Intensity dependence of the linewidth enhancement factor and its implications for semiconductor lasers

The linewidth enhancement factor is shown to become intensity dependent when the intraband relaxation effects responsible for nonlinear gain and index changes are incorporated in the theory of semiconductor lasers. The intensity dependence of the linewidth enhancement factor influences many laser characteristics such as the frequency chirp, the modulation response, the injection-locking range, and the phase noise. In particular, it leads to a power-independent contribution to the laser linewidth. Furthermore, for semiconductor lasers detuned to operate away from the gain peak, the nonlinear index changes can even lead to a rebroadening of the laser linewidth at high-output powers.>