An optical circulator is a three- or four-port
optical device designed such that
light 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 electronic
circulator. Fiber-optic circulators are used to separate optical signals that travel in opposite directions in an
optical fiber, for example to achieve bi-directional transmission over a single fiber.[1] Because of their high
isolation of the input and reflected optical powers and their low
insertion loss, optical circulators are widely used in advanced
communication systems and
fiber-optic sensor applications.
Optical circulators are non-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 external
magnetic field. A
Faraday 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.
History
In 1965, Ribbens reported an early form of optical circulator that utilized a
Nicol prism with a
Faraday rotator.[2] With the advent of
fiber and
guided-wave optics, waveguide-integrable and
polarization-independent optical circulators were later introduced.[3][4][5] The concept was later extended to
silicon photonic waveguide systems.[6][7][8][9] In 2016, Scheucher et al. have demonstrated a fiber-integrated optical circulator whose nonreciprocal behavior originated from the
chiral interaction between a single
85Rb atom and the confined light in a
whispering-gallery mode microresonator. The routing direction of the device is controlled by the internal
quantum state of the atom and the device is able to route
individualphotons.[10]
In 2013, Davoyan and
Engheta proposed a nanoscale
plasmonic Y-circulator based on three dielectric waveguides interconnected with a magneto-optical junction with plasmonic nanorods.[11]