<title>Effects of process control and exposure energy upon the inner structure and the optical properties of volume holograms in dichromated gelatin films</title>

Abstract

Holograms in dichromated gelatin (DCG) are of potential interest in many technical sectors. The current technology facilitates the manufacturing of large format (1 m<SUP>2</SUP>) holographic optical elements (HOE) operating between 400 nm and 2000 nm for various applications: photovoltaic two color solar concentrators, holographic stacks for daylighting and wideband passive shading devices for sun control, as well as spectrally matched concentrators for solar chemistry. For all these applications the optical properties of the HOEs have to be controlled carefully. By precise control of all the necessary manufacturing steps this can be achieved. The developing process has an high impact upon the optical properties of the HOE. Therefore, an excellent knowledge of the effect of the process on the inner structure of the gelatin is essential for achieving the desired optical properties. In this paper we describe the influence of the different developing baths and of the exposure energy on the inner structure of holograms in DCG. With increasing dehydration speed the swelling gradient becomes larger leading to broader reflection holograms and to asymmetric angular spectra of transmission gratings. With increasing exposure energy saturation of the refractive index modulation is attained. Higher exposure energies lead to a deformation of the refractive index profile and even `thick' transmission gratings exhibit higher diffraction orders with significant efficiencies. Models for the deformation of the refractive index profile and for inhomogeneous swelling across the layer of transmission holograms are presented. Theoretical simulations are compared to experiments and show excellent agreement.