Wavelength Division Multiplexing WDM

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Wavelength Division Multiplexing WDM Powered By Docstoc
					Optical Components for WDM
   Light-wave Networks

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

• 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
• IX. Experimental WDM Lightwave Networks
• X. Conclusion

               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

• The transmitter block
  – consists of one or more optical transmitters
• The transmitter
  – fixed to a single wavelength
    or 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.

• 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.

• Transmitter block        • Receiver block
  – fixed wavelength +       – demultiplexer +
    multiplexer                photodetector array
  – tunable across a         – tunable filter +
    range of wavelengths       photodiode

        II. Optical Fiber
                            • Rayleigh
                            • Peak loss
                              in 1400 nm
                              due to OH-
0.5dB                         impurities
0.2dB                         in the fiber

– low attenuation
  long distance (X00 KM)
  low bit error rate (BER=10-11)
– 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)
   A. Optical Transmission in Fiber
core = 纖核
cladding = 纖覆

                              Multimode               Singlemode

 • Fiber is a thin filament of glass that acts a a
    – 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

• Total internal reflection (完全內部反射)
   little loss

– Speed of Light
  • in vacuum:         cvac = 3 x 108 m/s
    in other material: cmat  cvac
– Refraction Index (n) 折射率
        nmat = cvac / cmat  1
– Snell‘s Law                                         nb
  • na sina = nb sinb                                na
  • nb / na = sina / sinb                a
  • 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

    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

• Two types of core-cladding implementations
  – step index
  – graded index
 B. Multi-mode versus Single-mode
core = 纖核
cladding = 纖覆

 • 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
• 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 LED‘s.
• 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
• 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
           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 transmitter
    A       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

          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

       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)

– 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 = n2k0L|E|2
          n2 = nonlinear coefficient for the index of
          k0 = 2/
          L = length of the fiber
          |E|2 = optical intensity

– 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.

• 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.

• 3) Stimulated Brillouin Scattering (SBS)
  – similar to SRS except that the frequency shift is
    caused by sound waves rather than moleculer
  – 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)

• 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
                         F. Couplers
• All devices that combine light into or split out
  of a fiber.
• Splitter
• Splitting Ratio 
   – the amount of power that goes to each output
   – Ex. 50:50 for a 1 x 2 splitter
• Return Loss (reflected in the opposite direction): 40-50 dB
  Insertion Loss (when directing the light into the fiber):

    1 x 2 splitter               2 x 1 combiner      2 x 2 coupler
      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 :

         III. Optical Transmitters
• A. How a Laser Works
  – Laser = Light Amplication by Stimulated Emission of
  – 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(準)-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.
• The general structure of a laser                                   28

  – (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)
– (5) Photons for which the frequency is an integral fraction of the 29
  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
– (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 = Planck‘s constant
                              Ei - Ef = Boltzmann distribution (temperature)

• 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 and
           extra holes in a p-type semiconductor.
• Forward bias the
  p-n junction
  electrons in ‚n‛
  with holes in
  emitting a beam
  of light

– 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
  • By confining the possible states of electrons and holes
    higher resolution
    low-linewidth (very narrow frequency range)

• 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

• 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
• 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 tunable
     Discretely tunable
– 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
• 4) Injection-Current Tuned Lasers:
   – a diffraction grating (柵) 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

• C. Optical Modulation
  – Analog: AM, FM, PM
    Digital: 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
   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 0’s or 1’s
     • 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
• B. Tunable Optical Filters                                                    38

  – 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 
• To increase the number of channels:
          Cascading filters with different FSR‘s
• Example:
  Cascading a high-resolution filter with a low-resolution
  filter, each with four channels within FSR,
  results in 16 unique channes.
– The Etalon                                                             40

  • 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
  • Modifications
     – Multipass: light passes the same cavity multiple times
     – Multicavity: multiple etalons with different FSR‘s are cascaded
       to increase finesse effectively
     – Fabry-Perot filter
       Single-cavity Fabry-Perot Etlon : max. number of channels 0.65F
       ( F = the finesse)
       Two-pass Fabry-Perot Etlon : max. number of channels 1.4F
       Two-cavity Fabry-Perot Etlon : max. number of channels 0.44F
     – Tuning range : virtually entire low-attenuation range
       Tuning time : order of milliseconds

– 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.
– Acousto-optic Filters     -not easy to filter out cross talk

   • RF waves are passed through a tranducer.
   • The sound waves change the transducer‘s refraction index.
   • Light incident upon the transducer will refract at an different
– Electro-optic Filters
   • Current changes the crystal‘s 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

• 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 (凹槽)
     • 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?

           V. Optical Amplifiers
• All-optical amplification
  Opto-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 bit
  3R (regeneration, reshaping, reclocking*) amplification
     * applied only to digitally modulated signals
     . Used by SONET, SDH

• 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.

• EDFA = Erbium(鉺)-Doped Fiber Amplifier
• PEFFA = Praseodymium(鐠)-Doped
  Fluoride(氟化物) Fiber Amplifier

• 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 %

• C. Doped-Fiber Amplifier
  – Length of fiber is doped with an element (rare
    earth, 稀土) that can amplify light
     • EDFA = Erbium(鉺)-Doped Fiber Amplifier
     • PEFFA = Praseodymium(鐠)-Doped Fluoride(氟化物)
       Fiber Amplifier

                                   Pump wavelength
                                            980 nm (10 dB/nW gain)
                                            1480 nm (5 dB/nW gain)

 – the 3-dB gain bandwidth = 35 nm
 – the gain saturation power = 10dB

           10 dB

                     35 nm

        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

• Two classes of switches:

  Relational 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 switching
  Logic Switches
     - the data signals control the state of the device
     - used for packet switching

• 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

         Cross state                    Bar state

• Two types of crossconnect technologies:

  (How to connect the input to the output?)

  – 1) Directive Switch
     • physically directed
  – 2) Gate Switch
     • using amplifier gates
                                                   54

                   • 1) Directive Switches

                     – (a) Directive Switches
Electrode 電極   ×
                     – (b) Reversed Data-Beta Coupler

                    – (c) Balanced Bridge
                           Interferometric Switch

                     – (d) Intersecting Waveguide
                          Switch
• 2) Gate Switches                                                  55

  – 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 ( < - 40 dB), bulky, expensive
       Integrated optical amplifer: 4  4 switch
             1550-nm, high polarization dependence (6-12 dB)
              fairly low cross talk ( < - 40 dB),

• 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

• B. Non-reconfigurable           Multiplexers   Demultiplexers
     Wavelength Router           DCBA                   PKFA
   – a stage of multiplexers +
     a tage of demultiplexers
   – hardwired                   HGFE                   LOBE
   – Example: 4  4 (Fig. 21)
     • Fixed routing matrix
                                 LKJI                   HCNI

                                 PONM                   DGJM
– WGR (Waveguide Grating Router)
  AWG (Arrayed Waveguide Grating)
  • Max. of N2 connections v.s. N for passve-star coupler
  •  Integrated device  low cost
                                            • Two passive-star
  •  fixed routing                            coupler
                                                – N  N‘ & N’
                                                  N, (N <<N’)
                                                – Seperated by
                                                  angles  & ‘
                                            • One grating array
                                                – N‘ waveguides
                                                – Lengths
                                                  l1 < l2 < … < lN‘
                                            • Different phase
                                              shift  =…
                                              Input transmitted to
                                              output with  = 2n
• C. Reconfigurable                                                     59

    Wavelength-Routing Switch (WRS)
    Wavelength-Selective Cross Connect (WSXC)
  – P  P Reconfigurable WRS
     • Photonic Switches inside,
       built from 2  2 optical crosspoint elements
       arranged in a banyan-based fabric

                     P  Demultiplexers M  Switches P  Multiplexers

• 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
– Staggering Switch                                               61

   • ‚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 ?

 • Yet to be
 • suffer from
   cross talk and
– 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.
   – HLAN Architecture                                                  63

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

• Guaranteed-bandwidth traffic  GBW segment
  Bandwidth-on-demand traffic  BOD segment
  Data is received  RCV segment
• 100 Gb/s or more
    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)

                                                            1 ?


• Fig. 28 (a)
   – Wavelength-continuity
   – Node 1 - Node 2 : 1
     Node 2 - Node 3 : 2
      Node 1 - Node 3 ?
• Fig. 28 (b)                 Wavelength-continuity constraint

   – Wavelength-conversion
   – Node 1 - Node 2 : 1
     Node 2 - Node 3 : 2
      Node 1 - Node 3
               2  1              Wavelength conversion

   – Improve the efficiency

• Wavelength Converter
  – s  c

• 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)
                     O/E                     E/O

                   Photodetector      Tunable Laser
      – 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)

(pump wavelength)
         (Continuous Wave)
– 2b) Wavelength Conversion Using Cross Modulation
   • Using optical-gating wavelength conversion techniques
     on 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
– SOA in XGM (Cross-Gain Modulation) mode                  69

   •  simple, 10 Gb/s    invertion


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

                                       (inverted | noninverted)
– Semiconductor Laser
   • inverted output
   • 10 Gb/s
   • Bandwidth 1 GHz

• B. Wavelength Conversion in Switches
   –  not very cost effective
       since not all the WC‘s may be required all the time

 M inputs   N wavelengths        MN                   M outputs

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

– Share-with-local switch                                                 73

   • 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

                                 O/E           E/O

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


                                          


  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

• A. Channels
  – Number of wavelengths to use
     • Total available bandwidth or spectral range of the
        –   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

• 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

• 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

• 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

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

• F. WDM WAN Design Issues
   – Electronic/optical networks v.s. all-optical networks
 light-paths to
 share each
 fiber links
–Wave length
IX. Experimental WDM Light-wave
• A. LAN Testbeds
  – Bellcore‘s LAMBDNET
    • each node:
      one fixed DBF laser transmitter + N fixed receivers
    • simple, multicast, not scalable, costly
  – IBM‘s Rainbow
    • 32 PS/2‘s + 32 200-Mbps WDM channels (1 Gb/s)
    • each node:
      a fixed DFB laser transmitter + a tunable Fabry-Perot
      filter receiver
    • When IDLE, scan for SETUP request
    • Not scale well

• 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 AT&T, Lucent,...
  – ONTC
    • Bellcore, Columbia University, Hughes Research Lab,...
  – AON
    • AT&T, DEC, MIT Lincoln Lab,...
– MONET : local exchange + cross connect + long distance

         (2  2)

         (ATM Switch)

Nationwide Backbone   – AON



                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.