Pulse shape measurements using differential optical gating of a picosecond free electron laser source with an unsynchronized femtosecond Ti:sapphire gate

We have measured the temporal pulse shape of a mid-infrared free electron laser using a 60 fs Ti:sapphire pulse as an optical gate. By employing a differential technique to simultaneously measure the instantaneous infrared intensity and its time derivative, we have reconstructed the pulse shape with sub-ps resolution, despite jitter the order of 50 ps between the two lasers. The experiment has been performed with two different types of gating methods: sum-frequency generation, which provides an instantaneous gate, and photo-induced plasma absorption, which supplies a step function gate. Differential optical gating can be generalized to include other gating methods, such as the optical Kerr effect or saturable absorption, thus permitting detailed pulse shape studies on a variety of optical processes, especially in the far-infrared, where conventional pulse shape measurement methods are more difficult to implement.