A dry polymeric mixture rendered red sensitive with a dye has been used to record interference gratings and holograms. Diffraction efficiencies of ∼10% can be attained with exposures of ∼94 mJ/cm2. If necessary the reading step can be done just after the exposure or after a fixing step performed with light from a mercury lamp.

Dry polymer for holographic recording.

Optical techniques for data storage have advanced rapidly during the last decade. Optical data storage presents many unique advantages, notably the high storage density and low access time, not attainable by conventional recording techniques. The special features of optical recording, the proposed materials and techniques, the components for optical storage as well as some representative optical bit-by-bit and holographic recording systems are reviewed. It is noted that in spite of the lack of widespread commercial success of the optical recording at the present, some unique devices have been successfully demonstrated. The knowledge accumulated in the development of optical data storage should be useful for many recent Applications such as video recording. It is expected that when the conventional magnetic memory technology reaches its limit, optical storage technology should be a competitive contender for the next generation data storage.

An overview of optical data storage technology

The general applicability of dynamic theory to the description of the recording and readout characteristics of volume (thick) hologram gratings is indicated. In dynamic theory (as opposed to static theory), the volume nature of the thick holographic grating allows the interference of an incident light beam with its own diffracted beam inside the recording medium. This effect causes the continuous recording of another grating that alters the initial one, producing a resultant grating that is not uniform through the thickness of the recording material and a grating whose writing and reading characteristics may vary dramatically depending on the recording material and the experimental conditions. A large number of diverse types of writing, reading, and angular selectivity behavior have been reported in the published literature. The dynamic theory of thick hologram writing and reading is shown to predict qualitatively all of these various types of experimental behavior.

Use of dynamic theory to describe experimental results from volume holography

A method for calculating arbitrary-order diffraction efficiencies of thick, lossless transmission gratings with arbitrary periodic grating shapes has been developed. A Fourier-series representation of the grating is employed, along with a coupled-mode theory of diffraction. For illustration, numerical values of the diffraction efficiencies at the first three Bragg angles are calculated for sinusoidal, square-wave, triangular, and saw-tooth gratings. Numerical results for the same grating shapes with the same parameters are also calculated for comparison, by extending Burckhardt's numerical method for analyzing thick sinusoidal gratings. The comparison shows that the coupled-mode theory provides results with relative computational ease and results that are in agreement with calculations obtained by extending the more-rigorous Burckhardt theory to nonsinusoidal grating shapes and to higher-order Bragg angles.

Calculation of arbitrary-order diffraction efficiencies of thick gratings with arbitrary grating shape*

Based on the theory of coupled waves the amplitude of the first-order diffracted wave and the diffraction efficiency is calculated for sequentially superposed plane gratings that are stored in dielectric transmission volume holograms. Using suitable approximations, both the Bragg-angle incidence and any deviations can be described. The grating interaction effects make it possible to couple the incident energy from one wave to another and to build up beam-couplers and splitters with certain coupling parameters. These coupling properties are adjustable in a definite manner by the recording process.

Diffraction Efficiency of Sequentially Stored Gratings in Transmission Volume Holograms

Many workers are turning away from holograms made with thin photographic emulsions and are investigating ‘thick’ holograms, with their exciting potential of high diffraction efficiency, increased information storage capacity, reduced noise, and increased spectral or angular discrimination. This article discusses the principles of thick hologram theory, and describes some of the materials which can be used as thick recording media. Advantages and limitations of thick holograms are illustrated by experimental examples. The article concludes with a survey of the existing and future applications for thick holograms.

M.R.B. Forshaw

Papers

Thick holograms: a survey