Holographic recording on a curved substrate: investigation of a method to increase the numerical aperture in holographic optical lenses

This study investigates methods to enhance the numerical aperture (NA) of holographic optical lenses (HOLs) through innovative holographic recording techniques. The research explores the limits of conventional holographic recording of high NA lenses and presents a novel curved-recording technique with the potential to go beyond these limits. The objective is to obtain high NA HOLs suitable for use with LED sources and other highly-diverging input beams. The work first outlines how the maximum numerical aperture lens achievable through holographic recording is limited by the recording geometry. Next, the effect of curving the substrate during holographic recording, and later returning it to a planar shape for use, is explored in order to demonstrate that this could provide a means of fabricating a higher numerical aperture lens. HOLs are then recorded experimentally, firstly using the standard approach for high NA; the overlap of two beams at a plane photosensitive film with as large a beam aperture as possible, and the focal point as close to the (planar) photopolymer as the geometry will allow. Then, a similar arrangement is used, but with curved photopolymer substrates. Diffraction angles and efficiencies of the resulting holographic lenses are compared with the theoretically predicted Bragg diffraction. HOLs with a 20 mm aperture and focal lengths as short as 12 mm are demonstrated.