Optical Components for WDM Light-wave Networks

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Optical Components for WDM Light-wave Networks

• Borella et al,Proceedings of the IEEE,August,
  1997.

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Contents

• I. Introduction
• II. Optical Fiber
• III. Optical Transmitters
• IV. Optical Receivers and Filters
• V. Optical Amplifiers
• VI. Switching Elements
• VII. Wavelength Conversion
• VIII. Designing WDM Networks Systems Considerati
  ons
• IX. Experimental WDM Lightwave Networks
• X. Conclusion

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I. Introduction

• The network medium may be
• a simple fiber link,
• a passive star coupler (PSC) (for boradcast and
  select network)
• a network of optical or electronic switch and
  fiber links

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• The transmitter block
• consists of one or more optical transmitters
• The transmitter
• fixed to a single wavelengthor tunable across a
  range of wavelengths
• consists of a laser, a laser modulator, or an
  optical filter for tuning purpose.
• For multiple optical transmitters, a multiplexer
  or coupler is needed to combine the signals onto
  a single fiber.

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• The receiver block may consist of
• tunable filter photodetector
• demultiplexer photodetector array
• Amplifiers are needed in various locations to
  maintain the strength of optical signals.

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• Transmitter block
• fixed wavelength multiplexer
• tunable across a range of wavelengths

• Receiver block
• demultiplexer photodetector array
• tunable filter photodiode

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II. Optical Fiber

• Rayleigh scattering
• Peak loss in 1400 nm due to OH- impurities in the
  fiber

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Advantages

• low attenuationlong distance (X00 KM)low bit
  error rate (BER10-11)
• small size, flexible,
• difficult to break, reliable in corrosive
  environments, deployable at short notice,
• immune to electomagnetic intererence and does not
  cause interference,
• made from sand (cheap, readily available and
  environmentally sound)

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A. Optical Transmission in Fiber

• Fiber is a thin filament of glass that acts a a
  waveguide.
• A waveguide is a physical medium or path that
  allows the propagation of electromagnetic waves,
  such as light.
• Two types of fiber multimode and single mode

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• Total internal reflection (??????)? little loss

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• Speed of Light
• in vacuum cvac 3 x 108 m/sin other
  material cmat ? cvac
• Refraction Index (n) ??? nmat cvac / cmat ? 1
• Snells Law
• na sin?a nb sin?b
• nb / na sin?a / sin?b
• when ?a ?, then ?b ?until ?b 90o, then sin?b
  1 sin?a nb / na ? 1 (i.e. nb ? na)
• Critical incident angle ?crit ? sin-1(nb /
  na)then No Refraction, i.e. Total Internal
  Reflection

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Numeric Aperture (N.A.)

• nair sin?air ncore sin(90 - ?crit) ncore (1 -
  sin2 ?crit )1/2
• sin ?crit nclad / ncore ? nair sin?air
  (n2core - n2clad)1/2

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• Two types of core-cladding implementations
• step index
• graded index

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B. Multi-mode versus Single-mode

• Normalized Frequency V k0 a (n2core -
  n2clad)1/2 where k0 2? / ? a radius of the
  core ? wavelength in vacuum
• The number of modes m ? 0.5 V2

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Multi-mode

• Advantages
• core diameter is relatively large
• injection of light into the fiber with low
  coupling loss can be accomplished by using
  inexpensive, large-area light source, such as
  LEDs.
• Disadvantages
• each mode
• due to different incident angles
• ? propagates at different velocity and arrives
  at the destination at different times
• ? a pulse spread out in the time domain
• ?intermodel dispersion ? distance of propagation
• reduced through the use of graded-index fiber

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Single-mode

• To reduce the intermodel dispersion
• to reduce the number of modes
• to reduce the core diameter
• to reduce the numerical aperture
• to increase the wavelength of the light
• ?single-mode fiber
• Advantages
• eliminating intermodel dispersion
• transmission over longer distance
• Disadvantages
• high concentration of light energy is needed,
  such as laser

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C. Attenuation in Fiber

• P(L) 10-AL/10 P(0)
• P(L) the power of the optical pulse at
  distance L km from the transmitterA the
  attenuation constant (in dB/km)P(0) optical
  power at the transmitter
• Lmax (10/A) log10 (P(0) / Pr)
• Lmax the maximum distance between the
  transmitter and the receiver
• Pr the receiver sensitivity

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D. Dispersion in Fiber

• The widening of a pulse duration as it travels
  through a fiber ?to interfere with neighboring
  pulses (bits)
• Three types of dispersions
• Intermodel Dispersion
• multiple modes propagates with differnet
  velocites
• Material or Chromatic Dispersion
• refraction index ? wavelength
• Waveguide Dispersion
• propagation of different wavelength ? waveguide
  characteristics

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E. Nonlinearities in Fiber

• 1) Nonlinear Refraction
• Index of refraction ? optical intensity of
  signals propagating through the fiber?The phase
  of the light at the receiver ?
• the phase of the light sent by the transmitter,
• the length of the fiber, and
• the optical intensity.
• Two types of Nonlinear Refraction
• Self-Phase Modulation (SPM)Cross-Phase
  Modulation (XPM)

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• Self-Phase Modulation (SPM)
• caused by variations in the power of an optical
  signal
• results in variations in the phase of the optical
  signal
• The amount of phase shift ?NL n2k0LE2where
  n2 nonlinear coefficient for the index of
  refraction k0 2?/? L length of the
  fiber E2 optical intensity

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• Cross-Phase Modulation (XPM)
• caused by the change in intensity of an optical
  signal propagating at a different wavelength
• results in a shift in the phase of the optical
  signal
• Advantage to modulate a pump signal at one
  wavelength from a modulated signal on a different
  wavelength. Such technique is used in Wavelength
  Conversion devices.

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• 2) Stimulated Raman Scattering (SRS)
• Light incident with molecules creates scattered
  light at a longer wavelength than that of the
  incident light.
• A portion of the light traveling at each
  frequency in a Raman-active fiber is downshifted
  across a region of lower frequencies the Stokes
  wave.
• The range of the frequencies occupied by the
  Stokes wave is determined by the Raman gain
  spectrum, which covers the range of around 40THz
  below the freq. of the input light. In silica
  fiber, max. gain at 13THz below input light.

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• 3) Stimulated Brillouin Scattering (SBS)
• similar to SRS except that the frequency shift is
  caused by sound waves rather than moleculer
  vibration.
• Stokes wave propagates in the opposite direction
  of the input light,SBS occurs at relatively low
  input powers for wide pulses (greater than 1?s)
  but has negligible effect for short pulses (less
  than 1?s)

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• 4) Four-Wave Mixing (FWM)
• Two wavelengths, operated at frequencies f1 and
  f2,mix to cause signals at 2f1-f2 and 2f2-f1
  sidebands
• Sidebands can cause interference if they overlap
  with frequencies used for data transmission
• can be reduced by using unequally spaced channels

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F. Couplers

• All devices that combine light into or split out
  of a fiber.
• SplitterCombinerCoupler
• Splitting Ratio ?
• the amount of power that goes to each output
• Ex. 5050 for a 1 x 2 splitter
• Return Loss (reflected in the opposite
  direction) 40-50 dBInsertion Loss (when
  directing the light into the fiber)

2 x 1 combiner
1 x 2 splitter
2 x 2 coupler
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PSC (Passive Star Coupler)

• Light coming from any input port is broadcasted
  to every output port.
• Pout Pin / N (excess loss is ignored)
• Example
• Using combiners, splitters, and couplers

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III. Optical Transmitters

• A. How a Laser Works
• Laser Light Amplication by Stimulated Emission
  of Radiation
• Stable atom whose electrons are in the lowest
  possible energy levels (ground state)
• When an atom absorts energy, it becomes excited
  and moves to a higher states (unstable atom). It
  moves quickly back to the ground state by
  releasing a photon.
• Quasi(?)-stable electrons staying in the excited
  states for a longer periods to time without
  constant excitation.By applying enough energy,
  population inversion occurs, i.e. more electrons
  are in the excited state than in the ground
  state. It emit more light than it sbsorts.

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• The general structure of a laser
• (1) The excitation device applies current to the
  lasing medium, which is made up of a quasi-stable
  substance.
• (2) The lasing medium is excited and emits a
  photon.
• (3) The photon reflects off the mirrors and
  passes back through the medium again.
• (4) When a photon passes very close to an excited
  electron, the electron releases its energy and
  return to the ground state and releases another
  photon, which will have the same direction and
  coherency (frequency) (stimulating photon)

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• (5) Photons for which the frequency is an
  integral fraction of the cavity length will
  coherently combine to build up light at the given
  frequency in the cavity until the energy is
  removed as rapidly as it is being applied.
• (6) The mirrors feed the photons back and forth,
  so further stimulated emission can occur and
  higher intensity of light is produced.
• (7) Some of the photons will escape through the
  partially transmitting mirror in the form of
  narrowly focused beam of light. The frequency can
  be adjusted by changing the length of the cavity.
• f (Ei - Ef) / h where Ei initial
  (quasi-stable) state of the electron Ef
  final (ground) state of the electron h
  Plancks constant Ei - Ef Boltzmann
  distribution (temperature)

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• Semiconductor Diode Laser
• 1) Bulk Laser Diode
• a p-n junction with mirrored edges
• Quasi-stable over-doped impurities to provide
  extra electrons in an n-type semiconductor
  andextra holes in a p-type semiconductor.

• Forward bias the p-n junction ?combine
  electrons in n with holes in p? emitting a
  beam of light (frequency ?)

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• 2) Multiple Quantum Well (MQW) Laser
• Thin alternating layers?confining the position
  of the electrons and holes to a smaller number of
  energy states
• The quantum wells are placed in the region of p-n
  junction
• By confining the possible states of electrons and
  holes?higher resolutionlow-linewidth (very
  narrow frequency range)

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• B. Tunable and Fixed Lasers
• Laser Characteristics
• Laser Linewidth the spectral width of the light
  gerneated by the laser
• affects the spacing of channels
• affects the amount of dispersion, thus, the
  maximum bit rate
• Frequency Instability variations in the laser
  frequency
• Mode Hopping a sudden jump in the laser
  frequency cuased by a change in the injection
  current above the a given threshold
• Mode Shift changes in frequency due to
  temperature changes
• Wavelength Chirp variations in the frequency
  due to variations in injection current

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• Number of Longitudinal Modes the number of
  wavelengths that are amplified by the laser
• wavelength ? 2L/n will be amplified,where L is
  the length of the cavity n is an integer
• the unwanted logitudianl modes may results in
  significant dispersion thus, it is desirable to
  have only a single logitudinal mode
• For tunable lasers
• Tuning Range ? the range of wavelengths over
  which the laser may be operated
• Tuning Time ? the time required for the laser
  to tune from one wavelength to another
• Continuously tunableDiscretely tunable

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• Tunable Lasers
• 1) Mechanically Tuned Lasers
• Fabry-Perot cavity that is adjacent to the lasing
  medium (external cavity) to filter out unwanted
  wavelengths
• physically adjusting the distance between two
  mirrors
• 2) Acousto-optically Tuned Lasers
• the index of refraction in the external cavity is
  changed by using sound waves
• 3) Electro-optically Tuned Lasers
• the index of refraction in the external cavity is
  changed by using electrical current

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• 4) Injection-Current Tuned Lasers
• a diffraction grating (?) inside or outside of
  the lasing medium, which consists of a waveguide
  in which the index of refraction alternates
  periodiclly between two values
• only wavelengths that match the period and
  indexes of the grating will be amplified
• tuned by injecting a currnet that changes the
  index of the grating
• DFB (Distributed Feedback) Laser grating is
  placed in the lasing medium, DBR(Distributed
  Bragg Reflector) Laser grating is moved to the
  outside of the lasing medium
• 5) Laser Arrays
• a set of fixed-tuned lasers, each with a
  different wavelength

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• C. Optical Modulation
• Analog AM, FM, PMDigital ASK, FSK, PSK
• Binary ASK (on-off keying, OOK)
• Simple ? Preferred
• Direct Modulation
• turning the laser on and off,
• External Modulation
• modulates the light coming out of the laser
• Mach-Zehnder (MZ) Interferometer
• a drive voltage is applied to one of the two
  waveguides,creating an electric field that
  cuases the signals in the two waveguides to be
  either in phase or 180o out of phases, resulting
  in the light being either passed or blocked.
• Up to 18 GHz
• Intergrated laser and modulation ? cost effective

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IV. Optical Receivers and Filters

• A. Photodetectors
• Direct detection
• converts a stream of light into a stream of
  electrons, the stream of electrons is then
  amplified and passed to a threshold device to
  determine a stream of digital 0s or 1s
• PN photodiode (a p-n junction)PIN photodiode (an
  intrinsic material between p- and n-type )
• light incident on the junction will create
  electron-hole pairs in both n and p regions,
  resulting a current flow.
• Coherent detection
• Phase information is used in the encoding and
  decoding
• the incident light is added to the local
  oscillator (a monochromatic laser), then is
  detected by a photodetector.
• more elaborate, difficult to maintain phase
  information

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• B. Tunable Optical Filters
• Filter Characteristics
• Tuning range ?
• Tuning time ?
• Free Spectral Range (FSR)
• the transfer function, or the shape of the filter
  passband, repeats itself after a certain period
• Finesse
• the ratio of the FSR to (3-dB)channel bandwidth
  (?f)
• Finesse? ? ?f ? ? number of channels ?

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• To increase the number of channels Cascading
  filters with different FSRs
• ExampleCascading a high-resolution filter with
  a low-resolution filter, each with four channels
  within FSR,results in 16 unique channes.

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• The Etalon
• consists of a single cavity formed by two
  parallel mirrors
• light from an input fiber enters the cavity and
  reflects a number of times between the mirrors
• by adjusting the distance of the mirrors
  (mechanically), a single wavelength can be chosen
  to propagate through the cavity
• Modifications
• Multipass light passes the same cavity multiple
  times
• Multicavity multiple etalons with different
  FSRs are cascaded to increase finesse
  effectively
• Fabry-Perot filterSingle-cavity Fabry-Perot
  Etlon max. number of channels 0.65F ( F the
  finesse) Two-pass Fabry-Perot Etlon max.
  number of channels 1.4FTwo-cavity Fabry-Perot
  Etlon max. number of channels 0.44F
• Tuning range virtually entire low-attenuation
  rangeTuning time order of milliseconds

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• MZ Chain
• A splitter a combiner a delay
• The adjustable delay controls one of the the path
  length,resulting in a phase difference when
  combined.
• Wavelengths with 180o phase difference are
  filtered out.
• By constructing a chain of these elements,a
  single desired optical wavelength can be selected.

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• Acousto-optic Filters -not easy to filter out
  cross talk
• RF waves are passed through a tranducer.
• The sound waves change the transducers
  refraction index.
• Light incident upon the transducer will refract
  at an different angle.
• Electro-optic Filters
• Current changes the crystals refraction index
• Liquid-Crystal Fabry-Perot Filters low power
  inexpensive
• The cavity consists of an LC.
• The refractive index of the LC is changed by an
  current

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• C. Fixed Filters may be used to implement
  optical multiplexers and demultiplexers or
  receiver devices
• Grating Filters
• a flat layer of transparent material (glass or
  plastic) with a row of parallel grooves (??)
• reflecting light at all angles
• at an angle, only a certain wavelength adds
  constructively
• place a filter at the proper angle to select a
  certain wavelength
• Fiber Bragg Gratings - low insertion loss, varied
  with temp.
• inducing a grating directly into the core of a
  fiber
• Thin-Film Interference Filters - poor thermal
  stability, high insertion loss, poor spectral
  frofile
• similar to fiber Bragg grating, except that the
  low index and high index materials onto a
  substrate layer?

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V. Optical Amplifiers

• All-optical amplificationOpto-electronic
  amplification
• 1R (regeneration) amplification total data
  transparency (independent of modulation
  format) - noise is amplified as well . Used by
  all-optical networks of tomorrow 2R
  (regeneration, reshaping) amplification
  reproduce the original pulse shape of each bit3R
  (regeneration, reshaping, reclocking)
  amplification applied only to digitally
  modulated signals . Used by SONET, SDH

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• A. Optical Amplifier Characteristics
• Gain (output power / input power)
• Gain Bandwidth
• Gain Saturation (3-dB gain)
• Polarization Sensitivity
• Amplifier Noise
• dominant source is ASE (amplified spontaneous
  emission), which arises from the spontaneous
  emission of photons in the active region of the
  amplifier.

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• EDFA Erbium(?)-Doped Fiber Amplifier
• PEFFA Praseodymium(?)-Doped Fluoride(???) Fiber
  Amplifier

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• B. Semiconductor-Laser Amplifier integratable
• A modified semiconductor laser.A weak signal is
  sent through the active region, which, via
  stimulated esission, results in a stronger
  signal.
• Fabry-Perot Amplifier
• reflectivity 30
• Traveling -Wave Amplifier (TWA)
• reflectivity 0.01

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• C. Doped-Fiber Amplifier
• Length of fiber is doped with an element (rare
  earth, ??) that can amplify light
• EDFA Erbium(?)-Doped Fiber Amplifier
• PEFFA Praseodymium(?)-Doped Fluoride(???) Fiber
  Amplifier

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• EDFA
• the 3-dB gain bandwidth 35 nm
• the gain saturation power 10dB

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VI. Switching Elements

• Electronic Switches - data switching with
  electro-optic conversion - electronic control of
  switching - flexibility, slow, extra
  delay.Optical Switches - data switching without
  electro-optic conversion - electronic control of
  switching - transparent switching

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• Two classes of switchesRelational Switches -
  relation between inputs and ouputs is a
  function of control signals applied to the
  device and is independent of the contents of
  the signal and data inputs (data
  transparency). - loss of flexibility - used for
  circuit switchingLogic Switches - the data
  signals control the state of the device - used
  for packet switching

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• A. Fiber Crossconnect Elements
• Wavelength insensitive(incapable of
  demultiplexing different wavelength signals on a
  given input fiber)
• Example 2 ? 2 cross-connect element
• cross state
• bar state

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• Two types of crossconnect technologies(How to
  connect the input to the output?)
• 1) Directive Switch
• physically directed
• 2) Gate Switch
• using amplifier gates

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• 1) Directive Switches
• (a) Directive Switches
• (b) Reversed Data-Beta Coupler
• (c) Balanced Bridge Interferometric Switch
• (d) Intersecting Waveguide Switch

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• 2) Gate Switches
• N ? N gate switch
• 1 ? N splitters N2 optical amplifiers N ? 1
  couplers
• controlling optical amplifier on or off to pass
  only selected signals to the outputs
• Example 2 ? 2 amplifier gate switch (Fig. 20)
• Semiconductor optical amplifer 8 ? 8
  switch 1300-nm, low polarization dependence (1
  dB) fairly low cross talk ( lt - 40 dB), bulky,
  expensive Integrated optical amplifer 4 ? 4
  switch 1550-nm, high polarization dependence
  (6-12 dB) fairly low cross talk ( lt - 40 dB),

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• Wavelength Routing Devices
• Routing is based on the wavelengths of the
  signals
• demultiplexing switching each wavelenght
  (optional) multiplexing
• Two types of wavelength routing
• Non-reconfigurable no switching stage between
  MUX and DEMUX
• Reconfigurable switching stage in between,
  controlled electronically

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• B. Non-reconfigurable Wavelength Router
• a stage of multiplexers a tage of
  demultiplexers
• hardwired
• Example 4 ? 4 (Fig. 21)
• Fixed routing matrix

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• WGR (Waveguide Grating Router)AWG (Arrayed
  Waveguide Grating)
• Max. of N2 connections v.s. N for passve-star
  coupler
• ? Integrated device ? low cost
• ? fixed routing

• Two passive-star coupler
• N ? N N ?N, (N ltltN)
• Seperated by angles ? ?
• One grating array
• N waveguides
• Lengthsl1 lt l2 lt lt lN
• Different phase shift ? ?Input transmitted
  to output with ? 2n?

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• C. Reconfigurable Wavelength-Routing Switch
  (WRS) Wavelength-Selective Cross Connect
  (WSXC)
• P ? P Reconfigurable WRS
• Photonic Switches inside,built from 2 ? 2
  optical crosspoint elements (tunable/reconfigarabl
  e) arranged in a banyan-based fabric

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• D. Photonic Packet Switches
• Composed of logic devices
• The switch configuration is a function of data on
  the input signal.
• Problem Resource Contention
• Multiple packets contend for a common resource in
  the switch.
• Contention is resolved through
• buffering for an electronic switch
• delay lines for an optical switch(a long length
  of fiber that introduces propagation delays that
  are on the order of packet transmission times)
• Examples
• Staggering Switch
• Contention Resolution by Delay Lines (CORD)
• HLAN Architecture

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• Staggering Switch
• almost-all-optical - fully optical data
  electronic control
• ? Transparent - the payload may be encoded in
  an arbitrary format or at an arbitrary data
  rate
• ? Lack of random-access optical memory ?

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• Contnesion Resolution by Delay Lines (CORD)
• A number of 2 ? 2 cross-connect elements and
  delay lines
• One packet may be switched to the delay line
  while the other packet is switched to the proper
  output.

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• HLAN Architecture
• A helical unidirectional bus is divided into
  three separated segments.
• A headend periodically generates equal-sized
  empty frame.

• Guaranteed-bandwidth traffic ? GBW
  segmentBandwidth-on-demand traffic ? BOD
  segmentData is received ? RCV segment
• 100 Gb/s or more

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VII. Wavelength Conversion

• Fig. 27
• two WDM cross connects (S1 and S2) and five
  access stations (A-E)three linepaths has been
  set up (C-A on ?1, C-B on ?2, and D-E on ?1 ?)
• Wavelength-continuity constraint(the same
  wavelength is allocated on all the links in the
  path)

65

• Fig. 28 (a)
• Wavelength-continuity
• Node 1 - Node 2 ?1 Node 2 - Node 3 ?2? Node
  1 - Node 3 ?
• Fig. 28 (b)
• Wavelength-conversion
• Node 1 - Node 2 ?1 Node 2 - Node 3 ?2? Node
  1 - Node 3 ?2 ? ?1
• Improve the efficiency

• Wavelength Converter
• ?s ? ?c

66

• A. Wavelength Conversion Technologies Two
  types - opto-electronic - all-optical -
  that employ coherent effects - that use cross
  modulation
• 1) Opto-electronic Wavelength Conversion
• up to 10 Gb/s
• more complex and more power consumption
• O/E affects data transparency (data format data
  rate)

67

• 2a) Wavelength Conversion Using Coherent Effects
• based on wave-mixing effects (Fig. 31)
• preserve data transparency (phase amplitude)
• the only approach that allows simultaneous
  conversion of a set of multiple input wavelengths
  to another set of multiple output wavelengths
• 100 Gb/s
• n 3 Four-Wave Mixing (FWM) 4th wave is
  generated fijk fi fj -fk n 2 Difference
  Frequency Generation (DFG)

68

• 2b) Wavelength Conversion Using Cross Modulation
• Using optical-gating wavelength conversion
  techniqueson active semiconductor optical
  devices such as a) SOA (semiconductor optical
  amplifier) in XGM (Cross-Gain Modulation)
  mode XPM (Cross-Phase Modulation) mode b)
  Semiconductor Laser

69

• SOA in XGM (Cross-Gain Modulation) mode
• ? simple, 10 Gb/s ? invertion

• SOA in XPM (Cross-Phase Modulation) mode
• ? power efficient

70

• Semiconductor Laser
• inverted output
• 10 Gb/s
• Bandwidth 1 GHz

71

• B. Wavelength Conversion in Switches
• ? not very cost effective since not all the
  WCs may be required all the time

72

• Fig. 36 (a)
• Share-Per-Node
• Converter Bank
• a collection of a few wavelength conveters
• accessed by any wavelength on any incoming fiber
  
• Fig. 36 (b)
• Share-Per-Link
• Converter Bank
• accessed by particular links

73

• Share-with-local switch
• Opto-electronic wavelength conversion is used
  either in
• the switch (Fig. 37)
• the network access station (Fig. 38)
• Share-with-local switch architecture (Fig. 37)
• O/E at RxE ? ESW ? E/O at TxE
• May be locally-added, or locally-dropped, or
  retransmitted on a different wavelength

74

• Share-with-local network access station
  architecture (Fig. 38)
• O/E E/O at the network access station

75
VIII. Designing WDM Networks Systems
Consideration

• Keep in mind
• Functionality of the networks
• Capabilities and limitations of the network
  componets
• Issues
• A. Channels
• B. Power Considerations
• C. Cross talk
• D. Additional Considerations
• E. Elements of Local-Area WDM Network Design
• F. WDM WAN Desig Issues

76

• A. Channels
• Number of wavelengths to use
• Total available bandwidth or spectral range of
  the components
• Fiber medium at 1300 and 1550 nm with 200 nm
  bandwidth each
• Amplifier 35-40 nm bandwidth
• Injection-current laser 10nm tuning range
• Fabry-Perot filter entire low-attenuation region
  tuning range
• electro-optic filter 16 nm tuning range
• Channel spacing
• Channel bit rates
• Optical power budget
• Non-linearities in the fiber
• Resolution of transmitters and receivers
• A higher number of channels ? more network
  capacity ? higher network costs and more
  complex protocols

77

• B. Power Considerations
• SNR
• Signal power degrades due to losses such as
• attenuation in the fiber
• splitter losses
• coupling losses
• optical amplifier losses
• Different characteristics for the three main
  applications for optical amplifiers
• Transmitter power booster immediately after the
  transmitter
• Receiver preamplifier before detection at a
  receiver photo-detector
• In-line amplifier used within the network

78

• C. Cross Talk
• Interband cross talk
• interference from signals on different
  wavelengths
• affects channel spacing
• can be removed by using appropriate narrow-band
  filers
• Intraband cross talk
• interference from signals on the same wavelengths
  on another fiber
• usually occur in switching nodes
• cannot easily be removed through filtering
• can accumulate over a number of nodes

79

• D. Additional Considerations
• Dispersion
• a pulse to broaden as it propagates along the
  fiber
• limits the spacing between bits the max.
  transmission rate the max. fiber distance for a
  given rate
• Architectural (topology) considerations
• choice of which transmitter-receiver pairs to
  operate on which wavelength
• fault tolerance and reliablity
• Standards

80

• E. Elements of Local-Area WDM Network Design
• Single-hop protocolMulti-hop protocol
• Network Medium
• PSC
• passive
• fairly reliable
• additional HW for routing
• no wavelength reuse
• Network Nodes
• Fixed Tx / Rx
• Fixed Tx / Rx

81

• F. WDM WAN Design Issues
• Electronic/optical networks v.s. all-optical
  networks

• Multiple light-paths to share each fiber links
• Wave length reuse

82
IX. Experimental WDM Light-wave Networks

• A. LAN Testbeds
• Bellcores LAMBDNET
• each nodeone fixed DBF laser transmitter N
  fixed receivers
• simple, multicast, not scalable, costly
• IBMs Rainbow
• 32 PS/2s 32 200-Mbps WDM channels (?1 Gb/s)
• each nodea fixed DFB laser transmitter a
  tunable Fabry-Perot filter receiver
• When IDLE, scan for SETUP request
• Not scale well

83

• B. WAN Testbeds
• MWTN (Multi-wavelength Transport Network)
• by RACE (Research and Development in Advanced
  Communications Technologies) in Europe
• MONET (Multi-wavelength Optical Networking)
• by ATT, Lucent,...
• ONTC
• Bellcore, Columbia University, Hughes Research
  Lab,...
• AON
• ATT, DEC, MIT Lincoln Lab,...

84

• MONET local exchange cross connect long
  distance

85

• ONTC

(2 ? 2)
(ATM Switch)
86

• AON

Nationwide Backbone
MAN
LAN
87
X. Conclusion

• As optical device technology continues to
  improve, network designers need to be ready to
  take advantage of new device capabilities while
  keeping in mind the limitations of such devices.