A bistable etalon, consisting of an optically nonlinear semiconductor spacer layer sandwiched between two Bragg mirrors, has been optimized for use in reflection, using a numerical model for a self-consistent carrier density based on published experimental data for the nonlinear absorption alpha(I,lambda) and refraction DELTA-n(I,lambda). The saturation behavior of the refractive index and the absorption in bulk GaAs has been investigated at different wavelengths. High saturation values of DELTA-n(s) almost-equal-to .080 were found 20 meV below the gap. 30 meV above the gap we observed a comparable index change, together with an absorption saturation feature, leading to a highly nonlinear response close to the bistability threshold in our unoptimized etalon.