Optical computing or photonic computing uses light waves produced by lasers or incoherent sources for data processing, data storage or data communication for computing. For decades, photons have shown promise to enable a higher bandwidth than the electrons used in conventional computers (see optical fibers). Most research projects focus on replacing current computer components with optical equivalents, resulting in an optical 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. However, optoelectronic devices consume 30% of their energy converting electronic energy into photons and back; this conversion also slows the transmission of messages. All-optical computers eliminate the need for optical-electrical-optical (OEO) conversions, thus reducing electrical power consumption. Application-specific devices, such as synthetic-aperture radar (SAR) and optical correlators, have been designed to use the principles of optical computing. Correlators can be used, for example, to detect and track objects, and to classify serial time-domain optical data.
Optical components for binary digital computer The fundamental building block of modern electronic computers is the transistor. To replace electronic components with optical ones, an equivalent optical transistor is required. This is achieved by crystal optics (using materials with a non-linear refractive index). In particular, materials exist where the intensity of incoming light affects the intensity of the light transmitted through the material in a similar manner to the current response of a bipolar transistor. Such an optical transistor can be used to create optical logic gates, which in turn are assembled into the higher level components of the computer's central processing unit (CPU). These will be nonlinear optical crystals used to manipulate light beams into controlling other light beams. Like any computing system, an optical computing system needs four things to function well:
optical processor optical data transfer, e.g. fiber-optic cable optical storage, optical power source (light source) Substituting electrical components will need data format conversion from photons to electrons, which will make the system slower.
Controversy Researchers dispute the future capabilities of optical computers; whether they will ultimately be able to compete with electronic computers in terms of speed or power consumption is currently unclear. Critics note that real-world logic systems require "logic-level restoration, cascadability, fan-out and input–output isolation", all of which are provided by electronic transistors at low cost, low power, and high speed. For optical logic to be competitive beyond niche applications, major breakthroughs in non-linear optical device technology would be required, or perhaps a change in the nature of computing itself.
Challenges A significant challenge to optical computing is that computation is a nonlinear process in which multiple signals must interact. Light, an electromagnetic wave, can interact with another electromagnetic wave only in the presence of electrons in a material, and the strength of this interaction is much weaker for electromagnetic waves, such as light, than for the electronic signals in a conventional computer. This may require processing elements with more power and larger dimensions than those for a conventional electronic computer. Since light can travel much faster than the drift velocity of electrons, and at frequencies measured in THz, optical transistors should be capable of extremely high frequencies. However, any electromagnetic wave must obey the transform limit, and therefore the rate at which an optical transistor can respond to a signal is limited by its spectral bandwidth. In fiber-optic communications, practical limits such as dispersion often constrain channels to bandwidths of tens of GHz, only slightly better than many silicon transistors. Obtaining dramatically faster operation than electronic transistors therefore requires practical methods of transmitting ultrashort pulses down dispersive waveguides.
Photonic logic
Photonic logic is the use of photons (light) in logic gates. Switching is obtained using nonlinear optical effects when two or more signals are combined. Resonators are especially useful in photonic logic, since they allow build-up of energy from constructive interference, thus enhancing optical nonlinear effects. Other approaches that have been investigated include photonic logic at a molecular level, using photoluminescent chemicals. Witlicki et al. demonstrated logical operations using molecules and SERS.
Unconventional approaches
Time delay The basic idea is to delay a signal in order to perform useful computations. Of interest would be to solve NP-complete problems as those are difficult problems for conventional computers. Two basic properties of light are used in this approach:
Light can be delayed by passing it through an optical fiber. Light can be split into multiple rays. This property allows multiple solutions to be evaluated concurrently. Solving a problem with time-delays involves the following steps:
Create a graph-like structure made from optical cables and splitters. Each graph has a start node and a destination node. Light enters through the start node and traverses the graph until it reaches the destination. It is delayed when passing through arcs and divided inside nodes. Light is marked when passing through an arc or through a node to identify that fact at the destination node. The destination node waits for a signal (fluctuation in the intensity of the signal) which arrives at a particular moment in time. If no signal arrives at that moment, it means no solution was found. Otherwise the problem has a solution. Fluctuations can be read with a photodetector and an oscilloscope. The first problem attacked in this way was the Hamiltonian path problem. The simplest problem is the subset sum problem. An optical device solving an instance with four numbers {a1, a2, a3, a4} is depicted below:
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