Gain saturation and the linewidth enhancement factor in semiconductor lasers

There is an asymmetry in the optical spectrum of the semiconductor laser un der weak Current modulation. It arises because the linewidth enhancement fa ctor that describes the proportionality between the real and imaginary part s of the optical susceptibility due to the differential gain is not appropr iate for the gain saturation. This asymmetry can be used to determine the p roportionality for the gain saturation term, and to measure the value of th e linewidth enhancement factor, a parameter that has been difficult to accu rately measure. Data for an oxide-confined vertical cavity surface-emitting laser is presented that shows a gain saturation term with a different, but nonnegligible, proportionality.