Influence of gain nonlinearity on the second order harmonic distortion in semiconductor lasers

The semiconductor laser is commonly used as a light source in fiber-optical telecommunication systems. In order to be able to send as much information as possible in a short time, it is important that the laser has a large modulation bandwidth, i.e., the turn-on and turn-off time should be as short as possible. In analogue fiber optic systems for transmission of radio or television signals, it is also important that the light from the laser increases linearly with driving current. Otherwise, the transmitted signal will become distorted. The modulation bandwidth and the modulation distortion are dependent both on the laser structure and the gain characteristics of the active material. In order to develop a correct laser model for calculation of, e.g., distortion one need an accurate model of the dependence of gain on carrier and photon density. We show that different gain models, fitted to give exactly the same modulation response can have significantly different distortion behavior.