Gain saturation and the linewidth enhancement factor in semiconductor lasers

There is an asymmetry in the optical spectrum of the semiconductor laser under weak current modulation. It arises because the linewidth enhancement factor that describes the proportionality between the real and imaginary parts of the optical susceptibility due to the differential gain is not appropriate for the gain saturation. This asymmetry can be used to determine the proportionality for the gain saturation term, and to measure the value of the linewidth enhancement factor, a parameter that has been difficult to accurately 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.