Daniel J. Coleman

Controlled shifting of the spectral response of reflection holograms

Dichromated gelatin (DCG) is regarded as one of the best holographic recording materials due to its grainless structure yielding high resolution, and its capability of reconstructing wave fronts with high diffraction efficiency (DE). This material, unfortunately, is insensitive to light of wavelength much longer than 540 nm. Much work has been done to extend the spectral sensitivity of DCG to the red wavelengths of the He-Ne and Kr lasers. Adding suitable dyes such as methylene green or methylene blue to the dichromate sensitizing solution yields dye-sensitized dichromated gelatin (DSDCG). High DE was reported for plates coated with these materials. It was found, however, that these high DE values could not be achieved in one of the construction geometries we use, namely, the back-mirror geometry, Fig. 1. The spherical wave front here represents the reference beam which passes through the photosensitized layer striking the mirror. The wave front reflected by this mirror forms the object beam. Recording in this geometry requires a double pass through the sensitized layer. Absorption at the construction (exposing) wavelength must be kept low to achieve a beam ratio compatible with needed fringe visibility and desired high DE. Simultaneously, though, high absorption of the DSDCG layer at the construction wavelength is needed for good photo-

Holographic optical configurations for eye protection against lasers

Holographic rejection (notch) filters are used for eye protection against laser radiation because they allow high visual transmission since they reflect only a narrow spectral bandwidth. Their angular dependence, typical of diffractive and interference filters, can be minimized or avoided when the filters also perform as holographic optical elements with particular optical configurations. These configurations are described for two eyewear geometries, and possible options between theoretical design, practical filter construction limitations, and spurious images are investigated.