optical network components

8/14/2019 Optical Network Components

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OPTICAL NETWORKOPTICAL NETWORK

COMPONENTSCOMPONENTS

: . ,redit Dr V Nagarajan.rofessor in dept of

,CE SSNCE


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Couplers, Splitters, Isolators,Circulators

Filters, Gratings, Multiplexors

Optical Amplifiers, Regenerators

Light Sources, Tunable Lasers,Detectors

Modulators

Overview


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Couplers and FiltersCouplers and Filters

What do they do?Combine wavelengths from many separatefibers into single fiber

Coupler

Separate wavelengths from single fiber tomany separate fibers

Decoupler

Highlights:

Completely passive devicesResponse is not linear across all wavelengthsripple

Some light power is lost wavelengths passthrough filters

Hard & expensive to manufacture demand


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Couplers, SplittersCouplers, Splitters

Coupler Splitter

DWDM System use Coupler + Splitter


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Optical CouplersOptical Couplers

Combines & splits signalsWavelength independent or selectiveFabricated using waveguides in integratedoptics = coupling ratio

Power(Output1) = Power(Input1)Power(Output2) = (1- ) Power(Input1)

Power splitter if =1/2: 3-dB coupler

Tap if close to 1

-selective if depends upon (used in EDFAs)


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CouplersCouplers (contd)(contd)

Light couples from one waveguide to aclosely placed waveguide because thepropagation mode overlaps the twowaveguides

Identical waveguides => complete couplingand back periodically (coupled modetheory)

Conservation of energy constraint:Possible that electric fields at two outputs havesame magnitude, but will be 90 deg out ofphase!

Lossless combining is not possible


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8x88x8 StarStar CouplerCoupler

Power fromall inputs equallysplit amongoutputs



CouplersCouplers (Contd)(Contd)



Optical CouplersOptical CouplersMultiplex and demultiplex optical channels into and outof a single fiber

One port for each DWDM channel common port to thefiber plant (outside)

Incorporates:

Monitoring taps for maintenance and troubleshooting

Variable Optical Attenuators (VOA) for received poweradjustment

Expansion ports for upgrades to add more wavelengths to asystemx1x2

.

.

Rx4x5RxN

Rx1x2Rx3

.

.

.

.

Tx4x5TxN

Tx3

1 , 2 . . . N

Tx1x2

.

.

Tx1x2TxN

Rx1x2RxN

.

.

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.

Rx1x2RxN

TxN

1 , 2 . . . N

Unidirectionnidirectionall Bidirectionaidirectional



Example: 16 DWDM CouplerExample: 16 DWDM CouplerArchitectureArchitecture

TxTx

Tx

RxRx

Rx

Tx

Coupler

Tx

and Splitting or Combining Coupler

idirectional AmplifierRxRxRx

TxTx

Tx

xpansion Ports

Coupler



8-port Splitter Made by Cascading Y-8-port Splitter Made by Cascading Y-CouplersCouplers



Isolators and CirculatorsIsolators and CirculatorsExtension of coupler concept

Non-reciprocal=> will not work same way ifinputs and outputs reversed

Isolator: allow transmission in one direction, butblock all transmission (eg: reflection) in the other

Circulator: similar to isolator, but with multipleports.



:ecall Polarization:Polarization Time course of the direction of the electric fiel

vector

, , , -Linear Elliptical Circular Non polar

Polarization plays an important role in the interaction of ligwith matter

Amount of light reflected at the boundary between twomaterials

, ,Light Absorption Scattering Rotation

Refractive index of anisotropic materials depends on( )polarization Brewster s law



Polarizing FiltersPolarizing Filters

SOP change Done using crystals called dichroics



Rotating PolarizationsRotating Polarizations

Crystals called Faraday Rotators can rotate thepolarization

without loss!



ptical Isolator



Polarization-dependentPolarization-dependentIsolatorsIsolators

Limitation: Requires a particular SOP for input light signal



o ar za on- n epen en-IsolatorsIsolators

SWP: Spatial Walk-off Polarizer (using birefringent crystals)Splits signal into orthogonally polarized components


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Multiplexers, Filters,Multiplexers, Filters,

GratingsGratings

Wavelength selection technologies


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ApplicationsApplicationsWavelength (band) selection,

Static wavelength crossconnects (WXCs)Equalization of gainFiltering of noise Ideas used in laser operation

Dispersion compensation modules


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Low insertion (input-to-output) loss

Loss independent of SOP:geometry of waveguides

Filter passband independentof temperature

Flat passbands

Sharp skirts on thepassband & crosstalkrejection

Cost: integrated opticwaveguide manufacture

Usually based uponinterference or diffraction

Characteristics of FiltersCharacteristics of Filters


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Device using

interference amongoptical signals fromsame source, but withdiff. relative phaseshifts

(i.e. different pathlengths)

Constructive

interference atwavelength andgrating pitch, a, if

a[sin(i) - sin(d)] =m

m = order of the grating

GratingsGratings


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Transmission vs ReflectionTransmission vs ReflectionGratingGrating

Note: etalon is a device where multipleoptical signals generated by repeatedtraversals of a single cavity

N arrow( )slits txvs narrow

re fle ctio nsurfaces

( )rx M a jo rity o f

d e v ice sa re la tte r( )typ e rx


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Diffraction GratingsDiffraction Gratings


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Grating principles (contd)Grating principles (contd)

Blazing: concentrating the refractedenergies at a different maxima other thanzero-th order

Reflecting slits are inclined at an angle to

the grating plane.


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Periodic perturbation (eg: of RI) written in thepropagation medium

Bragg condition: Energy is coupled from incident toscattered wave if wavelength is

0 = 2 neffwhere is period of grating If incident wave has wavelength 0, this wavelength is

reflected by Bragg grating

Bragg GratingsBragg Gratings


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Bragg Grating PrinciplesBragg Grating Principles


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Bragg Gratings (contd)Bragg Gratings (contd)

Uniform vs apodizedindex profile

Apodized: side lobes cutoff, but width of mainlobe increased

Reflection spectrum isthe F-transform of RI-

distribution

B/w of grating (1 nm)inversely proportionalto grating length (few

mm) Note: Lasers use Bragg

gratings to achieve asingle frequencyoperation


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Fiber GratingsFiber GratingsVery low-cost, low loss, ease of coupling (to

other fibers), polarization insensitivity, lowtemp coeff and simple packaging

Writing Fiber Gratings: Usephotosensitivityof certain types of fibers (eg: Silica

doped with Ge, hit with UV light => RI change) Use a phase mask (diffractive optical element)

Short-period(aka Bragg, 0.5 m) or long-periodgratings (upto a few mm) Short-period (Fiber Bragg): low loss (0.1dB), -accuracy

(0.05nm)

Long-period fiber gratings used in EDFAs to provide gaincompensation


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Fiber Bragg GratingFiber Bragg Grating

OADM El i h F BOADM El t ith F B


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OADM Elements with F-BOADM Elements with F-BGratingsGratings


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Fiber BraggFiber Bragg ChirpedChirpedGratingGrating

(U se d in d isp e rsio n co m p e n sa tio n it)tig h te n s th e p u lse w id th


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Long-period Fiber GratingsLong-period Fiber Gratings

Prin cip le o f o p e ra tio n slig h tly d iffe re n t fro m fib e rBragg

E n e rg y a fte r g ra tin g in te ra ctio n is co u p lin g in to otherfo rw ard p ro p a g a tin g m o d e s in th e cla d d in gin ste a d o f b e in g fu lly re fle cte d a s in Fib e r B ra g g

C la d d in g m o d e s ve ry lo ssy a n d q u icklyattenuated=> Couple energy O U Tof a desired wavelength band

optical network components

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