xWDM

  1. xWDM
    1. WDM
    2. Coarse WDM (CWDM)
    3. Dense DWM (DWDM)
  2. Optics
    1. Fiber
    2. Lasers
    3. Detectors
    4. Amplifier
      1. Erbium doped fiber amplifier (EDFA)
      2. Raman
    5. Wavelength
    6. Interference
  3. Multiplexing
    1. Prism diffraction
    2. Waveguide grating
    3. Arrayed waveguide grating
    4. Thin film filters

xWDM

  • FDM (usually in radio) in optics
  • transponder
    • colors the signal
    • sends wave to OADM (optical add-drop multimplexer)
  • static routing, based on wavelength
  • muxponder ≡ transponder + multiplexer
  • power equalization: channels need to be equalized by energy before amplifying, otherwise all amplification would be consumed be channel with highest energy
  • modes
    • transpaarent
      • transports grey signal, shifting it to carrier
      • not visible to client
    • native
      • protocol connection to client (e.g., Ethernet, FC), then transmission over WDM
      • protocol localization
  • compared to SDH/SONET
    • can transport generic payload
    • required less fiber and amplifiers, because wavelengths use same infrastructure
    • unconstrained by electronics: no need for optical-electrical-optical (OEO) conversion for amplifying
    • immune to crosstalk and interference: low error rate
    • less component weight

WDM

  • wideband WDM (2 channels): 1310 & 1550 nm or 850 & 1310 nm
  • narrowband WDM
    • 1550 nm
    • 2-8 channels: 400 GHz channel spacing
    • 16-40 channels: 100-200 GHz channel spacing
    • 64-160 channels: 25-50 GHz channel spacing
  • ROADM – reconfigurable OADM
  • optical service channel (OSC):
    • OOB communication between WDM nodes
      • data communication channel (DCC)
      • timing
      • channel with Ethernet payload
    • runs over same fiber as coloured signal, 1510nm (non-ITU)

Coarse WDM (CWDM)

  • ITU G.694.2
  • 16 channels, 20nm spacing
  • up to 60km
  • point-to-point, passive
  • EDFA does not work because of wide spacing: covers only 3 channels
  • cheap: less strict tolerance for lasers, temperature, optics

Dense DWM (DWDM)

  • ITU G.694.1
  • 40/80 channels, 0.4nm spacing ⇒ HW is more complex and expensive due to shallower channel spacing
  • supports EDFA, optical splitter, Y-cable
  • C-band: 0.1-0.8nm spacing
  • up to 5000 km
  • lasers have to be cooled – avoid wavelength drift

Optics

Fiber

  • low loss
  • components
    1. core:
      • 9 µm
      • conducts light
    2. cladding:
      • provides total internal reflection, Θ = arcsin(n₂/n₁)
      • 125 µm
    3. coating:
      • not optical, polymer
      • 250 µm
  • multimode:
    • several modes ≡ light paths → modal dispersion (disparity between light rays arrival time) ⇒ poor signal on Rx ≡ distance limit
    • type:
      • step-index: uniform refraction in fiber core ≡ step between core and cladding
      • graded-index: refractive index decreases from core outward; light in the center of the core travels slower than on the edge ≡ reduce modal dispersion
  • single mode
    • non-dispersion-shifted fiber (NDSF):
      • ITU-T G.652
      • TDM S-band, DWDM C-band (with dispersion compensator)
      • chromatic dispersion is near 1310 nm
    • dispersion shifted fiber (DSF):
      • ITU-T G.653
      • TDM C-band
      • zero-dispersion point at 1550 nm
      • not used in DWDM because of destructive non-linear effects
    • non-zero dispersion shifted fiber (NZ-DSF):
      • ITU-T G.655
      • TDM C-band, DWDM C+L bands
      • low (not zero) dispersion on 1550 nm: counter non-linear effects (four-wave mixing)
    • extended band:
      • G.652.C
      • TDM, TDM C-band, DWDM, CWDM

Lasers

  • stable and precise wavelength
  • types
    • monolithic Fabry-Perot
    • distributed feedback: preferred in DWDM
      • monochromatic light
      • high speed
      • high SNR
      • linear

Detectors

  • types
    • positive-intrinsic-negative photodiode (PIN): absorb photons, emit 1 electron for 1 photon
      • low cost
      • high reliability
    • avalanche photodiode (APD): 1 photon releases several electrons ≡ amplification
      • high sensitivity and accuracy
      • high cost
      • high current requirements
      • temperature sensitive

Amplifier

  • flat gain
  • 120 km max between amplifiers, if more – traffic must be regenerated
  • regeneration (3R): reshape, retime, retransmit

Erbium doped fiber amplifier (EDFA)

  • cable on bottom of the ocean
  • amplifies λ signal, energy is absorbed from λ’
  • fiber is doped with erbium
  • 1500-1600nm wavelength only: C-band, L-band

Raman

  • based on Raman scattering effect: if signals are 70-100 nm apart, signal from lower wavelength signal (1425 and 1452 nm) excites electrons and they pass energy to higher wavelength signal
  • any wavelength
  • common fiber
  • low gain compared to EDFA (< 15 dB)
  • requires long span fiber (short fiber not enough)
    • does not support OSC
  • noiseless amplifier
  • pump power: 100-450 mW
  • pump laser is sent against actual flow

Wavelength

  • window: minimum of power loss dB/km on this part of spectrum (≈ 0.25 dB/km)
    • 850nm: MMF
    • 1310nm: SMF, S-band
    • 1550nm: SMF, C-band
      • lower attenuation
      • higher dispersion: narrow linewidth, high-power lasers
      • allows EDFA
    • 1625nm: SMF, L-band

Interference

  1. attenuation:
    • ≈ 0.25 dB/km
    • dramatic increase after 1700 nm
  2. chromatic dispersion
    • spectrum drifts over time ⇒ pulses overlap with each other
    • ~ (bit rate)²
    • material dispersion in SMF: different wavelengths have different speeds
    • waveguide dispersion in SMF: different refractive indices of core and cladding
      • as wavelength increases, light spreads into cladding ≡ different delays for different wavelengths
  3. optical SNR: > 10dB required
  4. splice: 0.2 dB per connector
  5. patch panel, connector, filter, amplifier
  6. bends in fiber
  7. dirt on connectors
    • DCU: dispersion compensation unit
    • signal regeneration: optic → electrical → optic
    • lower interference compared to amplifier (increases both signal and noise)
  8. polarization mode dispersion (PMD)
    • wave shift along different axes (Nx ≠ Ny)
    • mitigated by regeneration or complex modulation
  9. four-wave mixing
    • inter-channel cross-talk (3 channels) creates new channels (4th channel)
    • limits channel capacity in DWDM
    • effect increases with the length of fiber
  10. cross-phase modulation, self-phase modulation
  11. Rayleigh scattering
    • caused by variations in the glass density because of temperature difference
    • variations are smaller than wavelength ≡ scattering objects
    • wavelengths below 800 nm cannot be used
  12. fiber aging: up to 2 dB loss

Multiplexing

Prism diffraction

  • based on dispersion
  • different medium from fiber: encourages dispersion instead of dampening it ≡ power loss at the border
  • lenses
    • cannot be made compact
    • focus length must be less than cm, such lenses cannot be made cheap
    • Fresnels lenses require high index contrasts
  • CWDM
Prism

Waveguide grating

  • based on diffraction
  • different λ are steered to different angles
Diffraction

Arrayed waveguide grating

  • based on interference
    1. incoming grey signal
    2. grey signal passes empty space
    3. grey signal is split over several paths of different length ≡ different phase shift, 2πn difference of wave front
    4. signals interfere with each other
    5. different wavelengths interfere in different locations
  • 1 → 5 ≡ demultiplexer, 5 → 1 ≡ multiplexer
  • uniform insertion loss: does not depend on channel count
  • temperature sensitive
  • depend on polarization
  • scalable channel count and spectral shape
  • cost effective mass productivity: semiconductor based
  • DWDM

Thin film filters

  • CWDM
  • low channel count WDM: loss due to reflection
  • TFF filters light serially: target λ is passed, the rest is reflected into next TFF
TFF