This Letter describes an experimental realization of a double-pass amplifier using rod-type fiber. In this device, the gain reaches 26 dB amplifying a 300 mW, 20 ps, 20 MHz seed up to 120 W, with an optical-to-optical efficiency of 50% and excellent beam quality. In addition, by design the output of the amplifier has a polarization extinction ratio of 33 dB. Besides these good performances, we report a marginal degradation of mode quality and degree of polarization followed by the so-called transverse mode instability which occurs at 120 W signal power. The first degradation is static, and by analyzing its two polarizations, we conclude it is caused by a coupling between modes due to the formation of a static thermal long-period grating, which in turn initiates the dynamic instability.

中文翻译：

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双通杆式光纤放大器中的静态和动态模式耦合

这封信描述了使用棒状光纤的双通放大器的实验实现。在该器件中，增益达到26 dB，将300 mW，20 ps，20 MHz的种子放大到120 W，光-光效率为50％，并且光束质量出色。另外，通过设计，放大器的输出具有33 dB的极化消光比。除了这些良好的性能外，我们还报告了模式质量和偏振度的轻微下降，随后是所谓的横向模式不稳定性，这种不稳定性在120 W信号功率下出现。第一个退化是静态的，并且通过分析其两个极化，我们得出结论，这是由于静态热长周期光栅的形成而导致的模式之间的耦合所引起的，而静态热长周期光栅又会引发动态不稳定性。