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TCOM 503 Fiber Optic Networks

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Title: TCOM 503 Fiber Optic Networks


1
TCOM 503Fiber Optic Networks
  • Spring, 2007
  • Thomas B. Fowler, Sc.D.
  • Senior Principal Engineer
  • Mitretek Systems

2
Topics for TCOM 503
  • Week 1 Overview of fiber optic communications
  • Week 2 Brief discussion of physics behind fiber
    optics
  • Week 3 Light sources for fiber optic networks,
    cable types and propagation of light in fiber
  • Week 4 Fiber optic components fabrication and
    use
  • Week 5 Modulation of light, its use to transmit
    information
  • Week 6 Noise and detection
  • Week 7 Optical fiber fabrication and testing of
    components

3
Week 2 Brief discussion of physics behind fiber
optics
  • Brief history of the physics of light
  • Nature of light
  • Basic principles of optics
  • Reflection and refraction
  • Interference and diffraction
  • Types of optical fiber
  • Devices used in fiber optics

4
Brief history of the physics of light
  • Atomists in ancient Greece (5th c. BC)
    formulated an emission theory
  • Pictured light as torrent of minute, high-speed
    particles
  • Aristotle (4th c. BC)
  • Added fifth element to traditional four (fire,
    air, earth, water) the aether
  • All void had to be filled with something, hence
    the aether
  • Proposed that human vision arises from movement
    of the aether produced by the body we perceive

5
Brief history of the physics of light (continued)
  • Robert Hooke (1660s) proposed that only a wave
    could account for observed properties of light
  • Pattern of colors in (thin films) soap bubbles
  • Fact that two beams of light can cross without
    scattering
  • Light is self-sustaining vibration of some medium
    without transport of matter
  • Newton favored corpuscular theory, primarily
    because of optics
  • Beams of light dont diverge, as he thought wave
    theory required
  • Didnt realize small size of light waves
  • Theory required speed of light to be greater in
    water than in air

6
Brief history of the physics of light (continued)
  • In 19th century, wave theory reborn with work of
    Thomas Young (1773-1829)
  • Discovered interference
  • Augustin Fresnel (1788-1827) did extensive
    experiments on interference and diffraction
  • Put wave theory on mathematical basis
  • Showed that rectilinear propagation of light due
    to short wavelength
  • 1850 Foucault measured speed of light in water
    and showed it was less than in air
  • 1860 Maxwell develops electromagnetic theory,
    shows that light is form of electromagnetic
    radiation
  • 1887 Hertz confirms Maxwells theory, but also
    discovers photoelectric effect
  • Ability of light to dislodge electrons
  • Found to be independent of intensity, dependent
    on frequency

7
Brief history of the physics of light (continued)
  • 1905 Einstein explains photoelectric effect by
    reverting to particle theory of light
  • Light particles called photons
  • Energy given by famous formula E hf
  • h Plancks constant
  • Also in 1905, Einstein proposes Special Theory of
    Relativity
  • Speed of light, c, is universal constant
  • 1920s Development of Quantum Mechanics
    elucidates nature of light and matter

8
Speed of light
  • Galileo attempted to measure, but his equipment
    was too crude, leading to assumption of infinite
    speed
  • 1675 Römer measured using eclipses of Jupiters
    moons
  • Obtained result of 125,000 miles/sec (2.02 x 108
    m/sec)
  • Used incorrect value for diameter of earths
    orbit
  • 1849 Fizeau measured speed using lenses and
    mirrors
  • Obtained value of 3.133 x 108 m/sec
  • 1850 Foucault measures speed with improved
    method
  • Obtained value of 2.98 x 108 m/sec

9
Speed of light (continued)
  • 1926 Michaelson used similar method
  • Obtained value of 2.99786 x 108 m/sec
  • Current value 299,792,458 m/sec

source
observer
10
Nature of light
  • Sometimes a particle, sometimes a wave
  • Electromagnetic spectrum
  • Particles ray optics, lenses, reflection,
    refraction
  • Waves interference, diffraction

11
Visible spectrumNewtons experiment
  • Demo
  • http//micro.magnet.fsu.edu/primer/java/scienceopt
    icsu/newton/

12
Types of waves
  • Longitudinal (sound waves)

Source C. R. Nave, Hyperphysics, Georgia State
Univ.
13
Types of waves (continued)
  • Transverse (light, waves on rope)

Source C. R. Nave, Hyperphysics, Georgia State
Univ.
14
Types of waves (continued)

Wave motion demo
http//www.matter.org.uk/schools/Content/seismolog
y/longitudinaltransverse.html
15
Light is type of electromagnetic radiation
  • Electric, magnetic fields orthogonal to each
    other and direction of propagation
  • Eye (and most other things) affected primarily by
    electric field
  • Magnetic field much weaker, by factor of c
  • E cB

16
Electromagnetic Spectrum
17
Electromagnetic spectrum in vicinity of visible
light used for fiber optics
  • Seven regions, called windows, lie at infrared
    wavelengths of relatively low attenuation in
    glass
  • First at 850 nm
  • First developed
  • Used now only for short distance multimode fiber
  • Second (O band) at 1310 nm (1260-1310 nm)
  • Second developed
  • Lower attenuation than 850 window
  • Third (C band) at 1550 nm (1530-1565 nm)
  • Third developed
  • Superior to other two
  • Fourth (L band) at 1625 (1565-1625) nm
  • Currently under development

18
Electromagnetic spectrum in vicinity of visible
light (continued)
  • Others
  • E band (1360-1460 nm)
  • S band (1460-1530 nm)
  • U band (1625-1675 nm)

19
Electromagnetic spectrum in vicinity of visible
light (continued)

Source Cisco
20
Electromagnetic spectrum in vicinity of visible
light (continued)
  • All the bands

Source Networkmagazine.com
21
Electric and magnetic fields of light wave

Source Dutton, Figure 3
22
Propagation of light 3d view

Source Dutton, Figure 4
23
Propagation of electromagnetic waves
  • Demonstration

http//www.phy.ntnu.edu.tw/java/emWave/emWave.html

24
Polarization

Source Hecht, Physics
25
Polarization
  • Demo
  • http//micro.magnet.fsu.edu/primer/java/polarizedl
    ight/filters/index.html

26
Circular polarization
  • Electric, magnetic fields can rotate as wave
    propagates
  • Referred to as circular polarization

Source Dutton, Figure 5
27
Basic principles of optics
  • Light propagation
  • Law of refraction

28
Refraction
  • Demo
  • http//micro.magnet.fsu.edu/primer/java/refraction
    /index.html

29
Reflection

Source Zona Land, http//id.mind.net/zona/index.
html
30
Reflection
  • Demo
  • http//micro.magnet.fsu.edu/primer/java/specular/i
    ndex.html

31
Basic relationships
  • Frequency n or f (Hz or cycles/second)
  • Angular frequency w 2pf
  • Wavelength l (m, cm, nm)
  • Wave number k (dimensionless), proportional to
    number
  • of waves per unit length
  • Period T (seconds, msec, microsecond,
    nanosecond)
  • Amplitude A
  • Velocity v (m/sec)
  • v f l
  • k 2p/l
  • Propagation of a wave
  • y(x,t) A sin (kx-kvt) A sin (kx wt)

32
Refraction (continued)
  • Snells law

33
Law of refraction

34
Refraction of light rays

n2 lt n1
n1
Source Tipler, Physics
35
Refraction and total internal reflection

Source Tipler, Physics
36
Total internal reflection

37
Total internal reflection
  • Demo

http//www.phy.ntnu.edu.tw/java/propagation/propag
ation.html
38
Optical fiber construction
n2 lt n1
Source Nortel
39
Optical fiber construction (continued)

Source Corning
40
Light propagation in a glass fiber

Source Hecht, Physics
41
Total internal reflection in fiber optic cables
  • Note that, in the case of optical fiber (and most
    other cases), cladding is not a conductor
  • Therefore electric and magnetic fields of light
    wave penetrate some distance into it
  • Sharp cutoff assumes ray optics, not actual wave
    optics and quantum mechanics

42
Dispersion
  • Newton's experiments illustrated the dispersion
    of sunlight into a spectrum (and recombination
    into white light).
  • Sunlight consists of a mixture of light with
    different wavelengths.
  • A dispersive medium is one in which different
    wavelengths of light have slightly different
    indices of refraction
  • Crown glass is a dispersive medium since the
    index of refraction for violet light in crown
    glass is higher than for red light
  • This is responsible for chromatic aberration
  • Manufacturers of optical glass customarily
    specify the refractive index of a material for
    yellow sodium light, the D line

43
Dependence of index of refraction on l
  • Index of refraction not constant
  • Since index of refraction is determined by speed
    of light in the medium, follows that speed of
    light in medium is function of l
  • Shorter wavelengths travel slower because index
    of refraction is greater
  • Will lead to dispersion of information bearing
    light waves over distance
  • Called material dispersion

44
Dependence of index of refraction on l

Source Hecht, Physics
45
Dispersion (continued)
  • Waveguide dispersion
  • Light travels in both core and inner cladding at
    slightly different speeds (faster in cladding)
  • Material and waveguide dispersion opposite
    effects
  • Can be balanced to allow for zero dispersion at a
    particular wavelength between 1310nm and 1650 nm
  • Total effect called chromatic dispersion

Source Corning
46
Effect of chromatic dispersion

Source Nortel
47
Interference and Diffraction
  • Extremely important for fiber optics
  • Both effects limit performance of optical fiber

48
Interference
  • Interference from two point sources
  • Originates because waves from two sources are in
    phase or out of phase, depending on position
    (distance from the two sources
  • Gives rise to series of alternating light and
    dark bands on target at fixed distance from the
    sources
  • Basic relationships
  • Maxima at angle q given by d sin q ml, m 0,
    1, 2
  • Minima at angle q given by d sin q (m1/2)l, m
    0, 1, 2

49
Interference Youngs experiment

Nowadays this would be replaced by a laser
Source Dutton, Figure 6
50
Intensity of interference pattern

Source Dutton, Figure 7
51
Intensity of interference pattern (continued)

Source Hecht, Physics
52
Interferencedemonstration
  • Basic relationships
  • Maxima at angle q given by d sin q ml, m 0,
    1, 2
  • Minima at angle q given by d sin q (m1/2)l, m
    0, 1, 2
  • For light of 650 nm (red), d .2 mm 1 x 10-4 m
  • Maxima at q .00325 radians .186o
  • At 10 m, distance to first maximum 3.25 cm
  • For light of 650 nm (red), d .1 mm 1 x 10-4 m
  • Maxima at q .0065 radians .372o
  • At 10 m, distance to first maximum 6.5 cm

53
Light reflection at boundary

Source Dutton, Figure 8
54
Light reflection at boundary (continued)

Source Dutton, Figure 9
55
Light reflection at boundary

56
Diffraction
  • Origin
  • Wave nature of light at sharp boundaries
  • Significant when opening l or when large
    magnifications are involved
  • Large magnifications amplify problem
  • Ultimately limits resolution of microscopes,
    telescopes

57
Calculation of diffraction relationships

Source Tipler, Physics
58
Diffraction

Opening l
Opening gtgt l
Source Tipler, Physics
59
Diffraction mathematical results
  • Basic relationships
  • a sin q ml, m 1, 2, 3, 4 gives angles of
    minimum intensity
  • Solving for angle q, q sin-1(ml/a)
  • I I0 sin (j/2)/(j /2)2
  • j (2p/l) a sin q
  • If a ltlt l, then angles for first several minima
    large
  • Note that if a gtgt l, then angles for first
    several minima (m1, 2) very small

60
Diffraction mathematical results
  • Basic trigonometry y/d tan q, so that y d
    tan q
  • For light of 650 nm (red), opening a .1 mm 1
    x 10-4 m
  • Computing angle, q sin-1(ml/a) sin-1(1 x 650
    x 10-9/10-4) sin-1(6.5 x 10-3) ? 0.0065 radians
  • First minimum at q .0065 radians .3724o
  • At D 10 m, distance to first minimum y 10m
    tan 0.0065 ? 10m x 0.00655 0.065 m 6.5 cm

first minimum
a
y
q
Laser
D
61
Typical diffraction pattern

Source Tipler, Physics
62
Diffraction gratings
  • Use large number of lines to amplify diffraction
    effects
  • Result is to sharpen diffraction maxima, minima
  • More importantly, pattern shifts to repeating
    light, dark bands with little or no fall off of
    intensity
  • Provides way to separate wavelengths of light
    (and information they are carrying)

More lines
Source Tipler, Physics
63
Diffraction gratings (continued)
  • Diffraction gratings specified by number of
    lines/mm
  • Calculation of maximum for diffraction gratings
    follows two slit interference formula, since the
    grating looks like a long row of slits
  • d sin q ml, where d is line (slit) separation
    1/lines per mm
  • This gives angle to mth maximum
  • Projection onto target at distance D gives y ??
    mlD/d
  • Be careful to keep all distances in same units
    (mm, cm, or m)

first maximum
y
q
Laser
D
64
Diffraction gratings (continued)
  • Example 500 l/mm gt d 1/500 mm 2 x 10-6 m
  • l 650 nm, D 10 m, m 1
  • Then y mlD/d 1 x 650 x 10-9 x 10/(2 x 10-6)
    3.25 m
  • Below is diagram of what would happen to a
    mixture of blue and red light incident on a
    diffraction grating

Source http//hyperphysics.phy-astr.gsu.edu/hbase
/phyopt/gratcal.html
65
Scattering
  • Definition Photons interact with material in
    propagation medium
  • Nonlinear cannot generally be compensated
  • Problems can only be fixed by making better fiber
  • Types
  • Impurities in fiber light exits fiber at high
    angles or is absorbed

66
Scattering (continued)
  • Rayleigh cause by small variations in density of
    glass as it cools
  • Variations smaller than l, leading to scattering
  • Stimulated Brillouin scattering (SBS)
    scattering of light backwards to transmitter
  • Caused by mechanical (actually acoustical)
    vibrations in fiber inducing changes in RI
  • In effect, fiber becomes a diffraction grating
  • Mainly a problem at high power levels, narrow
    linewidth

67
Scattering (continued)
  • Stimulated Raman scattering (SRS) similar to SBS
  • Effect originates in molecular rather than
    acoustical vibrations
  • Primarily a problem with multiple wavelength
    systems at high powers

68
Summary of phenomena associated with light and
their effects
  • Refraction basis of optical fiber through total
    internal reflection
  • Variation of refraction with l leads to
    dispersion in fiber and limits its length
  • Interference affects design of optical
    components, especially when light enters or
    leaves a medium
  • Forms basis for design of some filters
  • Diffraction limits all optical performance
  • Forms basis for many devices which allow
    separation of light waves
  • Scattering limits long distance propagation of
    light
  • Dispersion limits long distance propagation of
    light signals
  • Polarization limits long distance propagation of
    light signals at high speed

69
Optical phenomena and their impact on optical
fiber performance
  • Scattering scattering of light in fiber from
    various sources leading to gradual attenuation
    with distance
  • Dispersion speed in fiber varies with l, leading
    to blurring of pulses
  • Can be partially or totally compensated
  • Polarization fiber supports two orientations,
    orthogonal, which vary leading to smearing of
    pulses
  • Refraction/reflection determines diameter of
    core, index required for cladding to achieve
    single mode, multimode
  • Mixing different wavelengths interact in fiber,
    causing signal degradation
  • Can be partially compensated

70
Main types of optical fiber in common use today
  • Multimode
  • More than one path for light as it travels down
    fiber
  • Core 50, 62.5, 100 micrometers
  • Primarily for short distances
  • Single mode
  • Only a single path for light as it travels down
    fiber
  • Core 8.3-10 micrometers

Source Arcelect.com
71
Multimode fiber construction
  • Modern multimode fibers all use graded index
    (GI) technology
  • Operates in second transmission window, around
    1310 nm
  • Idea is to gradually decrease index of refraction
    of core outwards
  • Pulses travel slower in regions of higher index
    of refraction
  • Decrease is from center (highest r.i.) to outer
    edge of core (lowest r.i.)
  • Since pulses travel longer distances when
    bouncing off of cladding, they travel faster
    there, resulting in less dispersion
  • 200 MHz bandwidth over 2 km

72
Types of single mode fiber
  • Non-dispersion-shifted (NDSF)
  • ITU spec G.652
  • 95 of deployed plant
  • TDM in 1310 nm, DWDM in 1550 nm regions
  • Chromatic dispersion zero at 1310 nm
  • Dispersion-shifted (DSF)
  • ITU spec G.653
  • TDM in 1550 nm regions
  • Chromatic dispersion zero point shifted up to
    1550 nm
  • Used for soliton transmission

73
Types of single mode fiber (continued)
  • Nonzero-dispersion-shifted (NZ-DSF)
  • ITU spec G.655
  • TDM, DWDM in 1550 nm regions
  • No zero dispersion point in operating regions,
    but uses chromatic dispersion to compensate other
    problem of four wave mixing

74
Calculation of material dispersion
  • Dispersion measured in units of time, usually
    picoseconds (ps)
  • Tells how much smearing out in time a pulse will
    suffer
  • Fiber specifications given in units ps/nm-km
  • nm refers to spectral width of the source
  • This is a physical characteristic of the laser or
    LED used
  • km refers to the length of the fiber
  • You must determine this from your physical
    installation
  • May be read as picoseconds of pulse spreading
    per nanometer of source spectral width and per
    kilometer of path length

75
Calculation of dispersion (continued)
  • Example
  • You have a 10 km fiber, with dispersion specified
    as 5 ps/nm-km at the wavelength youre using
  • You are using a laser with spectral width 12 nm
  • Your pulses are 200 ps long to start
  • Dispersion 5 ps/nm-km x 12 nm x 10 km 600 ps
  • This means that your pulses have spread out by
    600 ps, for a total length of 200 ps 600 ps
    800 ps
  • As this is 4 times the starting length of the
    pulses, the system probably would not work

76
Calculation of dispersion (continued)
  • When reading dispersion off of a graph, for these
    calculations, use absolute value of dispersion
  • If dispersion is negative, means shorter
    wavelengths travel more slowly
  • If dispersion is positive, means longer
    wavelengths travel more slowly
  • Pulse smears in either case
  • Sign is important if youre trying to compensate
    for dispersion

77
Fibers and windows
850 nm 1310 nm 1550 nm 1625 nm
Multimode ?
NDSF ? (TDM-single channel) ? (DWDM-multiple channels if used with dispersion compensators)
DSF ? (TDM-single channel)
NZ-DSF ? (TDM DWDM)

78
Optical fiber devices
  • Splitters
  • Combiners
  • Filters
  • Light sources
  • Detectors
  • Switches
  • Opto-electronic converters
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