What’s Fused Fiber Optic Coupler
Fused fiber optic coupler is a kind of fiber optic couplers, which is formed based on Fused Biconical Taper (FBT) technology. Thus, it is also known as FBT coupler. As an important passive component in fiber optic communication systems, the perform functions of fused fiber optic coupler include light branching and splitting in passive networks, wavelength multiplexing / de-multiplexing, filtering, polarization selective splitting and wavelength independent splitting.
Fused Biconical Taper (FBT) Process
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 (Figure 1). This process is known as the Fused Biconical Taper (FBT) process. The fused biconical taper is the most widely used menthod in facture of optical fiber coupler, with many advantages of low excess loss, precise coupling ratio, good consistency and stability.
Figure 1. The Fused Biconical Taper process
How Does Fused Fiber Optic Coupler Work
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.
Figure 2. Light propagating down an optical fiber. The red region represents the evanescent wave.
For example, here is a 2×2 50/50 coupler (Figure 3.), assume that we launch 1mW into port 1 and 1mW into port 4. Obviously, we will measure 1mW at each output port, the light form each input port having split into two equal parts. In other word, if we launch 1mW into port 1 and 2mW into port 4, each path gets split into two equal parts again, so now we end up with 1.5 mW at each output port (0.5 mW contribution from port 1 and 1 mW contribution from port 4). Similarly, if it is a 1 x2 coupler and we launch 2 mW into port 1, we will end up with 1 mW at ports 2 and 3.
Figure 3. 1×2 or 2×2 50/50 coupler
As we know, optical fiber couplers allow bi-directional coupling and can be used to either split or combine signals. This is what we call “Bidirectionality”. Through the above example, we may have an idea of reversing the launch direction on a 2×2 standard coupler. In fact, the process is completely bi-directional. However, confusion arises sometimes when presented with a 1×2 coupler. The apparent non-symmetry of the device creates the false impression that the device somehow works differently. Continuing the 1×2 coupler, what happens if light is launched into one of the two “output” ports (ie. ports 2 or 3 above)? Does 100% of the light exit at port 1? I am so sorry to tell you the answer is not. Light still “wants” to exit from port 4 as well. So if we consider 1 mW launched into port 2, we will have just 0.5 mW exiting from port 1. Or, if we launch 1 mW into port 2 and 2 mW into port 3, we will have 1.5 mW exiting from port 1. Why? Because a 1×2 coupler is just a 2×2 coupler with one fiber cut short, crushed (to reduce back reflection from the end facet), and tucked away inside the housing of the coupler. In this case we can easily understand how a fused fiber optic coupler works.
Types of Fused Fiber Optic Coupler
Fused fiber optic couplers should be selected based on the window type or fiber type. Regardless of the port types used, fiber optic couplers can be designed for single window, dual window or even three window (wideband).
In addition, according fiber types, there are couplers with 1×2, 1×3, 1×4, 1×5, 1×6, 1×8, 1×12, 1×16, 1×18, 1×20, 1×24, 2×2, 2×4 configurations with single mode or multimode fiber.
Applications of Fused Fiber Optic Coupler
As an important passive components in fiber optic communication systems, there is a wide range of application of FBT couplers. In addition, with its advantage of small size, FBT coupler is available individually or integrated into modules for fiber protection switching, MUX/DMUX, optical channel monitoring, and add/drop multiplexing applications.