Improvement in holographic storage capacity by use of double-random phase encryption

Reduction of phase error on phase-only volume-holographic disc rotation with pre-processing by phase integral

In this paper, we present a study of observation of phase error of a volume holographic storage disc during the reading process when the disc is rotated or displaced in the theoretical calculation and the corresponding experiment. This additional phase error will dramatically decrease the bit error rate of a phase-only signal, even applying double-frequency shearing interferometry to retrieve the stored phase signal. Then we propose a novel approach to solve the problem. The stored signal is pre-processed by phase integral along the shearing direction so that applying the integral process to decode the phase signal is not necessary in the readout process. The proposed approach effectively reduces the error in phase retrieval and will be useful when applying double-frequency shearing interferometry in the readout process for volume holographic storage.

Volume holographic recording in Irgacure 784-doped PMMA photopolymer

Photoinitiator plays a crucial role on photopolymer. Unlike photoinitiator phenanthrenequinone (PQ), the solubility of Irgacure 784 dissolved in MMA is very high. In this paper, we use Irgacure 784 as photoinitiator doped in poly(methyl methacrylate)(PMMA)to make a bulk photopolymer with high photoinitiator concentration for holographic data storage. The effect of concentration of photoinitiator and record intensity are experimentally investigated. The results reveal the material has an optimum condition in holographic recording. A comparison between our material and the material of PQ doped PMMA is carried out by experiment. It is found that our material has better performance on diffraction efficiency, refractive index modulation and recording sensitivity. Besides, this material also has polarization sensitivity, which can be applied to polarization holography. With the capacity of recording polarization holography and conventional intensity holography simultaneously, the Irgacure 784 doped PMMA material is expected to be applied in holographic data storage.

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.

Randomly displaced phase distribution design and its advantage in page-data recording of Fourier transform holograms

For Fourier transform holography, an effective random phase distribution with randomly displaced phase segments is proposed for obtaining a smooth finite optical intensity distribution in the Fourier transform plane. Since unitary phase segments are randomly distributed in-plane, the blanks give various spatial frequency components to an image, and thus smooth the spectrum. Moreover, by randomly changing the phase segment size, spike generation from the unitary phase segment size in the spectrum can be reduced significantly. As a result, a smooth spectrum including sidebands can be formed at a relatively narrow extent. The proposed phase distribution sustains the primary functions of a random phase mask for holographic-data recording and reconstruction. Therefore, this distribution is expected to find applications in high-density holographic memory systems, replacing conventional random phase mask patterns.

Quantitative security evaluation of optical encryption using hybrid phase- and amplitude-modulated keys

In the increasing number of system approaches published in the field of optical encryption, the security level of the system is evaluated by qualitative and empirical methods. To quantify the security of the optical system, we propose to use the equivalent of the key length routinely used in algorithmic encryption. We provide a calculation method of the number of independent keys and deduce the binary key length for optical data encryption. We then investigate and optimize the key length of the combined phase- and amplitude-modulated key encryption in the holographic storage environment, which is one of the promising solutions for the security enhancement of single- and double-random phase-encoding encryption and storage systems. We show that a substantial growth of the key length can be achieved by optimized phase and amplitude modulation compared to phase-only encryption. We also provide experimental confirmation of the model results.

Fast double-phase retrieval in Fresnel domain using modified Gerchberg-Saxton algorithm for lensless optical security systems

A novel fast double-phase retrieval algorithm for lensless optical security systems based on the Fresnel domain is presented in this paper. Two phase-only masks are efficiently determined by using a modified Gerchberg-Saxton algorithm, in which two cascaded Fresnel transforms are replaced by one Fourier transform with compensations to reduce the consumed computations. Simulation results show that the proposed algorithm substantially speeds up the iterative process, while keeping the reconstructed image highly correlated with the original one.

Introducing secure modes of operation for optical encryption

We analyze optical encryption systems using the techniques of conventional cryptography. All conventional block encryption algorithms are vulnerable to attack, and often they employ secure modes of operation as one way to increase security. We introduce the concept of conventional secure modes to optical encryption and analyze the results in the context of known conventional and optical attacks. We consider only the optical system "double random phase encoding," which forms the basis for a large number of optical encryption, watermarking, and multiplexing systems. We consider all attacks proposed to date in one particular scenario. We analyze only the mathematical algorithms themselves and do not consider the additional security that arises from employing these algorithms in physical optical systems.

Spread-space spread-spectrum technique for secure multiplexing

A novel technique for multiplexing complex images is proposed in which each image may be demultiplexed only if a set of random encryption keys is known. The technique utilizes the ability of the double random phase encoding method to spread a signals' energy in both the space and the spatial frequency domains in a controlled manner. To multiplex, images are independently encrypted with different phase keys and then superimposed by recording sequentially on the same material. Each image is extracted by using the particular key associated with it. During decryption the energy from the other images is further spread, making it possible to minimize its effects by using suitable filters. Wigner analysis is applied to the technique, and numerical results are presented.

Fast algorithm of phase masks for image encryption in the Fresnel domain

A high performance lensless optical security system based on the discrete Fresnel transform is presented. Two phase-only masks are generated with what we believe to be a novel and efficient algorithm. Their position coordinates and the wavelength are used as encoding parameters in the encryption process. Compared with previous studies, the main advantage of this proposed encryption system is that it does not need any iterative algorithms to produce the masks, and that makes it very efficient and easy to implement without losing the encryption security.

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.

Collinear technology for a holographic versatile disk

A novel reading and writing technology for a holographic versatile disk (HVD) system called collinear technology is developed. With this method a two-dimensional data page can be recorded as volumetric holograms generated by a reference beam and a signal beam that are bundled on the same axis and that are irradiated on the recording medium through a single objective lens. The multiplex recording and reconstruction process is demonstrated, and it is shown that the optical configuration and the dichroic medium disk structure are suitable for a compact system. With the HVD's special structure, the system can use a servo to focus, track, and locate the reading and writing addresses. A unique selectable-capacity recording format of a HVD and its standardization activity are also introduced. This method willenable us to construct a small HVD system with CD and DVD upper compatibilities.

Collinear holography

A novel reading and writing technology for a holographic storage system called collinear holography is developed. With this method, two-dimensional page data can be recorded as volumetric holograms generated by a reference beam and a signal beam that are bundled on the same axis and irradiated on the recording medium through a single objective lens. The multiplex recording and reconstructing process is demonstrated, and it is presented that optical configuration and the dichroic media disk structure are suitable for a compact system. This method enables us to construct a small volumetric optical disk storage system with CD and DVD upper compatibility.