1. Fiber, Cables, and Connectors 13
Fibers are bundled into cables for transmission over large distances.
There are two types of cable construction: (1) tight-buffered that is used
for intrabuilding backbones that connect data centers, master controllers,
and telecommunications closets and patch panels; (2) and loose-tube
design that is used for outside plant environments and interbuilding
1.2 Basic Terminology
When specifying a fiber link, simplex refers to a one-directional link;
duplex, which uses two fibers, is required for two-way communication
(it is possible to design duplex links using different wavelengths over a
single fiber, which is a topic we will discuss in a later chapter).
The numerical aperture (NA) of a fiber measures the amount of light
the fiber can capture. It refers to the maximum angle for which light
incident on a fiber endface can still be refracted into the fiber and then
undergo total internal reflection. It can be calculated by Equation 1.4.
sin ≈ = n
= NA (1.4)
where is the cone of the acceptance angle, NA is the numerical aperture,
is the index of refraction of the medium where the light originates, n
is the index of refraction of core, and n
is the index of refraction of the
cladding. Single-mode fibers have an NA of about 0.1, while the NA of
multimode fibers vary from 0.2 to 0.3 .
The coupling efficiency, , is defined as
is the power accepted by the fiber, and P
is the input power,
is the radiance of the light source in watts per area and steradian, and
is the area of the fiber core, and is a correction for the reduction
due to reflection loss.
As can be seen, the area of the fiber core is very important in coupling
efficiency. Figure 1.9 shows several examples of coupling efficiency
between two optical fibers, including lateral misalignment, axial misalign-
ment, and angular misalignment. Coupling efficiency can be improved
by the use of a lens between the source and the fiber. The lens matches
the output angle of the source to the acceptance angle of the fiber. In this