Optically controlled image storage in azobenzene liquid-crystalline polymer films

We report on optically controlled image storage based on photo-induced alignment in azobenzene liquid-crystalline polymer films. Measurements reveal that the polymer film possesses photo-induced birefringence of large magnitude and the characteristic of long-term optical storage. The photo-induced alignment process is used for storing images. The stored image is read from the film placed between two crossed polarizers. It is demonstrated clearly that the alignment of the azo chromophores as well as the stored image can be easily controlled by choosing the appropriate polarization of the irradiating light or by rotating the film. Received: 20 October 1998 / Revised version: 7 December 1998 / Published online: 31 March 1999     

Topological stability of stored optical vortices

We report an experiment in which an optical vortex is stored in a vapor of Rb atoms. Because of its 2pi phase twist, this mode, also known as the Laguerre-Gauss mode, is topologically stable and cannot unwind even under conditions of strong diffusion. For comparison, we stored a Gaussian beam with a dark center and a uniform phase. Contrary to the optical vortex, which stays stable for over 100 micros, the dark center in the retrieved flat-phased image was filled with light after a storage time as short as 10 micros. The experiment proves that higher electromagnetic modes can be converted into atomic coherences and that modes with phase singularities are robust to decoherence effects such as diffusion. This opens the possibility to more elaborate schemes for classical and quantum information storage in atomic vapors.     

Image multiplexing and color images with theta modulation of the speckle patterns

We describe a technique of image multiplexing which employs theta modulation of the speckle patterns using the same diffuser to modulate all the signals. This technique is applicable for binary signals. The basic advantage of this technique is that a great number of signals can be stored without cross-talk, because each area of the film is exposed only once. In addition to this, if the development film is processed with white light, color images of the signals will be retrieved.     

Simulated quantum-optical object recognition from high-resolution images

A holographic experimental procedure assuming use of quantum states of light is simulated. It uses merely interference-based image storage and nonunitary image retrieval realized by wave function collapse. Successful results of computational view-invariant recognition of object images are presented. As in neural net theory, recognition is selective reconstruction of an image from a database of many concrete images (simultaneously stored in an associative memory) after presentation of a different version of that image. That is, in the first step, we store many high-resolution images of objects into quantum memory (a hologram). In the second step, we present a “nonlearned” noisy image version. We thereby trigger memory-influenced reorganization of the state of the system so that it finally encodes those corrected object images that correspond to the newly presented version. The holographic procedure seems to be implementable with present-day quantum optics.     

Design of a very high frame rate camera based on an asynchronous CCD driving method

A very high frame rate camera is designed based on an innovative CCD driving method. The CCD driving method is mainly implemented on frame transfer CCDs. Asynchronous drive timing sequences are applied in the image and storage section of the CCDs. Several rows of the charge in the image section are binned onto the same row in the storage section, and there are the same number of images to be stored in the storage section before they are read out. Based on the new driving method, the frame transfer CCDs can work at a very high frame rate in acquiring burst images though the reading speed remains at a lower level. A very high frame rate camera is designed in this paper. The innovative CCD driving method is mainly of concern. An e2v's CCD60 is adopted in the camera system, whose full size resolution is 128 × 128, and the up most frame rate is 1000 Hz in the conventional CCD driving method. By using the presented method, the CCD60 based imager is capable of operating at up to 40000 frames per second (fps) at a recognizable resolution of 128 × 32. Comparing cameras using traditional binning and region of interest technologies, the frame rate is normally less than 5000 fps while the resolution is only 32 × 32 left.     

Interference of multimode photon echoes generated in spatially separated solid-state atomic ensembles

High-visibility interference of photon echoes generated in spatially separated solid-state atomic ensembles is demonstrated. The solid-state ensembles were LiNbO(3) waveguides doped with erbium ions absorbing at 1.53 microm. Bright coherent states of light in several temporal modes (up to 3) are stored and retrieved from the optical memories using two-pulse photon echoes. The stored and retrieved optical pulses, when combined at a beam splitter, show almost perfect interference, which demonstrates both phase preserving storage and indistinguishability of photon echoes from separate optical memories. By measuring interference fringes for different storage times, we also show explicitly that the visibility is not limited by atomic decoherence. These results are relevant for novel quantum-repeater architectures with photon-echo based multimode quantum memories.     

Storing images in warm atomic vapor

Reversible and coherent storage of light in an atomic medium is a promising method with possible applications in many fields. In this work, arbitrary two-dimensional images are slowed and stored in warm atomic vapor for up to 30 micros, utilizing electromagnetically induced transparency. Both the intensity and the phase patterns of the optical field are maintained. The main limitation on the storage resolution and duration is found to be the diffusion of atoms. A technique analogous to phase-shift lithography is employed to diminish the effect of diffusion on the visibility of the reconstructed image.     

Experimental investigation of light storage of diffraction-free and quasi-diffraction-free beams in hot atomic gas cell

In this article we report on the experimental investigation of light storage for several types of diffractionfree beams (Bessel and Airy beams) and quasi-diffraction-free beams by utilizing electromagnetically induced transparency (EIT) technique in a hot atomic gas cell. The experimental results show that the diffraction-free and quasi-diffraction-free beams have better storage performances when compared with ordinary images possessing similar spatial profiles. Meanwhile, the Bessel beams and the quasidiffraction-free images are able to maintain their spatial profiles with a long storage time while the sidelobes of the Airy beam are gradually depleted with the increment of the storage time. We quantitatively analyze the storage results and give physical explanations behind these phenomena. Furthermore, the self-healing of the retrieved diffraction-free beams is verified, signifying that their characteristics preserve well after storage.     

Optimization of a holographic memory setup using an LCD and a PVA-based photopolymer

Holographic data pages were stored in a polyvinyl alcohol (PVA)/acrylamide (AA) photopolymer. This material is formed of AA photopolymers which are considered interesting materials for recording holographic memories. A liquid crystal device was used to modify the object beam and store the data pages in the material. During the storage process, some parameters like exposure time, beam ratio and reading beam intensity were controlled to obtain high image quality after the reconstruction process. The bit error rate (BER) was calculated fitting the histograms of the images to determine what parameters improve the quality of the images.     

Modulation coding for pixel-matched holographic data storage

We describe a digital holographic storage system for the study of noise sources and the evaluation of modulation and error-correction codes. A precision zoom lens and Fourier transform optics provide pixel-to-pixel matching between any input spatial light modulator and output CCD array over magnifications from 0.8 to 3. Holograms are angle multiplexed in LiNbO(3):Fe by use of the 90 degrees geometry, and reconstructions are detected with a 60-frame/s CCD camera. Modulation codes developed on this platform permit image transmission down to signal levels of ~2000 photons per ON camera pixel, at raw bit-error rates (BER's) of better than 10(-5). Using an 8-12-pixel modulation code, we have stored and retrieved 1200 holograms (each with 45,600 user bits) without error, for a raw BER of <2x10(-8).     

Conditional detection of pure quantum states of light after storage in a Tm-doped waveguide

We demonstrate the conditional detection of time-bin qubits after storage in and retrieval from a photon-echo-based waveguide quantum memory. Each qubit is encoded into one member of a photon pair produced via spontaneous parametric down-conversion, and the conditioning is achieved by the detection of the other member of the pair. By performing projection measurements with the stored and retrieved photons onto different bases, we obtain an average storage fidelity of 0.885±0.020, which exceeds the relevant classical bounds and shows the suitability of our integrated light-matter interface for future applications of quantum information processing.     

大容量高密度体全息数据存储

基于晶体体全息存储机理 ,采用半导体泵浦固体激光器取代氩离子激光器作为系统光源 ,设计并构建了小型体全息存储器 ,实现了 15 0 0幅等衍射效率高分辨率全息图的记录和复现 ,其存储容量大于 1 1Gbit ,存储密度大于1 4Gbit/cm3 。     

Controllable beating signal using stored light pulse

We experimentally study the controllable generation of a beating signal using stored light pulses based on electromagnetically induced transparency(EIT) in a solid medium. The beating signal relies on an asymmetric procedure of light storage and retrieval. After storing the probe pulse into the spin coherence under the EIT condition, two-color control fields with opposite detunings instead of the initial control field are used to scatter the stored spin coherence. The controllable beating signal is generated due to alternative constructive and destructive interferences in the retrieved signal intensities. The beating of the two-color control fields is mapped into the beating of weak probe fields by using atomic spin coherence. This beating signal will be important in precise atomic spectroscopy and fast quantum limited measurements.     

Storage of multiple coherent microwave excitations in an electron spin ensemble

Strong coupling between a microwave photon and electron spins, which could enable a long-lived quantum memory element for superconducting qubits, is possible using a large ensemble of spins. This represents an inefficient use of resources unless multiple photons, or qubits, can be orthogonally stored and retrieved. Here we employ holographic techniques to realize a coherent memory using a pulsed magnetic field gradient and demonstrate the storage and retrieval of up to 100 weak 10?GHz coherent excitations in collective states of an electron spin ensemble. We further show that such collective excitations in the electron spin can then be stored in nuclear spin states, which offer coherence times in excess of seconds.     

Manipulating stored images with phase imprinting at low light levels

Coherent manipulation of stored images is performed at low light levels based on enhanced cross-Kerr nonlinearity in a four-level N-type electromagnetically induced transparency (EIT) system. Using intensity masks in the signal pulse, quadratic phase shifts with low nonlinear absorption can be efficiently imprinted on the Fraunhofer diffraction patterns already stored in the EIT system. Fast-Fourier-transform-based numerical simulations clearly demonstrate that the far-field images of the retrieved probe light can be flexibly modulated by applying different signal fields. Our studies could help advance the goals of nonlinear all-optical processing for spatial information coherently stored in EIT systems.     

Signal and reference wave dually encrypted holographic memory with shift multiplexing

We propose a secure holographic storage system with signal and reference waves dually encrypted for shift multiplexing. Two random phase masks are used to encrypt the images in the input and the Fourier planes. The reference beam is phase encoded by a fiber optic faceplate. The encrypted data is stored in a LiNbO₃:Fe crystal and decrypted during the readout process. We experimentally demonstrate encryption and decryption of multiple images and the results show high quality and good fault tolerance.     

Off-resonant defocus-contrast imaging of cold atoms

We demonstrate the retrieval of column-density images of cold atoms, using a noninterferometric phase-recovery technique based on a single off-resonant and defocused intensity image. The quantitative column density is retrieved via Fourier inversion and remains robust with respect to detuning and defocus. The technique offers excellent prospects for simple, nondestructive imaging of atoms in magnetic and optical traps and condensates.     

Design of a very high frame rate camera based on an asynchronous CCD driving method

A very high frame rate camera is designed based on an innovative CCD driving method. The CCD driving method is mainly implemented on frame transfer CCDs. Asynchronous drive timing sequences are applied in the image and storage section of the CCDs. Several rows of the charge in the image section are binned onto the same row in the storage section, and there are the same number of images to be stored in the storage section before they are read out. Based on the new driving method, the frame transfer CCDs can work at a very high frame rate in acquiring burst images though the reading speed remains at a lower level. A very high frame rate camera is designed in this paper. The innovative CCD driving method is mainly of concern. An e2v's CCD60 is adopted in the camera system, whose full size resolution is 128×128, and the up most frame rate is 1000 Hz in the conventional CCD driving method. By using the presented method, the CCD60 based imager is capable of operating at up to 40000 frames per second (fps) at a recognizable resolution of 128×32. Comparing cameras using traditional binning and region of interest technologies, the frame rate is normally less than 5000 fps while the resolution is only 32 × 32 lett.     

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.     

Optical Diffractive Memories: Angular Selectivity and Diffraction Efficiency in Dichromated Gelatin

Two thousand images of resolution 512×512 pixels as a regular matrix pattern of 10×10 elements are stored, where each element is angularly multiplexed 20 times in a 25 μm thickness of dichromated gelatin emulsion without cross-talk effect. The surface area of the matrix is 1 cm². We show good concordance of the angular selectivity between the experimental result and theory. The diffraction efficiency of each 20 multiplexed images is measured and has nearly the same value. Examples of reconstructed images for multiple applications are given, for example, storage of 160,000 images on a 3′1/2 floppy disc format, which is about 100 min of black and white film. Application can be made to automobile cartography and storage of X-ray images as well as weather forecast images. Colored diffractive images are also possible and are illustrated.