Optical Fiber Communications

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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

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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
• 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. lt 2 dB/km
• 1980 optical cable refined affordable
  optical communication system
• 1990 0.16dB/km loss

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History

• 1988 long haul transmission system
• 1988 SONET
• 1990 optical voice and data network are common

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Advantages

• Wider bandwidth
• 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.

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Advantages

• Environmental Immunity
• 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

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Advantages

• Lower transmission loss
• lesser loss compared to metallic cables
• 0.19 dB/km loss _at_ 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 systems cost

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Disadvantages

• Interfacing cost
• 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

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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

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Optical Spectrum
13
Optical Communication systems
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types of fiber

• Optical fiber construction

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types of fiber

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

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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

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light propagation

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

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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

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light propagation

• Optical power
• flow of light energy past a given point in a
  specified time

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light propagation

• Optical power
• generally stated in decibel to define power
  level (dBm)
• Question
• 10 mW in dBm?

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light propagation

• Velocity of Propagation
• in vacuum 3 x 108 m/s
• but slower in a more dense material than free
  space
• when is passes through different medium such
  from one medium to another denser material the
  ray change its direction due to the change of
  speed

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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

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light propagation

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

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light propagation

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light propagation

• Snells Law
• how a light ray reacts when it meets the
  interface of two transmissive materials that have
  different indexes of refraction

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light propagation

• Snells Law
• angle of incident
• 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

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light propagation

• Snells Law

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light propagation

• Question
• medium 1 glass 1.5
• medium 2 ethyl alcohol 1.36
• angle of incident 30o
• determine the angle of refraction

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light propagation

• Critical Angle
• the angle of incident ray in which the refracted
  ray is 90o and refracted along the interface

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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

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light propagation

• Critical Angle

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light propagation

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

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light propagation

• Acceptance Angle

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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

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light propagation

• Numerical Aperture - NA

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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

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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

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Optical Fiber Configurations

• Index Profile
• step index single mode
• step index multimode
• graded index - multimode

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optical fiber classification

• Single Mode Step Index
• dominant widely used in telco system and
  network
• the core is significantly smaller in diameter
  than multimode fiber

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optical fiber classification

• Multimode Mode 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

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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

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optical fiber classification

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

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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

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losses in optical fiber

• Attenuation
• power loss reduction in the power of light
  wave as it travels down the cable
• effect on systems performance by reducing
• systems bandwidth
• information tx rate
• efficiency
• overall systems capacity

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losses in optical fiber

• Attenuation

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losses in optical fiber

• Attenuation
• depends on signals wavelength
• generally expressed as decibel loss per km
• dB/km

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losses in optical fiber

• Attenuation

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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

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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 glasss
  atom converted into random mechanical
  vibrations - heating
• ion resonance caused by OH- in in the
  material. OH- trapped in the glass during
  manufacturing process

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losses in optical fiber

• Absorption Loss

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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

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losses in optical fiber

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losses in optical fiber

• Chromatic Wavelength, Dispersion Loss
• many wavelengths being tx 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

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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

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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

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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

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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 displacements 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

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losses in optical fiber

• Coupling Losses
• angular displacement
• less than 2o, the loss will typically less than
  0.5 dB
• imperfect surface
• end fiber should be polished and fit together
  squarely

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losses in optical fiber

• coupling loss

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Sources

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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

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Sources

• LED
• p-n junction diode
• made from a semiconductor (AlGaAs)
• emits light by spontaneous emission

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Sources

• Homojunction 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

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Sources
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Sources
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Sources
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Detectors

• PIN diodes
• 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

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Detectors

• Characteristic of Light detectors
• responsivity
• 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

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optical fiber system link budget
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optical fiber system link budget

• Typical losses
• cable loss
• depend on cable length, material and purity.
  dB/km
• connector loss
• mechanical connector to connect two fibers.
• up to 2 dB loss per connector
• source to cable interface loss
• small percentage power loss between source and
  fiber
• cable to light detector interface loss
• small percentage power loss between fiber and
  detector
• splicing loss
• splices not perfect. Up to several dB loss
• cable bends
• bending more that allowed bending radius. Up to
  several dB to total loss

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optical fiber system link budget

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optical fiber system link budget

• Question
• LED output power 30 mW
• four sections of optical cable, 5 km each with
  0.5dB/km loss
• three cable to cable connector, 2 dB loss each
• no cable splice
• light source to fiber loss 1.9 dB
• Fiber to light detector loss 2.1 dB
• no bending loss
• determine the optical power received in dBm and
  watts for 20km optical link