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Volume 43 Issue 5
Sep.  2019
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Compact solid-state lasers with high peak power used for remote laser rangefinders

  • Received Date: 2018-11-11
    Accepted Date: 2018-11-21
  • In order to meet the application requirements of military long-distance laser rangefinders for solid-state lasers with small volume and high peak power, a small air-cooled LD-pumped Nd:YAG solid-state laser was designed by using short cavity length and orthogonal polarization electro-optic Q-switched technology. The prototype was verified by experiments. The results show that, the laser with weight of 630g, repetition frequency of 20Hz, output energy of 85mJ, pulse width of 3.9ns, laser peak power of 21.8MW and beam divergence angle of 1.9mrad meets the requirements of miniaturization and high peak power. The laser has strong environmental adaptability and has been applied in engineering.
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    McCARTHY J C, YOUNG Y E, DAY R C, et al. Athermal, lightweight, diode-pumped, 1-micron transmitter[J]. Proceedings of the SPIE, 2005, 5707:237-242. doi: 10.1117/12.589994
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    KALLENBACH R, MURPHY E, GRAMKOW B, et al. Space-qualified laser system for the BepiColombo laser altimeter[J]. Applied Optics, 2013, 52(36):8732-8746. doi: 10.1364/AO.52.008732
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    CREPY B, CLOSSE G, CRUZ J D, et al. Athermal diode-pumped laser designator modules for targeting application[J]. Proceedings of the SPIE, 2012, 8541: 85410R1. doi: 10.1117/12.977854
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    NIEUWSMA D E, WANG J. Design of an advanced diode-pumped solid state laser for high-altitude airborne operations[J]. Proceedings of the SPIE, 2005, 5659:163-170. doi: 10.1117/12.580348
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    GOLDBERG L. Compact laser sources for laser designation, ranging and active imaging[J]. Proceedings of the SPIE, 2007, 6552:65520G. doi: 10.1117/12.722143
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    ZHOU Sh H, ZHAO H, TANG X J. High average power laser diode pumped solid-state-laser[J]. Chinse Journal of Lasers, 2009, 36(7):1605-1618(in Chinese). doi: 10.3788/CJL20093607.1605
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    CHEN H T, CHE X H, XU H W, et al. Study on high-power laser diodes as pumping source at high operating temperature[J]. Chinse Journal of Lasers, 2010, 37(11):2799-2802(in Chinese). doi: 10.3788/CJL20103711.2799
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    LIU Y P, PENG X J, ZHAO G. Structure design and analysis of cooling parts of compact lasers [J]. Laser Technology, 2017, 41(6):886-889(in Chinese).
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    沈阳化工大学材料科学与工程学院 沈阳 110142

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Compact solid-state lasers with high peak power used for remote laser rangefinders

  • 1. Southwest Institute of Technical Physics, Chengdu 610041, China
  • 2. Navy Equipment Department, Beijing 100071, China

Abstract: In order to meet the application requirements of military long-distance laser rangefinders for solid-state lasers with small volume and high peak power, a small air-cooled LD-pumped Nd:YAG solid-state laser was designed by using short cavity length and orthogonal polarization electro-optic Q-switched technology. The prototype was verified by experiments. The results show that, the laser with weight of 630g, repetition frequency of 20Hz, output energy of 85mJ, pulse width of 3.9ns, laser peak power of 21.8MW and beam divergence angle of 1.9mrad meets the requirements of miniaturization and high peak power. The laser has strong environmental adaptability and has been applied in engineering.

引言
  • 脉冲激光测距机是利用射向目标的激光脉冲测量目标距离的一种距离测量仪,具有测程远、体积小等特点,多用于军事上对各种战场目标测距。军用激光测距技术以其测程远、精度高等特点被广泛用于各种军用测距机,对提高防空、海上作战、中近程精确打击及陆上武器的攻击的命中精度方面起到关键作用。激光测距机是激光器在军事上最早应用的项目。激光测距机至今已积累了40多年的研制经验和30多年的使用经验,是目前技术最成熟的一种军用激光仪器。军用激光测距仪装备军队以来,经过多年的研制和装备实践,目前跟激光器有关的发展趋势如下:(1)人眼安全激光测距机。重点发展1.57μm光学参量振荡器(optical parametric oscillator,OPO)激光测距机与1.54μm铒玻璃激光测距机;(2)小型化远程激光测距机。在很多战术应用中,随着探测、成像及武器攻击系统性能的提升,要求能测20km以上的远程激光测距机。目前小型化远程激光测距机还处于研发阶段,离实用阶段还有一段距离。

    本文中结合小型化远程激光测距机研发中的实际需求,设计了一种高峰值功率、小束散紧凑型风冷激光器。经测试,在-40℃~+65℃宽工作温度条件下,其性能优异、环境适应性强,满足复杂环境下的使用要求[1-2]

1.   激光器高峰值功率实现依据
  • 脉冲测距机测距方程如下:

    式中, Prec为最小可探测功率;Pout为激光输出峰值功率; D为接收光学系统口径;t为接收光学系统透过率;θ为激光发射束散角;r为目标反射率;A为目标等效截面积;R为最大测程;σ为大气吸收系数。

    从(1)式可看出,在其它参量不变情况下,要提高测程,只能提高激光输出峰值功率、减小激光发射束散角。

    激光器输出峰值功率取决于激光器输出能量与激光脉冲宽度,方程如下:

    式中,Eout为激光输出能量;tp为激光脉冲宽度。要提高激光器输出峰值功率,在输出能量保持不变情况下,需压窄激光脉冲宽度。

    激光器主动调Q方式中,影响脉宽的主要因素是腔长、谐振腔内增益。根据Degnan调Q理论,可以得到激光器脉冲宽度表达式:

    式中,a=(z-1)[zlnz],其中无量纲数z=2g0lc/Lg0是小信号增益系数,lc是激光晶体长度,l是谐振腔光程, L是单程损耗(包括吸收、散射、衍射损耗等),tr为振荡往返时间。令tr=2l/c,当z≥5时:

    从上式可知:激光脉冲宽度tp与谐振腔光程l成正比,与z成反比。所以,为获得窄脉宽激光器,应尽可能提高激光器增益,缩短谐振腔腔长[3-4]

2.   LD侧面抽运形式优化
  • 激光器采取激光二极管(laser diode, LD)抽运环形侧面抽运的方案可以使工作物质内的增益分布趋于高斯状,有利于低阶模的起振,从而得到小束散角的光束输出[5-6]

    图 1是模拟实际抽运情况下的激光工作物质截面的增益分布。

    Figure 1.  Gain distribution of laser working material cross section

    采用环形侧面抽运能获得更均匀的抽运增益,同时在采用环形侧面抽运条件下,选择适当的抽运距离,可以增加整个晶体的抽运吸收均匀性。若距离过远,则无法获得足够的抽运功率密度。针对上面的分析,作者采用左右半圆周交错均匀抽运的方式,抽运面到晶体表面距离采用2.5mm。

3.   调Q方式及电光晶体选取
  • 激光器输出高峰值功率激光,势必造成激光器谐振腔内功率密度的提升,这对激光器封锁能力及电光晶体损伤阈值提出更高要求。本文中针对高峰值功率输出激光器,采取正交偏振加压式调Q技术,且选用抗损伤阈值高的磷酸钛氧铷晶体(rubidium titanyl phosphate crystal,RTP)电光晶体,满足了激光器高峰值功率输出的要求。

    正交偏振片加压式调Q技术如图 2所示。通过偏振方向相互正交的两块偏振片实现激光器封锁功能,调Q晶体仅仅起到了加压触发作用。偏振片的高偏振性能保证了高峰值功率输出激光器在调Q封锁阶段的可靠,避免了因波片旋转误差造成的封锁性能不稳定问题[7-15]

    Figure 2.  Orthogonal polarization voltage on Q-switching

    激光器输出高峰值功率激光,亦对腔内电光晶体的损伤阈值提出了更高的要求,常用的电光调Q晶体RTP、磷酸二氘钾(DKDP)和LiNbO3的性能如表 1所示。

    parameters RTP DKDP LiNbO3
    refractive index 1.9 1.5 2.2
    half-wave-plate voltage(1:1)/kV 8 9 8.5
    half-wave-plate voltage
    temperature coefficient/(%·℃-1)
    little large little
    damage threshold/(MW·cm-2) 1000 500 280
    piezoelectric ringing no yes yes
    temperature stability fine bad bad
    non-hygroscopic no yes no

    Table 1.  Typical performance parameters of electro-optical crystal

    RTP晶体电光开关采用温度补偿式设计结构,每个电光开关由两块RTP晶体构成。相比其它两种常用电光晶体,通过表 1中可见,RTP晶体电光开关具备以下优异性能:高抗光损伤阈值、无压电振荡效应、低插入损耗、自动温度补偿、不潮解。

    考虑到高峰值功率输出激光器对电光晶体的高抗损伤性能要求,本文中选用RTP作为激光器电光晶体。

4.   LD侧面抽运高峰值功率激光器设计
  • 本文中设计的激光器物理腔长110mm,采用正交偏振调Q技术,调Q方式由2个偏振片组成的正交偏振加压式电光调Q,调Q晶体使用RTP电光晶体,尺寸为7mm×7mm×14mm。采用两套二极管交错对称抽运模块,两套二极管抽运模块中的Nd:YAG激光晶体分别铟焊在钨铜合金热沉上。激光器光路图如图 3所示。

    Figure 3.  Laser cavity diagram of high peak power laser

    激光器采用环形激光二极管侧面交错抽运结构,抽运源为LD半环形阵列交错封装。整个抽运模块共有40个bar条,单bar最高功率100W,808nm@55℃;激光介质为两根Nd:YAG晶体,尺寸为Ø4mm×20mm,掺杂原子数分数为0.008,两端镀1064nm增透膜,侧面在半圆周面上镀金,分别铟焊在钨铜热沉上。激光二极管抽运模块通过热电制冷器将废热传至散热器散热。激光晶体通过铜热沉传导散热至激光器外壳将热量散至外界环境中。采用杭州大和电子有限责任公司型号为9500/131/150B、尺寸为30.0mm×60.0mm×3.5mm的热电制冷器。冷却风机采用德国ebmpapstRLF35-8/12N风机,功率3.6W。激光二极管抽运模块如图 4所示。激光晶体及热沉如图 5所示。

    Figure 4.  Laser diode pump module

    Figure 5.  Laser crystal and heat sink

    为了压缩激光器输出激光束散角,激光器谐振腔采用非稳腔技术。激光谐振腔由曲率半径为尺寸为1800mm凹面全反镜与1000mm凸面输出镜组成,输出镜透过率65%。兼顾谐振腔内激光高增益、压缩激光束散角与谐振腔的热不敏感性。

5.   高峰值功率固体激光器实验结果与分析
  • 采用环形激光二极管侧面交错抽运结构及正交偏振加压式电光调Q技术,实现高峰值功率的1.06μm激光输出。使用美国相干公司EPM2000型激光能量计,在激光电源注入电压77V、电流65A情况下,获得85mJ激光输出能量,电光效率达到8.5%。激光输出能量与注入电流关系如图 6所示。从图中可以看出,激光器输出能量随着抽运电流的增大成线性增加。

    Figure 6.  Relationshop between laser output energy and input current

    军用测距机工作环境温度为-40℃~+65℃范围,本文中设计的激光器按照该要求在注入电流65A、常温、激光器输出能量85mJ下,进行了全温度范围且激光器60s工作时长时输出平均能量测试,激光输出能量与温度的关系如图 7所示。

    Figure 7.  Relationship between laser output energy and ambient temperature

    从图中可以看出,激光器在高温65℃时, 能量略有降低,全温度范围内,激光器输出能量基本保持恒定。高温能量略降低,考虑为激光器散热系统在高温环境下,因外界温度过高,激光器废热不易快速散出造成散热效率下降,进而二极管抽运模块温度偏离最佳工作点。

    采用美国TEK公司500MHz宽带示波器和带宽大于1GHz的P6701B型光电探头对输出激光脉冲宽度进行测试,测试得到激光脉宽约3.9ns,激光波形如图 8所示。

    Figure 8.  Values of laser output pulse width

    激光器通过焦距为3m的聚焦镜,对激光器输出光束质量及激光束散角进行测量,激光光斑呈标准高斯光束分布,激光发散角1.9mrad。焦点处激光光斑形状如图 9所示。

    Figure 9.  Laser far field pattern

    该激光器在-40℃~+65℃宽工作温度范围内,获得了稳定的1.06μm高峰值功率激光输出,以20Hz重复频率工作,激光输出能量85mJ左右,激光能量在整个温度范围内的变化小于5%,激光脉冲宽度达到3.9ns,激光束散角1.9mrad,满足军用激光测距机对高峰值功率激光器要求。

6.   结论
  • 采用短腔长及正交偏振加压式RTP电光调Q技术,解决了激光窄脉宽与高峰值功率造成的腔内光学器件损伤难题。采用两套二极管交错对称抽运模块及激光晶体铟焊在钨铜合金热沉,解决了激光器光斑均匀性及晶体高重频工作下冷却难题。抽运方式的优化及谐振腔的合理设计获得了较高的光光转换效率和高光束质量的1.06μm高峰值功率激光输出。激光器重频20Hz、光斑尺寸4mm,输出激光能量85mJ左右、脉冲宽度3.9ns、束散1.9mrad。该激光器体积小重量轻,激光器(含风机)外形尺寸:120mm×52mm×50mm,重量630g,输出能量和光轴指向稳定、环境适应性强,已实现小型化远程军用测距机上的工程化运用。

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