Journal of Optoelectronics and Biomedical Materials Volume 1, Issue 1, March 2009, p

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1 Journal of Otoelectronics and iomedical Materials Volume 1, Issue 1, March 009, Analysis of a novel stimulated rillouin scattering suression mechanism rough self hase modulation rocess in e high ower short ulse fiber amlifier ZHO LEI *, NING JIING, CHEN CHENG, HAN QN, ZHANG WEIYI, WANG JNTAO College of recision Instrument & Otoelectronics Eng.Tianjin niv.china Otoelectronic Information Science &Technology Lab The automatic Stimulated rillouin Scattering (SS) suression mechanism in low reetition rate, nanosecond level ulsed EDFA and assive fiber is analyzed in is aer.o based on e Self hase Modulation (SM) caused frequency sectrum broadening which will deease e SS gain and inease e SS reshold ower in e main ower amlifier stage of e Master Oscillator ower-amlifier configuration. sing e Slit ste Fourier and Local error meod (SSFM_LEM) to solve e modified nonlinear Shroedinger equation (NLSE) and our analysis data show at is meod will suress e SS ectively. (Received Aril 5, 009; acceted May 6, 009) Keywords: Stimulated rillouin Scattering; Self hase Modulation; Master Oscillator ower-amlifier; Slit ste Fourier; Fiber ulse amlifier 1. Introduction The high ower fiber short ulse amlifier wi high eak ower and narrow ulse wid have been develoed and researched widely for its imortant alications in many fields, for instance: laser radar, laser cutting, and sace laser communication [1, ]. The high eak ower in e fiber ulse amlifier may lead to many nonlinear henomena in e fiber such as SS, SRS, and SM. Among all e above, e SS ect will be e first and easily simulated for it has e lowest reshold ower, once e SS occurred, it will transfer e energy from e signal to e backward stokes wave and consume e inverted oulation of e gain medium [3]. So it restrains e ulse energy and deeases e outut signal ower significantly, in addition SS can cause e ulse distortion in e time domain, leading to e amlifier erformance degradation [4]. As a result, e suression of SS becomes e main limiting factor esecially in e high ower ulse amlifier and e suression of SS is e main concern in e system design of e high ower ulsed fiber amlifier. The leading framework of high ower fiber ulse amlifier called MOA (Master Oscillator ower-amlifier) as deicted in Fig.1, while e Master Oscillator art often include a seed laser source and one or two stage EDFA as e reamlifiers (Stage 1) to boost e ower to several hundred milliwatt level, en e signal will go into e main ower-amlifier art (Stage ) to obtain e required ower and ulse energy.

2 158 Zhou Lei *, Ning Jiing, Chen Cheng, Han Qun, Zhang Weiyi, Wang Juntao Stage 1 Stage Outut Seed reamlifiers ower amlifier Fig.1 High ower fiber amlifier based on MOA configuration There has been several ective choices to solve is roblem: ere will be considerable ermal gradients caused by heat generation because of e high um ower, is ermal gradients will broaden e rillouin gain and inease e SS reshold seriously [5-6] ; by alying different temerature distributions along e fiber to change e rillouin frequency downshift [7] or choosing e backward um scheme for e fiber leng exerienced by high ower ulses is significantly shorter [8] In is aer, we resent a novel automatic SS suression mechanism in e EDFA stage and e deeer suression in e assive fiber bo based on e SM caused frequency sectrum broadening. According to e nonlinear fiber otics eory, The SM will always occur as e otical ulse transmits along e fiber, e main ect of SM is to broaden e frequency sectrum of e otical ulse, and e broadening extent is decided by e ulse eak ower and fiber leng. First, we use e otimized Slit Ste Fourier Meod (SSFM) wi error control meod to analyze e variation of e ulse frequency sectrum in e EDFA by solving e modified NLSE wi Fast Fourier Transform algorim (FFT);en we analyze e furer sectrum broaden in e assive fiber by solving e NLSE;at last we select e tyical double clad Er-Yb co-doed fiber arameters to resent e SS suression extent and show at is meod is a useful and ective way to suress e SS when design e high ower low reetition rate (10kb/s-30kb/s) ulsed amlifier.. Theory mechanism As e seed otical ulse go rough e EDFA stage e SM ect will occur, in addition, ulse signal wi narrow ulse wid (ns) has a relative high eak ower an e continuous signal, so e SM will cause frequency sectrum broaden greatly and become useful to suress e SS ect at may haen in e next ower-amlifier stage for at e SS gain coicient deends on e signal frequency sectrum wid. The well-known SS gain coicient can be desibed by e following: [9] ' 1 g = g 1 + Δυ / Δυ (1) Where ' g is e maximum gain which is of e order of 5 Δυ m/w, is e uming light frequency sectrum wid and Δ υ is e intrinsic linewid of e rillouin rocess range from 0MHz to 100MHz decided by e different fiber [10]. SS has e reshold character, e estimate equation for e reshold inut ower is g L / A 19 (), and e factor 19 is used to relace e used aroximated factor 1, because modern fibers have much lower loss an e fibers at at time [11].

3 Analysis of a novel stimulated brillouin scattering suression mechanism rough self hase modulation rocess 159 Where is e eak ower of e um light, A is e ective area, in a single mode,sted-index fiber, e field of e fundamental mode can be aroximated by a Gaussian function wi e beam radius, so A = πw ( λ), w( λ) is e mode field diameter(mfd)of e fiber at e waveleng λ [1]. L is e ective fiber leng, where L = ( 1 ex( α l)) / α,l is e fiber leng,α is e fiber loss. Substitute Equ. (1) Into Equ. (), we obtained e equation for e reshold inut ower = 19 A [1 + Δ v ( g L ) / Δ v ] (3) When e um light is olarization Indeendent totally e will inease 50% [13], namely: 8.5 A [1 + Δ v / Δ v ] = (4) ( g L ) We can get e conclusion at if we can broaden e um light linewid roerly, e will inease to a considerable level and suress e SS ect. The common meod to inease e Δ υ is to add anoer high frequency carrier to inease e RF comonent oft e signal [14] or hase modulation using e additional hase modulator [15]. The main concet is to broaden e frequency sectrum of e signal and distribute e high eak ower to a relative wider frequency sectrum, but ey all have to use e extra energy inut wi e signal. Since e SM ect can broaden e frequency sectrum automatically when e signal ulse transfers along e fiber, so it can suress SS itself when we choose e otimal fiber leng wiout e extra energy. The SM ect in e EDFA transient model can be desibed by e modified nonlinear Shroedinger equation [16] : z = α i β + i8 / 9 G ( z ) γ T (5) Where = A( z, T) / is e normalized otical field, is e otical ulse eak ower,γ is fiber nonlinear coicient.,α is fiber absortion coicient. G(z) is e EDFA gain along e fiber leng z. 3. Simulation and Analysis Fig. is e EDFA transient gain versus EDF leng using e Otisystem simulation environment : using e arameter EDF C600 roduced by e Coractive comany ;bidirectional uming configuration wi 00 mw and300mw uming ower at each end, signal waveleng is 1560nm,e signal ulse wid is 10ns, reetition rate 10kb/s and 30kb/s Gaussian ulse signal resectively. We can get e arameter in Equ. (5) G (z) According to e Fig..en we use e traditional and ective Slit ste Fourier meod (SSFM) to solve e Equ.(5), furer more and use e advanced ste control meod called Local Error Meod (LEM) [17] for e sake of imroving e recision and e seed of e SSFM. Finally, we use e FFT algorim to analyze e variation of e frequency sectrum.

4 160 Zhou Lei *, Ning Jiing, Chen Cheng, Han Qun, Zhang Weiyi, Wang Juntao Fig. e EDFA transient gain versus EDF leng If we don t consider e frequency sectrum broadening, e at e entry ort of e main ower amlifier stage is about 4w according to e Equ. (4) sing e double clad fiber arameter: A =707um, L=8m, L =7.68m, Δ υ =0MHz, Δ υ =100MHz. Now if we consider e SM ect in e EDFA, Fig.3 is e eak ower variety of e mw average ower, 10kb/s, 10ns ulse wid inut signal versus e fiber leng. We choose e fiber leng 0m, e eak ower inease from e origin 0w to e 950w when e signal asses along e EDFA, and e signal frequency sectrum will broaden to 1.4GHz, leading to e inease to 8.79KW accordingly. Comared to e 4W above, e greatly. has been boosted Fig. 3. eak ower variety versus EDF leng Furer more, if e ulse ower is too high, and e SM ect caused frequency sectrum broadening during e EDFA stage suress e SS is not enough.we can add anoer standard single mode assive fiber between e first EDFA stage and e second double clad fiber stage to suress e SS deely which will occur in e next high ower amlifier stage. Since

5 Analysis of a novel stimulated brillouin scattering suression mechanism rough self hase modulation rocess 161 e otical ulse signal doesn t get e gain when it transfers along e assive fiber, we only considers e SM ect, we can use e NLSE wiout e gain arameter: z = α i β T + i γ (6) Also using e SSFM_LEM to solve e Equ. (6), and FFT to analyze e frequency sectrum broadening similarly as above. Otical ulse signal waveleng 1560nm, e stand single mode fiber arameter we used: g = m/w. γ = (1/W.m) e eak ower from e EDFA is 950W,Fig.4 is e frequency sectrum broadening against e fiber leng and Fig.5 is e waveleng broadening accordingly. Fig. 4 signal frequency sectrum broadening against e fiber leng Fig.5 e waveleng broadening versus fiber leng

6 16 Zhou Lei *, Ning Jiing, Chen Cheng, Han Qun, Zhang Weiyi, Wang Juntao Fig. 6 SS ineasing versus fiber leng We can see from Fig.4 at e frequency sectrum has broadened from 3.5GHz to 18GHz as e assive fiber leng varied between 0m and 100m. At e same time, e signal waveleng only broadens from 0.05nm to 0.145nm in Fig.5; is minute change in e waveleng scale can be neglectable in many alication environments. ut we can see from Fig.6 at as e uming light frequency sectrum broadens along e fiber e SS has ineased tremendously from 0KW to 110KW (olarization indeendent) and 13KW to 70KW (olarization comletely).is high level SS reshold ower can suress e SS ect at will haen in e high ower amlifier stage sufficiently. Even if e ulse eak ower is very high, such high SS ect reshold ower can also rohibit it from stimulating or at least restrain e SS ower in an accetable extent and won t influence e erformance of e amlifier. Just as e otical ulse signal transfer along e EDFA e frequency sectrum will be broadened, when e signal transfer in e main ower amlification stage, its eak ower is becoming higher and higher,and e frequency sectrum will be broaden again also due to e SM ect as e double clad fiber leng inease, leading to e SS reshold ower inease more, as a result,e SS reshold ower inease automatically as well, leading to e automatic SS suression again, if we select e fiber arameter carefully, e SS in e high ower ulse amlifier may be suressed absolutely. Table 1. Simulation arameters Signal Gaussian ulse wid 10ns eak ower 0w Reetition rate 100kHz rillouin intrinsic linewid 0MHz Signal bandwid(1) 0MHz Signal bandwid() 1.5GHz Er-Yb co-doed Double clad fiber leng 10m Forward um ower at 915nm 100w Core diameter 10 μ m Ytterbium concentration.685e 6 irons/m 3 Erbium concentration 3.08e 5 irons/m 3

7 Analysis of a novel stimulated brillouin scattering suression mechanism rough self hase modulation rocess 163 We simulated e high ower amlification account for e transient gain in e Er-Yb co-doed double clad fiber(dcf),e main DCF and e system arameters we used were showed in Table 1.e model which included e gain and e stokes wave ower evolution we selected is according to [3]. Fig. 7 Initial Gaussian ulse The initial inut Gaussian ulse wi 0w eak ower is showed in Fig.7, when we assuming its bandwid is 0MHz, after it is amlified by e double clad fiber, e eak ower is about 4kW as in Fig.8 (a), next, we consider e bandwid is 1.5GHz, e eak ower after amlifier is 8kW as in Fig.8 (b), e eak ower ineased very much as e sectrum broadening, namely, signal wi 1.5GHz, ineased e SS reshold tremendously. a b Fig.8 (a) outut Gaussian ulse wi bandwid 0MHz (b) outut Gaussian ulse wi bandwid 1.5GHz 4. Conclusion We have studied e novel SS suression mechanism in high ower ulsed fiber

8 164 Zhou Lei *, Ning Jiing, Chen Cheng, Han Qun, Zhang Weiyi, Wang Juntao amlifier based on e SM caused frequency broadening. y solving e modified NLSE in e EDFA and NLSE in e assive fiber using e SSFM_LEM.Our simulation results show at is meod will inease e in e ower amlification stage of e MOA configuration to dozens of kilowatt ough choosing e roer fiber leng, such high level SS reshold ower can suress e SS sufficiently even comletely. We use a model included e gain and SS ect in e double clad fiber to simulated e Gaussian ulse amlifier, e result showed e sectrum broadening will inease e eak ower greatly. Carefully selected e fiber arameters is meod will lead to a very simle and easy way to suress e SS in e high ower ulse amlifier. References [1] M. W. Wtight, G. C. Valley, J. Lightwave Technol., 3, (005). [] V.hiliov, C. Codemard, Y.Jeong, J.Ot.Lett., 9, (004) [3] Naan A.riliant, J.Ot.Soc.Am., 19, (00) [4] G.Kulcsar, Y.Jaouen, G.Canat. IEEE hoton.technol.lett., 15, (003) [5] V. I. Kovalev, R. G. Harrison, Ot.Lett.31, 161(006). [6] Y. Jeong, J. K. Sahu, D.. S. Soh, C. A. Codemard, J.Nilsson, Ot.Lett. 30, 997(005) [7] J. Hansryd, F. Dross, M. Westlund,. A. Andrekson, Member, IEEE, S.N.Knudsen, J.Lightwave Technol., 19, 1691(001). [8] Yong Wang, Member, IEEE, and Hong o,member, OSA, J.Lightwave Technol. 1,69(003) [9] E.Lichtman, A.A.Friesem, Ot.Commun. 65, (1987) [10] Nikes M., Thevenaz L. and Robert. A. J.Lightwave Technol. 15, (1997) [11] C. C. Lee and S. Chi, IEEE hoton. Technol. Lett. 1, 67, (000) [1] T.Schneider, Nonlinear Otics in Telecommunications, 98, (004) [13] G..Agrawal, Nonlinear Fiber Otics, Third Edition & Alications of Nonlinear Fiber Otics. [14] Norcia S,Tonda-Goldstein S, Dolfi D,et al. ECOC00 [C].Coenhagen, enmark, COM, (00) [15] Takashi Kotanigawa OFC[C] (004.) [16] C.R.MENYK. Journal of Engineering Maematics, 36, (1999) [17] Oleg V.Sinkin. J.Lightwave Technol. 1, 61-68(003.) * Corresonding auor: zhoulei_tju@tju.edu.cn