Thermally-induced-birefringence effects of highly Nd3+-doped Y3Al5O12 ceramic lasers

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  • ceer

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    nce,

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    achieved laser oscillation of Y3ScAl4O12 ceramics [5], andfemtosecond pulse operation with passively mode locking[6,7].

    eects

    We developed a pump-probe method for accuratelymeasuring the depolarization caused by the thermally-induced birefringence [8], which is dened by the ratio ofthe depolarized power to the initially polarized power as

    * Corresponding author. Fax: +81 3 3817 1847.E-mail address: ishoji@elect.chuo-u.ac.jp (I. Shoji).

    Optical Materials 29 (20071. Introduction

    The ceramic laser has been intensively studied as a can-didate for the next-generation high-performance laser sincehighly transparent Nd3+:Y3Al5O12 (Nd:YAG) ceramicswere successfully grown and its laser oscillation was dem-onstrated [1]. We succeeded in oscillation of a microchipNd:YAG ceramic laser and constructed a green laser byintracavity frequency doubling [2]. The fundamental opti-cal properties were also investigated [3], and it was alsofound that Nd:YAG ceramics are suitable for direct pump-ing into the emitting level at 885 nm [4]. Furthermore, we

    One of the most signicant advantages of Nd:YAGceramics is that Nd3+ can be highly doped into YAGceramics (up to 10%) without degradation of their homo-geneity and thermal conductivity [2]. This indicates thatNd:YAG ceramics can be highly ecient and high-powermicrochip laser materials because the pump absorptioncoecient becomes larger in proportion to the Nd3+ con-centration [3]. In this paper, we report the thermally-induced-birefringence eects in Nd:YAG ceramics whichare important for their high-power operation.

    2. Measurement of the thermally-induced-birefringenceAbstract

    Thermally-induced-birefringence eects of Nd:YAG ceramic lasers are accurately measured. Depolarization for the ceramics is foundto be nearly the same with that for (111)-cut single crystals if Nd3+ concentration is the same. Under non-lasing condition, the thermally-induced birefringence becomes larger as Nd3+ concentration increases due to severer quenching. Under lasing, however, depolarization ismeasured to be one-third of that under non-lasing because the stimulated emission reduces the nonradiative process, which decreases thethermal load. Moreover, direct pumping into the emitting level by use of a 885 nm pump source intrinsically reduces the heat generationcompared with the conventional 808 nm pumping; we have experimentally veried that the depolarization and the thermal load arereduced by 30% by the direct pumping. 2006 Elsevier B.V. All rights reserved.

    PACS: 42.70.Hj; 78.20.Fm; 78.20.Nv; 42.55.Rz; 78.55.Hx; 42.25.Ja; 42.25.Lc

    Keywords: Ceramic laser; Thermally-induced birefringence; Depolarization; Direct pumping; Nd:YAGThermally-induced-birefringenY3Al5O12 c

    Ichiro Shoji a,*, Takunoa Department of Electrical, Electronic, and Communication Engineerin

    b Laser Research Center, Institute for Molecular Sciec Poly-Techno Co., Ltd., 2-4-1 Mutsu

    Received 17 October 2005; received in revised foAvailable online0925-3467/$ - see front matter 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.optmat.2005.12.012eects of highly Nd3+-dopedamic lasers

    Taira b, Akio Ikesue c

    huo University, Kasuga 1-13-27, Bunkyo-ku, Tokyo 112-8551, Japan

    38 Nishigonaka, Myodaiji, Okazaki 444-8585, Japan

    Atsuta-ku, Nagoya 456-8587, Japan

    21 November 2005; accepted 12 December 2005eptember 2006

    www.elsevier.com/locate/optmat

    ) 12711276

  • 0.03

    0.02

    0.01

    0

    Dep

    olar

    izatio

    n D p

    ol

    543210Nd Concentration (at.%)

    Nd:YAG CeramicsNd:YAG Single Crystal

    h = 0.24

    h=0.69

    0.58

    0.410.38

    Lasing

    Fig. 4. Dependence of depolarization on Nd concentration under non-lasing at the absorbed pump power of 1 W. The solid line shows thecalculated depolarization, while the dotted line indicates the calculationunder the ideal lasing condition.

    1272 I. Shoji et al. / Optical MaterDpol = P?/Pinitial. The experimental setup is shown inFig. 1. A cw Ti:sapphire laser was used as the pump beam,which was focused onto the sample with the radius of35 lm. A linearly polarized HeNe laser which was colli-mated to a 1 mm radius, was used as the probe. After pass-ing through the sample, the probe beam was reected fromthe lter with high reectivity at 633 nm, and went backthrough the sample again. We detected only the depolar-ized (i.e. X-polarized) component of the probe beam usingthe analyzer. In addition, we inserted plane mirrors to con-gure the cavity when we measured the depolarizationunder lasing conditions.

    3. Depolarization under non-lasing conditions

    First we measured the depolarization under non-lasing

    Fig. 1. Experimental setup for the measurement of depolarizationresulting from thermally-induced birefringence.conditions. Fig. 2 shows the depolarized beam patternsfor the (111)-cut 1.0 at.% Nd:YAG single crystal and the3.5 at.% Nd:YAG ceramics, which were observed with aCCD camera. Since the thermally-induced birefringencein the (111) plane of a Nd:YAG single crystal is circularlysymmetric and occurs between the radial and the azimuthal

    Fig. 2. Depolarized beam patterns under non-lasing. (a) (111)-cut1.0 at.% Nd:YAG single crystal; (b) 3.5 at.% Nd:YAG ceramics. Theabsorbed pump power, Pabs, were both 1.7 W.nr

    X

    YNd:YAG(111) Plane

    (Analyzer)

    (Polarizer)InitialPolarization

    n

    r

    r0

    wawp

    Fig. 3. The principal axes and the thermally-induced birefringence nr andnU of the index ellipse which is formed in a (111) plane of a Nd:YAGsingle crystal. wp: the pump-beam radius; wa: the probe-beam radius.

    ials 29 (2007) 12711276directions as shown in Fig. 3, the depolarized beam forms afour-leaf-like pattern (Fig. 2(a)). Although the same pat-tern was observed for the 3.5 at.% Nd:YAG ceramics(Fig. 2(b)), its depolarization was enhanced compared withthat of the single crystal at the same absorbed pump power.Fig. 4 shows the dependence of the depolarization on theNd3+ concentration at the absorbed pump power of 1 W.It was found that the depolarization is nearly the samefor ceramic and single-crystal YAG when their Nd3+ con-centrations are the same. Because the YAG ceramics con-sist of many single-crystal grains (their sizes are in severaltens of micrometers) with random orientations, the ther-mally-induced birefringence eect is the average of thosegrains. Although the depolarization in the planes otherthan (111) signicantly depends on the direction of polar-ization as shown in Fig. 5 [9], the average, and the depolar-ization for the ceramic YAG, are close to that for the (111)plane [10].

    Moreover, it was found that the depolarization becamelarger in the samples with higher Nd3+ concentrations even

  • aterif the absorbed pump power was the same. This is mainlybecause the thermal load increases as Nd3+ concentrationincreases. The thermal load is given by the fraction of theabsorbed pump power that is converted into heat in thelaser medium as [11]

    gh 1 gp1 glgrkp=kf glkp=kl; 1where gp is the pump quantum eciency (1), gl is the stim-ulated emission eciency which is the fraction of excitedions that are extracted by stimulated emission, gr is the radi-ative quantum eciency, kp is the pump wavelength, kf isthe average uorescence wavelength (1038 nm), and kl isthe laser oscillation wavelength (1064 nm). Since gl = 0under non-lasing, Eq. (1) is simplied to gh = 1 gr(kp/kf). For a highly Nd

    3+-doped sample, in which the inter-action between Nd3+ ions is signicant, the concentrationquenching decreases the value of gr. The thermal load, gh,then becomes larger as Nd3+ concentration increases,indicated in Fig. 4 [12]. We calculated the amount ofdepolarization at each Nd3+ concentration using these val-ues of gh.

    The amount of depolarization is represented by [13]

    0.025

    0.020

    0.015

    0.010

    0.005

    0

    Dep

    olar

    izatio

    n D

    pol

    360270180900Polarization Angle p (degree)

    (111) (100) (110)Absorbed Pump Power: 1.5W

    Fig. 5. Dependence of depolarization on the direction of polarization for(111), (100), and (110)-cut single crystals.

    I. Shoji et al. / Optical MDpol 1pw2a

    Z wa0

    Z 2p0

    Dr;UrdrdU; 2

    which is the integration of the depolarization

    Dr;U sin2 2U sin2 Wr2

    3

    at each point (r,U) in Fig. 3 over the whole probe beam.Here we assume that the thermally-induced-birefringenceeect in ceramics is the same with that in a (111)-cut singlecrystal. W(r) is the phase dierence caused by the ther-mally-induced birefringence, and written as

    Wr 2Z t0

    2pknrr nUrdz XArS00; 4

    where t is the thickness of the sample, the factor of 2 meansthat the probe beam makes the round trip through the sam-ple, and X, A(r), S00 are given byX 13n301 mp11 p12 4p44; 5

    Ar 2 1 2r2

    Z r0

    E12r02

    w2p

    !r0 dr0 E1 2r

    2

    w2p

    !" #; 6

    S00 al1 m

    ghP abs8jk

    ; 7

    respectively. The values of those parameters used for thecalculation are shown in Table 1. E1(z) is the exponentialintegral function. The solid line in Fig. 4 is the numericalcalculation of the depolarization, which agrees well withthe experimental data. It is not desirable that the depolar-ization becomes more signicant as Nd3+ concentration in-creases. When lasing occurs, however, the stimulatedemission eciency, gl, is ideally 1 in Eq. (1), which reducesthe thermal load to a small constant value gh = 1 kp/kl = 0.24. The depolarization is then expected to remainsmall regardless of Nd3+ concentration as indicated bythe dotted line in Fig. 4.

    4. Depolarization under lasing conditions

    We measured reduction of depolarization under lasingconditions by use of the 3.5 at.% Nd:YAG ceramic andthe 1.0 at.% Nd:YAG single crystal samples. The pumpwavelengths for the ceramic and the single crystal were806 and 808 nm, respectively, in order for the absorptioncoecients to be nearly the same. Fig. 6 shows the depen-dence of the output power on the absorbed pump power.

    Table 1Parameters used for calculation of depolarization

    wp: pump beam radius 35 lmwa: probe beam radius 1 mmk: probe wavelength 0.6328 lmn0: refractive index of Nd:YAG 1.83al: linear expansion coecient 6.9 106 K1

    j: thermal conductivity 10 W/mKm: Poissons ratio 0.3pij: photoelastic coecient p11 = 0.029

    p12 = 0.009p44 = 0.062

    ials 29 (2007) 12711276 1273The slope eciency of 71% was obtained with the singlecrystal. On the other hand, the slope eciency of the cera-mic was 47% presumably because the highly Nd3+-dopedYAG still generated more heat and suered from largerthermal-lens eect than the lower concentrated YAG,reducing the mode-matching eciency. However, theamount of depolarization was found to be greatly reducedto the value of 1/3 that under non-lasing, as shown inFig. 7.

    The dotted lines in Fig. 7 are the calculation of depolar-ization under non-lasing, in which 0.69 and 0.38 were usedas the values of gh for the 3.5 and 1.0 at.% Nd:YAG,respectively. However, we have to experimentally obtainthe values of gh under lasing conditions because gl doesnot become 1 in practical cases. gl can be expressed by

  • ter2.0

    1.5

    1.0

    0.5

    0

    Out

    put P

    ower

    Pout (W

    )

    210Absorbed Pump Power Pabs (W)

    1.0

    0.8

    0.6

    0.4

    0.2

    0.0

    Stimulated Em

    ission Eff. l

    s=71%

    s=47%

    3.5at.% Ceramic (p=806nm)1.0at.% Single Crystal (p=808nm)

    Nd:YAG

    Fig. 6. Dependence of the laser output power Pout of the 1.0 at.%Nd:YAG single crystal and the 3.5 at.% Nd:YAG ceramic on the

    1274 I. Shoji et al. / Optical Mause of the output power, Pout, and the absorbed pumppower, Pabs, as

    gl nstimulatednexcited

    1gcgp

    nlasernpump

    1gcgpgq

    P outP abs

    ; 8

    where nlaser is the number of the output laser photons,npump is the number of the absorbed pump photons,gq = kp/kl, and gc is the coupling eciency. The outputpower can also be represented by

    P out gcgpgqgmP abs P th; 9where gm is the mode-matching eciency and Pth is thethreshold absorbed pump power. Eqs. (8) and (9) givethe expression of gl as

    gl gmP abs P th

    P abs: 10

    0.10

    0.08

    0.06

    0.04

    0.02

    0

    Dep

    olar

    izatio

    n D p

    ol

    210Absorbed Pump Power Pabs (W)

    Lasing

    Lasing

    1.0at.% Single Crystal ((111)-cut, p=808nm)

    Nd:YAG3.5at.% Ceramic (p=806nm)

    Fig. 7. Dependence of depolarization of the 1.0 at.% Nd:YAG singlecrystal ((111)-cut) and the 3.5 at.% Nd:YAG ceramic on the absorbedpump power under lasing and non-lasing conditions. The dotted curvesindicate the calculated depolarization under non-lasing conditions usingthe constant thermal load and the parameters shown in Table 1. On theother hand, the solid curves indicate the calculation under lasingconditions using the estimated values of the thermal load shown in Fig. 8.

    absorbed pump power Pabs. The stimulated emission eciencies gl as afunction of the absorbed pump power, which were estimated by use of Eq.(8), are also shown.It is obvious from Eq. (10) that gl cannot be 1 because thethreshold has a nite value and the mode-matching e-ciency is usually smaller than 1. We then estimated the val-ues of gl under lasing at each absorbed pump power withEq. (8). Here gc is given by gc = ln(1/R)/[Li + ln(1/R)],and the reectivity of the output mirror, R, the cavitylosses, Li, for the 3.5 at.% Nd:YAG ceramic and the1.0 at.% Nd:YAG single crystal, were 0.91, 0.029, and0.0096, respectively. gp was assumed to be 1. The opensquares and the open circles in Fig. 6 shows the estimatedvalues of gl for the ceramic and the single crystal, respec-tively, and Fig. 8 shows the dependence of the thermal loadon the absorbed pump power under lasing conditionswhich was obtained by the values of gl with Eq. (1).Although gh is large around the threshold, it becomes smal-ler as the pump and the output power increase. The solidlines in Fig. 7 are the calculated depolarization using the re-

    1.0

    0.8

    0.6

    0.4

    0.2

    0

    Ther

    mal

    Loa

    d h

    210Absorbed Pump Power Pabs (W)

    1.0at.% Nd:YAG Single Crystal 3.5at.% Nd:YAG Ceramic

    Lasing

    l=1

    l=0

    l=0

    3.5at.%Ndr=0.39

    1.0at.%Ndr=0.80Lasing

    Fig. 8. Dependence of the thermal load gh of the 1.0 at.% Nd:YAG singlecrystal and the 3.5 at.% Nd:YAG ceramic on the absorbed pump powerunder lasing condition, which was obtained by the values of gl shown inFig. 6. The constant values of gh under non-lasing (gl = 0) and the ideallasing (gl = 1) conditions are also shown by the dotted lines.

    ials 29 (2007) 12711276sults of Fig. 8, which agreed well with the experimentaldata. This means that we can approximately estimate theamount of the thermal load from the laser inputoutputcharacteristics. Moreover, it was found that highly Nd3+-doped YAG ceramics are suitable for high-power cw orhigh-repetition-rate Q-switched operations because lasingreduces the thermally-induced-birefringence eect.

    5. Reduction of the thermal load by the direct pumping of

    highly Nd3+-doped YAG ceramics

    The direct pumping is the scheme of pumping a Nd3+-doped material from the ground or the hot-band levels tothe emitting levels [4], reducing the quantum defect becausethe pump wavelength shifts from conventional 808 nm to885 nm. This makes the upper limit of the eciencyhigher; 80% slope eciency was achieved by the directpumped Nd:YVO4 [14]. Although the diode pumping issuitable for direct pumping of Nd:YAG because theabsorption linewidth around the 885 nm band is as broadas 3 nm, the absorption coecient for 1 at.% Nd:YAG is

  • only 1.5 cm1. Highly Nd3+-doped YAG ceramics whichhave large absorption coecients are then appropriatefor highly ecient and high-power direct pumped lasermaterials.

    In the case of 3.5 at.% Nd:YAG under non-lasing, thedierence of the thermal load between with the conven-tional (gh = 0.70) and with the direct pumping (gh = 0.67)is only 4%. Under lasing conditions, however, the thermalload is expected to be reduced by 30% from gh = 0.24 (con-ventional) to gh = 0.17 (direct pumping) if gl = 1 isassumed. We experimentally veried the reduction of thethermal load in the 3.5 at.% Nd:YAG ceramics by thedirect pumping.

    Fig. 9 shows the dependence of the laser output poweron the absorbed pump power with the conventional(kp = 806 nm) and the direct (kp = 886 nm) pumping. Theslope eciency increased to 66% by direct pumping, pre-sumably because less heat generation reduced the ther-mal-lens eect, improving the mode-matching. Thedepolarized beam patterns for the conventional and thedirect pumping are shown in Fig. 10, which shows thatthe depolarization with the direct pumping is smaller than

    with the conventional pumping even at the same absorbedpump power. This is quantitatively shown in Fig. 11, inwhich the depolarization is plotted in a log scale so thatthe dierence can be clearly seen. While little dierencewas measured under non-lasing conditions, the depolariza-tion with the direct pumping became 30% smaller than withthe conventional pumping under lasing conditions. Thecalculated depolarization, which was indicated by the dot-ted lines in Fig. 11, satisfactorily agreed with the experi-mental data. This result justies the values of the thermalload shown in Fig. 12, which were used for the calculationof the depolarization, and Fig. 12 veries that the heat gen-eration was reduced by 30% with the direct pumping.

    6. Summary

    We investigated the thermally-induced birefringence inNd:YAG ceramics. It was found that highly Nd3+-dopedYAG ceramics are promising for highly ecient, high-

    2.0

    1.5

    1.0

    t Pow

    er P

    out (W

    )

    1.0

    0.8

    0.6

    0.4

    Stimulated Em

    ission

    3.5at.% Nd:YAG Ceramic

    s=66%

    p=886nmp=806nm

    0.8

    0.6

    0.4

    0.2

    0

    Ther

    mal

    Loa

    d h

    210Absorbed Pump Power Pabs (W)

    p=806nmp=886nm

    Lasing

    (p=806nm)l=1l=1

    l=0

    (p=886nm)

    (p=806nm)l=0 (p=886nm)

    Fig. 12. Dependence of the thermal load gh of the 3.5 at.% Nd:YAGceramic on the absorbed pump power under lasing by the conventionaland the direct pumping, which were obtained by the values of gl shown inFig. 9. The constant values of gh under non-lasing (gl = 0) and the ideallasing (gl = 1) conditions are also shown by the dotted lines.

    I. Shoji et al. / Optical MaterFig. 10. Depolarized beam patterns under lasing for the 3.5 at.%

    0.5

    0

    Out

    pu

    210Absorbed Pump Power Pabs (W)

    0.2

    0.0

    Eff.l

    s=47%TO.C.=9%

    Fig. 9. Dependence of laser output power on the absorbed pump powerfor the conventional (lled square) and direct (lled circle) pumping. Theestimated stimulated emission eciencies for the direct and conventionalpumping are also shown by open circles and squares, respectively.Nd:YAG ceramics. The pumping wavelengths were (a) 806 and (b)886 nm. The absorbed pump power, Pabs, were both 1.7 W.68

    0.001

    2

    468

    0.01

    2

    468

    0.1

    Dep

    olar

    izatio

    n D p

    ol

    210Absorbed Pump Power Pabs (W)

    Lasingp=806nmp=886nm

    3.5at.% Nd:YAG Ceramic

    (p=806nm)

    (p=886nm)

    l=1

    l=1

    } l=0

    Fig. 11. Depolarization as a function of the absorbed pump power underlasing and non-lasing conditions for the direct and conventional pumping.The dashed and solid curves are the calculated result.

    1.03.5at.% Nd:YAG Ceramic

    ials 29 (2007) 12711276 1275power cw or high-repetition-rate Q-switched microchiplaser materials. Highly Nd3+-doped ceramic YAG is also

  • regarded as one of the best materials for the direct pumpingbecause 30% reduction of the thermal load is possible.

    Acknowledgements

    This work was partially supported by the Ministry ofEducation, Culture, Sports, Science and Technology,Grant-in-Aid for Scientic Research (A), 15206073, andthe Special Coordination Funds for Promoting Scienceand Technology.

    References

    [1] A. Ikesue, T. Kinoshita, K. Kamata, K. Yoshida, J. Am. Ceram. Soc.78 (1995) 1033.

    [2] T. Taira, A. Ikesue, K. Yoshida, in: W.R. Bosenberg, M.M. Fejer(Eds.), Advanced Solid-State Lasers, vol. 19, Optical Society ofAmerica, Washington, DC, 1998, p. 430.

    [3] I. Shoji, S. Kurimura, Y. Sato, T. Taira, A. Ikesue, K. Yoshida, Appl.Phys. Lett. 77 (2000) 939.

    [4] V. Lupei, A. Lupei, N. Pavel, T. Taira, I. Shoji, A. Ikesue, Appl.Phys. Lett. 79 (2001) 590.

    [5] J. Saikawa, Y. Sato, T. Taira, A. Ikesue, Appl. Phys. Lett. 85 (2004)1898.

    [6] J. Saikawa, Y. Sato, I. Shoji, T. Taira, A. Ikesue, Advanced Solid-State Photonics on CD-ROM, Optical Society of America, Wash-ington, DC, 2004, paper TuB17.

    [7] J. Saikawa, Y. Sato, T. Taira, A. Ikesue, Appl. Phys. Lett. 85 (2004)5845.

    [8] I. Shoji, Y. Sato, S. Kurimura, V. Lupei, T. Taira, A. Ikesue, K.Yoshida, Opt. Lett. 27 (2002) 234.

    [9] I. Shoji, Y. Sato, S. Kurimura, T. Taira, A. Ikesue, K. Yoshida, in:Proceedings of the International Conference on Lasers 2001, Tucson,2001, p. 239.

    [10] E.A. Khazanov, Opt. Lett. 27 (2002) 716.[11] T.Y. Fan, IEEE J. Quantum Electron. 29 (1993) 1457.[12] V. Lupei, T. Taira, A. Lupei, N. Pavel, I. Shoji, A. Ikesue, Opt.

    Commun. 195 (2001) 225.[13] M.E. Innocenzi, H.T. Yura, C.L. Fincher, R.A. Fields, Appl. Phys.

    Lett. 56 (1990) 1831.[14] Y. Sato, T. Taira, N. Pavel, V. Lupei, Appl. Phys. Lett. 82 (2003)

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    1276 I. Shoji et al. / Optical Materials 29 (2007) 12711276

    Thermally-induced-birefringence effects of highly Nd3+-doped Y3Al5O12 ceramic lasersIntroductionMeasurement of the thermally-induced-birefringence effectsDepolarization under non-lasing conditionsDepolarization under lasing conditionsReduction of the thermal load by the direct pumping of highly Nd3+-doped YAG ceramicsSummaryAcknowledgementsReferences

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