chapter04 fiber characteristics
DESCRIPTION
physicsTRANSCRIPT
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Chapter 4 FIBER CHARACTERISTICS
Lecturer: Tran Thi Ngoc Dung
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Content
Energy Loss in Fiber Fiber Dispersion Data Rate
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Fiber Losses
Three fundamental causes1. Material loss. Absorption the energy
by material2. Light Scattering. Light scattered by
the molecules of the material, by structural imperfections, and impurities.
3. Waveguide and bend losses. Losses by imperfections and deformations of the fiber structure.
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Loss
Pin Pout
in
outdB
in
out
PPLoss
PPLoss
log10)( =
=
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Material Losses
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Scattering loss
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Waveguide and Bend loss
Change in fiber diameter
Ray leaves fiber
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DISPERSION
2/121
22 )( pp ttt =
Cause of Pulse broadening Output Pulse is wider than
Input Pulse
t
tp1
tp2
Input pulse
Output pulse
t
Dispersion
Longer fiber causes more pulse brodeningDispersion for a given length:
)/( kmdispersionLt =
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Intermodal Dispersion
1
3 212
3
n1
For Mode zero, travel time:
For critical mode:
Dispersion
===
===
==
cLn
cnnnLn
cLn
cnLnttt
cnLn
cLn
nc
L
t
cLn
ncLt
oc
c
cc
o
1
2
2111
2
21
2
211
1
1
1
)(
sin/sin
/
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Intermodal Dispersion (cont.)
2
1
2
1
1
1
)(2
1
)(2
2
NAcnL
t
NAcn
LcLnt
nNA
=
==
=
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Example 4-4
A train of light pulses is transmitted through a 400-m fiber with ncore=1.4 and nclad=1.36. Sketch the output pulses for
(1) a pulse rate of 10 x 10^6 pulses per second (10Mb/s)
(2) a pulse rate of 20 x 10^6 pulses per second (20Mb/s)
(3) Find the dispersion per kilometer. Assume that the input pulse is of near zero width.
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Ex 4-4 (cont.)
ns
cnnnLnttt oc
9.5410336.1
)36.14.1(4004.1
)(
8
2
211
=
=
==
Dispersion
a) Pulse rate 10.10^6 pulses/sPeriode of pulse: T=100ns
t(ns)50 100 150 200
Relative Amplitude
T=100nsPulses are clearly distingushable
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Ex 4-4 (cont.)
50 100 150 200
Relative Amplitude
T=100ns
b) Pulse rate 20.10^6 pulses/s/ Periode of pulse: T=50ns
t(ns)
kmnskmns
kmLt /2.137
4.09.54
)(==
c) Dispersion per kilometer
Pulses are merged and nearly indistingushable
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Intramodal Dispersion (Chromatic dispersion) Related to the line width of the light
source, and caused by variations in the refractive index as a function of wavelength n()
Intramodal is often given in terms of picoseconds per kilometer per nanometer of line width.
At 1300nm, intramodal dispersion is minimum
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Total Dispersion
2/12int
2int ][ raertot ttt +=
EXAMPLE 4-5/p64
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Ex 4-5/p64
A fiber has the following specifications:
DispersionIntermodal 5ns/kmIntramodal100ps/(km x
nm)Linewidth 40nmFiber length 5km1. Find the total intermodal
dispersion2. Find the total intramodal
dispersion3. Find the total dispersion.
.255/5int nskmkmnst er ==
.20405100int nsnmkmnmkmpst ra =
=
nst
ttt raer322025
)(22
2/12int
2int
=+=
+=
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CHOICE OF WAVELENGTH
For better performance, the choice of wavelength must be based on minimizing loss and minimizing dispersion.
3 low loss Windows (820-880),(1200-1320), (1550-1610) (nm)
Low intramodal dispersion: 1300nm. Dispersion-shifted fibers: minimum
dispersion is shifted to about 1550nm.
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Questions
8. What is the difference between intermodal and intramodal dispersion
10. What are low loss windows? Give the wavelengths for the three low loss windows.
11. Why is it desirable to operate at a wavelength of about 1.3 m?
12. Why is it advantageous to operate at 1.55 m?
13. What is dispersion-shifted fibers?
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Problems solving
Do all problems on pp 68,69
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PROBLEMS
1. Find the loss, in decibels, for a fiber with input power of 200 W and output power of 10W.
2. Finb the output power for a 5-km fiber with a 4dB/km loss and with Pin=500 W.
3. A fier, 8km in length, has an input power of 300W and an output power of 10 W. What is the loss per kilometer?
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4. Two fibers, one 3km long with 4dB/km loss and the other 9km long with 1.2 dB/km loss, are spliced to form a 12km fiber. Ignore losses due to splicing
a) What is the average dB/km loss?b) For an input of 260W, what is the
power out?
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5. Given a fiber 5km long, with ncore=1.44 and nclad=1.4, find the intermodal dispersion.
6. Find the intermodal dispersion per kilometer for a fiber with =2% and n1=1.5
7. A fiber has a specified N.A. of 0.22 and ncore=1.4. Find the intermodal dispersion for a 500m length.
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8. A system introduces pulse broadening of 50ns. The input pulse is 20ns wide. Find the output pulse width.
9.
Chapter 4 FIBER CHARACTERISTICS ContentFiber LossesLoss Material LossesScattering lossWaveguide and Bend lossDISPERSIONIntermodal DispersionIntermodal Dispersion (cont.)Example 4-4Ex 4-4 (cont.)Ex 4-4 (cont.)Intramodal Dispersion (Chromatic dispersion) Total DispersionEx 4-5/p64CHOICE OF WAVELENGTHQuestionsProblems solvingPROBLEMSSlide Number 21Slide Number 22Slide Number 23