Spontaneous Rayleigh Seed for Stimulated Rayleigh Scattering in High Power Fiber Amplifiers

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 thermooptic 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.