Generating green to red light with semiconductor lasers

0.15 MB·English

by  Gabriele Ferrari

Generating green to red light with
semiconductor lasers
GabrieleFerrari
EuropeanLaboratoryforNonlinearSpectroscopy,IstitutoNazionaleFisicaNucleare,
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INFM-CNR,PoloScientifico-Universita` diFirenze,50019SestoFiorentino,Italy
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[email protected]fi.it
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Abstract: Diode lasers enable one to continuously cover the 730 to
a
J 1100nmrangeaswellasthe370to550nmrangebyfrequencydoubling,
4 but a large part of the electro-magnetic spectrum spanning from green to
red remains accessible only through expensive and unpractical optically

] pumped dye lasers. Here we devise a method to multiply the frequency
s
c of optical waves by a factor 3/2 with a conversion that is phase-coherent
i and highly efficient. Together with harmonic generation, it will enable
t
p
one to cover the visible spectrum with semiconductorlasers, openingnew
o
avenuesinimportantfieldssuchaslaserspectroscopyandopticalmetrology.
.
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c © 2008 OpticalSocietyofAmerica
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OCIScodes:(190.0190)Nonlinearoptics;(230.4320)Nonlinearopticaldevices;(190.2620)
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Frequencyconversion;(120.3940)Metrology.
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Referencesandlinks

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14. Thereflectivityat1006.5nmand2013nmishigherthan99.98%,whilethetransmissionat671nmis90%.The
concavemirrorshavea100mmradiusofcurvature,theirdistanceis130mm,andthetwononlinearcrystalare
alignedalongthisarmclosetothesmallerwaistofthecavity.Thepathbetweenthetwoconcavemirrorspassing
throughtheplanemirrorsis400mmlong.
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18. ThelambdameterisaCoherentWaveMasterTM with0.005nmaccuracyand0.001nmresolution.TheFabry-
Perotspectrometerhasaconfocalgeometrywith1.5GHzfreespectralrange,afinesseof200at671nmandit
isnotsensitiveto1m mradiation.
19. Theasymetricintensityofthetwofrequencymodesisduetotheunbalancedconversioninthefrequencysum-
mingcrystal.
20. Inaconfocalresonatorthefamiliarformulaforthemodespacing(thefreespectralrange,FSR=c/4Lwithcthe
speedoflight,andLthelengthofthecavity)resultsfromthespacingofc/2Lamongboththeevenandtheodd
transversemodes,andarelativedispacementofc/4Lbetweenthetwoclasses.SeealsoA.E.Siegman,Lasers
(UniversityScienceBooks,MillValley,California,1986),pp.763.
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22. Themeasureofthepumppowercoupledintothecavityisimmunetospuriouseffectsassociatedwiththenon
optimizedcouplingofthepumpbeamintotheopticalresonator,likegeometricandimpedencematching.
23. L.E.Myers,R.C.Eckardt,M.M.Fejer,R.L.Byer&W.R.Bosenberg “Multigrating quasi-phase-matched
opticalparametricoscillatorinperiodicallypoledLiNbO3,”Opt.Lett.21,591(1995).
24. T. M. Ramond, S. A. Diddams, L. Hollberg & A. Bartels “Phase-coherent link from optical to microwave
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20,1842(2002).
1. Introduction
Nonlinearopticsiscommonlyusedtoextendthespectrumcoveredbylasersoverunaccessible
regions[1].Forinstance,secondharmonicgenerationnowisawellestablishedprocessapplied
in frequency conversion, and with continuous wave diode lasers typically it is implemented
insideresonantenhancementopticalcavities[2].Third-anduptofifth-harmonicgenerationis
nowobtainedwithpulsedlaserseasilyaccessingtheUVspectralregionwithfamiliarinfrared
diode-pumpedsolid-statelasers.Theproductionofsub-harmonics,ontheotherhand,hasim-
portant applications in metrology and quantum optics. Division in 3:1 ratio is achieved with
activephasestabilization[3]and,morerecently,dynamicalsignaturesofself-phase-lockingfor
thesameprocesswereobserved[4].Concerningthe2:1ratio,bothpassiveandactivemethods
for the phase stabilization were applied [5, 6, 7]. More generally,frequencydownconversion
withOPO’soffersaratherflexiblewaytoaccesswideregionsoftheinfraredandnear-infrared
spectrum,buttogeneratecontinuous-waveandsingle-frequencyradiationoneemployssingle
resonantOPO’s,whichrequiremultiWattspumplasers[8],ordoubleresonantOPO’swhich,
withamodestelectronicstabilizationofthecomposingelements,showaconsiderablyreduced
threshold[5,9].
We report on the first demonstration of optical frequency multiplication by a factor 3/2.
We show that our frequency multiplier, based on a multi-resonant OPO, is inherently phase
coherent,preservingthesinglelongitudinalcharacteroftheincidentfieldwithoutactivephase
stabilization,efficient,witha 30%slopeefficiencyandfewtensofmilliWatts threshold,and
stableontimescalesoftheorderofseveralminutes.
2. The3/2frequencymultiplier
The converter is based on an OPO where the pump, the signal, and the idler fields are all
resonant in the cavity and which is operated at frequency degeneracymaking the signal and
idlerfrequenciestocoincide.TheOPOgeneratedfieldhasthenhalfthefrequencyofthepump,
and by inserting in the cavity a nonlinearcrystal for summingthe pump and the OPO fields,
we are able to generate radiation at 3/2 the pump frequency. Exact degeneracy operation is
Fig. 1. 3/2 frequency multiplier experimental setup. A continuous wave and single fre-
quencypumplaserdelivering400mWof1006.5nmisconvertedinto40mWradiationat
671nm.Thepumplaserisresonantlycoupledintoacavitywhere20mmlongperiodically
poledKTP[17]nonlinearcrystalsaresetsotosatisfyquasiphase-matchingfordegenerate
frequencydown-conversion(OPO),andsumfrequencygenerationbetweenthepumpand
down-converted light at 2013 nm (SFG). The wedged surfaces of the crystals are cut at
anangleof100mradwithrespecttothecrystalaxis.Theinput(output)facetoftheOPO
(SFG)crystalisatnormalincidence.Thetwoinclinedsurfacesfacingeachotherareparal-
lel.Thetransversedisplacementofthenonlinearcrystalsprovidesanindependentcontrol
overthecavitydispersion,insuringsimultaneousresonanceofthetwoinfraredfields.
obtainedowingtothedoublegainoftheindistinguishablesplittingprocesswithrespecttoall
theotherprocessesoriginatingsignalandidlerphotons[5].
Thetripleresonanceconditionhastheadvantageofreducingthethresholdofoscillationon
thepumpintensitydowntothemilliWattslevel[10]andallowsactivestabilizationofthecavity
length with respect to the pumpfrequency.On the other hand,the dispersive behaviorof the
opticalelementsofthecavity,i.e.mirrorsandnonlinearcrystals,preventsonefromcontrolling
thefrequenciesoftheOPOgeneratedfieldsindependently,whichhassofarmadesinglemode
operationintriplyresonantOPO’shardtoachieve.Inoursystemthetriplyresonantcondition
allows to actively stabilize the cavity length against the pumping laser, strongly relaxing the
requirements on the passive stabilization. We observed an oscillation threshold as low as 40
mW.ByintroducinganindependentcontrolontheOPOfrequencymodesviaafinetuningof
the relative phase accumulated between the pump and OPO-generated fields over one cavity
roundtrip we achieve the simultaneous resonance of the pump and OPO fields at frequency
degeneracy.
Wedemonstratethe3/2frequencymultiplierproducingradiationat671nmstartingfroma
lasersourceat1006.5nm,asschematicallyreportedinFig.1.Thepumplaseriscomposedbya
semiconductorMaster-OscillatorPower-Amplifiersystem.Themasterlaserisanantireflection
coateddiodelaserstabilizedonanextendedcavityintheLittrowconfiguration[11,12]deliver-
ing30mWat1006.5nmonasinglelongitudinalmodewithlessthan500kHzlinewidth.This
laser is then amplifiedto 400mW preservingits spectralpropertiesthrougha semiconductor
taperedamplifier[13].Thepumpradiationiscoupledintoanopticalcavitycomposedbyhighly
reflectivemirrorsat1006.5nmand2013nm,andhighlytransmittingat671nm[14].Theinput
mirrorhasa 10%transmissionat1006.5nmin orderto maximizethe couplingof thepump
fieldintothecavityunderresonance.Oneofthefoldingmirrorsismountedonapiezoelectric
transducer (PZT) to actively stabilize the cavity length to the pump field resonance. To this
purposetheerrorsignalisprovidedbythepolarizationanalysisofthereflectedpump[15],and
byinsertingintothecavityaverticalpolarizer.
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a(cid:13)
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Fabry-Perot cavity length (arb. un.)(cid:13)
Fig. 2.Transmission spectra of thefrequency multipliedlight through aconfocal Fabry-
Perot(FP)spectrumanalyzer.Displacingthenonlinear crystalstransversallywetunethe
cavitydispersioninordertoimposesinglefrequencyemission(b),ormultimodeemission
(a). c) The Gaussian beam profile of the 3/2 frequency multiplied output is verified by
coupling the single frequency radiation mainly into the fundamental transverse mode of
theFPcavity,whichresultsindoublingthespacingamongtheresonancepeaks[20].
TheindependentcontrolontheOPOcavityfrequencymodesunderpumpresonancecondi-
tionsisobtainedbycuttingthecrystalswithawedgedshape[16](seeFig.1).Displacingthe
crystalsalongthedirectionofthewedgeenablesonetochangetheopticalpathinthecrystal
and,duetothedispersion,itallowsafinetuningoftheOPOresonancemodeswhilekeeping
thecavityresonantwiththepumpfield.Thetwononlinearcrystalsare20mmlong,2×1mm2
cross section, periodically poled KTP [17] that insure quasi-phase-matchingfor linearly and
identicallypolarizedfields.TheOPOandSFGcrystalshaveapolingperiodofL =38and
OPO
L =19.5m mrespectively,andtheyareidenticallycutinanasymmetricwaysuchthatone
SFG
surfaceisatnormalincidence,whiletheotherhasananglef of100mradwithrespecttothe
crystalaxis.Intheresonatorthecrystalshavethewedgedsidefacingandparallelsuchthatthe
opticalaxiscoincide.Thisconfiguration,whileitallowstocontroltherelativephasebetween
the pumpand OPO fields, it insuresa negligibledeviationof the beam propagationat differ-
ent wavelengths, and hence simultaneousresonance of the pump and degenerateOPO fields.
Toreachthedoubleresonanceandfrequencydegeneratecondition,weobservea400m mpe-
riodicity on the crystal transverse position. This is consistent with the calculated periodicity
L /f =380m m.Thecrystalsurfacesareallanti-reflectioncoatedsuchthatthereflectivity
OPO
persurfaceis0.1%at1006.5nmand2013nm,and0.3%at671nm.
3. Spectralpropertiesandconversionefficiency
Thespectralpropertiesofthegeneratedredlightareanalyzedbothwithalambda-meterforthe
rough wavelength determination, and a confocal Fabry-Perot spectrometer (FP) to check the
singlelongitudinalmodeoperation[18].Asexpected,thespectrumofthegeneratedredlight
dependson cavity dispersion. When we change the transverse position of the crystal by tens
ofmicronsweareabletoswitchbetweensinglefrequencyemissionattheexpectedvalueand
multifrequencyemissionwithcentralwavelengthdisplacedasmuchas0.08nmfrom671nm,
withasimultaneousreductionintheoutputpower.Figure2reportsthetypicalspectrafromthe
Fabry-PerotanalyzerwhentheOPOoperatesclosetothedegeneratepoint.Dependingonthe
transverseposition of the crystals, the system emitssingle (spectrum2b) or multi(2a) longi-
tudinalmoderadiationwithastabilityoftheorderofminutes.Inthemulti-longitudinalmode
operation,energy conservationresults in the symmetric positioningof the frequencycompo-
nentswithrespecttothedegeneratemode[19].Thespatialmodeoftheredlighthasanearly
Gaussianprofile.AsacheckwecarefullyalignedtheFPanalyzerinordertodiscerntheeven
and oddstransversemodesof the cavity [20]. As reportedin Fig. 2c, we can couple97 %of
thepowerintotheeventransversemodes,indicatingthatatleast94%ofthegeneratedpower
is in the fundamentaltransverse mode. While multi-longitudinalmode operationis stable on
hours,whentheconverteremitssinglefrequencyradiationitprovestobestableontimescale
oforderofseveralminutes.Suchastabilityrequiresnoactivestabilizationofthecrystalspo-
sition.Figure3depictstheamplitudeofthegeneratedredlightwhenthefrequencymultiplier
worksinsinglelongitudinalmode.Themeasuredamplitudenoiseis1.4%RMSona50kHz
bandwidth.
The single longitudinalmode emission proves that the OPO works at frequency degeneracy,
anditisknownthatfortype-Iphasematching(asprovidedbytheperiodicallypoledcrystals)
infrequencydegenerateOPO’sthepumpanddownconvertedfieldsarephaselockedandthat
theymayexhibitp phasejumps[5].Ontheotherhandthephaseofthefundamentalfieldinthe
cavityislockedtothatoftheincidentbeambecauseofthepump-cavityresonancecondition.
Sincethefrequencysumprocessshouldnotaddanyrelevantphasenoise,wehaveanevidence
thatthe3/2multiplicationprocessisphasecoherent.Acomparisonwithindependentlygener-
ated phase coherentfields will allow a thoroughcharacterizationof the stability of the phase
transfer[21].
Wedeterminetheconversionefficiencybyvaryingthepumppowerandmeasuringthegen-
eratedpowerintheredasafunctionoftheIRpowercoupledintothecavity[22].Weobserve
athresholdforOPOoscillationsmallerthan50mWandobtaina30%incrementalefficiency
above150mWpumppowercoupledintothecavity(seeFig.4).Reducingtheintensityofthe
pumpbelow2/3ofthefullpowerraisestheamplitudenoiseintheoutput,andmakesthesys-
temmorecriticaltooperateonasinglelongitudinalmode.Suchadegradingcanbeovercome
by using a different geometry optimized for lower pump levels, with better focussing of the
cavity mode on the nonlinear crystals, and choosing different crystals with higher nonlinear
polarizability[10].
Thewavelengthtunabilityofthesourcecanbelimitedeitherbythetunabilityofthefunda-
mentallaser,orbythatofthe3/2frequencymultiplier.Typicallyanti-reflectioncoatedinfrared
semiconductorlasershaveatunabilityoffewpercentinwavelength,andinourcasethelaser
canemitfrom990nmto1040nm.Concerningthemultiplier,thenonlinearcrystalscanbetem-
peraturetunedto satisfy quasi-phase-matchingatdifferentwavelengths.With our crystals, to
generateradiationat670nm,onenmshorterwavelength,wehavetotunemasterlaserto1005
nm,cooltheOPOcrystalby5Celsius,andcooltheSFGcrystalby20.Withagivenchoiceof
grating periods,a reasonabletemperaturetunabilityof the multiplier is 0.5 % in wavelength.
Thiscanbeextended,withoutlossofefficiency,tothefull5%tunabilityofthepumpbyusing
multichannelperiodicallypoledcrystals[23],whichincludethe10gratingperiodsnecessaryto
accesstherelevantwavelengthintervals.Themirrorsofthecavityhaveaflatresponsebeyond
thewindowaccessiblethroughthemasterlaser.
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int 94(cid:13) 94(cid:13) time (ms)(cid:13)
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time (s)(cid:13)
Fig. 3. Frequency multiplier amplitude stability on 3 minutes and 10 ms (inset) time
timescaleunder singlelongitudinal modeemission.ThemeasuredRMSamplitudenoise
atfullpoweris1.4%on50kHzbandwidth.Undermultilongitudinalmodeoperationthe
amplitudestabilitydoesnotnot changequalitativelywhenmeasuringonthesameband-
width.
4. Conclusion
Tosummarize,wedemonstratedforthefirsttimea schemetomultiplythefrequencyofcon-
tinuous wave optical radiation by a factor 3/2 and preserving its single frequency character.
Thefrequencymultiplierisphasecoherent,hasahighconversionefficiency,isverystable,and
itsstabilityonhourstimescalecouldeasilybeachievedbyoptimizingthedesignoftheopto-
mechanicalapparatus.Employingexistingtechnology,theschemewilleasilyfindapplications
in many disciplines requiring laser in the green to red spectral interval, such a spectroscopy.
Together with integer harmonicgeneration,3/2 frequencymultiplication will allow to access
thecompletevisiblespectrumviaharmonicgenerationofsemiconductorlasers.Itmakespos-
sibletoestablishphasecoherentlinksamongspectralregionsdistant2/3ofanoctave[24],and
itmayconsiderablysimplifytherealizationofRGBlasersystems.
Itisworthnotingthatthefrequencymultiplieralsoactsasaparitydiscriminatoronthepump
resonant mode. In fact, neglecting cavity dispersion, the frequency degenerate and resonant
down conversion can take place only when the pump is resonant in the cavity with an even
numberofmodes.Thisisconfirmedbythesinglefrequencyemissionoftheconverterwitha
twofoldperiodicitywhensteppingthecavitylengthbetweenadjacentpumpresonances.
Acknowledgment
WethankM.Artoni,G.OppoandN.Poliforacriticalreadingofthemanuscript,R.Ballerini,
M.DePas,M.GiuntiniandA.Hajebfortechnicalassistance.WeareindebtedwithG.M.Tino,
R.Grimm,F.SchreckandLaser&Electro-OpticSolutionsforthegeneralsupportandtheloan
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stable operation(cid:13)
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50(cid:13) 100(cid:13) 150(cid:13) 200(cid:13)
coupled power (mW)(cid:13)
Fig.4.Extractedpowerat671nmasfunctionofthepumppowercoupledintothecavity.
Theverticalgraylineindicatesthethresholdvalueforastablesinglefrequencyoperation
oftheconverter.TheerrorbarscorrespondtotheRMSamplitudenoise.
ofpartsoftheapparatus.WealsoacknowledgestimulatingdiscussionswithC.Salomon.This
workwassupportedbyEUundercontractRII3-CT-2003-506350,andEnteCassadiRisparmio
diFirenze.