Self-developing, glycerin-containing, thick-layer bichromated gelatin as a medium for recording volume holograms

The synthesis of layers of glycerin-containing, self-developing bichromated gelatin between 100 and 500 μm thick is described and the holographic characteristics of this light-sensitive material are discussed. Experimental data obtained by measuring the diffraction efficiency of holograms of two plane waves recorded using a symmetric system for layers of different thickness and various ammonium bichromate concentrations showed that the optimum layers for hologram recording are around 200 μm thick and have an ammonium bichromate concentration of 2–2.5% by weight of dry gelatin. The sensitivity of these layers is 5–10 J/cm². Pis’ma Zh. Tekh. Fiz. 25, 64–69 (March 12, 1999)     

Multiplexed counter-beam speckle holograms in bichromated gelatin

Experimental data on the recording and multiplexing of shifted counter-beam holograms with a speckle reference beam are presented. The experiment confirmed the possibility of multiplexing shear speckle holograms by shifting the photosensitive material by a distance equal to the reference beam speckle size. As the number of multiplexed holograms increases, their lifetime in a layer of bichromated gelatin significantly decreases.     

Recording volume holograms in dye-activated aqueous gelatin gel

Holographic characteristics of a thick photosensitive material based on a dye-activated aqueous gelatin gel have been studied. Holograms recorded using second harmonic radiation of YAG:Nd³⁺ laser with a pulse duration of 17 ns showed a diffraction efficiency of ～70% and an angular selectivity of ～20′.     

A thin-layer bichromated gelatin for holography sensitive in the red spectral range

A variant of thin-layer optically sensitive material for holographic applications, based on a bichromated gelatin with glycerol and Methylene Blue dye additives, is suggested. The material is sensitive in the red spectral range, possesses the property of self-development, and provides for the possibility of the realtime reconstruction of holographic images. The bichromated gelatin is prepared using potassium bichromate.     

Polarization recording of holograms in partially polarized recording light

A theoretical approach is considered for media whose scalar and anisotropic responses are of opposite sign. The nondiffracted beam, and the imaginary and real images formed by a polarization hologram are analyzed under these conditions. It is shown that the imaginary image has its polarization transformed compared with the object field while a pseudoscopic object field reconstructed in terms of polarization state and degree is formed in the real image. Pis’ma Zh. Tekh. Fiz. 23, 38–42 (February 12, 1997)     

Methylene-blue-sensitized dichromated gelatin holograms with antihumidity polymer coatings

An efficient method of resisting humidity for methylene-blue-sensitized dichromated gelatin (MBDCG) holograms is reported. The method uses a viscous liquid pure poly(styrene) solution with a low degree of polymerization as the coating material. On the basis of this coating, the diffraction efficiency of treated MBDCG holograms exposed to a high-humidity (relative humidity of 80-85%) environment was not obviously changed during observation over 3 mon.     

Transient volume holograms in polymethylmethacrylate

New transient volume holographic recording materials are described in this report. These consist of either coronene or 1,2;5,6-dibenzanthracene incorporated into polymethylmethacrylate. The largely amplitude, volume holograms formed as a result of triplet-state absorption have lifetimes of 6.4 and 0.9 s respectively and diffraction efficiencies close to the predicted values of 10⁻⁴. Production of permanent holograms has been prevented by careful control of the polymerization conditions to produce a matrix of sufficient rigidity to prevent reactions between the triplet states and the polymer. By using spectrally well separated write and read wavelengths, erasure of the holograms by the read beam is avoided.     

Duplication of phase key for random-phase-encrypted volume holograms

Holographic memory encrypted by an optical random-phase key and decrypted by either the original phase key or a duplicate key is proposed and demonstrated. The duplicate key is made by recording the encryption wave front with angle multiplexing during writing of the hologram. The amount of three-dimensional shifting that is tolerable in the duplicate key is analyzed.     

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.     

Polarization-selective volume holograms: general design

General design of polarization-selective volume holograms is discussed in detail. The required diffraction angles for these highly polarization-selective elements are calculated. An effective indexmodulation parameter is defined and used to calculate the required index-modulation value at any specified operating wavelength. Design examples of operating wavelengths at 780, 830, 1050, 1300, and 1550 nm are given. Highly polarization-selective substrate-mode grating pairs for 780-nm operation were fabricated to verify the idea. These elements are suitable for applications in optical switching networks and magneto-optic data storage systems.     

Control of the spectral position of dichromated gelatin reflection holograms

Dichromated gelatin (DCG) is one of the best light-sensitive materials for holographic recording. However, strict technological control should be observed during deposition, gelation, and drying of DCG plates and the chemical processing of the recorded structures. Usually, for reflection holograms the gelatin layer should be sealed in order to protect it from ambient humidity, which affects the value and position of the reflection maximum. Herein we give three different recipes for tuning the reflection maximum from 430 to 700 nm by swelling the hologram thickness with a filler material that is commercially available. The mechanism of the hologram swelling, which changes the period of the recorded holographic structure, is also briefly discussed.     

Model for the effects of material shrinkage on volume holograms

A model for analyzing the effects of material shrinkage on volume holograms is presented. This model is based on the fringe-plane rotation model used for describing the effects of plane-wave grating holograms that undergo shrinkage. A computer was used to exercise the model for a simple input object typical of those used in digital holographic memory applications and stored as a Fourier-transform hologram. The theoretical formulation of the model is presented as well as the results of the numerical analysis.     

Double-layer method for increasing the bandwidth of UV spectrally responsive holograms in dichromated gelatin

We present a method for increasing the bandwidth of reflection holograms in dichromated gelatin. The recording plate has two dichromated-gelatin layers of different hardness so that their shrinkage at the end of processing is different. We use this method for doubling the bandwidth of holographic mirrors that are designed for solar UV energy concentration. We also study the superposition of two double-layer holograms, which results in UV holographic mirrors with a bandwidth of 60 nm.     

Silver halide sensitized gelatin holograms in Slavich PFG-01 red-sensitive emulsion

Abstract

Silver halide sensitized gelatin (SHSG) is an interesting technique for the production of holographic optical elements. It combines the high sensitivity of photographic emulsions with the well-known low scattering and high diffraction efficiency corresponding to dichromated gelatin. In this paper we describe SHSG holograms recorded on Slavich PFG-01 emulsions. We will show that real high diffraction efficiencies can be obtained (as high as 93% allowing for reflections) when using this material for recording phase transmission holograms. The influence of the bleach bath temperature and a hardening bath in the processing was studied and the procedure was optimized for these two parameters. Our investigations reveal that PFG-01 emulsion is not only a substitute for Agfa material, but also offers better results, at a moment in which Agfa has stopped its production of holographic material.     

Digital recording and numerical reconstruction of holograms: an optical diagnostic for combustion

Holographic interferometry (HI) has proved to be a useful tool for nonintrusive temperature measurements in flames (and thereafter for inference of the local composition based on the state relationship approach) with high spatial and temporal resolution. Digital holographic interferometry (DHI) is a relatively new imaging and measurement technique that electronically records a hologram (e.g., on a CCD) and reconstructs it by a numerical method. Cumbersome chemical processing of the hologram is avoided in DHI, which thereby provides greater flexibility, speed, and the potential for real-time processing. In conventional holography, fringes that are neither bright nor dark on a hologram cannot be accurately resolved. The DHI technique has not yet to our knowledge been used for combustion applications. Herein we evaluate its efficacy for making temperature measurements in flames and assess its applicability through a simulation. Each part of a double exposure associated with the holographic technique is considered to be recorded by a hypothetical CCD sensor at a separate time from the other part. We applied the principles of Fourier optics to develop two numerical methods for hologram reconstruction, and we show that both methods provide an accurate reconstruction of the phase image associated with a flame. Because of the periodic nature of the wave function, the reconstructed phase values are limited to the interval [-pi/2, pi/2]. Thus an unwrapping algorithm is provided that produces a continuous phase distribution based on the condition that the reconstructed phase is jumped by a value of -pi or pi. We have also developed an iterative calculation method to adjust the value of the digital reference wave parameters that determines the phase imaging reconstruction in DHI.     

Spherical beam volume holograms for spectroscopic applications: modeling and implementation

The spherical beam volume hologram, recorded by a plane wave and a spherical beam, is investigated for spectroscopic applications in detail. It is shown that both the diffracted and the transmitted beam can be used for spectroscopy when the hologram is read with a collimated beam. A new method is introduced and used for analysis of the spherical beam volume hologram that can be extended for analysis of arbitrary holograms. Experimental results are consistent with the theoretical study. It is shown that the spherical beam volume hologram can be used in a compact spectroscopic configuration when the transmitted beam is monitored. Also, on the basis of the properties of the spherical beam hologram, the response of a hologram recorded by a plane wave and an arbitrary pattern is predicted. The information can be used to optimize holographic spectrometer design.     

Controllable Fabry-Perot interferometer based on dynamic volume holograms

We present experimental results for a high-resolution interferometer based on dynamic volume holography in photorefractive materials. The proposed interferometer is able to detect small angular and phase deviations separately. It has no free spectral range limitations and is comparatively insensitive to light intensity fluctuations.     

Content-addressable data storage by use of volume holograms

Data stored as volume holograms-optical interference patterns imprinted into a photosensitive storage material-can be accessed both by address and by content. An optical correlation-based search compares each input query against all stored records simultaneously, a massively parallel but inherently noisy analog process. With data encoding and signal postprocessing we demonstrate a holographic content-addressable data-storage system that searches digital data with high search fidelity.     

Electric-field multiplexing of volume holograms in paraelectric crystals

Electric-field multiplexing (EFM) results from the tuning of the effective wavelength of the light beam inside a photorefractive crystal. This tuning results from the application of an external electric field to the crystal during holographic recording. We demonstrate the high Bragg selectivity of this multiplexing technique in paraelectric crystals and compare it with the selectivity obtained in the ferroelectric phase. The effects of the two major physical parameters of working in the paraelectric phase, the temperature and the external electric field applied during the writing stage, are investigated. Experimental results of the EFM of three image-bearing holograms recorded in reflection geometry are presented along with a qualitative analysis of the Bragg selectivity in paraelectric crystals.