Increasing mode instability thresholds of fiber amplifiers by gain saturation

Arlee V Smith,Jesse J Smith

doi:10.1364/oe.21.015168

Abstract

We show by numerical modeling that saturation of the population inversion reduces the stimulated thermal Rayleigh gain relative to the laser gain in large mode area fiber amplifiers. We show how to exploit this effect to raise mode instability thresholds by a substantial factor. We also demonstrate that when suppression of stimulated Brillouin scattering and the population saturation effect are both taken into account, counter-pumped amplifiers have higher mode instability thresholds than co-pumped amplifiers for fully Yb3+ doped cores, and confined doping can further raise the thresholds.

Highlights

In earlier papers we described a stimulated thermal Rayleigh scattering process (STRS) that can account for observed modal instability in large mode area fiber amplifiers, and we described in detail our numerical model of this process [1,2,3,4,5]
The essence of the STRS process responsible for mode instability is that laser gain necessarily deposits quantum defect heat in the core of the amplifier fiber, and the asymmetric heating produced by the asymmetric signal irradiance profile due to interference between modes LP01 and LP11 leads to an asymmetric thermal lens that couples light between those two modes
An additional requirement is that there be a phase shift between the temperature grating and the signal irradiance grating which, in our STRS model, is created by the time lag between an irradiance grating traveling along the fiber and the temperature grating that it creates

Summary

Introduction

In earlier papers we described a stimulated thermal Rayleigh scattering process (STRS) that can account for observed modal instability in large mode area fiber amplifiers, and we described in detail our numerical model of this process [1,2,3,4,5]. Our model computes the mode coupling gain and the laser gain for fiber amplifiers with practically achievable step index profiles operating at realistic pump and signal powers. The essence of the STRS process responsible for mode instability is that laser gain necessarily deposits quantum defect heat in the core of the amplifier fiber, and the asymmetric heating produced by the asymmetric signal irradiance profile due to interference between modes LP01 and LP11 leads to an asymmetric thermal lens that couples light between those two modes. Hansen et al and Dong showed that using their assumed heat profile the mode coupling gain is related in a simple way to the laser gain This would imply that the mode instability threshold is largely determined by the net laser gain, and can be adjusted only by changing the modal profiles and their overlap with the Yb3+ doping profile.

Transverse hole burning

Without hole burning

With hole burning

Scaling for other LP11 starting powers

Findings

Conclusion