How Do Different Fiber Optic Couplers Work?

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What
Is Fiber Optic Coupler?

Fiber
optic coupler is one type of fiber optic component that allows for the
redistribution of optical signals. It covers a wide range of fiber optic
devices such as optical splitters, optical combiners, and optical couplers. A
fiber optic coupler is a device that 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.

 

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. For instance, with a 1 x 2
fiber optic coupler, each output is less than one-half of the power of the
input signal (over a 3dB loss).

 

Different
Fiber Optic Coupler Types

A
basic fiber
optic coupler has N input ports and M output ports. N and M typically
range from 1 to 64. The number of input ports and output ports varies depending
on the intended application for the coupler. Under different principles, fiber
optic couplers can be categorized differently. The different types of optical
couplers will be explored in the following part.

 

Classified
by Manufacturing Technologies

Technologies
used for constructing optical couplers can be complex and difficult to
understand. Three major manufacturing techniques are micro-optics, fused-fiber,
and planar waveguide.

 

Micro-optics
couplers use individual optical elements such as prisms, lens, mirrors, etc. to
construct an optical route. These elements divide the input optical signal into
two or more separated light beams.

 

Fused-fiber
couplers used the most basic material–optical fiber. Two or more fiber cores
are twisted, fused and tapered together in a length.

 

Planar
waveguides are more like a semiconductor. A planar wafer is used to make a
waveguide coupler, and the reflections occur only in y-directions. Planar
waveguides are more often used to make high port count couplers, such as 1 x 12
PLC splitter, 1 x 24 PLC splitter.

 

Classified
by Shape

If
we see optical couplers by shape, there is Y coupler, T coupler, X coupler,
star coupler and tree coupler, which split the optical signal based on the
power.

 

Y
Coupler

 

A
Y coupler resembles the letter Y. Y coupler also called optical tap coupler.
The input signal is split into two output fibers. Sometimes, to meet users’
specific applications, the power distribution ratio also can be controlled
precisely.

 

T
Coupler

 

Unlike
the Y coupler, a T coupler has an uneven power distribution. The power of one
output signal is greater than the other output signal. Popular splitting ratios
include 10:90 percent and 20:80 percent. This optical coupler is often used in
small networks with less port counts.

 

X
Coupler (2×2)

 

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 2 x 2 coupler.

 

Star
Coupler

 

A
star coupler generally has several input and output port combinations, in which
the optical power is distributed from more than two input ports among several
output ports. The number of input and output ports may or may not be equal in
star couplers such as 2×4, 4×4, 8×16, etc. However, in all possible input and
output port combinations, the distribution of power among the output ports
remains equal.

 

Tree
Coupler

 

A
tree coupler is also a multiport coupler. It splits optical power from one
input fiber to more than two output fibers. A tree coupler may also be used
reversely to combine the optical signal from more than two input fibers to one
output fiber.

 

Classified
by Bandwidth or Window

Bandwidth
or Window is also a factor to consider when users choose the optical coupler
types. Regardless of the port types used, fiber optic couplers can be designed
for single window, dual wavelength or wideband transmissions.

 

Single
window couplers are designed to incorporate a single wavelength within a narrow
wavelength window.

 

Dual-wavelength
couplers have wider wavelength windows and can work with two wavelengths at a
time.

 

Wideband
couplers can be designed to work with a single wavelength covering a wider
range of wavelengths.

 

In
addition, fiber optic coupler also can be classified to single mode optical
coupler and multimode optical coupler.

 

Applications
of Fiber Optic Couplers

In
local area network (LAN) applications, fiber optic couplers are used in either
bus architecture or star architecture.

 

In
a star network topology, the stations branch off from a central hub, much like
the spokes on a wheel. Each network device connected by the star coupler can
communicate with each other. Besides, the star coupler makes it easy to expand
the number of workstations. For example, changing from a 4 x 4 to 8 x 8 doubles
the system capacity.

 

Bus
architecture utilizes T couplers to connect a series of stations to a single
backbone cable. 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 typically used in cascaded 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 (outside enclosure/terminal box). If there’s a need to further
divide the signal, more tree couplers can be added. Of course, other
amplification or compensation modules are required to ensure the transmission.