Fused fiber optic coupler is a kind of optical light distribution devices, technically based on Fused Biconical Taper (FBT) technology, so also called FBT coupler. As an important passive optical component in fiber optic communication system, the main functions of a fused fiber optic coupler is light branching and splitting in fiber access networks, wavelength multiplexing / de-multiplexing, filtering, polarization selective splitting and wavelength independent splitting.
A fused fiber optic coupler is a structure formed by two independent optical fibers. These two parallel optical fibers are twisted, stretched and fused together so that the coupling substantially takes place through interaction between the cladding modes. During the operation, the power output values form the output ports are monitored, and the process can be stopped at any desired coupling ratio. This process is known as the Fused Biconical Taper (FBT) process. The fused biconical taper is the most widely used method in make of optical fiber coupler, with many advantages of low excess loss, precise coupling ratio, good consistency and stability.
Before talking about the working principle of FBT couplers, we firstly understand the evanescent wave. An evanescent wave is a near-field wave with an intensity that exhibits exponential decay without absorption as a function of the distance from the boundary at which the wave was formed (Figure 2. The red tails are the evanescent wave). In the FBT process the cores of two identical parallel fibers are so close to one another that the evanescent wave can “leak” from one fiber core into the other core which allows an exchange of energy. The FBT couplers work as a result of energy transfer between the optical fiber cores and the energy transfer is dependent on the core separation (d) and the interaction length (L). It is easy to see that if the coupling length is long enough, a complete transfer of energy can take place from one core into the other. If the length is longer still, the process will continue, shifting the energy back into the original core.