Dark self-enhancement in dichromated-gelatin gratings: a detailed study

Strong dark self-enhancement, the increase in diffraction efficiency after irradiation, has been reported previously for dichromated-gelatin gratings. Here a more-detailed study of this self-enhancement effect is presented. The self-enhancement in dichromated-gelatin gratings was of the order of 10-100, depending on the pH and on the dichromate concentration of the solution used for the preparation of dichromated-gelatin films. The highest self-enhancement gain occurred when pH was high and dichromate concentration was low. The increase in the diffraction efficiencies of the gratings continued over 3 months.     

Fabrication of three-dimensional photonic crystals with two-beam holographic lithography

A holographic technique used to fabricate three-dimensional photonic crystals with a two-beam interference method is presented. In the optical setup of fabrication one beam is incident on the recording plate in the direction of the plate normal and the other beam with an angle to the normal. Three exposures were taken. Between each exposure, the recording plate was rotated 120 degrees on axis until three exposures were completed. Good three-dimensional lattice structures have been obtained. Theoretical analysis, computer simulations, and experimental results are presented.     

Computer-generated holograms on a metal ion-doped polymer system: contact copying

Dichromated poly(acrylic acid) films with dimethyl formamide have been used as a real-time recording medium for volume transmission holograms. In this study a simple computer-generated holographic grating with a sinusoidal amplitude profile is copied onto this recording material by the contact-copying technique. The experimental setup for the contact copying of a computer-generated hologram (CGH) is explained, and the phase modulation is observed for a different exposure time. Theoretical and experimental diffraction efficiency values for the CGH copy are also evaluated. Photographs of the original and copy of the CGH on the photopolymer film are presented.     

“Hidden” recording of phase holograms with self-amplification in a polymer with photoinduced diffusion

The possibility of minimizing the diffraction efficiency of a volume phase holographic grating while recording in a medium with diffusion self-amplification is considered. The recording process is modeled to demonstrate that the diffraction efficiency can be varied from 0.02 to 0.003 (50–250 gain). The results of the analysis are confirmed by the experimental data.     

Effect of Temperature Change and Storage Time on Holographic Gratings in KBr Crystals with Colour Centres

Abstract

Holographic gratings have been recorded in additively and electrolytically coloured KBr crystals at wavelengths of 514·5 and 632·8 nm respectively. The storage properties of additively coloured crystals have been investigated for 7 years. It has been proved that the storage times less than 4 years are reasonable. It has also been verified that temperature changes during the recording process are the main reason for low diffraction efficiencies. Finally the influence of the relaxational self-enhancement of the diffraction efficiency on the storage properties of these crystals has been studied.     

Digital-holographic interferometry with an image-intensifier system

A method for recording digital holograms on an image intensifier coupled with a CCD sensor is presented. The advantage of the image intensifier is that it can be gated (electronic shutter action produced by controlling of the image intensifier's photocathode voltage). This allows us to record holograms with a short exposure time. Two holograms of an object submitted to dynamical displacements (e.g., vibrations) are recorded by two short exposures. The phase of the wave front recorded at different times is calculated from the recorded intensity by use of a digital Fourier-transform method. By comparison of the phases recorded it is possible to get the displacement of the object during a short interval. Experimental results are presented, and the problems related to the noise and to the spatial resolution are discussed.     

Frequency and phase swept holograms in spectral hole-burning materials

A new hologram type in spectral hole-burning systems is presented. During exposure, the frequency of narrow-band laser light is swept over a spectral range that corresponds to a few homogeneous linewidths of the spectrally selective recording material. Simultaneously the phase of the hologram is adjusted as a function of frequency-the phase sweep function. Because of the phase-reconstructing properties of holography, this recording technique programs the sample as a spectral amplitude and phase filter. We call this hologram type frequency and phase swept (FPS) holograms. Their properties and applications are summarized, and a straightforward theory is presented that describes all the diffraction phenomena observed to date. Thin FPS holograms show strongly asymmetric diffraction into conjugated diffraction orders, which is an unusual behavior for thin transmission holograms. Investigations demonstrate the advantages of FPS holograms with respect to conventional cw recording techniques in freq ncymultiplexed data storage. By choosing appropriate phase sweep functions, various features of holographic data storage can be optimized. Examples for cross-talk reduction, highest diffraction efficiency, and maximal readout stability are demonstrated. The properties of these FPS hologram types are deduced from theoretical considerations and confirmed by experiments.     

Analysis of the absorptive behavior of photopolymer materials. Part I. Theoretical modeling

Photopolymers have received a great deal of attention due to their broad range of applications. The variation of their absorptive behavior during exposure is pivotal to the study of such materials. A model combining the associated electromagnetics and photochemical kinetics is presented to describe these absorptive processes. Such a model is critical in describing both self-modulations during holographic recording and also self-focusing effects. To describe the photophysical and photochemical changes taking place, a modulated equivalent electrical conductivity is introduced. Temporal variations of the concentrations of dye, monomer, and polymer are then predicted using the modified nonlocal photopolymerization driven diffusion model. The numerical convergence of the model is examined. Comparisons between the predictions of the model and experimental results, for both acrylamide/polyvinyl alcohol and Phenanthrenequinone doped poly(methyl methacrylate) photopolymer materials, are presented and analyzed in Part II of this paper.     

High-rate coding system for magnetic recording

A coding system for magnetic recording channels combining a high-rate error detection code with a list Viterbi algorithm (LVA) is proposed. This coding system can provide a coding gain of up to 2-3 dB for a modified extended extended partial response class IV (EEPR4) channel. A pipelined implementation of the LVA is presented, and an accurate estimation technique for the LVA performance is developed     

Scanned-picture-beam holography for nonrigid objects

A technique is described for holographically recording nonrigid objects by scanning the picture beam in a small, bright spot across the object in a series of short exposures. The reference beam is incident across the entire recording medium during each exposure. This technique can freeze the motion of objects subject to ambient forcings, such as acoustic vibrations. Experiments are described in support of this method. Exposures are in the 1- to 10-ms range with 100 separate object regions. An analysis of certain factors affecting image quality, such as diffraction efficiency, is also presented.     

Statistical Model for Noise Gratings Recorded in Volume Holograms

Abstract

A statistical model is used to study the formation of noise gratings in holograms and the mechanism that causes radiation scattering in them. The theoretical model presented describes noise scattering in terms of diffraction due to the high number of spurious gratings formed during the recording stage by the interference of a single reference beam with its own scattered radiation. Numerical results calculated from the theory showing the dependence of noise grating efficiency on exposure are compared with experimental results obtained with bleached emulsions, and good agreement is found.     

Hologram recording in polyvinyl alcohol/acrylamide photopolymers by means of pulsed laser exposure

The variation in transmittance produced when a photopolymer is irradiated with a pulsed laser is analyzed and experimental results obtained when diffraction gratings are stored using pulsed exposure are presented. In either case, the influence of the energy of the irradiation pulse, the number of pulses, and the pulse repetition rate were studied. The photopolymer used was an acrylamide/polyvinyl alcohol dry film with a yellow eosin-thiethanol-amine mixture as a photoinitiator system. The recording of the gratings was performed by use of a holographic copying process. The samples were exposed and holograms recorded with a collimated beam from a frequency-doubled Nd:YAG (532 nm) Q-switched laser. Our initial results show that it is possible to obtain diffraction gratings with a diffraction efficiency of 60% and a refractive index modulation up to 2.8 x 10(-3). The energetic sensitivities achieved are close to those obtained with the same material and continuous irradiation without a preprocessing of the gratings.     

Fourier analysis of high-spatial-frequency holographic phase gratings

Plane-wave holograms were recorded on Agfa–Gevaert 8E75HD holographic plates, in a wide range of bias exposures and fringe visibilities. Plates were processed by developer AAC and bleaching agent R-9. Phase gratings were studied by phase-contrast microscopy, using a high-power immersion (100×) objective. Phase-contrast photomicrographs were Fourier analysed. Thus first-, second- and third-order modulations of the refractive index as functions of bias exposure and visibility of the recording interference pattern could be determined. Relative amplitudes of the higher-order modulations to that of the first-order modulation can serve as a measure of the nonlinearity of the holographic recording. The results presented here can be used to check the validity of grating profile calculations based on higher-order coupled-wave theory.     

Multichannel holographic recording method for three-dimensional displays

A multichannel holographic recording method is presented for three-dimensional (3D) displays, utilizing pixel-based recording instead of image-based recording in order to realize parallel processing. The proposed approach is composed of two main stages. In the first stage, each two-dimensional (2D) image acquired from multiple viewpoints is partitioned by holographic recording channels (HRC) into nonoverlapping subimages. In the second stage, the corresponding pixels of the subimages are rearranged to constitute an encoding image. The encoding images are recorded simultaneously by each HRC, respectively, so the recording speed is improved significantly. The experimental results have demonstrated that the three-channel system is feasible and the full-parallax hologram reconstructed with white light is acceptable in quality. The three-channel system saves approximately 60% of the recording time in comparison with the single-channel system. More importantly, the proposed method can accomplish a large-scale final hologram composed of multichannel holograms without sacrificing the hologram quality. Several 3D imaging applications such as medical diagnosis and advertisements could benefit from this research.     

Measurement of mode patterns in a high-power microwave cavity

A wet thermal paper method for measuring of mode patterns and heat distributions in a high-power microwave multimode cavity is developed. The exposure time of the paper is evaluated. It is shown that this method allows measuring and recording mode patterns in the loaded and unloaded cavity, and the heat distribution inside the cavity with a load movement. The mode patterns and heat distributions along horizontal and vertical planes of the cavity are presented. Possible applications of the method in medicine and biology are discussed, and a calibration protocol of a microwave oven for microwave radiation exposure on cell cultures in the cavity is given.     

Esorex 2005

The European Study on Occupational Radiation Exposure called ESOREX was initiated by the European Commission in the 1997 year. The objectives of this European study are: (1) to provide the European Commission and the national competent radiation protection authorities with reliable information on how personal radiation monitoring, reporting and recording of dosimetric results is structured in European countries; (2) to collect reliable and directly comparable data on individual and collective radiation exposure in all occupational sectors where classified workers are employed. Therefore, it is important to receive information about the levels of individual personal radiation doses to workers in the different sectors and the trends and developments of these doses over a period of several years; (3) at present, all 25 European Union Member States, plus Bulgaria, Iceland, Norway, Romania and Switzerland, participate in the study; and (4) the study was executed under the leadership of German BfS in co-operation with Czech SUJB. First results and analyses based on the data collected in the previous studies are presented in the paper 'Frasch, Petrová: Dose trends in occupational radiation exposure in Europe-Results from the ESOREX project'. As a result of a call for tender of the European Commission/DGTREN in the year 2003, the new ESOREX study called 'ESOREX2005' has been initiated. This study will end at the year 2007 and its main objectives are-to finalise the updating of the country-wise reports by describing the current situation in the field of occupational exposure control, evaluation and registration of personal doses of radiation workers and as a second part of the study, to collect dosimetric data for the period 2001-2005.     

Dynamics of optical data recording in thin layers of chalcogenide glassy semiconductors

Temporal variation and spatial distribution of the temperature in thin films of chalcogenide glassy semiconductors (CGSs) during laser data recording have been studied. A model describing the dynamics of CGS film heating by laser pulses is proposed and a comparative analysis of theoretical results and experimental data is presented. Methods for determining some physical characteristics of CGS films relevant to the optical data recording are considered.     

Influence of background exposure on detection and determination limits for a TL dosimetry system based on LiF:Mg,Cu,P(GR-200A)

Thermoluminescence dosemeters are widely used to monitor personal doses. For these low dose range applications, it is important to determine the detection limit L(D) and the determination limit L(Q) of the dosimetric system. The influence of background exposure on these limits for LiF:Mg,Cu,P(GR-200A) based TL dosimetry was investigated. Both the conventional analysis and the glow curve analysis methods were used to determinate these limits. The detection limit L(D) was compared with the recording level and the investigation level. A systematic error can occur in the occupational dose evaluation when the detection limit L(D) is more than the recording level. It was found that the L(D) of the dosimetric system-based LiF:Mg,Cu,P(GR-200A) was less than the recording level for exposure time tau > or = 10 days considering an annual dose limit of 1 mSv for the public recommended in ICRP Publication 60.     

Noise filtering in the interference pattern by dynamic holographic recording in photorefractive Bi(12)TiO(20) crystals

The possibilities of stabilization of the interference pattern by filtration of a random-phase noise caused by vibrations, turbulence, and other local changes in the wave front in interferometric measurements are investigated. Dynamic holographic recording in photorefractive Bi(12)TiO(20) crystals is used. The parameters of the holographic recording are presented for determination of the dynamic range for filtering. Noise filtering takes place in real time and contributes to the enhancement of the contrast and the signal-to-noise ratio of the interference pattern. This results in a considerable increase in the sensitivity and the accuracy of the interferometric measurements.