双晶电光Q开关消光比的研究【增强内容出版】
Electro-optic (EO) Q-switching technology has been extensively used to fabricate pulsed lasers. Its advantages of a faster switching rate, better hold-off ability, and controllable repetition rates enable the generation of energetic short laser pulses. To date, practical EO crystals include LiNbO3 (LN), LiTaO3 (LT), KD2PO4 (DKDP), and RbTiOPO4 (RTP). To achieve a low driving voltage, the laser must propagate along the non-optical-axis direction of these crystals, which introduces additional phase retardation induced by natural birefringence. Using a second crystal that is rotated 90° with respect to the first crystal is necessary to compensate for the natural birefringence and its strong thermal fluctuations. However, achieving double-crystal EO Q-switches with a high extinction ratio is difficult because both crystals should have high transverse optical homogeneity and should be carefully matched. The matching quality may be affected by numerous factors, including the optical inhomogeneity of the crystals, optical processing accuracy, and temperature changes. To date, factors affecting the extinction ratio of double-crystal EO Q-switches have not been systematically studied, limiting the development and application of double-crystal EO Q-switches. In this study, we comprehensively analyze the factors affecting the extinction ratio and fabricated double-crystal LT EO Q-switches.
First, using double-crystal LT EO Q-switches as examples, we analyze the factors that affect the extinction ratio of double-crystal EO Q-switches. To maximize the EO effect, an LT Q-switch is fabricated from two x-cut LT crystals with light propagating along the x axis and voltage applied along the z axis. A set of analytical phase-shift formulas that consider the optical inhomogeneity, crystallographic orientation deviation, length deviation, and temperature change of the two matching crystals are derived. Combined with the transmittance formula for the parallel-polarization system, the tolerances of these factors are calculated using an extinction ratio of 100∶1. Accordingly, we fabricate two LT EO Q-switches with aperture of 9 mm×9 mm and lengths of 10 mm and 5 mm, respectively. The matching crystals are polished using the same polishing lap to ensure that the deviations in length and orientation satisfy the requirements. Each face of the crystals is finely ground, and the x surface is precisely polished and coated with anti-reflection films at 1064 nm. The z surface is then plated with gold and chromium. The two matching crystals are packaged in an elastic holder. The matching quality and extinction ratios are measured and characterized.
According to univariate analysis, the extinction ratio of double-crystal EO Q-switches is strongly related to the optical inhomogeneity, crystallographic orientation deviation, and the length and temperature differences of the two matching crystals. The extinction ratio is inversely proportional to the square of the optical inhomogeneity [Fig. 2(b)] and those of the length and temperature differences when the other parameters are kept constant. In addition, the crystal length significantly affects the extinction ratio. When the optical inhomogeneity, crystallographic orientation deviation, and temperature difference are set, the extinction ratio is inversely proportional to the square of the crystal length [Fig. 2(a)]. In addition, even the same change in temperature in the two matching crystals may affect the extinction ratio when a difference in crystal length is observed. To achieve an extinction ratio of 100∶1 under a crystal length of 10 mm, laser spot radium of 2.5 mm, and wavelength of 1064 nm, the optical homogeneity must be better than 6.8×10-6/cm, the x and z orientation deviations should be less than 1.3° and 3.1°, respectively, the length difference should be less than 4.6 μm, and the temperature difference must be less than 0.16 ℃. For the two prepared LT EO Q-switches (Fig. 4), the shorter switch exhibits a better matching quality and higher extinction ratio (Fig. 6). The extinction ratio of the shorter LT Q-switch is approximately three times that of the longer switch (Table 1). However, the extinction ratios of both Q-switches are low due to poor optical homogeneity (Fig. 5).
Using double-crystal LT EO Q-switches as an example, we systematically analyze the factors affecting the extinction ratio and calculate their tolerances. Based on the theoretical results and practical difficulties of crystal growth and optical processing, the optical inhomogeneity and length and temperature differences are verified as critical factors that must be strictly controlled. In addition, the extinction ratio is found to be inversely proportional to the square of the crystal length when the optical inhomogeneity, crystallographic orientation deviation, and temperature difference are constant. Accordingly, we prepare two double-crystal LT EO Q-switches with different lengths. A shorter LT Q-switch is verified to have a better matching quality and higher extinction ratio. However, the extinction ratios of both Q-switches are low due to poor optical homogeneity. Double-crystal LT-EO Q-switches with high extinction ratios can be achieved using crystals with high optical quality. This work can be of significance in guiding the development of double-crystal EO Q-switches with high extinction ratios.
1 引言
高峰值功率窄脉宽激光在激光加工、光电探测、非线性光学和医疗等领域中都有着广泛的应用[1-4]。调Q技术是获得脉冲激光的主要方式之一,其中,电光调Q技术由于具有开关速率高、关断能力强、主动可控、激光器与其他仪器容易实现高精度同步等优势,在短脉冲激光系统中得到了广泛应用[5-6]。
电光晶体是决定电光调Q开关性能的关键元件。目前,已实现实用化的电光晶体主要有铌酸锂(LiNbO3, LN)、磷酸二氘钾(KD2PO4, DKDP)和磷酸钛氧铷(RbTiOPO4, RTP)晶体[7]。LN晶体和DKDP晶体作为单轴晶体,用作电光Q开关时多采用沿光轴方向通光的调制方式,采用单块晶体即可实现电光调Q,但这种方式无法利用晶体的最大有效电光系数,导致半波电压较高[8]。若沿非光轴方向通光,则存在自然双折射且受温度影响较大,需要采用两块晶体配对使用以补偿自然双折射及温度效应的影响[9]。RTP晶体是近年来新投入使用的一种电光晶体,其作为双轴晶体,应用时也需要采用两块晶体配对以补偿自然双折射[10]。此外,钽酸锂(LiTaO3, LT)晶体和LN晶体结构类似,也有着优良的电光性能,生长技术成熟,容易生长成大尺寸高光学质量的晶体,并且其损伤阈值远高于LN晶体[11]。但由于LT晶体的电光系数(γ22)(约为0.1 pm·V-1)很小[12],故无法与LN调Q开关一样,采用沿光轴方向通光、沿x轴方向施加电场的调制方式。当沿其他方向通光时,也必须采用两块晶体配对以补偿自然双折射。目前关于LT晶体用作电光调Q器件的报道较少。
综上可见,受晶体生长技术和晶体自身属性的限制,实现实用化的电光晶体较少,双晶体匹配是制备低半波电压电光Q开关的重要手段,但这种方式对晶体的光学质量、加工精度、温控精度等提出了更高的要求。目前关于双晶电光Q开关消光比的影响因素及其容差范围的研究较少。董磊等[13]仅分析了两晶体通光方向偏差及偏振器件透振方向偏差对双晶电光开关消光比的影响。李清连等[14]估算了制备双晶LT电光Q开关时,通光方向晶向角偏差和尺寸偏差应满足的指标要求,但未给出具体的分析过程。以上分析不够系统,限制了双晶电光Q开关的开发及应用。
本文以双晶LT电光Q开关为例,系统研究了影响双晶电光Q开关消光比的各因素,确定了晶体光学质量、加工参数、温控精度等应满足的指标要求。在此基础上,采用光学冷加工工艺,利用同盘抛光方式制备了两种不同尺寸的双晶LT电光Q开关,并对Q开关的双晶匹配质量和消光比进行了检测。
2 理论分析
为了利用最大的有效电光系数,根据LT晶体在不同电场方向下的电光特性[15],采用沿z轴方向施加电场、沿x轴方向通光的调制方式。施加电场后,晶体的电感应主轴仍和原主轴重合。为补偿自然双折射,两块晶体绕x轴相对旋转90°,外加电场极性相反,如
图 1. 双晶LT电光Q开关结构示意图
Fig. 1. Structural diagram of double-crystal LT electro-optic Q-switch
在实际应用中,受晶体光学不均匀性、加工偏差、温度变化等的影响,两块晶体中不同区域的光学路径长度和双折射率都可能存在差异,从而导致由自然双折射引起的相位差无法完全补偿,进而影响调Q效果,下面逐一进行分析。
2.1 光学不均匀性的影响
LT晶体是一种非化学计量比固液同成分共熔化合物,晶体缺陷结构复杂,存在光学不均匀性[16]。通常,采用提拉法生长的晶体沿生长方向的光学均匀性更差。为简化分析,仅考虑沿生长方向的光学不均匀性。本实验所用LT晶体为z向生长,设光学不均匀性为ϕ,取晶体1的晶轴坐标系为统一的物理坐标系,则两晶体中的自然双折射可分别表示为
不加电压时,激光通过两晶体后的总相位差为
式中:
式中:io、i分别为输入光束和输出光束的光强分布。激光光强一般呈准高斯分布,这里仅考虑TEM00模,设输入激光光强分布为
式中:Io为入射总光强;ro为高斯光束的光斑半径。则输出光强I为
令
当在晶体上施加1/4波电压时,激光通过两晶体后的总相位差将附加π/2,代入
因此,双晶电光Q开关的消光比(Ext)为
一般地,电光Q开关的消光比达到100∶1以上时可以满足应用需求。设激光波长为1064 nm,激光光斑半径为2.5 mm,由
可见,当光学均匀性一定时,消光比基本与晶体长度的平方成反比。同样地,当晶体长度一定时,消光比基本与光学不均匀性的平方成反比。由
图 2. 消光比计算结果。(a)消光比随晶体长度的变化;(b)消光比随光学不均匀性的变化
Fig. 2. Calculation results of extinction ratio. (a) Extinction ratio versus crystal length; (b) extinction ratio versus optical inhomogeneity
2.2 晶向偏离的影响
受加工精度的影响,晶轴和晶体端面法线之间总会存在偏差,一方面相位差会受到影响,另一方面晶体中的本征偏振方向也会发生偏离,进而影响消光比。这里讨论两种特殊的情况,即仅有相位差变化和仅有本征偏振方向变化。
2.2.1 仅有相位差变化
当激光在晶轴坐标系xoy面或xoz面内传播时,只有相位差发生变化。设激光传播方向与两晶体的x轴的夹角分别为θ1、θ2,在yoz面的投影与晶体1的y轴的夹角为φ,由折射率椭球理论[17]可以求得激光通过Q开关后的相位差为
式中:
无外加电压与施加1/4波电压时的透过率的比值即为Q开关的消光比。结合式(
2.2.2 仅有本征偏振方向变化
设激光严格沿着x轴方向传播,但两晶体的z轴不严格正交,晶体1的y轴和晶体2的z轴之间存在夹角
图 3. 两晶体z轴不正交时本征偏振方向示意图
Fig. 3. Schematic of eigen polarization directions when z axes of two crystals are not orthogonal
式中:
由此可求得透过率
2.3 温度和晶体尺寸的影响
设两块晶体的长度分别为L1、L2,温度变化分别为ΔT1、ΔT2,LT晶体的双折射率温度系数为κ,沿x轴方向的热膨胀系数为α。则不加电场时,温度变化下的总相位差为
式中:ΔL=L2-L1;ΔT=ΔT2-ΔT1;二阶小项已忽略不计。式(15)中第一项反映了两晶体长度偏差对相位差的影响,第二项反映了晶体长度、两晶体温度变化、长度偏差和温差等因素对相位差的综合影响。当两块晶体的温度变化相同时,若两晶体长度没有偏差,则温度变化不会影响相位差,因此不会影响消光比。但若两块晶体长度存在偏差,则温度变化也会影响消光比。若两块晶体的温度变化不同,则必然会影响消光比,且影响程度和单块晶体长度相关。此外,可以证明,当施加电场时,由电场引起的附加相位差随温度的变化可以忽略不计[21]。下面对各情况进行定量分析。
2.3.1 两晶体温度变化相同但存在长度偏差
当两晶体无温度变化,仅存在长度偏差时,由式(15)和透过率公式可以求出,要使消光比大于100∶1,两晶体的长度偏差必须在5.6 μm以下,消光比基本与长度偏差的平方成反比。在相关应用领域,调Q激光系统往往要经历一个较宽的温度范围[22],工作温度范围通常为-40~65 ℃,当存在长度偏差时,该温度变化将影响消光比。假设两晶体的长度偏差为5 μm,LT晶体的双折射率温度系数κ约为10-5 ℃-1,沿x轴方向的线膨胀系数α约为10-6 ℃-1,则当温度变化为-65 ℃时,消光比由127∶1变为85∶1。要使Q开关的消光比在此温度变化下始终不低于100∶1,则两晶体的长度偏差应小于4.6 μm。
2.3.2 两晶体温度变化不同
设两块晶体尺寸没有偏差,但温度变化不同。当温差为0.1 ℃时,由式(15)和透过率公式可以求得,当单块晶体长度为10 mm时,消光比为286∶1,单块晶体长度为5 mm时,消光比为1147∶1,可见消光比和晶体长度的平方成反比。同样地,可以求得消光比与温差的平方成反比。要使消光比大于100∶1,当晶体长度为10 mm时,温差应小于0.16 ℃,当晶体长度为5 mm时,温差应小于0.33 ℃,即允许的温差范围和晶体长度成反比。
3 实验研究
3.1 双晶LT电光Q开关的制备
基于上述分析,制备了双晶LT电光Q开关。为了使配对用晶体的光学均匀性尽可能一致,两块晶体选自同一晶棒的同一纵向深度且径向对称的位置。综合考虑通光口径、半波电压、消光比等因素,制备了两种规格的调Q开关,单块晶体尺寸分别为9 mm×9 mm×10 mm(y×z×x)和9 mm×9 mm×5 mm(y×z×x),分别标记为LT1和LT2。采用同盘抛光光学加工工艺保证两块匹配晶体的长度偏差、晶向偏离满足应用要求。加工完成后,采用准静态测量仪对z轴正负向进行了标定,并在晶体两z面镀制Ti/Au电极,在两x面镀制1064 nm增透膜。
采用“半固定”弹性支架对两晶体进行装配固定,其由两个夹角为90°的V形槽和一个底座构成。调节V形槽的俯仰角使两晶体的通光方向重合,外加电场方向相互垂直。在两电极面引出导线用以施加电场。由于两晶体的温差容许量较小,为保证两晶体温度变化的一致性,同时减少可能诱发激光损伤的晶体端面污染,在装调好的Q开关外加罩子,装配好的LT调Q开关如
图 4. 装配好的双晶LT电光Q开关。(a)半固定弹性支架装配;(b)外加罩子封装
Fig. 4. Assembled double-crystal LT electro-optic Q-switch. (a) Semi-fixed elastic holder; (b) external cover
3.2 锥光干涉检测
采用锥光干涉技术[23]对晶体的光学质量以及两晶体的匹配程度进行检测。光源采用波长为632.8 nm的氦氖激光,利用毛玻璃使激光变为发散光束,待测晶体置于正交偏振镜间,偏振镜的透振方向与两晶体的y轴或z轴方向成45°角。由于LT晶体的双折射率比较小,测试时尽量使毛玻璃靠近晶体,使发射角足够大,以产生完整的干涉图样。平移晶体,观察通光面内不同区域的锥光干涉图样,从而对单块晶体的光学均匀性以及两块晶体的匹配质量进行判断。
3.3 消光比测试
采用偏光法测量了LT电光Q开关的动态消光比。光源采用波长为1064 nm的Nd∶YAG脉冲激光,重复频率为1 Hz,光斑直径为5 mm,发散角约为6 mrad,输出能量波动小于3%。激光为偏振光,偏振方向沿水平方向。激光依次通过LT电光Q开关、检偏镜后,用激光能量计测量透过能量。在电光Q开关上施加可调直流高压,研究透过能量随外加电压的变化,最大透过能量(Emax)与最小透过能量(Emin)的比值即为开关的动态消光比。改变检偏镜的透振方向,使其分别与入射激光的偏振方向平行和垂直,分别测量平行偏光和正交偏光下的动态消光比。
4 实验结果与分析
4.1 晶体光学质量
图 5. LT晶体通光面不同区域的锥光干涉图样。(a)(b)y轴方向差异最大的干涉图样;(c)(d)z轴方向差异最大的干涉图样
Fig. 5. Conoscopic interference patterns in different regions of light passing surface of LT crystal. (a)(b) Interference patterns with maximum difference along y-axis; (c)(d) interference patterns with maximum difference along z-axis
4.2 双晶体匹配质量
图 6. 双晶LT电光Q开关通光面不同区域差异最大的锥光干涉图样。(a)(b)LT1;(c)(d)LT2
Fig. 6. Conoscopic interference patterns with maximum difference in different regions of light passing surface of double-crystal LT electro-optic Q-switch. (a)(b) LT1; (c)(d) LT2
4.3 消光比
首先对系统的消光比进行了测量。光路中先不插入LT调Q开关,调节使检偏镜的透振方向(P2)分别与激光偏振方向(P1)平行和垂直,平行时透过能量Emax与垂直时透过能量Emin的比值即为系统消光比。
表 1. 系统消光比和LT电光Q开关的动态消光比
Table 1. System extinction ratio and dynamic extinction ratios of LT electro-optic Q-switches
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5 结论
以双晶LT电光Q开关为例,系统分析计算了光学不均匀性、晶向偏离、晶体尺寸、温度变化、两晶体长度偏差及温差等因素对双晶电光Q开关消光比的影响,明确了消光比为100∶1时各因素的容差范围。结合晶体生长和光学加工的实际难度,探明了光学不均匀性、两晶体长度偏差和温差是影响消光比的关键因素,当仅考虑单一变量时,消光比与此变量的平方成反比,因此必须严格控制。同时,晶体长度也对消光比有显著影响,当光学不均匀性、晶向偏离、两晶体温差一定时,消光比与单块晶体长度的平方成反比。基于上述研究,为了保证两晶体长度偏差、晶向偏离等满足应用需求,采用同盘抛光光学加工工艺制备了两种不同长度的双晶LT电光Q开关。通过检测晶体的光学质量、Q开关的双晶匹配质量和消光比,证实晶体通光方向的长度越短,两晶体匹配越好,消光比越高。但是,受晶体光学不均匀性的影响,制备的两块LT调Q开关的消光比均不太高。后续通过提高晶体的光学均匀性,有望获得高消光比双晶LT电光Q开关。研究结果对高消光比双晶电光Q开关的研制具有重要指导作用。
[1] 孙小燕, 梁昶, 张伟, 等. 超短脉冲激光加工在微电/光互连领域的应用研究进展[J]. 中国激光, 2022, 49(10): 1002502.
[2] Zhang X W, Li H S, Guo Q M, et al. Modeling of laser optical detection system for detecting target’s sensitivity and analysis of the impact of optical heterodyne detection performance[J]. Optik, 2021, 243: 167411.
[4] 申玉, 宗楠, 温雅, 等. 新颖的中红外6.45 μm激光医疗应用及光源研究进展(特邀)[J]. 光电技术应用, 2022, 37(1): 10-18.
[5] 吕世威, 朱占达, 刘大鹏, 等. 高效率LD角侧泵浦Nd∶YAG电光调Q激光器[J]. 中国激光, 2022, 49(17): 1701004.
[7] 郑大怀, 吴婧, 商继芳, 等. 电光调Q晶体研究进展[J]. 中国科学: 技术科学, 2017, 47(11): 1139-1148.
Zheng D H, Wu J, Shang J F, et al. Progress on electro-optic crystals for Q-switches[J]. Scientia Sinica: Technologica, 2017, 47(11): 1139-1148.
[8] Shang J F, Yang J F, Hao H S, et al. Compact low-voltage electro-optic Q-switch made of LiNbO3[J]. Optics Express, 2020, 28(15): 22287-22296.
[9] 吴婧, 李清连, 张中正, 等. 基于双晶匹配的低压铌酸锂电光削波器的研究[J]. 中国激光, 2022, 49(7): 0708001.
[10] Yu Y J, Chen X Y, Wang C, et al. High repetition rate 880 nm diode-directly-pumped electro-optic Q-switched Nd:GdVO4 laser with a double-crystal RTP electro-optic modulator[J]. Optics Communications, 2013, 304: 39-42.
[11] Roth M, Tseitlin M, Angert N. Oxide crystals for electro-optic Q-switching of lasers[J]. Glass Physics and Chemistry, 2005, 31(1): 86-95.
[12] Salvestrini J P, Abarkan M, Fontana M D. Comparative study of nonlinear optical crystals for electro-optic Q-switching of laser resonators[J]. Optical Materials, 2004, 26(4): 449-458.
[13] 董磊, 卓壮, 赵圣之. 影响双块晶体电光开关消光比因素的分析[J]. 激光技术, 2008, 32(1): 15-17.
[14] 李清连, 商继芳, 吴婧, 等. 钽酸锂电光调Q晶体开关的制备研究[J]. 人工晶体学报, 2019, 48(5): 812-816.
Li Q L, Shang J F, Wu J, et al. Fabrication of lithium tantalate crystals electro-optic Q-switch[J]. Journal of Synthetic Crystals, 2019, 48(5): 812-816.
[15] 方启万. 铌酸锂电光调制的研究[J]. 邮电研究, 1979(2): 39-65.
Fang Q W. Study on electro-optic modulation of lithium niobate[J]. Study on Optical Communications, 1979(2): 39-65.
[16] Kostritskii S M, Aillerie M, Bourson P, et al. Raman spectroscopy study of compositional inhomogeneity in lithium tantalate crystals[J]. Applied Physics B, 2009, 95(1): 125-130.
[17] 母国光, 战元龄. 光学[M]. 3版. 北京: 高等教育出版社, 2023.
MuG G, ZhanY L. Optics[M]. 3rd ed. Beijing: Higher Education Press, 2023.
[18] 宋哲, 刘立人, 周煜, 等. 入射光偏振方向对LiNbO3晶体近光轴电光调制的影响[J]. 中国激光, 2005, 32(3): 319-322.
[19] 尼科咯相. 非线性光学晶体: 一份完整的总结[M]. 王继扬, 译. 北京: 高等教育出版社, 2009.
NikogosyanD N. Nonlinear optical crystals: a complete survey[M]. WangJ Y, Transl. Beijing: Higher Education Press, 2009.
[20] 廖延彪. 偏振光学[M]. 北京: 科学出版社, 2003.
LiaoY B. Polarization optics[M]. Beijing: Science Press, 2003.
[21] Shang J F, Sun J, Li Q L, et al. Single-block pulse-on electro-optic Q-switch made of LiNbO3[J]. Scientific Reports, 2017, 7: 4651.
[22] Jundt D H. Temperature-stable lithium niobate electro-optic Q-switch for improved cold performance[J]. Proceedings of SPIE, 2014, 9251: 92510F.
[23] 商继芳, 孙军, 张勇军, 等. 一种锥光干涉和近光轴调制结合测量晶体电光系数的方法[J]. 人工晶体学报, 2015, 44(11): 2925-2930.
Shang J F, Sun J, Zhang Y J, et al. A method to measure electro-optic coefficients of crystals by combining conoscopic interference and near optical axis electro-optic modulation[J]. Journal of Synthetic Crystals, 2015, 44(11): 2925-2930.
[24] 尚辉. 激光器DKDP开关晶体电光性能的研究报告[D]. 济南: 山东大学, 2008.
ShangH. Research report on electro-optical properties of laser DKDP switching crystal[D]. Jinan: Shandong University, 2008.
Article Outline
商继芳, 李清连, 孙兴, 陈铃, 杜文静, 李留帮. 双晶电光Q开关消光比的研究[J]. 中国激光, 2024, 51(8): 0801002. Jifang Shang, Qinglian Li, Xing Sun, Ling Chen, Wenjing Du, Liubang Li. Research on Extinction Ratio of Double-Crystal Electro-Optic Q-switch[J]. Chinese Journal of Lasers, 2024, 51(8): 0801002.