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What is Optical Circulator in Optical Communication?


Optical circulator is a multi-port optical device with nonreciprocal property. It is based on the nonreciprocal polarization of an optical signal by Faraday effect. When an optical signal is input from any port, it can be output from the next port sequentially with very low loss, and the loss from this port to all other ports is very large, so these ports are not communicating with each other.


That means that optical circulator is a three- or four-port optical device designed such that light entering any port exits from the next. 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. 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.


Structure Principle
It consists of a Faraday rotator and two polarizing prisms on both sides. When polarized light passes through a Faraday rotator, its polarization plane can rotate 45°under the action of an external magnetic field. As long as the optical axes of the two polarizing prisms are set at an appropriate angle to each other, the insertion loss of the inter-connected optical paths can be very low and the isolation of the disconnected optical path is very large.


The optical circulator can also be formed by utilizing the characteristics of the single-mode fiber will produce the Faraday rotation effect under the action of an external magnetic field. The insertion loss and isolation of the polarization-independent optical circulator are independent of the polarization state of the incident light.


Technical Parameters
The technical parameters of optical circulator include insertion loss, isolation, crosstalk, polarization dependent loss(PDL), polarization mode dispersion(PDM) and return loss, etc. The definitions of insertion loss, isolation, polarization dependent loss and polarization mode dispersion of optical circulators are basically the same as those of optical isolators, except that for an optical circulator, it refers to a specific index between two adjacent ports.


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