Optical Fiber Basics

1
Optical Fiber Basics

• Lighthouse Chapter SCTE
• September 17th and 18th, 2008
• Jay Lazorcik
• Jay.Lazorcik_at_arrisi.com
• 814-466-9300

2
Agenda

• Basics Principles
• Loss Mechanisms
• Connectors and Couplers
• WDM Basics
• Amplification

3
Basic Principles
4
What is Light ?

• Most people think of light as the portion of the
  electromagnetic spectrum visible to the naked
  eye, however, in fiber optic communications it
  has a slightly broader definition.
• Light is the portion of the electromagnetic
  spectrum from approximately 300nm to 2000nm.

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Spectral Characteristics
Optical Region
wavelength
850 nm

• Light
• Ultra-Violet (UV)
• Visible
• Infrared (IR)
• Communication Wavelengths
• 850, 1310, 1550 nm
• Low Loss Wavelengths
• Specialty Wavelengths
• 980, 1480, 1625 nm

980 nm
1310 nm
1480 nm
1550 nm
1625 nm
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A Fiber Optic Chronology

• Circa 2500 B.C. Earliest known glass
• Roman Times Glass is drawn into fibers
• 1841 Daniel Colladon demonstrates light guiding
  in jet of water in Geneva
• 1930 Heinrich Lamm assembles first bundle of
  transparent fibers to carry an image of an
  electric lamp filament.
• 1959 American Optical draws fibers so fine they
  transmit only a single mode of light recognizes
  the fibers as single-mode waveguides.
• 1960 Theodore Maiman demonstrates first laser at
  Hughes Research Laboratories.
• 1975 First non-experimental fiber-optic link
  installed by Dorset (UK) police after lightning
  knocks out communications

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Optics Fundamentals
Reflection
Reflection
Refraction
Light reflects inside medium
Light reflects inside medium
Light passes through medium boundary
Light
Air Glass
Air Glass
Light is refracted
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Refraction

• When light waves propagate through a medium other
  than free space, the velocity (or speed) of the
  wave is reduced.
• The change in speed from one medium to the other
  causes the light to be bent from its original
  direction.

Original Direction of Propagation
Transition Boundary
For any given time interval the wave front
travels slower inside the new medium
New Direction of Propagation
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Index of Refraction

• The index of refraction (n) is the ratio of the
  speed of light in a vacuum (c) to the speed of
  light in the material (v). This is written as n
  c/v
• Simply, Index of Refraction is a relative measure
  of the propagation speed of the signal.
• For a vacuum n1 Air n1.0003 Water
  n1.333
• Also, different wavelengths have different
  indices of refraction. This is why a prism
  divides the visible colors of the spectrum.

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Dispersion

• Because different colors have different indices
  of refraction, they will travel at different
  speeds through the fiber.
• This is referred to as dispersion.
• If two pulses are launched simultaneously at two
  different colors into the fiber, they will arrive
  at the receiver at different times.

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Snells Law

• Snells Law is the property used to determine the
  new direction of propagation through a transition
  boundary.

n1 refractive index of medium 1 n2
refractive index of medium 2 q1 Angle of
incidence q2 Angle of refraction
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Total Internal Reflection

• Beyond some maximum incident angle the ray of
  light cannot pass through the boundary of the two
  materials and the ray is completely reflected.
• When the angle of incidence exceeds the maximum
  angle or Critical Angle, we have Total Internal
  Reflection.
• Total Internal Reflection is the property that
  allows fiber optic communication to occur.

Critical Angle
13
Fiber Construction

• Optical Fiber is a cylindrical waveguide made of
  a high purity fused silica.
• The core has a refractive index slightly higher
  than the cladding which allows the propagation of
  light via total internal reflection.
• A single-mode core diameter is typically 5-10?m.
• A multimode core diameter is typically over 100
  ?m.

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Single and Multimode Fiber

• Multimode fiber has a larger core and allows
  several modes to propagate while single mode only
  allows the first (or fundamental) mode to
  propagate.

15
Fiber Construction

• Fiber is manufactured using a drawing process by
  which a silicon fiber cylinder is heated and then
  stretched to the proper diameter.
• Once the fiber has the proper diameter, it is
  given a protective coating that is dried in place
  using ultraviolet light.

16
Waveguide Propagation

• To better describe some optical phenomena, it is
  important to remember that light is actually a
  traveling electromagnetic wave.
• As light propagates through a fiber, it creates a
  standing wave across the diameter of the fiber
  core. This is called waveguide propagation.
• A small portion of the power also penetrates into
  the cladding.

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Mode-Field Diameter

• Since some power propagates in the cladding,
  fiber manufacturers typically refer to the
  effective diameter of the core or Mode Field
  Diameter.
• Mode Field diameter is defined as the width of
  the field at e-1 ( 1/3) of the maximum amplitude.

18
Loss Mechanisms
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Fiber Attenuation Standard SMF

• Due to the characteristic attenuation curve of
  fiber, there are two regions typically used for
  communications.

20
Rayleigh Scattering

• Rayleigh scattering is a fundamental loss
  mechanism caused by local microscopic
  fluctuations in density.
• The density fluctuations cause random refractive
  index fluctuations on a scale much smaller than
  the optical wavelength.
• Rayleigh scattering is more predominant at short
  l.

21
Macro and Micro-bends

• Macrobend refers to loss caused by bending the
  fiber beyond a minimum bend radius.
• Microbend refers to small bends or minute
  deviations in the core/cladding interface

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Other Types of Attenuation

• Attenuation also commonly occurs at the junction
  interface between two pieces of fiber.

23
Connectors and Couplers
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Connectors Splices

• Frequently there is the need to temporarily or
  permanently joint two pieces of fiber. To
  achieve this, many different types and styles of
  mechanical assemblies have been designed.
• Many types of connectors were designed based on
  cost and performance as the key objectives.
• Merits of each connector are in the ability to
  hold the close tolerances necessary for good
  connection and in the repeatability of multiple
  connections.

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Common Connector Types

• FC and SC connectors are the most common in
  domestic market, ST connectors are mostly used
  in telephony applications and E-2000 is popular
  overseas.

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Physical Contact Connectors

• The differences in connectors types are mainly in
  the mechanical assembly that holds the ferrule in
  position against another, identical ferrule.
• The FC, SC and ST are all examples of physically
  contacting (PC) connectors. Physically contacted
  is where the fiber ends are mated in physical
  contact with each other.

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Physical Contact Connectors

• Physical Contact connectors are polished with a
  convex shape that is meant to slightly deform
  when two connectors are brought together.

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UPC Ferrule Configuration

• Ultra Polished connectors (UPC) have a flat end
  surface where both the ceramic and fiber are
  polished to the same plain.
• The UPC connector provides very low insertion
  losses, however, whatever light is reflected will
  propagate back toward the source.
• UPC connectors are typically used near receivers,
  since and back reflections will be attenuated as
  they propagate back through the fiber.

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APC Ferrule Configuration

• Angled Polished connectors (APC) are polished
  with an 8? end surface angle.
• The APC has slightly higher insertion losses than
  the UPC style, however the angled end directs
  back reflections into the cladding.
• APC connectors are typically used near sources
  (i.e. transmitters or EDFAs) where back
  reflections will quickly reduce system
  performance.

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Common Connector Faults

• Angular misalignment caused by mating an APC
  connector to a UPC connector is extremely common
  and will cause approximately 3-5 dB of
  attenuation.

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Connector Dos and Donts

• Proper connector care and fiber cleanliness is
  the number one cause of fiber faults and
  connector related signal loss.
• ALWAYS
• Protect connector ends with end caps.
• Clean the connector tip before mating connectors.
• Ensure both mating surfaces are clean. Dirty
  connectors mated to clean connectors can damage
  both connector faces.
• NEVER
• Use matching gels. After first connection, gel
  can act as an adhesive for abrasive particles.
• Over-tighten mating connectors Over-tightening
  can distort and damage fiber ends
• Touch or blow on connector end.
• Use metallic or sharp objects on fiber ends.

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Connector Cleaning

• Ø Mildly abrasive tape cartridge
• Extended use can damage the high polish on
  physical contact connectors
• Ø Wet clean method
• High grade (gt99) isopropyl alcohol
• Low grade alcohol leaves an oil residue
• Lint free Kimwipe or lint free cotton
  polishing swab
• First pass wet with alcohol to clean
• Second pass dry to remove moisture and to polish
• Optional compressed, filtered, non-residue air

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Fusion Splices

• Fusion splices are created by heating both fiber
  ends with a high current electric arc and then
  bringing them into contact with one another.
• Fusion splices are permanent and have extremely
  low loss
• Splice loss is typically 0.02db
• Fusion Splice equipment is expensive and can be
  difficult to use in the field

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Mechanical Splices

• Fiber ends are butted together and mechanically
  locked into place in a funneled capillary.
• Inside the tube is a small amount of Index
  Matching Gel that reduces the amount of
  reflection at the fiber ends interface.
• Mechanical Splices can be permanent or reusable,
  but splice loss is typically 0.1db to 0.2db and
  degrades with each use.
• Splice tubes are less than 10.00

35
Splitters and Couplers

• Similar to an RF splitter, an Optical Splitter is
  a signal path divider that divides the signal
  into two or more outputs.
• Coupler or Directional Coupler is a term commonly
  used interchangeably to describe splitters.

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Splitters and Couplers

• Ideal optical couplers are basically power
  dividers that direct but do not consume signal.
  In other words, the output power from all legs
  must equal the input power to the device.

• Non-ideal couplers include approximately 0.25 dB
  of consumed power loss.

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Optical Couplers

• Like RF taps, optical couplers can be made into
  virtually any splitting ratio or any number of
  outputs.
• 1XN couplers are easily created by cascading
  multiple two output couplers.
• Split ratios on two output couplers are typically
  available in standard 5 increments, such as 95/5
  or 75/25.
• Other commonly available splits are 99/1, 97/3
  and 67/33
• The two most common types of couplers available
  are Directional and Planar.

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Wave Division Multiplexing
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Wave Division Multiplexing

• Breaking down the Concept
• Wave
• The wavelength of light or frequency of the
  channel
• Think of it as a CATV channel or Color of light
• Division
• Dividing the Spectrum
• Think of it as the same as the CATV channel plan
• Multiplexing
• Combining or Breaking out of specific channels
• Think of it as a CATV headend combining network
• Can be channel specific like a filter
• Can be Broadband and non-channel specific

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ITU Channel Window
V
A
C
NTSC 6 MHz Channel Windows using Frequency
Division Multiplexing
ITU Windows using Wave Division Multiplexing

• Wave division Multiplexing does the same thing,
  but at Optical frequencies (or wavelengths).

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Frequency and Wavelength

• Frequency is a measure of the number of cycles
  that a voltage signal completes in one second
• Frequency is measured in Hertz (Hz) or cycles per
  second and is inversely proportional to
  wavelength (l)

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Wavelengths of Light

• Light travels farther in fiber at certain
  wavelengths
• Those wavelengths are used for transmission
  systems

1530 1560

• 1310nm Region
• Used extensively for metropolitan area systems
  and analog video transport

• 1550nm Region
• Light travels farther than at 1310
• Components are more expensive
• Used mostly for long distance

• DWDM - Between 1530 and 1560
• Wavelengths must be very specific
• Extra components needed to lock wavelengths to
  specific color

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Evolution of WDM Systems
WDM one 1310 and one 1550 channel
CWDM 18 Windows, 20nm spacing with 1 carrier
per window
Dense WDM 20 - 40 Windows, 100-200 GHz spacing
with 1 carrier per window
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CWDM

• Course Wave Division Multiplexing
• Standard channel plan developed by the ITU
• International Telecommunications Union
• 20 nanometer spacing between channels
• Starting at 1270nm and going thru 1610nm
• 18 Channels

1270 nm
1610 nm
1510 nm
1570 nm
1290 nm
1490 nm
1530 nm
1550 nm
1590 nm
1310 nm
1330 nm
1350 nm
1370 nm
1390 nm
1410 nm
1430 nm
1450 nm
1470 nm
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Illustrating the Optical Spectrum
Some wavelengths are more suitable for certain
services than others
Water Peak
Low Dispersion Area
Wide Availability of CWDM GbE SFPs
ITU G.695 CWDM Wavelength Allocation Plan
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DWDM

• Dense Wave Division Multiplexing
• Standard channel plan developed by the ITU
• International Telecommunications Union
• 400, 200, 100, and now 50 GHz spacing between
  channels
• Starting at 1530nm and going thru 1560nm

1530 to 1560 DWDM
1310
1270 nm
1610 nm
1510 nm
1570 nm
1290 nm
1490 nm
1530 nm
1550 nm
1590 nm
1310 nm
1330 nm
1350 nm
1370 nm
1390 nm
1410 nm
1430 nm
1450 nm
1470 nm
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ITU Grid (100GHz Spacing)

• C-Band roughly extends from 1530nm to 1560nm
• ITU Channel number related to frequency

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Amplification
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Optical Line Amplification
l l l l l l l l l
4
5
7
8
1
2
3
6
16
Attenuated Channels
Amplified Channels
All Wavelengths Amplified with One Amplifier
50
EDFA Operation
amplified spontaneous emissions
amplified spontaneous emissions
Erbium Doped Fiber
1550 nm band signal output
1550 nm band signal input
pump signal output (980 and 1480 nm)
(980 and 1480 nm) pump signal input
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EDFA Operation
4
F
650
nm
9
/
2
4
I
800
nm
9
/
2
4
I
980
nm
11
/
2
4
I
13
/
2
1536
nm
4
I
15
/
2

• Pump lasers (typically 980nm and 1480nm) are used
  to excite the Erbium doped fiber to a higher
  energy state.

• Pump lasers (typically 980nm and 1480nm) are used
  to excite the Erbium doped fiber to a higher
  energy state.
• The Erbium fiber resides in the 4I-13/2 state
  until triggered by a passing 1550nm photon, at
  which time a duplicate photon is released.

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EDFA

• Erbium Doped Fiber Amplifiers (EDFAs) are the
  amplification device most commonly used in fiber
  systems.

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Constant Power EDFA

• As number of wavelengths increase, the total
  power remains constant
• However, the power per wavelength decreases

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Constant Gain EDFA

• As the number of wavelengths increase, the total
  output power increases
• However, the power per wavelength remains constant

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Constant Gain EDFA

• Care must be taken in constant gain mode such
  that the total number of wavelengths does not
  cause the total output power to exceed the EDFA
  capability.

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Summary

• Basics Principles
• Loss Mechanisms
• Connectors and Couplers
• WDM Basics
• Amplification

57
Thanks for your time!
Jay Lazorcik Jay.Lazorcik_at_arrisi.com