Generation of chirp-free picosecond pulses

The frequency spectrum of moderately chirped laser pulses depends
upon the portion of the beam which is accepted by the spectrometer.
Observation of the development of the chirp in a mode-locked pulse
train allows to determine the small incipient chirp of early
pulses. A product, bandwidth times pulse duration, of 0.47 ± 0.03
is consistently observed for single pulses switched from a
passively mode-locked Nd-glass system.