Modal instabilities in high power fiber laser oscillators.

V Scarnera,M N Zervas,F Ghiringhelli,C A Codemard,A Malinowski,M K Durkin

doi:10.1364/oe.27.004386

Abstract

Transverse mode competition and instabilities in high-power fiber oscillators have been studied experimentally by monitoring the dynamic power exchanges and characteristic frequencies of the transmitted fundamental mode (FM) and scattered high-order modes (HOMs) of the fiber laser cavity under CW and pulsed pumping. The FM and HOM power evolution indicates the presence of two competing effective laser cavities which result in rich output dynamics and full chaotic operation. The thermal and inversion related contributions to the observed instabilities have been identified by monitoring the associated characteristic instability frequencies under pulsed pumping. It is shown that in the transient regime, both inversion and thermal effects contribute successively to the observed power instabilities. Increasing the pump power leads to full chaotic response through an interplay between transverse and longitudinal mode instabilities.

Highlights

Transverse mode instability (TMI) is a recently observed nonlinearity [1] proving to be a major power-scaling limitation in high-power fiber lasers and amplifiers [2,3,4]
The fundamental mode (FM) and high-order modes (HOMs) power evolution indicates the presence of two competing effective laser cavities which result in rich output dynamics and full chaotic operation
TMI is widely attributed to a refractive index (RI) grating formation due to transverse mode beating and coupling between the amplified fundamental mode (FM) and high order modes (HOM)

Summary

Introduction

Transverse mode instability (TMI) is a recently observed nonlinearity [1] proving to be a major power-scaling limitation in high-power fiber lasers and amplifiers [2,3,4]. In addition to TMI power threshold, which is shown to be inversely proportional to the fiber core diameter [1,2,3,19,20], another important parameter in the study of TMI is the associated frequency, which is related to the characteristic time scale of the dominant physical mechanism responsible for the instability.

Results

Conclusion