Speckle-reduced three-dimensional volume holographic display by use of integral imaging

We propose a method to implement a speckle-reduced coherent three-dimensional (3D) display system by a combination of integral imaging and photorefractive volume holographic storage. The 3D real object is imaged through the microlens array and stored in the photorefractive crystal. During the reconstruction process a phase conjugate reading beam is used to minimize aberration, and a rotating diffuser located on the imaging plane of the lens array is employed to reduce the speckle noise. The speckle-reduced 3D image with a wide viewing angle can be reconstructed by use of the proposed system. Experimental results are presented and optical parameters of the proposed system are discussed in detail.     

Ebert-Fastie型双层结构平面全息光栅双单色仪的光学设计

为满足空间紫外遥感高精度光谱辐射测量工作的要求,设计了一种Ebert-Fastie 型双层结构平面全息光栅双单色仪,由球面准直聚光镜、平面和屋脊转向镜、平面全息光栅及入射、出射和中间狭缝组成,扫描波长范围160 ～400 nm .这种双层结构的特点在于两块完全相同的平面全息光栅安装在同一转轴上做到同轴转动,不但把机构基本上简化为一个单色仪的结构,而且确保了两块光栅同步地进行光谱扫描,色散相加,光谱分辨率小于0.15 nm .此外,前后两单色系统被隔成基本封闭的腔体,用来割断两单色系统杂散光之间的相互影响,抑制整个系统的杂散光,整个系统的杂散光水平可达10 -6 ,满足空间紫外遥感高精度光谱辐射测量的要求.     

Secure optical storage that uses fully phase encryption

A secure holographic memory system that uses fully phase encryption is presented. Two-dimensional arrays of data are phase encoded. Each array is then transformed into a stationary white-noise-like pattern by use of a random-phase mask located at the input plane and another at the Fourier plane. This encrypted information is then stored holographically in a photorefractive LiNbO(3):Fe crystal. The original phase-encoded data can be recovered, by use of the two random-phase masks, with a phase-conjugate readout beam. This phase information can then be converted back to intensity information with an interferometer. Recording multiple images by use of angular multiplexing is demonstrated. The influence of a limited system bandwidth on the quality of reconstructed data is evaluated numerically. These computer simulation results show that a fully phase-based encryption system generally performs better than an amplitude-based encryption system when the system bandwidth is limited by a moderate amount.     

Spatio-temporal operator formalism for holographic recording and diffraction in a photorefractive-based true-time-delay phased-array processor

We present a spatio-temporal operator formalism and beam propagation simulations that describe the broadband efficient adaptive method for a true-time-delay array processing (BEAMTAP) algorithm for an optical beamformer by use of a photorefractive crystal. The optical system consists of a tapped-delay line implemented with an acoustooptic Bragg cell, an accumulating scrolling time-delay detector achieved with a traveling-fringes detector, and a photorefractive crystal to store the adaptive spatio-temporal weights as volume holographic gratings. In this analysis, linear shift-invariant integral operators are used to describe the propagation, interference, grating accumulation, and volume holographic diffraction of the spatio-temporally modulated optical fields in the system to compute the adaptive array processing operation. In addition, it is shown that the random fluctuation in time and phase delays of the optically modulated and transmitted array signals produced by fiber perturbations (temperature fluctuations, vibrations, or bending) are dynamically compensated for through the process of holographic wavefront reconstruction as a byproduct of the adaptive beam-forming and jammer-excision operation. The complexity of the cascaded spatial-temporal integrals describing the holographic formation, and subsequent readout processes, is shown to collapse to a simple imaging condition through standard operator manipulation. We also present spatio-temporal beam propagation simulation results as an illustrative demonstration of our analysis and the operation of a BEAMTAP beamformer.     

Optoelectronic implementation of a 256-channel sonar adaptive-array processor

We present an optoelectronic implementation of an adaptive-array processor that is capable of performing beam forming and jammer nulling in signals of wide fractional bandwidth that are detected by an array of arbitrary topology. The optical system makes use of a two-dimensional scrolling spatial light modulator to represent an array of input signals in 256 tapped delay lines, two acousto-optic modulators for modulating the feedback error signal, and a photorefractive crystal for representing the adaptive weights as holographic gratings. Gradient-descent learning is used to dynamically adapt the holographic weights to optimally form multiple beams and to null out multiple interference sources, either in the near field or in the far field. Space-integration followed by differential heterodyne detection is used for generating the system's output. The processor is analyzed to show the effects of exponential weight decay on the optimum solution and on the convergence conditions. Several experimental results are presented that validate the system's capacity for broadband beam forming and jammer nulling for linear and circular arrays.     

Measuring the coherence function of continuous-wave lasers by a photorefractive grating method

We demonstrate a simple, adjustment-insensitive technique for measuring the temporal coherence function of cw multimode and monomode semiconductor lasers, using two-beam coupling in photorefractive InP and CdTe crystals. The emission spectra of the diodes are measured independently. The coherence functions are also calculated from these spectra and agree with the photorefractive measurements. Coupling of two partially coherent waves in low-speed photorefractive media is described theoretically. The range of the experimental parameters in which the method of coherence measurement is correct is given.     

Dynamic evolution of dissipative holographic solitons in photorefractive materials via two-wave mixing with a moving grating

The dynamic evolution and stability of bright dissipative holographic solitons supported by photorefractive materials via two-wave mixing with a moving grating are investigated numerically. Results show that these solitons are stable relative to small perturbations. The state of such solitons can be easily controlled by adjusting some system parameters, such as the linear loss of the crystal and the frequency detuning between the pump beam and the self-trapping beam. Potential amplification in optical switches, repeaters or splitters is discussed.     

Selective erasure of speckle-multiplexed holograms by use of a double Mach-Zehnder interferometric arrangement

A novel method to selectively erase and update speckle-multiplexed holograms in photorefractive crystals by use of a double Mach-Zehnder (DMZ) interferometric arrangement is presented. The DMZ arrangement automatically produces a pair of pi-phase-shifted interference patterns used for holographic recording, erasure, and update operations with a fairly simple optical configuration that consists of a commonly used dielectric multilayer beam splitter and two mirrors. The recording and the erasure conditions required for erasing a photorefractive hologram quickly and completely are discussed by calculating the temporal property of the hologram buildup and decay using the time-dependent coupled-wave equations. An experiment is also performed, in which arbitrary holograms in speckle-multiplexed holograms are selectively erased and updated with the simple DMZ optical configuration.     

Holographic interferometers with photorefractive recording media

We discuss two types of holographic interferometer that contain photorefractive recording media. The first type contains two beams interacting in a photorefractive medium. The second type utilizes a single beam and relies on self-pumped phase conjugation from a photorefractive crystal to make phase changes appear as intensity changes. We show both theoretically and experimentally that the first type can be analyzed in a straightforward manner; however, the second type cannot be approximated as simply a special case of the first type, as one may n?ively suspect.     

Optical gain by a simple photoisomerization process

Organic holographic materials are pursued as versatile and cheap data-storage materials. It is generally assumed that under steady-state conditions, only photorefractive holographic media exhibit a non-local response to a light-intensity pattern, which results in an asymmetric two-beam coupling or 'gain', where intensity is transferred from one beam to the other as a measure of writing efficiency. Here, we demonstrate non-local holographic recording in a non-photorefractive material. We demonstrate that reversible photoisomerization gratings recorded in a non-photorefractive azo-based material exhibit large optical gain coefficients beyond 1,000 cm(-1), even for polarization gratings. The grating characteristics differ markedly from classical photorefractive features, but can be modelled by considering the influence of the Poynting vector on the photoisomerization. The external control of the Poynting vector enables manipulation of the gain coefficient, including its sign (the direction of energy exchange), a novel phenomenon we refer to as 'gain steering'. A very high sensitivity of about 100 cm(2) J(-1) was achieved. This high sensitivity, combined with a high spatial resolution, suggests a great technical advantage for applications in image processing and phase conjugation.     

Viewing-angle enhancement of speckle-reduced volume holographic three-dimensional display by use of integral imaging

In a conventional integral imaging system the viewing angle is limited by the f-number of the microlens. To overcome this limitation we employ a phase-conjugate beam to read out elemental images, which are stored in photorefractive volume holographic storage, while the rotating diffuser reduces the speckle noise. In the proposed system the viewing angle can be enhanced over the f-number limitation. Experimental results and discussions of viewing parameters are presented.     

Holographic imaging and interferometry with non-Bragg diffraction orders on volume and surface-relief gratings in lithium niobate and photo-thermoplastic materials

The recording of holographic volume and surface-relief gratings in a photorefractive crystal using a photo-thermoplastic (PTP) holographic camera with an image-bearing signal beam leads to the appearance of two Bragg and two or more non-Bragg diffracted beams that show the transformed images in each beam (rotation and angular amplification of images). Using this real-time mode of interferometry, the hologram is retrieved with a deformed object beam, resulting in the appearance of fringes with a proper phase shift in each of four diffracted beams. This one-shot (one-exposure) phase-shifting interferometry results in clarification of the object wave-front information (for example, from surface deformation) and solution of the sign ambiguity problem. This procedure demonstrates that high-resolution holographic imaging of the PTP holographic camera static deformations in the order of ～0.1?mm can be revealed on the diffusion reflection surface. In addition, it was demonstrated that using the PTP materials could achieve holographic recording and imaging through phase aberration, with the image appearing in the non-Bragg diffraction order.     

Dual holographic interferometry for measuring the three velocity components in a fluid plane

A technique that allows one to measure simultaneously the three velocity components in a fluid plane is presented. One obtains the quantitative information from only one holographic recording by combining two different reconstruction processes. As both processes use an interferometric comparison of two waves, we refer to this technique as dual holographic interferometry. The far-field fringe pattern that is obtained when reconstruction is made with an expanded laser beam allows one to determine the in-plane velocity components. The image-field fringe pattern that is obtained when a pointwise laser beam is used for reconstruction contains information about an out-of-plane velocity component. As the two reconstruction processes have different sensitivities, two different ways to combine them are proposed. The system has been demonstrated in a fluidlike solid object and in a convective flow.     

Single-beam data encoding using a holographic angular multiplexing technique

We propose a novel configuration for angular multiplexing holographic encoding in which the signal beam and the reference beam are combined into a single beam. By using a spatial light modulator based on twisted nematic liquid crystals, the signal and the reference beams are modulated in amplitude mode and phase mode, respectively. The multiplexed interference patterns with the reference beams of different incident angles are recorded near the Fourier transform plane, and then the signals are selectively reconstructed by the corresponding reference beam. Both the simulation and the experiment of single-beam angular multiplexed holography are performed with consistent results. Compared with the traditional angular multiplexing holographic recording system, the single-beam configuration is more compact, easier to adjust, and less sensitive to the vibration of the environment. Therefore, it will be more attractive for potential applications in many fields, such as high-density signal recording and data encryption.     

Encrypted optical storage with wavelength-key and random phase codes

An encrypted optical memory system that uses a wavelength code as well as input and Fourier-plane random phase codes is proposed. Original data are illuminated by a coherent light source with a specified wavelength and are then encrypted with two random phase codes before being stored holographically in a photorefractive material. Successful decryption requires the use of a readout beam with the same wavelength as that used in the recording, in addition to the correct phase key in the Fourier plane. The wavelength selectivity of the proposed system is evaluated numerically. We show that the number of available wavelength keys depends on the correlation length of the phase key in the Fourier plane. Preliminary experiments of encryption and decryption of optical memory in a LiNbO(3):Fe photorefractive crystal are demonstrated.     

Experimental realization of a multiproduct photorefractive correlation system for temporal signals

The design and operation of a multidimensional photorefractive correlator is described. With a photorefractive medium as the integrator, this system effects the correlation of up to four time-evolving signals. The holographic correlation volume is viewed by projection onto a two-dimensional detector. The correlator is designed for use with binary-phase signals, especially pseudorandom noise sequences.     

Observation of high gain in a liquid-crystal panel with photoconducting polymeric layers

A novel, to our knowledge, liquid-crystal panel suitable for real-time holographic purposes has been prepared. A nematic liquid-crystal layer sandwiched between photoconducting polymeric layers, when exposed to a sinusoidal light-intensity pattern, shows efficient formation of refractive-index gratings. The unique feature of the presented panel is its ability to switch energy from beam to beam in a manner similar to the charge-diffusion-controlled photorefractive effect. In a two-wave-mixing experiment multiple orders of diffraction are present, and a very high two-beam coupling-gain ratio (2.5) and a net exponential gain coefficient of ? = 931 cm(-1) have been measured. This gain was achieved in samples biased by a dc external electric field and tilted with respect to the beam-incidence bisector at 45 degrees . The time constants for grating formation and erasure in the studied system are functions of the applied voltage and can be made as short as a few milliseconds under favorable conditions. The mechanism of beam coupling is linked with an electric-field-driven reorientation of the nematic director as a result of a spatially modulated space-charge field created by light in a photoconducting poly(3-octyl)thiophene polymeric layer.     

Holographic zone plates for f x theta and collimating lenses

A holographic zone plate generated by subtracting the spherical wave phase is proposed, that is, a hologram generated by using two divergent or convergent spherical waves. This holographic zone plate can correct the increase in focal power in a radial direction in contrast with the interferometric zone plate. Use of this holographic zone plate for f x O and collimating lenses is analyzed. For an f x O lens, it is shown that an optimum combination of recording spherical waves exists that satisfies both scan linearity and image field flatness. For a collimating lens for a laser diode, it is shown that aberration can be corrected for different playback wavelengths from that used during recording.     

Simultaneous determination of the refractive index and wedge angle of an optical wedge plate using a photorefractive holographic interferometer

Abstract

A simple and effective method for simultaneously determining the refractive index and the wedge angle of an optical wedge plate is described. The method is based on a real-time holographic interferometer which uses a photorefractive crystal as the recording and reconstruction medium. The wedge sample under test is inserted into a rectangular cell that is placed in the object light beam of the holographic interferometer. The interference patterns produced before and after a reference liquid is poured into the cell are received by a CCD camera and stored in a computer, respectively. The refractive index and the wedge angle of the wedge sample are determined by measuring the number of fringes falling inside a fixed aperture. The principle of the method is analysed and some experimental results with adequate accuracy are given.     

Optical implementation of a wavelet transform by the use of dynamic holographic recording in a photorefractive material

An optical system that employs holographic recording in a photorefractive material is proposed and experimentally demonstrated for the implementation of a wavelet transform of two-dimensional mages. A scaling operation, to derive the family of wavelet filters from a mother wavelet filter, is performed by the use of an optical feedback loop. The selection of a desired wavelet filter from the family and the correlation for a wavelet transformation are made by the use of a holographic recording in a photorefractive material. The principle of operation of the system relies on the frequency detuning introduced inside the loop and the subsequent variation in the holographic grating diffraction. Experimental results on wavelet-filter selection and wavelet transformation are presented. This nonlinear optical wavelet-transform system is advantageous for pattern recognition applications.