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28-01-2010, 04:20 PM
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OPTICAL COMPUTING[.docx (Size: 221.17 KB / Downloads: 12)
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
Current trends in optical computing emphasize communications, for
example the use of free space optical interconnects as a potential solution to
remove ‘Bottlenecks’ experienced in electronic architectures. Optical technology
is one of the most promising, and may eventually lead to new computing
applications as a consequence of faster processing speed, as well as better
connectivity and higher bandwidth.
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.
The optical computer comes as a solution of miniaturization
problem.Optical data processing can perform several operations in parallel much
faster and easier than electrons. This parallelism helps in staggering
computational power. For example a calculation that takes a conventional
electronic computer more than 11 years to complete could be performed by an
optical computer in a single hour. Any way we can realize that in an optical
computer, electrons are replaced by photons, the subatomic bits of
electromagnetic radiation that make up light.
SOME KEY OPTICAL COMPONENTS FOR COMPUTING
The major breakthroughs on optical computing have been centered on the
development of micro-optic devices for data input.
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.
OPTICAL INTERCONNECTION OF CIRCUIT BOARDS USING
VCSEL AND PHOTODIODE
VCSEL convert the electrical signal to optical signal when the light beams
are passed through a pair of lenses and micromirrors. Micromirrors are used to
direct the light beams and this light rays is passed through a polymer waveguide
which serves as the path for transmitting data instead of copper wires in
electronic computers. Then these optical beams are again passed through a pair
of lenses and sent to a photodiode. This photodiode convert the optical signal
back to the electrical signal.
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 efficientnonlinear
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.
Organic materials have many features that make them desirable for use in
optical devices such as
1. High nonlinearities
2. Flexibility of molecular design
3. Damage resistance to optical radiations
Some organic materials belonging to the classes of phthalocyanines and
polydiacetylenes are promising for optical thin films and wave guides. These
compounds exhibit strong electronic transitions in the visible region and have
high chemical and thermal stability up to 400 degree Celsius. Polydiacetylenes
are among the most widely investigated class of polymers for nonlinear optical
applications. Their subpicosecond time response to laser signals makes them
candidates for high-speed optoelectronics and information processing.
To make thin polymer film for electro-optic applications, NASA scientists
dissolve a monomer (the building block of a polymer) in an organic solvent. This
solution is then put into a growth cell with a quartz window, shining a laser
through the quartz can cause the polymer to deposit in specific pattern.
ADVANCES IN PHOTONIC SWITCHES
Logic gates are the building blocks of any digital system. An optical logic
gate is a switch that controls one light beam by another; it is ON when the device
transmits light and it is OFF when it blocks the light.
To demonstrate the AND gate in the phthalocyanine film, two focused
collinear laser beams are wave guided through a thin film of phthalocyanine.
Nanosecond green pulsed Nd:YAG laser was used together with a red continuous
wave (cw) He-Ne beam. At the output a narrow band filter was set to block the
green beam and allow only the He-Ne beam. Then the transmitted beam was
detected on an oscilloscope. It was found that the transmitted He-Ne cw beam
was pulsating with a nanosecond duration and in synchronous with the input
Nd:YAG nanosecond pulse. This demonstrated the characteristic table of an
AND logic gate.