There are two major techniques to measure ps - fs pulses of the mode-locked lasers:
I) Steak camera
II) Optical autocorrelation
According to the space-time transformation in second-order autocorrelator, what is the function of the nonlinear crystal ( KDP or BBO) leading to the interferometric autocorrelation to display the amplitude-coherence behavior of laser sources?
our optical detectors response time will be in the range of nano second so it is not possible to resolve the time less than the nano second. In the measurement pulse width of pico and femto second laser pulses, we we will use second order correlations by the space-time transformations. basically we spatially separate the two beams along the longitudinal direction of laser beams (in the propagation direction as like Michelson interferometer) and then make interfere the two beams within the nonlinear crystal. Harmonic generation is intensity dependent process (second order correlation) and we will vary the optical path difference in the range of pulse width in the spatial domain. in the scanning process we can measure the intensity of Harmonic signal as a function of path difference between the two beams and the temporal conversion of this gives the pulse width of laser beam.
The NL crystal is used to generate the SHG signal. For temporally separated pulses the SHG signal is constant in intensity, irrespective of the magnitude of the pulse delay. In case the pulses start to overlap in time and space on the crystal and the interferometer is stable, the SHG signal is oscillatorily modulated with respect to the pulse delay due to the interference of the two pulses. The generated signal is instantaneous, as such the temporal duration of your signal reflects the pulse duration best. If the SHG signal is separately detected behind the crystal, the result is the second order autocorellation function which you can easily use to calculate the pulse duration, phase modulation and spectrum.
If your question has practical purpose, then you can consider direct detection of pulses with duration > 20 ps by photodiode and oscilloscope. There is a lot of fast photodiodes at the market with rise time
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