An optical switch is a device that selectively switches optical signals on or off or from one channel to another. The former is known as an optical (time-domain) switch or an optical modulator, while the latter is called an optical space switch or an optical router. Since the switching can be temporal or spatial, such switches are analogous to one-way or two-way switches in electrical circuits. In general, optical modulators and routers can be made from each other.
The word applies on several levels. In commercial terms (such as "the telecom optical switch market size") it refers to any piece of circuit switching equipment between fibers. The majority of installed systems in this category actually use electronic switching between fiber transponders. Systems that perform this function by routing light beams are often referred to as "photonic" switches, independent of how the light itself is switched. Away from telecom, an optical switch is the unit that actually switches light between fibers, and a photonic switch is one that does this by exploiting nonlinear material properties, such as semiconductor-based materials, to steer light (i.e., to switch wavelengths, intensities, or directions) . Hence a certain portion of the optical switch market is made up of photonic switches. These will contain within them an optical switch, which will, in some cases, be a photonic switch.
An optical switch may operate by mechanical means, such as physically shifting an optical fiber to drive one or more alternative fibers, or by electro-optic effects, magneto-optic effects, or other methods. Slow optical switches, such as those using moving fibers, may be used for alternate routing of an optical switch transmission path, such as routing around a fault. Fast optical switches, such as those using electro-optic or magneto-optic effects, may be used to perform logic operations; also included in this category are semiconductor optical amplifiers, which are optoelectronic devices that can be used as optical switches and be integrated with discrete or integrated microelectronic circuits.
The functionality of any switch can be described in terms of the connections it can establish. As stated in Telcordia GR-1073, a connection is the association between two ports on a switch and is indicated as a pair of port identifiers (i, j ), where i and j are two ports between which the connection is established. A connection identifies the transmission path between two ports. An optical signal can be applied to either one of the connected ports. However, the nature of the signal emerging at the other port depends on the optical switch and the state of the connection. A connection can be in the on state or the off state. A connection is said to be in the on state if an optical signal applied to one port emerges at the other port with essentially zero loss in optical energy. A connection is said to be in the off state if essentially zero optical energy emerges at the other port.
Connections established in optical switches can be unidirectional or bidirectional. A unidirectional connection only allows optical signal transmission in one direction between the connected ports. A bidirectional connection allows optical signal transmission in both directions over the connection. Connections in passive and transparent optical switches are bidirectional, i.e., if a connection (i, j ) is set up, optical transmission is possible from i to j and from j to i.
A device is optically "transparent" if the optical signal launched at the input remains optical throughout its transmission path in the device and appears as an optical signal at the output. Optically transparent devices operate over a range of wavelengths called the passband.
A passive optical switch does not have optical gain elements. An active optical switch has optical gain elements. An all-optical switch is a transparent optical switch in which the actuating signal is also optical. Thus, in an all-optical switch, an optical signal is used to switch the path another optical signal takes through the switch.
Various parameters are defined and specified to quantify the performance of optical switches. The steady state performance of an optical switch (or optical switching matrix) is measured by its ability to effectively transmit optical power from an input port to any one of N output ports over the "on" state transmission path, and its ability to effectively isolate input power sources from all non-active ports over the "off" state transmission paths. Other key optical performance parameters include transmission efficiency over a range of wavelengths, the ability to minimize input optical power reflected back into the input fiber, transmission balance, and bidirectional transmission. The optical switch (or switching matrix) transient behavior is another important characteristic that is specified by its speed of response to control stimulation via the time interval it takes to either transmit or block the optical signal on any given output port.
Two rates can be associated with switches: the switching rate and the signal transmission rate. The switching rate is the rate at which a switch changes states. The signal transmission rate is the modulation rate of information passing through a switch. The signal transmission rate is usually much greater than the switching rate. (If the switching rate approaches or exceeds the transmission rate, then the switch can be called an optical modulator.)
A switch's ability to sustain its steady state and transient performance specifications under stressful environmental conditions and over time is also an important characteristic.
Optical switching technology is driven by the need to provide flexibility in optical network connectivity. Prime applications are optical protection, test systems, remotely reconfigurable add-drop multiplexers, and sensing. Possible future applications include remote optical provisioning and restoration.
Current switching applications include passive protection switching for service restoration following a disruption, such as a fiber cut. One common application for switches is in Remote Fiber Test Systems (RFTSs) that can monitor and locate a fault on a fiber transmission line. An emerging application of optical switches is optical cross-connection. Optical cross-connects utilize optical switching fabrics to establish an interconnection between multiple optical inputs and outputs.
A 2011 search on "optical switch"  yielded some 8,000 patents, roughly categorized as follows: