Optical Fiber Communication.

TRANSMISSION CHARACTERISTICS OF
OPTICAL FIBER.
Prof. Amogha B,
Dept. of EC&E,
JIT, Davangere.

OUTLINE
 Introduction,
 Attenuation,
 Absorption,
 Scattering losses,
 Bending loss,
 Dispersion,
 Intra model dispersion,
 Inter model dispersion.
12/23/2017Dept. of EC&E, JIT, Davangere.
2

INTRODUCTION
 Four losses are discussed- Absorption, Scattering,
Bending and dispersion.
 During the transmission the attenuation and
bandwidth are considered
 1966 a optical fiber using glass.
 Attenuation is caused due to material impurities.
 Bandwidth is reduced due to dispersion of signals.
3
Video: Showing on Plastic Tube- Red is longest wavelength VideoFiber
Optics Live! Fiber Attenuation.mp4

ATTENUATION
 One of the main reason why the optical
fiber is included in the communication
field.
 The loss of optical fiber should be less
than metal conductors 5dB/Km.
 Gradual decrease in intensity of any kind
of quantity passing through the medium or
reduction in the signal strength during
transmission.
4

CONTI.
 Signal attenuation within optical fibers is
usually expressed in the logarithmic unit of
the decibel and used to compare power
levels.
 Formulae for calculating Attenuation.
5
Video: Attenuation VideoFiber Optics Live!
Fiber Attenuation.mp4

Material Absorption Losses In Silica Glass
Fiber
 Material absorption is a loss mechanism
related to both the material composition
and the fabrication process for the fiber.
 Losses are scattering, splices, connector
loss etc.
 Two types intrinsic and extrinsic(ion
impurities).
6

INTRINSIC ABSORPTION.
 The main reason of this absorption is usage of pure
glass without adding any impurities.
 Choosing the perfect material is the solution for the
problem. Ex: nano-oxides losses occurs at longer
wavelength (50um) and less attenuation.
 The electronic absorption bands are associated with the
band gaps of amorphous glass materials.
 Absorption occurs when a photon interacts with an
electron in the valance band and excites it to a higher
energy level.
7

INTRINSIC ABSORPTION.
8
Intrinsic loss in pure Silica

EXTRINSIC ABSORPTION
 Major extrinsic loss mechanism is caused
by absorption due to water (as the
hydroxyl or OH-) and metallic ions;
introduced in the glass fiber during fiber
pulling by means of oxy-hydrogen flame.
 To overcome this problem refining
technique is used which almost decreases
the metallic impurities.
9

TABLE SHOWING ABSORPTION RATES
10
Absorption losses in one part in 10^9.

SCATTERING LOSS
 This is the one of the parameter which is
responsible for attenuation of transmitted signal.
 Responsible for changing the mode and loose
the radiation in the form of heat.
 The wave doesn’t undergo TIR if scattering
losses becomes more as shown in previous
video.
 Linear losses: Frequency of optical carrier
remains same if light changes from one mode to
11

TYPES OF LINEAR LOSS
 Rayleigh Scattering: the selective scattering of the shorter
wavelengths of visible light (violet and blue) by atmospheric
gases. rayleigh.PNG
 Mie Scattering. all
 mie.PNG
12
Video for Rayleigh Scattering VideoOptical Fibre and
Rayleigh Scattering.mp4
Live Video: VideoRayleigh Scattering - EXFO
animated glossary of Fiber Optics.mp4

RAYLEIGH SCATTERING
 It is one of the major intrinsic loss which is caused due to usage of pure silica.
 It scatters the light by particles much smaller than wavelength of light.
 Dominant in low absorption window between ultraviolet and infrared absorption
tails.
 Results from random in-homogeneities that are small in size compared with the
wavelength.
 Compositional variations are reduced from improved techniques in fabrication.
 Rayleigh scattering of light is due to small localized changes in the refractive
index of the core and cladding material.
 Scattering due to density fluctuations produces an attenuation proportional to
1/λ^4 following Rayleigh Formula.
13

FORMULA
14
Thermal Equilibrium ( Fictive
Temperature).

MIA SCATTERING
 Linear scattering occur at in-homogeneities in
size of the waveguide.
 Reason is due to non-perfect cylindrical
structure. Irregularities at core-cladding
interface
 Cases: Refractive index of core and cladding
along the fiber length, diameter fluctuations,
bubbles.
 When a scattering in-homogeneity size is
greater than λ^4/10, the scattering intensity
which has an angular dependence can be very
large.
 Scattering caused by such in-homogeneities is
15

TO REMOVE IN-HOMOGENEITIES
 Removing imperfections due to glass
manufacturing process
 Controlled extrusion (a manufacturing process
where billet of material is pushed and drawn
through a die to create sharp rod ) and coating
of fiber.
 Increasing the fiber guidance by increasing the
refractive index difference.
16

NON-LINEAR SCATTERING LOSS
 Optical power won’t be linear.
 During the high power levels: The attenuation
is going to happen due to scattering.
 At different frequency the optical power is
transformed in either forward or backward
direction.
 Optical power density is the main reason for
non-linear scattering and it is significant
when it exceeds above the threshold point.
17

TYPES OF NON-LINEAR SCATTERING
 Stimulated Brillouin Scattering.
 Stimulated Raman Scattering.
18

STIMULATED BRILLOUIN SCATTERING (SBS)
19
Heat is transferred to other molecules through vibration
or free electrons.
It is modulation of light through thermal molecular vibration
within fiber.
The modulation frequency separates the fallen light into upper
(Optical) and lower (acoustical) side bands from the incident light.
Incident photon produces a phonon and scattered photon.

STIMULATED BRILLOUIN SCATTERING (SBS)
20
 Acoustic and optical branch
 Acoustic : The displacement of both atoms
has the same wavelength, amplitude
direction and phase.
 Optical: Displacement of both the atoms
move opposite to each other and amplitude
is greater.

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21

CONT.
 Photon generation produces an optical
frequency shift which varies with scattering
angle because the frequency of the scattering
varies with acoustic frequency.
 This frequency shift is a maximum in backward
direction than in forward direction hence
Stimulated Brillouin Scattering is a backward
process.
 Brillouin scattering is only significant above a
threshold power PB which is given as:
22

STIMULATED RAMAN SCATTERING (SRS)
 Similar to Stimulated Brillouin Scattering
except that high frequency optical phonon
rather than an acoustic phonon is generated in
scattering process.
 Also Stimulated Raman Scattering can occur
in both forward and in backward direction in a
fiber.
 Have threshold three orders higher than
Stimulated Brillouin Scattering .
23

BENDING LOSS
24
This happens due to energy entering to vanishing field during bend,
exceeding the velocity of light in rarer medium (cladding) which is practically
impossible hence ray radiate outside the fiber.

BENDING LOSS
 The sharp bend of a fiber causes significant
radiation losses and there is also possibility of
mechanical failure.
 As the core bends the normal will follow it and
the ray will now find itself on the wrong side of
critical angle and will escape. The sharp bends
are therefore avoided.
 The losses are generally represented by
radiation attenuation coefficient
 𝛼𝑟 = c1 𝑒 (−c2 𝑅)
25Video: Bending loss, 2:22 VideoFiber_All.mp4

CONTI.
 R is the radius of curvature of bend and c1
c2 are constants depend on R.
 The radiation loss from a bent fiber depends
on
 Field strength of certain critical distance 𝑐𝑐 from
fiber axis where power is lost through radiation.
 The higher order modes are less tightly bound to
the fiber core, and radiate out of the fiber first.
26

CONTI.
Macro bending losses can be reduced by
 Designing fiber with large relative refractive
index difference.
 Operating with short wavelength.
27

DISPERSION
 Group velocity delay.
 Video is showing it VideoChromatic
Dispersion - EXFO animated glossary of
Fiber Optics.mp4
28

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29

DISPERSION
 The pulse gets distorted as it travels along the
fiber lengths. Pulse spreading in fiber is referred
as dispersion
 Dispersion of optical signal causes distortion for
both digital and analog signal.
 Dispersion is caused by difference in the
propagation times of light rays that takes
different paths during the propagation.
 The light pulses traveling down the fiber
encounter dispersion effect because of this the
pulse spreads out in time domain.
 Dispersion limits the information bandwidth
30

31

FIGURE
 After travelling some distance, Pulses starts
broadening and overlap with the neighboring
pulses.
 Covering certain distance the pulses are not
even distinguishable this effect is known as
inter-symbol interference (ISI)
32

FIGURE
33
As Distance increases the pulses gets broadened

CONTI.
 To avoid overlapping of light pulses down on
an optical fiber link the digital bit rate B𝜏 must
be less than the reciprocal of the broadened
pulse duration (2𝜏). Hence ℬ𝜏 = 1/ 2𝜏
 Here we represent that pulse broadening due
to dispersion on the channel as well as input
pulse duration as “𝜏”.
 This is another more accurate estimation
method of maximum bit rate for an optical
channel.
 The dispersion may be obtained by considering
the light pulses at the output to have a
Gaussian shape with an rms width of σ.
34

CONTI.
 Maximum bit rate is given by
ℬΤ = 0.2/  𝑏𝑖𝑡𝑠/s.
  represent the rms response for the
channel.
35

CONVERSION OF BIT-RATE TO BANDWIDTH.
36
Digital Coding Format
To convert to bandwidth from bit rate the digital
coding format is used.
Non-Return to Zero

NON RETURN TO ZERO
 For metallic conductors
 When a non return to zero code is employed,
binary one level is held for the whole period 𝜏.
Here there are two bit periods in one wavelength
as shown in the figure.
 Hence the maximum bandwidth B is one half
of the maximum data rate
 𝐵Τ (max)= 2 B
37

RETURN TO ZERO
 When return to zero is considered as shown
in the figure the binary one level is held for
only part (usually half ) the bit period .
 For this signaling scheme the data rate is
equal to bandwidth in hertz.
38

FIBER STRUCTURE
 Three common optical fiber structures
 Multimode step index
 Multimode graded index
 Single mode step index
39

FIGURE
40

CONTI.
 Multimode step index fiber exhibits the greater
dispersion of a transmitted.
 Multimode mode graded index fiber gives a
considerable improved performance.
 Single mode step index fiber gives minimum pulse
broadening thus capable of greater transmission
bandwidth which is in gigahertz.
 Number of optical signal pulses which may be
transmitted in a given period , and therefore the
information carrying capacity of the fiber is restricted
by the amount of pulse dispersion per unit length .
41

CONT.
 In the absence of filtering, the pulse
dispersion or pulse broadening increases
linearly with fiber length and thus the
bandwidth is inversely proportional to
distance.
 This leads to adoption of a parameter for the
information – carrying capacity of an optical
fiber which is known as bandwidth – length
product (Bopt * L).
42

INTRA MODEL DISPERSION
 The optical pulse broadening results from the
finite spectral linewidth of the optical source
and the modulated carrier.
43

INTERMODEL DISPERSION
 In simple terms its signal distortion due to
path.
44

TYPES OF INTRAMODAL DISPERSION
 Material Dispersion.
 Waveguide Dispersion.
45

DERIVATION FOR MULTIMODAL STEP INDEX AND
GRADED INDEX FIBER
46
Multimode Step Index Fiber:

DERIVATION FOR MULTIMODAL STEP INDEX AND
GRADED INDEX FIBER
47
Multimode Graded Index Fiber:

GRADED INDEX OPTICAL FIBER
 Intermodal dispersion in multimode fibers are
decreased hence increase bandwidth.
 Due to curved path of rays the bandwidth
can be increased.
 Group velocity is inversely proportional to
refractive index
 Time delay is present due to rays chose
different path of travel.
48

MODAL NOISE
 Speckle or modal noise is the speckle
pattern generated which is greater than the
resolution time of detector.
C:UsersAmoghaDesktopspeckleoflaser.jpg
 Speckle patterns affected by vibrations,
connectors, splices and during
source/detector coupling.
49

REASONS FOR GENERATION OF MODAL NOISE.
 A coherent source with narrow spectral width
and long coherence length.
 Disturbances along the fiber which generates
differential mode delay.
 Phase correlation between the modes.
50

TO OVERCOME THE MODAL NOISE
 Generating the broad spectrum source
 Increase the width of single longitude mode and
hence decrease its coherence time.
 Increasing the number of longitudinal modes and
averaging out of interference pattern.
 Fiber with large numerical aperture.
 Usage of single mode optical fiber.
 The design of fiber should sustain speckle
generated by mechanical vibrations.
51

SUMMARY
 Attenuation
 Material absorption in silica glass
 Intrinsic.
 Extrinsic.
 Scattering losses
 Linear Scattering losses and its types.
 Non linear Scattering losses and its types.
 Bending loss
 Dispersion
52

53
THANK YOU

Optical Fiber Communication.

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