Transverse-modal-instability gain in high power fiber amplifiers: Effect of the perturbation relative phase

Abstract

Local transverse modal instability (TMI) gain has been derived by carrying out a stability analysis of the fundamental mode amplification in high power fiber amplifiers, in the presence of spatial (transverse) and temporal perturbations, taking into account the gain saturation and resulting thermal loading. We have shown that the relative phase between the fundamental fiber mode and the transverse perturbation significantly affects the local TMI gain. By controlling the relative phase, the TMI gain can be reduced significantly, with the gain variation being more pronounced as the core diameter increases. This finding can be used in conjunction with other proposed approaches to develop efficient strategies for mitigating TMI in high power fiber amplifiers and lasers.