OBSERVATION AND CHARACTERIZATION OF FREQUENCY-CHIRPED OPTICAL PULSES ON THE MARK-III FREE-ELECTRON LASER

In an experimental program to develop and operate a chirped-pulse free-electron laser at Duke University, we have accelerated similar to 2 ps electron bunches in the Mark III rf linac with an offset of 17 degrees from the phase that minimizes the energy spread, and have observed (via optical autocorrelation measurements) compression of the energy-chirped electron bunches in the dispersive elements of the transport system by an amount consistent with theoretical predictions. The resulting 1.0 ps optical pulses from the laser exhibited a spectral width at 3.2 mu m equal to 3.6 times the transform limit, and were over-compressed by a factor of 2.7 in a dispersive delay line comprised of four sapphire, Brewster angle prisms. The measured time-bandwidth product and the observed over-compression both indicated a minimum compressed pulse width of approximately 150 fs. We present a detailed description of the experimental methods and results.