Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
1999-08-13
2002-07-09
Edun, Muhammad (Department: 2653)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
C369S112100, C369S112150
Reexamination Certificate
active
06418106
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to holographic storage devices and more particularly to a method and device for storing a plurality of volume holograms within a spinning, disk-shaped, photorefractive medium.
BACKGROUND OF THE INVENTION
Holographic techniques for storing images are well known. Such techniques are commonly used to store images in a wide variety of different applications. Additionally, various methodologies for utilizing such holographic techniques to store digital data for use in computer systems are currently being explored.
The technique for forming holograms comprises splitting the highly coherent output beam of a laser into separate reference and object beams. The reference beam is directed onto the holographic storage medium, e.g., a photorefractive material, while the object beam is directed onto the object whose image is to be stored. Light from the object is directed to the photorefractive medium wherein an interference pattern is formed due to the interaction of the reference beam with the object beam.
When utilized in digital data storage applications, the object beam typically passes through a spatial light modulator, e.g., a liquid crystal shutter matrix, rather than being reflected off an object, in order to form the holographic image. The spatial light modulator adds the desired digital data to the object beam to facilitate storage of the digital data in the hologram formed therefrom.
Regardless of the application (i.e., the storage of images or data), subsequently directing a reference beam onto the holographic storage medium results in the reconstruction of an image representative of the originally illuminated object or stored digital data.
Also known are techniques for storing a plurality of such images within a single photorefractive medium via angle-multiplexing of the reference beam. Such angle-multiplexing is discussed in, “THEORY OF OPTICAL INFORMATION STORAGE IN SOLIDS”,
Applied Optics
, Vol. 2, No. 4, pg. 393 (1963). The method of angle-multiplexing generally involves maintaining a constant angle for the object beam, while varying the angle of the reference beam for each sequential exposure, i.e., the formation of each separate hologram. Angle-multiplexing thus allows a large number of holograms to be stored within a common volume of photorefractive medium, thereby greatly enhancing the storage density thereof.
Also known are techniques for storing a plurality of such holograms within a spinning drum or disk shaped photorefractive medium. Examples of some holographic memories which utilize drum or disk shaped medium are provided in U.S. Pat. Nos. 3,610,722; 3,737,878; 3,848,096; 4,104,489; 4,224,480; 4,420,829; 4,449,785; 4,929,823; 5,111,445; 5,128,693; 5,285,438; 5,339,305.
However, one problem commonly associated with such contemporary disk and drum based holographic memories is that the geometry of the system is not optimized with respect to the crystalline structure of the storage medium. Further, such contemporary systems do not utilize effective path-length monitoring so as to assure the integrity of holograms within the medium and to assure reliable read-out of a plurality of different sets of angle-multiplexed holograms.
As such, although the prior art has recognized to a limited extend the problem of storing volume holograms in a spinning disk medium, the proposed solutions, to date, have been ineffective in providing a satisfactory remedy.
SUMMARY OF THE INVENTION
The present invention specifically addresses and alleviates the above-mentioned deficiencies associated with the prior art. More particularly, the present invention comprises a volume holographic memory comprising a disk comprised of photorefractive medium and configured to spin about a central axis thereof. The spin axis is perpendicular to a central opening formed within the disk such that the disk spins in a manner similar to that of a contemporary CD-ROM.
Object beam optics are configured to direct an object beam through the outer edge of the disk and reference beam optics are similarly configured to direct a reference beam through the outer edge of the disk. The object beam and the reference beam intersect within the photorefractive medium wherein they cooperate so as to sequentially form a plurality of separate volume holograms within the spinning disk. Such volume holograms may be written to, erased from, or read from the disk while the disk is spinning, so as to provide a fast, high density memory.
An angle multiplexer varies the angle at which either the object beam or the reference beam, preferably the reference beam, is directed through the outer edge of the disk. The angle multiplexer preferably comprises a galvanometer mirror. The storage density of the photorefractive medium is substantially enhanced via the use of such angle multiplexing.
Both the object beam and the reference beam are preferably directed into the center opening of the disk after cooperating to form a hologram, and are then reflected from the center opening of the disk via a reflecting element, preferably a pair of beam splitters. During write and erase operations, both the object and reference beams may be terminated, preferably via beam blocks, after exiting the disk, since their task has been completed and they are both no longer needed. During read-out operations, one of the two beam splitters disposed within the central opening of the disk directs the reference beam from the central opening of the disk to a phase conjugator.
The phase conjugator forms a conjugate reference beam which is directed back through the beam splitter and into the photorefractive medium of the spinning disk. As the conjugate reference beam is transmitted through the photorefractive medium, a previously stored hologram formed therein causes the conjugate reference beam to be transformed into a conjugate object beam which is representative of the hologram effecting such transformation. Thus, the conjugate reference beam excites a conjugate object beam from the stored hologram. The excited conjugate object beam is then read by a sensor, preferably a two-dimensional array charge coupled device (CCD), so as to provide an electrical signal representative of the originally stored data.
As those skilled in the art will appreciate, the phase conjugator removes distortions introduced into stored holograms in a manner which facilitates the use of inexpensive, fast, (i.e., low f/#) object beam optics. Thus, the effects of distortion, such as spreading of the object beam due to undesirable diffraction, are substantially reversed by the phase conjugator.
According to the preferred embodiment of the present invention, a high-power pulsed laser is utilized for the write, erase, and read-out operations. The energy density obtained by such a high-power pulsed laser, when focused, is sufficient to cause ionization of the air in the immediately vicinity of the focus. For this reason, a pressure cell is preferably disposed at the focus of the reference beam optics, where the power density is greatest, so as to inhibit such ionization. Those skilled in the art will appreciate that air ionization is inhibited at increased pressure.
The present invention preferably comprises a liquid Stimulated Brillouin Scattering (SBS) phase conjugator, preferably comprised of methanol. Those skilled in the art will appreciate that various other phase conjugating materials are likewise suitable.
According to the preferred embodiment of the present invention, a Pockels cell is utilized to rotate the polarization of the laser beam from which the object and reference beams are formed to an orientation suitable for writing, erasing, and reading of holograms, according to well-known principles.
A spatial light modulator (SLM) is utilized for modulating, i.e., applying digital data to, the object beam. According to the preferred embodiment of the present invention, a 1024×1024 pixel reflecting type spatial light modulator is utilized to facilitate the storage of 1.18 terabits of data, as discussed in detail b
Edun Muhammad
Northrop Grumman Corporation
Stetina Brunda Garred & Brucker
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