Generation and amplification of temporally `square´ optical pulses for the FEL photoelectric injector

The authors discuss the application of self-phase modulation and grating pulse compression to the generation of optical pulses suitable for controlling the electron beam emittance and energy spread in a FEL (free-electron laser) photoelectron injector. Pulse compression in a single-stage pulse compressor, with background reduction based on the nonlinear birefringence of the optical fiber, yields 5-10-ps Gaussian pulses. Temporarily square pulses with 15-ps FWHM (full width at half maximum) and 5-ps risetime have been obtained through self-phase modulation and group-velocity dispersion of the 5-ps pulses in a second fiber. Pulse shaping through the use of nonlinear birefringence in optical fibers is also discussed