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Meng Peibei, Shi Wenzong, Jiang Shuo, Qi Ming, Deng Yongtao, Li Xu. Influence of Gaussian mirror parameters on LD-pumped Nd:YAG laser[J]. Infrared and Laser Engineering, 2021, 50(4): 20200127. doi: 10.3788/IRLA20200127
Citation: Meng Peibei, Shi Wenzong, Jiang Shuo, Qi Ming, Deng Yongtao, Li Xu. Influence of Gaussian mirror parameters on LD-pumped Nd:YAG laser[J]. Infrared and Laser Engineering, 2021, 50(4): 20200127. doi: 10.3788/IRLA20200127

Influence of Gaussian mirror parameters on LD-pumped Nd:YAG laser

doi: 10.3788/IRLA20200127
  • Received Date: 2020-04-12
  • Rev Recd Date: 2021-02-11
  • Available Online: 2021-04-30
  • Publish Date: 2021-04-30
  • The influence of eccentricity and laser performance of LD-pumped Nd:YAG laser was investigated experimentally at different parameter Gaussian mirrors. Largest energy, narrowest width and smallest divergence can be obtained simultaneously only when the optical axis, laser crystal axis and Q-switch axis were in agreement, furthermore the optical axis went through the reflectivity center of Gaussian mirror. When eccentricity appeared, the energy, pulse width and divergence degraded more with smaller reflectivity radius or larger center reflectivity of Gaussian mirror. For 2.5 mm reflectivity radius and 30% central reflectivity Gaussian mirror, energy decreased 7%, pulse width increased 33%, and divergence increased 20% under 0.5 mm eccentricity. For laser performance, the smaller the reflectivity radius or center reflectivity of Gaussian mirror, the better the beam quality and the smaller the optical-to-optical efficiency. Considering the eccentricity influence and laser performance, 2.75 mm reflectivity radius and 20% center reflectivity Gaussian mirror was optimum. When the pump energy was 984 mJ, output energy of 128 mJ, pulse width of 7.3 ns, and beam quality M2 factor of 4.6 at 1064 nm were achieved, corresponding to the optical-to-optical efficiency of 13%. The experimental results in this paper can be a reference of the laser design and alignment.
  • [1] Sawruk N W, Burns P M, Edwards R E, et al. ICESat-2 laser Nd: YVO4 amplifier[C]//SPIE, 2018, 10513: 105130X.
    [2] Meng Peibei, Yan Fanjiang, Li Xu, et al. Influence of boundary condition and pump scheme on thermal effects of laser crystal [J]. Infrared and Laser Engineering, 2015, 44(1): 3216-3222. (in Chinese)
    [3] Shi Xiangchun, Chen Weibiao, Hou Xia. Application of all solid state laser in space [J]. Infrared and Laser Engineering, 2005, 34(2): 127-131. (in Chinese) doi:  10.3969/j.issn.1007-2276.2005.02.001
    [4] Cheng Yong. Development and progress of adjust-free solid state laser [J]. Infrared and Laser Engineering, 2006, 35(3): 297-301. (in Chinese) doi:  10.3969/j.issn.1007-2276.2006.03.011
    [5] Stysley P R, Coyle D B, Kay R B, et al. Long term performance of the High Output Maximum Efficiency Resonator (HOMER) laser for NASA's Global Ecosystem Dynamics Investigation (GEDI) lidar [J]. Optics & Laser Technology, 2015, 68: 67-72.
    [6] Dai Qin, Zhang Shanchun, Yang Fan, et al. Research on the high beam quality of Gaussian unstable resonators in solid state lasers [J]. Chinese Optics, 2019, 12(3): 559-566. (in Chinese) doi:  10.3788/co.20191203.0559
    [7] Zou Lu, Jin Qian, Zhou Ping, et al. Unstable resonator design for high power solid-state laser [J]. Ordnance Industry Automation, 2014, 33(7): 16-19. (in Chinese)
    [8] Li Kuohu, Ma Jingjie. Mode analysis of the circular piano-concave resonator with a deformed Gaussian-reflectivity mirror [J]. Optical Technique, 2010, 36(5): 192-195. (in Chinese)
    [9] Wang Wentao, Liu Yang, Wang Chao, et al. Study on beam-quality of slab laser by the usage of Gaussian mirrors [J]. Laser & Infrared, 2012, 42(9): 980-982. (in Chinese) doi:  10.3969/j.issn.1001-5078.2012.09.003
    [10] Liu Xu, Wang Xiaobing, Cheng Yong, et al. Design of diode pumped all-solid-state laser for SLA [J]. Infrared and Laser Engineering, 2007, 37(7): 794-800. (in Chinese)
    [11] Wang Canzhao, Li Li, Shang Wendong, et al. Study on pulsed solid-state lasers with positive branch confocal unstable resonators [J]. Laser Technology, 2013, 37(4): 441-444. (in Chinese)
    [12] Meng Peibei, Shi Wenzong, Yan Fanjiang, et al. Influence of resonator misalignment on performance of diode-pumped Nd:YAG laser [J]. Infrared and Laser Engineering, 2017, 46(6): 0605001. (in Chinese) doi:  10.3788/IRLA201746.0605001
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Influence of Gaussian mirror parameters on LD-pumped Nd:YAG laser

doi: 10.3788/IRLA20200127
  • Key Laboratory for Space Laser Information Perception Technology of CAST, Beijing Institute of Space Mechanics & Electricity,Beijing 100094, China

Abstract: The influence of eccentricity and laser performance of LD-pumped Nd:YAG laser was investigated experimentally at different parameter Gaussian mirrors. Largest energy, narrowest width and smallest divergence can be obtained simultaneously only when the optical axis, laser crystal axis and Q-switch axis were in agreement, furthermore the optical axis went through the reflectivity center of Gaussian mirror. When eccentricity appeared, the energy, pulse width and divergence degraded more with smaller reflectivity radius or larger center reflectivity of Gaussian mirror. For 2.5 mm reflectivity radius and 30% central reflectivity Gaussian mirror, energy decreased 7%, pulse width increased 33%, and divergence increased 20% under 0.5 mm eccentricity. For laser performance, the smaller the reflectivity radius or center reflectivity of Gaussian mirror, the better the beam quality and the smaller the optical-to-optical efficiency. Considering the eccentricity influence and laser performance, 2.75 mm reflectivity radius and 20% center reflectivity Gaussian mirror was optimum. When the pump energy was 984 mJ, output energy of 128 mJ, pulse width of 7.3 ns, and beam quality M2 factor of 4.6 at 1064 nm were achieved, corresponding to the optical-to-optical efficiency of 13%. The experimental results in this paper can be a reference of the laser design and alignment.

    • LD泵浦的全固态激光器结构紧凑、效率高、稳定好、寿命长,在星载激光雷达领域有广泛的应用[1-5]。星载激光雷达由于作用距离远,且接收口径受限,激光器能量通常为几十毫焦至百毫焦,同时为了获得高的测量精度,通常采用扩束镜头压缩激光器输出的发散角至几十至百微弧度。当激光器光束质量越好时,相同束腰情况下,发散角越小,则扩束镜头的扩束倍率越小,扩束镜头的口径和激光雷达系统体积越小,因此高光束质量是星载激光器的发展趋势。相对端面泵浦,侧面泵浦更容易注入高的泵浦功率,获得大能量输出。但侧面泵浦时,激光增益模式大,要实现高光束质量输出,需要采用大体积基模谐振腔。非稳腔具有良好的空间选模特性,能产生大体积基模。然而传统硬边镜非稳腔由于输出镜中心镀全反膜,边缘镀增透膜,其基模衍射损耗较大,输出光束近场为环状光斑,远场光斑有衍射环,能量分布分散,不利于应用。高斯镜沿径向渐变的反射率轮廓减少了光学谐振腔的边缘衍射效应,一定程度上消除了输出光束近场的衍射纹波,抑制了远场的旁瓣,改善了空间模式。但由于高斯镜的反射率中心无法引出和测量,光轴和反射率中心的对准只能通过结构设计和装调保证,易出现反射率中心与光轴的偏离(简称“偏心”)。目前关于高斯镜非稳腔的研究主要集中于模式分析[6-8]及激光器实验[9-11],没有对不同高斯镜参数下,激光器的输出特性及偏心的影响进行系统性的研究,而这对于激光器设计和装调是极有意义的。

      为了研究高斯镜参数对激光器的影响,文中设计了凹凸结构的非稳腔,搭建LD泵浦Nd:YAG电光调Q激光器,实验对比了不同高斯镜参数下,偏心对激光器输出特性的影响和激光器的输出特性。

    • 实验装置如图1所示。Nd:YAG晶体尺寸为Ф6×100 mm,掺杂浓度为1at.%,端面镀1064 nm增透膜(反射率<0.1%)。Nd:YAG晶体通过铟箔包裹置于热沉上。12支激光二极管阵列(LDA)分成4沿晶体轴交叉排布,每组的3支LDA均匀排布在晶体半侧面。每支LDA包含6个巴条,其慢轴和快轴发散角分别为10°和40°(FWHM)。LDA安装热沉通过热电制冷器控温并保持温度在293 K,以使LDA发出光中心波长在808 nm,与Nd:YAG晶体吸收峰匹配。LDA为脉冲工作模式,脉冲重复频率为5 Hz,脉冲宽度为210 μs,峰值功率可调。电光Q开关由偏振片、1/4波片和KD*P普克尔盒组成。谐振腔为凹凸非稳腔,由全反镜M1和输出镜M2ii=1,2,3,4,5)组成。谐振腔腔长为265 mm。M1为平凹镜,曲率半径为2.6 m,内表面镀1064 nm高反膜(反射率>99.9%)。输出镜M2i为弯月透镜,第一面曲率半径为−2 m,第二面曲率半径为2 m,厚度为3 mm。第一面的反射率分布为:

      式中:R0为中心反射率;r为距反射率中心的距离;ω0为反射率降到峰值的1/e2时的径向距离,后简称“反射率半径”。当ω0为∞时,即为普通的均匀反射率镜。M2i第一面的反射率参数如表1所示。M2i第二面镀1064 nm增透膜(反射率<0.1%)。M2i安装于二维平移台的镜架上,可以调整M2i XY方向的位置和角度。

      Figure 1.  Diagram of experimental setup

      ParameterM21M22M23M24M25
      ω02.5 mm2.75 mm3 mm2.75 mm
      R030%30%30%20%30%

      Table 1.  Reflectivity parameters of M2i first surface

    • 图1所示,M1曲率中心O1与M2i曲率中心O2的连线作为光轴,M2i的反射率中心与O1O2偏离称为偏心。

      由于高斯镜的反射率中心无法确定,因此,于M2i不同位置处,对激光器分别按照最大能量(简称state1)和最小脉冲宽度(简称state2)进行准直,并用采用能量计(探头型号:Ophir PE50-DIF-ER-C)测试激光输出能量(Eout),采用光电探测器(型号:Thorlabs DET025A,上升时间150 ps)和示波器(型号:Keysight DSO9104A)测试脉冲宽度(τ),采用平行光管(焦距1 m)和CCD相机(位于平行光管焦面,型号:Spiricon SP620)测量远场光斑分布和发散角(θ)。为了减小测试误差影响,采用多次测试取平均值进行数据处理。实验发现,输出镜为M25时,在确保有效通光的任意位置,通过调整输出镜角度可以实现最大能量和最窄脉冲宽度同时输出;而输出镜为M21,M22,M23,M24时,存在最佳位置(如表2所示),在该位置可以同时实现最大能量和最窄脉冲宽度输出。

      DirectionM21M22M23M24
      X3.8 mm3.6 mm3.7 mm3.7 mm
      Y5.1 mm5 mm3.7 mm5.3 mm

      Table 2.  Best position of output mirror

      X方向为例,泵浦能量(Epump)为856 mJ,且M2iY方向位于最佳位置时,state1和state2两种准直状态下,X方向位置对输出能量和脉冲宽度的影响如图2所示。可以看出,偏离最佳位置越多,即偏心量越大,则能量越小,脉冲宽度越宽。相同偏心量情况下,ω0越大或R0越小,则偏心影响越小。X方向偏心量(Δx)为0.5 mm时,能量和脉冲宽度情况如表3所示。

      Figure 2.  Influence of X on energy and pulse width: (a) state1; (b) state2

      Output mirrorStateEnergy/mJPulse width/ns
      M21state1106.3-104.27.2-9.6 (33%)
      state2106.5-99.1 (7%)~7.2
      M22state1128-126.67.3-9 (23%)
      state2128.2-121 (6%)~7.2
      M23state1~1277.3-8.3 (14%)
      state2128.2-123.6 (5%)~7.3
      M24state1~917.3-7.9 (3%)
      state291.4-88.6 (3%)~7.7

      Table 3.  Energy and pulse width at Δx=0.5 mm

      发散角方面,以M21为例,Y方向位于5.1 mm时,X方向位置对激光器发散角的影响如图3所示。可以看出,对于state2,偏离最佳位置对发散角影响不大,X由3.8 mm变化到3.3 mm时,发散角为1.18 mrad~1.19 mrad。对于state1,发散角随偏心量(Δx)的增大而增大,X由3.8 mm变化到3.3 mm时,发散角由1.17 mrad增大到1.4 mrad(20%)。远场光斑分布如图3所示,偏离最佳位置时,X方向的发散角增大。

      Figure 3.  Influence of X on divergence for M21

      出现上述现象的原因为,一定激光器参数下,当光轴与激光晶体中心轴、Q开关中心轴一致,且经过高斯镜的反射率中心,即输出镜位于最佳位置时,激光器处于对准状态,能量最大,脉冲宽度最窄,发散角最小。当出现偏心,即输出镜移动至位置1时(如图1所示),曲率中心由O2移动至O2’,反射率中心由P移动至P’。对于均匀反射率镜(M25),通过调整输出镜角度将O2’调整至O2’’,光轴则与调整前一致,同时反射率均匀分布,输出特性不受影响。对于高斯镜(M21、M22、M23、M24),反射率分布不均匀,输出能量受输出镜反射率的影响[12],脉冲宽度主要由Q开关决定,发散角主要由光轴与激光晶体中心轴的匹配决定。因此,输出能量最大(state1)时,光轴接近或经过高斯镜的反射率中心,此时与Q开关和激光晶体中心轴存在一定角度,因此脉冲宽度和发散角增大;输出脉冲宽度最小(state2)时,光轴与Q开关中心轴一致,而激光晶体中心轴与Q开关中心轴一致,则此时发散角最小,而平均反射率下降,因此输出能量减小。

    • 当M2i位于最佳位置时,不同泵浦能量下,对激光器输出能量、脉冲宽度和发散角进行测试,同时在输出镜后20 mm处采用CCD相机(型号Spiricon SP620)和缩束镜头(缩束倍率4倍)对近场光斑直径(D)进行测试,由于激光器输出光为发散光,近场光斑近似为激光束腰,测试结果如图4所示。可以看出,当R0为30%时,ω0越大,则平均反射率越大,对应的阈值越小。相比M21、M22和M23,M25为输出镜时,激光器的阈值最小。M24平均反射率最小,因此,M24为输出镜时,激光器的阈值最大。斜率效率方面,M24和M25为输出镜时斜率效率最大,M21为输出镜时,斜率效率最低。随着泵浦能量的增加,发散角和近场光斑逐步增大,M25增长最快,ω0越小,限模作用越明显,发散角和近场光斑直径增长越慢。

      Figure 4.  Comparison of output performance: (a) Eout and τ; (b) θ and D

      相同输出能量时的性能对比如表4图5所示。可以看出,相同输出能量下,脉冲宽度类似。M21对应的泵浦能量最大,光光效率(η)最低(12.6%),相比M22、M23和M25R0相同),θD最小,光束质量最好。M25为输出镜时,光光效率最高(16%),但θD最大,光束质量最差。相对M21,M22为输出镜时的光束质量接近,但是光光效率明显增大。相比M22,M24为输出镜时的光光效率较低(13%),但是θ减少9%,光束质量较好,M2因子约为4.6,对应的脉宽轮廓和近场光斑分布如图6所示。可以看出,脉冲宽度为7.3 ns,CCD相机测试光斑直径为1.28 mm,由于缩束镜头缩束倍率为4倍,因此实际光斑直径D约为5.1 mm。

      ParameterM21M22M23M24M25
      Eout /mJ 128.3 128.2 127.2 128 128.1
      Epump/mJ 1020 856 856 984 802
      η 12.6% 15% 14.9% 13% 16%
      τ/ns 7 7.2 7.5 7.3 7.3
      θ/mrad 1.31 1.34 1.48 1.22 1.5
      D/mm 5.2 5.2 5.5 5.1 5.7

      Table 4.  Properties under the same output energy

      Figure 5.  Far field beam distribution: (a) M21; (b) M22; (c) M23; (d) M24; (e) M25

      Figure 6.  Performance with M24 as output coupler: (a) pulse trace; (b) near field beam distribution

    • 文中对比了使用不同参数的高斯输出镜时,高斯镜的偏心量对激光器输出特性的影响及激光器的输出特性。仅当光轴与激光晶体、Q开关中心轴一致,且经过反射率中心时,可同时实现最大能量、最窄脉冲宽度和最小发散角输出。出现偏心时,高斯镜反射率半径越小或中心反射率越大,则能量下降越多,脉冲宽度和发散角增大越大。对于反射率半径为2.5 mm及中心反射率为30%的高斯输出镜,偏心0.5 mm时,能量降低7%,脉冲宽度变宽33%,发散角增大20%。输出镜反射率均匀分布时,偏心对激光输出特性影响小。激光性能方面,相同中心反射率情况下,高斯镜反射率半径越小,则激光阈值越大,光束质量越好,但是光光效率低。中心反射率对激光阈值及光束质量均有影响。综合考虑偏心影响和激光性能,反射率半径为2.75 mm及中心反射率为20%的高斯镜作为输出镜最佳,泵浦能量为984 mJ时,输出能量为128 mJ,脉冲宽度为7.3 ns,发散角为1.22 mrad,光束质量M2因子约为4.6,对应光光转换效率为13%。后续可以通过增加谐振腔腔长或优化LDA和晶体的参数及分布,提升增益均匀性而进一步优化光束质量。

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