In the wireless access network, the circulator is mainly used to isolate the output signal from the input signal of the base station antenna. Circulators and isolators are a core device in 5G base stations, and with the large increase of 5G Massive MIMO, the application demand of optical fiber circulators will also increase significantly.
The main feature of optical fiber circulator is that it can realize bidirectional optical signal transmission on a single fiber.The signal transmission direction of the circulator is irreversible, and the optical signal can only be guided from one port to another in one direction at a time. Although the optical signal can be redirected, it must pass through the port in a sequence along one direction. For example, a three-port circulator optical signal must be transmitted from port 1 to port 2 and then to port 3.
The schematic diagram of the three-port circulator is shown below. If the signal is input from port 1, it is output from port 2. While the signal is input from port 2, it will be output from port 3, and the output loss is small. When light is input from port 2, it loses a lot when output from port 1, and when light is input from port 3, it loses a lot when output from port 1, 2.
Fiber optic circulators can be classified by the number of ports, and can also be classified by whether the polarization is related. According to the port can be divided into 3 ports, 4 ports and 6 ports. The 3-port and 4-port types are common. It can be divided into polarization-preserving circulators (PM) and polarization-independent circulators (PI). Polarizing fiber circulators are commonly used in polarizing areas such as 40Gbps high-speed systems or Raman pumping applications, dual-pass amplifiers and dispersion compensators (DCM). Polarization-independent fiber circulators, along with fiber grating and other reflective devices, are widely used in coarse wavelength division multiplexing (DWDM) systems, high-speed systems and bidirectional communication systems.
Optical fiber circulators are commonly used in passive optical networks, (wavelength division multiplexing) WDM networks, polarization mode dispersion, dispersion compensation, optical plug-in modules (OADM), optical amplifiers, optical time domain reflectometry (OTDR), and optical fiber sensing applications.
There are several applications for fiber optic circulators
FIG. 1 Basic application structure of optical circulator in transmission system
FIG. 2 Application of optical circulator in EDFA pump
Figure 3 DWM demultiplexing structure composed of optical circulator and fiber grating
FIG. 4 Application of optical circulator in dispersion compensation
According to the estimated budget, within the 5G construction cycle (2019-2025), the number of 5G macro base stations will be 1.3-1.5 times that of the 4G period, reaching 5 million stations in 2025, and the domestic 5G base station ring market space is expected to reach 11.5 billion yuan, and the peak market size will reach 3 billion yuan. Optical circulator is an important device in advanced optical network DWDM, with the increase of base stations, the circulator will also have a huge market space.
