Working principle, definition, characteristics and application fields of fiber optic circulator

With the surge in the density of 5G base stations and the accelerated implementation of quantum communications, it has become a rigid demand for a single optical fiber to transmit bidirectional signals at the same time. The core device that achieves this breakthrough is the fiber optic circulator, which is called the “one-way valve” of optical communication. It is like a precise traffic commander, allowing optical signals to strictly follow the one-way traffic rules in complex networks, and building a technical barrier for the efficient operation of optical communication systems.

Definition of fiber optic circulator:

Fiber optic circulator is a non-reciprocal optical device based on the Faraday magneto-optical effect, and its core feature is the unidirectional conductivity between ports. Taking a three-port circulator as an example, when the optical signal is input from port 1, it can be transmitted to port 2 with low loss; when the signal is input from port 2, it can only be transmitted unidirectionally to port 3. The isolation of the reverse transmission path (such as port 2→1 or port 3→2) generally exceeds 30dB, which is equivalent to attenuating the reverse signal strength to less than one thousandth. This feature distinguishes it from ordinary optical couplers and makes it a key component for signal isolation and routing in optical networks.

 Working principle of fiber circulator:

The unidirectional conduction mechanism of the circulator originates from the synergy of the Faraday rotator and the polarization beam splitter. When linearly polarized light enters the circulator, it first passes through the polarizer to form a beam with a specific polarization direction. Subsequently, the Faraday rotator causes the light polarization plane to rotate irreversibly (usually 45°) under the action of the magnetic field. This rotation angle has nothing to do with the direction of light propagation and is only determined by the magnetic field strength and material properties. When the rotated light passes through the polarization beam splitter, its polarization state matches the transmission axis of the beam splitter and is thus guided to the next port. During reverse propagation, due to the non-reciprocity of the Faraday effect, the polarization plane of the light rotates in the opposite direction, resulting in the polarization state being misaligned with the transmission axis of the beam splitter and finally being reflected to the isolation port.

Characteristics and advantages of fiber circulator:

1. Ultra-low transmission loss: Modern circulators use aspheric lens collimation technology and air gap packaging to control the insertion loss below 0.5dB. In Huawei’s latest 5G fronthaul solution, the measured loss of its circulator at a wavelength of 1310nm is only 0.32dB, which is 40% higher than that of traditional devices.

2. Excellent isolation performance: By optimizing the angle matching between the Faraday rotator and the polarization beam splitter, the isolation exceeds the 50dB mark. In the quantum key distribution system, this high isolation characteristic can effectively suppress Rayleigh backscattering noise and extend the secure key distribution distance to more than 500 kilometers.

3. Precise control of polarization state: The polarization-maintaining circulator adopts PANDA fiber and stress-induced birefringence design, and the polarization extinction ratio (PER) exceeds 25dB. In coherent optical communication systems, this feature ensures the polarization matching between the local oscillator light and the signal light, and improves the receiving sensitivity by 3dB.

Application areas of fiber circulators:

1. 5G base station optical module: In the Massive MIMO antenna system, the circulator realizes the isolation of the transmit signal and the receive signal. The measured data of a certain operator showed that after the use of circulators, the noise coefficient of the base station uplink was reduced by 1.2dB and the coverage radius was increased by 15%.

2. DWDM network: The demultiplexer is constructed in conjunction with the fiber Bragg grating (FBG), which can separate the 100G signal into 10 10G channels. In a backbone network transformation project of China Telecom, the circulator array reduced the equipment volume by 60%, and the insertion loss fluctuation was controlled within ±0.1dB.

3. Quantum communication: In the quantum repeater, the circulator constructs the photon routing channel, and its low polarization-dependent loss characteristics ensure the fidelity of the entangled photon pairs. After the latest quantum satellite ground station of KUST Guodun adopted the circulator, the bit error rate of the satellite-to-ground link was reduced from 10⁻³ to 10⁻⁶.

4. Fiber optic sensing: In the distributed strain monitoring system, the circulator is combined with the Raman scattering module to achieve a measurement accuracy of 0.1με at a sensing distance of 100km. A PetroChina oil pipeline monitoring project showed that this solution reduced costs by 40% compared to traditional OTDR systems.