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CHAPTER 4
LOSSES IN FIBER
OPTIC SYSTEM
Signal Degradation in the Optical Fiber
 Signal Attenuation
- It determines the maximum unamplified
repeaterless distance between transmitter
receiver.
 Signal Distortion
or
and
- Causes optical pulses broaden.
- Overlapping with neighboring pulses, creating errors in
the receiver output.
- It limits the information carrying capacity of a fiber.
The Basic Attenuation Mechanisms in a Fiber
1. Absorption
 It is related to the fiber material
2. Scattering
 It is associated both with the fiber material
and with the structural imperfections in the optical
waveguide.
3. Radiation losses/ Bending losses:
 It originates from perturbation (both microscopic and
macroscopic) of the fiber geometry.
Absorption
 Light travels best in clear substances. Impurities
such as metal particles or moisture in the fiber can
block some of the light energy, it absorb the light and
dissipate it in the form of heat energy, which caused
absorption loss.
 The solution is to use ultra-pure glass and dopant
chemicals to minimize impurities, and to eliminate
loss at the peak wavelength during the process of
fiber manufacturing.
Absorption
Absorption is caused by three different mechanisms:
1- Impurities in fiber material - occurs due to electronic
transitions between the energy levels and because of charge
transitions from one ion to another. A major source of
attenuation is from transition of metal impurity ions such
as iron, chromium, cobalt, and copper
2- Intrinsic absorption- Intrinsic absorption results from
electronic absorption bands in UV region and from atomic
vibration bands in the near infrared region. Absorption
occurs when a photon interacts with an electronic in the
valance band and excites it to a higher energy level
3- Atomic defects -imperfections in the atomic structure of the
fiber materials such as missing molecules, high density
clusters of atom group.
Absorption
Absorption
Atomic Defects
Extrinsic
(Impurity atoms)
Intrinsic
Absorption
Absorption in
Ultraviolet region
Absorption in
Infrared region
Scattering Loss
 Small variation in material density, chemical
composition, and structural inhomogeneity scatter
light in other directions and absorb energy from
guided optical wave.
 Scattering losses in glass arise from microscopic
variation in the material density from
1. Compositional fluctuations
2. Inhomogeneities or defects occurring during fiber
manufacture
Rayleigh Scatter
 Rayleigh scatter occurs at random when there are
small changes in the refractive index of materials in
which the light signal travels.
 In this case, it is the changes in the refractive index
of the core and the cladding of the fiber optic cable.
 This loss is caused by the miniscule variation in the
composition and density of the optical glass material
itself, which is related to the fiber manufacturing
process.
Rayleigh Scatter
 Diagram-2: Light scattered during transmission
Bending Loss
 Bending losses occurs in
two forms macrobending and
microbending. When a
cable is bent and it
disrupts the path of the
light signal. The tighter
the bends of a cable, the
greater it is of the light
loss.
Bending Loss
 Radiative losses occur whenever an optical fiber undergoes a
bend of finite radius of curvature.
 Macrobending:
Light lost from the optical core due to macroscopic effects such
as tight bends being induced in the fiber itself.
Macrobending losses are normally produced by poor handling of
fiber .
Poor reeling and mishandling during installation can create
severe bending of the fiber resulting in small but important
localized losses
 Microbending:
Light lost from the optical core due to microscopic effects
resulting from deformation and damage to the core cladding
interface. Occurs when a fiber is sheathed within a protective
cable. The stresses set up during the cabling process cause small
axial distortions (microbends)
Microbending loss
Attenuation
Attenuation
Scattering
Losses
Absorption
Intrinsic
Absorption
Extrinsic
(Impurity
atoms)
Absorption
in
Infrared
region
Absorption
in
Ultraviolet
region
Radiative
losses/ Bending
losses
Atomic
Defects
Inhomogeneities Compositional
or defects
fluctuations
in fiber
in material
Microscopic Macroscopic
bends
bends
Signal Distortion in Fibers
Signal Distortion in Fibers
 Optical signal weakens from attenuation
mechanisms and broadens due to distortion effects
 Eventually these two factors will cause neighboring
pulses to overlap.
 After a certain amount of overlap occurs, the
receiver can no longer distinguish the individual
adjacent pulses and error arise when interpreting
the received signal.
Dispersion
 Dispersion is the spreading out of a light pulse as it
travels through the fiber.
 Attenuation only reduces the amplitude of the
output waveform which does not alter the shape of
the signal.
 Dispersion distorts both pulse and analog
modulation signals
 Three types:



Modal Dispersion
Chromatic Dispersion
Polarization Mode Dispersion (PMD)
Dispersion
Modal Dispersion
 Modal Dispersion
 Spreading of a pulse because different modes (paths) through
the fiber take different times
 Only happens in multimode fiber
 Reduced, but not eliminated, with graded-index fiber
Chromatic Dispersion
 Different wavelengths travel at different speeds
through the fiber
 This spreads a pulse in an effect named chromatic
dispersion
 Chromatic dispersion occurs in both single mode and
multimode fiber


Larger effect with LEDs than with lasers
A far smaller effect than modal dispersion
Polarization Mode Dispersion
 Light with different polarization can travel at
different speeds, if the fiber is not perfectly
symmetric at the atomic level
 This could come from imperfect circular geometry or
stress on the cable, and there is no easy way to
correct it
 It can affect both singlemode and multimode fiber
Modal Distribution
 In graded-index fiber, the off-axis modes go a longer
distance than the axial mode, but they travel faster,
compensating for dispersion

But because the off-axis modes travel further, they suffer more
attenuation
Insertion Losses
 Most important performance indicator of a fiber optic
interconnection. This is the loss of light signal, measured
in decibels (dB), during the insertion of a fiber optic
connector.
 Some of the common causes of insertion losses includes:
(i) the misalignment of ferrules during connection,
(ii) the air gap between two mating ferrules, and
(iii) absorption loss from impurities such as scratches
and oil contamination
 Insertion loss can be minimized by proper selection of
interconnect materials, good polishing and termination
process of fiber connectors
Types of insertion losses
 Coupling Losses
- The connector assembly must maintain stringent
alignment tolerances to ensure low mating losses.
- The losses should be around 2 to 5 percent (0.1 to
0.2 dB) and must not change significantly during
operation and after numerous connects and
disconnects.
Types of insertion losses
 Splicing Losses
- Optical power loss at the splicing point of two ends
of optical fiber is known as splice loss.
- It is important to remember that actual splice-loss is
the measured splice-loss in both directions divided
with two.
Types of insertion losses
 Connector Loss
- Optical loss at connection points in fiber cables
- Connector losses are associated with the coupling of
the output of one fiber with the input of another
fiber, or couplings with detectors or other
components.
LOSSES CALCULATION
• Total Loss = Lsplice + Lfiber + Lconn. + Lnon-linear
Lsplice = Splice Loss
Lfiber = Fiber Loss
Lconn. = Connector Loss
Lnon-linear= Non-linear Loss
 Gain,G = Gainamp + Gnon-linear
Gainamp = Amplifier Gain
Gnon-linear = Non-linear Gain
Formula
 COUTION !!!
 in dBm or dBW
 PTX  in dBm or dBW
 Total Losses  in dB
 Total Gain  in dB
 PMARGIN  in dB
 PRX
dB = 10 log (Po/Pin)
= 10 log (PRX / PTX)
dBm = 10 log (P/1mW)
P  PRX atau PTX
dBm = 10 log (P/1mW)
P  PRX atau PTX