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光纤激光器是指以光纤作为工作介质的激光器，具有光束质量好、转换效率高、热管理方便、结构紧凑、可柔性操作等优势，是激光与光电子学等领域的研究前沿和热点，并在生物医疗、智能制造、能源勘探和国防安全等领域得到广泛应用[1-5]。近年来，随着增益光纤、无源器件、泵浦光源性能的不断提高，以及热管理、非线性效应调控等技术的快速发展，光纤激光器的输出功率水平得以飞速提升，特别是1 μm波段光纤激光器的表现尤为突出。2009年，1 μm波段高功率光纤激光器实现单模万瓦级功率输出；2013年，实现单纤20 kW级激光输出，但多年来功率量级尚无进一步突破的报道[6-11]。研究表明，热致模式不稳定、热透镜等热效应是限制高功率光纤激光器性能进一步提升的重要因素之一[12-15]。
量子亏损(Quantum Defect，QD)表示激光增益过程中泵浦光子与信号光子之间的能量差，是高功率光纤激光器内废热的重要来源之一[16-18]。降低量子亏损可减少光纤激光器中的产热，提高热致模式不稳定等效应的发生阈值，以及激光器的光光转换效率，提升系统长期运行安全性。1995年，Zellmer等[19]报道了首个十瓦级光纤激光输出；该激光器基于掺钕光纤构建，泵浦光波长和信号光波长分别约为810 nm和1060 nm，对应量子亏损约为23.58%。1999年，Dominic等[20]基于掺镱光纤实现了首个百瓦级光纤激光器；其泵浦光波长和信号光波长分别为915 nm和1120 nm，量子亏损约为18.30%。2004年，Jeong等[21]报道了首个千瓦级掺镱光纤激光器；该激光器泵浦光波长和信号光波长约为975 nm和1100 nm，量子亏损约为11.36%。为了降低光纤激光器中的量子亏损和热负载，提高泵浦亮度，实现高功率输出，研究人员提出级联泵浦方案。2009年，IPG Photonics公司[6, 22]基于级联泵浦方案实现了首台万瓦级光纤激光器；其泵浦波长和信号波长分别为1018 nm和1070 nm，相应量子亏损约为4.86%。可以说，光纤激光器功率提升的过程也是不断与废热等作“斗争”的过程；量子亏损作为高功率光纤激光器中废热的重要来源之一，其降低方法自然受到研究人员的广泛关注。文中主要梳理近年来在进一步减小1 μm波段光纤激光器量子亏损方面的研究工作（为了描述方便，文中约定低量子亏损光纤激光器的量子亏损≤4.50%、超低量子亏损光纤激光器的量子亏损≤1%），以期为促进高功率光纤激光器性能发展提供参考。
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1 μm波段光纤激光的最高输出功率由掺镱光纤激光器实现。在降低掺镱光纤激光器量子亏损方面，研究人员开展了大量工作，如通过级联泵浦技术获得低量子亏损掺镱光纤激光，采用强泵浦、多组分掺杂等技术实现超低量子亏损掺镱光纤激光输出等。
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级联泵浦又叫同带泵浦或二次泵浦，即以半导体激光器泵浦的光纤激光器作为二次泵浦源耦合到掺杂光纤中提供增益、获得激光输出[23-24]。与半导体激光直接泵浦方案相比，采用级联泵浦技术的光纤激光器可以减小激光器中泵浦光波长与信号光波长间隔、降低量子亏损引入的废热，提升泵浦激光的亮度，实现对激光器中的热效应的抑制，提高其功率提升潜力。该技术最早应用于掺饵光纤激光器中，此后还在掺铥、掺钬等光纤激光器中得到应用[25-27]；在基于级联泵浦技术的1 μm波段掺镱光纤激光器中，研究人员尝试采用较长波长的泵浦光、较短波长的信号光以减小激光器的量子亏损。
2011年，德国耶拿大学的Wirth等[28]设计了基于级联泵浦技术的空间结构光纤放大器，结构如图1所示。利用1030 nm盘片激光器泵浦大模场双包层掺镱光纤，成功得到2.9 kW 1071 nm光纤激光，对应的量子亏损为3.83%，斜率效率为73%。
2012 年，英国南安普顿大学的Yao等[29]搭建全光纤结构的光纤放大器，利用掺镱铝硅酸盐光纤作为增益介质，通过级联泵浦技术，输出2.5 W 1040 nm光纤激光，斜率效率超过90%，量子亏损仅为1.92%。2014年，该课题组的Chang等[30]将级联泵浦掺镱光纤放大器的输出功率提高至5.7 W，量子亏损保持在1.9%左右，对应斜率效率为64%，实验结构如图2所示。此外，他们测量出具有量子亏损为2%的级联泵浦磷硅酸盐掺镱光纤放大器纤芯的产热量为3%，这是首次对光纤激光纤芯内热负载的测量。
2018年，笔者单位的Ma等[31]搭建了如图3所示的级联泵浦全光纤窄线宽光纤放大器，其采用主振荡器功率放大器 (MOPA)结构，种子激光为输出功率为60 mW的低噪声、单频(线宽低于20 kHz) 1064 nm光纤激光，通过调制和三级放大后注入大模场双包层掺镱光纤(纤芯/内包层直径30/250 μm)中，泵浦源采用总输出功率可达6 kW的1018 nm全光纤激光器，成功获得线宽为0.89 nm、最高功率接近4 kW的光纤激光输出，对应的量子亏损为4.32%左右，光束质量因子M2为1.86。
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采用级联泵浦技术可有效降低1 μm波段掺镱光纤激光器的量子亏损，但总体来说难以实现超低量子亏损（QD≤1%）光纤激光输出。为了进一步降低掺镱光纤激光器的量子亏损，研究人员还开展了基于强泵浦、多组分掺杂等方面的研究工作。
2007年，日本福井大学的Matsubara等[32]报道了如图4所示的空间结构超低量子亏损掺镱光纤激光器。使用半导体激光器(中心波长978 nm，输出功率290 mW)泵浦一段单模掺镱光纤(纤芯/包层直径6.2/125 μm，长度90 mm)，通过构建强泵浦条件（~1 MW/cm2），分别实现了980.6 nm 和983 nm的光纤激光输出，对应量子亏损分别低至0.26%和0.5%，其中983 nm光纤激光的输出功率为18 mW、斜率效率为15%。
2014年，英国南安普顿大学的Yao等[33]对双包层掺镱铝硅酸盐光纤激光器和放大器的量子亏损极限进行理论研究，指出量子亏损与光纤的纤芯-内包层面积比有很大关系，该面积比越小，激光器越容易实现较低的量子亏损输出。在纤芯泵浦情况下，掺镱光纤激光器可达到的最低量子亏损为0.6%。
2018年，美国伊利诺伊大学的Yu等[34]报道了一种量子亏损<1%的光纤激光器，其增益光纤为自制的掺镱多组分氟硅酸盐光纤。搭建了由半导体激光器泵浦的光纤激光谐振腔，结构如图5所示。在976.6 nm和981 nm泵浦波长条件下，该光纤激光器分别输出了约400 mW的985.7 nm激光和约150 mW的989.8 nm激光，转换效率分别为62.1%和56.8%，对应的量子亏损分别为0.92%和0.90%。
2021年，该课题组首次采用双波长脉冲泵浦掺镱氟硅酸盐光纤，其两个泵浦波长分别为由半导体激光器输出的976.6 nm斯托克斯泵浦波长和掺镱光纤激光器输出的989.6 nm反斯托克斯泵浦波长，成功产生了985.6 nm光纤激光，对应量子亏损低至0.51%[35]。
总的来说，基于级联泵浦方案的低量子亏损掺镱光纤激光器已实现数千瓦功率输出，但是目前公开报道的最低量子亏损为1.92%，难以进一步降低。基于多组分掺杂、强泵浦等方案可实现超低量子亏损光纤激光输出，但是目前公开报道的最高输出功率仅为400 mW。
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拉曼光纤激光器利用无源光纤中的受激拉曼散射（SRS）提供增益，信号光波长主要受限于泵浦光波长，具有增益带宽较宽、波长可调谐、无光子暗化、结构简单等特点[36-41]。此外，与传统的半导体泵浦掺镱光纤激光器相比，工作在1 μm波段的拉曼光纤激光器具有较小的量子亏损（以泵浦光波长为1070 nm为例，利用硅基光纤中的受激拉曼散射提供增益可获得1120 nm左右的信号光，量子亏损约为4.46%左右）。因此，拉曼光纤激光器也是获得1 μm波段低量子亏损光纤激光的重要技术方案。
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近年，随着掺镱光纤激光技术及相关亮度增强技术的发展，拉曼光纤激光器的输出功率快速提升[42-47]。
在基于阶跃折射率无源光纤的低量子亏损拉曼光纤激光器方面，欧洲南方天文台的Feng等[48]于2009年利用中心波长1070 nm、最高输出功率200 W的掺镱光纤激光泵浦30 m硅基光纤，首次实现了百瓦级拉曼光纤激光输出，其实验结构如图6所示。在泵浦功率增长到180 W时，该激光器成功实现153 W的1120 nm拉曼光纤激光输出，对应量子亏损为4.46%，转换效率达到85%。2018年，笔者单位Zhang等[49]利用1070 nm掺镱光纤激光器泵浦半开腔结构随机拉曼光纤激光器，实现491 W功率输出。该激光器信号光波长为1120 nm，对应的量子亏损为4.46%。2022年，笔者单位Zhang等[50]利用可调谐放大的自发辐射（ASE）光源后向泵浦拉曼光纤激光器，获得低量子亏损的随机拉曼光纤激光输出。该激光器最高输出功率11.71 W，泵浦光波长1066 nm、信号光波长1080 nm，对应量子亏损为1.3%。
此外，基于渐变折射率光纤的拉曼光纤激光器也以其亮度提升能力和光束净化效果得到了国内外研究人员的广泛关注和深入研究[51-55]。目前，基于渐变折射率光纤的拉曼激光器的最高输出功率已达到3 kW，由笔者单位Chen等[56]于2021年实现。该激光器结构如图7所示，采用金属铝涂覆层的拉曼光纤搭建，最终获得了3083 W输出功率，在2364 W功率时输出M2为3.12，在最高功率条件下输出M2为5.72，对应的亮度提升倍数分别为9和2.9左右；泵浦光波长为1080 nm、信号光波长为1130 nm，对应量子亏损为4.42%。
在包层泵浦低量子亏损拉曼光纤激光器方面，英国南安普顿大学Yao等[57]于2015年设计并制作了一款双包层拉曼光纤(纤芯/内包层直径14.6 μm /38 μm，数值孔径0.1/0.3)，并利用这款光纤搭建了由975 nm半导体激光器泵浦的空间结构拉曼光纤激光器（结构如图8所示），得到了6 W 1019 nm激光输出，对应的量子亏损为4.32%。2018年，以色列Soreq NRC研究中心Glick等[58]采用三包层硅基光纤（纤芯/内包层直径25 μm/45 μm，数值孔径0.065/0.22）光纤搭建了拉曼激光器，其中泵浦源为1070 nm大模场光纤激光器，被动光纤长度为80 m，成功实现了1.2 kW的1120 nm拉曼光纤激光输出，光束质量因子M2为2.75，亮度提升了7倍左右，对应转换效率为85%，量子亏损为4.46%。这是目前包层泵浦低量子亏损拉曼光纤激光器的最高输出功率记录。
2020年，笔者单位的Chen等[59]报道了首台全光纤包层泵浦拉曼放大器。该放大器由总功率170 W的1130 nm激光光源提供种子激光，利用输出功率超过3 kW的1080 nm光纤激光器去泵浦自制的三包层硅基光纤(纤芯/内包层/外包层直径31 μm/55.55 μm/360 μm，数值孔径0.06/0.22/0.46，光纤长度38 m)，成功得到762.6 W激光输出，最佳光束质量因子M2为1.9，对应的量子亏损为4.42%。
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由于硅基光纤的拉曼增益谱较宽、在低频移区有较弱的增益，研究人员报道了基于硅基光纤的超低量子亏损拉曼光纤激光器工作。2008年，加拿大拉瓦尔大学的Bélanger等[60]利用1064 nm的掺镱光纤激光器作为泵浦源，一段长度为125 m的单模光纤提供拉曼增益，波长可调谐光纤布拉格光栅提供反馈，实现了中心波长1075 nm、量子亏损约为1%的光纤激光输出，但其最高功率仅为1.1 W。2023年，笔者单位的Ma等[61]利用可调谐超荧光光源泵浦基于硅基光纤的拉曼光纤激光器，通过优化硅基光纤长度和低反光栅反射率等参数，实现量子亏损0.56%、输出功率6.2 W、斜率效率47.7%的拉曼光纤激光输出。总的来说，硅基光纤在低频移区的拉曼增益相对较弱，受增益竞争因素制约，基于硅基光纤的超低量子亏损拉曼光纤激光器的输出功率仍未突破十瓦量级。
此外，掺磷光纤的拉曼增益谱中除了具有与硅元素相关的13.2、14.7 THz频移的增益峰，以及与磷元素相关的~40 THz频移的增益峰，还具有因振动态密度过剩而存在的频移约为3 THz 左右的玻色峰[62-63]，如图9所示。在此基础上，笔者单位设计实现了基于掺磷光纤中玻色峰的超低量子亏损(QD≤1%)拉曼光纤激光器。
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玻色峰通常指非晶材料中太赫兹频率附近的超出德拜定律的过剩振动态密度，可以在其子散射谱、拉曼散射谱以及热导率谱中观测到。玻色峰的研究历史可以追溯到20世纪50年代。1949年，英国牛津大学的Berman[64]在测量非晶玻璃材料的导热率时发现，在温度10 K时其导热率比晶体材料低几个数量级。1953年，印度科学院的Krishnan等[65]在测量石英玻璃的拉曼散射谱时发现，相比于晶体材料，石英玻璃在频移30~120 cm−1有较强的拉曼散射峰，即为玻色峰。在过去的几十年里，研究人员围绕非晶材料中玻色峰的产生机理开展了大量研究[66-69]。1986年，俄罗斯科学研究院Malinovsky等[70]对非晶材料中的玻色峰展开了理论与实验研究，他们认为玻色峰的产生与玻璃材料微观结构的中程有序度有关。此外，他们通过实验发现，相较于制备时缓慢退火冷却的玻璃材料，在冰水中骤然退火冷却的玻璃材料有着更强的玻色峰，如图10所示。
1996年，西班牙Fayos等[71]通过测量并对比乙醇在液态、晶态、非晶态等不同状态下的静态结构因子，证实了玻色峰与材料微观结构的中程有序度之间的关联。1997年，英国剑桥大学的Dove等[72]通过对比非晶氧化硅与方石英的弹性中子散射谱推断玻色峰源于一种横向声学振动模式。2004年，美国伦斯勒理工大学Schroeder等[73]研究了环境温度与压强对不同玻璃材料中玻色峰的影响，通过测量不同温度与压强下玻璃材料的拉曼散射谱，他们发现玻色峰的强度随温度升高而增强，而玻色峰的频移则随压强增大而增大。2012年，墨西哥Salceda-Delgado等[74]从理论仿真与实验测量两方面同时分析了掺磷硅基光纤中磷元素的浓度对其拉曼增益谱的影响。图11给出了不同掺杂浓度下掺磷硅基光纤的拉曼增益谱，可以看出，低频移区玻色峰的强度随着磷元素浓度的增加而增加。该实验结果对掺磷硅基光纤中玻色峰的调控有重要指导意义。
2019年，法国巴黎综合理工学院Shcheblanov等[69]基于Classical Molecular Dynamics (CMD)与Reverse Monte Carlo (RMC)等理论模型分别对磷酸盐玻璃中的中子谱、X射线谱、拉曼增益谱、近红外吸收谱以振动态密度谱进行了仿真计算与实验验证，研究结果表明相较于CMD模型，基于RMC模型的仿真计算结果与实验测量结果吻合更好。
图 12给出了基于RMC模型计算得到的振动态密度谱及其中各个峰所对应的振动模式。他们认为磷酸盐玻璃中低频移区的玻色峰起源于单个PO4四面体晶胞的旋转振动。
2022年，日本东京大学Hu等[75]对三种二维玻璃材料的振动特性进行了理论分析与仿真计算。他们首先计算材料的横向与纵向振动态密度，横向振动态中，在玻色峰频率附近有明显的过剩振动态，纵向振动态中则未出现，由此推断玻色峰源于横向振动态。随后，对横向振动态中扩展态与准局域态的占比及场分布进行计算，在玻色峰频率附近，准局域态密度更高，其场分布更随机，进一步推断玻色峰源于横向准局域振动态。进一步的，他们计算了不同q值时横向与纵向动态结构因子，对于横向动态结构因子，不同q值时，玻色峰附近均有一个明显的特征峰；而纵向动态结构因子中，不存在该固定特征峰，该结果进一步证实了玻色峰源于横向准局域振动态。最后，为了探究玻色峰的准局域态产生原因，他们计算了粒子对不同频率过剩振动态密度的贡献的场分布，仿真结果如图13所示。根据图示仿真结果，他们推断玻色峰源于线状和环状分子基团的横向准局域振动。2023年，英国格拉斯哥大学Jimenez等[76]通过对液态四正丁氧基硅烷的振动模式进行实验表征与分子动力学模拟计算，推断其玻色峰源于约20个分子基团的准局域振动。
总的来说，目前主流观点认为玻色峰源于材料中数十个分子组成的基团的准局域振动。但是，对玻色峰的形成机理及调控方法尚未形成统一认识，亟需开展进一步的深入研究。
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2020年，笔者单位Zhang等[77]搭建了如图14所示的基于掺磷光纤的超低量子亏损拉曼光纤激光器。利用掺磷光纤中3.65 THz频移的玻色峰提供增益，采用中心波长为1066 nm的ASE光源作为泵浦源，获得了12.5 W的1080 nm激光输出，对应的量子亏损为1.3%，转换效率为67.2%，光谱纯度高达96.03%。另外，通过将泵浦波长优化至1072 nm，该激光器获得10.7 W的1080 nm拉曼激光输出，光谱纯度达到82.82%，量子亏损低至0.74%。这项工作首次从实验上验证了基于掺磷光纤的玻色峰实现超低量子亏损拉曼激光输出的可行性。
2021年，笔者单位Ma等[78]通过进一步优化泵浦光波长、线宽，以及谐振腔低反光栅反射率、掺磷光纤长度等系统参数，实现了最高功率100.9 W的光纤激光输出，泵浦光波长1069.5 nm、信号光波长1080 nm，对应的量子亏损为0.97%。该研究还指出：较宽的泵浦带宽会降低掺磷光纤中硅基拉曼增益峰的增益、抑制增益竞争和高阶拉曼效应、促进信号光功率提升；较高反射率的低反光栅可提高信号光在谐振腔内的功率密度，促进泵浦功率吸收转化，但同时也促进了高阶拉曼效应的产生，从而使低量子亏损信号光的功率降低；输出功率的提升主要受限于高阶拉曼效应、三波混频效应等非线性效应和增益竞争。
2022年，Ma等[79]开展了包层泵浦超低量子亏损拉曼光纤激光器的可行性研究。搭建包层泵浦拉曼激光器，采用可调谐ASE光源提供泵浦光，利用纤芯/包层直径为10 μm/20 μm的三包层掺磷光纤提供拉曼增益，分析比较了包层/纤芯泵浦方案对超低量子亏损拉曼光纤激光输出性能的影响。实验结果表明，采用纤芯泵浦方案时得到的信号光功率比包层泵浦方案低、效率也较低，这可能是不同泵浦方案时光纤内部光束传输路径不同（如图15所示）、功率分布特性不同，进而导致非线性效应发生差异所致。通过优化激光器工作参数，获得最低量子亏损为0.78%、对应功率为23.7 W的包层泵浦超低量子亏损拉曼光纤激光输出。同年，笔者单位Zhang等[80]还研究了泵浦方向等因素对拉曼光纤激光器中拉曼辅助的四波混频、高阶非线性效应阈值、光谱纯度等的影响，提出基于后向泵浦获得高光谱纯度超低量子亏损拉曼光纤激光输出的方案。该研究可为超低量子亏损拉曼光纤激光器性能优化提供重要参考。
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目前，1 μm波段低量子亏损光纤激光主要采用稀土掺杂增益和拉曼增益两种体制。在稀土掺杂光纤激光器中，通过采用级联泵浦、多组分掺杂、强泵浦等技术实现了量子亏损的降低，量子亏损≤1%的掺镱光纤激光器已实现400 mW功率输出；在拉曼光纤激光器方面，通过采用特殊掺杂、泵浦光谱调控、增益竞争抑制等技术，量子亏损≤1%的拉曼光纤激光器已实现百瓦级功率输出。后续工作中，通过优化光纤掺杂组分、调控增益谱、采用包层泵浦技术等，有望进一步降低光纤激光器的量子亏损，获得高功率、高效率的超低量子亏损光纤激光输出，并拓展其在低噪声、高稳定性等方面的性能。
Research progress of low-quantum-defect fiber laser at 1 μm band (invited)
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摘要: 量子亏损对高功率光纤激光器内的废热产生和光光转换效率具有重要影响，光纤激光器输出功率的提升过程可以视为不断与量子亏损作斗争的过程。文中梳理了近年来1 μm波段低量子亏损光纤激光的重要进展，重点介绍了稀土掺杂增益和拉曼增益两种体制的光纤激光器在实现低量子亏损输出方面的相关工作。在稀土掺杂光纤激光器中，采用级联泵浦、多组分掺杂、强泵浦等技术可降低激光器的量子亏损，其中量子亏损≤1%的掺镱光纤激光器已实现400 mW功率输出。在拉曼光纤激光器中，通过采用特殊掺杂、泵浦光谱调控、增益竞争抑制等技术，量子亏损≤1%的拉曼光纤激光器已实现百瓦级功率输出，并成功验证包层泵浦方案的可行性，表明其在实现高功率低量子亏损输出方面具有重要潜力。Abstract:
Significance Power scalability of fiber lasers have attracted a great deal of attention for its remarkable features, such as excellent beam quality, high conversion efficiency, flexible operation, and wide applications in biomedicine, intelligent manufacturing, energy exploration, defense and security. However, there have been no reports of significant operation power breakthroughs of near-single-mode fiber laser since the first demonstration of 10 kW-level system in 2009. The wasted heat accumulation, which can induce thermal lens and transverse mode instability effects, is one of the most important limitation factors. Quantum defect, defined as (1-λp/λs), where λp is the pump wavelength and λs is the lasing wavelength, has always been a key parameter in high-power fiber lasers. High quantum defect not only limits the conversion efficiency but also increases the thermal load in fiber lasers. In hence, much research on low quantum defect fiber laser has been reported in the past decades. Progress This paper first introduces the performance exploration of high-power fiber laser at 1 μm band, including the power scaling and corresponding quantum defect decrease. It can be said that the power scaling progress of fiber lasers is also a continuous struggle against waste heat and other factors. As to low quantum defect fiber laser, the reported works mainly focus on two different technical schemes based on rare earth doped fiber and passive fiber. For the convenience of description, this article stipulates that the quantum defect of low quantum defect fiber lasers is ≤ 4.50%, and the quantum defect of ultra-low quantum defect fiber lasers is ≤ 1%. Then, ytterbium-doped fiber lasers with low quantum defect are summarized. In 2011, Wirth et al. demonstrated a 2.9 kW fiber laser operating at 1071 nm that is tandem-pumped by a 1030 nm thin-disk laser, and the corresponding quantum defect is about 3.83% (Fig.1). In 2014, Chang et al. presented a fiber laser with a maximal output power of 5.7 W and a quantum defect of 1.9% (Fig.2). To further reduce the quantum defect of fiber lasers, some specially designed active fibers and high pumping intensity methods are adopted. For example, in 2018, Yu et al. demonstrated a 400 mW-level fiber lasers with less than 1% quantum defect via ytterbium-doped multicomponent fluorosilicate fibers (Fig.5). Additionally, Raman fiber lasers with low quantum defect are reviewed. Based on common silicon fiber, a maximal power of 3 kW-level with a quantum defect of 4.42% (Fig.7) and a maximal power of 6.2 W with a quantum defect of 0.56% were achieved. To further improve the operation power of ultra-low quantum defect fiber laser, the scheme enabled by boson peak in phosphosilicate fiber was presented and realized by Zhang et al. in 2020 (Fig.14). In 2021, Ma et al. demonstrated a 100 W-level ultra-low quantum defect fiber laser with a quantum defect of 0.97%. What's more, cladding pump scheme was also been validated (Fig.15). Conclusions and Prospects The important progress of low quantum defect fiber laser operating at 1 μm band is reviewed. And the manuscript mainly focuses on two different technical schemes based on rare earth doped fiber and passive fiber. In rare earth doped fiber based lasers, the utilization of tandem-pumping, multi-component doping and strong pumping schemes can reduce the quantum defect, and the related ytterbium-doped fiber lasers with quantum defect ≤1% have achieved 400 mW-level output power. In Raman fiber lasers, the maximal output power of 100 W-level with a quantum defect of ≤1% has been demonstrated with the aid of techniques such as special doping, pump spectrum regulation, and gain competition suppression. The feasibility of cladding pumping scheme has also been verified successfully, indicating its significant potential in achieving high-power and low quantum loss output. -
Key words:
- fiber laser /
- high power /
- low quantum defect /
- ytterbium-doped fiber laser /
- Raman fiber laser
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