Effect of nonlinear gain on the intermodulation distortion in DFB lasers

Harmonics and intermodulation distortion in the AM response of SCH-MQW laser diodes can be very detrimental to the performance of analog optical communication systems. Sources of distortion, often discussed in the literature, are those due to gain suppression, spontaneous emission and relaxation oscillations. In this work the effect of the logarithmic dependence of the material gain on carrier density is investigated both theoretically and experimentally. Our analysis is based upon an extension of a Green´s function method to calculate the intermodulation distortions. Contrary to the intuitive expectations that the nonlinear gain dependence on carrier density would add to the other sources of distortion leading to an increase in the total distortion