1.55um Wavelength Operation of E(3+,r)-Doped Optical Fiber Bistability

Wuhan University Journal of Natural Sciences Vol. 4 No. 4 1 9 9 9 . 4 3 5 ~ 4 3 8

Article ID:

1007-1202(1999)04-0435-04

1.55 Wavelength Operation of E3+-Doped Optical Fiber Bistability" S O N G Q i a n ~, Z H A N G

Yuan-cheng 2

l. College of Electronics Information. W u h a n University, Wuhan 430072. China 2. Department of Physics. W u h a n University. Wuhan 430072, China

Abstract : An all-optical all-fiber optical bistability operation has been realized in an all-fiber cavity consisted with Er-doped fiber and optical fiber-loop mirrors. The experimental bistability threshold is consistent with the theory.

Key words: optical nonlinearity; optical bistability; Er-doped fiber; all*fiber device CLC number : O 437 Document code : A

0

Introduction

As is well known, optical bistability device (OBD) is a broad applicable device with flexbility. It may be used for severial optical signal processing functions such as optical amplification, wave reshaping, regeneration, and etc [1-42. Recently, as a nonlinear optical material, optical fiber is more and more noteceable. Using optical fiber as the optical medium of OBD, the necessary threshold optical power may be low owing to its extra thin and long geometry. Fiber optical bistability (FOB)operated firstly in 1983ES3,but the threshold power is in the magitude order of 100 W in order that common optical fiber and a couple of bulk optical mirrors are used in the experiment. It is known that the Kerr coefficient n2 of Er-doped optical fiber is higher in 4 magnitude orders then common fiber at 514.5 nm wavelength of Ar-laser~6i,so the threshold power must be low correspondingly;besides,owing to the well known resonance enhancement effect ,the optical nonlinearity of Er-doped fiber at 1.55 ,um wavelength must be higher because this wavelength is just the most absorption peak of Er-ion. So that the threshold of an Er-doped FOBD may reach the level that is used in an optical communication sys-

tern.

Severial kinds of all-fiber optical cavity with Er-doped fiber have been proposed and studied c73, all of their threshold powers may be less than a milliwatt. In this letter, a 1.55/~m wavelength operation of Er-doped FOB using optical loolDmirrors is reported. An accompanied pulse compression with wave-reshaping can be relized simultaneouslyE83 when an OBD amplifies an optical digital signal. Certainly this effect is most interesting for optical fiber communications: amplitude amplification compensates for the attenuation due to fiber absorption, while pulse compression and wave-reshaping eliminate the pulse broadening and distortion caused by the fiber dispersion, i. e., the OBD' s amplification is naturely a digital signal amplification. Here the fiber OBD is an all-fiber device furthermore, so it is possible to expect that some kind of "quasi-one-dimentional"(along fiber) integrate optical device, which integrete all-fiber laser, OBD amplifier, OBD regenerator, and etc. on a segment of optical fiber to genarate, amplify, reshape, and regenerate an optical digital signal in very high speed and large capacity, without any low-speed electrical intermediate processing. In other words, this is a really all-optical communica-

Received date: 199!I 05-12 * Foundation item: Supported by the National Natural Science Foundation of China(69487007) Biography:SONC; Qian (1937).female. Professor. Research direction :nonlinear optics and all-optical communication.

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Wuhan University Journal o f Natural Sciences

tion system with an all-optical all-fiber direct repeater.

I

Principle

Vol. 4

E(1 - - 2 k z ) z ( 1 - - 7 z ) z] Icu(z) =4IT(1 - k z ) k z e x p [ - - a ( z - L ) ] / (1 - - 2 k z ) z

(9) (10)

where the Eqs. ( t ) . ( 2 ) have been used. FM2

FM~

Fig. 1 is an all-fiber optical cavity consisted of a couple of fiber loop mirrors and a segment of Erdoped optical fiber with length L. The fiber loop mirror F M , ( i = 1,2) is made of an optical directional coupler and a short segment of optical fiber. Let the coupling coefficient, loss excess, and the length of the fiber of the ith mirror are respectively ki,)'i and l , The intensity reflectivity and transmittance are [73 : Ri = 4k, ( 1 - ki ) (1 - 7~) Zexp( -- 2eli) for i = 1,2 (1) Ti = (1 - 2k,)2(1 - Y/)%xp(- 2eli) for i = 1,2 (2) respectively, where a is the amplitude absorptive coefficient of the fiber. As a Fabry-Perot optical cavity with bulk optical mirrors, this fiber-loop mirror cavity (FMC) is a standing-wave resonance cavity, and has a similar modulation characterastic: TM(~P) ~ Ir/I1 -= To/{1 + Fsin2[(r - n)/2]} = 2pL (3) where I1,I-r and ~ are the input and transmissive intensity, and the round-trip phase shift of the cavity respectively. Here rt is corresponding to the phase lag undergone in the two fiber couplers. The peak transmittance To and fineness factor F of the cavity are To = T l T 2 e x p ( - 2eL)~(1 - R ) 2 (4) F = 4R/(1 -- R) 2 (5) where R is effective refrectivety of the cavity mirrors : R = (RiRz)l/Zexp( - 2eL) (6) Let z be the coordinate along the Er-doped fiber as shown in Fig. 1. Noting the second fiberloop mirror F M z ( z = L ) , for the forward and backward travelling waves L ~ ( z ) and Icb(z) in the cavity we have Ia(L) =IT/T 2 (7) Icb(L) = I c f ( L ) R 2 (8) Thus the two travelling wave in the fiber cavity are I a ( z ) = IT exp[-2a(L + lz -- z ) ] / 9

kl

Yl

k2

|

o Fig. l

)'2 i

..

L Z OBI) with fiber mirror cavity

Under nonlinear condition, the nonlinear phase shift at a little segment dz of the Er-doped fiber is d~L = ( 2 n / X ) n z I r (11) where nz is the Kerr coefficient of the core of the Er-doped fiber, ,~ is the wavelength of the light source, and I t ( z ) = Icf(z) + Icb(Z) (12) is the total intensity in the cavity. The Er-doped fiber used in the experiment is made by the Optical Fiber Section, Wuhan Research Institute of Posts and Telecom, the MPT of China. Its intensity absorptive coefficient is 2a=1.06 dB/m and may be regards as a constant in the experimental conditions in accordance with our experimental measurment. An integretion of Eq. (11) from z = 0 to z = L introduces a total nonlinear phase shift in the cavity 7~L =-~0-- % = 2(2nnz/2)

f, o

[Ir

+ Ir

= 2 n n z l x [ e x p ( 2 a L ) -- 13(1 + R z ) / ( a a T 2 )

(13) where the nonlinear phase shift and loss in the two short segments of fiber in FMz are neglected. In Eq. (13) % = (M2n)noL (14) is initial (low intensity) phase shift and no is refrective index of the core of the Er-doped fiber at low intensity. A feedback charecteristic Tr(~) = Ir/I1 = K ( ~ o - ~ ) (15) may be obtained from Eq. (13), where K = 2 a T z e x p ( - 2aL)/{27rnz[1 - e x p ( - 2aL)] (1 + Rz)I1} (16) The feedback characteristic[-Eq. (15)3 is a cluster of stright line shot from the initial phase point with a slope K which is inversely propertion-

No. 4

S()NG Qian

et

al: 1.55 ,um Wavelength Operation of'..

al to the input intensity I~. The nonlinear all-fiber optical cavity is described by the modulation characteristic [Eq. (3)] and the feedback characteristic simultaneously. i . e . , the steady states in the system are decided by all the intersection points of the two characteristics. From this there is no difficulty to understand the formation of bistability hysteresis. The minimum threshold intensity Lh,, is corresponding to the maximum slope Km of the modulation characteristic. lthm = 2 a T 2 e x p ( - 2ag)/{2~n211 - exp(-- 2ag)] X (1 + Re)K,,} (17) It is easy to know K m = ToF[X(1 - X)]~/z/(1 ~- F X ) 2 (18) with X = sin2G, = {(3F + 2) - [(31: ~ 2) z -- 8F]~/2}/(4F) (19) is the coordinlate of the point where the second differential of modulation characteristic is zero.

2

Experiment

On the basis of our prior experimental works on OBD '--8-112 an Er-doped fiber optical bistability experiment has been completed. The experiment setup is shown in Fig. 2. The FMC is an asymmetric cavity with mirror parameters Rt = 0. 77, T I = 0 . 18,R2--0. 87 and 2"2=0.08. It is constructed from 0.80 m single-mode Er-doped fiber F and two fiber-loop mirrors whose fiber directional couplers were fabricated by Wuhan Research Institute of Posts and Telecom, the MPT of China. The Erdoped fiber in the cavity coils several times around each of two perpendicular piezoelectrical cylinders whose ends have been plated with electrodes to form a piezoelectrical transducer PT, so that a variable direct current voltage may be apply across the electrodes to change the length I. of the Erdoped fiber in the cavity and fine tune the initial phase shift ~ of Eq. (14), then some hysteresises with different width can be obtained, tt is abvious that an one-wavelength-variety of L is just enough to change % over 27r. In Fig. 2, S is the source. It is a multiple quantum well ( M Q W ) distribution feedback

437

(DFB) single-mode semiconductor laser made by the Semiconductor Institute of Chinese Academy of Sciences, and has a central wavelength 1 552.6 nm at 20 C and linewidth 10 MHz, ISO is a fiber isolator. In order to observate hysteresises of the cavity H~C

@

t'i~. 2

Experimental setup

it is necessary that the intensity of the input beam must change between 0 to a value beyond the threshold. Thus an electrical triangle wave generator TWG is used as a driven source to modulate the intensity. This modulated beam is then splited into two beams by a Y-type fiber coupler C. The stronger one is coupled into the FMC cavity as the input light of the cavity; the weaker one is fed to a photoelectrical detector D1 to transform into an electrical signal. It is propertional to the input intensity and sent to X input terminal of a KIKUSUI 5021 double-trace oscillascope OSC as a scanning signal horizontally. The output optical signal from the nonlinear FMC cavity is sent to another detector Dz. After amplification by an amplifier A this electical signal, which is propertional to the transmissive intensity of the cavity, is fed to the Y input terminal of the oscillascope OSC. Adjusting the direct current voltage to set some diffirent initial phase %, some experimental hysteresises with different width and different threshold powers can be obtained. Four experimental oscillascope photographs are shown in Fig. 3. They are corresponding to bistability threshold powers 0.08, 0. 12, 0.17 and 0.21 mW respectively. The calculated minimum threshold power for this device is 0. 12 mW according to Ref. [7] where nz value at 514. 5 nm as in Ref. [ 6 ] is used . Threshold of any hysteresis with nonvanished width is appropriately larger than this minimum ,so the above observational results show that n2 value in our experiment is more than that in Ref. [6] and accords with our measurment result in Ref. [12]. An obvious resonance enhancement effect may be

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Wuhan University ,lournal o f Natural Sciences

due to the fact that the wavelength 1 552.6 nm of the light source is near the absorptive peak wavelength 1 560. 0 nm of the Er-doped fiber used in the experiment. 3

Conclusion Er-doped fiber optical bistability has been re-

Fig. 3

References

Vol. 4

lized successfully using optical fiber-loop mirrors construction. The experimental results is consistant with the theory. The bistability threshold power is in the magnitude order of mW, far lower then the experiment value in Ref. ES~ where a common optical fiber and bulk optical mirrors was used. The observational response time is short than 1 ~ts.

Expertmental oscilloscope photographs

9

[-1] Venkateson T. All-optical data switching in an optical fiber link using a GaAs OBD[J]. Opt I.ett,1984.9 (7) : 279. [2! Tooley F A P. High gain signal amplification in InSb transphaser at 77 KEJ-]. Appl Phys Lett, 1983o43 (9) :807. [3] HE J. Ail optical bistable switching and signal regeneration in semiconductor layered distributive feedback/FabryPerot structure [J-]. Appl Phy.~ Lett, 1993.63(7 ) : 886. [4] Jaczuet J. 2.5 G bit/s optical triggering with signal reshaping using bistable laser[(;]. ECOC'93 European Conference on Optical Communication Proceedings. 1993. 2: 293. [5] Nakatsuka H. Asaka S, hob H, et al. Observation of bifueation to chaos in an all-optical bistable system i-J]. Phys Rev Lett, 1993,50(2) : 109. [6] Betts R A, Gibbs H M. Nonlinear refrective index in

Eubilum doped fiber : theory and experiment [J]. 1EEE J Quan Elect, 1993,QE-27(4):908. [7] ZHANG Yuan-cheng , SONG Qian. Nonliner optical fiber resonator: optical fiber bistability[-J-]. Wuhan University .Journal o f Natural Sciences, 1996,1 (2) : 187. [8-] ZHANG Yuan-cheng, SONG Qian, Optical signal amplificalion and processing using semiconductor laser diode ()BD[-J~. Chin .I I.a.~ , 1993,A20(7):805. [9-] ZHANG Yuan-cheng, SONG Qian. Optical pulse signal amplification with an optical bislable device[J]. Chin Phys /.as, 1988,15(6) :456. [-10] SONG Qian, ZHANG Yuan-cheng. Nonlinear optical fiber loop: 2. All-fiber oplical bistability experiment [JA. Acta Opt Sin, 1977,17(7): 855. E11] SONG Qian, ZHANG Yuan-cheng, I.I Ji-xing, et al. Optical bistability of Er-doped fiber under Ar-laser irradiation[J]. Chin d Las,1996,A23(8):219. [-12n. SONG Qian, ZHANG Yuan-eheng,LI Ji-xing. Dispersive nonlinearity in Er-doped optical fiber EJ]Wuhan Uviver~ity Journal o f Natural Sciences, 1999, 4(3) : 307.