CASCADED 2ND-ORDER EFFECTS IN THE FEMTOSECOND REGIME IN BETA-BARIUM BORATE - SELF-COMPRESSION IN A VISIBLE FEMTOSECOND OPTICAL PARAMETRIC OSCILLATOR

Cascaded second-order effects are studied in the femtosecond regime in beta-barium borate by use of the Z-scan technique. Large nonlinear phase shifts can be obtained near the second-harmonic-generation phase-matching condition for a Ti:sapphire laser. Solution of the nonlinear propagation equations for femtosecond pulses yields good agreement with experiment and also demonstrates that the description of cascaded effects by an effective nonlinear refractive index is no longer valid in the ultrafast domain and that these effects are less efficient for ultrashort pulses. The cascade-induced negative nonlinear phase shift in beta-barium borate is shown to be responsible for the self-compression observed in a prismless femtosecond visible optical parametric oscillator.