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8/12/2019 Lect 14 Dispersion in Optical Fiber
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High-Speed Circuits and Systems LaboratoryOptoelectronics and Photonics
Lect 14: Dispersion in Optical Fiber
vg (m/s)
(1/s)
TE1
TE2
2.0810 8
2.0710 8
2.0610 8
2.0510 8
0 110 15 210 15 31015
cut-off= 2.31014
c/n2
c/n1
d=10mn1=1.455
n2=1.440
n2
Modal or intermodal)
dispersion:
Waveguide can support
several modes having
different values of v
g
Solution: Use single
mode waveguide
Group velocity depends onFrequency: intramodal dispersion
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8/12/2019 Lect 14 Dispersion in Optical Fiber
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High-Speed Circuits and Systems LaboratoryOptoelectronics and Photonics
Lect 14: Dispersion in Optical Fiber
Dispersion in single-mode waveguide:
Group velocity depends on frequency.
Limitation on data rate and transmission distance.
t
Intensity
Spectrum
Intensity
f
t
Spread
Intensity
Dispersive SMF
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8/12/2019 Lect 14 Dispersion in Optical Fiber
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High-Speed Circuits and Systems LaboratoryOptoelectronics and Photonics
Lect 14: Dispersion in Optical Fiber
n
500 700 900 1100 1300 15001700 1900
1.44
1.45
1.46
1.47
1.48
1.49
Wavelength (nm)
Refractive Index for Silica
- Material or chromatic) dispersion
Refractive index of any materialdepends on frequency(wavelength)
vg depends on frequency
(wavelength)
Why does single mode waveguide
have dispersion?
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8/12/2019 Lect 14 Dispersion in Optical Fiber
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High-Speed Circuits and Systems LaboratoryOptoelectronics and Photonics
Lect 14: Dispersion in Optical Fiber
Waveguide dispersion
Even if refractive indexdoes not change,vg depends onfrequency (wavelength)
dn1
n2
n2Why does single mode waveguide
have dispersion?
2y
dX k=
2
dY =
2
2
2 2 2 2 2
0 1 2( )2
dX Y k n n
+ =
tanY X X=
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8/12/2019 Lect 14 Dispersion in Optical Fiber
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High-Speed Circuits and Systems LaboratoryOptoelectronics and Photonics
Lect 14: Dispersion in Optical Fiber
Waveguide dispersion
Even if refractive index
does not change,vg depends onfrequency (wavelength)
d=10mn1=1.455
n2=1.440
n2
vg (m/s)
(1/s)
TE1
2.0810 8
2.0710 8
2.0610 8
2.0510 8
0 110 15 210 15 31015
14
c/n2
c/n1
Why does single mode waveguide
have dispersion?
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8/12/2019 Lect 14 Dispersion in Optical Fiber
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High-Speed Circuits and Systems LaboratoryOptoelectronics and Photonics
Lect 14: Dispersion in Optical Fiber
Dispersion coefficient, D, for silica fiber with a=4.2mDm: material (chromatic) dispersion only
Dw: waveguide dispersion only
Dm+Dw: total dispersion
0
1.2 1.3 1.4 1.5 1.61.1
-30
20
30
10
-20
-10
(m)
Dm
Dm+ Dw
Dw0
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8/12/2019 Lect 14 Dispersion in Optical Fiber
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High-Speed Circuits and Systems LaboratoryOptoelectronics and Photonics
Lect 14: Dispersion in Optical Fiber
20
-10
-20
-30
10
1.1 1.2 1.3 1.4 1.5 1.6 1.7
0
30
(m)
Dm
Dw
Dch
= Dm
+ Dw
1 2
n
r
Thin layer of cladding
with a depressed index
D
It is possible to control D by changing waveguide structure
Dispersion Flattened Fiber)
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8/12/2019 Lect 14 Dispersion in Optical Fiber
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High-Speed Circuits and Systems LaboratoryOptoelectronics and Photonics
Lect 14: Dispersion in Optical Fiber
How do we model dispersion
0 0
0
22
0 0 2
2
0 1 0 2 0
20
0
1( ) ( ) 2
1 ( ) ( ) ( )
2
1 1 1 = ( )( ) 2
==> Dispersion exists if is not not linear with
In Silica
g gv v
+ = + + +
+ +
+ +
i i
i i
i i
2
2
2
fiber, 20 ps /km at =1.5 m
(With 0, increases as increases)gv
0)
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8/12/2019 Lect 14 Dispersion in Optical Fiber
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High-Speed Circuits and Systems LaboratoryOptoelectronics and Photonics
Lect 14: Dispersion in Optical Fiber
Exercises:
Prob. 8, 9, 10, 14
