Features of a phase-only shaper set for a long rectangular pulse

The generation of electron pulses of high peak current and low emittance in a 3-GHz radio-frequency electron gun requires the illumination of the photocathode by a rectangular-shaped 10-ps light beam with a rise time of less than 1 ps. The generation of this kind of laser pulse needs the insertion of a programmable pulse shaper into a femtosecond laser system. The shaping system that we discuss is a spectral phase-only filter. A genetic algorithm, in conjunction with a proper cost function, is used to determine the phase filter function for the desired square-pulse generation. Simulations show that the system is highly sensitive to the properties of the input signal and to alignment. Third-harmonic generation, as required by photoemission, leads to some relaxation in the settings of the system.