Theory of mode-locked semiconductor lasers.

We present a theoretical study of semiconductor mode-locked lasers at a phenomenological level. We use the slow absorber model of New and Haus, but extend the analysis by taking into account the shift in the gain maximum due to the changing number of carriers. In our analysis of the resulting equation, we show that due to stability requirements the shortest attainable pulse duration is limited. The use of self-phase modulation due to a slow medium can at most lead to a shortening of the pulse by a factor of 2. We show that the shifting of the gain maximum due to the changing number of carriers in the laser is a crucial aspect in the operation of mode-locked semiconductor lasers. We further find that the end mirrors must be reflecting more than about half of the light intensity to prevent a self-pulsation type instability similar to the relaxation oscillation

Self-diffusion of particles interacting through a square-well or square-shoulder potential

The diffusion coefficient and velocity autocorrelation function for a fluid of particles interacting through a square-well or square-shoulder potential are calculated from a kinetic theory similar to the Davis-Rice-Sengers theory and the results are compared to those of computer simulations. At low densities the theory yields too low estimates due to the neglect of correlations between subsequent partial collisions of identical pairs; in particular, the neglect of boundstate effects appears important. At intermediate densities the theory makes reasonable predictions and at high densities it produces too high values, due to the neglect of ring terms and other correlated collision events. The results for the square-shoulder potential generally exhibit better agreement between theory and simulations than do those for the square-well potential