Femtosecond pulse phase measurement by spectrally resolved up-conversion: application to continuum compression

The authors have developed an extension of the usual cross-correlation method in order to get the amplitude and phase of a femtosecond light pulse. This is done by analyzing the spectra of the upconverted pulse obtained by mixing the initial pulse with the reference pulse delayed with different times. The theory is worked out in closed form for Gaussian pulses and then checked with simulations for arbitrary fields. It is shown that the phase derivative of any probed pulse can be directly recovered in the limit where the reference is narrowband. This technique is used to measure the chirp induced at the output of a femtosecond Ti:sapphire oscillator. This technique is then applied to the determination of the exact pulse shape of a 21-fs compressed continuum at 770 nm using, as a reference, the initial fundamental 65-fs pulse at 620 nm