INFLUENCE OF SPATIOTEMPORAL REFRACTIVE-INDEX CHANGES IN A GAAS ETALON ON THE GENERATION OF ULTRASHORT PULSES IN A ND-PHOSPHATE GLASS-LASER

It is shown for the first time that self-phase modulation due to a carrier-dependent refractive index change in a GaAs etalon used for optical limiting has to be taken into account in ultrashort pulse forming arrangements along with its spatial counterpart, i.e. self-defocusing. Self-phase modulation leads to a spectral broadening and an asymmetric frequency shift. Self-defocusing combined with diffraction at an aperture is responsible for pulse shaping. Optical limiting restricts the intracavity intensity to values of about 200 MW/cm(2) preventing the Kerr nonlinearity in the laser rod to be effective. Pulses with a duration of 2 ps and an optimum spatial profile have been found in the later stage of the lengthened pulse train.