Recording of 6000 holograms by use of spectral hole burning

Experiments verifying a new method of storing spectral hole-burning holograms, which yields reduced cross talk as compared with standard spectral hole-burning holograms, have been conducted. Results demonstrating the reduced width of this type of hologram in both frequency and the applied electric-field dimension are presented. Analytic solutions for the spectral width and diffraction efficiency of these holograms are presented. Using this exposure technique, we have recorded 6000 holograms in a single spectral hole-burning sample.     

Holographic optical elements recorded in silver halide sensitized gelatin emulsions. Part 2. Reflection holographic optical elements

Silver halide sensitized gelatin (SHSG) holograms are similar to holograms recorded in dichromated gelatin (DCG), the main recording material for holographic optical elements (HOEs). The drawback of DCG is its low energetic sensitivity and limited spectral response. Silver halide materials can be processed in such away that the final hologram will have properties like a DCG hologram. Recently this technique has become more interesting since the introduction of new ultra-fine-grain silver halide (AgHal) emulsions. In particular, high spatial-frequency fringes associated with HOEs of the reflection type are difficult to construct when SHSG processing methods are employed. Therefore an optimized processing technique for reflection HOEs recorded in the new AgHal materials is introduced. Diffraction efficiencies over 90% can be obtained repeatably for reflection diffraction gratings. Understanding the importance of a selective hardening process has made it possible to obtain results similar to conventional DCG processing. The main advantage of the SHSG process is that high-sensitivity recording can be performed with laser wavelengths anywhere within the visible spectrum. This simplifies the manufacturing of high-quality, large-format HOEs, also including high-quality display holograms of the reflection type in both monochrome and full color.     

Gratings formed during nonlinear recording of superimposed holograms

Nonlinear recording of superimposed holograms has been theoretically studied. In the case of holograms recorded by method of angular or phase multiplexing, nonlinearity of the recording process leads to the appearance of gratings of two types, both meeting Bragg’s condition in the course of reconstruction. The gratings of one type change the hologram diffraction efficiency, while those of the other give rise to diffractional cross-talk. The strength of gratings responsible for the cross-talk decreases until vanishing with increasing mutual orthogonality of the hologram object waves.     

Dammann gratings as phase diffusers in Fourier holography

Fourier-transform holograms of periodic data display intense diffraction orders. To provide accurate storage, the holographic recording medium must have a large dynamic range. Dammann gratings can be used as deterministic phase diffusers to redistribute spatially the irradiance pattern of the hologram and thereby decrease the required dynamic range.     

Holographic optical elements recorded in silver halide sensitized gelatin emulsions. Part I. Transmission holographic optical elements

Silver halide sensitized gelatin (SHSG) holograms are similar to holograms recorded in dichromated gelatin (DCG), the main recording material for holographic optical elements (HOE's). The drawback of DCG is its low sensitivity and limited spectral response. Silver halide materials can be processed in such a way that the final hologram will have properties like a DCG hologram. Recently this technique has become more interesting since the introduction of new ultra-high-resolution silver halide emulsions. An optimized processing technique for transmission HOE's recorded in these materials is introduced. Diffraction efficiencies over 90% can be obtained for transmissive diffraction gratings. Understanding the importance of the selective hardening process has made it possible to obtain results similar to conventional DCG processing. The main advantage of the SHSG process is that high-sensitivity recording can be performed with laser wavelengths anywhere within the visible spectrum. This simplifies the manufacturing of high-quality, large-format HOE's.     

Holographic image storage and multiple hologram storage in a planar Methyl Red-doped liquid crystal film

Holographic images are recorded and the diffraction characteristic of the hologram is investigated in a planar Methyl Red-doped liquid crystal film without an external electric field. Multiple holograms are stored in this film with no external electric field, and peristrophic multiplexing, angular multiplexing, and spatial multiplexing are demonstrated to be suited for the implementation of azo-dye doped liquid crystal films among the multiplexing techniques. The diffraction efficiency is theoretically studied for multiple storage. Optical reconstruction for peristrophic multiplexed holograms and angular multiplexed holograms is discussed, which involves the peristrophic rotation angles and the recording angles, respectively.     

Reflection volume holographic scanners with field-curvature corrections

A new type of polygonal holographic scanner that combines a reflection volume hologram with a computer-generated hologram (CGH) is described. The scanner is free from the aberration of field curvature. Such a scanning system can allow for a compact folded version of the scanner and Bragg diffraction into only a single order. The equations expressing the spatial-variable image distance are derived and are fit to the phase function designated by polynomials incorporated into a CGH in terms of the least-squares method. A reflection scanner with field-curvature correction is made by interfering a diffracted wave front from this CGH with a spherical wave front having scanning focal power through a second plane hologram. Experiments demonstrating the feasibility of this scanner are presented. Raster-scan patterns using a multifaceted scanner are shown. Helpful data on the diffraction efficiency and the spectrally diffracted intensity of reflection holograms are also presented.     

Experimental analysis in recording transmission and reflection holograms at the same time and location

Holographic recording media with a reflection layer are useful because they make it possible to maintain backward compatibility with CDs and DVDs, and a conventional servo system is easily attachable. The incident beam is fed back to the recording layer by the reflection layer, so there are four beam pairs to record the transmission and reflection holograms. We analyze the basic property of the transmission and reflection holograms and evaluate the problem when the transmission and reflection holograms are recorded at the same time. It is shown that the shrinkage in the photopolymer medium has a different effect on each hologram, so the readout image from the two holograms is misaligned. Those diffraction beams make the interference pattern, and the signal-to-noise ratio (SNR) of the output image decreased. Taking into account the difference in wavelength selectivity between the transmission and the reflection holograms, we propose a way to select one hologram to get the diffraction beam and eliminate the interference pattern using the tuning readout wavelength. By using this method, we can eliminate the diffraction beam from the reflection hologram and keep a high SNR.     

Two methods of reducing the dynamic range of a holographic filter

Two methods of reducing the dynamic range of the transmittance of computer-generated interconnection holograms are presented and compared. The holograms are used in an optical implementation of an associative memory to connect the input and the output planes but are representative of more generalN(4) interconnection holograms. Because the holograms play a double correlation-reconstruction role, the standard spectrum-smoothing techniques (e.g., random phase) cannot be applied. We show, in computer simulations and optical experiments, that by using deterministic phase functions that can be realized in the optical system (defocusing the hologram or controlling the phases of the diffraction spots of a Dammann grating used in the system input) the hologram dynamic range can be lowered, reducing the errors during the hologram binarization and increasing the hologram's diffraction efficiency.     

Coding of Many Phase Objects Using Computer-generated Binary Holograms

A method for coding many phase objects by means of a binary synthetic hologram is presented. The paper gives a vectorial analysis of the spectrum of multi-exposure synthetic holograms and the analysis of the storage capacity of the method. On the basis of theoretical considerations and experimental results, the errors caused by the overlapping of the first diffraction order reconstructed wavefront with beat frequencies are discussed.     

Optimal recording wavelength for maximum diffraction efficiency of thermal fixing in LiNbO3:Fe

Optimal recording wavelength for maximum diffraction efficiency of thermal fixing in LiNbO(3):Fe crystal is investigated. Holographic gratings are recorded using three typical recording wavelengths including 488, 514, and 633 nm. Optimal switching from recording to thermal fixing is taken into consideration. The fixed holograms are developed by an original recording setup. Diffraction efficiencies of recording and thermal fixing are measured by a two-wave coupling technique. The theoretical and experimental results are analyzed and compared. With a blue beam, the nonvolatile hologram with maximum fixing efficiency is achieved. This work can obtain high persistent diffraction of the nonvolatile holographic storage in LiNbO(3):Fe crystals.     

Method for the characterization of hologram processing

Abstract

A novel method for the characterization of the processing of both absorption and phase holograms is proposed. Differently from the previous models, the square root of the diffraction efficiency of the processed hologram was directly related to the amplitude of the optical density modulation obtained at the developed step. This characteristic is a good indicator of the degree of nonlinearity of the hologram processing. While the Lin functions of phase holograms are similar to those of absorption holograms, the shape of the proposed function is completely different. The optical density and diffraction efficiency of holograms recorded using Agfa–Geveart 8E75HD plates and processed with AAC developer and a solvent bleach without a fixation step were measured and used to demonstrate the method.     

Photopolymer system and its application to a color hologram

We present a new type of photopolymer system for recording reflection holograms. The photopolymer system with radical and cationic photopolymerization controlled by the wavelength of light gives us enhanced diffraction efficiencies and a balanced recording sensitivity (20-60 mJ/cm(2)) in the blue to red region of the spectrum. In color hologram recordings, diffraction efficiencies of ? 60% are obtained when a photopolymer film is used that is composed of different spectrally sensitive photopolymer layers.     

Photorefractive and Photovoltaic Effects in Doped LiNbO 3

Photo-induced currents and voltages determine the photorefractive process in doped LiNbO 3, which can be utilized for the reversible storage of thick phase holograms. Optical recording and erasure are studied in the spectral region between 1·5 and 4 eV (lambda; = 0·8 ? 0·3 μm). The influence of various transition metal dopants, chemical treatments, temperature and external electric fields on the storage properties is investigated. The results yield consequences for write-read-erase applications and for read-only storage of stacked phase holograms.     

Characteristics of optical recording on thin films of quaternary glasses Cu-As-Se-I

The characteristics of optical recording and changes in the hologram parameters of the amorphous quaternary system Cu-As-Se-I are described. The dependence of the relative contrast of recording on the exposure and transparency properties of the film before and after erasure is the basis for discussion of these processes. Investigation into the dependence of diffraction efficiency of the holograms indicates that these quantities are determined by the thin film composition.     

Moiré pattern encoded extended fractional Fourier transform security hologram

This paper describes a simple method for making extended fractional Fourier transform (EFRT) based Moiré pattern encoded security holograms. These security holograms contain multifold concealed and encoded anticounterfeit security features that can only be read through a key hologram and periodic patterns in the final reading process. The encoded features in these holograms are concealed and unknown to the counterfeiter. These features are encoded separately for each individual recording in angularly multiplexed extended fractional Fourier transform hologram (EFRTH). The principle of recording and reconstruction of the proposed security hologram along with experimental results are presented.     

Suppression of undesired diffraction orders of binary phase holograms

A method to remove undesired diffraction orders of computer-generated binary phase holograms is demonstrated. Normally, the reconstruction of binary Fourier holograms, made from just two phase levels, results in an undesired inverted image from the minus first diffraction order, which is superposed with the desired one. This can be avoided by reconstructing the hologram with a diffuse light field with a pseudorandom, but known, phase distribution, which is taken into account for the hologram computation. As a consequence, only the desired image is reconstructed, whereas all residual light is dispersed, propagating as a diffuse background wave. The method may be advantageous to employ ferroelectric spatial light modulators as holographic display devices, which can display only binary phase holograms, but which have the advantage of fast switching rates.     

Comparison of diffraction efficiencies for single-exposure and unity-contrast multiple-exposure holograms

The diffraction efficiency for a single-exposure holographic recording is compared with that of a multiple-exposure hologram recorded with maximum signal-to-bias ratio. For the multiple-exposure recording the object is illuminated point by point, and the reference wave amplitude for each subexposure is adjusted to produce a unity-contrast interferogram in the holographic recording plane. It is shown that, for the same M-point object and the same average exposure, the ratio of the single-exposure diffraction efficiency to the mulitple-exposure diffraction efficiency is proportional to M/R, where R is the reference-to-object beam ratio in the single-exposure case. This result is compared with that obtained in the multiple-exposure case when the reference wave amplitude assumes a constant value. Application areas include optical interconnects, holographic data storage, and volumetric multiplexed holograms for display.     

Selective erasure of speckle-multiplexed holograms by use of a double Mach-Zehnder interferometric arrangement

A novel method to selectively erase and update speckle-multiplexed holograms in photorefractive crystals by use of a double Mach-Zehnder (DMZ) interferometric arrangement is presented. The DMZ arrangement automatically produces a pair of pi-phase-shifted interference patterns used for holographic recording, erasure, and update operations with a fairly simple optical configuration that consists of a commonly used dielectric multilayer beam splitter and two mirrors. The recording and the erasure conditions required for erasing a photorefractive hologram quickly and completely are discussed by calculating the temporal property of the hologram buildup and decay using the time-dependent coupled-wave equations. An experiment is also performed, in which arbitrary holograms in speckle-multiplexed holograms are selectively erased and updated with the simple DMZ optical configuration.     

Optimally tuned spatial light modulators for digital holography

Digital holography and comparative digital holography are applications that require computer-addressable modulators for the optical reconstruction of digital holograms. The quality of the reconstructed holograms depends on the modulator's properties. Therefore a characterization of the modulators is required. We show the result of a modulator characterization and the modulator's influence on the quality of the reconstructed hologram. We then compare qualitatively and quantitatively the optical reconstruction of phase and amplitude holograms by considering their level of detail and their diffraction efficiency.