Fiber Optic Essentials

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