What Is an Optical Circulator?

Andrew Kirmayer

A device used in optical communications systems, an optical circulator is built to pass light from one optical fiber to another. The movement occurs in the same direction the light is traveling, from the first port to the second, or the second to the third port. Most of these devices have three ports, and the incoming light beams can’t go back to other ports, which means the device is non-reciprocal. These optical devices are used to pass light from a unidirectional to a duplex fiber communication link and also in optical testing instruments such as optical time-domain reflectometers.

Woman holding a disc
Woman holding a disc

An optical circulator is similar to an optical isolator in that it is used to move light forward, although an isolator also works to prevent the light beam from reversing direction. There is typically some loss of light energy in the isolator, but the circulator directs all of the light to the output port and into the next optical fiber. Optical circulators typically have three ports, but many applications only require two, so they can be built to block any light that hits the third port.

Light rays in these devices can be redirected by using components such as a Faraday rotator. A spatial walk-off polarizer splits the light into horizontal and vertical beams. Light also passes through a half wave plate and all of the components in the circulator pass the beam toward the output port. The separated light rays in the optical circulator are recombined before getting to the exit port by a polarizing beamsplitter cube and a reflector prism, which condition the light for continuation through the next fiber.

When an optical circulator is used in a communication system, engineers don’t have to use the series of transmitters, receivers, and amplifiers that would otherwise be necessary. It is a more expensive component but fewer parts are needed and the fiber optic infrastructure ends up being more simple and reliable as well. A circulator, transmitter, and receiver can be built into the same device if need be.

A fiber optic system is also made more efficient because the optical circulator minimizes the loss of light. The beam goes in one direction with effective separation of the signals coming in and going out. Systems with isolators and beamsplitters tend to drain some of the light’s energy. An optical circulator is, however, an efficient means for conveying a light signal and makes the design of communication systems more economical.

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Discussion Comments


@nony - I worked in the telecommunications industry for ten years. I never got close to the fiber myself, but I did know that it was the efficient way to build out our network.

I agree with the article; you don’t need repeaters with this kind of technology. Repeaters in telecommunications are devices that repeat signals every so often to reduce the possibility of signal loss. Signals tend to degrade overtime and so you need to repeat and amplify them.

Fiber virtually eliminates that. The article rightly points out that there is some loss of light, but it’s nothing near the degradation you have with traditional telecommunications signals.

Fiber is the way to go. We have tons of underground unlit fiber in the nation, waiting to be activated and put to good use.


I’ve seen demonstrations of fiber optics in science shows but never understood all the details.

The only permanent impression that I’ve had is that light travels through the fiber, and that as fiber is bent, so is the light so to speak. That in and of itself is a fascinating thing to see.

If I understand the article correctly, the optical circulator is a traffic cop of sorts, routing light from one fiber to another. I’ve never seen that demonstrated in any of the science shows I’ve been to, because they usually demonstrate one fiber strand, not a network of fibers.

It makes sense however. I don’t suppose that a single strand of fiber would necessarily be very long, and so at some point the light would need to hop on to another fiber in the network.

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