Influence of refractive index nonlinearities on modulation and noise properties of semiconductor lasers

The modulation response and the spectral linewidth of singlemode semiconductor lasers are analysed by taking into account the nonlinear gain and the nonlinear refractive index in the rate equations. It is shown that the effect of nonlinear gain and index can be included through an effective linewidth enhancement factor alpha _eff that is different for frequency modulation and for spectral linewidth. The effect of the nonlinear index is particularly strong in the case of frequency modulation where alpha _eff can become zero or even negative for lasers operating on the red side of the gain peak. In the case of laser noise, alpha _eff causes linewidth saturation but no rebroadening at high output powers. The authors´ results indicate that gain and index nonlinearities are not the cause of linewidth rebroadening.