Recent Developments In Holographic Optical Elements

Abstract

Dichromated gelatin layers facilitate the design and fabrication of holographic optical elements (HOE) of high efficiency and quality. The research efforts are aimed at the development and evaluation of processes, such as exposure, film development and thermochemical after-treatment, which ensure the attainment of the desired diffraction efficiency, bandwidth and Bragg-shift. The emphasis is placed on the realization of homogeneous diffraction efficiency across the aperture of the HOE. These objectives are achieved by means of a precise control of the thickness of the holographic layer while maintaining simultaneously the capability to modify the refractive index modulation over a wide range. The diffraction efficiencies of transmissive and reflective gratings are unique functions of the state of polarization of the reconstruction wave. The transmissive and the reflective holographic gratings will diffract the perpendicularly polarized component while the parallel component will pass through the hologram without diffraction. Thus, holographic gratings of the transmissive and reflective types can serve as polarizing beamsplitters with an extinction ratio of 1000:1. The experimentally determined diffraction efficiencies show a departure from the predictions of Kogelnik's coupled wave theory.