Optical computers pdf

DEPTT. OF COMPUTER SCIENCE OPTICAL COMPUTERS

GYAN VIHAR
ScHool of ENGINEERING & TEcHNoloGY

A
Seminar Report On

OPTICAL COMPUTERS
Submitted in Partial Fulfilment for The Award of Degree
B.Tech. (Computer Science & Engineering)
By
Rajasthan Technical University, Kota
Session 2009-10

Submitted to: - Submitted by:
Mr. Naveen Hemrajani Sudhanshu Shekhar
Head of the Department B.Tech. IV Year,
Computer Science Engineering (VIII Semester

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Contents
Overview of Optical computers
1 Components of Optical computers. . . . . . . . . . . . . . . . . . . . . . 9
1.1 Hard Disk
1.2 CPU
1.3 Memory
1.4 Cache Memory
1.5 Main Memory
1.6 Screen
1.7 Power Supply

2 Need of Optical Computers . . . . . . . . . . . . . . . . . . . . . . . . . 15
3 Optical Components for Computing . . . . . . . . . . . . . . . . . . 20
3.1 VCSEL
3.2 SLM
3.3 WDM
3.4 Optical Memory

4 Fibre Optics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
4.1 Use of Fibre Optics in Computing
4.2 Why use Fibre Optics
5 An Optical Computer Powered by Germanium Laser. . . . 40

6 Concept of Picosecond (By NASA) . . . . . . . . . . . . . . . . . . . . 44
7 Optical computer Bus. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

Application
Merits
Drawback

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Some current research
Future Trends
References
PREFACE

An optical computer (also called a photonic computer) is a device that uses the
photons of visible light or infrared (IR) beams, rather than electric current, to
perform digital computations. An electric current creates heat in computer systems.
As the processing speed increases, so does the amount of electricity required; this
extra heat is extremely damaging to the hardware. Light, however, creates
insignificant amounts of heat, regardless of how much is used. Thus, the
development of more powerful processing systems becomes possible.

An optical desktop computer could be capable of processing data up to 100,000
times faster than current models because multiple operations can be performed
simultaneously.

On October 4, 1993, the eminent Soviet physicist Prof. U. Kh. Kopvillem would
have been 70 years old. However, he died prematurely on September 24, 1991.

His research was the foundation of several areas of nonlinear optics, quantum
acoustics, and radioacoustics. The breadth of the subject matter of this issue,
ranging from studies on the role of photon modes in high-temperature
superconductivity to the propagation of ullxashort pulses (of the order of one
period), only partially reflects the wide specmam of the scientific interests of U.
Kh. Kopvillem.

Optical computing where the processing of electrical energy is replaced by light
quanta is very attractive for future technologies. The replacement of wires by
optical pathways is of special interest because light can cross without interference
and thus, the complex wiring of modern computers may be appreciably simplified.
Moreover, optical computers can operate at very high rates because there are not
the problems of electrical computers such as inductivities of wires and loading of
parasitic capacitors.

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AN OVERVIEW OF OPTICAL COMPUTING

Computers have become an indispensable part of life. We need
computers everywhere, be it for work, research or in any such field. As the use of
computers in our day-to-day life increases, the computing resources that we need
also go up. For companies like Google and Microsoft, harnessing the resources as
and when they need it is not a problem. But when it comes to smaller enterprises,
affordability becomes a huge factor. With the huge infrastructure come problems
like machines failure, hard drive crashes, software bugs, etc. This might be a big
headache for such a community. Optical Computing offers a solution to this
situation.
An Optical Computer is a hypothetical device that uses visible light or
infrared beams, rather than electric current, to perform digital computations. An
electric current flows at only about 10 percent of speed of light.
By applying some of the advantages of visible and/or IR networks at the device
and component scale, a computer can be developed that can perform operations
very much times faster than a conventional electronic computer.

Optical computing describes a new technological approach for
constructing computer’s processors and other components. Instead of the current
approach of electrically transmitting data along tiny wires etched onto silicon.
Optical computing employs a technology called silicon photonics that uses laser
light instead.

This use of optical lasers overcomes the constraints associated with
heat dissipation in today’s components and allows much more information to
be stored and transmitted in the same amount of space. Optical computing means
performing computations, operations, storage and transmission of data using light.
Optical technology promises massive upgrades in the efficiency and speed of
computers, as well as significant shrinkage in their size and cost. An optical
desktop computer is capable of processing data up to 1,00,000 times faster than
current models.

An optical computer (also called a photonic computer) is a device that
uses the photons of visible light or infrared (IR) beams, rather than electric current,

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to perform digital computations. An electric current creates heat in computer
systems. As the processing speed increases, so does the amount of electricity
required; this extra heat is extremely damaging to the hardware.

For decades, silicon, with its talent for carrying electrons, has been
the mainstay of computing. But for a variety of reasons (see "The Coming Light
Years"), we're rapidly approaching the day when electrons will no longer cut it.
Within 10 years, in fact, silicon will fall to the computer scientist's triple curse:
"It's bulky, it's slow, and it runs too hot." At this point, computers will need a new
architecture, one that depends less on electrons and more on... well...what else?

Computer of 2010

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Optics. With the assistance of award-winning firm frogdesign (the geniuses behind
the look of the early Apple and many of today's supercomputers and workstations),
Forbes ASAP has designed and built (virtually, of course) the computer of 2010.

Whenever possible, our newly designed computer replaces stodgy old
electrons with shiny, cool-running particles of light--photons. Electrons remain,
doing everything they do best (switching), while photons do what they do best
(traveling very, very fast). In other words, we've brought the speed and bandwidth
of optical communications inside the computer itself. This mix is called
optoelectronics, another buzzword we encourage you to start using immediately.

The result is a computer that is far more reliable, cheaper, and more compact
—the entire thing, believe it or not, is about the size of a Frisbee--than the all-
electronic solution. But above all, optoelectronic computing is faster than what's
available today.How fast ? In a decade, we believe, you will be able to buy at your
local computer shop the equivalent of today's supercomputers.

How likely is it that this computer will be built ? Some of its components
are slightly pie-in-the-sky. But many others have already been developed or are
being developed by some of the best scientific minds in the country. Sooner or
later, and probably sooner, an optoelectronic computer will exist .

Okay, so we've built a revolutionary new optical computer just in
time for 2010. What do we do with it now? Everything. Because it's small (about
the size of a Frisbee) and because it has the power of today's supercomputer, the
2010 PC will become the repository of information covering every aspect of our
daily life. Our computer, untethered and unfettered by wires and electrical outlets,
becomes something of a key that unlocks the safety deposit box of our lives.

When we plug our 2010 PC into the wall of our home, our house will
become smart, anticipating our every desire. At work, we'll plug it into our desk,
which will become a gigantic interactive screen. When it communicates wirelessly
with a small mobile device, we'll have a personal digital assistant—on steroids.

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Standard, electrical-based, computers rapidly approach fundamental
limitation. Alternative principles should be explored in order to keep computing
developments at the current pace or even faster. Optical computing has major
potential in providing a solution through its use of photons to perform
computations instead of electrons. This workshop will be an opportunity to
bring people together from optics and computer science who are interested in
establishing important principles and in developing optical computers. This will
also be an opportunity to meet with pioneering figures and to discuss the future of
optical supercomputing.
Computers have enhanced human life to a great extent. The speed of
conventional computers is achieved by miniaturizing electronic components to a
very small micron-size scale so that those electrons need to travel only very short
distances within a very short time. 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. Last year, the smallest-to
date dimensions of VLSI reached 0.08 e m by researchers at Lucent Technology.
Whereas VLSI technology has revolutionized the electronics industry and
established the 20th century as the computer age, increasing usage of the Internet
demands better accommodation of a 10 to 15 percent per month growth rate.
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. 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 2 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

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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 donot 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.

SECURITY

The PC will be protected from theft, thanks to an advanced biometric scanner
that can recognize your fingerprint.
INTERFACE
You'll communicate with the PC primarily with your voice, putting it truly at
your beck and call.
The Desktop as Desk Top
In 2010, a "desktop" will be a desk top...in other words, by plugging our
computer into an office desk, its top becomes a gigantic computer screen--an
interactive photonic display. You won't need a keyboard because files can be
opened and closed simply by touching and dragging with your finger. And for
those throwbacks who must have a keyboard, we've supplied that as well.
A virtual keyboard can be momentarily created on the tabletop, only to
disappear when no longer needed. Now you see it, now you don't.
Your Digital Butler
What do we do with our 2010 computer when we arrive home after a long
day's work? The computer becomes the operating system for our house, and our
house, in turn, knows our habits and responds to our needs. ("Brew coffee at 7,
play Beethoven the moment the front door opens, and tell me when I'm low on
milk.")
Your Home
The PC of 2010 plugs into your home so your house becomes a smart
operating system.

Optical Computing Technology
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An optical computer (also called a photonic computer) is a device that uses
the photons of visible light or infrared (IR) beams, rather than electric current, to

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perform digital computations. An electric current creates heat in computer systems.
As the processing speed increases, so does the amount of electricity required; this
extra heat is extremely damaging to the hardware. Light, however, creates
insignificant amounts of heat, regardless of how much is used. Thus, the
development of more powerful processing systems becomes possible. By applying
some of the advantages of visible and/or IR networks at the device and component
scale, a computer might someday be developed that can perform operations 10 or
more times faster than a conventional electronic computer.

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. Electric currents must be guided around
each other, and this makes three-dimensional wiring necessary. Thus, an optical
computer, besides being much faster than an electronic one, might also be smaller.

Most research projects focus on replacing current computer components
with optical equivalents, resulting in an photonic digital computer system
processing binary data. This approach appears to offer the best short-term
prospects for commercial optical computing, since optical components could be
integrated into traditional computers to produce an optical/electronic hybrid. Other
research projects take a non-traditional approach, attempting to develop entirely
new methods of computing that are not physically possible with electronics.

Optical computing where the processing of electrical energy is replaced by
light quanta is very attractive for future technologies . The replacement of
wires by optical pathways is of special interest because light can cross without
interference and thus, the complex wiring of modern computers may be
appreciably simplified. Moreover, optical computers can operate at very high rates
because there are not the problems of electrical computers such as inductivities of
wires and loading of parasitic capacitors. Chemical structures are required for the
handling of light and this has to be done by suitable chromophores. Organic
materials are preferred because of their chemical variability and uncritical
recycling for mass production. There are mainly three obstacles for the
development of optical computers: firstly the preservation of the optical energy,
secondly the low light-fastness of many active optical components and thirdly
the comparably long wavelengths of light of about 0.5 m. The former two
problems can be solved by the application of highly light-fast fluorescence dyes
where the fluorescence quantum yield is a measure of the preservation of light-
energy; light fast fluorescent dyes with 100% fluorescence quantum yield are

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known .
The third problem sets a lower limit to the size of conventional optical
components and hinders the construction of an optical computer on a molecular
scale. However, the development of molecular optics would reduce the size of such
components by a factor of 500.
The limitation of resolution by the wavelengths of light may be overcome
by the transport of the energy of light instead of the emission and absorption of
light quanta. This corresponds to the use of the alternating current (50 Hz) with a
problematic wavelength of some 6000 km where the electrical energy is handled
on a human scale or even lower.
In analogy to such a transport of electrical energy an energy transfer
between chromophores can replace the absorption and emission of light quanta in
optical signal processing components. The transfer will proceed rapidly if the
distance between the two chromophores lies within the F¨orster radius, that means
between 2 and 3 nm for most combinations of similarly absorbing chromophores.
On the other hand, this F¨orster radius would be the natural lower limit for the size
of complex arrangements of switching components for handling energy transfer
because going below this limit would spread energy over many chromophores
without control; a solution of this limiting problem would be the prerequisite for
the development of optical computers with very high densities of integration.

COMPONENTS OF OPTICAL COMPUTER

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• Hard Disk
• CPU
• Memory
• Cache Memory
• Main Memory
• Screen
• Power Supply

(1) HARD DISK (STORES PROGRAMS AND FILES)

To build our 2010 computer (see previous page) we first need to
build the hard disk. The disk will be holographic and will
somewhat resemble a CD-ROM or DVD. That is, it will be a
spinning, transparent plastic platter with a writing laser on one side
and reading laser on the other, and it will hold an astounding
terabyte (1 trillion bytes) of data, just a tad more than we get
today--1,000 times more, to be exact. With such capacity, you'll be
able to store every ounce of information about your life. But
beware.
If your computer is stolen or destroyed , you might actually start
wondering who you are.

WHERE ARE WE?
A holographic disk might be the easiest
component here to build since it exists in the lab today.

WHO'S WORKING ON IT?

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Stanford University, Lucent Technologies, and
cutting-edge Silicon Valley optics company Siros Technologies.

TIME OF COMPLETION? 2005, for a commercial product.

(2) THE CENTRAL PROCESSING UNIT (CPU)

Our 2010 CPU will operate on the same principle as today's PCs.
But instead of electronic microprocessors providing the brains and
brawn, our future CPU will have optoelectronic integrated circuits
(chips that use silicon to switch but optics to communicate). This
will give us huge increases in speed and efficiency. Why? Because
the CPU of today spends far too much time waiting around for data
to process. Instantaneous on-chip optical communication, and
memory running as fast as the processor, will guarantee a
continuous stream of data processing within the CPU. With
communication between components no longer bottlenecked by
electronic transmission, we can probably push the clock rate to 100
gigahertz.
Our universal appliance of tomorrow also has a
hexagonal optoelectronic processor surrounded by a ring of fast
cache, so that data for any part of the chip can be fetched from the
closest part of the cache. The result will be computer
performance--or, at any rate, delivery of computational results--
comparable to today'ssupercomputers .

WHERE ARE WE?
Optoelectronic integrated circuits do exist
today, on a small scale and for specialized purposes. Getting from
the current state of the art to a complete and superfast
optoelectronic CPU will require tremendous effort and the
accumulation of an entirely new body of intellectual property.

WHO'S WORKING ON IT?

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Scientific-Atlanta, Lucent, and Nortel. Advanced work
in optical interconnection is now being done at Stanford. Intel,
through its purchase of Danish optoelectronics company GIGA,
intends to have the fast track outofthegate.

TIME OF COMPLETION? 2010,If we're really lucky.

(3) MEMORY(RAM)

When we stir optical communication into the old-fashioned
electronic computer, some of the greatest potential gains will
involve your computer's short-term memory. In the long-gone days
(1980) of the 80286, computers enjoyed a design advantage that
we've never had since. The memory bus speed--that is, the speed at
which data flowed between CPU and memory--was the same as the
CPU's clock rate, or how fast it operates . (Of course, they were
both 8 megahertz , but we said this was a long time ago.) Data
reached the CPU as fast as the chip could process it, which
kept the CPU from waiting around being bored.
We've never reached that pinnacle again, and since then, the
situation has gotten steadily worse. A reasonably fast computer
today has a CPU clock of 600 megahertz and a memory bus speed
of 133 megahertz. Despite various clever technical feats, the CPU
still spends half to two-thirds of its time just waiting around for
data from memory.
Optoelectronics will knock this problem out of the park. With a
properly designed optical memory bus, speed of fetch from
memory can once again equal CPU clock rate.
Of course, this also will require that processing in RAM be very
quick, so we'll need a faster RAM architecture, which luckily is--or
will be--available. A large cache (see below) made of superfast,
nonvolatile magnetic RAM will hold information that the CPU
needs quickly and repeatedly. It will be backed up by a much
larger area of holographic (pure optical) main RAM that will hold
programs, files, images, etc., while you work with them.

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(4) FAST MEMORY (CACHE)

To build our new fast cache, we'll need to get rid of the
inefficiencies of today's product, which requires the computer to
constantly refresh it, just like short-term memory in humans needs
to be constantly refreshed or it's forgotten. The inefficiencies in
cache are so bad, in fact, that once you know the speed of your
cache you can assume that its real-world performance will be about
a third of that--the missing two-thirds being sacrificed to refresh
cycles.

Enter 2010's semiconductor technology, which, instead of using
today's silicon memory, will rely upon magnetic memory on a
molecular scale. Because tiny elements will be magnetized to
represent zeros, or demagnetized to represent ones, the information
can be easily and quickly refreshed with just a quick electrical
signal. The whole process will be much faster than today's silicon
memory--which is a good thing, because satisfying the demands of
a CPU running at 100 gigahertz will definitely mean no coffee
breaks.

Let's install a gigabyte of fast cache--1,000 times as much as the
megabyte that serves an Intel Pentium III today. And, to put the
whole system in overdrive, we'll hitch it directly to the CPU with a
multiplexed optical bridge. Get ready for warp speed!

WHERE ARE WE? Mostly in the experimental stage.

WHO'S WORKING ON IT? U.S. government laboratories and
IBM, which probably knows more about magnetic memory than
any other company.

TIME OF COMPLETION? 2010, with just a small leap of faith.

(5) MAIN MEMORY

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Our main RAM will be purely optical, in fact, holographic.
Holographic memory is three-dimensional by nature, so we can
stack up any number of memory planes into a rectangular solid to
create 256 gigabytes of optical main memory, 1,000 times as much
as a really powerful desktop computer today.

WHERE ARE WE? Holographic memory exists, but it is slow,
bulky, extremely difficult to build in quantity, and has quality-
control problems.

WHO'S WORKING ON IT?University laboratories.
TIME OF COMPLETION? 2009,or maybe a tad earlier.

(6) POWER SUPPLY

One of the biggest advantages of photonic circuitry is an extremely
low power requirement. A long, sticklike lithium battery, bent into
a doughnut and installed in the periphery of the computer, will run
it for a couple of weeks. But fresh power is as close as the charging
cradle on the nearest wall, which resembles the one for today's
cordless or cellular phones.

WHERE ARE WE? Pretty close. We've come a long way in
battery development in the past few years.

WHO'S WORKING ON IT? Hewlett-Packard.

TIME OF COMPLETION? 2007.

(7) THE SCREEN

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Size does matter in our 2010 computer screen. It will either be very
large, literally the desk top of your desktop, or very small, a
monocle you hold up to your eye. For the bigger version, our
computer screen will depend on some kind of photonically excited
liquid crystal, with power requirements significantly lower than
today's monitors. Colors will be vivid and images precise (think
plasma displays). In fact, today's concept of "resolution" will be
largely obsolete. Get ready for pay-per-view Webcasts.

WHERE ARE WE? This design, if fully realized, depends on a
technology that doesn't exist today. Optical excitement of a liquid
crystal is the stuff of research papers. More likely is that our
computer will end up with a less ambitious display, one like our
current PCs possess but much, much better. We've got 10 fruitful
years to develop it, after all.

WHO'S WORKING ON IT? Sharp Electronics, a world leader in
color LCD technology, which is also investing heavily in
optoelectronics. Sony, Toshiba, and IBM are the current leaders in
flat-panel displays.

TIME OF COMPLETION? 2010, if we're lucky.

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NEED OF OPTICAL COMPUTERS
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 digital
computers are still some years away, however a number of devices that can
ultimately lead to real optical computers have already been manufactured,
including optical logic gates, optical switches, optical interconnections, and optical
memory. The most likely near-term optical computer will really be a hybrid
composed of traditional architectural design along with some portions that can
perform some functional operations in optical mode.

With today’s growing dependence on computing technology, the need for
high performance computers (HPC) has significantly increased. Many performance
improvements in conventional computers are achieved by miniaturizing electronic
components to very small micron-size scale so that electrons need to travel only
short distances within a very short time. This approach relies on the steadily
shrinking trace size on microchips (i.e., the size of elements that can be ‘drawn’
onto each chip). 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 0.08 mm.
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.

Technologies lead to breakthroughs in engineering and manufacturing in a
wide range of industries. With the help of virtual product design and development,
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costs can be reduced; hence looking for improved computing capabilities is
desirable. Optical computing includes the optical calculation of transforms and
optical pattern matching. Emerging technologies also make the optical storage of
data a reality.

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 (Einstein’s principle that signals cannot propagate
faster than the speed of light) 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. In an optical computer, electrons are
replaced by photons, the subatomic bits of electromagnetic radiation that make up
light.
Optics, which is the science of light, is already used in computing, most
often in the fiber-optic glass cables that currently transmit data on communication
networks much faster than via traditional copper wires. Thus, optical signals might
be the ticket for the fastest supercomputers ever. Compared to light, electronic
signals in chips travel at snail speed. Moreover, there is no such thing as a short
circuit with light, so beams could cross with no problem after being redirected by
pinpoint-size mirrors in a switchboard. In a pursuit to probe into cutting-edge
research areas, optical technology (optoelectronic, photonic devices) is one of the
most promising, and may eventually lead to new computing applications as a
consequence of faster processor speeds, as well as better connectivity and higher
bandwidth. 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. For example, a one-foot length of wire
produces approximately one nanosecond (billionth of a second) of time delay.
Extreme miniaturization of tiny electronic com- Optical computing includes the
optical calculation of transforms and optical pattern matching. Emerging
technologies also make the optical storage of data.

These and other obstacles have led scientists to seek answers in light itself.
Light does not have the time response limitations of electronics, does not need
insulators, and can even send dozens or hundreds of photon signal streams
simultaneously using different color frequencies. Those 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
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essentially no interference or cross talk. They are compact, lightweight, and
inexpensive to manufacture, as well as more facile with stored information than
magnetic materials. By replacing electrons and wires with photons, fiber optics,
crystals, thin films and mirrors, researchers are hoping to build a new generation of
computers that work 100 million times faster than today’s machines.

The fundamental issues associated with optical computing, its advantages
over conventional (electronics-based) computing, current applications of optics in
computers are discussed in this part. In the second part of this article the problems
that remain to be overcome and current research will be discussed.

Optical computing was a hot research area in the 1980s. But the work
tapered off because of materials limitations that seemed to prevent optochips from
getting small enough and cheap enough to be more than laboratory curiosities.
Now, optical computers are back with advances in self-assembled conducting
organic polymers that promise super-tiny all-optical chips.

[1]. Advances in optical storage device have generated the
promise of efficient, compact and large-scale storage devices

[2]. Another advantage of optical methods over electronic
ones for computing is that parallel data processing can frequently be done much
more easily and less expensively in optics than in electronics

[3]. Light does not have the time response limitations of
electronics, does not need insulators, and can even send dozens or hundreds of
photon signal streams simultaneously using
different color frequencies. Parallelism, the capability to execute more than one
operation simultaneously, is now common in electronic computer architectures.
But, most electronic computers still execute instructions
sequentially; parallelism with electronics remains sparsely used. Its first
widespread appearance was in Cray supercomputers in the early 1980’s when two
processors were used in conjunction with one shared memory. Today, large
supercomputers may utilize thousands of processors but communication overhead
frequently results in reduced overall efficiency

[4]. On the other hand for some applications in input-output
(I/O), such as image processing, by using a simple optical design an array of pixels
can be transferred simultaneously in parallel from one point to another. Optical
technology promises massive upgrades in the efficiency and speed of computers,

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as well as significant shrinkage in their size and cost. An optical desktop computer
could be capable of processing data up to 100,000 times faster than current models
because multiple operations can be performed simultaneously. Other advantages of
optics include low manufacturing costs, immunity to electromagnetic interference,
a tolerance for lowloss transmissions, freedom from short electrical circuits and the
capability to supply large bandwidth and propagate signals within the same or
adjacent fibers without interference.
One oversimplified example may help to appreciate
the difference between optical and electronic parallelism. Consider an imaging
system with 1000 t 1000 independent points per mm2 in the object plane which
are connected optically by a lens to a corresponding number of points per mm2 in
the image plane; the lens effectively performs an FFT of the image plane in real
time. For this to be accomplished electrically, a million operations are required.
Parallelism, when associated with fast switching speeds, would result in staggering
computational speeds. Assume, for example, there are only 100 million gates on a
chip, much less than what was mentioned earlier (optical integration is still in its
infancy compared to electronics). Further, conservatively assume that Optical
technology promises massive upgrades in the efficiency and speed of computers,
as well as significant shrinkage in their size and cost.
An optical desktop computer could be capable of
processing data up to 100,000 times faster than current models because multiple
operations can be performedsimultaneously. 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 time could require less than one hour by an optical one. But building an
optical computer will not be easy. A major challenge is finding materials that can
be mass produced yet consume little power; for this reason, optical computers may
not hit the consumer market for 10 to 15 years.
Another of the typical problems optical computers
have faced is that the digital optical devices have practical limits of eight to eleven
bits of accuracy in basic operations due to, e.g., intensity fluctuations. Recent
research has shown ways around this difficulty. Thus, for example, digital
partitioning algorithms, that 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. Nevertheless, many
problems in developing appropriate materials and devices must be overcome
21


before digital optical computers will be in widespread commercial use. In the near
term, at least, optical computers will most likely be hybrid optical/electronic
systems that use electronic circuits to preprocess input data for computation and to
post-process output data for error correction before outputting the results.
The promise of all-optical computing remains highly
attractive, however, and the goal of developing optical computers continues to be a
worthy one. Nevertheless, many scientists feel that an all-optical computer will not
be the computer of the future; instead optoelectronic computers will rule where the
advantages of both electronics and optics will be used. Optical computing can also
be linked intrinsically to quantum computing. Each photon is a quantum of a wave
function describing the whole function. It is now possible to control atoms by
trapping single photons in small, superconducting cavities

[5]. So photon quantum computing could become a future
possibility.
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.

22


Optical Components for Computing
The major breakthroughs on optical computing have been centered on the
development of micro-optic devices for data input. Conventional lasers are known
as ‘edge emitters’ because their laser light comes out from the edges. Also, their
laser cavities run horizontally along their length. A vertical cavity surface emitting
laser (VCSEL – pronounced ‘vixel’), however, gives out laser light from its
surface and has a laser cavity that is vertical; hence the name. VCSEL is a
semiconductor vertical cavity surface emitting microlaser 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. They emit at 850 nm and have
rather low thresholds (typically a few mA). They are very fast and can give mW of
coupled power into a 50 micron core fiber and are extremely radiation hard.
VCSELS can be tested at the wafer level (as opposed to edge emitting lasers which
have to be cut and cleaved before they can be tested) and hence are relatively
cheap. In fact, VCSELs can be fabricated efficiently on a 3-inch diameter wafer. A
schematic of VCSEL is shown in Figure 1.

23


Fig.- 1

The principles involved in the operation of a VCSEL are
very similar to those of regular lasers. As shown in Figure , 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
semiconductor with differing compositions create these mirrors, and each mirror
reflects a narrow range of wavelengths back into the cavity in order to cause light
emission at just one wavelength.

Spatial light modulators (SLMs) play an important role in
several technical areas where the control of light on a pixel-bypixel basis is a key
element, such as optical processing, for inputting information on light beams, and
displays. 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.

24


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 the 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.
Figure 2(a) shows a reflective 256x256 pixel device based
on SRAM technology. Several technologies have contributed to the development
of SLMs. These include micro-electro-mechanical devices, such as, acousto-optic
modulators (AOMs), and pixelated electrooptical devices, such as liquid-crystal
modulators (LCMs).
Figure 2(b) shows a simple AOM operation in deflecting
light beam direction. Encompassed within these categories are amplitudeonly,
phase-only, or amplitude-phase modulators. Broadly speaking, an optical computer
is a computer in which light is used somewhere. This can means fiber optical
connections between electronic components, free space connections, or one in
which light functions as a mechanism for storage of data, logic or arithmetic.
Instead of electrons in silicon integrated circuits, the digital optical computers will
be based on photons. Smart pixels, the union of optics and electronics, both
expands the capabilities of electronic systems and enables optical systems with
high levels of electronic signal processing. Thus, smart pixel systems add value to
electronics through optical input/output and interconnection, and value is added to
optical systems through electronic enhancements which include gain, feedback
control, and image processing and compression.

25


Fig.- 2 (a)

26


Fig.- 2 (b)

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

27


emitting diode (LED) under the control of a field-effect transistor (FET), 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 (when mass-production techniques take over) lead to cheaper, lighter,
circuitry that can be printed rather than etched. Scientists at Bell Labs have made
300-micron-wide pixels using polymer FETs and LEDs made from a sandwich of
organic materials, one of which allows electrons to flow, another which acts as
highway for holes (the absence of electrons); light is produced when electrons and
holes meet. The pixels are quite potent, with a brightness of about 2300
candela/m2, compared to a figure of 100 for present flat-panel displays . A
Cambridge University group has also made an all-organic device, not as bright as
the Bell Labs version, but easier to make on a large scale .

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.
4
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.
5

28


SLM (SPATIAL LIGHT MODULATORS)
SLM play an important role in several technical areas where the control of
light on a pixel-by-pixel basis is a key element, such as optical processing and
displays.

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.
6
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. WDM can transmit up to 32 wavelengths through a single
fiber, but cannot meet the bandwidth requirements of the present day
communication systems. So nowadays DWDM (Dense wavelength division
multiplexing) is used. This can transmit up to 1000 wavelengths through a single
fiber. That is by using this we can improve the bandwidth efficiency.
29


8
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. 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.
The field of optical computing is considered to be the most multidisciplinary
field and requires for its success collaborative efforts of many disciplines, ranging
from device and optical engineers to computer architects, chemists, material
scientists, and optical physicists. On the materials side, the role of nonlinear
materials in optical computing has become extremely significant. Nonlinear
materials are those, which interact with light and modulate its properties. For
example, such materials can change the color of light from being unseen in the
infrared region of the color spectrum to a green color where it is easily seen in the
visible region of the spectrum. Several of the optical computer components require
efficient nonlinear materials for their operation. What in fact restrains the wide-
spread use of all optical devices is the inefficiency of currently available nonlinear
optical materials, which require large amounts of energy for responding or
switching. In spite of new developments in materials, presented in the literature
daily, a great deal of research by chemists and material scientists is still required to
30


enable better and more efficient optical materials. Although organic materials have
many features that make them desirable for use in optical devices, such as high
nonlinearities, Flexibility of molecular design, and damage resistance to optical
radiation, their use in devices has been hindered by processing difficulties for
crystals and thin films. Our focus is on a couple of these materials, which have
undergone some investigation in the NASA/MSFC laboratories, and were also
processed in space either by the MSFC group, or others. These materials belong to
the classes of phthalocyanines and polydiacetylenes. These classes of organic
compounds are promising for optical thin films and waveguides. Phthalocyanines
are large ring-structured porophyrins for which large and ultrafast nonlinearities
have been observed. These compounds exhibit strong electronic transitions in the
visible region and have high chemical and thermal stability up to 400°C. We
measured the third order susceptibility of phthalocyanine, which is a measure of its
nonlinear efficiency to be more than a million times larger than that of the standard
material, carbon disulfide. This class of materials has good potential for
commercial device applications, and has been used as a photosensitive organic
material, and for photovoltiac, photoconductive, and photoelectrochemical
applications.

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.

OPTICAL NAND GATE

In an optical NAND gate the phthalocyanine film is replaced by a hollow
31


fiber filled with polydiacetylene. Nd:YAG green picosecond laser pulse was sent
collinearly with red cw He-Ne laser onto one end of the fiber. At the other end of
the fiber a lens was focusing the output on to the narrow slit of a monochrometer
with its grating set for the red He-Ne laser. When both He-Ne laser and Nd:YAG
laser are present there will be no output at the oscilloscope. If either one or none
of the laser beams are present we get the output at the oscilloscope showing
NAND function.
11

OPTICAL MEMORY
In optical computing two types of memory are discussed. One consists of
arrays of one-bit-store elements and other is mass storage, which is implemented
by optical disks or by holographic storage systems. This type of memory promises
very high capacity and storage density. The primary benefits offered by
holographic optical data storage over current storage technologies include
significantly higher storage capacities and faster read-out rates. This research is
expected to lead to compact, high capacity, rapid-and random-access, and low
power and low cost data storage devices necessary for future intelligent spacecraft.
The SLMs are used in optical data storage applications. These devices are used to
write data into the optical storage medium at high speed. More conventional
approaches to holographic storage use ion doped lithium niobate crystals to store
pages of data.
For audio recordings ,a 150MBminidisk with a 2.5- in diameter has been
developed that uses special compression to shrink a standard CD’s640-MB storage
capacity onto the smaller polymer substrate. It is rewritable and uses magnetic field
modulation on optical material. The mini disc uses one of the two methods to write
information on to an optical disk. With the mini disk a magnetic field placed
behind the optical disk is modulated while the intensity of the writing laser is held
constant. By switching the polarity of the magnetic field while the laser creates a
state of flux in the optical material digital data can be recorded on a single layer.
As with all optical storage media a read laser retrieves the data.

Fiber Optics: -

32


Definition:
A basic fiber optic system consists of a transmitting device, which generates
the light signal; an optical fiber cable, which carries the light; and a receiver, which
accepts the light signal transmitted. The fiber itself is passive and does not contain
any active, generative properties.

History:
Many individuals throughout the history of the world have recognized the
value of using light to to communicate. Early defense warning systems were set up
on the Great wall of China with signal fires to warn of enemies approaching. In the
late 1700's the "optical telegraph" was invented by a French engineer named
Claude Chappe which, similar to the fire signals, used semaphores mounted on
towers, where human operators relayed messages from one tower to the next. In
1870, John Tyndal demonstrated the principle of total internal reflection by shining
a light into a water tank, poking a hole in the side, and as the water ran out in an
arc, the light took the shape and followed the water down. Ten years later,
Alexander Graham Bell patented an optical telephone system "Photophone" which
he imagined sound waves carried by light. It wasn't until many years later through
numerous advances in thinking and technical discovery's that Tyndal's and Bell's
basic concepts came together to what we now know as fiber optics. Through the
invention of the continuouswave helium-neon laser and enhancements to optical
fiber, researchers Dr. Robert Maurer, Peter Schultz, and Donald Keck of Corning
Incorporated lead the way in development of Silica manufactured fiber optics and
in 1970 were successful in manufacturing 20dB/km, cable that was tested and used
successfully in Britain. Today optical fiber is manufactured at .25dB/km, which is
an indicator of the purity of the silica and how much loss of light occurs over
distance.

Technical Info:

Optical fiber for telecommunications is made up of three parts including the
core, cladding & coating. The core is the central part of the fiber which transmits
the light. The cladding surrounds the core and keeps the light in the core because it
is made of material with a lower index of refraction. The core and cladding are
inseparable because they are made up of a single piece of glass silica, treated to
create the differences needed in refraction. Finally, a coating generally made of
UV protective acrylate is put on a fiber during the draw process to protect it.

33


Fiber optic systems can carry both analog and digital signals over
light waves. A system consists of a signal generator, (e.g. computer, video, audio)
an encoder, a fiber optic cable, and a decoder, and a receiving device (e.g. tv,
computer network, etc.) Fiber optics have many advantages over copper cable.
They have become a desired standard for networking backbones and hubs because
of the advantages they have over copper to achieve the speed and bandwidth
capacity. A single fiber optic cable can transmit the same amount of data as
approximately 600 pair traditional copper telecommunications wire, an transmit
data further with less boosting of the signal, it is not effected by electrical
anomalies such as lightning, it is small, light weight and easy to install.

Year2000:
34


With the highly purified and streamlined manufacturing process, the current
speeds of data transfer are around 5millionbps. The biggest challenge remaining is
the economic challenge. Today telephone and cable television companies generally
bring in fiber links (backbones)to remote sites serving many customers, but then
use twisted wire pair or coaxial cables from optical network units to individual
homes. This technology is often referred to "broadband" and is becoming
increasingly popular, but considerably limited to the potential of complete fiber
optic networks directly linked to individual homes. Only time will tell how long it
will take before the technology becomes reasonably economical and enough
demand is given to take that next step.
1
Uses of Optics in Computing

Currently, optics is used mostly to link portions of computers, or more
intrinsically in devices that have some optical application or component. For
example, 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 analyzers. The early work in optical signal processing and computing
was basically analog in nature.
In the past two decades, however, a great deal of effort has been expended in
the development of digital optical processors. Much work remains before digital
optical computers will be widely available commercially, but the pace of research
and development has increased through the 1990s. During the last decade, there
has been continuing emphasis on the following aspects of optical computing:

 Optical tunnel devices are under continuous development
varying from small caliber endoscopes to character
recognition systems with multiple type capability.

 Development of optical processors for asynchronous
transfer mode.

Development architectures for optical neural networks. Development of high
accuracy analog optical processors, capable of processing large amounts of data in
parallel.
Since photons are uncharged and do not interact with one another as readily
as electrons, 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

35


through the loop changes. Further, high frequency or fast switching pulses will
cause interference in neighboring wires.
On the other hand, signals in adjacent optical 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. 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 are now very common in home and office
computers. Current trends in optical computing emphasize communications, for
example the use of free-space optical interconnects as a potential solution to
alleviate bottlenecks experienced in electronic architectures, including loss of
communication efficiency in multiprocessors and difficulty of scaling down the IC
technology to sub-micron levels. Light beams can travel very close to each other,
and even intersect, without observable or measurable generation of unwanted
signals. Therefore, dense arrays of interconnects can be built using optical systems.
In addition, risk of noise is further reduced, as light is immune to electromagnetic
interferences. Finally, as light travels fast and it has extremely large spatial
bandwidth and physical channel density, it appears to be an excellent media for
information transport and hence can be harnessed for data processing. This high
bandwidth capability offers a great deal of architectural advantage and flexibility.
Based on the technology now available, future systems could have 1024 smart
pixels per chip with each channel clocked at 200MHz (a chip I/O of 200Gbits per
second), giving aggregate data capacity in the parallel optical highway of more
that 200Tbits per second; this could be further increased to 1000Tbits. Free-space
optical techniques are also used in scalable crossbar systems, which allow arbitrary
interconnections between a set of inputs and a set of outputs. Optical sorting and
optical crossbar inter-connects are used in asynchronous transfer modes or packet
routing and in shared memory multiprocessor systems.
In optical computing two types of memory are discussed. One consists of
arrays of one-bit-store elements and the other is mass storage, which is
implemented by optical disks or by holographic storage systems. This type of
memory promises very high capacity and storage density. The primary benefits
offered by holographic optical data storage over current storage technologies
include significantly higher storage capacities and faster read-out rates. This
research is expected to lead to compact, high-capacity, rapid- and random-access,
radiation-resistant, low-power, and low-cost data storage devices necessary for
future intelligent spacecraft, as well as to massive-capacity and fast-access
36


terrestrial data archives. As multimedia applications and services become more and
more prevalent, entertainment and data storage companies are looking at ways to
increase the amount of stored data and reduce the time it takes to get that data out
of storage. The SLMs and the linear array beam steerer are used in optical data
storage applications. These devices are used to write data into the optical storage
medium at high speed.
The analog nature of these devices means that data can be stored at much
higher density than data written by conventional devices. Researchers around the
world are evaluating a number of inventive ways to store optical data while
improving the performance and capacity of existing optical disk technology. While
these approaches vary in materials and methods, they do share a common
objective: expanded capacity through stacking layers of optical material. For audio
recordings, a 150-MB minidisk with a 2.5-in. diameter has been developed that
uses special compression to shrink a standard CD’s 640-MB storage capacity
onto the smaller polymer substrate. It is rewritable and uses magnetic field
modulation on optical material. The minidisk uses one of two methods to write
information onto an optical disk. With the minidisk, a magnetic field placed behind
the optical disk is modulated while the intensity of the writing laser head is held
constant. By switching the polarity of the magnetic field while the laser creates a
state of flux in the optical material, digital data can be recorded on a single layer.
As with all optical storage media, a read laser retrieves the data. Along with
minidisk developments, standard magneto-optical CD technology has expanded the
capacity of the 3.5-in. diameter disk from 640 MB to commercially available 1 GB
storage media. These conventional storage media modulate the laser instead of the
magnetic field during the writing process. Fourth-generation 8,5.25 in.diameter
disks that use the same technology have reached capacities of 4 GB per disk. These
disks are used mainly in ‘jukebox’ devices. Not to be confused with the musical
jukebox, these machines contain multiple disks for storage and backup of large
amounts of data that need to be accessed quickly.
Beyond these existing systems are several laboratory systems that use
multiple layers of optical material on a single disk. The one with the largest
capacity, magnetic super-resolution (MSR), uses two layers of optical material.
The data is written onto the bottom layer through a writing laser and magnetic field
modulation (MFM). When reading the disk in MSR mode, the data is copied from
the lower layer to the upper layer with greater spacing between bits. In this way,
data can be stored much closer together (at distances smaller than the read beam
wavelength) on the bottom layer without losing data due to averaging across bits.
This method is close to commercial production, offering capacities of up to 20 GB
on a 5.25 in. disk without the need for altering conventional read-laser technology.

37


Advanced storage magnetic optics (ASMO) builds on MSR, but with one
exception.
Standard optical disks, including those used in MSR, have grooves and lands
just like a phonograph record. These grooves are used as guideposts for the writing
and reading lasers. However, standard systems only record data in the grooves, not
on the lands, wasting a certain amount of the optical material’s capacity. ASMO
records data on both lands and grooves and, by choosing groove depths
approximately 1/6 the wavelength of the reading laser light, the system can
eliminate the crosstrack crosstalk that would normally be the result of recording on
both grooves and lands. Even conventional CD recordings pick up data from
neighboring tracks, but this information is filtered out, reducing the signal-to-noise
ratio. By closely controlling the groove depth, ASMO eliminates this problem
while maximizing the signal-to-noise ratio. MSR and ASMO technologies are
expected to produce removable optical disk drives with capacities between 6 and
20 GB on a 12-cm optical disk, which is the same size as a standard CD that holds
640 MB. Magnetic amplifying magneto-optical systems (MAMMOS) use a
standard polymer disk with two or three magnetic layers. In general terms,
MAMMOS is similar to MSR, except that when the data is copied from the bottom
to the upper layer, it is expanded in size, amplifying the signal. According to
Archie Smith of Storagetek’s Advanced Technology Office (Louisville, CO),
MAMMOS represents a two-fold increase in storage capacity over ASMO.
Technology developed by Call/Recall Inc. (San Diego, CA) could help bridge the
gap between optical disk drives and holographic memories. Called 2-photon
optical storage technology (which got its start with the assistance of the Air Force
research laboratories and DARPA), the Call/Recall systems under development use
a single beam to write the data in either optical disks with up to 120 layers, or into
100-layer cubes of active-molecule-doped MMA polymer. In operation, a mask
representing data is illuminated by a mode-locked Nd:YAG laser emitting at 1064
nm with pulse durations of 35 ps. The focal point of the beam intersects a second
beam formed by the second harmonic of the same beam at 532 nm. The second
beam fixes the data spatially and temporally. A third beam from a He Ne laser
emitting at 543 nm reads the data by causing the material to fluoresce. The
fluorescence is read by a chargecoupled device (CCD) chip and converted through
proprietary algorithms back into data. Newer versions of the system use a
Ti:Sapphire laser with 200-fs pulses. Call/Recall’s Fredrick McCormick said the
newer and older approaches offer different strengths. The YAG system can deliver
higher-power pulses capable of storing megabits of data with a single pulse, but at
much lower repetition rates than the Ti:Sapphire laser with its lower-power pulses.
Thus, it is a trade-off. Call/Recall has demonstrated the system using portable
apparatus comprised of a simple stepper-motor-driven stage and 200-microwatt
38


HeNe laser in conjunction with a low-cost video camera. The company estimates
that an optimized system could produce static bit error rates (BER) of less than 9
10–13. McCormick believes that a final prototype operating at standard CD
rotation rates would offer BERs that match or slightly exceed conventional optical
disk technology. Researchers such as Demetri Psaltis and associates at the
California Institute of Technology are also using active-molecule-doped polymers
to store optical data holographically.
Their system uses a thin polymer layer of PMMA doped with
phenanthrenequinone (PQ). When illuminated with two coherent beams, the
subsequent interference pattern causes the PQ molecules to bond to the PMMA
host matrix to a greater extent in brighter areas and to a lesser extent in areas where
the intensity drops due to destructive interference. As a result, a pair of partially
offsetting index gratings is formed in the PMMA matrix. After writing the
hologram into the polymer material, the substrate is baked, which causes the
remaining unbounded PQ molecules to diffuse throughout the polymer, removing
the offsetting grating and leaving the hologram. A uniform illumination is the final
step, bonding the diffuse PQ throughout the matrix and fixing the hologram in the
polymer material.
Storagetek’s Archie Smith estimates that devices based on this method could
hold between 100 and 200 GB of data on a 5.25-in diameter polymer disk.
More conventional approaches to holographic storage use irondoped
lithium niobate crystals to store pages of data. Unlike standard magneto-optical
storage devices, however, the systems developed by Pericles Mitkas at Colorado
State University use the associative search capabilities of holographic memories.
Associative or content-based data access enables the search of the entire memory
space in parallel for the presence of a keyword or search argument. Conventional
systems use memory addresses to track data and retrieve the data at that location
when requested. Several applications can benefit from this mode of operation
including management of large multimedia databases, video indexing, image
recognition, and data mining.
Different types of data such as formatted and unformatted text, gray scale
and binary images, video frames, alphanumeric data tables, and time signals can be
interleaved in the same medium and we can search the memory with either data
type. The system uses a data and a reference beam to create a hologram on one
plane inside the lithium niobate. By changing the angle of the reference beam,
more data can be written into the cube just like pages in a book. The current
systems have stored up to 1000 pages per spatial location in either VGA or VGA
resolutions. To search the data, a binary or analog pattern that represents the search
argument is loaded into a spatial light modulator and modulates a laser beam. The
light diffracted by the holographic cube on a CCD generates a signal that indicates
39


the pages that match the sought data. Recent results have shown the system can
find the correct data 75 percent of the time when using patterns as small as 1 to 5
percent of the total page. That level goes up to 95 to 100 percent by increasing the
amount of data included in the search argument.2

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.
We are in an era of daily explosions in the development of optics and optical
components for computing and other applications. The business of photonics is
booming in industry and universities worldwide. It is estimated that photonic
device sales worldwide will range between $12 billion and $100 billion in 1999
due to an ever-increasing demand for data traffic.
According to KMI corp., data traffic is growing worldwide at a rate of 100%
per year, while, the Phillips Group in London estimates that the U.S. data traffic
will increase by 300% annually. KMI corp. also estimates that sales of dense-
wavelength division multiplexing equipment will increase by more than quadruple
its growth in the next five years, i.e. from $2.2 billion worldwide in 1998 to $9.4
billion 2004. In fact, Future Communication Inc., London, announced this year to
upgrade their communication system accordingly. The companyÕs goal is to use
wavelength division multiplexing at 10 Gb/s/channel to transmit at a total rate of
more than 1000 Tb/s.
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 alloptical components
require a high level of laser power to function as required. A group of researchers
from the university of southern California, jointly with a team from the university
of California Los Anglos, have developed an organic polymer with a switching
frequency of 60 GHz. This is three times faster than the current industry standard,

40


lithium niobate crystal-based devices. The California team has been working to
incorporate their material into a working prototype. Development of such a device
could revolutionize the information superhighway and speed data processing for
optical computing. Another group at Brown University and the IBM.
Almaden Research Center (San Jose, CA) have used ultrafast laser pulses to
build ultrafast datastorage devices. This group was able to achieve ultrafast
switching down to 100ps. Their results are almost ten times faster than currently
available Òspeed limitsÓ. Optoelectronic technologies for optical computers and
communication hold promise for transmitting data as short as the space
between computer chips or as long as the orbital distance between satellites. A
European collaborative effort demonstrated a high-speed optical data input and
output in free-space between IC chips in computers at a rate of more than 1 Tb/s.
Astro Terra, in collaboration with Jet Propulsion Laboratory (Pasadena, CA) has
built a 32-channel 1-Ggb/s earth Ðto Ðsatellite link with a 2000 km range. Many
more active devices in development, and some are likely to become crucial
components in future optical computer and networks.
The race is on with foreign competitors. NEC (Tokyo, Japan) have
developed a method for interconnecting circuit boards optically using Vertical
Cavity Surface Emitting Laser arrays (VCSEL). Researchers at Osaka City
University (Osaka, Japan) reported on a method for automatic alignment of a set of
optical beams in space with a set of optical fibers.
As of last year, researchers at NTT (Tokyo, Japan) have designed an optical
back plane with free Ðspace optical interconnects using tunable beam deflectors
and a mirror. The project had achieved 1000 interconnections per printed-circuit
board, with throughput ranging from 1 to 10 Tb/s.
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. Parallelism first
appeared in Cray super computers in the early 1980s.
Two processors were used in conjunction with the computer memory to
achieve parallelism and to enhance the speed to more than 10 Gb/ s. It was later
realized that more processors were not necessary to increase computational speed,
41


but could be in fact detrimental. This is because as more processors are used, there
is more time lost in communication. On the other hand, 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 mmin the object plane which are connected optically by a
lens to a corresponding 1000x 1000 points per mm in the image plane. For this to
be accomplished electrically, a million nonintersecting and properly isolated
conduction channels per mm 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, much less than what was mentioned earlier (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 6 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 fullduplex 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 dramaticopportunities 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 in home and office computers.

42


OPTICAL DISK
13
WORKING
The 780nm light emitted from AlGaAs/GaAs laser diodes is collimated by a lens
and focused to a diameter of about 1micrometer on the disk. If there is no pit where
the light is incident, it is reflected at the Al mirror of the disk and returns to the
lens, the depth of the pit is set at a value such that the difference between the path
of the light reflected at a pit and the path of light reflected at a mirror is an integral
multiple of half-wavelength consequently, if there is a pit where light is incident,
the amount of reflected light decreases tremendously because the reflected lights
are almost cancelled by interference. The incident and reflected beams pass
through the quarter wave plate and all reflected light is introduced to the
photodiode by the beam splitter because of the polarization rotation due to the
quarter wave plate. By the photodiode the reflected light, which has a signal
whether, a pit is on the disk or not is changed into an electrical signal.

An Optical Computer Powered by Germanium Laser
One of the issues of current chip design is the excessive power needed to
transport and store ever increasing amounts of data. A possible solution is to use
optics not just for sending data, but also to store information and perform
calculations, which would reduce heat dissipation and increase operating speeds.
Disproving previous beliefs in the matter, MIT researchers have demonstrated the
first laser built from germanium which can perform optical communications... and
it's also cheap to manufacture.

43


As Moore's law keeps giving us faster and faster computers, chip builders
also need higher-bandwidth data connections. But excessive heat dissipation and
power requirements make conventional wires impractical at higher frequencies,
which has lead researchers to develop new ways to store, transmit and elaborate
optically-encoded information.

If optical-based data elaboration is to have a future, researchers will need to
find a cheap and effective way to integrate optical and electronic components onto
silicon chips.

The solution found by the MIT team and detailed in a paper published in the
journal Optics Letters is notable not only because it achieves these objectives, but
also because it changes the way physicists have been looking at a class of materials
that were previously thought to be unsuitable for manufacturing lasers.

In a semiconductor, electrons that receive a certain amount of energy enter a
"conduction band" and are free to conduct electrical charge. Once they fall out of
this excited state, the electrons can either release their energy as heat or as photons.
Materials such as the expensive gallium arsenide are thought to be the best for
manufacturing lasers, because their excited electrons tend to go fall back into the
photon-emitting state.

However, the MIT team demonstrated that materials such as germanium,
whose electrons would normally tend to go in the heat-emitting state, can be
manipulated to emit photons and used to produce lasers that are cheap not only
because of the cost of the materials, but also because the processes used to build
them are already very familiar to chip manufacturers.

The researchers found two ways to make germanium "optics-friendly". The
first is a technique called "doping," which involves implanting very low
concentrations of a material such as phosphorous to force more electrons in the
conduction band and modify the electrical properties of the material.

The second strategy was to "strain" the germanium, pulling its atoms slightly
farther apart than they would be naturally by growing it directly on top of a layer
of silicon. This makes it easier for electrons to jump into the photon-emitting state.

The team now needs to find a way to increase the concentration of
phosphorus atoms in the doped germanium to increase the power efficiency of the
44


lasers, making them more attractive as sources of light for optical data
connections and, one day, for computing as well.

First germanium laser could pave way for optical computers
By Dario Borghino

18:34 February 14, 2010

First germanium laser could pave way for optical computers

One of the issues of current chip design is the excessive power needed to
transport and store ever increasing amounts of data. A possible solution is to use
optics not just for sending data, but also to store information and perform
calculations, which would reduce heat dissipation and increase operating speeds.
Disproving previous beliefs in the matter, MIT researchers have demonstrated the
first laser built from germanium which can perform optical communications... and
it's also cheap to manufacture.

As Moore's law keeps giving us faster and faster computers, chip builders
also need higher-bandwidth data connections. But excessive heat dissipation and
power requirements make conventional wires impractical at higher frequencies,
45


which has lead researchers to develop new ways to store, transmit and elaborate
optically-encoded information.

If optical-based data elaboration is to have a future, researchers will need to
find a cheap and effective way to integrate optical and electronic components onto
silicon chips.

The solution found by the MIT team and detailed in a paper published in the
journal Optics Letters is notable not only because it achieves these objectives, but
also because it changes the way physicists have been looking at a class of materials
that were previously thought to be unsuitable for manufacturing lasers.

In a semiconductor, electrons that receive a certain amount of energy enter a
"conduction band" and are free to conduct electrical charge. Once they fall out of
this excited state, the electrons can either release their energy as heat or as photons.
Materials such as the expensive gallium arsenide are thought to be the best for
manufacturing lasers, because their excited electrons tend to go fall back into the
photon-emitting state.

However, the MIT team demonstrated that materials such as germanium,
whose electrons would normally tend to go in the heat-emitting state, can be
manipulated to emit photons and used to produce lasers that are cheap not only
because of the cost of the materials, but also because the processes used to build
them are already very familiar to chip manufacturers.

The researchers found two ways to make germanium "optics-friendly". The
first is a technique called "doping," which involves implanting very low
concentrations of a material such as phosphorous to force more electrons in the
conduction band and modify the electrical properties of the material.

The second strategy was to "strain" the germanium, pulling its atoms slightly
farther apart than they would be naturally by growing it directly on top of a layer
of silicon. This makes it easier for electrons to jump into the photon-emitting state.

The team now needs to find a way to increase the concentration of
phosphorus atoms in the doped germanium to increase the power efficiency of the
lasers, making them more attractive as sources of light for optical data connections
and, one day, for computing as well.

46


The work is part of the Si-Based-Laser Initiative of the Multidisciplinary
University Research Initiative (MURI), and was sponsored by the Air Force Office
of Scientific Research (AFOSR).

Concept of Picosecond (By NASA)

NASA scientists are working to solve the need for computer speed using
light itself to accelerate calculations and increase data bandwidth.

Watches tick in seconds. Basketball games are timed in 10ths of a second,
and drag racers in 100ths. Computers used to work in milliseconds (1,000ths), then
moved up to microseconds (millionths), and now are approaching nanoseconds

47


(billionths) for logic operations - and picoseconds (trillionths!) for the switches and
gates in chips.

"That's great in theory," says Dr. Donald Frazier of NASA's Marshall Space
Flight Center. "Except that electronic signals, even with Very Large Scale
Integration (VLSI) and maximum miniaturization, are bogged down by many
aspects of the solid materials they travel through. So we've had to find a faster
medium for the signals - and the answer seems to be light itself!" Above: Dr.
Donald Frazier monitors a blue laser light used with electro-optical materials.

Light travels at 186,000 miles per second. That's 982,080,000 feet per
second -- or 11,784,960,000 inches. In a billionth of a second, one nanosecond,
photons of light travel just a bit less than a foot, not considering resistance in air or
of an optical fiber strand or thin film. Just right for doing things very quickly in
microminiaturized computer chips.

"Entirely optical computers are still some time in the future," says Dr.
Frazier, "but electro-optical hybrids have been possible since 1978, when it was
learned that photons can respond to electrons through media such as lithium
niobate. Newer advances have produced a variety of thin films and optical fibers
that make optical interconnections and devices practical. We are focusing on thin
films made of organic molecules, which are more light sensitive than inorganics.
Organics can perform functions such as switching, signal processing and frequency
doubling using less power than inorganics. Inorganics such as silicon used with
organic materials let us use both photons and electrons in current hybrid systems,
which will eventually lead to all-optical computer systems."

"What we are accomplishing in the lab today will result in development of
super-fast, super-miniaturized, super-lightweight and lower cost optical computing
and optical communication devices and systems," Frazier explained.

The speed of computers has now become a pressing problem as electronic
circuits reach their miniaturization limit. The rapid growth of the Internet,
expanding at almost 15% per month, demands faster speeds and larger bandwidths
than electronic circuits can provide. Electronic switching limits network speeds to
about 50 Gigabits per second (1 Gigabit (Gb) is 109, or 1 billion bits).

Dr. Hossin Abdeldayem, a member of Frazier's optical technologies
research group, states that Terabit speeds (1 Terabit, abbreviated "Tb", is 1012, or
1 trillion bits) are needed to accommodate the growth rate of the Internet and the
48


increasing demand for bandwidth-intensive data streams. Optical data processing
can perform several operations simultaneously (in parallel) much faster and easier
than electronics. This "parallelism" when associated with fast switching speeds
would result in staggering computational power. For example, a calculation that
might take a conventional electronic computer more than eleven years to complete
could be performed by an optical computer in a single hour.

All-optical switching using optical materials can relieve the escalating
problem of bandwidth limitations imposed by electronics," says Dr. Abdeldayem.
"In 1998, Lucent Technologies introduced a lithographic submicron technology to
further miniaturize electronic circuits and enhance computer speed. Additional
miniaturization of electronic components only provides a short-term solution to the
problem. There are also physical problems accompanied by miniaturization that
might affect the computer's reliability. "
Drs. Frazier and Abdeldayem and their group in Huntsville, AL, have
designed and built all-optical logic gate circuits for data processing at Gigabit and
Terabit rates, and they are also working on a system for pattern recognition.

Dr. Hossin Abdeldayem of NASA/Marshall works with lasers to develop a
system for pattern recognition. "We have also developed and tested nanosecond
optical switches, which can act as computer logic gates," says Dr. Abdeldayem,
who recently presented the group's research paper entitled "All-Optical Logic
Gates for Optical Computing" at The Pittsburgh Conference in New Orleans, LA.
"Picosecond and nanosecond all-optical switches, which act as AND and partial
NAND logic gates were demonstrated in our laboratory," explains Dr.
Abdeldayem. "Such logic gates are members of a large family of gates in
computers that perform logic operations such as addition, subtraction and
multiplication. They are vital for the development of optical computing and optical
communication. Our all-optical logic gates were made using a thin film of metal-
free phthalocyanine compound and a polydiacetylene polymer in a hollow fiber"

Logic gates are the building blocks of any digital system," he continues.
"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."

"Our phthalocyanine switch operates in the nanosecond regime (i.e.,
Gigabits per second), functioning as an all-optical AND logic gate. To demonstrate
it, we waveguided a continuous (cw) laser beam co-linearly with a nanosecond
pump beam through a thin film of metal-free phthalocyanine. The output was sent
to a fast photo-detector and to an oscilloscope. The cw beam was found to pulsate
49


synchronously with the pump beam, showing the characteristic table of an AND
logic gate."

A schematic of the nanosecond all-optical AND logic gate setup. More
schematics and illustrations are available in "Recent Advances in Photonic Devices
for Optical Computing" by NASA/Marshall's Hossin Abdeldayem, Donald O.
Frazier, Mark S. Paley, and William K. Witherow.

"Our setup for the picosecond switch was similar, except that the
phthalocyanine film was replaced with a hollow fiber coated from inside with a
thin polydiacetylene film. Both collinear laser beams were focused on one end of
the tube, and a lens at the other end focused the output onto a monochrometer with
a fast detector attached. The product of the two beams demonstrates three of the
four characteristics of a NAND logic gate."

"Optical bistable devices and logic gates such as these are the equivalent of
electronic transistors," concludes Dr. Abdeldayem. "They operate as very high
speed on-off switches and are also useful as optical cells for information storage."

According to Dr. Frazier, these all-optical computer components and thin-
films developed by NASA are essential to the current worldwide work in electro-
optical hybrid computers - and will help to make possible the astounding organic
optical computers that will be the standard of future terrestrial and space
information, operating and communication systems.

Recent Advances in Photonic Switches at NASA/MSFC

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 ÒONÓ when the
device transmits light and it is ÒOFFÓ when it blocks the light. Recently we
demonstrated in our laboratory at NASA/Marshall Space Flight Center two fast all-
optical switches using phthalocyanine thin films and polydiacetylene fiber. The
phthalocyanine switch is in the nanosecond regime and functions as an all-optical
AND logic gate, while the polydiacetylene one is in the picosecond regime and
exhibits a partial all-optical NAND logic gate.
To demonstrate the AND gate in the phthalocyanine film, we waveguided
two focused collinear beams through a thin film of metal-free phthalocyanine film.
The film thickness was ~ 1 m and a few millimeters in length. We used the second
50


harmonic at 532 nm from a pulsed Nd:YAG laser with pulse duration of 8 ns a
long with a cw He-Ne beam at 632.8 nm. The two collinear beams were then
focused by a microscopic objective and sent through the phthalocyanine film.
At the output a narrow band filter was set to block the 532 nm beam and
allow only the He-Ne beam. The transmitted beam was then focused on a fast
photo-detector and to a 500 MHz 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. The setup described above
demonstrated the characteristic table of an AND logic gate.
The setup for the picosecond switch was very much similar to the setup in
figure 3 except that the phthalocyanine film was replaced by a hollow fiber filled
with a polydiacetylene. The polydiacetylene fiber was prepared by injecting a
diacetylene monomer into the hollow fiber and polymerizing it by UV lamps. The
UV irradiation induces a thin film of the polymer on the interior of the hollow fiber
with a refractive index of 1.7 and the hollow fiber is of refractive index 1.2. In the
experiment, the 532 nm from a mode locked picosecond laser was sent collinearly
with a cw He-Ne laser and both were focused onto one end of the fiber. At the
other end of the fiber a lens was focusing the output onto the narrow slit of a
monochrometer with its grating set at 632.8 nm. A fast detector was attached to the
monochrometer and sending the signal to a 20 GHz digital oscilloscope. It was
found that with the He-Ne beam OFF, the Nd:YAG pulse is inducing a week
fluorescent picosecond signal (40 ps) at 632.8 nm that is shown as a picosecond
pulse on the oscilloscope. This signal disappears each time the He-Ne beam is
turned on. These results exhibit a picosecond respond in the system and
demonstrated three of the four characteristics of a NAND logic gate .

A comparison of a scanning electron micrographs of 1 m thick films of
copper phthalocyanine deposited by physical vapor transport in the 3M PVTOS
flight (STS-20) and ground control experiments. In microgravity the filmÕs
microstructure is very dense compared to that produced in unit gravity in the
presence of convection. This difference in microstructure has a significant affect
on the macroscopic film optical properties.

A comparison of a ground-grown polydiacetylene film with a microgravity-
grown one. The aggregates are impeded into the film by the fluid convection on the
ground, while the microgravity film is almost free of these aggregates where
convection is almost absent.

A schematic of the nanosecond all-optical AND logic gate setup. A
schematic of the all-optical NAND logic gate setup.
51


Optical computer bus
with dynamic bandwidth allocation
A signal communication device for use within a computer includes a set of
optical fibers configured to form an optical computer bus between a set of
computer sub-system elements of a computer. A set of input optical connector
cards are connected to the set of optical fibers. Each of the input optical connector
cards includes a transmitting dynamic bandwidth allocator responsive to an optical
bus clock signal operating at a multiple of a computer system clock signal such
that a set of bus time slots are available for each computer system clock signal
cycle. The transmitting dynamic bandwidth allocator allows a light signal to be
applied to the optical computer bus during a dynamically assigned bus time slot. In
this way, the optical computer bus bandwidth can be dynamically allocated to
52


different computer sub-system elements during a single computer system clock
signal cycle.
1. A method of signal communication within a computer, the method
comprising the following steps:

(a) operating a bus clock signal at a multiple of a computer system
clock signal, said multiple being greater than one, such that a set of bus time slots
are available for each computer system clock signal;

(b) dynamically assigning at least one bus time slot to given ones of a
plurality of computer sub-system elements during a single computer system clock
signal cycle;

(c) applying a signal from a computer sub-system element to a
computer bus during a dynamically assigned bus time slot.

2. The method of claim 1, wherein step (c) includes: applying a signal from
one of said plurality of computer sub-system elements to said computer bus during
said at least one bus time slot, a plurality of dynamically assigned bus time slots
being divisible among said plurality of computer sub-system elements in
accordance with a bandwidth requirement of one of the computer sub-system
elements.

3. The method of claim 1, wherein step (c) includes a step of alternately
producing a uniform, a random, and a dedicated division of bus time slots between
said plurality of computer sub-system elements.

4. The method of claim 1, wherein step (c) includes a step of converting said
signal from said computer sub-system element to a light signal.

5. The method of claim 4, wherein step (c) includes coupling said light
signal to an optical computer bus.

6. The method as recited in claim 1 further comprising determining a
receiver skew value measured between a received signal and said bus clock signal.

7. The method as recited in claim 6 further comprising reducing a transmitter
skew value between a transmitted signal and said bus clock signal based upon said
receiver skew value.

53


BRIEF DESCRIPTION OF THE INVENTION

This invention relates generally to computer buses. More particularly, this
invention relates to a computer bus that is implemented with optical fibers to avoid
the physical limitations associated with traditional computer bus designs.

BACKGROUND OF THE INVENTION

A computer bus is a communication link used to connect multiple computer
subsystems. For example, a computer bus is used to link the memory and
processor, and to link the processor with input/output (I/O) devices. Computer
buses are traditionally classified as follows: processor-memory buses, I/O buses, or
backplane buses. Processor-memory buses are short, generally high speed, and
matched to the memory system so as to maximize memory-processor bandwidth. I/
O buses, by contrast, can be lengthy, can have many types of devices connected to
them, and often have a wide range in the data bandwidth of the devices connected
to them. Backplane buses are designed to allow processors, memory, and I/O
devices to coexist on a single bus. Backplane buses balance the demands of
processor-memory communication with the demands of I/O device-memory
communication. Backplane buses received their name from the fact that they are
typically built into a computer backplane--the fundamental interconnection
structure within the computer chassis. Processor, memory, and I/O boards plug into
a backplane and then use the backplane bus to communicate.

Processor-memory buses are often design-specific, wile both I/O buses and
backplane buses are frequently standard buses with parameters established by
industry standards. The distinction between bus types is becoming increasingly
difficult to specify. Thus, the present application generically refers to computer
buses to encompass all processor-memory buses, I/O buses, and backplane buses.

The problem with computer buses is that they create a communication
bottleneck since all input/output must pass through a single bus. Thus, the
bandwidth of the bus limits the throughput of the computer. Physical constraints
associated with existing computer buses are beginning to limit the available
performance improvements generally available in computers.

The physical operation and constraints of existing computer bus designs are
most fully appreciated .A computer bus 20 positioned on a backplane 22. The
computer bus 20 is a set of wires, effectively forming a transmission line. A
random number of system cards (or cards) 24A-24N are attached to the computer
54


bus 20. By way of example, the cards 24 may include a video processing card, a
memory controller card, an I/O controller card, and a network card. Each card 24 is
connected to the computer bus 20 through a connector 26. Thus, each card 24 is
electrically connected to the set of wires forming the computer bus 20. As a result,
one card, say card 24A, can communicate with another card, say card 24N, by
writing information onto the computer bus 20. Only one card 24 can write
information onto the computer bus 20 at a time, thus a computer bus 20 can
generate a performance bottleneck as different cards 24 wait to write information
onto the bus 20.

Another problem associated with a traditional computer bus 20, is that its
performance is constrained by complicated electrical phenomenon. For example,
the connectors 26 effectively divide the bus into transmission line segments,
resulting in complicated transmission line effects. Note that the transmission line
segments will vary depending upon the number of cards 26 connected to the bus
20. This periodic loading of the bus 20 makes it difficult to optimize bus
performance. In addition, each connector 26 produces a lumped discontinuity with
parallel capacitance and series inductance, thereby complicating the electrical
characteristics of the bus 20. Note also that "T-connections" are formed between
the wires of a computer bus 20 and the wires to a connector 26. The T-connections
complicate the electrical characteristics of the computer bus 20.

Each card 24 includes a transceiver circuit 28 connected to a card logic
circuit 30, which performs the functional operations associated with the card 24.
The transceiver circuit 28 is used to read and write information on the bus 20. That
is, the transceiver circuit 28 reads information from the bus 20, the card logic
circuit 30 processes the information, and then the transceiver circuit 28 writes
processed information to the bus 20. Additional electrical complications arise with
the transceiver circuits 28. For example, transmission line segments are formed
between each connector 26 and each bus transceiver 28 circuit. In addition, the
transceiver circuits 28 present an impedance at their package pins that depends
upon the circuit design, the electrical state of the transceiver, and the packaging.

In sum, the computer bus 20 constitutes a transmission line with complicated
electrical interactions caused by such factors as transmission line segments and
connectors forming lumped discontinuities with parallel capacitance and series
inductance. The bus 20 may be terminated with termination resistors (R) to reduce
transmission line effects, such as reflections and mismatches. Nevertheless,
solutions of this sort do not overcome all transmission line problems associated
with a computer bus 20.
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Given these complicated electrical interactions, signals on the bus 20 do not
experience a uniform rise. That is, if the bus 20 was a perfect transmission line,
then high signals (digital ONES) written to the bus 20 would experience a uniform
rise. However, in view of the complicated electrical interactions on the bus 20,
high signals frequently experience one or more spurious signal transitions before
reaching a final peak value that can be processed. Waiting for signals to settle
causes delays. Another problem is that the complicated electrical interactions on
the computer bus 20 require higher powered drive signals, and thus more power
dissipation.

It is difficult to avoid these problems by changing the electrical
characteristics of the bus 20. That is, it is difficult to design a bus with improved
transmission line properties in view of the complicated factors that establish bus 20
performance. Thus, it would be highly desirable to design a new type of bus whose
performance is not contingent upon complicated transmission line effects
associated with prior art buses.

SUMMARY OF THE INVENTION

An embodiment of the invention includes a set of optical fibers configured
to form an optical computer bus between a set of computer sub-system elements of
a computer. A set of input optical connector cards are connected to the set of
optical fibers. Each of the input optical connector cards includes a transmitting
dynamic bandwidth allocator responsive to an optical bus clock signal operating at
a multiple of a computer system clock signal such that a set of bus time slots are
available for each computer system clock signal cycle. The transmitting dynamic
bandwidth allocator allows a light signal to be applied to the optical computer bus
during a dynamically assigned bus time slot. In this way, the optical computer bus
bandwidth can be dynamically allocated to different computer sub-system elements
during a single computer system clock signal cycle.

The optical computer bus is extremely fast, with bus signals moving at
approximately the speed of light ((index of refraction of the fiber)31 1×the speed
of light). The operation of the bus is not compromised by transmission line effects
associated with prior art computer buses. Further, the optical computer bus of the
invention does not suffer from electrical noise problems. The optical computer bus
is compact and is therefore ideal for space-constrained modern computers. Despite
its radically different design and configuration, the computer bus of the invention
otherwise operates in a standard manner. Thus, the computer bus can be used in
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existing computers and system designers can still rely upon known bus design
techniques.

DETAILED DESCRIPTION OF THE INVENTION

A digital gate computer bus 40, also called a chip bus, in accordance with
the invention. The chip bus 40 of the invention uses digital circuits 30 to perform
the function executed by a conventional computer bus. That is, the chip bus 40 of
the invention is used to perform a set of logical OR operations with digital gates so
that these operation do not have to be performed as wired OR operations on the
wires of a computer bus. In this way, the transmission line problems associated
with prior art computer buses are eliminated.

The operation of the invention is more fully appreciated with a simple example.
Typically, each card attached to a computer bus has N communication bits
corresponding to the N wires forming the computer bus. Thus, for example, if four
cards are attached to a computer bus, then each card has a designated bit that reads
and writes signals to a designated wire of the computer bus. If any card on the bus
writes a digital ONE to this designated wire of the computer bus, then all cards on
the bus read a digital high signal for this designated bit. This is a logical OR
operation performed by a hardwired circuit (the wire of the bus). The present
invention eliminates the physical wires of traditional computer buses and executes
the operation associated with such wires with digital gates. That is, the chip bus 40
of the invention performs logical OR operations with digital gates in order to
eliminate the transmission line problems associated with prior art computer buses.

The chip bus 40 is positioned on a backplane 22. Chip bus communication
lines 42 are electrically connected to the chip bus 40. In one embodiment, chip bus
input lines 44 carry input signals to the chip bus 40, the chip bus performs logical
OR operations on the input signals and generates output signals which are applied
to chip bus output lines 46. The chip bus communication lines 42 are electrically
connected to connectors 48, which in turn are electrically connected to cards 49.
The connectors 48 and cards 49 may be of the type known in the art. Thus, the chip
bus 40 of the invention can be used with prior art computer configurations.

Therein is a single bit embodiment of the chip bus 40 of the invention. In
particular, the figure illustrates a chip bus bit processor 50. The chip bus bit
processor 50 includes a logical OR circuit 51, illustratively shown as a wired OR
circuit. In this embodiment of the invention, the chip bus bit processor 50 also

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includes a card signal driver 52 with a bus input signal driver 54, implemented as
an inverter, and a bus output signal driver 56, also implemented as an inverter.

Thus, it can be appreciated that the chip bus bit processor 50 of receives a
single bit input signal from four cards (49A, 49B, 49C, 49D). In particular, each
single bit input signal is driven by the bus input signal driver 54 and applied to the
logical OR circuit 51. If any single bit input signal is a digital ONE on the logical
OR circuit 51, then a high output is generated at all output nodes. For the
embodiment, the high output signal is seen by the card logic circuit 66 after
processing by inverters 56 and 64.

In one embodiment of the invention, the card 49B may include a card
transceiver 60B. In this embodiment, the card transceiver 60B includes a logic
output signal driver 62, implemented as an inverter, and a logic input signal driver
64, also implemented as an inverter. The signals from the card transceiver 60B are
then processed by a logic circuit 66 in a conventional manner.

Therein are the same components shown , but the components are
rearranged to more fully describe the invention. In addition, the logical OR circuit
51 as being implemented with a four input OR gate. Thus, it is seen that each card
(49A, 49B, 49C, 49D) generates a single bit signal that is respectively applied to
the chip bus input lines (44A, 44B, 44C, 44D). The four signals are routed to the
four input OR gate 51. The output of the four input OR gate 51 is then routed back
to the cards (49A, 49B, 49C, 49D) through their respective chip bus output lines
(46A, 46B, 46C, 46D).

A four bit digital gate computer bus in accordance with the invention. The
four bit digital gate computer bus is used in conjunction with four processing cards
(49A, 49B, 49C, 49D). The four bit digital gate computer bus includes a chip bus
package 70 with package pins 72. Standard packaging techniques may be used to
form this structure. Within the package 70 are four chip bus bit processors (50A,
50B, 50C, 50D). The package 70 is positioned on a backplane 22.

Each processing card (49A, 49B, 49C, 49D) generates a single bit signal that
is applied to one of the chip bus bit processors 50. In particular, each processing
card generates a single bit signal that is applied to a chip bus input line 44 formed
on backplane 22. The signal reaches a package pin 72 and is then routed to a chip
bus bit processor 50 via a package internal trace 74. After processing by the chip
bus bit processor 50 is completed, the output signals are applied to chip bus output

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lines 46 formed in the backplane 22. The chip bus output lines 46 route the output
signals to their respective cards for processing in a standard fashion.

The invention has now been fully described. Attention presently turns to a
discussion of various implementation issues. Implementations of the chip bus 40 of
the invention will have the shared portion of a physical bus implemented with
digital gates and will use point-to-point wiring to connect the daughterboards
(cards 24). As used herein, point-to-point wiring refers to wiring running directly
between pins of two packages, without "T-connections", "Y-connections", or
related configurations or sources which complicate signal transmission.

The preferred embodiment of the invention uses separate chip bus input lines
44 and chip bus output lines 46. However, it is possible to use bidirectional wires
to make these connections. The bidirectional wires save a factor of two in signals,
but cannot reach the speed attainable by the unidirectional technique, unless
special transceivers are used that can simultaneously send and receive on the same
line. For the highest speed systems, it may be advantageous to use differential
simultaneous bidirectional signaling to reduce system noise.

Simultaneous bidirectional transceiver technology has been available in
emitter coupled logic for many years. The technology depends on having very high
performance differential amplifiers to subtract the outgoing signal from the signal
on the pin to recover the incoming signal. Simultaneous bidirectional signaling has
been demonstrated in CMOS, but is harder to implement because the close
matching and high gain of the bipolar devices is not available.

If the chip bus 40 is used in a synchronous system which is properly
arbitrated, it is not necessary to provide any control signals to control who is
driving the bus. While additional control signals are not required, the chip bus 40
of the invention does require more wires on the backplane than the traditional bus
structure, and also requires IC packages with many pins.

The logical OR circuit 51 may be implemented in any number of manners.
For example, wide fan-in pseudo-NMOS gates with four to six inputs have been
successful. For a chip bus bit processor 50 that process more than six signals, a
gate tree is generally required.

If the electrical distance from the card 49 to the chip bus 40 is less than
about half the transition time, the signal can be unterminated, and the driver can be
quite small. If the line is long enough to be terminated, it is possible to operate in
59


the 50 to 100 Ohm regime, rather than the sub-20 Ohm regime associated with a
heavily loaded conventional bus. Note that the termination can be done by
correctly sizing the driver transistors.

One way to use the chip bus 40 is as a drop-in replacement for a traditional
bus structure. In this mode, the chip bus provides the advantages of lower power
because it is easier to drive the lines, there are smaller propagation delays because
the point-to-point wiring is not periodically loaded, and the bus topology is
decoupled from the electrical behavior. The chip bus does not suffer from the
multiple reflection noise and settling delays associated with classical bus
implementations.

In a second implementation of the chip bus 40, a constraint is placed on the
wire lengths. It is easiest to think about this in the context of the unidirectional
implementation with all wire lengths equal. In this case, under the assumption of
no clock skew, the signal duration may be set to the minimal width to ensure
recognition. The electrical distance from the card 49 to the chip bus 40 enters into
latency, but no longer influences the maximum signaling speed.

The chip bus 40 of the invention is extremely fast. Simulated chip bus 40
designs have shown bit rates of 2.4 Gbits/sec per line. The delay through the chip
bus 40 is only 330 pS. A portion of the chip bus's speed is attributable to the fact
that input signals to the bus 40 can be pipelined, four input signals A, B, C, and D
are respectively carried by chip bus input lines 44A, 44B, 44C, and 44D at time
To. The pulse width of each signal is equivalent to the pulse width of the signal
clock, shown as Tp.. 6B illustrates the progression of the four input signals after a
clock cycle, that is, at time T=To Tp. the same signals on the chip bus output lines
46A, 46B, 46C, and 46D. The signals appear on the chip bus output lines at a time
T=To nTp, where n is the number of clock cycles required to drive the signals
through the chip bus 40. that it is possible to have an input signal to the bus and an
output signal from the bus every cycle. This pipelining capability results in
extremely high processing speeds that are not possible with traditional bus
architectures.

Current processor designs have about twenty gates between latches. The sum
of the setup and hold time of the latches is around 10% of the cycle time, or two
unit gate delays. The bus chip can be modeled as a pure delay, it doesn't change
pulse width. This implies that up to ten bus signals could be stacked in one
processor cycle. If some margin is allowed for timing tolerances, a practical limit
near eight transactions per cycle might be obtained with very careful design.
60


The real limitations on the speed of the system are clock skew and bit-to-bit
skew within a single package. Careful design of the wiring on the backplane 22
allows wire skew to be reduced to below all other skews in the system. Clock skew
can be kept low by using self-compensating clock drivers.

If the bus is wide enough that more than one package 70 is required, two
elements will contribute to the bit-to-bit skew. One is the difference in average
total delay between the parts, and the other is the spread in delay between the pins
within a single part. The traditional way of coping with part-to-part variations is to
bin the parts. Note that this does not cause yield loss, it just requires that any
particular board be populated with parts that have the same total delay dash-
number. It is also possible to build active compensation circuits into the parts to
force the average delay to match, for example, a reference delay printed on the
board.

Bit-to-bit skew within a part is controlled by a combination of the vendor's
process control, and what special efforts were taken during the design and layout
of the chip to minimize the sensitivity of the part to random variations in the
processing.

The clock protocol also influences the effect of delay variations. If the
signalling is source synchronized on a chip-by-chip basis, that is each group of bits
that is carried by a single bus chip carries its own clock, the sensitivity to interchip
delay variations may be minimized. This does add some complexity to the receiver
design to ensure that all the bit groups are correctly realigned. The source clock
may be used to provide the reference input for delay lock loops to compensate
these errors. Errors in arrival time of signals at the inputs of a single bus chip can
directly subtract from the signal pulse width.

Each signal can carry its own clock, for example, by using Manchester
coding as the synchronization protocol. Any method that carries the clock on the
same line as the signal will pay some overhead in bandwidth and latency. One
advantage of using a self clocking protocol is that all inputs to a chip bus can be
individually actively delay compensated by choosing one of the inputs as a
reference for all the others. This can be made to work both for the chip buses and
the system chips, and provides a global clock synchronization method as a side
effect of minimizing skew errors in the interconnect.

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The available bandwidth in a chip bus system in accordance with the
invention can be reduced by parasitic reactances in the IC packages and in the
interconnect. Reactances in the package can reduce the bandwidth by two
mechanisms: low pass filtering the signals and introducing noise. If the IC
packages are designed with close attention to the parasitics, it is possible to resolve
these problems. For example, a flip chip circuit can be used for very low series
inductance, and to maintain a controlled impedance right up to the pads.

Simultaneous switching noise caused by inductance in the ground return
path in the chip packages (ground bounce) and crosstalk between signals also
introduce uncertainty in when the transitions are recognized. The same measures
that are used to reduce package parasitics to avoid bandwidth reduction will also
help reduce these noise sources.

The ability to swap out boards in servers without powering down the system
or stopping the clock has become a requirement for new server designs. This is
rather difficult to implement using a traditional bus structure because both the
insertion and removal of the board produces electrical transients on the backplane.
The chip bus of the invention provides an elegant solution to this problem. A
disable pin for each port on a bus chip can be provided to force the corresponding
port into an idle state where the output is not driven and the input is ignored. This
isolates a board being removed or inserted from the bus. The control of these
disable signals can be derived from variable length fingers on the backplane
connectors.

It is possible to provide small state machines on the chip bus to perform
arbitration or protocol functions. If protocol or arbitration logic is embedded in the
chip bus, two problems arise. The first is that the gate depth rises beyond the
minimum needed to accommodate the fanout. The second is that connections are
required between the chip buses to coordinate control. Both factors increase
latency and reduce bandwidth. These problems may be reduced by pipelining the
bus protocol and arbitration. The pipelining can be done through central or
distributed arbitration. In the case of central arbitration, a special arbiter chip is
placed on the backplane near the chip buses. To match the performance of the chip
buses, all connections to the arbiter must be point-to-point, and the length matched
to the signal lines. The shortest pipeline sequence is: request, resolve, grant,
transfer. Distributed arbitration can be accomplished by running the same state
machine on each of the devices present on the bus. This usually requires dedicating
N request lines, where N is the number of devices. Pipelining the arbitration is still
required.
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When state machines or other intelligence is not used, the chip bus is
logically equivalent to passive wires on a backplane. This allows them to run at the
maximum speed that the technology will support and permits bit-slicing the bus to
accommodate real world packaging constraints.

In the bi-directional communication line implementation, a package would
require control pins to control the signal direction. A package would typically
require one power or ground pin per two signal pins. Standard pin versus speed
tradeoffs may be made when designing a package 70. The chip bus 40 of the
invention may be clocked at up to eight times the processor clock speed.

To control clock skew it may be advantageous to use a commercially
available clock distribution chip. Such chips compensate for skew by measuring
the phase of a reflected signal relative to an internal reference clock.

The pin count required for a package 70 can be reduced as far as desirable
by relying upon multiple chips. A 70 bit bus supporting 16 cards can be
implemented with 8 chip buses 40 of the type that use separate input and output
lines. Each chip bus 40 could be formed in a 432 pin package with the processor
and bus running at the same clock speed. This has the advantage of requiring no
control signals to the bus chips and can provide more bandwidth if the bus were
run at a multiple of the processor clock.

An optical bus bit processor 80 in accordance with another embodiment of
the invention. From a logic standpoint, the optical bus bit processor 80 operates in
the same manner as the previously described bus bit processor 50. The difference
between the devices is that the optical bus bit processor 80 processes light signals.
That is, a digital high state is represented by a light pulse, while a digital low state
is represented by an absence of a light pulse. The optical bus bit processor 80
performs a logical OR operation on light signals.

The optical bus bit processor, also called a star coupler, 80 includes a set of
N input optical fibers 82A-82N carry input signals to an optical fiber link 84,
shown in this embodiment as a fiber ring. A set of N output optical fibers 86A-86N
carry output signals. Consistent with previous embodiments of the invention, if a
single input signal is digitally ON (equivalent to a light pulse), then the fiber link
84 will cause each output optical fiber 86A-86N to carry a digitally ON signal.
Thus, the apparatus 80 performs a logical OR operation, consistent with the
previous embodiments of the invention.
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An optical computer bus 90 in accordance with an embodiment of the
invention. The optical computer bus 90 includes a stack of optical bus bit
processors 80. Each optical bus bit processor 80 is formed on a substrate 92. A set
of substrates are combined to form a stack 93. A stack of eight substrates to
provide a system that transmits eight bit words. A stack structure is a convenient
configuration, other physical configurations are also possible.

A system card (not shown) is attached to a connector 96. The system card
may be memory card, a local input/output card, a network input/output card,
graphics card, etc. Thus, each system card is typically in the form of a computer
sub-system. Alternately, all computer sub-systems can be contained on a single
card.

A set of signal lines 98 electrically link the connector 96 to an input optical
connector card 100. The input optical connector card 100 includes a set of signal
drivers 102. The signal drivers 102 process electrical signals from the connector 96
and convert them into appropriate drive signals for an array of light producing
devices 104. The array 104 is preferably implemented as a set of Vertical Cavity
Surface Emitting Lasers (VCSELs). Each VCSEL is optically connected to a single
input fiber 82 of a single substrate 92. Thus, the system card attached to connector
96 can apply eight separate signals to the optical computer bus 90. In particular,
each signal of the eight separate signals is applied to the first input fiber 82 of each
optical bus bit processor 80 of the stack 93 of optical bus bit processors forming
the optical computer bus 90. Alternately, the output of the array 104 may be
applied to a fiber optic ribbon cable, which is connected to the first input fiber 82
of each optical bus bit processor 80 of the stack 93.

It should be appreciated that the embodiment allows for three additional
input connectors to be respectively linked to the three input fibers 82B, 82C, and
82D. Thus, in the example system , four elements are connected to the optical bus
90, thus forming a four word bus that processes four eight bit words. Given this
configuration, the optical computer bus 90 can be considered a "two-dimensional
bus".

Each optical bus bit processor 80 is capable of processing four input bits and
producing four output bits. The output signals on output fibers 86A-86D are
applied to a set of output connector cards. For the sake of simplicity, a single
output optical connector card 110. The output optical connector card 110 includes
a light signal receiver array 112, which may be implemented using a set of
64


photodiodes or a set of VCSELs. An array of drivers 114 is connected to the light
signal receiver array 112. The array of drivers 114 generates a set of electrical
signals that are applied to the connector 96 for processing in a standard manner.

The optical bus bit processor 80 is positioned on a substrate 92. The optical
bus bit processor 80 may also be implemented as a fused coupler or in free space.
In the free space embodiment, an individual light source of a set of light sources at
a sending end is capable of transmitting a signal through space. The single signal
generates an output signal at a set of light receiving sources. The light sources and
light receiving sources are controlled by dynamic bandwidth allocators of the type
described below.

The optical bus bit processor 80 may also be implemented as
lithographically produced polymer or silica planar waveguides. Preferably, the
output optical connector card 110 is DC coupled and has fast recovery from
overload. This is important because several input pulses may overlap at the
receiver array 112 during system start-up or during a fault.

Those skilled in the art will appreciate that the optical computer bus 90 of
the invention is very fast. The optical and electrical implementations of the
invention allow the bus clock to operate at a multiple of the system clock. This
operation illustrates a four word bus which makes connections to four input optical
connector cards 100, although only one is shown for the sake of simplicity.
Similarly, the bus is connected to four output optical connector card 110, although
only one is shown. Relying upon this example, if the clock for the optical bus 90 is
operated at four times the speed of the system clock, then four bus time slots exist.
Each of the four connector cards can transmit an eight bit data word in a bus time
slot.

Waveform 120 illustrates the system clock signal. Waveform 122 illustrates
the bus clock signal, which is four times faster than the system clock signal.
Waveform 124 illustrates that a first input optical connector card transmits data (an
eight bit word in this example) during the first bus time slot, which corresponds to
the first bus clock signal cycle. The second input optical connector card transmits
data during the second bus clock cycle, the third input optical connector card
transmits data during the third bus clock cycle, and the fourth input optical
connector card transmits data during the fourth bus clock cycle, during one system
clock cycle and four bus clock cycles (bus time slots), each optical connector card
is allowed to transmit data on the system bus. This process may be repeated for
subsequent clock cycles.
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A flat or uniform allocation of optical bus bandwidth. Observe that during
the course of a single computer system clock signal cycle, every node gets to send
one message in its own bus time slot. This functionality is a superset of a crossbar
because every node can observe all of the transmissions. This allows for the
implementation of snoopy cache coherence methods that have a lower overhead
than the directory based methods required for a classic crossbar.

The invention can also be implemented by dynamically allocating optical
bus bandwidth. During the first system clock cycle, all available bus bandwidth is
assigned to the first input optical connector card, as illustrated with waveform 132.
This allocation may be viewed as a dedicated division of bus bandwidth resources
to a single computer sub-system. During the second system clock cycle, the bus
bandwidth is split between the second input optical connector card and the third
input optical connector card, as respectively shown with waveforms 134 and 136.
This division of bandwidth resources may be viewed as being random. In the final
system clock cycle, the bus bandwidth is assigned to the fourth input optical
connector card, as shown with waveform 138. Thus for each system clock cycle
the bus bandwidth can be divided among the input nodes in any number of ways.
This feature allows nodes with heavy traffic to dominate the bus bandwidth for
improved overall system performance.

One technique for implementing the foregoing functionality. An input
optical connector card 100 of the type. The input to the card 100 is from the signal
lines 98, which are linked to the connector 96. The output from the card 100 is
applied to the optical bus 90. As previously discussed, the input optical connector
card 100 includes a laser array 104 and a set of signal drivers 102. In accordance
with an embodiment of the invention, the signal drivers 102 may be implemented
as a set of transmitting dynamic bandwidth allocators 150A-150N. The signal
drivers 102 are connected to a transmission mask register array 148, which is an
array of registers, with each register storing a transmission mask signal indicating
which computer sub-system signals are to be transmitted during a bus clock cycle.
The signal drivers 102 are also attached to a buffer array 144, which stores data
from the signal lines 98. In particular, each buffer in the buffer array 144 stores
data for a corresponding transmitting dynamic bandwidth allocator 150.

A transmitting dynamic bandwidth allocator 150, in accordance with an
embodiment of the invention. The circuit 150 includes a transmission circuit 152
which receives a single data bit from the buffer array 144, a transmission mask bit
from the transmission mask register array 148, and a bus clock signal. The data bit
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is applied to an input node of a flip-flop 160. The flip-flop 160 is enabled if the bus
clock signal is high and the transmission mask bit is set to a digital high value. In
this case, the logical AND gate 162 generates a digital high value, or flip-flop
enable signal, to enable the flip-flop 160. Thus, it can be appreciated that the
transmission mask bit controls the output from the transmission circuit 152. The
output from the transmission circuit 152 is used as a drive signal for the laser array
104. Preferably, a deskew circuit 154 and a drive circuit 156 are used at the output
end of the transmission circuit 152.

An output optical connector card 110 has a similar configuration to that of
the input optical connector card 100 . In particular, an output optical connector
card 110 has a receiver array 112 connected to a driver array 114, which includes a
set of receiving dynamic bandwidth allocators, which are controlled by receive
mask signals stored in receive mask registers. The output from the driver array 114
is applied to a buffer array.

The processing of a bit signal between a transmitting dynamic bandwidth
allocator 150 of an input optical connector card 100 and a receiving dynamic
bandwidth allocator 170 of an output optical connector card 110. As discussed
above, the transmitting dynamic bandwidth allocator 150 includes a transmission
circuit 152, a transmission signal mask register 148A of the transmission mask
array 148, a deskew circuit 154 and a driver 156. Similarly, the receiving dynamic
bandwidth allocator 170 includes a driver 172 and a receiver circuit 174, which is
controlled by a receiving mask bit from the receiving mask register 176 of a
receiving mask register array (not shown). The receiver circuit 174 operates in the
same manner as the transmission circuit 152. The receiving dynamic bandwidth
allocator 170 also includes a skew compare circuit 178, which identifies skew
between the received signal form the optical bus and the bus clock signal. The
skew value is then sent to the deskew circuit 154 of the transmitting dynamic
bandwidth allocator 150 so that future signals are sent with reduced skew.

It should be appreciated that the disclosed dynamic bandwidth allocation
concept of the invention is also applicable to the disclosed digital gate computer
bus. When implemented in reference to the digital gate computer bus embodiment
of the invention, light array transmitters 104 and receivers 112 are omitted.

The foregoing descriptions of specific embodiments of the present invention
are presented for purposes of illustration and description. They are not intended to
be exhaustive or to limit the invention to the precise forms disclosed, obviously
many modifications and variations are possible in view of the above teachings. For
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example, a traditional backplane 22, connectors 48, and cards 49 need not be used.
The embodiments were chosen and described in order to best explain the principles
of the invention and its practical applications, to thereby enable others skilled in
the art to best utilize the invention and various embodiments with various
modifications as are suited to the particular use contemplated.
APPLICATIONS :-

1. High speed communications :The rapid growth of
internet,expanding at almost15% per month, demands faster speeds
and larger bandwidth than electronic circuits can provide. Terabits
speeds are needed to accommodate the growth rate of internet
since in optical computers data is transmitted at the speed of light
which is of the order of 3 10*8 m/sec hence terabit speeds
areattainable.

2. Optical crossbar interconnects are used in asynchronous transfer
modes and shared memory multiprocessor systems.

3. Process satellite data.

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MERITS :-

1. Optical computing is at least 1000 to 100000 times faster than
today’s silicon machines.
2. Optical storage will provide an extremely optimized way to store
data, with space requirements far lesser than today’s silicon chips.
3. Super fast searches through databases.

4. No short circuits, light beam can cross each other without
interfering with each other’s data.
5. Light beams can travel in parallel and no limit to number of
packets that can travel in the photonic circuits.
6. Optical computer removes the bottleneck in the present day
communication system

1

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DRAWBACKS

1. Today’s materials require much high power to work in consumer
products, coming up with the right materials may take five years or
more.
2. Optical computing using a coherent source is simple to compute
and understand, but it has many drawbacks like any imperfections
or dust on the optical components will create unwanted
interference pattern due to scattering effects. Incoherent processing
on the other hand cannot store phase information.
17

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SOME CURRENT RESEARCH

High performance computing has gained momentum in recent years , with
efforts to optimize all the resources of electronic computing and researcher brain
power in order to increase computing throughput. Optical computing is a topic of
current support in many places , with private companies as well as governments in
several countries encouraging such research work.

A group of researchers from the university of southern California , jointly
with a team from the university of California , los angles , have developed an
organic polymer with a switching frequency of 60 Ghz . this is three times faster
than the current industry standard , lithium niobate crystal baswed device.

Another group at brown university and the IBM , Almaden research center
has used ultrafast laser pulses to build ultra fast data storage devices . this groupe
was able to achivie ultra fast switching down to 100 picosecond .

In Japan , NEC has developed a method for interconnecting circuit boards
optically using VCSEL arrays .Another researchers at NTT have designed an
optical backplane with free-space opical interconnects using tunable beam
deflectors and mirrors. Theproject achieved 1000 interconnections per printed
circuit board;with a throughput ranging from 1 to 10 Tb/s.

8

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FUTURE TRENDS

The Ministry of Information Technology has initiated a photonic
development program. Under this program some funded projects are continuing
in fiber optic high-speed network systems. Research is going on for developing
new laser diodes, photodetectors, and nonlinear material studies for faster switches.
Research efforts on nanoparticle thin film or layer studies for display devices are
also in progress. At the Indian Institute of Technology (IIT), Mumbai, efforts are in
progress to generate a white light source from a diodecase based fiber amplifier
system in order to provide WDM communication channels.

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REFERENCES

1. Debabrata Goswami , “ article on optical computing, optical components
and storage systems,” Resonance- Journal of science education pp:56-71
July 2003

2. Hossin Abdeldayem,Donald. O.Frazier, Mark.S.Paley and William.K,
“Recent advances in photonic devices for optical computing,”
science.nasa.gov Nov 2001

3. Mc Aulay,Alastair.D , “Optical computer architectures and the application
of optical concepts to next generation computers”

4. John M Senior , “Optical fiber communications –principles and practice”

5. Mitsuo Fukuda “Optical semiconductor devices”

6. www.sciam.com

7. www.msfc.com

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