Image Number 5 for United States Patent #5079772.
A passively mode-locked laser (110) is disclosed that includes a resonant cavity (112, 114) having a gain medium (116) therein. A transmissive element (116), which may be the gain medium, is also located in the cavity and is formed from a material which varies the two-dimensional, lateral spatial profile of the beam with respect to intensity due to the Kerr effect. The resonator is arranged such that the round trip gain of the system increases with respect to the intensity of the beam so that mode-locking operation can be achieved. In one approach, the variation in the spatial profile is used to increase the extraction of energy from the gain medium. In another approach, the variation in spatial profile which occurs with an increase inintensity is used to decrease the loss in the system so that pulse operation is favored. In an illustrated embodiment, wherein the gain medium (116) is formed from titanium-sapphire, the self-phase modulation of the crystal is also used to compress the pulses into the femtosecond range. In addition, a prism set (136, 138) can be used to balance the group velocity dispersion and self-modulation of the gain crystal to achieve stable, soliton-like operation.