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与传统的红外激光和可见光波段激光相比，266 nm连续波单频紫外激光具有波长短、能量分布集中、加工分辨率高等优点[1]，而且紫外激光在某些特殊材料上具有更高的吸收系数，使其在科学研究和医疗等领域具有广泛的应用前景[2-4]。近年来，高功率连续波单频紫外激光器在光学数据储存、光谱分析、光通讯、大气污染检测、高分辨率光谱检测、光印刷光刻等应用领域不断拓展，引起了人们的广泛关注[5-9]。
目前获得266 nm连续波单频紫外激光器的方法主要是通过基于非线性光学频率变换的外腔倍频获得。其中，基于连续波单频1064 nm全固态激光的外腔共振增强倍频技术产生四次谐波成为低功率条件下获得连续波266 nm单频激光输出的重要方法[10-14]。外腔共振增强倍频技术与腔内倍频和腔外单通倍频不同，是指在基频激光外部设计独立的环形共振倍频腔，通过设计倍频腔合适的腔参数，利用电学反馈系统精确控制倍频腔长，使得倍频腔光学长度为基频光波长的整数倍，从而使注入腔内的基频激光功率密度通过共振得到极大地增强，并且能多次通过倍频晶体从而增加倍频次数，提高倍频转换效率[15-18]。与成本高、结构庞大及长期稳定性差的准分子、离子激光器相比，这种全固态紫外激光器体积小、结构紧凑，具有稳定性高、光束质量好、线宽窄、可靠性高等优点，使之更加广泛地应用于科研、产品制造和工业加工等领域中[19]。
文中综合比较了用于产生高功率266 nm紫外激光的非线性光学晶体性能，结合笔者课题组研究成果全面综述了基于H-C频率锁定和PDH频率锁定共振腔技术的266 nm单频连续波激光研究进展情况，并对高性能266 nm单频连续波紫外激光器发展亟需解决的问题及可能的解决途径进行了分析和展望。
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在全固态266 nm紫外激光器中，非线性光学晶体是构成激光器的核心要素之一[20]。在紫外激光器设计中，非线性光学晶体的光学性能和质量直接影响了紫外激光的输出功率和光束质量[21]。非线性光学晶体的发展是倍频激光技术进步的基础，因而选择合适的、物化性能优良的非线性光学晶体对获得高功率、高稳定性连续波266 nm激光至关重要。
在传统的倍频晶体中，如KTiOPO4 (KTP)、LiB3O5 (LBO)等晶体具有物理化学性能稳定、抗潮解能力较强、光学透明度范围大、有效非线性转换系数大等优点，并且易于高质量、大尺寸生长，被广泛应用在高功率、大能量532 nm和355 nm激光器中。然而，KTP晶体的紫外透过截止波长为350 nm，无法作为四倍频晶体用于产生266 nm紫外激光。LBO晶体虽然紫外透光波长低于200 nm，但其双折射率小，无法满足产生266 nm倍频的相位匹配条件[22]。以β-BaB2O4 (BBO)[23-25]为代表的硼酸盐晶体的发现极大促进了高功率紫外激光器的发展。硼酸盐晶体具有光学波段宽、非线性光学效应优异、物化性能稳定、激光损伤阈值高等优势，且易于大尺寸晶体生长及加工，这些优点使其作为优秀的倍频晶体被广泛应用于高功率266 nm深紫外激光器中。随着非线性光学晶体材料的研究和制备工艺提升，诸如CsLiB6O10 (CLBO)[26-29]、RbBe2BO3F2(RBBF)[30-31]、 KBe2BO3F2 (KBBF)[32]、K2Al2B2O7 (KABO)[33-34]等晶体在实现掺钕1064 nm激光器的三倍频、四倍频及五倍频过程都发挥着各自独有的优势，加快了紫外及深紫外激光器的发展。同时，一些诸如YAl3(BO3)4 (YAB)[35-37]和 NaSr3Be3B3O9F4 (NSBBF)[38-40]等新型晶体同样也展现了不俗的非线性转换能力，并逐渐成为深紫外激光领域的研究热点，但因这些晶体存在有效非线性系数低和激光损伤阈值小等因素，未在高功率紫外激光器领域中得到广泛应用[21]。
BBO晶体是中国科学院福建物质结构研究所发明的一种性能优异的非线性频率转换晶体。BBO晶体的透光波长范围为190~3500 nm，具有稳定的物理化学性质、低潮解性、光学均匀性高、激光损伤阈值高等独特优势，并且具有很高的有效非线性频率转换系数，非常适合作为激光四倍频或五倍频的非线性光学晶体。同时，BBO晶体易于生长加工，使其在工业中的应用更为广泛。此外，BBO晶体还具有低色散、大双折射率、宽相位匹配范围及高光学质量，因此在光参量放大(OPA)和光参量振荡(OPO)等领域的应用也比较成熟。
CLBO晶体透光范围为180~2750 nm，由于宽的透光范围，该晶体常用于产生四次谐波和五次谐波，是一种性能优良的紫外非线性光学晶体。相比于其他的硼酸盐晶体，CLBO晶体更容易生长得到大尺寸和高光学质量晶体。但是CLBO 晶体极易潮解的性质使其难以在空气中维持长时间工作，只能够在密闭的环境中或者在150 ℃的温度下保存，限制了其在高功率全固态紫外激光器中的发展。
KBBF晶体在紫外区域的透光截止波长为155 nm，在红外区域的截止波长为3.66 μm，是目前紫外透光截止波长最短的非线性光学晶体，可实现六倍频177.3 nm深紫外激光输出，使其在深紫外激光领域具有重要的发展潜力。但该晶体的莫氏硬度为2.66，坚固的层状结构特导致晶体生长困难，且晶体具有严重的解离性，加工难度大，使其在激光器中的应用受到了极大限制。
RBBF晶体在紫外区域透光截止波长为160 nm，在红外区域截止波长为3.55 μm，且有较大的双折射相位匹配空间，适用于四次、五次谐波产生。其莫氏硬度与KBBF晶体相同，但RBBF 晶体机械性能较差，容易断裂，使其应用也受到限制。
KABO晶体的透光范围为180~3600 nm，晶体具有稳定的物理化学性质，潮解性低，能够实现四次、五次谐波产生。但是KABO晶体在200~300 nm波段存在严重的吸收效应，限制了其在高功率紫外激光器上的应用。
表1主要列举了几种常见的四倍频非线性光学晶体的光学性能。
表 1 常见紫外非线性光学晶体性能
Table 1. Properties of common nonlinear optical crystals
Crystal Space group Transmission range/nm Birefringence Δn@1064 nm Nonlinear coefficient dij/pm·V−1 Shortest PM λ/nm BBO R3C 190-3 500 0.12 d22=1.6
d31=0.96205 CLBO I-42d 180-2 750 0.05 d36=0.95 238 KBBF R32 155-3 660 0.080 d11=0.49 161 RBBF R32 160-3 550 0.075 d11=0.45 170 KABO P321 180-3 600 0.068 d11=0.48 225 -
目前，产生单频连续波266 nm全固态激光的方法主要有两种。一种是采用高光束质量的单频532 nm连续波激光直接腔外倍频获得连续波266 nm单频激光。由于倍频晶体的有效非线性系数较小，因此对泵浦光功率密度要求苛刻，如果利用泵浦光单次通过倍频晶体，倍频效率低下。2016年，西班牙ICFO科学研究院Kavita Devi等[41]采用单通级联多晶体方案，利用单频532 nm连续波激光器作为基频光源，在基频光功率为9.2 W时，获得37.7 mW的连续波266 nm单频激光输出，倍频效率仅为0.41%，功率稳定性为0.12%，实验装置和功率特性如图1所示。由于该方案对基频光光束质量和倍频晶体光学质量具有较为苛刻的要求，因此不适用于工业用高功率单频连续波266 nm激光器需求。第二种方法则是采用共振增强腔的外腔倍频技术，通过搭建外腔共振增强环形腔，利用电学反馈系统控制压电陶瓷，满足倍频腔长度为泵浦光波长的整数倍来获得高的功率密度，并且基频光能多次通过倍频晶体，倍频转换效率得到大大提高，从而可以获得高功率、输出稳定的单频连续波266 nm激光。
对共振增强外腔倍频技术，由于激光器系统共振腔长受到温度、湿度以及机械振动等外部环境干扰会发生改变，从而破坏倍频腔的共振条件，因此采用电学反馈控制系统实现对腔长进行精确、稳定和实时的控制非常关键。腔长电学反馈控制系统是将光学、电学与机械学紧密结合的一项综合技术，通常分为光电转换与探测、电学反馈控制和电机驱动部分，而电学反馈控制是其核心部分。为了将激光频率锁定在F-P参考腔中心频率，目前主要采用H-C频率锁定、PDH频率锁定和边模偏频锁定等稳频方法。下文对H-C法和PDH法两种稳频方法获得稳定单频连续波266 nm紫外激光进行系统介绍和总结。
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H-C频率锁定是由美国斯坦福大学的Hänsch和Couillaud[42]于1980年首次提出的一项通过测量腔对光偏振态的变化来获得误差信号的激光稳频技术。在倍频腔内放置一块倍频晶体作为光学偏振器件，由于输入腔内激光平行于偏振器件起偏方向的分量在经过倍频晶体后的损耗小，而垂直于偏振器件起偏方向的分量损耗极大，所以平行分量的线偏振光会在腔内实现共振，而垂直分量的线偏振光则全部反射到腔外。由于反射光的偏振态会随着腔内振荡光与腔模的失配而发生改变，使平行偏振分量的相位变化与腔模线型色散呈正比关系，因此通过探测环形腔入射镜的反射光信号，就可以获得稳频的误差信号。H-C频率锁定技术所需搭建的光路简单，也无需增加额外的相位调制，通过对PID控制系统各种参数的优化，能够获得信噪比高的误差信号。该系统已被广泛应用于外腔共振增强腔的非线性频率转换过程。
1999年，英国Microlase光学系统有限公司Angus S. Bell等人[43]通过搭建四镜环形共振腔获得单频连续波266 nm紫外激光输出。该激光系统以高功率单频532 nm激光器作为泵浦源，利用沿布儒斯特角切割的BBO晶体作为倍频晶体，以H-C锁频得到误差信号反馈驱动PZT位移调整激光腔长，满足腔内532 nm基频光共振匹配条件。在基频光功率为5 W时，获得了1.5 W单频连续波266 nm激光输出，光束质量优于1.2。同年，德国LAS公司Eckhard Zanger等[44]采用由两个反射镜和一个棱镜组成的Δ型DeltaConcept外腔作为倍频共振腔，非线性光学晶体利用布儒斯特角切割BBO晶体，在5 W泵浦功率时获得了1.5 W单频连续波266 nm紫外激光高效稳定输出，激光功率不稳定度小于2%。Δ型DeltaConcept外腔设计如图2所示，当棱镜沿其对称轴移动时，谐振腔内的光束路径不变。因此，DeltaConcept是一种几何不变的腔体设计。光路长度的变化对谐振腔起到重要的调谐作用，当棱镜向其对称轴方向移动时，光程长度会发生一定的变化，实现腔内基频光的共振增强。
2004年，日本Cyber激光公司Jun Sakuma等[45]使用布儒斯特角切割CLBO晶体作为四倍频晶体，通过对环形共振腔频率锁定获得5.0 W的单频连续波266 nm紫外激光。随后将获得的266 nm激光与1064 nm激光进行和频，获得了106 mW的单频连续波213 nm激光，该激光器成为第一台基于掺钕激光器的五次谐波单频连续波激光器。2014年，国防科学技术大学陈国柱[46]等理论分析了束腰尺寸对倍频晶体转换效率的影响，并进行了实验验证，实现了超过180 mW的单频连续波紫外激光输出，倍频效率达到18%，但当继续升高基频光功率时，由于光折变效应与热透镜效应的双重影响，导致倍频光功率呈现下降趋势，并造成倍频晶体损坏。2020年，山西大学赵彪等[47]使用自主研发的连续单频波532 nm激光器作为基频源，倍频晶体采用I类相位匹配BBO晶体，在基频光功率3 W时，实现了810 mW的高光束质量、高稳定性的连续波单频紫外266 nm激光输出，激光器3 h内功率稳定性优于1.5%，激光光束质量优于1.5，实验装置如图3所示。
笔者课题组持续开展了一系列单频连续波266 nm紫外激光器实验研究。最近，利用532 nm单频连续波激光器作为基频光源，采用5 mm长I类相位匹配BBO晶体作为倍频晶体，基于H-C锁频技术实现了单频连续波266 nm紫外激光稳定输出，激光器结构示意图如图4所示。当532 nm基频光功率为4.9 W时，获得了最高功率1.1 W的单频连续波266 nm紫外激光输出，非线性光光转换效率为22.5%，激光输出功率不稳定性RMS<1.5%，光束质量M2<1.7。
图 4 (a)实验装置图；(b) 266 nm激光输出功率；(c)功率稳定性曲线
Figure 4. (a) Experimental setup; (b) Output power of 266 nm laser; (c) Power stability curve
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PDH频率锁定技术[48]是对注入的基频激光进行相位调制，通过将腔反射信号与相位调制后的调制信号进行混频来获得高信噪比的误差信号，过程中需要对激光频率进行高频调制解调，图5(a)为PDH锁频原理示意图[49]。PDH技术首先利用电光相位调制器（EOM）对激光进行射频电光相位调制，在激光频率两侧各产生一个幅值相等、相位相反的边频带，如图4(b)所示。利用F-P腔标准频率作为参考，通过混频器（DBM）对混频信号进行解调从而获得误差信号，如图5(c)所示。将解调获得的误差信号利用PID模块处理，通过反馈伺服系统反馈到环形腔PZT上来精确控制环形倍频腔的腔长，从而将环形倍频腔的共振频率锁定在激光器的中心频率上。PDH锁频所需要的光路复杂，需要对单频激光源进行相位调制从而获得调制信号。但由于PDH调制频率通常在MHz以上，能够过滤大部分低频噪声，从而可以获得高信噪比的误差信号，使激光器更容易锁定在中心频率上，激光器输出的长期稳定性更好。
1994年，日本索尼公司Liu等[51]以单频连续波532 nm激光器作为基频光源，在泵浦激光功率为3.5 W时，获得800 mW的单频连续波266 nm激光输出，其中倍频晶体为BBO晶体，紫外激光器采用PDH法进行锁频，激光输出功率不稳定性为5%。次年，日本索尼公司Michio Oka等[52]利用一种新型音圈电机(VCM)作为共振腔位移调制驱动器，在2.9 W的基频光功率驱动下，获得了1.5 W单频连续波266 nm紫外激光输出，倍频效率获得明显提升，但由于晶体镀膜表面退化等原因，激光器长期稳定性受到影响，运行100 h后的紫外激光功率下降30%。1998年，该课题组Michio Oka等[53]通过优化BBO晶体生长条件，改善表面抛光和晶体镀膜工艺，同时通过改进四镜环形腔参数优化了腔内激光功率密度，最终实现了连续波单频266 nm激光器长期稳定运行。该激光器在500 mW绿光泵浦下获得了200 mW高稳定性单频连续波266 nm激光输出。1999年，日本索尼公司Naoya Eguchi 等[54]通过PDH锁频技术实现了超5000 h的高稳定运转单频连续波266 nm紫外激光器，并利用该激光器研制出了分辨率小于0.1 μm的高分辨率深紫外显微镜，成功实现脱离真空室的测量。2008年，日本索尼公司Thomas Südmeyer等[55]将连续波单频1064 nm激光器作为基频光源，通过搭建双环形共振腔，以LBO晶体作为二倍频晶体获得了单频连续波532 nm的二次谐波输出，利用BBO晶体作为四倍频晶体最终实现了12.2 W的单频连续波266 nm激光输出，实验装置如图6所示。该激光器运用PDH法和VCM驱动器对双环形共振腔进行锁定，获得了良好的激光输出稳定性，激光倍频效率高达83.0%，四倍频转化效率为50.6%。
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文中总结了用于产生高功率266 nm紫外激光的非线性光学晶体性能，详细分析了H-C频率锁定和PDH频率锁定方法的优缺点，并结合本课题组研究成果对全固态单频连续波266 nm紫外激光器研究进展情况进行了汇总。从目前发展情况来看，全固态单频连续波266 nm激光器主要采用共振增强腔的外腔倍频技术获得，而且已经实现一定程度的产品化，但是如何进一步提升功率等性能还存在一些问题亟待解决。下文，针对存在的问题和解决路径进行简要分析。
（1）倍频晶体的激光损伤阈值及使用寿命问题。随着深紫外激光技术的不断发展，激光器输出功率也不断提高，倍频晶体存在的抗激光损伤能力较差以及晶体表面镀膜容易老化等问题对激光器发展的限制也越来越突出。要解决这个问题，首先可以通过改进非线性光学晶体材料的生长工艺、提高晶体加工和镀膜技术，来优化倍频晶体的质量；其次，采用布儒斯特角切割倍频晶体，既可以实现分光又可以不用镀膜，从而提高晶体的抗激光损伤能力，保障激光器运转的长期稳定性。
（2）深紫外激光光束质量差的问题。由于非线性转换过程中倍频晶体存在严重的走离效应以及非线性光学晶体在生长中存在一定的质量问题，这严重影响激光输出的光束质量，可以通过适当改变晶体结构来补偿走离效应，提高深紫外激光器的光束质量。
（3）深紫外激光器倍频效率低的问题。由于非线性频率转换效率与光功率密度成正比，可以通过提高激光功率密度来提高激光器的倍频转换效率，但这与晶体的抗激光损伤问题产生冲突，需要综合考虑激光功率及晶体处束腰光斑大小，在不引起损伤晶体的同时获得大的倍频转换效率；此外，非线性光学晶体存在的光学走离效应和光折变效应也是影响倍频效率的重要因素。
（4）紫外激光器稳定性问题。深紫外单频激光器的长期稳定性取决于基频激光器功率和模式稳定性，以及伺服电路和压电陶瓷工作稳定性。全固态532 nm激光器在长时间使用过程中容易出现因腔体老化导致跳模进而影响环形倍频腔腔模匹配问题，从而导致紫外激光器功率不稳定和失锁。光纤激光器模式稳定性更好，使得基于光纤激光的深紫外激光器功率运转更为稳定。压电陶瓷因制备工艺及材料本征特性存在电压与位移成非正比关系的性质，加上PZT搭载的负载会对PZT伸缩运动存在影响，使得反馈电压驱动PZT位移实现的腔长改变量存在微小误差，从而影响深紫外激光器稳定性及转换效率。采用高精度音圈电机可以更精确控制倍频谐振腔腔长，通过进一步优化腔模匹配，可以实现更稳定的深紫外激光输出。
Research progress of high-power 266 nm all-solid-state single-frequency CW laser
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摘要: 高功率单频连续波266 nm激光在大容量信息存储、高分辨光谱监测及高精度紫外光刻等领域具有重要应用价值，近年来已成为国内外紫外激光领域的研究热点之一。文中首先综合比较了用于产生高功率266 nm紫外激光的非线性光学晶体基本性能，并根据主要的激光器频率锁定方法，重点分析了Hänsch-Couillaud (H-C)频率锁定和Pound-Drever-Hall (PDH)频率锁定方法的优缺点以及连续波单频266 nm激光器发展现状，介绍了本课题组最新研究成果，即基于H-C频率锁定方法实现了功率1.1 W的单频连续波266 nm紫外激光稳定输出。最后，针对进一步提升全固态单频连续波266 nm激光器性能亟需解决的问题和可能解决路径进行了简要分析和展望。
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关键词:
- 全固态单频连续波激光器 /
- 266 nm /
- 共振增强 /
- 频率锁定
Abstract:Significance High-power continuous wave (CW) single-frequency ultraviolet (UV) lasers have the advantages of narrow linewidth and concentrated energy distribution, and have shown promising applications in scientific research, industrial production and manufacturing, medical diagnosis and treatment, and civil life, including semiconductor lithography, fine material processing, and high-precision spectral analysis. Compared with traditional excimer lasers, ion lasers and free-electron lasers that produce ultraviolet lasers, all-solid-state ultraviolet lasers have more compact structure, lower cost, higher long-term stability and better beam quality. These advantages make people pay more attention to all-solid-state ultraviolet lasers, and all-solid-state continuous-wave single-frequency ultraviolet lasers will continue to develop towards high power and high reliability. Progress The basic properties of nonlinear optical crystals used to produce high-power 266 nm ultraviolet laser are comprehensively compared (Tab.1). In the design and manufacturing of ultraviolet laser, the selection of nonlinear optical crystals and their optical properties and quality will directly affect the output power and beam quality of ultraviolet laser. In order to obtain higher-performance UV laser output, β-BaB2O4 (BBO) crystal, CsLiB6O10 (CLBO) and other borate crystals with wide optical band, excellent nonlinear optical effect, stable physical and chemical properties, and high laser damage threshold have been discovered successively, greatly promoting the development of high-power ultraviolet lasers. At present, the method to obtain 266 nm CW single-frequency ultraviolet laser is mainly obtained by external cavity frequency doubling based on nonlinear optical frequency conversion. Among them, the external cavity resonance enhanced frequency doubling technology based on continuous wave single-frequency 1064 nm all-solid-state laser to generate fourth harmonic has become an important method to obtain continuous wave 266 nm single-frequency laser output under low power conditions. For the resonance enhanced external cavity frequency doubling technology, because the resonance enhanced cavity length of the laser system will change under the interference of external environment such as external temperature, air humidity and mechanical vibration, the resonance state of the frequency doubling cavity will be damaged, resulting in poor laser output stability and even lower laser output power, so it is very important to use the electrical feedback control system to achieve accurate, stable and real-time control of the cavity length. At present, the frequency stabilization methods such as Hänsch-Couillaud (H-C) frequency locking, Pound-Driver-Hall (PDH) frequency locking and side-mode bias frequency locking are used for electrical feedback and control of the laser resonator. According to different laser frequency locking methods, this study mainly summarizes the development status of continuous wave single-frequency 266 nm laser using H-C frequency locking and PDH frequency locking methods at home and abroad. Compared with H-C frequency locking method with simple optical path, PDH frequency locking method is easier to obtain error signals with high signal-to-noise ratio, which is conducive to more stable UV laser output. Through comprehensive investigation, the development trend of all-solid-state ultraviolet laser is prospected at the end of this paper, aiming to provide reference for the development and research of all-solid-state ultraviolet laser technology. And this study introduces the latest research result of our research group, which is the stable output of a 1.1 W single-frequency continuous wave 266 nm ultraviolet laser based on the H-C frequency locking method. Conclusions and Prospects The all-solid-state CW single-frequency ultraviolet laser is developing rapidly with the efforts of researchers. The all-solid-state single-frequency CW 266 nm laser has achieved a certain degree of productization, but there are still some problems to be solved for its development towards high power, mainly focusing on the poor anti-damage ability of frequency doubling crystal and the low frequency doubling efficiency caused by the intrinsic characteristics of crystal, and it is difficult to achieve higher power laser output. This study aims to provide some references for the design and optimization of all-solid-state ultraviolet lasers in the future. In order to meet higher production requirements and fully realize the commercialization of ultraviolet lasers, all-solid-state ultraviolet lasers will eventually develop towards a more stable and higher power direction. -
图 4 (a)实验装置图；(b) 266 nm激光输出功率；(c)功率稳定性曲线
Figure 4. (a) Experimental setup; (b) Output power of 266 nm laser; (c) Power stability curve
表 1 常见紫外非线性光学晶体性能
Table 1. Properties of common nonlinear optical crystals
Crystal Space group Transmission range/nm Birefringence Δn@1064 nm Nonlinear coefficient dij/pm·V−1 Shortest PM λ/nm BBO R3C 190-3 500 0.12 d22=1.6
d31=0.96205 CLBO I-42d 180-2 750 0.05 d36=0.95 238 KBBF R32 155-3 660 0.080 d11=0.49 161 RBBF R32 160-3 550 0.075 d11=0.45 170 KABO P321 180-3 600 0.068 d11=0.48 225 
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