Measurements of the optical power required to switch on‐and‐off an optical bistable diode laser amplifier as a function of detuning and current from threshold are presented. Good quantitative agreement between the theory for a nonlinear Fabry–Perot and the experimental results are obtained. A linewidth enhancement factor of 3.1 and a value for the change of the refractive index versus carrier density of −1.8×10−20cm3are extracted from the measurements.