Contrast measurement of ultra intensity laser pulses

A third order correlator is built based on the technique of cross correlation between the fundamental and second harmonic waves of femtosecond lasers. Using this correlator, the contrast ratio of femtosecond laser pulses from a typical CPA laser system is measured. The measurements show that the contrast ratio of the system is about 10６. There is a strony ASE beside the main pulse, lasting for a long time. The seed contrast ratio is about 10５. There is a little pulse at the 2ps before the main pulse and a small stage at the two sides of main pulse．