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Losses in Fiber Optics
Intramodel, bend loss-micro macro
scattering losses-Linear, Non linear,
Link Budget, Power Budget
Block diagram and working of OTDR
Attenuation means loss of light energy as the light
pulse travels from one end of the cable to the other.
It is also called as signal loss or fiber loss.
It also decides the the number of repeaters required
between transmitter and receiver.
Attenuation is directly proportional to the length of the
Attenuation is defined as the ratio of optical output
power to the input power in the fiber of length L.
α= 10log10 Pi/Po [in db/km]
where, Pi= Input Power
Po= Output Power, α is attenuation constant
The various losses in the cable are due to
The loss which exists when an optical fiber undergoes
bending is called bending losses.
There are two types of bending
i) Macroscopic bending
Bending in which complete fiber undergoes bends
which causes certain modes not to be reflected and
therefore causes loss to the cladding.
ii) Microscopic Bending
Either the core or cladding undergoes slight bends at
its surface. It causes light to be reflected at angles
when there is no further reflection.
ISO 9001 : 2008 certified
Absorption of light energy due to heating of ion
impurities results in dimming of light at the end of the
1. Intrinsic Absorption
2. Extrinsic Absorption
Caused by the interaction with one or more
components of the glass
Occurs when photon interacts with an electron in the
valence band & excites it to a higher energy level near
the UV region.
Also called impurity absorption.
Results from the presence of transition metal ions like
iron, chromium, cobalt, copper & from OH ions i.e. from
As an optical signal travels along the fiber, it becomes
This distortion is a sequence of intermodal and
1. Intermodal Dispersion
2. Intramodal Dispersion
Pulse broadening due to intermodal dispersion results
from the propagation delay differences between modes
within a multimode fiber.
It is the pulse spreading that occurs within a single
1) Material Dispersion:
Also known as spectral dispersion or chromatic
Results because of variation due to Refractive Index
of core as a function of wavelength, because of which
pulse spreading occurs even when different
wavelengths follow the same path.
2) Waveguide Dispersion:
Whenever any optical signal is passed through the
optical fiber, practically 80% of optical power is
confined to core & rest 20% optical power into
It occurs due to microscopic variations in the material
density, compositional fluctuations, structural in
homogeneities and manufacturing defects.
Rayleigh Scattering losses
Mie Scattering Losses
Waveguide Scattering Losses
Stimulated Brillouin Scattering(SBS)
Stimulated Raman Scattering(SRS)
i) Linear Scattering
a) Rayleigh Scattering Losses:
These losses are due to microscopic variation in the
material of the fiber.
Unequal distribution of molecular densities or atomic
densities leads to Rayleigh Scattering losses
Glass is made up of several acids like SiO2, P2O5,etc.
compositions, fluctuations can occur because of these
several oxides which rise to Rayleigh scattering losses
b) Mie Scattering Losses:
These losses results from the compositional
fluctuations & structural inhomogenerics & defects
created during fiber fabrications, causes the light to
scatter outside the fiber.
c) Waveguide Scattering Losses:
It is a result of variation in the core diameter,
imperfections of the core cladding interface, change in
RI of either core or cladding.
ii) Non-linear Scattering
a) SBS Scattering:
Stimulated Brillouin Scattering(SBS) may be regarded
as the modulation of light through thermal molecular
vibrations within the fiber.
α dB v watts
where, λ= operating wavelength μm
d= fiber core diameter μm
v = source bandwidth in GHz
b) SRS Scattering:
Stimulated Raman Scattering is similar to SBS except
that high frequency optical phonon rather than
acoustic phonon is generated in scattering processes.
λα dB watts
Collective excitation in a periodic arrangement of atoms
or molecules in solid.
What is OTDR?
It is a trouble shooting device to find faults, splices
and bends in fiber optic cable.
It is used to measure time and intensity of light
reflected on an optical fiber.
It can detect light loss and pinpoint trouble areas
making repair easy.
OTDR test can be anywhere along the length of
fiber from ten seconds to three minutes
Principle of Operation
OTDR emits a high-power pulse that hits the fiber
and bounces back.
What comes back is measured, factoring in time
and distance, and results in “trouble spots” which
can be targeted for repair.
The more quickly trouble areas are identified and
addressed the less fiber optic network will suffer
from data transfer problems.
A light pulsed is launched into the fiber in
forward direction from an injection laser using
a coupler or beam splitter.
Beam splitter or coupler makes possible to
couple the optical excitation power impulse
into the tested fiber and to deviate the
backscattered power to the optical receiver.
The backscattered light is detected using an
Avalanche Photodiode receiver.
Output of photodiode receiver drives an
Integrator improves SNR by giving an arithmetic
average over a number of measurements taken at
This signal is fed to Logarithmic amplifier and
average measurements for successive points within
the fiber are plotted as a Chart Recorder.
Overall link length can be determined from the
time difference between reflection from the fiber
input and output end faces.
The below fig shows the possible backscatter
plot for the fiber under test.
ISO 9001 : 2008 certified
ACS Important Questions
6. Losses in fiber optics (M=8)
1. With a neat diagram explain working of OTDR. (Twice)
2. Explain in brief the two fiber band losses.
a) Reflection b) Diffraction c) Absorption
4. Explain Dispersion with help of light theory.
5. Explain the losses due to scattering in FOC & state applications.
6- What is dispersion? Explain inter model dispersion.
7. ) An optical fiber communication is to be designed to operate over an 8
KM length without using repeaters. The use times of the chosen
component are source (LED) 8ns. Fiber inter model 5ns KM –1(Pulse
broadening) intramodel 1ns KM –1.Detector (p-I-n photodiode) 6 ns
b) The following parameters are established for a long haul single – mode
optical fiber operating at a wavelength of 1.3 um.
Mean power launched from the laser Transmitter –3 dbm
Cable fiber loss 0.4 db KM –1
Splices loss 0.1 db KM –1
Connector losses at the Transmitter & Receiver
When operating at 35 Mbits-1 (BER 10-9) –55 dbm
When operating at 400 Mbits-1 (BER 10-9) –44 dbm
Required safety margin 7db
i) The maximum possible link length without repeaters when operating at
35 Mbits-1 (BER 10-9). It may be assumed that there is no dispersion
equalization penalty at this bit rate.
ii) The maximum possible link length without repeaters when operating at
400 Mbits-1 (BER 10-9). & assuming no dispersion equalization penalty.
8. Describe attenuation in optical fiber