Components for WDM Networks

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Components for WDM Networks

• Xavier Fernando
• ADROIT Group
• Ryerson University

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Passive Devices

• These operate completely in the optical domain
  (no O/E conversion) and does not need electrical
  power
• Split/combine light stream Ex N X N couplers,
  power splitters, power taps and star couplers
• Technologies - Fiber based or
• Optical waveguides based
• Micro (Nano) optics based
• Fabricated using optical fiber or waveguide (with
  special material like InP, LiNbO3)

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10.2 Passive Components

• Operate completely in optical domain
• N x N couplers, power splitters, power taps, star
  couplers etc.

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Fig. 10-3 Basic Star Coupler
May have N inputs and M outputs

• Can be wavelength selective/nonselective
• Up to N M 64, typically N, M lt 10

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Fig. 10-4 Fused-fiber coupler / Directional
coupler

• P3, P4 extremely low ( -70 dB below Po)
• Coupling / Splitting Ratio P2/(P1P2)
• If P1P2 ? It is called 3-dB coupler

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Definitions
Try Ex. 10.2
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Coupler characteristics
? Coupling Coefficient
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Coupler Characteristics

• By adjusting the draw length of a simple fused
  fiber coupler,
• power ratio can be changed
• Can be made wavelength selective

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Wavelength Selective Devices

• These perform their operation on the incoming
  optical signal as a function of the wavelength
• Examples
• Wavelength add/drop multiplexers
• Wavelength selective optical combiners/splitters
• Wavelength selective switches and routers

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Filter, Multiplexer and Router
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A Static Wavelength Router
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Fig. 10-11 Fused-fiber star coupler
Splitting Loss -10 Log(1/N) dB Excess Loss 10
Log (Total Pin/Total Pout) Fused couplers have
high excess loss
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Fig. 10-12 8x8 bi-directional star coupler by
cascading 3 stages of 3-dB Couplers
?1, ?2
?1, ?2 ?5, ?6
?1, ?2
?3, ?4 ?7, ?8
(12 4 X 3) Try Ex. 10.5
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Fiber Bragg Grating

• This is invented at Communication Research
  Center, Ottawa, Canada
• The FBG has changed the way optical filtering is
  done
• The FBG has so many applications
• The FBG changes a single mode fiber (all pass
  filter) into a wavelength selective filter

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Fiber Brag Grating (FBG)

• Basic FBG is an in-fiber passive optical band
  reject filter
• FBG is created by imprinting a periodic
  perturbation in the fiber core
• The spacing between two adjacent slits is called
  the pitch
• Grating play an important role in
• Wavelength filtering
• Dispersion compensation
• Optical sensing
• EDFA Gain flattening and many more areas

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Fig. 10-16 Bragg grating formation
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FBG Theory

• Exposure to the high intensity UV radiation, the
  refractive index of the fiber core (n)
  permanently changes to a periodic function of z

z Distance measured along fiber core axis ?
Pitch of the grating ncore Core refractive index
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Reflection at FBG
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Fig. 10-17 Simple de-multiplexing function
Peak Reflectivity Rmax tanh2(kL)
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Wavelength Selective DEMUX
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Dispersion Compensation using FBG
Longer wavelengths take more time
Reverse the operation of dispersive fiber
Shorter wavelengths take more time
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ADD/DROP MUX
FBG Reflects in both directions it is
bidirectional
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Fig. 10-27 Extended add/drop Mux
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Advanced Grating Profiles
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FBG Properties

• Advantages
• Easy to manufacture, low cost, ease of coupling
• Minimal insertion losses approx. 0.1 db or less
• Passive devices
• Disadvantages
• Sensitive to temperature and strain.
• Any change in temperature or strain in a FBG
  causes the grating period and/or the effective
  refractive index to change, which causes the
  Bragg wavelength to change.

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Interferometers
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Interferometer

• An interferometric device uses 2 interfering
  paths of different lengths to resolve wavelengths
• Typical configuration two 3-dB directional
  couplers connected with 2 paths having different
  lengths
• Applications
• wideband filters (coarse WDM)
• separate signals at1300 nm from those at 1550 nm
• narrowband filters
• filter bandwidth depends on the number of
  cascades (i.e. the number of 3-dB couplers
  connected)

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Fig. 10-13 Basic Mach-Zehnder interferometer
Phase shift of the propagating wave increases
with ?L, Constructive or destructive
interference depending on ?L
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Mach-Zehnder interferometer

• Phase shift at the output due to the propagation
  path length difference
• If the power from both inputs (at different
  wavelengths) to be added at output port 2, then,
• Try Ex. 10-6

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Mach-Zehnder interferometer
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Fig. 10-14 Four-channel wavelength multiplexer
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Mach-Zehnder interferometer
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Mach-Zehnder interferometer
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MZI- Demux Example
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Fiber Grating Filters

• Grating is a periodic structure or perturbation
  in a material
• Transmitting or Reflecting gratings
• The spacing between two adjacent slits is called
  the pitch
• Grating play an important role in
• Wavelength filtering
• Dispersion compensation
• EDFA Gain flattening and many more areas

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Reflection grating

• Different wavelength can be separated/added

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Arrayed wave guide grating
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Phase Array Based WDM Devices

• The arrayed waveguide is a generalization of 2x2
  MZI multiplexer
• The lengths of adjacent waveguides differ by a
  constant ?L
• Different wavelengths get multiplexed
  (multi-inputs one output) or de-multiplexed (one
  input multi output)
• For wavelength routing applications multi-input
  multi-output routers are available

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Diffraction gratings
source impinges on a diffraction grating ,each
wavelength is diffracted at a different
angle Using a lens, these wavelengths can be
focused onto individual fibers. Less
channel isolation between closely spaced
wavelengths.
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Arrayed Waveguide Grating

• -- good performance -- more cost
  effective
• -- quicker design cycle time --- higher
  channel count

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Multi wavelength sources

• Series of discrete DFB lasers
• Straight forward, but expensive stable sources
• Wavelength tunable lasers
• By changing the temperature (0.1 nm/OC)
• By altering the injection current (0.006 nm/mA)
• Multi-wavelength laser array
• Integrated on the same substrate
• Multiple quantum wells for better optical and
  carrier confinement
• Spectral slicing LED source and comb filters

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Tunable Filters

• At least one branch of the coupler has its length
  or ref. index altered by a control mechanism
• Parameters tuning range (depends on amplifier
  bandwidth), channel spacing (to minimize
  crosstalk), maximum number of channels (N) and
  tuning speed

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Fig. 10-23 Tunable optical filter
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Fig. 10-21 Tunable laser characteristics

• Typically, tuning range 10-15 nm,
• Channel spacing 10 X Channel width

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Summary

• DWDM plays an important role in high capacity
  optical networks
• Theoretically enormous capacity is possible
• Practically wavelength selective (optical signal
  processing) components decide it
• Passive signal processing elements are attractive
• Optical amplifications is imperative to realize
  DWDM networks