Optical Computing
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INTRODUCTION
With the growth of computing technology the need of high performance computers (HPC) has significantly increased. Optics has been used in computing for a number of years but the main emphasis has been and continues to be to link portions of computers, for communications, or more intrinsically in devices that have some optical application or component (optical pattern recognition etc.)Optical computing was a hot research area in 1980’s.But the work tapered off due to materials limitations that prevented optochips from getting small enough and cheap enough beyond laboratory curiosities. Now, optical computers are back with advances in self-assembled conducting organic polymers that promise super-tiny of all optical chips.Optical computing technology is, in general, developing in two directions. One approach is to build computers that have the same architecture as present day computers but using optics that is Electro optical hybrids. Another approach is to generate a completely new kind of computer, which can perform all functional operations in optical mode. In recent years, a number of devices that can ultimately lead us to real optical computers have already been manufactured. These include optical logic gates, optical switches, optical interconnections and optical memory.Current trends in optical computing emphasize communications, for example the use of free space optical interconnects as a potential solution to remove ‘Bottlenecks’ experienced in electronic architectures. Optical technology is one of the most promising, and may eventually lead to new computing applications as a consequence of faster processing speed, as well as better connectivity and higher bandwidth.
HISTORY OF OPTICAL COMPUTING
Optical computing, or the use of an optical device/phenomenon to perform information processing or computations, is a relatively new technology. Early research into optical signal processing resulted from attempts to process large amounts of data from radar systems. However, results from such research were rudimentary in nature, and it wasn’t until 1960 with the advent of the laser that optical computing stumbled ahead. The laser allowed signal processing, or analog operations. Data could be manipulated with two spatial degrees of freedom, rather than the single temporal degree as in electronic systems; thus, the data is said to be a two-dimensional image. Using the light from a laser, it is possible to perform a Fourier transform on the 2-d data, which results in easier manipulation of the image, in terms of functions and extrapolating values. As a result of the rapid development of electronic computers with digital logic, most notably the Von Neumann computers, research into optical means of computing and storage was hindered. Much in science and technology iscontrolled by economics, and electronic computers were the choice of businesses, with their accuracy, usability, and practicality. Optical technologies were more difficult to swallow, especially with their complicated and unsure operation. To further hinder the development of optical computing and storage came the transistor and integrated circuit. These sped electronic computing technologies way ahead, and left optical engineers searching for elusive materials with odd properties. However, research into optical data storage has been quite successful in the past decade or so, and much of the discovered technology is already available on the market. Apart from the optical storage devices currently familiar to society, many other ideas are being developed that will lead to giant leaps in modern trends.
BACKGROUND
To process, store, and transfer data in today’s modern computers electrons are sent through millions of transistor switches along metal wires. In order to create faster data transfer and processing the computer industry has repeatedly made smaller transistors and put more of them on a chip. Right now, it is possible for companies to fit 300 million transistor switches on one chip. Scientists even predict that in the coming decades computer technology will become atomic in size. But this process of using electrons and metal wires has some disadvantages that fiber optics will completely eliminate.Optical fibers are small glass wires used to send light pulses. They are basically made up of a center glass core, a cladding that makes sure the light doesn’t escape the core, and a buffer coating which protects the inside fibers. When the light enters the core, it is reflected off the walls, which are mirror-lined so the light continues all the way down the fiber. The process of the light reflecting down the fiber is called total internal reflection Because optical fibers transmit light, the transfer speed is very fast, a great amount faster than that of the copper wires we use today. Also, when sending information over copper wires, it must put the data in small groups called packets. Copper wires can only send one of these packets at a time because the electrical signals cannot run parallel. Light, on the other hand, has no problem with having other data run parallel with it. This means you can send and receive vast amounts of data at the same time.Optical Fibers also have other advantages, for example, optical wires are cheaper, thinner (which lets you have a higher carrying capacity than copper wires), more power efficient, have less signal degradation, clearer signals, are optimal for carrying digital signals, lightweight, and flexible. Along with these, optical fibers can also help benefit many occupations such as medical imaging, mechanical imaging, and even in plumbing to examine the sewer lines.
NEED FOR OPTICAL COMPUTING
The pressing need for optical technology stems from the fact that today’s computers are limited by the time response of electronic circuits. A solid transmission medium limits both the speed and volume of signals, as well as building up heat that damages components.One of the theoretical limits on how fast a computer can function is given by Einstein’s principle that signal cannot propagate faster than speed of light. So to make computers faster, their components must be smaller and there by decrease the distance between them. This has resulted in the development of very large scale integration (VLSI) technology, with smaller device dimensions and greater complexity. The smallest dimensions of VLSI nowadays are about 0.08mm. Despite the incredible progress in the development and refinement of the basic technologies over the past decade, there is growing concern that these technologies may not be capable of solving the computing problems of even the current millennium. The speed of computers was achieved by miniaturizing electronic components to a very small micron-size scale, but they are limited not only by the speed of electrons in matter but also by the increasing density of interconnections necessary to link the electronic gates on microchips. The optical computer comes as a solution of miniaturization problem. Optical data processing can perform several operations in parallel much faster and easier than electrons. This parallelism helps in staggering computational power. For example a calculation that takes a conventional electronic computer more than 11 years to complete could be performed by an optical computer in a single hour. Any way we can realize that in an optical computer, electrons are replaced by photons, the subatomic bits of electromagnetic radiation that make up light.Optical interconnections and optical integrated circuits 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.
SOME KEY OPTICAL COMPONENTS FOR COMPUTING
The major breakthroughs on optical computing have been centered on the development of micro-optic devices for data input.
VCSEL (VERTICAL CAVITY SURFACE EMITTING LASER)
VCSEL (pronounced ‘vixel’) is a semiconductor vertical cavity surface emitting laser diode that emits light in a cylindrical beam vertically from the surface of a fabricated wafer, and offers significant advantages when compared to the edge-emitting lasers currently used in the majority of fiber optic communications devices. The principle involved in the operation of a VCSEL is very similar to those of regular laserThere are two special semiconductor materials sandwiching an active layer where all the action takes place. But rather than reflective ends, in a VCSEL there are several layers of partially reflective mirrors above and below the active layer. Layers of semiconductors with differing compositions create these mirrors, and each mirror reflects a narrow range of wavelengths back in to the cavity in order to cause light emission at just one wavelength.
OPTICAL INTERCONNECTION OF CIRCUIT BOARDS USINGVCSEL AND PHOTODIODE
VCSEL convert the electrical signal to optical signal when the light beams are passed through a pair of lenses and micro mirrors. Micro mirrors 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.
SLM (SPATIAL LIGHT MODULATORS)
SLM play an important role in several technical areas where the control of light on a pixel-by-pixel basis is a key element, such as optical processing and displays.
SLM FOR DISPLAY PURPOSE
For display purposes the desire is to have as many pixels as possible in as small and cheap a device as possible. For such purposes designing silicon chips for use as spatial light modulators has been effective. The basic idea is to have a set of memory cells laid out on a regular grid. These cells are electrically connected to metal mirrors, such that the voltage on the mirror depends on the value stored in the memory cell. A layer of optically active liquid crystal is sandwiched between this array of mirrors and a piece of glass with a conductive coating. The voltage between individual mirrors and the front electrode affects the optical activity of liquid crystal in that neighborhood. Hence by being able to individually program the memory locations one can set up a pattern of optical activity in the liquid crystal layer.
SMART PIXEL TECHLOLOGY
Smart pixel technology is a relatively new approach to integrating electronic circuitry and optoelectronic devices in a common framework. The purpose is to leverage the advantages of each individual technology and provide improved performance for specific applications. Here, the electronic circuitry provides complex functionality and programmability while the optoelectronic devices provide high-speed switching and compatibility with existing optical media. Arrays of these smart pixels leverage the parallelism of optics for interconnections as well as computation. A smart pixel device, a light emitting diode under the control of a field effect transistor can now be made entirely out of organic materials on the same substrate for the first time. In general, the benefit of organic over conventional semiconductor electronics is that they should lead to cheaper, lighter, circuitry that can be printed rather than etched..
WDM (WAVELENGTH DIVISION MULTIPLEXING)
Wavelength division multiplexing is a method of sending many different wavelengths down the same optical fiber. Using this technology, modern networks in which individual lasers can transmit at 10 gigabits per second through the same fiber at the same time.WDM can transmit up to 32 wavelengths through a single fiber, but cannot meet the bandwidth requirements of the present day communication systems. So nowadays DWDM (Dense wavelength division multiplexing) is used. This can transmit up to 1000 wavelengths through a single fiber. That is by using this we can improve the bandwidth efficiency.
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. 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. 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. Organic materials have many features that make them desirable for use in optical devices such as1. High nonlinearities2. Flexibility of molecular design3. Damage resistance to optical radiationsSome organic materials belonging to the classes of phthalocyanines and polydiacetylenes are promising for optical thin films and wave guides. These compounds exhibit strong electronic transitions in the visible region and have high chemical and thermal stability up to 400 degree Celsius. Polydiacetylenes are among the most widely investigated class of polymers for nonlinear optical applications. Their subpicosecond time response to laser signals makes them candidates for high-speed optoelectronics and information processing.To make thin polymer film for electro-optic applications, NASA scientists dissolve a monomer (the building block of a polymer) in an organic solvent. This solution is then put into a growth cell with a quartz window, shining a laser through the quartz can cause the polymer to deposit in specific pattern.
ADVANCES IN PHOTONIC SWITCHES
\Logic gates are the building blocks of any digital system. An optical logic gate is a switch that controls one light beam by another; it is ON when the device transmits light and it is OFF when it blocks the light. The two fast all – optical AND logic gate, demonstrated using phthalocyanine thin films and polydiacetylene fiber. The phthalocyanine switch is in the nanosecond regime and functions as an all-optical AND logic gate, while the polydiacetylene one is in the picoseconds regime and exhibits a partial all-optical NAND logic gate.
OPTICAL AND GATE
To demonstrate the AND gate in the phthalocyanine film, two focused collinear laser beams are wave guided through a thin film of metal-free phthalocyanine film. The film thickness was ~ 1 m and a few millimeters in length. 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 nanosecond duration and in synchronous with the input Nd:YAG nanosecond pulse. This demonstrated the characteristic table of an AND logic gate.
OPTICAL NAND GATE
In an optical NAND gate the phthalocyanine film is replaced by a hollow fiber filled with polydiacetylene. Nd:YAG green picoseconds laser pulse was sent collinearly with red cw He-Ne laser onto one end of the fiber. At the other end of the fiber a lens was focusing the output on to the narrow slit of a monochrometer with its grating set for the red He-Ne laser. When both He-Ne laser and Nd:YAG laser are present there will be no output at the oscilloscope. If either one or none of the laser beams are present we get the output at the oscilloscope showing NAND function.
OPTICAL PROCESSORS
Probably the most important piece of hardware in your computer is your processor. It is the “brain” of your computer. It performs all the tasks on your computer from running games to spell checking word documents. And as games and applications become more complex, they take more computation power. This has not been a problem though because the computer companies like Intel and AMD are always releasing faster and more powerful processing chips. But eventually these companies will have to find another way to make faster and affordable chips because of the transistor problem previously stated.The answer to this problem is optics. Through optics, computers will be able to reach new speeds people never dreamed possible before. This will be made possible by the ability of optical wires to send data as light, and to send the data in huge packets, called solitons. There will be two different types of optical computers, a pure optical computer and an electro-optic computer.The pure optical computer will use different wavelengths, or multiple frequencies, to transmit data. Because of the optical fibers’ ability to send data parallel, the optical computer will be able to send various streams of data simultaneously. Multi-tasking will become much easier with the optical computer’s processor. In comparison to today’s electric computers a calculation that would take 1000 hours on current electric computers could take an hour or less on the future optical computer. The hardest part about this is that to use it you must build a device that can read the different wavelengths. Today’s computers use binary code. Scientists are working on building a totally optical computer, but don’t expect to see one in your home for at least another 10 years.The second type of optical computer will be the electro-optic computer. This will be a hybrid of the two, using optical fibers, but also using electric parts to read the data and direct it. Unlike the pure optical computer, the electro-optic computer will use light pulses to send information. When the processor sends a 1 or a 0, a device will make the code into a light pulse and, using an LED or laser, will send it to the next location, whereupon it will be decoded by an electronic device, back into a 1 or 0. So when sending data, if the light is on it will be translated as a 1, if the light is off, it will be translated as a 0. It should be around 3 to 5 years before we start seeing electro-optic computers on the market for home use.One major benefit of optical computers is the pattern recognition system. With this someone can put in a picture or symbol and the computer will examine it against a reference picture to see if it is valid. For example, if the police get a fingerprint off a crime scene and want to find out whose it is, they can shine a laser through the fingerprint. Then the beam goes through a special lense that will project and implimentation the picture onto a large board to see if it corresponds to any of the pictures on it. If it does then it can send out a matrix with a “1” being the picture it corresponded to. Then the police look at the large board, find out who the “1” was, and have the person they were looking for. They can also use this for face recognition and credit card validation. The hope to eventually use pattern recognition with artificial intelligence so that the robots would be able to identify things they see.To the bottom is a picture of the way credit card validation would be made possible using pattern recognition. After project and implimentationing the card and its reference image, the detector array decides if the images correspond with each other. If they do then it outputs a signal, if it does not correspond then it does not output a signal.
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.
HOLOGRAPHIC MEMORY
When Optical Computers are available, a new type of storage device will be used in the new computers. The new Holographic hard drives will store massive amounts of information in a sugar cube sized area. They will be able to do this by storing data in hologram form. This will be achieved using a precise technique of laser shining, as shown in the diagram to the right. A blue-green argon laser will be shined through a beam splitter. The original beam, which will have been split into a signal beam and a reference beam, will take different paths towards a lithium-niobate crystal. The signal beam will be reflected by a mirror into a spatial light modulator (SLM), which is a liquid crystal display (LCD) that shows pages of raw binary data as clear and dark boxes. The information from the page of binary code is carried by the signal beam around to the light-sensitive crystal. Meanwhile, the reference beam will take another path to the crystal. When the two beams meet, the interference created between them will be stored in a specific area of the crystal, as a hologram.
OPTICAL DISK
For audio recordings, a150MB minidisk with a 2.5- in diameter has been developed that uses special compression to shrink a standard CD’s640-MB storage capacity onto the smaller polymer substrate. It is rewritable and uses magnetic field modulation on optical material. The mini disc uses one of the two methods to write information on to an optical disk. With the mini disk a magnetic field placed behind the optical disk is modulated while the intensity of the writing laser is held constant. By switching the polarity of the magnetic field while the laser creates a state of flux in the optical material digital data can be recorded on a single layer. As with all optical storage media a read laser retrieves the data.
WORKING
The 780nm light emitted from AlGaAs/GaAs laser diodes is collimated by a lens and focused to a diameter of about 1micrometer on the disk. If there is no pit where the light is incident, it is reflected at the Al mirror of the disk and returns to the lens, the depth of the pit is set at a value such that the difference between the path of the light reflected at a pit and the path of light reflected at a mirror is an integral multiple of half-wavelength consequently, if there is a pit where light is incident, the amount of reflected light decreases tremendously because the reflected lights are almost cancelled by interference. The incident and reflected beams pass through the quarter wave plate and all reflected light is introduced to the photodiode by the beam splitter because of the polarization rotation due to the quarter wave plate. By the photodiode the reflected light, which has a signal whether, a pit is on the disk or not is changed into an electrical signal. APPLICATIONS
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 are attainable.Optical crossbar interconnects are used in asynchronous transfer modes and shared memory multiprocessor systems.Process satellite data.Optical computers can also used for face recognition and credit card validation.
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. Compact, lightweight, and inexpensive to manufacture.
7. optical computer removes the bottle neck in the present day communication system.
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.
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 ofcurrent support in many places , with private companies as well as governments in several countries encouraging such research work.A group of researchers from the university of southern California , jointly with a team from the university of California , los angles , have developed an organic polymer with a switching frequency of 60 Ghz . this is three times faster than the current industry standard , lithium niobate crystal baswed device. Another groupe at brown university and the IBM , Almaden research center has used ultrafast laser pulses to build ultra fast data storage devices . this group was able to archivie ultra fast switching down to 100 pico second .In japan , NEC has developed a method for interconnecting circuit boards optically using VCSEL arrays .Another researchers at NTT have designed an optical backplane with free-space opical interconnects using tunable beam deflectors and mirrors. The project and implimentation achieved 1000 interconnections per printed circuit board ;with a throughput ranging from 1 to 10 Tb/s.
FUTURE TRENDS
The Ministry of Information Technology has initiated a photonic development program. Under this program some funded project and implimentations are continuing in fiber optic high-speed network systems. Research is going on for developing new laser diodes, photo detectors, and nonlinear material studies for faster switches. Research efforts on nano particle 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 diode- case based fiber amplifier system in order to provide WDM communication channels.
CONCLUSION
Research in optical computing has opened up new possibilities in several fields related to high performance computing, high-speed communications. To design algorithms that execute applications faster ,the specific properties of optics must be considered, such as their ability to exploit massive parallelism, and global interconnections. As optoelectronic and smart pixel devices mature, software development will have a major impact in the future and the ground rules for the computing may have to be rewritten.
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Optical Computing




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Introduction

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.


Some Key Optical Components for Computing

The major breakthroughs on optical computing have been centered
on the development of micro-optic devices for data input.
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).


Uses of Optics in Computing

Currently, optics is used mostly to link portions of computers,
or more intrinsically in devices that have some optical application
or component. For example, much progress has been
achieved, and optical signal processors have been successfully
used, for applications such as synthetic aperture radars, optical
pattern recognition, optical image processing, fingerprint enhancement,
and optical spectrum analyzers. The early work in optical
signal processing and computing was basically analog in nature.
In the past two decades, however, a great deal of effort has
been expended in the development of digital optical processors.
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