The document summarizes key aspects of optical fiber communication including:
1) It describes the advantages of optical fiber communication over copper wire communication such as smaller size, lower transmission loss, higher bandwidth, and immunity to electromagnetic interference.
2) The basic components of an optical fiber are described including the core, cladding, buffer, and jacket. Total internal reflection is explained as the mechanism that guides light through the fiber.
3) Different types of optical fibers are discussed including plastic optical fiber, single-mode fiber, multimode step-index fiber, and multimode graded-index fiber.
5. Fiber vs. Copper
Optical fiber transmits light pulses
Can be used for analog or digital transmission
Voice, computer data, video, etc.
Copper wires (or other metals) can carry the same
types of signals with electrical pulses
6. Advantages of
OFC over conventional Copper Wire Communication
Small Size and light Weight
Abundant Raw Material Availability
Higher Band Width Low Noise
Low transmission loss and high signal security
Highly Reliable and easy of maintainance
Fibers Are Electrically Isolation
Highly transparent at particularo Wave Length
No possiblityof ISI and echoes cross talketc.
Immunity to natural hazardous.
7. Index of Refraction(n)
When light enters a dense medium like glass
or water, it slows down
The index of refraction (n) is the ratio of the
speed of light in vacuum to the speed of light
in the medium n=c/v
Water has n = 1.3
Light takes 30% longer to travel through it
Fiber optic glass has n = 1.5
Light takes 50% longer to travel through it
8. Material Used
Basic materials are plastic and Sand(Sio2)
The Dopents used increase or decrease RI
are:-
GeO2 and P2O5--- To increase RI
B2O3------ To Decrease RI
10. Optical Fiber
Core
Glass or plastic with a higher index of
refraction than the cladding
Carries the signal
Cladding
Glass or plastic with a lower index of
refraction than the core
Buffer
Protects the fiber from damage and
moisture
Jacket
Holds one or more fibers in a cable
11. Plastic Optical Fiber
Large core (1 mm) step-index multimode fiber
Easy to cut and work with, but high attenuation (1
dB / meter) makes it useless for long distances
12. Singlemode Fiber
Singlemode fiber has a core diameter of 8 to 9
microns, which only allows one light path or mode
Images from arcelect.com (Link Ch 2a)
Index of
refraction
13. Multimode Step-Index Fiber
Multimode fiber has a core diameter of 50 or 62.5
microns (sometimes even larger)
Allows several light paths or modes
This causes modal dispersion – some modes take
longer to pass through the fiber than others because
they travel a longer distance
See animation at link Ch 2f
Index of
refraction
14. Step-index Multimode
Large core size, so source power can be
efficiently coupled to the fiber
High attenuation (4-6 dB / km)
Low bandwidth (50 MHz-km)
Used in short, low-speed datalinks
Also useful in high-radiation
environments, because it can be made with pure
silica core
15. Singlemode FIber
Best for high speeds and long distances
Used by telephone companies and CATV
16. Multimode Graded-Index Fiber
The index of refraction gradually changes across the
core
Modes that travel further also move faster
This reduces modal dispersion so the bandwidth is
greatly increased
Index of
refraction
17. Step-index and Graded-index
Step index multimode was developed first, but
rare today because it has a low bandwidth (50
MHz-km)
It has been replaced by graded-index multimode
with a bandwidth up to 2 GHz-km
19. Total Internal Reflection
Snells Law :
n1 sin1 n2 sin2
Re flection Condition
1 3
When n1 n2 and as 1 increases eventually 2
goes to 90 deg rees and
n
n1 sinc n2 or sinc 2
n1
c is called the Critical angle
For 1 c there is no propagating refracted ray
21. Acceptance Angle
The acceptance angle (i) is the
largest incident angle ray that
can be coupled into a guided ray
within the fiber
The Numerical Aperture (NA) is
the sin(i) this is defined
analogously to that for a lens
1 1 1
NA = (n12 - n2 2 2
) = 2 2
(2D n ) = n(2D ) 2
n1 - n2 n + n2
Where D º and n º 1 f# º
f
=
f
n 2 D FullAccep tan ceAngle
1
=
2 ×NA
22. Sources and Wavelengths
Multimode fiber is used with
LED sources at wavelengths of 850 and 1300 nm
for slower local area networks
Lasers at 850 and 1310 nm for networks running at
gigabits per second or more
23. Sources and Wavelengths
Singlemode fiber is used with
Laser sources at 1300 and 1550 nm
Bandwidth is extremely high, around 100 THz-km
24. Fiber Optic Specifications
Attenuation
Loss of signal, measured in dB
Dispersion
Blurring of a signal, affects bandwidth
Bandwidth
The number of bits per second that can be sent
through a data link
Numerical Aperture
Measures the largest angle of light that can be
accepted into the core
26. Measuring Bandwidth
The bandwidth-distance product in units of
MHz×km shows how fast data can be sent
through a cable
A common multimode fiber with bandwidth-
distance product of 500 MHz×km could
carry
A 500 MHz signal for 1 km, or
A 1000 MHz signal for 0.5 km
From Wikipedia
27. Numerical Aperture
If the core and cladding have almost the same
index of refraction, the numerical aperture will be
small
This means that light must be shooting right
down the center of the fiber to stay in the core
29. Three Methods
Modified Chemical Vapor Deposition (MCVD)
Outside Vapor Deposition (OVD)
Vapor Axial Deposition (VAD)
30. Modified Chemical Vapor Deposition
(MCVD)
A hollow, rotating glass tube is
heated with a torch
Chemicals inside the tube
precipitate to form soot
Rod is collapsed to crate a
preform
Preform is stretched in a
drawing tower to form a single
fiber up to 10 km long
32. Outside Vapor Deposition (OVD)
A mandrel is coated with a porous preform in a
furnace
Then the mandrel is removed and the preform is
collapsed in a process called sintering
Image from csrg.ch.pw.edu.pl
33. Vapor Axial Deposition (VAD)
Preform is fabricated
continuously
When the preform is long
enough, it goes directly to
the drawing tower
Image from csrg.ch.pw.edu.pl
34. Drawing Apparatus
The fiber is drawn from the preform
and then coated with a protective
coating
36. Attenuation
Modern fiber material is very pure, but there is still some
attenuation
The wavelengths used are chosen to avoid absorption
bands
850 nm, 1300 nm, and 1550 nm
Plastic fiber uses 660 nm LEDs
Image from iec.org (Link Ch 2n)
37. Optical Loss in dB (decibels)
Power In Power Out
Data Link
If the data link is perfect, and loses no power
The loss is 0 dB
If the data link loses 50% of the power
The loss is 3 dB, or a change of – 3 dB
If the data link loses 90% of the power
The loss is 10 dB, or a change of – 10 dB
If the data link loses 99% of the power
The loss is 20 dB, or a change of – 20 dB
dB = 10 log (Power Out / Power In)
38. Absolute Power in dBm
The power of a light is measured in milliwatts
For convenience, we use the dBm
units, where
-20 dBm = 0.01 milliwatt
-10 dBm = 0.1 milliwatt
0 dBm = 1 milliwatt
10 dBm = 10 milliwatts
20 dBm = 100 milliwatts
39. Three Types of Dispersion
Dispersion is the spreading out of a light pulse as
it travels through the fiber
Three types:
Modal Dispersion
Chromatic Dispersion
Polarization Mode Dispersion (PMD)
40. Modal Dispersion
Modal Dispersion
Spreading of a pulse because different modes
(paths) through the fiber take different times
Only happens in multimode fiber
Reduced, but not eliminated, with graded-index
fiber
41. Chromatic Dispersion
Different wavelengths travel at different speeds
through the fiber
This spreads a pulse in an effect named chromatic
dispersion(group Delay)
T=1/vg =1/L *dВ/dώ
Chromatic dispersion occurs in both singlemode and
multimode fiber
Larger effect with LEDs than with lasers
A far smaller effect than modal dispersion
42. Modal Distribution
In graded-index fiber, the off-axis modes go a
longer distance than the axial mode, but they
travel faster, compensating for dispersion
But because the off-axis modes travel further, they
suffer more attenuation
43. Equilibrium Modal Distribution
A long fiber that has lost the high-order modes is
said to have an equilibrium modal distribution
For testing fibers, devices that can be used to
condition the modal distribution so that
measurements will be accurate
44. Mode Stripper
An index-matching substance is put on the
outside of the fiber to remove light travelling
through the cladding
45. Mode Scrambler
Mode scramblers mix light to excite every
possible mode of transmission within the fiber
Used for accurate measurements of attenuation
Figure from
fiber-optics.info
(Link Ch 2o)
47. Source Characteristics
Important Parameters
Electrical-optical conversion efficiency
Optical power
Wavelength
Wavelength distribution (called linewidth)
Cost
Semiconductor lasers
Compact
Good electrical-optical conversion efficiency
Low voltages
Los cost
48. Semiconductor Optoelectronics
Two energy bands
Conduction band (CB)
Valence band (VB)
Fundamental processes
Absorbed photon creates an electron-hole pair
Recombination of an electron and hole can emit a photon
Types of photon emission
Spontaneous emission
Random recombination of an electron-hole pair
Dominant emission for light emitting diodes (LED)
Stimulated emission
A photon excites another electron and hole to recombine
Emitted photon has similar wavelength, direction, and phase
Dominant emission for laser diodes
50. Semiconductor Material
Semiconductor crystal is required
Type IV elements on Periodic Table
Silicon
Germanium
Combination of III-V materials
GaAs
InP
AlAs
GaP
InAs
…
51. Direct and Indirect Materials
Relationship between energy and momentum for electrons and holes
Depends on the material
Electrons in the CB combine with holes in the VB
Photons have no momentum
Photon emission requires no momentum change
CB minimum needs to be directly over the VB maximum
Direct band gap transition required
Only specific materials have a direct bandgap
52. Light Emission
The emission wavelength depends on the energy band gap
Eg E2 E1
ch 1.24
Eg E
Semiconductor compounds ghave different
Energy band gaps
Atomic spacing (called lattice constants)
Combine semiconductor compounds
Adjust the bandgap
Lattice constants (atomic spacing) must be matched
Compound must be matched to a substrate
Usually GaAs or InP
53. Common Semiconductor
Compounds
GaAs and AlAs have the same lattice constants
These compounds are used to grow a ternary compound that
is lattice matched to a GaAs substrate (Al1-xGaxAs)
0.87 < < 0.63 (mm)
Quaternary compound GaxIn1-xAsyP1-y is lattice
matched to InP if y=2.2x
1.0 < < 1.65 (mm)
Optical telecommunication laser compounds
In0.72Ga0.28As0.62P0.38 (=1300nm)
In0.58Ga0.42As0.9P0.1 (=1550nm)
54. Optical Sources
Two main types of optical sources
Light emitting diode (LED)
Large wavelength content
Incoherent
Limited directionality
Laser diode (LD)
Small wavelength content
Highly coherent
Directional
55. Avoiding
losses in LED
Carrier Photon
confinement Confinement
Band-gap and refractive index engineering.
Heterostructured LED
56. Double Heterojunction LED
(important)
Fiber
Optics
Epoxy
Double
Metal contact
heterostructure
n AlGaAs
p GaAs (active region)
Burrus type
p Al GaAs LED
n+ GaAs
Metal contact
Shown
bonded to a fiber
with index-
matching epoxy.
57. Double Heterostructure
The double heterostructure is invariably used for
optical sources for communication as seen in the
figure in the pervious slide.
Heterostucture can be used to increase:
Efficiency by carrier confinement (band gap engineering)
Efficiency by photon confinement (refractive index)
The double heterostructure enables the source
radiation to be much better defined, but further, the
optical power generated per unit volume is much
greater as well. If the central layer of a double
heterostructure, the narrow band-gap region is made
no more than 1mm wide.
58. Photon confinement -
Reabsorption problem
Source of electrons
Active region (micron in thickness)
Source of holes
Active region (thin layer of GaAs) has smaller band gap, energy of photons
emitted is smaller then the band gap of the P and N-GaAlAs hence could not
be reabsorbed.
59. Carrier confinement
electrons
holes
n+-AlGaAs p-GaAs p+-AlGaAs
Simplified band diagram of the ‘sandwich’ top show carrier confinement
60. Light Emitting Diodes (LED)
Spontaneous emission
dominates
Random photon emission
Spatial implications of random
emission
Broad far field emission pattern
Dome used to extract more of the
light
Critical angle is between
semiconductor and plastic
Angle between plastic and air is
near normal
Spectral implications of random
emission
Broad spectrum 1.452 kT
p
61. Laser Diode
Stimulated emission dominates
Narrower spectrum
More directional
Requires high optical power density in the gain region
High photon flux attained by creating an optical cavity
Optical Feedback: Part of the optical power is reflected back into the cavity
End mirrors
Lasing requires net positive gain
Gain > Loss
Cavity gain
Depends on external pumping
Applying current to a semiconductor pn junction
Cavity loss
Material absorption
Scatter
End face reflectivity
62. Laser Diode
Stimulated emission dominates
Narrower spectrum
More directional
Requires high optical power density in the gain region
High photon flux attained by creating an optical cavity
Optical Feedback: Part of the optical power is reflected back into the cavity
End mirrors
Lasing requires net positive gain
Gain > Loss
Cavity gain
Depends on external pumping
Applying current to a semiconductor pn junction
Cavity loss
Material absorption
Scatter
End face reflectivity
63. Optical Feedback
Easiest method: cleaved end faces
End faces must be parallel
Uses Fresnel reflection
n 1
2
R
n 1
For GaAs (n=3.6) R=0.32
Lasing condition requires the net cavity gain to be one
R1 R2 expg a L 1
g: distributed medium gain
a: distributed loss
R1 and R2 are the end facet reflectivity's
64. Phase Condition
The waves must add in phase as given by
2 L z 2 m
Resulting in modes given by
2Ln
m
Where m is an integer and n is the refractive index of the cavity
66. Longitudinal Modes
The optical cavity excites various longitudinal
modes
Modes with gain above the cavity loss have the
potential to lase
Gain distribution depends on the spontaneous
emission band
Wavelength width of the individual longitudinal
67. Mode Separation
Wavelength of the various modes
2Ln
m
1 1
m m1 2 L n
m m 1
The wavelength separationof the modes is
2 Ln 2
A longer cavity 2
m 2 Ln
Increases the number of modes
Decrease the threshold gain
There is a trade-off with the length of the laser cavity