Nonvolatile storage in photorefractive crystals

Optical data encryption using time-dependent dynamics of refractive index changes in LiNbO3

We present a method for optical encryption of information, based on the time-dependent dynamics of writing and erasure of refractive index changes in a bulk lithium niobate medium. Information is written into the photorefractive crystal with a spatially amplitude-modulated laser beam which when overexposed significantly degrades the stored data making it unrecognizable. We show that the degradation can be reversed and that a one-to-one relationship exists between the degradation and recovery rates. It is shown that this simple relationship can be used to determine the erasure time required for decrypting the scrambled index patterns. In addition, this method could be used as a straightforward general technique for determining characteristic writing and erasure rates in photorefractive media.

Holographic technique for measurement of the kinetic constant and optical modulation of photosensitive materials

We use a holographic technique to measure simultaneously and separately the temporal evolution of the refractive-index and the absorption coefficient modulations induced by light in a photosensitive material. The technique is phase sensitive, allowing separation of the signals from the phase and from the amplitude grating. The refractive-index and the absorption coefficient modulations as well as the kinetic constant of the photoreaction in the positive photoresist SC 1827 were measured at three different wavelengths. The results were compared with independent measurements, performed under homogeneous exposition. The good accord demonstrates the applicability of the technique to study photosensitive materials.

Reversal of degradation of information masks in lithium niobate

We report on the reversal of degradation of information masks stored in self-defocusing lithium niobate. After a long writing time, the image degradation appears as the splitting of refractive-index patterns stored in the medium. The reversal is achieved simply by illuminating the crystal with incoherent light from a halogen lamp. The reversal occurs because the refractive-index changes responsible for the splitting are of a smaller magnitude and are therefore erased first during incoherent illumination. Additionally, we gain insight into the storage, degradation, and erasure dynamics using a time-dependent numerical model of the photorefractive effect in this medium. Since the data can be recovered from a degraded state in which the original data are unrecognizable, this technique could be utilized in such applications as image scrambling or encryption.

Polychromatic reconstruction for volume holographic memory

We propose a method of reconstructing an image from a volume hologram at a wavelength different from the recording one. Spectrally broad but spatially coherent light was used as a probe beam. Each angular spectral component of the recorded hologram could be Bragg matched at one particular wavelength within the broadband spectrum. We experimentally demonstrated that a whole image could be reconstructed by using polychromatic light, whereas only a partial image was obtained by using single-mode laser light. We discuss the required bandwidth of the probe beam and the deformation of the reconstructed image.

Rainbow volume holographic imaging

We present a new form of volume holographic imaging with rainbow illumination. High depth resolution is obtained because each quasi-monochromatic band of the rainbow acts as a depth-selective confocal slit. The color slits work in parallel to achieve a wide field of view (FoV) and so the need to scan in one lateral dimension is eliminated. Our experiments demonstrated <250-micro m depth resolution over an approximately equal to 15 degree FoV at a 50-mm working distance.

N-ocular volume holographic imaging

Volume holographic imaging utilizes Bragg selectivity to optically slice the object space of the imaging system and measure four- (three spatial and one spectral) dimensional object information. The N-ocular version of this method combines multiple-volume holographic sensors and digital postprocessing to yield high-resolution three-dimensional images for broadband objects located at long working distances. We discuss the physical properties of volume holography pertinent to imaging performance and describe two computational algorithms for image inversion based on filtered backprojection and least-squares optimization.

Broadband volume holographic imaging

We demonstrate transmission geometry volume holograms working under broadband illumination. We show that increased illumination bandwidth enhances the lateral field of view of planar reference holograms. We exploit this phenomenon to design volume holographic spectrum analyzers and present results from an experimental prototype. Furthermore, we show that there is a trade-off involved, because an improvement in the field of view results in a reduction of image contrast as a function of depth. We experimentally demonstrate this trade-off and discuss possible ways to overcome it.

Qualitative demonstration of spectral diversity filtering using spherical beam volume holograms

We investigate the feasibility of designing spectral diversity filters using spherical beam volume holograms. Our experimental results qualitatively show the separation of the information of different incident wavelength channels using spherical beam volume holograms. The major trade-off in using these holograms is between the degree of spatial spectral diversity and the number of allowed spatial modes (or the divergence angle) of the incident beam.

Volume holographic imaging in transmission geometry

We address the performance of transmission geometry volume holograms as depth-selective imaging elements. We consider two simple implementations using holograms recorded with spherical and plane beams. We derive the point-spread function (PSF) of these systems using volume diffraction theory and use the PSF to estimate depth resolution. Furthermore, we show that appropriately designed objective optics can significantly improve the depth resolution or the working distance of plane-wave reference holographic imaging systems. These results are confirmed experimentally and demonstrated for objects with millimeter axial features, imaged from the 5- to 50-cm range.

All-optical pattern recognition for digital real-time information processing

To recognize digital streams of digital data, all-optical and passive techniques able to discriminate optical bit words in real time are presented. Discrimination capability of different correlators, both in free space architectures and in delay lines structures, is theoretically and experimentally analyzed. Experimental performances in word recognition are shown in the case of a volume holographic correlator, in the case of a lithographic phase-only-filter correlator, and in the case of a novel coherent delay lines correlator operating at the wavelength 1550 nm and at the bit rate of 2.5 Gbit/s.

Real-time spectral imaging in three spatial dimensions

We report what is to our knowledge the first volume-holographic optical imaging instrument with the capability to return three-dimensional spatial as well as spectral information about semitranslucent microscopic objects in a single measurement. The four-dimensional volume-holographic microscope is characterized theoretically and experimentally by use of fluorescent microspheres as objects.

Holographic data storage with arbitrarily misaligned data pages

An improved postprocessing algorithm that can compensate for arbitrary misregistrations between a detector array and the coherent image of a pixelated two-dimensional data page is described. Previously [Opt. Lett. 26, 542 (2001)], an algorithm was reported in which both linear and quadratic interpixel cross-talk contributions are reallocated to the appropriate neighboring pixels. However, page misalignments close to +/-0.5 pixels could not be corrected to an acceptable bit-error rate because of propagation in the iterative procedure. An improved algorithm is reported in which an intentional magnification error is introduced optically and then corrected during postprocessing. Experimental results from a pixel-matched megapel volume holographic system are presented, showing that the dependence of bit-error rate on transverse detector alignment is entirely removed. This improved procedure can completely bypass constraints on page registration, optical distortion, and material shrinkage that currently hamper page-oriented holographic storage systems.

Diffraction properties of transmission photorefractive volume gratings in a cerium-doped potassium sodium strontium barium niobate crystal

The diffraction efficiency of volume gratings written by two-wave mixing in a cerium-doped potassium sodium strontium barium niobate (Ce:KNSBN) photorefractive crystal is studied. It is found that the diffraction efficiency strongly depends on the polarization of writing beams and exhibits loop behavior with respect to the fringe modulation. The fringe modulations before and behind the crystal are compared. Modified coupled-wave theory is used to fit the experimental data. This research presents data that are relevant to the application of Ce:KNSBN crystals to holographic recording and optical information processing.

Two-color holography in reduced near-stoichiometric lithium niobate

We explored a number of factors affecting the properties relevant to holographic optical data storage by using a two-color recording scheme in reduced, near-stoichiometric lithium niobate. Two-color, or photon-gated, recording is achieved by use of 852-nm information-carrying beams and 488-nm gating light. Readout at 852 nm is nondestructive, with a gating ratio of ~10(4). Recording sensitivity, gating ratio, dynamic range, and dark decay were measured for crystals of differing stoichiometry, degree of reduction, wavelength of the gating light, temperature, and optical power density. The two-color sensitivity per incident photon is still somewhat less than that of the one-color process at 488 nm for ~1 W/cm(2) of gating light but is essentially the same in terms of absorbed photons. Two-color recording is an attractive way of achieving nondestructive readout in a read-write material, and it allows selective optical erasure.

Nonvolatile photorefractive spectral holography

We demonstrate nonvolatile storage of femtosecond pulses in a photorefractive LiNbO(3) crystal with recording and readout of spectral holograms at wavelengths of 460 and 920 nm, respectively. No degradation was observed after 24 h of continuous readout. We also show that we can realize the time-lens effect with our system by appropriately setting the ratio of the recording and the reconstruction wavelengths and the spectral resolution of the recording and the reconstruction processes.

Storage of 1000 holograms with use of a dual-wavelength method

We demonstrate the storage of 1000 holograms in a memory architecture that makes use of different wavelengths for recording and readout to reduce the grating decay while retrieving data. Bragg-mismatch problems from the use of two wavelengths are minimized through recording in the image plane and using thin crystals. Peristrophic multiplexing can be combined with angle multiplexing to counter the poorer angular selectivity of thin crystals. Dark conductivity reduces the effectiveness of the dual-wavelength method for nonvolatile readout, and constraints on the usable pixel sizes limit this method to moderate storage densities.

Digital quasi-phase-matched two-color nonvolatile holographic storage

Unwanted erasure during readout of holographic data can be reduced or eliminated by use of a different wavelength for reading than that which was used for writing. To prevent distortion and Bragg mismatch that would be unacceptable for digital data storage, one can format data to account for the wavelength difference. Techniques to format data and the results of this formatting are presented. Varying the formatting parameters is investigated to optimize diffraction efficiency.

High-speed, acousto-optically addressed optical memory

A page-oriented, angle-multiplexed volume holographic optical-memory recording system has been constructed. This memory is addressed by the use of an acousto-optic deflector with a random-access time of 16 µs per page. This enables data transfer rates of 5.28 Gbits/s when pages of binary data are being stored. The reconstruction quality of images stored as memory pages is assessed with the quality achieved with the acousto-optic device compared with that achieved with the original recording optics.

Shift-multiplexed holographic memory using the two-lambda method

We present theoretical and experimental results on the application of the two-lambda method for prolonged readout of holographic memories to shift multiplexing implemented with a spherical-wave reference beam.

Volume gratings for holographic storage applications written in high-quality germanosilicate glass

Volume holographic gratings are written with ultraviolet light in high-optical-quality, commercially available Ge-doped silica films and in Ge-doped optical-fiber preform sections loaded with molecular hydrogen. In the film samples, peak refractive-index changes exceeding 10(-2) and a sensitivity (index change/absorbed energy density) of 0.4 × 10(-7) cm(3)/J are measured. Angular multiplexing of up to 51 gratings is demonstrated in the preform samples.

Angle-dependent diffraction efficiency in a thick photorefractive hologram

The diffraction from a thick photorefractive hologram is shown to be angular dependent, which originates mainly from the angle-dependent effective electro-optic coefficient of a photorefractive crystal. The angle dependency of the diffraction causes a nonuniform diffraction over the pixel positions or the spatial frequency contents of a hologram image in a page-oriented holographic system, resulting in a deteriorated reconstructed image. In addition, owing to the angular variations in diffraction, the wavelength-multiplexing scheme should be a better choice than angular one.

Optical parallel-access shared memory system: analysis and experimental demonstration

An optical implementation of a parallel-access shared memory uses a single photorefractive crystal and can realize the set of memory modules in a digital shared memory computer. This implementation uses two arrays of sources that are individually coherent but mutually incoherent from region to region across each array, and it permits incoherent/coherent double angular multiplexing of data in the crystal. A complete instruction set for its memory access consists of four operations, READ, WRITE, SELECTIVE ERASE, and REFRESH, which can be applied to any memory module independent of (and parallel with) instructions to the other memory modules. In addition, a memory module can execute a sequence of READ operations simultaneously with the execution of a WRITE operation to accommodate differences efficiently in optical recording and readout times common to optical volume storage media. An experimental shared memory system demonstrates two memory modules, each consisting of up to two 5-bit data blocks, implemented in a single Fe:LiNbO(3) crystal. The projected performance of the optical parallel-access shared memory system is analyzed and compared with conventional page-addressed volume holographic memories.