Effects of dispersion in additive-pulse mode-locked lasers

The transient process of pulse shortening in a laser with a nonlinear external cavity is studied by numerical calculation. The effect of normal fiber dispersion on the steady-state output parameters is investigated. The main features of the pulse-shortening process can be understood by simple arguments. The numerical simulations indicate that pulse shortening is accompanied by an increase in the chirp of the master resonator pulse. It is shown that a chirped master resonator pulse in the presence of dispersion diminishes the pulse shortening and results in longer stationary pulse. If the chirp is strong enough, pulse shortening is even reversed into pulse lengthening. Our conclusions are supported by experimental observations.