Generation of frequency-chirped optical pulses in a large-slippage free-electron laser

An optical output survey of free-electron lasers for infrared applications reveals the prominent role of a slow ramp of the electron beam energy in inducing a fast picosecond frequency chirp. Analysis of the optical pulses with a second-order autocorrelator and a double-grating pair shows a linear dependence of the chirp on the cavity desynchronization. The ramp technique gives spectral bandwidths that appear to be directly related to the sweep available to the electron beam energy within the cavity decay time. The sweep rate may be maximized by tinkering with the laser amplification process.