Caustic surfaces in a Keplerian two-lens beam shaper

The refractive, aspheric beam shaper in its Keplerian configuration has a focus between the two lenses and near that focal point are caustic surfaces on which the optical intensity, in the geometrical optics approximation, is infinite. We use the intensity law of geometrical optics to find the caustic surfaces and the irradiance everywhere in the caustic region. The theoretical results are in good agreement with measurements on an aspheric beam shaping lens using a CCD camera.