Page "Optical autocorrelation" Paragraph 16

The generation of the second harmonic in crystals is a nonlinear process that requires high peak power, unlike the previous setup.

However, such high peak power can be obtained from a limited amount of energy by ultrashort pulses, and as a result their intensity autocorrelation is often measured experimentally.

Another difficulty with this setup is that both beams must be focused at the same point inside the crystal as the delay is scanned in order for the second harmonic to be generated.