An optical fiber coupler is a kind of optical fiber passive device used to transmit and distribute optical signals. It is one of the most used optical passive devices in optical fiber systems, and it occupies a pivotal position in the field of optical fiber communication and optical fiber sensing. Fiber couplers generally have the following characteristics: First, the device is composed of optical fibers, which are all-fiber devices; the second is that the demultiplexing and multiplexing of the optical field are mainly realized through mode coupling; the third is that the optical signal transmission is directional.
According to the principle of light coupling, many kinds 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 widespread use of various optical fiber communications and optical fiber sensor devices, the position and function of optical fiber couplers have become more and more important, and they have become an indispensable part of the field of optical fiber communications and optical fiber sensing. Designing fiber couplers with low insertion loss, high coupling efficiency, adjustable splitting ratio, and special coupling has always been the focus pursued by scientific researchers in the optical field and the goal of the industry.


The fusion taper method is the most common technology for making couplers. The fusion taper optical fiber coupler removes the coating of two (or more than two) optical fibers. Side stretching, a special waveguide structure in the form of a double cone is formed in the heating zone to realize optical power coupling. Controlling the length of the stretching cone coupling zone can control the power coupling ratio (split ratio) of the two ports.
Star-shaped coupler
The star coupler refers to a coupler with N×N input and output ports. The star coupler can be formed by twisting multiple optical fibers and heating and melting the taper. For single-mode optical fibers, these multi-core fused-taper star-shaped couplers need to accurately adjust the coupling between multiple optical fibers, which is very difficult, so another splicing method is usually used to construct N×N star couplers. As shown in Figure 5.2.3, 4 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 a 16 ×16 coupler, etc.
Tree-shape couplers
In the case of tree couplers, the main input and output ports have a 1×N type coupler. This type of coupler is mainly used for optical power distribution occasions and is used for optical distribution networks in access networks. Using a similar method, 1×2 or 2×2 couplers can be spliced successively to form 1×N or 2×N.

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