What is a Fiber Optic Circulator?

A Fiber Optic Circulator is a three or four port optical device that directs the flow of an optical signal from an input port to an output port in a manner that is not reciprocal. For example, if a 4-port circulator is used, a signal introduced through Port 1 exits through Port 2, and the signal introduced through Port 2 exits through Port 3. Similarly, a signal introduced through Port 3 exits through Port 4 and if introduced to Port 4, it exits through Port 1.

Fiber optic circulators are employed to separate optical signals that move in opposite directions within an optical fiber. This is done, for example, to enable bi-directional transmission over a single fiber. Due to their exceptional capability to isolate input and reflected optical powers, along with their low insertion loss, these devices find extensive use in advanced communication systems and applications involving fiber-optic sensors.

Optical circulators are classified as non-reciprocal optics, implying that alterations in the characteristics of light that traverse the device are not reversed when the light is passed through in the opposite direction. The only way this can happen is if the symmetry of the system is disturbed, such as through an external magnetic field. Another instance of a non-reciprocal optical device is a Faraday rotator, and it is possible to build an optical circulator by utilizing a Faraday rotator.

Optical Circulator in a bidirectional transmission system

In a bidirectional system, optical circulators play an important role in routing incoming and outgoing signals without interference or loss of signal. A typical setup involves two circulators and a fiber. The incoming signal is fed into the Port 1 of the first circulator, where it is directed to Port 2. From here, the signal travels through the fiber to the Port 2 of the second circulator. Then the outgoing signal is directed to the Port 3 of the second circulator. This non-reciprocal nature of the circulators ensures that the incoming and outgoing signals are routed to their respective ports without interfering with each other. This arrangement of circulators and fiber allows for efficient bidirectional transmission without loss of signal or interference.

Optical Circulator with a reflective erbium-doped fiber amplifier

At a wavelength of 1550 nm, the signal light is fed into the system through Port 1, where it travels through Port 2 with negligible loss. Following this, the signal light is combined with the pump light, operating at a wavelength of 980 nm, using a WDM coupler. Both lights are then launched into an erbium-doped fiber, where the signal is amplified. The signal and residual pump light are reflected by a mirror and pass through the erbium-doped fiber again, amplifying the signal further. At the WDM coupler, the signal light at 1550 nm passes through, while the pump light at 980 nm is directed into the pump laser (and absorbed by a built-in isolator). Finally, the signal light is guided into Port 3 by the circulator.

By using a double pass through the erbium-doped fiber, the length of the required fiber is decreased. Additionally, the unused pump power is reused and employed to boost the pump efficiency.

Types of Optical Circulators

Polarization-Dependent Circulator: A polarization-dependent circulator is a device that operates effectively only with light having a specific polarization state. Ordinary optical fibers do not maintain the polarization state of light due to fiber imperfections, thereby limiting the applications of polarization-dependent circulators. These devices are used in certain applications like free-space communication between satellites and optical sensing. Also, other devices, such as optical modulators and nonreciprocal devices, exhibit polarization-dependent traits that result in a reduced coupling efficiency of optical fibers.

Polarization-Independent Circulator: A polarization-independent circulator is necessary to enhance the optical power transferred from optical fibers as the light usually has different polarization components. Unlike polarization-dependent circulators, these devices operate independently of the polarization state of light. A four-port polarization-independent circulator that employs ferrite materials is designed to achieve lower losses and higher light efficiency compared to other four-port circulators. 

By appropriately selecting the input and output port sizes, connection area size, and ferrite material's shape and dimensions at the center of the circulator, the electromagnetic wave is directed in the desired direction in the four-port ferrite optical circulator. Polarization-independent circulators find extensive use in fiber optic telecom networks.

Full Circulator: A full circulator is a device that allows light to pass through all ports in a complete circle. This means that light from the last port is transmitted back to the first port, completing a full loop. The circular transmission path is established by using a combination of magnetic fields and waveguides. Full circulators are useful in various applications, including optical communication systems and radar systems. They enable efficient routing of signals and reduce the need for additional optical components.

Quasi-Circulator: A quasi-circulator is a device that allows light to pass through all ports sequentially, but unlike a full circulator, the light from the last port is lost and cannot be transmitted back to the first port. This is due to the absence of a circulator's circular transmission path, which is typically established using magnetic fields and waveguides. Quasi-circulators are used in various applications such as in fiber-optic sensors and measurement systems.

Applications of Optical Circulator

Telecommunication systems use optical circulators to improve the transmission capacity of existing networks. A simple approach to double the capacity of a bidirectional transmission system is to incorporate optical circulators. These devices are also used to extract optical signals from reflective apparatus. When combined with a mirror, they can efficiently transmit optical elements in a double pass, thus amplifying the efficacy of reflective erbium-doped fiber amplifiers (EDFAs).

Optical circulators are also used for add-drop multiplexing to add or drop wavelengths. Optical circulators find application in fiber sensors, bidirectional pumping, bidirectional signal transmission systems, and coupling in-line chromatic dispersion devices.