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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
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. -
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 -
[1] Tang J, Liao J H, Meng H Y, et al. Ultraviolet laser and its application in laser processing [J]. Laser & Optoelectronics Progress, 2007, 44(8): 52-56. [2] Guillong M, Horn I, Gunther D. A comparison of 266 nm, 213 nm and 193 nm produced from a single solid state Nd: YAG laser for laser ablation ICP-MS [J]. Journal of Analytical Atomic Spectrometry, 2003, 18(10): 1224-1230. doi: 10.1039/B305434A [3] Neev J, Tadir Y, Ho P D, et al. Laser zona dissection using short-pulse ultraviolet lasers[C]//Proceedings of SPIE-The International Society for Optical Engineering, 1992, 1650: 61-69. [4] Angelov D, Beylot B, Spassky A. Origin of the heterogeneous distribution of the yield of guanyl radical in UV laser photolyzed DNA [J]. Biophysical Journal, 2005, 88(4): 2766-2778. doi: 10.1529/biophysj.104.049015 [5] Zhao S Y, Xiao L, Wang X, et al. Study on a practical 266 nm ultraviolet laser [J]. Laser & Infrared, 2012, 42(8): 883-886. (in Chinese) [6] Li S, Li P X, Yang M, et al. The 266-nm ultraviolet-beam generation of all-fiberized super-large-mode-area narrow-linewidth nanosecond amplifier with tunable pulse width and repetition rate [J]. Chinese Physics B, 2022, 31(3): 034207. doi: 10.1088/1674-1056/ac192b [7] Wang N, Zhang J, Yu H, et al. Sum-frequency generation of 133 mJ, 270 ps laser pulses at 266 nm in LBO crystals [J]. Optics Express, 2022, 30(4): 5700-5708. doi: 10.1364/OE.451262 [8] Li Q, Ruckstuhl T, Seeger S. Deep-UV laser-based fluorescence lifetime imaging microscopy of single molecules [J]. The Journal of Physical Chemistry B, 2004, 108(24): 8324-8329. doi: 10.1021/jp0375160 [9] Su P. Design and tolerance analysis of the zoom system in 365 nm UV lithography illumination system [J]. Infrared and Laser Engineering, 2022, 51(7): 20210524. (in Chinese) doi: 10.3788/IRLA20210524 [10] Ge Q, Yu L, Jia X J, et al. Extracativy frequency doubled red laser with single frequency [J]. Chinese Journal of Lasers, 2009, 36(7): 1744-1748. (in Chinese) doi: 10.3788/CJL20093607.1744 [11] Li H, Feng J X, Wan Z J, et al. Low noise continuous-wave single frequency 780 nm laser high-efficiently generated by extra-cavity-enhanced frequency doubling [J]. Chinese Journal of Lasers, 2014, 41(5): 0502003. (in Chinese) doi: 10.3788/CJL204741.0502003 [12] Ge Y, Guo S, Han Y, et al. Realization of 1.5 W 780 nm single-frequency laser by using cavity-enhanced frequency doubling of an EDFA boosted 1560 nm diode laser [J]. Optics Communications, 2015, 334: 74-78. doi: 10.1016/j.optcom.2014.08.013 [13] Xu X F, Lu Y H, Zhang L, et al. Technical study of 8.7 W continuous wave single frequency green laser based on extra-cavity frequency doubling [J]. Chinese Journal of Lasers, 2016, 43(11): 1101010. (in Chinese) [14] Wei J, Cao X, Jin P, et al. Diving angle optimization of BRF in a single-frequency continuous-wave wideband tunable titanium: sapphire laser [J]. Optics Express, 2021, 29(5): 6714-6725. doi: 10.1364/OE.419580 [15] Beskrovnyi V N, Chirkin A S. Squeezed state of light at doubled frequency in an external ring cavity [J]. Quantum Electronics, 1995, 25(12): 1194. doi: 10.1070/QE1995v025n12ABEH000564 [16] Emery Y, Fleischhauer A, Walther T, et al. Angle-tuned type II external-cavity frequency doubling without temperature stabilization [J]. Applied Optics, 1999, 38(6): 972-975. doi: 10.1364/AO.38.000972 [17] Bhawalkar J D, Mao Y, Po H, et al. High-power 390-nm laser source based on efficient frequency doubling of a tapered diode laser in an external resonant cavity [J]. Optics Letters, 1999, 24(12): 823-825. doi: 10.1364/OL.24.000823 [18] Sun X G, Switzer G W, Carlsten J L. Blue light generation in an external ring cavity using both cavity and grating feedback [J]. Applied Physics Letters, 2000, 76(8): 955-957. doi: 10.1063/1.125903 [19] Görtler A, Strowitzki C. Excimer lasers–The powerful light source in the UV and VUV [J]. Laser Technik Journal, 2005, 2(2): 46-50. doi: 10.1002/latj.200790037 [20] Li Z H, Li Y, Luo N N, et al. Research progress of deep-ultraviolet nonlinear optical crystals [J]. Journal of Shandong Normal University (Natural Science), 2021, 36(3): 234-252. (in Chinese) [21] Liu Q, Yan X P, Chen H L, et al. New progress in high-power all-solid-state ultraviolet laser [J]. Chinese Journal of Lasers, 2010, 37(9): 2289-2298. (in Chinese) doi: 10.3788/CJL20103709.2289 [22] Devi K, Parsa S, Ebrahim-Zadeh M. Birefringent-multicrystal, single-pass, continuous-wave second-harmonic-generation in deep-ultraviolet[C]//Nonlinear Optics and its Applications IV. International Society for Optics and Photonics, 2016, 9894: 98940R. [23] Bhandari R, Taira T, Miyamoto A, et al. > 3 MW peak power at 266 nm using Nd: YAG/Cr4+: YAG microchip laser and fluxless-BBO [J]. Optical Materials Express, 2012, 2(7): 907-913. doi: 10.1364/OME.2.000907 [24] Kumar S C, Casals J C, Wei J, et al. High-power, high-repetition-rate performance characteristics of β-BaB2O4 for single-pass picosecond ultraviolet generation at 266 nm [J]. Optics Express, 2015, 23(21): 28091-28103. doi: 10.1364/OE.23.028091 [25] Rao A S, Chaitanya N A, Samanta G K. High-power, high repetition-rate, ultrafast fibre laser based source of DUV radiation at 266 nm [J]. OSA Continuum, 2019, 2(1): 99-106. doi: 10.1364/OSAC.2.000099 [26] Kojima T, Konno S, Fujikawa S, et al. 20-W ultraviolet-beam generation by fourth-harmonic generation of an all-solid-state laser [J]. Optics Letters, 2000, 25(1): 58-60. doi: 10.1364/OL.25.000058 [27] Wang G, Geng A, Bo Y, et al. 28.4 W 266 nm ultraviolet-beam generation by fourth-harmonic generation of an all-solid-state laser [J]. Optics Communications, 2006, 259(2): 820-822. doi: 10.1016/j.optcom.2005.09.061 [28] Kohno K, Orii Y, Sawada H, et al. High-power DUV picosecond pulse laser with a gain-switched-LD-seeded MOPA and large CLBO crystal [J]. Optics Letters, 2020, 45(8): 2351-2354. doi: 10.1364/OL.389017 [29] Orii Y, Kohno K, Tanaka H, et al. Stable 10, 000-hour operation of 20-W deep ultraviolet laser generation at 266 nm [J]. Optics Express, 2022, 30(7): 11797-11808. doi: 10.1364/OE.454643 [30] Wang L R, Wang G L, Zhang X, et al. Generation of ultraviolet radiation at 266 nm with RbBe2BO3F2 crystal [J]. Chinese Physics Letters, 2012, 29(6): 064203. doi: 10.1088/0256-307X/29/6/064203 [31] Liu L, Zhou H, He X, et al. Hydrothermal growth and optical properties of RbBe2BO3F2 crystals [J]. Journal of Crystal Growth, 2012, 348(1): 60-64. doi: 10.1016/j.jcrysgro.2012.03.025 [32] Wang L, Zhai N, Liu L, et al. High-average-power 266 nm generation with a KB2BO3F2 prism-coupled device [J]. Optics Express, 2014, 22(22): 27086-27093. doi: 10.1364/OE.22.027086 [33] Liu C, Liu L, Zhang X, et al. Crystal growth and optical properties of non-UV absorption K2Al2B2O7 crystals [J]. Journal of Crystal Growth, 2011, 318(1): 618-620. doi: 10.1016/j.jcrysgro.2010.11.175 [34] Wang Y, Wang L, Gao X, et al. Growth, characterization and the fourth harmonic generation at 266 nm of K2Al2B2O7 crystals without UV absorptions and Na impurity [J]. Journal of Crystal Growth, 2012, 348(1): 1-4. doi: 10.1016/j.jcrysgro.2012.03.038 [35] Liu Q, Yan X, Gong M, et al. High-power 266 nm ultraviolet generation in yttrium aluminum borate [J]. Optics Letters, 2011, 36(14): 2653-2655. doi: 10.1364/OL.36.002653 [36] Ilas S, Loiseau P, Aka G, et al. 240 kW peak power at 266 nm in nonlinear YAl3(BO3)4 single crystal [J]. Optics Express, 2014, 22(24): 30325-30332. doi: 10.1364/OE.22.030325 [37] Zheng L, Ren J, Loiseau P, et al. >1 MW peak power at 266 nm in nonlinear YAl3(BO3)4 (YAB) single crystal[C]//2015 Conference on Lasers and Electro-Optics (CLEO), IEEE, 2015: 1-2. [38] Huang H, Yao J, Lin Z, et al. NaSr3Be3B3O9F4: A promising deep-ultraviolet nonlinear optical material resulting from the cooperative alignment of the [Be3B3O12F]10- anionic group [J]. Angewandte Chemie, 2011, 123(39): 9307-9310. doi: 10.1002/ange.201103960 [39] Fang Z, Hou Z, Yang F, et al. High-efficiency UV generation at 266 nm in a new nonlinear optical crystal NaSr3Be3B3O9F4 [J]. Optics Express, 2017, 25(22): 26500-26507. doi: 10.1364/OE.25.026500 [40] Chen X D, Liu L, Wang L, et al. Fourth-harmonic-generation of 266-nm ultraviolet nanosecond laser with NaSr3Be3B3O9F4 crystal [J]. Optical Engineering, 2020, 59(11): 116110. [41] Devi K, Parsa S, Ebrahim-Zadeh M. Continuous-wave, single-pass, single-frequency second-harmonic-generation at 266 nm based on birefringent-multicrystal scheme [J]. Optics Express, 2016, 24(8): 8763-8775. doi: 10.1364/OE.24.008763 [42] Hansch T W, Couillaud B. Laser frequency stabilization by polarization spectroscopy of a reflecting reference cavity [J]. Optics Communications, 1980, 35(3): 441-444. doi: 10.1016/0030-4018(80)90069-3 [43] Bell A S, Malcolm G P A, Maker G T. High-power continuous-wave UV generation[C]//Solid State Lasers VIII. International Society for Optics and Photonics, 1999, 3613: 151-154. [44] Zanger E, Müller R, Liu B, et al. Diode-pumped cw all solid-state laser at 266 nm[C]//Advanced Solid State Lasers, Optical Society of America, 1999: MB4. [45] Sakuma J, Asakawa Y, Obara M. Generation of 5-W deep-UV continuous-wave radiation at 266 nm by an external cavity with a CsLiB6O10 crystal [J]. Optics Letters, 2004, 29(1): 92-94. doi: 10.1364/OL.29.000092 [46] Chen G Z, Shen Y, Liu Q, et al. Generation of 266 nm continuous-wave with elliptical Gaussian beams [J]. Acta Physica Sinica, 2014, 63(5): 171-175. (in Chinese) [47] Zhao B, Qin W X, Li F Q, et al. All-solid-state CW single-frequency deep UV 266 nm laser [J]. Journal of Quantum Optics, 2020, 26(2): 194-201. (in Chinese) [48] Drever R W P. Laser interferometer gravitational radiation detectors[C]//AIP Conference Proceedings. American Institute of Physics, 1983, 96(1): 336-346. [49] Peng Y, Zhao Y, Li Y, et al. Three methods to lock the second harmonic generation for 461 nm [J]. Chinese Journal of Lasers, 2010, 37(2): 345-350. (in Chinese) doi: 10.3788/CJL20103702.0345 [50] 李超. 边带调制PDH激光稳频技术的研究[D]. 杭州: 中国计量大学, 2017. Li C. Research of sideband modulation PDH laser frequency stabilization technology[D]. Hangzhou: China Jiliang University, 2017. (in Chinese) [51] Liu L Y, Oka M, Wiechmann W, et al. Longitudinally diode-pumped continuous-wave 3.5-W green laser [J]. Optics Letters, 1994, 19(3): 189-191. doi: 10.1364/OL.19.000189 [52] Oka M, Liu L Y, Wiechmann W, et al. All solid-state continuous-wave frequency-quadrupled Nd: YAG laser [J]. IEEE Journal of Selected Topics in Quantum Electronics, 1995, 1(3): 859-866. doi: 10.1109/2944.473671 [53] Oka M, Takeda M, Kashiwagi T, et al. An all-solid-state continuous-wave 266 nm laser for optical disk mastering[C]//Optical Data Storage, Optica Publishing Group, 1998: TuA. 2. [54] Eguchi N, Oka M, Imai Y, et al. New deep-UV microscope[C]//Optical Engineering for Sensing and Nanotechnology (ICOSN'99), SPIE, 1999, 3740: 394-397. [55] Südmeyer T, Imai Y, Masuda H, et al. Efficient 2nd and 4th harmonic generation of a single-frequency, continuous-wave fiber amplifier [J]. Optics Express, 2008, 16(3): 1546-1551. doi: 10.1364/OE.16.001546