Holography plays an important role in optical data storage. In this review, a detailed description of the existing recording material is given under four major classifications based on the holographic recording mechanism and nature as: photopolymerizable systems, photocrosslinkable systems, doped polymer systems and other systems.

Photopolymerizable formulations exhibiting a fair sensitivity in the near IR were developed. These materials were formulated of a mixture of acrylate multifunctional monomers and oligomers with an initiating system working through a photoredox process. Depending on the spectrum of the sensitizers, this system absorbs various laser lines in the red (Kr⁺ at 676, 752 and 799 nm or laser diode at (lambda) > 750 nm). In particular, it was holographed with a monomode laser diode emitting in the 780 nm region, and bright transmission gratings were obtained. Typically, an exposure of ca. 500 mJ/cm² is necessary to reach complete inactivation of the material and to produce gratings with diffraction efficiency exceeding 40% with a spatial frequency of 800 lines/mm. The quality of the holographic recording was studied by recording multiple series of holograms of a resolution test target. In addition, these materials that contain crosslinkable monomers and convert to a tridimensional network upon irradiation, show a self-processing character allowing storage of optical information in situ. The applicability of this material to real time holographic interferometry was demonstrated.

New holographic recording material based on photopolymerizable systems have contributed significantly to recent growth of holographic applications. Previously, we report that on photopolymerizable system with the presence of the difunctional monomer, ethylene glycol dimethacrylate improves the behavior of the system and particularly to elucidate the role played by eosin ester with oxooxime group in the production of amine initiator radicals. This comparative study has been carried out in our laboratory using differential scanning photocalorimetry and holography. The results of the new photosensitive recording material for holography indicate that this system can be used for the formation of very promising photopolymer with better performance. The aim of this work has been changed the crosslinking monomer to decrease the energetin sensitivity and know the rest of the behavior. The new photopolymerizable mixture containing pentaethritol triacrylate in the same relation with the ethylene glycol dimethacrylate mixture. A diffraction efficiency of 80% is achieved with a energetic sensitivity of 3 J/cm² at 514 nm, and the spatial resolution is up to 2000 l/mm.

Volume holograms may be used to make rejection filters which have narrow spectral bandwidths. A unique, and potentially useful, feature of such holographic filters is that the reflecting planes (Bragg planes) need not be parallel to the substrate. For example, spherical holograms (holograms of a spherical mirror) can be made on flat substrates. The slant angle, defined as the angle between the Bragg planes and the substrate, in this case will increase continuously from the center of the hologram to the edge. It has been observed, however, that when such holograms are made in the DuPont photopolymers the playback wavelength is a function of the slant angle. Under identical exposure and processing conditions the playback wavelength increases with increasing slant angle. The degree of shift with slant angle is affected by the presence and type of cover film or cover material used to protect the photopolymer during exposure and processing. In addition to the anomalous spectral shift, the line shape of the reflection band is distorted and the distortion is a function of the playback angle. The results of the experimental observations are reported and discussed.

A number of helmet mounted displays have been designed in recent years. The majority of these designs have been dismissed on the grounds of safety when considering the optical configuration. The principle of the design discussed in this paper is that of a display projected using the visor, where it acts not only as the display visor but also provides windblast, laser, and nuclear flash, protection. The display is projected by means of a holographic combining element on the visor. This paper addresses the design and manufacture of the holographic components employed, discussing the use of DuPont photopolymer material for this application. Off-axis holograms configured around the geometry of the head/helmet, give a wide field of view, with good eye relief. Well defined reflective and transmissive properties of the hologram are required to give full system compatibility. The resultant system does not compromise the safety of the user, whilst maintaining a fully integrated man/machine interface, i.e. low mass, and manageable center of gravity.

DuPont has developed holographic photopolymer films capable of producing high diffraction efficiency in full-color volume holograms and holographic optical elements. The properties of these films allows for a greater range of applications than has been considered feasible for color holograms. The recording mechanism and methods for holographic color recording in these materials is considered. Methods for full-color mastering and copying to achieve optimal performance in three-color recording are discussed. Examples of full-color holograms are presented.

The precise knowledge of the mean refractive index and the thickness of holographic films is important in applications such as polarization-sensitive holographic optical elements or substrate-mode holography. We present results of the measurements of the mean refractive index and the thickness of dichromated gelatin films before exposure, uniformly exposed films, and holograms. The measurements are based on the thin film resonance method. The interference between the two waves reflected at the air-film surface and the film-substrate interface modulates the reflectivity of the holographic film as a function of the angle of incidence. The frequency and the amplitude of this modulation are analyzed in order to determine the optical parameters. With the measured mean refractive index and the thickness as input we simulated the angular response of volume gratings and compared the results of the modelling with experimental data. The excellent agreement between the simulated and measured diffraction efficiencies confirm the applicability of the method to holographic films.

Independent characterization of changes in film thickness and refractive index is necessary for accurate prediction of the Bragg playback conditions of volume holograms after processing. We have developed a method which uses weak holographic mirrors to characterize processing- induced swelling or shrinkage and index change in volume holographic films, and have applied this method to DuPont OmniDex^TM holographic recording films. Results of these measurements are presented.

Color holograms recorded in panchromatic, single-layer ultra-high-resolution silver-halide emulsions have been previously reported. Color reflection holography presents no problems as regards the geometry of the recording setup, but the final result is highly dependent on the recording material used. The processing of such emulsions is critical in order to obtain high diffraction efficiency and good color rendering. In particular, preventing emulsion shrinkage is extremely important. The recording procedure and the processing steps will be described.

The sensitivity of a fine-grained silver halide VRP photoplates exposed by laser radiation over a wide range of durations (10² - 10^-3 sec) is studied. The drastic deviation of sensitivity in the time range from 10^-9 sec to 10^-13 sec is found and discussed.

Holography has shown its suitability for optical storage technology by developing high information density and fast access type memories with a high read-out efficiency. In this work, we are presenting the results obtained on poly(methyl methacrylate) matrices doped with five new azo-dyes. Dynamic holograms with reasonably high diffraction efficiency have been recorded at 488 nm and these photosensitive holographic matrices are capable of self- erasing as soon as the writing lights are cut off. The efficiency of the write, read and erase (WRE) cycles in these azo-dye doped polymer materials was determined. A complete WRE cycle can be performed within a time interval of 10 to 20 seconds. Numerous (> 250 cycles) WRE cycles have been recorded on the same area of the holographic medium without noticeable fatigue or alteration in the diffraction efficiency. The maximum real-time diffraction efficiencies achieved reached up to 10%.

The performance of photorefractive polymers has improved considerably over the last few years, leading to the recent observation of 86% diffraction efficiency and a net two-beam coupling gain of more than 200 cm^-1 in a poly(N-vinylcarbazole)-based polymer doped with the azo dye 2,5-dimethyl-4-(p-nitrophenylazo)anisole. This performance by far surpasses the other organic PR materials reported to date. We report experimental results that evaluate the suitability of this polymer composite for dynamic holography and other applications.

The photon-gated intensity effect upon holographic storage for thin layers of polyvinyl alcohol matrices doped methyl orange dyes is investigated. The dynamic curves for grating growing and decay are given for different gating intensity. The time that grating gets to saturation becomes short. The maximum diffraction efficiency of the grating gets little with the rise of gating light intensity. The proper intensity Ar-ion laser can erase grating. The erasing velocity is proportional to erasing light intensity. The modulation transfer function is measured for writing beams having different polarization states.

An experimental study to ascertain the recornbination action of zinic chloride ZnC1) and triethanolarnjne(TEA) on improving photosensitivity in rnethylene blue sensitized dichrornated gelatin(MBDCG) holograms has been carried out. The required unslanted volunie holographic transmission gratings in MBDCG have been recorded using 633nm light from a He— Ne laser. The experiments show three important results: First, the ZnCI and TEA have great recombination action on the photosensitivity of MBDCG. Next, the diffraction efficiency vs light exposure energy graph is bell—shaped. Last, the maximum diffraction efficiency reaches 83 under 200rnj/ctn2 of light exposure. Keywords: volume hoigratu, diffraction efficiency, dichrotaated gelatins red- sensitive hoigraphic uaierial, photosensitivity, modulation of refractive iadex, holographic transuission grating.

The studies have been performed on new volume (about 1 mm thick) recording medium based on porous glasses and light-sensitive silver compounds. It was shown that the materials of the sort makes it possible to create holograms with high values of phase modulation not only in visible, but also in near infrared spectral regions. The essential feature of the media is the fact that the parameters of the developed holograms are independent of impregnation of free volume with immersion filler having the refractive index equal to that of the framework. That makes it possible to obtain high efficiency holograms with low level of light scattering and high optical quality of reconstructed wavefront.

A method for producing a large format (multiple square meters) holographic multistereogram on the high sensitive photoresist REFO 200 M will be presented. The basic images are acquired by video, movie film or generated by computer systems. They will be used to drive a light modulator. On examples it will be shown that this light modulator can be photographic film, a LCD screen or a scanning system. This method requires a high number of multiple exposures on the recording medium. This requirement and a possible solution will be discussed as the main subject of this paper.