FiberOptic Systems

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Fiber-Optic Systems
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Basic System

• Transmitter, receiver, fiber with splices and/or
  connectors.
  
• No repeaters.
• Maximum distance may be limited by loss or
  dispersion --- check both.

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Loss Budget

• Express transmitter power in dBm, losses in dB.
• Received power (dBm)
• transmitter power (dBm) - losses (dB)
• System should have a margin of about 5-10 dB to
  allow for extra splices, equipment aging, etc.

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Rise-time Budget

• Transmitter, receiver and fiber all contribute to
  pulse spreading (splices and connectors have
  negligible effect).
  
• Dispersion of a fiber can be expressed in terms
  of the rise time of a pulse.
  
• Rise time of transmitter and receiver is given
  directly in specifications.
  

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Pulse-Spreading in Fiber

• Rise time for fiber is approximately equal to
  dispersion.
  

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System Rise Time

• Transmitter, fiber and receiver make contributions

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Maximum Data Rate

• Pulse spreading reduces maximum data rate.
• Pulses interfere with each other when they are
  close enough together that they start to overlap
  after spreading.
  
• This is called inter-symbol interference.
• Maximum bit rate depends on code used and the
  amount of inter-symbol interference that can be
  tolerated.

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Approximate Maximum Bit Rates

• RZ Code

• NRZ Code

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Analog Transmission

• Electrical bandwidth is smaller than optical
  bandwidth because optical power in fiber is
  proportional to electrical current (not power) at
  receiver.

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For Multi-mode Fiber
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Repeaters and Optical Amplifiers

• When transmission distance is limited by
  dispersion, regenerative repeaters are required.
  
• When transmission distance is limited by losses,
  an optical amplifier can be used
  
• Optical amplifiers, like their electronic
  equivalent, are analog devices.

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Regenerative Repeater

• Signal must be converted to electrical form.
• Electrical signal is decoded, recoded.
• Signal is then converted back to optical for
  transmission.
  
• Repeaters can extend the range of an optical
  system indefinitely.

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Optical Amplifiers

• Most common type is erbium-doped fiber amplifier
  (EDFA).
  
• Pump laser used to add power to the optical
  signal in the fiber.
  
• No electrical parts (except laser power supply).
• No need to convert between optical and electrical
  signals.

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Commercial EDFA
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Properties of Erbium

• When erbium is excited by photons at 800 nm
  or 980 nm, it has a non-radiative decay (energy
  drops without producing light) to a state where
  it can stay excited for relatively long periods
  of time - on the order of 10ms.
• When a photon at about 1550 nm interacts with
  an atom with an electron in the excited state,
  that electron returns to the valence band,
  emitting a photon of the same wavelength.
• Erbium can also be excited by photons at 1480nm,
  but this is typically undesirable as it is too
  close to the signal wavelength.
  

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Erbium Energy States
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Non-radiating Transitions
Pump Laser Provides Energy
Radiating Transition triggered by photon
Not used
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EDFA Amplification

• Photons emitted due to stimulation by signal
  cause other atoms to emit more photons.
  
• Result is high gain (up to 40 dB) and power
  output (up to 20dBm).
  
• Most EDFA amplifiers work in the range around
  1550 nm.

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Typical EDFA Specifications
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Wavelength-Division Multiplexing (WDM)

• Use of two or more optical sources with different
  wavelengths.
  
• Corresponding number of receivers is needed.
• Latest trend is to dense wavelength-division
  multiplexing (DWDM) with many different
  wavelengths.

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DWDM

• Current state of the art is 80 wavelengths on one
  fiber in 1550 nm range (36-40 is more common).
  
• ISO has a standard for 100 GHz (approx. 1 nm)
  spacing. New standards 50, 25 GHz.
  
• All wavelengths can be amplified by one EDFA but
  available output power is divided by number of
  wavelengths.

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DWDM Applications

• Optical switches can be used to route optical
  signals from fiber to fiber by microscopic
  mirrors, without any electrical switching.
  
• DWDM may make terabit electrical switches
  unnecessary.
  

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Submarine Cables

• Short distances covered without repeaters or
  amplifiers where possible for easier
  maintenance.
  
• Long runs formerly used repeaters, systems now
  being built use DWDM and EDFAs with no
  repeaters.
  
• Fiber with almost no dispersion is required and
  is now available.

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First Optical Transatlantic Cable

• TAT-8 (1988) (TATs 1-7 were copper).
• 109 repeaters at 70 km intervals.
• 295.6 Mb/s (allows 40,000 phone calls).
• Wavelength of 1300 nm.

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Second Generation Transatlantic Fiber

• TAT-10 (1992).
• Repeater spacing approx. 100 km.
• Data rate doubled from TAT-8.
• Wavelength 1550 nm.

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Third Generation Transatlantic Fiber

• TAT12/13 has no repeaters.
• Two cables in a ring configuration with one
  service pair and one restoration (spare) pair of
  fibers on each.
  
• Allows continuation of service even if one cable
  is lost.
  
• 5 Gb/s in each direction.
• Uses 1550 nm wavelength.
• EDFAs at 45 km spacing with 10 dB gain each.

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TAT12/13 Transatlantic Fiber-Optic Cable
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Atlantic Ocean
 
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Europe-Indian Ocean-Asia
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Asia-Pacific
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Syncronous Optical Network (SONET)

• Method of multiplexing digital signals into high
  bit-rate streams using TDM.
  
• Replaces T-1 etc. in North America and E-1 etc.
  in Europe for high-speed networks.
  
• Compatible with European Synchronous Digital
  Hierarchy (SDH).
  
• Works at multiples of 51.840 Mb/s.

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Sonet Terminology

• Synchronous Transport Signal (STS-1 etc)
• equivalent to DS-1 etc (but higher bit rate).
• Optical Carrier (OC-1 etc.).
• signal on a fiber, equivalent to T-1, etc.
• Synchronous Transport Module (STM-1 etc).
• line rates actually used in synchronous networks.

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SONET Frame

• 8000 frames per second as in conventional digital
  telephony.
  
• Frame has duration of 125 microseconds.
• Each frame of STS-1has 6480 bits (810 bytes).

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Sonet Frame Structure

• Overhead alternates with data.
• More overhead than DS-signals allows more
  addressing, etc.
  
• Think of frame as a table, starting in upper left
  corner and proceeding left to right and top to
  bottom.
  
• Higher level frames are similar but with more
  columns.

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SONET STS-1 Frame
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SONET Overhead

• 27 bytes of an STS-1 frame are transport
  overhead.
  
• Used for info on routing, composition of the
  signal, billing, etc.
  
• Also used for synchronizing.
• 9 bytes of an STS-1 frame are path overhead (info
  on fiber routing etc.)

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Synchronization

• Digital signals moving over long, but different,
  distances, will not be synchronized with each
  other at far end, even if they begin
  synchronized.
• This causes difficulties in switching.
• Conventional way to fix this is to add bits to
  the earlier signal to delay it (bit stuffing).

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SONET Synchronization

• SONET uses a pointer, included in the transport
  overhead, to point to the start of a frame.
  
• This allows the signals to be aligned for
  switching without adding any extra bits.
  
• If signals are at slightly different bit rates or
  the path changes, the pointer can be moved to
  adjust this.

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Fiber in LANs

• Two main fiber LAN systems
• Fiber Distributed Data Interface (FDDI).
• Ethernet over Fiber.

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LAN Topology

• 3 Basic Types
• Star
• Ring
• Bus
• Bus is difficult with fiber because of the
  difficulty of splitting the optical signals
  building a fiber tee is tricky.
  
• FDDI uses ring, Ethernet on fiber uses physical
  star (logical bus)

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FDDI

• Token-ring architecture.
• Multimode fiber at 1300 nm.
• Each node acts as a regenerative repeater.
• 2 fibers for redundancy.
• 100 Mb/s.
• Up to 2 km between nodes, 200 km total network
  length.
  

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Ethernet on Fiber

• 10 Mb/s and 100 Mb/s use LEDs or lasers.
• Multimode glass fiber, 62.5/125 micron.
• Wavelength 1300 nm.
• Logical bus but physical star using a hub similar
  to Ethernet on twisted pair wiring.
  
• 2 Fibers used between terminal and hub, one for
  each direction.
  

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Gigabit Ethernet on Fiber

• For short distances (500 m), laser diode at 850
  nm used with multimode fiber.
  
• For longer distances (5 km), laser diode at 1300
  nm used with single-mode fiber.
  

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Local Telephony

• Fiber is almost always used in new long distance
  and interoffice trunks.
  
• Trunks use single mode fiber with laser diode.
• Older systems 1300 nm.
• Some newer systems 1550 nm.
• For short-distance trunks, attenuators may be
  needed.
  

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Fiber in the Loop (FITL)

• Fiber runs right to individual residence.
• Still too expensive for wide use.
• In some experimental setups in new communities.
• Advantage one fiber pair has plenty of bandwidth
  for all conceivable applications to the home
  
• telephone, CATV, internet, interactive video, etc.

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FITL continued

• Disadvantages
• Cost rewiring areas that already have copper is
  expensive.
  
• Lack of ability to provide power for in-home
  equipment.
  
• Most areas are already served by CATV, and ADSL
  is reasonably priced, provides fairly high speed
  internet, and works on twisted pair wiring.

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Fiber to the Curb

• Run fiber to a neighbourhood concentrator.
• Copper from concentrator to individual homes.
• One fiber pair can handle hundreds of houses.
• Does not have to be able to handle all customers
  simultaneously.
  
• One fiber pair replaces hundreds of copper pairs
  to the C.O.

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FTTC
Twisted pairs to individual customers
Central Office
Concen-trator
Fiber pair
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CATV Applications

• Most common is to use fiber for analog trunks.
• Laser, single mode fiber, analog modulation.
• One fiber can easily carry all CATV signals.

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Fiber used for trunks
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Experimental Techniques

• Many experimental techniques being developed.

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Solitons

• Short high-level pulses at slightly more than the
  zero dispersion wavelength can travel long
  distances with virtually no dispersion.
  
• Higher frequency components at pulse edges
  propagate faster than flat top.
  
• Large amplitude required so EDFAs needed at
  fairly close intervals (25 to 50 km).
  
• Solitons are being implemented.

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Heterodyne Techniques

• Idea use a laser as a local oscillator in an
  optical receiver.
  
• Mix two optical signals to get a microwave
  signal.
  
• Use a conventional microwave receiver which could
  have much better performance than a PIN diode or
  APD.

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Heterodyne Techniques

• Problem L.O. laser must be very stable and
  narrowband.
  
• Current laser diodes are not good enough.

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Fiber Test Equipment

• Most loss measurements use calibrated source and
  optical power meter.
  
• Both source and power meter must be calibrated
  for the wavelength used.
  
• Continuity testing can be done with a specialized
  flashlight with a lens to couple to the fiber.

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Optical Time-Domain Reflectometry

• Differs from conventional copper TDR.
• Send pulse down fiber.
• Fiber scatters light back into fiber
  (backscatter) throughout its length.
  
• The rate of decrease in scattered power with time
  gives fiber loss.
  

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OTDR continued

• Splices show up as a sudden drop of power which
  allows loss of splice to be measured.
  
• Connectors show up as a glitch due to the
  reflection of light at a connector (usually due
  to an air gap).
  

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OTDR
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OTDR