Real-time Holography Research Articles

Digital image holography

Digital image holography (DIH) is an interferometric method that allows one to superpose the states of the device investigated at different times. The interference fringes produced are similar to those obtained by real-time holography and are interpretable in the same way. High-contrast interference fringes are reliably obtained even under nonideal experimental conditions. In DIH a CCD camera captures the superposition of a phase-variant object beam and a temporally constant reference beam. The phase difference between these two beams is calculated. The way this is done differs from digital speckle pattern interferometry and other related methods. The temporal phase shift of the object beam is calculated from the phase difference between object and reference beam at two different times. The resulting interference intensity depends on the phase shift of the object beam.

Photosensitive Materials for Holography

Photosensitive polymers are becoming increasingly important for holographic applications. Commercially available polymers have proved essential to the development of holography as a tool to prevent counterfeiting, for holographic optical elements, as media for display holography, as well as media for waveguiding and to make optical interconnects. Simple polymers such as PVA, PAA, PVCz and PMMA were shown to be quite useful in the field of polarization holography, real-time holography, and non-linear optics. High real-time diffraction efficiency as achieved with dye doped dichormated polyvinyl alcohol (DCPVA) and polyacrylic acid (DCPAA) opens the door to a more extensive usage of these polymers for specific applications. However, development procedures and fabrication techniques have to be established to permit broad utilization of these simple and inexpensive recording media.

Characterization of dye-doped media for real-time holography

Studies of saturable dyes used as holographic media where saturation gives rise to both amplitude and phase holograms are presented. Using a simple absorption model together with the Kramers-Kronig relation, a theory for the prediction of holographic diffraction efficiencies over a range of read-wavelengths was developed. Results of experimental tests are presented as support for the model.

Real-time enhancement of defects in periodic patterns by use of a bacteriorhodopsin film

We present a real-time optical system to enhance defects in periodic patterns by use of nonlinear spatial filtering by real-time holography. A straight and equispaced grating written in a bacteriorhodopsin film is read by the Fourier transform of a periodic pattern to be inspected. Because the diffraction efficiency of the grating depends on the intensity of the reading beam, only the defect component can be selectively diffracted and imaged. This system is applicable even to moving objects. We present experimental results that show enhancement of defects as small as 10 microm in a photomask with a pixel pitch of 150 microm used for a liquid-crystal display.

Survey of optical diagnosis dedicated to fluid physics and materials science

The evolution of scientific requirements to improve knowledge and understanding of sophisticated phenomena occurring during microgravity experiences leads to implement a new generation of hardware with more than classical instrumentation such as temperature or pressure measurement; optical diagnosis for in situ characterization is the unavoidable answer to this problem. However, there are many ways to “observe” one experiment, and optical specifications are so different among the great panel of microgravity experiments. This paper deals with a survey, performed by CNES, for identifying each unique optical system with respect to each optical diagnosis selected to satisfy experimental scenarios such as crystal growth, directional solidification, fluid physics and physical chemistry. The survey pointed out the needs of high resolution video data links and “real time holography”. A new generation of advanced technology was raised from this study, especially the use of photorefractive crystals and polymer resins for hologram recordings. Such optical diagnosis will be implemented in a new multi-user facility (CHEOPS) developed by CNES for the next Columbus Orbital Facility.

並列計算機を用いたリアルタイム電子ホログラフィの高速化

CGHの1次元ホログラム上の負荷を各プロセッサに均等に分散する並列化アルゴリズムを提案する.従来の2倍の128プロセッサを用いた実験の結果, プロセッサ台数に比例した処理速度の向上, および従来に対するプロセッサ台数比を上回る高速化を達成した.

Photochromic Liquid Hydrogels as Hosts for Holographic Materials

AbstractThe goal of this project is to develop, fabricate, and test advanced optical materials for potential applications to real-time holography based on liquid crystalline polymer hydrogels. In this project, we are investigating the feasibility of increasing holographic capacity and lifetime by coupling a photochromic spyropyran dye to in a liquid crystalline polymer in which cholesteric order has been ‘captured.’ ‘Capture’ is being approached using a unique in-plane poling process with the helical polypeptide poly(α-benzyl-L-glutamate), PBLG, a biopolymer which is capable of maintaining cholesteric order in a liquid crystalline state. Subsequent in situ crosslinking of this aligned biopolymer is projected to offer increased birefringence of the host in the writing of a hologram. Given that a key issue is the magnitude of the real component of the refractive index, increasing the bireflingence may be a useful approach. In writing the hologram, the liquid crystals (LC's) go from isotropic to an ordered dispersion, a property which can be captured via crosslinking to improve holographic lifetime. In the following, the characterization of an aligned host LC system based on the biopolymer poly(α-benzyl-L-glutamate), PBLG, is presented. Inplane alignment is shown to depend on a number of variables, most notably the choice of solvent, polymer molecular weight, and field strength. The results show that optimal alignment of the PBLG LC is achieved with a 2.5% (w/w) concentration of a 118kD biopolymer in methylene chloride in an applied field of 10 kV/cm. Subsequent work will exploit this system as a host for a spiropyran dye for improved holographics.

Dye-doped liquid crystal composite for real-time holography

In this paper we report the results of dynamic phase grating formation in an anthraquinone derivative dichroic dye-doped nematic liquid crystal in which an external DC electric field was used to change the planar alignment of liquid crystal molecules into a near homeotropic one. This field was also needed to produce an efficient grating via photoconductivity-induced molecular reorientation in a degenerate two-wave mixing experiment with He - Ne (632.8 nm) laser light serving as a low power excitation source. The characteristic of the measured effect shows that the mechanism of grating formation is evidently a non-photorefractive one. A diffraction efficiency of up to 20% has been measured in the system. Fast hologram recording and erasing times (1 ms) make this system attractive for image processing applications.

Real-time optical image processing by synthesis of the coherence function using real-time holography

We have proposed an optical information processing technique by synthesis of the optical coherence function utilizing direct frequency modulation of a laser diode. In the former system, however, a silver halide holographic plate was used, so wet developing process was required. In this letter, real-time holography is adopted by using a liquid crystal spatial light modulator, which enables real-time processing. This system does not require any mechanically moving part nor electrical calculation in the processing procedure. Selective extraction of a 2-dimensional image from a 3-dimensional object is successfully demonstrated in real-time.

Self-induced phase gratings due to the inhomogeneous structure of acrylamide photopolymer systems used as holographic recording materials

We report the observation of self-induced gratings or noise gratings in an acrylamide photopolymer for use in real time holography. The possibilities of this noise source as an optimization technique for this type of material are pointed out. Noise gratings in these polymer films were created upon exposure to a He–Ne laser collimated beam at 633 nm without any subsequent processing step. The influence of intensity on recording noise gratings and angular selectivity are reported showing its influence on the recording of this type of noise source in real time holographic materials.

Nonstationary mass transfer under particle magnetophoresis in diluted ferrocolloids

Nonstationary mass transfer under nanoparticle magnetophoresis in diluted ferrocolloids is experimentally investigated. Measurements performed by using the real time holography technique indicate a difference between the concentration boundary layer parameters found in the experiment and those calculated by using the approximation of nonstationary magnetodiffusion of colloidal particles. We suppose that the final stationary concentration distribution in the boundary layer is caused by a magnetic convection. Approximative calculations of concentration magnetic convection give the mass transfer relaxation time close to the exprimentally determined one.

Simultaneous acquisition of π/2 phase-stepped interferograms with a photorefractive Bi_12GeO_20 crystal: application to real-time double-pulse holography

We present a novel method for real-time analysis of vibrations in double-pulse laser holography. Two pi/2 phase-stepped interferograms are obtained simultaneously. An experimental demonstration gives a phase measurement with an accuracy of 4 degrees .

Photoinduced refractive-index changes in fulgide-doped PMMA films

As an example for a photochromic system we study an active fulgide in a polymer matrix. For theoretical description a two-state model is suggested. Absorption spectroscopy,m-line spectroscopy and UV real-time holography are tools to characterize the optical properties of the guest/host system. The photoinduced refractive-index change is about 7.5×10−4 per % dye concentration. The diffraction efficiency of the holographic grating induced by UV-illumination reaches a high value for photochromic materials of about 7%. The obtained gratings are nonlinear in the case of higher exposure energies.

Spiropyran-doped poly(vinyl carbazole): a new photopolymer recording medium for erasable holography

The photosensitive organic system of poly(vinyl carbazole) doped with spiropyran is studied for real-time reversible holography. The photoinduced reversible color change between thermally stable and metastable states of spiropyran molecules can modulate the absorption and the refractive index of the spiropyran-doped polymer film. Erasable holograms can be recorded in either stable or metastable states of such films of thickness 10 and 50 μm prepared by a gravity settling method. The grating constant achieved was δ= 0.58 μm, using the 350-nm UV line of a krypton-ion laser. It has been found that write-read-erase cycles can be performed repeatedly. High-efficiency erasable holographic recording at 350 nm has been performed in these films, and a diffraction efficiency greater than 10% has been achieved.

All-optical nonlinear joint Fourier transform correlator

We present the first all-optical nonlinear joint transform correlator based on a square-law receiver in the Fourier plane. Our device uses a photorefractive limiting quadratic processor. The compressional nonlinearity associated with the transfer function of the limiting quadratic processor enables the correlator to detect signals embedded in Gaussian and non-Gaussian noise. In the limiting region this device correlates the phase-only information of the input. This is the first time to our knowledge that photorefractives or real-time holography has been used in the correlation of the phase-only information. We demonstrate the operation of this device experimentally, and we evaluate its performance throughcomputer simulation for various forms of noise.

Real-time optical multiplication with high accuracy

A method of performing matrix multiplication using optical processing is presented. It combines speed due to the inherent parallelism of optics and high accuracy due to the digital representation of the multiplied numbers. The multiplication of numbers is performed by spatially convolving their binary digital representations. The spatial convolution is realized by frequency-domain multiplication. Real-time recording is necessary. An architecture for vector-vector multiplication is also proposed. The real-time recording is performed by real-time holography using fourwave optical mixing.

Dynamic observation of magnetic domains by on-line real-time electron holography

Dynamic behavior of magnetic domains in a thin permalloy film has been observed by on-line real-time electron holography. Electron holograms formed in a transmission electron microscope equipped with an electron biprism are detected by a TV camera and transferred to a liquid-crystal spatial light modulator (LC-SLM) located at the output port of a Mach-Zehnder interferometer. Interference micrographs, where magnetic flux lines are directly visualized, are obtained by superimposing a plane wave onto an object wave reconstructed from the hologram in the LC-SLM. Using this system, dynamic behavior of magnetic domains in the magnetization or demagnetization processes is clearly observed.

Volume phase gratings in DMPA-doped polymers

The photosensitive organic system Poly(Methyl MethAcrylate)/DiMethoxyPhenylAcetophenon (PMMA/DMPA) is studied by real-time holography. Thin film samples (thickness ≃ 50 μm) are prepared by solution deposition on glass substrates using different solvents. Besides this, block samples with a thickness of about 2 mm are investigated. The achieved grating constant varies between 1.1–2 μm using the 363.8 nm UV line of an argon-ion laser. DMPA concentration varies between 0.5–10% and affects the temporal stability of the gratings, dynamics of the diffraction efficiency and the sensitivity of the photopolymer system. The gratings are also characterized by measuring the angular sensitivity. With a functional polymer as a matrix material stable gratings could be obtained.

Bulk Photorefractive Semiconductors

The photorefractive (PR) effect has been studied for more than 25 years and many applications for optical signal processing such as correlation, real-time holography, dynamic interconnections, and optical memories have been developed. The main focus of study for the PR effect has been oxides (ferroelectrics and sillenites) in which the useful spectral range lies in the visible. Applications for telecommunication systems and eye-safe devices have required extending the spectral range into the near infrared (1.0 to 1.5 μm), and so the exploration of different materials. It has been shown that the bulk semi-insulating III-V semiconductors GaAs and InP, and more recently the II-VI compound CdTe, were efficient materials for this spectral range. III-V materials offer the advantage of availability as bulk semiinsulating materials of high crystalline perfection and homogeneity regarding their electrical properties due to their importance as substrate materials in micro and optoelectronic technology. However, these materials have not been optimized for PR applications, so quantitative analyses of PR experiments related to the specific material defect properties are necessary for further developments. It has equally been shown that the PR effect can be used as an efficient tool for materials characterization.

Submicrometer defect detection in periodic structures by photorefractive holography: system design and performance

We have built an adaptive system capable of detecting submicrometer defects in periodic structures by using real-time holography in photorefractive crystals. We summarize the design and optimization of the defect-enhancement system. We present representative results of 0.2- to 0.5-µm diameter defect detection on two different periodic substrates: chrome-on-glass masks and patterned silicon wafers. On patterned silicon we detect 94% of the 0.5-µm diameter defects with three false positives, while inspecting an area greater than 1 mm(2) in 20 s or less. The throughput on the glass masks is somewhat less. To the best of our knowledge, these defects have an area 100 times smaller than those previously detected with any real-time holographic technique.