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)     

Thick-layer glycerin-containing bichromated gelatin for recording volume holograms

A thick-layer light-sensitive material consisting of glycerin-containing bichromated gelatin has been prepared and used to record real-time volume holograms with 0.44 μm helium-cadmium laser radiation. The thickness of the layers is 400–600 μm. The holographic sensitivity of the material is 1 J/cm². The highest diffraction efficiency of holograms recorded using a symmetric system with parallel beams is 32%. Pis’ma Zh. Tekh. Fiz. 23, 62–65 (April 12, 1997)     

Time modulation of the input signal during recording of holograms on hydrogenated amorphous silicon-liquid crystal structures

It is shown that the resolution and diffraction efficiency of p-i-na-Si:H-liquid crystal structures can be improved without causing any deterioration in the other parameters, by introducing amplitude modulation of the recording light beam synchronous with the supply voltage. Pis’ma Zh. Tekh. Fiz. 23, 20–25 (April 26, 1997)     

Digital recording and reconstruction of holograms in hologram interferometry and shearography

The fundamentals of digital recording and mathematical reconstruction of Fresnel holograms are described. The object is recorded in two different states, and the holograms are stored electronically with a charge-coupled-device detector. In the process of reconstruction the digitally sampled holograms are applied to the different coherent optical methods as hologram interferometry and shearography. If the holograms are superimposed and reconstructed jointly, a holographic interferogram results. If a shearing is introduced in the reconstruction process, a shearogram results. This means that the evaluation technique, e.g., hologram interferometry or shearography, can be influenced by numerical methods.     

Digital recording and numerical reconstruction of lensless fourier holograms in optical metrology

Advantages of the lensless Fourier holography setup for the reconstruction of digitally recorded holograms in holographic interferometry are presented. This very simple setup helps to achieve a maximum lateral resolution of the object under investigation. Also, the numerical-reconstruction algorithm is very simple and fast to compute. A mathematical model based on Fourier optics is used to describe discretization effects and to determine the lateral resolution. The recording and the reconstruction processes are regarded as an optical imaging system, and the point-spread function is calculated. Results are verified by an experimental setup for a combined shape and deformation measurement.     

Fixed holograms in iron-doped lithium niobate: simultaneous self-stabilized recording and compensation

We analyze the mechanisms leading to a highly diffractive fixed hologram in photorefractive Fe-doped lithium niobate crystals by simultaneous self-stabilized holographic recording and compensation at moderately high temperatures. We show that a partially compensated running hologram is produced during recording under this condition and discuss the performance of the process in terms of the operating temperature, the degree of oxidation ([Fe(3+)]/[Fe(2+)] ratio) of the sample, and the effect of the absorption grating arising from the spatial modulation of the Fe(2+) concentration produced during photorefractive recording. We experimentally measure the evolution of the uncompensated remaining hologram during recording and the evolution of the diffraction efficiency of the fixed hologram during white-light development and show that the maximum fixed grating modulation to be achieved is roughly limited by Fe-dopant saturation. A reproducible eta approximately 66% efficiency fixed grating was obtained on a sample exhibiting an otherwise maximum fixed eta approximately 3% when using the classical three-step (recording at room temperature--compensating at high temperature--developing at room temperature) process.     

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.     

Determination of threshold exposure and intensity for recording holograms in thick green-sensitive acrylamide-based photopolymer

For optical data storage applications, it is essential to determine the lowest intensity (also known as threshold intensity) below or at which no data page or grating can be recorded in the photosensitive material, as this in turn determines the data capacity of the material. Here, experiments were carried out to determine the threshold intensity below which the formation of a simple hologram--a holographic diffraction grating in a green-sensitized acrylamide-based photopolymer--is not possible. Two main parameters of the recording layers--dye concentration and thickness--were varied to study the influence of the density of the generated free radicals on the holographic properties of these layers. It was observed that a minimum concentration per unit volume of free radicals is required for efficient cross-linking of the created polymer chains and for recording a hologram. The threshold intensity below which no hologram can be recorded in the Erythrosin B sensitized layers with absorbance less than 0.16 was 50 μW/cm(2). The real-time diffraction efficiency was analyzed in the early stage of recording. It was determined that the minimum intensity required to obtain diffraction efficiency of 1% was 90 μW/cm(2), and the minimum required exposure was 8 mJ/cm(2). It was also determined that there is an optimum dye concentration of 1.5 × 10(-7) mol/L for effective recording above which no increase in the sensitivity of the layers is observed.     

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)     

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.     

High-efficiency holograms fixed in lithium niobate after recording using a digital fringe stabilization system

We used a digital feedback control loop system to produce reproducible fixed volume transmission holograms of high diffraction efficiency. Different strategies were investigated to obtain holograms of good quality and the highest refractive index modulation depth. Using this control system, we were able to record holograms with stationary fringes. Additionally to using the stationary fringe recording, a double recording-fixing schedule resulted in being the most appropriate one to produce reproducible holograms of better characteristics. This strategy is discussed and compared with other already established ones.     

“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.     

Digital recording and numerical reconstruction of holograms: a new method for displaying light in flight

We present a new method for displaying light in flight. Fresnel holograms are recorded directly on a CCD sensor, electronically stored, and numerically reconstructed. Experimental results are shown. From different parts of a single holographic recording, different views of a wave front can be reconstructed. This means that the temporal evolution of a wave front can be observed by numerical methods.     

High-efficiency volume holograms recording on acrylamide and N,N′methylene-bis-acrylamide photopolymer with pulsed laser

In order to achieve a better understanding of the mechanisms of hologram formation and higher diffraction efficiencies in volume gratings stored in acrylamide based photopolymers, a crosslinker (N,N′methylene-bis-acrylamide) has been incorporated in the photopolymer to record holograms by pulsed laser exposure. The presence of this component increases the polymerization rate and refractive index modulation. The recording was performed using a holographic copying process. The original was a grating of 1000?lines/mm processed using silver halide sensitized gelatin. First, the effect of the pulse fluence was investigated. When the pulse fluence was optimized, the results obtained using the new composition of material were compared with those using the composition without a crosslinker. Using a pulsed laser at 532?nm the photopolymer without crosslinker presented diffraction efficiencies slightly less than 60%. On the other hand, when the crosslinker was introduced in the photopolymer composition, the diffraction efficiencies achieved were higher than 85%. The non-linearity of the material's response was also studied comparing the energetic sensitivity, diffraction efficiencies and index modulation of gratings recorded with pulsed and continuous laser exposure. This study was performed fitting the angular scan of each grating using Kogelnik's theory.     

Coupled-wave analysis of vector holograms. 2. Reflective gratings formed in photoanisotropic medium with uniaxial birefringence

The diffraction properties of reflective anisotropic gratings, which can be recorded in photoanisotropic media with uniaxial birefringence by three-dimensional vector holography, were characterized through the use of coupled-wave analysis (CWA). By investigating the perturbation of the dielectric tensor, we demonstrated that the gratings with sinusoidal distribution of the azimuthal angle of the optic axis diffract polarized light in which the ordinary and extraordinary components are converted for incident light. The polarization conversion was consistent with that calculated by a numerical method. In addition, it was shown that CWA enables highly accurate calculation of the diffraction efficiency with wavelength dispersion when the amplitude of the azimuthal angle is small.     

Stibnite-liquid crystal structure: A strongly nonlinear reversible recording medium

It is shown that stibnite can be used as a material for the photosensitive layer in an optically controlled structure of the photosemiconductor-liquid crystal type. The spectral characteristic of stibnite allows a highly sensitive recording medium to be obtained for writing holograms using He-Ne laser radiation. The maximum diffraction efficiency achieved with this medium was 37%. The modulation characteristic of the stibnite-liquid crystal structure exhibits pronounced nonlinearity, with the slope of both rising and falling regions being dependent on the applied bias voltage.     

Influence of the purity of the starting materials on the recording of dynamic holograms in Bi12TiO20 crystals

The parameters of Bi₁₂TiO₂₀ crystals grown with bismuth oxide of different degrees of purity were measured by a holographic recording technique in an external ac electric field and were compared with a crystal specially doped with chromium. Various crystal parameters such as the diffusion length of the photoexcited carriers and the Debye screening length were determined. It was found that the absorption spectrum of the material and the holographic recording efficiency were strongly influenced by light chromium doping and by insufficient purity of the starting mixture. Pis’ma Zh. Tekh. Fiz. 23, 27–32 (March 12, 1997)     

Salient features of recording of the optical rectification signal from nonlinear optical crystals

Analysis and derived formulas were employed to examine the salient features of the process when the current and voltage due to optical rectification in crystals were used to measure the electric parameters of continuous, pulsed, and pulsed-periodic lasers, with allowance for spatial dispersion. The errors of measurement were evaluated. Meters based on optical rectification show promise for use in laser fusion facilities and for measuring the efficiency of harmonic generation. Translated from Izmeritel'naya Tekhnika, No. 5, pp. 8–14, May, 1997.