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脉冲测距机测距方程如下:
$ {P_{{\rm{rec}}}} = \frac{{{P_{{\rm{out}}}}rA{D^2}t}}{{{R^4}{\rm{ \mathsf{ π} }}{\theta ^2}}}\exp ( - 2\sigma R) $
(1) 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:
$ {P_{{\rm{out }}}} = {E_{{\rm{out }}}}/{t_{\rm{p}}} $
(2) 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:
$ {t_{\rm{p}}} = \frac{{{t_{\rm{r}}}}}{L}\left\{ {\frac{{\ln z}}{{z[1 - a(1 - \ln a)]}}} \right\} $
(3) 式中,a=(z-1)[zlnz],其中无量纲数z=2g0lc/L,g0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:
$ {t_{\rm{p}}} \propto {t_{\rm{r}}}/(zL) $
(4) 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]。
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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]。
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。
Table 1. Typical performance parameters of electro-optical crystal
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 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。
用于远程激光测距机的小体积高功率固体激光器
Compact solid-state lasers with high peak power used for remote laser rangefinders
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摘要: 为了满足军用远程激光测距机对小体积高峰值功率固体激光器的应用需求, 采用短腔长、正交偏振电光调Q技术, 设计了小型风冷LD抽运Nd:YAG固体激光器, 并对样机进行了实验验证。结果表明, 激光器重量630g、重频20Hz、输出能量85mJ、脉冲宽度3.9ns、激光峰值功率21.8MW、束散角1.9mrad, 满足了小型化和高峰值功率的要求。该激光器具备较强环境适应性, 目前已实现工程化应用。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.
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Key words:
- lasers /
- remote laser rangefinder /
- pulsed laser /
- electro-optic Q-switched /
- peak power /
- divergence angle /
- energy stability
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Table 1. Typical performance parameters of electro-optical crystal
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 -
[1] MENDE S, ANGELOPOULOS V, FREY H U, et al. Timing and location of substorm onsets from THEMIS satellite and ground based observations[J]. Annales Geophysicae, 2009, 27(7):84-92. [2] STEINVALL O. Review of laser sensing devices and systems[J]. Proceedings of the SPIE, 2005, 5983:598303. doi: 10.1117/12.627442 [3] WAND J G, SUN Zh, JIANG M H.A high-power 1ns Nd:YAG laser system[J]. Journal of Optoelectronics·Laser, 2012, 23(7):1257-1262(in Chinese). [4] KOECHNER W. Solid-state laser engineering[M]. Beijing: Science Press, 2002: 410-448(in Chinese). [5] VOLODIN B L, DOLGY S V, MELNIK E D, et al. Wavelength stabilization and spectrum narrowing of high-power multimode laser diodes and arrays by use of volume Bragg gratings[J]. Optics Letters, 2004, 29(16):1891-1893. doi: 10.1364/OL.29.001891 [6] KREBS D J, NOVO-GRADAC A M, LI S X, et al. Compact, pa-ssively Q-switched Nd:YAG laser for the MESSENGER mission to Mercury[J]. Applied Optics, 2005, 44(9):1715-1718. doi: 10.1364/AO.44.001715 [7] 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 [8] 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 [9] 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 [10] 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 [11] 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 [12] 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 [13] 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 [14] 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). [15] ZHAO G, LI J, PENG X J. Compact repetitive diode pump slab laser without thermoelectric cooler [J]. Laser Technology, 2016, 40(5):625-629(in Chinese).