Manufacturing and reproduction of holographic optical elements in dichromated gelatin films for operation in the infrared
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 and high modulation capacity. Such properties ensure the attainment of the desired diffraction efficiency, bandwidth and operation in the IR. These objectives are achieved by means of precise control of the thickness and of the hardness of the holographic film 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 geometry, of the reconstruction wavelength, and of the refractive index modulation. For a given geometry and constant layer thickness, it is the index of refraction modulation that determines the IR shift in the operating wavelength of the hologram. The desired film thickness of 10 micrometers is achieved by means of nozzle deposition and exhibits 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 for IR operation.