EFFECT OF GAIN AND INDEX NONLINEARITIES ON SINGLE-MODE DYNAMICS IN SEMICONDUCTOR-LASERS

The finite intraband relaxation time in semiconductor lasers leads to gain saturation at high laser powers. The nonperturbative solution of the single-mode density-matrix equations shows that both the optical gain and the refractive index become intensity dependent as a result of intraband relaxation dynamics. We include the gain and index nonlinearities in the rate equations and study how the modulation response and noise characteristics of semiconductor lasers are affected by such nonlinearities. The intensity dependence of the frequency and the damping rate of relaxation oscillations leads to a fundamental limit imposed on the small-signal modulation bandwidth; our analysis provides an expression for the ultimate modulation bandwidth in terms of the material parameters. In the case of large-signal modulation the rise and fall times associated with the optical pulse increase considerably because of gain nonlinearities while the frequency chirp is affected by index nonlinearities. The intraband-relaxation effects also lead to saturation and rebroadening of the laser linewidth at higher operating powers.