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Volume 41 Issue 1
Jan.  2017
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Laser research of output wavelength controlled common aperture 0.532μm/1.064μm/3.9μm

  • Received Date: 2015-12-15
    Accepted Date: 2016-01-13
  • In order to achieve the laser of high power, high frequency and controllable output with the same aperture, using the method of laser amplifier, high frequency tuning Q, optical parametric oscillator(OPO), frequency doubling and scanning reflection mirror, theoretical analysis and experimental verification were carried out. Selective laser outputs 0.532μm of 40W, 1.064μm of 100W, 3.9μm of 12.6W were gotten under the condition of power supply current of 42A, Q frequency of 10kHz and the same aperture. The results show that the laser with high power, high frequency, controllable output and the same aperture can be achieved by this experimental device.
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    CHO K H, RHEE B K. Intracavity infrared OPO using periodically poled Mg-doped stoichiometric LiTaO3 for generating high average power[J]. Proceedings of the SPIE, 2008, 6875:68751A. doi: 10.1117/12.761494
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    YAN B X, BI Y, ZHOU M, et al. Highly efficient continuous-wave mid-infrared intracavity singly resonant optical parametric oscillator based on MgO:PPLN[J]. Chinese Physical Letters, 2010, 27(12):124203. doi: 10.1088/0256-307X/27/12/124203
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    KONNO S, KOJIMA T, FUJIKAWA S, et al. High-brightness 138W green laser based on an intracavity-frequency-doubled diode-side-pumped Q-switched Nd:YAG laser[J]. Optics Letters, 2000, 25(2):105-107.
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通讯作者: 陈斌, bchen63@163.com
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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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Laser research of output wavelength controlled common aperture 0.532μm/1.064μm/3.9μm

  • 1. School of Science, Hubei University for Nationalities, Enshi 445000, China
  • 2. Ministry of Eduction Key Laboratory of Weak Light Nonlinear Photonics, Nankai University, Tianjin 300457, China

Abstract: In order to achieve the laser of high power, high frequency and controllable output with the same aperture, using the method of laser amplifier, high frequency tuning Q, optical parametric oscillator(OPO), frequency doubling and scanning reflection mirror, theoretical analysis and experimental verification were carried out. Selective laser outputs 0.532μm of 40W, 1.064μm of 100W, 3.9μm of 12.6W were gotten under the condition of power supply current of 42A, Q frequency of 10kHz and the same aperture. The results show that the laser with high power, high frequency, controllable output and the same aperture can be achieved by this experimental device.

引言
  • 多波段可控输出激光器在光电对抗领域有重要应用。这类激光器输出波段范围跨度大,激光器设计、镀膜难度较大,本文中对该类激光器进行了实验探索。实验中采用一级激光放大技术、非线性周期极化钽酸锂(periodically poled LiTaO3, PPLT)晶体光参量振荡(optical parametric oscillator, OPO)技术、双MgO:LN电光开关激光切换技术及电控扫描反射镜技术, 实现了多种波长共孔径激光的高功率输出,试验为该类激光器的工程应用积累了经验。

1.   1.064μm基频光设计
  • 由于激光器要求输出3种波长,而在这3种波长中,0.532μm和1.064μm技术成熟,实现较为简单,3.9μm激光波长跨度大,激光转换效率低,因此1.064μm激光器在功率选择由3.9μm激光输出来确定。1.064μm激光通过OPO转换到3.9μm输出效率可达10%以上,因此要实现十几瓦的3.9μm激光输出,至少需要100W左右的1.064μm激光输出。为了降低1.064μm激光器中元器件的功率负荷,在1.064μm激光实现上,采用一级放大装置,在调Q选择上,选择容易实现高频输出的声光开关,由于倍频及OPO需要偏振光,在光路中增加了起偏器[1]。为了防止杂散光对振荡级的影响,在输出镜放大级中添加了隔离器,其光路原理如图 1所示。其工作原理如下:激光器工作时激光电源触发二极管阵列放电,产生在钇铝石榴石(yttrium aluminum garnet, YAG)激光晶体吸收带内808nm的光谱辐射。这些光谱辐射由聚光腔[2]反射聚焦到激光工作物质上,完成基态离子的跃迁。为了提高激光脉冲能量及峰值功率,这时以声光Q开关组成的开关关闭,阻止腔镜的反馈,进而阻断谐振腔激光振荡。当激光工作介质激发态能级储存的反转粒子数达到极大值时,Q开关迅速打开,腔内形成激光振荡,产生高峰值功率1.064μm波长激光脉冲。再经由放大级种子光放大,产生大功率的激光脉冲[3-5]。实验过程中对聚光腔和声光调Q开关采用循环液体冷却,水温为22℃。

    Figure 1.  Experimental light path diagram of fundamental frequency light

2.   0.532μm激光产生设计
  • 用于1.064μm激光非线性变换常用的晶体有磷酸钛氧钾(KTiOPO4,KTP)、三硼酸锂(LiB3O5, LBO)、偏硼酸钡(BaB2O4, BBO)等。KTP晶体光学性能好、透光范围大、走离小、价格低廉。在1.064μm的二次谐波时可产生70%左右的效率。在本方案中选用了Ⅱ类临界相位匹配KTP晶体进行腔外倍频[6],KTP晶体两端面镀1.064μm和0.532μm增透膜,尺寸为6mm×6mm×20mm, 在切割角度θ=90°,φ=26.5°条件下,有效非线性系数为3.28pm/V。本方案中晶体在较高温度切割,主要是提高其抗损伤能力和提高激光的稳定性。在光路实现上,使用基频1.06μm激光一次通过实现,其光路示意图如图 2所示。

    Figure 2.  Principle diagram of laser frequency doubling

3.   3.9μm激光产生设计
  • 本方案中选用基频1.06μm激光抽运非线性周期极化钽酸锂(periodically poled LiTaO3,PPLT)晶体OPO技术来产生3.9μm激光。OPO中的波长转换需要同时满足能量和动量守恒(相位匹配)方程[7],守恒方程如下:

    式中,λp为抽运光波长;λi为闲频光波长;λs为信号光波长;T为晶体温度;np(λp, T)为抽运光在温度T时的折射率;ni(λi, T)为闲频光在温度T时的折射率;ns(λs, T)为信号光在温度T时的折射率;Λ(T)为非线性晶体的极化周期。

    当晶体温度控温在25℃、抽运光为1.06μm、信号光为1.46μm时,输出3.9μm激光,由上式计算出PPLT晶体的极化周期Λ=29.2μm。原理光路如图 3所示。

    Figure 3.  Principle diagram of OPO

4.   波长可控的共孔径高功率、高重频激光器实验
  • 综上所示,最终设计的激光器总体光路如图 4所示。图中, Ⅰ为1.064μm基频激光器,其主要作用是产生高功率高频1.064μm激光; Ⅱ为波长输出可控的共孔径输出激光器核心部分,主要用于完成0.532μm和3.9μm激光的产生、1.064μm激光的分光以及3种波长的共孔径选择输出[7-9]

    Figure 4.  General layout of experimental light path (Ⅰ—fundamental frequency light path diagram; Ⅱ—the output wavelength controllable common aperture laser output optical path; 1—total reflection mirror; 2—acousto-optic Q switch; 3, 7—pump chambers; 4, 9, 17—polarizer; 5—output mirror; 6—isolator; 8, 16—electro-optical crystal(LN); 10—focusing lens; 11—OPO total reflection mirror; 12—PPLT; 13—OPO output mirror; 14, 19—splitter mirror; 15, 21—dump; 18—KTP; 20, 22, 23—45°reflection mirror)

    图 4中1,2,3,4,5组成激光器振荡级,用于产生1.06μm激光。其中1为平凹的基频激光器全反镜,凹面镀1.064μm高反介质硬膜(反射率R>99.5%),曲率半径ρ1=2m;2为声光调Q开关,频率10kHz;3为激光二极管(laser diode,LD)侧抽运模块,由激光二极管抽运阵列和激光增益介质Nd:YAG组成;4为偏振片;5为输出镜,平平镜,对1.06μm透过率t=20%;6为隔离器;7为与3同样功率的LD侧抽运模块,用于激光能量的放大。

    图 4中8,9,16,17组成光路切换装置,主要用于基频1.06μm激光到3.9μm和0.532μm激光光路的切换。其中9,17为偏振器,8,16为掺镁铌酸锂晶体,该晶体掺锂0.486mol,掺镁0.065mol,掺锌0.04mol,晶体内部均匀性Δne < 3×10-5,晶体大小为8mm×8mm×16mm。

    图 4中10~15组成3.9μm激光产生光路,用于产生3.9μm激光。其中10为聚焦透镜,镀1.064μm增透介质硬膜,透镜曲率半径ρ2=-300mm。11,12,13组成光参量振荡器,实现3.9μm波段激光输出。11为中红外OPO腔全反镜,靠近抽运源端镀1.064μm增透介质硬膜(t>99.9%),靠近PPLT晶体的面镀1.4μm~1.6μm高反膜(R> 99.9%)和3.7μm~4.0μm高反膜(R> 99%)。13为中红外OPO腔输出镜,内侧镀1.06μm介质硬增透膜(t>98%),1.4μm~1.6μm高反介质硬膜(R>99.8%)和3.7μm~4.0μm部分透过介质硬膜(t=20%)。12为周期极化晶体PPLT晶体,尺寸为50mm×10mm×1mm, 极化周期为29.2μm,用于实现2阶非线性频率变换产生3μm~5μm波段激光输出。当1.06μm抽运光入射OPO腔,抽运周期极化晶体PPLT晶体,产生1.46μm信号光,信号光在腔镜的反馈下开始振荡,其和1.06μm抽运光在PPLT内发生光参量振荡效应$\left( {\frac{{2{\rm{ \mathsf{ π} }}}}{{3.9{\rm{ \mathsf{ μ} m}}}} = \frac{{2{\rm{ \mathsf{ π} }}}}{{1.06{\rm{ \mathsf{ μ} m}}}} - \frac{{2{\rm{ \mathsf{ π} }}}}{{1.46{\rm{ \mathsf{ μ} m}}}}} \right)$,产生3.9μm闲频光[10]。14为谐波反射镜,对3.9μm高反,对剩余未转换激光增透。15为光吸收器主要吸收剩余光[11]

    图 4中18,19,21组成0.532μm激光产生光路,用于产生0.532μm激光。其中18为KTP晶体, 尺寸为10mm×10mm×7mm; 19为谐波反射镜; 21为光吸收器,用以吸收剩余1.064μm激光。

    图 4中22,23组成1.064μm激光耦合光路,用于将1.064μm激光反射进电控扫描反射镜20,实现共孔径输出。其中22和23为1.064μm 45°反射镜,反射率为99.9%。

    当激光器需要输出3.9μm激光时,8号晶体加半波电压,1.064μm激光偏振面产生旋转,激光经过偏振器9反射,光进入3.9μm OPO激光光路产生3.9μm激光,再经谐波反射镜14反射后,进入反射镜20然后输出(电机控制的公用45°反射镜,镀3种波长全反膜,当需要那个波长输出,电机控制反射镜对准那个激光输出口),剩余未转换激光通过吸收器吸收[12-13]

    当激光器需要输出0.532μm激光时,16号晶体加半波电压,1.064μm激光偏振面产生旋转,激光经过偏振器17反射,光进入0.532μm激光光路产生0.532μm激光,经谐波反射镜19反射后,进入反射镜20然后输出,剩余未转换激光通过吸收器吸收[13-14]

    当激光器需要输出1.064μm激光时,8号和16号晶体不加半波电压,激光经过反射镜22,23反射,进入反射镜20然后输出。

    当电源电流42A、调Q频率为10kHz的时,在反射镜20后测得的1.064μm激光最高输出功率为100W。当8号晶体加电,在反射镜20后测得的3.9μm激光输出功率如图 5所示。实验中发现, PPLT晶体出现绿、红及黄闪烁现象,这是因为晶体产生了2阶非线性效应,出现红绿黄光所致。

    Figure 5.  3.9μm laser output power

    当16号晶体加半波电压时,在反射镜20后测得的光斑如图 6所示。0.532μm激光输出功率为40W,如图 7所示。

    Figure 6.  0.532μm laser image

    Figure 7.  0.532μm laser output power

5.   结论
  • 利用激光放大技术,非线性周期极化PPLT晶体OPO技术,双MgO:LN电光开关激光切换技术及扫描反射镜技术, 可以实现多种波长同口径激光高功率输出,实验结果表明:在电源电流42A、调Q频率10kHz及共孔径条件下,可实现40W的0.532μm、100W的1.064μm、12.6W的3.9μm激光选择性输出。

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