The present invention relates to an apparatus and a method for recording information on and/or reproducing information from optical information recording media by using holography.
Optical disc products with a recording density of about 50 GB are being commercialized that are based on Blu-ray Disc (BD) standard and High Definition Digital Versatile Disc (HD DVD) standard using a blue semiconductor laser.
Optical discs are expected to have an increased capacity of as large as 100 GB to 1 TB, comparable to that of HDD (Hard Disc Drive), in the future.
However, to realize such an ultrahigh density with the current optical discs, a novel storage technology is required, different from the conventional trend of high density technologies that attempts to increase the storage capacity by shortening a wavelength and increasing NA of an object lens.
With a wide-ranging studies on next generation storage technologies under way, a hologram recording technology is available that records digital information using holography.
Among the hologram recording technologies is one disclosed in JP-A-2004-272268. This patent document describes a so-called angle-multiplexing recording method which focuses a signal beam flux on an optical information recording medium through a lens and at the same time throws a reference beam of collimated rays to the medium to cause interferences to record a hologram and displays different pages of data on a spatial light modulator by changing an incidence angle of the reference beam to the optical recording medium to realize multiplex recording. The patent document also discloses a technology that puts an aperture (spatial filter) at a beam waist of a lens-focused signal beam to shorten the intervals of adjoining holograms, thereby increasing the recording density and capacity, compared with those of the conventional angle-multiplexing recording method.
Another hologram recording technology is disclosed in, for example, WO2004-102542. This document describes an example of shift multiplexing hologram recording method which, in one spatial light modulator, focuses a light from inner pixels as a signal beam and a light from outer ring-like pixels as a reference beam onto an optical recording medium through one and the same lens to cause interferences between the signal beam and the reference beam at near the focus plane of the lens to record a hologram.
There is an encoding method used for the above hologram recording, such as one disclosed in JP-A-9-197947. This patent document describes a 2-dimensional encoding method for hologram recording which throws at least one light wave through a 2-dimensional spatial light modulator to determine information to be recorded, characterized in that four adjoining pixels or 4-multiples of pixels in the 2-dimensional spatial light modulator are taken as one set and that one fourth of the number of pixels making up each set is made to pass the light and the remaining three fourths are made to interrupt it.
Another example of the conventional technology is JP-A-2005-190636, which provides “a holographic recording method, a holographic memory reproducing method, a holographic recording apparatus and a holographic memory reproducing apparatus, designed to improve an encoding rate by preventing variations in reproduced imaged intensity even if pixel blocks of different numbers of ON pixels are mixedly used.”