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.

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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.

A novel secure holographic memory system with polarization encoding is proposed. Two-dimensional original data are encoded as a two-dimensional polarization distribution. The polarization state at each pixel is scrambled by a mask that changes the polarization state into a random state. The mask can rotate the direction of the principal axes of the elliptically polarized light and can change the phase retardation at each pixel. The light with the random polarization state is stored as a hologram that can produce the vector phase-conjugate beam. In the decryption the vector phase-conjugation readout can recover the original polarization state by use of the same mask used in the encryption. Experimental results of encryption and decryption with a bacteriorhodopsin film are presented.

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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.

In the era of information explosion, the demand of data storage is increased dramatically. Holographic data storage technology is one of the most promising next-generation data storage technologies due to its high storage density, fast data transfer rate, long data life time and less energy consumption. Collinear holographic data storage technology is the typical solution of the holographic data storage technology which owns a more compact, compatible and practical system. This paper gives a brief review of holographic data storage, introduces collinear holographic data storage technology and discusses phase modulation technology being used in the holographic data storage system to achieve higher storage density and higher data transfer rate.