Use of polycrystalline Nd:YAG rods to achievepure radially or azimuthally

polarized beams from high-average-power lasersInon Moshe,* Steven Jackel, Yaakov Lumer, Avi Meir, Revital Feldman, and Yehoshua Shimony

Applied-Physics Division, Soreq NRC, Yavne 81800, Israel*Corresponding author: inon@soreq.gov.il

Received April 29, 2010; revised June 9, 2010; accepted June 28, 2010;posted July 6, 2010 (Doc. ID 127534); published July 19, 2010

We report maintenance of perfect radial-polarization purity in high-power, cw pump chambers using strengthened,polycrystalline Nd:YAG laser rods. Although the cubic symmetry of single-crystal rods caused threefold symmetricbirefringence due to shear stresses at the ends of the pumped zone, polycrystalline rods macroscopically behaved asisotropic material and enabled polarization preservation. Elimination of this source of depolarization prevents themajor source of bifocusing aberrations in a chain of amplifiers. © 2010 Optical Society of AmericaOCIS codes: 140.3570, 140.6810, 260.1180.

High-average-power radially or azimuthally polarized la-sers are important for applications such as industrial cut-ting and drilling [1,2]. Radially or azimuthally polarizedbeams in rod-based high-power lasers also improve thebeam quality and efficiency by bypassing the effects ofthermally induced birefringence [3]. In this case, onemust consider effects that degrade polarization purityin such lasers. Serious polarization degradation couldarise, for instance, from a nonradially symmetric pumpdistribution that induces azimuthal aberrations [4]. As ra-dial and azimuthal polarizations are not space invariant,these aberrations deteriorate polarization purity as thebeam propagates, with maximum depolarization occur-ring close to the far field of the aberration plane. Anotherdepolarization source arises from mismatching of thebeam propagation axis and the rod central axis. Our anal-ysis showed that a lateral alignment error of up to�50 μm is acceptable for good-beam-quality-laser sys-tems based on 10 mm diameter rods.The amplifiers used in our laser system were based on

Soreq’s stripe through apertured reflector (STAR) 3 kWpump chambers. The pump chambers were designed toside pump Nd:YAG rods (8 or 10 mm diameter by205 mm long with 150 mm pump length) using seven hor-izontal diode arrays, emitting at a wavelength of 808 nm.Output power of 3 kWwas achieved from a single 10 mmSTAR in a short-cavity multimode oscillator pumped with7 kW of diode light. Slope and overall light-to-light effi-ciencies were 48% and 43%, respectively. To minimize azi-muthal aberrations, the laser pump chambers weredesigned to have a perfect radially symmetric distribu-tion. Figure 1(a) presents the measured fluorescenceat the laser rod cross section to demonstrate the radialsymmetry of the pump distribution. Optical path differ-ence for the pumped rod measured using a Shark–Hartman sensor from Imagine Optics (HASO32) showedspherical aberrations but negligible azimuthal aberra-tions. Figure 1(b) presents the measured wavefront (WF)distortion in a 7:5 mm diameter output beam after pas-sage through a 10 mm STAR amplifier pumped by 6 kWlight power, while focusing was subtracted. Figure 1(c)presents the WF distortion after the spherical aberrationswere corrected using a wave-plate corrector. The resi-

dual azimuthal aberrations caused relatively low WF de-formation with peak to valley <0:25 μm. Such pumpchambers minimized the depolarization induced by azi-muthal aberrations. In cases where azimuthal aberrationsdid exist (induced by diode pump arrays with unmatchedwavelength and/or power) WF correction was applied inthe rod’s near-field (or in a relay-imaged plane) to elim-inate propagation-induced depolarization. Near-field de-polarization due to phase-front aberrations is estimatedto be negligible for the pump chambers used in this work,even when pumped to maximum power. This is becausedepolarization induced by phase-front aberrations alsomanifests itself as an azimuthally dependent intensityprofile in the far field. The intensity profile, in fact, re-mained invariant.

More serious depolarization occurred in stronglypumped single-crystal Nd:YAG rods. Two-dimensional(2D) polarization measurements of an azimuthally polar-ized beam that passed through a pumped Nd:YAG rodshowed a sixfold depolarization at the rod principal plane[5]. This depolarization was negligible at low pumppower but became significant at pump powers greaterthan 1:8 kW (120 W=cm) and reached a value of 8% atpump power of 5:25 kW (350 W=cm). Analysis showedthat the source of this depolarization was the shear stressat the boundary between the pumped and unpumped re-gions along the laser rod axis [5]. In cubic crystals wherethe photoelastic tensor is not isotropic, the effect of these

Fig. 1. (Color online) Measured fluorescence distributionthrough the rod cross section that demonstrates (a) a radiallysymmetric pump distribution and (b) the resultant WF distor-tion of an azimuthally polarized beam for a pump power of6 kW. The WF distortion is dominated by spherical aberrationsin (b). (c) Measured WF after correction of the spherical aber-rations with an aspheric phase plate to demonstrate low azi-muthal aberrations.

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stresses on the refractive index does not vanish (as itdoes in isotropic materials) but induces threefold bire-fringence. Such birefringence is complex and difficultto compensate. In addition to the resulting depolariza-tion, it affects the beam quality in two ways: the minoreffect is by inducing threefold WF deformation (trefoilaberration), while the major indirect effect that is rele-vant in an amplifier chain is the interaction of the depo-larized beam with bifocal lensing of the followingamplifier rods.A straightforward solution to eliminate this depolariza-

tion effect is the use of isotropic rod material. The rele-vant material for our pump chambers is polycrystalline(ceramic) Nd:YAG that is composed of randomly or-iented nanocrystals that macroscopically behave as anisotropic material [6–8]. Polycrystalline rods werestrengthened by a process that included grinding to a finemicroroughness followed by chemical etching in hotortho-phosphoric acid [9]. This procedure enhancedthe strength of the polycrystalline ceramic Nd:YAG ele-ments in a way similar to the strengthening of single-crystal Nd:YAG by removing the outer defective layerand, thus, reducing the size of the surface flaws [10].The optical setup for 2D polarization measurement

is presented in Fig. 2. An azimuthally polarized probebeam was propagated through the pumped rod(thermal focus ¼ 12 cm) along a symmetrical path withequal input and output beam diameters (7:5 mm) andwith a large fill factor (maximum beam diameter≈8:5 mm). The beam cross section at the output principalplane was relay imaged onto a CCD camera after passingthrough a half-wave plate and a cube polarizer. Fourvideo frames were captured at 0, 45, 90, and 135 deg po-larization orientations, and the 2D polarization orienta-tion was calculated using Stocks parameters [11].Depolarization measurement results for an initially94.7% azimuthally polarized beam that has passedthrough single-crystal and polycrystalline Nd:YAG rodsas a function of heat power are presented in Fig. 3.The heat load was determined on the basis of thermalfocus measurements (Pheat=Pdiode ¼ 0:25). One can seethat strong polarization deterioration resulted in single-crystal Nd:YAG, while polarization preservation wasachieved in polycrystalline Nd:YAG over the full heatpower range. Figure 4 presents the depolarized beam in-tensity profiles at maximum pump power as derived fromthe 2D polarization measurements. Figures 4(a) and 4(b)are the measured and simulated depolarized intensity dis-tributions for the case of single-crystal rods. Figure 4(c)shows the result for a polycrystalline rod. The depolar-ized beam profiles contained circular rings and sixfoldsymmetry structures. The source of the rings is the ra-dially symmetric birefringence caused by the rod heatingand cooling that induced phase delay between the azi-muthal- and radial-polarized parts of the 94% polarization

pure beam (approximately equal to the polarization pur-ity of the oscillator). The sixfold symmetry results fromthe threefold birefringence induced by the edge effect incubic crystals. Depolarization of 8% with sixfold symme-try was measured with the single-crystal rod. The poly-crystalline rod resulted in a 0.3% depolarization thatshowed no sixfold symmetry. The measurement accu-racy was �0:5%, so we conclude that no depolarizationwas measured in the polycrystalline rod. These resultsdemonstrate the superiority of polycrystalline rods oversingle-crystal rods in terms of depolarization elimination.

The advantage of polycrystalline Nd:YAG rods inmaintaining polarization purity of radially/azimuthallypolarized beams was tested in a master-oscillatorpower-amplifier (MOPA) configuration. The oscillatorwas designed to produce radially polarized modes whilesuppressing azimuthally polarized modes, based on thethermally induced bifocusing effect [3,12]. Details onsuch oscillators based on the STAR pump chamber aredescribed in [13]. To achieve a stable oscillator beamwith a smooth profile, the resonator was designed in asimple configuration with no spherical aberration correc-tion and was run with a relatively low power of 60 W. Theoutput beam was a TEM01

� doughnut mode with M2 ¼2:5 that was measured to be 94.2% radially polarized.To experience less thermal lensing in the amplifier chain,a 90° quartz rotator was used extracavity to convert theradial polarization to azimuthal polarization. The oscilla-tor beam then passed through three relay-imaged ampli-fiers. The pump light power to each amplifier was 6 kW.The polarization was measured at the exit plane of eachamplifier, and the result is presented in Fig. 5, wherebeam intensity in the required polarization is compared

Fig. 2. Setup for measuring the thermal aberrations in thepumped rods.

Fig. 3. Polarization measurement results for an initially 94.7%azimuthally polarized beam after passage through a STAR pumpchamber as a function of the heat load inside the laser rod.

Fig. 4. (Color online) Depolarized intensity in an azimuthallypolarized beam after passage through a single-crystal Nd:YAGrod: (a) measured, (b) simulated results, and (c) measuredresults after passage through a polycrystalline rod.

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to the intensity in the orthogonal polarization. The outputbeam polarization from the three amplifiers was 93.8%azimuthally polarized, so no depolarization was observedwithin the measurement accuracy.The output power from the MOPA was 1:62 kW, while

the first, second, and third amplifiers contributed 210,555, and 800 W, respectively. The third amplifier contrib-uted half of the maximum available power at saturation(taking into account the overlap efficiency of the beamwith the rod volume). The amplified oscillator beam pro-vided some cooling to the amplifier rods within the laserbeam volume by converting stored energy to laser energybefore it could be converted into heat. This may alter thetemperature and thermal-stress profiles inside the rod. Inour case, however, of a radially symmetric beam, this ef-fect will not affect the polarization symmetry of the rod.Thus, the purity of radially or azimuthally polarizedbeams will be maintained. Running the oscillator a higheroutput power of 550 W to produce an azimuthally polar-ized beam of moderate M2, we observed no depolariza-tion and obtained 4:0 kW of MOPA output power in anazimuthally polarized beam.

To summarize, polycrystalline (“ceramic”) Nd:YAG la-ser rods were found to be superior to single-crystalrods in terms of eliminating depolarization to radially/azimuthally polarized beams. While the crystallographicsymmetry of single-crystal YAG is cubic, it yields aphotoelastic tensor that is anisotropic and that resultsin sheer-stress-induced depolarization. Polycrystallinerods behave macroscopically as an isotropic materialand suffer no such depolarization. Polarization preserva-tion was measured for azimuthally polarized beams in in-dividual polycrystalline Nd:YAG rods pumped with 6 kWof diode light. The polycrystalline rods showed no mea-surable depolarization (<0:3%), in contrast to the 8% de-polarization measured using single-crystal rods. Threepolycrystalline rod-based amplifiers were used in aMOPA to produce a high-power pure azimuthally polar-ized beam.

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Fig. 5. (Color online) Beam intensity in required and orthogo-nal polarization states at the exit of each MOPA stage. Beamintensities were produced based on 2D polarization measure-ment results. The same intensity scale has been maintainedbetween the pairs of images with orthogonal polarizations. Out-put powers from the oscillator first, second, and third amplifierswere 60, 270, 825, and 1625 W, respectively.

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