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
applications.
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.
n
1
sin = n
1
2
n
2
2
= NA (1.4)
where is the cone of the acceptance angle, NA is the numerical aperture,
n
0
is the index of refraction of the medium where the light originates, n
1
is the index of refraction of core, and n
2
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 [9].
The coupling efficiency, , is defined as
=
P
acc
P
input
=
L
s
A
f
NA
2
P
input
(1.5)
where P
acc
is the power accepted by the fiber, and P
input
is the input power,
L
s
is the radiance of the light source in watts per area and steradian, and
A
f
is the area of the fiber core, and is a correction for the reduction
due to reflection loss.
= n
1
n
0
/n
1
+n
0

2
(1.6)
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
14 Fiber Optic Essentials
Optical fiber cores
(a)
(b)
Δ
(c)
ΔZ
(d)
θ
Figure 1.9 Different coupling cases (courtesy of Academic Press).
case, the previous formula must be multiplied by the lens magnification
factor M.
From a transmission point of view, the two most important fiber param-
eters are bandwidth and attenuation. The fundamental reason we are
using fiber instead of copper cable is the increased bandwidth. Band-
width is the difference between the highest and the lowest frequency
information that can be transmitted by a system. A higher bandwidth
implies a greater capacity for a channel to carry information. Bandwidth
is tested using a very fast laser and a sensitive receiver. Software analyzes
the difference between the input and the output pulses, and calculates the
bandwidth of the fiber.
Bandwidth is also design dependent—for example, the bandwidth of
a step-index multimode fiber 125 MHz is lower than for a graded-
index multimode fiber 500 MHz. Table 1.1 shows bandwidth varia-
tions on two 100 m lengths of FDDI-grade fiber with different mid-span
connections—connector, mechanical splice, and fusion splice. As noted
earlier, multimode fibers are generally specified by their bandwidth in
a 1 km length, which ranges from 100 MHz to 1 GHz. For example, a

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