  We study the impact of two-photon absorption (2PA) and fifth-order nonlinear
loss such as 2PA-induced free-carrier absorption in semiconductors on the
performance of Stimulated Brillouin Scattering devices. We formulate the
equations of motion including effective loss coefficients, whose explicit
expressions are provided for numerical evaluation in any waveguide geometry. We
find that 2PA results in a monotonic, algebraic relationship between
amplification, waveguide length and pump power, whereas fifth-order losses lead
to a non-monotonic relationship. We define a figure of merit for materials and
waveguide designs in the presence of fifth-order losses. From this, we
determine the optimal waveguide length for the case of 2PA alone and upper
bounds for the total Stokes amplification for the case of 2PA as well as
fifth-order losses. The analysis is performed analytically using a small-signal
approximation and is compared to numerical solutions of the full nonlinear
modal equations.
