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超连续谱光源是利用一束固定波长的抽运激光与非线性材料相互作用,基于非线性频率变换效应将抽运激光的光谱向短波及长波方向扩展,实现一种连续谱超宽带光输出。其最大的优点是输出光可以连续覆盖极宽的光谱范围,如直接覆盖可见光波段的白光超连续谱光源及覆盖近红外至远红外波段的红外超连续谱光源已成为近年来的研究热点。为实现抽运激光光谱的大范围扩展,一般采用fs、ps等高峰值功率的超短脉冲激光抽运高非线性光纤(特别是光子晶体光纤)或具有大非线性系数的非线性光学晶体,通过有效激发各种非线性效应,如四波混频(four-wave mixing, FWM)、受激喇曼散射(stimulated Raman scattering, SRS)、受激布里渊散射(stimulated Brillouin scattering, SBS)、自相位调制(self-phase modulation, SPM)、交叉相位调制(cross-phase modulation, XPM)、自陡峭(self-steepening)、高阶色散、皮秒脉冲调制不稳定性(modulation instability, MI)、飞秒脉冲孤子分裂(soliton fission, SF)、二次谐波产生(second-harmonic generation, SHG)、三次谐波产生(third-harmonic generation, THG)等,进行激光频率的多种非线性变换,从而实现光谱的大范围扩展。
为获得有效覆盖8μm~14μm波段的超连续谱长波红外光源,首先抽运激光波长应尽量靠近或处于8μm~14μm波段内,当前可利用的抽运激光通常为中红外fs/ps光参量激光、差频激光及CO2皮秒激光等;其次,采用的非线性材料必须对抽运激光及8μm~14μm辐射透明,且同时具备高非线性、高抗激光损伤及可大尺寸制备特性。当前硫、碲系玻璃光纤等由于在中红外区域具有良好的透过性能及高的非线性成为实现中红外超连续谱辐射的首选材料。如2014年, PETERSEN等人基于4.5μm和6.3μm的飞秒差频激光,抽运85mm长的阶跃折射率硫系玻璃光纤分别获得了光谱范围覆盖1.5μm~11.7μm和1.4μm~13.3μm的中红外超连续谱光源,最大平均功率约150μW[24]。2015年, YU等人利用波长4μm,峰值功率3kW,脉宽330fs的激光抽运11cm长阶跃折射率硫属化物光纤获得超连续光源光谱范围为1.8μm~10μm[25]。2017年, ZHAO等人采用波长7μm的飞秒光参量放大激光抽运14cm长Ge-Te-AgI碲化物玻璃光纤,实现了2μm~16μm的超连续激光产生[26]。2018年, CHAUHAN等人报道了基于4μm飞秒激光抽运9mm长Ga-Sb-S硫系玻璃光子晶体光纤,实现了1μm~14μm超连续谱产生[27]。另外,红外非线性晶体也是一种良好的超连续谱产生介质。如2014年PIGEON等人报道了基于10.6μm皮秒CO2激光抽运67mm长的GaAs晶体获得光谱范围为2μm~20μm的超连续谱激光产生[28]。
受制于中红外波段抽运光源及非线性材料的发展,8μm~14μm中红外超连续谱输出功率较低,主要用于光学相干断层成像、宽带光谱学、敏感物的检测等。
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非线性变频中红外固体激光器是基于非线性光学晶体的二阶非线性光学效应,如差频(difference frequency generation, DFG)、光参量振荡(optical parametric oscillation, OPO)、光参量放大(optical parametric amplification, OPA)及光参量产生(optical parametric generation, OPG)等,将现有的固体激光(波长一般短于3μm)进行频率下转换,实现固体激光向中红外波段的扩展。
8μm~14μm非线性变频固体激光的产生依赖于非线性红外晶体、固体激光抽运源及频率下转换技术3个方面。理想的红外晶体应在整个8μm~14μm波段均具有良好的透过性能(包括吸收及散射损耗小、光学均匀性好),可基于性能优良成熟的固体激光(如1μm波段Nd, Yb激光等)抽运,同时具备大的非线性系数、宽的相位匹配范围、高损伤阈值、高热导率、机械强度高、易于加工、物理化学性质稳定及易于大尺寸生长等。当前,可实现8μm~14μm长波红外激光输出的红外晶体材料主要有ZnGeP2(ZGP), AgGaS2(AGS), LiInSe2(LISe), BaGa4S7(BGS), AgGaSe2(AGSe), GaSe, CdSe,BaGa4Se7(BGSe), 定向图案磷化镓(orientation patterned GaP, OP-GaP), OP-GaAs等,其主要性能参量如表 1所示。其中ZGP, AGS, LISe, BGS, OP-GaP均在约8.5μm后存在本征吸收峰导致晶体线性吸收严重,难以应用至高功率/大能量8.5μm~14μm波段,且ZGP需不小于2μm的激光抽运,AGS损伤阈值较低,BGS非线性系数较小。AGSe, GaSe, CdSe, BGSe及OP-GaAs在8μm~14μm波段均具有优异的透光性能,同时具有大的非线性系数,但AGSe, GaSe, CdSe, OP-GaAs由于严重的双光子吸收均不能采用成熟的1μm激光直接抽运,且AGSe损伤阈值较低,GaSe具有严重层状结构导致器件厚度受限。OP-GaP, OP-GaAs为准相位匹配结构,其调谐范围不受双折射相位匹配的限制并可利用最大的非线性系数,但制备难度很大,晶体通光口径严重受限。BGSe是近年我国发明并拥有自主知识产权的一种新型红外晶体,在8μm~14μm波段具有优异的透光性能,同时非线性系数较大,损伤阈值较高,易于大尺寸生长,特别是其可采用1μm激光直接抽运,因此极具应用前景。在固体激光抽运源方面,1μm波段近红外激光(Nd, Yb激光等)是目前最有效、发展最成熟的固体激光,被认为是非线性红外晶体频率下转换的最佳抽运源。然而由于许多红外晶体只能采用波长不小于1.5μm的激光抽运,陆续发展了1.54μm Er激光、2μm Tm/Ho:YAG激光、2.79μm Er, Cr:YSGG激光及1μm激光抽运的光参量激光等短波红外固体激光抽运源。但是上述抽运激光在输出功率、光束质量、整体效率及系统紧凑性与可靠性等方面均不如1μm激光,如Er, Cr:YSGG晶体热导率较低,且激光产生量子效率低,导致热效应极其严重,难以实现高功率高重频输出。在频率下转换技术方面,DFG, OPO, OPA及OPG均被广泛用于产生宽波段连续调谐中远红外相干辐射。DFG基于两束高强度激光(其中一束波长需连续调谐)抽运非线性晶体,新产生一束频率为入射的两束激光频率差值的长波闲频光输出,可在脉冲(fs, ps, ns等)及连续波(continuous wave, CW)模式下运转。如BEUTLER等人采用1.38μm~1.98μm及4.1μm~2.2μm的两束可调谐光参量超快激光抽运AGSe晶体,基于差频效应实现了5μm~17μm连续调谐的中红外相干辐射产生,实验光路如图 2所示。其中8μm~14μm波段内ps, 80MHz运转时输出功率范围为70mW~8mW,fs, 53MHz运转时输出功率范围为30mW~5mW[29]。DFG由于需要两台激光器作为抽运源,同时要求两束激光在时间、空间及偏振上要满足严格的匹配,使得系统较为复杂。后续出现了一种只需一束宽带脉冲激光的脉冲内差频技术(intra-pulse DFG, IP-DFG),如GAIDA等人采用1.3μm~2.4μm的宽带Tm:fiber激光抽运GaSe晶体,基于脉冲内差频效应,实现了3.7μm~18μm的中红外输出[30],但输出功率较低。OPO基于一束固定波长激光抽运置于谐振腔内的非线性晶体,产生的信号光与闲频光在谐振腔内多次通过非线性晶体实现高效光放大,可在脉冲及连续波模式下运转。其中ns-OPO效率高且结构紧凑简单,是目前发展最成熟的器件[31],哈尔滨工业大学QIAN等人采用2.09μm Ho:YAG激光抽运的ZGP-OPO,实现了8.2W的20kHz、ns级8.3μm激光输出,进一步通过ZGP-OPA将功率提升至11.4W,实验光路如图 3所示,这是目前该波段激光的最高输出[32]。KOSTYUKOVA等人采用1μm Nd:YAG激光抽运的BGSe-OPO,实现了2.7μm~17μm的ns, 10Hz超宽调谐中红外相干辐射输出,其中8μm~14μm脉冲输出能量范围高达0.1mJ~0.6mJ[33]。ps, fs-OPO需采用同步抽运方式(synchronously pumped OPO, SP-OPO),CW-OPO需采用内腔谐振方式及具有大非线性系数的晶体(如准相位匹配结构晶体),一定程度上增加了系统复杂性。由于OPO需从量子噪声中产生参量种子,阈值较高,因此需要较高的抽运强度。对于8μm~14μm波段OPO器件,谐振腔腔镜及红外晶体所必需的多色宽带红外激光薄膜制备目前尚不成熟,在高功率/大能量运转时极易损伤,同时输出光束质量的恶化成为当前制约8μm~14μm波段OPO向前发展的最大问题。OPA基于一束高强度激光抽运及一束高质量种子激光(信号光或闲频光)注入非线性晶体,可实现种子激光的高效放大,同时产生另一束闲频光/信号光输出,一般以fs, ps, ns脉冲模式运转。种子激光可大幅降低OPA器件阈值,并提升输出光的频率、时间及空间特性。作者采用1μm Nd:YAG激光作为抽运源,在国际上率先实现了基于BGSe晶体的大能量ps中红外宽调谐输出[34],并且最近实现了输出能量范围高达140μJ~230μJ的ps连续调谐8μm~14μm长波红外相干辐射输出。由于OPA需要一束可调谐或者宽谱激光作为种子,尽管低强度调谐种子激光相比于DFG所需的高强度调谐激光易于获得,但是种子光仍一定程度上增加了整个系统的复杂性。OPG只需一束激光抽运非线性晶体材料,结构最为简单,示意图如图 4所示。其需要极高的抽运强度及较长的晶体作用距离,通常以ps、fs模式运转,并且输出光束质量及光谱特性很差,效率很低,当前使用较少。表 2中分类列出了近年来8μm~14μm范围内非线性变频中红外激光输出的研究进展。
表 1 8μm~14μm波段主要红外非线性晶体
晶体 透光波段/μm 实用红外限/μm 抽运波长/μm 非线性系数/ (pm·V-1) ZnGeP2[35] 0.74~12 ≈8.5 ≥ 2 ≈70 AgGaS2[35] 0.5~13 ≈8.5 ≥ 1 ≈10 LiInSe2[35] 0.47~13.7 ≈8.5 ≥ 1 ≈10 BaGa4S7[36] 0.35~13.7 ≈8.5 ≥ 0.8 ≈5 AgGaSe2[35] 0.7~19 ≈15 ≥ 1.5 ≈20 GaSe[35] 0.65~20 ≈14 ≥ 1.5 ≈50 CdSe[37] 0.75~24 ≈16.5 ≥ 1.5 ≈20 BaGa4Se7[38] 0.47~18 ≈14 ≥ 1 ≈20 OP-GaP[39] 0.57~12 ≈8.5 ≥ 1 ≈70 OP-GaAs[39] 0.9~17 ≈12 ≥ 1.9 ≈90 表 2 8μm~14μm波段非线性变频中红外固体激光进展
晶体 抽运激光 调谐范围/μm 8μm~14μm内能量/功率 脉冲体制 产生方式 LiInSe2 Ti:S抽运SP-OPO(1.33~1.51)μm & (1.7~2.0)μm 4~11.5 (7-2.5)mW@ (8~11.5)μm fs, 80MHz DFG[40] GaS0.4Se0.6 Ti:S抽运SP-OPO(1.33~1.51)μm & (1.7~2.0)μm 4~12 > 0.5mW@ (8~12)μm fs, 80MHz DFG[41] LiInSe2 Yb-fiber抽运SP-OPO(1.38~1.98)μm & (4.1~2.2)μm 5~12 (35~5)mW@ (8~12)μm fs, 53MHz DFG[42] AgGaSe2 Yb-fiber抽运SP-OPO(1.38~1.98)μm & (4.1~2.2)μm 5~17 (30~5)mW@ (8~14)μm fs, 53MHz DFG[29] AgGaSe2 Yb-fiber抽运SP-OPO(1.38~1.98)μm & (4.1~2.2)μm 5~18 (70~8)mW@ (8~14)μm ps, 80MHz DFG[29] LiInSe2 Yb-fiber抽运SP-OPO(1.38~1.98)μm & (4.1~2.2)μm 5~12 (10~2)mW@ (8~12)μm ps, 80MHz DFG[42] CdSe Nd:YAG抽运KTP-SP-OPO(1.87~2.03)μm & (2.23~2.45)μm 10~21 (0.5~7)mW@ (10~14)μm ps, 2.5kHz DFG[43] GaSe Tm:fiber(1.3~2.4)μm宽带光 3.7~18 数值孔径@(8~14)μm fs, 400kHz IP-DFG[30] LiGaS2 1.3μm Yb:YAG-fiber(125nm谱宽) 6.7~18 数值孔径@(8~14)μm fs, 100MHz IP-DFG[44] CdSe Ho:YAG(2.09ΜM) 10.24~12.07 (200~85)mW@(10.24~12.07)μm ns, 1.2kHz OPO[45] CdSe Ho:YLF (2.05μm) 10.2 320mW@ 10.2μm ns, 5kHz OPO[46] CdSe Cr, Er:YSGG (2.79μm) 8.5~12.3 (1.2~2.4)mJ@(8.5~12.3)μm ns, 10Hz OPO[47] OP-GaAs Tm-fiber(1.95μm) 10.6 800mW@10.6μm ns, 50kHz OPO[48] BaGa4Se7 Nd:YAG(1.064μm) 2.7~17 (0.6~0.1)mJ@(8~14)μm ns, 10Hz OPO[33] ZnGeP2 Ho:YAG(2.09μm) 8.3 11.4W@8.3μm ns, 20kHz OPO+OPA[32] OP-GaP Yb laser (1.04μm) 5~12 (15~7.5)mW@(8.5~11.8)μm fs, 100MHz SP-OPO[49] CdSe Nd:YLF抽运PPLN-SP-OPO(1.85~1.97)μm 9.1~9.7 10.8mW@9.1μm ps, 120MHz SP-OPO[50] LiInSe2 Nd:YAG(1.064μm)种子光(1.255~1.168)μm 7~12 (110~20)μJ@(8~12)μm ps, 10Hz OPA[51] BaGa4Se7 Nd:YAG(1.064μm)种子光(1.28~1.17)μm 6.4~11 (90~38)μJ@(8~11)μm ps, 10Hz OPA[34] GaS0.4Se0.6 Nd:YAG(1.064μm)种子光(1.175~1.4)μm 5~11 (4~1)μJ@(8~11)μm ps, 10Hz OPA[52] HgGa2S4 Nd:YVO4 MOPA(1.064μm) 3.85~10 数值孔径 ps, 250kHz OPG[53] 非线性变频中红外固体激光具有波长宽范围连续可调谐、热效应极小、光束质量好、运转体制丰富灵活(重频1Hz~GHz,脉宽ns~fs或连续波)、全固化、结构紧凑、可靠性高等优点,被认为是发展实用化与精密化8μm~14μm宽调谐激光源的最有效途径之一。随着输出功率/能量的不断提升,其将获得广泛的应用。
8μm~14μm长波红外相干辐射技术
8μm~14μm long-wave infrared coherent radiation technology
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摘要: 8μm~14μm长波红外相干辐射在大气激光遥感与通信、高分辨分子指纹光谱及国家安全等诸多领域具有重要应用价值。概述了几种可获得8μm~14μm相干辐射的光源, 主要有CO2气体激光器、半导体量子级联激光器、红外自由电子激光器、红外超连续谱光源和非线性变频中红外固体激光器等。分别介绍了不同技术的基本原理及国内外进展, 并分析比较了各自的优缺点、发展方向及应用适用范围。其中, 非线性变频中红外固体激光器具有8μm~14μm连续调谐, 可飞秒、皮秒、纳秒及连续波运转, 重频1Hz~GHz可调的突出优势, 且全固化结构、可靠性高、光束质量好, 将成为发展实用化与精密化长波红外相干辐射光源的最佳选择。
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关键词:
- 激光技术 /
- 8μm~14μm相干辐射 /
- 频率变换 /
- 非线性晶体
Abstract: 8μm~14μm long-wave infrared coherent radiation has important application value in many fields such as atmospheric laser remote sensing and communication, high-resolution molecular fingerprint spectrum and national security. Several kinds of optical sources which can obtain 8μm~14μm coherent radiation are summarized, including CO2 gas laser, semiconductor quantum cascade laser, infrared free electron laser, infrared supercontinuum spectrum source and non-linear variable frequency mid-infrared solid-state laser. The basic principles of different technologies and their progress at home and abroad are introduced. The advantages and disadvantages, development direction and application scope of each method are analyzed and compared. Nonlinear variable frequency mid-infrared solid-state lasers have outstanding advantages of 8μm~14μm continuous tuning, femtosecond, picosecond, nanosecond and continuous wave operation, and 1Hz~GHz tunable repetition frequency. Full solid structure, high reliability and good beam quality are also obtained. It will be the best choice for developing practical and precise long-wave infrared coherent radiation sources.-
Key words:
- laser technique /
- 8μm~14μm coherent radiation /
- frequency conversion /
- nonlinear crystal
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表 1 8μm~14μm波段主要红外非线性晶体
晶体 透光波段/μm 实用红外限/μm 抽运波长/μm 非线性系数/ (pm·V-1) ZnGeP2[35] 0.74~12 ≈8.5 ≥ 2 ≈70 AgGaS2[35] 0.5~13 ≈8.5 ≥ 1 ≈10 LiInSe2[35] 0.47~13.7 ≈8.5 ≥ 1 ≈10 BaGa4S7[36] 0.35~13.7 ≈8.5 ≥ 0.8 ≈5 AgGaSe2[35] 0.7~19 ≈15 ≥ 1.5 ≈20 GaSe[35] 0.65~20 ≈14 ≥ 1.5 ≈50 CdSe[37] 0.75~24 ≈16.5 ≥ 1.5 ≈20 BaGa4Se7[38] 0.47~18 ≈14 ≥ 1 ≈20 OP-GaP[39] 0.57~12 ≈8.5 ≥ 1 ≈70 OP-GaAs[39] 0.9~17 ≈12 ≥ 1.9 ≈90 表 2 8μm~14μm波段非线性变频中红外固体激光进展
晶体 抽运激光 调谐范围/μm 8μm~14μm内能量/功率 脉冲体制 产生方式 LiInSe2 Ti:S抽运SP-OPO(1.33~1.51)μm & (1.7~2.0)μm 4~11.5 (7-2.5)mW@ (8~11.5)μm fs, 80MHz DFG[40] GaS0.4Se0.6 Ti:S抽运SP-OPO(1.33~1.51)μm & (1.7~2.0)μm 4~12 > 0.5mW@ (8~12)μm fs, 80MHz DFG[41] LiInSe2 Yb-fiber抽运SP-OPO(1.38~1.98)μm & (4.1~2.2)μm 5~12 (35~5)mW@ (8~12)μm fs, 53MHz DFG[42] AgGaSe2 Yb-fiber抽运SP-OPO(1.38~1.98)μm & (4.1~2.2)μm 5~17 (30~5)mW@ (8~14)μm fs, 53MHz DFG[29] AgGaSe2 Yb-fiber抽运SP-OPO(1.38~1.98)μm & (4.1~2.2)μm 5~18 (70~8)mW@ (8~14)μm ps, 80MHz DFG[29] LiInSe2 Yb-fiber抽运SP-OPO(1.38~1.98)μm & (4.1~2.2)μm 5~12 (10~2)mW@ (8~12)μm ps, 80MHz DFG[42] CdSe Nd:YAG抽运KTP-SP-OPO(1.87~2.03)μm & (2.23~2.45)μm 10~21 (0.5~7)mW@ (10~14)μm ps, 2.5kHz DFG[43] GaSe Tm:fiber(1.3~2.4)μm宽带光 3.7~18 数值孔径@(8~14)μm fs, 400kHz IP-DFG[30] LiGaS2 1.3μm Yb:YAG-fiber(125nm谱宽) 6.7~18 数值孔径@(8~14)μm fs, 100MHz IP-DFG[44] CdSe Ho:YAG(2.09ΜM) 10.24~12.07 (200~85)mW@(10.24~12.07)μm ns, 1.2kHz OPO[45] CdSe Ho:YLF (2.05μm) 10.2 320mW@ 10.2μm ns, 5kHz OPO[46] CdSe Cr, Er:YSGG (2.79μm) 8.5~12.3 (1.2~2.4)mJ@(8.5~12.3)μm ns, 10Hz OPO[47] OP-GaAs Tm-fiber(1.95μm) 10.6 800mW@10.6μm ns, 50kHz OPO[48] BaGa4Se7 Nd:YAG(1.064μm) 2.7~17 (0.6~0.1)mJ@(8~14)μm ns, 10Hz OPO[33] ZnGeP2 Ho:YAG(2.09μm) 8.3 11.4W@8.3μm ns, 20kHz OPO+OPA[32] OP-GaP Yb laser (1.04μm) 5~12 (15~7.5)mW@(8.5~11.8)μm fs, 100MHz SP-OPO[49] CdSe Nd:YLF抽运PPLN-SP-OPO(1.85~1.97)μm 9.1~9.7 10.8mW@9.1μm ps, 120MHz SP-OPO[50] LiInSe2 Nd:YAG(1.064μm)种子光(1.255~1.168)μm 7~12 (110~20)μJ@(8~12)μm ps, 10Hz OPA[51] BaGa4Se7 Nd:YAG(1.064μm)种子光(1.28~1.17)μm 6.4~11 (90~38)μJ@(8~11)μm ps, 10Hz OPA[34] GaS0.4Se0.6 Nd:YAG(1.064μm)种子光(1.175~1.4)μm 5~11 (4~1)μJ@(8~11)μm ps, 10Hz OPA[52] HgGa2S4 Nd:YVO4 MOPA(1.064μm) 3.85~10 数值孔径 ps, 250kHz OPG[53] -
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