The Information Age has brought about an explosion of information available to all users. The decreasing cost and increasing capacity of storage devices have been among the most crucial contributors. While current storage needs are being met, the storage technologies must continue to improve, so as to keep up with the rapid growing demand.
The optical data storage has been a popular storage media for many decades owing to its low price, large capacity, and portability. The first widely-adopted system was CD-ROM, which was introduced and adapted to data storage in 1980s. Even nowadays, the Compact Disc is the universal standard for audio recordings, though its place for other multimedia recordings and data storage has largely been superseded by DVD. The DVD, an acronym for Digital Versatile Disc, was the mass market successor to CD. It was introduced in 1996 and raised the capacity of optical storages to a
Gigabyte level. As of 2007, DVD is the de facto standard for pre-recorded movies, and popular storage of data beyond the capacity of CD.
With the development of high definition television, and the popularization of broadband, a further format development took place, which involved two camps:
(High-Definition) HD-DVD and Blu-ray, both based on a switch from red to blue-violet laser and tighter engineering tolerances. They further extended the capacity of optical data storage to tens of Gigabytes. In 2007, both camps have significant releases in the pre-recorded movie sector, while little impact has yet been made on the global market for data storages. In this format war, both camps are backed up by several powerful players respectively, while the result of this battle is still hard to predict.
Yet beyond HD-DVD and Blu-ray, a category of more advanced optical storage is at the same time under development based on revolutionary ideas, and it promises the prospect of a Terabyte class storage system. As indicated by strong evidences, the conventional optical data storage technologies, in which each individual bit is stored as a distinct optical change on the surface of a recording medium, are approaching physical limits beyond which individual bits may be too small or too difficult to store.
Those fundamental limits may be hard to overcome, such as the wavelength of light and the thermal stability of stored bits. As a result, an alternative approach has been
proposed for next-generation optical memories, whose idea is to store data in three dimensions.
Storing information throughout the volume of a medium, while not just on the surface layer, provides a novel high-capacity solution. This category of storage systems are named as the page-oriented optical memory, since it stores information in the format of cascaded two-dimensional pages, as opposed to the traditional bit-serial fashion. Among the page-oriented optical memories, one of the most promising candidates is the holographic data storage. Although invented decades ago, holographic data storage has made recent progress toward feasibility with the maturity of recording material technology. It possesses two main advantages over the conventional so-called surface storages. First, the throughput rate that can be achieved would be much higher owing to the approach of accessing data in page format. Secondly, by storing data throughout the volume of the storage medium, it has the potential to extend the optical storage roadmap up to Terabyte class.
Currently, there are also two major camps competing in this advanced field of next-generation optical storage, and they have both introduced their own format: HVD (Holographic Versatile Disc), as proposed by Optware Corp. in Japan, and TapestryTM, as proposed by InPhase Technologies Inc. in America.
The creator of HVD format, Optware Corp., was established in 1999 by two
leading scientists at Sony. This company has been developing the HVD systems based on its proprietary CollinearTM holography. Conventionally, the implementation of most holographic data storages requires two laser beams of different incident angles, which usually leads to highly complex optical systems to line them up at the exact point at which they are supposed to intersect. At the same time, this feature results in the incompatibility with the common CD and DVD systems. Therefore, the CollinearTM technique suggests a system in which the laser beams travel in the same axis and strike the recording medium at the same angle. This method requires a less complicated system of optics and thus enables a smaller optical pick-up which is more suitable for consumer use. The basic principles of traditional two-axis holography and the CollinearTM holography are illustrated as in Figure 1.1.
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Figure 1.1 CollinearTM holography and the traditional two-axis holography
Besides its own continuous development, in 2005 Optware Corp. founded the HVD FORUM (previously named as HVD Alliance), which is a coalition of corporations, intending to provide an industry forum for testing and technical discussion of all aspects of HVD design and manufacturing. As of February 2006, the HVD forum consists of corporations including Optware Corp., CMC Magnetics Corp., Fuji Photo Film Company, LiteOn Technology, etc. They have been working on the standardization of holographic data storage system with the Technical Committee 44 (TC44) of Ecma international, which is an international private standards organization for information and communication systems.
Originally, the HVD format is claimed to be able to store up to 3.9 Terabytes.
Then, Optware was expected to release a 200 GB disc in early June 2006. However, there have been no further news or products on market since the announcement. It seems that the HVD FORUM, led by Optware Corp., is gradually showing less influence in this field, while its competitor, InPhase Technologies may start to dominate.
InPhase Technologies was founded in 2000 as a Lucent Technologies venture, spun out from Bell Labs research. This company also promises Terabyte storage eventually, as Optware Corp. does. At present it has devised a three-stage product roadmap, as shown in Figure 1.2, and the first generation of product, tapestryTM300r is
expected to be available on market by the 4th quarter of 2007. This initial commercialized version is already able to hold up to 300 GB, which is 462 times the capacity of a CD-ROM, 64 times the capacity of a DVD, 10 times the capacity of a dual-layer HD-DVD disc, or 6 times the capacity of a dual-layer Blu-ray disc. In addition to the huge capacity, the throughput rate of tapestryTM300r also greatly exceeds that of other current optical storages. A comparison of storage capacity and the throughput rate has been provided in Figure 1.3.
At present, the major partners of InPhase Technologies include Hitachi Maxell, ALPS Electric, DisplayTECH, etc. With the commercialization of tapestry300r, we believe that more corporations will join this camp.
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Figure 1.2 Product roadmap of InPhase Technologies (WORM: Write Once Read Many, HDS-300R is the old name for tapestryTM300r)
Figure 1.3 Comparison of the existing optical storage technologies