Suppressing transverse mode instability through multimode excitation in a fiber amplifier

TL;DR

This paper introduces a multimode excitation technique in fiber amplifiers to suppress transverse mode instability, enabling high-power, narrow-band, and high-quality laser output with improved stability and coherence.

Contribution

The study proposes using equal modal excitation in highly multimode fibers to reduce mode coupling and instability, a novel approach compared to traditional single-mode suppression methods.

Findings

Significant reduction in mode coupling and instability predicted by simulations.

High spatial coherence maintained with narrow seed bandwidth.

Achieves high power, narrow spectral width, and good beam quality simultaneously.

Abstract

High-power fiber laser amplifiers have enabled an increasing range of applications in industry, medicine and defense. The power scaling for narrow-band amplifiers is currently limited by the transverse modal instability. Various techniques have been developed to suppress the instability in a single or few-mode fiber in order to output a clean, collimated beam. Here we propose to use a highly multimode fiber and equal modal excitation to suppress the thermo-optical nonlinearity and instability. Our numerical simulations and theoretical analysis predict a significant reduction of dynamic coupling among the fiber modes with such excitation. When the bandwidth of a coherent seed is narrower than the spectral correlation width of the multimode fiber, the amplified light maintains high spatial coherence and can be transformed to any target pattern or focused to a diffraction-limited spot by a spatial mask at either input or output end of the amplifier. Our method simultaneously achieves high average power, narrow spectral width, and good beam quality, which are desired for fiber amplifiers in many applications.