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

We have measured the temporal pulse shape of a mid-infrared free electron l aser using a 60 fs Ti:sapphire pulse as an optical gate. By employing a dif ferential technique to simultaneously measure the instantaneous infrared in tensity and its time derivative, we have reconstructed the pulse shape with sub-ps resolution, despite jitter the order of 50 ps between the two laser s. The experiment has been performed with two different types of gating met hods: sum-frequency generation: which provides an instantaneous gate, and p hoto-induced plasma absorption, which supplies a step function gate. Differ ential optical gating can be generalized to include other gating methods, s uch as the optical Kerr effect or saturable absorption, thus permitting det ailed pulse shape studies on a variety of optical processes, especially in the far-infrared, where conventional pulse shape measurement methods are mo re difficult to implement. (C) 1998 Elsevier Science B.V. All rights reserv ed.