19-01-2013, 04:51 PM
OPTICAL COMPUTING TECHNOLOGY
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INTRODUCTION
With the growth of computing technology the need of high performance computers (HPC) has significantly increased. Optics has been used in computing for a number of years but the main emphasis has been and continues to be to link portions of computers, for communications, or more intrinsically in devices that have some optical application or component (optical pattern recognition etc.)
Optical computing was a hot research area in 1980’s.But the work tapered off due to materials limitations that prevented optochips from getting small enough and cheap enough beyond laboratory curiosities. Now, optical computers are back with advances in self-assembled conducting organic polymers that promise super-tiny of all optical chips.Optical computing technology is, in general, developing in two directions. One approach is to build computers that have the same architecture as present day computers but using optics that is Electro optical hybrids. Another approach is to generate a completely new kind of computer, which can perform all functional operations in optical mode. In recent years, a number of devices that can ultimately lead us to real optical computers have already been manufactured. These include optical logic gates, optical switches, optical interconnections and optical memory.
NEED FOR OPTICAL COMPUTING
The pressing need for optical technology stems from the fact that today’s computers are limited by the time response of electronic circuits. A solid transmission medium limits both the speed and volume of signals, as well as building up heat that damages components.
One of the theoretical limits on how fast a computer can function is given by Einstein’s principle that signal cannot propagate faster than speed of light. So to make computers faster, their components must be smaller and there by decrease the distance between them. This has resulted in the development of very large scale integration (VLSI) technology, with smaller device dimensions and greater complexity. The smallest dimensions of VLSI nowadays are about 0.08mm. Despite the incredible progress in the development and refinement of the basic technologies over the past decade, there is growing concern that these technologies may not be capable of solving the computing problems of even the current millennium. The speed of computers was achieved by miniaturizing electronic components to a very small micron-size scale, but they 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.
VCSEL (VERTICAL CAVITY SURFACE EMITTING LASER)
VCSEL (pronounced ‘vixel’) is a semiconductor vertical cavity surface emitting laser diode that emits light in a cylindrical beam vertically from the surface of a fabricated wafer, and offers significant advantages when compared to the edge-emitting lasers currently used in the majority of fiber optic communications devices. The principle involved in the operation of a VCSEL is very similar to those of regular lasers.There are two special semiconductor materials sandwiching an active layer where all the action takes place. But rather than reflective ends, in a VCSEL there are several layers of partially reflective mirrors above and below the active layer. Layers of semiconductors with differing compositions create these mirrors, and each mirror reflects a narrow range of wavelengths back in to the cavity in order to cause light emission at just one wavelength
SLM FOR DISPLAY PURPOSES
For display purposes the desire is to have as many pixels as possible in as small and cheap a device as possible. For such purposes designing silicon chips for use as spatial light modulators has been effective. The basic idea is to have a set of memory cells laid out on a regular grid. These cells are electrically connected to metal mirrors, such that the voltage on the mirror depends on the value stored in the memory cell. A layer of optically active liquid crystal is sandwiched between this array of mirrors and a piece of glass with a conductive coating. The voltage between individual mirrors and the front electrode affects the optical activity of liquid crystal in that neighborhood. Hence by being able to individually program the memory locations one can set up a pattern of optical activity in the liquid crystal layer.
SMART PIXEL TECHNOLOGY
Smart pixel technology is a relatively new approach to integrating electronic circuitry and optoelectronic devices in a common framework. The purpose is to leverage the advantages of each individual technology and provide improved performance for specific applications. Here, the electronic circuitry provides complex functionality and programmability while the optoelectronic devices provide high-speed switching and compatibility with existing optical media. Arrays of these smart pixels leverage the parallelism of optics for interconnections as well as computation. A smart pixel device, a light emitting diode under the control of a field effect transistor can now be made entirely out of organic materials on the same substrate for the first time. In general, the benefit of organic over conventional semiconductor electronics is that they should lead to cheaper, lighter, circuitry that can be printed rather than etched.
WDM (WAVELENGTH DIVISION MULTIPLEXING)
Wavelength division multiplexing is a method of sending many different wavelengths down the same optical fiber. Using this technology, modern networks in which individual lasers can transmit at 10 gigabits per second through the same fiber at the same time.
ROLE OF NLO IN OPTICAL COMPUTING
The role of nonlinear materials in optical computing has become extremely significant. Non-linear materials are those, which interact with light
and modulate its properties. Several of the optical components require efficient- nonlinear materials for their operations. What in fact restrains the widespread use of all optical devices is the in efficiency of currently available nonlinear materials, which require large amount of energy for responding or switching.