What are Fiber Optic Couplers?

A fiber optic coupler is an optic component that allows the redistribution of optical signals. A fiber optic coupler is can distribute the optical signal from one fiber among two or more fibers, or combine the optical signal from two or more fibers into a single fiber. The device allows the transmission of light waves through multiple paths. It covers a range of fiber optic devices such as optical splitters, optical combiners, and optical couplers. Combiners combine two signals and provide one output. While splitters supply two outputs while making use of one optical signal. Usually, optical signals are attenuated more in an optical coupler than in a connector or a splice because the input signal is not directly transmitted from one fiber to another, but divided among the output ports. An external power source is required for active fiber optic couplers, whereas no power is required for the operation of passive fiber optic couplers.

There are many benefits of using fiber optic couplers. They have a low excess loss, high stability, dual operating window, high reliability, and low polarization-dependent loss. They also have high directivity and low insertion loss.

Fiber Optic Coupler Types: If we see optical couplers by shape, there is a Y coupler, T coupler, X coupler, star coupler, and tree coupler, which split the optical signal based on the power as described below.

Y Coupler

Y coupler is also called a tap coupler. This type of coupler simply divides the signal into two outputs. The power distribution ratio between two outputs can be precisely controlled, such as 10/90 percent, 20/80 percent, 30/70 percent, 40/60 percent, or 50/50 percent.

T Coupler

The T coupler functions the same as the Y coupler. T couplers can be used to connect multiple terminals on a network, as shown in the figure above. The split ratio between two outputs should be 10/90 percent or 20/80 percent to have enough power left for the next terminal in the link.

X Coupler

X couplers carry out the function of a splitter and a combiner in one package. The X coupler combines and divides the optical power from the two input fibers between the two output fibers. Another name for the X coupler is the 2 x 2 coupler.

Star Couplers

Star couplers comprise of multiple inputs and outputs. The input and output port numbers could be the same or multiples, such as 2×2, 4×4, etc as shown in the figure above. The input power is uniformly distributed among the output fibers using a star coupler.

Non-Directional Star Coupler

This type of star coupler is non-directional i.e., it takes inputs from all fibers and then distributes them among all fibers, both input, and output, as shown in the figure above. In a non-directional coupler, the light going in any one port emerges from all the ports, including the input. A non-directional coupler is made by reflecting the input light using a mirror, into all output ports. 

Using star coupler networks with more than 3 or 4 terminals can be coupled with lower loss. This is because the star coupler requires only one input connector and suffers only one excess loss. The larger number of terminals, the more significant the loss benefits.

Tree Couplers

Tree couplers as shown above are also called 1xN couplers because they take one or two inputs and split them into multiple outputs. These couplers distribute the input power uniformly among the output fibers. The common configurations include 1×4, 1×8, 1×16, 1×32 and 2×4, 2×8, 2×16, 2×32 port ratios. Tree couplers are used to split and mix optical signals in community antenna television (CATV), local area networks (LAN), and all other kinds of optical communication systems.

Wavelength Division Multiplexing (WDM) Couplers

WDM couplers separate optical signals according to their wavelengths. They split the incoming power into two different wavelengths into two outputs, e.g. 1310/1550nm and 980/1550nm couplers.

WDM couplers are used to separate wavelengths transmitted for different purposes through the same fiber, such as separating the light pumping an optical amplifier from the amplified signal. They should block wavelengths from entering the wrong output port.

The coupler Fabrication method is very important in fiber optical coupler communication, some of the fabrication methods are discussed below:

Biconical Taper Coupler (or Fused Coupler)

A fused coupler can be fabricated if the claddings of two or more fibers are partially removed and the fibers are placed in close proximity over some length applying tension, and heating the junction as shown above. Then some light will couple from one fiber into the others. The fraction that couples can be controlled by the thickness of the remaining cladding and the length of the region where the fibers are in proximity. 

Fused fiber coupler operation depends on whether the fibers are multimode or single mode. In a multimode coupler, the higher-order modes leak into the cladding and the core of the other fiber; the degree of coupling depends on the length of the coupling zone. However, in single-mode fibers, light transfers between the two cores in a resonant interaction that varies with length. If all the light enters in one fiber, it gradually transfers completely to the other, then transfers back as it travels farther, shifting back and forth cyclically. The distance over which the cycling takes place depends on the coupler design and the wavelength. So Single Mode WDM coupler can be made this way.

Mode-Mixing Rods

A mode-mixing rod is a glass rod of a few millimeters in diameter with sufficient length to allow the light from all input locations to fully expand to uniformly illuminate the end of the rod. In addition to this transmissive configuration, the rod can be cut in half, a mirrored reflecting surface can be applied, and the output fibers can be moved to the input side of the device to make a reflective system.

Planar Waveguide Couplers

The planar waveguide can be a strip deposited on the top of a flat substrate. Air and the substrate combine to serve the function of the cladding in a fiber. A simple type of waveguide coupler is a Y-shaped structure that divides the input waveguide into two outputs as shown in the figure above. If the output waveguide is split at equal angles, the light divides equally between them.

Fiber Optic Coupler Applications

Fiber Optic Couplers are used in local area network (LAN) applications, star architecture or bus architecture. In a star network topology, the stations branch off from a central hub, much like the bars on a wheel as shown in the figure below. Each network device connected by the star coupler can communicate with each other. The star coupler makes it easy to expand the number of workstations e.g., from a 4 x 4 to 8 x 8 doubles the system capacity.

On the other hand, bus architecture utilizes T couplers to connect a series of stations to a single backbone cable as shown in the figure above. In a typical bus network topology, the T coupler at each node splits off part of the power from the bus and carries it to the attached equipment.

Tree couplers are used in Passive optical network (PON) architecture. The first tree coupler is directly connected to the optical line terminal (OLT) port in the central office, then each of the output fibers is routed to a tree coupler in other sites like an outside enclosure or terminal box. Other amplification or compensation modules are attached to the required module to ensure transmission.