An optical transistor is a device intended for the switching or amplification of optical signals. Light incident on an optical transistor’s input changes the intensity of light emitted from the transistor’s output which 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 which forms the basis of modern electronic devices. An optical transistor provides 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 outperform electronics in terms of speed and power consumption.
Since photons are inherently non-interacting particles, an optical transistor must employ an operating medium to mediate interactions between photons. This is done without the need for optical to electronic signal conversion as an intermediate step. Several implementations of optical transistors using a variety of operating mediums have been proposed and experimentally demonstrated. However, their potential 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 widely used to transfer data over large distances at high speed, tasks such as signal routing are done electronically. This requires optical-electronic-optical conversion leading to bottlenecks in communication networks. In principle, all-optical digital signal processing and routing is achievable using multiple optical transistors arranged into photonic integrated circuits. The same devices could also 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 binary digital computer in which components of a computer processor operate using photons rather than electrons (see: Optical computing). 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.
Comparison with electronics
The most commonly argued case for optical logic is that the 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.
In many implementations of optical transistors, it is questionable whether the designs can be optimized in such a way that the energy required to switch a single transistor is 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 the proposed single-photon transistors  for quantum information processing.
Perhaps the most significant advantage of optical logic over electronic logic is 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 realistically compete with existing 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 the following:
- Fan-out - An optical transistor’s output must be in the correct form and of sufficient power to operate the inputs of at least two transistors. This implies that the input and output wavelengths, beam shapes and pulse shapes must be compatible.
- Logic level restoration - The signal needs to be ‘cleaned’ by each transistor. Noise and degradations in signal quality must be removed so that they do not propagate through the system and accumulate to a errors.
- Logic level independent of loss - In optical communication, the signal intensity decreases over distance due to absorption of light in the fiber optic cable. Therefore, a simple intensity threshold cannot distinguish between on and off signals for arbitrary length interconnects. To make the transistors operate independent of loss, some other scheme is required such as encoding zeros and ones in different frequencies or, using differential signaling where the ratio or difference in two different powers carries the logic signal.
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
- electromagnetically induced transparency where the transmission of an optical cavity or a microresonator is controlled by a weaker flux of gate photons.
- a system of indirect excitons (composed of bound pairs of electrons and holes in double quantum wells with a static dipole moment) - indirect excitons, which are both created by light and decay to emit light, strongly interact due their dipole alignment and provide a means to mediate interactions between photons in an optical transistor.
- a system of microcavity polaritons (exciton-polaritons inside an optical microcavity) where, similar to the exciton based optical transistors, polaritons facilitate effective interactions between photons 
- photonic crystal cavities with an active Raman gain medium.
- silicon microrings placed in the path of an optical signal - gate photons heat the silicon microring causing a shift in the optical resonant frequency, leading to a change in transparency at a given frequency of the optical supply.
- Optical Network on Chip
- Optical interconnect
- Parallel optical interface
- Optical communication
- Optical fiber cable
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