Holographic Storage Technology

The theory of holography was developed by Dennis Gabor, a Hungarian physicist, in the year 1947. His theory was originally intended to increase the resolving power of electron microscopes. Gabor proved his theory not with an electron beam, but with a light beam. The result was the first hologram ever made. The early holograms were legible but plagued with many imperfections because Gabor did not have the correct light to make crisp clear holograms as we can today . Gabor needed laser Light. In the 1960s two engineers from the University of Michigan: Emmett Leith and Juris Upatnieks, developed a new device which produced a three dimensional image of an object. Building on the discoveries of Gabor, they produced the diffuse light hologram. Today, we can see holograms, or 3D images, on credit cards, magazine covers and in art galleries. Yet this unique method of capturing information with lasers has many more applications in the industrial world and is on the verge of revolutionising data storage technology as we know it. A project at Lucent Technologies Bell Laboratories could result in the first commercially viable holographic storage system. Leveraging advances across a number of technologies from micromirror arrays to new non linear polymer recording media, the team hopes to spin the project off into a startup. This technology not only offers very high storage densities, it could access that data at very high rates, due to the fact that holographic methods read an entire page of data in one operation. While conventional optical storage techniques read and write data by altering an optical medium on a per bit basis, holographic storage records an entire interference pattern in a single operation. This technique makes unique demands on both the light source and the recording medium. While a conventional optical disk system can get by with a relatively low power laser diode and a single detector, holographic techniques require high quality laser sources and detector arrays. However, these types of components have been getting cheaper. For example, CMOS pixel sensors offer the potential for the low cost detection of data arrays, while digital micromirrors can be used for data input from electronic systems. The biggest challenge has been devising a suitable optical medium for storing the interference patterns. The team turned to non linear polymers in its search for that key component. What is needed is a medium that can support the overlap of megabyte data pages, each with a high enough diffraction efficiency to enable high transfer rates. These two demands sound reasonably simple, but it really leads to a very long list of pretty stringent criteria for what a material has to do. The researchers have found what they believe is a suitable candidate, an acrylic polymer compound that polymerises in response to light. In addition to having the required optical performance properties, the new material, being a polymer, is easy to form into thick films. Film thickness directly relates to storage capacity and inorganic nonlinear materials, which are crystalline, are difficult to build in thick films. The researchers have built a prototype system using off the shelf components such as camera lenses and digital micromirrors from Texas Instruments. Many novel technologies are being pursued in parallel towards accomplishing higher capacities per disk and higher data transfer rates. Several unconventional long term optical data storage techniques promise data densities greater than 100 Gb/in 2 and perhaps even exceeding Tb/in 2 . These include near field and solid immersion lens approaches, volumetric (multi layer and holographic) storage, and probe storage techniques. A solid immersion lens approach using MO media pursued by Terastor in the United States promises at least 100 Gb/in 2 areal density. This technique relies on flying a small optical lens about 50 nm above the storage medium to achieve spot sizes smaller than the diffraction limit of light. Since the head is now lighter, this type of technology may lead to access times comparable with hard drives. Several Japanese companies are intrigued by the approach and are involved in Terastor's activities. Similar objectives are pursued by Quinta, a Seagate Company, where increasing amounts of optical technologies including optical fibers and fiber switches are used to reduce the size and weight of the head, which is non flying, but still placed quite near to the disk medium. Multi layer storage is pursued both in Japan and the United States. In Japan the effort concentrates on increasing the number of storage layers in a PC based DVD disk. Some researchers also envision adapting multi layer recording to MO media by simultaneously reading and computing the data on several layers. Both approaches, however, have limited scalability in the number of layers. In the United States, Call/Recall, Inc. is using a fluorescent disk medium to record and read hundreds of layers. Also in the United States, significant effort is being put into developing holographic storage, aiming for areal densities exceeding 100 Gb/in 2 . Companies both in the United States and Japan are exploring the use of parallel heads to speed up data transfer rates. Finally, both in Japan and in the United States, optically assisted probe techniques are being explored to achieve areal densities beyond a Tb/in 2 . In summary, a fast growing removable data storage market governed by optical storage has resulted from substantial progress that has been made in optical disk storage techniques. These advances have come through a combination of laser wavelength reduction, increases in the objective lens numerical aperture, better crosstalk management, and coding improvements under the constant pull of new applications. Undoubtedly, emerging applications will pull optical storage techniques to reach new performance levels. There is room for advances in storage capacity, as transitions to blue lasers, near field optical recording, and multi layer systems will occur.

Solid State Memory Storage

OmniDimensional Systems plans to create a 2 Gigabyte solid state memory by integrating thin film transistors and diodes onto a substrate that is formed from the flexible foil used to store information optically on CD-ROM. The intent is to substitute thin film electronics for the slow and unreliable mechanical parts used in optical drives, enabling subsystems that promise virtually instantaneous access to very large databases. The company is combining the solid state memory with a special encoding technique that it says can pack three times the normal amount of information into a given space. The company uses the basic data encoding and compression scheme, called autosophy, in its data communications products. What they've done is marry two mature technologies to fill a need for cheap, large associative memories. Autosophy can do the same thing for rewritable optical memories, by using a secondary grid. CD-ROM and similar rewritable optical media modify the surface of a thin sheet of foil with various aberrations, which are normally sensed by a photodiode's picking up light from a laser diode on the read head. In the OmniDimensional approach, the read head is replaced with an array of integrated thin film transistors and diodes of the kind used in active matrix liquid crystal displays (LCD). Autosophy encodings simplify the reading electronics by ensuring that only one output row activates at a time. The company believes the associative operation of the memory will enable autosophy theory to expand from the RAM based telecommunications code it is today to become a mainstream solid state memory technology. Autosophy theory enables graphical data to be compressed but requires the use of associative memories to do real time lookups of dictionary entries. The process is simpler for serial telecommunications, because the bit stream is artificially divided into 8 bit characters (plus start and stop bits), which can be kept in a dynamically maintained library in RAM. For instance, autosophy as used in telecommunications only transmits a full set of information once. The second time, only the address of the information is transmitted. But with graphical data which is two dimensional and not neatly divided into characters, autosophy needs associative memories to perform real time lookup in a dictionary of pieces of the image. As in telecommunications, the first time an image is sent there is no savings, but the second time only the addresses of the tiles from which it is made need be sent. With premade associative ROM's installed in TVs, perfect error corrected digital HDTV sized images could be sent over ordinary TV channels. Autosophy permits you to build enormous systems. The larger the memory, the larger the compression with autosophy, even though every item can still be retrieved in nanoseconds. You can play it forward or backward, skip around or just let the memory act like a normal RAM. The normal RAM mode divides the 64 inputs and outputs from the associative ROM into address and data lines. Then the 32 bit address can be input and 32 bit data retrieved from the ROM merely by ignoring the input data lines and the output address lines. Because of the associative operation, data can be entered, and the memory will retrieve its address. The autosophy algorithm also enables the memory technology to map automatically around defects in the foil, just as it error corrects in telecommunications systems. The first level of the dictionary comprises the letters of the alphabet, but has expanded the entries in the dictionaries beyond serial data to parallel data with live video.

Protonic Memory

One of the minor horrors of the computer age is to be working on a document not yet saved to the hard drive and lose everything because of a power outage or a system crash that forces the operator to shut down the computer. Attempts to create circuits that store the information when the power is interrupted have used high voltages, which quickly wear down computer electronic components, and have been expensive. Now scientists at Sandia and at France Telecom have applied for a patent on a prototype memory retention device that is inexpensive, low-powered, and simple to fabricate. To transmit data, the device uses embedded protons, which remain where they are when the power turns off, thus preserving the information. In devices such as DRAM's (dynamic random access memory), typically based on electron flow, the information is lost when the power is turned off. To create the memory retentive chip, only a few steps must be added to the hundreds currently used to fabricate microchips. The key additional step is to bathe the hot microchip in hydrogen gas. The gas, permeating the chip, breaks up into single ions - protons - at defects in the silicon dioxide. (The defects are created by the heat of the manufacturing process.) The protons can roam only within the chip's central layer of silicon dioxide, where they are trapped by two layers of silicon that sandwich the silicon dioxide. The Sandia researchers found that: A positive low voltage applied to one side of the silicon repels the protons to the far side of the silicon dioxide. A negative low voltage applied to the silicon attracts the protons to the near side of the silicon dioxide. If the power is turned off, the protons stay where they are, retaining information in the chip circuit. First observation of the effect that protons remain in silicon when it is baked at high temperatures in hydrogen gas came as part of a systematic study at Sandia and France Telecom of the effects of hydrogen on silicon.