Ho∶YLF激光泵浦的长波红外ZnGeP2光参量振荡器 
下载： 1201次
Objective The 8--12μm long-wave infrared laser is just within the transmission bands of atmosphere and widely used for gas composition detection and electro-optic countermeasure. As traditional long-wave lasers, CO2 lasers can output lasers with a specific wavelength within the range of 9--10μm. Beyond them, a long-wave infrared optical parametric oscillator (OPO) shows an enormous advantage because of its wavelength tuning. However, OPO-based lasers with wavelengths longer than 8μm and high optical-to-optical conversion efficiency are still scarce. Herein, we construct a ZnGeP2 OPO and experimentally test its long-wave infrared output. The experimental result shows that a long-wave laser with high conversion efficiency is obtained, which provides a reference to engineer the laser based on ZnGeP2 OPO.
Methods The scheme of the OPO-based long-wave infrared laser pumped by a 2μm laser is discussed, in which the selection of a 2μm laser crystal and a long-wave infrared nonlinear crystal is included. In this scheme, the selected nonlinear crystal is ZnGeP2, and the selected pumping laser source is 2.05μm Ho∶YLF laser with a maximum output power of 27W (10kHz). The two end faces of the ZnGeP2 crystal are polished and coated with an antireflection film at 2.05, 2.7, and 8.2μm bands, which are the key processes for reducing the optical loss in the crystal and for reducing the risk of damage. The resonator of the ZnGeP2 OPO is a flat cavity and the resonant mode is double resonance OPO. The Ho∶YLF laser is linearly polarized, which is helpful for ZnGeP2 OPOs to achieve a high optical-to-optical conversion efficiency. The Ho∶YLF laser, pulsed using an acousto-optic Q-switch, is pumped by a Tm∶YAP laser (CW) with a wavelength of 1.94μm and a maximum output power of 62W. Without damaging the elements of the Ho∶YLF laser, the laser’s repetition rate is minimized, the OPO’s threshold is reduced, and the conversion efficiency is improved. The ZnGeP2 crystal, Ho∶YLF crystal, and Tm∶YAP crystal are all wrapped in thin indium foils and placed in copper heat sinks to collect the heat absorbed by them. During the operation of the experimental apparatus, there is the water flow with 20 ℃ in the Q-switch and all heat sinks, and a microchannel structure for the water flow is indicated. Finally, the typical parameters of the long-wave infrared laser, including average power, wavelength, laser beam quality, repetition rate, and pulse duration, are measured.
Results and Discussions The laser experimental apparatus (corresponding to the scheme mentioned above) achieves good experimental results with high power, efficiency, and repetition rate. The long-wave laser is generated when the 2.05μm pulsed laser with an average power of 10.5W is injected. The maximum output power of the long-wave laser is 3.2W when the 2.05μm pulsed laser with an average power of 26.68W is injected. Meanwhile, the corresponding optical-to-optical conversion efficiency is up to 12% and the slope efficiency is up to 19.3%. A spectrum analyzer is used to measure the spectrum of the long-wave laser with an output power of 3.2W and the peak wavelength of 8.135μm is disclosed. A CCD laser beam analyzer is used to measure the laser beam quality factor of the long-wave laser with an output power of 3.2W. The focusing lens method is used for these measurements. After the measurements, the quality factor is 4.5 in the X direction and 4.2 in the Y direction. The laser parameters including repetition rate of 10kHz and pulse duration of 27.11ns are measured using a photoelectric detector. The simple calculation shows that the single pulse laser energy is 0.32mJ and the peak power is 11.8kW.
Conclusions We verify that a ZnGeP2 OPO is feasible to realize high efficiency and tunable long-wave laser output. First, the phase-matching mode and the phase-matching angle of the ZnGeP2 crystal are analyzed and designed according to the principle that the output laser wavelength of a ZnGeP2 OPO corresponds to its phase-matching angle. Second, to realize the 8μm laser, the ZnGeP2 crystal is processed according to the above phase-matching angle. Third, the experimental apparatus is set up and the effect of the long-wave ZnGeP2 OPO laser is verified, and the ZnGeP2 OPO laser pumped by the 2.05μm Ho∶YLF pulsed laser can generate a long-wave laser output with a specific wavelength, high efficiency, and high power. In the future, long-wave infrared lasers with wavelengths longer than 8μm can be achieved just by reducing the phase-matching angle of the ZnGeP2 crystal (changing its cutting angle as an example) or reducing the incident angle of the pump laser.
1 引言
8~12μm波段处在大气层的透射窗口,该波段激光可应用于气体成分检测和光电对抗等多个领域,因此该波段的激光器逐渐成为研究热点之一。在该波段范围内,CO2激光器作为传统的长波激光源,其输出波长主要位于9~10μm范围内,而且波长值为某些特定值。光参量振荡/放大(OPO/OPA)等非线性频率变换方法可调谐长波激光器的输出波长,特别是随着性能优异的新型非线性晶体的出现,长波红外激光器的输出波长不断向长波、甚长波方向拓展。
近年来,国内外在长波红外激光材料非线性频率变换技术方面开展了很多研究,尤其是硒化镉晶体(CdSe)和磷锗锌晶体(ZnGeP2或ZGP)等非线性材料及OPO/OPA等非线性频率变换技术取得了很多进展。2018年,Wang等[1]以重复频率为5kHz、平均功率为18.06W的2.05μm Ho∶YLF激光为光源,泵浦了CdSe 光参量振荡器,获得了平均功率为320mW的10.20μm闲频光输出。2020年, Chen等 [2]以2.1μm脉冲激光泵浦了CdSe 光参量振荡器,实现了1.05W的10.1μm长波激光,光光转换效率达4.69%,光束质量因子分别为2.25(X方向)和2.12(Y方向)。2016年, Fonnum等[3]以Ho∶YLF脉冲激光为光源,采用V型环形ZnGeP2 光参量振荡谐振腔,最终实现了8μm激光输出,单脉冲能量为1.8mJ,光束质量因子为2.6。2019年,Liu等[4]以100W@10kHz的Ho∶YAG激光为光源,泵浦了ZGP 光参量振荡器/放大器,实现了平均功率为12.6W、中心波长为8.2μm、光光转换效率达12.6%的激光输出。2015年,Yu等[5]采用2.09μm的Ho∶YAG脉冲激光,泵浦了环形 ZnGeP2光参量振荡谐振腔,其中ZnGeP2晶体按I类匹配角切割,最终激光输出波长为8μm,光束质量因子为1.2(X方向)/1.22(Y方向)。
本文采用1.94μm Tm∶YAP激光作为泵浦源,以掺钬氟化钇锂晶体(Ho∶YLF)为工作物质,通过声光调Q和端面泵浦方案,获得了2.05μm的线偏振脉冲激光,利用该激光泵浦长波ZGP 光参量振荡器,最终激光波长为8.1μm,输出平均功率为3.2W@10kHz。
2 实验装置
长波红外激光器结构如
反射镜M4为平平镜,与2.05μm光路呈45°放置,镜片表面镀有2μm波段高反射@45°膜层,M3输出的Ho∶YLF激光经过M4反射后进入第二光束整形系统,然后进入由M5、M6和ZGP晶体组成的OPO结构。调节第二光束整形系统参数,使ZGP 光参量振荡器具有合适的振荡阈值和转换效率。ZGP 光参量振荡器采用平平腔和双谐振工作方式,入射镜M5镀有2.05μm高透膜及信频光和闲频光高反膜;M6所镀膜层对2.05μm高透,在信频光和闲频光波段的透过率为40%。ZGP晶体两端面镀有2.05 ,2.70 ,8.20μm三波段高透膜层以减少参量光损耗。
为了保证实验装置稳定运行,需要对Ho∶YLF晶体、Q-switch和ZGP晶体等产生废热的光学元件进行控温,控温方式均采用水冷,其中Ho∶YLF晶体和ZGP晶体均采用铜热沉夹持,晶体与热沉的接触面上覆有厚度为0.2mm的铟箔,晶体热沉和Q-switch的内部均有水流微通道结构,激光器运行时,保持微通道内有(20±0.5)℃的水流循环。
3 方案分析
3.1 Ho∶YLF晶体的特性
Ho∶YLF材料具有优秀的物理化学性能,其化学性能稳定,热导率较高,抗光学损伤能力较强[6]。此外,Ho∶YLF晶体在紫外光谱区的吸收损耗小,具有较高的光存储容量。另外,YLF材料的声子能量低,无辐射跃迁几率低,使Ho掺杂的YLF晶体同其他基质相比,更容易获得较高的转换效率。更为重要的是,YLF材料的双折射特性使得Ho∶YLF激光能保持很好的线偏振特性,这有助于简化Ho∶YLF激光器的谐振腔结构。同时,YLF特有的负折射率温度系数使Ho∶YLF晶体在高功率泵浦光抽运时具有更小的热透镜效应,有助于提高激光器工作的稳定性。
YLF材料中掺杂的Ho3+离子具有发射截面大(0.9×10-20 cm2)和上能级寿命长(~15ms)的特点[7],这非常有利于上能级的储能,而且适合采用连续激光作为泵浦源。另外,Ho3+离子稳定的三能级结构非常适合调Q脉冲运转。综合来看,Ho∶YLF晶体在高功率、高效率的2μm激光输出方面具有优良的性能,Ho∶YLF激光器能够为长波光参量振荡器/放大器提供优质的泵浦光源。
图 2. Ho∶YLF晶体的吸收谱和发射谱 [8]。(a)吸收谱;(b)发射谱
Fig. 2. Absorption and emission spectra of Ho∶YLF [8]. (a) Absorption spectra; (b) emission spectra
3.2 ZGP及相位匹配
与其他的长波红外非线性晶体相比,ZGP晶体的性能优势明显[9-10],包括透明范围宽(0.7~12μm)、摩氏硬度高(5.5)、损伤阈值高(30GW/cm2)、非线性系数大[d36=(75±8)pm/V]和热导率较高[~0.35W/( cm·K)]等。因此,当研究人员希望通过非线性频率变换方式获得长波红外激光输出时,ZGP晶体往往成为最佳选项。
基于目前的ZGP晶体加工水平,经过退火和辐照等一系列工艺,ZGP晶体在2.05μm处的吸收系数不超过0.1db/cm-1,在8μm处的吸收系数不超过0.05db/cm-1。
实现长波激光可调谐的可行方式有两个:一是改变ZGP晶体的温度,使通过晶体的参量光的折射率发生变化,进而在新的相位匹配下获得新的输出波长,然而晶体折射率随温度的变化过于缓慢,因此调节晶体温度不是理想的波长调谐方式;二是通过改变ZGP晶体的相位匹配角度来改变输出激光的波长,可以通过改变晶体放置角度来改变泵浦光入射角,进而实现晶体相位匹配角的调节,这种方式简单可行,因此调节晶体的相位匹配角成为首选的波长调谐方式。接下来要做的就是获得输出波长与相位匹配角之间的关系。
当波长为λ的激光在ZGP晶体内传播时,激光在ZGP晶体内的折射率遵循椭球公式[11]:
式中:θ为光轴和晶体主轴之间的夹角;no为ZGP晶体内部垂直光轴方向的折射率,ne为ZGP晶体内沿光轴方向的折射率[11]。
作为正单轴晶体,ZGP对o光和e光的折射率[11]分别为
ZGP晶体内的三波相互作用过程遵循能量守恒公式:
式中:ω为光的频率;n为光在介质内的折射率;p代表泵浦光;s代表信频光;i代表闲频光。
综合(1)~(4)式可以得出2.05μm激光泵浦ZGP光参量振荡器时的相位匹配曲线。
图 4. 2.05μm激光源泵浦ZGP光参量振荡器的相位匹配曲线
Fig. 4. Phase-matching curve of ZGP OPO pumped by 2.05μm laser
有效非线性系数是决定非线性频率变换器件运行效率的关键参数,对ZGP晶体而言,相位匹配的效果与相位角θ和方位角φ均有关,其中,θ决定能否实现相位匹配,而φ决定有效非线性系数的大小。
图 5. ZGP晶体的有效非线性系数曲面。(a) I类;(b) II类
Fig. 5. Effective nonlinear coefficient surface of ZGP crystal. (a) Type I; (b) type II
4 实验结果及分析
4.1 长波激光输出功率
对长波激光的输出功率进行测试,结果如
图 6. 长波激光输出功率与2.05μm激光泵浦功率的关系
Fig. 6. Relationship between output power of long-wave laser and pump power of 2.05μm laser
4.2 长波激光输出光谱
使用光谱分析仪对输出平均功率为3.2W时的长波激光的光谱进行测量,结果如
4.3 长波激光光束特性
使用光束分析仪测量输出平均功率为3.2W的长波激光的光束质量因子,测试方法选用聚焦透镜法,近场二维光斑数据如
5 结论
讨论了长波红外激光工作方案,研究了2μm波段激光晶体和长波红外非线性晶体的选择问题。采用水冷方式,最终输出平均功率为3.2W@10kHz,光光转换效率达12%,斜效率达19.3%,光束质量因子为4.5(X方向)/4.2(Y方向),激光波长峰值为8.135μm,激光脉宽为27.11ns。验证了通过2.05μm Ho∶YLF激光泵浦长波ZGP光参量振荡器来获得高效率、高重复频率长波红外激光输出的可行性,为实现长波固体激光器工程化奠定了基础。
[3] FonnumH, BakklandA, Haakestad MW. Optical parametric oscillator at 8 μm with high pulse energy and good beam quality[C]∥High-Brightness Sources and Light-Driven Interactions, March 20-22, 2016, Long Beach, California, United States. Washington, D.C.: OSA, 2016: MS4C. 5.
[5] Yu KK, Liang ZQ, Yan XS. Experimental studies on beam quality-improving of 8μm ZGP optical parametric oscillator[C]∥2015 International Conference on Optoelectronics and Microelectronics (ICOM), July 16-18, 2015, Changchun, China. New York: IEEE, 2015: 34- 37.
[6] Hemmer M, Sánchez D, Jelínek M, et al. 2-μm wavelength, high-energy Ho∶ YLF chirped-pulse amplifier for mid-infrared OPCPA[J]. Optics Letters, 2015, 40(4): 451-454.
[8] Walsh B M. Barnes N P, di Bartolo B. Branching ratios, cross sections, and radiative lifetimes of rare earth ions in solids: application to Tm 3+ and Ho 3+ ions in LiYF4[J]. Journal of Applied Physics, 1998, 83(5): 2772-2787.
[9] Zawilski K T, Setzler S D, Schunemann P G, et al. Increasing the laser-induced damage threshold of single-crystal ZnGeP2[J]. JOSA B, 2006, 23(11): 2310-2316.
[10] 夏士兴, 雷作涛, 王猛, 等. 大尺寸ZnGeP2晶体生长与中红外光参量振荡[J]. 人工晶体学报, 2011, 40(2): 541.
[11] 韩隆, 苑利钢, 陈国, 等. 26W中波红外固体激光器[J]. 中国激光, 2015, 42(3): 0302004.
Article Outline
魏磊, 吴德成, 刘东, 赵书云, 陈国, 李宝, 方聪, 韩隆, 王英俭. Ho∶YLF激光泵浦的长波红外ZnGeP2光参量振荡器[J]. 中国激光, 2021, 48(1): 0101002. Lei Wei, Decheng Wu, Dong Liu, Shuyun Zhao, Guo Chen, Bao Li, Cong Fang, Long Han, Yingjian Wang. Long-Wave Infrared ZnGeP2 Optical Parametric Oscillator Pumped by Ho∶YLF Laser[J]. Chinese Journal of Lasers, 2021, 48(1): 0101002.
PDF全文
