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激光自20世纪问世以来，以其卓越的方向性、相干性、单色性及高亮度等特性得到人们的广泛关注，而激光技术的发展也极大地推动了科学研究、国防、医疗、工业等诸多领域和行业的革新。波长是激光光束的重要参数之一，对激光器的应用起到至关重要的作用，例如，可见光波段激光常用于数据存储、光通信和新一代显示技术等领域[1-5]，近红外波段激光常用于激光武器、临床手术、精密加工和加密通讯等领域[6-10]，中长波红外激光波段常用于大气光谱检测和红外光电对抗[11-15]。当然，激光的应用通常并不局限于某一个特定的波段，如大气透过窗口在紫外、可见光和红外波段均有所覆盖[14,16]，针对不同材料和加工需求也通常采用不同波长的激光[17-18]；在某些特定领域的应用中，有时也需要激光工作在某个特定波长且具有满足特定的线宽，如钠导星需要589 nm波长的窄线宽激光[19-20]。
近年来，激光技术的发展不断推动医学眼底治疗、精密测量、雷达等方面逐步实现质的飞跃，单一波长激光器已难以满足更高的应用需求，多波长激光光源研制和应用已逐渐成为研究的热点（图1）[21-31]。在医学领域，多波长激光治疗仪可针对眼底病变的程度与范围进行分区治疗 [9,22,32]。在通信领域，波分复用技术的快速发展通常要求产生光源具有波长间隔小、线宽窄、功率谱平坦等特点，其中较为典型的光源就是掺饵多波长激光器[33-34]。在科研领域，利用非线性光学晶体将两个波长相近的激光进行混频成为产生太赫兹波的重要技术路径之一，由于需要进行相位锁定，因此利用单一光源直接产生双波长激光的稳定性要优于采用两台独立波长的激光器[35-36]。此外，多波长激光器常用于差分吸收雷达、干涉彩虹全息等，同时也在军事、智能装备等领域有着重要应用[37-40]。
常见实现多波长激光输出的方法包括：利用工作物质固有的多条发射谱线直接实现多波长辐射、利用光栅等器件将不同波长激光进行光束合成、通过非线性光学频率变换将单一波长拓展至若干个波长等。利用工作物质直接辐射包括通过激活离子不同能带之间的跃迁产生大频率间隔的多波长激光，或通过相同能带不同能级间的跃迁产生小频率间隔的多波长激光[41]。为克服工作物质的增益竞争，往往需要控制谐振腔中不同跃迁谱线的损耗以达到目标波长的激发阈值，如调节腔镜的透过率、谐振腔内损耗 (如插入标准具、滤光片等）、改变受激发射截面的大小等。人们现已实现546 nm/550 nm[42]、1061 nm/1064 nm[43]、1074 nm/1112 nm[44]、1112 nm/1116 nm/1123 nm[45]以及2064 nm/2066 nm[46]等不同组合的多波长激光。但是不同发射波长之间增益竞争的存在，使得基于单一工作物质的多波长激光器的输出参数往往难以实现理想调控。光束合成技术是将若干独立输出的不同波长激光通过分光或衍射等器件合成一束激光，其特点在于参与合成的各光源参数独立控制，避免了单一工作物质实现多波长运转出现的增益竞争[47-50]。近年来，人们已经实现了诸多不同频率和波长间隔的高功率光束合成输出[51-53]。光束合成技术的难点在于系统结构较为复杂，且通过空间组合的方式难以实现多波长光束的同轴输出。非线性光学频率变换技术可获得丰富波长的辐射，是目前获得多波长激光输出的重要途径，常见的手段包括以倍频、和频和光参量振荡器（OPO）等为代表的二阶非线性效应[54-57]，以及受激拉曼散射（SRS）和受激布里渊散射（SBS）等为代表的三阶非线性效应[58-61]。对比二阶非线性效应，SRS和SBS不受相位匹配条件的限制，因此只要工作波长在材料的透明区域并达到激发阈值即可工作，结合SRS和SBS固有的级联频率转换特性，拉曼和布里渊激光器能够在一个增益介质中产生若干频率的Stokes光输出[60,62-65]。同时，SRS和SBS固有的相位共轭特性使其能够实现高亮度高光束质量的输出[66-68]。此外，SRS的频移量通常比SBS高两三个数量级（以晶体材料为例），即拉曼激光器的波长跨度范围大，因此，基于SRS的拉曼激光器通过光学镀膜即可实现对不同波长激光参数的增益和损耗控制。综上，拉曼激光器在实现多波长激光方面具有显著的优势，其中基于晶体拉曼转换的多波长激光器以其结构紧凑、性能稳定等优点成为人们关注的重点。
文中介绍了拉曼激光器的工作原理，归纳了自由空间拉曼振荡器的常见结构和典型的拉曼晶体特性，综述了基于晶体拉曼转换的多波长激光技术发展现状，拟通过该文为开展多波长激光技术研究和应用的人员提供有价值的参考。
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当以高强度的相干激光入射拉曼增益介质时，激发增益介质内部分子的振动，导致相干光被散射的同时产生受激光学声子（SBS产生的为声学声子），受激光学声子将继续参与相干散射过程并极速增加所形成的一种雪崩过程，在该过程中产生的具有相同频率的相干散射光即为SRS，其过程如图2所示[69-71]。
图 2 SRS激发过程示意图
Figure 2. Schematic diagram of SRS excitation process
SRS作为一种非弹性弹射也可实现从一阶到级联的频率变换，即当一束光入射拉曼增益介质并达到激发阈值时，首先会产生与入射光存在一定频率差的Stokes光，即一阶Stokes光；当一阶Stokes光功率密度不断增加并达到下一阶阈值时，会激发二阶Stokes光；以此类推，随着功率密度不断上升，可逐级形成三阶、四阶乃至更高阶次的Stokes光，在此过程中通常低阶的Stokes光会维持稳态。级联SRS过程示意图如图3所示[35,71]，对于拉曼转换，其频率满足ωs1=ωp−ωv，其中ωs1、ωp、ωv分别为一阶Stokes光、泵浦光和分子振动的频率。因此，利用谐振腔所造成的光能量损耗设计输入/输出镜对各个阶次光的反射率，可实现多阶次Stokes光即多波长激光输出，即二阶Stokes光为ωs2= ωs1 − ωv，三阶Stokes光为ωs3 =ω s2−ωv 。
图 3 级联受激拉曼散射过程示意图
Figure 3. Schematic diagram of the cascaded SRS process
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拉曼激光器的输出波长是由泵浦光波长和拉曼增益介质的固有频移决定的，即在拉曼增益介质的透过光谱范围内，通过改变泵浦波长即可获得不同波长的一阶拉曼光输出，进而可以结合级联SRS实现多波长输出。但是，受限于拉曼增益介质有限的长度，通常需要借助振荡器这一载体实现拉曼激光器的波长选择和转换效率提升。值得一提的是，拉曼激光器中不存在粒子数反转激光器中的“空间烧孔”效应，因此，在波长拓展的同时还可通过优化设计实现窄光谱或单纵模运转[72-75]。
拉曼激光器宏观上可根据激光工作物质与拉曼增益介质是否在同一个振荡器中，分为内腔拉曼激光器和外腔拉曼激光器，而当振荡器中的激光工作物质与拉曼增益介质为相同材料时，通常将其定义为自拉曼激光器[76]。图4展示了几种典型的拉曼振荡器结构，由于笔者综述内容主要针对块状的晶体材料，故讨论的结构和研究进展不包含光纤、波导、片上等导波拉曼激光器。
图 4 典型拉曼激光振荡器的结构示意图。（a）外腔型；（b）内腔型；（c）自拉曼
Figure 4. Schematic diagram of typical Raman lasr oscillator structure. (a) External cavity; (b) Intracavity; (c) Self-Raman
图4(a)为外腔型拉曼激光器的结构示意图，该结构中的激光工作物质与拉曼增益介质均独立成腔，其优点是拉曼振荡器的泵浦光不受激光振荡器的制约，无论在拉曼增益介质选择、腔型结构设计还是系统热管理方面都更加灵活，且输出参数可控性强。图4(b)为内腔型拉曼激光器的结构示意图，激光晶体与拉曼晶体介质放置在同一个振荡器内，该结构的特点在于腔内的高功率密度有助于拉曼增益介质充分利用泵浦功率，因此相对于外腔拉曼振荡器具有更高的拉曼转换效率，且结构紧凑。但是内腔型拉曼也面临着结构设计复杂、振荡器腔镜镀膜难度高、系统热管理复杂等问题。图4(c)为自拉曼激光器的结构示意图，该结构中的材料需要兼具激光工作物质与拉曼增益介质的功能，因此需要具有较高的拉曼增益系数，常见的自拉曼晶体材料包括Nd:YVO4、Nd:GdVO4等。该结构的特点在于可大幅缩短腔长、结构更加紧凑，但是也面临着材料热负载严重、输出功率不高、输出参数单一且难以控制的瓶颈。
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目前，人们利用气态、液态和固态介质均已实现拉曼激光运转。气态拉曼介质往往是分子型活性材料，常见有氢气（H2）、氧气（O2）、氮气（N2）、甲烷（CH4）和二氧化碳（CO2）等气体[77-81]。气态拉曼介质具有拉曼频移高、透过谱宽、光学均匀性好且成本低等特点。但是，气体的使用存储过程往往需要高压密闭装置且介质热导率通常较低。常见的液态受激拉曼介质有苯、乙醇、水、二硫化碳等[82-85]。液态拉曼介质具有拉曼增益较高、抗光损伤阈值高、拉曼频移丰富、光谱宽等特点。但是，液态拉曼介质往往也存在挥发性、部分有毒性、分子不稳定等固有缺陷。固态拉曼介质目前应用较为广泛，近年来拉曼晶体介质展现出散热性好、透过光谱宽、热导率高、拉曼增益高等优点，因此，越来越多基于拉曼晶体的激光器被研发与应用[61]。目前，常用的拉曼晶体介质包括金刚石[86-88]、硝酸盐[89-90]、钨酸盐[91-95]、钒酸盐[96-98]等。表1列出了几种典型固体拉曼晶体介质及其特性。相较于气态/液态介质，拉曼晶体不仅具有拉曼频移量大、热导率高、输出光束质量良好等显著优势，而且拉曼晶体中高度对称排列的原子和分子使得拉曼晶体抑制谱线加宽能力强，抗干扰能力强。
表 1 室温下典型拉曼晶体特性
Table 1. Properties of typical Raman crystals at room temperature
Material Frequency shift/
cm−1Raman gain coefficient/
cm·GW−1Thermal conductivity/
W·m−1·K−1Thermal expansion/
×10−6 K−1Transmission range/
μmDiamond 1332.3 ~15 2 000 1.1 >0.23 Ba(NO3)2 1047.3 11 1.17 13 0.35-1.8 BaWO4 924/332 8.5 3 - 0.25-5.1 SrWO4 921 5.0 2.9 - 0.3-2.7 KGW 89/901/768 3.5 2.6 4.0 0.34-5.5 YVO4 259/376/816/838/890 4.5 5.2 4.43 0.4-5.0 GdVO4 885 4.5 10.5 1.5 0.35-5.0 LiIO3 822/770 4.8 4 28 0.31-4.0 KY(WO4)2 905/765 3.6 3.3 1.83 0.35-5.5 Silicon 521 - 153 3 >1.1 -
综上，根据激光工作物质与拉曼增益介质的相对位置不同将拉曼激光器分为内腔、外腔和自拉曼激光器三种。为了更好地实现其细分，文中结合具体的空间腔型结构将多波长拉曼激光器分为线形腔、环型和折叠型腔、微型腔三类，并对其发展现状进行总结。
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线形腔结构是最常见的一种实现多波长输出的谐振腔型，其特点在于引入的光学元件少，腔内等效热效应处理难度低，有利于提高输出光功率及光-光转换效率和实现小型化。
2012年，山东大学Shen H.等人[99]利用BaWO4晶体作为拉曼增益介质，通过激光二极管（LD）侧面泵浦声光调Q的内腔拉曼振荡器结构，实现了一阶1180 nm和二阶1325 nm 的双波长级联拉曼输出，在重复频率为15 kHz时，获得了双波长最大输出功率分别为8.30 W和2.84 W、脉冲宽度分别为20.5 ns和5.8 ns，作者在实验中观察到了锁模脉冲, 实验装置见图5。2012年，江苏师范大学的Huang H.等人[100]利用KTP晶体与KTA晶体作为拉曼增益介质，基于LD端面泵浦Nd:YAG/Cr4+:YAG键合晶体的内腔拉曼振荡器，实现了1091 nm和1095 nm正交偏振双波长输出，对应双波长的最大输出功率分别为170 mW和150 mW，脉冲宽度为3.3 ns，重复频率为11.2 kHz。2014年，山东大学的Zhang H.等人[101]实现了LD端面泵浦主动调Q的Nd:YAG/BaWO4内腔拉曼激光器的双波长输出，一阶Stokes光和二阶Stokes光的波长分别为1240 nm和1376 nm，重复频率为10 kHz时获得的最大输出功率分别为869 mW和512 mW。2015年，台湾交通大学Huang H.J.等人[102]利用KTP和KTA晶体作为拉曼增益介质，基于LD端面泵浦Nd:YAP的声光调Q内腔拉曼振荡器结构，分别实现了1478 nm和1503 nm双波长，以及1474 nm和1480 nm的双波长人眼安全激光输出。2016年，中国科学院大学的Sun Y.等人[103]用KGW晶体中768 cm−1和901 cm−1的两个正交偏振拉曼偏移，经旋转 Yb:GAB 激光晶体90°分别实现1133.1、1156.6 nm和1 137.8 、1 151.9 nm的正交偏振双波长输出。2020年，暨南大学Tu Z.等人[104]基于主动调Q的Nd:YLF/KGW的内腔拉曼振荡器结构，经旋转KGW晶体90°分别实现波长1470、1490 nm和1461、1499 nm的正交偏振双波长激光输出。2020年，温州大学的Duan Y.等人[105]报道了一台声光调Q的Nd:YAP/YVO4级联拉曼激光器，结合BBO晶体角度调谐，实现了539.9、567.2、597.4、631.0、668.5 nm五种波长的激光输出，该方案为实现多波长可切换激光输出提供了一种新思路。2020年，扬州大学樊莉等人[106]设计了一款Nd:YVO4/BaWO4连续波多波长拉曼激光器，利用BaWO4晶体中的925 cm−1和332 cm−1的频移量和YVO4晶体中的890 cm−1频移，获得1103.6 nm、1175.9 nm和1180.7 nm的三个一阶Stokes光和1145.7 nm和1228.9 nm的两个二阶Stokes光的输出。
金刚石作为一种具有超高热导率和极宽光谱透过范围的拉曼晶体，在实现高功率多波长激光输出方面具有显著优势[107-110]。2014年，澳大利亚麦考瑞大学McKay A.等人[111]利用纳秒脉冲泵浦外腔金刚石拉曼振荡器，在36.5 kHz脉冲重复频率泵浦时产生总功率14.5 W的1240 nm一阶和1485 nm二阶拉曼激光输出。2021年，河北工业大学的白振旭等人[24]报道了一台可实现1.2 μm和1.5 μm双波长输出的百瓦级外腔金刚石拉曼激光器，1.2 μm和1.5 μm的稳态功率分别为72 W和110 W，且输出的光谱相对于泵浦光的均出现一定的窄化，实验装置如图6所示。近期，该团队研制了一台532 nm绿光泵浦的多波长级联金刚石拉曼激光器，通过将一阶Stokes 黄橙光(573 nm)锁定在振荡器中，实现了620 nm、676 nm 和743 nm的级联拉曼激光输出，对应三个波长的脉冲宽度分别为10.41 ns、3.75 ns和2.45 ns，总峰值功率为70.7 kW。
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非线形腔结构主要包括环形腔和折叠腔两种，此类腔型由三个及以上的腔镜组成，其区别在于折叠腔与线形腔均为驻波腔，而环形腔内的光束为行波传输。环形腔的特点是易于实现光束的单向传播，且便于形成腔增强结构以提高振荡器内的功率密度[112-114]；折叠腔典型结构包括“V”字型、“Z”字型腔等，其相比于线形腔具有腔内模式设计灵活、易于进行双端泵浦等[115-117]。
2014年，澳大利亚麦考瑞大学的Warrier A.M.等[118]报道了一台同步泵浦的1240 nm和1485 nm皮秒金刚石拉曼激光器，实现了1240 nm一阶Stokes光功率2.75 W的输出，并通过结合四波混频和单通SRS获得了1485 nm的二阶Stokes光功率为1.0 W的输出。2020年，麦考瑞大学的Li M.等人[119]报道了一台可调谐钛宝石连续波激光器谐振泵浦的金刚石拉曼环形腔激光器，获得了波长为964.9 nm的一阶Stokes光和1101.3 nm的二阶Stokes光，并实现了单纵模的运转，实验装置如图7所示。随后，笔者利用数学模型表征了高阶Stokes系统的损耗和斜效率，提出二阶Stokes光输出功率可通过改进光束的传播方向来优化[120]。2021年，捷克布拉格技术大学的Frank M.等人[121]利用混合掺杂的Pb(MoO4)0.2(WO4)0.8作为拉曼晶体，结合环型谐振腔结构，实现了在1128~1360 nm光谱范围内的12个短波长间隔的激光输出，并提出优化拉曼晶体的掺杂，腔镜的反射率有望提高多波长转换效率。
2010年，麦考瑞大学的Eduardo G.等人[122]报道了一台级联连续波锁模KGW拉曼振荡器，利用脉冲宽度为28 ps的532 nm激光作为泵浦源，基于“Z”型腔结构，实现了一阶559 nm和二阶589 nm的拉曼转换输出，对应脉冲宽度为6.5 ps和5.5 ps，功率分别为2.5 W和1.4 W。2011年，英国思克莱德大学Parrotta D. C.等人[123]利用LD泵浦InGaAs的半导体圆盘激光器为金刚石拉曼激光器提供泵浦，实现了波长1217~1244 nm范围内的可调输出，其中波长1227 nm时的一阶Stokes光输出功率为1.3 W。虽然该研究并非真正意义的多波长拉曼激光器，但是作者验证了基于晶体拉曼实现连续可调波长变换的可行性，对后续折叠腔实现多波长激光具有一定的参考意义，后续国内外多个团队围绕波长可调的晶体拉曼激光器开展了相关研究工作[74,87,124]。在2018年，英国思克莱德大学同一团队的Casula R.等人[125]研制了一台基于KGW晶体的多波长拉曼激光器，通过旋转放置在谐振腔内的双折射滤光片，最终实现1.32、1.50、1.73 μm的三波长级联Stokes 光输出，每个波长功率均实现了瓦级输出，实验装置如图8所示。
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微型谐振腔结构是由不同透过率的输入/输出耦合镜直接与晶体紧密接触形成的激光器结构。微型谐振腔多波长激光器具有腔长短、易于实现短脉冲等优点。由于参与键合的晶体往往需要是相同或相近的基质，因此目前基于微腔的多波长拉曼激光器主要是利用YVO4晶体与激光晶体键合。
2016年，厦门大学Wang X. L.等人[35]报道一台LD泵浦Yb:YAG/Nd:YVO4多波长连续波微片拉曼激光器，采用a切Nd:YVO4晶体作为拉曼转换介质，实现了1.05 μm和1.08 μm的双波长激光输出。2018年，该团队[126]利用Nd:GdVO4/Cr4+:YAG/ YVO4拉曼微片激光器实现了1164.4 nm和1174.7 nm的同步脉冲双波长输出，双波长激光脉冲宽度为825 ps、峰值功率超过1 kW，实验装置如图9所示。
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表2总结了基于拉曼转换多波长激光器的参数。结合研究现状不难发现，线形腔仍是目前最常用的产生多波长拉曼激光的谐振腔结构，且围绕脉冲激光的研究占比最高；相比于内腔拉曼振荡器，外腔拉曼振荡器的平均和峰值功率更高，展现了更为强大的功率拓展性；微腔拉曼激光器目前输出功率与转换效率均较低，但其具有高重复频率与小型化等特点。
表 2 多波长拉曼激光器研究现状
Table 2. Research status of multi-wavelength Raman laser
Year Cavity type Cavity structure Pumping method Pump wavelength Raman crystal Output wavelength Output power Ref. 2012 Intracavity Linear cavity Pulsed 808 nm BaWO4 1180 nm
1325 nm8.30 W
2.84 W[99] 2012 Intracavity Linear cavity Pulsed 808 nm KTP/KTA 1091 nm
1095 nm170 mW
150 mW[100] 2014 Intracavity Linear cavity Pulsed 808 nm BaWO4 1240 nm
1376 nm869 mW
512 mW[101] 2014 External cavity Linear cavity Pulsed 1064 nm Diamond 1240+1485 nm 14.5 W [111] 2015 Intracavity Linear cavity Pulsed 808 nm KTP/KTA 1478 nm
1503 nm117 mW
389 mW[102] 2016 Intracavity Linear cavity Pulsed 976 nm KGW 1133+1156 nm
1137+1151 nm155 mW
154 mW[103] 2020 Intracavity Linear cavity Pulsed 808 nm KGW 1470+1490 nm
1461+1499 nm2.6 W
2.4 W[104] 2020 Intracavity Linear cavity Pulsed 804 nm YVO4 539.9+567.2+597.4+
631.0+668.5 nm800,340,460,
190,326 mW[105] 2020 Intracavity Linear cavity CW 879 nm BaWO4+YVO4 1103.6+1175.9+1180.7+
1145.7 +1228.9 nm1.24 W
(MAX)[106] 2021 External cavity Linear cavity QCW 1064 nm Diamond 1240 nm
1485 nm72 W
110 W[24] 2023 External cavity Linear cavity Pulsed 532 nm Diamond 620 nm
676 nm
743 nm12.5 kW*
40.8 kW*
17.4 kW*2010 External cavity Z-fold CW 1064 nm KGW 559 nm
589 nm2.5 W
1.4 W[122] 2014 External cavity Ring-cavity Pulsed 1064 nm Diamond 1240 nm
1485 nm2.75 W
1.0 W[118] 2020 External cavity Ring-cavity CW 845-930 nm Diamond 965 nm
1101 nm400 mW
364 mW[119] 2021 External cavity Ring-cavity Pulsed 1063 nm Pb(MoO4)0.2
(WO4)0.81128 nm
1360 nmwatt level [121] 2018 External cavity Z-fold CW 808 nm KGW 1.32 μm
1.50 μm
1.73 μm6.1 W
1.1 W
1.1 W[125] 2016 Intracavity Microcavity CW 808 nm Nd:YVO4 1.05 μm
1.08 μm260 mW [35] 2019 Intracavity Microcavity Pulsed 880 nm YVO4 1164 nm
1175 nm40 mW [126] *Peak power -
文中简述了SRS的工作原理、拉曼振荡器的基本结构以及拉曼晶体的特性，对基于晶体拉曼转换的多波长激光的研究现状进行了综述。虽然围绕晶体拉曼转换的多波长激光器的研究仅有十余年，但是诸多新晶体、新结构和新波长不断被探索，尤其近年来随着金刚石晶体等新晶体材料趋于成熟，使多波长拉曼激光器在功率提升、波长拓展和小型化等方面有着不凡的表现。笔者认为，为了全面提升多波长拉曼激光器的性能，今后的研究将主要围绕以下方面展开，即：优化泵浦光参数和优化振荡器设计提高转换效率，避免级联阶次的增加导致的量子效率降低；拓展多波长激光的输出光谱范围，实现波长和波段的参数可控输出；高功率运转下的热管理，提高系统的稳定性和光束质量；与结构光产生技术相结合，拓展多波长激光器的维度。未来，基于晶体介质的拉曼激光器有望成为多波长激光产生和应用的主力军。
Review of multi-wavelength laser technology based on crystalline Raman conversion (invited)
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摘要: 近年来，在光电对抗、激光雷达、精密测量、医疗等诸多应用的牵引下，能够同时或交替输出不同波长的激光器得到广泛关注，但是受到激光工作物质中激活粒子固有发射谱及其增益强度的限制，实现多波长激光的功率、波长和时频域的高效可控辐射具有较大难度。非线性光学频率变换技术是拓展激光波长的有效手段，具有系统灵活性强、波长调节范围宽和功率拓展性强等特点。作为一种三阶非线性光学效应，受激拉曼散射(SRS)通过介质内部的分子或晶格振动使入射的泵浦光产生一定的频移，结合其固有的放大、相位共轭、级联转换等特性，基于SRS的拉曼激光器在获得高功率、高光束质量、多波长激光输出中具有显著优势，尤其是以晶体作为拉曼增益介质的多波长激光器一直是激光领域研究的热点。文中介绍了SRS和级联拉曼转换的基本原理，归纳了典型晶体拉曼激光器的分类和基本结构，综述并讨论了基于晶体拉曼转换的多波长激光技术的研究现状。Abstract:
Significance Multi-wavelength lasers that can simultaneously or alternately output different wavelengths have various applications in optoelectronic countermeasures, LiDAR, and medical treatment. However, achieving controllable and efficient multi-wavelength laser radiation is challenging due to the limitations of the emission spectrum and intensity of the laser materials. Nonlinear optical frequency conversion technology, especially stimulated Raman scattering (SRS), is an effective way to expand the laser wavelength range and enhance the laser power. SRS is a third-order nonlinear optical effect that shifts the frequency of the pump through molecular or lattice vibrations in the medium. Raman lasers can obtain high-power, high-beam-quality, and multi-wavelength laser output by utilizing the characteristics of phase conjugation, amplification, and cascade conversion of SRS. This paper introduces the basic principles of SRS and cascaded Raman conversion, summarizes the classification and structure of typical crystal Raman lasers, and reviews the current status, challenges, and opportunities of multi-wavelength laser technology based on crystal Raman conversion. Progress The working principle of the stimulated Raman scattering (Fig.2) and the excitation principle of cascaded Raman scattering (Fig.3) are first outlined in this article. Then the basic structure of Raman lasers was discussed (Fig.4), which can be classified into intracavity and external cavity based on the location of the Raman gain medium relative to the laser working material. A special case of intracavity Raman lasers is self-Raman lasers, where the laser working material and the Raman gain medium are the same. Next, the characteristics of different types of Raman gain media, including gas, liquid, and solid are analyzed. Among them, Raman crystals are regarded as a promising medium for multi-wavelength lasers due to their advantages such as high gain, compact structure, and good stability. Typical crystal Raman gain media were compared and their parameters are summarized (Tab.1). Finally, the current research status of multi-wavelength crystalline Raman lasers as well as their features are summarized. Based on the above research status, it is not difficult to find that linear cavities are still the most commonly used resonant cavity structure for generating multi-wavelength Raman lasers, and pulse lasers account for the highest proportion of the research. In addition, compared to intracavity Raman oscillators, external cavity Raman oscillators exhibit higher average and peak power, demonstrating stronger power scalability. Although microcavity Raman lasers currently have low output power and conversion efficiency, they have the characteristics such as high repetition rate and miniaturization. Conclusions and Prospects In conclusion, research on multi-wavelength lasers based on crystalline Raman conversion has made significant progress in the past decade, with the discovery of new crystals, structures, and wavelengths. The use of new crystal materials such as diamond has led to a remarkable performance in power enhancement, wavelength expansion, and miniaturization of multi-wavelength Raman lasers. Future research should focus on optimizing pump parameters and oscillator design to improve conversion efficiency, expand multi-wavelength lasers' output spectral range, and improve thermal management under high-power operation to enhance system stability and beam quality. With these advancements, we can expect that multi-wavelength solid-state lasers based on crystalline Raman conversion will play a major role in future applications. -
图 4 典型拉曼激光振荡器的结构示意图。（a）外腔型；（b）内腔型；（c）自拉曼
Figure 4. Schematic diagram of typical Raman lasr oscillator structure. (a) External cavity; (b) Intracavity; (c) Self-Raman
表 1 室温下典型拉曼晶体特性
Table 1. Properties of typical Raman crystals at room temperature
Material Frequency shift/
cm−1Raman gain coefficient/
cm·GW−1Thermal conductivity/
W·m−1·K−1Thermal expansion/
×10−6 K−1Transmission range/
μmDiamond 1332.3 ~15 2 000 1.1 >0.23 Ba(NO3)2 1047.3 11 1.17 13 0.35-1.8 BaWO4 924/332 8.5 3 - 0.25-5.1 SrWO4 921 5.0 2.9 - 0.3-2.7 KGW 89/901/768 3.5 2.6 4.0 0.34-5.5 YVO4 259/376/816/838/890 4.5 5.2 4.43 0.4-5.0 GdVO4 885 4.5 10.5 1.5 0.35-5.0 LiIO3 822/770 4.8 4 28 0.31-4.0 KY(WO4)2 905/765 3.6 3.3 1.83 0.35-5.5 Silicon 521 - 153 3 >1.1 
下载: 导出CSV
表 2 多波长拉曼激光器研究现状
Table 2. Research status of multi-wavelength Raman laser
Year Cavity type Cavity structure Pumping method Pump wavelength Raman crystal Output wavelength Output power Ref. 2012 Intracavity Linear cavity Pulsed 808 nm BaWO4 1180 nm
1325 nm8.30 W
2.84 W[99] 2012 Intracavity Linear cavity Pulsed 808 nm KTP/KTA 1091 nm
1095 nm170 mW
150 mW[100] 2014 Intracavity Linear cavity Pulsed 808 nm BaWO4 1240 nm
1376 nm869 mW
512 mW[101] 2014 External cavity Linear cavity Pulsed 1064 nm Diamond 1240+1485 nm 14.5 W [111] 2015 Intracavity Linear cavity Pulsed 808 nm KTP/KTA 1478 nm
1503 nm117 mW
389 mW[102] 2016 Intracavity Linear cavity Pulsed 976 nm KGW 1133+1156 nm
1137+1151 nm155 mW
154 mW[103] 2020 Intracavity Linear cavity Pulsed 808 nm KGW 1470+1490 nm
1461+1499 nm2.6 W
2.4 W[104] 2020 Intracavity Linear cavity Pulsed 804 nm YVO4 539.9+567.2+597.4+
631.0+668.5 nm800,340,460,
190,326 mW[105] 2020 Intracavity Linear cavity CW 879 nm BaWO4+YVO4 1103.6+1175.9+1180.7+
1145.7 +1228.9 nm1.24 W
(MAX)[106] 2021 External cavity Linear cavity QCW 1064 nm Diamond 1240 nm
1485 nm72 W
110 W[24] 2023 External cavity Linear cavity Pulsed 532 nm Diamond 620 nm
676 nm
743 nm12.5 kW*
40.8 kW*
17.4 kW*2010 External cavity Z-fold CW 1064 nm KGW 559 nm
589 nm2.5 W
1.4 W[122] 2014 External cavity Ring-cavity Pulsed 1064 nm Diamond 1240 nm
1485 nm2.75 W
1.0 W[118] 2020 External cavity Ring-cavity CW 845-930 nm Diamond 965 nm
1101 nm400 mW
364 mW[119] 2021 External cavity Ring-cavity Pulsed 1063 nm Pb(MoO4)0.2
(WO4)0.81128 nm
1360 nmwatt level [121] 2018 External cavity Z-fold CW 808 nm KGW 1.32 μm
1.50 μm
1.73 μm6.1 W
1.1 W
1.1 W[125] 2016 Intracavity Microcavity CW 808 nm Nd:YVO4 1.05 μm
1.08 μm260 mW [35] 2019 Intracavity Microcavity Pulsed 880 nm YVO4 1164 nm
1175 nm40 mW [126] *Peak power
下载: 导出CSV
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