Abstract
This article describes holographic data storage means Holographic memory as a viable alternative to magnetic disk data storage. Currently data access times are extremely slow for magnetic disks when compared to the speed of execution of CPUs so that any improvement in data access speeds will greatly increase the capabilities of computers, especially with large data and multimedia files. Holographic memory is a technology that uses a three-dimensional medium to store data and it can access such data a page at a time instead of sequentially, which leads to increases in storage density and access speed. Holographic data storage systems are very close to becoming economically feasible. Obstacles that limit holographic memory are hologram decay over time and with repeated accesses, slow recording rates, and data transfer rates that need to be increased. Photorefractive crystals and photopolymers have been used successfully in experimental holographic data storage systems. Such systems exploit the optical properties of these photosensitive materials along with the behavior of laser light when it is used to record an image of an object. Holographic memory lies between main memory and magnetic disk in regards to data access times, data transfer rates, and data storage density.
Introduction
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.
As the computer evolves, so do the applications that computers are used for. 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.
Holographic memory is a promising technology for data storage because it is a true three-dimensional storage system, data can be accessed an entire page at a time instead of sequentially, and there are very few moving parts so that the limitations of mechanical motion are minimized. Holographic memory uses a photosensitive material to record interference patterns of a reference beam and a signal beam of coherent light, where the signal beam is reflected off of an object or it contains data in the form of light and dark areas. The nature of the photosensitive material is such that the recorded interference pattern can be reproduced by applying a beam of light to the material that is identical to the reference beam. The resulting light that is transmitted through the medium will take on the recorded interference pattern and will be collected on a laser detector array that encompasses the entire surface of the holographic medium. Many holograms can be recorded in the same space by changing the angle or the wavelength of the incident light. An entire page of data is accessed in this way.
The three features of holographic memory that make it an attractive candidate to replace magnetic storage devices are redundancy of stored data, parallelism, and multiplexing. Stored data is redundant because of the nature of the interference pattern between the reference and signal beams that is imprinted into the holographic medium. Since the interference pattern is a plane wave front, the stored pattern is propagated throughout the entire volume of the holographic medium, repeating at intervals. The data can be corrupted to a certain level before information is lost so this is a very safe method of data storage. Also, the effect of lost data is to lower the signal to noise ratio so that the amount of data that can be safely lost is dependent on the desired signal to noise ratio. Stored holograms are massively parallel because the data is recorded as an optical wave front that is retrieved as a single page in one access. Since light is used to retrieve data and there are no moving parts in the detector array, data access time is on the order of 10 ms and data transfer rate approaches 1.0 Gbytes/sec [2]. Multiplexing allows many different patterns to be stored in the same crystal volume simply by changing the angle at which the reference beam records the hologram.
Currently, holographic memory techniques are very close to becoming technologically and economically feasible. The major obstacles to implementing holographic data storage are recording rate, pixel sizes, laser output power, degradation of holograms during access, temporal decay of holograms, and sensitivity of recording materials. An angle multiplexed holographic data storage system using a photorefractive crystal for a recording medium can provide an access speed of 2.4 ms, a recording rate of 31 kB/s and a readout rate of 10 GB/s, which is between the typical values for DRAM and magnetic disk. At an estimated cost of between $161 and $236 for a complete holographic memory system, this may become a feasible alternative to magnetic disk in the near future.
Implementation
A holographic data storage system consists of a recording medium, an optical recording system, and a photodetector 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. 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. One characteristic of the recording medium that limits the usefulness of holographic storage is the property that every time the crystal is read with the reference beam the stored hologram
at that “location” is disturbed by the reference beam and some of the data integrity is lost. With current technology, recorded holograms in Fe- and Tb- doped LiNbO3 that use UV light to activate the Tb atoms can be preserved without significant decay for two years.
The most common holographic recording system uses laser light, a beam splitter to divide the laser light into a reference beam and a signal beam, various lenses and mirrors to redirect the light, a photorefractive crystal, and an array of photodetectors around the crystal to receive the holographic data. To record a hologram, a beam of laser light is split into two beams by a mirror. These two beams then become the reference and the signal beams. The signal beam interacts with an object and the light that is reflected by the object intersects the reference beam at right angles. 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 photoreactive material and may be retrieved at a later time by using a beam that is identical to the reference beam (including the wavelength and the angle of incidence into the photoreactive material). This is possible because the hologram has the property that if it is illuminated by either of the beams used to record it, the hologram causes light to be diffracted in the direction of the second beam that was used to record it, thereby recreating the reflected image of the object if the reference beam was used to illuminate the hologram. So, the reflected image must be transformed into a real image with mirrors and lenses that can be sent to the laser detector array.
There are many different volume holographic techniques that are being researched. The most promising techniques are angle-multiplexed, wavelength-multiplexed, spectral, and phase-conjugate holography. Angle- and wavelength- multiplexed holographic methods are very similar, with the only difference being the way data is stored and retrieved, either multiplexed with different angles of incidence of the reference beam, or with different wavelengths of the reference beam. Spectral holography combines the basic principles of volume holography using a photorefractive crystal with a time sequencing scheme to partition holograms into their own subvolume of the crystal using the collision of ultrashort laser pulses to differentiate between the image and the time-delayed reference beam. Phase-conjugate holography is a technique to reduce the total volume of the system (the system includes recording devices, storage medium, and detector array) by eliminating the need for the optical parts between the spatial light modulator (SLM) and the detector. The SLM is an optical device that is used to convert the real image into a single beam of light that will intersect with the reference beam during recording. Phase-conjugate holography eliminates these optical parts by replacing the reference beam that is used to read the hologram with a conjugate reference beam that propagates in the opposite direction as the beam used for recording. The signal diffracted by the hologram being accessed is sent back along the path from which it came, and is refocused onto the SLM which now serves as both the SLM and the detector.
There are two main classes of materials used for the holographic storage medium. These are photorefractive crystals and photopolymers (organic films). The most commonly used photorefractive crystals used are LiNbO3 and BaTiO3. During hologram recording, the refractive index of the crystal is changed by migration of electron charge in response to the imprinted three-dimensional interference pattern of the reference and signal beams. As more and more holograms are superimposed into the crystal, the more decay of the holograms occurs due to interference from the superimposed holograms. Also, holograms are degraded every time they are read out because the reference beam used to read out the hologram alters the refractive nature of the crystal in that region. Photorefractive crystals are suitable for random access memory with periodic refreshing of data, and can be erased and written to many times. Photopolymers have been developed that can also be used as a holographic storage medium. Typically, the thickness of photopolymers is much less than the thickness of photorefractive crystals because the photopolymers are limited by mechanical stability and optical quality. An example of a photopolymer is DuPont’s HRF-150. This film can achieve 12 bits/mm2 with a 100 mm thickness, which is greater than DVD-ROM by a factor of two. When a hologram is recorded, the interference pattern is imprinted into the
photopolymer by inducing photochemical changes in the film. The refractive index modulation is changed by changing the density of exposed areas of the film. Stored holograms are permanent and do not degrade over time or by readout of the hologram, so photopolymers are suited for read-only memory (ROM).
Holographic Memory Vs. Existing Memory Technology
In the memory hierarchy, holographic memory lies somewhere between RAM and magnetic storage in terms of data transfer rates, storage capacity, and data access times. The theoretical limit of the number of pixels that can be stored using volume holography is V2/3/l2 where V is the volume of the recording medium and l is the wavelength of the reference beam. For green light, the maximum theoretical storage capacity is 0.4 Gbits/cm2 for a page size of 1 cm x 1 cm. Also, 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. Modern magnetic disks have data transfer rates in the neighbourhood of 5 to 20 MB/s. Typical DRAM today has an access time close to 10 – 40 ns, and a recording rate of 10 GB/s (See Table1).
Table 1. Comparison of data for holographic, RAM, and magnetic disk
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 |
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.
Future of Holographic Memory
Today holographic memory is very close to becoming a reality. The basic theory behind it has been shown to be reliable and has been implemented in numerous experiments. Materials research has yielded some promising results in photorefractive crystals such as LiNbO3 and BaTiO3, especially for use with rewritable, refreshed random access memory. Also, a read only version of holographic data storage is certainly feasible with some of the photopolymer films available today. For holographic memory to truly become the next revolution in data storage, data transfer rates must be improved, hologram decay must become negligible, and hologram recording time must be reduced. Then it will be economical for holographic memories to be produced for mass consumption.
Holographic Versatile Disc
One of the most important research in the field of holographic memory is the HVD (Holographic Versatile Disc). Holographic Versatile Disc is an advanced optical disc technology still in the research stage which would greatly increase storage over Blu-ray and HD DVD optical disc systems. It employs a technique known as collinear holography, whereby two lasers, one red and one blue-green, are collimated in a single beam. The blue-green laser reads data encoded as laser interference fringes from a holographic layer near the top of the disc while the red laser is used as the reference beam and to read servo information from a regular CD-style aluminium layer near the bottom. Servo information is used to monitor the position of the read head over the disc, similar to the head, track, and sector information on a conventional hard disk drive. On a CD or DVD this servo information is interspersed amongst the data.
A dichroic mirror layer between the holographic data and the servo data reflects the blue-green laser while letting the red laser pass through. This prevents interference from refraction of the blue-green laser off the servo data pits and is an advance over past holographic storage media, which either experienced too much interference, or lacked the servo data entirely, making them incompatible with current CD and DVD drive technology. These disks have the capacity to hold up to 3.9 terabytes (TB) of information, which is approximately 6000 times the capacity of a CD-ROM, 830 times the capacity of a DVD and 160 times the capacity of single-layer Blu-ray Discs. The HVD also has a transfer rate of 1 Gbyte/s.
- Green writing/reading laser (532nm)
- Red positioning/addressing laser (650nm)
- Hologram (data)
- Polycarbon layer
- Photopolymeric layer (data-containing layer)
- Distance layers
- Dichroic layer (reflecting green light)
- Aluminium reflective layer (reflecting red light)
- Transparent base
P. PIT
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