Optical: systems and elements – Holographic system or element – Hardware for producing a hologram
Reexamination Certificate
2003-02-19
2004-08-24
Boutsikaris, Leo (Department: 2872)
Optical: systems and elements
Holographic system or element
Hardware for producing a hologram
C359S025000, C365S125000, C430S001000
Reexamination Certificate
active
06781725
ABSTRACT:
The present invention relates to a storage method and system for storing mutually non-overlapping volume holograms and to a confocal holographic optical data storage system with non-overlapping records. The invention pertains to the technical field of holographic optical data storage systems with high density storage capabilities. In the present patent application, a new method and system for storing data in the form of mutually non-overlapping three-dimensional holograms are disclosed.
The invention provides a solution to the technical problem of improving the manner in which data is stored in the holographic optical storage medium.
Optical storage is largely used today, e.g. in the form of music and data CDs, DVD discs and magneto-optical systems. All these systems are two-dimensional (2-D), which means they only store data in one plane or two at the most, the said two planes being both accessible from one side of the storage medium. The storage density is limited, by the nature of light as electromagnetic radiation, to approx. 1 bit per square area having the side equal in length to the wavelength of the light being used. For storing greater amounts of data—the capacity of 1000 CD-ROMs being set as a desired target—all three dimensions of the storage material must be put to use. The data density should be of 1 bit per volume of a cube having the side equal in length to the wavelength of the light being used. This would mean that into a cube with the side of 1 cm, using blue-green light with the wavelength lambda0 in vacuum of 500 nm, 10
12
Bytes of data could be stored, thus exceeding the capacity of 1000 CD-ROMs. Since the refractive index of any storage material is greater than 1, the wavelength lambda inside said material is shorter than the wavelength lambda0 in vacuum. With a shorter wavelength, the volume data storage density is even greater than in the above example.
Of all the systems for three-dimensional data storage, holographic optical storage is the most promising. A general description of holographic optical storage is to be found among others in the U.S. Pat. Nos. 4,920,220, 5,450,218, 5,440,669 and 5,377,176, FR Pat. No. 2770912, GB Patent No. 2332754, and in the book “Nonlinear Optical Effects and Materials”, Springer, 2000, Ed. P. Günter, in the chapter “Photorefractive Memories for Optical Processing”, written by M. Duelli, G. Montemezzani, M. Zgonik, and P. Günter.
To store one page of data, containing typically 1000×1000 bits in the form of a chessboard image with transparent and opaque pixels, the following procedure is used. The laser light beam is split into two coherent beams. The signal light beam is modulated with data and focused onto the optical storage material. Simultaneously, the said material is illuminated from the same or any other convenient direction by another light beam, referred to as the reference beam. While the hologram is being written, the signal and the reference beams interfere, and their interference pattern is recorded as a variation in the optical characteristics of the material, which is called a hologram. The data is read back by illuminating the same hologram with only the reference light beam, which, upon being refracted on the hologram, reconstructs the signal beam and, consequently, the recorded data.
The advantages of holographic optical storage over other systems are high data density and parallel read/write operation. Although said advantages have been known for nearly 40 years, these systems, despite all the efforts, have not become largely used. In recent years, however, the interest in the development and the commercialization of these systems has been growing again, owing to rising necessities and to the advancements made in the supporting optoelectronic technologies.
DESCRIPTION OF THE PRIOR ART
By using the three-dimensional technique of hologram recording, multiple holograms may be stored in the same volume. This is called multiplexing and, so far, a variety of multiplexing methods have been investigated, such as: angular multiplexing, wavelength multiplexing, phase correlation multiplexing and, finally, spatial multiplexing. Fundamentally, these methods are all based upon the Bragg diffraction selectivity on a three-dimensional hologram and they all use relatively wide reference beams to write and read holograms. The methods of multiplexing are relatively complex and, moreover, the writing of multiple holograms in the same space causes the diffraction efficiency of individual holograms to decline rapidly. The main reason for this is that the writing of each new hologram partially erases all the previous holograms. By conveniently selecting the hologram writing time sequence, it can be achieved that at the end of the writing procedure all the holograms have equal diffraction efficiency. In this case, their diffraction efficiency declines at a rate of 1/N
2
, where N is the number of holograms written in the same volume, which practically means that not more than 1000 holograms can be stored in the same volume. A further disadvantage with multiplexed holograms is that selective erasing of single holograms is difficult. Although selective erasing is possible, it poses such high requirements upon the mechanical stability of the storage system as a whole that it is extremely uneconomical. Another possibility for erasing a single hologram is to erase all the multiplexed holograms and discard the superfluous one, while rewriting the rest to a new spot. As a consequence of all the described problems related to multiplexing, all known methods of holographic data storage are still greatly limited in their use and still very far from reaching the theoretically predicted storage densities.
If the so-called photo refractive crystals are used as the storage material, a further serious drawback of the optical holographic storage emerges. A description of the characteristics and advantages of photo refractive crystals is found in the aforementioned book, “Nonlinear Optical Effects and Materials”, Springer, 2000. The problem consists in the unwanted erasure of the holograms while they are being read out, which affects multiplexed holograms even more severely because of the low diffraction efficiency of single holograms. They must be read out by means of a stronger reference beam, which in turn erases all the holograms that are multiplexed in the same volume as the chosen one.
A search has been run through the patent databases by the keywords “holographic storage”. In the European Patent Office database at http://ep.espacenet.com there were 295 hits when running the keyword search through title and abstract only. In the U.S. Patent Office at http://www.uspto.gov/patft/ there are 490 hits for the years 1999-2000 alone when running the keyword search through all fields. The most relevant patents and specialized bibliography of the field have been sifted. The most promising recent methods for simplifying the system of multiplexing are disclosed in the following patents. In the U.S. Pat. No. 5,844,700 data is recorded on a cylindrically shaped storage material, multiplexing is done by a translation of the cylinder along and a rotation about its axis, there being a considerable overlapping of consecutive holograms. In the U.S. Pat. No. 5,949,558 data is written onto a plate-shaped material, multiplexing is carried out by shifting the plate, while consecutive holograms likewise overlap considerably. A system similar to the one previously described is disclosed also in the patent CN 1159046. In the U.S. Pat. No. 6,040,930 a method of minimizing the volume of the hologram is disclosed by the authors, the results being, however, still far from what is theoretically possible.
The usual manner of optimizing the digital method of data storage is to minimize the space needed to store a single bit of information. For optical three-dimensional storage systems this would mean that, for each bit of information, the exact volume where the bit is stored is known, the minimal volume being a cube with its side equal to the wavelength of t
Boutsikaris Leo
Connolly Bove & Lodge & Hutz LLP
Institut Jozef Stefan
Wyche Myron K.
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