Optical computing means performing computations, operations, storage and transmission of data using light. Instead of silicon chips optical computer uses organic polymers like phthalocyanine and polydiacetylene.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.
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 1980â„¢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 ËœBottlenecksâ„¢ 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.
2. NEED FOR OPTICAL COMPUTING
The pressing need for optical technology stems from the fact that todayâ„¢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 Einsteinâ„¢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 miniaturizing 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.
3. 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.
1. VCSEL (VERTICAL CAVITY SURFACE EMITTING LACER)
VCSEL (pronounced Ëœvixelâ„¢) is a semiconductor vertical cavity surface emitting laser diode that emits light in a cylindrical beam vertically from the surface of a fabricated wafer, and offers significant advantages when compared to the edge-emitting lasers currently used in the majority of fiber optic communications devices. The principle involved in the operation of a VCSEL is very similar to those of regular lasers.
There are two special semiconductor materials sandwiching an active layer where all the action takes place. But rather than reflective ends, in a VCSEL there are several layers of partially reflective mirrors above and below the active layer. Layers of semiconductors with differing compositions create these mirrors, and each mirror reflects a narrow range of wavelengths back in to the cavity in order to cause light emission at just one wavelength.
2. 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. For display purposes the desire is to have as many pixels as possible in as small and cheap a device as possible.
3. 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.
4. 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.
OPTICAL INTERCONNECTION OF CIRCUIT BOARDS USING VCSEL AND PHOTODIODE
VCSEL convert the electrical signal to optical signal when the light beams are passed through a pair of lenses and micromirrors. Micromirrors are used to direct the light beams and this light rays is passed through a polymer waveguide which serves as the path for transmitting data instead of copper wires in electronic computers. Then these optical beams are again passed through a pair of lenses and sent to a photodiode. This photodiode convert the optical signal back to the electrical signal.
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.
4. ROLE OF NLO IN OPTICAL COMPUTING
The role of nonlinear materials in optical computing has become extremely significant. Non-linear materials are those, which interact with light and modulate its properties. Several of the optical components require efficient nonlinear materials for their operations. What in fact restrains the widespread use of all optical devices is the in efficiency of currently available nonlinear materials, which require large amount of energy for responding or switching.
Organic materials have many features that make them desirable for use in optical devices such as
1. High nonlinearities
2. Flexibility of molecular design
3. Damage resistance to optical radiations
Some organic materials belonging to the classes of phthalocyanines and polydiacetylenes are promising for optical thin films and wave guides. These compounds exhibit strong electronic transitions in the visible region and have high chemical and thermal stability up to 400 degree Celsius. Polydiacetylenes are among the most widely investigated class of polymers for nonlinear optical applications. Their subpicosecond time response to laser signals makes them candidates for high-speed optoelectronics and information processing.
To make thin polymer film for electro-optic applications, NASA scientists dissolve a monomer (the building block of a polymer) in an organic solvent. This solution is then put into a growth cell with a quartz window, shining a laser through the quartz can cause the polymer to deposit in specific pattern.
5. 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.
EXPLANATION OF COMPONENTS USED IN OPTICAL AND GATE
1. He-Ne laser
This is the most widely used laser with continuous power output (cw laser) in the fraction of 1mW to 100mW. It is relatively easy to construct and is reliable in operation.
This was the first gas laser consists of a fused quartz tube with a diameter of about 1cm and length about 80cm. The tube is filled with a mixture of helium and neon gases in the ratio 10:1. The ends of the tube have Brewster windows. There are two reflectors and pumping takes place due to the electron impact.
A few energy levels of He&Ne atoms which correspond to the important laser transitions are shown in figure.
When the RF discharge at about 30MHz is passed through He-Ne gas mixture, the He atoms colliding with the electrons receive the energy and they are excited to levels F2 and F3 from F1. The energy levels F2&F3 are metastable. Ne atoms also have energy levels E4&E6 which have nearly the same energy as F2&F3. Unexcited Ne atoms colliding with the excited He atoms are excited and taken to the metastable states E4&E6. Population density in E4&E6 increases at this stage and appropriate photons can initiate laser emission. Stimulated emission takes place from E6 to E5, E6 to E3, and E4 to E3. From the level E3 by spontaneous emission the atoms come to the level E2 and there after by colliding with the walls, de-excitation takes place and the atoms come to the ground state. The emitted photons move two and fro within the gas between the end mirrors. Through the partially reflecting surface laser output is obtained.
2. Nd:YAG laser
Neodymium:Yttrium Aluminium Garnet is a solid state four level laser system.
The optically excited Neodymium ions from the bands E1&E2 quickly decay to the metastable upper laser level. The difference in energy is transferred to the crystal lattice. The upper lifetime is about 230micro second. It is quite long, so population can be accumulated over a relatively long time during the pumping cycle. In this level (upper laser level), the ions are stimulated by photons due to spontaneous emission, to emit on the main 1.064micrometer laser. The ions are dropped to a lower laser level. They quickly leave again by transferring the energy to the crystal lattice.
3. CONVEX LENS
These are used to converge the light after passing through the phthalocyanine film.
Narrow band filter was set to block the Nd:YAG laser beam and pass only He-Ne laser beam.
The purpose of the detector is to convert the optical laser (He-Ne laser) signal into electrical signal.
Oscilloscope is used to view the AND gate properties of the two laser beams, after it was passed through a phthalocyanine film.
OPTICAL NAND GATE
In an optical NAND gate the phthalocyanine film is replaced by a hollow fiber filled with polydiacetylene. Nd:YAG green picosecond laser pulse was sent collinearly with red cw He-Ne laser onto one end of the fiber. At the other end of the fiber a lens was focusing the output on to the narrow slit of a monochrometer with its grating set for the red He-Ne laser. When both He-Ne laser and Nd:YAG laser are present there will be no output at the oscilloscope. If either one or none of the laser beams are present we get the output at the oscilloscope showing NAND function.
6. 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.
1. High speed communications
The rapid growth of internet, expanding at almost 15% 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 310*8 m/sec hence terabit speeds are attainable.
2. Optical crossbar interconnects are used in asynchronous transfer modes and shared memory multiprocessor systems.
3. Process satellite data.
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.
1. Optical computing is at least 1000 to 100000 times faster than todayâ„¢s silicon machines.
2. Optical storage will provide an extremely optimized way to store data, with space requirements far lesser than todayâ„¢s silicon chips.
3. Super fast searches through databases.
4. No short circuits, light beam can cross each other without interfering with each otherâ„¢s data.
5. Light beams can travel in parallel and no limit to number of packets that can travel in the photonic circuits.
1. Todayâ„¢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.
10. FUTURE TRENDS
The Ministry of Information Technology has initiated a photonic development program. Under this program some funded projects are continuing in fiber optic high-speed network systems. Research is going on for developing new laser diodes, photodetectors, and nonlinear material studies for faster switches. Research efforts on nanoparticle thin film or layer studies for display devices are also in progress. At the Indian Institute of Technology (IIT), Mumbai, efforts are in progress to generate a white light source from a diode-case based fiber amplifier system in order to provide WDM communication channels.
Research in optical computing has opened up new possibilities in several fields related to high performance computing, high-speed communications. As opto electronic and smart pixel devices mature, software development will have a major impact in future.
1. RESONANCE-A Journal Of Science Education
2. OPTIC FIBER COMMUNICATION PRINCIPLE AND PRACTISE
Wilson.J , J.F.B.Hankes
2. NEED FOR OPTICAL COMPUTING
3. SOME KEY OPTICAL COMPONENTS FOR COMPUTING
4. ROLE OF NLO IN OPTICAL COMPUTING
5. ADVANCES IN PHOTONIC SWITCHES
6. OPTICAL MEMORY
9. DRAW BACKS
10. FUTURE TRENDS