Spectrum broadening in femtosecond pulses by cascaded second-order nonlinearities and the influence of temporal walk-off

Pulse compression is a well-established technique for shortening pulses generated by oscillators or amplifiers. In practice, pulses can be compressed using two stages, one for spectrum broadening and the other for dispersion compensation. A commonly used technique consists of self-phase modulation in an optical fiber and the group-velocity dispersion provided by gratings or prisms. Such compressors, however, have several drawbacks, such as coupling loss, limitation on pulse energies, etc. Recent research has shown that cascaded second-order nonlinearities can generate nonlinear phase shift similar to the optical Kerr effect. We report spectrum broadening in femtosecond pulses by cascaded nonlinearities, and the numerical study on it. We particularly investigated the influence of the temporal walk-off between fundamental and second harmonic pulses. A mode-locked Ti:sapphire oscillator and a regenerative amplifier generate a 120 fs, 800 nm pulse at 1 kHz repetition rate. The BBO crystal is 7x7x7 mm/sup 3/ size and cut for type I phase matching angle.