High-Power Laser Beam Transfer through Optical Relay Fibers for a Laser Guide Adaptive Optics System

摘要

Wearedevelopingalaserguidestaradaptiveoptics(LGSAO)systemfortheSubaruTelescopeonMaunaKea,Hawaii.TheAO188systemwilldramaticallyincreasetheobservableskyarea.Thissystemdiffersfromsystemsusedatotherlargetelescopesinthatitutilizesthecombinationofanall-solid-statemode-lockedsum-frequencygeneration(SFG)laserasalightsourceandsingle-modeopticalfiberforbeamtransference.Opticalfiberstransferlaserlightfromthesource,locatedattheNasmythplatform,tothelaserlaunchingtelescopemoreflexiblyandmoreeasilythandomirrorrelayoptics.However,opticalfibersinducenonlinearscatteringeffects,suchasstimulatedRamanscattering(SRS)andstimulatedBrillouinscattering(SBS),beyondcertainthresholdlevelsinhigh-powerlasers.Wemeasuredthelasertransmissioncharacteristicsofaphotoniccrystalfiber(PCF)whosemodefielddiameter(MFD)was11$$mu$$m,andastepindexfiber(SIF)cablewhoseMFDwas4.2$$mu$$mtoevaluatethethresholdlevelsfornon-lineareffects.WeobservedSRSinthe200-m-longSIFwhenweinput1.3a€‰W.However,SRSandSBSwerenotinducedinthe200-m-longPCF,evenforaninputpowerof5.3a€‰W.Asaresult,weestimatedthethresholdofSRStobe33a€‰Wforthe35-m-longPCFdesignedfortheSubaruLGSAOsystem.Giventhatthefiberwillcarryalaserbeamofabout30a€‰Wwithapulsewidthoflessthan1ns,weconcludethatnonlinearscatteringwillposenoproblemsforthisapplication.