Fiber Optics
Introduction
Optical fiber is a long thin transparent dielectric material which carries EM
waves of visible and IR frequencies from one end to the other end of the fiber by
means of Total Internal Reflection (TIR).
NOTE: Glass or Plastic is used as Dielectric material.
Optical fibers works as Wave guides in optical television signals, digital data
to transmit voice television signals, digital data to any desired distance from one
end to the other end of the fiber.
What is Fiber Optics?
• Fiber optics (optical fibers) are long, thin strands of very pure glass about the
diameter of a human hair.
• They are arranged in bundles called optical cables and used to transmit light
signals over long distances.
• Optical Fiber are Composed of either glass or plastic and sometimes both are
very flexible
Optical fiber consists of three sections
1. Core
2. Cladding 3. Protective Jacket
Core: It is an inner cylindrical material made up of glass or plastic.
Cladding: It is a cylindrical shell of glass or plastic material in which Core is
inserted.
Protective Jacket: The Cladding is enclosed in polyurethane jacket and it protects
the fiber from surroundings.
NOTE: The RI of core is slightly greater than the RI of Cladding. The normal
standard values are 1.48 and 1.46 respectively.
Structure of an Optical fiber
How Does Optical Fiber Transmit Light??
Principle:
Optical fiber works on the principle of Total Internal Reflection (TIR). Once
light ray enters into core, it propagates by means of multiple TIR’s at corecladding interface.
The light in a fiber-optic cable travels through the core (hallway) by
constantly bouncing from the cladding (mirror-lined walls), a principle called total
internal reflection.
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
Acceptance Angle
The maximum angle of incidence at the end face of an Optical fiber for
which the light ray can be propagated along Core-Cladding interface is known as
maximum Acceptance angle. It is also called Acceptance cone half angle.
Applying Snell’s law for Air-Core media
n0 sin  i  n1 sin  r ..............(1)
from the right angle triangle ABC
 r    90 0
 r  90 0  
n0 sin  i  n1 sin( 90 0   )
n0 sin  i  n1 cos 
sin  i 
n1
cos  .........( 2)
n0
applying ' snell ' s ' law' at ' core' claddiing ' boundary
n1 sin   n2 sin 90
sin  
n2
n1
cos   1 
n2 2
n1 2
from' equation' (2)' we' get
sin  i 
2
n1
n
1  22
n0
n1
n1  n2
2
n
sin  i  1
n0
n1
n1  n2
2
sin  i 
2
2
n0
For air medium n0  1
sin  i  n1  n2
2
2
 i  sin 1 n1  n2
2
2
This is the expression for the acceptance angle.
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.
Numerical Aperture:
• The light gathering capacity of an optical fiber is known as Numerical Aperture
and it is proportional to Acceptance Angle.
• It is numerically equal to sine of minimum Acceptance Angle.
The NA is related to the critical angle of the waveguide and is defined as:
NA  sin  max
NA  sin  i   n12  n22
Fractional Index Difference  
NA  n12  n22 
n1  n2
n1
n1  n2 n1  n2  
2 n12
n1  n2
2
sin  max 
n1  n2
n1
2
n0
NA  n1  n2
2
I
2
NA  (n1  n2 )( n1  n2 )

n1  n2
n1
NA  n1 (n1  n2 )
n1  n2
NA  n1 2
2
NA  n1 2
Modes of Propagation :
Even though the light ray is incident with acceptance angle but there is no
guarantee for that light to propagate through the fiber. The directions along
which the incident ray propagates through the fiber are called modes of
propagation. If the fiber supports only one allowed direction it is called a single
mode fiber (SMF) otherwise it is said to be multi mode fiber (MMF).
Incase of SMS the light ray propagates along the axis of the fiber, it is called axial
ray. Incase of MMF the light rays are called Zigzag rays. In fact the number of
modes is decided by d/λ value. Where‘d’ the diameter of the core and ‘ λ’ is the
wavelength of the light. Incase of SMF ‘d’ is of the order of 4 µm where as incase
of MMF it is nearly 100 µm.
Single mode fibers are characterized by Step Index (SI) Where as Multimode
fibers are characterized by Graded Index (GRIN). Incase of SI fiber the refractive
index from core to cladding changes through a single step and incase of GRIN
fiber it changes through several steps gradually. It is achieved by taking the core
as concentric layers, where the refractive index decreases from centre to
outward.
• The optical fiber support a set of discrete modes
• Qualitatively these modes can be thought of as different propagation angles
Number of Modes:
• The number of modes can be characterized by the normalized frequency
V
2

a n12  n22
• Most standard optical fibers are characterized by their numerical aperture
NA  n12  n22
• Normalized frequency is related to numerical aperture
• The optical fiber is single mode if V<2.405
V 
2

a NA
• For large normalized frequency the number of modes is approximately
# Modes 
Normalized frequency for Fiber
4

2
V2
V  1
Numerical Aperture Examples :
Optical Fiber Attenuation:
Reduction of signal magnitude, or loss, normally measured in decibels. Fiber
attenuation is normally measured per unit length in decibels per kilometer. The
decrease in signal strength along a fiber optic waveguide is caused by absorption
and scattering. Attenuation is usually expressed in dB/km.
The extremely low attenuation or transmission loss of optical fibers is one of
the most important factors in bringing its wide acceptance as a medium of
transmission. Signal transmission within optical fibers, as with metallic
conductors, is usually abbreviated as dB. The decibel (dB) is a convenient way of
comparing two divergent power levels, say, P1 and P2. This is defined as
Optical fiber attenuation is the measurement of light loss between input and output.
Total attenuation is the sum of all losses. So it is the sum of material absorption,
Rayleigh scattering in the fiber and waveguide imperfections.
There are other factors which could also cause light loss, such as light leakage
when the fiber is under micro bending. Attenuation limits how far a signal can
travel through a fiber before it becomes too weak to detect.
:: Material Absorption
Material absorption can be divided into two categories. Intrinsic absorption losses
correspond to absorption by fused silica (material used to make fibers) whereas
extrinsic absorption is related to losses caused by impurities within silica.
A) Intrinsic Absorption
Any material absorbs at certain wavelengths corresponding to the electronic and
vibrational resonances associated with specific molecules. For silica molecules,
electronic resonances occur in the ultraviolet region (wavelength < 0.4um),
whereas vibrational resonances occur in the infrared region (wavelength > 7um).
Because of the amorphous nature of fused silica, these resonances are in the form
of absorption bands whose tails extend into the visible region. The following
picture shows that intrinsic material absorption for silica in the wavelength range
0.8~1.6um is below 0.1dB/km. In fact, it is less than 0.03dB/km in the 1.3 to
1.6um wavelength range which are commonly used for light wave systems.
B) Extrinsic Absorption
Extrinsic absorption results from the presence of impurities. Transition-metal
impurities such as Fe, Cu, Co, Ni, Mn, and Cr absorb strongly in the wavelength
range 0.6~1.6um. Their amount should be reduced to below 1 part per billion to
obtain a loss level below 1dB/km. Such high-purity silica can be obtained by using
modern techniques.
The main source of extrinsic absorption in state-of-the-art silica fibers is the
presence of water vapors. A vibrational resonance of the OH ion occurs near
2.73um. Its harmonic and combination tones with silica produce absorption at the
1.39um, 1.24um and 0.95um wavelengths. The three spectral peaks seen in above
figure occur near these wavelengths and are due to the presence of residual water
vapor in silica.
In new kind of glass fiber, known as dry fiber, the OH ion concentration is reduced
to such low levels that the 1.39um peak almost disappears. This is show in the
below picture. Such fibers are used to transmit WDM (Wavelength Division
Multiplexer) signals over the entire 1.30um to 1.65um wavelength range.
:: Rayleigh Scattering
Rayleigh scattering is a loss mechanism arising from local microscopic fluctuation
in density. Silica molecules move randomly in the molten state and freeze in place
during fiber fabrication. Density fluctuation lead to random fluctuations of the
refractive index on a scale smaller than the optical wavelength. Light scattering in
such a medium is known as Rayleigh scattering.
Scattering depends not on the specific type of material but on the size of the
particles relative to the wavelength of light. The closer the wavelength is to the
particle size, the more scattering. In fact, the amount of scattering increases rapidly
as the wavelength decreases.
:: Waveguide Imperfections
An ideal single mode fiber with a perfect cylindrical geometry guides the optical
mode without energy leakage into the cladding layer. But in reality, imperfections
at the core-cladding interface, such as random core-radius variations, can lead to
additional losses which contribute to the total fiber loss.
The physical process behind such losses is Mie scattering, occurring because of
index inhomogeneities on a scale longer than the optical wavelength.
This has been taken good care of in optical fiber manufacturing and the core radius
is made sure not to vary significantly along the fiber length.
What are the advantages of optical fiber communication?
1) Fiber-optic cable is a HOLLOW pipe made up of silica glass
or silicon-di-oxide.
2) It is coated with mirror polish on its inner wall, so that light
rays from its one end are reflected inside and come to other
end.
3) It uses light wave for communication of extremely high
frequency signal of about (3 x 106GHz). Hence, it has highest velocity also.
4) Since frequency of light is extremely high, it can carry large amount of
information.
5) So its bandwidth is highest.
6) It is very small in size.
7) Due to small size, very large number of optical fibers can be fitted in small
cable.
8) It is very light in weight.
9) Hence, it is very useful in airplanes, space shuttles etc. where less weight is
important.
10) It requires very simple transmitter and receiver circuits.
11) As it uses light wave for communication, so the signals are NOT affected by
external electrical noise.
12) It has negligible attenuation over a very long distance, unlike copper cable.
13) It uses light waves for communication hence it is shockproof.
14) Since it is shockproof, it is very useful in sensitive areas like petroleum
industries, oil and natural gas industries, cotton industries etc.
15) It CANNOT be tapped unlike copper cables.
16) There is NOT any leakage of signals so communication is secured.
17) It is very strong, flexible and can work on high temperature.
18) It does NOT have corrosion due to water, chemicals and high humidity etc.
19) It is cost effective and maintenance free.
20) It is very easy to install. It does NOT require skilled labor.
Block diagram of fiber-optic communication system –
Following is the block diagram as per syllabus. The working of each block
(in brief) is also given below (do NOT draw diagram of computer and telephone in
examination) –
Block diagram of fiber-optic communication system
Transmitter section – its main function is to transmit the information signals like
voice, video or computer in the form of light signals. As shown above, the
information at input is converted into digital signals by coder or converter circuit.
This circuit is actually ADC (analog to digital converter). Thus, it converts analog
signals into proportional digital signals. If the input signals are computer signals,
they are directly connected to light source transmitter circuit.
The light source block is a powerful light source. It is generally a FOCUS type
LED or low intensity laser beam source or in some cases infrared beam of light is
also used. The rate at which light source turns ON/OFF, depends on frequency of
digital pulses. Thus, its flashing is proportional to digital input.
In this way, digital signals are converted into equivalent light pulses and
focused at one end of fiber-optic cable. They are then received at its other end.
Fiber–optic cable – when light pulses are fed to one end of fiber-optic cable, they
are passed on to other end. The cable has VERY LESS attenuation (loss due to
absorption of light waves) over a long distance. Its bandwidth is large; hence, its
information carrying capacity is high.
Receiver section – at receiving end, a light detector or photocell is used to detect
light pulses. It is a transducer, which converts light signals into proportional
electrical signals. These signals are amplified and reshaped into original digital
pulses, (while reshaping, distortion & noise are filtered out) with the help of
shaper circuit.
Then the signals are connected to decoder. It is actually ADC circuit (Analog
to Digital Converter), which converts digital signals into proportional analog
signals like voice, video or computer data. Digital signals for computer can be
directly taken from output of shaper circuit.
Thus, this total unit is used fiber optic communication system. However if
the distance between transmitter and receiver is very large, then REPEATER
UNITS are used. Due to repeaters signals attenuation is compensated. For this,
light signals at far end are converted into electrical signals, amplified and
retransmitted. Such repeater unit is also called RELAY STATION.
Optical Fiber Sensor
Optical fiber sensor: A sensor that measures a physical quantity
based on its modulation on the intensity, spectrum, phase, or
polarization of light traveling through an optical fiber.
Advantages of optical fiber sensors
Compact size
Multi-functional
Remote accessible
Multiplexing
Resistant to harsh environment
Immunity to electro-magnetic interference
Optical Fiber Sensor Types:
Intrinsic: the effect of the measurand on the light being transmitted
take place in the fiber
Extrinsic: the fiber carries the light from the source and to the
detector, but the modulation occurs outside the fiber
Optical Fiber Sensor Types
Point sensor: detect
measurand variation
only in the vicinity of
the sensor
Optoelectronics
Sensing
element
Output, M(t)
Multiplexed sensor:
Optoelectronics
Multiple localized sensors
are placed at intervals along
Output, M(t, Zi)
the fiber length.
Distributed sensor:
Sensing is distributed
along the length of the
fiber
Optoelectronics
Output, M(t,z)
Optical Fiber Sensor Types:
Intensity-based: measure physic measurand based on the
intensity of the light detected through the fiber, e.g. fiber break,
OTDR
Interferometric (phase modulation):
– Fabry-Perot Interferometry
Grating based (wavelength modulation)
Fiber Bragg Grating (FBG)
Long Period Fiber Grating (LPFG)
Intensity-based Optical Fiber Sensor:
Advantages:
• Simple signal processing
• Inexpensive measurement instrument
Disadvantages:
• Susceptible to power fluctuation of the light source
• Susceptible to fiber bending losses
• Variation in modal power distribution in Multi-mode fiber (MMF)
Intensity-based Optical Fiber Sensor
Reference: “Split-spectrum intensity-based optical
fiber sensors for measurement of
microdisplacement, strain, and pressure”, by Anbo
Wang et al.