TCOM 503 Fiber Optic Networks

1
TCOM 503 Fiber 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
• Week 4 Fiber optic components fabrication and
  use
• Week 5 Fiber optic components, modulation of
  light
• Week 6 Optical fiber fabrication and testing of
  components
• Week 7 Noise and detection

3
Topics for final exam (revised)

• Principles of fiber optic cable and devices
  (reflection, refraction, interference,
  diffraction)
• Types of fiber optic cable
• Types of distortion and other problems involved
  with optical fiber
• Operation of LEDs and lasers
• Operation of detectors
• Operation of EDFAs
• Resonant couplers/wavelength selective couplers
  splitters
• Other optical devices
• Isolators - Fabry-Perot filters
• GRINs - Dielectric filters
• FBGs - Modulators Modulation types
• Optical fiber construction, fabrication
• Optical test instruments and how to interpret
  their displays

4
Optical fiber standard dimensions

• Core, cladding, jacketing standardized
• Jacket 245 mm

Source Corning
5
Requirements for fabricating useful optical fiber

• Materials must be extremely pure
• Impurity lt 1 part per billion for metals
• Impurity lt 1 part per 10 million for water
• About 1000 times more pure than traditional
  chemical purification techniques allow
• Dimensions must be controlled to extremely high
  degree
• Core size, position, cladding size tolerances 1
  micron or less
• Roughly 1 wavelength of light
• Refractive indices must also be very precisely
  controlled
• Must be made in long lengths
• Must have tensile strength

6
Purification of silica a two step process

• First use distillation
• Heat silica to boiling point (2230o C), condense
  gas
• Metals are heavier and do not boil at this
  temperature
• Yields impurity levels of 10-6
• Second stage takes place when fiber fabricated

7
Fiber fabrication process

• Called Outside Vapor Deposition Process or OVD
  process
• Stages
• Laydown
• Consolidation
• Drawing

8
First stage Laydown

• Vapor deposition from ultrapure vapors
• Soot preform made when vapors exposed to burner
  and form fine soot particles of silica and
  germanium

(From particles of silica and germania)
Source Corning
9
Laydown (continued)

• Particles deposited on surface of rotating bait
  rod
• Core first
• Then silica cladding
• Vapor deposition process purifies fiber material
  as impurities do not deposit as rapidly
• Preform is somewhat porous at this stage

10
Second stage consolidation

• Bait rod removed
• Placed in high-temperature consolidation furnace
• Water vapor removed
• Preform sintered into solid, dense, transparent
  glass
• Has same cross-section profile as final fiber,
  but is much larger (1-2.5 cm, final 125 mm
  .0125 cm)

11
Third stage drawing

• Done in draw tower
• Glass blank from consolidation stage lowered into
  draw furnace
• Tip heated until gob of glass falls
• Pulls behind it a thin strand of glass
• Gob cut off
• Strand threaded into computer-controlled tractor
  assembly
• Sensors control speed of drawing to make precise
  diameter

1850-2000o C
Source Corning
12
Drawing (continued)

• Diameter measured hundreds of times per second
• Ensures precise outside dimension
• Primary and secondary coatings (jackets) applied
• At end, fiber wound onto spools for further
  processing

Gob forming, Source Corning
13
Draw tower

Source Axsys
14
Other methods used to make fiber

• Vapor phase axial deposition (VAD)
• Batch process
• Preforms can be drawn up to 250 km
• Flame hydrolysis
• Soot formed and deposited by torches

15
VAD process (continued)
Source Dutton
16
Other methods used to make fiber (continued)

• Modified chemical vapor deposition (MCVD)
• Silica formed inside silica tube in gaseous phase
  reaction
• Soot deposited on inside of tube
• Burners traverse tube
• Sinters soot
• Produces highly controllable RI profile
• At end, tube evacuated, sides collapse

17
MCVD process
Source Fotec
18
Types of optical fiber

• Single mode glasslong distance communications
• Multimode glassshort distance communications
• Plasticconsumer short distance, electronics
  cars
• Hybrid or polymer clad (glass core, plastic
  cladding)lighting, consumer applications

19
Basic structure of all optical fiber

• Corecarries most of light
• Claddingconfines light to core
• In some fibers, substrate glass layer to add
  strength
• Inner jacket or primary buffer coatingmechanical
  protection
• Outer jacket or secondary buffer
  coatingmechanical protection

Source Optical Cable Corporation
20
Plastic optical fiber (POF)

• 1000 mm diameter, 980 mm core
• Strong
• Uses LEDs in visible range, 650 nm
• Not suitable for long-distance uses
• Does not transmit infrared

Source Pofeska/Mitsubishi Rayon Co.
21
Numerical aperture

• Light must fall at an angle such that it can
  enter fiber core, before total internal
  reflection takes over
• This angle is called numerical aperture
• www.corning.com/opticalfiber/discovery_center/tuto
  rials/fiber_101/aperture.asp

22
Basic cable construction types

• Tight buffered
• No room for fibers to move inside of cable
• Loose tube
• Multiple fibers loose inside of outer plastic
  tube
• Advantage is that with extra length of fiber
  inside tube due to curling, less likelihood of
  damage in sharp bends
• Loose tube with gel filler
• Multiple fibers immersed in gel inside of plastic
  tube

Source Dutton
23
Typical indoor cable

• Single core or double core
• Utilize substrate for additional strength (aramid
  or fiberglass)

Source Dutton
24
Tight buffered indoor cable

• Application building risers
• 6 or 12 fibers typically
• Central strength member supports weight of cable
• Tight buffering means that fibers are not put
  under tension due to their own weight

Source Dutton
25
Outdoor cable

• More rugged, larger number of fibers per cable
• 6 fibers/tube, 6 tubes 36 fibers
• 8 fibers/tube, 12 tubes 96 fibers
• Steel or plastic used for strength member
• Outer nylon layer in locations where termites are
  a problem

Source Dutton
26
Outdoor cable (continued)
Source Dutton
27
Submarine cable

• Smaller number of fibers because mechanical
  requirements much greater
• 4 to 20 typically
• Must withstand high pressure, damage from
  anchors, trawlers, etc.
• Cables for shallow water are in greatest danger
• Typically heavily armored

Source Dutton
28

MM 0.90/ftSM 0.53/ft
Indoor/OutdoorArmoredDirect bury
MM 6.65/ftSM 2.45/ft
Source Mohawk/CDT
29
Splicing fiber geometry parameters

• Always necessary to splice fiber
• 3 parameters are most important to making good
  splices
• Cladding diameter tolerance
• Must be tight so that cores meet
• Typical spec is 125 mm 1.0 mm, removes this as
  problem
• Core/cladding concentricity
• Must be tight so that cores meet
• Fiber curl
• Must be minimal so that cores meet

30
Fiber geometry parameters (continued)

31
Cable connecting and splicing

• Problem of splicing and joining fibers
• Core is very small
• Any irregularity can lead to significant loss of
  power or complete failure
• Light is not like electricity
• Travels in waveguide and is a guided wave
• Requirements for good connection
• Precisely square cuts
• Ends polished flat
• Ends butted together
• Nearly exact matchup

32
Ways of joining fibers

• Fusion splicing (welding)
• Commonly done in field
• Index matching epoxy glue
• Common done in field
• Mechanical connectors
• Used in field, but connectors attached in factory
• Not suitable for fiber breaks

33
Dangers to fiber optic cable

• Excessive tension
• Bends of small radius
• Not generally problem with outdoor or undersea
  cables because physical size keeps bend radius to
  3 feet
• Physical damage from animals, earth moving
  equipment
• Installation damage
• Lifting
• Pulling through conduits
• Water inflo
• Lightning

34
Cabling environments

• Long-haul outdoor
• Usually direct bury
• Campus area outdoor
• Direct bury
• Conduit
• Outdoor overhead
• Undersea
• Most difficult environment
• Indoor
• Benign environment
• Installation usually most difficult problem

35
Fusion splicing

• After cutting and polishing, ends are butted and
  then fused by heat
• Requires high temperature
• Can yield losses as low as 0.1 db (loss on 1 km
  of fiber)

Source Dutton
36
Steps in fusion splicing

• Strip primary coating on each fiber
• Cleave ends square
• Position ends a few mm from each other and clamp
• Align ends and bring closer together
• Electric arc started and melts glass, joining
  fibers

37
Problems with splicing

• When fiber ends melted and touched together,
  surface tension effects tend to align outside of
  cladding
• Result is OK for multimode fibers with larger
  cores
• May not work for single mode fibers
• Single mode fibers require more precise alignment
• Use of laser shining into one fiber with detector
  in the other
• One fiber moved with precision actuator to
  position it
• Other problems can occur during join
• Surface tension can change fiber position as join
  is made
• Other systems use magnified image of fiber ends
  displayed on a screen

38
Problems with splicing (continued)

• Heating is difficult
• Best results when glasses melt and fuse
• Can change refractive index and hence cause
  losses
• Idea is to melt only a very thin layer on each end

39
Joining with epoxy glue

• Ends cleaved and polished, space between them
  filled with epoxy resin
• Same RI as fiber core
• Fibers held in place mechanically
• Low cost but subject to many problems
• Concentricity
• Differing outside diameters
• Circularity of outside
• Tolerances of alignment device
• Long-term stability of epoxy

40
Mechanical spicing and bonding

• Fibers inserted into silica sleeves
• Aligned as with welding method
• Bonded with epoxy
• High quality but costly

41
Losses in fiber splices

• Extrinsiccaused by joining method but unrelated
  to fiber properties
• IntrinsicCaused by some inherent property of the
  fiber

42
Extrinsic losses

• Longitudinal misalignment
• Some light not within NA
• Endfaces form Fabry-Perot interferometer
• Lateral misalignment
• 2.5 microns 1 db loss
• Ends not square
• Surfaces cannot be joined closely
• Angular misalignment
• Losses due to NA
• 2o 1 db
• Fiber end rough or irregular
• Scattering
• No close contact

Source Dutton
43
Intrinsic losses

• Concentricity
• Axes of core and fiber differ
• Greater for SM
• Core shape
• Not problem for MM
• For SM, causes fiber to be birefringent
• Different RIs for different polarizations
• Leads to PMD
• Core diameter
• Losses traveling from large to small diameter
• Cladding diameter
• If diameters differ, cores cannot be aligned
• NA and Refractive Index
• Some light reflected if these differ

Source Dutton
44
Purely mechanical connectors

• Todays most common interconnection device
• Not fitted in field, as equipment expensive
  (100K) and process difficult
• Early connectors were poor
• Latest generation much better
• Components
• Ferrule long thin cylinder for alignment
• Connector body holds ferrule
• Cable attachment mechanism holds cable in body
• Coupling device where cables mate
• Fiber optic cables generally do not use
  male/female connection method common for
  electronic cables

45
Purely mechanical connectors
Source Goff
46
Connector evolution

Source Goff
47
Commonly used fiber optic connectors
Source Goff Fotec
48

Source Goff Fotec
49
Connectors (continued)
L to R SC-DC, LC, MT-RJ, SC, Volition, Opti-Jack
Source Fotec
50
Fiber optic connector selection guide
51
Typical data sheet

• hansonfiber.com/pdf/DS106320MTRJ.pdf

Source Johanson
52
Typical fiber optic cable prices

• Indoor, multimode, duplex, with connector 50
  0.50/ft
• Outdoor, MM, 6 fibers/cable 0.90/ft
• Outdoor, MM, 96 fibers/cable 6.65/ft
• Outdoor, SM, 6 fibers/cable 0.53/ft
• Outdoor, SM, 96 fibers/cable 2.45/ft
• Submarine 65,000/km installed

53
Testing of optical fiber

• Overview
• Standards
• Equipment
• Cable plant testing
• Network testing

54
Overview of testing

• Definition process of verifying the performance
  parameters of fiber optic components, links,
  systems and networks and troubleshooting their
  problems
• Basic measurements
• Optical power emanating from a fiber
• Continuity or optical loss of fiber, cable,
  connectors and splices
• Bandwidth or dispersion
• Determines the information carrying capacity of
  fiber or cable
• Most tests must be repeated to determine changes
  in measured parameters under environmental stress
• Testing also includes finding problems in
  installed fiber optic cable plants

55
Tests and equipment
Test Equipment used
Optical power Fiber optic power meter
Attenuation or loss Fiber optic power meter source
Source wavelength, backscatter Optical loss test set, optical spectrum analyzer
Fault location Optical time domain reflectometer, visual cable fault locator
Bandwidth dispersion Bandwidth tester, simulation software

56
Fiber optic power meters

• Measure average optical power emanating from
  fiber
• Basic components
• Detector (Si, Ga, or InGaAs)
• Signal conditioning circuitry
• Digital display
• Connectors for coupling to fiber
• Calibration
• Normal power units mw, microw, nw (linear scale)
• Power referenced to 1 mw or microw in db (dbm)
• Broad dynamic range (1 million to 1 or 60 db)
• Normal range of measurements 0 dbm to 50 dbm

57
Fiber optic power meters (continued)

• Measurements require source with known duty
  cycle, or knowledge of duty cycle of signal
• Typically source has 2 kHz square wave output
• Uncertainty typically 5

58
Typical optical power meter
59
(No Transcript)
60
Optical multimeter
Source Tektronix
61
Fiber optic test source

• Standard signal source
• Compatible with type of fiber in use
• Single mode
• Multimode
• Wavelength
• Ability to connect to fiber and couple signal
• Same light source (LED or laser)

62
Typical fiber optic test source
63
Optical time domain reflectometer (OTDR)

• Uses phenomenon of fiber backscattering to
  characterize fibers, find faults and optimize
  splices
• OTDR sends out into the fiber a high powered
  pulse and measures the light scattered back
  toward the instrument
• Works since scattering is one of primary loss
  factors in fiber (the other being absorption)
• Pulse is attenuated on outbound leg and
  backscattered light is attenuated on the return
  leg
• Returned signal is function of twice the fiber
  loss and the backscatter coefficient of the fiber

64
OTDR (continued)

• If one assumes the backscatter coefficient is
  constant, OTDR can be used to measure loss
• Gives a graphic display of status of fiber being
  tested
• Requires access to only one end of the fiber, as
  opposed to both ends for power meters
• Not a new device models exist for microwave
  (CATV) and similar electronic applications

65
Typical OTDR
66
OTDR typical display
67
OTDR specifications
68
Tektronix OTDR
69
Use of OTDRs

• Basic architecture

Pulse
Source Fotec
Backscattered light
70
Operation of OTDRs

• Pulse from high-powered laser sent down fiber
• Pulse width 200 mm/ns
• Response measured on time scale
• Backscatter from pulse is what is sensed
• Longer distances require longer pulses
• Decreases accuracy

71
OTDR trace
Reflection from first connector
Noise typically seen here
Source Fotec
72
More OTDR traces
Source Fotec
73
More OTDR traces
Source Fotec
74
OTDR performance limitations

• Accuracy limited by several factors
• Pulse width
• Long pulse for long cables 100 ns or longer
• Short pulse for short cables 10 ns
• Speed of light variations among fibers
• 1-2 10-20 m/km
• Backscatter not constant between fibers
• Initial saturation means minimum distance
  measurement 100 to 1000 m unless precautions
  taken
• Requires launch cable to allow transients to
  die out
• Long fibers resolution 250-500 m
• Short fibers 5-10 m
• Limited usefulness for in-building testing

75
Signal analyzer

• Used to measure performance parameters of optical
  systems
• Q-factor
• Extinction ratio
• Optical power
• Signal-to-noise ratio
• Jitter
• Duty cycle
• Eye diagrams
• High-performance models directly calculate
  parameters
• Work on Return-to-zero (RZ) and
  non-return-to-zero (NRZ) signals

76
Typical high-performance signal analyzer
Source Tektronix
77
Visual cable tracers and fault locators

• Many problems in connection of fiber optic
  networks are related to making proper connections
• Since light used in systems is invisible, one
  cannot see the system transmitter light
• By injecting light from a visible source, such as
  a LED or incandescent bulb, one can visually
  trace the fiber from transmitter to receiver
• To insure correct orientation
• To check continuity
• Simple instruments that inject visible light are
  called visual fault locators

78
Typical fault tracers
Source Fotec
79
Fiber optic inspection microscopes

• Inspecting connector finish during polishing
• Inspecting installed connectors for dirt or
  scratches
• Inspecting bare fiber before splicing.
• Assures only properly cleaved fibers are used for
  splices

80
Inspection microscope
Source Fotec
81
Optical spectrum analyzer

• Shows graphically how power is distributed as
  function of wavelength or frequency

Source Agilent
82
Typical unit
Source Agilent 86140B www.agilent.com/cm/rdmfg/os
a/86140b/index.shtml
83
Close up of trace
Source Agilent 86140B
84
Controls
Source Agilent 86140B
85
(No Transcript)
86
(No Transcript)
87
Typical traces from spectrum analyzer
EDFA
FP laser
Source Agilent
88
Optical power in common communications
applications
Source Fotec
89
Causes of error in fiber optic power measurement

• Imperfect optical coupling
• Reflection from coupled surfaces
• Dirt and other contaminants on optical surfaces
• Wavelength calibration
• Most meters are calibrated for particular
  wavelength or set of wavelengths
• Excessively high or low levels
• Exceed linear range of meter
• High levels saturation
• Low levels noise interference
• Field measurements typically made to 0.1 db
  accuracy

Source Fotec
90
Fiber types
Source Fotec
91
Testing attenuation

• Basic setup
• Method
• Insert modal condition(if multimode fiber)
• Measure power at end
• Calculate
• If multimode, removeconditioner by cuttingback
  and remeasure

Source Fotec
92
Modal distribution problems

• In multimode fiber, strength of modes is function
  of length
• In single mode fiber, there is only one mode
• Zero order mode straight-through path
• Higher-order modes paths which bounce off of
  cladding
• Travel longer distance
• Experience more attenuation because path is
  longer
• Equilibrium modal distribution(EMD) refers to
  modes in longfiber, where higher-order
  modeslost through attenuation

Source Fotec
93
Modal distribution problems (continued)

• Modal distribution depends on many factors
• Source
• Fiber
• Components encountered (splices, connectors,
  etc.)
• Accurate and reproducible measurement of power in
  multimode fiber depends on knowing (or
  standardizing) modal distribution
• Also a function of type of coupling used
• Mode conditioners
• Strippers (remove unwanted cladding mode light)
• Scramblers (equalize power in modes)
• Filters (remove higher-order modes to simulate
  long distances)

94
Fiber mode distribution attenuation

Source Fotec
95
Testing bandwidth and dispersion

• Modal dispersion
• Use narrow spectral width laser, high-speed
  receiver
• Modulate laser with sine waves (sweep generator)
• Measure attenuation
• Alternate method measure degradation of pulse
  risetime
• Chromatic dispersion
• Compare pulse transit times as function of
  wavelength
• Requires several different laser sources of
  different wavelengths
• Equipment needed is very expensive

96
Fiber optic link performance parameters
Link type Source/Fiber Wave-
Transmit Receiver Margin
length (nm)
Power (dbm) Sensitivity (dbm) (dB)
Telecom laser/SM 1300
3 to -6 -40 to -45 34 to 48
1550
0 to 10 -40 to -45
40 to 45 Datacom LED/MM 850
-10 to 20 -30 to -35 10
to 25
1300 -10 to -20 -30 to -35
10 to 25 CATV(AM) laser/SM 1300
10 to 0 0 to -10
10 to 20
Source Fotec
97
Fiber optic link testing and troubleshooting

• Use power source and meter to test end-to-end
  power loss and power levels
• If there is a problem, work back from receiver to
  transmitter
• Types of tests

98
Fiber optic link testing and troubleshooting
(continued)

• Many systems and components can be tested with
  loopback