History of holographic memory
When two beams of light overlap, the crests and troughs of their waves interact to produce patterns of interference. You can see this effect in the beautiful colors on the surface of a soap bubble, and in the supernumerary arcs of a rainbow. The use of interference for storing and reproducing images is not a new idea. It was first demonstrated in 1810 by Thomas See beck, and in 1908 the French physicist Gabriel-Jonas Lippman was awarded the Nobel Prize for his work on 'interference heliography', a method of color photography. But accurate control of interference was impossible without extremely pure light, and it was not until the invention of the laser that the science of holography became practical.
The idea of holographic memory was first proposed by Pieter J. van Heerden in the early 1960s. Some work was done on it a decade later. Scientists at RCA Laboratories demonstrated the technology by recording 500 holograms in an iron-doped lithium-niobate crystal, and 550 holograms of high-resolution images in a light-sensitive polymer material. But the lack of cheap parts and the advancement of magnetic and semiconductor memories placed the development of holographic data storage on hold.
What is Holography?
Holographic memory - something that can revolutionize the future seemed to have started strengthening its moves though slowly.
Wide possibilities in this case are provided by technology of optical recording, it's known as holography: it allows high record density together with maximum data access speed. It's achieved due to the fact that the holographic image (hologram) is coded in one big data block which is recorded at one access. And while reading this block is entirely extracted out of the memory. For reading and recording of the blocks kept holographically on the light-sensitive material (LiNbO3 is taken as the basic material) they use lasers.
Need of Holographic memory
Holographic memory is an optical storage method that will use the volume of the recording medium for storage, instead of only the surface area.
In order to increase storage capabilities, scientists are now working on a new optical storage method, called holographic memory that will go beneath the surface and use the volume of the recording medium for storage, instead of only the surface area.
As processors and buses roughly double their data capacity every three years (Moore’s Law), data storage has struggled to close the gap. CPUs can perform an instruction execution every nanosecond, which is six orders of magnitude faster than a single magnetic disk access. Much research has gone into finding hardware and software solutions to closing the time gap between CPUs and data storage. Some of these advances include cache, pipelining, optimizing compilers, and RAM.
HOLOGRAPHIC MEMORY vs. EXISTING MEMORY TECHNOLOGY
| Storage Medium | Access Time | Data Transfer Rate | Storage Capacity |
| Holographic Memory | 2.4 ms | 10 GB/s | 400 Mbits/cm2 |
| Main Memory (RAM) | 10 - 40 ns | 5 MB/s | 4.0 Mbits/cm2 |
| Magnetic Disk | 8.3 ms | 5 - 20 MB/s | 100 Mbits/cm2 |
Table 1: This table shows the comparison of access time, data transfer rates (readout), and storage capacity (storage density) for three types of memory; holographic, RAM, and magnetic disk.
Two-dimensional data storage systems such as CDs and DVDs record and retrieve data in a serial manner -- one bit at a time. Three-dimensional holographic memory store data in parallel, an entire page of data at a time. That gives you more access speed.
Holographic memory has an access time somewhere between main memory and magnetic disk, a data transfer rate that is an order of magnitude better than both main memory and magnetic disk, and a storage capacity that is higher than both main memory and magnetic disk. Certainly if the issues of hologram decay and interference are resolved, then holographic memory could become a part of the memory hierarchy, or take the place of magnetic disk much as magnetic disk has displaced magnetic tape for most applications.
Holographic data storage systems
Here are the main components of holographic data storage systems (HDSS):
· Blue-green argon laser
· Beam splitters to spilt the laser beam
· Mirrors to direct the laser beams
· Liquid crystal display (LCD) panel (spatial light modulator)
· Lenses to focus the laser beams
· Lithium-niobate crystal or photopolymer
· Charge-coupled device (CCD) camera
“When the blue-green argon laser is fired, a beam splitter creates two beams. One beam called the object or signal beam, will go straight, and bounce off one mirror and travel through a spatial-light modulator (SLM). An SLM is a liquid crystal display (LCD) that shows pages of raw binary data as clear and dark boxes. The information from the page of binary code is carried by the signal beam around to the light-sensitive lithium-niobate crystal. Some systems use a photopolymer in place of the crystal. A second beam, called the reference beam, shoots out the side of the beam splitter and takes a separate path to the crystal. When the two beams meet, the interference pattern that is created stores the data carried by the signal beam in a specific area in the crystal -- the data is stored as a hologram”. “To carry data, one of the beams would have a pattern imposed on it: either an image or a checkerboard of light and dark squares representing 1's and 0's”.
The key component of any holographic data storage system is the angle at which the second reference beam is fired at the crystal to retrieve a page of data. It must match the original reference beam angle exactly. A difference of just a thousandth of a millimeter will result in failure to retrieve that page of data.
What is holograms?
Basically how the holographic memory will work is a hologram will be recorded on a photographic crystal.
A hologram is “a light wave interference pattern of reference beam and signal beam 90° to each other recorded on photographic film that can produce a 3-dimentsional image when illuminated properly” (Amateur Holography). Since the hologram produces a 3-dimentsional image it allows for 3-dimentsional storage.
Working Of Holographic Memory
A holographic data storage system consists of a recording medium, an optical recording system, and a photo detector array. A beam of coherent light is split into a reference beam and a signal beam which are used to record a hologram into the recording medium. The recording medium is usually a photorefractive crystal such as LiNbO3 or BaTiO3 that has certain optical characteristics. These characteristics are high diffraction efficiency, high resolution, and permanent storage until erasure, and fast erasure on the application of external stimulus such as UV light. A ‘hologram’ is simply the three-dimensional interference pattern of the intersection of the reference and signal beams at 90° to each other. This interference pattern is imprinted into the crystal as regions of positive and negative charge.
HOW DO HOLOGRAPHIC MEMORY SYSTEMS RECORD DATA?
The most common holographic recording system uses laser light, a beam splitter to divide the laser light.
One of the beams of coherent laser light is called the "reference" beam and the other is called the "signal" or "object" beam. The signal beam contains data encoded by a Spatial Light Modulator (SLM). The signal beam interacts with an object and the light that is reflected by the object intersects the reference beam at right angles. When the reference and signal light beams intersect in a three-dimensional, photosensitive media such as a crystal or polymer, then a chemical reaction engraves the hologram in the media for later retrieval.
The resulting interference pattern contains all the information necessary to recreate the image of the object after suitable processing. The interference pattern is recorded onto the photo reactive material and may be retrieved at a later time by using a beam that is identical to the reference beam.
HOW DO HOLOGRAPHIC MEMORY SYSTEMS READ DATA?
To retrieve the stored hologram, a beam of light that has the same wavelength and angle of incidence as the reference beam is sent into the crystal and the resulting diffraction pattern is used to reconstruct the pattern of the signal beam. Many different holograms may be stored in the same crystal volume by changing the angle of incidence of the reference beam. While reading the data the reference beam must fall at the same angle at which the recording was made; alteration of this angle mustn't exceed 1 degree. It allows obtaining high data density: measuring the angle of the reference beam or its frequency you can record additional pages of data in the same crystal.
With three-dimensional holographic storage media, one can compare patterns of data using parallel processing and compare entire page patterns in an instant. This makes holographic memory systems superior for associative retrieval properties and searching large quantities of data to identify relationships.
The ability of holographic memory systems to access millions of bits of information at a time via parallel processing allows data transfer rates, in both recording and retrieval, of several gigabits per second. This is much faster than two-dimensional media that requires much more mechanical movement and rotating media to access different sectors sequentially.
Advantages
§ The first and major advantage of holographic memory is that it is capable to store much more data in the same physical space.
§ Another advantage to holographic memory, other than being able to store more, is that “an entire page of data can be retrieved quickly and at one time. So it gives very high speed.
§ The decreasing cost of storing data, and the increasing storage capacities of the same small device footprint, has been key enablers of this revolution.
§ The main advantage is that mechanical components are practically absent (those that typical for current storage devices). It ensures not only a fast data access, less probability of failures.
§ It also lowers power consumption, since today a hard disc is one of the greatest power-consuming elements of a computer.
§ Since the interference patterns fill up the whole substance uniformly, it gives another useful property to the holographic memory - high reliability of the recorded information.
Disadvantages
§ There is a draw back to this though, since “the key component of any holographic data storage system is the angle at which the second reference beam is fired at the crystal to retrieve a page of data. It must match the original reference beam angle exactly. A difference of just a thousandth of a millimeter will result in failure to retrieve that page of data”
§ Another draw back is that “once the hologram is recorded it cannot be erased, which is why the medium is intended for write-once, read-many-times applications”
§ recording rate
Holographic memory has an access time near 2.4 ms, a recording rate of 31 kb/s, and a readout rate of 10 GB/s. This is much slower.
Conclusion
Recently large binary files containing sound or image data have become commonplace, greatly increasing the need for high capacity data storage and data access. A new high capacity form of data storage must be developed to handle these large files quickly and efficiently.
Imagine being able to record 100 movies on a disk the size of a CD - - or one day recording the contents of the Library of Congress on such a disk. These are the promises of holographic data storage
But still it is at discovery stage, It is not being used in the market. But when it will come in the market, the world of storage will get the new horizon of capacity and speed.
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