UNIT III
Optical fibers
Introduction: Optical fiber
• Optical fiber is a long thin strand of transparent dielectric
material (glass or plastic) about the diameter of a human hair.
• Optical fiber carries electromagnetic waves of visible and
infrared frequencies from one end to the other end of the fiber
by means of Total Internal Reflection.
• Optical fibers are arranged in bundles called optical cables
that can transmit large amounts of information at the speed of
light.
Constructionally, an optical fiber is a solid cylindrical glass rod called the core,
through which light in the form of optical signals propagates. This rod is
surrounded by another coaxial cylindrical shell made of glass of lower refractive
index called the cladding. This basic arrangement that guides light over long
distances is shown in
Structure of an Optical fiber
fiber core
glass or plastic
cladding
plastic jacket
Polyurethane is a unique material
that offers elasticity of rubber
combined with toughness and
durability of metal
Optical fiber consists of three sections
1) Core: It is an inner cylindrical material made up of glass or plastic.
2) Cladding: It is a cylindrical shell of glass or plastic material in which
Core is inserted.
3) Protective Jacket: The Cladding is enclosed in polyurethane jacket
and it protects the fiber from surroundings.
NOTE: The refractive index of core is slightly greater than the
refractive index of Cladding. The normal standard values are 1.48 and
1.46 respectively.
Fiber Optic Cables
How Does Optical Fiber Transmit Light??
Principle: Optical fiber works on the principle of Total internal reflection.
Once light ray enters into core, it propagates by means of multiple total
internal reflection’ s at core-cladding interface.
The light energy in the form of optical signals propagates inside the
core-cladding arrangement and throughout the length of the fiber by
a phenomenon called the Total Internal Reflection (TIR) of light.
This phenomenon occurs only when the refractive index of core is
greater than the refractive index of cladding and so the cladding is
made from glass of lower refractive index.
Because the cladding does not absorb any light from the
core, the light wave can travel great distances.
However, some of the light signal degrades within the
fiber, mostly due to impurities in the glass. The extent
that the signal degrades depends on the purity of the
glass and the wavelength of the transmitted light
Total Internal Reflection
Critical Angle
The critical angle is the angle of incidence above which total
internal reflection occurs.
From Snell’s law of refraction the angles of incidence and
refraction are related to each other and to the refractive indices
of the mediums as :
n1 sin  i  n2 sin  r
where  i is angle of incidence
 r is angle of refraction
   c   r  900
sin  c 
n2
sin 900
n1
n2
sin  c  ;
n1
 c  Arc sin
n2
n1
Transmission of a light ray in a perfect optical fiber
Any discontinuities or imperfections at the core–cladding interface
would probably result in refraction rather than total internal
reflection, with the subsequent loss of the light ray into the
cladding.
Acceptance angle
Numerical Aperture
Acceptance angle
 The maximum angle of incidence at the end face of an Optical
fiber for which the light ray can be propagated along CoreCladding interface is known as Acceptance angle. (The Max
value of for which the internal rays will strikes at critical angle)
 It is also called Acceptance cone half angle.
θi
Core-Cladding interface
θr
θi A
B
θ
Core n1
θr
C
Fiber axis
Cladding n2
Incident light ray
n1 >n2
Applying Snell’ s law of refraction at the point of entry of the ray into the core
n0 sin  i  n1 sin  r ..............(1)
from the right angle triangle ABC
 r    900
 r  900  
n0 sin  i  n1 sin(900   )
n0 sin  i  n1 cos
n1
sin  i  cos .........(2)
n0
Note: n0 is the refractive index of the medium from which the light
ray enters the fiber
when   critical angle( c )   i   max
sin  m 
n1
cos c ................(3)
n0
according to law of refraction
n1 sin i  n2 sin r
For core-cladding
interface
i   c  r  900
sin  c 
n2
sin 900
n1
n2
sin  c 
n1
 n2 
cos c  1  sin  c  1   
 n1 
2
n1 n2
............(4)
n1
2
cos c 
2
2
Contd...
substitute equation (4) in (3)
sin  max
n1 n2
n1
2
n1

n0
2
if the medium surrounding the fiber is air , then n0  1
sin  max  n1 n2
2
2
 max  sin 1 n1 n2
2
2
Which is the required expression for Acceptance Angle in
optical fibers.
Acceptance cone

Rotating the Acceptance angle about the fiber axis describes
the Acceptance Cone of the fiber.
Light launched at the fiber end within this Acceptance Cone
alone will be accepted and propagated to the other end of the
fiber by total internal reflection.
θmax
θmax
Acceptance Cone
Numerical Aperture
NA  sin  max
• The light gathering capacity of an
optical fiber is known as Numerical
Aperture and it is proportional to
Acceptance Angle.
sin  max
• It is numerically equal to sine of
Acceptance Angle.
NA  (n1  n2 )(n1  n2 )
The ratio between the difference
in refractive indices of Core and
Cladding to that of core is called the
fractional change Δ.
n1  n2

n0
2
NA  n1  n2
2

2
2
n1  n2
n1
NA  n1(n1  n2 )
n1  n2
NA  n1 2
2
NA  n1 2
Types of Fibers
TYPES OF OPTICAL FIBRES
On the basis of variation of RI of core, the optical fibers
are mainly classified into following types. i.e.,
1.Step Index fiber 2.Gradex Index fiber
Based on Mode of propagation, the fibers are further
divided into
1. Single Mode and 2.Multi Mode.
Single and multimode fiber
 Single-mode fiber
 Narrow core allow only one wavelength (or mode )
to pass
 NA and Acceptance angle is small.
 Require high power laser sources.
 Provide high speed, large bandwidth & long
distance transmissions
 Multimode fiber
 Relatively wide core allow different waves
modes to pass through.
 LED(incoherent) is used as the optical source.
 In multimode fibers there are two types of rays
travelling along the core:
 Axial ray(near the axis)
 Marginal ray (near the fiber surface)
Refractive index discontinuity at
core-clad boundary
Refractive index gradually deceases as
we move away from the center
The refractive index profile & ray transmission in
step index fibers
(a) multimode step index fiber; (b) single-mode step index fiber
Single Mode Step Index fiber
The RI is constant for the core in this fiber. As we go radically from center
of the core, the RI undergoes a step change at core-cladding interface .
The core diameter of this fiber is about 8 to 10µm and the outer diameter
of cladding is 60 to 70µm.
There is only one path for light ray propagation. Hence it is called single
mode step index fiber.
It is a reflective fiber since light is transmitted from one end to the other
end of a fiber by TIR.
These are extensively used because distortion and transmission losses are
very less.
Refractive index profile of
single mode step index fiber
60 to 70 µm
8 to 10 µm
RI
Radial distance
SINGLE MODE STEP INDEX FIBER
CORE
RAY
PROPAGATION
CLADDING
Multimode Step Index Fiber
The construction of this fiber is similar to Single mode step
index fiber but dimensions of Core and Cladding are much
larger to have more number of paths for light propagation.
The Core diameter varies from 50 to 200µm and the
Cladding diameter varies from 100 to 250µm.
It is also a reflective fiber since light is propagated in the
form of multiple TIRS.
 Multimode fiber
 Relatively wide core allow different waves modes
to pass through.
 LED(incoherent) is used as the optical source.
 In multimode fibers there are two types of rays
travelling along the core:
 Axial ray(near the axis)
 Marginal ray (near the fiber surface)
REFRACTIVE INDEX PROFILE OF MULTI MODE STEP INDEX FIBRE
100 to 250 µm
50 to 200 µm
RI
Radial distance
GRADED INDEX FIBRE
In this fiber , Radially the RI of Core continuously decreases
from center to the surface.
The RI is maximum at the center of Core and Minimum at
the Surface.
This fiber can be a single mode or Multimode ,the diameters
of core and cladding varies from 50-200µm and 100-250µm
respectively.
LIGHT PROPAGATION IN MUTI-MODE
GRADED INDEX FIBRE
Using
the
concepts
of
geometric optics, the gradual
decrease in refractive index
from the center of the core
creates many refractions of the
rays as they are effectively
incident on a large number or
high to low index interfaces.
• As RI changes continuously radially in Core, the light rays
suffers continuous refraction with in the Core from its center
to surface.
• Thus the propagation of light rays are not due to TIR but
by refraction. Therefore it is called Refractive fiber.
• In this fiber, the light rays travel at different speeds in
different parts.
• Near the surface RI is least so, the light rays travel faster
compared to the light rays near the axis. Because of this all
the rays almost arrive at the same time at the other end of the
fiber.
Advantages of optical fibers
•
•
•
•
•
•
Can carry much more information
Much higher data rates
Much longer distances than co-axial cables
Immune to electromagnetic noise
Light in weight
Unaffected by atmospheric agents
Disadvantage of fiber optic over copper wire cable
• Optical fiber is more expensive per meter
than copper
• Optical fiber can not be join together as
easily as copper cable. It requires training
and expensive splicing and measurement
equipment.