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An optical transistor is a device that switches or amplifies optical signals. Light incident on an optical transistor’s input changes the intensity of light emitted from the transistor’s output. Output power is supplied by an additional optical source. Since the input signal intensity may be weaker than that of the source, an optical transistor amplifies the optical signal. The device is the optical analog of the electronic transistor that forms the basis of modern electronic devices. Optical transistors provide a means to control light using only light and has applications in optical computing and fiber-optic communication networks. Such technology has the potential to exceed the speed of electronics, while consuming less power.
Since photons inherently do not interact with each other, an optical transistor must employ an operating medium to mediate interactions. This is done without converting optical to electronic signals as an intermediate step. Implementations using a variety of operating mediums have been proposed and experimentally demonstrated. However, their ability to compete with modern electronics is currently limited.
Optical transistors could be used to improve the performance of fiber-optic communication networks. Although fiber-optic cables are used to transfer data, tasks such as signal routing are done electronically. This requires optical-electronic-optical conversion, which form bottlenecks. In principle, all-optical digital signal processing and routing is achievable using optical transistors arranged into photonic integrated circuits. The same devices could be used to create new types of optical amplifiers to compensate for signal attenuation along transmission lines.
A more elaborate application of optical transistors is the development of an optical digital computer in which components process photons rather than electrons. Further, optical transistors that operate using single photons could form an integral part of quantum information processing where they can be used to selectively address individual units of quantum information, known as qubits.
The most commonly argued case for optical logic is that optical transistor switching times can be much faster than in conventional electronic transistors. This is due to the fact that the speed of light in an optical medium is typically much faster than the drift velocity of electrons in semiconductors.
Optical transistors can be directly linked to fiber-optic cables whereas electronics requires coupling via photodetectors and LEDs or lasers. The more natural integration of all-optical signal processors with fiber-optics would reduce the complexity and delay in the routing and other processing of signals in optical communication networks.
It remains questionable whether optical processing can reduce the energy required to switch a single transistor to be less than that for electronic transistors. To realistically compete, transistors requiring a few tens of photons per operation are required. It is clear, however, that this is achievable in proposed single-photon transistors  for quantum information processing.
Perhaps the most significant advantage of optical over electronic logic is reduced power consumption. This comes from the absence of capacitance in the connections between individual logic gates. In electronics, the transmission line needs to be charged to the signal voltage. The capacitance of a transmission line is proportional to its length and it exceeds the capacitance of the transistors in a logic gate when its length is equal to that of a single gate. The charging of transmission lines is one of the main energy losses in electronic logic. This loss is avoided in optical communication where only enough energy to switch an optical transistor at the receiving end must be transmitted down a line. This fact has played a major role in the uptake of fiber optics for long distance communication but is yet to be exploited at the microprocessor level.
Besides the potential advantages of higher speed, lower power consumption and high compatibility with optical communication systems, optical transistors must satisfy a set of benchmarks before they can compete with electronics. No single design has yet satisfied all these criteria whilst outperforming speed and power consumption of state of the art electronics.
The criteria include:
Several schemes have been proposed to implement all-optical transistors. In many cases, a proof of concept has been experimentally demonstrated. Among the designs are those based on: