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 ∼ 2 ps electron bunches in the Mark III rf linac with an offset of 17° 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 μm 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.