Components for WDM Networks

Xavier Fernando

ADROIT Group

Ryerson University

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)

10.2 Passive Components

• Operate completely in optical domain

• N x N couplers, power splitters, power taps, star couplers etc.

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

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

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

)(sin 201 zPP

)(cos202 zPP : Coupling Coefficient

Coupler Characteristics

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

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

Filter, Multiplexer and Router

A Static Wavelength Router

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

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

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 and many more areas

Fig. 10-16: Bragg grating formation

uv )2/sin(2

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

Reflection at FBG

Fig. 10-17: Simple de-multiplexing function

Reflected Wavelength 2B effn

Peak Reflectivity Rmax = tanh2(kL)

Wavelength Selective DEMUX

Dispersion Compensation using FBG

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

Fig. 10-27: Extended add/drop Mux

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

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 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)

Fig. 10-13: 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

Mach-Zehnder interferometer

Fig. 10-14: Four-channel wavelength multiplexer

Mach-Zehnder interferometer

Mach-Zehnder interferometer

MZI- Demux Example

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

Reflection grating

Different wavelength can be separated/added

Arrayed wave guide grating

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 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.

Arrayed Waveguide Grating

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

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

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

Fig. 10-23: Tunable optical filter

Fig. 10-21: Tunable laser characteristics

Typically, tuning range 10-15 nm,

Channel spacing = 10 X Channel width

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