Phase filters for correlators with incoherent light

Optimized phase filters can be designed and incorporated into hybrid electronic-optic correlators that exploit incoherent light. These filters lead to high optical efficiency and little, if any, loss of information that normally result from discarding amplitude data. The optimization procedure was experimentally tested by recording optimized filters in photo resist and incorporating them into the correlator. The optimization algorithm, the filter recording procedures, and the experiment correlation results demonstrating the efficacy of our design are presented. These results are superior when compared to those of correlators using holographic filters.     

Transmission type twisted nematic liquid crystal display for three gray-level phase-modulated holographic data storage systems

Gray-level data pages enhance the storage capacity of holographic data storage systems. A balanced three gray-scale data page in phase mode produces a homogenized Fourier spectrum which is regarded to be necessary for suitable exploitation of the holographic recording medium. A commercially available transmission type twisted nematic liquid crystal display has been characterized for use as a phase spatial light modulator with phase modulation in the range 0–3π/2 at 532 nm wavelength, such that it may be used for holographic data storage with binary as well as three gray-level phase-modulated data pages. Experimental results of the phase and amplitude modulation characteristics of the SLM, Fourier plane homogeneity, and recording of phase-modulated binary, three gray-level data pages and their reconstruction with a real-time holographic interferometric method are presented.     

Selective phase masking to reduce material saturation in holographic data storage systems

Emerging networks and applications require enormous data storage. Holographic techniques promise high-capacity storage, given resolution of a few remaining technical issues. In this paper, we propose a technique to overcome one such issue: mitigation of large magnitude peaks in the stored image that cause material saturation resulting in readout errors. We consider the use of ternary data symbols, with modulation in amplitude and phase, and use a phase mask during the encoding stage to reduce the probability of large peaks arising in the stored Fourier domain image. An appropriate mask is selected from a predefined set of pseudo-random masks by computing the Fourier transform of the raw data array as well as the data array multiplied by each mask. The data array or masked array with the lowest Fourier domain peak values is recorded. On readout, the recorded array is multiplied by the mask used during recording to recover the original data array. Simulations are presented that demonstrate the benefit of this approach, and provide insight into the appropriate number of phase masks to use in high capacity holographic data storage systems.     

Hybrid multinary modulation using a phase modulating spatial light modulator and a low-pass spatial filter

We propose a method for performing binary intensity and continuous phase modulation of beams with a spatial light modulator (SLM) and a low-pass spatial filtering 4-f system. With our method it is possible to avoid the use of phase masks in holographic data storage systems or to enhance the phase encoding of the SLM by making it capable of binary amplitude modulation. The data storage capabilities and the limitations of the method are studied.     

Efficient unstationary holographic recording in photorefractive crystals under an external alternating electric field

The theoretical analysis of unstationary holographic recording in photorefractive crystals under an external alternating electric field is presented. In particular, a step-like ac field resulting in a shifted phase hologram with an amplitude exceeding that of a hologram recorded under an external dc field is shown to be most efficient. The experimental curves for the gain factor Γ obtained in Bi₁₂TiO₂₀ (λ = 0.63μm) under an ac step-like field illustrate the field and spatial frequency dependencies of the hologram amplitude characteristics of the unstationary mechanisms of holographic recording.     

Holographic investigation of the Ag-As2S3 sandwich-like structure

The evolution of diffracted power is followed in real time during the recording of simple holographic gratings in Ag-As₂S₃ sandwich structures by means of a He-Ne laser. Measurements are made of the kinetics in several diffraction orders in which the holographic exposure is combined with uniform pre- or post-illumination of the sample. The results obtained suggest the presence of two recording mechanisms: one, predominantly phase, due to photodiffusion of silver into the sulphide layer and another, mainly amplitude, due to the bleaching of the silver deposit.     

Use of phase transparency to reconstruct the phase structure of a field

A method for reconstructing an optical field is developed. It is based on special shaping of the structure of the information on the amplitude and phase pattern of the recording field. The amplitude information is recorded by a standard method, such as photography. The phase information is recorded after visualization on a phase-sensitive material. The field structure is reconstructed by passing a quasimonochromatic space-coherent plane wave through the obtained transparencies. The method was implemented experimentally for phase-modulated fields with the phase information visualized by the Zernike method.Translated from FIAN Preprint No. 227, Lebedev Physics Institute, Moscow, 1988.     

Two-dimensional shift-orthogonal random-interleaving phase-code multiplexing for holographic data storage

A novel phase encoding technology for phase-code multiplexing in holographic data storage (HDS) system called two-dimensional shift-orthogonal random-interleaving (2-D SORI) phase encoding is proposed. Compared with the traditional one-dimensional orthogonal phase-code multiplexing methods, the 2-D method is less sensitive to the variations of the diffraction amplitude and to the phase error of the phase mask. Phase masks for the 2-D SORI phase-code multiplexing can be generated by shifting an elaborately designed phase plate at a certain step, which can avoid the use of a high-cost phase spatial light modulator for the generation of multiple orthogonal phase masks. The cross-talk arising from the systematic phase defects of the static phase modulator is eliminated by the shift operation of the phase plate. Phase codes are interleaved under a predetermined random mapping rule to prevent unauthorized users from accessing the data and eliminate the Bragg degeneration cross-talk. A 2-D SORI phase plate with the size of 5.12 mm × 10.24 mm is designed and fabricated, from which 128 orthogonal phase patterns are generated. The feasibility of the 2-D SORI phase-code multiplexing method for HDS is experimentally demonstrated.     

Time-average holographic interferometry with a sinusoidally modulated laser diode

Laser diodes have the features of a single-mode operation and a frequency tunability. Holographic interferometry is described for investigating the vibrating amplitude and phase with a frequency-modulated laser diode. Sinusoidal wavelength change by varying laser injection current produces the sinusoidal phase modulation of relative phase difference between the reference and vibrating object paths. Holograms are made by time-average exposure of three-type recording modes with a laser diode. The phase of vibrating object is extracted from the measurements of Bessel-type fringe irradiances in three kinds of time-average holographic reconstruction. Experimental results of phase and amplitude for a vibrating cantilever object are shown.     

Thermomechanical modification of diffraction gratings

The most accurate approaches to fabrication of diffraction gratings are known to be the lithographic and holographic methods. The lithographic methods allow fabrication of arbitrarily chirped gratings whose performance, however, is degraded by stitching errors. The holographic methods are free from stitching errors; however, they are limited in the achievable spatial variations of their grating periods. We suggest a method of diffraction grating modification by nonuniform heating and stretching that is much more flexible than the holographic approach and does not suffer from the problem of stitching error. We demonstrate our approach for quartz phase masks that have a characteristic grating period of 1 microm and a length of several centimeters. Our approach allows the grating periods of the phase masks to vary in a range from a few picometers to a few nanometers and a spatial resolution of a few millimeters. It is shown that the grating period can be modified with a negligible effect on the profile of the gratings.     

Formation of random and regular relief-phase structures on silver halide photographic emulsions by holographic methods

The formation of diffusers and microlens rasters on silver halide emulsions by holographic methods is considered. Two techniques for converting amplitude holographic recording to relief-phase recording, selective curing and irradiation of the emulsion gelatin by short-wavelength UV radiation, are compared.     

Spectral selectivity of holographic mirrors recorded in an attenuating medium

Numerical simulation of the optical properties of unslanted reflection holographic gratings (holographic mirrors) is made by the characteristic matrix method. The holograms are recorded in an attenuating medium but the reconstruction is that of pure phase structures in which the refractive index depends exponentially on the exposure. The dependence of the hologram spectral selectivity on medium absorption, amplitude ratio of the two interference waves and recording linearity is studied.     

Holographic particle image velocimetry: analysis using a conjugate reconstruction geometry

Holographic recording techniques have recently been studied as a means to extend two-component, planar particle image velocimetry (PIV) techniques for three-component, whole-field velocity measurements. In a similar manner to two-component PIV, three-component, holographic PIV (HPIV) uses correlation-based techniques to extract particle displacement fields from double-exposure holograms. Since a holographic image contains information concerning both the phase and the amplitude of the scattered field it is possible to correlate either the intensity or the complex amplitude. In previous work we have shown that optical methods to compute the autocorrelation of the complex amplitude are inherently more tolerant to aberrations introduced in the reconstruction process, Coupland, Halliwell, Proc. Roy. Soc. 453 (1960) (1997) 1066. In this paper we introduce a new method of holographic recording and reconstruction that allows a constant image shift to be introduced to the particle image displacement. The technique, which we call conjugate reconstruction, resolves directional ambiguity and extends the dynamic range of HPIV. The theory of this method is examined in detail and a relationship between the image and object displacement is derived. Experimental verification of the theory is presented.     

Photochemically deposited surface relief gratings of an azo-dye-labeled phospholipid from the aqueous phase

Surface relief holographic gratings are fabricated on the polybutadiene-coated walls of a cell filled with an aqueous solution of an azo-dye-labeled phospholipid. A low power (2 mW) 488 nm argon ion laser wavelength is used. Laser-excited azo dye reacts to produce a permanent surface-relief pattern on the polybutadiene substrate. Gratings are recorded for varying concentrations of the phospholipid solution as well as laser intensity. Lithographic masks are used to show that the photochemical pattern on the substrate is an exact replica of the light intensity distribution, and so the technique can be used for holographic recording as well as for biomolecular applications.     

Holographic properties of chalcogenide-metal photosensitive materials

A new type of photosensitive material using a chalcogenide-metal system can be used for recording holograms. This hologram is thin, recorded by amplitude and/or phase variations; a relief hologram is also available. It is shown that this material does not require development and fixation procedures and can be used for real-time holographic interferometry.     

Experimental implementation of multiplexing/demultiplexing in digital images using virtual phase conjugation for holographic data storage

We experimentally implemented and proved the concept of multiplexing and demultiplexing in digital images using virtual phase conjugation, which was proposed in our previous study. In the experiment, we concluded that two digital images multiplexed in a single signal beam are recorded in a holographic medium, and these images are independently and successfully reproduced. In this method, the digital images are multiplexed by superimposing them on a complex amplitude, and not using volume hologram’s multiplexing. Thus, the exposure amount in the holographic medium is constant regardless of the number of multiplexing of digital images, and the method has great potential for achieving high recording density.     

Unequally spaced four levels phase encoding in holographic data storage

Holographic data storage system is a candidate for the information recording due to its large storage capacity and high transfer rate. We propose an unequally spaced four levels phase encoding in the holographic data storage system here. Compared with two levels or three levels phase encoding, four levels phase encoding effectively improves the code rate. While more phase levels can further improve code rate, it also puts higher demand for the camera to differentiate the resulting smaller grayscale difference. Unequally spaced quaternary level phases eliminates the ambiguity of pixels with same phase difference relative to reference light compared to equally spaced quaternary levels. Corresponding encoding pattern design with phase pairs as the data element and decoding method were developed. Our encoding improves the code rate up to 0.875, which is 1.75 times of the conventional amplitude method with an error rate of 0.13 % according to our simulation results.     

体全息成像系统中非周期体光栅的深度选择性研究

由一束球面波和一束平面波或者两束球面波干涉所形成的体全息光栅,由于其光栅矢量在全息图体积内是随位置的变化而变化的,所以被称为非周期型的体光栅。主要研究了用于体全息三维成像系统的非周期体光栅的深度选择特性。根据叠加的原理,将非周期体光栅看作多个固定周期的基元体光栅的叠加,结合耦合波理论分析非周期体光栅的衍射特性。采用这一方法,利用Matlab软件模拟,研究了两束记录光夹角对所记录的非周期体光栅深度选择性的影响和在两束记录光夹角相同时,球面参考光体全息成像系统及平面参考光体全息成像系统深度选择性的差别,最后在光折变晶体材料中进行非周期型和周期型体光栅的记录和再现,对模拟结果进行了实验验证。     

采用紫外光提高双掺杂铌酸锂晶体中全息记录的灵敏度和光栅强度

提出了一种在双掺杂铌酸锂晶体中用调制的双紫外光进行非挥发全息记录的方法。与通常的用紫外光敏化的非挥发全息记录相比,这种方法可以大幅度地提高光栅强度和记录灵敏度。联立双中心物质方程和双光束耦合波方程,数值分析了光栅强度和衍射效率随时间的变化并讨论了掺杂浓度和记录光强对紫外光非挥发全息记录机制下光折变效应的影响。研究发现,紫外光记录得到的深浅中心的光栅具有相同的相位,总的光栅（深浅中心光栅的叠加）强度为两光栅强度之和,固定过程中深中心的光栅得到增强;增大深浅中心掺杂的浓度可以提高光栅强度,增大记录紫外光的光强可以增加光栅的强度和记录灵敏度。理论模拟可以证实并预测实验结果。     

On the Fresnel diffraction pattern of phase holographic gratings

The Fresnel diffraction pattern of phase diffraction gratings is studied on the example of idealized sinusoidal relief-phase plane holographic thermoplastic gratings. It is shown that, in the region of low spatial frequencies, which are more than two times lower than the inverse wavelength of the reading coherent radiation, the intensities of the main diffraction maxima, the diffraction efficiency, and the light-scattering power are independent of the spatial frequency of the phase gratings under investigation, which are characterized by a sinusoidal law of modulation of the reading-radiation phase. It is established that the range of spatial frequencies at which a plateau is observed in these dependences increases with decreasing amplitude of the phase modulation of the reading light. These dependences are also observed to have similar character for the thermoplastic holographic gratings under study. Calculations were performed to analyze the influence of real profiles of thermoplastic strain on the diffraction efficiency and light-scattering power of such gratings. On the basis of analysis of the results obtained, recommendations on using holographic and raster techniques of recording data on phase carriers are given that make it possible to improve the quality of the restored optical image.