1. Optical Fiber Communications

2. Outline History Types of fiber Light propagation
Losses in optical fiber
Optical fiber classification
Sources
Detectors
Optical fiber system link budget

3. Introduction
EM waves are guided through media composed of transparent material
Without using electrical current flow
Uses glass or plastic cable to contain the light wave and guided them
Infinite bandwidth – carry much more information

4. History Photophone Smoke signals and mirrors Uncoated fiber cables
Alexander Graham Bell
Mirrors and detectors transmit sound wave via beam light
Awkward, unreliable, no practical application
Smoke signals and mirrors
Uncoated fiber cables
1930, J.L. Baird and C.W. Hansell
scanning and transmitting TV image

5. History
1951 – light transmission via bundles of fibers – leads to fiberscope – medical field
1958 – light amplification – stimulated emission
1960 – laser invention
1967 – fiber cable with clad
1970 – low loss optical cable. < 2 dB/km
1980 – optical cable refined – affordable optical communication system
1990 – 0.16dB/km loss

6. History 1988 – long haul transmission system 1988 – SONET
1990 – optical voice and data network are common

7. Advantages Wider bandwidth Immunity to crosstalk
Better than metallic cables
Up to several thousand GHz
Speed up to several Gbps
Immunity to crosstalk
glass fiber/plastic are non-conductor to electrical current
immune to adjacent cables
Immunity to static interference
immune to static noise – EMI, lightning etc.

8. Advantages Environmental Immunity Safety and convenience
more resistant to environment, weather variations
wider temperature range operation
less affected by corrosive liquids and gases
Safety and convenience
safer and easier to install and maintain
no current and voltage associated
no worry about explosion and fire caused
lighter and compact, flexible, lesser space required

9. Advantages Lower transmission loss Security Durability and reliability
lesser loss compared to metallic cables
0.19 dB/km 1550 nm
amplifiers can be spaced more farther apart
Security
virtually impossible to tap into a fiber cable
Durability and reliability
last longer, higher tolerance to changes in environment and immune to corrosion
Economics
Approximately the same cost as metallic cables
less loss between repeaters. Lower installation and overall system’s cost

10. Disadvantages Interfacing cost Strength Remote electrical power
Optical cable – transmission medium
Needs to be connected to standards electronics facilities – often to be expensive
Strength
lower tensile strength
can be improved with kevlar and protective jacket
glass – fragile – less required for portability
Remote electrical power
need to be include electrical line within fiber cable for interfacing and signal regeneration

11. Disadvantages Loss due to bending
bending causes irregularities in cable dimension – the light escapes from fiber core – loss of signal power
prone to manufacturing defect
Specialized tools, equipment and training
tools to splice, repair cable
test equipment for measurements
skilled technicians

12. Optical Spectrum

13. Optical Communication systems

14. Types of fiber
Optical fiber construction

15. Types of fiber Optical fiber construction
special lacquer, silicone, or acrylate coating – outside of cladding – to seal and preserve the fiber’s strength, protects from moisture
Buffer jacket – additional cable strength against shocks
Strength members – increase a tensile strength
Outer polyurethane jacket

16. Types of fiber fiber cables – either glass, plastic or both
Plastic core and cladding (PCP)
Glass core – plastic cladding (PCS)
Glass core – glass cladding (SCS)
Plastic core – more flexible - easier to install
but higher attenuation than glass fiber – not as good as glass
Glass core – lesser attenuation – best propagation characteristics
but least rugged
Selection of fiber depends on its application – trade off between economics and logistics of particular application

17. Physics of light Physics of light
Einstein and Planck – light behaves like EM wave and particles – photon – posses energy proportional to its frequency

18. light propagation the lowest energy state – grounds state
energy level above ground state – excited state
if energy level decays to a lower level – loss of energy is emitted as a photons of light
The process of decaying from one level to another – spontaneous decay or spontaneous emission
Atoms can absorbs light energy and change its level to higher level – absorption

19. light propagation Optical power
flow of light energy past a given point in a specified time

20. light propagation Question Optical power
generally stated in decibel to define power level (dBm)
Question
10 mW in dBm?

21. light propagation Velocity of Propagation in vacuum – 3 x 108 m/s
but slower in a more dense material than free space
when it passes through different medium or from one medium to another denser material – the ray changes its direction due to the change of speed

22. light propagation
from less dense to more denser material – the ray refracted closer to the normal
from more denser material to less denser material – the ray refracted away from the normal

23. light propagation Refraction Refractive Index
Occurs when the light travels between two different material density and changes it speed based on the light frequency
Refractive Index
the ratio of the velocity of propagation of a light ray in a given material

24. light propagation

25. light propagation Snell’s Law
how a light ray reacts when it meets the interface of two transmissive materials that have different indexes of refraction

26. light propagation Snell’s Law angle of incidence angle of refraction
angle at which the propagating ray strike the interface with respect to the normal
angle of refraction
the angle formed between the propagating ray and the normal after the ray entered the 2nd medium

27. light propagation
Snell’s Law

28. light propagation Question medium 1 – glass = 1.5
medium 2 – ethyl alcohol = 1.36
angle of incident – 30o
determine the angle of refraction

29. light propagation Critical Angle
the angle of incident ray in which the refracted ray is 90o and refracted along the interface

30. light propagation Critical Angle
the minimum angle of incident at which the refracted angle is 90o or greater
the light must travel from higher refractive index to a lesser refractive index material

31. light propagation
Critical Angle

32. light propagation Acceptance Angle
the maximum angle in which external light rays may strike the air/glass interface and still propagate down the fiber

33. light propagation
Acceptance Angle

34. light propagation Numerical Aperture - NA
to measure the magnitude of the acceptance angle
describe the light gathering or light-collecting ability of an optical fiber
the larger the magnitude of NA, the greater the amount of external light the fiber will accept

35. light propagation
Numerical Aperture - NA

36. Optical Fiber Configurations
Mode of propagation
single mode
only one path for light rays down the fiber
multimode
many higher order path rays down the fiber

37. Optical Fiber Configurations
Index Profile
graphical presentation of the magnitude of the refractive index across the fiber
refractive index – horizontal axis
radial distance from core – vertical axis

38. Optical Fiber Configurations
Index Profile
step index – single mode
step index – multimode
graded index - multimode

39. optical fiber classification
Single Mode Step Index
dominant – widely used in telecommunications and data networking industries
the core is significantly smaller in diameter than multimode cables

40. optical fiber classification
Multimode Step Index
similar to single mode – step index fiber
but the core diameter is much larger
light enters the fiber follows many paths as it propagate down the fiber
results in different time arrival for each of the path

41. optical fiber classification
Multimode Mode Graded Index
non uniform refractive index – decreases toward the outer edge
the light is guided back gradually to the center of the fiber

42. optical fiber classification
Comparison
Single mode step index
(+) minimum dispersion – same path propagation – same time of arrival
(+) wider bandwidth and higher information txn. rate
(-) small core – hard to couple light into the fiber
(-) small line width of laser required
(-) expensive – difficult to manufacture

43. optical fiber classification
Comparison
Multimode step index
(+) relatively inexpensive, simple to manufacture
(+) easier to couple light into the fiber
(-) different path of rays – different time arrival
(-) less bandwidth and transfer rate
Multimode graded index
intermediate characteristic between step index single and multimode

44. losses in optical fiber
Attenuation
power loss – reduction in the power of light wave as it travels down the cable
effect on system’s performance by reducing:
system’s bandwidth
information tx rate
efficiency
overall system’s capacity

45. losses in optical fiber
Attenuation

46. losses in optical fiber
Attenuation
depends on signal’s wavelength
generally expressed as decibel loss per km
dB/km

47. losses in optical fiber
Attenuation
optical power in decibel units is
P(dBm)= Pin(dBm)-A(dB)
P= measured power level (dBm)
Pin =transmit power (dBm)
A= cable power loss, attenuation (dB)

48. losses in optical fiber
Question
Single-mode optical cable
input power 0.1 mW light source
0.25 dB/km cable loss
determine
optical power 100 km from the transmitter side

49. losses in optical fiber
Absorption Loss
absorption due to impurities – absorb lights and convert it into heat
contributors:
Ultraviolet – ionized valence electron in the silica material.
infrared – photons of light absorbed by glass’s atom – converted into random mechanical vibrations - heating
ion resonance – caused by OH- in in the material. OH- trapped in the glass during manufacturing process

50. losses in optical fiber
Absorption Loss

51. losses in optical fiber
Material – Rayleigh, Scattering Losses
permanent submicroscopic irregularities during fiber drawing process
when the light propagates and strike one of the impurities, they are diffracted – causes the light to disperse and spread out
some continues down the fiber, some escapes via cladding – power loss

52. losses in optical fiber

53. losses in optical fiber
Chromatic – Wavelength, Dispersion Loss
many wavelengths being txn. from LED
each wavelength travels at different velocity
arrives at end of fiber at different time
resulting in chromatic distortion
solution: using monochromatic light source

54. losses in optical fiber
Radiation Losses
loss due to small bends and kinks in the fiber
two types of bend:
microbend – difference in the thermal contraction rates between core and cladding. Geometric imperfection along the axis.
constant radius bend – excessive pressure and tension during handling and installation

55. losses in optical fiber
Modal Dispersion Losses
pulse spreading
difference in the propagation times of light rays that take different path
occur only in multimode fiber
solution: use graded index fiber or single mode step index fiber

56. losses in optical fiber
Coupling Losses
imperfect physical connection
three types of optical junctions:
Light source to fiber connection
Fiber to fiber connection
Fiber to photodetector connection
Caused by:
Lateral displacement
Gap dispalcement
Angular displacement
Imperfect surface

57. losses in optical fiber
Coupling Losses
Lateral Displacement
axis displacement between 2 pieces of adjoining fiber cable
amount of loss – couple tenth to several decibels
Gap displacement – miss alignment
end separation
the farther apart, the greater the light loss
if the two fiber is spliced, no gap between fiber
if the two fiber is joined with a connector, the ends should not touch each other

58. losses in optical fiber
Coupling Losses
angular displacement (misalignment)
less than 2o, the loss will typically less than 0.5 dB
imperfect surface finish
end fiber should be polished and fit together squarely

59. losses in optical fiber
coupling loss

60. Light Sources Light source for optical communication system
efficiently propagated by optical fiber
sufficient power to allow light to propagate
constructed so that their output can be efficiently coupled into and out of optical fiber

61. Light Sources

62. Light Sources LED p-n junction diode
made from a semiconductor (AlGaAs)
emits light by spontaneous emission

63. Sources Homojunction LED Heterojunction LED
p-n junction
two different mixture of the same type of atoms
Heterojunction LED
made from p type semiconductor material from one set of atom and n type semiconductor material from another set
Burrus Etched well surface emitting LED
for higher data rate
the well helps concentrate the emitted light ray
allow more power to be coupled into the fiber
ILD
Injection Laser Diode

64. Sources

65. Sources

66. Sources

67. Light Detectors PIN diodes APD
light doped material between two heavily doped n and p type semiconductor
most common as light detector
APD
avalanche photo diode
more sensitive than PIN diode
require less additional amplification

68. Detectors Characteristic of Light detectors responsivity dark current
a measure of conversion efficiency of photodetector
ratio of output current to the input optical power
dark current
the leakage current that flows through photodiode when there is no light input
transit time
time of light induced carrier to travel across the depletion region of semiconductor
spectral response
the range of wavelength values that a given photodiode will respond
light sensitivity
the minimum optical power a light detector can receive and still produce a usable electrical output signal

69. Lasers
LASER-Light amplification stimulated by the emission of radiation
--laser technology deals with the concentration of light into a very small, powerful beam
--there are 4 types of lasers
1)Gas lasers: Helium and Neon enclosed in a glass tube laser, CO2 lasers
--Output is continuous mono chromatic (one colour)
2)Liquid lasers: organic dye enclosed in a glass tube for an active medium
--A powerful pulse of light excites the organic dye
3)Solid lasers: solid, cylindrical crystal such as ruby, for the active medium. Ruby is excited by a tungsten lamp tied to an alternating-current power supply.
--Output is continuous
4)Semiconductor lasers: Made from semiconductor p-n junctions and are commonly called Injection laser diodes (ILD’s).
-- a direct-current power supply controls the amount of current to the active medium

70. Laser characteristics
All lasers use
an active material to convert energy into laser light
a pumping source to provide power or energy
optics to direct the beam through the active material to be amplified
optics to direct the beam into a narrow powerful cone of divergence
a feedback mechanism to provide continuous operation
an output coupler to transmit power out of the laser