Study of dopant concentrations on thermal induced mode instability in high power fiber amplifiers

TL;DR

This study investigates how ytterbium dopant concentrations affect mode instability thresholds in high power fiber amplifiers, revealing that doping levels do not influence MI thresholds when fiber length is adjusted to maintain pump absorption.

Contribution

The paper demonstrates that dopant concentration does not impact MI threshold if fiber length is varied to keep pump absorption constant, enabling heavy doping to suppress nonlinear effects without reducing MI thresholds.

Findings

MI threshold is independent of dopant concentration when fiber length is adjusted.

Heavy doping can suppress nonlinear effects without decreasing MI threshold.

Experimental results agree with theoretical predictions.

Abstract

Dependence of mode instabilities (MI) on ytterbium dopant concentrations in high power fiber amplifiers has been investigated. It is theoretically shown that, by only varying the fiber length to maintain the same total small-signal pump absorption, the MI threshold is independent of dopant concentration. MI thresholds of gain fibers with ytterbium dopant concentration of 5.93X10^25/m3 and 1.02X10^26/m3 have been measured, which exhibit similar thresholds and agree with theoretical results. The result indicates that heavy doping of active fiber can be adopted to suppress nonlinear effects without decreasing MI threshold, which provides a method of maximizing the power output of fiber laser, taking into account the stimulated Brillouin scattering, stimulated Raman Scattering, and MI thresholds simultaneously.