Xiaodi Tan

Analysis of a collinear holographic storage system: introduction of pixel spread function

Image formation in a collinear holographic storage system was analyzed. The wavefront from each pixel of a spatial light modulator was regarded as a plane wave in the recording medium, and its wave vector was determined by the position of the pixel. The hologram in the recording medium was treated as the summation of all gratings written by all combinations of two plane waves. The image of a data page was formed by diffraction of the reference waves by all gratings. The results of the simulation showed good agreement with experiment. We introduced the pixel spread function to describe the image formation characteristics. Analysis of the pixel spread function reveals that a radial-line pixel pattern for reference waves gave a sharper image than other reference pixel patterns. It is also shown that a random phase modulation applied to each reference pixel improved the image formation.

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 HVDs 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.

Holographic Information Storage System: Today and Future

Holographic information storage systems using Collinear Technologies is proposed by Optware Corporation, Yokohama, Kanagawa, Japan, and in which the information and reference beams are modulated coaxially by the same spatial light modulator (SLM). With this unique configuration, the optical pickup can be designed as small as the DVDs and can be placed on one side of the recording disk. In a Holographic Versatile Disc (HVD) structure, the preformatted metadata reflective layer is used for the focus/tracking servo and reading address information, and the dichroic mirror layer is used for detecting holographic recording information without interfering with the preformatted information. A two-dimensional (2-D) digital page data format is used, and the shift-multiplexing method is employed to increased recording density of HVD. Experimental and theoretical studies suggest that the holographic material is very effective to increased recording density of the system. As the servo technology is being introduced to control the objective lens to be maintained precisely to the disk in the recording and the reconstructing process, a vibration isolator is no longer necessary. HVD will be compatible with existing disk storage systems, like CD and DVD, and enable us to expand its applications into other optical information storage systems

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

We show that a double-random encryption technique can improve the storage capacity of an angular-multiplexed holographic memory system. In the holographic memory system, input binary images are encrypted into white-noise-like images by use of two random phase masks located at the input and the Fourier planes. These encrypted images are stored as holograms in a photorefractive medium by use of angular multiplexing. All the images are encrypted by different sets of random phase masks. Even when the angle separation between adjacent images is small enough to cause cross talk between adjacent images, original binary data can be recovered with the correct phase mask; the other reconstructed images remain white-noise-like images because incorrect masks are used. Therefore the capacity of the proposed system can be larger than that of a conventional holographic memory system without the random phase encryption technique. Numerical evaluation and experimental results are presented to confirm that the capacity of the system with random phase masks is larger than that of the conventional memory system.

Stable injection locking of diode lasers through a phase-modulated double phase-conjugate mirror

The stable injection locking of 0.8-mum diode lasers with a double phase-conjugate mirror (DPCM) was achieved. Phase modulation by piezoelectric transducers allowed us to keep two input beams of the DPCM mutually incoherent during locking. We preserved the high performance of the DPCM and retained stable locking for more than an hour.

Dual-channel recording based on the null reconstruction effect of orthogonal linear polarization holography

We report on dual-channel recording within polarization holography written by orthogonal linear polarization waves. The null reconstruction effect (NRE) of linear polarization holography was experimentally achieved at a large cross-angle of π/2 inside the polarization-sensitive media. Based on the NRE, two polarization encoded holograms were recorded in a dual-channel recording system with negligible inter-channel crosstalk. The two polarization multiplexed holograms could then be sequentially or simultaneously readout by shifting the polarization state of reference wave with the best signal-to-noise of 18:1 obtained within the experiment.

Shift Selectivity of the Collinear Holographic Storage System

A numerical simulations of the signal decay with media shift were performed. Shift selectivity of the page did not depend on the media thickness, but mainly determined by the width of the reference pixel areaWe propose a paraxial solution for pixel shift selectivity, which can simulate the pixel shift selectivity in two dimensions and in wide range easily. Thus, the effect of different reference patterns can be calculated in detail. From the simulation result, we conclude that the pixel shift selectivity get worse for amplitude modulation reference patterns. Making no modulation is the best reference pattern for pixel shift selectivity, however the point spread function will be worst in this case. To get an optimum system in both pixel shift selectivity and point spread function, the reference pattern with phase modulation will be the best choice.

Collinear technology for holographic versatile disc (HVD) system

Holographic Versatile Disc (HVDTM) using CollinearTM Technologies is proposed by OPTWARE Corporation, in which the information and reference beams are displayed co-axially by the same SLM. With this unique configuration the optical pickup can be designed as small as the DVDs, and can be placed on one side of the recording disc. In HVDTM structure, the pre-formatted meta-data reflective layer is used for the focus/tracking servo and reading address information, and the dichroic mirror layer is used for detecting holographic recording information without interfering with the preformatted information. A 2-dimensional digital page data format is used and the shift-multiplexing method is employed to increased recording density of HVDTM. Experimental and theoretical studies suggest that the holographic material is very effective to increased recording density of the system. As the servo technology is being introduced to control the objective lens to be maintained precisely to the disc in the recording and the reconstructing process, a vibration isolator is no longer necessary. HVDTM will be compatible with existing disc storage systems, like CD and DVD, and enable us to expand its applications into other optical information storage systems.

Calculation of the Pixel Spread Function with a Simple Numerical Model for the Collinear Holographic Storage System

A numerical simulation method to evaluate the collinear holographic memory is proposed. A pixel spread function is defined and the resolving power of the page data image increases with the increase of the media thickness.

Duplication Technology for Secured Read-Only Holographic Versatile Disc

A duplication technology for secured Read-Only Holographic Versatile Disc (HVD-ROM), using collinear technology, is proposed. With this method a large amount of HVD-ROMs can be duplicated, but the HVD-ROM can not be duplicated again.