Optical or photonic computing uses photons produced by lasers or diodes to calculate them. For decades, photons have promised to allow greater bandwidth than electrons used in conventional computers. Most research projects focus on replacing current computer components with optical equivalents, resulting in an optical digital binary data processing system. This approach seems to offer the best short-term prospects for commercial optical computing as optical components could be integrated into traditional computers to produce an optical-electronic hybrid. However, optoelectronic devices lose 30% of their energy which converts electronic energy into photons and back; this conversion also slows down the transmission of messages. Fully optical equipment eliminates the need for optical-optical-electric (OEO) conversions, thus reducing the need for electrical power.
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 optical data in series over time. The fundamental building block of modern electronic computers is the transistor. To replace electronic components with optics, an equivalent optical transistor is required. This is achieved by using materials with a non-linear refractive index. In particular, there are materials in which the incoming light intensity affects the intensity of the light transmitted through the material in a manner similar 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 top-level components of the computer's CPU. These will be nonlinear optical crystals used to manipulate light beams in the control of other light beams.