Misurata University
Faculty of Information Technology
Optical Communication Networks ( CN 326 )
Lecture 4: Optical fibers and cables
Instructor : Osama Mohamed Elrajubi
Spring 2024
1
Introduction
In order to plan the use of optical fibers in a
variety of line communication applications it is
necessary to consider the various optical fibers
currently available.
This chapter is a summary of the dominant
optical fiber types with an indication of their
general characteristics.
2
Introduction
• The values quoted are based upon both
manufacturers’ and suppliers’ data, and practical
descriptions for commercially available fibers.
• Hence in some cases the fibers may appear to
have
somewhat
poorer
performance
characteristics than those stated for the
equivalent fiber types produced by the best
possible techniques and in the best possible
conditions (in the laboratory).
3
Introduction
• This section therefore reflects the maturity of
the technology associated with the production
of both Multimode fibers and Single-mode
fibers, and also plastic optical fibers.
• In particular, a variety of high-performance
silica-based single-mode fibers for operation
over the 1.260 to 1.625 μm wavelength range,
are now widely commercially available.
4
Types of Optical Fibers
1. Multimode step index fibers
2. Multimode graded index fibers
3. Single-mode fibers
4. Plastic-clad fibers
5
5. Plastic optical fibers
Types of Optical Fibers
Singlemode step-index fibre
no air
n1 core
n2 cladding
Multimode step-index fibre
n1 core
n2 cladding
Multimode graded-index fibre
n variable
6
Application areas for fibre and optical
transmitters
Plastic fibre
LED 660 nm
Multimode fibre
LED 850 nm
Multimode fibre
LED 1300 nm
102
103
Singlemode fibre
Laser diode
Transmission rate [Mbit/s]
103
102
101
100
10-1
10-2
100
7
101
104
Transmission distance [m]
1. Multimode step index fibers
8
1. Multimode step index fibers
• Multimode step index fibers may be fabricated
from multicomponent glass compounds
(glass/glass (glass-clad glass)), or fabricated
from doped silica (silica/silica (silica-clad
silica)).
• These fibers can have large core diameters
and large numerical apertures to facilitate
efficient coupling to incoherent light sources
such as LEDs.
9
1. Multimode step index fibers
• The performance characteristics of this fiber
type may vary considerably depending on the
materials used and the method of
preparation; the doped silica fibers exhibit the
best performance.
• A typical structure for a glass multimode step
index fiber is shown in Figure 1.
10
Figure 1: Typical structure for a glass
multimode step index fiber
11
Multimode step index fiber
12
Software: Determine numbers of the Modes
And their Angles
13
2. Multimode graded index fibers
14
2. Multimode graded index fibers
• These multimode fibers which have a graded
index profile may also be fabricated using
multicomponent glasses or doped silica.
• However, they tend to be manufactured from
materials with higher purity than the majority
of multimode step index fibers in order to
reduce fiber losses.
15
2. Multimode graded index fibers
• The performance characteristics of multimode graded
index fibers are therefore generally better than those
for multimode step index fibers due to the index
grading and lower attenuation.
• Multimode graded index fibers tend to have smaller
core diameters than multimode step index fibers,
although the overall diameter including the buffer
jacket is usually about the same. This gives the fiber
greater rigidity to resist bending. A typical structure is
illustrated in Figure 2.
16
Parameters
• Structure
• Core diameter: 50 to 100 μm
• Cladding diameter: 125 to 150 μm
• Coating diameter: 200 to 300 μm (e.g. 245 ± 5
μm for Corning fibers)
• Buffer jacket diameter: 400 to 1000 μm
• Numerical aperture: 0.2 to 0.3.
17
Figure 2: Typical structure for a glass
multimode graded index fiber
18
3. Single-mode fibers
19
3. Single-mode fibers
• Single-mode fibers can have either a step index or
graded index profile.
• The benefits of using a graded index profile are to
provide dispersion-modified single-mode fibers.
20
3. Single-mode fibers
• Although single-mode fibers have small core
diameters to allow single-mode propagation, the
cladding diameter must be at least 10 times the
core diameter to avoid losses from the
evanescent field.
• Hence with a coating and buffer jacket to provide
protection and strength, single-mode fibers have
similar overall diameters to multimode fibers.
21
Figure 3: Typical structure for a singlemode step index fiber
22
Types of Single-mode fibers
1. Standard single-mode fiber (SSMF)
2. Low-water-peak nondispersion-shifted fiber
(LWPF).
3. Loss-minimized fiber.
4. Nonzero-dispersion-shifted fiber.
23
4. Plastic-clad Fibers
24
4. Plastic-clad Fibers
• Plastic-clad fibers are multimode and have
either a step index or a graded index profile.
• They have a plastic cladding (often a silicone
rubber) and a glass core which is frequently
silica (i.e. plastic-clad silica (PCS) fibers).
25
4. Plastic-clad Fibers
• Hard-clad silica fibers with a tougher plastic
cladding are also commercially available which
provide for increased durability.
• Plastic-clad fibers are generally slightly
cheaper than the corresponding glass fibers,
but usually have more limited performance
characteristics.
26
Figure 4: Typical structure for a plasticclad silica multimode step index fiber
27
5. Plastic optical fibers
28
5. Plastic optical fibers
• Plastic or polymeric optical fibers (POFs) are fabricated
from organic polymers for both the core and cladding
regions exhibiting large core and cladding diameters.
• Hence there is a reduced requirement for a buffer
jacket for fiber protection and strengthening. These
fibers are usually cheaper to produce and easier to
handle than the corresponding silica based glass
variety.
• However, their performance is restricted, giving them
limited use in communication applications.
29
Figure 5: Typical structure for a PMMA
plastic fiber
30
Optical fiber cables
31
Optical fiber cables
• Bare glass fibers are brittle and have small crosssectional areas which make them very
susceptible to damage when employing normal
transmission line handling procedures.
• It is therefore necessary to cover the fibers to
improve their tensile strength and to protect
them against external influences. This is usually
achieved by surrounding the fiber with a series of
protective layers, which is referred to as coating
and cabling.
32
Optical fiber cables
• The initial coating of plastic with high elastic
modulus is applied directly to the fiber
cladding.
• It is then necessary to incorporate the coated,
and buffered fiber into an optical cable to
increase its resistance to mechanical strain
and stress as well as adverse environmental
conditions.
33
Functions of the optical cable
The functions of the optical cable may be
summarized into four main areas. These are as
follows:
1. Fiber protection. The major function of the
optical cable is to protect against fiber damage
and breakage both during installation and
throughout the life of the fiber.
34
Functions of the optical cable
2. Stability of the fiber transmission characteristics.
Optical fiber cables must be designed so that the
transmission characteristics of the fiber are
maintained after the cabling process and cable
installation.
Increases in optical attenuation due to cabling are
quite usual and must be minimized within the
cable design.
35
Functions of the optical cable
3. Cable strength. Optical cables must have similar
mechanical properties to electrical transmission
cables in order that they may be handled in the
same manner.
These mechanical properties include tension,
torsion, compression, bending, squeezing and
vibration. Hence the cable strength may be
improved by incorporating a suitable strength
member and by giving the cable a properly
designed thick outer sheath (‫)غالف‬.
36
Functions of the optical cable
4. Identification and jointing of the fibers
within the cable. This is especially important
for cables including a large number of optical
fibers.
If the fibers are arranged in a suitable geometry
it may be possible to use multiple jointing
techniques rather than jointing each fiber
individually.
37
Cable Design
38
Cable design
• The design of optical fiber cables must take
account of the functions of the optical cable.
• Nevertheless, cable design may generally be
separated into a number of major
considerations. These can be summarized into
the categories of fiber buffering, cable
structural and strength members, and cable
sheath and water barrier.
39
1. Fiber buffering
The fiber is given a primary coating during production in
order to prevent abrasion of the glass surface and
subsequent flaws in the material.
The primary coated fiber is then given a secondary or
buffer coating (jacket) to provide protection against
external mechanical and environmental influences.
This buffer jacket is designed to protect the fiber from
microbending losses and may take several different
forms. These generally fall into one of three distinct
types which are illustrated in figure 6.
40
Figure 6: Techniques for buffering of optical
fibers: (a) tight buffer jacket; (b) loose tube buffer
jacket; (c) filled loose tube buffer jacket
41
1. Fiber buffering
• A tight buffer jacket is shown in Figure 6 (a),
which usually consists of a hard plastic and is
in direct contact with the primary coated fiber.
• An alternative and now common approach,
which is shown in Figure 6 (b), is the use of a
loose tube buffer jacket.
42
1. Fiber buffering
• As the buffer tube is smooth inside, it exhibits a
low resistance to movement of the fiber. In
addition, it provides the benefit that it can be
easily stripped for jointing or fiber termination.
• Filled loose tube buffer jacket: A variation on the
loose tube buffering in which the over sized
cavity is filled with a moisture-resistant
compound is showed in Figure 6 (c).
43
2. Cable structural and strength
members
• One or more structural members are usually
included in the optical fiber cable. This
approach, which is referred to as stranding, is
illustrated in Figure 7.
• It may be observed that the cable elements in
Figure 7(a) and (b) are stranded in one, two or
indeed several layers around the central
structural member.
44
Figure 7: Optical fiber cable structures: (a) one-layer
cable incorporating single-fiber loose tube buffers;
(b) layer cable incorporating single-fiber loose tube in
two layers; (c) unit cable construction.
45
Figure 8: Slotted core cable with four fiber
ribbons incorporating a total of 100 fibers
46
3. Cable sheath, water barrier and cable core
• The cable is normally covered with a
substantial outer plastic sheath in order to
reduce abrasion and to provide the cable with
extra protection against external mechanical
effects such as crushing.
47
Examples of fiber cables
48
Figure 9: Single-fiber cables [Refs 35, 39]: (a) tight buffer jacket
design; (b) loose buffer jacket design
49
Figure 10: Indoor cables: (a) interconnect cable incorporating
two optical fibers; (b) 6-fiber subunit of a 48-fiber cable
50