Get ready for
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
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.