光电子课件Lesson_5

3 18 2004 Q J Zhang 1 ReviewofLastLesson Example Nd YAGLaserOscillatorLongitudinalandTransverseModesandModeSelection 3 18 2004 Q J Zhang 2 OverviewofThisLesson Fabry PerotResonatorSpikingandRelaxationOscillations 3 18 2004 Q J Zhang 3 Fabry PerotResonators Applications EssentialelementofthelaserModeselectionCanbeusedasaninstrumenttomeasurelaserlinewidth2 Basicequations Phasedifference When When 3 18 2004 Q J Zhang 4 Finesse Forsmall FreeSpectralRange 3 18 2004 Q J Zhang 5 3 Example 3 18 2004 Q J Zhang 6 4 Gaussiantransversemodefrequencies Transversemodefrequency Alltheeven symmetrytransversemodesareexactlydegenerateattheaxialfrequencies Alltheodd symmetrytransversemodesareexactlydegenerateatthe half axial positionsmidwaybetweentheaxialmodes TransversemodefrequenciesinvariousstableGaussianresonators ConfocalCavity 3 18 2004 Q J Zhang 7 ScanningFabry PerotInterferometers PracticalDifficulties Misalignmentorimpropermode matchwillexcitedifferenthigherordertransversemodes causingmultiplespuriousorunwantedfrequencycomponentsSolution Carefulalignmentandmodematch adelicateanddifficulttask 3 18 2004 Q J Zhang 8 ScanningFabry PerotInterferometer ConfocalCavity ConfocalCavity Advantages Allthehigher ordertransversemodesareexactlydegenerateinfrequencyNomodematchneeded inputbeamcanbemisalignedandmismatchedDisadvantage FSRisreducedinhalf 3 18 2004 Q J Zhang 9 Mode Match Purpose toexciteoneofthetransversemodesofthecavity usuallythelowest ordertransversemode TEM00 TEM00 mosthighlyconfined lowestleakage anddiffractionlossesHigherordertransversemodes shiftedresonances higherdiffractionlosses lowerQ Curvedmirrors welldefined well behaved low losstransversemodes HermiteGaussian dependsoncurvaturesandmirrorspacing Planarmirror moreirregulartransversemodes non gaussian largerdiffractionlossesorpowerleakages dependsonthesizeandshapeofthemirror 3 18 2004 Q J Zhang 10 5 SpectralCharacteristicsofLaserOutput Forkaxialmodes thebandwidthbetweenthetwoextrememodesis Forsinglemodeoperation thelaserbandwidthis 3 18 2004 Q J Zhang 11 Schematicofoutputofalaseroperatingattwo three andnlongitudinalmodes 3 18 2004 Q J Zhang 12 TwoLongitudinalModes NLongitudinalModes Inmanyapplications highspectralpurityisrequired theimportanceofsingle modeoperation Temporalcoherence thepathlengthdifferenceoverwhichtheradiationcanstillinterferewithitselfsingle mode Coherencelength Example Forsingle moderubylaser 3 18 2004 Q J Zhang 13 6 AxialModeControl Interferometermodeselectiona Resonantreflectors Fabry Perotresonatorwithfixedspacing Etalon 3 18 2004 Q J Zhang 14 Analysisofathree plateresonantreflector 3 18 2004 Q J Zhang 15 IntracavityResonanceUsinglaserrodasanetalonc IntracavitytiltedetalonDecouplestheinternaltransmissionetalonfromtheresonatorEtalonactsasabandpasstransmissionfilterDisadvantage needdielectriccoatingsdamagethresholdUsedmainlyinCWlasers EnhancementofLongitudinalModeSelectionCoolingoftheactivemedium reducethelinewidthofgainmediumShorteningoftheopticalresonatorLengtheningofthepulsebuilduptimeReductionofspatialholeburningTravelingwaveringcavityGeneratingcircularlypolarizedlightinthelaserrodMechanicalmotionElectro opticphasemodulation 3 18 2004 Q J Zhang 16 SpikingandRelaxationOscillations Spiking discrete sharp largeamplitudepulsesthattypicallyoccurduringtheinitialturnonphaseofthelaserRelaxationOscillations small amplitude quasi sinusoidal exponentiallydampedoscillationsaboutthesteady stateamplitudewhichoccurwhenacontinuouslyoperatinglaserisslightlydisturbed orintowhichtheinitialspikingbehaviorgenerallyevolve 1 Thedefinitions 2 Therubylaser Spikingbehaviorinthefirstrubylaser Rubylaserspikingonanexpandedtime scale 3 18 2004 Q J Zhang 17 3 Nd YAGlaser Spikingturn onbehaviorinthreetypicalNd YAGlasers 3 18 2004 Q J Zhang 18 LaserRateEquation Chronologyofasinglelaserspiking Nonlinearequations 3 18 2004 Q J Zhang 19 RelaxationOscillation Intherelaxationoscillationregime 3 18 2004 Q J Zhang 20 a NonspikingLasers b StrongSpikingLasers 2 100us atomicdecayrate ismuchslowerthan c 100ns cavitydecayrate 3 18 2004 Q J Zhang 21 Examples 1 Nd YAGLaser pumpedto50 abovethreshold r 1 5 Conditionforspikingandrelaxationoscillation 3 18 2004 Q J Zhang 22 Summary SpikingisingeneralmoreofanuisancethanausefulphenomenonItdoesprovideanillustrationofthevalidityoftherateequationsoflaseroscillatorsMeasurementof sp calculateSpikingindiodelaser generatesingleshortpulse 100ps 3 18 2004 Q J Zhang 23 Homework 1 ToestimatetheeffectofsurfacequalityonaFabry Perotinterferometers consideraninerferometerthathasan