Optical fiber coupler is a kind of optical fiber passive device used for transmitting and distributing optical signal. Optical fiber couplers generally have the following characteristics: First, the device is composed of optical fiber, which is an all-fiber device; second, the demultiplexing and combining of the optical field are mainly realized by mode coupling; third, the optical signal transmission is directional.

According to the coupling principle of light, a variety of fiber coupler structures have been designed. Including: X-type fiber coupler, star fiber coupler, double-clad fiber coupler, fiber grating coupler, long-period fiber grating coupler, Bragg fiber coupler, photonic crystal fiber coupler, etc.

With the wide use of various optical fiber communication and optical fiber sensing devices, the status and role of fiber optic couplers are becoming more and more important, and have become an indispensable part of the field of optical fiber communication and optical fiber sensing. Designing a fiber coupler with low insertion loss, high coupling efficiency, adjustable splitting ratio and special coupling has always been the focus of researchers in the field of optics and the goal of industry insiders.

Optical fiber coupler structure and principle analysis
The fused cone method is the most common technique for making couplers. The fused taper type fiber coupler removes the coating layer from two (or more than two) optical fibers, moves them closer together in a certain way, and fuses them under high temperature heating. Side stretching, a special waveguide structure in the form of a double cone is formed in the heating area to realize optical power coupling, and the length of the stretching cone coupling area can be controlled to control the power coupling ratio (splitting ratio) of the two ports. The fused taper and taper fiber coupler is shown in Figure 5.2.2.

star coupler
A star coupler refers to a coupler with an N×N type of input and output ports. The star coupler can be formed by twisting multiple fibers, heating and melting them. For single-mode fibers, this multi-core fused-cone star A star coupler needs to precisely adjust the coupling between multiple fibers, which is difficult, so another splicing method is usually used to construct an N×N star coupler. As shown in Figure 5.2.3, four 2×2 basic units can be used to form a 4×4 coupler, 12 2×2 basic units can be used to form an 8×8 coupler, and 8 4×4 basic units can be used to form 16 ×16 coupler, etc.

N×N Star Coupler are quite useful in high-speed, multiple-access optical networks. FIBERWDM can support provide 8X8 star fiber coupler, witch having multiple 8 channels input and 8 channels output ports. It is used for uniformly distributing each input signal to the output ports.

tree coupler
When the tree-shaped coupler is used, the main input and output ports have a 1×N-type coupler. This coupler is mainly used for optical power distribution occasions, and is used in the optical distribution network in the access network. Using a similar method, 1×2 or 2×2 couplers can be spliced successively to form 1×N or 2×N, and the splicing scheme is shown in Figure 5.2.4.