8/2/2019 Fiber Optics1
1/29
FiberFiber
OpticsOptics
8/2/2019 Fiber Optics1
2/29
Introduction
Optical fiber is a long thin transparentdielectric material which carries EMwaves of visible and IR frequenciesfrom one end to the other end of the
fiber by means ofTIR.
NOTE: Glass or Plastic is used asDielectric material.
Optical fibers works as Wave guides inoptical television signals, digital datato transmit voice television signals,
digital data to any desired distance
8/2/2019 Fiber Optics1
3/29
Optical fiber consists of three
sections1. Core 2. Cladding 3. Protective
Jacket
Core: It is an inner cylindrical material made up ofglassor plastic.
Cladding: It is a cylindrical shell of glass or plasticmaterial in which Core is inserted.
Protective Jacket: The Cladding is enclosed inpolyurethane jacket and it protects the fiber from
surroundings.
8/2/2019 Fiber Optics1
4/29
Structure of anOptical fiber
Poly urethane protective jacket
Cladding
Core
Principle:Optical fiber works on the principle of TIR. Once
light ray enters into core ,it propagates by means of multiple
TIR s at core-cladding interface.
8/2/2019 Fiber Optics1
5/29
c=
c
NORMALRERAR MEDIUM
DENSAR MEDIUM
8/2/2019 Fiber Optics1
7/29
1
2
0
1
2
0
21
sin
90sinsin
90
sinsin
nn
n
n
r
rnn
refractionoflawtoaccording
c
c
c
=
=
==
=
8/2/2019 Fiber Optics1
8/29
Acceptance Angle
The maximum angle of incidence at the end face of
an Optical fiber for which the light ray can bepropagated along Core-Cladding interface is knownas maximum Acceptance angle. It is also called
Acceptance cone half angle.
8/2/2019 Fiber Optics1
9/29
Core-Cladding interface
Fiber axis
Core n1
Cladding n2
A
B
C
ri
r
Incident light ray
8/2/2019 Fiber Optics1
10/29
Applying Snell s law for Air-Core media
)2.........(cossin
cossin
)90sin(sin
90
90
)1....(..........sinsin
0
1
10
0
10
0
0
10
n
n
nn
nn
ABCtriangleanglerightthefrom
nn
i
i
i
r
r
ri
=
=
=
=
=+
=
8/2/2019 Fiber Optics1
11/29
)4..(..........cos
)(1sin1cos
sin
90sinsin
90
sinsin
)3......(..........cossin
)(
1
2
2
2
1
2
1
22
1
2
0
1
2
0
21
0
1
n
nn
n
n
n
n
n
n
ri
rnin
refractionoflawtoaccording
n
n
anglecriticalwhen
c
cc
c
c
c
cm
mic
=
==
=
=
==
=
=
==
8/2/2019 Fiber Optics1
12/29
2
2
2
1
1
max
2
2
2
1
0
1
22
21
0
1
sin
sin
1,
sin
)3()4(
nn
nn
nthenairisfiberthegsurroundinmediumtheif
n
nn
n
n
inequationsubstitute
m
m
=
=
=
=
Which is required expression for Maximum
Acceptance Angle in optical fibers.
8/2/2019 Fiber Optics1
13/29
Acceptance Cone
Rotating the Acceptance angle about the
fiber axis describes the Acceptance Cone ofthe fiber.
Light launched at the fiber end within thisAcceptance Cone alone will be accepted and
propagated to the other end of the fiber bytotal internal reflection.
m
m
Acceptance Cone
8/2/2019 Fiber Optics1
14/29
Numerical Aperture
The light gathering capacity of an
optical fiber is known as NumericalAperture and it is proportional toAcceptance Angle.
It is numerically equal to sine of
minimum Acceptance Angle.
=
=
+=
=
+=
=
=
=
2
2
)(
))((
sin
sin
1
2
1
21
211
1
21
2121
2
2
2
1
0
2
2
2
1
max
max
nNA
nNA
nn
nnnNA
n
nn
nnnnNA
nnNA
n
nn
NA
The ratio between the difference
in RIs of
Core and Cladding to that of RI of
core is called the fractional change.
8/2/2019 Fiber Optics1
15/29
TYPES OF OPTICAL FIBRES
On the basis of variation of RI of core, theoptical fibers are mainly classified into
following types. i.e.,1.Step Index fiber 2.Gradex Index fiber
NOTE: Based on Mode of propagation, thefibers are further divided into Single Modeand Multi Mode.
8/2/2019 Fiber Optics1
16/29
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 stepchange at core-cladding interface .
The core diameter of this fiber is about 8 to 10m and theouter diameter of cladding is 60 to 70m.
There is only one path for light ray propagation. Hence it iscalled single mode step index fiber.
It is a reflective fiber since light is transmitted from oneend to the other end of a fiber byTIR.
These are extensively used because distortion andtransmission losses are very less.
8/2/2019 Fiber Optics1
17/29
Refractive index profile ofsingle mode step index fiber
Radial distance
60 to 70 m
8 to 10 m
RI
8/2/2019 Fiber Optics1
18/29
SINGLE MODE STEP INDEX FIBER
CORE
CLADDING
RAY
PROPAGATION
8/2/2019 Fiber Optics1
19/29
Multimode Step Index Fiber
The construction of this fiber is similar toSingle mode step index fiber butdimensions of Core and Cladding are
much larger to have more number ofpaths for light propagation.
The Core diameter varies from 50 to
200m and the Cladding diameter variesfrom 100 to 250m.
It is also a reflective fiber since light is
propagated in the form of multiple TIRS.
8/2/2019 Fiber Optics1
20/29
Radial distance
100 to 250 m
50 to 200 m
RI
REFRACTIVE INDEX PROFILE OF MULTI MODE STEP INDEX FIBRE
8/2/2019 Fiber Optics1
21/29
GRADED INDEX FIBRE
In this fiber , Radially the RI of Corecontinuously decreases from center to thesurface.
The RI is maximum at the center of Coreand Minimum at the Surface.
This fiber can be a single mode orMultimode ,the diameters of core andcladding varies from 50-200m and 100-250m respectively.
8/2/2019 Fiber Optics1
22/29
Radialdistnce
60 to 70 m
8 to 10 m
RI
REFRACTIVE INDEX PROFILE OF SINGLE MODE GRADED INDEX FIBER
8/2/2019 Fiber Optics1
23/29
Radialdistance
100 to 250 m
50 to 200 m
RI
REFRACTIVE INDEX PROFILE OF MULTIMODE GRADED INDEX FIBRE
8/2/2019 Fiber Optics1
24/29
As RI changes continuously radially inCore, the light rays suffers continuous
refraction with in the Core from its centerto surface.
Thus the propagation of light rays are
not due to TIR but by refraction. Thereforeit 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 lightrays travel faster compared to the lightrays near the axis. Because of this all the
8/2/2019 Fiber Optics1
25/29
CLADDING
CORE
LIGHT PROPAGATION IN MUTI-MODEGRADED INDEX FIBRE
8/2/2019 Fiber Optics1
26/29
Optical fiber Communication System
An efficient optical fiber communication systemrequires high information carrying capacity suchas voice signals, video signals over long
distances with a minimum number of repeaters.It essentially consists of following parts.
1.Encoder 2. Transmitter 3.Wave guide
4.Receiver 5.Decoder
8/2/2019 Fiber Optics1
27/29
1.Encoder: It converts electric signal corresponding toanalog information such as voice, figures, objects etc intoa binary data. This binary data corns out in the form ofstream of electrical pulses.
2.TRANSMITTER: It mainly consists of drive circuit and alight source. Drive circuit supplies the electric pulses tothe light source from the encoder.
NOTE: LED or diode laser is used as light source and it
converts electrical signals are infected into optical signals.These optical signals are injected into wave guide.
8/2/2019 Fiber Optics1
28/29
Electrical
signal
ENCODER
DRIVE SOURCE LIGHT SOURCE
TRANSMITTER
OPTICAL SIGNAL
AMPLIFIER
PHOTO
DETECTOR
SIGNAL
RESTORER
DECODER
Wave guide
receiver
8/2/2019 Fiber Optics1
29/29
Attenuation or Power loss in Opticalfibers
The power of the light at the out put end isfound to be always less than the power
launched at the input end.
Attenuation is found t be a function of fibermaterial, wavelength of light and length of
the fiber and it is measured in terms of thedecibel.
Attenuation mainly three types.
1.Scattering losses
2 Absorption losses
Top Related