Three-dimensional shifting selectivity of random phase encoding in volume holograms

Optical servo with high design freedom using spherical-wave Bragg degeneracy in a volume holographic optical element

A novel optical sensor is proposed and demonstrated with the use of spherical-wave Bragg degeneracy of a volume holographic optical element. Bragg degeneracy can be performed perfectly for a volume hologram written by two plane waves or spherical waves. But only spherical reference light is useful to perform severe shifting selectivity in one direction and Bragg degeneracy in another direction. The experiment corresponding to theoretical simulation was demonstrated in a volume holographic optical element, which was an optical servo for a collinear volume holographic disc. Through clever design, the displacement response of the tracking beam was observed for a magnification magnitude of 980% to the displacement of the probe beam so that the resolution requirement of the photosensor can be reduced. Besides, a high linearity of the displacement response was observed. To discover the design freedom, a theoretical study of the displacement response was done for the servo by two spherical waves. The linearity, the magnification magnitude, and the diffraction efficiency were examined and the property of the Bragg degeneracy was figured out.

Analysis of a lens-array modulated coaxial holographic data storage system with considering recording dynamics of material

In the first time, a simulation model with considering the recording dynamics of material is built and is used to simulate evolution of the grating strength of the recorded hologram in a coaxial volume holographic memory system. In addition, phase modulation by lens array in the reference is introduced and observed to perform better diffracted signal quality and higher shifting selectivity, in both simulation and experiment. The use of lens array is found to provide multiple advantages in volume holographic memory system. The new simulation model potentially can be used to precisely design the system to obtain higher diffracted signal quality, higher shifting selectivity, and reduction of M# consumption and increase of storage capacity.

Correlation-Based Multiplexing of Complex Amplitude Data Pages in a Holographic Storage System Using Digital Holographic Techniques

Holographic recording media can store the amplitude and the phase, or the complex amplitude, of a beam on the basis of holography. Owing to this characteristic, digital data can be encoded onto the complex amplitude of a signal beam in holographic data storage. However, most of conventional holographic storage systems encode digital data onto the amplitude alone because there are difficulties for modulating and detecting the phase. To solve the difficulties, a holographic storage system using digital holographic techniques has been proposed. With the help of digital holographic techniques, it is possible to modulate and detect the complex amplitude of a signal beam. Moreover, the proposed system can modulate the complex amplitude of a reference beam. In this paper, by making use of the capability, a correlation-based multiplexing with uncorrelated reference beams is demonstrated in the proposed system. Multiple holograms can be recorded in the same volume of a recording medium with no need for mechanical movements. Experimental results show that the proposed system with a correlation-based multiplexing can improve the storage capacity and can utilize the full potential of a recording medium without crosstalk noise stem from the optical setup.

Cyphertext-only attack on the double random-phase encryption: Experimental demonstration

We demonstrate experimentally that the traditional double random phase encoding (DPRE) technique is vulnerable to the cyphertext-only attack (COA). With the statistical ergodic property of the speckle, we show that the plaintext image can be recovered from the cyphertext alone owing to the fact that their energy spectral density functions are identical. Our result reveals the most serious security issue with the DRPE as it suggests that even the one-time-pad does not guarantee its security. This will open up new inside understanding of current optical security techniques.

Digital super-resolution holographic data storage based on Hermitian symmetry for achieving high areal density

Digital super-resolution holographic data storage based on Hermitian symmetry is proposed to store digital data in a tiny area of a medium. In general, reducing a recording area with an aperture leads to the improvement in the storage capacity of holographic data storage. Conventional holographic data storage systems however have a limitation in reducing a recording area. This limitation is called a Nyquist size. Unlike the conventional systems, our proposed system can overcome the limitation with the help of a digital holographic technique and digital signal processing. Experimental result shows that the proposed system can record and retrieve a hologram in a smaller area than the Nyquist size on the basis of Hermitian symmetry.

RCWA-EIS method for interlayer grating coupling

The grating coupling efficiencies for interlayer connection (overlaid chips) were previously calculated using the new rigorous coupled-wave analysis equivalent-index-slab (RCWA-EIS) method. The chip-to-chip coupling efficiencies were determined for rectangular-groove (binary) gratings. In the present work, the search algorithms used in the RCWA-EIS method are optimized giving rise to improved definition of equivalent indices. Further, the versatility of the RCWA-EIS method is demonstrated by extending it to (nonbinary) parallelogramic gratings, sawtooth gratings, and volume gratings. The finite-difference time-domain method is used to verify the results. This demonstrates the flexibility of the RCWA-EIS method in analyzing arbitrary 1D gratings.

Multilayer recording holographic data storage using a varifocal lens generated with a kinoform

A multilayer recording method using a varifocal lens generated with a kinoform is presented. In this recording method, a focus position is axially displaced by adding a defocus phase to a phase modulation pattern, which consists of a random phase mask and a computer-generated reference pattern. Shift selectivity and multiplexed recording are experimentally investigated in coaxial holographic data storage. Experimental results show that the proposed method allows the recording of holograms along an optical axis without any mechanical movement.

Multiplexed recording with uncorrelated computer-generated reference patterns in coaxial holographic data storage

A computer-generated reference pattern (CGRP) allows improvement in light efficiency and the quality of reconstructed data in coaxial holographic data storage. In this Letter, a multiplexed recording method with uncorrelated CGRPs is proposed. With this method, crosstalk from adjacent holograms is suppressed without shifting a medium. To confirm the feasibility of the proposed method experimentally, shift selectivity is investigated, and then multiplexed recording is performed. Experimental results show that the proposed method enables high-density recording compared with conventional shift multiplexing. In addition, a theoretical analysis implies that at least 100 uncorrelated CGRPs can be designed and used for multiplexed recording.

Review of Random Phase Encoding in Volume Holographic Storage

Random phase encoding is a unique technique for volume hologram which can be applied to various applications such as holographic multiplexing storage, image encryption, and optical sensing. In this review article, we first review and discuss diffraction selectivity of random phase encoding in volume holograms, which is the most important parameter related to multiplexing capacity of volume holographic storage. We then review an image encryption system based on random phase encoding. The alignment of phase key for decryption of the encoded image stored in holographic memory is analyzed and discussed. In the latter part of the review, an all-optical sensing system implemented by random phase encoding and holographic interconnection is presented.

Security optical data storage in Fourier holograms

We have proposed and demonstrated a holographic security storage system that is implemented with a shift multiplexing technique. The security function of this storage system is achieved by using a microdiffuser (MD) for random phase encoding of the reference beams. The apparatus of random phase encoding in this system offers an additional and flexible function during the recording processes. The system can generate holographic security memory or nonsecurity holographic memory via using the MD or not. The storage capacity and the average signal-to-noise value of the security storage system are 16 bits/μm(2) and 3.5, respectively. Lateral shifting selectivity in this holographic security storage system is theoretically analyzed and experimentally investigated.

Multiple-image encryption by compressive holography

We present multiple-image encryption (MIE) based on compressive holography. In the encryption, a holographic technique is employed to record multiple images simultaneously to form a hologram. The two-dimensional Fourier data of the hologram are then compressed by nonuniform sampling, which gives rise to compressive encryption. Decryption of individual images is cast into a minimization problem. The minimization retains the sparsity of recovered images in the wavelet basis. Meanwhile, total variation regularization is used to preserve edges in the reconstruction. Experiments have been conducted using holograms acquired by optical scanning holography as an example. Computer simulations of multiple images are subsequently demonstrated to illustrate the feasibility of the MIE scheme.

Increase of signal-to-noise ratio of a collinear holographic storage system with reference modulated by a ring lens array

We propose an approach of a reference modulation in a collinear holographic storage system to increase the signal-to-noise ratio (SNR). We derive a solution that shows that the point-spread function is related to the autocorrelation function of the reference pattern times a defocusing phase term. Accordingly, the lens-array phase modulation is proposed and demonstrated theoretically. The SNR of the system can be dramatically enhanced to 63.2, rather than 2.3 in the traditional approach.

Iterative data reconstruction in a thin photonic data storage medium using three-dimensional absorbers in a scattering volume medium

Iterative data reconstruction in a thin photonic data storage medium using three-dimensional (3D) absorbers in a highly scattering medium is presented for secure data storage. 3D absorbers are used as data. A highly scattering medium protects data to measure 3D absorbers without knowledge of the scattering coefficient distribution because the phase information is lost during multiple scattering. This prevents an interferometer from measuring the volume medium. To recover the 3D absorbers, a 3D scattering coefficient distribution model is used as prior information and then an iterative method is applied to reduce the difference between the measured and numerical output intensity distributions. A preliminary numerical evaluation is presented.

Optical authentication method using a three-dimensional phase object with various wavelength readouts

An optical system for authentication using a 3D (3D) random phase object with various wavelength readouts is proposed. The 3D phase object without surface modulation is secure when the scattering is strong enough because it prevents from the interferometric measurement. The identification is implemented by the correlation between a measured speckle pattern of the 3D phase object and stored speckle patterns. For accurate identification, two speckle patterns of the 3D object obtained by illuminating two wavelengths are used. Experimental demonstrations and numerical evaluations of wavelength selectivity are presented.

Enhanced rotational Bragg selectivity by use of random phase encoding in volume holographic filter

An out-of-plane angular detection scheme with random phase encoding is proposed. A ground glass is attached on a rod, which is rotated around a center point, so that the rotation of the rod induces the displacement of the ground glass in a circular path. To enhance the rotational sensitivity we adjust the Bragg selectivity of the volume holographic optical element encoded by random phase. Therefore, the rotational sensitivity can be tuned over a large range from several degrees to ten thousandth degrees by changing the radius of rotation. The theoretical calculation, as well as experiment, is demonstrated.

Volume-holographic filters for rotational sensing of three-dimensional objects

A highly precise rotational filter based on a volume-holographic optical element is proposed and demonstrated. We present a clear theoretical calculation of the rotation sensitivity of the volume-holographic filter used to sense the rotation of a spherical ground glass object. By introducing the longitudinal displacement of the scattering point across the sphere, the sensitivity of the filter is greatly improved to 350 times that of a general case for a planar ground glass.

Encrypted holographic memory using an encoded reference wave

A secure holographic memory system is proposed by use of an encoded reference beam. The reference beam is encrypted by a fiber-optic faceplate, which serves as a phase mask. There are seven keys in the system including the position and direction of the fiber bundle and the direction of the incident beam. The experiment shows that the total key length is larger than 1.8 x 1019. The method can be used directly in a shift-multiplexing system with high selectivity.

Volume holographic optical elements for point-to-point imaging with local cross talk

A novel volume holographic optical element serving as an atypical mirror with a lateral magnification of 1 in both the horizontal and the vertical directions is proposed. Optical imaging is performed by point-to-point imaging with local cross talk, which prevents ghost diffraction spots from the holographic element of multiple gratings.

Improvement to human-face recognition in a volume holographic correlator by use of speckle modulation

We show that a speckle-modulation technique can improve the parallelism and the recognition accuracy of volume holographic correlators. The object patterns are modulated by a speckle pattern generated by a diffuser. These modulated patterns are stored as Fourier holograms by use of angular-fractal multiplexing. With the speckle modulation the sidelobes are completely suppressed, the cross talk is negligible, and the correlation peak becomes a bright sharp spot. Thus higher recognition accuracy is achieved. The angular separation between adjacent patterns in the multiplexing could be much smaller, resulting in larger capacity and higher parallelism of the correlator. Also, this technique can be combined with other methods such as wavelet filtering to achieve a large invariant tolerance range. Theoretical analysis, numerical evaluation, and experimental results are presented to confirm that sidelobes and cross talk are sharply suppressed by the speckle modulation.

Enhancement of the angular selectivity in encrypted holographic memory

Enhancement of the angular selectivity in a double random-phase encoded holographic memory by use of reference plane waves is presented. In the storage algorithm, the angular selectivity is improved with the effect of random-phase masks. We show that the angular selectivity achieved by this scheme is more sensitive than that for Bragg angle detuning in a 90 degrees geometry. Both theoretical and experimental results are presented and analyzed.

Duplication of phase key for random-phase-encrypted volume holograms

Holographic memory encrypted by an optical random-phase key and decrypted by either the original phase key or a duplicate key is proposed and demonstrated. The duplicate key is made by recording the encryption wave front with angle multiplexing during writing of the hologram. The amount of three-dimensional shifting that is tolerable in the duplicate key is analyzed.