Coherence-induced instabilities in single-frequency fiber amplifiers

Single-frequency fiber laser systems have been applied within a wide range of fields the last couple of decades. In some of these applications, frequency modulation (FM) of the seed light is used, and ever higher output powers are demanded. However, the FM can induce a power instability in the fiber amplifiers of the systems, and this is a potential limitation for power scaling. The instability manifests itself as periodic elastic backscattering of the frequency-modulated signal in one or more of the fiber amplifiers. It can be detrimental for the output power stability, and ultimately lead to catastrophic failure of the system through component damage. The purpose of the research presented in this thesis is to gain an understanding of this instability. The research includes both experiments, theoretical modeling and simulations. The phenomenon is characterized experimentally for a doubleclad Yb-doped fiber amplifier. Numerous features are described, and some of them motivate the development of a coupled-mode model that provides a simplified description of the phenomenon. A separate experiment is designed and performed to estimate a material parameter central to this model. The model is then used to numerically study the interaction in the aforementioned fiber amplifier of a main single-frequency signal with a weak signal originating from a reflection of the main signal. It is shown that the coupled-mode model can explain most of the qualitative features that are observed experimentally, despite the simplifications made.