wdm concept and components ee 8114 course notes. part 1: wdm concept
TRANSCRIPT
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WDM Concept and Components
EE 8114Course 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
2
c
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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 @1300 nm band• 120 nm or 15 THz @ 1550 nm • Discrete wavelengths form individual channels that can
be modulated, routed and switched individually• These operations require variety of passive and active
devices
2
c
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 WDMPassive Optical Components• Wavelength Selective Splitters• Wavelength Selective CouplersActive 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
• 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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Fused-Biconical coupler OR 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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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 = L+W
• 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
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
)(sin202 zPP
)(cos201 zPP
: 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) dBExcess 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
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
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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
uv )2/sin(2
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FBG TheoryExposure to the high intensity UV radiation
changes the fiber core n(z) permanently as a periodic function of z
)]/2cos(1[)( znnzn core
z: Distance measured along fiber core axis: Pitch of the gratingncore: Core refractive indexδn: Peak refractive index
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Reflection at FBG
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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
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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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 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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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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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 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
1 2
1 12 effn L
2 effnL
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Four-Channel Wavelength Multiplexer
• By appropriately selecting ΔL, 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 Gratingssource 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 (Δν): 25 THz (or 200nm) for the
whole 1330 nm to 1500 nm. With EDFA normally Δλ = 35 nm centered at 1550 nm
• Channel Spacing (δν): the min. separation between channels selected to minimize crosstalk (30 dB or better)
• Maximum Number of Channels (N = Δν/ δν):• 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