Generation of flat-top picosecond pulses by means of a two-stage birefringent filter

We describe the type of pulse shaper for production of flat-top pulses that is used at the Photo Injector Test Facility (PITZ) at DESY. This shaper consists of a two-stage birefringent filter. Using a simple model, this pulse shaper can be regarded as a system that splits the Gaussian input pulse into four mutually delayed replicas and stacks them in a phase-coherent way. Although a variety of different pulse shapes can be generated by this method, the system is particularly suited for the generation of flat-top output pulses with duration between 10 and 100 ps. When operating in combination with an actively modelocked Nd:YLF oscillator, it can shape picosecond pulses arranged in long trains, as required for photocathode lasers for the TESLA-type linear accelerators. Using trains of equally shaped picosecond pulses has an additional benefit: It permits measuring the shape of these pulses by cross-correlation in real-time using a so-called optical sampling system. Such a system is employed as a measurement tool for appropriate alignment of the individual components of the pulse shaper.