Spontaneous Rayleigh seeding of stimulated Rayleigh scattering in high power fiber amplifiers

TL;DR

This paper estimates the spontaneous Rayleigh seeding power in high-power fiber amplifiers caused by thermal fluctuations, which can influence mode instability thresholds and improve STRS model accuracy.

Contribution

It provides a new estimate for spontaneous Rayleigh seeding power due to thermal fluctuations in fiber amplifiers.

Findings

Spontaneous Rayleigh seed can surpass quantum noise levels.

Thermal fluctuations can induce mode coupling in fiber amplifiers.

The estimate improves understanding of mode instability thresholds.

Abstract

We estimate the Stokes wave starting power for stimulated thermal Rayleigh scattering (STRS) produced by thermal fluctuations in the fiber core that transiently alter the refractive index profile in the core. A transverse temperature gradient creates a transverse refractive index gradient via the thermo optic effect, and if the fluctuation frequency lies in the STRS gain band, it can couple light from mode LP$_{01}$ to LP$_{11}$ to seed STRS. This spontaneous Rayleigh seed may be stronger than the quantum background and may affect the mode instability thresholds of fiber amplifiers. This new seed estimate can be incorporated in STRS models to improve threshold calculations.