A red‐shift in the diode modes with increasing optical feedback is observed in external‐cavity AlxGa1−xAs injection lasers. This shift is due to a change in the refractive index in the active region resulting from a reduction in the population‐inversion induced by the optical feedback. This leads to a new determination of the carrier dependence of the refractive index at the laser wavelength in the active region of semiconductor lasers independent of current‐induced thermal effects.