Holographic Versatile Disc (HVD) is an optical disc technology which would hold up to 3.9 terabytes (TB) of information. It employs a technique known as collinear holography, whereby two lasers one red and one green are collimated in a single beam. The 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 green laser while letting the red laser pass through. This prevents interference from refraction of the 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 discs have the capacity to hold up to 3.9 terabytes (TB) of information, which is approximately 5,500 times the capacity of a CD-ROM, 830 times the capacity of a DVD, 160 times the capacity of single-layer Blu-ray Discs, and about 7 times the capacity of standard computer hard drives as of 2007. The HVD also has a transfer rate of 1 gigabit/s (125 megabytes/s).
Holographic Versatile Disc Structure
- Green writing/reading laser (532 nm)
- Red positioning/addressing laser (650 nm)
- 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
How Holographic Memory Work
Devices that use light to store and read data have been the backbone of data storage for nearly two decades. Compact discs revolutionized data storage in the early 1980s, allowing multi-megabytes of data to be stored on a disc that has a diameter of a mere 12 centimeters and a thickness of about 1.2 millimeters. In 1997, an improved version of the CD, called a digital versatile disc (DVD), was released, which enabled the storage of full-length movies on a single disc.
CDs and DVDs are the primary data storage methods for music, software, personal computing and video. A CD can hold 783 megabytes of data, which is equivalent to about one hour and 15 minutes of music, but Sony has plans to release a 1.3-gigabyte (GB) high-capacity CD. A double-sided, double-layer DVD can hold 15.9 GB of data, which is about eight hours of movies. These conventional storage mediums meet today's storage needs, but storage technologies have to evolve to keep pace with increasing consumer demand. CDs, DVDs and magnetic storage all store bits of information on the surface of a recording medium. 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.
Three-dimensional data storage will be able to store more information in a smaller space and offer faster data transfer times. In this article, you will learn how a holographic storage system might be built in the next three or four years, and what it will take to make a desktop version of such a high-density storage system.
Basics of Holographic Memory
The first step in understanding holographic memory is to understand what "holographic" means. Holography is a method of recording patterns of light to produce a three-dimensional object. The recorded patterns of light are called a hologram.
The process of creating a hologram begins with a focused beam of light -- a laser beam. This laser beam is split into two separate beams: a reference beam, which remains unchanged throughout much of the process, and an information beam, which passes through an image. When light encounters an image, its composition changes . In a sense, once the information beam encounters an image, it carries that image in its waveforms. When these two beams intersect, it creates a pattern of light interference. If you record this pattern of light interference -- for example, in a photosensitive polymer layer of a disc -- you are essentially recording the light pattern of the image.
3-D image of the Death Star created by holography
To retrieve the information stored in a hologram, you shine the reference beam directly onto the hologram. When it reflects off the hologram, it holds the light pattern of the image stored there. You then send this reconstruction beam to a CMOS sensor to recreate the original image.
Most of us think of holograms as storing the image of an object, like the Death Star pictured above. The holographic memory systems we're discussing here use holograms to store digital instead of analog information, but it's the same concept. Instead of the information beam encountering a pattern of light that represents the Death Star, it encounters a pattern of light and dark areas that represent ones and zeroes.
Encoded page data
HVD offers several advantages over traditional storage technology. HVDs can ultimately store more than 1 terabyte (TB) of information -- that's 200 times more than a single-sided DVD and 20 times more than a current double-sided Blu-ray. This is partly due to HVDs storing holograms in overlapping patterns, while a DVD basically stores bits of information side-by-side. HVDs also use a thicker recording layer than DVDs -- an HVD stores information in almost the entire volume of the disc, instead of just a single, thin layer.
The other major boost over conventional memory systems is HVD's transfer rate of up to 1 gigabyte (GB) per second -- that's 40 times faster than DVD. An HVD stores and retrieves an entire page of data, approximately 60,000 bits of information, in one pulse of light, while a DVD stores and retrieves one bit of data in one pulse of light.
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