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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ContentsOverview of Optical computers1 Components of Optical computers. . . . . . . . . . . . . . . . . . . . . . 9

1.1 Hard Disk1.2 CPU1.3 Memory1.4 Cache Memory 1.5 Main Memory1.6 Screen1.7 Power Supply

2 Need of Optical Computers . . . . . . . . . . . . . . . . . . . . . . . . . 15

3 Optical Components for Computing . . . . . . . . . . . . . . . . . . 203.1 VCSEL3.2 SLM3.3 WDM3.4 Optical Memory

4 Fibre Optics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294.1 Use of Fibre Optics in Computing4.2 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 Some current researchFuture Trends

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http://lasermodule.blogspot.com/2010/03/optical-computer-powered-by-germanium.html


ReferencesPREFACE

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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http://en.wikipedia.org/wiki/Photon


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, to perform digital computations. An electric current creates heat in computer

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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 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.

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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 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 ofwires 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 thirdlythe 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

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where the fluorescence quantum yield is a measure of the preservation of light-energy; light fast fluorescent dyes with 100% fluorescence quantum yield areknown . 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.

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COMPONENTS OF OPTICAL COMPUTER

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? Stanford University, Lucent Technologies, and

cutting-edge Silicon Valley optics company Siros Technologies.

TIME OF COMPLETION? 2005, for a commercial product.

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(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? 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.

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(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.

(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.

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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

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.

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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

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.

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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.

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

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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 technologiesmay 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, costs can be reduced; hence looking for improved computing capabilities is desirable. Optical computing includes the optical calculation of transforms andoptical 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

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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 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.

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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 usingdifferent 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 instructionssequentially; 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, 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 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

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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 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

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[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 solidtransmission medium limits both the speed and volume of signals, as well asbuilding 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.

Optical Components for Computing

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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.

Fig.- 1

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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.

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

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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.

Fig.- 2 (a)

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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

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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 opticcommunications devices. The principle involved in the operation of a VCSEL isvery 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 aVCSEL there are several layers of partially reflective mirrors above and belowthe active layer. Layers of semiconductors with differing compositions createthese mirrors, and each mirror reflects a narrow range of wavelengths back in tothe cavity in order to cause light emission at just one wavelength.4OPTICAL 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

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

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 differentwavelengths 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

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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.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 nonlinearities2. Flexibility of molecular design3. 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 scientistsdissolve a monomer (the building block of a polymer) in an organic solvent. Thissolution 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

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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 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.

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OPTICAL NAND GATE In an optical NAND gate the phthalocyanine film is replaced by a hollowfiber filled with polydiacetylene. Nd:YAG green picosecond laser pulse was sentcollinearly with red cw He-Ne laser onto one end of the fiber. At the other end ofthe fiber a lens was focusing the output on to the narrow slit of a monochrometerwith its grating set for the red He-Ne laser. When both He-Ne laser and Nd:YAGlaser are present there will be no output at the oscilloscope. If either one or noneof 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 beendeveloped 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.

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Fiber Optics: -

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.

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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.

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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:

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.

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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, opticalpattern 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 developmentvarying 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 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 ofcoherent light have dropped rapidly in price due to mass production.

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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 electromagneticinterferences. 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 morethat 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 andoptical 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 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

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world are evaluating a number of inventive ways to store optical data while improving the performance and capacity of existing optical disk technology. Whilethese 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 capacityonto 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. 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

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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 atmuch 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 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 opticaldisk 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.

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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 irondopedlithium 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 oneplane 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 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?

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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, 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

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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 spacebetween 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, 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

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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 donot 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.

OPTICAL DISK13WORKING

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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.

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.

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http://lasermodule.blogspot.com/2010/03/optical-computer-powered-by-germanium.html


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.

First germanium laser could pave way for optical computers

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

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http://www.gizmag.com/author/dario-borghino/

http://www.gizmag.com/mit-germanium-laser-optical-computing/14173/picture/110599/


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.

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).

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http://www.opticsinfobase.org/ol/upcoming_pdf.cfm?id=121712


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

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"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 increasing demand for bandwidth-intensive data streams. Optical data processing can perform several operations simultaneously (in parallel) much faster and easier

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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

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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-opticalAND 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.

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The film thickness was ~ 1 m and a few millimeters in length. We used the second 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.

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

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

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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.

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.

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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.

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.

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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 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.

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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.

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.

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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 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

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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 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

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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 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

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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 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

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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.

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

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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.

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

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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.

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

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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.

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

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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 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

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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.

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.

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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 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

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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 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.

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3. Process satellite data.

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 daycommunication system

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1

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.

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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 fasterthan the current industry standard , lithium niobate crystal baswed device.

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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 boardsoptically 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.

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FUTURE TRENDS

The Ministry of Information Technology has initiated a photonic development program. Under this program some funded projects are continuingin 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

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http://www.blogger.com/rearrange?blogID=4959731529319975820&widgetType=Profile&widgetId=Profile1&action=editWidget


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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