Optical: systems and elements – Holographic system or element – For producing or reconstructing images from multiple holograms
Reexamination Certificate
2002-02-07
2004-09-07
Robinson, Mark A. (Department: 2872)
Optical: systems and elements
Holographic system or element
For producing or reconstructing images from multiple holograms
C359S025000
Reexamination Certificate
active
06788443
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates in general to information storage media, and more particularly, to holographic data storage systems (HDSS). More specifically, this invention relates to methods and devices for associative, or content-addressable, data retrieval. The invention can also be applied to a device that does not support associative retrieval.
BACKGROUND
Holographic data storage is accomplished by intersecting two coherent light beams in a photosensitive medium in order to record a hologram. Data retrieval is accomplished by illuminating the hologram with a replica of one of the beams, thereby causing a replica of the other beam to be reconstructed through the physics of holography. In the case of sufficiently thick media, a plurality of holograms (a hologram “stack”) may be recorded in the same location (or in overlapping locations).
A hologram is a volume or film of photosensitive material that records the interference pattern of two light sources. To create a hologram, laser light is first split into two beams, an object beam and a reference beam. The object beam is then manipulated and sent into the photosensitive material. Once inside this material, it intersects the reference beam and the resulting interference pattern of laser light is recorded in the photosensitive material, resulting in a hologram. Once a hologram is recorded, it can be viewed with the reference beam alone. The reference beam is projected into the hologram at the exact angle it was projected during recording. When this light hits the recorded diffraction pattern, the object beam is regenerated out of the diffracted light. An exact copy of the object beam is sent out of the hologram and can be read by optical sensors. For example, a hologram that can be obtained from a toy store illustrates this idea. Precise laser equipment is used at the factory to create the hologram. A recording material that can recreate recorded images out of natural light is used so the consumer does not need high-tech equipment to view the information stored in the hologram. Natural light becomes the reference beam and human eyes become the optical sensors. Generally, white light holograms use a variation in the recording process, not special media.
In order for holographic technology to be applied to computer systems, it must store data in a form that a computer can recognize. In current computer systems, this form is binary. In the previous section, it was mentioned that the source beam is manipulated. In common holograms, this manipulation is the creation of an optical image such as a ball or human face. In computer applications, this manipulation is in the form of bits. The next section explains the spatial light modulator, a device that modulates laser light with binary data.
Typically in the prior art, the two beams are assigned distinct roles. The “object beam” is modulated in some manner (e.g., with a spatial light modulator, or “SLM”) that allows it to carry data. A spatial light modulator is used for creating binary information out of laser light. The SLM is a 2D plane, consisting of pixels that can be turned on and off to create binary 1's and 0's. An illustration of this is a window and a window shade. It is possible to pull the shade down over a window to block incoming sunlight. If sunlight is desired again, the shade can be raised. A spatial light modulator contains a two-dimensional array of “windows” which are only microns wide. These windows block some parts of the incoming laser light and let other parts go through. The resulting cross section of the laser beam is a two dimensional array of binary data, exactly the same as what was represented in the SLM. This is correct right in the plane of the SLM, but as the beam propagates diffraction could cause the cross section to evolve into other shapes. After the laser beam is manipulated, it is sent into the hologram to be recorded. This data is written into the hologram as page form. It is called this due to its representation as a two dimensional plane, or page, of data.
The other beam, the “reference beam,” is drawn from an enumerable set of possible reference beams (e.g., plane waves incident at differing angles) designed to have characteristics favorable to the holographic recording process. A plurality of data-bearing object beams may be recorded in the same volume of medium and retrieved independently provided that each is paired with a distinct reference beam during recording.
Holographic storage media take advantage of the photorefractive effect described by David M. Pepper et al., in “The Photorefractive Effect,” Scientific American, October 1990 pages 62-74. The index of refraction in photorefractive materials can be changed by light that passes through them. By controllably changing the index of refraction in such materials, information can be stored in the photorefractive material in the form of interference patterns (or holograms). Holographic storage systems allow for high-density, high-capacity, and high-speed storage of information in photorefractive and photopolymers (or holographic) storage media.
A hologram stores data in three dimensions and reads an entire page of data at one time, which is unlike an optical CD disk that stores data in two dimensions and reads one bit at a time. The advantages of recording a hologram are high density (storage of hundreds of billions of bytes of data), high speed (transfer rate of a billion or more bits per second) and ability to select a randomly chosen data element in 100 microseconds or less. These advantages arise from three-dimensional recording and from simultaneous readout of an entire page of data at one time.
A hologram is a pattern, also known as a grating, which is formed when two laser beams interfere with each other in a light-sensitive material (LSM) whose optical properties are altered by the intersecting beams. Before the bits of data can be imprinted in this manner in the LSM, they must be modulated by a SLM to be represented as a pattern of clear and opaque squares on a display such as a liquid crystal display (LCD) screen, a miniature version of the ones in laptop computers. A blue-green laser beam, for example, is shined through this crossword puzzle-like pattern called a page, and focused by lenses to create a beam known as an object beam. A hologram of the page of data is created when the object beam meets another beam, called the reference beam, in the LSM. The reference beam could be collimated, which means that all its light rays propagate in the same direction. The term for “synchronized” light is “coherent,” and coherence is necessary for holography. Such waves are known as plane waves. The grating created when the signal and reference beams meet is captured as a pattern of varying refractive index in the LSM.
After recording the grating, the page can be holographically reconstructed by shining the reference beam into the LSM from the same angle at which it had entered the LSM to create the hologram. As it passes through the grating in the LSM, the reference beam is diffracted in such a way that it recreates the original object beam and the information contained on it. The reconstructed object beam is then focused into an image of the original page onto a detector such as an array of electrooptical detectors that sense the light-and-dark pattern, thereby reading all the stored information on the page at once. The data can then be electronically stored, accessed or manipulated by any conventional computer.
As explained above, in the typical holographic storage system, two coherent light beams are directed onto a photosensitive storage medium. The first coherent light beam is an object beam, which is used to encode data. The second coherent light beam is a reference light beam. The two coherent light beams intersect within the storage medium to produce an interference pattern. The photosensitive storage medium records this interference pattern by changing its index of refraction to form a diffraction grating.
The recorded information, stored as a
Ayres Mark
Curtis Kevin
Riley Brian
Amari Alessandro
Inphase Technologies, Inc.
Morrison & Foerster / LLP
Robinson Mark A.
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