Optimal input excitations for suppressing nonlinear instabilities in multimode fibers

TL;DR

This paper introduces a linear programming-based method to optimize multimode fiber input excitations, significantly suppressing nonlinear instabilities like SBS and TMI, enabling higher power fiber laser operation.

Contribution

It formulates the suppression of SBS and TMI as optimization problems and develops an efficient method to find the globally optimal multimode input excitation.

Findings

Order of magnitude power enhancement over fundamental mode excitation

Robustness of optimal mode content to perturbations

Effective suppression even with mode-dependent loss and gain

Abstract

Wavefront shaping has become a powerful tool for manipulating light propagation in various complex media undergoing linear scattering. Controlling nonlinear optical interactions with spatial degrees of freedom is a relatively recent but growing area of research. A wavefront-shaping-based approach can be used to suppress nonlinear stimulated Brillouin scattering (SBS) and transverse mode instability (TMI), which are the two main limitations to power scaling in high-power narrowband fiber amplifiers. Here we formulate both SBS and TMI suppression as optimization problems with respect to coherent multimode input excitation in a given multimode fiber. We develop an efficient method for finding the globally optimal input excitation for SBS and TMI suppression using linear programming. We theoretically show that optimally exciting a standard multimode fiber leads to roughly an order of magnitude enhancement in output power limited by SBS and TMI, compared to fundamental-mode-only excitation. We find that the optimal mode content is robust to small perturbations and our approach works even in the presence of mode dependent loss and gain. Optimal mode content can be excited in real experiments using spatial light modulators, creating a novel platform for instability-free ultrahigh-power fiber lasers.