  We use a semi-analytical model of stimulated thermal Rayleigh scattering to
present theoretical study of seed power on mode instability with the presence
of photodarkening. The behavior of mode instabilities as a function of seed
power has been investigated. The nonlinear dependence of the threshold with
lower seed powers is ascribed to the influence of gain saturation while the
reduction behavior of the threshold with higher seed powers is put down to the
effects of photodarkening, which agrees well with the experimental results.
