Abstract
We present a coupled-mode model of transverse mode instability in high-power fiber amplifiers, which takes the effect of gain saturation into account. The model provides simple semi-analytical formulas for the mode instability threshold, which are valid also for highly saturated amplifiers. The model is compared to recently published detailed numerical simulations of mode instability, and we find reasonably good agreement with our simplified coupled-mode model.
Highlights
The recent discovery of transverse mode instability (TMI) in high-power fiber amplifiers [1] has motivated experimental [2,3,4] and theoretical [5–13] efforts to characterize and explain this phenomenon
TMI manifests itself as an instability in which the signal power fluctuates on a ms timescale between the fundamental mode (FM) and the first few higher-order modes (HOMs) when the average signal power is increased above a certain threshold
This approximation leads to an overestimate of the strength of the thermally induced long period grating (LPG), which in turn leads to an underestimate of the TMI threshold power
Summary
The recent discovery of transverse mode instability (TMI) in high-power fiber amplifiers [1] has motivated experimental [2,3,4] and theoretical [5–13] efforts to characterize and explain this phenomenon. We have proposed an alternative model of TMI in terms of a coupled-mode model [11, 12] While this model has the advantage of numerical efficiency, and even allows for semi-analytical solutions, it does rely on a number of approximations, the most limiting of which is the neglect of spatiotemporal oscillations in the gain coefficient due to gain saturation. This approximation leads to an overestimate of the strength of the thermally induced LPG, which in turn leads to an underestimate of the TMI threshold power. We show how this important effect can be included to lowest order in a coupled-mode formulation of TMI in fiber amplifiers
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