Abstract

The recent observation^1,2 of self-mode-locking in Ti:Al₂O₃ lasers has shown that solid-state lasers can generate femtosecond pulse trains with an average power exceeding the watt level. At such a power level, it can be estimated that the nonlinear phase shift due to the Kerr nonlinearity in the laser rod can be as high as unity; hence, self-focusing from the spatial variation of this phase shift is likely to modify the beam shape to favor the generation of short pulses. In this paper, we describe the operation of such lasers using a numerical model, where propagation, self-focusing, and gain reshaping are treated exactly, and an approximate ray matrix model which assumes Gaussian beams. For appropriate resonator parameters, the average power increases with laser intensity, a situation favoring self-mode-locking; the beam shape is then modified and the saturated gain profile is such that a cw signal falls below threshold. At large intensities, the output power drops and beam filamentation may develop. Experiments have revealed that the output power and beam size are different in the mode-locked and cw regimes.

© 1991 Optical Society of America