What is Optical Circulator?

Optical Circulator

What is Optical Circulator?

It is a nonreciprocal device that directs an optical signal from one port to the next, in only one direction at a time. While the direction of the optical signal may be redirected as needed, the optical signal must pass through ports sequentially (i.e. from port 1, to port 2, before traveling to port 3). Additionally, optical circulator can be used to achieve bi-directional transmission over a single fiber. Because of its high isolation of the input and reflected optical powers and its low insertion loss, optical circulator is widely used in advanced communication systems and fiber optic sensor applications.

How does Optical Circulator Work?

Optical circulator operates in a similar way to optical isolator which also uses Faraday rotators, however, its construction is more complex. Its reverse propagating lightwave is directed to a third port for output, instead of being lost. Figure 1(a) shows a three-port optical circulator. An input signal (λ1) at Port 1 exits at Port 2, an input signal (λ2) at Port 2 exits at Port 3, and an input signal (λ3) at Port 3 exits at Port 1. In the same way, in a four-port optical circulator, as shown in Figure 1(b), one could ideally have four inputs and four outputs. In practise, many applications do not need four inputs and four outputs. Therefore, in a four-port circulator, it is common to have three input ports and three output ports. This is done by making Port 1 an input-only port, Port 2 and Port 3 input and output ports, and Port 4 an output-only port.

An optical circulator can be analogous to an electronic circulator, with similar functions. Optical circulators are three-port devices and light can only travel in one direction. The signal input from port 1 is output from port 2 with low loss; The signal input from port 2 is output from port 3 with low loss. The signal input from port 2 will have a large loss at port 1, and the signal input from port 3 will have a large loss at ports 2 and 1. Optical circulator is an irreversible optical device. This means that any property changes caused by light passing through the device will not be reversed when the light travels in reverse.

Because of its high isolation and low insertion loss, fiber optic circulators are widely used in advanced communication systems, such as forked multiplexers, bidirectional pumping systems and dispersion compensation devices.

The following figure depicts the separation of optical channels from a DWDM system using a circulator and Fiber Bragg grating (FBG). The DWDM input signal is coupled from port 1 of the device, and the FBG device is connected to port 2. The single wavelength reflected in the FBG re-enters the circulator from port 2 and then goes to port 3. The rest of the signal is output from the top fiber through the FBG.

Circulators can also be used to send optical signals bidirectional through a single fiber. Each end of the optical fiber is connected with a ring. Each circulator adds a signal in one direction and eliminates it in the other. See the diagram below.