A fiber optic coupler is an optical component that is widely used for distributing optical signals over the network. It is designed to distribute signals from one fiber to two or more fibers. In general, optical signals are attenuated more when used in an optical coupler. It is because of the fact that the input signal is not transmitted from one fiber to another directly but divided among different output ports.
When it comes to defining an optical fused coupler specifically, it is important to understand that it is made of two parallel optical fibers that are twisted, stretched, and fused together to ensure that their cores stay in close proximity.
How does an optical fused coupler work?
The intensity profile of optical signals traveling in a single mode (SM) fiber is said to be Gaussian. Meaning, the intensity of light is greatest at the center and tapers off as the core or cladding interface ends. The rear ends of the Gaussian profile slightly goes further across the core and into the cladding. This extended tail at both ends is known as an evanescent wave.
In an optical fused coupler, the cores of two identical parallel fibers are so close that the evanescent wave can leak from one fiber core to the core of another fiber. This, in turn, allows an exchange of energy which is similar to the energy exchange that takes place in two coupled pendulums.
The amount of energy that gets exchanged varies depending on the closeness of two fused cores and the length over which energy exchange occurs. If the coupling length is long enough, complete energy may transfer from one core to another. If the length is even longer, the process will continue, transferring the energy back into the original core. Hence, with the selection of proper length over which energy exchange occurs, manufacturers can achieve any given power transfer ratio.
When the light is launched into an input port during the manufacturing process of a fused optical coupler, the output power that comes out of each output port is rigorously monitored. When the desired coupling ratio is achieved, the fully automated manufacturing process is also stopped. This results in a coupler made of one fiber with two cores that lie very close to each other. This process is called the Fused Biconical Taper (FBT) process.
Depending on the type of optical fused coupler, it is used in a variety of applications, such as CATV systems, optical fiber communication systems, testing instruments, FTTH and LAN optical networks, digital, hybrid, and AM-video systems, fiber sensors, mini EDFA, and small transmitter/receiver modules.