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5:00pm-5:15pmWY5 Accurate fhst wavelength switching of :in erbium-doped fiber laserwith a Vabry-Perotsemiconductor fi1ic.I

Past wavclcngth tuning in crhium-doped tihcr iilser isveiy important l o r sonlc applications such :is inrccoiikigurablc Iiigh-siicctl w;iveleiigtli-divisi~rii-niiiltiplcxctl systems ;uid rictwoiks. Ilcci~osc theIccoveiy time o f th e excited-state poptilation ii i ancrhium-tlopctl fiticr is sulistciiitially longel. tlmi theplioioii liictirnc i i i tlic cavity, rclaxaticrn oscillationsultiinatcly limit the tuning speed of the laser [ 1,21.Ihercforc,a fast wavelength-switcliiiig spccd requiresn o t only rapid tuning of the cavity l o s s Iiut alsosuplrrcssion 01 thr, relaxation oscillations. Hy using aiacousio-optic tun;ihlc liltcr tu pirwidc active fecdliackt o suppress th e rclaxation oscillations, ii wiivclength-tuning spccd of tlic order ( 1 1 ens 01 niicrosccmtls hashccn echievcd 111. An all-optical passive techniqueliascd on dam pin g the rclaxatiori riscillalions with anonlinear f ilm ainplillci loop niiimr has also hccnreported LZJ . In this papcI, we report a r iwel approachto suppress the relaxation oscillations and, Iicnce,achicvc fast wavclcngth switching.

,\c drtvrFig.1. l k p e r i r i r e i r ~ drenip of tire wovelengrl r~ t l rn rrb lp erhiir~ir-d o p ed J i b e r lo.se,: IC: polmimtion cor i tm l le r : WLIM:j i b e , tl,rrgg grittirrgs.~80,~,n/l .550,~,,1~ t r v e l e , l j i l i l ~ d i v i . ~ i , , , ~, it lit il, iexer, mm, r t i m

lhc configurati on (i f tlic fiber laser is shown inFig.1. A standing cavity was k l with a lahry-Pcrot scniiconductor tilter (PPSl;) and two cascaded1BGs (FBGI and FBGZ). llie FBGs Iiad ii Braggwavelength o f 1552.5nr11, bandwidth of 0.3nni, an da peak rcflectivity of >YY%> The Bragg w;ivelerigthsof the FUG, could ha adjusted independently todcsircd values by applying suitable straiii to them. A1.5pin laser diode, o[ated Irelow its threshoki(-3RmA), was iiscd as the FPSF:.Lhc nicrde siiaciiig ofthe FPSF wiis -1 .hrini. A polarization controller (PC)w as p k l G C d in the cavity to control the polarizationstate of light injected into tlic PPSF.

Lhc principle of th e laser is a s follows. ?henarrow-lrandFHGs, each o i wliich ciin ac t as a cavitymin.or, sct the wavclcngth references for tlic laser.The lasing wavelength is selected froi~n tliescwavclcngths by tun ing the rcllection spectrum of hci;ISF, It is known that tlic rctlcction spectrum o l th e

I~ISt:can hc sliiftcd towards ii shoilcr w;ivclcngth byincreasing tlic iiijectioii currciit 131. Ilccanse tlicrefractive inclcx o f a sciniconductol. can be Iiiodulatotlby the injection current at ii high spccd ( o i th c ortlerof IGHz), ihc rctlcctiori spcctriirn t i l tlic FlSl; ciuialso 11c turicd iit i i high spcctl Iry c1i;uiging tlicinjection curmiit. Wlicn the w;ivclcngtli separations oFth e 1 M i s ir c clioscn p r ~ i l i ~ l yn relation to the niodcspacing [if the FPSi, the lasing wavclcngtli ciin tictuned lioni o m Rragg wavcicngth or th c 1;OC;s toanothcr Iiy tlic vernier ellcct. Figure 7 illusll-atcs tlicincchanisiii of wavclcngtli tuning Irctwccii tw nwavclcngths. As showii i i i Pig.2, when tlic ciiricntapplied to th e i1,Sl s I , , lic Ilragg wavclcngth o lFHGI, A i , falls within ii reflection peak of th e FISF,while that of PUG2, &, is neiir it rcllccti,on trough . 111that case, the cavity I(iss fo r A, s low. while that for& is high, atid the lascr emits at A l . By changing tlicinjection current to 12 ,however, the sitwltioiis lor tlicwavelengths A I a ~ ~ d iirc reversed, as shown i nFig.2. The lasing wavclcngtli c u i thus hc switchedfrom A, o &by changing the injection ctiircnt Iroiii /Ito 12 .

W v c l c ig ttiFig.2. Wnveiengtlr tiriiiiig Orised ON ( i i ig~i i t ig he 1-ejlert imi p r k s ,fthe PISl; 11, 111v Urrrfi,ywituelr irg l l is of [he f?lG.< once U lime hyr.lrrmgitig the itrjccriwi CIOIEN - t i re i~ r s in f i ovrle,rgtli is swirrhrti/ r o w dl lo d2 v h ~ ~ i rhe iiijeclion ciirrenl i s rwi l i ~ l re d / roni, (I h.

I:igurc 2 a p p to indicate that the liiscr emits atA, s long a s th e Bragg wavclciigth AI is within therange defined hy the points ;I ;ind a on areilection order of th c bISF..Siniilarly, the Iiisor emitsat A+ iis long ti s th e Uragg wavelength & is within thcfringe dctincd by the points 1 1 sn d b on anothcrreflection order rrf th e IISF. 111otlicr words, the liisorcould ripcuatc on cither side of :I rctlcctirin peak ol th eFPSI;, having Iiositivc or ncgativc s h p c s ,

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rcspectively. Howcvcr, wc claim that the laser isstable with suppresscd relaxation oscillations onlywhen it is operated on the positive slope of areflection peek of thc IjPSF; it becomes unstable whenoperated on the negative slope.When a FPSF is DC biased, an increase i n theintensity o f light injected into the FPSF can increasestimulated emission and thus reduce the cawierdensity (the carrier depletion effect) [4]. reductionin thc carrier density increases the refractive iiidex ofthe PPSF and causes the entire reflection spectrum toshift towards a longer wave lengt h, Whe n the laser isoperated on the positive slope of a reflection peak ofth e I'PSI', a shift in the reflection spcctrum of theFPSF towards a longer wavelength can decrease thereflectivity of the FPSF at the lasing wavelength andhencc increase the cavity l oss . An increase in theintensity of light injected into the FPSF can thus causean inci-ease in the cavity l oss , an d hencc a dccwase inthe light intensity in the cavity. l'his all-opticalnegative lecdback mechanism c i i n stabilize theintensity of light in the laser cavity. Because theresponse time of the refractive index to carrier densityin a senticonductor (typically -Ins) is much shorterthan the rclaxation oscillation periods of a fiber lascr(tens of microseconds), the negative feedbackmcchanism is expected to be effective for thesuppression of relaxation oscillations. On the otherhand, if the laser is operated on the uegative slop e of iireflection peak of the FPSF, positive fecdback on thecavity light intensity will be built up and the laser willbecome unstable. To confirm experimentally that therctlcction spectrum of th e FPSF can indced he shiftedby changing the intensity of the injected light, laserlight with. a spectral bandwidth -0. lnni at thewavelength 1549.7nm was injected into the FPSF.When the current applied to the FPSF was 26mA, thcwavelength of the FPSF mode increased by 0.24ntnwhen the injectcd light powci. was increased hornOtnW to 0.94rnW .Figure 3 shows the measured switching dynamicsof th e laser when the Bragg wavelengths of F BGl a n dPBG2 were set at Al=I552.O8nin and &=1554.25nin,respectively. l'hc laser was operated on the positiveslope of a reflection peak of the FPSF to ensure stableoperation. l'he FPSF was U C biased at l!l.61nA anddrivcn by a square w ave with 0.4V amplitudc. Whenthe current was switched from lY.6niA to 22.41nA,the lasing wavclcngth was switched from&=1554,25nm to I l=15 52.08n m. Thc cxpcrin ien ta lresults in Fig.3(a) show that relaxation oscillations atAI were almost completely eliminated with only amoderate o vershoot, A build-up time of 23.811s wasachieved for I,. he long dccay time for & of 2O.Ipswas due to the low finesse of the FPSF and hence thesinall difference in the cavity loss between the on an doff states. l'he output powcr of the laser at I, o r &was about 0.3mW . Switching of the lascr wavelengthfrom I, o & by changing the injection cunent from22.4mA to 19.6mA is shown in Fig.3(b) . Relaxation

oscillations at & were again suppresscd and cven theovershoot prcviously observed was elimina ted. Wcbelieve that the better switching-on perforinancc forA,was the result o i a stronger carrier-depletion cffcct(and hence a strongcr negative feedback) duc to thelarger injection current fo r &.

I F - 2 3 . 8 ~ ~

r

d !. . . . . . . . . . . .........................

-. 1 (iii)Driving 1-c tir ren.-. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .~;-I y r . . . -?'i i i i c ( I 0ps Id i v .)

Fig.3. Switcliinfi dynamics of /l ie loser with A,=ISS2.081itir wid A1=/554.25,1rn ivhetz llie loser is a%vilchrd u)froni h, In &; ar id (b)fi7m , IOL. urves ( i ) , (ii), and ( i i i ) ~ I I J W lr e /userO S I ~ I U I . ~if 1,o r i d A2 orid the i r i j ec t i on cu r ren r rg tlze W S P , revpedively.

In conclusion, we have demonstrated ii simplepassive method for realizing fast accurate wavelengthswitching in a fiber lascr. W e have shown that theoptical nonlinearity in the FPSF can suppressrclaxation oscillations effectively an d stabilize thelaser output. A clean wavelength switching in lcssthan 25ps has been achieved.Refercnccs:I .

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M. Y . Frankel, K.U. Esnian, and J. F. Weller, / IPhoton. Techriol. L e f t . ,6, 591)1994.Y. T, Chieng, G. J . Cowlc, IEEE Pliofun. Technol.Lett. , 7 , (485)1995.J. Maning, R. Olshansky, an d I:.. Su, Shari Wey,IEEEJ. Qiionfitm U e c f r o n . , 9, (1512)1983.S. Li , K. ?'. Chan, and C . Lou, IEEE Plroron.Technol. Lett. , 10, (1094)1998.616