Kuznetsov Alexey G, Terentyev Vadim S, Simonov Victor A, Rizk Hiba A, Nemov Ilya N, Bronnikov Kirill A, Dostovalov Alexander V, Babin Sergey A
Institute of Automation and Electrometry SB RAS, Novosibirsk 630090, Russia.
Department of Physics, Novosibirsk State University, Novosibirsk 630090, Russia.
Micromachines (Basel). 2024 Jul 24;15(8):940. doi: 10.3390/mi15080940.
Multimode fibers are attractive for high-power lasers if transverse modes are efficiently controlled. Here, a dielectric thin-film mirror (R20%) is micro-fabricated on the central area of the end face of a 1 km multimode 100/140 µm graded-index fiber and tested as the output mirror of a Raman laser with highly multimode (M34) 940 nm diode pumping. In the cavity with highly reflective input FBG, Raman lasing of the Stokes wave at 976 nm starts at the threshold pump power of 80 W. Mode-selective properties of mirrors with various diameters were tested experimentally and compared with calculations in COMSOL, with the optimum diameter found to be around 12 µm. The measured Raman laser output beam at 976 nm has a quality factor of M2 near the threshold, which confirms a rather good selection of the fundamental transverse mode. The power scaling capabilities, together with a more detailed characterization of the output beam's spatial profile, spectrum, and their stability, are performed. An approximately 35 W output power with an approximately 60% slope efficiency and a narrow spectrum has been demonstrated at the expense of a slight worsening of beam quality to M~3 without any sign of mirror degradation at the achieved intensity of >30 MW/cm. Further power scaling of such lasers as well as the application of the proposed technique in high-power fiber lasers are discussed.
如果能够有效控制横向模式，多模光纤对于高功率激光器具有吸引力。在此，在1 km长的100/140 µm渐变折射率多模光纤端面的中心区域微制造了一个介质薄膜镜（反射率R20%），并将其作为具有高多模（M34）940 nm二极管泵浦的拉曼激光器的输出镜进行测试。在具有高反射率输入光纤布拉格光栅（FBG）的腔中，976 nm斯托克斯波的拉曼激光在约80 W的阈值泵浦功率下开始产生。对各种直径的镜子的模式选择特性进行了实验测试，并与COMSOL中的计算结果进行了比较，发现最佳直径约为12 µm。在阈值附近测得的976 nm拉曼激光输出光束的质量因子M2，这证实了对基模横向模式有相当好的选择。研究了功率缩放能力，以及对输出光束的空间分布、光谱及其稳定性进行更详细的表征。已经证明，在光束质量略有恶化至M3的情况下，能够实现约35 W的输出功率、约60%的斜率效率和窄光谱，并且在达到的强度>30 MW/cm时没有任何镜子退化的迹象。还讨论了此类激光器的进一步功率缩放以及所提出技术在高功率光纤激光器中的应用。
