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Anoptical circulator is a three- or four-portoptical device designed such thatlight entering any port exits from the next. This means that if light enters port 1 it is emitted from port 2, but if some of the emitted light is reflected back to the circulator, it does not come out of port 1 but instead exits from port 3. This is analogous to the operation of an electroniccirculator. Fiber-optic circulators are used to separate optical signals that travel in opposite directions in anoptical fiber, for example to achieve bi-directional transmission over a single fiber.[1] Because of their highisolation of the input and reflected optical powers and their lowinsertion loss, optical circulators are widely used in advancedfiber-optic communications andfiber-optic sensor applications.
Optical circulators arenon-reciprocal optics, which means that changes in the properties of light passing through the device are not reversed when the light passes through in the opposite direction. This can only happen when the symmetry of the system is broken, for example by an externalmagnetic field. AFaraday rotator is another example of a non-reciprocal optical device, and indeed it is possible to construct an optical circulator based on a Faraday rotator.
In 1965, Ribbens reported an early form of optical circulator that utilized aNicol prism with aFaraday rotator.[2] With the advent offiber andguided-wave optics, waveguide-integrable andpolarization-independent optical circulators were later introduced.[3][4][5] The concept was later extended tosilicon photonic waveguide systems.[6][7][8][9] In 2016, Scheucher et al. have demonstrated a fiber-integrated optical circulator whose nonreciprocal behavior originated from thechiral interaction between a single85Rb atom and the confined light in awhispering-gallery mode microresonator. The routing direction of the device is controlled by the internalquantum state of the atom and the device is able to routeindividualphotons.[10]
In 2013, Davoyan andEngheta proposed a nanoscaleplasmonic Y-circulator based on three dielectric waveguides interconnected with a magneto-optical junction with plasmonic nanorods.[11]