Theory of spatial mode competition in a fiber amplifier

TL;DR

This paper develops a theoretical framework for understanding spatial mode competition in fiber amplifiers, revealing how cross-modal gain influences mode growth and dominance, with analytical illustrations in waveguide systems.

Contribution

It introduces a new theoretical model incorporating cross-modal gain effects, explaining unusual amplification behaviors in waveguide arrays.

Findings

One mode can dominate and suppress others due to cross-modal gain.

Conditions identified where lower-gain modes become dominant.

Analytical examples demonstrate mode competition dynamics.

Abstract

Theory of monochromatic wave field amplification in a waveguide array based on expansion of the wave field in terms of guided array modes is developed. Equations for the expansion coefficients include cross-modal gain, which completely changes behavior of the amplified wave field. Analysis of two-mode amplification reveals new unusual features in characteristics of two-mode amplifier. Instead of unlimited growth of both modes for incoherent fields, one of the modes grows with no limit and suppresses the lower-power mode. Effects associated with the cross-modal gain are illustrated analytically on a system of two thin parallel planar waveguides. Conditions are found when a mode with lower gain can become the dominant one at the output of amplifier.