Modified pinhole spatial filter producing a clean flat-topped beam

Abstract

The expanded beam from a laser has a sharply peaked intensity profile. As a result, in many applications where uniform illumination of an extended field is required, it is only possible to use the central part of the beam. We show that if the pinhole normally used to spatially filter the beam is replaced by an annular phase mask, it should be possible to obtain a clean beam providing very nearly uniform illumination over an extended area, with a minimal loss of light.