenide glasses Interest in chalcogenide materials, particularly in the form of glass, is continuing to increase as their usual and varied properties are ever more understood. Chalcogenide materials are generally classified as those in which sulfur, selenium, or tellurium form one of the basic constituents, usually in the form of crystal, ceramic, or glass. The transmission window for chalcogenide glasses extends from 1 um to longer infrared wavelengths compared with oxide and fluoride glasses. The highrefractive index, enhanced mid-IR transmission in chalcogenide glasses with reduced maximum phonon energy and ability to exhibit localized reversible or irreversible material changes when irradiated with light suggest these glasses are indeed interesting candidates for thin film development.117"181 A.K.Mairaj and co-workers described inverted spin coating technique to deposite Ga:La:S films (see Fig. 9.4).[19]

9.2 Glass Waveguides Fabrication and Optical Properties stage 1 depositon boron nitride vacuum chuck substrate (dadding glass)

stage 2 withdrawal withdrawal of substrate

309

stage 2 withdrawal spinning of molten glass

: * I msniseus

thin film is formed

molten class

Fig. 9.4 Three-step process for inverted deposition spin coating of thin film glass waveguides. After Ref. [19].

The fabrication of spin-coated amorphous thin films of Ga:La:S glass can be separated into three stages: the deposition of fluid onto the substrate, the withdrawal of the coated substrate from the melt, and spinning of the coated substrate. The surface uniformity obtained has a variation of 0.5 jim across 25 mm. The propagation loss at 1.064 um is 0.3 dB/cm over a 15 mm path length, making this technique attractive for a number of optical applications. This technique allows the deposition of thin Ga:La:S films ranging in thickness from 1 rnm to 10 ^m, by varying temperature and spinning speed. B. Luther-Davies and his group used high-repetition rate short-pulse laser to deposit chalcogenide thin films for waveguide application.[20"21] Pulse laser deposition (PLD) is a process that evaporates a target material by high intensity laser irradiation and deposition of the evaporated materials onto a substrate. A unique advantage of PLD is its stoichiometric transferring of target composition to the deposited film. However, conventional PLD encounters the problem of depositing small clusters or particles onto the substrate. The Australian group successfully overcame the problem by using ultra-fast lasers for PLD. In ultra-fast PLD, the laser pulse energy is lowered so that it is no longer possible to create large particles. On-target intensity is chosen so that sufficient energy is available to ablate the surface to a depth equal to the penetration of the thermal wave. This ensures that the ablation process reaches its maximum efficiency and occurs without significant heating of the target. They used second harmonic light of a mode-locked Nd:YAG laser (532 nm, 50 ps pulse duration) at 76 MHz repetition rate (pulse

310


energy 80 nJ) to deposit As2S3 films. After focusing the light onto the target, the peak intensity reached was about 5xl0 8 W/cm2.

Fig, 9.5 SEM micrographs showing the profile of AS2S3 waveguides etched by ICP using photoresist mask (a) AS2S3 waveguide with photoresist mask (b) coating with polysiloxane cladding, its width is well controlled as required. After Ref. [21].

The deposition rate was 2 nm/second. As-deposited film had propagation loss of 0.1 dB/cm at 1550 nm wavelength. Film was also dry-etched by using either helicon plasma etching or ion-coupled plasma (ICP) etching. Fig. 9.5 shows the SEM images of the etched waveguide. The side wall smoothness was better than 50 nm, the channel waveguide had propagation loss of 0.25 dB/cm at 1550 nm for 4 and 5 um wide waveguides. 9.3 Organic/inorganic Hybrid Glass Waveguide Materials Organic/inorganic hybrid materials refer to a class of materials that combine organic and inorganic moieties at the molecular level. These materials are also known as polycerams, ormosils or ormocers.[22] Hybrid materials are synthesized at low temperatures by wet-chemical techniques, and the resulting products can be produced with exceptional purity and homogeneity. The chemistry of hybrid materials typically involves chemical reactions (hydrolysis/condensation) between metal alkoxides and functionalized polymers. Their structures can be varied, depending on the nature of the constituents and the processing parameters. Methacryl-oxypropyl trimethoxysilane (MAPTMS),[23"24] Hyroxyl-terinated polydimethylsiloxane (PDMS)[25] are among the first few of hybrid materials for optical waveguides fabrication. Hybrid materials are interesting waveguide materials because they keep the good

9.3 Organic/inorganic Hybrid Glass Waveguide Materials

thermo-stability and chemical stability of inorganic materials, whiles the organic entities enable thick film forming and incorporation with functional organic dopants. In the following, we will take MAPTMS as an example. MAPTMS consists a silane end group and an organic end group with C=C double bond that can be polymerized under heat or light exposure. MAPTMS is hydrolyzed together with zirconium propoxide (ZPO) in acidic environment. ZPO is introduced to adjust the refractive index of the final product, ZPO also helps in producing uniform films with good optical quality. After hydrolysis and condensation, structures such as -Si-O-Siand -Si-O-Zr- are formed. Fig. 9.7 is the flow chart of preparing the organic/inorganic hybrid sol.

Fig. 9.6 Flow chart for preparation of hybrid waveguide materials.

After hydrolysis and condensation, films as thick as 10 um can be spin-coated on silicon wafer. UV light exposure then polymerize the hybrid andfilmsbecome glassy hard. Spin-coated film thickness depends on the viscosity of the solutions. A convenient way to control film thickness is let the solvent (here it is propanol) to evaporate in vacuum so as to adjust the viscosity of the final solution. Fig. 9.7 shows for two MAPTMS/ZPO ratio (one used as waveguide cladding material, the other for waveguide core), the relation between thickness and residual solution weight after evaporation.

311


312

;

!

i

icladdingj/

!

1

|

1/

L cor©

:î2fer: 100

95

90

S5

80

75

Solution weight ratio [%] Fig. 9.7 Spin-coated film thickness versus residual solution weight after evaporation.

The spin speed was kept at 600 tums/min. We found that this method is more controllable than the generally accepted method of changing spin speed.

o

Fig. 9.8 Film morphology of hybrid film from AFM.

Fig. 9.8 is the film surface morphology obtained from AFM measurement. The fluctuation in height is around 3 nm. Fig. 9.9 shows the absorption spectrum of the hybrid thin film in the near infrared region. The absorbing structure at 1400 nm is due to the residual water in the film. A sharp peak at 1610 nm is the residual C=C vibrational mode absorption, and the structures in the 1700 nm band are C-H absorptions.

9.3 Organic/inorganic Hybrid Glass Waveguide Materials

313

1100 1200 1300 1400 1500 1600 1700 1800

Wavelength [ran] Fig. 9.9 Absorption spectrum of a MAPTMS/ZPO hybrid film.

The thermal optical property of the hybrid materials was also measured. The dependence of a material's refractive index on temperature is called the thermo-optic effect. This effect is very useful in the fabrication of optical waveguide switches, which are key devices in intelligent optical communication network systems. According to Prod'home's theory, the thermo-optic coefficient relates to the density and electronic polarizability changes of a material with temperature :[26] dn (n2-\)(n2+2) 6n ~dT

(«>-/?),

(9.9)

where

400

450

500

Ik 550

600

650

0.5 0.4 0.3 0.2

•a

§

0.0005 -2.500000

0.0000

0.000000

2.500000

5.000000

Radius (mm)

Fig. 10.11 The refractive index changes after exposure to 266 nm laser.

10.1.3 Pb-doped glass X. C Long found that the maximum An could reach 0.21 in a lead silicate glass with 57 mol% PbO concentration. In his work, 25 mJ/cm2, 10 ns, 10 Hz, 266 nm YAG laser was used.[13] We observed close result. An of0.25 was measured in a 50 mol% PbO lead silicate glass after the glass was irradiated by light pulses from a 50 mJ/cm2, 10Hz, 266 nm Nd:YAG laser.[44] The knowledge about the mechanism of the photosensitivity in Pbdoped glass is very limited. But it was believed that bond breaking and structure changes play the key roles.[13] Although color centers were reported to be formed in lead-silica glass through two-photon absorption of 532 nm photons coming from a Nd:YAG laser, no photosensitivity was observed in these bulk glasses at laser intensities up to its damage threshold^451 We observed large negative refractive index change in lead silicate glasses with different lead concentrations (from 30 mol% to 50 mol% PbO) by irradiation with fourth harmonic output of a Q-switched Nd:YAG laser (266 nm, 10 Hz repetition rate) at the energy density of 50 mJ/cm2. The largest An was -0.25 ± 0.04 in lead silicate glasses with 50 mol% PbO.

354

Glass Photosensitivity and Fiber Gratings 1.90

1

10

100

Time (min)

Fig. 10.12 The refractive index change versus irradiating time in the lead silicate glass with 50 mol% PbO.

The relationship between photoinduced index changes of refraction and PbO concentration in samples was shown in Fig. 10.13. An increases exponentially to the PbO concentration. Similar results were also obtained by other groups.[13'46]

PbO mol%

Fig. 10.13 The relationship between refractive index changes and PbO contents.

Energy density (mJ/cm ,

Fig. 10.14 The relationship between absorption coefficients at 600 nm and 266 nm pulse energy density. The sample was a lead silicate glass with 43mol% PbO contented. The irradiation time was 1 minute.

10.2 Principles of Fiber Gratings

355

UV-visible absorption spectra of the lead silicate glass with 43 mol% PbO were measured after exposure to the 266nm laser beams with energy density from 50 mJ/cm2 to 350 mJ/cm2. We found that (see Fig. 10.14) the absorption coefficients in visible wavelength increased suddenly when the energy density of the laser beam was larger than a threshold value. This indicates that the photoinduced structural changes of lead silicate glasses are different with varying of energy densities. 10.2 Principles of Fiber Gratings 10.2.1 Basic properties of uniform Bragg grating The simplest form of a fiber Bragg grating is a periodic modulation of the refractive index in the core of a single-mode optical fiber (see Fig. 10.15). Uniform fiber gratings are fundamental building blocks for most Bragg grating structures. Light propagating along the optical fiber will be scattered by each grating plane. If the light wavelength satisfies the Bragg condition ~kt+lc = ~kr,

(10.3)

light will be reflected by the grating. In Eq. (10.3), kt is the incident wave vector. K=2n/A is the grating wave vector that has a direction normal to the grating planes. A is the grating spacing. kr is the reflected wave vector. Bragg grating

Fig. 10.15 Illustration of a uniform Bragg grating with constant refractive index modulation amplitude and period. After reference [6].

356

Glass Photosensitivity and Fiber Gratings

In the first-order Bragg condition for uniform fiber gratings, the center wavelength of the back-reflected peak is called Bragg wavelength, defined as XB=2neffA, (10.4) where neff is the effective refractive index of the fiber mode at wavelength XB. 10.2.1.1 Bragg grating reflectivity Consider a uniform Bragg grating with an average refractive index n0 in the core. The refractive index profile can be simplified as 2.7ZX

n(x) = n0+An cos(

), (10.5) A where An is the amplitude of the induced refractive index (typically 10~5~10~2) and x is the distance along the fiber longitudinal axis. The reflectivity of this grating can be given by coupled-mode theory as the following expression:[3'4] R(L,A)=

f ^f , 2 2 AA:2sinh2(5Z) + 5 2 c o s h 2 ( ^ )

(10.6)

where R(L, X) is the reflectivity, it is a function of the grating length L and wavelength A, Q=nAn/AB is the coupling coefficient, Ak = k~n/A is the detuning wave vector, k = 2rni0/A is the propagation constant and s2 = ff-Ak2. At the Bragg wavelength XB, there is no wave-vector detuning, so Ak = 0. Therefore, the reflectivity reaches the maximum: Rmm=R(L,AB) = tanh2(^).

(10.7)

According to Eq. (10.6), the fiber grating reflectivity increases as An or L increases. A calculated reflection spectrum as the function of wavelength detuning is shown in Fig. 10.16.


1548.8

1550.0

357

1550.2

Wavelength (nm)

Fig. 10.16 Bragg grating reflection spectrum as the function of wavelength detuning. After reference [6].

The approximate full width at half-maximum bandwidth is given by[5]

AA =

ABaA>+(±)\

,

(10.8)

2n

o

where L/A is the number of the grating plane. The parameter a ~1 for strong modulated gratings (for grating with near 100% reflection) whereas a -0.5 for weak modulated gratings. 10.2.1.2 Strain and temperature sensitivity^6'47] The Bragg grating wavelength XB, which is the center wavelength of the light back reflected from a Bragg grating, depends on the refractive index of the core and the periodicity of the grating. Both parameters are sensitive to strain and temperature. According to the Eq. (10.4), the shift in AB due to strain and temperature change is given by dn dAs dn dA (10.9) AXB = 2 ( A — + n—)AL + 2 ( A — + n—)AT dT 8T 8L dL The first term in Eq. (10.9) represents the strain effect on an optical fiber, corresponding to a change in the grating spacing and the strain induced change in refractive index. The above strain effect term can be expressed as A/LB=/lB(l-peK,

(10.10)

358


where p e is an effective strain-optic constant defined as P.=n2[p12+Kpn+p12)]/2,

(10- 11 )

in which pn and pn are components of the strain-optic tensor, n is the refractive index of the core, and v is the Poisson's ratio. For a typical optical fiber pn=0.113, pi2=0.252, v=0.16 and n=1.482, using these parameters and the above equations, the expected sensitivity at 1550 nm is 1.2pm/|j.£. The second term in Eq. (10.9) represents the temperature effect on an optical fiber. AB is shifted when the grating spacing change due to thermal expansion and the refractive index change due to thermo-optic effect. The wavelength shift for a temperature change AT can be written as Al^ = ^(a

+ £)AT,

(10.12)

where a=(l/A)(dL/dT) is the thermal expansion coefficient for the fiber, which is 0.55 XI0~ 6 for silica. The quantity £=(l/n)(dn/dT) represents the thermo-optic coefficient and is about 8.6 X 10~6/°C for the Ge-doped silica fiber core. Clearly, the index change is the dominant effect. From Eq. (10.12), the expected sensitivity at -1550 nm is approximately 13.7pm/°C. 10.2.2 Types of fiber Bragg gratings There are several distinct types of fiber Bragg grating structures such as the common Bragg reflector, the blazed Bragg grating and the chirped Bragg grating. These fiber Bragg gratings are distinguished by their grating pitch (spacing between grating planes) or tilt angle (between grating planes and fiber axis). 10.2.2.1 Common Bragg reflector Uniform Bragg gratings are the simplest and most frequently used fiber gratings. Depending on the parameters such as grating length L and magnitude of refractive index change An, the Bragg reflectors can function as narrow-band transmission/reflection filters or broadband reflectors. In combination with other Bragg reflectors, these gratings can


359

be arranged as band-pass filters. An important application of Bragg reflector is in fiber lasers or external cavity laser diodes, where fiber gratings are used at one or both ends of the laser cavity and tune the laser wavelength by varying the Bragg resonance feedback.'481 Bragg reflectors are considered as excellent strain and temperature sensing devices because the measurements are wavelength encoded'491 to avoid amplitude or intensity fluctuations. Fiber Bragg grating lasers can also be used as sensors where the Bragg reflector serves the dual purpose of wavelength tuning and sensing. A series of Bragg gratings can be written on the same fiber, each having a distinct Bragg wavelength. This configuration is used for wavelength division/multiplexing or multipoint sensing. 10.2.2.2 Blazed Bragg gratings Tilting the Bragg grating planes at angles to the fiber axis will couple the light satisfying Bragg condition into loosely bounded guiding modes or leaking modes. The tilt of the grating planes and the refractive index modulation depth determine the coupling efficiency and bandwidth of the tapped out light. For blazed gratings, not only different wavelengths emerge at different angles, but different modes at the same wavelength also emerge at slightly different angles due to their different propagation constants. Therefore the blazed fiber grating acts as a spectrometer and mode discriminator. Multiple blazed gratings were used to flatten the gain spectrum of erbium doped fiber amplifiers.'50' Another interesting application of blazed gratings is mode conversion.'511 Cladding

/

Topped Light

-r© Blazed index modufcrtlon

Core

Fig. 10.17 Schematic diagram of a blazed grating. Light is directed either upward or downward depending on the propagation direction of the bound mode. After reference [6].


360

10.2.2.3 Chirped Bragg grating A chirped Bragg grating is a grating that has a monotonically varying grating period. This can be realized by axially varying either the period of the grating or the refractive index of the core or both. In a linearly chirped grating, the ^ B is a linear function of the axial position along the grating so that different frequencies are reflected at different points acquiring different delay times. Chirped gratings, therefore, can be used as a dispersion-correction and compensation devices like compressing temporally broadened pulses for high-bit-rate transmission over long distance. Other applications include chirped pulse amplification, chirp compensation of gain-switched semiconductor lasers, sensing, higherorder fiber dispersion compensation, ASE suppression, amplifier gain flattening, and band-blocking and band-pass filters.[7] M

Core

. ^

re*

Oocfclng /

shottX» tongX* (b)

longX-"

•"• "

^ = 3
.SB=1557.585nm, corresponds to the slow-axis mode.

*

1

1

1

1

1

'

1

1

|

6 1558- i^ /;ciT=0.0088(nm/'C) SB so

a >

|

1

|

1

-__*-—•"-"*""""^ "

1557A

.

—**

0.79

1556-

• - , 0.78 \ .

!%FB/;nT=0.0093(iWC) 1 ™

SO 00 l l

1

IqâTNJ.OOOSfiWC)

'ra ° - 7 5

1555-

"^

m

; . -

0.74 0.73

-

>0

1554-60

- 1

-40

-20

20

-40

-20 1

40

0 1

20 1

60

40 .

60 1

80

80 1

100

Temperature (°C) Fig. 10.28 The temperature dependence of the two Bragg wavelengths under a normal atmosphere. (•) slow-axis mode, (A) fast-axis mode, lines are the linear fittings. Inset: Bragg wavelength difference A.SB-A,FB dependence on the surrounding temperature.

The temperature characteristics of the FBG were studied experimentally. When the temperature rose from -50 °C to 80 °C, both of the Bragg wavelengths red-shifted linearly, but the difference of the two

370


Bragg wavelengths decreased (see Fig. 10.28). The temperature sensitivities of these two Bragg wavelengths were measured to be KFT=0.0093 nm/°C and KsT=0.0088 nm/°C, respectively. The reflection spectra were recorded for the pressure change from 0 to 10 MPa (see Fig. 10.29 for the measurement results). The two curves showed good linearity and the linear fit error was less than 0.3%, which mainly came from the errors of the barometer. The pressure sensitivities of both Bragg wavelengths were almost the same, K P F =K S F =0.020 nm/MPa.

-,

0

1

1

2

1

1

4

1

,

1

6

1

8

,

f—

10

Pressure change |nP£"MPa£©

Fig. 10.29 The gas pressure dependence of the two Bragg wavelengths at room temperature. (•) slow-axis mode, (A) fast-axis mode, lines are the linear fittings. Inset: Bragg wavelength difference X,SB-A.FB dependence on the applied gas pressure.

From the above equations and data, the applied gas or fluid pressure (in MPa) and temperature (in °C) could be obtained from the following equations: AT=2000(AA. FB -AX S B),

(10.15)

AP=930A2iSB-880A?iFB.

(10.16)

We measured temperature and gas pressure change simultaneously using the Hi-Bi fiber Bragg grating. Fig. 10.30 shows the typical measurement results when the gas pressure applied on the fiber grating

References

371

was changed at the two different temperatures (-50 °C and 80 °C). The deviations are less than 1 °C and 0.5 MPa respectively.

0

2

4

6

8

10

0

2

4

6

8

10

Pressure change AP (MPa) Fig. 10.30 The gas pressure dependence of the two Bragg wavelengths at two different temperature of-50°C and 80°C. (•) slow-axis mode, (A) fast-axis mode.

References 1. K.O. Hill, Y. Fujii, D.C. Johnson, and B. Kawasaki, Appl. Phys. Lett. 32 (1978) 647. 2. G. Meltz, W. W. Morey, and W. H. Glenn, Opt. Lett. 14 (1989) 823. 3. D. K. W. Lam and B. K. Garside, Appl. Opt. 20 (1981) 440. 4. P. St J. Russell, J.-L. Archambault, and L. Reekie, Phys. World 6 (1993) 41. 5. Agrawal G P, Radic S, IEEE Phot. Tech. Lett, 6 (1994) 995. 6. A. Othonos, Rev. Sci. Instrum. 68 (1997) 4309. 7. Raman, Kashyap, "Fiber Bragg Gratings", (Publication: San Diego Elsevier, 1999). 8. J. Stone, J. Appl. Phys. 62 (1987) 4371. 9.P. J. Lemaire, R. M. Atkins, V. Mizrahi, and W. A. Reed, Elect. Lett. 29 (1993) 1191. 10. F. Bilodeau, B. Malo, J. Albert, et al, Opt. Lett. 18 (1993) 953. 11. D. L. Williams, B. J. Ainslie, R. Armitage, et al, Electron. Lett. 29 (1993) 45. 12. G. Brambilla and V Pruneri, IEEE J. On Sel. Topics in Quan. Elect., 7 (2001) 403. 13. X.C. Long and S.R.J. Brueck, Opt. Lett. 24 (1999) 1136. 14. P. Niay, M. Douay, P. Bernage, et al, Opt. Mat. 11 (1999) 115. 15. K. Gaff, A. Durandet, T. Weijers, et al, Electron. Lett. 36 (2000) 842. 16. M.V. Bazylendo, D. Moss, and J. Canning, Opt. Lett. 23 (1998) 697.

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17. Jarvis RA, Love JD, Durandet A, et al, Electron. Lett. 32 (1996) 550. 18. R.M. Atkins, P.J. Lemaire, T. Erdogan, and V. Mizrahi, Electron. Lett. 29 (1993) 1234. 19. D.P. Hang and P.S.J. Russell, Opt. Lett. 15 (1990) 102. 20. R. M. Atkins, V. Mizrahi, and T. Erdogan, Electron. Lett. 29 (1993) 385. 21. B.G. Potter, Jr., and K. Simmons-Potter, Nucl. Instr. And Meth. B 166 (2000) 771. 22. M. Fujimaki, T. Watanabe, T. Katoh, et al, Phys., Rev. B 57 (1998) 3920. 23. M. Essid, J. Albert, J.L. Brebner, K. Awazu, J. Non-cryst. Solids 146 (1999) 39. 24. T. Uchino, M. Takahashi, and T. Yoko, Phys. Rev. B 84 (2000) 1475. 25. C. Fiori and R. A. B. Devine, Mater. Res. Soc. Symp. Proc. 61 (1986) 187. 26. M.V. Bazylenko and M. Gross, J. Appl. Phys. 81 (1997) 7497. 27. M.G. Sceats, G.E. Atkins, and S.B. Poole, Annu. Rev. Mater. Sci. 23 (1993) 381. 28. A.I. Gusarov and D.B. Doyle, Opt. Lett. 25 (2000) 872. 29. D. L. Williams, B. J. Ainslie, R. Kashyap, et al, Proc. SPIE 2044 (1993) 55. 30. M. G. Sceats and S. B. Poole, Aust. Conf. Opt. Fiber Technol., (1991) 302. 31. K. Noguchi, N. Shibata, N. Uesugi, and Y. Negishi, J. Lightwave Technol. LT-3 (1985)236. 32. L. Dong, J. L. Cruz, L. Reekie, et al, IEEE Photon. Technol. Lett., 7 (1995) 1048. 33. G. Brambilla, V. Pruned, L. Reedie, et al, Opt. Lett. 25 (2000) 1153. 34. L. Dong, J.L. Cruz, J.A. Tucknott.et al, Opt. Lett. 20 (1995) 1982. 35. G. Brambilla, V. Pruned, and L. Reekie, Appl. Phys. Lett. 76 (2000) 807 36. Anedda, CM. Carbonaro, A. Serpi, et al, J. Non-Cryst. Solids 280 (2001) 287. 37. N. Chiodini, F. Meinardi, F. Morazzoni, et al, Phys. Rev. B 58 (1998) 9615. 38. N. Chiodini, F. Meinardi, F. Morazzoni, et al, J. Non-Cryst. Solids 261 (2000) 1. 39. N. Chiodini, S. Ghidini, and A. Paleari, Phys. Rev. B 64 (2001) 073102. 40. Chen GH, Li YG, Liu LY, He YJ, Xu L, Wang WC, J. Appl. Phys. 96 (2004) 6153. 41. R.M. Atkins and R.P. Espindola, Appl. Phys. Lett. 70 (1997) 1068. 42. G. G. Karapetyan, A. V. Daryan, D. M. Meghavoryan, N. E. Gevorgyan, Opt. Commun. 205(2002)421. 43. N. Chiodini, A. Paleari, G. Spinolo, et al, J. Non-Cryst. Solids, 311 (2002) 217. 44. Jia HZ, Chen GG, Wang WC, J. Non-cytal. Solids 347 (2004) 220. 45. K. W. Delong, V. Mizrahi, G. I. Stegeman, et al, J. Opt. Soc. Am. B 7 (1990) 2210. 46. S. Mailis, A. A. Anderson, S. J. Barrington, et al, Opt. Lett. 23 (1998) 1751. 47. W. W. Morey, G. Meltz, and W. H. Glenn, Proc. SPIE 1582 (1992) 36. 48. G. A. Ball and W. W. Morey, Opt. Lett. 17 (1992) 420. 49. Byoungho Lee, Optical Fiber Technology, 9 (2003) 57. 50. R. Kashyap, R. Wyatt, and P. F. Mckee, Electron. Lett. 29 (1993) 1025. 51. K. O. Hill, F. Bilodeau, B. Malo, and D. C. Johnson, Electron. Lett. 27 (1991) 1548. 52. K. O. Hill, B. Malo, F. Bilodeau, et al, Appl. Phys. Lett 62 (1993) 1035. 53. B. Malo, K. O. Hill, F. Bilodeau, et al, Electron. Lett. 29 (1993) 1668. 54. M. G. Xu, J.-L. Archambault, L. Reekie and J. P. Dakin, Electron. Lett., 30 (1994) 1085. 55. H. J. Patrick, G. M. Williams, A. D. Kersey, et al, IEEE Photo. Tech. Lett., 8(1996) 1223. 56. Y. Liu, Z. Guo, Y. Zhang, K. S. Chiang and X. Dong, Electron. Lett., 16 (2000) 564.

References

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57. Chen GH, Liu LY, Jia HZ, Yu JM, Xu L, Wang WC, Opt. Comm. 228 (2003) 99. 58. Chen GH, Liu LY, Jia HZ, et al, IEEE Photo. Technol. Lett. 16 (2004) 221. 59. M. Sudo, M. Nakai, K. Himeno, S. Suzaki, A. Wada, and R. Yamauchi, Proc. 12th Int. Conf. on Optical Fiber Sensors, IEEE/LEOS and OS A (1997) 170. 60. L. A. Ferreira, F. M. Araujo, J. L. Santos, F. Farahi, Opt. Eng. 39 (2000) 2226.

Chapter 11

Glass Fibers for Photonic Crystals

Photonics plays an important role in modern technological systems, such as telecommunications and sensing systems. Traditionally, the manipulation of optical photons has relied in general on the mechanism of total internal reflection (TIR).[1] Since the 1980s, the research in new purpose-built materials has opened up the possibilities of localizing and controlling light in cavities and waveguides by a new physical mechanism/2'3] namely the photonic band-gap (PBG) effect, there has been a growing interests in the realization of photonic crystals or photonic band-gap structures as optical components and circuits/4' Photonic band gap that excludes light transmission in all directions for a specific wavelength range, is analogous to the electronic band structure of semiconductor materials,'51 where the periodic arrangement of ions on a lattice gives rise to the energy band structure and the energy bands control the motion of charge carriers through the crystal. The periodic arrangement of refractive index variation in photonic crystal controls how photons are able to move through the crystal. A simple example is a simple-cubic lattice, whose lattice points are occupied by dielectric spheres. The crux of photonic crystals is that the lattice constant lies in the order of an optical wavelength/1' The most striking effect is that for a certain range a frequencies (colors) light can not propagate in the photonic crystal. Natural examples can be found, such as opal. Light of frequency within the photonic band-gap of the opals is reflected, which causes the stone to appear colorful, although the stone is made of transparent silicon dioxide. The photonic band-gap edges in an opal shift as a function of direction, thus its color changes when you look at in different ways. The beautiful iridescent wings of some butterflies and 375

376


beetles are also caused by photonic band-gap incorporated into the wing surface. The main concept of photonic band-gap structure or photonic crystal was first proposed by Yablonovitch[2] as a possible way of inhibiting spontaneous emission and JohnP] for localization of light in strong scattering dielectric structure. Photonic crystals are a promising technology platform for realizing compact optical components and advanced/novel functions in integrated photonic circuits. They have many potential applications, such as quantum optical devices, optoelectronics, inhibition of the spontaneous emission, single-mode light emitting diodes, waveguides in the optical domain, filters, polarizers, planar antenna substrates. Photonic crystals have three structural forms: one-dimensional (ID), two-dimensional (2D), and three-dimensional (3D) periodic structures. A one-dimensional photonic crystal is repetitive along one direction, forming a stack of dielectric sheets. They are used as interference filters or in distributed Bragg reflectors. A three-dimensional photonic band-gap crystal has periodic structure in all directions and possesses one or more complete photonic band-gaps. They are described as stacked cheesecloth or stacked wood. Since the periodicity of the medium must be comparable to the wavelength of the electromagnetic waves to inhibit their propagation, photonic band-gap in optical or infrared range requires sub-micron size structures. However, the fabrication of photonic crystals with threedimensional (3D) periodicity is still a challenge at this tiny scale. Two-dimensional photonic band-gap crystals are structures that exhibit photonic band-gaps for waves traveling within a two-dimensional plan. Two-dimensional photonic crystals can be fabricated in two different forms: photonic crystal fibers (PCFs) and planar photonic crystals. Optical fibers are an excellent approximation of twodimensional (2D) structures because they are effectively infinite in the third dimension. Optical fibers are nominally invariant along their length, with all interfaces parallel to the fiber axis. Russell first proposed the concept of PCF[6'7] and Knight and co-worker first reported a fabricated example in 1996.[8] The new class of optical fibers is featured by an array of air holes running along the fiber length, so it also named holey fiber or microstructure fiber. By using this particular structure, various unique

11.1 Light Guidance in PCF

m

features, which are not realized in conventional fibers, ~ J such as the single-mode cut-off wavelength from the UV to IR, numerical aperture (NA) values ranging from very lov/ to about 0.9[!5] and air core guidance, extremely low bending loss, manipulated dispersion and nonlinear characteristics, can be provided. The design flexibility for PCFs is very large, and designers can use many different, fascinating, and odd air-hole patterns to achieve specific PCF parameters. Furthermore, doped glasses are now used in a variety of fiber lasers and amplifiers; these can be combined with the unique capabilities of PCFs to provide even more useful devices. Hence, PCFs are now receiving interests from areas as diverse as spectroscopy, metrology, imaging, telecommunication, and industrial machining. 11.1 Light Guidance in PCF PCFs contain a hollow core or solid core and an arrangement of air holes that ran along the fiber length, and example is shown in Fig. 11.1.[9'16] Light can be guided using either one of two quite different mechanisms. [11-13] ^ | j e r i igj-gg aij-.hoieg are arranged in a periodic lattice, guidance can be obtained through photonic band-gap effects and it is therefore called PBG-PCF or hollow core PCF.

Fig. 11.1 Cross-section of photonic crystal fibers (a) hollow core PCF, (b) index-guiding PCF. After References [16, 9].

378


The second form of guidance arises from TIR, so the core must have a higher average refractive index (solid core) than the holey cladding; and it does not rely on any periodicity of the air holes. This fiber is thus called TIR-PCF or index-guiding PCF. 11.1.1 Total internal reflection Traditional optical fibers or step-index fibers have a silica glass core surrounded by a cladding layer. The cladding has a lower refractive index than the core (n cladding < ncore), so that light can be reflected at the boundary by total internal reflection. Index-guiding PCF guides light following the principle of TIR as well. [11~131 Holes act to lower the effective refractive index in the cladding region, and light is confined in the solid core, which has a relatively higher index. Unlike conventional fiber, index-guiding PCF can be made entirely from single material, typically un-doped silica. The basic operation of index-guiding fibers does not depend on having a periodic array of holes. However, practically the holes are arranged typically on a hexagonal lattice, so these fibers are sometimes referred to as photonic crystal fibers. The parameters that characterize a PCF profile are the hole-to-hole spacing A, the hole diameter d as shown in Fig. 11.2 and the number of rings of holes used to define the cladding region. The average refractive index of the cladding is lowed by the air holes.

Photonitic crystal fiber

Step index fiber

Fig. 11.2 Cross-section of a photonic crystal fiber and step-index fiber.

11.1 Light Guidance in PCF

379

A P-parameter has traditionally been applied to derive the higherorder mode cutoff as well as the mode field diameters for step-index fibers. In a standard step-index fiber with core radius a, and core and cladding indices ncore and nciadding at the operation wavelength X, the number of guided modes is determined by the Vsi value: n.

2a7r / AJnc

-n

(11.1)

cladding

Vs\ must be less than 2.405 for the fiber to be single mode. Thus single-mode fibers are in fact multimode for light of sufficiently short wavelength. Core diameter for step-index single mode fibers of silica is about 4-11.0 urn. In analogy with conventional step-index fibers, a normalized frequency parameter (F-parameter) for a PCF can be defined . [9,17-18]

as:

VPCF = Ink I ^ncore2{X)-ndadding\X).

(11.2)

The F-parameter is shown on the left side of Fig. 11.3 for a photonic crystal fiber having a core formed from a single missing hole. The condition for higher-order mode cut-off can be formulated as VPCF = 7T.[18] In contrast to step-index fibers, the effective index of the cladding of a PCF is strongly wavelength dependent. If the ratio of the wavelength of the guided mode to the hole-to-hole distance, X/A, approaches zero, then the effective cladding index approaches the effective core index. 1.451.44I 1.43-

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Fig. 11.3 (A) Effective index of photonic crystal cladding and (B) single-mode and multimode parameter space. After Reference [17].

380


These unusual dispersion properties of the cladding, taken together with the single-mode condition of Fig. 11.3(a), lead to the behaviors as shown in Fig. 11.3(b). Here, the curved line across the lower-right side of the plot indicates where the single-mode condition is met. Above the line, only single-mode propagation is possible. For any fiber whose d/A ratio is less than 0.45, light with all wavelength will propagate in a single mode. Such fibers are termed endlessly single mode. The unusual guiding property is due to that the air holes (diameter d, spacing A) are considered as a modal filter or modal sieve.[6] Due to the evanescence of light in air, the holes are strong barriers and can be analogous to the wire mesh of a sieve. The fundamental transverse mode with a lobe dimension of ~2A fits into the core and cannot escape through the too-narrow silica gaps between the air holes. However, the scaling of the core size is limited by the increasing propagation loss. If the F-parameter of a PCF is smaller than 1, confinement of the mode is too weak and leakage occurs through the finite cladding. On the other hand, if X/A becomes too small (25% would easily crack. Therefore, high air-fill fraction glass fiber can be obtained from hydrofluoric acid etched low high air-fill fraction preforms.

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Extrusion technique suits for fabricating the structured perform of compound glass with low-softening temperatures."' J In this process, a glass billet is forced by using a die at elevated temperature near the glass softening point. The perform geometry is determined by the die orifice. Furthermore, drilling method also is used to fabricate the perform. [29'30] The advantage of the extrusion and drilling techniques are allowance of fiber drawing directly from bulk glass. It works for many materials, including chalcogenides, polymers, and compound glasses. By comparison, stack and draw methods are limited to closest-packed geometries such as triangular or honeycomb lattices and cannot easily generate peculiar circular patterns. Extrusion technique provides design freedom, but is typically limited to the glasses with low glass softening point. Drilling method allows adjustment of both the hole size and spacing, but is generally limited to a small number of holes and restricted to circular shapes. Furthermore, drilling of preforms leads to roughened surfaces along the air hole so that extra steps of etching and polishing of

386


the inner surfaces are desired. Sol-gel method accompanies large weight loss during drying, moreover, attenuation due to absorption of hydroxy bonds and roughened surfaces is large. Several designs such as fibers for low-bend loss, dispersion flattened designs or birefringent fibers require independent spacing, hole size or even noncircular holes. Different methods provide additional design flexibility that will be necessary for different types of fibers. 11.3 Properties of PCFs and Device Applications The presence of wavelength-scale holes in the transverse profile of a PCF can lead to novel optical properties that cannot be achieved in conventional optical fibers. It opens new prospects in various fields such as spectroscopy, metrology, telecommunication, nonlinear optics, laserinduced guidance, quantum optics. One important property of optical fiber is it extremely low propagation loss. The complex structure of PCFs generates several new loss mechanisms. The losses of PCFs depend strongly on their structures, the number of rings, and local variations of structure and surface roughness of the fiber. Early results yielded propagation loss of PCFs in the order of 100 dB/km, but the loss has been reduced dramatically. The loss of index-guiding PCF reached to the current record level of 0.28 dB/km [31] and the hollow core-PCF loss was ofl.2dB/km. [32] 11.3.1 Index-guiding PCFs Due to an operation based on TIR, the properties of index-guiding PCFs in many respects resemble those of step-index fibers. However, very important differences occur as a result of the complex geometry of the cladding structure and the large refractive index contrast between silica core and the air-filled cladding. Hence, depending on PCF design, the presence of wavelength-scale holes in the transverse profile of a PCF can lead to novel optical properties that cannot be achieved in more conventional forms of optical fiber.133"381 Examples of such properties include very-large-core (up to 25 um or larger) with endless single-mode

11.3 Properties of PCFs and Device Applications

387

guidance, ultra-small effective areas (down to approx. 1 um) and the extremes of fiber non-linearity, and a range of remarkable dispersion properties, including broadband flattened dispersion, anomalous dispersion below 1.3 um, and large normal dispersion values at 1.55 urn, polarization maintenance and high birefringence and very high numerical apertures (up to 0.9). Non-silica based PCFs [27"30] have been identified as particularly promising candidates for highly nonlinear applications due to the high nonlinear index of the materials. PCFs are ideally suited for applications requiring large nonlinearity, broadband operation range with single-mode guidance, large mode areas. 11.3.1.1 Dispersion Characteristics Dispersion is usually defined as a parameter proportional to the second derivative of the effective refractive index of a waveguide, and is determined both by the material properties and by the geometry. Since PCF has greater freedom in fiber design as d and A can be independently decided, a dispersion shifted and dispersion flattened PCF can be achieved with a proper design. By increasing the relative air-hole size d/A, the zero dispersion wavelength can be shifted to shorter wavelength down to 560 nm.[36'38] Shifting the zero-dispersion wavelength to regimes where there are convenient high-power femtosecond laser source (Ti: sapphire at 800nm, Yb-fiber and Nd:YAG at 1060nm) also allows the development of efficient supercontinuum sources. On the other hand, wavelength converters based on modulation of cross-phase and optical threshold devices based on self-phase-modulation require small normal dispersion to minimize coherence degradation, In addition, parametric oscillation and wavelength converters based on four-wave-mixing require small normal dispersion to achieve efficient phase matching. For dispersion compensation of standard fibers, a large normal dispersion is needed at 1550nm. The slop of the dispersion is also important and ultraflat dispersion at the wavelength range of interest enhances the useful spectral bandwidth of nonlinear devices. Several methods have been developed to design PCFs with ultra-flat dispersion of a certain value and for a certain wavelength range. For example, Saitoh et al. proposed a PCF structure as shown in Fig. 11.9,[39] the central core is perturbed with


388

an extra air-hole with diameter dc. By a judicious choice of the geometrical parameters d/A and dJA, this PCF structure can exhibit ultraflattened dispersion characteristics from 1100 to 1800 nm with low confinement losses and small effective area. Rarity et al. used a singlemode photonic crystal fiber and pumped in the normal dispersion regime,1401 they obtained a PCF source of correlated photon pairs at 839 nm and 1392 nm. This single-mode source of pair-photons will have wide application in quantum communications.

(a)

(b)

Fig. 11.9 (a) Cross-sectional profiles of the PCF with ultra-flattened dispersion characteristics and (b) dispersion as a function of wavelength and diameter of cladding air-holes. After Reference [39].

11.3.1.2 Polarization characteristics Polarization maintaining (PM) fibers are optical fibers that preserve the polarization state of the light. PCFs take advantage that is able to fabricate anisotropic claddings, cores with high ellipticity and the high index contrast between silica and air. PM-PCFs have been fabricated based on structures of circular air holes with different diameters along two orthogonal axes near the core region,[33,34,381 or on asymmetric cores obtained by placing two adjacent rods. The values for birefringence reported for these structures range from 9.3x10"* to 3.7 xlQ"3, and the configuration using two large air holes near the core region has become the standard for the kind of structures'411 as shown in Fig. 11.10(a). Fig.

11.3 Properties ofPCFs and Device Applications

389

11.10(b) shows the wavelength dependence of the fiber birefringence. Its modal birefringence is 1.4xl0"3 at 1550 nm and three times larger than that of existing PN- fiber.

b

Fig. 11.10 (a) Cross-sectional profiles of the polarization maintaining-PCF. (b) wavelength dependence of the birefringence. After Reference [41].

A PM large mode area photonic crystal fiber has been demonstrated.1423 The birefringence was introduced using stress-applying parts. The fibers with mode field diameters from about 4 to 6.5 um exhibited a typical birefringence of 1.5x10"4 and were both single mode at any wavelength and had a practically constant birefringence for any wavelength. Another interesting application of PM-PCF is absolutely single polarization PCF.I43] Single-polarization fibers are optical fibers that guide only one polarization state in a specific wavelength range. Compared with PM-fibers that guide both polarization states, singlepolarization fibers typically have a higher polarization extinction ratio, which is independent of the length. Single-polarization fibers have various applications in gyroscopes, fiber polarizers, and fiber lasers and amplifiers to ensure linearly polarized output. Single-polarization fibers based on fibers with air-holes have been demonstrated. For example, Folkenberg et al. reported a single-mode photonic crystal fiber with mode-field diameter of 15.5 um,[43] it supported only one polarization state in a 220-nm broad spectral region centered at 727 nm.

390


11.3.1.3 Nonlinear characteristics ofPCF The high index difference between the silica core and the air-filled microstructure enables tight mode confinement and a significantly larger numerical aperture (NA) resulting in a low effective area and thereby a high nonlinear coefficient.133'34,36"38] The air-filled region also results in strong wavelength dependence and is responsible for the large waveguide dispersion possible in such fibers as mentioned above. Small core PCFs can obtain short zero dispersion wavelengths in the infrared and visible range and such fibers are ideally suited for pumping with Ti: Sapphire femtosecond pulse sources since the low fiber dispersion provides excellent phase matching for creating wideband supercontinuum.[50] Some nonlinear PCFs were designed with a small core to get a high nonlinear coefficient as shown in Fig. 11.7. By choosing the dispersion profile carefully, the fibers can be tailored to facilitate different nonlinear processes. Nonlinear effects can be used for a wide range of optical processing applications including optical data regeneration, wavelength conversion, optical demultiplexing and Raman amplification. Specially, broad phase-stabilized frequency combs can be generated by launching femtosecond pulses from a Ti:Sapphire laser into nonlinear PCF and phase-locking the resulting output frequency comb.[44] It has led to technologically important applications in the field of precision metrology. Hu and Wang et al. reported that a random array of fused silica waveguide wires in a microstructure fiber could frequency converts unamplified ultrashort laser pulses in a broad frequency range with high efficiency.145^71 A cross-sectional view of the fiber is shown in Fig. 11.11.[46] There are two sections in the microstructured part of the fiber, inner microstructure part and outer microstructured part with radically different sizes of air holes (Fig. 11.11(a)). The size of air holes in the outer section is typically about 20 um. This outer microstructured part serves as a cladding, confining light to the inner microstructured part of the fiber (Fig. 11.11(a)). The sizes of air holes and fused silica channels in the inner section vary from 0.3 up to 2 (am and from 0.6 up to 1.5 |am, respectively. Each of an array of fused silica channels can be viewed as a fiber core surrounded by a random holey cladding, providing waveguide

11.3 Properties qfPCFs and Device Applications

391

due to the TIR. There are two typical geometries of waveguide channels in the fiber as shown in Fig. 11.11(b). Thefirsttype is bounded by a triad of air holes and has a triangular shape, such as channels 2, 3 in Fig. 11.11(b). The second type is bounded by four air holes, such as channels 1, 4 in Fig. 11.11(b). Dispersion can be switched in such waveguide arrays by coupling the pump field into waveguide wires with different diameters.

Fig. 11,11. Cross-sectional profiles of the random-hole microstracture fiber: (a) general view and (b) a close-up view. After Reference [46].

Fig. 11.12 Output beam patterns of anti-Stokes-shifted emission from channels 1 - 6 in Fig. 11.11(b) of the random microstructurefiber.After Reference [46].

This microstructured fiber integrated random arrays of waveguides with different diameters can frequency-convert unamplified subnanojoule Ti: sapphire laser pulses to any wavelength within a broad

392


spectral range from 400 up to 700 nm. As shown in Fig. 11.12, when a Ti: sapphire laser pump pulses were coupled into submicron channels of the 3-cm-long microstructure fiber, the generation of supercontinuum was observed. Fig. 11.12 (l)-(6) are output beam patterns of anti-Stokesshifted emission and display the basic colors of palette ((1) blue, (2) orange, (3) celadon, (4) red, (5) green, (6) Cambridge blue) produced by waveguide channels 1 - 6 in Fig. 11.11(b). Zheng and Hou et al. reported that holey fibers with random cladding distribution have special ability of localizing light and controlling the group velocity dispersion.[48] A supercontinuum extending from 350 nm to more than 1700 nm was observed in the fiber by using a Ti: sapphire femtosecond laser. The maximum total power of the supercontinuum was 63 mW with 288 mW pump power. The wavelength and power, polarization states and waveguide modes of the visible light ranging in the supercontinuum could be tuned by adjusting the pump incident point or incident angle. Due to the small cores of the above described dispersion engineered/phasematched nonlinear fibers, output pulse energy is still limited and may be too low for applications like hyperspectral laser radar, hyperspectral imaging and speckle-free illumination. Such limitations are overcome in the recently demonstrated supercontinuum generation in a 25|j.m core large mode area-type fiber.[34] 11.3.2 Laser active PCFs One of the most promising applications for PCFs is in high-power fiber lasers. Doping of the core of PCFs with rare-earth elements such as Nd, Yb or Er produces fibers for laser and amplifier applications.118'34'35' 37,38,49, so] Traditionally, fiber lasers are constructed using dual-clad, stepindex fibers with a polymer outer cladding and a core doped with such rare-earth ions. [18] Unfortunately, it is difficult to extend this design to produce single-mode output at higher pump and output powers, as was discussed in Chapter 7 of this book, high power generated in small core size fiber will lead to detrimental nonlinear effects. The special and outstanding characteristics of PCFs are: very high numerical aperture (NA) [15] with double-clad structures for efficient pump light use and large core sizes improving the destruction limit for high power


393

applications. The combination of very large core size and high NA makes it possible to fabricate lasers and amplifiers with very high power and short fiber lengths. For example as shown in Fig. 11.13,t50] it is a rod-type photonic crystal fiber, the inner cladding of the fiber is surrounded by an air-cladding region, which produces a significantly greater index difference between the inner and surrounding region, and therefore a higher NA than that can be achieved by conventional polymer coated fibers, and effectively confines the pump light to a silica multimode pump core, commonly referred to as the inner cladding.

(A)

(B)

Fig. 11.13 (A). Microscope image of the rod-type photonic crystal fiber with a diameter of 1.7mm and (B) Close-up of the hexagonal inner cladding with a diameter of 117 \xm and core region with a diameter of 35 |im. After Reference [50].

The higher NA permits efficient pumping with relatively inexpensive high-power large-emitting-area pump diodes. Within the inner cladding, another microstructured rare-earth-doped core defines the laser-beam output parameters. The single-mode core can be expanded to a large mode area to facilitate high power levels in a single mode while avoiding nonlinearities and providing a good overlap between the pump mode and the laser mode. 11.3.2.1 CW high powerfiberlasers Continuous wave (CW) high power fiber lasers based on Yb-doped large mode area PCF (LMA PCF) have been the focus of considerable research. Li et al. obtained a 50 W fiber laser with single transverse mode operation using an Yb-doped LMA PCF and a Fabry-Perot (F-P)

394


configuration. J Output powers up to 1.53 kW based on a LMA PCF has been obtained.118'34] The rod-type photonic crystal fiber as shown in Fig. 11.13(A) has a large outer diameter of the fiber (1.7 mm), it provides enough mechanical stability and the need for protective coating is eliminated, thereby improves the thermal properties of the fiber. Fig. 11.13(B) shows a close-up of the inner cladding and core region. The hexagonal inner cladding has a diameter of 117 um (face to face) and a numerical aperture of around 0.6. The signal core has a diameter of 35 urn and the hole-diameter to pitch ratio is ~ 0.33. The fiber with length of 48cm was pumped with 165 W at 976 nm resulting in 120 W laser output at 1035 nm with a slope efficiency of 74%. This is equivalent to a power extraction of 250 W/m.[50] An analysis based on a single-mode core with a mode-field diameter of 35 um reveals that a power level of «10kW with diffraction-limited beam quality is possible with established fiber technology.149'53] The first step in this direction has been done with a 1.53 kW emission out of an ytterbium-doped photonic crystal fiber.[18' 34] PCFs have been developed in which the cores have also been co-doped with photosensitive materials, enabling fiber-Bragg gratings to be inscribed right into the core using UV laser light, thus further simplifying the architecture.[53] 11.3.2.2 Pulsed high-power fiber lasers Due to the short fiber length, large mode area (reduced nonlinearity) and relatively high-energy storage capacity, PCF is also very suitable for Qswitched laser operation. Roser et al. reported a LMA Yb doped-fiber based chirped-pulse-amplification system (CPA) generating up to 131 W of average power of 220 femtosecond pulses at 1040 nm center wavelength in a diffraction limited beam (M2 < 1.2).[51] The pulse repetition rate was 73 MHz, corresponding to a pulse energy of 1.8 uJ and a peak power as high as 8.2 MW. The high average power femtosecond laser system consisted of a passively mode-locked, diodepumped solid-state laser system, a gold-coated diffraction grating stretcher, a two-stage Yb-doped single-mode PCF amplifier, and a fusedsilica transmission grating compressor. The experimental setup is shown in Fig. 11.14.


395

*as &i, rasas

Fig. 11.14 Schematic setup of the high average power fiber CPA system. After Reference [51].

The femtosecond seed source was a passively mode-locked Yb:KGd (WO^ (Yb:KGW) laser system producing 150 femtosecond pulses at a 73 MHz repetition rate and a 1040 nm center wavelength. These pulses were stretched to 120 ps using a conventional gold-coated 1200 line/mm diffraction grating. The preamplifier and the power amplifier were constructed with identical PCFs. The PCFs possessed an active core diameter of 40 um (NA, 0.03) and an inner cladding diameter of 170 urn (NA=0.62). This structure had a pump light absorption of 15 dB/m at 976 nm. Therefore, the fiber length used v/as just 1.2 m. The core parameters leaded to a calculated mode-field diameter of 35 um, resulting in an effective mode-field area of -1000 um2. The output power of the fiber laser system was limited by available pump power and not by degradation in pulse quality owing to nonlinearity or even by fiber damage. Thus, further scaling of the output power is possible with the present system. 11.3.3 Properties of hollow core-PCFs Hallow core photonic crystal fibers have become the most advanced manifestation of two dimensional photonic band-gap structures. Due to the hollow core guidance and the resulting small overlap between the propagating mode and the silica fiber material, these fibers exhibit lower Rayleigh scattering and absorption in the glass, lower nonlinearity and a range of interesting prospects such asiWMWSfl high damage threshold,

396


radiation insensitive, easy access to rilling the core or cladding with gasses or liquids, gas/liquid sensors with long interaction lengths, low Fresnel reflection from the fiber facets, large span of dispersion values achievable: from highly negative to highly positive, bend insensitive. It is possible to guide light in a hollow core with low attenuation on kilometer length scales and provide excellent potential for applications such as power delivery and gas lasing or gas-based nonlinear, laser-induced particle guidance, stimulated Raman scattering (SRS) and laser frequency metrology.

Fig. 11.15 Optical micrograph of a hollow core-PCF. The other end of the fiber is illuminated with white light. The fiber showed guides blue and green light in the lowindex core. After Reference [10].

The first PCF in which light guidance was by a true full twodimensional photonic band gap was reported in 1998.[10] This PCF had a honeycomb lattice of air-holes, with an additional air-hole at the center, and light was guided in narrow glass regions adjacent to the central airhole as shown in Fig. 11.15. Theory predicts that the guided mode in this photonic band-gap fiber is highly dispersive and exists only for narrow wavelength ranges. This was confirmed by experiments, when illuminated with white light, a bright colored mode appears in the core the modes of this fiber came in yellow, blue and green. A calculation revealed that there is also photonic gap in two-dimensional amorphous photonic materials, which does not possess any long-range order but have only short-range order.[55] Yang and Chen et al. fabricated a hollow core holey fiber with a random distribution of air holes in the cladding, and this fiber also demonstrated many features that previously attributed


397

to photonic crystal fibers with perfect arrangement of air holes, J such as the fiber behaving a second guided mode with two-lobe pattern and different colors when a white light was launched. 11.3.3.1 Loss The minimum optical attenuation of-0.15 dB/km in conventional fibers is determined by fundamental scattering and absorption processes in the high-purity glass. However, over 99% of the light in hollow core PCFs can propagate in air and avoid these loss mechanisms, making hollow core PCFs promising candidates as a new generation ultra-low loss telecommunication fibers.[32] Nevertheless the lowest loss reported in hollow core PCFs is 1.2 dB/km. Direct leakage of light from the core is easily suppressed by incorporating enough holes in the cladding. Fiber non-uniformity then becomes the key loss mechanism. The minimum loss in hollow-core photonic crystal fibers is limited by scattering due to surface roughness from frozen-in surface capillary waves. It can be mitigated through fiber design, and attenuation of the order of 0.1 dB/km is plausible. Design improvements to eliminate surface modes, material processes to increase surface tension or alternative glasses transparent at longer wavelengths may further reduce the attenuation of hollow core PCFs. Silica based hollow core photonic crystal fibers could be useful for many mid-IR applications.'571 The spectral region of 3 to 5 urn, in the mid-infrared (mid-IR), is currently of growing interest because the development of a new generation of laser sources promises to open this spectral window for applications in the near future. Many gases exhibit strong molecular absorption at these wavelengths, especially in the wavelength range of 3 to 3.5 um. For example, CH4 has a strong absorption band around 3.3 um. In bulk silica the material loss above 3 um is greater than 60 dB m"1. For practical purposes this means that standard silica fibers are unusable in this wavelength range. Due to the low overlap of the guided light with glass in hollow core photonic crystal fibers, which can be less than 1%,[32] the effect of the relatively high material loss of silica at these wavelengths is minimized, giving a loss of 2.6 dB m"1 in the wavelength range 3.1-3.2 um in an effectively single-

398


mode fiber. This level of loss means that the current fiber is suitable for applications where short lengths of fiber are required. However, with improvements in fiber design lower losses around 0.5 dB/m were predicted and such fibers will be suitable for a much wider range of applications. 11.3.3.2 Gas in PGBfibers Gas phase materials are used in a variety of laser-based applications,'541 for example, in high-precision frequency measurement, quantum optics and nonlinear optics. Their full potential has however not been realized because of the lack of a suitable technology for creating gas cells that can guide light over long distance in a single transverse mode while still offering a high level of integration in a practical and compact set-up or device. Gas-filled hollow-core photonic crystal fibers demonstrated substantially enhanced stimulated Raman scattering[21'54] and exhibited high performance, excellent long-term pressure stability and ease of use. There are two different devices: a hydrogen-filled cell for low threshold stimulated Raman scattering (SRS); and acetylene-filled cells for absolute frequency-locking of diode lasers with very high signal to noise ratios. The generation of vibrational SRS in hydrogen using a Kagome hollow core-PCF with a pump threshold 100 times lower than any previously reported in single-pass or multi-pass cells, and rotational SRS using a band-gap hollow core-PCF with thresholds some 1 million times lower than its corresponding experiments and with photon conversion reaching almost the quantum limit.'211 Acetylene represents an excellent frequency standards source for the optical communications wavelength range, and the long interaction length offered by hollow core-PCF leaded to unprecedented enhancement in the signal to noise ratio, it offered a comb of stable and regularly spaced ro-vibrational overtone transitions around 1.55 um. With the two isotopes of carbon 12C and 13C, the spectrum spans a range between -1.510 um and 1.560 um and exhibits more than 50 strong lines, thereby providing a wide grid of frequency references. The stable performance of these compact gas-phase devices could permit, for example, gas-phase laser devices incorporated in a 'credit card' or even in a laser pointer.


399

11.3.3.3 Particles guidance Small particles can be propelled and suspended against gravity using only the force of radiation pressure.[14'58] The use of the radiation pressure principle provides a useful means for non-intrusive manipulation of microscopic objects such as biological objects, particles etc, it has been employed in different areas such as biology, chemistry, atomic physics and engineering. However, these applications were intrinsically limited by the diffraction of the laser beam to micrometer length scales, as strong lateral confinement requires tight beam focusing. Overcoming this limitation is of particular interests in many areas where transportation of micro-sized objects over longer distance is required. For stable guidance, including cornering, it requires constant beam intensity focused to a small spot over many Rayleigh lengths. Up until now, the only possibility is to use fiber capillaries as the waveguide. Unfortunately, two inherent light guiding properties of capillaries set a limit to the guidance length: fundamentally leaky and power loss. Unlike hollow fiber capillaries, hollow core-PCF guides light without leakage using a photonic band gap, enabling a much longer guidance length to be allied with stable strong transverse particle confinement by using a small hollow core. Benabid et al. demonstrated particle guidance in a hollow core-PCF.[581 The hollow core diameter of the fiber is 20 um and the loss was measured to be 10 dB/m at 514 nm. An Argon ion laser beam operating at a wavelength of 514 nm with a power of 80 mW was sufficient to levitate a 5 urn diameter polystyrene sphere and guided it through a -150 mm long hollow-core crystal photonic fiber. The speed of the guided particle was measured to be around 1 cm/s. Such a strong gradient force with a comparable laser power would only be attainable over a distance of 0.6 mm using a focused beam in free space, or over 12 mm using a standard fiber capillary. With loss below 10 dB/km, the possible guidance lengths in a hollow core-PCF would increase to a few 100 m.

400


11.4. Non-Silica Glasses for PCFs Non-silica glasses, often referred to as compound or soft glass, such as silicate glasses, [28 ' 29 ' 60] phosphate glass, [59] chalcogenide glasses, [27] tellurite glasses,[60] and other heavy metal oxide glasses. It offers unique material properties, which cannot be provided by SiC>2 glass. Typically, they have special properties like[60] highly linear refractive index, high transparency from the near-infrared (near-IR) to the mid-IR region, high rare-earth solubility, low phonon energy, low melting temperature, etc. Thus, non-silica glass PCFs were proposed as noteworthy candidates for highly nonlinear applications, novel devices for mid-IR laser transmission and short active fiber devices. Capillary-stacking techniques and the extrusion techniques have already been utilized for fabricating non-silica glass PCFs. The highest nonlinearity in optical fibers, 640 W"1 km"1,[28] which is more than 600 times that of standard single mode silica fiber, was reported in the extruded index-guiding PCFs based on Schott SF57 glass (Si02-PbO glass). Supercontinuum generation, i.e., the broadening of the ultrashort pulse signal after passing through a nonlinear media, was also demonstrated in an extruded nonsilica glass index-guiding PCFs based on Schott SF6 glass.[61] Because non-silica glasses typically possess higher refractive indexes (n=1.5~2.8) than that of pure silica glass (n=1.44), photonic band-gaps can be easily observed in band-gap PCFs based on high-index nonsilica glass.'62'631 It has been demonstrated that a robust photonic band-gap exists in high-index glass («>2.0) when the air-filling fraction is only about 60%. As mentioned in section 11.1.2, however, a high air filling factor (D/A>90%) is required to achieve a broad bandwidth and low-loss in silica hollow core PCFs. Using chalcogenide glasses, which have high index (2.0-2.8) and high transparency in mid-IR regions (2-1 Oum), very broad band-gaps in the mid-IR region was predicted in hollow core-PCFs for the potential applications for C0 2 laser transmission.'621 Large mode area index-guiding PCFs with mode areas as large as 430 urn2 have been developed using phosphate glass, which is of particular interest for Er-Yb-codoped fiber laser and amplifiers at 1550 nm.[59] For an only 11 cm long cladding-pumped fiber laser, more than 3 W of continuous wave output power was demonstrated, and near single-mode

References

401

beam quality was obtained for an active core area larger than 400 um 2 . An Nd3+ doped silicate glass photonic crystal fiber with a large fiber core of 58 um as shown in Fig. 11.16(a) has been demonstrated.164,651 Optical measurement demonstrated that the fiber sustain a single mode at least over wavelength range from 660 nm to 1550 nm as shown in Fig. 11.16 (b). By pumping with an 808 nm laser diode, the fiber exhibited an amplified spontaneous emission.

Fig. 11.16 (a) Cross-sectional profiles of the Nd3+ doped silicate glass PCF and (b) a 3D intensity distribution of the guided mode from the output end of the PCF excited by an 808nm laser diode.

PCFs are expected to have various applications, such as higher-power fiber lasers, power delivery, creating supercontinuum, wavelength tunable light source, and distribution compensation. Much more works remain to be explored. There is no doubt that the impact of PCF will spread to numerous applications in research, biomedicine, sensing, and material process.

References 1. J. Joannopoulos, R. Meade, and J. Winn, "Photonic Crystals: Molding the Flow of Light", (Princeton Press, Princeton, New Jersey, 1995). 2. E. Yablonovitch, Phys. Rev. Lett., 58 (1987) 2059. 3. S. John, Phys. Rev. Lett., 58 (1987) 2486.

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4. P. Russell, D. Atkin, and T. Birks, "Bound modes of two-dimensional photonic crystal waveguides," Quantum Optics in Wavelength Scale Structures, 1996. 5. S. John, "Photonic Band Gap Materials", (C. M. Soukoulis, Ed., NATO ASI Series: Series E, Applied Sciences, vol. 315, NATO Scientific Affairs Division, Kluwer, Dordrecht, 1996) pp. 563-666. 6. P.Russell, Science, 299 (2003) 358. 7. T. Birks, P. Roberts, P. Russell, D. Atkin and T. Shepherd, Elect. Lett., 31 (1995) 1941. 8. J. Knight, T. Birks, P. Russell and D. Atkin, postdeadline paper at OFC'96; Opt. Lett., 21 (1996) 1547. 9. T. Birks, J. Knight, and P. Russell, Opt. Lett., 22 (1997) 961. 10. J. Knight, J. Broeng , T. Birks T, et al., Science, 282 (1998) 1476. 11. J. Knight, Nature, 424 (2003) 847. 12. D. Ouzounov, F. Ahmad, D. Muller, et al., Science, 301 (2003) 1702. 13. A. Bjarklev, J. Broeng, and A. Bjarklev., "Photonic Crystal Fibers." (Dordrecht: Kluwer; 2003). 14. F. Benabid and P. Russell, Proc. of SPIE, 5733 (2005) 176. 15. W. Wadsworth, J. Knight, T. Birks, and P. Russell, Proc. of SPIE, 5246 (2003) 362. 16. F. Couny, H. Sabert, P. Roberts, et al., Opt Express, 13 (2005) 534. 17. N. Mortensen, J. Folkenberg, M. Nielsen, and K. Hansen, Opt. Lett., 28 (2003) 1879. 18. A. Tunnermann, T. Schreiber, F. Roser, et al., J. Phys. B: 38 (2005) S681. 19. M. Nielsen, J. Folkenberg, and N. Mortensen, Electron. Lett., 39 (2003) 1802. 20. M. Yan, and P. Shum, Opt. Lett., 30(2005) 1920. 21. F. Benabid, J. Knight, G. Antonopoulos, and P. Russell, Science 298 (2002) 399. 22. W. Chen, J. Li, S. Li, et al., Study on Opt. Commun., 129 (2005) 47 (In Chinese). 23. S. Li, L. Hou, Y. Ji, and G. Zhou, Chin. Phys. Lett., 20 (2003) 1300. 24. G. Zhou, Z. Hou, S. Li, and L. Hou, Chin. Phys. Lett., 22 (2004) 1162. 25. Q. Zhou, X. Lu, J. Qiu, D. Chen, X. Jiang, and C. Zhu, Chin. Opt. Lett., 3 (2005) 686. 26. R. Bise, W. Monberg, F. DiMarcello, et al., IEEE, 2 (2004) 643. 27. T. Monro, Y. West, D. Hewak, et al., Electr. Lett., 36 (2000) 1998. 28. P. Petropoulos, H. Heidepriem, V. Finazzi, et al, Opt. Express, 11 (2003) 3568. 29. X. Feng, T. Monro, P. Petropoulos, et al., Opt. Express, 11(2003) 2225. 30. J. Canning, E. Buckley, K. Lyttikainen, and T. Ryan, Optics Comm., 205 (2002) 95. 31. J. Zhou, K. Tajima, K. Nakajima, et al., Optical Fiber Technology, 11 (2005) 101. 32. P. Roberts, F. Couny, H. Sabert, et al., Opt. Express, 13 (2005) 236. 33. S.Kawanishi, Proc. of SPIE, 5596 (2004), 280. 34. R.Kristiansen, K.Hansen, J. Broeng, et al., 4a Reunion Espanola de Optoelectronica, OPTOEL'05, CI-5(2005), 37. 35. K. Mori, H. Muller, J. Kirchhof, et al., Proc. of SPIE, 5595 (2004) 66. 36. J. Eichenholz, http://newport.com/Support/Magazine_Features, Optoelectron. World (2004). 37. T. Monro, and David J. Richardson, C. R. Physique, 4 (2003) 175. 38. H. Ebendorff-Heidepriem, K. Furusawa, et al., Proc. of SPIE, 5350 (2004) 35. 39. K.Saitoh, N. Florous, and M. Koshiba, Opt. Express, 13 (2005) 8365. 40. J. Rarity, J. Fulconis, J. Duligall, et al, Opt Express, 13 (2005) 534.

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K. Suzuki, H. Kubota, S. Kawanishi, et al., Opt. Express, 37 (2001) 1399. J. Folkenberg, M. Nielsen, N. Mortensen, et al., Opt. Express, 12 (2004) 956. J. Folkenberg, M. Nielsen, and C. Jakobsen, Opt. Lett., 30 (2005) 1446. J. Stenger, H. Schnatz, C. Tamm, and H. Telle, Phys. Rev. Lett., 88 (2002) 073601. M. Hu, C. Wang, L. Chai, and A. Zheltikov, Opt. Express, 12(2004) 1932. M. Hu, C. Wang, Y. Li, et al., Opt. Express, 12 (2004) 6129. M. Hu, C. Wang, Y. Li, L. Chai, and A. Zheltikov, Opt. Express, 13 (2005) 5974. Y. Zheng, Y. Zhang, X. Huang, et al, Chin. Phys. Lett., 21 (2004) 750. J. Limpert, T. Schreiber, and A. Tunnermann, "Fiber Based High Power Laser Systems", (RP Photonics), http://www.highpowerlasers.rp-photonics.com/ fibersy stems. Html. J. Limpert, N. Deguil-Robin, I. Manek-Honninger, et al, Opt. Express, 13 (2005) 1055. F. Roser, J. Rothhard, B. Ortac, A. Liem, et al., Opt. Lett., 30 (2005) 2754. K. Li, Y. Wang, W. Zhao, et al, Chin. Opt. Lett., 3 (2005) 457. J. Canning, Optics and Lasers in Engineering, in press (2006). F. Benabid, F. Couny, J. Knight, T. Birks, and P. Russell, Nature, 434 (2005) 488. C. Jin, X. Meng, B. Cheng, Z. Li, and D. Zhang, Phys. Rev. B, 63 (2001) 195107. L. Yang, D. Chen, N. Da, et al., Chin. Phys. Lett., 22(2005) 2592. J. Shephard, W. MacPherson, R. Maier, et al, Opt. Express 13 (2005) 7139. F. Benabid, J. Knight, and P. Russell, Opt. Express, 10 (2002) 1195. L. Li, A. Schiilzgen, V. Temyanko, et al., Opt. Lett., 30 (2005) 1141. X. Feng, A. Mairaj, D. Hewak, and T. Monro, J. Lightwave Tech., 23 (2005) 2046. V. Kumar, A. George, W. Reeves, et al., Opt. Express, 10 (2002) 1520. B. Temelkuran, S. Hart, G. Benoit, et al., Nature, 420 (2002) 650. J. Pottage, D. Bird, T. Hedley, et al., Opt. Express, 11 (2003) 2854. L.Yang, D. Chen, L. Da, X. Jiang, C. Zhu, and J. Qiu, Opt. Mater., in press (2006). L.Yang, D. Chen, J. Xia, et al, Chin. Phys. Lett., 22 (2005) 388.

Chapter 12

Functional Microstructures in Glass Induced by a Femtosecond Laser

12.1. Introduction In 1994, Prof. Hirao of Kyoto University, Japan proposed a basic research idea of "induced structure".1'1 He paid attention to the fact that glass is metastable from the viewpoint of thermodynamics. A metastable state of glass is easily changed to other states in an intensive external electromagnetic field. If we can control the external electromagnetic field induced structure and the concentration, variety and valence state of active ions in glass; in particular, if we can space-selectively control the induced microstructures or 3-dimensionally distributed electronic structure in glass, we expect that novel optical functions of the glass will be achieved. From the viewpoint of practical applications, we expect to obtain a glass with properties superior to corresponding single crystal. Based on this idea, we applied various electromagnetic fields such as X-ray, ultra-violet light, electron beam and laser to make microscopic modifications to glass structure, and observed many interesting phenomena and discussed the promising applications of the observed phenomena.'2"81 We selected a femtosecond laser as a powerful tool to make microscopic modifications to glass structure. Compared with CW and long pulsed lasers, femtosecond laser has two apparent features: (1) elimination of the thermal effect due to extremely short energy deposition time, and (2) participation of various nonlinear processes enabled by highly localization of laser photons in both time and spatial 405

406


domains. Due to the ultra-short light-matter interaction time and the high peak power, material processing with the femtosecond laser is generally characterized by the absence of heat diffusion and, consequently molten layers.[9] The nature of ultra-short light-matter interaction permits femtosecond laser to overcome the diffraction limit.[10] The first important discovery of femtosecond laser induced structure inside a transparent material was made by Dr. Misawa of Masuhara MicroChemistry Project, Japan Science and Technology Corporation. He occasionally observed the formation of femtosecond laser-induced micro-spot in a slide glass in 1994, and suggested that the observed phenomenon can be used for the fabrication of 3-dimensional optical memory with ultrahigh storage density.[U] Mazur group at Harvard University, Gaeta group at Cornell University, Misawa group at Tokushima University and Hokkaido University, Hosono group at Tokyo Institute of Technology, Itoh group at Osaka University and Tunnermann group at the Friedrich Schiller University have made great contributions to the application of femtosecond laser micro-processing of glass. We started the systematic investigations on the femtosecond laser induced microstructures in glasses and applications in micro-optics at the end of 1994. The reason for using this laser is that the strength of its electric field in the focal point of the laser beam can reach lOTW/cm2, which is sufficient for inducing various nonlinear physicochemical reactions in glasses by using a focusing lens, when the pulse width is lOOfs and the pulse energy is 1 u J. The photo-induced reactions are expected to occur only near the focused part of the laser beam due to multiphoton processes. In the past several couple of years, a lot of research efforts have been devoted to the field of 3 dimensional microscopic modifications to transparent materials by using femtosecond laser. Promising applications have been demonstrated for the formation of 3dimensional optical memory[11"14] and multicolor image,[15] and fabrication of optical waveguide,[1617] coupler [18"19] and photonic crystal.[20]

12.2 Micro-Structural Changes Induced by Femtosecond Lasers

407

12.2 Micro-Structural Changes Induced by Femtosecond Lasers 12.2.1 Various femtosecond laser induced localized microstructures in glasses Considerable research has been carried out on the writing of Bragg gratings inside optical fibers since the first observation of photo-induced refractive index change in glass fiber by Hill et al. in 1978.[21] The reaction between light and glass is usually induced by irradiating an area in a glass to achieve various types of light-induced structural changes. It is difficult to produce an interaction effect between glass and light by a one-photon process when the wavelength of excitation light differs from the resonant absorption wavelength of the glass. But, due to the ultrashort laser pulse and ultrahigh light intensity, femtosecond laser may induce different microstructures from those induced by CW, nano- and picosecond pulsed laser in glass. A Typical optical setup for irradiation of the femtosecond laser is shown in Fig. 12.1. 200 kHz

A I CCD[—

V

2 glass -60.1), that the oxygen defects were formed in the regions corresponding to dark domains of the BE image, which reduce the average molecular weight in these regions (S1O2-X ~60.1-16x). To test this suggestion, we carried out Auger spectra mapping of silicon and oxygen on the same surface with 10 nm spatial resolution. The Auger signal of the oxygen in the regions corresponding to dark domains in the BE image is lower compared to other regions as shown in Fig. 12.19, indicating low oxygen concentration in these domains. Furthermore, the intensity of the oxygen signal is stronger in the regions between the dark domains of the BE image. On the other hand, the intensity of the silicon signal is the same in the whole imaged region. These results indicate that the periodic structure observed in the BE image consists of periodically distributed oxygen-deficient regions (Si02-x). The Auger signal intensity is

12.5 Novel Phenomena Induced by Femtosecond Lasers

437

proportional to the concentration of element constituting the surface, which gives an estimate to the value x ~ 0.4.

Fig. 12.19 Auger spectra maps and corresponding line scans of oxygen (a) and silicon (b) on the silica glass surface polished close to the depth of focal spot.

We observed the decrease of the grating period with an increase of the exposure time. The grating periods were about 240 nm, 180 nm and 140 nm for the number of light pulses of 5 x 104, 20 x 104 and 80 x 104 respectively and for the pulse energy of 1 uJ, corresponding to intensity of 2x1014 W/cm2. This indicated a logarithmic dependence of the grating period A on the number of light pulses. The dependence of the observed periodic nanostructures on pulse energy for a fixed exposure time was also investigated and an increase of the period with the pulse energy was observed. Grating periods of 180 nm, 240 nm and 320 nm were measured at pulse energies of 1 uJ, 2 uJ and 2.8 uJ respectively and for the number of light pulses of 20 x 104. The following explanation of the observed phenomenon is proposed. Once a high free electron density is produced by multiphoton ionization the material has the properties of plasma and will absorb the laser energy via one-photon absorption mechanism of inverse Bremsstrahlung (Joule)

438


heating. The light absorption in the electron plasma will excite bulk electron plasma density waves. These are longitudinal waves with the electric field component parallel to the direction of propagation. Such electron plasma wave could couple with the incident light wave only if it propagates in the plane of light polarization. Initial coupling is produced by inhomogeneities induced by electrons moving in the plane of light polarization. The coupling is increased by a periodic structure created via a pattern of interference between the incident light field and the electric field of the bulk electron plasma wave, resulting in the periodic modulation of the electron plasma concentration and the structural changes in glass. A positive gain coefficient for the plasma wave will lead to an exponential growth of the periodic structures oriented perpendicular to the light polarization, which become frozen within the material. The observed increase of the grating period with the pulse energy is in agreement with this theoretical prediction. The observed formation of stripe-like regions with low oxygen concentration can be also explained as follows. The plasma electrons are created in the process of breaking of Si-O-Si bonds via multi-photon absorption of light which is accompanied by the generation of a Si-Si bonds, non-bridging oxygen-hole centers (NBOHC, ^Si-O") and interstitial oxygen atoms (Oi). Such oxygen atoms are mobile and can diffuse from the regions of high concentration. Negatively charged oxygen ions can be also repelled from the regions of high electron concentration. The photoluminescence and electron spin resonance spectra confirmed the presence of non-bridging oxygen defects and E' centers in the irradiated samples. The small thickness of these regions, compared to the period of the grating, could be explained by a highly nonlinear dependence of the structural changes on the electron concentration. Major changes in composition take place after the attainment of thermal equilibrium, involving formation and decay of defect states, such as oxygen vacancies. Detailed mechanism of the structural changes responsible for the nano-grating formation is under investigation.

12.5 Novel Phenomena Induced by Femtosecond Lasers

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12.5.2 Moving of hole Watanabe et al. observed and demonstrated optical movement of a void and merger of two voids along the optical axis by translation of the focal spot of a focusing lens with femtosecond laser pulses.[52] The laser pulses propagated along the optical axis (+z direction). They focused the femtosecond laser pulses inside silica glass to create local structural changes. They used high-NA objectives to avoid self-trapping effects and create a void. The void can be seized and moved to the negative side of the +z direction. First, they created three voids along the y axis at a depth of 300^m beneath the surface, with a spacing of 5|j,m between successive voids. Then, they moved the focal point and translated the focal point to the -z direction by 0.5^m with one exposure. By repeating this step, they moved the void further. The visible trajectory of the void after translation of the focal point implies structural changes. These changes were considered to have a connection with the refractive index change that is utilized for fabrication of waveguides. They also demonstrated that two voids can be merged to form one bigger void. The mechanism of void generation and its properties are open questions. 12.5.3 Formation of periodic nanovoid structures Recently, Kanehira et al. observed the formation of periodic nanosized voids inside a glass sample along the propagation direction of the femtosecond laser beam.[53] Two hundred and fifty pulses of the femtosecond laser were launched in the interior of a glass sample. The laser beam was focused at a depth of 750um from the entrance surface. Sideview scanning electron microscope photograph shows there is an aligned void structure below the focal point along the propagation direction of the femtosecond laser beam. The formed voids have almost a spherical shape, and the neighboring two voids are independent of each other. No microcracks or catastrophic collapses are observed around the voids or the focal point. An interesting feature is that the aligned void structure contains a region with periodically aligned voids without connected voids or cracks, which are located at a distance of ~90 (im from the bottom surface of the glass sample. A void with a diameter of

440


1.6um is formed at the focal point. The distance between the void at the focal point and the next void is 7.2 ^m. Both the diameter of the voids decrease gradually with the closing of the bottom surface of the glass sample, and approach limiting values at a distance of ~90 um from the bottom surface. Namely, the periodic part exists at a range of ~90 urn from the bottom surface. The void size and intervoid separation in the periodic part are 380nm and 1.7 (im, respectively. They observed that the entire length of the periodic void structure gradually increases with increasing pulse numbers and exhibits a constant value of 130 when the pulse number is increased above 125. The phenomenon is explained as follows. Microexplosion takes place around the bottom surface and create a nanosized void at first. When the next femtosecond laser pulse propagates from the focal point, it is trapped in the heated region around the void formed beforehand, resulting in the production of new high temperature region and the formation of the next void. The size of the voids, and the period and the entire length of the viod structure could be controlled by varying the pulse energy, pulse number, and focal point of the femtosecond laser. Though the mechanism remains an open question and the nature of the phenomenon should be further examined, this discovery is promising for the fabrication of controllable periodic void structure such as three-dimensional photonic crystals. Conclusion We have observed and discussed the mechanisms of the femtosecond laser induced various localized microstructures in glasses. We demonstrated the possibility of direct writing three-dimensional optical circuit by using femtosecond laser-induced refractive index change, space-selective control of valence state of active ions (rare-earth, transition metal ions), precipitation and control of metal and functional crystals, defect manipulation, 3-dimensional micro-drilling, coherent field femtosecond laser induced structures and formation of nano-grating by the femtosecond laser single beam, etc. The 3-dimensional direct writing technique by using the femtsecond laser will be useful in the fabrication of three-dimensional multicolored

References

441

industrial art object, rewriteable optical memory with ultrahigh storage density, integrated optical circuit and micro-optical devices. Due to the page limitation, we did not introduce nonlinear coherent femtosecond laser field induced second harmonic generation.[54] For details, please refer the cited references. We are convinced that femtosecond laser will open new possibilities in micro-optics, material sciences, physics, chemistry and bioscience fields. Acknowledgements The author thanks Professor K. Hirao of Kyoto University for his continuous encouragement and guidance. Drs. T. Mitsuyu, K. Miura, J. Si, Y. Kondo, Y. Himeii, H. Inouye, Y. Shimotsuma, Mrs. T. Nakaya, M. Shirai of Hirao Active Glass Project and Photon Craft, ICORP, JST, Prof. C. Zhu, Drs. X. Jiang, S. Qu, Q. Zhao and H. Zeng of Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, and Prof. P. Kazansky of the Southampton University, UK are also appreciated for their kind cooperation. J.Q. would like to acknowledge the financial support provided by National Science Foundation of China (No. 50125208).

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

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Functional Microstructures in Glass Induced by A Femtosecond Laser

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443

Index

arrayed waveguide grating, 315 amorphous, 17, 40 amplifier fiber, 191 waveguide, 329

fiber laser, 227, 366 end pumping, 243 high power, 227 RE-doped, 247, 250 fiber grating attenuator, 415 birefringence, 367 fabrication, 261 filter, 363 principles, 355 flame hydrolysis deposition (FHD), 302 fluoride glasses, 112 Ho3+ doped, 107 Nd3+ doped, 108 Tm3+ doped, 107 Yb3+doped, 108 fluoride glass fiber, 112 fullerene, 144

beam quality, 286 Bragg grating, 353 chalcogenide, 53, 126 DFB, 290 Dammann micrograting, 413 degenerate four wave mixing(DFWM), 118 energy transfer between laser dyes, 281 rare earth doped glasses, 194 rare earth ions, 106 activated ions, 97

Ge-doped glass photosensitivity, 341 second order nonlinearity, 170 glass borate, 17, 78, 172 chalcogenide, 53, 126 fluoride, 112 non-oxide, 307 non-silica, 400 organic-inorganic, 131, 261, 310 oxide, 124 phosphate, 77, 193 RE-TM, 40 rare-earth doped, 236, 329

femtosecond laser, 62, 405 fiber amplifier, 191,365 double clad, 232 Er doped, 193 fabrication, 234 fluoride glass, 112 numerical aperture, 230 phosphate glass, 193, 211 photonic crystal, 375 rare-earth doped, 110, 236, 239 silica glass, 110 445

446 silica, 110, 167,315 silicate, 17 soft, 171 tellurate, 185 hybrid dye laser glass, 261 gel, 278 organic-inorganic, 261, 310 waveguide, 310, 318 ions ion-exchange, 304 rare-earth, 239, 419 transition metal, 417 Kerr effect, 122 laser cavity, 286 dye, 262, 263 fiber, 227, 366 glass, 18 photonic crystal, 392 solid state dye, 261 tunable, 263, 286, 326 waveguide, 329 laser spectroscopy Nd doped, 77 Yb doped, 84 light scattering, 3 luminescence avalanche luminescence, 101 cooperative luminescence, 99 nonlinear, 94 super luminescence, 110 metallo-phthalocyanine, 148 microstructure femtosecond laser induced, 405 maganetization, 48 magnetic anisotropy, 48 magneto-optical property, 41 storage, 40 nano-composite fullerence, 144

Index glass, 136 metallic, 144 semiconductor, 137 nano pariticle copper, 144 gold, 426 silver, 424 nano grating,435 nano void, 439 nonlinear susceptibility, 118, 131 nonlinearity second order, 153 third order, 117 non-silica glass, 400 optical dispersion, 3 glass, 1 nonlinearity, 5, 117, 153 switch, 318 optical data storage, 40, 409 optical limiting effect, 146 metallo-phthalocyanines, 148 ormosil, 146 sol-gel, 149 organic-inorganic laser, 261 waveguide, 310 ormosil, 146, 269, 291 phase change, 54, 58, 61,62 mask, 361 matched, 187 phosphate glasses, 77 phosphate glass fiber, 193 photonic crystal, 375 glass, 1 photonic crystal fiber band-gap, 380 Hollow core, 395 Index-guiding, 386 non-silica glass, 400 laser, 392 property, 386 photostability, 273

447 photosensitivity Ge doped glass, 341 Pb doped glass, 353 Sn-doped glass, 344 photodegradation, 273 poling, 167, 168 rare-earth ions, 239, 419 Er3+, 193 Nd3+, 77, 112 Yb3+, 77, 194 rare-earth transition metal(RE-TM), 40 second harmonic generation, 158, 187 second order optical nonlinearity characterization, 161 chalcohalide glass, 168 Ge-doped silica, 170 high refractive index glass, 171 lead borate glass, 172 measurement, 158 mechanism, 164 tellurate glass, 185 semiconductor micro-crystallites, 137 sol gel, 267, 290, 305 tellurate glasses, 185 two photon, 291 thermal dynamics, 54 optical effect, 70 thin film amorphous, 17, 40 chalcogenide, 53, 126 hybrid, 290

metallic, 53 RE-TM, 40 third order nonlinearity, 117 waveguide amplifier, 326 arrayed waveguide grating, 315 fabrication, 302 laser, 329 non-oxide, 307 organic-inorganic, 310 principles, 300 properties, 302 sensor, 332 switch, 318 variable optical attenuator, 322 up-conversion avalanche, 101 cooperative, 91 emission, 97 energy transfer, 97 excited absorption, 84 fluorescence, 94 luminescence, 106

waveguide fabrication chemical vapor deposition (CVD), 302 flame hydrolysis deposition (FHD), 302 ion-exchange, 303 sol-gel, 305 sputtering, 302 zirconium, 291 z-scan, 120

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