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. We can creat binary information out of laser light by the help of spatial light modulator (SLM).
Page Data Access
Because data is stored as page data in a hologram, the retrieval of this data must also be in this form. Page data access is the method of reading stored data in sheets, not serially as in conventional storage systems. It was mentioned in the introduction that conventional storage was reaching its fundamental limits. One such limit is the way data is read in streams. Holographic memory reads data in the form of pages instead. For example, if a stream of 32 bits is sent to a processing unit by a conventional read head, a holographic memory system would in turn send 32 x 32 bits, or 1024 bits due to its added dimension. This provides very fast access times in volumes far greater than serial access methods. The volume could be one Megabit per page using a SLM resolution of 1024 x 1024 bits at 15-20 microns per pixel.
Once one can store a page of bits in a hologram, an interface to a computer can be made. The problem arises, however, that storing only one page of bits is not beneficial. Fortunately, the properties of holograms provide a unique solution to this dilemma. Unlike magnetic storage mechanisms which store data on their surface, holographic memories store information throughout their whole volume. After a page of data is recorded in the hologram, a small modification to the source beam before it reenters the hologram will record another page of data in the same volume. This method of storing multiple pages of data in the hologram is called multiplexing. The thicker the volume becomes, the smaller the modifications to the source beam can be.
When a reference beam recreates the source beam, it needs to be at the same angle it was during recording. A very small alteration in this angle will make the regenerated source beam disappear. Harnessing this property, angular multiplexing changes the angle of the source beam by very minuscule amounts after each page of data is recorded. Depending on the sensitivity of the recording material, thousands of pages of data can be stored in the same hologram, at the same point of laser beam entry. Staying away from conventional data access systems which move mechanical matter to obtain data, the angle of entry on the source beam can be deflected by high-frequency sound waves in solids. The elimination of mechanical access methods reduces access times from milliseconds to microseconds.
Used mainly in conjunction with other multiplexing methods, wavelength multiplexing alters the wavelength of source and reference beams between recordings. Sending beams to the same point of origin in the recording medium at different wavelengths allows multiple pages of data to be recorded. Due to the small tuning range of lasers, however, this form of multiplexing is limited on its own.
Spatial multiplexing is the method of changing the point of entry of source and reference beams into the recording medium. This form tends to break away from the non-mechanical paradigm because either the medium or recording beams must be physically moved. Like wavelength multiplexing, this is combined with other forms of multiplexing to maximize the amount of data stored in the holographic volume. Two commonly used forms of spatial multiplexing are peristrophic multiplexing and shift multiplexing.
- Peristrophic Multiplexing: This form of spatial multiplexing rotates the recording medium as the light source beams remain in fixed positions. For instance, a holographic cube could be rotated so each of its six sides could take in a source beam. This would provide six times the number of pages which could be stored in the volume. Certain problems arise when implementing this method of multiplexing. The rotational axes need to be positioned in a way which does not interfere with the laser beams. As with all spatial multiplexing, bringing the recording media back to its original position for data retrieval would need to be very precise. This is much easier to maintain when the media remains static. Angular multiplexing is a beam steering device is used to modulate the angle of incidence of the reference beam.
- Shift Multiplexing: Shift multiplexing alters the point of entry on one surface of the recording media. The recording optics or media could be repositioned to allow the source beam to enter multiple positions on a surface. Depending on the size of the laser beam and the sensitivity of the recording media, the points of entry the source beam takes into it can be immense. This form of multiplexing combined with peristrophic multiplexing could cover a very large percentage of the hologram. The original point of entry of the source beam is denoted at A. The holographic cube is shifted along B and the new point of entry is at C.
Phase - Encoded Multiplexing
The form of multiplexing farthest away from using mechanical means to record many pages in the same volume of a holograph is called phase-encoded multiplexing. Rather than manipulate the angle of entry of a laser beam or rotate/translate the recording medium, phase-encoded multiplexing changes the phase of individual parts of a reference beam. The main reference beam is split up into many smaller partial beams which cover the same area as the original reference beam. These smaller -beamlets vary by phase which changes the state of the reference beam as a whole. The reference beams intersects the source beam and records the diffraction relative to the different phases of the beamlets. The phase of the beamlets can be changed by non-mechanical means, therefore speeding up access times.
Combining Multiplexing Methods
No single multiplexing method by itself is the best way to pack a hologram full of information. The true power of multiplexing is brought out in the combination of one or more methods. Hybrid wavelength and angular multiplexing systems have been tested and the results are promising. Recent tests have also been formed on spatial multiplexing methods which create a hologram the size of a compact disc, but which hold 500 times more data.
It is inevitable that storing massive amounts of data in a small volume will be error prone. Factors exist in both the recording and retrieval of information which will be covered in the following subsections, respectively. In order for holographic memory systems to be practical in next generation computer systems, a reliable form of error control needs to be created.
When data is recorded in a holographic medium, certain factors can lead to erroneously recorded data. One major factor is the electronic -noise generated by laser beams. When a laser beam is split up (for example, through a SLM), the generated light bleeds into places where light was meant to be blocked out. Areas where zero light is desired might have minuscule amounts of laser light present which mutates its bit representation. For example, if too much light gets recorded into this zero area representing a binary 0, an erroneous change to a binary 1 might occur. Changes in both the quality of the laser beam and recording material are being researched, but these improvements must take into consideration the cost-effectiveness of a holographic memory system. These limitations to current laser beam and photosensitive technology are some of the main factors for the delay of practical holographic memory systems.
Page - Level Parity Bits
Once error-free data is recorded into a hologram, methods which read data back out of it need to be error free as well. Data in page format requires a new way to provide error control. Current error control methods concentrate on a stream of bits. Because page data is in the form of a two dimensional array, error correction needs to take into account the extra dimension of bits. When a page of data is written to the holographic media, the page is separated into smaller two dimensional arrays. These sub sections are appended with an additional row and column of bits. The added bits calculate the parity of each row and column of data. An odd number of bits in a row or column create a parity bit of 1 and an even number of bits create a 0. A parity bit where the row and column meet is also created which is called an overall parity bit. The sub sections are rejoined and sent to the holographic medium for recording.
When data is read back from storage, another row and column are added called parity check bits. Because the row of parity bits evens out the data, the addition or subtraction of a bit of stored data will cause two of the parity check bits to become a one. The overall parity check bit becomes a one and the place of error is calculated. The calculation occurs by finding where the column parity check bit and the row parity check bit meet up in the original data. This erroneous bit is flipped and the data is read out error free. If there happens to be two or more errors in the original data, the overall parity check bit becomes a zero and the page is re-read.
Like error control, the I/O interface to modern computer systems needs to be tailored to data retrieval in page format. Bits are no longer read from a stream, they are sent to the computer as sheets. Clearly the I/O interface needs to be changed to accommodate for this. One of the problems with such large amounts of data being fed to a processor is that the incoming data may exceed the processor's throughput. This is where interfacing needs to bridge the data in a coherent fashion between memory and processor. In the following subsections, two kinds of interfacing are covered which vary in a unique way.
Smart interfacing is a method of controlling the way data is sent to the processor from holographic memory by a pre-defined set of logical commands. These logical commands come from outside the stored memory and are provided to control the way data is managed before going to the processor. An example of these pre-defined instructions is the fixed set of rules used by error detection and correction. Because these rules stay the same throughout memory retrieval, they can be hard coded into the smart interfacing agent.
Seemingly the same as smart interfacing by name, intelligent interfacing is different in one important way. Intelligent interfacing has external control signals which can be manipulated to transform incoming data in a non-static manner. These signals create a way for the intelligent interfacing agent to reduce the incoming data in a meaningful way. For example, a data mining system could utilize these control signals to ignore certain data which is not a part of the pattern being searched for. Intelligent interfacing agents can contain the functionality of smart interfaces such as error control, but have the added feature of dynamically changing the way data passes through it.
There are many possible applications of holographic memory. Holographic memory systems can potentially provide the high-speed transfers and large volumes of future computer systems. One possible application is data mining. Data mining is the process of finding patterns in large amounts of data. Data mining is used greatly in large databases which hold possible patterns which can't be distinguished by human eyes due to the vast amount of data. Some current computer systems implement data mining, but the mass amount of storage required is pushing the limits of current data storage systems. The many advances in access times and data storage capacity that holographic memory provides could exceed conventional storage and speed up data mining considerably. This would result in more located patterns in a shorter amount of time. Another possible application of holographic memory is in petaflop computing. A petaflop is a thousand trillion floating point operations per second. The fast access in extremely large amounts of data provided by holographic memory systems could be utilized in a petaflop architecture. Clearly advances are needed in more than memory systems, but the theoretical schematics do exist for such a machine. Optical storage such as holographic memory provides a viable solution to the extreme amount of data which is required for petaflop computing.
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