Fiber Attenuation &
Dispersion
Attenuation
Attenuation in optical fibers-It is the
reduction in intensity of the light
beam or optical power as light travels
down a fiber. It is measured in
decibels (dB/km).
Attenuation in optical fibers is caused
primarily by :
Light Scattering
Absorption
Scattering and Absorption
LIGHT SCATTERING
Rough and irregular surfaces, even at the
molecular level of the glass, can cause
light rays to be reflected in many random
directions. Another term commonly used
for this type of reflection is “diffuse
reflection”.
Light scattering depends on the
wavelength of light being scattered.
Absorption
Absorption is a major cause of
signal loss in an optical fiber.
Absorption is defined as the portion
of attenuation resulting from the
conversion of optical power into
another energy form, such as heat.
Contd.
Absorption in optical fibers is
explained by three factors:
Imperfections in the atomic structure
of the fiber material.
The intrinsic or basic fiber-material
properties.
The extrinsic (presence of impurities)
fiber-material properties
Contd……
Imperfections in the atomic structure
induce absorption by the presence of
missing molecules or oxygen defects.
Intrinsic Absorption: Intrinsic absorption is
caused by the interaction of the propagating
waves with one or more major components
of glass. It is due to basic fiber-material
properties.
Contd.
Extrinsic Absorption: caused by impurities
introduced into the fiber material such as iron,
nickel, and chromium, are introduced into the
fiber during fabrication. Extrinsic absorption is
caused by the electronic transition of these metal
ions from one energy level to another.
It is also due to OH- ions in glass.
BROADENING:
FIBER MATERIAL
FIBER STRUCTURE
SOURCE SPECTRAL WIDTH
DISPERSION
Dispersion is the "spreading" of a light pulse as it travels
down a fiber. As the pulses spread, or broaden, they tend
to overlap, and are no longer distinguishable by the
receiver as 0s and 1s.
Light pulses launched close together (high data rates)
that spread too much (high dispersion) result in errors and
loss of information. Thus the smaller the pulse dispersion,
the greater will be the information –carrying capacity of
the system.
MECHANISMS OF DISPERSION
INTERMODAL DISPERSION: in multimode
fibers
MATERIAL DISPERSION : in single mode
fibers
WAVEGUIDE DISPERSION : in single mode
fibers
Order of magnitude :
Intermodal dispersion ~ ns/km
Material and waveguide dispersion ~ ps/km
Modal dispersion is significant in multimode
applications, where the various modes of light traveling
down the fiber arrive at the receiver at different times,
causing a spreading effect.
Chromatic ( Material) dispersion occurs as a result of
the range of wavelengths in the light source. Light from
lasers and LEDs consists of a range of wavelengths. Each
of these wavelengths travels at a slightly different speed.
Over distance, the varying wavelength speeds cause the
light pulse to spread in time. This is of most importance in
single-mode applications.
Contd…
Waveguide dispersion occurs because the mode
propagation constant is a function of the size of the
fiber's core relative to the wavelength of operation.
Waveguide dispersion also occurs because light
propagates differently in the core than in the cladding.
Waveguide dispersion and material dispersion have
opposite signs, so they tend to counteract one another.
1.3um is dispersion free wavelength in single mode
fiber.
MATERIAL DISPERSION
Arises due to wavelength dependence of the
refractive index of the material.
n = n(w)
Wave number k(w) =w/c *n(w)
If length of medium is L
Time taken τ(λ) = L dk/d λ
= L/c [ n(λ) - λ *dn/d λ]
Each source has a finite spectral width and each wavelength
travels with a different group velocity. So different λ take
different time τ(λ)
∆ τ m =d τ /d λ *∆ λ
∆ τ m = - L/c (λ2 d2n/d λ 2 ) ∆ λ / λ
Dispersion coefficient
Tm = ∆ τ m /(L ∆λ) ps/km.nm
= - (λd2n/d λ 2 ) / c
Example:
For pure silica
λ 2 d2n/d λ 2 = 0.0289 at λ = 0.85μm
Tm =113 ps/km.nm
• For LED source ∆ λ =25nm
So ∆ τ m = 2.8ns/km
λ 2 dn2/d λ 2 = -0.010 at λ = 1.55 μ m
Tm = 22ps/km.nm
Empirical formula
• For pure silica
n(λ) = [ 1+ A λ 2 /(λ 2 – B2) + C λ 2 /(λ 2 – D2)
+E λ 2 /(λ 2 – F2) ] ½
A = 0.6961663 B = 0.0684043
C = 0.4079426 D = 0.1162414
E = 0.897494 F = 9.8961610
Applicable for wavelength region of interest
for FOCS
Variation of Tm for silica
Τm
Ps/Km.nm
λ(Tm =0) = 1.274 μm
1 1.1 1.2 1.3 1.6
λ( μm)
INTERMODAL DISPERSION
This dispersion is arises due to different propagation
velocity of an optical signal in different modes as it
travels through a fiber.
θ2 ray1
θ1
ray2
θ2 > θ 1
Ray 2 takes more time than Ray 1
Pulse Dispersion
In a multimode fiber since many modes propagates at
the same time, they take variable length of
time to reach the detector. This results in a pulse
stretching characteristics and is known as pulse
dispersion.
It limits the rate at which data can be practically
transmitted.
Output pulses has reduced amplitude as well as increased
width.
The greater the length of fiber, the less is the distortion
effect.
Reduction of dispersion
As, ‘V’ parameter is defined
V = k0*a* √n12-n22
For single mode 0 a cladding
Where,
∆ =(n12 - n22 )/2n12
n1=refractive index of core
n2=refractive index of cladding
a = radius of core
q= ∞ - Step index
q= 2 - Parabolic index
q= 2-2 ∆ - Optimum profile
β-parameter
β =n(r) cos θ(r)
n(r) – refractive index at a distance r from the axis
θ(r) – the angle that the ray makes with the axis
Parameter β is a characteristic of a guided ray in a graded index
fiber.A ray moves in such a way that β remains constant
n2 λc = 1.05μm and V λ = 2.88 μm
Tm
T ps/Km.nm
20 Tch
0
Tw
-10
1.0 1.1 1.31 1.6 1.7
λ(μm)
Dispersion shifted fiber
*Zero dispersion wavelength shifted to
1550nm
*Minimum loss and very low dispersion
*High bandwidth systems with very long
repeater less transmission
30
20
10
-10
-20 .
-30
-40
Dispersion compensation fibers
Specially designed fibers
Dispersion coefficient is negative and large
at 1550nm
Compensation of dispersion at a
wavelength around 1550nm in a 1310nm
optimized single mode fiber
Dispersion compensation efficiency is
given by Figure of merit
FOM=|D|/Total fiber loss
Why Dispersion Compensation?
At 1550nm – attenuation lower
EDFA developed at 1550nm
Hence up gradation of existing links
installed for 1310nm operation to
1550nm operation
Obviously replacing the fibers
involves huge costs