Generation of high-average-power visible lightin periodically poled nearly stoichiometric

lithium tantalate

Allen J. Tracy,1,* Camilo Lopez,1 Allen Hankla,1 Douglas J. Bamford,2

David J. Cook,2 and Scott J. Sharpe2

1Lockheed Martin Coherent Technologies, 135 South Taylor Avenue, Louisville, Colorado 80026, USA2Physical Sciences Inc., 2110 Omega Road Suite D, San Ramon, California 94583, USA

*Corresponding author: allen.tracy@lmco.com

Received 6 August 2008; revised 19 December 2008; accepted 19 December 2008;posted 9 January 2009 (Doc. ID 99577); published 4 February 2009

Sum-frequency generation (SFG) of >16W of 589nm light has been achieved by a single pass through a20mm long, undoped, periodically poled, nearly stoichiometric lithium tantalate (PPSLT) crystal. This,to our knowledge, represents the highest reported average power in the visible produced by a single-passSFG crystal and the highest visible average power produced by PPSLT. The stoichiometric lithiumtantalate crystal was grown directly from the melt without magnesium doping. © 2009 Optical Societyof America

OCIS codes: 190.0190, 190.2620, 190.4400, 140.3580.

1. Introduction

Periodically poled nearly stoichiometric lithiumtantalate has been the subject of recent researchinvolving quasi-phase-matched (QPM) nonlinear fre-quency generation of visible light. The advantages ofnearly stoichiometric lithium tantalate over othernonlinear materials, such as congruent lithium nio-bate (CLN) and potassium titanyl phosphate (KTP)include a higher optical damage threshold, greaterUV transparency, less susceptibility to photochromiceffects such as gray-tracking, and a lower coercivefield. In order to reduce the density of defects primar-ily responsible for photochromic and photorefractiveeffects and common in congruently grown crystals,various techniques for producing crystals with com-positions closer to stoichiometry have been devel-oped, including vapor-transport equilibrium (VTE)[1] and growth directly from a lithium-rich melt[2]. Material produced by VTE has been calledvapor-transport equilibrated nearly stoichiometric

lithium niobate (VSLT) [1] to distinguish it frommaterial grown directly from the melt (referred tohenceforward as SLT). These growth processes resultin fewer defects and a more stoichiometric material,as indicated by the relatively low coercive field.Periodically poled VSLT has been used to generate5W of average power at 532nm [1], while periodi-cally poled SLT doped with 1mol:% of magnesiumhas been used to generate 7W of average power at542nm [3]. The SLT crystal that was tested in thepresent effort, which was grown at Deltronic CrystalIndustries under conditions which have beendescribed previously [4], had a coercive field of∼2:5kV=mm. Although this is much lower thanthe coercive field of congruent lithium tantalate(∼20kV=mm) [2], it is higher than the smallestcoercive field measured in VSLT (<100V=mm) [1].Since the coercive field is a linear function of thedeviation from perfect stoichiometry [2], the crystalused in this work had a Li=ðLiþ TaÞ ratio between49.8% and 49.9%. A 0:5mm thick, 76:2mm diameterwafer of SLT was patterned at Physical Sciences Inc.(PSI) using techniques that have been describedelsewhere [5] to produce periodically poled nearly

0003-6935/09/050964-05$15.00/0© 2009 Optical Society of America

964 APPLIED OPTICS / Vol. 48, No. 5 / 10 February 2009

stoichiometric lithium tantalate (PPSLT). The waferwas diced to produce the 20mm × 10mm crystal usedfor most of this work. Antireflection coatings wereapplied to both end faces. The crystal contained eightdifferent QPM gratings with periods ranging from10:5 μm to 11:2 μm. The effort described here was pri-marily focused on the grating with a period of10:8 μm, which had a phase-matching temperatureof 46:6 °C for the generation of 589nm radiation bysum-frequency generation (SFG).

2. Experimental Setup

The crystal was tested in conjunction with the devel-opment of a commercial sodium guide star laser thatwas delivered to the Gemini North Observatory inMarch 2005. The baseline design of the system calledfor single-pass SFG of 589nm light by mixing IRwavelengths of 1319 and 1064nm. The guide starlaser system includes two custom mode-lockedoscillators, designed and built by Lockheed MartinCoherent Technologies (LMCT) and based on thedesign shown in Fig. 1 [6].Several researchers have achieved significant

technical steps along the development path ofsodium guide star laser systems [7–9]. The laserarchitecture of the LMCT-built system is based onCW mode-locked solid-state lasers, and is simplerand more robust than current Nd:YAG SFG schemesbased on either CWor macropulse/micropulse lasers,making it more compatible with turnkey deploymentand minimal maintenance in the observatory envir-onment. The mode-locked format provides signifi-cantly higher peak intensity than CW, enablingmore efficient SFG conversion, and dispenses withcomplex resonant intensity enhancement systemsand injection-locking electronics. The laser is alsofree of the thermal and intensity transients thatare inherent in the macropulse format. The chosenformat is also compatible with upgrade paths forthe higher power levels and complex pulse formatsrequired for multiconjugate adaptive optics (MCAO)systems on 8–100m telescopes.The 1064nm laser operated at a nominal pump

power of 104W, generating 26W of mode-lockedTEM00 1064nm power for a 25% optical-to-opticalefficiency while the 1319nm laser generated 12Wof TEM00 mode-locked power for an efficiency of 11%.Both lasers exhibited approximately 2% depolariza-tion and good beam quality with anM2 measurementof 1.2 for 1064nm and 1.1 for 1319nm (Fig. 2). Whenmode locked the 1319nm laser typically generated

pulses of 560ps duration separated by 12:7ns. Thepulse bandwidth was 840MHz full width at half-maximum (FWHM), resulting in a nearly transformlimited time-bandwidth product of 0.47. The1064nm laser typically generated 600ps pulses witha FWHM bandwidth of 690MHz, resulting in a time-bandwidth product of 0.41.

Efficient sum-frequencymixing of two input beamsis particularly challenging because it requires bothpump beams to be overlapped in time and space.For this particular system, these requirements aremade even more difficult as the system is mountedon the telescope and performance must be main-tained as the telescope rotates and tilts to 60° fromzenith.

A requirement of less than 200ps relative pulse de-lay (for 1ns input pulses) was determined based oninitial SFG modeling using Sandia Nonlinear Optics(SNLO) code [10]. The model was later verified ex-perimentally. This requirement necessitated the im-plementation of an active timing stabilization circuitthat resulted in a relative delay of the two beams ofless than 50ps RMS, corresponding to less than a 2%change in conversion efficiency.

Timing stability control is required to keep the twomode-locked lasers in temporal and spatial synchro-nization. A card in the PXI (PC-based computer plat-form) controller generates a programmable RFsource. This signal is amplified to drive two AOMssimultaneously. The generator card also has a digitalreference output that feeds to a phase comparatorcircuit on a custom designed board. After some signalconditioning and amplification, the mode-lockedpulses are used for the other side of the emitter-coupled logic (ECL) phase comparator. Low-passfiltering of the phase comparator output producesa signal that is proportional to the phase error

Fig. 1. Schematic of 1064nm cavity layout. HR, high reflector;DM, dichroic mirror; QR, quartz rotator; OC, output coupler;FM, fold mirror.

Fig. 2. Measurements of the 1319nm laser mode locked at 12Wshowing an M2 of 1.1.

10 February 2009 / Vol. 48, No. 5 / APPLIED OPTICS 965

between the RF reference source and the laser mode-locked pulses. The error signal is sampled by a cardin the PXI and processed via a proportional-integral(PI) controller implemented in LabVIEW. The PI loopcontroller output is converted to pulses that drive astepper motor stage on which an HR mirror is at-tached. This is the mechanism that controls thephase of the mode-locked pulses. By setting an offsetin the PI controller set point, the relative phase of thetwo mode-locked lasers can be adjusted. This servowill then keep the two lasers tracking over operatingtemperatures, which will assure good SFG conver-sion of the two overlapped beams.The two IR beams were combined via custom

coated, high-reflectivity mirrors. Spatial overlap ofthe two beams was ensured by using an InGaAs cam-era to image both beams at several planes within thecrystal. The beams were focused at the midpoint ofthe crystal and the focusing optics were designedaccording to the Boyd–Kleinman [11] criteria, suchthat the Rayleigh length of the beams are approxi-mately one-half of the crystal length. The crystalwas placed in an oven manufactured by SuperOptronics. The 589nm power was monitored by athermopile detector.

3. Experimental Results

Modeling was performed using SNLO prior to the la-boratory work in order to get a basis as to expectedconversion efficiency. This allowed researchers toascertain that experimental conditions, such as tem-perature control, input polarization, and beam over-lap, were nominal. Staying in a low-power regime,the pulse-timing feedback loops were optimized toachieve nominal temporal overlap of the pulses with-in the crystal. After this was accomplished, laserparameters, such as spot size, M2, pulsewidth, andbandwidth, were measured. These parameters werethen inserted into the model and the nonlinear coef-ficient was varied until the model matched theexperimental data points. Once the model and datawere well matched across the input power parameterrange, confidence in the model’s ability to predict thesystem conversion efficiency and 589nm outputpower was high.The first set of experiments involved measuring

the power at 589nm as a function of temperaturein order to compare the phase-matching propertieswith predictions based on the published Sellmeierequation for SLT [12]. The experimental phase-matching temperature (as shown in Fig. 3) was46:6 °C with a temperature acceptance of 4:9 °C.The phase-matching temperature was significantlylower than the predicted value of 65:0 °C for first-order QPM in a grating with a period of 10:8 μm.Better agreement between experimental and pre-dicted phase-matching temperatures was obtainedfor QPM gratings with shorter periods (higherphase-matching temperatures), as shown in Fig. 4.Similar discrepancies between the experiment and

the predictions of the published Sellmeier equation

have been seen for second-harmonic generation(SHG) of a 1064nm laser in other PPSLT crystalsproduced from the same wafer [13]. The symmetryof the bandwidth profile assures us that the crystalis heated evenly. The differences between the experi-mental and predicted phase-matching temperaturesand temperature acceptance suggest that the opticalproperties of the SLT crystal used in this work arenot identical to the optical properties of the SLTcrystals used to construct the Sellmeier equation[4], which came from earlier generations of crystalgrowth.

The output power at 589nm as a function of totalnear-IR input power for the QPM grating with a per-iod of 10:8 μm is shown in Fig. 5. Photon balance (i.e.,a ratio of 1.0 between the number of 1064nm photonsper unit time incident upon the PPSLT crystaland the number of 1319nm photons per unit time

Fig. 3. Relative power at 589nm as a function of temperature fora 20mm long PPSLT crystal.

Fig. 4. QPM temperature for poling periods from 10.5 to 10:8 μmas calculated from the Sellmeier equation and as measured.

966 APPLIED OPTICS / Vol. 48, No. 5 / 10 February 2009

incident upon the crystal) was maintained for thetwo near-infrared (NIR) beams up to 33W of totalNIR power, at which point the average power ofthe 1319nm laser could not be increased any further.At the highest power level tested, the number of1064nm photons per unit time incident upon thePPSLT crystal was 1.2 times greater than the num-ber of 1319nm photons per unit time incident uponthe crystal. In this experiment, the pulse widths ofthe two input NIR beams were matched at∼560ps and both beams were focused to a beam dia-meter of ∼60 μm FWHM. The maximum measured589nm power was 16:5W with an IR total inputpower of 37:1W, resulting in a conversion efficiencyof 44.6%. The roll-off in conversion efficiency at thehighest power levels can be attributed to the lackof photon balance. To our knowledge, this is the high-est reported average power in the visible created bysingle-pass SFG in PPSLT.To simulate actual operating conditions when used

as a laser guide star, the crystal was subjected to lim-ited stability testing during three separate 14h runsover a three-day period, averaging ∼11:0W of589nm power. The power stability during the lastof these runs was 10:8� 0:6W RMS. The beam pro-file of the output beam was also monitored over the42h test period. The beam was very stable and themeasured M2 values of the beam were 1.1 and 1.5in the horizontal and vertical dimensions, respec-tively. The larger M2 value in the vertical directionwas due to the enlargement of the input IR beamsin that dimension in order to avoid possible PRD.

4. Conclusions

PPSLT, produced from SLT that is grown directlyfrom a lithium-rich melt without magnesium doping,is a viable material for generating high-average-power visible light. By avoiding complicationsassociated with adding a dopant or performinghigh-temperature postgrowth processing, a simplerand more reliable crystal-growth procedure can beused. With an input average NIR power of 37:1W,a PPSLT crystal converted up to 45% of the NIRpower to 589nm, yielding over 16:5W with excellent(M2 ¼ 1:1 × 1:2) beam quality. The nonlinear opticalperformance of the PPSLT crystal matched the

results of numerical simulations based on SNLO [2]when an effective nonlinear coefficient of 5:3pm=Vwas assumed. The d33 coefficient of undoped SLThas been measured to be 14:9pm=V [14], which im-plies that deff should be 9:5pm=V in a sample with aperfect QPM grating. The deviation between thevalue of deff that fits the experimental data andthe predicted value of deff can be attributed to var-ious nonidealities in the experiment, including im-perfections in the QPM grating and imperfectspatial and temporal overlap of the interactingbeams. Phase-matching conditions differed signifi-cantly from predictions based on the published Sell-meier equation of SLT [3], suggesting that thematerial used in this work had slightly different op-tical properties from the material used to derive theSellmeier equation.

Short-term life tests of the crystal for QPM periodsof 10:7 μm and shorter (corresponding to phase-matching temperatures of 90 °C and higher) resultedin slow degradation of beam quality and conversionefficiency. Therefore, the crystal was more stablewith respect to optical damage at lower tempera-tures than it was at higher temperatures. This is asurprising finding, because photorefractive damage(PRD) in ferroelectric crystals is generally less harm-ful at high temperatures than it is at lower tempera-tures. For example, the first demonstration ofhigh-average-power green generation in periodicallypoled lithium niobate (PPLN) was carried out at atemperature of 200 °C to avoid the harmful effectsof PRD that were observed at lower temperatures[15]. A suggested explanation for the observed beha-vior is that photorefractive damage is too small to beobserved at any of the temperatures tested, whilesome other damage mechanism becomes weaker asthe temperature is decreased. Possible factors contri-buting to better behavior at lower temperaturesinclude higher thermal conductivity and lower in-trinsic absorption at the parasitic wavelengths of407nm and 379nm (produced by SFG between thenear-IR input beams and the 589nm beam). Futurework will focus on further investigation of the causesof the degradation and on characterizing PPSLTcrystals that are more stoichiometric than the sample used in this work.

This material is based upon work supported by theNational Science Foundation (NSF) and the UnitedStates Air Force Research Laboratory Directed En-ergy Directorate under contract F29601-03-C-0044.Any opinions, findings, conclusions or recommenda-tions expressed in this material are those of theauthors and do not necessarily reflect the views ofthe NSF or the U.S. Air Force.

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