Ilavarsan module 2 part 2 optical foptical fibreibre

Module – 2
Signal Degradation

Signal degradation in fiber cable
2 Dr. T.Ilavarasan,AssociateProfessor,SENSE, VIT

Fiber Dispersion
⦁ Fiber dispersion results inoptical pulse broadening andhence digitalsignaldegradation.
Dispersion mechanisms:
 Modal(orintermodal)dispersion
 Chromaticdispersion(CD)
 Polarizationmodedispersion(PMD)

Dispersion in digital systems

Pulse broadening limits fiber bandwidth (data rate)

Dispersion types
⦁ Intermodal (modal) Dispersion
⦁ Intramodal Dispersion
⦁ Chromatic Dispersion
 MaterialDispersion
 WaveguideDispersion
⦁ Polarization Mode Dispersion

Modal dispersion
⦁ When numerous waveguide modes are propagating, they all travel with different net
velocities withrespect tothe waveguide axis.
⦁ An input waveform distorts during propagation because its energy is distributed among
several modes, each traveling atadifferent speed.
⦁ Parts of the wave arrive at the output before other parts, spreading out the waveform.
This isthus known asmultimode (modal) dispersion.
⦁ Multimode dispersion does not depend on the source linewidth (even a single
wavelength can besimultaneously carriedby multiple modes inawaveguide).
⦁ Multimode dispersion would not occur if the waveguide allows only one mode to
propagate -the advantage ofsingle-mode waveguides!

Inter modal dispersion (meridional rays)

Inter modal dispersion (meridional rays)
Shortest path  i = 0
path length = L
Longest path   = c
path length = L / Sin c
Velocity V  c / n1
Time delay Tmod = Path length difference / Velocity
= [ ( L / Sin c) – L ] / [c / n1]
Sin c = n2 / n1  Tmod = ( L / c ) ( n1
2 / n2 )  = Ln1  / c

RMS pulse broadening
RMS pulse spread for a MMSIF:
σs = Tmod / (2 √3 ) = L n1  / (c 2 √3 )
RMS pulse spread for a MMSIF considering mode coupling:
σsc = Tsc / (2 √3 ) = (L Lc )1/2 n1  / (c 2 √3 )
Lc  characteristic length of fiber
RMS pulse spread for a MMGIF:
Tgi = Ln1 2 / 2c [ using ray theory approach ]
= Ln1 2 / 8c [ using mode theory approach ]
= Ln1 2 / Dc [ where D varies from 2 to 10 ]
σgi = Tgi / (2 √3 )
= L n1 2 / (20 c √3 ) = σs (  / 10 )
RMS pulse width σ = τ / (2 √3 )

Attenuation units – Numerical problem
Compare the rmspulse broadening per kilometer forthefollowing three fibers:
(i).amultimode step index fiberwithcore indexn1 =1.49 andΔ=1.0%,
(ii).agraded indexfiberhaving anoptimum parabolic index profile andthe same n1 andΔ
as in(i),
(iii).thesame type ofgraded index fiberasin(ii)but with Δ=0.5%.

Attenuation units – Numerical problem
Compare the rmspulse broadening per kilometer forthefollowing three fibers:
(i).amultimode step index fiberwithcore indexn1 =1.49 andΔ=1.0%,
(ii).agraded indexfiberhaving anoptimum parabolic index profile andthe same n1 andΔ
as in(i),
(iii).thesame type ofgraded index fiberasin(ii)but with Δ=0.5%.
(i) σstep = 14.3 ns / km
(ii) σgi = 14.3 ps / km
(iii) σgi = 3.58 ps / km

Dispersion Characteristics of optical fiber

Bit-rate distance product
How much will a light pulse spread after traveling along 1 km of a step-index
fiberwhoseNA = 0.275andncore = 1.487?

How doesmodaldispersionrestrictsfiberbit rate?
Suppose wetransmit atalow bitrateof10Mb/s
Pulse duration =1/107 s=100ns
Usingthe above e.g.,each pulse willspread uptoapprox. 100ns(i.e.approx. pulse duration
!)every km
The broadened pulses overlap! (Intersymbol interference (ISI))
*Modal dispersion limits the bitrateofafiber-optic link to~10Mb/s.
(acoaxial cable supports thisbitrateeasily!)

⦁ We can relate the pulse broadening ΔT to the information-carrying capacity of the fiber
measured through thebitrateB.
⦁ Although a precise relation between B and ΔT depends on many details, such as the
pulse shape, it is intuitively clear that ΔT should be less than the allocated bit time slot
givenby 1/B.
⦁ An order-of-magnitude estimate of the supported bit rate is obtained from the condition
BΔT<1.
⦁ Bit-rate distance product (limited by modal dispersion)
⦁ This condition provides a rough estimate of a fundamental limitation of step-index
multimode fibers.
(the smaller isthe NA, the largeristhe bit-rate distance product)


1
n
c
BL

⦁ The capacity of optical communications systems is frequently measured in terms of the
bitrate-distance product.
⦁ e.g. If a system is capable of transmitting 10 Mb/s over a distance of 1 km, it is said to
have abitrate-distance product of10(Mb/s)-km.
⦁ This may be suitable forsome local-area networks (LANs).
⦁ Note that the same system can transmit 100 Mb/s along 100 m, or 1 Gb/s along 10 m, or
10Gb/s along 1m,or100Gb/s along 10cm, 1Tb/s along 1cm

Single-mode fiber eliminates modal dispersion
⦁ The main advantage of single-mode fibers is to propagate only one mode so that modal
dispersion isabsent.
⦁ However, pulse broadening does not disappear altogether. The group velocity associated
with the fundamental mode is frequency dependent within the pulse spectral linewidth
because ofchromatic dispersion.

Ilavarsan module 2 part 2 optical foptical fibreibre

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