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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
• Can be wavelength selective/nonselective
• Up to N =M = 64, typically N, M < 10
May have N inputs and M outputs
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Fig. 10-4: Fused-fiber coupler / Directional coupler
• P3, P4 extremely low ( -70 dB below Po)• Coupling / Splitting Ratio = P2/(P1+P2)• If P1=P2 It is called 3-dB coupler
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Definitions
2 1 2Splitting (Coupling) Rat = )i (o P P P
0 1 2=10 LogExcess Lo [ss ( ] )P P P
=1In 0 sert Log[ion Loss ] in outP P
3 0= 10 LoCrosstalk g( P P )
Try Ex. 10.2
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Coupler characteristics
)(sin 201 zPP
)(cos202 zPP : 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
c 2Number of 3-dB CouN
N = log N 2
plers (12 = 4 X 3)Try Ex. 10.5
1, 2
1, 2
1, 2 5, 6
3, 4 7, 8
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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
uv )2/sin(2
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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
)]/2cos(1[)( znnzn core
z: Distance measured along fiber core axis: Pitch of the gratingncore: Core refractive index
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Reflection at FBG
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Fig. 10-17: Simple de-multiplexing function
Reflected Wavelength 2B effn
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
Shorter wavelengths take more time
Reverse the operation ofdispersive fiber
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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 PropertiesAdvantages
• 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.
neff
TT
neffneff
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Interferometers
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InterferometerAn interferometric device uses 2 interfering paths of
different lengths to resolve wavelengthsTypical configuration: two 3-dB directional couplers
connected with 2 paths having different lengthsApplications:— 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
1 2
1 12 effn L
2 effnL
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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