WDM Concept and Components

EE 8114Course Notes

Part 1: WDM Concept

Evolution of the Technology

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

Wavelength Division Multiplexing

Each wavelength is like a separate channel (fiber)

TDM Vs WDM

Ex: SONET

Wavelength Division Multiplexing

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

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

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

ITU-T Standard Transmission DWDM windows

2

c

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

Nortel OPTERA 640 System

64 wavelengths each carrying 10 Gb/s

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

Part II: WDM Devices

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

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

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)

Filter, Multiplexer and Router

Basic Star Coupler

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

May have N inputs and M outputs

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

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

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

Coupler characteristics

)(sin202 zPP

)(cos201 zPP

: Coupling Coefficient

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

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

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

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

Fiber Bragg Grating

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

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

Bragg Grating formation

uv )2/sin(2

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

Reflection at FBG

Simple De-multiplexing Function

Reflected Wavelength 2B effn

Peak Reflectivity Rmax = tanh2(kL)

Wavelength Selective DEMUX

Dispersion Compensation

Longer wavelengths take more time

Shorter wavelengths take more time

Reverse the operation ofdispersive fiber

ADD/DROP MUX

FBG Reflects in both directions; it is bidirectional

Extended Add/Drop Mux

FBG for DFB Laser

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

Advanced Grating Profiles

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

Unique Application of FBG

Resonance Cavity with FBG

Transmission Characteristics

Experimental Set-Up

• What is the wavelength separation when RF separation 50 MHz?

Interferometers

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)

Basic Mach-Zehnder Interferometer

Phase shift of the propagating wave increases with L, Constructive or destructive interference depending on L

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

Four-Channel Wavelength Multiplexer

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

MZI- Demux Example

Arrayed Wave Guide Filters

Each waveguide has slightly different length

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

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.

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

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

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

Tunable Laser Characteristics

Typically, tuning range 10-15 nm,

Channel spacing = 10 X Channel width

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

Tunable Optical Filters

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)

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)

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