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近年来，随着激光材料的迅速发展和以半导体激光器为代表的泵浦技术的持续进步，全固态激光器正在向结构紧凑、性能更优及输出特性更稳定的方向发展[1]。其中，中红外波段的全固态激光器因其在军事、科研、工业等领域的特殊应用，已成为激光领域的研究热点之一[2−4]。2 μm波段的激光位于人眼安全波段，具有较高的大气传输透过率和水吸收特性，目前被广泛应用于激光雷达、大气环境监测、激光医疗、光谱分析等领域[5−7]。
在高相干探测应用领域，激光需要具有窄光谱线宽、高频率稳定度的特点，这些都需要单纵模激光来实现[8-10]。与多纵模运转的激光器相比，单纵模激光器不仅具有窄输出线宽、良好的相干性等优点，同时其稳定性更强、光束质量也更好，因此在激光雷达、高分辨率气体探测器等特定的激光应用中更具优势。近年来，中红外激光技术开始展现出独特的应用前景，2 μm单纵模激光器作为相干差分吸收激光雷达、多普勒测风雷达等应用场景的优质光源，在大气温室气体探测、工业以及国防等领域发挥着重要的作用[11−16]。2 μm激光器按工作介质可分为半导体激光器、光纤激光器和固体激光器三种。其中，半导体激光器和光纤激光器均难以直接获得较高的输出能量。相比之下，固体激光器不仅输出能量高，而且稳定性好、工作寿命长、输出光束质量好、波长覆盖范围广。此外，相较于闪光灯泵浦的方式，以半导体激光器为泵浦源的全固态激光器在能量转换效率、激光光束质量、抗干扰能力、寿命和可靠性等方面都有显著的提升[17−20]。
光参量振荡（optical parametric pscillator，OPO）等非线性光学频率变换技术和直接泵浦掺杂Tm3+或Ho3+的增益介质，均可以获得2 µm固体激光输出。然而，OPO等光参量技术中存在较为严重的光谱线宽展宽效应，从而难以实现激光单纵模输出[21-22]。采用直接泵浦的方式获得2 µm单纵模激光，这就需要对掺Tm3+或掺Ho3+激光器采取特殊的选模方法，如对谐振腔进行特殊设计或插入特定选模元件。具体的选模方法主要有环形腔法、扭转模腔法、体布拉格光栅（volume Bragg grating，VBG）法、短腔法、标准具法等。为了提高单纵模激光脉冲输出能量，通常采用种子注入激光技术：由上述方法所实现的单纵模激光器均可作为种子光源，通过将单纵模种子激光注入到从属激光器中，可以实现单纵模运转的高能量脉冲激光输出[23-24]。对于单纵模脉冲激光器来说，其输出特性主要由中心波长、输出能量、脉冲宽度、频谱半峰全宽（full width at half maximum, FWHM）、脉冲重复频率、频率稳定度等参数表征。
文中结合2 μm单纵模全固态脉冲激光器在各领域的应用，综述了2 μm全固态脉冲环形腔激光技术、扭转模腔激光技术、VBG激光技术、种子注入激光技术的研究进展，总结分析了各种技术的输出特性，并对2 μm单纵模全固态脉冲激光器的未来发展作了展望，期望能够为从事全固态激光技术及中红外激光的研究人员提供参考。
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激光增益介质的理化性质主要由其基质材料所决定，而其光谱特性则主要取决于掺杂的激活离子。这些特性包括激光介质的吸收波长、吸收截面、发射波长和发射截面等。激光增益介质的这些参数将会直接影响到整个激光器的性能。Tm3+和Ho3+激活离子的发射波长均在2 μm附近，其激光跃迁在基态的上斯塔克能级结束，属于准三能级系统。能级结构如图1所示，Tm3+和Ho3+分别在3F4-3H6和5I7-5I8跃迁产生2 μm激光[25]。目前，2 μm固体激光器常用的增益介质主要包括Tm:YAG、Tm:YAP、Tm:YLF、Ho:YAG、Ho:YLF等。
Tm:YAG晶体是在钇铝石榴石（Y3Al5O12，YAG）基质中掺入一定浓度的Tm3+形成的，为各向同性的单轴晶体，硬度较高且热性能优良。Tm:YAG晶体吸收峰在785 nm附近，吸收截面为7.5×10−21 cm2，因此可以使用中心波长为785 nm的半导体激光器作为其泵浦源。发射波长覆盖1.87~2.16 μm，分别在1882 nm、1960 nm和2 014 nm处有三个强发射峰。对于常用的2014 nm发射波段，其发射截面为2.9×10−21 cm2，荧光寿命约为11 ms。
Tm:YAP为掺杂Tm3+的铝酸钇（YAlO3，YAP）晶体，是具有自然双折射特性的双轴晶体，有着良好的热力学和光谱特性。Tm:YAP晶体的吸收波段在795 nm附近，吸收截面3.7×10−21~8.5×10−21 cm2。Tm:YAP有较宽的发射带宽，分别在1940 nm和1980 nm处存在发射峰，受激发射截面在5×10−21 ~6×10−21 cm2之间，荧光寿命在4.4~7.7 ms之间。
Tm:YLF晶体为掺杂Tm3+的氟化钇锂（LiYF4，YLF）介质，可实现不同波长的π偏振输出和σ偏振输出。在2 μm固体激光器中，通常利用其σ偏振输出。Tm:YLF晶体在792 nm处存在吸收峰，吸收截面为5.5×10−21 cm2，发射峰则在1908 nm附近，发射截面2.3×10−21 cm2，荧光寿命16 ms。
Ho:YAG晶体具有高熔点和高密度，可以长时间在高功率下泵浦，是一种性能良好的2 μm激光增益介质。Ho:YAG晶体的吸收峰主要位于1908 nm处，对应的吸收截面为1.09×10−20 cm2。因此，发射峰值为1908 nm的Tm:YLF固体激光器常被用作Ho:YAG激光器的泵浦源。最大发射峰位于2090 nm处，对应的发射截面为1.14×10−20 cm2，荧光寿命为7 ms。
Ho:YLF晶体是一种各向异性晶体，可直接实现线偏振光输出。Ho:YLF晶体在1940 nm附近具有最大吸收峰，对应吸收截面为1.2×10−20 cm2，通常采用发射峰值为1940 nm的Tm:YAP固体激光器作为其泵浦源。Ho:YLF在2060 nm附近存在较大的发射峰，发射截面1.8×10−20 cm2，荧光寿命约为10 ms。
表1总结了几种常用的2 μm固体激光增益介质的光谱特性，其中，Tm3+掺杂晶体的吸收波长与GaAlAs半导体激光器的工作波长相匹配，且发射波长在1.8~2.1 μm间可实现连续调谐，但发射截面较小，上能级寿命较短。与之相比，Ho3+掺杂晶体发射截面较大，荧光寿命较长，易实现高脉冲能量2.1 μm激光输出，但是缺少成熟的商用半导体激光器，常用掺Tm3+激光器作为泵浦源。
表 1 常见2 μm固体激光增益介质光谱特性
Table 1. Spectral characteristics of commonly used gain media for 2 μm solid-state laser
Crystal Typical absorption wavelength/
nmAbsorption section/
cm2Typical emission wavelength/
nmEmission section/
cm2Fluorescence lifetime/
msTm:YAG 785 7.5×10−21 2 014 2.9×10−21 11 Tm:YAP 795 3.7×10−21-8.5×10−21 1 940, 1 980 5×10−21-6×10−21 4.4-7.7 Tm:YLF 792 5.5×10−21 1 908 2.3×10−21 16 Ho:YAG 1 908 1.09×10−20 2 090 1.14×10−20 7 Ho:YLF 1 940 1.2×10−20 2 060 1.8×10−20 10 -
目前，针对固体激光器的单纵模选择技术主要包括环形腔法、扭转模腔法、VBG法和种子注入法等。以下是对这些选模方法的原理和特点的简要介绍。
在激光谐振腔中，理论上，由于增益饱和效应引起的模式竞争，可以实现单纵模在腔内振荡。然而，由于腔内驻波场引起的空间烧孔效应，光强在空间中分布不均匀，导致激光器会输出多纵模激光。为了消除空间烧孔效应，通常会在谐振腔内加入隔离器等单向选通元件，谐振腔一般采用环形腔结构，以保证振荡器稳定的单向运转，即形成行波谐振腔，如图2(a)所示，从而实现激光单纵模输出[26]。
扭转模腔法也是消除空间烧孔效应的常用方法，其结构如图2(b)所示。扭转模激光器的单纵模输出是通过在谐振腔内插入一个偏振片和分别位于增益介质两端的两个四分之一波片来实现的。两个波片的快轴相互垂直，且与偏振片的选偏方向成45°角，这样，增益介质中的光场不再处于驻波模式，两个波片之间传播的激光为旋向不同的圆偏振光，光强分布均匀，空间烧孔效应从根本上被消除，从而实现激光单纵模输出。扭转模腔全固态激光器以其简单的结构、较低的阈值电压，较高的单纵模率及较好的频率稳定性，已经成为一种良好的单纵模选择方案[27-28]。
图 2 结构示意图。(a) 环形腔；(b) 扭转模腔
Figure 2. Structure diagram. (a) Ring cavity; (b) Twisted-mode cavity
VBG是一种常用的光谱滤波元件，具有良好的角度选择性和波长选择性。任何不符合布拉格条件的角度或波长变化都将导致其衍射效率的显著下降、腔内损耗增加，从而使特定波长在腔内振荡、实现纵模的选择。在单纵模激光器中，反射式和透射式VBG都可以用于模式选择，使用具有高衍射效率的VBG可以实现在特定波长处的高反射/透射率，将输出波长锁定并压缩线宽。此外，可以通过光栅周期的横向啁啾实现可调谐波长输出[29-30]。
种子注入技术是将稳定的单纵模种子激光注入到从属激光器中进行选模和放大的技术[7−8]，是获得单纵模激光最重要的技术之一。其基本原理为：将单纵模种子激光注入谐振腔中，并通过压电陶瓷换能器（piezoelectricity transducer，PZT）等器件控制谐振腔长度变化。由于注入的种子光比腔内自由振荡光更具优势，因此从腔中与种子光频率最接近的模式会优先起振。当检测到谐振腔内种子光和振荡光谐振匹配时，Q开关工作，从而输出窄线宽、高能量的单纵模脉冲激光[23-24]。常见的种子注入技术有扫描-触发技术（ramp-fire）、扫描-保持-触发技术（ramp-hold-fire）等[31-33]。
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2006年，美国兰利研究中心的Yu等[34]在具有主振荡功率放大器（master oscillator power amplifier，MOPA）结构的Ho:Tm:LuLiF4激光器中实现了波长2.053 μm，单脉冲能量达到1.1 J的激光输出，振荡器使用稳定的环形谐振腔来获得接近高斯的空间轮廓光束。2010年，Koch等[35]研制了一台2 µm波长的相干多普勒测风激光雷达，由具有光学放大器的Ho:Tm:LuLiF4激光器产生重复频率5 Hz，能量250 mJ的激光，并经外场试验表明，此高脉冲能量2 µm多普勒激光雷达的研制已趋于成熟。2014年，Bai等[36]设计了一种用于星载激光雷达的单纵模脉冲Ho:YLF激光器，系统总泵浦功率为40 W时，振荡器的输出脉冲能量在100 Hz重频下为40 mJ，在200 Hz重频下为34 mJ，放大器峰值功率大于1 MW，脉冲宽度32 ns，线宽小于20 MHz。
2017年，哈尔滨工业大学的Dai等[37]采用声光调制器和两块半波片实现了Ho:YLF环形腔激光输出，当入射泵浦功率为16.4 W时，在波长2063.8 nm处获得了3.73 W的输出功率，斜效率为27.1%，光束质量因子1.12，且增加声光调制器的射频功率可以进一步提高单纵模输出功率。2019年，同课题组的Wang等[38]报道了2052.96 nm单纵模脉冲Ho:YVO4-MOPA系统，该系统由图3所示的单向环形被动调Q振荡器和单通放大器组成。通过在环形Ho:YVO4谐振腔中插入隔离器、半波片和Cr2+:ZnS，获得了平均输出功率1.02 W、脉冲宽度910 ns、脉冲重复频率67 kHz的单纵模激光输出。利用振荡器的剩余泵浦光作为放大器的泵浦源，采用单通结构的Ho:YVO4放大器获得了1.67 W的输出功率，系统总的光-光效率达到14.3%。
表2总结了近年来基于单向环形腔的2 μm单纵模全固态脉冲激光器的输出特性，可以看出，基于单向环形腔的单纵模激光器腔形设计灵活，可以在较宽的波长范围内获得较高的输出功率。但其结构复杂且光路元件较多，较多的反射镜也会造成一定的激光能量损耗[39−40]。
表 2 环形腔2 μm单纵模全固态脉冲激光器输出特性
Table 2. Output characteristics of 2 μm single-longitudinal-mode all-solid-state pulsed laser with ring cavity
Year Institution Wavelength/nm Repetition rate Power/W Energy Pulse width/ns 2006[34] NASA Langley Research Center 2053 - - 1.1 J - 2010[35] NASA Langley Research Center 2053 5 Hz 1.25 250 mJ - 2014[35] NASA Langley Research Center 2050.967 100 Hz - 40 mJ 32 2017[37] Harbin Institute of Technology 2063.8 - 3.73 - - 2017[41] Harbin Institute of Technology 2053.9 - 0.941 - - 2019[38] Harbin Institute of Technology 2052.96 67 kHz 1.67 24.9 μJ 910 -
2020年，哈尔滨工业大学的Dai等[42]研制了一种如图4所示的扭转模腔单纵模Ho:YAG激光器，在波长2097.46 nm处获得了0.76 W的最大连续波单纵模输出功率，对应28.9%的斜效率，并通过在谐振腔中插入标准具，可以将波长从2096.94 nm调谐到2098.48 nm。利用电光Q开关实现了激光器的脉冲输出，在2 kHz的脉冲重复频率下，获得单脉冲能量0.2 mJ，脉冲宽度116.5 ns，激光器在x和y方向上的光束质量因子分别为1.15和1.10。
由于基于扭转模腔的2 μm单纵模全固态脉冲激光器的研究相对较少，表3列出近年来部分相关连续波激光器的输出特性。从表中可以看出，由于基于扭转模腔的单纵模激光器对腔内偏振态的变化十分敏感，因此，受到增益介质热致双折射等因素的影响，激光器的单纵模率和高功率输出受到了一定的限制[43]。然而，随着扭转模腔技术研究的不断发展，通过结合其他选模方法解决晶体热效应等问题，扭转模腔法将逐渐成为单纵模激光技术的研究热点。
表 3 扭转模腔2 μm单纵模全固态激光器输出特性
Table 3. Output characteristics of 2 μm single-longitudinal-mode all-solid-state laser with twisted-mode cavity
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目前，VBG已被广泛应用于1 µm激光器，但其在2 µm激光器中应用的报道相对较少。
2015年，西北大学的Jin[47]等研制了一台基于VBG的Tm：YAP单纵模被动调Q激光器，其在重复频率96.2 kHz时，中心波长为1988.8 nm、线宽4.2 MHz、平均输出功率为724 mW，脉宽2.2 μs，单脉冲能量7.5 μJ。
2018年，哈尔滨工业大学的Duan等[48]研制了一种在连续波和脉冲运转下的窄线宽Ho:CaF2激光器，使用VBG在波长2100.5 nm处获得的最大连续波输出功率为6.94 W，斜效率57.9%。当泵浦功率为13.2 W时，在3 kHz的重复频率下获得了最小脉冲宽度54 ns、最大单脉冲能量1.9 mJ和最大峰值功率达35.3 kW的激光脉冲输出。2019年，该课题组[49]在电光调Q腔倒空Ho:SSO激光器中利用VBG压缩线宽，泵浦功率19.3 W时，在100 kHz的重频下获得了6.33 W的最大平均输出功率，对应48.5%的斜效率和32.8%的光-光转换效率，脉冲宽度3.6 ns，输出波长2100.5 nm。
2020年，法国巴黎萨克雷大学的Berthomé等[50]研制了一种如图5所示的脉冲单纵模波长可调谐Tm:YAP激光器，通过使用VBG作为输出耦合器，并在腔中插入YAG标准具，在1 kHz的重复频率下可以获得能量为230 µJ、脉宽50 ns的稳定脉冲输出，并且光谱线宽可以压缩到4 pm以下，又由于布拉格光栅周期的横向啁啾，输出波长可以从1 940 nm调谐到1 960 nm。
表4总结了近年来基于VBG的2 μm单纵模全固态脉冲激光器的输出特性，可以看出，由于VBG较高的选模精度、较窄的谱线宽度、良好的热稳定性等特点，VBG激光器已经成为了获得窄线宽激光的有效途径。然而，VBG的缺点也很明显，比如光栅孔径难以做大、损伤阈值较低、价格较高等[30]。
表 4 VBG法2 μm单纵模全固态脉冲激光器输出特性
Table 4. Output characteristics of 2 μm single-longitudinal-mode all-solid-state pulsed laser with VBG
Year Institution Wavelength/nm Repetition rate/kHz Energy Pulse width Linewidth 2015[47] Northwest University 1988.8 96.2 7.5 μJ 2.2 μs 4.2 MHz 2018[48] Harbin Institute of Technology 2100.5 3 1.9 mJ 54 ns - 2019[49] Harbin Institute of Technology 2100.5 100 63.3µJ 3.6 ns - 2020[50] Paris-Saclay University 1960 1 230 µJ 50 ns <4 pm -
在2 μm单纵模种子注入系统中，需要稳定的单纵模激光器作为种子。微片激光器、标准具激光器、环形腔激光器、非平面环形腔（non-planar ring oscillator，NPRO）激光器以及分布式反馈（distributed feedback，DFB）半导体激光器等均可作为该系统的种子源，而种子源的选择将直接影响系统的性能和稳定性。
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对于一个参数固定的激光谐振腔，其纵模间隔与腔长成反比，因此通过缩短腔长增加纵模间隔，使得增益曲线中只有一个纵向模式达到振荡阈值，获得单纵模激光输出，这种方法被称为短腔法。由于一般的固体激光器腔长较长，纵模间隔较小，为了通过短腔法实现单纵模输出，固体激光器通常需要毫米级的腔长。所以，可以直接在增益介质的通光表面镀上相应的膜层，将其作为谐振腔的腔镜制成微型激光腔，因此这类激光器也常被称为微片激光器[51−53]。
虽然短腔法具有结构紧凑的优点，但这种微片结构的腔长和增益介质尺寸较小，导致微片激光器难以获得较大能量的单纵模输出[54-55]。因此，需要将微片激光器作为种子源，利用种子注入技术和MOPA系统来实现高能量窄线宽激光输出。
1997年，美国兰利研究中心的Singh等[56]研制了一种用于测风的2 μm固态激光雷达发射机，该系统由Ho:Tm:YLF微片激光种子源、环形腔振荡器和五个激光放大器组成。重频10 Hz时，振荡器产生35 mJ脉冲能量输出，脉宽为400 ns，放大器最大单脉冲能量达700 mJ。1998年，Singh等[57]利用微片种子注入技术实现了输出能量125 mJ，重频6 Hz，脉冲宽度170 ns的单纵模激光，并在放大后获得了600 mJ的单脉冲能量。
2012年，哈尔滨工业大学的Dai等[58]研制了一台Tm:Ho:YAP种子注入调Q激光器，使用单纵模Tm:Ho:YAP微片激光器作为种子，在2130.7 nm处产生37 mW的单纵模种子激光。通过种子注入技术，在100 Hz时获得了2.8 mJ的输出能量，脉冲宽度289 ns，并通过外差法测得线宽为4.5 MHz。
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法布里-珀罗（Fabry-Perot，F-P）标准具法是目前常用的固体激光器纵模选择方法，通过在谐振腔内插入基于干涉原理的F-P标准具，利用标准具的窄线宽透射谱，增大腔内其他纵模的损耗，使透射率最高的纵模在腔内振荡，从而可以实现激光单纵模输出[59−62]。在2 μm固体激光器中，由于介质增益曲线较宽，通常需要两个或更多的标准具组合使用。
基于标准具的单纵模激光器整体结构简单、紧凑性高，且通过调整标准具的角度可以实现单纵模激光可调谐输出[63-64]。然而，标准具较大的插入损耗很难以高效率直接产生高能量单纵模激光。因此，需要将插入标准具的单纵模激光器作为种子源进行功率放大。
2012年，哈尔滨工业大学的Dai等[65]通过将两个F-P标准具插入激光谐振腔，在波长2090.9 nm处获得了60 mW的Tm:Ho:YAG种子激光。然后利用种子注入技术，由Tm:YLF激光器泵浦Ho:YAG，在100 Hz时获得了输出能量7.6 mJ，脉冲宽度132 ns的单纵模激光。利用外差法测得线宽为3.5 MHz，可以将其用于多普勒测风激光雷达系统。2018年，Dai等[66]在Tm:Ho:YLF激光谐振腔中插入两个F-P标准具，并通过PZT改变腔长，实现了如图6所示的可调谐单纵模种子激光器，波长可以从2050.962 nm微调到2051.000 nm，且在2050.967 nm的CO2吸收峰处，获得了76 mW的单纵模激光。然后采用种子注入技术实现了脉冲输出能量4.4 mJ的Ho:YLF单纵模激光，脉冲宽度65 ns，重复频率100 Hz，光束质量因子1.07，接近衍射极限，线宽4.1 MHz。
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2011年，南非科学与工业研究理事会的Strauss等[67]使用Ho:YLF和Ho:LuLF晶体研制了一种单纵模2 µm板条激光放大系统。环形腔种子激光器在50 Hz下产生73 mJ的最大输出能量，脉冲宽度365 ns。单通放大器采用可扩展的板条结构，在2064 nm中心波长下获得210 mJ的输出功率，脉宽350 ns。2013年，Strauss等[68]利用双通放大技术，用50 mJ的能量进行种子注入，在波长2064 nm，重复频率50 Hz时，得到放大后的最大激光脉冲能量333 mJ，脉冲宽度保持在350 ns。
2018年，日本国家信息与通信技术研究所的Mizutani等[39]研制了用1.94 μm掺Tm光纤激光器泵浦的Ho:YLF激光器，由环形谐振腔振荡器和放大器组成的激光系统在室温下以200~5000 Hz的重复频率工作。在输出波长为2.064 μm、200 Hz的最小重复频率下，获得了脉冲宽度150 ns、最大脉冲能量21 mJ的激光输出。然后利用种子注入技术，在重频为300 Hz时可得到16 mJ的单纵模激光输出，并成功将其用于图7所示多普勒测风激光雷达中。
2019年，哈尔滨工业大学的Wang[69]等设计了一种用于CO2差分吸收激光雷达的Ho:YLF双波长种子注入调Q激光器，以具有双角立方体结构的单纵模Ho:YLF环形激光器作为种子，波长在2064.414 nm的CO2吸收峰处，100 Hz的重复频率下，放大器输出单脉冲能量16.1 mJ，脉宽221.3 ns，线宽3.87 MHz。2023年，该课题组[70]采用具有双角立方体结构的Ho:YAG种子激光器和从属激光器，在100 Hz的重复频率下可获得7.3 mJ的单脉冲能量，脉宽161.2 ns，经放大器放大后能量可达33.3 mJ，线宽4.12 MHz。
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NPRO由单块特殊加工的激光晶体、泵浦源和外加磁场组成，其结构如图8所示。整块增益介质即是激光谐振腔，通过外加磁场导致的法拉第旋转效应和增益介质不同面的反射特性来实现激光的单向运转，从而消除空间烧孔效应，输出单纵模激光[71-73]。由于构成NPRO激光器的激光晶体必须具有磁光效应，所以目前主要使用的是YAG晶体。
图 8 NPRO结构示意图
Figure 8. Structure diagram of NPRO
NPRO激光器结构紧凑，具有高稳定性，在2 μm波段可实现线宽MHz量级、重频kHz量级的单纵模激光输出。因此，NPRO激光器常作为种子源用于种子注入系统[74]。此外，通过控制NPRO激光器的晶体温度可以实现激光频率调谐，但调谐速率慢且范围较窄。
2012年，哈尔滨工业大学的Dai等[75]采用Ho:YAG NPRO作为种子，注入到Ho:YAG激光放大器中获得11 mJ的单脉冲能量，脉冲宽度110 ns，重频110 Hz，线宽4.8 MHz，光束质量在x和y方向上分别为1.09和1.04。
2016年，北京理工大学的Gao等[76]使用NPRO种子注入技术实现了Ho:YAG陶瓷激光器的单纵模输出。以140 mW的2.09 μm单纵模Ho:YAG NPRO作为种子激光器，采用“ramp-hold-fire”谐振探测技术，最终在200 Hz重频下得到最大输出能量14.76 mJ，脉冲宽度121.6 ns，线宽3.84 MHz。2018年，Zhang等[77]同样采用Ho:YAG NPRO作为种子，实现了一个高重频单纵模Ho:YAG MOPA系统，在1.25 kHz的重复频率下获得13.76 mJ的最大能量输出，脉宽178.9 ns，线宽2.65 MHz，光束质量因子在x和y方向上分别为1.16和1.20，此高重频单纵模Ho:YAG MOPA系统将成为相干多普勒测风激光雷达的理想光源。
2023年，哈尔滨工业大学的Yan等[78]使用2090.6964 nm的Ho:YAG NPRO作为种子，从属激光器由两个角立方体构成，如图9所示。在100 Hz的重复频率下可获得6.8 mJ的单脉冲能量，脉冲宽度166 ns。Ho:YAG单通放大器可将能量放大到32.3 mJ，线宽2.84 MHz，输出单纵模激光具有高稳定性。
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DFB半导体激光器通过在半导体激光器中放置布拉格光栅来实现单纵模输出，输出激光线宽可以达到MHz量级，十分适合用于种子注入以及雷达系统的集成[20]。而2 μm的DFB半导体激光器常作为种子源，主要用于测量CO2气体的相干差分吸收激光雷达（coherent differential absorption lidar，CDIAL）中[79]。
2015年，法国国家科研中心动力气象实验室的Gibert等[79]设计了一种如图10所示的种子注入单纵模Ho:YAG脉冲激光系统。以DFB半导体激光器为种子源，重频2 kHz时，单脉冲能量达10 mJ，脉宽40 ns，线宽10 MHz。在相干差分吸收激光雷达中使用此激光系统来测量大气CO2吸收系数，能够显著提高此类测量的时间和空间分辨率。
2020年，中国科学院上海光学精密机械研究所的Chen等[80]设计了一种基于Tm:Ho:LuLiF4的2 µm单纵模激光系统，种子激光器由DFB半导体激光器和三级放大器组成，以10 Hz的重复频率输出100 µJ脉冲，线宽小于0.05 nm。通过六级双通放大结构后，在2051.9 nm的中心波长下获得了5.6 mJ的放大脉冲能量，脉冲宽度为429.7 ns，重频10 Hz，使用外差法检测到的激光光谱线宽为1.24 MHz。
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种子注入技术因其选模精度高、输出模式好、性能稳定、谱线宽度窄等特点，被视为获得高能量单纵模窄脉冲激光的重要技术手段，己成为单纵模激光领域的研究热点之一[81-82]。表5总结了近年来基于种子注入技术的2 μm全固态脉冲激光器的输出特性，可以看出，最高330 mJ的单脉冲能量是由环形腔激光器作为种子源所实现的[67]，而DFB半导体激光器种子源则实现了2 kHz的最高重复频率、40 ns的最窄脉宽[79]以及1.24 MHz的最窄线宽[80]，由此可以看出DFB半导体激光器作为种子源的巨大优势。但种子注入的结构及控制系统比较复杂，因此应当根据实际使用情况选择合适的种子源和种子注入技术[83-84]。
表 5 种子注入2 μm单纵模全固态脉冲激光器输出特性
Table 5. Output characteristics of 2 μm single-longitudinal-mode all-solid-state pulsed laser with injection-seeded
Year Institution Wavelength /nm Repetition rate Energy /mJ Pulse width /ns Linewidth /MHz 1997[56] NASA Langley Research Center 2050 10 Hz 35 400 - 1998[57]
2011[67]NASA Langley Research Center
Council for Scientific and Industrial Research, South Africa2050
20646 Hz
50 Hz125
210170
350-
-2012[58] Harbin Institute of Technology 2130.7 100 Hz 2.8 289 4.5 2012[61] Harbin Institute of Technology 2090.9 100 Hz 7.6 132 3.5 2012[74] Harbin Institute of Technology 2090 110 Hz 11 110 4.8 2013[85] Harbin Institute of Technology 2118 100 Hz 8 151 3.7 2013[68] Council for Scientific and Industrial Research, South Africa 2064 60 Hz 330 - - 2015[79] French National Centre for Scientific Research 2050 2 kHz 10 40 10 2016[76] Beijing Institute of Technology 2090.2912 200 Hz 14.76 121.6 3.84 2017[86] Beijing Institute of Technology 2100 200 Hz 44 113 3.98 2018[39] National Institute of Information and Communications Technology, Japan 2064 200 Hz 21 150 - 2018[66] Harbin Institute of Technology 2050.967 100 Hz 4.4 65 4.1 2018[77] Beijing Institute of Technology 2090 1.25 kHz 13.76 178.9 2.65 2019[69] Harbin Institute of Technology 2064.414 100 Hz 16.1 221.3 3.87 2020[80] Shanghai Institute of Optics and Fine Mechanics 2051.9 10 Hz 5.6 429.7 1.24 2020[87] Harbin Institute of Technology 2064.414 100 Hz 24.2 250 2.81 2023[77] Harbin Institute of Technology 2090.6964 100 Hz 32.3 166 2.84 2023[70] Harbin Institute of Technology 2096.667 100 Hz 33.3 161.2 4.12 -
文中结合2 μm单纵模全固态脉冲激光器在各领域的应用，对环形腔、扭转模腔、VBG以及种子注入方式的2 μm单纵模全固态脉冲激光技术研究进展进行了综述。目前利用环形腔法已经可以实现焦耳量级的单脉冲能量[34]，利用VBG法可以实现百kHz的重复频率[47,49]，利用种子注入法可以实现亚MHz量级的线宽[80]。近年来，激光泵浦、单纵模选择、以及高能量激光脉冲等激光技术均取得了显著进步，2 μm单纵模全固态脉冲激光器正朝着高稳定、高效率、窄线宽、大能量的方向快速发展。
然而，在当前2 μm单纵模全固态脉冲激光技术发展的过程中，仍存在一些问题亟待解决：首先，激光器通常需要复杂的放大光路来实现大能量激光输出，而环形腔和种子注入单纵模激光器本身结构就较为复杂，这使得激光器装置庞大，不够紧凑和便携。其次，目前2 μm单纵模全固态脉冲激光器的输出脉冲宽度仍较宽，限制了其在激光雷达等应用中的使用。最后，由于Tm3+、Ho3+均为准三能级系统，受到再吸收和上转换效应的影响，会导致较大的热效应问题，尤其是扭转模腔单纵模激光器对偏振态变化敏感，热效应对偏振态的影响会降低单纵模出模率，使得激光器的效率和稳定性下降。
针对此，国内外科研人员陆续开展了一些相关技术的研究，例如：利用具有紧凑结构的碟片激光器（disk laser），可以在更大的散热面积、更小的实验装置上实现高能量激光输出[88-89]；或者采用受激布里渊散射（Stimulated Brillouin Scattering，SBS）技术，在保持激光高能量单纵模的前提下获得更窄的脉宽[90−94]；抑或是采用有效的热管理方法来减弱热效应[95]，如使用液冷薄片激光器结构[96]、将液态金属作为热接触材料[97]、探索具有高热稳定性的新基质材料[98]、均匀化泵浦光强[99]。目前，在激光振荡器内引入非线性增益竞争也是固体激光器实现单纵模窄线宽激光运转的手段[100−102]，但是目前针对2 μm波段的研究鲜有报道，但未来有望成为拓展中红外激光的波长和获得高相干激光输出的重要方向。此外，在实际应用中，当单一选模技术无法有效实现单纵模输出时，可以采用多种纵模选择技术结合的方式进行纵模选取。
相信在不久的未来，在日益增长的应用需求推动下，随着纵模选择、脉宽压缩、热管理等技术的不断发展以及对新型增益介质和激光器结构的不断探索，2 μm单纵模全固态脉冲激光技术将在更广阔的领域中实现更高的应用价值。
Advances in 2 μm single-longitudinal-mode all-solid-state pulsed lasers (cover paper·invited)
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摘要: 2 μm激光属于人眼安全波段，具有高大气透过率和水吸收特性，能覆盖CO2等温室气体的吸收峰，因此在大气环境监测、光通信、激光雷达、材料加工、医疗手术等领域有广泛应用。其中，单纵模运转的全固态2 μm脉冲激光器以其高稳定性、窄线宽和优良的光束质量等优势，可作为多普勒测风、相干差分吸收等激光雷达应用的优质光源，在工业、国防和科研等领域具有重要意义。目前，实现2 μm激光输出的主要方法有光参量技术和直接泵浦法。相比光参量技术，直接泵浦法更具高效率、可调节性和集成性等优点，已成为2 μm全固态激光的主流方式。文中总结了常用的2 μm固体激光增益介质，分析了空间结构振荡器单纵模选择的原理和特点，综述了2 μm单纵模全固态脉冲激光的研究进展，并对不同结构激光器的输出特性进行了比较，最后对2 μm单纵模全固态脉冲激光的发展前景进行了展望。Abstract:
Significance The 2 µm single-longitudinal-mode (SLM) all-solid-state pulsed laser has attracted much attention for its applications in lidar, gas monitoring, laser medicine, material processing and scientific research, owing to its high stability, narrow spectral linewidth and other advantages. For instance, the 2 µm SLM laser features high atmospheric transmittance and eye-safety, making it an ideal emission source for Doppler wind lidar. Moreover, the 2 µm laser covers the absorption peaks of various gases such as H2O, CO2 and CH4, enabling it to be used as the emitter of differential absorption lidar for atmospheric greenhouse gas monitoring. By combining the 2 µm laser with other sensors, a comprehensive atmospheric environment monitoring system can also be established. In the field of material processing, the 2 µm laser can interact with many materials, greatly simplifying the processing steps. Furthermore, the 2 µm laser has diverse applications in medical surgery, such as tissue cutting, stone crushing and eye surgery. Through the characteristics of its working wavelength, the 2 µm laser can achieve precise tissue treatment, while reducing the damage to the surrounding tissue, offering a safer and more effective option for medical surgery. The 2 µm SLM all-solid-state pulsed laser also plays a vital role in the field of military defense. The 2 µm laser output can be obtained by using nonlinear frequency conversion or directly pumping gain medium doped with Tm3+ or Ho3+. However, the linewidth of the 2 µm laser output generated by nonlinear frequency conversion is relatively wide, so it is extremely difficult to achieve SLM laser output. In contrast, compared with the nonlinear frequency conversion technique using 1 µm lasers as the pump source of optical parametric oscillators, Tm3+ or Ho3+ doped Q-switched lasers typically involve using a special resonator design or introducing mode selection elements, which have more compact structure and higher stability in achieving a 2 µm SLM pulsed laser. With the significant development of laser technologies such as laser pump technology, single longitudinal mode selection technology, and high energy laser pulse technology, the 2 µm SLM all-solid-state pulsed laser is developing towards smaller size, better performance, and more stable output performance. In recent years, researchers at home and abroad have designed and fabricated various 2 µm SLM all-solid-state pulsed lasers. According to the specific application scenario, the most suitable SLM selection scheme is chosen, and researchers have obtained 2 µm SLM pulsed lasers with different characteristics and successfully applied them to several fields. However, there are still some technical challenges to be overcome in the development of the current 2 µm SLM all-solid-state pulsed laser technology. In this paper, the common 2 µm single-mode all-solid-state pulsed laser technologies with the ring cavity, twisted-mode cavity, volume Bragg grating and injection-seeded method are analyzed and summarized. Progress This paper reviews the research progress of 2 µm SLM all-solid-state pulsed laser technology, in conjunction with its applications across various fields. It introduces the working principles and characteristics of SLM selection techniques such as the ring cavity, twisted-mode cavity, volume Bragg grating, and injection-seeded method. The laser output characteristics of different structures, including central wavelength, output energy, pulse width, full width at half maximum (FWHM) of the spectrum, pulse repetition rate, and beam quality factor, are summarized based on different SLM selection techniques. The results indicate that the 2 µm SLM all-solid-state pulsed laser has made significant strides in single pulse energy, spectral line width, and stability. It can achieve high-energy SLM laser output with a line width on the order of MHz and pulse repetition frequency on the order of kHz. However, the output pulse width remains wide (on the order of nanoseconds), the structure is complex, and the thermal effect is pronounced. Finally, the paper analyzes the current technical bottlenecks, provides corresponding solutions, and prospects the future development of 2 µm SLM all-solid-state pulse lasers. Conclusions and Prospects Driven by the escalating demand for practical applications, 2 µm SLM all-solid-state pulsed lasers are evolving rapidly towards miniaturization, enhanced stability, high efficiency, narrow spectral linewidth, and substantial output energy. Future development trends are expected to focus on further advancements in output performance and the exploration of innovative methods for realizing 2 µm SLM all-solid-state pulsed lasers. Moreover, with the progression of laser technologies such as longitudinal-mode selection, pulse width compression, and thermal management, coupled with the continuous exploration of new gain media and laser structures, the comprehensive performance of 2 µm SLM all-solid-state pulse lasers is anticipated to be further improved to cater to diverse application requirements. -
Key words:
- all-solid-state laser /
- 2 μm /
- pulse /
- single-longitudinal-mode (SLM) /
- mode selection
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图 2 结构示意图。(a) 环形腔；(b) 扭转模腔
Figure 2. Structure diagram. (a) Ring cavity; (b) Twisted-mode cavity
表 1 常见2 μm固体激光增益介质光谱特性
Table 1. Spectral characteristics of commonly used gain media for 2 μm solid-state laser
Crystal Typical absorption wavelength/
nmAbsorption section/
cm2Typical emission wavelength/
nmEmission section/
cm2Fluorescence lifetime/
msTm:YAG 785 7.5×10−21 2 014 2.9×10−21 11 Tm:YAP 795 3.7×10−21-8.5×10−21 1 940, 1 980 5×10−21-6×10−21 4.4-7.7 Tm:YLF 792 5.5×10−21 1 908 2.3×10−21 16 Ho:YAG 1 908 1.09×10−20 2 090 1.14×10−20 7 Ho:YLF 1 940 1.2×10−20 2 060 1.8×10−20 10 
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表 2 环形腔2 μm单纵模全固态脉冲激光器输出特性
Table 2. Output characteristics of 2 μm single-longitudinal-mode all-solid-state pulsed laser with ring cavity
Year Institution Wavelength/nm Repetition rate Power/W Energy Pulse width/ns 2006[34] NASA Langley Research Center 2053 - - 1.1 J - 2010[35] NASA Langley Research Center 2053 5 Hz 1.25 250 mJ - 2014[35] NASA Langley Research Center 2050.967 100 Hz - 40 mJ 32 2017[37] Harbin Institute of Technology 2063.8 - 3.73 - - 2017[41] Harbin Institute of Technology 2053.9 - 0.941 - - 2019[38] Harbin Institute of Technology 2052.96 67 kHz 1.67 24.9 μJ 910
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表 3 扭转模腔2 μm单纵模全固态激光器输出特性
Table 3. Output characteristics of 2 μm single-longitudinal-mode all-solid-state laser with twisted-mode cavity
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表 4 VBG法2 μm单纵模全固态脉冲激光器输出特性
Table 4. Output characteristics of 2 μm single-longitudinal-mode all-solid-state pulsed laser with VBG
Year Institution Wavelength/nm Repetition rate/kHz Energy Pulse width Linewidth 2015[47] Northwest University 1988.8 96.2 7.5 μJ 2.2 μs 4.2 MHz 2018[48] Harbin Institute of Technology 2100.5 3 1.9 mJ 54 ns - 2019[49] Harbin Institute of Technology 2100.5 100 63.3µJ 3.6 ns - 2020[50] Paris-Saclay University 1960 1 230 µJ 50 ns <4 pm
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表 5 种子注入2 μm单纵模全固态脉冲激光器输出特性
Table 5. Output characteristics of 2 μm single-longitudinal-mode all-solid-state pulsed laser with injection-seeded
Year Institution Wavelength /nm Repetition rate Energy /mJ Pulse width /ns Linewidth /MHz 1997[56] NASA Langley Research Center 2050 10 Hz 35 400 - 1998[57]
2011[67]NASA Langley Research Center
Council for Scientific and Industrial Research, South Africa2050
20646 Hz
50 Hz125
210170
350-
-2012[58] Harbin Institute of Technology 2130.7 100 Hz 2.8 289 4.5 2012[61] Harbin Institute of Technology 2090.9 100 Hz 7.6 132 3.5 2012[74] Harbin Institute of Technology 2090 110 Hz 11 110 4.8 2013[85] Harbin Institute of Technology 2118 100 Hz 8 151 3.7 2013[68] Council for Scientific and Industrial Research, South Africa 2064 60 Hz 330 - - 2015[79] French National Centre for Scientific Research 2050 2 kHz 10 40 10 2016[76] Beijing Institute of Technology 2090.2912 200 Hz 14.76 121.6 3.84 2017[86] Beijing Institute of Technology 2100 200 Hz 44 113 3.98 2018[39] National Institute of Information and Communications Technology, Japan 2064 200 Hz 21 150 - 2018[66] Harbin Institute of Technology 2050.967 100 Hz 4.4 65 4.1 2018[77] Beijing Institute of Technology 2090 1.25 kHz 13.76 178.9 2.65 2019[69] Harbin Institute of Technology 2064.414 100 Hz 16.1 221.3 3.87 2020[80] Shanghai Institute of Optics and Fine Mechanics 2051.9 10 Hz 5.6 429.7 1.24 2020[87] Harbin Institute of Technology 2064.414 100 Hz 24.2 250 2.81 2023[77] Harbin Institute of Technology 2090.6964 100 Hz 32.3 166 2.84 2023[70] Harbin Institute of Technology 2096.667 100 Hz 33.3 161.2 4.12
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