Accurate focal spot diagnostics based on a single shot coherent modulation imaging

A single-shot method based on coherent modulation imaging is presented for the diagnostics of the focal spot of laser facilities. The laser beam to be measured first illuminates a highly random phase plate with a known structure and subsequently the intensity of the resulting diffraction pattern is recorded by a charge-coupled device positioned behind the phase plate. Intensity distribution at the focus of the laser beam is accurately reconstructed with the coherent modulation imaging method. The feasibility of this method is demonstrated with an experiment involving a He–Ne laser. (letter)