Abstract
There is no doubt that the concept of volume holography has led to an incredibly great amount of scientific research and technological applications. One of these applications is the use of volume holograms as optical memories, and in particular, the use of a photosensitive medium like a photopolymeric material to record information in all its volume. In this work we analyze the applicability of Kogelnik’s Coupled Wave theory to the study of volume holograms recorded in photopolymers. Some of the theoretical models in the literature describing the mechanism of hologram formation in photopolymer materials use Kogelnik’s theory to analyze the gratings recorded in photopolymeric materials. If Kogelnik’s theory cannot be applied is necessary to use a more general Coupled Wave theory (CW) or the Rigorous Coupled Wave theory (RCW). The RCW does not incorporate any approximation and thus, since it is rigorous, permits judging the accurateness of the approximations included in Kogelnik’s and CW theories. In this article, a comparison between the predictions of the three theories for phase transmission diffraction gratings is carried out. We have demonstrated the agreement in the prediction of CW and RCW and the validity of Kogelnik’s theory only for gratings with spatial frequencies higher than 500 lines/mm for the usual values of the refractive index modulations obtained in photopolymers.
Highlights
Photopolymers are useful for different applications due to the refractive index variations and relief profiles generated [1,2,3,4,5,6,7,8]
A stable formulation of the method was proposed by Moharam et al in 1995 [35]. We have chosen this version of the Rigorous Coupled Wave theory (RCW) to simulate the propagation of the light in the gratings recorded in photopolymers
In our comparison with experimental data we analyzed the holographic behavior of an acrylamide photopolymer in layers that range in thickness from 20 to 90 μm and spatial frequency between 500 and 1125 lines/mm
Summary
Photopolymers are useful for different applications due to the refractive index variations and relief profiles generated [1,2,3,4,5,6,7,8]. It is possible to multiplex many gratings in the material, trying to achieve high values of diffraction efficiency, and to measure the signal-to-noise ratio Using this method to characterize photopolymers as holographic materials the fittings of the gratings stored in them are of great importance. Using these fittings, parameters such as the refractive index modulation, optical thickness and the scattering coefficients can be calculated. In this work we will study the applicability of Kogelnik’s Coupled Wave theory to gratings recorded in photopolymers for usual values of parameters such as thickness or refractive index modulation. To carry out the experimental work we have used a polyvinyl-alcohol/acrylamide (PVA/AA) based photopolymer
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