Timing sychronization between a colliding‐pulse mode‐locked dye laser and a gain‐switched Fabry–Perot‐type AlGaAs laser diode has been achieved with less than 40 fs of relative timing jitter by using a pulsed optical phase lock loop. The relative timing jitter was measured using the error voltage of the feedback loop which has a 5 kHz bandwidth. This technique of measuring the relative timing jitter is accurate since the frequencies of all the timing fluctuations fall within the loop bandwidth. The novel element in the implementation of the pulsed optical phase lock loop is the time delay discriminator which is based on a cross correlation between the two pulse trains. Under locked conditions, the output of the cross correlator operates quiescently about a point of nonzero temporal slope.