unit04_focs.pdf

Fiber Optic
Communication Systems
Dr. Fahim Aziz Umrani
Department of Telecommunication, Room # 213
Institute of Information & Communication Technologies (IICT),
Mehran UET, Jamshoro
https://sites.google.com/a/faculty.muet.edu.pk/fau/focs

Fiber Optic Communication Systems By Dr. Fahim Aziz Umrani
Department of Telecommunication, Mehran UET
How Are Optical Fibers Made
 Optical fibers are made of extremely pure optical glass.
 The thicker the glass gets, the less transparent it becomes due to impurities in the
glass.
 Making optical fibers requires the following steps:
 Making a preform glass cylinder
 Drawing the fibers from the preform
 Testing the fibers
 A variation of the refractive index inside the fiber cable is a fundamental necessity
in the fabrication of fibers for light transmission. Hence at least two different
materials which are transparent to light over the current operating wavelength
range (0.8 to 1.6 μm) are required.
 In practice these materials must exhibit relatively low optical attenuation and
dispersion.
 glasses (or glass like materials) and
 mono-crystalline structures (certain plastics). only used for the fabrication of step-index fibers.
2

Making the Preform Blank
 Step – 1: Oxygen is bubbled through solutions of silicon chloride (SiCl4), germanium
chloride (GeCl4) and/or other chemicals.
The precise mixture governs the various physical and optical properties (index of refraction,
coefficient of expansion, melting point, etc.).
 Step – 2: The gas vapors are then conducted to the inside of a highly refined synthetic
silica or quartz tube (cladding) in a special lathe. As the lathe turns, a torch is moved up
and down the outside of the tube. The extreme heat from the torch causes two things
to happen:
 The silicon and germanium react with oxygen, forming silicon dioxide (SiO2) and germanium dioxide (GeO2).
 The silicon dioxide and germanium dioxide deposit on the inside of the tube and fuse together to form glass.
 Step – 3: The lathe turns continuously to make an even coating and consistent blank.
 The purity of the glass is maintained:
 by using corrosion-resistant plastic in the gas delivery system (valve blocks, pipes, seals) and
 by precisely controlling the flow and composition of the mixture. The process of making the preform blank is
highly automated and takes several hours. After the preform blank cools, it is tested for quality control
(index of refraction).
3

Drawing Fibers from the Preform Blank
 Once the preform blank has been tested, it gets loaded into a fiber
drawing tower.
 The blank gets lowered into a graphite furnace (3,452 to 3,992 degrees
Fahrenheit or 1,900 to 2,200 degrees Celsius) and the tip gets melted until
a molten glob falls down by gravity.
 As it drops, it cools and forms a thread.
 The operator threads the strand through a series of coating cups (buffer
coatings) and ultraviolet light curing ovens onto a tractor-controlled spool.
 The tractor mechanism slowly pulls the fiber from the heated preform
blank and is precisely controlled by using a laser micrometer to measure
the diameter of the fiber and feed the information back to the tractor
mechanism.
 Fibers are pulled from the blank at a rate of 33 to 66 ft/s (10 to 20 m/s)
and the finished product is wound onto the spool. It is not uncommon for
spools to contain more than 1.4 miles (2.2 km) of optical fiber.
4

Testing the Finished Optical Fiber
 The finished optical fiber is tested for the following:
 Tensile strength - Must withstand 100,000 lb/in2 or more
 Refractive index profile - Determine numerical aperture as well as screen for optical
defects
 Fiber geometry - Core diameter, cladding dimensions and coating diameter are uniform
 Attenuation - Determine the extent that light signals of various wavelengths degrade
over distance
 Information carrying capacity (bandwidth) - Number of signals that can be carried at
one time (multi-mode fibers)
 Chromatic dispersion - Spread of various wavelengths of light through the core
(important for bandwidth)
 Operating temperature/humidity range
 Temperature dependence of attenuation
 Ability to conduct light underwater - Important for undersea cables
5

Inside Vapor Phase Oxidation (IVPO) Process
 There are a number of variations in actually realizing the glass soot
formation.
 Developed by Bell Labs, also known as Modified Chemical Vapor
Deposition (MCVD).
 The process involves a rotating quartz tube, heated by burners from
the outside which move along a length of the tube.
 The high temperature created causes oxidation of gases inside the
tube with glass soot deposition on the inner surface of the tube.
The traversing burners deposit a thin layer along the length of the
tube. The next pass of burners deposits another layer. Thus the
preform is built layer by layer.
 A total of 60 to 70 layers may be deposited. The tube is finally
collapsed under high temperature (2000ºC) to form a solid preform.
In this process core glass is deposited and the tube acts as cladding.
6

Modified Chemical Vapor Deposition (MCVD)
7

Outside Vapor Phase Oxidation (OVPO) Process
 The Outside Vapor Phase Oxidation (OVPO) process
involves of a seed rod and feeding of raw materials through
a set of burners as shown in Figure.
 The rotating seed rod collects layer of glass soot formed
through the flame hydrolysis process, with every pass of
the burner along the length of the seed rod.
 Later layer by layer, first the core and then the cladding
glass if deposited. The preform grown over the seed rod is
then removed. The porous preform is consolidated under
dry high-temperature conditions for removal of hydroxyl
ions as well as formation of solid preform.
8

Department of Telecommunication, Mehran UET 9

Optical Fiber Cables
 The function of the optical cable may be summarized into four main
areas:
 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 fiber.
 Stability of the fiber transmission characteristics: the cabled fiber must
have good stable transmission characteristics which are comparable with
the uncabled fiber. Increases in optical attenuation due to cabling are
quite usual and must be minimized.
 Cable strength: optical fibers 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.
10

Optical Fiber Cables
 Identification and jointing of 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 joining each fiber
individually.
11

Fiber strength and durability
 The silica or multi-component glass are brittle and exhibit
almost perfectly elasticity until their breaking point is
reached. The bulk material strength of flawless glass is
quite high and may be estimated for individual materials
using the relationship:
where St = cohesive strength
γp =surface energy of the material
E = (stress/strain) = young’s modulus for the material
(stress/strain), and
la = atomic spacing or bond distance.
12
a
p
t
l
E
S
4



Fiber strength and durability
 In order to treat surface flaws in glass analytically, the Griffith
theory is normally used.
 This theory assumes that the surface flaws are narrow cracks with
small radii of curvature at their tips.
 It postulates that the stress is concentrated at the tip of the crack
which leads to crack growth and eventually catastrophic failure.
 The Griffith theory gives a stress intensity factor KI as:
where S = macroscopic stress on the fiber
Y = constant dedicated by the shape of crack, and
C = depth of the crack
 Further, the Griffith theory gives an expression for the critical stress
intensity factor KIC where fracture occurs as:
13
C
SY
KI 
p
IC E
K 
2



C
Y
E
S
p
f 2
2 


Stress Corrosion
 It is due to slow growth of flaws under the action of stress and
water. Stress corrosion occurs due to the molecular bands at the
tips of the crack which are attracted by water when they are under
stress.
 The stress corrosion is usually predicted using empirical relationship
for the crack velocity in terms of the applied stress intensity factor,
KI:
where n = stress corrosion susceptibility
A = constant for fiber material
 Therefore, from a combination of fiber testing and stress corrosion,
information estimates of the maximum allowable fiber strain can be
made available to the cable designer.
15
n
I
c AK
v 

Fiber Optic Connections
 Fiber optic connections permit the transfer of optical
power from one component to another.
 A system connection may require a:
Fiber optic splice – a permanent connection between two
fibers.
• Mechanical splice
• Fusion splice
Connector - Fiber optic connectors sometimes resemble
familiar electrical plugs and sockets.
Coupler - Fiber optic couplers distribute or combine optical
signals between fibers.
16

Optical Fiber connections
 Optical fiber links, like any wired common system, have a
requirement for both jointing and termination of the transmission
medium. The number of intermediate fiber connections or joints is
dependent on the link length (between repeaters), the continuous
length of fiber cable manufactured, and the length of the fiber that
may be practically or conveniently installed as a continuous section
on the link. Current practices allow single length of fiber cable of
several kilometers, especially for submarine systems.
 The two major categories of fiber joint are:
 Fiber splices: these are semi-permanent or permanent joints which
find major use in most optical fiber telecommunication systems
(similar to electrical soldered joints).
 Demountable fiber connectors or simply connectors: these are
removable joints which allow easy fast manual coupling and
uncoupling of fibers (similar to electrical plugs and sockets).
17

Fresnel Reflection
 A major consideration with all types of fiber-fiber connection is the optical loss
encountered at the interface. Even when the two joined fiber ends are smooth and
perpendicular to the fiber axes, and the two fiber axes are perfectly aligned, a small
proportion of the light may be reflected back into the transmitting fiber causing
attenuation at the joint. This phenomenon, known as Fresnel reflection.
 The magnitude of this partial reflection of the light transmitted through the interface
may be estimated using the classical Fresnel formula for light of normal incidence
and is given as:
where r is the fraction of the light reflected at a single interface, 𝑛1 is the refractive index
of the fiber core and 𝑛 is the refractive index of the medium between the two jointed
fibers (i.e., for air 𝑛 = 1).
 However, in order to determine the amount of light reflected at a fiber joint, Fresnel
reflection at both fiber interfaces must be taken into account. The loss in decibels
due to Fresnel reflection at a single interface is given by:
18
𝑟 =
𝑛1 − 𝑛
𝑛1 + 𝑛
2
𝐿𝑜𝑠𝑠𝐹𝑟𝑒𝑠 = −10𝑙𝑜𝑔10 1 − 𝑟

Fiber alignment and joint loss
 Any deviations in the geometrical and optical parameters of the two optical fibers,
which are jointed, will affect the optical attenuation (insertion loss) through the
connection. Hence there are inherent connection problems when jointing fibers
with, for instance:
 Different core and/or cladding diameters;
 Different numerical apertures and/or relative refractive index differences;
 Different refractive index profiles;
 Fiber faults (core ellipticity, core concentricity etc).
 Even if the two fibers to be jointed are exactly the same, there is still the problem
of fiber alignment. The misalignment losses in the fiber-to-fiber joints due to
some imperfection in splicing are:
 Core misalignment and imperfections
 Lateral (axial) misalignment
 Angular misalignment
 Gap between ends contact
 Non-flat ends due to cleaving
 Cladding alignment
19

Alignment Losses
20

Couplers & Splitters
Fiber, connectors, and splices rank as the most important passive
devices. However, closely following are tap ports, switches,
wavelength-division multiplexers, bandwidth couplers and splitters.
These devices divide, route, or combine multiple optical signals.
 Some of the most common applications for couplers and splitters
include:
 Local monitoring of a light source output (usually for control
purposes).
 Distributing a common signal to several locations simultaneously.
An 8-port coupler allows a single transmitter to drive
eight receivers.
 Making a linear, tapped fiber optic bus. Here, each splitter would be
a 95%-5% device that allows a small portion of the energy to be
tapped while the bulk of the energy continues down the main
trunk.
21

Couplers
Fiber optic couplers either split optical signals into multiple paths or combine
multiple signals on one path.
 Optical signals are more complex than electrical signals, making optical
couplers trickier to design than their electrical counterparts. Like electrical
currents, a flow of signal carriers, in this case photons, comprise the
optical signal.
 However, an optical signal does not flow through the receiver to the
ground. Rather, at the receiver, a detector absorbs the signal flow. Multiple
receivers, connected in a series, would receive no signal past the first
receiver which would absorb the entire signal.
 Thus, multiple parallel optical output ports must divide the signal between
the ports, reducing its magnitude. The number of input and output ports,
expressed as an N x M configuration, characterizes a coupler. The letter N
represents the number of input fibers, and M represents the number of
output fibers.
 Fused couplers can be made in any configuration, but they commonly use
multiples of two (2 x 2, 4 x 4, 8 x 8, etc.).
22

Fused-fiber coupler
 In this type of coupler, two or more fibers are
twisted together and melted in a flame.
23

Splitters
 The simplest couplers are fiber optic splitters. These devices possess at
least three ports but may have more than 32 for more complex devices.
 Figure 1 illustrates a simple 3-port device, also called a tee coupler. It can
be thought of as a directional coupler. One fiber is called the common
fiber, while the other two fibers may be called input or output ports.
 The coupler manufacturer determines the ratio of the distribution of light
between the two output legs. Popular splitting ratios include 50%-50%,
90%-10%, 95%-5% and 99%-1%; however, almost any custom value can be
achieved.
 For example, using a 90%-10% splitter with a 50 µW light source, the
outputs would equal 45 µW and 5 µW. However, excess loss hinders that
performance. All couplers and splitters share this parameter. Excess loss
assures that the total output is never as high as the input. Loss figures
range from 0.05 dB to 2 dB for different coupler types.
 An interesting, and unexpected, property of splitters is that they are
symmetrical. For instance, if the same coupler injected 50 µW into the
10% output leg, only 5 µW would reach the common port.
24

Connectors
Different connector types have different characteristics, different advantages and
disadvantages, and different performance parameters. But all connectors have the
same four basic components.
26
BASIC COMPONENTS OF CONNECTORS
The Ferrule: The fiber is mounted in a long, thin cylinder, the ferrule, which acts as a fiber alignment mechanism. The
ferrule is bored through the center at a diameter that is slightly larger than the diameter of the fiber
cladding. The end of the fiber is located at the end of the ferrule. Ferrules are typically made of metal or
ceramic, but they may also be constructed of plastic.
The Connector Body: Also called the connector housing, the connector body holds the ferrule. It is usually constructed of metal
or plastic and includes one or more assembled pieces which hold the fiber in place. The details of these
connector body assemblies vary among connectors, but bonding and/or crimping is commonly used to
attach strength members and cable jackets to the connector body. The ferrule extends past the connector
body to slip into the coupling device.
The Cable: The cable is attached to the connector body. It acts as the point of entry for the fiber. Typically, a strain-
relief boot is added over the junction between the cable and the connector body, providing extra strength
to the junction.
The Coupling Device: Most fiber optic connectors do not use the male-female configuration common to electronic connectors.
Instead, a coupling device such as an alignment sleeve is used to mate the connectors. Similar devices
may be installed in fiber optic transmitters and receivers to allow these devices to be mated via a
connector. These devices are also known as feed-through bulkhead adapters.

Connector
Insertion
Loss
Repeatability Fiber Type Applications
FC
0.50-1.00 dB 0.20 dB SM, MM
Datacom,
Telecommunication
FDDI
0.20-0.70 dB 0.20 dB SM, MM Fiber Optic Network
LC
0.15 db (SM)
0.10 dB (MM)
0.2 dB SM, MM
High Density
Interconnection
MT Array 0.30-1.00 dB 0.25 dB SM, MM
High Density
Interconnection
SC
0.20-0.45 dB 0.10 dB SM, MM Datacom
SC Duplex
0.20-0.45 dB 0.10 dB SM, MM Datacom
ST
Typ. 0.40 dB
(SM)
Typ. 0.50 dB
(MM)
Typ. 0.40 dB (SM)
Typ. 0.20 dB
(MM)
SM, MM
Inter-/Intra-Building,
Security, Navy

Installing fiber optic connectors
The following steps are given as a reference for the basics of optical fiber interconnection.
 Cut the cable one inch longer than the required finished length.
 Carefully strip the outer jacket of the fiber with “no nick” fiber strippers. Cut the exposed strength
members, and remove the fiber coating. The fiber coating may be removed two ways:
 by soaking the fiber for two minutes in paint thinner and wiping the fiber clean with a soft, lint-free cloth,
 or by carefully stripping the fiber with a fiber stripper. Be sure to use strippers made specifically for use with fiber rather than metal wire
strippers as damage can occur, weakening the fiber.
 Clean the bared fiber with isopropyl alcohol poured onto a soft, lint-free cloth such as Kimwipes®.
NEVER clean the fiber with a dry tissue. Note: Use only industrial grade 99% pure isopropyl alcohol.
 The connector may be connected by applying epoxy or by crimping. If using epoxy, fill the
connector with enough epoxy to allow a small bead of epoxy to form at the tip of the connector.
Insert the clean, stripped fiber into the connector. Cure the epoxy according to the instructions
provided by the epoxy manufacturer.
 Anchor the cable strength members to the connector body. This prevents direct stress on the fiber.
Slide the back end of the connector into place (where applicable).
 Prepare the fiber face to achieve a good optical finish by cleaving and polishing the fiber end.
Before the connection is made, the end of each fiber must have a smooth finish that is free of
defects such as hackles, lips, and fractures.
28

Cleaving
Cleaving (also called the scribe-and-break method ) involves
cutting the fiber end flush with the end of the ferrule. The
steps listed below outline one procedure for producing good,
consistent cleaves:
 Place the blade of the cleaver tool at the tip of the ferrule.
 Gently score the fiber across the cladding region in one
direction. If the scoring is not done lightly, the fiber may
break, making it necessary to re-terminate the fiber.
 Pull the excess, cleaved fiber up and away from the ferrule.
 Carefully dress the nub of the fiber with a piece of 12-
micron alumina-oxide paper.
 Do the final polishing.
29

Cable Design
 The fiber cable must be designed so that the strain
on the fiber cable does not exceed 0.2%.
 In practice, these constraints can be overcome in
various ways which are, to some extent, dependent
upon the cable’s application.
 Nevertheless, cable design may generally be
separated into a number of major considerations
such as:
 Fiber Buffering
 Cable Strength and Structural Members
 Cable Sheath
30

Fiber Buffering
 The buffer layers of a fiber optic cable protect optical fibers against
ambient stresses that can cause weakening, fatigue and breakage. The
ideal buffering system has the following properties:
 Concentricity
 Flexibility
 Thermal stability
 Crush resistance
 High tensile strength
 Several types of coatings are used depending on the application. The most
durable is polyimide but acrylate, silicone; fluoropolymers and aluminum
are also used.
 The two types of buffering mechanisms for optical fiber cable are:
 Loose buffered (outside plant and some inside plant cables)
 Tight buffered (inside plant and underground plant cables)
31

Loose-tube buffering
 Loose-tube buffering systems decouple the fiber from the buffer to
minimize stress transfer.
 A gel layer between the fiber and buffer absorbs shock and high impact
stresses. Cables incorporating loose-tube buffering system are much larger
than those with tight-tube buffers and often are difficult to terminate.
32
 Loose-buffered cables have the following
characteristics:
More robust than tight buffered cables for outdoor
applications.
Optimized and proven for long outdoor runs.
Less expensive than indoor cable per fiber-meter.
Have high fiber counts.
Have better packing density.

Tight-buffered Cables
 In a tight-tube buffering system, a thermoplastic buffer material is extruded
directly onto the fiber. This results in a package that is small in size but susceptible
to micro and macro bending.
 Because the buffer layers are in close contact with the fiber, any stress applied to
the outside of the buffer is transferred to the fiber core.
 These cables are usually more sensitive than loose buffered cables to adverse
temperatures, and outside forces.
33
 Tight buffered cables are desirable for:
 Increased physical flexibility.
 Smaller bend radius for low fiber-count cables.
 Easier handling characteristics in low fiber counts
 The two typical constructions of tight-buffered cables
are: Distribution design, which has a single jacket
protecting all the tight buffered fibers, and Breakout
design, which has an individual jacket for each tight-
buffered fiber.

Cable Strength and Structural Members
 Optical fibers produced with synthetic fused silica have remarkable strength.
 The fiber has a theoretical strength of about 2,000kpsi, which is stronger than steel. In
practice the observed strength is considerably lower (typically 700kpsi) due to the presence
of small flaws in the bulk and on the surface of the silica.
 The dynamic strength of an optical fiber refers to the force required for an instantaneous
break (as opposed to a delayed failure).
 Fiber-optic cables use strength members (central support members) to increase the strength
of the cable and protect the fiber from strain. Strength and support members must be light
and flexible. The materials used for strength and support include
 steel wire and textile fibers (such as nylon and arimid yarn), carbon.
 Cable assemblies can be made with various fiber sizes (<5µm to >2000µm diameters), fiber types and NAs, covering a
wavelength range from 190nm to 2500nm. Typically, single and multiple fiber cables can be designed to work over a
range from –50ºC to 200ºC in almost any environmental conditions. Jacket and strength member materials are often
selected from Hytrel (polyester), Tefzel (ETFE), Polyurethane, Nylon, PFA, TFE, PVC, PVC monocoil, Kevlar (aramid
yarn), and stainless steel or others based on the application.
 The choice of buffering techniques depends on the intended application. In large fiber count
commercial applications, manufacturers use the loose-tube buffers. In commercial building
and Navy applications, manufacturers use tight buffers.
34

Cable Sheath
 The properties of cables are more or less determined by the
materials used in the design. Choices of materials for
insulations and sheaths are therefore vital for meeting
specified requirements. The fire properties, chemical
resistance and environmental performance of a cable are to a
high degree determined by the sheath. Below you will find a
summary of the most important sheath materials and their
properties.
 PVC (Polyvinyl Cloride)
 Halogenfree compounds
 Polythylene (PE)
 Rubber
35

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