Figure 6. Schematic of a two-colour optical cross-correlator.

The input pulses with arbitrary temporal overlap are combined with a dichroic mirror (DM) for collinear propagation and focused into a type-I phase-matched BBO crystal. In this ‘first stage’, the intensity of the SFG signal depends on the timing of the input pulses. The SFG signal is separated using another DM and measured with ‘detector 1’. The input pulses then pass through a quartz plate, are retro-reflected and pass the quartz plate again, so that one of the pulses is delayed with respect to the other by a fixed amount due to the different group velocities of the different wavelengths in quartz. When they are focused a second time back into the BBO crystal, another SFG signal is produced in the ‘second stage’ and measured with ‘detector 2’. Combination of the two signals gives the relative delay without sign ambiguity. The influence of amplitude fluctuations cancels out in a feedback loop, which keeps the temporal relation of the pulses fixed such that the combined signal is zero.