MODULE – 1
FIBER
OPTIC COMMUNICATIONS
PREPARED BY,
MR. SREE SANKAR J MENON
ASSISTANT PROFESSOR, AIET
WHAT IS FIBER OPTIC CABLE ?
A fiber optic cable is a network
cable that contains strands of glass
fibers inside an insulated casing.
They're designed for long-distance,
high-performance data
networking, and
telecommunications.
Compared to wired cables, fiber
optic cables provide higher
bandwidth and transmit data over
longer distances.
Fiber optic cables support much of
the world's internet, cable
television, and telephone systems.
Fiber optic cables
NEED FOR OPTICAL FIBER
COMMUNICATION
• In long distance communication there is an
increased need for lossless transmission
• There is a need for compact and weightless
transmitters and receivers
• There is a need for increased span of
transmission or repeater spacing
• There is a need for increased bit-rate distance
product
Fiber optics satisfies all these needs hence
emerging as the technology of the hour
HISTORICAL DEVELOPMENT
Generation Bit rate Repeater Operating
Spacing Wavelength
1st Generation 45 mb/s 10 km 0.8 micro
meter
2nd Generation 100 mb/s 50 Km 1.3 micro
to meter
1.7 Gb/s
3rd Generation 10 Gb/s 100 Km 1.55 micro
meter
4th Generation 10 Tb/s > 10,000 Km 1.45 to 1.62
micro meter
5th Generation 40 to 160 Tb/s 24,000 Km to 1.53 to 1.57
35,000 Km micro meter
Computer networking and cable
television- OFC applications
THE GENERAL SYSTEM
A DIGITAL OPTICAL LINK
ADVANTAGES OF OPTICAL FIBER
COMMUNICATION
Long distance transmission- optical fibers has low losses compared
to the conventional copper wires
Large information capacity – wider bandwidth than copper wires
Small size and light weight- not like other heavy bulky wire cables.
Optical fibers– diameter even less than human hair.
Immune to interference and cross talks- free from EMI and RFI
Electrical isolation- optical fibers fabricated from glass or sometimes
plastics-which are insulators. This makes suitable for electrically
hazardous environments
Signal security- the transferred signals cannot be obtained in a non-
invasive manner
System flexibility and ease of maintenance – life time for optical
components is 20 to30 years
DISADVANTAGES OF
OPTICAL FIBER COMMUNICATION
• Lack of bandwidth demand
• Radiation darkening
• Difficult in splicing
• High investment cost
• Complex testing procedure
APPLICATIONS OF OPTICAL FIBER
COMMUNICATION
• Public Network applications
• Military applications
• Civil applications
• Consumer applications
• Industrial applications
• Optical sensor system
• Local Area Network
OPTICAL FIBER WAVEGUIDE
OPTICAL FIBER WAVEGUIDE
Optical fiber cable consists of – core, cladding and
buffer coating
Core - central region
Core is surrounded by a transparent lower refractive
index cladding
Cladding reduces scattering loss, adds mechanical
strength to fiber and protects core from absorbing
contaminants
Buffer coating/ jacket layer /protective coating –
plastic coating – further adds strength to the fiber
OPTICAL FIBER WAVEGUIDE
The light guiding characteristics of an optical
fiber waveguide is dependent on refractive
index of core and cladding
n1- refractive index of the core
n2- refractive index of the cladding
Always n1 > n2
Optical fiber cable works on the principle of
Total Internal Reflection
RAY THEORY TRANSMISSION
There is a conception that light always travels at the
same speed but this fact is not simply true
The speed of light depends upon the material in
which it is moving
In free space light travels at its maximum speed of
3 x 108 m/s
When light propagates through a medium it exhibits
some characteristic behavior explained by the laws
of Reflection and Refraction
REFLECTION
• The laws of
Reflection states
that angle of
incidence is equal
to angle of
reflection
• The incident ray ,
the normal and
the reflected ray
all lie on the
same plane
REFRACTION
• Refraction occurs
when the light rays
passes from one
medium to another
i.e. light rays
changes its
direction at the
interface
• The bending of light
ray occurs due to
the difference in
the densities of the
medium
REFRACTIVE INDEX
• It is defined as the ratio of velocity of light in free
space to the velocity of light in that medium
• It is denoted by ‘n’
• n= C/V
• C 3 x 108 m/s and V Velocity of light in any
medium under consideration
• Refractive index is also known as index of
refraction
• For air n= 1.0
• For glass n= 1.5
• For water n= 1.3
Snell’s Law
• Snell’s law defines how the light ray reacts
when it meet the interface of two media
having different index of refraction
• Let n1 and n2 be the refractive index of two
medium and let φ1 be the angle of incidence
and φ2 be the angle of refraction then
according to Snell's law:
n1Sinφ1 = n2Sinφ2
CRITICAL ANGLE
When the angle of incidence
(φ1) is progressively
increased then there will be
progressive increase of the
refractive angle(φ2 ).
At some condition of angle of
incidence angle (φ1) , the
angle of refraction (φ2 )
becomes 900 To the normal
When this happens the
refracted light ray travels
along the interface. The angle
of incidence (φ1) at which the
refracted angle (φ2 )becomes
TOTAL INTERNAL REFLECTION
• When the incident ray is increased beyond the
critical angle then the light ray will be
reflected back into the originating dielectric
medium
• This gives an effect of a mirror that is existing
between the interface of two medium.
ACCEPTANCE ANGLE
The maximum angle to the axis at
which the light may enter the
fiber in order to be propagated
along the fiber is referred to as
acceptance angle for the fiber.
ACCEPTANCE ANGLE
Derivation for Acceptance angle
NUMERICAL APERTURE
• The NA of a fiber is a figure of merit
which represents the light gathering
capability
• Larger the NA greater will be the amount
of light accepted by the fiber
Formula NA= Sin θa(max)
Where θa(max) is the maximum acceptance angle
MERIDIONAL RAYS AND SKEW RAYS
CYLINDRICAL FIBERS: MODES
The light rays will propagate as an EM
wave along the optical fiber
The mode of a fiber refers to the number
of paths for the light rays within the cable
According to the modes the optical fibers
are classified into two types:
(1)Single mode fibers
(2)Multi mode fibers
FIBER BASED ON MODES
SINGLE MODE FIBERS
• It allows propagation of light ray by only one path
• Single mode fiber are best for retaining fidelity of
each light pulse over longer distance and also they
do not exhibit dispersion caused by multiple
modes
• Advantage – higher bandwidth when compared to
multi mode fibers
• Disadvantages – (1) small core diameter makes
coupling of light more difficult
(2) more precision is required for single more
connectors and splices
MULTI MODE FIBERS
• It allows propagation of numerous modes
simultaneously
• Multi mode fibers have much larger diameter
when compared to single mode fibers
• Advantages – (1) more than one modes can
be transmitted
(2) suitable for short distances transmission
• Disadvantages – (1) Modal noise is present
(2) Greater intermodal dispersion
FIBER INDEX PROFILES
Fiber index profile is a graphical
representation of value of refractive
index across the core diameter
One way of classifying fibers is by
according to the index profiles
There are two basic types of index
profiles
(1) Step Index Profile
(2) Graded Index Profile
STEP INDEX FIBER
STEP INDEX (SI) FIBER
• The step index fiber is a cylindrical wave guide
core with central or inner core that has
uniform refractive index n1 and the core is
surrounded by outer cladding with uniform
refractive index of n2
• In the refractive index profile of the step index
fibers the refractive index is plotted on the
horizontal axis and the radial distance from
the core is plotted in the vertical axis
• The propagation of light rays - meridional rays
STEP INDEX (SI) FIBER
• The step index fiber has
Core diameter of 50 – 80 µm
Cladding diameter -- 125 µm
• The refractive index profile for SI Fiber is
defined by
GRADED INDEX FIBER
GRADED INDEX FIBER (GRIN)
• GRIN fiber has a core made from many layers of
glass
• In GRIN the RI is not uniform within the core, it is
highest at the center and decreases smoothly and
continuously with the distance towards the cladding
• In GRIN fibers the light waves bent by refraction
towards the core axis and they follow the curved
path down the fiber length.
• This results because of the change in refractive
index as moved away from the center of the core
• GRIN-lower coupling efficiency and higher
bandwidth than SI fibers
GRADED INDEX FIBER
• Refractive index variations of GRIN is given as
Where, r- radial distance from fiber axis
a - core radius
α -- shape of the index profile
n1 - RI of Core
n2 - RI of Cladding
∆ -Relative Refractive index Difference
Possible fiber refractive index
profiles for different values of α
OPTICAL FIBER CONFIGURATIONS
Based on mode and Profile optical
fibers are classified into 3 types
1. Single Mode Step Index Fiber
2. Multi Mode Step Index Fiber
3. Multi Mode Graded Index Fiber
SINGLE MODE STEP INDEX FIBER
SINGLE MODE STEP INDEX FIBER
• Also known as mono mode step index fiber
• Single mode step index has a core that is very
small so that there is essentially only one path
for the light ray to pass through the fiber
• Core diameter 2 µm to 10 µm
• Advantage low intermodal dispersion i.e..
broadening of pulses is low as there is only one
path for light pulses to travel
• Disadvantage because of small size
interconnection of cables and interfacing with
source is difficult
MULTIMODE STEP INDEX FIBER
MULTIMODE STEP INDEX FIBER
• It is widely used optical fiber
• Core diameter around 50 µm or greater
• The core diameter is large enough to allow the
propagation of many modes within the fiber core
• Light rays travel in zig - zag manner
• Major disadvantage considerable amount of
dispersion occurs due to differing group velocities
of the propagation modes and this in turns restricts
the bandwidth
• Advantage larger NA and core diameter
facilitates the easy coupling with optical sources
MULTIMODE GRADED INDEX FIBER
MULTIMODE GRADED INDEX FIBER
• Multi mode graded index fiber has a parabolic
index profile
• It can be observed that the meridional rays
appears to follows a curved path through the fiber
• Using the concepts of geometric optics the gradual
decrease in refractive index from the center of the
core creates many refractions of the rays
• It exhibits less inter modal dispersion
• Core size 50 µm to 100 µm
• Advantage used for long distance
communication
EXPANDED RAY DIAGRAM
FIBER MATERIALS
Requirements of materials for fabrication of
optical fibers
1. Must be possible to make out
Long, Thin, Flexible fibers from the materials
2. Material must be transparent at particular
wavelength
3. Physically compatible materials that have slightly
different refractive indices for core and cladding
must be available
Materials that satisfy these
requirements are glasses and plastics
FIBER MATERIALS
Majority of glass fibers are made from silica (SiO 2)
or silicate
Variety of glass fibers are available ranging fro
moderate loss using for short distance transmission
to low loss fibers using for long haul applications
Plastics fibers are less widely used because of their
substantially higher attenuation than glass fibers
Main use of plastic fibers is in short distance
applications and in abusive environments
Based on materials, fibers are classified into
1.Glass fibers 2.Active glass fibers
3.Plastic optical fiber
GLASS FIBERS
• Glass is generally made by fusing mixtures of
metal oxides, sulphides, selenides etc.
• The materials so formed by a mixture of
oxides will have a randomly connected
molecular network
• To produce two compatible transparent
materials with different refractive indices
Silica is doped with Fluoride (F) or other
oxides such as GeO2 , P2O5 ,B2O5 etc.
• Core refractive index must be greater than
cladding refractive index.
GLASS FIBERS
• When Glass is heated up from room
temperature, it remain as a hard solid up to
several hundreds degrees centigrade.
• As temperature increases further the glass
gradually begins to soften and becomes
viscous liquid
• The melting temperature at which glass
becomes viscous liquid is used for
manufacturing glass
GLASS FIBERS
GLASS FIBERS
• Addition of GeO2 or P2O5 increases the
refractive index
• Where as doping the silica with Fluorine or
B2O5 decreases the refractive index
• Examples of fiber compositions are
GeO2-SiO2 Core ; SiO2 Cladding
P2O5 -SiO2 Core ; SiO2 Cladding
ACTIVE GLASS FIBERS
• In order to achieve new optical and
magnetic properties the rare earth
elements i.e. atomic number 57 – 71
is added into passive glass.
• The resulting material allows in
improving the characteristics of fibers
PLASTIC OPTICAL FIBER
• The plastic fiber core is made up of
polymethyl methacrylate or Perfluorinated
Polymer and is referred as PMMA POF and
PFPPOF respectively
• These fibers meet the growing demand for
delivering high speed services directly to work
stations and have very high bandwidth. Hence
these fibers are called graded index polymer
fibers.
• But these polymer fibers exhibits greater
signal attenuation compared to glass fibers
PHOTONIC CRYSTAL FIBERS
• In early 1990’s the researchers demonstrated a
new type of optical fiber structure
• Initially it is called holey fibers then later became
known as Photonic Crystal Fibers or a Micro
structured fiber
• The major differences between the PCF and the
conventional optical fibers is that the cladding in
some cases the core regions of a PCF contains
holes which runs along the entire length of the
fiber
PHOTONIC CRYSTAL FIBERS
• The material properties of the core and
cladding define the light transmission
characteristics of the conventional fibers.
• In PCF the internal microstructure offers
another dimension of light control in the fiber
• The two basic PCF categories are
(1) Index Guiding PCF
(2) Photonic Band Gap fibers
INDEX GUIDING PCF
INDEX GUIDING PCF
• This fiber has a solid core that is surrounded by a
cladding region which contains air holes running
along the length of the fiber
• The holes have a diameter ‘d’ and a pitch ‘ꓥ’
• Although the core and cladding are made up of
the same materials the presence of air holes
lowers the effective refractive index of refraction
in the cladding region
• Since n=1 for air and n= 1.45 for silica,
consequently the microstructure arrangement
creates a step index optical fibers
PHOTONIC BAND GAP - PCF
PHOTONIC BAND GAP - PCF
• In contrast to an index guiding PCF
here the fiber has a hollow core that
is surrounded by a cladding region
which contains air holes running
along the length of the fiber
• The holes have a diameter ‘d’ and a
pitch ‘ꓥ’
PHASE & GROUP VELOCITY
With in all Electromagnetic waves there are points
of constant phases for plane waves these constant
phase points form a surface which is referred to as a
wave front.
As a monochromatic light wave propagates along a
wave guide in the Z-direction these points of
constant phase travel at a phase velocity
PHASE & GROUP VELOCITY
There are situations where a group of waves with
closely similar frequencies propagate so that their
resultant forms a packet of waves. The formation of
such a wave packet forms a combination of two
waves with nearly equal frequencies
This wave packet does not travel at the phase
velocity of the individual waves but it is observed to
move at a group velocity
PHASE & GROUP VELOCITY
