Increased mode instability thresholds of fiber amplifiers by gain saturation

TL;DR

This paper demonstrates through numerical modeling that gain saturation in large mode area fiber amplifiers can significantly increase mode instability thresholds, especially when combined with suppression of stimulated Brillouin scattering and optimized doping strategies.

Contribution

It introduces a method to exploit gain saturation to raise mode instability thresholds and compares configurations to optimize amplifier stability.

Findings

Gain saturation reduces stimulated thermal Rayleigh gain.

Counter-pumped amplifiers have higher thresholds than co-pumped ones.

Confined doping further increases mode instability thresholds.

Abstract

We show by numerical modeling that saturation of the population inversion reduces the stimulated thermal Rayleigh gain relative to the laser gain in large mode area fiber amplifiers. We show how to exploit this effect to raise mode instability thresholds by a substantial factor. We also demonstrate that when suppression of stimulated Brillouin scattering and the population saturation effect are both taken into account, counter-pumped amplifiers have higher mode instability thresholds than co-pumped amplifiers for fully Yb$^{3+}$ doped cores, and confined doping can further raise the thresholds.