Fiber fuse effect can occur spontaneously and propagate along optical fibers to cause widespread damage; it threatens all applications involving optical fibers. This paper presents two results. First, it establishes that the initiation of fiber fuse (IFF) in silica fibers is caused by virtual-defect-induced absorption. Critical temperatures and critical optical powers for IFF are simulated for the first time using a 3D solid-state heat transfer model with heat source generated by the virtual-defect-induced absorption. In this method, formation energies of the virtual defects can be uniquely determined, which offers critical information on the chemical reasons for fiber fuse. Second, this paper offers a method to evaluate operating temperatures of fiber lasers. General analytical solutions of the operating temperatures along gain fibers are deduced. Results of 976-nm laser-diode-pumped and 1018-nm tandem-pumped ytterbium-doped fiber (YDF) amplifiers using 10/130-μm YDFs are calculated. Potential limits caused by fiber fuse are discussed.

在脉冲大电流直线驱动装置中，电枢和轨道的接触状态会改变电枢起动特性，而电枢起动过程将直接影响整个发射系统的效率和寿命，因此有必要对电枢起始阶段的滑动接触状态进行研究。搭建发射实验平台，通过高速相机观察电枢起动状态，并结合有限元软件 ANSYS，对电枢的预紧力、初始接触状态以及电磁压力、电流密度进行仿真分析，研究电枢表面形貌对电枢起动的影响。结论表明：开槽电枢增加了电枢本身的柔顺性，使得预紧力增大，同时由于电流趋肤效应使得电流密度分布更加均匀，从而电磁压力增大，电枢起动变慢，接触电阻变小。实验和仿真结果对于改善电枢起动过程的接触状态，减轻烧蚀具有重要意义。

为揭示液体电介质击穿过程中形成的气体放电通道对液体电介质放电过程的影响，以针—板电极间隙变压器油为研究对象，基于等离子体流体力学模型，引入了液体电介质放电过程中气相放电通道对电离机制及自由电荷迁移率的影响，建立了用于模拟脉冲电压下液体电介质放电过程的两相流体模型，仿真研究了纳秒脉冲下针板电极流注放电的起始与发展过程。仿真结果表明：采用Heaviside方程可以在模型的不同区域同时实现气相物理过程和液相物理过程的模拟与计算。气相物理过程的引入导致流注尾部电场显著降低，流注头部电场进一步增强，使流注通道的发展速度要高于传统液相模型，有助于加深对纳秒脉冲下液体电介质中预击穿流注的起始、发展过程的认识和理解。

In this paper, we reported both the experimental demonstration and theoretical analysis of a Raman fiber laser based on a master oscillator–power amplifier configuration. The Raman fiber laser adopted the dual-wavelength bidirectional pumping configuration, utilizing 976 nm laser diodes and 1018 nm fiber lasers as the pump sources. A 60-m-long $25/400~\unicode[STIX]{x03BC}\text{m}$ ytterbium-doped fiber was used to convert the power from 1070 to 1124 nm, realizing a maximum power output of 3.7 kW with a 3 dB spectral width of 6.8 nm. Moreover, we developed a multi-frequency model taking into consideration the Raman gain spectrum and amplified spontaneous emission. The calculated spectral broadening of both the forward and backward laser was in good agreement with the experimental results. Finally, a 1.5 kW, 1183 nm second-order Raman fiber laser was further experimentally demonstrated by the addition of a 70-m-long germanium-doped passive fiber.