Optical Computers.doc (Size: 941.5 KB / Downloads: 1716)
Computers have enhanced human life to a great extent.The goal of improving on computer speed has resulted in the development of the Very Large Scale Integration (VLSI) technology with smaller device dimensions and greater complexity. Ã‚Â¬Ã‚Â¬Ã‚Â¬
VLSI technology has revolutionized the electronics industry and additionally, our daily lives demand solutions to increasingly sophisticated and complex problems, which requires more speed and better performance of computers.
For these reasons, it is unfortunate that VLSI technology is approaching its fundamental limits in the sub-micron miniaturization process. It is now possible to fit up to 300 million transistors on a single silicon chip. As per the Mooreâ„¢Ã‚Â¬s law it is also estimated that the number of transistor switches that can be put onto a chip doubles every 18 months. Further miniaturization of lithography introduces several problems such as dielectric breakdown, hot carriers, and short channel effects. All of these factors combine to seriously degrade device reliability. Even if developing technology succeeded in temporarily overcoming these physical problems, we will continue to face them as long as increasing demands for higher integration continues. Therefore, a dramatic solution to the problem is needed, and unless we gear our thoughts toward a totally different pathway, we will not be able to further improve our computer performance for the future.
Optical interconnections and optical integrated circuits will provide a way out of these limitations to computational speed and complexity inherent in conventional electronics. Optical computers will use photons traveling on optical fibers or thin films instead of electrons to perform the appropriate functions. In the optical computer of the future, electronic circuits and wires will be replaced by a few optical fibers and films, making the systems more efficient with no interference, more cost effective, lighter and more compact. Optical components would not need to have insulators as those needed between electronic components because they donâ„¢t experience cross talk. Indeed, multiple frequencies (or different colors) of light can travel through optical components without interfacing with each others, allowing photonic devices to process multiple streams of data simultaneously.
1.1 Why Use Optics for Computing
Optical interconnections and optical integrated circuits have several advantageous over their electronic counterparts. They are immune to electromagnetic interference, and free from electrical short circuits. They have low-loss transmission and provide large bandwidth; i.e. multiplexing capability, capable of communicating several channels in parallel without interference. They are capable of propagating signals within the same or adjacent fibers with essentially no interference or cross-talk. They are compact, lightweight, and inexpensive to manufacture, and more facile with stored information than magnetic materials.
Most of the components that are currently very much in demand are electro-optical (EO). Such hybrid components are limited by the speed of their electronic parts. All optical components will have the advantage of speed over EO components. Unfortunately, there is an absence of known efficient nonlinear optical materials that can respond at low power levels. Most all optical components require a high level of laser power to function as required.
Optics has a higher bandwidth capacity over electronics, which enables more information to be carried and data to be processed arises because electronic communication along wires requires charging of a capacitor that depends on length. In contrast, optical signals in optical fibers, optical integrated circuits, and free space do not have to charge a capacitor and are therefore faster.
Another advantage of optical methods over electronic ones for computing is that optical data processing can be done much easier and less expensive in parallel than can be done in electronics. Parallelism is the capability of the system to execute more than one operation simultaneously. Electronic computer architecture is, in general, sequential, where the instructions are implemented in sequence. This implies that parallelism with electronics is difficult to construct. Using a simple optical design, an array of pixels can be transferred simultaneously in parallel from one point to another. To appreciate the difference between both optical parallelism and electronic one can think of an imaging system of as many as 1000x1000 independent points per mm2 in the object plane which are connected optically by a lens to a corresponding 1000x1000 points per mm2 in the image plane. For this to be accomplished electrically, a million nonintersecting and properly isolated conduction channels per mm2 would be required. Parallelism, therefore, when associated with fast switching speeds, would result in staggering computational speeds.
Assume, for example, there are only 100 million gates on a chip (optical integration is still in its infancy compared to electronics). Further, conservatively assume that each gate operates with a switching time of only 1 nanosecond (organic optical switches can switch at sub-picosecond rates compared to maximum picosecond switching times for electronic switching). Such a system could perform more than 1017 bit operations per second. Compare this to the gigabits (109) or terabits (1012) per second rates which electronics are either currently limited to, or hoping to achieve. In other words, a computation that might require one hundred thousand hours (more than 11 years) of a conventional computer could require less than one hour by an optical one.
Another advantage of light results because photons are uncharged and do not interact with one another as readily as electrons. Consequently, light beams may pass through one another in full-duplex operation, for example without distorting the information carried. In the case of electronics, loops usually generate noise voltage spikes whenever the electromagnetic fields through the loop changes. Further, high frequency or fast switching pulses will cause interference in neighboring wires. Signals in adjacent fibers or in optical integrated channels do not affect one another nor do they pick up noise due to loops. Finally, optical materials possess superior storage density and accessibility over magnetic materials.
Obviously, the field of optical computing is progressing rapidly and shows many dramatic opportunities for overcoming the limitations described earlier for current electronic computers. The process is already underway whereby optical devices have been incorporated into many computing systems. Laser diodes as sources of coherent light have dropped rapidly in price due to mass production. Also, optical CD-ROM discs have been very common or even outdated in home and office computers