Abstract

By taking advantage of the two-photon absorption (TPA) in a semiconductor device^1,2 the loss in the laser-resonator is controlled with picosecond precision (optical limiting). We have used a thin GaAs-etalon for loss control in the Nd-glass laser resonator (TEM_00q-mode) which stabilizes the intensity at about 200 MW/cm² inside the laser cavity. The experimental set-up comprising the active element, the saturable absorber, the aperture for mode-selection as well as pulse shortening and the nonlinear element is shown in Fig. 1. We have observed a modification of the characteristics of the pulse depending on the number of round- trips in the laser cavity due to intensity- dependent refractive index changes in the GaAs-etalon. The refractive index modification originating from the Kerr-nonlinearity in the rod and GaAs, the electron- hole-pair generation by TPA as well as the heat generation in GaAs has been estimated. Comparing the intensity-dependent contributions to the refractive index regarding to the propagation length in the medium, one finds a net negative refractive index change due to the free carriers. This causes a phase shift giving rise to a deviation both from the unperturbed beam propagation (self-defocusihg) and from the original frequency spectrum (self-phase modulation) of the pulse. Moreover, two- photon absorption is responsible for flattening of the spatial beam profile.

© 1994 Optical Society of America