Infrared UpconversionUpconversion for Astronomy
Robert W.W. Boyd Department of PhysicsPhysics University of California Berkeley, California 94720
Abstract The fieldfield ofof infraredinfrared upconversionUpconversion forfor astronomy astronomy is is reviewed. reviewed. The basic theory of upconversionUpconversion is presented, along with a brief historical summary of upconversionUpconversion techniques.techniques. SeveralSeveral investi-investi gators have employed upconverters in astronomical studies, but have met withwith onlyonly modestmodest success.success. UpconversionUpconversion willwill becomebecome IR a useful detection method for astronomy only if substantial butbut perhaps forseeable improvementsimprovements can be realized. Introduction A novel solutionsolution toto the problem ofof infraredinfrared detectiondetection isis affordedafforded by thethe processprocess ofof upconversion. Upconversion. TheThe upconversionUpconversion processprocess isis shown schematicallyschematically inin FigureFigure 1.1. InfraredInfrared radiationradiation of frequency
Figure 2.2. ImagingImaging property ofof the the upconversion Upconversion process. process. ConservationConservation of photonphoton momentummomentum (hk)(hk) requiresrequires thatthat the the sum sum frequencyfrequency photon be emitted inin aa unique direction. rections of propagation. In fact,fact, toto aa goodgood approximation,approximation, the Nonlinear crystal angle of propagation ofof thethe sumsum frequencyfrequency radiationradiation isis relatedrelated to the angleangle of propagation of thethe infraredinfrared radiationradiation byby
Figure 1.1. Schematic description of thethe upconversion Upconversion process.process. InfraredInfrared vIR 05°IR (1) radiation of frequencyfrequency vIR^|R isis mixedmixed with with an an intense intense laser laser beambeam ofof fre-fre PL quency vLv\_ inin aa nonlinearnonlinear crystal,crystal, producingproducing aa signalsignal at the sum frequency PS."s- where OS6$ andand #JRBIR are defined inin FigureFigure 2.2. Thus, if the optical sys-sys tem which collects the infraredinfrared radiationradiation isis designeddesigned soso as toto map vIRv^ is mixed withwith anan intenseintense visible visible oror near near-infrared -infrared laserlaser beambeam each point in the fieldfield ofof viewview into aa planeplane wavewave at the crystalcrystal of frequencyfrequency vLv^ in a nonlinear crystal.crystal, The nonlinearity causescauses a with aa directiondirection ofof propagationpropagation BIR, #JR, Eq.Eq. (1) predicts that the sum signal toto be generated at thethe sumsum frequency,frequency, vs,^§, whichwhich is in the frequency response will be aa perfectperfect imageimage ofof thethe infraredinfrared field,field, visible region;region; thus the infrared signalsignal is converted to thethe visible,visible, demagnified byby thethe factor vIR^IR/^L. /vL where sensitive, lowlow noisenoise detectors areare readilyreadily available.available. The This article will reviewreview thethe variousvarious instrumentsinstruments for astronomy conversion efficiencyefficiency ofof infraredinfrared photonsphotons to visible photonsphotons at which have been constructed usingusing thethe upconversionUpconversion process.process. De-De the sumsum frequencyfrequency cancan approachapproach 100100%. %.11 Furthermore,Furthermore, the up- spite the potentialpotential ofof thethe technique, technique, only only modest modest results results havehave conversion processprocess isis inherentlyinherently noise free in the sense that ener-ener been achieved toto date. OneOne seriousserious problem has been thatthat the gy conservation prohibits the generationgeneration ofof aa responseresponse atat thethe sumsum quantum conversion efficiency hashas been quite low forfor upcon-upcon- frequency in thethe absenceabsence ofof aa signalsignal atat thethe infraredinfrared frequency.frequency. verters employing continuouscontinuous wave wave lasers.lasers. AnAn additional problem A related use of the upconversionUpconversion processprocess isis toto convert convert images images is thatthat manymany workersworkers inin thethe fieldfield havehave beenbeen limitedlimited byby noisenoise from the infraredinfrared spectralspectral regionregion toto thethe visible. visible. AsAs illustrated in sources largerlarger thanthan expected. As a result,result, thethe techniquetechnique hashas Figure 2, thethe upconversionUpconversion process,process, inin conservingconserving photon mo-mo yielded only marginal resultsresults inin terms of astronomical detection. mentum (hic),(hk), insures that aa oneone-to-one -to -one correspondencecorrespondence exists be-be However, sincesince the the problemsproblems toto datedate do not appear to be ones ofof tween infrared directions of propagationpropagation andand sumsum frequencyfrequency di-di- principle, it is hoped that upconversionUpconversion maymay becomebecome aa useful de-de tection technique at somesome future time.time. Work partially supported byby NASANASA GrantsGrants NGL NGL 05- 05-003-272 003 -272 and NGRNCR 05-05-003-452. 003 -452. Upconversion Theory IR-105IR -105 receivedreceived June June 8,8, 1977;1977; revisedrevised JuneJune 30, 1977.1977. Some of thethe theoreticaltheoretical aspects aspects ofof the the upconversion Upconversion processprocess willwill
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be considered in this section. Only those fundamentals needed to where OkAk is the propagation vector mismatchmismatch understand the descriptiondescription ofof various various upconversionup conversion systemssystems dis-dis cussed later will be provided. The excellent treatments ofof Hulme2Hulme2 AkOlc = = llt IkSs -£ - kLL -lc -kIRIIR l , (6) and of MidwinterMidwinter and Zernike3Zernike3 are recommendedrecommended forfor thosethose de-de I?£ isis thethe length of thethe nonlinearnonlinear crystal,crystal, cc isis thethe velocityvelocity ofof light;light; siring additionaladditional information. nIR,nIR> nL,nL' andanc* nSnS areare the^e indicesmdices of°f refractionrefraction forfor the the three three waves; waves; When transparenttransparent mattermatter is subjected to intenseintense electromag-electromag AJRAIR andand XsX§ are the vacuum wavelengths ofof thethe sum andand infraredinfrared netic radiation, the response of the matter (measured(measured by the po-po fields, ILIL isis the laser power per unit area,area, andand deftdeff isis thethe effec-effec larization, oror dipoledipole momentmoment perper unitunit volume) volume) ceases ceases toto bebe linear-linear tive value of dykdijk obtained by carryingcarrying outout thethe summationsummation indi-indi ly dependentdependent onon thethe incidentincident fieldfield amplitudeamplitude andand displays displays non-non cated in Eq.Eq. (2}I.(2), For For thethe casecase ofof perfectperfect phasematching thethe term linear effects.effects. TheThe lowestlowest orderorder nonlinearitynonlinearity gives gives riserise toto thethe sec- sec in squaresquare brackets in Eq. (5) isis equal to one.one. We note that thethe ond order polarization, aadar^d it is tinstis nonlinearity that isis exploited system quantum efficiency is proportional to ddeff2eff2 , toto thethe laserlaser in mostmost upconverters.upconverters. IfIf EL and EIREj^ areare thethe electricelectric fieldfield ampli-ampli power per unitunit area,area, andand toto the the square square ofof the the lengthlength ofof thethe crys-crys tudes associatedassociated with the laserlaser beam and infraredinfrared field,field, respec-respec tal. This lastlast resultresult followsfollows fromfrom thethe phasematching phasematching conditioncondition tively, thethe nononlinearity linearity willwill induceinduce aa secondsecond orderorder polarizationpolarization which allows the individualindividual dipoles within the crystalcrystal to radiate of amplitude P withinwithin thethe AI nonlinear onlinear crystalcrystal at the sumsum andand dif-dif coherently. ference frequencies, where PP isis given by The infrared bandpassbandpass ofof anan upconverterupconverter isis limitedlimited to thethe ex-ex -JR tent thatthat thethe termterm inin square square bracketsbrackets inin Eq.Eq. (5)(5) fallsfalls toto zerozero asas Pi =22dkijk E.EIk. (2) OkAk increasesincreases as aa resultresult ofof the the infrared infrared frequency frequency being being varied varied from itsits "centralcentral value. The actualactual valuevalue ofof the the bandpass bandpass will will de-de dijkis-is the nonlinear coefficient of the medium,medium, and thethe indicesindices pend upon thethe detailsdetails ofof the the dispersion dispersion of of the the refractive refractive indicesindices i, j,1, andand k refer to cartesian components. InIn practice,practice, dijkdyj, cancan al-al for any particular crystal,crystal butbut aa goodgood rulerule ofof thumbthumb isis thatthat thethe most never bebe calculated,calculated, andand isis regardedregarded asas anan empiricalempirical con-con infrared bandpass inin cmcm ï is numerically equal to thethe inverseinverse of stant. the lengthlength of thethe crystalcrystal inin cm,cm, andand itft isis generallygenerally truetrue thatthat thethe Not all optically transparent materials allow this lowest order bandpass is inversely proportionalproportional to the lengthlength ofof thethe nonlinearnonlinear nonlinearity to exist.exist. OnlyOnly materialsmaterials whichwhich areare notnot symmetricsymmetric crystal. under inversioninversion can give a second order polarization, andand thisthis re-re For anan imagingimaging upconverter, thethe questionsquestions ofof angular angular resolu-resolu striction limits thethe classclass ofof crystalscrystals ofof interestinterest inin upconversionupconversion tion andand fieldfield ofof view view areare alsoalso raised.raised. WhileWhile thethe phasematchingphasematching work. MaterialsMaterials withwith inversion symmetrysymmetry areare stillstill ofof interest in conditions place aa seriousserious constraintconstraint onon thethe conecone angleangle ofof radia-radia nonlinear optics, however, as theythey can participate in higher orderorder tion ofof anyany particularparticular wavelengthwavelength thatthat cancan bebe efficientlyefficiently upcon- nonlinear interactions. Since these higher order interactionsinteractions areare verted, it hashas beenbeen foundfound thatthat the the field field of of an an imaging imaging upconverter generally lessless intense intense thanthan thethe lowest order nonlinearnonlinear interaction,interaction, can be quitequite largelarge inin thatthat different different infrared infrared wavelengths wavelengths areare up-up- they havehave only recentlyrecently beenbeen exploitedexploited inin experimentalexperimental studies.studies. converted at differentdifferent anglesangles within thethe fieldfield ofof view.4view.4 WhileWhile The time varying polarization P within the nonlinear mediummedium this may be an undesirable feature forfor somesome spectroscopic work, can radiate energyenergy efficiently onlyonly ifif thethe various various dipolesdipoles cancan actact it should notnot presentpresent a a problem problem in in the the imaging imaging ofof thermalthermal sources.sources. as a phasedphased array,array, andand thisthis requiresrequires thatthat thethe condition Under certain conditions, the angularangular resolutionresolution ofof anan upcon-upcon KLitL ++KIR f,R ==k's Ks (3)(3) verter willwill be limited onlyonly byby diffraction.diffraction. InIn particular, particular, if if a a single single transverse modemode laser isis used to illuminateilluminate an optically perfect between the propagationpropagation vectorsvectors ofof thethe threethree waves waves bebe satisfied,satisfied, crystal with faces flat toto X/10X/10 atat all all relevant relevant wavelengths,wavelengths, the as well asas thethe conditioncondition defining thethe sum frequency configuration discussed earlier in whichwhich collimatedcollimated infraredinfrared isis upconverted to thethe visible visible willwill resultresult in in visible visible imagesimages whosewhose vL"L + vIR"IR ==vS "S (4) sharpness is degraded only by thethe uncertaintyuncertainty inin propagation propagation angle due to the diffraction of the infrared beam. Thus, no in In general, these conditions are incompatible. For a birefringent angle due to the diffraction of the infrared beam. Thus, no in- In general, these conditions are incompatible. For a birefringent formation is lost in the upconversionupconversion process.process. crystal, however, thethe indexindex of refraction isis dependent upon the direction of the EE vector,vector, andand thusthus for for certain certain choices choices ofof polari-polari zation and propagationpropagation directionsdirections Eqs.Eqs. (3)(3) andand (4)(4) cancan bebe simul-simul Historical Survey of Upconversion taneously satisfied, andand under thisthis condition thethe interactioninteraction isis In thisthis section,section, thethe historical historical developmentdevelopment ofof upconversionupconversion forfor said toto be phasematched. When phasematching is obtained by roro- detection of infraredinfrared radiation will be outlined. No attemptattempt at tating the crystalcrystal soso asas to vary thethe directiondirection of propagationpropagation withinwithin completeness will be made;made; rather,rather, onlyonly somesome ofof thethe mostmost sig- sig the crystal,crystal, thethe upconverterupconverter isis saidsaid toto bebe angleangle tuned.tuned. InIn somesome nificant theoreticaltheoretical andand experimentalexperimental resultsresults willwill bebe presented.presented. cases thethe refractive indices of thethe nonlinear crystal are sufficiently In thethe followingfollowing sectionsection anan accountaccount ofof all all reported reported applicationsapplications temperature dependent that thethe phasematchingphasematching conditioncondition cancan of upconversion to astronomy willwill bebe presented. be met byby accuratelyaccurately controlling the crystalcrystal temperature, which Nonlinear optics is almost as oldold asas thethe laserlaser itself.itself. TheThe firstfirst is referred to asas temperaturetemperature tuning.tuning. InIn practice,practice, itit is is almost almost al- al working laser waswas constructedconstructed by by Maiman5 Maiman5 followingfollowing aa sugges- sugges ways the case that ifif thethe sumsum frequencyfrequency generation generation processprocess is tion ofof SchawlowSchawlow andand Townes.6Townes. 6 By 1961,1961, Franken et allal7 hadhad phasematched, thethe difference frequency process will be badly succeeded in observing the second harmonicharmonic ofof rubyruby laserlaser lightlight mismatched, and hence no appreciableappreciable powerpower willwill bebe generatedgenerated using crystal quartzquartz asas aa nonlinearnonlinear material.material. TheThe necessitynecessity of i at the differencedifference frequency.frequency. phasematching if highhigh conversionconversion efficiencyefficiency werewere toto bebe achievedachieved Perhaps the most importantimportant parameterparameter describingdescribing anan upcon-upcon was apparent, and inin 19621962 Giordmaine8Giordmaine8 andand Maker Maker etet al9al9 inde-inde version system is thethe quantum efficiency, or the ratio of the numnum- pendently succeededsucceeded in utilizingutilizing birefringencebirefringence to realizerealize thethe ber of sumsum frequencyfrequency photonsphotons producedproduced to thethe numbernumber ofof in-in phasematching condition. frared photonsphotons enteringentering thethe nonlinearnonlinear crystal.crystal. SinceSince GaussianGaussian Armstrong et al10al10 consideredconsidered thethe generalgeneral casecase ofof sumsum fre-fre electrostatic units areare the units most oftenoften usedused in nonlinear op-op quency generation in addition toto thethe degenerate degenerate casecase ofof secondsecond tics, thisthis conventionconvention will be followedfollowed here. For thethe (unfortu-(unfortu harmonic generation, and developeddeveloped a generalgeneral theory ofof thesethese nately) usual casecase ofof smallsmall quantumquantum efficiency,efficiency, the quantumquantum processes, treatingtreating the nonlinearnonlinear materialmaterial usingusing quantum mech-mech efficiency is given by anical perturbation theorytheory and and using using Maxwell's Maxwell's equationsequations toto de-de scribe the opticaloptical field.field. Furthermore,Furthermore, theythey suggested suggested usingusing thethe 5125127r ?TSs déffd*ff ILIL 522e2 rsin(QOk/2)1ce Ak/2)" 2 upconversion process as a method ofof detecting infraredinfrared radiation. 71= (5) The firstfirst experimentalexperimental studiesstudies of upconversionupconversion were those of nIRnLnSXSXIRc L Rok/2J Johnson andand DuardollDuardo11 andand Midwinter Midwinter and and Warner.12 Warner, 12 MidwinterMidwinter
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and Warner upconverted 1.71.7 µmptm infraredinfrared radiationradiation inin a a 6 6 cm cm" -11 bandpass to thethe visiblevisible byby mixingmixing with aa pulsed ruby laser beam in a temperaturetemperature tunedtuned lithiumlithium niobateniobate crystal. crystal. TheyThey achievedachieved aa maximum conversion efficiency of 1%1% at theirtheir peakpeak powerpower den-den sity ofof 11 MWMW/cm /cm22 . .The The noise noise performanceperformance of their upconverter was worseworse thanthan theoretical due to anan unidentifiedunidentified sourcesource of noise at the sumsum frequency.frequency. TheThe sourcesource ofof noise noise waswas assumedassumed toto bebe up-up- converted infrared radiation becausebecause thethe noisenoise waswas polarizedpolarized inin the samesame sense as thethe sum frequencyfrequency radiationradiation andand appearedappeared toto be phasematched. They speculated that thethe sourcesource ofof infraredinfrared noise was either dust particles heated by thethe laserlaser beam,beam, fluor-fluor escence ofof one ofof theirtheir opticaloptical components,components, oror aa higherhigher orderorder nonlinear process,process, butbut they could not isolateisolate thethe actualactual source.source. Motivated by this observation of anan unexpectedunexpected noisenoise source,source, Smith and Townes13Townes13 investigated several possible higher order processes whichwhich couldcould produceproduce noise, the most importantImportant beingbeing the parametricparametric process Inin whichwhich aa laserlaser photonphoton isis annihilated,annihilated, creating an infrared photon andand aa differencedifference frequencyfrequency photon, with the infrared photon beingbeing subsequently upconverted byby the usual phasematchedphasematched upconversionupc on version process.process. UsingUsing semiclassicalsemiclassical arguments, SmithSmith andand Townes estimated that whilewhile this process could be an appreciable source of noise, it could not havehave pro-pro Figure 3. Upconverted image of aa resolutionresolution testtest pattern pattern by by Stappaerts Stappaerts duced as much noise as that measuredmeasured byby MidwinterMidwinter andand Warner.Warner. et al.23al. 23 They estimateestimate that thethe originaloriginal photographphotograph consistsconsists of atat leastleast They also concluded thatthat thesethese higherhigher orderorder noisenoise sourcessources could 1000 resolvable spots. be rendered negligiblynegligibly smallsmall byby aa suitable choicechoice ofof systemsystem param-param eters and hence upconversion was a potentiallypotentially attractive method radiation to .455.455 µm/irn withwith a aquantum quantum efficiency efficiency of of 3 3%.%. TheirTheir of high sensitivitysensitivity infrared detection. TheseThese resultsresults were corrob-corrob final images contained 10001000 resolutionresolution elementselements (see(see FigureFigure 3),3), orated byby Tang,14Tang, 14 who repeatedrepeated thesethese calculationscalculations using using aa fullyfully and again,again, the usefulnessusefulness ofof the the device device forfor astronomical astronomical applica-applica quantized fieldfield approach. tions was limited byby thethe requirementrequirement thatthat theirtheir laserlaser bebe aa pulsed Imaging upconversion waswas firstfirst reportedreported by Midwinter who source. achieved 50 lines of resolution acrossacross his field of view. TheoreticalTheoretical Another new technique is that discusseddiscussed by Bethune etet alala,24 , discussions of imagingimaging upconversion werewere givengiven byby Midwinter,16Midwinter, 16 in whichwhich sumsum frequencyfrequency generationgeneration isis inducedinduced viavia quadrupolequadrupole Warner,Warner,1717 and and Firester.Firester.1818 Warner Warner pointed pointed out out thatthat the angular transitions in sodiumsodium vapor.vapor. WhereasWhereas sumsum frequencyfrequency generationgeneration field of viewview forfor upconversionupconversion ofof monochromaticmonochromatic infraredinfrared cancan is disalloweddisallowed inin the dipoledipole approximationapproximation forfor isotropicisotropic mediamedia be made quite largelarge forfor certaincertain geometriesgeometries inin whichwhich the threethree such as atomicatomic vapors,vapors, thisthis processprocess isis allowedallowed forfor quadrupolequadrupole waves areare notnot colinearly propagating. This suggested thatthat a scene transitions. Being a lower order nonlinear process thanthan the third actively illuminated byby a 10.610.6 µm//m CO2CO2 laserlaser couldcould bebe usefullyusefully order allowed process of Bloom et al,al, itit isis foundfound to to be be of of corn com - studied by imaging upconversion.upconversion. Experimental investigationsinvestigations of parable intensity.intensity. ThisThis processprocess reliesrelies onon aa resonantresonant enhancementenhancement upconversion imaging ofof actively illuminatedilluminated scenesscenes have been to increaseincrease the magnitudemagnitude ofof thethe nonlinearnonlinear interaction,interaction, andand thusthus carried out byby Lucy19Lucy 19 andand Tseng.20Tseng. 20 Firester18Firester18 clarified the role requires thethe useuse ofof tunable tunable laser laser sources. sources. ThisThis techniquetechnique hashas not of laserlaser beambeam divergencedivergence inin limiting the angularangular resolution ofof anan yet beenbeen exploited for infrared detection. imaging upconverter.upconverter. InIn particular, aa singlesingle transverse mode laser beam need not have plane wavefronts in orderorder for thethe upconverter to achieveachieve maximummaximum resolution,resolution, inIn disagreementdisagreement with thethe pre-pre Astronomical Instruments vailing viewview at at thatthat time. AA curvaturecurvature toto thethe laser laser wavefronts wavefronts The first reported useuse ofof anan upconverterupconverter forfor astronomical astronomical workwork will simply displace thethe focus of the upconverted image.image. was that ofof GurskiGurski etet aí.25al. 25 ThisThis systemsystem usedused aa pulsedpulsed Nd:YAGNd:YAG Falk and Yarborough21Yarborough21 first succeeded inIn detecting room tem-tem laser pumped lithium iodateiodate upconverter,upconverter, yieldingyielding aa peakpeak quan-quan perature thermalthermal radiationradiation withwith theirtheir Nd Nd:YAG :YAG laserlaser pumpedpumped tum conversionconversion efficiency ofof KO1CT 3. . PhasematchingPhase matching waswas achievedachieved proustite upconverter.upconverter. GurskilGurski1 succeededsucceeded inin obtainingobtaining almostalmost by angle tuning the 55 cmcm longlong crystal;crystal; extremelyextremely fortuitousfortuitous dis-dis 100% conversionconversion ofof 3.393.39 ju.mµm radiation intoInto visiblevisible radiationradiation withwith persion of lithium iodate allowed.allowed the extremelyextremely widewide bandpassbandpass his ruby laserlaser (0.6943 µm)/irn) pumpedpumped lithiumlithium iodateiodate upconverter.upconverter. of 1.81.8 pmjtirn extendingextending fromfrom 3.23.2 µmjum toto 5.05,0 µm.jurn. GurskiGurski et al found AA. potentiallypotentially greatgreat improvementimprovement inin upconverterupconverter sensitivitysensitivity it convenientconvenient toto pulsepulse theirtheir laser laser atat aa 0.50.5 HzHz repetionrep et ion rate,rate, yield-yield- is possiblepossible using the techniquetechnique developeddeveloped byby HarrisHarris andand hishis co-co- ingIng a duty cyclecycle ofof 10'1CT 3for for the the 2.0 2.0 ms ms pulse pulse duration. duration. The The sys- sys workers atat Stanford University,University, in whichwhich thethe thirdthird orderorder nonnon- - tem waswas capablecapable of operatingoperating atat aa 6% 6% duty duty cycle, cycle, however. however. linearity ofof aa metalmetal vaporvapor isIs utilized.22 utilized,.22 AsAs atomicatomic transitionstransitions With this system,system,, Gurski et al werewere able to detect thethe nearnear tend toto bebe quitequite narrow,narrow, itit isIs possiblepossible toto workwork veryvery closeclose to infrared flux from the Moon, Venus, aa Gri,Ori, andand a.a Tau at a 2 a resonance, with a correspondingcorresponding increaseIncrease inin thethe efficiencyefficiency ofof level Inin 100 seconds of realreal time for thethe firstfirst twotwo objectsobjects and in the nonlinear coupling. Bloom et al22al22 reportedreported thethe operation of 200 seconds for thethe secondsecond twotwo objects.objects. TakingTaking their dutyduty cyclecycle such a device toto convert 9.26 Mµmm infrared radiation to the near into account,account, thethe actualactual datadata-taking -taking tooktook placeplace in 0.10.1 andand 0.20.2 ultraviolet atat 0.3305 pm,/irn, byby mixingmixing with 0.68560.6856 µmjum opticaloptical sec. respectively. An excess noise source was found whichwhich they parametric oscillator radiation (3 kwkw peakpeak power)power) inin aa sodiumsodium attributed toto anan absorptionabsorption bandband inin theirtheir crystal. crystal. However,However, sky cell. The nonallowednonallowed 3s3s-3d -3d transitiontransition ofof sodium is pumped by and telescope noise were the predominant sourcessources ofof noise.noise. Tak-Tak the secondsecond harmonic of thethe opticaloptical parametricparametric oscillator, oscillator, givinggiving ing photomultiplier quantumquantum efficiencyefficiency andand thethe imperfectimperfect trans-trans a resonant enhancement to thethe conversionconversion process.process. DespiteDespite the mission of their opticsoptics intointo account, account, Gurski Gurski etet al al estimateestimate losseslosses fact that nono efforteffort waswas mademade totophasernatch phasematch thethe process, aa photonphoton of^aof a factor of 100,100, yielding a peak system quantum efficiency ofof conversion efficiency of 58%58% was reported. SuchSuch aa devicedevice would 101-5,10~5 , and and a a timetime averaged systemsystem quantumquantum efficiencyefficiency ofof 1CT168.8 . have limited useuse inin astronomy due to thethe relativelyrelatively lowlow dutyduty An infrared upconverter withwith anan inherentlyinherently narrownarrow bandpass cycle (~10~(^ -11T6)6 ) of of their their optical optical parametricparametric oscillator. for spectroscopicspectroscopic applicationsapplications hashas beenbeen describeddescribed byby Smith.26 Smith.26 Stappaerts et al23al23 havehave constructedconstructed anan imagingimaging upconverterupconverter His systemsystem utilizedutilized the nonlinearity ofof aa 55 cmcm longlong lithiumlithium nio-nio operating on similarsimilar principles.principles. Their device converts 2.9 jumµm bate crystal to mixmix aa temperaturetemperature tuneabletuneable (2.8(2.8 pm(Jim toto 4.24.2 pm)jum)
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infrared signalsignal of of 11 cm"cm'1 bandpassbandpass withwith aa oneone watt,watt, cw,cw, argonargon with thethe laserlaser beambeam toto produceproduce upconvertedupconverted photons.photons. VacuumVacuum ion laser pump (0.5145 µm),JUKI), toto yieldyield aa sumsum frequencyfrequency atat aboutabout fluctuations provide electromagnetic couplingcoupling between the .450 pm.p,m. Smith'sSmith's upconversionupconversion quantumquantum efficiencyefficiency waswas 10-410~4 , alal- crystalline ground statestate andand infraredinfrared levels levels whichwhich allowsallows the procproc- though he achievedachieved 3x103xlCT -33 under under conditionsconditions notnot optimum for ess toto be coherent andand phasephase matched.matched. ThisThis processprocess isis moremore in-in coupling to aa telescope.telescope. HisHis losseslosses of aboutabout 10001000 yielded a system tense thanthan upconversionupconversion of infraredinfrared radiated by thethe crystalcrystal quantum efficiencyefficiency ofof 10-7. 10~7 . HisHis measuredmeasured NEPNEP waswas 10-1310~ 13 watts/ itself.itself. Hz112Hz172 , Abbas etet alaí2828 havehave reported reported thethe construction of aa systemsystem sim-sim With this apparatus, SmithSmith was ableable to detect the Moon, aa ilar toto thatthat ofof Smith, Smith, but but employing employing aa choppedchopped infraredinfrared beambeam Ori, and a Boo.Boo. AA spectrumspectrum of aa OriOri isis shownshown inin FigureFigure 4.4. Al-Al- and thethe useuse ofof phase phase sensitive sensitive detectiondetection ofof thethe sumsum frequencyfrequency radiation. This procedure minimizesminimizes thethe effects of drifts inin theirtheir system parameters,parameters, and allows themthem to obtainobtain aa systemsystem NEP of 1(F14W10~ 14 W/Hz /Hz1/2.1/2 . They They have have not not yetyet used their upconverter forfor as-as 300 - tronomical detection, butbut theirtheir published published paperpaper presentspresents aa dis-dis cussion of the useuse ofof suchsuch aa systemsystem forfor astronomicalastronomical applications.applications. A laboratory spectrogram of methane obtainedobtained withwith theirtheir up-up converter isis shownshown in.in Figure 5.5.
lOcm10 cm CELLCELL CH4CH 4 203203TORR TORR I300°K BLACKBLACK BODYBODY 2+3 4 1 2 Y -4 N 6 5 +6 rw 16.7cm-I z
O I I 3.39µ3.39/1 X=3.3IA=3.31±0.01µ ±0.01,1 3.24/13.24µ±.05 ±.05 X 2950cm2950cm" I1 302lcm~'3021cm-I 3086 cm- cnrT I1
Figure 5. The absorption spectrumspectrum ofof methanemethane measuredmeasured withwith the up-up converter of Abbas,Abbas, etet al,26al, 28 showingshowing thethe P,P, Q,Q, andand RR branches.branches. The upconupcon- verter spectral resolutionresolution waswas ~2.7-2.7 cm'cm '.. 40 40 minutes minutes of of integration integration were were required to take this spectrum.
Infrared imaging of astronomicalastronomical sourcessources byby upconvertingupconverting their 1010 µmjLtm radiationradiation has has beenbeen reportedreported by by Boyd.29 Boyd.29 HisHis system,system, shown in Figure 6,6, usesuses an 0.250.25 wattwatt cwcw kryptonkrypton ionion laserlaser beambeam at 0.75250.7525 µmfJLm toto pump pump a a1 -cm1-cm-long -long proustite crystal.crystal. AnAn infra-infra red bandpass of 22 cm-cm" 1l isis tunable fromfrom 99 pmjum toto 1111 µm/im byby angleangle tuning the proustite crystal.crystal. TheThe upconversionupconversion quantumquantum efficien- efficien-
--1 Image Imageformed formed by astronomical by asfronomical telescope felesco,
tI} Germaniumwindow J L Aperture stop Centimeters Ca ra \\ Figure 4. a Orionis spectrum, takentaken in oneone-half -half hourhour onon thethe 120" teletele ) scope atat Lick Observatory by Smith.26Smith. 26 TheThe featuresfeatures are all telluric, asas can J Interferencenterference ^Proustite` Proustite crystalcrystc be seenseen by by comparisoncomparison withwith the lower curve, taken at a different,different, drierdrier site.site. Image filters - ———J------intensifieri tube tensifier tube Monochromator-*-Monochromator though the feature detected is of tellurictelluric origin, and hence not of particularparticular astronomicalastronomical interest, thisthis observationobservation indicatesindicates that upconvertersupconverters areare nearlynearly capablecapable ofof providing providing usefuluseful spec-spec- troscopic information regardingregarding astronomical sources.sources. Krypton ion laserlaser Smith also was troubled byby anan unexpectedunexpected sourcesource ofof noisenoise inin (0.2510.25 WW at 0.75250.7525 p.m)/trr his system, and in fact thisthis additionaladditional noisenoise sourcesource waswas thethe pri-pri mary limitation to hishis systemsystem NEP.NEP. SmithSmith mademade aa concertedconcerted effort to ascertainascertain thethe causecause ofof thisthis noisenoise contribution, andand con-con cluded that nonenone ofof the the usual usual explanations explanations (crystal (crystal emissivity, emissivity, Figure 6.6. 10 µmMm imagingimaging upconverterupconverter ofof Boyd.29Boyd. 29 TheThe monochromatormonochromator radiation from his crystal oven, fluorescence of optical compo is used toto eliminate backgroundbackground lightlight from from the the laser laser discharge discharge tube.tube. Col-Col- radiation from his crystal oven, fluorescence of optical compo- limated infraredinfrared radiationradiation isis mixedmixed withwith thethe laser laser beambeam inin thethe proustiteproustite nents) could contributecontribute thethe measuredmeasured amount.amount. SmithSmith andand crystal. The interferenceinterference filters passpass the sum frequency while rejectingrejecting Townes27Townes27 have have developeddeveloped a a theorytheory whichwhich couldcould accountaccount forfor the laserlaser frequency, providing a factorfactor ofof 1018 10 18 discriminationdiscrimination betweenbetween the unexpected noise sources encounteredencountered byby soso many of the thethe two frequencies.frequencies. The sum frequencyfrequency imageimage is amplified by thethe imageimage workers inin upconversion. InIn this theorytheory thermalthermal energyenergy mixesmixes intensifier tubetube andand recordedrecorded photographically.
566 // OPTICAL OPTICAL ENGINEERING ENGINEERING / /Vol. Vol. 16 16 No. No. 6 6/ November/ November-December -December 19771977 Downloaded From: http://opticalengineering.spiedigitallibrary.org/ on 02/27/2015 Terms of Use: http://spiedl.org/terms INFRARED UPCONVERSIONUPCONVERSION FORFOR ASTRONOMY
cy is 2 xx 10-', 10" 7 and, and the the system system quantum quantum efficiency efficiency is is 1 1x x 10-9 10~9 . Should upconversion systems become comparablecomparable with otherother The angular resolution ofof the.the3 system is very nearly diffractiondiffraction infrared systems in termsterms ofof theirtheir sensitivity,sensitivity, thisthis easeease of opera-opera limited; laserlaser inducedinduced heatingheating ofof thethe proustite crystalcrystal distorts thethe tion couldcould makemake upconvertersupconverters thethe preferredpreferred infraredinfrared detectors.detectors. sum frequency wavefronts so as to degrade system resolution to Furthermore, mostmost upconvertersupconverters havehave anan inherentlyinherently narrownarrow in-in 75% of theoretical.theoretical. SumSum frequencyfrequency picturespictures containcontain approxi-approxi frared bandpass , andand ifif spectralspectral informationinformation regarding regarding anan astro-astro mately 300 resolution elements. nomical source is desireddesired no additionaladditional losseslosses need be sufferedsuffered As an astronomical device, the system is mounted at the focus by using aa monochromatormonochromator inin frontfront of the detector. of thethe 1.51.5 mm McMath McMath SolarSolar TelescopeTelescope ofof KittKitt PeakPeak NationalNational Ob-Ob servatory, yielding aa field of view of 40 seconds of arc, with aa Upconversion systems appear mostmost attractiveattractive inin termsterms ofof infra-infra resolution ofof 2.52.5 secondsseconds of of arc. arc. ImagesImages were obtained ofof thethe Sun,Sun, red imaging. With thethe exception ofof thethe workwork ofof Westphal Westphal etet aí30,al30 , Moon, Mercury, and thethe starstar VY VY CanisCanis Majoris,Majoris, inin limitinglimiting ex-ex infrared imagingimaging devices are stillstill not commoncommon instrumentsinstruments inin as-as posure times of 2 sec,sec, 22 min,min, 1 min, andand 1515 minutes,minutes, respec-respec tronomy, duedue mainlymainly toto the the expense expense of of two two-dimensional -dimensional detector tively. Results are shown in Figure 7. Comparisons of astronom-astronom arrays andand mechanicalmechanical instabilitiesinstabilities inin rasterraster scanning scanning systems.systems. ical seeing at 1010 pmjum andand atat visiblevisible wavelengths were also obtained. More sensitive, diffraction limitedlimited infraredinfrared imagingimaging upconverters could easily outperform otherother infraredinfrared systemssystems forfor highhigh angularangular resolution work. Sky Sun MH Diffraction-Diffraction-limited limited resolutionresolution One possible direction for furtherfurther improvementsimprovements inin upconver-upconver sion techniquestechniques wouldwould bebe thethe eliminationelimination ofof thethe unexplainedunexplained noise source thatthat hashas afflicted afflicted several several workers, workers, as as discussed discussed pre-pre viously in this article. Since these noise sources do not seem to be ofof aa fundamentalfundamental nature,nature, theythey can can probably probably be be eliminated, eliminated, perhaps with the fabricationfabrication ofof moremore perfectperfect crystals.crystals. Conversion efficiencyefficiency isis proportionalproportional to laserlaser power per unit area forfor thethe standardstandard upconverter upconverter utilizing utilizing three three-wave -wave mixing;mixing; for thethe fourfour-wave -wave mixing technique of BloomBloom etet al22,al22 , thethe con-con version efficiency scales as the squaresquare ofof thisthis quantity.quantity. AA signifi-signifi a. Solar limb b. Lunar limblimb cant improvementimprovement inin upconverterupconverter performance performance could could be be achievedachieved by thethe developmentdevelopment of cw laser sources inin the 1010 to 40 watt ^Visible,Visible appearance range, or higher, asas opposed toto the 11 toto 44 wattwatt range range currentlycurrently
Illllpill' available. IfIf tunable lasers of suchsuch powerpower becomebecome available,available, the resonant techniquestechniques of BloomBloom etet al22al22 andand BethuneBethune etet al24al24 maymay be exploited forfor astronomicalastronomical applications.applications.
Conversely, a significant improvement couldcould bebe realizedrealized with il existing laser systems if crystalscrystals withwith aa largerlarger nonlinearnonlinear coef-coef ficient dd becamebecame available.available. ItIt shouldshould bebe recalledrecalled thatthat thethe conver-conver sion efficiency depends onon the squaresquare of d.d. TheThe propertiesproperties of a number ofof crystalscrystals ofof interestinterest in in upconversion upconversion studies studies areare listedlisted in TableTable 1.1. KDP,KDP, usedused inin manymany ofof the the early early experiments experiments inin non-non linear optics, isis includedincluded forfor comparison. comparison. OfOf thethe rest,rest, onlyonly lithiumlithium c. Mercury d. VY CanisCanis MajorisMajoris niobate, lithium iodate andand proustiteproustite havehave beenbeen usedused inin upcon-upcon verters for astronomy. It willwill bebe notednoted thatthat several several of the other Figure 7. Infrared imagesimages of a number of astronomical objects fromfrom the materials have valuesvalues ofof dd significantly largerlarger thanthan thosethose ofof the work ofof Boyd."Boyd. 29Computer Computer-generated -generated plots plots of of digitized digitized photographic photographic nega- nega tives are shown. In eacheach case the the fieldfield of viewview isis round,round, andand aa spuriousspurious crystals currently beingbeing employedemployed inin astronomicalastronomical upconverters.upconverters. spot is introduced at the center of the fieldfield fromfrom the hole inin thethe collimatingcollimating Cinnabar could potentially provideprovide aa factorfactor ofof 44 improvementimprovement inin mirror shown in FigureFigure 6. NoteNote thethe enhancementenhancement in thethe signalsignal from thethe quantum efficiencyefficiency overover proustite,proustite, and and ZnGeP2 ZnGeP2 couldcould provideprovide aa subsolar pointpoint on Mercury. TheThe detectiondetection ofof VY VY Canis Canis Majoris Majoris is is marginal.marginal. factor ofof 2525 improvement.improvement. NeitherNeither of of these these crystals crystals isis currentlycurrently available inin large samples with good transmission, but withwith suf-suf ficient work theythey couldcould perhaps perhaps bebe fabricated. fabricated. CdGeAs2CdGeAs2 hashas aa Future ofof UpconversionUpconversion in Astronomy nonlinear coefficient whichwhich predictspredicts aa conversionconversion efficiencyefficiency 400 It isis clear that toto datedate upconversion Upconversion hashas notnot proved proved toto bebe aa par-par times greater thanthan that ofof proustite.proustite. ThisThis crystalcrystal isis notnot trans-trans ticularly useful technique in astronomy. Only the very brightest parent in thethe visible,visible, andand thusthus isis not not of of use use for for upconversion. upconversion. infrared celestial sources can even be detecteddetected withwith existingexisting up-up- Its largelarge valuevalue ofof dd isis veryvery suggestive,suggestive, however,however, that significantsignificant conversion systems,systems, andand no spectroscopicspectroscopic information hashas yet improvement in mixing crystals is possible. been obtained fromfrom thesethese studies.studies. However,However, upconversionUpconversion meth-meth ods have steadily improvedimproved inin their sensitivity and it isis likelylikely Furthermore, it shouldshould bebe notednoted thatthat the the resonantlyresonantly enhancedenhanced that thethe techniquetechnique willwill continuecontinue toto improve.improve. ItIt seemsseems usefuluseful toto gas phase upconversion techniques ofof BloomBloom etet al22 al22 andand Bethune list here some of thethe advantagesadvantages and some of thethe limitationslimitations ofof et al24al24 have hardly been exploited forfor sensitivesensitive infrared detection, upconversion detectiondetection systems, and to studystudy thethe possible possible im-im and significant improvements in upconverter sensitivitysensitivity maymay bebe provements inin upconversion techniques that seemseem possible at realized by these methods. this time. Since the timetime ofof the the early early suggestions10 suggestions10 thatthat upconversionupconversion be used in infrared detection systems,systems, infrared photoconductive detectors have become increasingly more sensitive, andand thusthus the potential competitive advantage of upconversionupconversion systemssystems in Acknowledgments terms ofof NEPNEP isis limited.limited. UpconversionUpconversion systemssystems do havehave thethe in-in The authorauthor acknowledgesacknowledges usefuluseful discussions discussions regardingregarding upconver-upconver herent advantage thatthat they need not bebe cooledcooled toto cryogeniccryogenic tem-tem sion techniquestechniques withwith C.C. H.H. Townes,Townes, J.J. Falk,Falk, T.T. Kostiuk,Kostiuk, H.H. A.A. peratures, in contrast to mostmost otherother low low noise noise infraredinfrared systems.systems. Smith, J. H.H. Lacy,Lacy, D.D. N.N. Matsakis,Matsakis, and J.W.V.J.W.V. Storey.
November-DecemberNovember -December 1977 1977 / Vol./ Vol. 16 16 No. No. 6 6/ OPTICAL/ OPTICAL ENGINEERING ENGINEERING / / 567 Downloaded From: http://opticalengineering.spiedigitallibrary.org/ on 02/27/2015 Terms of Use: http://spiedl.org/terms ROBERT W. BOYD
Table 1. Properties of Some Nonlinear Crystals ofof Interest in Upconversion
Approximate Refractive Birefringence Measured Values Crystal Transparency Index nonQ no-none -nQ of d in units Reference Name of Crystal Point Group Range (nm)(jam) at 11 ,umjum at 11 gmMm of 101CT 9 esu Number
Potassium dihydrogen 42m 0,2-1.00.2-1.0 1.5 -0.0397 d36d36 = 1.5±20% 31 phosphate (KDP)
Lithium niobate d15d ls -= 18±35%18±35% (LiNb0(LiNbO33 ) 3m 0.4-4.5 2.2487 -0.084 d12d22 = 10±35%10±35% 32
Lithium iodate (LiIO(LilO33 ) 6 0.3-5.5 1.8517 -0.1349 d31d31 =- 17±30%17±30% 33
Proustite d31d31 = 40 (Ag3AsS3)(Ag3 AsS3 ) 3m 0.6-13 2.8264 -0.2362 dd2222 =60= 60 34
Pyragyrite d31d31 = 30±30% (Ag3(Ag3 SbSSbS3)3 ) 3m 0.7-13 2.973 -0.152 d22d22 = 32±30%32+30% 35
Cinnabar (HgS)(HgS) 32 0.6-15 2.7120 +0.293 ddttn == 120±30%120±30% 36
Zinc germanium phosphide 42m 0.75-12 3.2478 +0.0476 d14d 14 =- 270±45% 37 (ZnGeP(ZnGeP2)2 )
Cadmium germanium arsenide 42m 33-17 17 --- - dd1414 == 1100±50% 38 (CdGeAs(CdGeAs2)2 )
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