Design of a dimple mirror system for uniform illumination from an MIR-FEL

In order to generate a flat-topped beam profile from a tunable, broadband, mid-infrared free-electron laser (MIR-FEL), we have proposed and designed a dimple mirror system (DMS), using a laser optics calculation code LOCCO. The principles of the DMS are similar to those of a micro-lens array system involving a principal focusing lens and a two-dimensional (2D) array of micro-lenses located ahead of the principal lens. The DMS is composed of a parabolic mirror, which acts as the principle lens and a 2D array of micro-dimples, which splits the incident FEL beam into many partial beams. All beamlets overlap near the focal point of the parabolic mirror, resulting in the generation of an approximately flat-topped intensity profile due to the interference of light. We proved that the DMS can generate a flat-topped profile even over a wide MIR waveband. Thus, we showed the applicability of the DMS for uniform illumination from a tunable, broadband MIR-FEL.