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Lecture 5b Fiber-Optics in Digital Communication Systems
& Electronic Interfaces
1. Introduction2. Geometric Optics3. Classification of Optical Fibers and Their Characteristics4. Multimode Graded Index Fiber5. Single Mode Step Index Fiber6. Dispersion7. Bandwidth of a Single Mode Optical Fiber8. Fiber Optic in Point-to-Point Communication Link9. Lasers10. Distributed Feedback Laser Diode11. Avalanche Photodiode12. Conclusion13. Appendix
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Frequency Wavelength 659 THz Violet 455 nm 612 THz Blue 490 nm 545 THz Green 550 nm 517 THz Yellow 580 nm 484 THz Orange 620 nm 375 THz Red 750 nm 216 THz IR 1300 nm 181 THz IR 1500 nm
1022
1021
1020
1019
1018
1017
1016
1015
1014
1013
1012
1011
1010
109
108
107
106
105
104
103
102
10
1
Visible Light
Cosmic Rays
Gamma Rays
X-Rays
Ultraviolet Light
Infrared Light
Microwave, Radar, UHF, Cellular, PCS
VHF Television, FM
Short wave
Radio
AM Radio
Sound
Subsonic
Wavelength = c
Frequency
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Geometric Optics (Physical Background)
• Dual Nature
A. Huygens's Principle and Ray Approximation
• Huygens's principle states that all points on a wavefront can be taken as point sources for the production of secondary wavelets.
We can assume that the wave travels through a medium in a straight line. This is the ray approximation, and it assumes that light behaves like particles traveling in a straight line.
B. Diffraction
• When light passes through an aperture, the ray approximation is valid if the light wavelength is much shorter than the dimensions of the aperture.
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(a) Straight line propagation
in a wide aperture, d >>
d
(b) Some diffraction at edges
when d =
d
(c) Full diffraction when d <<
5
C. ReflectionWhen light encounters a surface, some light will be absorbed by the surface, some light will be transmitted through the surface, and some light will be reflected by the surface.
(a) Specular reflection (b) Diffuse reflection
θ1 θ1́
θ1 = θ1́
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Light source
n1
Slower transmission
Faster transmission n2 < n1 a
L1
L2
L1́
L2́
Lightdestination
D. Format’s Principle Format’s principle states that when a light ray travels between two points via surface, its path will be the one that requires the least time. This effect is called refraction.
c
nL
c
nL
c
nL
c
nL 2'
21'
12
21
1
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Single-mode Step Index Fiber • long-haul telecommunications; 100 Gbps for 1 km; repeater spacing
of up to 300 km, but these capabilities continue to be improved.
• Axial transmission. For a given core diameter, there is a minimum wavelength c . A single mode fiber will transmit only the single
mode for all wavelengths greater than the cut-off wavelength c .
405.2
22 1
nac
nm1290m29.1405.2
023.02545.1105.12
023.0545.1
510.1545.1;m5.1
2
3
6
1
21
c
n
nnma
Cutoff wavelength c for a fiber with a 3-micrometer core diameter, a core
index of refraction of 1.545, and a cladding index of refraction of 1.510.
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• Single mode fiber has a core diameter 4-9 micrometers, about four times the wavelength of light, allows only one mode (single mode) to exist in the core. No bouncing, destructive or constructive interference occurs.
• Typical bit rate is 100 Gb/s/km. This is 5000 times the bit rate of multi-mode fiber. Theoretical BW limit is 100,000 GHz.
• Single mode is the highest bandwidth optical fiber and is used for long distance communications.
• The single mode fiber bandwidth limitations is due to different light wavelength traveling at slightly different speeds. This phenomena is called chromatic dispersion.
• Using single mode 100 km optical fiber between repeaters bit rate:
Bandwidth of a Single Mode Optical Fiber
sGbkm
kmsGb/0.1
100
//100
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Lasers Light Amplification by Stimulated Emission of Radiation
The laser uses several heavily doped layers of p‑type and n-type materials. When a large forward bias is applied, a large number of free holes and electrons are created in the immediate vicinity of the junction. When a hole and electron pair collide and recombine, they produce a photon of light.
Metallization
n - AlGaAs, Confinement
p - AlGaAs, Active Layer
p - AlGaAs, Confinement
p - GaAs, Contact layer
SiO2 - Insulation Metallization
GaAs Substrate
0.1 - 0.3 m
1 m
1 m
1 m
-
+
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Current (mA)
Optical Power (mW)
1
2
3
4
5
50 100 150 Ith
Threshold Current
Metallized layer
p-type
Grating
n-type
Active Layer
Cleaved Facet
Output
Metallized layer
typeprefractnndiffractioBragg
ofordermperiodgratingm
n
p
p
.,
,2
0
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Avalanche photodiode - +
+ + + + + +
- - - - - -
e-
e- e-
e- e- e-
e-
e- e-
e- e-
e- e-
e- e-
initial photon collision
electron-electron collisions depletion region (avalanche region)
photodiode ofcurrent p-i-nMch
PeMi
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Point-to-Point Communication Link
Optical Fiber (single-mode or multimode)
Output data
Driver (electronics)
Transmitter
Input data
Optical Source
External Modulator
(a) (b)
Connector
Connector
Photodetector Regenerator
Receiver
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Basics of semiconductor theory
21 EE
hc
(a) Absorption
incoming photon
(b) Emission
E1
E2
emitted photon
E1
E2
Ephoton + E1 = E2 Ephoton = E2 - E1
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Excess Electron
4 4 4 4
4 4 5 4
4 4 4 4
Silicon Atom
Arsenic Atom
Excess + Charge
4 4 4 4
4 3 4 4
4 4 4 4
Silicon Atom
Gallium Atom
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5 3 5 3
3 5 3 5
5 3 5 3
Arsenic Atom (5)
Gallium Atom (3)
5 3 5 3
3 5 3 3
5 3 3 3
hole
p-type material
(positively charged area)
5 3 5 3
5 5 3 5
5 3 5 3
n-type material
free electron
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Laser Beam External Modulation
c
Ln
v
Lt 1
11
c
Ln
v
Lt 2
22
)(
)(1
21
2121
21
nnL
nnc
L
c
Ln
c
Ln
cfTtt
2k /2, Max
(2k+1) /2, Min
)(2 Vn
=1500 nm; L=2cm
007.021 nn
L
035.021 nn
=1500 nm; L=2cm K=0,1,2….
2
1
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Transphasor.
Crystal block
Partially reflective surfaces
(a) Transphasor is off
Laser beam Output low (near zero)
Reverberating Waves
(b) Transphasor is on
Laser beam Output light
Amplified Waves Weaker beam
indium antimonide
1000 times faster
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By DWDM splitting of a spectrum into hundreds of channels, Decreasing of duration of pulses,
Speed of transfer of the information on a separate line (one single wavelength) now managed to be up to 10 Gb per second,
and 40-3200 Gb on main channel.
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