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Demand of high-capacity and high-speed telecommunication systems triggered the research in the area of all-optical signal processing techniques. The maximum speed of switching of electronic logic gates is of the order of 1010 Hz limiting the speed and bandwidth of telecommunication. On the other hand, switching speed of optical logic gates is limited only by the speed of light passing through it and of the order of 1014 Hz. All-optical logic gates are key elements in all-optical signal processing techniques. So far, several techniques have been investigated to realize various all-optical logic functions. These approaches have shown some advantages, but which are difficult to operate at very high speed data rate. Optical computing or photonic computing is intended to use the photons produced by the lasers or diodes in place of electrons, which may eventually lead to new computing applications as a consequence of faster processing speed as well as better connectivity and higher bandwidth. A numerous new technologies and optical processors had been designed and constructed for real applications in late 20th century, thus making the 1980-2000, the golden era for optical computing. Research in the field of photonic crystal structures shows that photonic crystals are the strong candidates for optical computing. These devices have been widely investigated since John and Yablonovitch reported in 1987. Very recently, Arun et. al. showed that a photonic crystal can work as optical switch and this property can be used to design flip-flop and logic gate. Today the research in optical computing has opened up new possibilities in several fields related to high speed computations, operations, storage and transmission of data using light. This paper enlightens the development in optical computing from early 60 years until today and brief survey of some of the ongoing and future research on this technology has been presented.
Keywords-Optical logic gates,Â Optical memory, Optical switches, Optical interconnects, Optical processors, Photonic crystals.
The computers today use the movement of electrons in-and-out of transistors to do logic.Â OpticalÂ orÂ Photonic computingÂ is intended to useÂ light particles called photons, produced by diodes or more commonly lasers, in place ofÂ electrons. Photons are much faster when compared to electrons as light travels about 30cm in aÂ nanosecond and have a higherÂ bandwidth too. Optical computing performs computations, operations, storage and the transmission of data using photons. Optical computing explains a new technological approach for constructing computer's processors using photonic gates, optical memory and optical interconnects. Optical computing employs a technology called silicon photonics that uses laser light instead of the current approach of electrically transmitting data along tiny wires etched into silicon. Thus, the use of optical lasers overcome the constraints associated with dissipation of heat in today's components and allows much more information to be stored and transmitted. Optical technology promises massive upgrading in the speed and efficiency of computers, as well as significant shrinkage in their size along with cost.
Optical computing uses an optical device/phenomenon for information processing or carrying computations, is a relatively new technology. This is approximately 70 years old technology and it has a well-defined and respected domain.Â Although, atleast as early as 1940, Von Neuman considered digital computing with the use of optical component. If lasers were available that time, the first digital computers may have used optics. In 1958, Townes and Schawlow gave the idea of laser  in visible region of electromagnetic spectrum. In 1960, Maiman invented theÂ ruby laserÂ  which is considered as the first laser. And thus with the advent of laser the optical computing stumbled ahead. The laser allowed signal processing, or analog operations.
Vander Lugt in 1963 proposed and demonstrated a technique for synthesizing the complex filter of a coherent optical processor using a Fourier hologram technique. In 1966, Weawer and Goodman presented the Joint Transform Correlator (JTC)  architecture that may be widely used in pattern recognition.
In 1966, Brown et al revolutionized holography by introducing the first computer generated hologram (CGH) . In 1969, a pure phase encoded CGH was proposed opening the way to the modern diffractive optical elements with a high diffraction efficiency. Until 1980, the CGHs encoding methods were limited by the power of the computers.
In the early 1960's and throughout the 1970's and 1980's, optics was employed for fourier transform computations of military images in the matched filtering operations. The applications are performed optically when bandwidth needs exceed electronic capability. In 1980's, optical computing was hot research area. But due to the material limitations the work tapered off that prevented opto chips from getting small and cheap enough for commercial applications. Optical technologies were more difficult to swallow, especially with their complicated and unsure operation. Also, the transistor and integrated circuits when came, they further hinderd the development of optical computing. These speed electronic computing technologies way ahead and optical engineers were left with the search for materials with odd properties. New researches and developments in optical transmission and non-linear materials in 1980's, opens new doors in the field of optical computing. The period from 1980 to till now is considered as the golden age of the optical computing.
There are different types of optical computing for example, digital optical computing , optical analog computing , quantum computing . A lot of research had been done and going on in this field  and future look very bright, there is funding for the programs in the area and the research effort is very intensive worldwide. Every year, several international conferences organized by different international societies on subjects related to optical computing. The journals had frequently a special issue on optical computing and information Processing. David Wineland of NIST and Professor Serge Haroche of the College de France have shared this year's (2012) Nobel Prize in Physics for their work in the field of quantum optics. They worked on the interaction of matter and light; a field which has seen considerable progress since the mid 1980s, and suggested that it can be used to design the optical computers. Thus have opened the door to a new era of quantum computing, which use photon in the processing.
Devices Used In Optical Computing
Optical logic Gates
Since electronic technology will very soon reach its speed limit, future computation and communication problems are inevitable. The networks have to perform optical signal processing such as binary addition, parity checking, optical pattern recognition, header recognition, demultiplexing, regenerating and switching with very high speeds, because all-optical signal processing can handle large bandwidth and large flow of information. All-optical logic gates are key elements in all-optical signal processing techniques. So far, several techniques have been investigated to realize various all-optical logic functions [7-20]. Optical logic gates have attracted increasing attention due to their practical utility in networks and systems, such as recognition for optical packet switching schemes [15, 16], and optical label swapping . In order to realize the gates, various configurations have been reported that utilize the nonlinear properties of the optics. Until now, all-optical gates reported in the literature [18-23] could be achieved with a semiconductor laser amplifier loop mirror (SLALOM), a semiconductor optical amplifier- (SOA) based Mach-Zehnder interferometer (SOA-MZI) , cross polarization modulation, a SOA based ultra fast nonlinear interferometer(UNI), and four-wave mixing (FWM) in SOAs, SOA with Optical filter, Periodically Poled Lithium Noibate (PPLN) waveguide. All these schemes have some advantages and disadvantages. In recent years, logic gates based on photonic crystals have attracted much attention because of their dimensions and low loss structures. Normally the Photonic Crystals (PC) are produced by artificially imparting periodic change to the refractive index of a structure that have band gap to prevent the propagation of certain frequency range of light. But the propagation of light inside the Photonic Crystals can be controlled by different propagation mechanisms such as negative refraction, super prism and nonlinear effects. When a non-linear refractive index material is introduced photonic crystal, the reflection and transmission properties of the structure got changed and the structure can be made reflective at high intensity in that region of frequency for which the composite material is transparent at low intensity thereby suggesting that it can work as a switch. In self-collimating effect , the collimated light beam insensitive to the divergence of the incident beam without applying a nonlinear effect. Based on the self collimated propagation in photonic crystal, Zhang et al. realized EXOR and OR gate . Very recently, Arun et. al. showed that a photonic crystal can work as optical switch and this property can be used to design flip-flop and logic gate. Logic gates based on some other technique are also proposed .
Memory is one of the key portions of a modern digital computer, in which it is able to store information so as to have the capability of utilizing this information later in various computations. Optical computing describes two types of memories. One consists of mass storage, which is implemented by optical disks or by holographic storage systems, and the other consists of arrays of one-bit store elements. The former 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 technique is expected to lead to compact, high capacity and low cost data storage devices. A hologram has a very large information storage capability . Holography offers a significant increase in bit packaging density over magnetic recording, since the bit size can approach the wavelength of light. In addition to improved density, holographic storage assures a reduction in access time over other mass storage systems. Recent advances in optical memory technology now make it appear to be the leading contender for the next generation of mass storage devices. Holographic data storage also provides a method to preserve and archive information.
Communication between the storage devices at a very high rate is possible through optical cables using optical interconnect. Compared to the traditional cables being used, optical cables are capable of very large bandwidth, more than 100 Gbit/s .
The brain of the computer is a processor. The development of various transistors and logic gate circuits in the sub-micron values are needed for the design o f an efficient and reliable processor for a computer. The optical transistors and logic gates have been still in developing conditions. Even though there are no all-optical processors available commercially, there are opto-electronic hybrid optical processors available . Thus an optical computer is a hypothetical device up till now, but most research are now focused to replace the current computer components with optical equivalents, leading to an optical digital computer system processing binary data. Optical parallel data processing is easier and less expensive than electronic. In addition, optical computing systems offer computational speeds more than 100,000 times faster than the currently fastest electronic systems. This means a computation that takes a conventional computer more than 11 years to solve would take an optical computer less than one hour.
Nonlinear materials for optical computing
Nonlinear materials play very important role in optical computing. The materials which interact with light and modulate its properties are the nonlinear materials. Many of the optical components require efficient nonlinear materials for their operations. What in fact restrains the widespread use of all optical devices is the inefficiency of the currently available non-linear materials, which require large amount of energy for responding or switching . Organic materials offered many features that make them desirable for use in optical devices. They have high nonlinearities, flexibility of molecular design and damage resistance to optical radiations. Their sub picoseconds time response to laser signals makes them candidates for high-speed optoelectronics and information processing .
Advantages & drawbacks of computing at light speed
The photonic transistor products are expected to replace much of the existing electronic infrastructure during the 21st century, because it can be made smaller, faster, cheaper and also more reliable, generate less heat, are immune to electromagnetic interference. Best available electronic device is slow, in comparison to photonic devices, because photons are faster than electrons. In comparison to electronic circuits, optical computing process information by manipulating light by light with the speed of light. The amount of information that can be handeled and processed in one second depends on the switching time of the components used. That switching time in photonic circuit is of the order of fs or even less. So, photonic transistors are able to manipulate information nearly 100,000 times faster than electronic transistors. Also, photonic circuit can be made smaller in comparison to electronic circuit. This results in the miniaturization.
There are some drawbacks also of optical computing. Unlike electronic circuits, conventional optical amplifiers and many nonlinear optical devices add noise to the signals passing through. Search of cheap and feasible practical switching material has been very difficult and the situation has not improved much in last few decades. So far the non-linear optical materials being developed required large optical power.
Applications of optical computing
Optical computing has tremendous applications including (1) High speed communication, (2) Optical signal processing, (3) Optical data storage (4) Optical crossbar interconnects are used in asynchronous transfer modes and shared memory multiprocessor systems, (5) Process satellite data, (6) Optical computing in VLSI and many more.
Some current research and future scope
In recent years, high performance computing attracted much attention. Too much emphasize have been given to optimize all the resources of existing electronic computing to increase computing throughput. Optical computing looks promising to increase the computing power largely. Wirld wide different research group are working on optical computing [27-31].
The future expectations of optical computers include the development of photonic transistor, all optical logic gates, transistors in sub-atomic sizes, which can have much higher speeds than that of the current electronic ones.
The development of the photonic transistors, all optical logic gates, optical storage devices and their fabrication in low costs create the optical computer readily available in the future. Recent Researches and development show that this future is not so far. The dream of very high data rates of more than hexabytes per second can be achieved by the development of optical computers.
Our attempt on this article is to give a survey and update some of the recent and ongoing research and development on optical computing. We have sufficient hope that we can convince the reader about the richness of this emerging field.
As you can see, there is a huge potential and also large class of problems which have not been explored in much detail. Clearly optical computing has played an important role in past and ongoing research, but we expect them to have its greatest impact in the years to come.
We wish to thank Dr. Ashok K. Chauhan, Founder President, Amity University, India for his constant interest in research and encouragement. We are also thankful to Lt. Gen. P. D. Bhargava, Director General, AITTM and Deputy Vice Chancellor, Amity University, Noida, UP, India for motivation.