<title>Development and investigation of dichromated gelatin film for the fabrication of large-format holograms operating at 400-900 nm</title>

Abstract

Dichromated gelatin layers facilitate the design and fabrication of holographic optical elements of high optical quality and diffraction efficiency. The research efforts are aimed at the development and evaluation of layer deposition techniques for the manufacturing of large- format holograms in dichromated gelatin. The emphasis is placed on the realization of DCG films that exhibit low scattering losses, high thickness constancy over the entire aperture, and especially high modulation capacity. Such properties ensure the attainment of the desired diffraction efficiency, bandwidth, and Bragg-shift. These objectives are achieved by means of precise control of the thickness of the holographic layer while maintaining the capability to modify the refractive index modulation over a wide range. The diffraction efficiency is a nonlinear function of the grating strength, i.e., of the layer thickness, of the wavelength and of the refractive index modulation. The phase of the transmitted light depends upon the spatial distribution of these parameters. The desired thickness of 10 micrometers is achieved by means of nozzle deposition and shows a thickness variation of +/- 1 micrometers over the 1 m2 aperture. The high capacity for index of refraction variation is realized by means of precise control of the flow velocity and of the water evaporation rate during the drying of the film. The exposure duration, the development process, and the subsequent thermal after-treatment of the film facilitate the attainment of the desired modulation characteristics. At present, we have achieved 90% diffraction efficiency at 900 nm for transmissive holographic gratings recorded at 514 nm. This technique permitted the attainment of 96% diffraction efficiency at 650 nm (maximum of the solar spectrum) and a band-width of 400 nm. Holographic solar concentrators are fabricated at present in sizes up to 0.5 by 0.5 m. Current developments will facilitate the fabrication of 1 m2 holographic solar concentrators. The results of the theoretical analysis are compared with the experimental data.