Suppressing self-induced frequency scanning of a phase conjugate diode laser arrayusing counterbalance dispersionMartin Lo/bel, Paul M. Petersen, and Per M. Johansen

Citation: Applied Physics Letters 72, 1263 (1998); doi: 10.1063/1.120605 View online: http://dx.doi.org/10.1063/1.120605 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/72/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Fast scanning cavity offset lock for laser frequency drift stabilization Rev. Sci. Instrum. 81, 075109 (2010); 10.1063/1.3455830 Laser frequency stabilizations using electromagnetically induced transparency Appl. Phys. Lett. 84, 3001 (2004); 10.1063/1.1713050 Narrowing of high power diode laser arrays using reflection feedback from an etalon Appl. Phys. Lett. 77, 1080 (2000); 10.1063/1.1289652 Stability of the single-mode output of a laser diode array with phase conjugate feedback Appl. Phys. Lett. 76, 535 (2000); 10.1063/1.125810 Characterization and stabilization of fiber-coupled laser diode arrays Rev. Sci. Instrum. 70, 2905 (1999); 10.1063/1.1149847

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APPLIED PHYSICS LETTERS VOLUME 72, NUMBER 11 16 MARCH 1998

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Suppressing self-induced frequency scanning of a phase conjugatediode laser array using counterbalance dispersion

Martin Lo”bel,a) Paul M. Petersen, and Per M. JohansenOptics and Fluid Dynamics Department, Riso” National Laboratory, DK-4000 Roskilde, Denmark

~Received 14 October 1997; accepted for publication 19 January 1998!

Experimental results show that angular dispersion strongly influences the self-induced frequencyscanning of a multimode broad-area diode laser array coupled to a photorefractive self-pumpedphase conjugate mirror. Prisms or a dispersive grating placed in the external cavity opposing thematerial frequency dispersion of the phase conjugate BaTiO3 crystal suppress the frequencyscanning and stabilize the center wavelength and the output power. We show that the dispersion ofthe crystal is crucial for the mechanism of the frequency scanning. ©1998 American Institute ofPhysics.@S0003-6951~98!02211-6#

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High-power laser diode arrays have a strong tendencoscillate in numerous spatial and longitudinal modes. Conquently, such laser arrays have an output with high divgence far from the diffraction limit and very low temporcoherence.1 It has been demonstrated that optical phase cjugate feedback of the incident beam generated by a phrefractive BaTiO3 crystal increases both the spatial and tenhancement of coherence of a 1 W diode laser lasing at 800nm.2 Increase in the spatial coherence and line width narring effects have also been demonstrated in dye lasers uphotorefractive phase conjugate mirrors~PCMs!.3–5 A majordrawback in such cases is, however, that the optical feedbfrom the photorefractive crystal causes self-inducedquency scanning of the lasing wavelength. This sescanning effect has also been observed in a GaAlAs dlaser6 and in an argon laser7 coupled to a PCM. When thisfrequency scanning phenomenon was reported,3–7 the originwas—and up to today still is—unknown. Three differepossible origins have been suggested:~1! mode competitionin coupled cavities,5 ~2! accumulated Doppler shifts stemming from reflection of spontaneously moving gratingsthe photorefractive crystal,3 and ~3! asymmetric Bragg fre-quency selectivity of the photorefractive gratings.6 Here werule out explanation~1! since frequency scanning has beobserved in a single resonator formed between an ordinmirror and a PCM.3 As pointed out in Ref. 6, one can alsrule out explanation~2! since the response time of thBaTiO3 crystal is too low to explain the high scan rates oserved. The phase conjugate feedback forces the specmany nanometers away from the wavelength at whichgain medium has the highest gain and, as a result, thesystem does not seem to seek the wavelength that halargest overall optical gain. Therefore explanation num~3! seems very likely; however, the question still remainwhat is the origin of the asymmetric Bragg frequency seltivity? A fundamental perception of the mechanism of tself-induced frequency scanning is crucial in the processdeveloping better coherence properties of high-power ladiode arrays. It has recently been observed that the frequ

a!Electronic mail: martin.lobel@risoe.dk

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scanning does not take place when a low-power diode lais coupled to a stimulated photorefractive backscatterphase conjugator.8 In this letter, however, we investigatehigh-power laser diode coupled to a self-pumped phase cjugator based on a BaTiO3 crystal arranged in the Cat geometry in which the beam undergoes total internal reflectionthe crystal.9 The experimental data show that it is possiblesuppress the self-induced frequency scanning using coubalance dispersion and that the material frequency disperof the BaTiO3 crystal plays an important role in the frequency scanning process.

The index of refraction of the BaTiO3 crystal is depen-dent on the optical frequency~dispersion! and in the presenletter we investigate how the frequency scanning is alteby the angular dispersion of the different optical elemeinserted into the experimental setup. We investigate the sinduced frequency scanning experimentally using the tsetups shown in Figs. 1~a! and 1~b! and it is shown how thescanning behavior can be significantly changed by disperprisms or a dispersive grating placed in front of the BaTi3

crystal. For reasons given below we will name the setup

FIG. 1. Experimental setups with prisms. Broad-area diode laser couplea phase conjugate mirror. L1: lens Thorlab C230TM-Bf 54.5 mm, L2:spherical lensf 580 mm, BS: beamsplitter, M: mirror, WP: half-wave plat~a! dispersion enhancement setup,~b! dispersion compensation setup.

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1264 Appl. Phys. Lett., Vol. 72, No. 11, 16 March 1998 Lo”bel, Petersen, and Johansen

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Fig. 1~a! the dispersionenhancementsetup and the setup iFig. 1~b! the dispersioncompensationsetup, respectivelyThe laser array is a broad-area diode laser, SDL-2432, laat 811 nm (l0) with a maximum output power of 0.5 W. Thspectrum has a full width at half-maximum of 1.2 nm. Tlight is collimated with a lens~L1! and is sent through fou45° BK7 prisms and one 60° SF10 prism that are all orienat minimum deviation to add angular dispersion to the stem. After passing through the prisms the beam is focusea lens~L2! and directed to the phase conjugate mirror whis made of a 0°-cut rhodium doped~800 ppm! BaTiO3 crystalthat measures 5.135.535.3 mm3 ~a3a3c axis!. The crys-tal is placed after the focal point of lens L2 and is arrangeda self-pumped Cat configuration9 with an angle of incidenceof 60°. The half-wave plate~WP! ensures that the incidenbeam at the crystal surface is extraordinarily polarized. Treflection off the beamsplitter~BS! is directed to a spectrometer so that the spectrum of the output of the laser arraybe monitored.

The compensation setup is almost identical to thehancement setup; the prisms, however, have been oriedifferently with respect to the laser array and the crystal. Tdispersion of the prisms has different signs for the enhanment and the compensation setups. Using ray tracing wOptica™ for Mathematica™ the angular dispersion of tbeam after refraction at the air crystal interface~f in Fig. 1!is calculated by inserting accurate information about theperimental setup. If the dispersion of the prisms is ignorthe angular dispersion after refraction at the air crystal inface itself can be estimated todf/dl5331025 rad/nm.The total angular dispersion of the prisms, the lenses andrefraction at the air crystal interface is calculated to31025 rad/nm for the enhancement setup and to2631025 rad/nm for the compensation setup, respectively.ter the laser array has been switched on and the crystilluminated, dynamic gratings start to form in the crystal anas a result, the reflectivity of the PCM is slowly increaseThe phase conjugate feedback forces the spectrum oflaser array to narrow down from approximately ten longidinal modes to a few modes and the center wavelength ospectrum starts to shift towards a longer wavelength. Tbehavior is identical for the enhancement and the competion setups. Moreover, it also occurs if all the prisms aremoved and, instead, the beam from the laser array is padirectly to crystal, which means that the initial stage of tself-induced frequency scanning process is due to propeof the formed gratings in the crystal and has nothing towith the prisms placed in front of the PCM. As the lasinwavelength of the output from the laser array scans towathe red, the angle of incidence~u in Fig. 1!, the angle ofrefraction ~f!, and the position of the beam at the cryssurface change due to the dispersion of the prisms andcrystal. For the compensation setup the dispersion ofprisms is sufficiently large to cancel the angular dispersionthe air crystal interface. If dispersion is the origin andresponsible for the frequency scanning, then one may exscanning for the enhancement setup and none or at leasduced scanning—or even scanning towards the blue—forcompensation setup.

Figure 2 shows the recorded center wavelength ofrticle is copyrighted as indicated in the article. Reuse of AIP content is s

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lasing spectrum from the laser array as a function of timFigure 2~a! is for the enhancement setup and Fig. 2~b! is forthe compensation setup. As seen in Fig. 2~a!, we do observethe well known self-induced frequency scanning for the ehancement setup, as was to be expected. Due to the pconjugate feedback, the spectrum scans away from the wlengthl0 , and the oscillating energy between the laser arand the crystal gradually decreases as the spectrum mfurther away. After some time the spectrum has scannederal nanometers and the reduced feedback level canlonger suppress the oscillation of the natural modes offreely running laser; the natural modes then emergeerase whatever grating is left in the crystal. Consequenthe reflectivity of the PCM is reduced to zero, and the cenwavelength jumps back tol0 . The cycle is thereafter repeated. Only scanning towards the red was observed.spectrum, however, would occasionally scan a fraction onanometer towards the blue or make a stop for a short tbefore resuming the scanning towards the red. The anglincidence, the position of the beam at the crystal surface,the intensity all affect the scan rate. For the data shownFigs. 2~a! and 2~b! the laser array was operated at two timthe threshold current (I th50.29 A). The power of the inci-dent beam at the crystal was 100 mW and the intensity250 mW/mm2.

Figure 2~b! displays the recorded center wavelength vsus time for the compensation setup. No regular scan cyare observed but after temporal buildup of the reflectivitythe PCM, the spectrum starts to scan until it reaches soequilibrium, where the effective round-trip gain of the cavihas become symmetric with respect to the center waveleof the spectrum, and the output power and spectrum becostable. The maximum phase conjugate reflectivity measuat the beamsplitter was 1.6% and 2.8% for the enhancemand the compensation setup, respectively. The higher pconjugate reflectivity observed in the compensation confiration is due to the stabilization of the center wavelength tleads to a larger index modulation in the crystal.

Figure 2~b! clearly shows that for the compensatiosetup the dispersion of the prisms has suppressed thequency scanning. We refer to this ascounterbalance disper-sion since the dispersion of the BaTiO3 crystal has beencompensated by the prisms. In order to suppress thequency scanning a certain amount of counterbalance dission must at least be applied. With less dispersion applied

FIG. 2. Center wavelength of the optical spectrum vs time:~a! dispersionenhancement setup with prisms;~b! dispersion compensation setup witprisms.

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1265Appl. Phys. Lett., Vol. 72, No. 11, 16 March 1998 Lo”bel, Petersen, and Johansen

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scan cycle continues. If the four BK7 prisms were removthe frequency scanning could not be suppressed.

In order to verify that the angular dispersion of thprisms is indeed responsible for the different behaviorserved with the enhancement and the compensation sewe replace the prisms with a dispersive grating. The expmental setups are shown in Figs. 3~a! and 3~b! and are alsonamed the enhancement setup and the compensation srespectively. The only difference between the enhancemand the compensation setups is that the crystal hasturned upside down. The point of incidence at the cryssurface remains the same. This is an exact equivalent toversing the sign of the external angular dispersion ofgrating. For the case with the prisms one can also changeenhancement setup to the compensation setup and visaby simply turning the crystal upside down as shown heThe dashed and solid lines in Figs. 3 correspond to‘‘blue’’ and ‘‘red’’ wavelength, respectively. One can sethat by turning the crystal upside down and maintainingpoint of incidence, the angle of incidence is interchangbetween the blue and the red wavelength. Figures 4~a! and4~b! show the center wavelength of the spectrum versus tfor the enhancement setup and the compensation setupspectively. The laser array was operated at two timesthreshold current and the incident beam intensity w120 mW/mm2. Again, it is observed that regular scan cycltake place with the enhancement setup and that the scanis suppressed with the compensation setup due to the cterbalance dispersion of the grating. The buildup timestabilization depends on many experimental parameters,as angle of incidence and spatial position of the beam atcrystal surface. The initial scan rate is therefore not alwhigher for the compensation configuration than for thehancement configuration, as it can be seen in Fig. 2~b!. The

FIG. 3. Experimental setup with grating. L1: lensf 54.5 mm, L2: sphericallens f 5100 mm, WP: wave plate, BS: beamsplitter, grating: 1200 lines/mruled with a blaze angle of 26.4 deg.~750 nm!; ~a! dispersion enhancemensetup;~b! dispersion compensation setup~BaTiO3 crystal is turned upsidedown, but the position of the beam at the surface is maintained!.

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maximum phase conjugate reflectivity measured at the besplitter was 3.2% and 5.5% for the enhancement andcompensation setups, respectively.

The completely different behavior observed for the copensation and the enhancement setups shows that the mrial frequency dispersion of the crystal plays a vital role fthe mechanism of the self-induced frequency scanningcan even be the very origin of the scanning effect.

In conclusion, we have shown that the self-induced fquency scanning of a broad-area laser with external phconjugate feedback from a self-pumped BaTiO3 crystal ar-ranged in the Cat configuration is significantly altered bydispersion of prisms or a dispersive grating placed inexternal cavity in front of the crystal. We have shown ththese prisms or the grating can even suppress theinduced frequency scanning and stabilize the output poof the broad-area diode laser lasing at 811 nm. The expmental results show that the material dispersion ofBaTiO3 crystal is an important effect for the self-inducefrequency scanning process.

M. Lo”bel thanks the Danish Research Academy fornancial support toward a Ph.D. dissertation~Grant No. 95-0058-SAM!.

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FIG. 4. Center wavelength of the optical spectrum vs time:~a! dispersionenhancement setup with grating;~b! dispersion compensation setup witgrating.

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