Abstract
A semi-analytical model of stimulated thermal Rayleigh scattering, which assumes that there is a frequency offset between the fundamental mode and the high-order mode at the input of the amplifier, has been employed to study the effects of seed power on thermal-induced mode instability. Both the effects of gain saturation and photodarkening have been taken into consideration. The dependence of the threshold power and the maximal coupling frequency on various seed powers has been investigated when different parameters of fiber amplifier systems, such as the core/pump cladding diameter, and the level of photodarkening, are changed. The nonlinear dependence of the threshold at lower seed powers is mainly ascribed to the influence of gain saturation, while the reduction behavior of the threshold at higher seed powers is put down to the effects of photodarkening. The maximal coupling frequency is shown to vary with different parameters of fiber amplifier systems, such as seed powers, the level of the photodarkening, and core/pump cladding diameters. Comparing the results with the recently published experimental results, reveals that the experimental observation can be explained within this framework.
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