Monday, January 28, 2002		On Hardware

Get ready for optical computer
S.S. Verma

WITH the rapid growth of computers in various applications of daily life, efforts are on to make them deliver at high speed and make them smaller in size. For example, the rapid growth of the Internet, expanding at almost 15 per cent per month, demands faster speeds and larger bandwidths. The computers that used to work in milliseconds (1,000ths) have moved up to picoseconds (trillionths!) for the switches and gates in chips. Speed of computers has now become a pressing problem as electronic circuits reach their miniaturisation limit. The electronic computers are limited not only by the speed of electrons in matter but also by the increasing density of interconnections necessary to link the electronic gates on microchips. Though, further miniaturisation of electronic circuits may enhance computer speed yet additional miniaturisation of electronic components only provides a short-term solution to the problem because, there are physical problems accompanied by miniaturisation that might affect the computer's reliability.

The limitation of speed is rather fundamental because the fastest possible speed for information transmission is, of course, the speed of light and the speed of an electron is already a substantial fraction of this. There is a need to find a faster medium for the signals - and the answer seems to be light itself. All optical switching using optical materials can relieve the escalating problem of bandwidth limitations imposed by electronics. Optical data processing can perform several operations simultaneously (in parallel) much faster and easier than electronics. This "parallelism" when associated with fast switching speeds would result in staggering computational power. For example, a calculation that might take a conventional electronic computer more than 11 years to complete could be performed by an optical computer in a single hour.

For more than 40 years, scientists and engineers have been working on the technologies of analog and digital optical computing, in which the information is primarily carried by photons rather than by electrons. With a speed of 3 x 108 m/sec photons of light travel just a bit less than a foot in one nanosecond, just right for doing things quickly in micro-miniaturised computer chips. Optical computing could, in principle, result in much higher computer speeds. Much progress has been achieved, and optical signal processors have been successfully used for applications such as synthetic aperture radars, optical pattern recognition, optical image processing, fingerprint enhancement and optical spectrum analysers. The early work in optical signal processing and computing was basically analog in nature. In the past two decades, however, a lot of effort has been expanded on the development of digital optical processors.

The major breakthroughs have been centred around the development of devices such as VCSELS (Vertical Cavity Surface-Emitting Lasers) for data input, SMLs (Spatial Light Modulators, such as liquid-crystal and acousto-optic devices) for putting information on the light beams, and high-speed APDs (Avalanche Photo-Diodes), or so-called Smart Pixel devices, for data output. Much work remains before digital optical computers will be widely available commercially, but the pace of research and development increased in the 1990s. One of the problems optical computers faced was the lack of accuracy.

Recent research has shown ways around this difficulty. Digital partitioning algorithms, which can break matrix-vector products into lower-accuracy sub products, working in tandem with error-correction codes, can substantially improve the accuracy of optical computing operations. The optical data storage devices will also be important in the development of optical computers. Technologies currently under investigation include advanced optical DC ROMs as well as write/read/erase optical memory technologies. Holographic data storage also offers a lot of promise for high-density optical data storage in future optical computers or for other applications such as archival data storage.

A group of researchers has developed an organic polymer with a switching frequency of 60 GHz - three times faster than the current industry-standard lithium niobate crystal-based devices. Commercial development of such a device could revolutionise the information superhighway and speed data processing for optical computing.

Another group of researchers used ultrafast laser pulses to build ultrafast data-storage devices, achieving switching down to 100ps - results that are almost 10 times faster than currently available speed limits. Newer advances have produced a variety of thin films and optical fibres that make optical interconnections and devices practical.

Scientists are working with lasers to develop a system for pattern recognition. Nanosecond optical switches are also being developed that can act as computer logic gates. These picosecond and nanosecond all-optical switches, which act as AND and partial NAND logic gates have been demonstrated at laboratory scale. As the Logic gates are the building blocks of any digital system, an optical logic gate is a switch that controls one light beam with another. It is "on" when the device transmits light, and "off" when it blocks the light. Such logic gates are members of a large family of gates in computers that perform logic operations such as addition, substraction and multiplication. They are vital for the development of optical computing and optical communication.