Abstract
A systematic variation of the Fresnel number has been performed when solving the nonlinear wave equation for a Fabry–Perot étalon filled with a saturable medium and including unsaturable background absorption. We show that the optimum Fresnel number for absorptive and defocusing bistability in media such as CdS, using the nonlinearity due to the bound exciton resonance, can be considerably greater than one and that bistability is completely lost for tight focusing.