Influence of thermal fluctuation noise on two-wave mixing in saturable absorbers

We have extended the description of nonlinear optical two-wave mixing in a three-level saturable absorbing medium to include a noise contribution from thermally driven fluctuations in the real part of the refractive index. An effective gain factor is introduced to describe this influence on the signal beam and numerical data are given for ruby. These data, which include saturation, show that the influence of noise on the signal beam near the saturation gain value is more than ten times larger than that calculated using the theory for a Kerr medium for the same gain value.