Mode-locked lasers with nonlinear external cavities

A nonlinear element, when introduced into an external cavity, is shown to improve the mode locking of lasers. Mode locking is achieved by inducing coupling between the cavity modes, thus permitting more efficient transmission of phase information. We begin by discussing a phenomenological laser model for homogeneously broadened systems. The model laser is coupled to an external cavity and contains a nonlinear element. The returning pulse from this external cavity is then mixed with a circulating pulse in the laser at the output mirror. We have considered two nonlinear elements, a saturable absorber and a saturable amplifier. Although these elements have quite different pulse-shaping effects, they both cause considerable improvement in mode-locked performance.