Second-harmonic generation frequency-resolved optical gating in thesingle-cycle regime

摘要

The problem of measuring broad-band femtosecond pulses by thentechnique of second-harmonic generation frequency-resolved opticalngating (SHG FROG) is addressed. We derive the full equation for the FROGnsignal, which is valid even for single-optical-cycle pulses. The effectnof the phase mismatch in the second-harmonic crystal, the implicationsnof the beam geometry, and the frequency-dependent variation of thennonlinearity are discussed in detail. Our numerical simulations shownthat, under carefully chosen experimental conditions and with a propernspectral correction of the data, the traditional FROG inversion routinesnwork well even in the single-cycle regime. The developed description ofnthe SHG FROG signal was applied to measure the white-light continuumnpulses in the spectral region of 500-1100 nm. The obtained spectralnphase of these pulses served as a target function for the pulsencompressor design. The pulses produced by compression around 800 nm werenalso characterized by SHG FROG. The resulting pulse duration measuresn4.5 fs which corresponds to ~2.5 optical cycles