Optical gating

Measuring ultrafast signal requires either a ultrafast detector (like a streak camera) or to exploit some properties of the signal.
The optical gating technique can be used when the signal is repeated periodically. Each pulse is mixed on a non-linear crystal with a delayed reference pulse (the gate) to generate a sum-frequency signal. For each value of the delay the time-averaged intensity of the sum-frequency is proportional to the product of the signal and the gate. Therefore, varying the delay it is possible to measure directly the cross-correlation between the signal and the gate. If the gate is extremely short (so that can be approximated to a Dirac delta) or has a known shape it is possible to recover the time profile of the signal with a very good accuracy and a femtosecond resolution using a slow detector.