OCN_Unit.2.pptx

EC8751
OPTICAL COMMUNICATION
M.PRABU, M.E, (Ph.D)
ASSISTANT PROFESSOR
DEPARTMENT OF ECE
MNM JAIN ENGINEERING COLLEGE
CHENNAI-97
EMAIL: mprabuvlsi@gmail.com

UNIT II TRANSMISSION CHARACTERISTIC OF OPTICAL FIBER
2
Attenuation - absorption – scattering losses - bending losses - core and cladding losses -
signal dispersion – inter symbol interference and bandwidth - intra model dispersion - material
dispersion - waveguide dispersion - polarization mode dispersion - intermodal dispersion -
dispersion optimization of single mode fiber-characteristics of single mode fiber - R-I Profile -
cutoff wave length - dispersion calculation - mode field diameter.

Signal Attenuation & Distortion in Optical Fibers
4
 What are the loss or signal attenuation mechanism in a fiber?
 Why & to what degree do optical signals get distorted as they propagate down a fiber?
 Signal attenuation (fiber loss) largely determines the maximum repeater less separation
between optical transmitter & receiver.
 Signal distortion cause that optical pulses to broaden as they travel along a fiber, the
overlap between neighboring pulses, creating errors in the receiver output, resulting in the
limitation of information-carrying capacity of a fiber.

FIBER LOSSES
5
 Fiber Losses: Optical fiber cables suffer few losses. They are classified as Attenuation and
Dispersion. These two are further classified into several other losses.
 Attenuation Coefficient: Signal attenuation or transmission loss is defined as the ratio of the input
transmission optical power 𝑃𝑖𝑛 into a fiber to the output (received) optical power 𝑃𝑜𝑢𝑡 from the fiber.
This ratio is a function of the operating wavelength.
 The symbol α𝑑𝐵 is commonly used to express the attenuation in decibels (dB) per kilometre (L).

Optical fiber attenuation vs. wavelength
8

Attenuation
9
The basic attenuation mechanisms in the fiber are:
 Absorption (Material Absorption)
 Scattering Losses
 Nonlinear/ Radiative Loses

Absorption
10
 Material absorption is a loss mechanism related to both the material composition and the
fabrication process for the fiber.
 The optical power is lost as heat in the fiber.
 The light absorption can be intrinsic (due to the material components of the glass) or extrinsic
(due to impurities introduced into the glass during fabrication).
 Pure silica-based glass has two major intrinsic absorption mechanisms at optical wavelengths:
Intrinsic Absorption: 1. Electronic Absorption in the UV region
2. Atomic absorption in the infra-red region.
Extrinsic Absorption: 1. Transition metal impurities
2. OH (water) Ions impurities

11
Electronic Absorption in the UV region:
 The band gap of fused silica is about 8.9 eV (~140 nm). This causes strong absorption of light
in the UV spectral region due to electronic transitions across the band gap.
 An amorphous material like fused silica generally has very long band tails. These band tails
lead to an absorption tail extending into the visible and infrared regions. Empirically, the
absorption tail at photon energies below the band gap falls off exponentially with photon energy.
 In fundamental UV absorption edge, the peaks are centered in the UV region. Fused silica
valence electrons absorb light and can be ionized to conduction electrons.
 This gives rise to an energy loss in the light field contributing to transmission loss. The
absorption loss increases with the decrease of wavelength. The UV edge of electron absorption
band in both crystalline and amorphous materials follows Urbach’s Rule
 Here C and E0 are empirical constants. E is the photon energy. 𝜆𝑢𝑣 = attenuation constant in
the UV region

12
Atomic absorption in the infra-red region:
 In the infrared region, the absorption of photons is accompanied by transitions between
different vibrational modes of silica molecules. The fundamental vibrational transition of fused
silica causes a very strong absorption peak at about 9 μm wavelength.
 Nonlinear effects contribute to important harmonics and combination frequencies
corresponding to minor absorption peaks at 4.4, 3.8 and 3.2 μm wavelengths.
 A long absorption tail extending into the near infrared, causing a sharp rise in absorption at
optical wavelengths longer than 1.6 μm.
 Fundamental IR and Far-IR absorption edge is due to the molecular vibrations (Si-O) and
absorption peaks may extend into the longer wavelengths.
 IR absorption occurs because the photons are absorbed by atoms within the glass molecules
and converted to random mechanical vibrations typical of heating.

14
 Most impurity ions such as OH-, Fe2+ and Cu2+ form absorption bands in the near infrared
region where both electronic and molecular absorption losses of the host silica glass are very low.
 Near the peaks of the impurity absorption bands, an impurity concentration as low as one part
per billion can contribute to an absorption loss as high as 1 dB km-1.
 Nowadays, impurities in fibers have been reduced to levels where losses associated with their
absorption are negligible, with the exception of the OH- radical.
Transition metal impurities

15
OH (water) Ions impurities:
 Major extrinsic loss mechanism is caused by absorption due to water (as the hydroxyl or OH-
ions) introduced in the glass fiber during fiber pulling by means of oxyhydrogen flame used for
the hydrolysis reaction of the SiCl4, GeCl4, and PoCl3. This leads to Ion – Resonance
Absorption.
 The lowest attenuation for typical silica-based fibers occur at wavelength 1.55 μm at about 0.3
dB/km and at 1.3 μm about 0.5 dB/km approaching the minimum possible attenuation at this
wavelength.

16
Imperfection in the atomic structure- missing molecules, high-density clusters of atom groups,
oxygen defects.
 negligible when compared with other two
 Radiation damages material by changing internal structure.
Absorption by atomic defects:

SCATTERING LOSSES
17
 Scattering results in attenuation (in the form of radiation) as the scattered light may not continue to
satisfy the total internal reflection in the fiber core.
 The scattered ray can escape by refraction according to Snell’s Law.
 Scattering is due to irregularity of materials.
 When a beam of light interacts with a material, part of it is transmitted, part it is reflected, and part
of it is scattered.

18
Linear Scattering
1. Rayleigh Scattering
2. Mie Scattering
Non Linear Scattering
1. Stimulated Brillouin Scattering
2. Stimulated Raman Scattering
Scattering

19
 Rayleigh scattering results from random in homogeneities that are small in size compared with
the wavelength. It takes place due to the variations in the refractive index in glass. The glass used
is amorphous one, prepared by allowing glass to cool from molten state at high temperature until it
freezes.
During this transition two defects may arise.
1. Glass being amorphous is composed to randomly connected network of molecules. And
therefore it may contain regions in which the molecular density is higher or lower than the
average density in the glass.
2. Since the glass is made up of several oxides, such as SiO2, GeO2 and P2O5, compositional
fluctuations may occur.
 For a single component glass, the Rayleigh scattering coefficient is given by
Rayleigh scattering:

20
 The fictive temperature of glass is defined as the temperature at which glass can reach a state
of thermal equilibrium and closely related to the anneal temperature.
 Sub microscopic variations in the glass density and doping impurities are frozen into glass
during manufacture and they act as the reflecting and refracting facets to scatter a small portion of
light through the glass.
 These defects may be in the form of trapped bubbles, unreacted starting materials and
crystallized regions in the glass.

21
Mie scattering:
 Linear scattering may occur at inhomogeneities which are comparable in size with the guided
wavelength.
 When the size of scattering inhomogeneity is greater than λ/10, the scattering intensity has an
angular dependence and can be quite large.
 The scattering occurring due to such inhomogeneity is mainly in the forward direction and is
known as Mie Scattering.
 Depending on the fiber material, design and manufacture, Mie scattering can cause considerable
power loss. The inhomogeneity can be minimized by
1. Reducing imperfection during glass manufacturing process.
2. Careful controlled extrusion and coating of the fiber
3. Increasing the fiber guidance by increasing the relative refractive index between core and
cladding.

22
Stimulated Brillouin Scattering (SBS):
 It may be regarded as the modulation of light through thermal molecular vibration within the fiber.
The incident photons of light undergo nonlinear interaction to produce vibrational energy or
phonons in the glass as well as the scattered light or photons.
 The scattered light is found to be frequency modulated by the thermal energy and both upward
and downward frequency shifts are observed.
 The amount of frequency shift and the strength of scattering vary as the function of the scattering
angle maximum occurring at the backward direction and the minimum or zero being observed in the
forward direction.
 Thus Brillouin scattering mainly occurs in the backward direction which directs the power to the
source and the power of the receiver is reduced.

23
Stimulated Raman Scattering (SRS):
 The non-linear interaction in Raman scattering produces a high frequency phonon and a scattered
photon, where as low frequency phonons are produced in Brillouin scattering.
 In Raman scattering, light is predominantly in the forward direction and thus the power is not
reduced in the receiver.
 The threshold power level for the significant Raman scattering to occur is given by
Where d is the diameter of the fiber in μm, λ is the wavelength emitted by the source in μm, 𝛼𝑑𝐵 is
the fiber loss in dB/km and PR is the threshold optical power.

BENDING LOSSES
24
 Radiative losses occur whenever an optical fibre undergoes a bend of finite radius of curvature.
 Fibers can be subject to two types of bends viz. Micro bending and Macro bending or Constant
Radius Bending.

25
 It is a microscopic bending with repetitive changes in the axis of the core and it takes place due
to the slightly different contraction rate between the core and the cladding materials.
 It occurs due to non uniform lateral pressure created during cabling.
 Losses in the micro bending take place because the small bends act as the scattering facets and
these facets cause mode coupling to occur.
 Energy from the guided modes is cross coupled to the leaky mode and is lost through the
cladding.
 Micro bending are randomly distributed over the length of the fiber.
 Careful precaution in manufacturing and handling of fibers will reduce the loss.
 One method to minimize is done by extruding a compressible jacket over the fiber which will be
able to take on external tension without deforming the core.
Micro Bending:

26
 Potential micro bending losses may be minimized by
1. Designing fibers with large relative refractive index differences between the core and the
cladding.
2. Operating at the shortest possible wavelength.

27
Macro Bending:
 It is also called Constant Radius Bending.
 Bends are introduced while installing cable ducts to join corners.
 Sometimes these bends are quite sharp.
 These large radius bends introduce losses in the fiber.
 The bending may provide incidence angles less than the critical angle thereby allowing a part of
the light energy to escape from the fiber through the cladding.
 It is therefore necessary to ensure that no sharp bends are introduced in the path of the fiber.

28
Critical radius of Bend
 Critical radius of Bend: The relationship between the radius of curvature of the bend and
radiation attenuation coefficient 𝜆𝑟 is given by
R = radius of curvature; C1 and C2 are constants independent of R.
 Large bending losses tend to occur in multi mode fiber at a critical radius of curvature R𝐶
given by

SIGNAL DISPERSION
33
Spreading of light pulse as it propagates through the fiber. It results in ISI and also limits information
carrying capacity.
Intermodal Dispersion
Intramodal Dispersion
Dispersion

34
Intramodal Dispersion: Intramodal dispersion refers to dispersion or spreading of the pulse that
occurs within a particular mode and it is generally find in all types of fibers.
Intermodal Dispersion: Intermodal dispersion is caused by the time delay between various
modes to travel to the destination point. Thus, it is found to be present only in a multimode fiber
which supports more than one mode to carry the optical power and thus the delay is caused by the
time difference between the lowest and highest order modes.

INTERSYMBOL INTERFERENCE AND BANDWIDTH
35

36
Bandwidth Distance Product:A measure of information capacity of an optical fiber for digital
transmission is usually specified by the bandwidth distance product in GHz.km. For multi-
mode step index fiber this quantity is about 20 MHz.km, for graded index fiber is about 2.5 GHz.km & for
single mode fibers are higher than 10 GHz.km.
L
BW 

Intramodal Dispersion
37
 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.
 It is of two types
1) Material Dispersion which is wavelength based effect caused by glass of which fiber is made
2) Waveguide Dispersion occurs due to change in speed of wave propagating through
waveguide

Material Dispersion
38
Waves in the guide with different free space wavelengths travel at different group velocities due to
wavelength dependence of n1. The waves arrives at the end of the fiber at different times and hence
result in a broadened output pulse.

Waveguide Dispersion
44
Waveguide dispersion is due to the dependency of the group velocity of the fundamental mode as well as other
modes on the V number. In order to calculate waveguide dispersion, we consider that n is not dependent on
wavelength. Defining the normalized propagation constant b as:

52
Waveguide dispersion in single mode fiber:

Polarization Mode dispersion
54

Mode coupling
66
 We have thus far considered the propagation aspects of perfect dielectric waveguides.
 Coupling of energy arises from one mode to another mode arises because of the following issues:
Deviations of the fiber axis from straightness (Cabling induced micro bend)
Variations in the core diameter
Irregularities at the core–cladding interface
Refractive index variations
 Mode coupling may change the propagation characteristics of the fiber.
 Below illustrates two types of perturbation. It may be observed that in both cases the ray no longer
maintains the same angle with the axis.

67
 Individual modes do not normally propagate throughout the length of the fiber without large
energy transfers to adjacent modes, even when the fiber is exceptionally good quality and is not
strained or bent by its surroundings.
This mode conversion is known as mode coupling or mixing. It is usually analyzed using coupled
mode equations which can be obtained directly from Maxwell’s equations.

DISPERSION OPTIMIZATION OF SINGLE – MODE FIBERS
68
DESIGN OPTIMIZATION OF SINGLE – MODE FIBERS
CHARECTRISTICS OF SINGLE – MODE FIBERS
 Intermodal dispersion is totally absent.
 Intramodal dispersion is present (Waveguide dispersion is large).
 Overall dispersion is very less in single mode dispersion than multi mode fibers.
Attributes:
 Long life time
 Very low attenuation
 High quality signal transfer
 Largest available bandwidth
Design Optimization:
 Cut off Wavelength
 Dispersion
 Mode-field diameter
 Bending loss

Refractive Index (RI) Profiles
69
Fact 1) Minimum distortion at wavelength about 1300 nm for single mode silica fiber.
Fact 2) Minimum attenuation is at 1550 nm for single mode silica fiber.
Strategy: shifting the zero-dispersion to longer wavelength for minimum attenuation and dispersion by
Modifying waveguide dispersion by changing from a simple step-index core profile to more complicated
profiles.
There are four major categories to do that:
 1300 nm optimized single mode step-fibers
 Dispersion shifted fibers.
 Dispersion-flattened fibers.
 Large-effective area (LEA) fibers.

70
1300 nm optimized single mode step-fibers

72
Dispersion-flattened fibers.

73
Large-effective area (LEA) fibers.

77
Dispersion for non-dispersion-shifted fibers (1270 nm – 1340 nm)
Dispersion for dispersion shifted fibers (1500 nm- 1600 nm)
2
0
0
0 )
(
2
)
( 



 


S
0
0 )
(
)
( S
D 

 
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Mode-Field Diameter
78
 The mode field diameter is an important parameter for characterizing single mode fiber
properties which takes into account the wavelength dependent fiber penetration into the fiber
cladding.
 This parameter can be determined from the mode-field distribution of the fundamental LP01
mode.

OCN_Unit.2.pptx

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