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Lecture 5b
Fiber-Optics in Digital Communication Systems
& Electronic Interfaces
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
Introduction
Geometric Optics
Classification of Optical Fibers and Their Characteristics
Multimode Graded Index Fiber
Single Mode Step Index Fiber
Dispersion
Bandwidth of a Single Mode Optical Fiber
Fiber Optic in Point-to-Point Communication Link
Lasers
Distributed Feedback Laser Diode
Avalanche Photodiode
Conclusion
Appendix
1
1022
Cosmic Rays
1021
1020
1019
1018
Gamma Rays
Frequency
X-Rays
1017
16
10
1013
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
Ultraviolet Light
1015
1014
Wavelength
Visible Light
Infrared Light
12
10
1011
1010
109
108
10
7
10
6
10
5
Microwave,
Radar, UHF,
Cellular, PCS
VHF Television,
FM
Radio
Short wave
AM Radio
104
103
Sound
c
102
Wavelength =
Frequency
10
1
Subsonic
2
Geometric Optics (Physical Background)
• Dual Nature
A. Huygens's Principle and Ray Approximation
•
B.
•
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.
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.
3
d
(a) Straight line
propagation
in a wide
aperture, d >>
d
(b) Some diffraction
at edges
when d =
(c) Full diffraction
when d <<
4
C. Reflection
When 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.
θ1 = θ΄1
θ1
θ΄1
(a) Specular reflection
(b) Diffuse reflection
5
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.
Light
destination
L2
Faster transmission
n2 < n1
a
L´2
n1
Slower transmission
L1
L´1
Light source
n1
n2
' n1
' n2
L1 L2
L1 L2
c
c
c
c
6
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 .
2 a n1 2
c
2.405
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.
a
3 m
n n
1.545 1.510
1.5 m ; 1 2
0.023
2
n1
1.545
2 1.5 106 1.545 2 0.023
c
1.29 m 1290 nm
2.405
7
Bandwidth of a Single Mode Optical Fiber
• 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:
100 Gb / s / km
1.0 Gb / s
100 km
8
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
GaAs Substrate
1 m
1 m
n - AlGaAs, Confinement
p - AlGaAs, Active Layer
p - AlGaAs, Confinement
1 m
p - GaAs, Contact layer
0.1 - 0.3 m
+
SiO2 - Insulation
Metallization
9
Optical Power (mW) 5
Threshold
Current
4
3
2
1
50
Ith 100
150 Current (mA)
Metallized layer
p-type
Grating
Active Layer
Cleaved Facet
Output
n-type
Metallized layer
0
2 n p
grating period , m order of
m
Bragg diffraction, n p refract . p type
10
Avalanche photodiode
-
+ + + +
+ +
e- initial photon collision
depletion region
(avalanche region)
ee-
e
e-
e-
electron-electron collisions
e-
e
e-
e-
-
e
-
-
-
-
e-
eee-
-
-
+
i M
eP
M current of p-i-n photodiode
hc
11
Point-to-Point Communication Link
(a)
Input
data
(b)
Driver
(electronics)
Optical
Source
Connector
External
Modulator
Optical Fiber
(single-mode or multimode)
Transmitter
Photodetector
Connector
Regenerator
Receiver
Output
data
12
Basics of semiconductor theory
E2
E2
emitted
photon
incoming
photon
E1
E1
Ephoton + E1 = E2
Ephoton = E2 - E1
(a) Absorption
(b) Emission
hc
E1 E 2
13
Silicon Atom
4
4
4
4
4
4
4
4
4
4
4
4
Number of
Valence
Electrons
Covalent Bond
14
Silicon Atom
4
4
4
4
Arsenic Atom
Silicon Atom
4
4
5
4
4
4
4
4
Excess Electron
4
4
4
4
4
3
4
4
4
4
4
4
Gallium Atom
Excess + Charge
15
Arsenic Atom (5)
5
3
5
3
3
5
3
5
5
3
5
3
Gallium Atom (3)
p-type material
5
3
5
3
3
5
3
3
5
3
3
3
5
3
5
3
5
5
3
5
5
3
5
3
hole
(positively
charged area)
n-type material
free electron
16
Laser Beam External Modulation
2
L
1
L Ln1
t1
v1
c
L Ln2
t2
v2
c
1 Ln1 Ln2 L
t1 t 2 T
( n1 n2 )
f
c
c
c
c
L( n1 n2 )
n2 (V )
2k /2, Max
(2k+1) /2, Min
n1 n2 0.007
=1500 nm; L=2cm
=1500 nm; L=2cm
n1 n2 0.035
K=0,1,2….
17
Transphasor.
indium antimonide
Partially reflective surfaces
Crystal block
Output low
(near zero)
Laser beam
Reverberating Waves
(a) Transphasor is off
1000 times faster
Output light
Laser beam
Weaker beam
Amplified Waves
(b) Transphasor is on
18
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.
19
Orbital angular moment of
photons
A
1011001001
B
C
D
E F
G
H
I
1800
Angular Re solution
220
8
A
A
20