WDM Concept and Components

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WDM Concept and Components

• EE 8114
• Course Notes

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Part 1 WDM Concept
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Evolution of the Technology
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Why WDM?

• Capacity upgrade of existing fiber networks
  (without adding fibers)
• Transparency Each optical channel can carry any
  transmission format (different asynchronous bit
  rates, analog or digital)
• Scalability Buy and install equipment for
  additional demand as needed
• Wavelength routing and switching Wavelength is
  used as another dimension to time and space

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Wavelength Division Multiplexing
Each wavelength is like a separate channel (fiber)
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TDM Vs WDM
Ex SONET
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Wavelength Division Multiplexing

• Passive/active devices are needed to combine,
  distribute, isolate and amplify optical power at
  different wavelengths

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WDM, CWDM and DWDM

• WDM technology uses multiple wavelengths to
  transmit information over a single fiber
• Coarse WDM (CWDM) has wider channel spacing (20
  nm) low cost
• Dense WDM (DWDM) has dense channel spacing (0.8
  nm) which allows simultaneous transmission of 16
  wavelengths high capacity

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WDM and DWDM

• First WDM networks used just two wavelengths,
  1310 nm and 1550 nm
• Today's DWDM systems utilize 16, 32,64,128 or
  more wavelengths in the 1550 nm window
• Each of these wavelength provide an independent
  channel (Ex each may transmit 10 Gb/s digital or
  SCMA analog)
• The range of standardized channel grids includes
  50, 100, 200 and 1000 GHz spacing
• Wavelength spacing practically depends on
• laser linewidth
• optical filter bandwidth

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ITU-T Standard Transmission DWDM windows
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Principles of DWDM

• BW of a modulated laser 10-50 MHz ? 0.001 nm
• Typical Guard band 0.4 1.6 nm
• 80 nm or 14 THz _at_1300 nm band
• 120 nm or 15 THz _at_ 1550 nm
• Discrete wavelengths form individual channels
  that can be modulated, routed and switched
  individually
• These operations require variety of passive and
  active devices

Ex. 10.1
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Nortel OPTERA 640 System
64 wavelengths each carrying 10 Gb/s
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DWDM Limitations

• Theoretically large number of channels can be
  packed in a fiber
• For physical realization of DWDM networks we need
  precise wavelength selective devices
• Optical amplifiers are imperative to provide long
  transmission distances without repeaters

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Part II WDM Devices
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Key Components for WDM

• Passive Optical Components
• Wavelength Selective Splitters
• Wavelength Selective Couplers
• Active Optical Components
• Tunable Optical Filter
• Tunable Source
• Optical amplifier
• Add-drop Multiplexer and De-multiplexer

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Photo detector Responsivity

• Photo detectors are sensitive over wide spectrum
  (600 nm).
• Hence, narrow optical filters needed to separate
  channels before the detection in DWDM systems

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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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Filter, Multiplexer and Router
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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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Fused-Biconical coupler OR 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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Fused Biconical Tapered Coupler

• Fabricated by twisting together, melting and
  pulling together two single mode fibers
• They get fused together over length W tapered
  section of length L total draw length LW
• Significant decrease in V-number in the coupling
  region energy in the core leak out and gradually
  couples into the second fibre

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

• power ratio between both output can be changed by
  adjusting the draw length of a simple fused fiber
  coupler
• It can be made a WDM de-multiplexer
• Example, 1300 nm will appear output 2 (p2) and
  1550 nm will appear at output 1 (P1)
• However, suitable only for few wavelengths that
  are far apart, not good for DWDM

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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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Fused-Fiber Star Coupler
Splitting Loss -10 Log(1/N) dB 10 Log (N)
dB Excess Loss 10 Log (Total Pin/Total
Pout) Fused couplers have high excess loss
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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
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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
• Single mode lasers and many more areas

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Bragg Grating formation
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FBG Theory

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

z Distance measured along fiber core axis ?
Pitch of the grating ncore Core refractive
index dn Peak refractive index
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Reflection at FBG
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Simple De-multiplexing Function
Peak Reflectivity Rmax tanh2(kL)
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Wavelength Selective DEMUX
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Dispersion Compensation
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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Extended Add/Drop Mux
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FBG for DFB Laser

• Only one wavelength gets positive feedback ?
  single mode Distributed Feed Back laser

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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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Unique Application of FBG
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Resonance Cavity with FBG
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Transmission Characteristics
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Experimental Set-Up
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• What is the wavelength separation when RF
  separation 50 MHz?

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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) that 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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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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Four-Channel Wavelength Multiplexer

• By appropriately selecting ?L, wavelength
  multiplexing/de-multiplexing can be achieved

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MZI- Demux Example
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Arrayed Wave Guide Filters
Each waveguide has slightly different length
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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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Generating Multiple Wavelength for WDM Networks

• Discrete DFB lasers
• Straight forward stable sources, but expensive
• Wavelength tunable DFB lasers
• 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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Discrete Single-Wavelength Lasers

• Number of lasers into simple power coupler each
  emit one fixed wavelength
• Expensive (multiple lasers)
• Sources must be carefully controlled to avoid
  wavelength drift

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Frequency Tuneable Laser

• Only one (DFB or DBR) laser that has grating
  filter in the lasing cavity
• Wavelength is tuned by either changing the
  temperature of the grating (0.1 nm/OC)
• Or by altering the injection current into the
  passive section (0.006 nm/mA)
• The tuning range decreases with the optical
  output power

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Tunable Laser Characteristics

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

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Tunable Filters

• Tunable filters are made by at least one branch
  of an interferometric filter has its
• Propagation length or
• Refractive index altered by a control mechanism
• When these parameters change, phase of the
  propagating light wave changes (as a function of
  wavelength)
• Hence, intensity of the added signal changes (as
  a function of wavelength)
• As a result, wavelength selectivity is achieved

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Tunable Optical Filters
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Tuneable Filter Considerations

• Tuning Range (??) 25 THz (or 200nm) for the
  whole 1330 nm to 1500 nm. With EDFA normally ??
  35 nm centered at 1550 nm
• Channel Spacing (d?) the min. separation
  between channels selected to minimize crosstalk
  (30 dB or better)
• Maximum Number of Channels (N ??/ d?)
• Tuning speed Depends on how fast switching needs
  to be done (usually milliseconds)

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Issues in WDM Networks

• Nonlinear inelastic scattering processes due to
  interactions between light and molecular or
  acoustic vibrations in the fibre
• Stimulated Raman Scattering (SRS)
• Stimulated Brillouin Scattering (SBS)
• Nonlinear variations in the refractive index due
  to varying light intensity
• Self Phase Modulation (SPM)
• Cross Phase Modulation (XPM)
• Four Wave Mixing (FWM)

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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 and nonlinear effects
  limit the performance
• Passive signal processing elements like FBG, AWG
  are attractive
• Optical amplifications is imperative to realize
  DWDM networks