Shift Multiplexing Research Articles

Π-shaped pixel pattern for shift multiplexing surface holograms in holographic data storage

In this study, a method for shift multiplexing surface holograms was proposed for data storage systems. Shift multiplexing of surface holograms is inhibited by noise specific to Raman–Nath diffraction. We proposed a model to analyze the influence of noise. Subsequently, we proposed a method for noise suppression to arrange signal and reference pixels in a shape similar to the Greek letter “Π.” The proposed method is generally applicable to thin holograms in which Raman–Nath diffraction occurs. The effectiveness of noise suppression using the proposed method was experimentally demonstrated. We theoretically estimated that a shift multiplexing surface holographic data storage system using the proposed method could achieve a significantly higher data transfer rate than conventional optical discs while maintaining a comparable storage density.

Interference-based multiple-image encryption and authentication via lateral shift multiplexing and sparsification

We report a novel optical method of interference-based multiple-image encryption and authentication via lateral shift multiplexing and sparsification. We encode multiple original images into two ciphertexts only containing partial information of the multiple images when the secret key generated randomly is laterally movable within a predefined interval, and we analytically derive the two other keys of each original image. Information discrimination for each original image as well as its silhouette is still prohibited in spite of the utilization of one, two, three, or even four masks for decryption. The three secret keys of each original image significantly improve the security of the proposed method, and binary amplitude masks are included in the decryption and authentication procedure. Moreover, the proposed scheme completely eliminates the cross-talk problem and destroys any high correlation between the plaintexts and ciphertexts. Numerical simulations are executed to prove the effectiveness and security of this scheme.

Electrically tunable diffuser for holographic multiplexing storage

A polymer-dispersed liquid crystal (PDLC) device is fabricated to serve as a diffuser for random phase encoding of reference beams in holographic storage system. By tuning the applied voltage on the PDLC device, three individual random phase reference waves which are orthogonal to each other can be generated. Therefore, holographic multiplexing scheme can be performed using voltage modulation on the PDLC diffuser. In addition, shift multiplexing technique still can be performed in the storage system. When compared with conventional shift multiplexing, using electrically tunable PDLC diffuser offers additional multiplexing mechanism in the presented technique. Using the proposed PDLC diffuser, the multiplexing numbers can be enhanced three times owing to voltage modulation. A holographic storage system implemented using shift multiplexing and voltage modulation on PDLC diffuser is demonstrated.

Analysis of crosstalk-free conditions for a cross-shift multiplexing method in holographic data recording.

There are various multiplexing methods in holographic data recording. A larger storage capacity is anticipated when combining two different multiplexing methods and then ensuring crosstalk-free recording. We have proposed a cross-shift multiplexing method that combines shift multiplexing and peristrophic multiplexing. Crosstalk-free conditions for shift multiplexing and peristrophic multiplexing were separately clarified. However, these conditions cannot be individually applied to cross-shift multiplexing. Then, we deduced the analytic formula of crosstalk for cross-shift multiplexing and verified its validity by comparing the theoretical and experimental results. Using this analytic formula, the shift pitch and rotation angle that attain crosstalk-free multiplexing can be determined.

Shift multiplexing with a spherical wave in holographic data storage based on a computer-generated hologram.

A holographic data storage system based on a computer-generated hologram (CGH) is simple and compact because a hologram of a data page is recorded through an imaging system without an additional optical path for a reference beam. In this paper, to improve the recording density of the holographic data storage based on a CGH, a shift multiplexing method using a spherical wave is proposed. A data page to be stored and a spherical wave are simultaneously reconstructed from a single CGH. This allows shift multiplexing by displacing a recording medium. Experimental results show that the proposed method can improve shift selectivity and enables us to implement shift multiplexing.

Numerical simulations on inter-page crosstalk characteristics in three-dimensional shift multiplexed self-referential holographic data storage

A three-dimensional (3D) shift multiplexing technique is applied to increase the number of multiplexed datapages in self-referential holographic data storage (SR-HDS), enabling holographic recording with purely one-beam geometry. In the 3D shift multiplexing technique, the recording medium is shifted not only along the x- and y-axes but also along the z-axis to multiplex the datapages. Because the shift directions in the 3D shift multiplexing technique are expanded compared with the conventional 2D technique, the number of multiplexed datapages is expected to be increased. We numerically clarify the appropriate 3D recording layout of holograms in which the effect of the inter-page crosstalk can be reduced after showing that datapages are multiplexed on shifting the recording medium along the z-axis. The results show 3D shift multiplexing can effectively realize high-density SR-HDS when appropriately designed layouts are used.

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.

Shift-Peristrophic Multiplexing for High Density Holographic Data Storage

Holographic data storage is a promising technology that provides very large data storage capacity, and the multiplexing method plays a significant role in increasing this capacity. Various multiplexing methods have been previously researched. In the present study, we propose a shift-peristrophic multiplexing technique that uses spherical reference waves, and experimentally verify that this method efficiently increases the data capacity. In the proposed method, a series of holograms is recorded with shift multiplexing, in which the recording material is rotated with its axis perpendicular to the material’s surface. By iterating this procedure, multiplicity is shown to improve. This method achieves more than 1 Tbits/inch2 data density recording. Furthermore, a capacity increase of several TB per disk is expected by maximizing the recording medium performance.

Multiple-image encryption in an interference-based scheme by lateral shift multiplexing

In this paper, a novel multiple-image encryption method is proposed by lateral shift multiplexing of phase-only mask. Multiple images are encrypted into three phase-only masks, one of which is generated in computer and saved as the secret key, and the other two can be obtained with a simple non-iterative algorithm. The optical scheme for decryption is quite simple, and different original images could be successfully retrieved at the output plane while the secret key is laterally shifted continuously with predefined intervals. In addition, the silhouette problem, which persecuted the previous interference-based encryption scheme, is simultaneously eliminated. The feasibility and effectiveness of the proposed method are demonstrated by numerical results.

Multilayer Collinear Holographic Memory with Movable Random Phase Mask

We proposed a new multilayer collinear holographic memory (MCHM) with a movable random phase mask that can act as an interlayer crosstalk reducer. First, to clarify the feasibility of our proposed device, we showed that it has an extremely sharp shift selectivity along the thickness direction of medium of nearly 5.0 µm when the numerical aperture of the objective lens is 0.60. Next, we demonstrated that the utilization of the movable random phase mask can suppress interlayer crosstalk sufficiently, resulting in the improvement in the quality of reconstructed signals. Moreover, we revealed that the MCHM, in which the parallel use of a multilayered technique and collinear shift multiplexing is performed, brings out the medium potential sufficiently in terms of signal to noise ratio and medium dynamic range consumption. Finally, we verified that the MCHM can achieve a nearly 2 times larger data density at least compared with a conventional collinear holographic memory.

Multilayer Collinear Holographic Memory with Movable Random Phase Mask

We proposed a new multilayer collinear holographic memory (MCHM) with a movable random phase mask that can act as an interlayer crosstalk reducer. First, to clarify the feasibility of our proposed device, we showed that it has an extremely sharp shift selectivity along the thickness direction of medium of nearly 5.0 µm when the numerical aperture of the objective lens is 0.60. Next, we demonstrated that the utilization of the movable random phase mask can suppress interlayer crosstalk sufficiently, resulting in the improvement in the quality of reconstructed signals. Moreover, we revealed that the MCHM, in which the parallel use of a multilayered technique and collinear shift multiplexing is performed, brings out the medium potential sufficiently in terms of signal to noise ratio and medium dynamic range consumption. Finally, we verified that the MCHM can achieve a nearly 2 times larger data density at least compared with a conventional collinear holographic memory.

Multiresolution Coding Using Amplitude and Phase Modulations for Holographic Data Storage

A new data coding method using both amplitude and phase modulations is proposed for increasing the coding rate in a holographic memory. The phase modulation code has a lower resolution than the conventional sparse block code used as the amplitude modulation code. The phase modulation code can add more than 1 bit to the amplitude modulation code. We show that the coding rate can be increased up to 0.78 from 0.67 in the 3:9 amplitude modulation code when the binary phase code is added. We also show that the bit error rate and signal-to-noise ratio in the amplitude data are improved by adding the phase modulation code in speckle shift multiplexing. The histogram of the phase distribution is also separated well.

Improvement of Storage Capacity Using Confocal Scheme in Reflection-Type Holographic Memory System with Speckle Shift Multiplexing

The storage capacity of a reflection-type holographic memory with three-dimensional speckle shift multiplexing embedded in a confocal scheme is presented. The confocal optical system can eliminate the interpage cross-talk noise in the speckle shift multiplexing. This leads to the increase in signal-to-noise ratio and the decrease in bit error rate. Numerical simulation results show that a maximum storage capacity can be increased up to 4.35 Tbyte/disk by using the confocal scheme. This value is derived numerically from the ideal situations of recording medium and evaluation of bit error rate in three-dimensional multiplexing.

Improvement of Storage Capacity Using Confocal Scheme in Reflection-Type Holographic Memory System with Speckle Shift Multiplexing

Improvement of Storage Capacity Using Confocal Scheme in Reflection-Type Holographic Memory System with Speckle Shift Multiplexing

Speckle-Multiplexed Hologram Using Speckle Filter with a Polymer-Dispersed Liquid Crystal

It is known well that speckle referencing enhances the shift selectivity of holograms. In this paper, the scheme of using speckle patterns produced by a filter with a polymer-dispersed liquid crystal is applied to multiplexing holograms. The speckle patterns can be reconfigured by applying an electric field to the filter. The experiment confirmed the feasibility of a hybrid scheme with shift multiplexing and speckle multiplexing.

Numerical estimation of storage capacity in reflection-type holographic disk memory with three-dimensional speckle-shift multiplexing

Maximum storage capacity in a reflection-type holographic memory with three-dimensional speckle shift multiplexing is investigated numerically. An explicit expression of storage capacity is derived on the basis of interpage crosstalk noise. We fabricate a simulator to evaluate reflection-type holographic data storage by calculating wave propagation, recording a hologram, and reconstruction by scalar diffraction. We calculate the properties of the resultant diffraction efficiency, that is the noise, at the first null in the speckle-shift multiplexing. Numerical results indicate that the storage capacity is proportional to the numerical aperture to the fourth power and to the volume of the recording medium and is inversely proportional to the wavelength to the third power. Achievable storage capacity is discussed.

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.

Three-dimensional shift selectivity in reflection-type holographic disk memory with speckle shift recording

Three-dimensional shift selectivity of a reflection-type hologram with speckle shift recording is investigated experimentally and numerically. We build an experimental setup consisting of lenses with numerical apertures of 0.28 and an iron-doped LiNbO(3) with a thickness of 0.5 mm. The experimental results show that three-dimensional selectivity has a size of 0.97 microm x 0.97 microm x 8.8 microm in diffraction efficiency. We also develop a volume holographic memory simulator to evaluate the experimental results. The simulator can quantitatively evaluate bit error rate, signal-to-noise ratio, and diffraction efficiency. Numerical results are in good agreement with the experimental results. The experimental and numerical results indicate that three-dimensional shift multiplexing can increase the storage capacity.

Reflection-type holographic disk memory with random phase shift multiplexing

A reflection-type holographic disk memory system with random phase shift multiplexing is proposed. The experimental results show that a binary data page of 18x17 bits is recorded successfully at intervals of 4 mum in a Fe:LiNbO3 crystal with a thickness of 0.5 mm when six data pages are superimposed. Numerical results show that random phase modulation can improve the shift selectivity in shift multiplexing recording as well as in data security. Experimental and numerical results show that reflection-type holographic disk memory has a high potential for terabyte storage capacity as in transmission-type memory.

Orthogonal polarization dual-channel holographic memory in cationic ring-opening photopolymer

A dual-channel holographic recording technique and its corresponding memory scheme in the cationic ring-opening photopolymer are presented. In the dual-channel technique, a pair of holograms are recorded simultaneously with two orthogonal polarization channels in the common volume of the material, and are reconstructed concurrently with negligible inter-channel crosstalk. The grating strengths of these two channels are investigated and the relevant parameters for equal diffraction intensity readout are optimized. Combining the dual-channel technique with speckle shift multiplexing, a high-density holographic memory is realized. This dual-channel scheme enables the users to interact with the storage medium from an additional channel. The simultaneous nature of the two channels also offers a faster data transfer rate in both the recording and reading processes.