<title>Influence of the dichromate concentration on the holographic properties of dichromated gelatin layers</title>

Abstract

Dichromated gelatin (DCG) is a commonly used material for recording holographic optical elements. DCG has good optical properties as for example low scattering and high resolution. A photochemical effect causes a change of the refractive index after exposure and development of the DCG- layer. An important parameter for the holographic properties of the DCG-layer, e.g. maximum attainable modulation of the refractive index and swelling of the layer after development, is the concentration of dichromate in the DCG- layer. A further advantage of DCG is the attainable high modulation of the refractive index. In this paper we present experimental results, which give the maximum modulation of the refractive index and the swelling of the DCG-layer as a function of the concentration of dichromate. The results are obtained by analyzing volume phase gratings. The 'coupled wave theory' of Kogelnik gives a formal connection between the modulation of the refractive index and the diffraction efficiency of volume phase gratings. The latter is determined by analyzing the dependance of the diffraction efficiency on reconstruction angle. The exposure energy was varied to attain the maximum modulation of the refractive index for every dichromate concentration. The angular deviation of the Bragg-angle compared with the recording parameters yields the swelling of the DCG-layer. The diffraction gratings were recorded with a holographic copying process. the knowledge of these dependencies is necessary to facilitate optimization of the DCG-layer for different applications of holographic optical elements.