12-11-2012, 03:07 PM
REPORT ON OPTICAL COMPUTING
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INTRODUCTION
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. ¬¬¬
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 Moore’¬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 don’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.
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.
OPTICAL COMPUTER
An optical computer (also called a photonic computer) is a device that uses the photons in visible light or infrared (IR) beams, rather than electric current, to perform digital computations. Optical computing could produce computers tens of thousands of times faster than today's computers, because light can travel that much faster than electric current. With all the advantages described for the optical circuits compared to the electronic counterparts, the need for optical computing is one of the main requirements in this century. Visible-light and IR beams, unlike electric currents, pass through each other without interacting. Several (or many) laser beams can be shone so their paths intersect, but there is no interference among the beams, even when they are confined essentially to two dimensions. This makes optical computers smaller also. Figure 1 and 2 shows the differences between the optical circuits and the electronic circuits.
Optical Transistor
The transistor is one of the most influential inventions of modern times and is ubiquitous in present-day technologies. To replace electronic components with optical ones, an equivalent "optical transistor" was required. The optical transistor is one of the most basic components of an optical computer. It does the same thing its electronic counterpart does, but with the difference of using photons instead of electrons. The Photonic Transistor is vacuum compatible, meaning that they can be operated in air or even in a vacuum where there light moves at the universal speed limit. The first ever photonic transistor was invented in 1989 by the Rocky Mountain Research Center, and then tested in the laboratories of the University of Montana, and Montana State, USA. The photonic transistor can be used to build up various gates and switches same as that its electronic counterpart does. Therefore it can be called as the basic building block of an optical computer
Laser Transistor (Single Molecule Laser Transistor)
Amplification in a conventional laser is achieved by an enormous number of molecules. By focusing a laser beam on only a single tiny molecule, the ETH Zurich scientists have been able to generate stimulated emission using just one molecule. They were helped in this by the fact that, at low temperatures, molecules seem to increase their apparent surface area for interaction with light In this case, the enlarged surface area corresponded approximately to the diameter of the focused laser beam.
For creating an optical transistor with a single molecule, the fact used is that a molecule’s energy is quantized: when laser light strikes a molecule that is in its ground state, the light is absorbed. As a result, the laser beam is quenched. Conversely, it is possible to release the absorbed energy again in a targeted way with a second light beam. This occurs because the beam changes the molecule’s quantum state, with the result that the light beam is amplified. This so-called stimulated emission, which is the basic working principle of Laser. By using one laser beam to prepare the quantum state of a single molecule in a controlled fashion, scientists could significantly attenuate or amplify a second laser beam. This mode of operation is identical to that of a conventional transistor, in which electrical potential can be used to modulate a second signal.