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.
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.