Optical fibre communication

1. ECE – IV SEM
UNIT-II
Optical Fiber
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2. Optical Fiber Classification
 Can be classified in a number of ways
 On the basis of manufacturing
 Single component/Multi component
 Glass core glass clad
 Doped silica core clad
 All plastic fiber
 On the basis of profile
 Step index
 Multi mode
 Mono mode
 Graded index
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3. Fiber modes
 Electromagnetic field propagating in fiber can be described by
Maxwell’s equations whose solution yields number of modes M.
For a step index profile
where a is the core radius and V is the mode parameter (or
normalized frequency of the fiber)
 Depending on fiber
parameters, number of
different propagating
modes appear
 For single mode fibers
 Single mode fibers do not
have mode dispersion
(see the supplementary
‘Mode Theory’ for
further details)
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4. Fiber modes (cont.)
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5. Inter-modal (mode) dispersion
 Multimode fibers exhibit modal dispersion that is caused by different
propagation modes taking
different paths:
mod max min
T T
  
/
/
v s t
v c n





1 2 1
( )/
n n n
   2 1(1 )
n n
  
1 1 2
cos cos(0)
n n
   1 2 1
cos / 1 /
n n L s
     
1
 1

Path 1
Path 2
core
cladding
cladding
L
1
/cos /(1 )
s L L

   
 
max 1
/ / (1 ) /
T s v L c n
   
 
mod max min 1 1
1 1
mod
/ (1 ) /
1
T T Ln c Ln c
Ln Ln
c c


     

 
  
 
 
 
min
1
/
L
T
c n

1 1 2 2
cos cos
n n
 

s
1
n
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Rachna
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of
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6. Step Index Fiber
 Core and Cladding are glass with appropriate optical properties
 Buffer is plastic for mechanical protection
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7. The Optical Fiber
 Fiber optic cable functions as a ”light guide,” guiding the
light from one end to the other end.
 Categories based on propagation: Single Mode Fiber (SMF)
 Multimode Fiber (MMF)
 Categories based on refractive index profile Step Index Fiber (SIF)
 Graded Index Fiber (GIF)
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8. Manav
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9. Single Mode Step Index Fiber
 Only one propagation mode is allowed in a given wavelength.
 This is achieved by very small core diameter (8-10 μm)
 SMF offers highest bit rate, most widely used in telecom
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10. Ray description of different fibers
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11. Refraction and Reflection Snell’s Law:
Snell’s Law: n1Sin Φ1= n2 Sin Φ2
When Φ2= 90, Φ1= Φc is the Critical Angle
Φc=Sin-1(n2/n1)
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12. Step Index Multimode Fiber
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15. Step and Graded Index Fibers
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16. Graded Index Fiber
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17. Total Internal Reflection
 TIR
(a) A ray in thinly stratified medium becomes refracted as it passes from
one layer to the next upper layer with lower n and eventually its angle
satisfies TIR
(b) In a medium where n decreases continuously the path of the ray bends
continuously
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18. Skew Rays
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27. Higher the Wavelength More the
Evanescent Field
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28. Light Intensity
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33. Modal Dispersion
dispersion
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45. Optical fibers - attenuation
 Traditionally two windows available:
 1.3 mm and 1.55 mm
 The lower window is used
with Si and GaAlAs
and the upper window
with InGaAsP compounds
 Nowadays these attenuation windows
no longer separate (water-spike
attenuation
region can be removed)
 There are single- and mono mode
fibers that may have step or
graded refraction index profile
 Propagation in optical fibers
is influenced by attenuation,
scattering, absorption, and dispersion
 In addition there are non-linear
effects that are important in
WDM-transmission
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53. Scattering loss: from index discontinuity
 Scatterers are much smaller than the wavelength: Rayleigh and
Raman scattering
 Scatterers are much bigger than the wavelength: geometric ray
optics
 Scatterers are about the same size as the wavelength: Mie
scattering
 Scatterers are sound waves: Brillouin scattering
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54. Raman scattering
 A small fraction of Rayleigh scattered light comes off at the difference
frequency between the applied light and the frequency of a molecular
vibration (a Stokes line)
 In addition, some scattered light comes off at the sum frequency
(anti-Stokes
 MIE Scattering
 Similar effect to microbending loss
 Mie scattering depends roughly on λ-2; scattering angle also depends
upon λ
 In planar waveguide devices, roughness on side walls leads to
polarization-dependent loss
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55. Couplers and connectors
 Connectors:
 Metallic system: Wire: Soldering: Lossless: Economical
 Fiber system: fiber: Splicing: Loss: Economical
 Splicing is the permanent connection of two optical fiber.
 Two mechanism of splicing:
 Fusion
 Mechanical
 Splices are of two types:
 Midspan: the connecting of two cables
 Pigtail: assembly of a fiber that has been factory-installed into
a connector in one end, with the other end free for splicing to
a cable.
 The quality of splicing is measured by the insertion and reflection
losses caused by the splice
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56. Couplers and connectors losses
 Connectors losses at any splice stem from the fact that no all light
from one fiber is transmitted to another. The loss results from:
 Mismatch: due to fiber’s mechanical dimensions and numerical
aperture. An improvement in splicing technique can not solve
 the problem. Also called intrinsic connection losses.
Misalignment: are caused by some imperfection in splicing,
that theoretically can be eliminated. An improvement in
splicing technique can solve the problem. Also called extrinsic
connection losses.
 Misalignment Losses in fiber-to-fiber:
 Core misalignment and imperfections.
 Lateral (axial) misalignment
 Angular misalignment
 gap between ends contact
 Non-flat ends
 Cladding alignment
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