Fiberoptic Communication Systems

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Fiberoptic Communication Systems

• P. M. Shankar
• Department of Electrical
• Computer Engineering

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Fiberoptic Communication Systems

• Why use fibers?
• large bandwidth, i.e., very high data rates
  possible
• no interference from electric or magnetic fields
• can carry analog, digital, data, alone or
  jointly
• reasonably low cost

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Fiberoptic Communication Systems

• Refractive index of a material n is
• Air has an index of 1
• Water has an index of 1.33
• Ordinary glass has an index of approximately 1.45
• Glass is a denser material than water. Air is a
  rarer material than water,

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Fiberoptic Communication Systems

• Snells Law

n2
q2
interface
n1
q1
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Fiberoptic Communication Systems

• Light propagation in fibersHow?

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Fiberoptic Communication Systems

• Fiber consists of a material of higher index
  (core) surrounded by a material of lower index
  (cladding)
• Light is confined in the core by TIR.
• Thus light can only be accepted within a certain
  cone of angle at the fiber end.

out
in
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Fiberoptic Communication Systems

• What is a fiber? A core material of higher index
  and a cladding material of lower index

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Fiberoptic Communication Systems

• Is the geometrical optics based view sufficient
  to explain light propagation in fibers?
• Use of wave optics or e.m. theory required.
• What are modes?

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Fiberoptic Communication Systems

• An optical mode refers to a particular solution
  to the equation governing the propagation of
  light inside the fiber, subject to the boundary
  conditions existing from the physical properties
  of the fiber such as the core diameter, index of
  the core, index of the cladding and the operating
  wavelength.
• The mode has the property that its spatial
  distribution does not change with distance.

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Fiberoptic Communication Systems

• A fiber can support many modes.
• We can also fabricate a fiber that only supports
  a single mode (single mode fiber).
• The number of modes supported in a fiber is
  determined by the indices, operating wavelength
  and the diameter of the core.
• The V parameter determines the number of modes.

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Fiberoptic Communication Systems

• Vlt2.405 corresponds to a single mode fiber.
• If we take a multimode fiber and reduce the
  radius of the fiber, the number of modes
  supported in the fiber goes down, and, it is
  possible to reach a point when only a single mode
  can be supported.

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Fiberoptic Communication Systems

• What are Modes?

core
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Fiberoptic Communication Systems

• Index profiles
• Light can be confined in the fiber by
  manipulating the refractive indexindex
  profiles.., i.e., how the refractive index varies
  across the diameter of the fiber.
• The profiles also determine how many modes are
  supported, how much dispersion (?) will be
  present, etc.

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Fiberoptic Communication Systems

• Single mode fibers
• Since modes travel with different velocities,
  fibers that support only a single mode will have
  less dispersion.
• Single mode fibers are therefore used in long
  haul communications.
• The amount of dispersion in a single mode fiber
  can be controlled by appropriately profiling the
  index.

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Fiberoptic Communication Systems

• Index profiles of optical fibers

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Fiberoptic Communication Systems

• Power distribution in a single mode fiber

Intensity (power) profile of the fundamental mode
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Fiberoptic Communication Systems

• What is the evanescent field?
• Couplers, connectors, splices?

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Fiberoptic Communication Systems

• Is a single mode fiber a true single mode one?
• By virtue of the circular symmetry, a single mode
  can support two orthogonal polarizations.

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Fiberoptic Communication Systems

• Use elliptical cross sections
• Polarization maintaining fibers

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Fiberoptic Communication Systems

• What limits the transmission capability?
• Attenuation
• Dispersion

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Fiberoptic Communication Systems

• Attenuation

Low loss window
Low loss window
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Fiberoptic Communication Systems

• Dispersion in fibers

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Fiberoptic Communication Systems

• What happens when dispersion is present?

Two pulses are injected into the fiber. Pulses
broaden as they travel down the fiber (a)
Closest to the input end (b) away from the input
end (c) further away (d) farthest from the
input end.
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Fiberoptic Communication Systems

• Dispersion..
• As pulses travel down the fiber, they spread and
  overlap.
• This produces inter symbol interference and makes
  it difficult to separate the pulses, i.e., it
  becomes difficult separate the data bits
  increasing the bit error rate.
• This will limit either the data rate (more
  separation between the pulses) or the maximum
  distance.
• Thus, dispersion limits the data transmission
  capability in terms of Gbit/s.Km. (data rate
  distance product)

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Fiberoptic Communication Systems

• Dispersion.
• Dispersion from multimodes.each mode travels
  with its own velocity
• Dispersion in single mode fibersthe refractive
  index is a function of the wavelength and we do
  not have single wave length sourcesall sources
  have a finite spectral width

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Fiberoptic Communication Systems

• Dispersion

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Fiberoptic Communication Systems

• Dispersion in single mode fibers

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Fiberoptic Communication Systems

• Data transmission capability

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Fiberoptic Communication Systems

• Typical FO systems

• A generic fiberoptic communication system
• A long distance fiberoptic communication system
  showing repeaters

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Fiberoptic Communication Systems

• What kind of source should be used?

Intensity (power) profiles of three different
types of lasers l0 is the mean wavelength and
sl is the standard deviation. (a) Low data rate
modulation (b) Higher data rate modulation (c)
Highest data rate modulation
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Fiberoptic Communication Systems

• Link Budget
• Calculate the maximum transmission distance

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Fiberoptic Communication Systems

• Link budget
• Transmit power
• Sensitivity.minimum power required at the
  receiver to maintain acceptable performance
• include losses from connectorssource and
  detector
• losses from splices
• losses from fiber
• include power penalties (dispersion
  compensation,..)
• POWER MARGIN (6dB)

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Fiberoptic Communication Systems

• Coherent FO systems

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Fiberoptic Communication Systems

• Only optical fibers can efficiently transfer
  large volumes of data
• How can we transmit still more data? Need to
  transmit large volumes of data (40-100 Gbit/s or
  more) over long distances
• Traditional approaches may not meet this growing
  need
• Is it possible to increase the volume using
  existing fiber trunks?

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Fiberoptic Communication Systems

• Fibers carry light of a single color. Each color
  or wavelength carries a certain volume of data
• Use of several fibers can carry multiply the data
  volume being transmitted.
• This may not be possible because additional costs
  involved such as real estate costs, installation
  costs, etc.
• Is it possible to transmit more than one color
  through the same fiber?

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Fiberoptic Communication Systems

• The fiber must behave identically for all the
  colors
• Any loss suffered must be almost same for all
  colors
• Any broadening suffered by the pulses also be
  same for all colors
• The colors should not mix

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Fiberoptic Communication Systems

• Increase the number of wavelengths (colors) in a
  single fiber
• Make them dense. Pack more colors in a fiber. 40
  colors is normally referred to as coarse
  Wavelength Division Multiplexing (WDM). More than
  80 colors is Dense WDM (DWDM)
• If a single fiber using a single color transmits
  40 Gbit/s, use of 40 colors in a fiber will
  increase the data rate to 1.6 Trillion bits/s.
  This is equivalent to about 20 million
  simultaneous conversations/fiber.
• If a cable contains 200 fibers, each using 40
  colors, the amount of data that can be carried is

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Fiberoptic Communication Systems

• How do you get different colors?
• Use laser diodes
• Each laser operates over a very narrow color
  range (0.4x10-9 m or 0.4 nano meters).

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Fiberoptic Communication Systems

• Multichannel systems (WDM)

Dl
Each band has a spectral width of
and
l
centered around
, n 1,2,..N.
n
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Fiberoptic Communication Systems

• Concept of WDM

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Fiberoptic Communication Systems

• Optical Amplifiers SRS

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Fiberoptic Communication Systems

• Stimulated Raman Amplifier
• EDFA

amplifier region
strong data
weak data
signal out
signal in
Fiber coupler
strong pump
signal in
weak
pump
signal out
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Fiberoptic Communication Systems

• Long Haul Systems based on SRS

Chain of amplifiers in a long haul fiberoptic
system is shown. The pump wavelength is shorter
than the wavelength of the information bearing
signal.
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Fiberoptic Communication Systems

• Eliminate dispersion? Solitons

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Fiberoptic Communication Systems

• If the pulse broadening introduced by the
  dispersive behavior of the fiber can be
  compensated by another phenomenon which can
  produce pulse compression, the pulses can travel
  without broadening, thus eliminating any
  intersymbol interference (ISI).
• Solitons or solitary pulses can travel through
  the fibers without undergoing any pulse
  broadening.

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Fiberoptic Communication Systems

• Fiberoptic communication systems based on
  solitons use SRS or other amplifiers to keep the
  signal energy to the minimum value required to
  produce the nonlinearities needed.
• Use of solitons and SRS mitigates the twin
  problems of attenuation and dispersion.

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Fiberoptic Communication Systems

• In addition to point-to-point communications,
  fibers can also be used in Local Area Networks
  (LAN), Metropolitan Area Networks (MAN) and Wide
  Area Networks (WAN).
• Fiber based networks provide improved security,
  smaller size, large bandwidth, lower weight,
  bidirectional capability on a single fiber etc.

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Fiberoptic Communication Systems

• STAR couplercombines optical signals entering
  its multiple ports and divide them equally among
  its output ports

tunable receivers
l1
l2
l3
l4
input
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Fiberoptic Communication Systems

• Typical star coupler

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Fiberoptic Communication Systems

• Wavelength router

Input
Output
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Fiberoptic Communication Systems

• Optical Routers provide the backbone of current
  and next generation internet
• Dense WDM concepts are used in these routers

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Fiberoptic Communication Systems

• Overview of the physics of optical fibers
• Properties
• Characteristics of FO communication systems
• Fiberoptic Components