Longitudinal mode spectrum of semiconductor lasers under high-speed modulation

Experimental observations of the lasing spectrum of a single-mode semiconductor laser under continuous microwave modulation reveal that the lasing spectrum is apparently locked to a single-longitudinal mode for optical modulation depths up to ～ 80 percent, beyond which the lasing spectrum breaks into multimode oscillation. The width of the envelope of the multimode spectrum increases very rapidly with further increase in modulation depth. These results are satisfactorily explained by a theoretical treatment which gives simple analytic results for the time evolution of the individual longitudinal modes. It also yields considerable insight into spectral dynamics, and enables one to predict the dynamic lasing spectrum of a laser from its CW lasing spectra at various output powers. The results can also be used to predict the amount of spectral envelope broadening under single or pseudo-random pulse modulation.