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