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Seppo Honkanen, Brian R. West,Seppo Honkanen, Brian R. West,

and Sanna Yliniemiand Sanna Yliniemi

Optical Sciences CenterOptical Sciences Center

University of ArizonaUniversity of Arizona

Recent Advances on Ion-Exchanged Glass Waveguidesand Devices


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GOMD Meeting, Florida, November 12, 2004GOMD Meeting, Florida, November 12, 2004 22

Acknowledgements Acknowledgements

David Geraghty, Jose CastroDavid Geraghty, Jose Castro

ECE, University of ArizonaECE, University of Arizona

Nasser Peyghambarian, Axel Schulzgen, Pratheepan Madasamy, MikeNasser Peyghambarian, Axel Schulzgen, Pratheepan Madasamy, MikeMorrell and Jason AuxierMorrell and Jason Auxier

Ray Kostuk, James Carriere, Jesse FrantzRay Kostuk, James Carriere, Jesse Frantz

OSC, University of ArizonaOSC, University of Arizona

NicholasNicholas BorelliBorelli

Corning, Inc.Corning, Inc.

Joseph HaydenJoseph Hayden

Schott North AmericaSchott North America

Funding: Arizona State Photonics Initiative, NSF, Corning, BAE SFunding: Arizona State Photonics Initiative, NSF, Corning, BAE Systemsystems


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OverviewOverview

•• Historical background and reviewHistorical background and review

•• Ion exchange processesIon exchange processes

•• Process modelingProcess modeling

•• Optical modeling of waveguidesOptical modeling of waveguides

•• Parameter extractionParameter extraction

•• Model validationModel validation

•• Channel waveguide examplesChannel waveguide examples•• Device DemonstrationsDevice Demonstrations

•• Outlook and ConclusionOutlook and Conclusion


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Ion ExchangeIon ExchangeHistorical BackgroundHistorical Background


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Historical BackgroundHistorical Background

•• First known useFirst known use – – coloring of earthenwarecoloring of earthenware(6(6thth century Egypt)century Egypt)

•• Staining of window glassStaining of window glass (Europe, middle ages)(Europe, middle ages)•• First engineering applicationFirst engineering application – – improvingimproving

surfacesurface--mechanical properties ofmechanical properties of

structural glassstructural glass (1913, 1960s)(1913, 1960s)

•• Related application: improving thermal shockRelated application: improving thermal shock

resistance of laser glassesresistance of laser glasses


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Historical BackgroundHistorical Background

•• First published results of opticalFirst published results of optical

waveguide fabrication by ion exchangewaveguide fabrication by ion exchange

(thermal exchange + field assisted burial)(thermal exchange + field assisted burial)Izawa, T., and H.Izawa, T., and H. NakagomeNakagome,, “ “Optical waveguide formed by electricallyOptical waveguide formed by electricallyinduced migration of ions in glass plates,induced migration of ions in glass plates,” ” Appl Appl. Phys.. Phys. LettLett.. 2121, pp. 584, pp. 584--

586 (1972).586 (1972).

•• First published results of silver ionFirst published results of silver ion--

exchanged waveguidesexchanged waveguidesGiallorenziGiallorenzi, T. G., E. J. West, R. Kirk, R., T. G., E. J. West, R. Kirk, R. GintherGinther, and R. A. Andrews,, and R. A. Andrews,

“ “Optical waveguides formed by thermal migration of ions in glass,Optical waveguides formed by thermal migration of ions in glass,” ”

Appl Appl. Opt.. Opt. 1212, pp. 1240, pp. 1240--1245 (1973).1245 (1973).


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Ion ExchangeIon Exchange

Waveguide CharacteristicsWaveguide Characteristics•• Low attenuationLow attenuation (


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StateState--of of --thethe--art Ionart Ion--ExchangedExchanged

Waveguide DevicesWaveguide Devices

Performance of 1XN splittersTypical Y-branch configuration

1 x N Splitters by Teem Photonics (France)


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Amplified splitter by Teem Photonics (France)

StateState--of of --thethe--art Ionart Ion--ExchangedExchanged

Waveguide DevicesWaveguide Devices


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Ion ExchangeIon ExchangeProcessesProcesses


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ProcessesProcessesThermal exchange from a molten saltThermal exchange from a molten salt

• Waveguide geometry is defined by openings in an oxidized metal mask

• The sample is placed in a melt containing silver ions

• Ag+ is driven into the substrate by a chemical potential gradient, andNa+ is released into the melt to preserve charge neutrality

• Ag+ is distributed within the substrate by thermal diffusion

Na+

Ag+


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ProcessesProcessesFieldField--assisted thermal exchangeassisted thermal exchangefrom a molten saltfrom a molten salt

• Ion exchange is enhanced by application of an electric field across thesubstrate

• Ion migration is dominated by drift rather than diffusion, resulting in amore step-like profile

Na+

Ag+

Va


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ProcessesProcesses

FieldField--assisted burialassisted burial

• Substrate is placed into a melt containing Na+ ions

• Under the influence of an applied field, Ag+ ions are driven further untothe substrate and are replaced by Na+ ions at the surface

• Buried waveguide has lower loss and birefringence (mode interactionwith the surface is reduced), and better mode matching to optical fiber(waveguide profile becomes rounded)

Na+

Na+ Ag+

Va


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ProcessesProcesses

Thermal annealingThermal annealing

• Substrate is held at an elevated temperature

• Silver ions are redistributed through thermal diffusion

• Birefringence is reduced as the waveguide becomes more circular


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Ion ExchangeIon ExchangeProcess ModelingProcess Modeling


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Process ModelingProcess ModelingBinary ion exchange is described by a nonlinear differential equation

Ag

Ag

Ag

Ag

Ag

Ag AgC

kT

q

C M

C M C

C M

D

t

C ext

2

2

)1(1

)()1(

)1(1

E

• C Ag is normalized concentration of silver ions (C Ag = 1 at glass/melt interface)

• D Ag is self-diffusion coefficient of silver ions in substrate glass

• D Na is self-diffusion coefficient of sodium ions in substrate glass

• M = D Ag / D Na

• Eext is applied electric field

• T = absolute temperature, k = Boltzmann’s constant, q = electron charge

thermal diffusioninternal field due to dissimilar D external applied field


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Process ModelingProcess ModelingSolving the Diffusion EquationSolving the Diffusion Equation

The most common situation requires solution in two (transverse)dimensions only - slow variation of waveguide geometry in the

propagation direction implies negligible ion transport


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Process ModelingProcess ModelingSolving the Diffusion EquationSolving the Diffusion Equation

The Peaceman-Rachford Alternating Direction Implicit(PR-ADI) method combines elements of both explicit

and implicit algorithms• Each time step is divided into two half-steps

• In the first half-step, derivatives are calculated explicitly in onedirection and implicitly in the other - in the second half-step, theprocedure is reversed

• As a result, one needs only to solve nx matrices of size ny by ny, then nymatrices of size nx by nx

• These matrices are tridiagonal – very efficient solution methods exist


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Process ModelingProcess ModelingBoundary Conditions

AgC

0 AgC

0 AgC

0 AgC

0

y

C Ag

(burial)0

exchange)(1 AgC


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Electrostatic ModelingElectrostatic Modeling

kT

qc D 2

Ionic conductivity is calculated using the Nernst-Einstein equation

Both species of mobile ions contribute to the conductivity

Ag Ag Na Ag Ag y xC y xC C y x ),(]),(1[),,(

Combining the above two equations

),(),(1

1),,(

2

0 y xC y xC

M kT

qc DC y x Ag Ag

Ag

Ag

Potential profile is described by a non-standard Laplace equation

0),(),,(),(),,( 2 y xC y x y xC y x Ag Ag


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Electrostatic ModelingElectrostatic Modeling

0

U d

hV U a

V a = applied voltageh = domain thickness

d = substrate thickness

0 y

Boundary Conditions

Electrical potential (color scale) and electric field lines

Space charge layer

0 x


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Electrostatic ModelingElectrostatic ModelingExample

Thermal Exchange

D Ag = 6 x 10-16 m2 /s M = 0.15 t = 20 min W m = 3 μm


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Electrostatic ModelingElectrostatic ModelingSelective Field-Assisted Burial

D Ag = 3 x 10-16 m2 /s M = 0.15 t = 45 min T = 523 K V a = 250 V d = 2 mm


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Optical ModelingOptical Modeling

of of IonIon--Exchanged WaveguidesExchanged Waveguides


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),()()(),,( 0sub y xC nn y xn Ag

For small changes in ion concentration, there is a linear relationshipbetween silver concentration and refractive index


of Ionof Ion--Exchanged WaveguidesExchanged Waveguides

This allows normalized values to be used for silver ion concentration

All eigenmodes and propagation constants (quasi-TE and -TM modes) ofthe waveguide can be solved for using the Helmholtz equation

nnn E E k 222

)(

),(2 y xnk

with


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of Ionof Ion--Exchanged WaveguidesExchanged WaveguidesSelectively-Buried Waveguide

(midpoint)

λ = 1550 nm, n sub = 1.507, Δ n 0 = 0.075

Fundamental mode – n eff = 1.5123


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Parameter ExtractionParameter Extraction


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•• To evaluate the diffusion equation, we must knowTo evaluate the diffusion equation, we must know D D Ag Ag andand M M

•• To convert the concentration profile to an index profile,To convert the concentration profile to an index profile,

we must knowwe must know Δ Δ n n 00•• These parameters are dependent on glass composition,These parameters are dependent on glass composition,

melt composition, and temperaturemelt composition, and temperature

Glass manufacturers do not routinely provide this dataGlass manufacturers do not routinely provide this data


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•• Ion exchange parameters can beIon exchange parameters can bedetermined by matching thedetermined by matching themeasured index profile of slabmeasured index profile of slabwaveguides (thermally exchanged)waveguides (thermally exchanged)to trial modeling resultsto trial modeling results

•• A simple, non A simple, non--destructive methoddestructive methodof determining index profile is byof determining index profile is byreconstructingreconstructing n n ((y y ) from mode) from modeindices measured using a prismindices measured using a prismcouplercoupler

•• In practice, it is more accurate toIn practice, it is more accurate tocalculate mode indices from a givencalculate mode indices from a givenindex profileindex profile

•• Previously, onlyPreviously, only brute forcebrute forcemethods have been used to adjustmethods have been used to adjustthe parametersthe parameters

Guess at parameters

Simulate IX

Calculate modes

Agree with measurement?

no

stop

yes

Subsequent guesses require a great deal of intuition


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Genetic AlgorithmGenetic Algorithm

•• Parameter optimization canParameter optimization canbe automated using abe automated using agenetic algorithmgenetic algorithm

•• The user needs only toThe user needs only tospecify aspecify a rangerange andandresolutionresolution for eachfor eachparameterparameter

•• Solution of the diffusionSolution of the diffusionequation in 1D (no appliedequation in 1D (no appliedfield) is fastfield) is fast

Create first generationof trial parameters

Simulate IX

Calculate modes

Acceptable FOM? no

stop

yes

Calculate figure of merit

Create nextgeneration of

trial parameters


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Genetic AlgorithmGenetic AlgorithmCalculate figure of merit

m

meff meff m n N wF 2

,,exp

• N eff,m are mode indices calculated from the simulated IX with trial parameters

• n eff,m are mode indices measured from fabricated slab waveguides

• w m are weights that determine the relative importance or confidence of each mode(modes that lie near the substrate index may be difficult to accuratelymeasure)

• Sum of squared errors ensures that errors of either sign provide a positivecontribution to F

• exponential factor serves to bias the following generation toward larger vales of F


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GOMD Meeting, Florida, November 12, 2004GOMD Meeting, Florida, November 12, 2004

3232

ModelModel Validation Validation


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3333

Model ValidationModel Validation

The parameter extraction, process modeling, and optical modeling havebeen validated by comparison with a fabricated waveguide

Parameter Extraction

n sub = 1.4574 λ = 1550 nm T = 300 C t = 1 h

D Ag = 1.1 X 10-15 m2 /s M = 0.72 Δ n 0 = 0.034

0.6

0.8

1

1.2

1.4

x 10-15

0.5

0.6

0.7

0.8

0.9

1

0.03

0.032

0.034

0.036

0.038

0.04

D Ag

M

n 0


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Thermal Exchange

D Ag = 1.1 x 10-15 M = 0.72 t = 60 min W m = 5 μm

Field-Assisted Burial

D Ag = 5 x 10-16 M = 0.72 t = 5 min V a = 275 V


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H e i g h t [ m ]

a) Width [m]

-10 -5 0 5 10

-10

-5

0

5

10

H e i g h t [ m ]

b) Width [m]

-10 -5 0 5 10

-10

-5

0

5

10

-10 -5 0 5 10

-10

-5

0

5

10

H e i g h t [ m ] )

a) Width [m])-10 -5 0 5 10

-10

-5

0

5

10

H e i g h t [ m ] )

b) Width [m]

Modeled Intensity Profile

n eff,0 = 1.4638

n eff,1 = 1.4575

Measured Intensity Profile

n eff,0 = 1.4637

n eff,1 = 1.4575


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Experiments withExperiments with

Buried IonBuried Ion--ExchangedExchanged

Channel WaveguidesChannel Waveguides


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Burial Depth vs. WaveguideBurial Depth vs. Waveguide

WidthWidth

-7.5

-7

-6.5

-6

-5.5

-5

-4.5

-4

0 1 2 3 4 5 6 7 8 9 10Mask opening width (µm)

B u r i a l d e p t h ( µ m )

Experimental

Modeling(M=0.2)

Modeling(M=0.5)

Modeling(M=1)

Linear fit(Exp)

Linear fit(M=0.2)

Lineat f it(M=0.5)

Linear f it(M=1)


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Waveguide Birefringence vs.Waveguide Birefringence vs.

Waveguide WidthWaveguide Width

0

5

10

15

20

25

0 2 4 6 8 10

mask opening w idth ( m )

n T E

- n T M (

1 0 - 6 )

model experiment


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Effect of Annealing on WaveguideEffect of Annealing on Waveguide

BirefringenceBirefringence

-10

-5

0

5

10

15

1 3 5 7 9 11

mask opening w idth (

m )

n T E

- n T M (

1 0 - 6 )

0 min 15 min 45 min 75 min 105 min @250 C


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Experiments with QuantumExperiments with Quantum

Dot Doped IonDot Doped Ion--ExchangedExchangedChannel WaveguidesChannel Waveguides


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PbSPbS QDQD--doped glassesdoped glasses

• Small bandgap - 0.4 eV (3m) @ room temperature• Strong quantum-confinement: R Bohr~18 nm

• Less prone to surface effects• Tunable from 3 m to visible

PbS doped glass

~ 5-10 nm

Thermal treatment of an oxide molten glass:• Sizes 2R = 5-10 nm, R ~ 5%

• n ~ 1-5 ·1016

/cm3


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Fabrication by K Fabrication by K ++ -- NaNa++ IonIon

ExchangeExchange

Single mode waveguides

Propagation loss: < 0.5 dB/cm

Measured Mode Profile


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Are the Are the QDsQDs destroyed?destroyed?

1100 1200 1300 1400

0.01

0.1

1

Wvgd

IonX

Sample2

Abs.

L uminescence - PbS QD-doped glass

I n t e n s i t y ( a . u . )

Wavelength (nm)

0.0

0.1

0.2

0.3

A l p h a ( c m - 1

)


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IonIon--Exchanged WaveguideExchanged WaveguideDevice ExamplesDevice Examples


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Multimode Interference (MMI)Multimode Interference (MMI)

DeviceDevice

MMI devices are a guided-wave analogy of Talbot imaging

• Guided modes are approximately periodic in the transverse (x -) direction

• Quadratic distribution of propagation constants (in the paraxial approximation)produces a series of self-imaging planes

• Used primarily for short power splitting applications (no lengthy s-bends required)

• Other applications in pump/signal multiplexing, mode coupling, sensing


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Multimode Interference DevicesMultimode Interference Devices

Self Self --Imaging TheoryImaging Theory

0

x

L MMI

x1

x2

x N

W in

W N

1 X N MMI Power Splitter

),()0,,( y xa z y x

y x y x

y x y x z y xa

dd),(

dd),()0,,(

2

Field of input waveguide can beexpressed as an orthogonal expansionof multimode waveguide modes

L3

)2(0

Propagation constants are quadratic in ν

10

L

After propagating a distance L , the

field distribution is

L

Li y xa L y x

3

)2(exp),(),,(

“Mode Propagation Analysis”


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WeaklyWeakly--Guided MMI DevicesGuided MMI Devices

When the multimode section is weakly guiding, or contains an index gradient(as with ion exchange):

• Higher-order modes have progressively larger effective widths

• Distribution of effective indices becomes sub-parabolic

• This results in a longitudinal shift of the self-imaging planes, as well as atransverse shift of the optimum positions of the output waveguides

An additional problem arises in buried ion-exchanged MMI devices, as theburial depth is dependent upon the waveguide width – there will be a verticaloffset between the wide multimode waveguide and the narrow access

waveguides.


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Designed by Genetic AlgorithmDesigned by Genetic AlgorithmThe nonlinear relation between MMI device geometry and operational characteristicssuggests that an iterative optimization algorithm would be beneficial in the design

0

x

L MMI

x1

x2

x N

W in

W N

Pre-determined parameters:

• MMI section width

• Fabrication process

Parameters to optimize:

• MMI section length L MMI • Input waveguide width W in • Output waveguide positions x i • Output waveguide widths W i

For symmetric devices, the number of parameters is reduced to N +2:

N odd: N /2 output waveguide positions & N /2 output waveguide widths

N even: (N -1)/2 output waveguide positions & (N +1)/2 output waveguide widths


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Designed by Genetic AlgorithmDesigned by Genetic Algorithm

Self-Imaging Design Genetic Algorithm Design

0.0010.016PDL (dB)

0.0071.225Power imbalance (dB)

1.9012.088Excess loss (dB)250 N/AGenerations

15 N/ASims per generation

6.65.0 x2 (μm)

15.115.0 x1 (μm)

4.55.0W 2 (μm)

6.05.0W 1 (μm)

11.310.0W in

(μm)

446.7395.7 L MMI (μm)

GAS-IParameter


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Device DemonstrationsDevice Demonstrations

•• Add Add--Drop Wavelength FilterDrop Wavelength Filter

•• ErEr--doped Waveguide laser arraydoped Waveguide laser array

•• Tapered ErTapered Er--doped Waveguide Laserdoped Waveguide Laser


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Add Add--Drop Wavelength FilterDrop Wavelength Filter

Output

AddDrop

Input

OutputInput

On Resonance

Off Resonance

Asymmetric Y branches- Wide guide excites even mode- Narrow guide excites odd mode

Tilted Grating- Breaks orthogonality of the 2 modes- UV written gratings


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Add Add--Drop Wavelength FilterDrop Wavelength Filter

• 20 dB Extinction

• 0.4 nm 3 dB Bandwidth

1562 1564 1566 1568-20

-15

-10

-5

0

R e l a t i v

e P o w e r ( d B )

Wavelength (nm)

1562 1564 1566 1568-20

-15

-10

-5

0

R e l a t i v

e P o w e r ( d B )

Wavelength (nm)

1562 1564 1566 1568-20

-15

-10

-5

0

R e l a t i v

e P o w e r ( d B )

Wavelength (nm)

PASS ADDDROP

Pass

AddDrop

Input


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ErEr--Doped Waveguide Devices byDoped Waveguide Devices by

Ag Ag--Film Ion ExchangeFilm Ion Exchange

+V

GND

a) Spin-coat Photo Resist b) Mask Patterning c) Ag film deposition

d) Electric Field Assisted

Ion-Exchangef) Thermal Diffusione) Ag Removal

PR

Er-Doped

Phosphate Glass

Ag

Ag

Surface Waveguide


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MultiMulti--Wavelength WaveguideWavelength Waveguide

Laser ArrayLaser Array

975 nm

pump

Laser

output

1

2

4

3

HR coating

Ion exchanged waveguides

Surface relief grating

Wavelength range: 2.1 nm (1548.6 - 1550.7 nm)

- covers 5 ITU grid wavelengths (50 GHz spacing )

Output Power: 11 mW

Threshold: 60 mW

Slope Efficiency: 13 %


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Tapered MMTapered MM--Pumped ErPumped Er--DopedDoped

Waveguide LaserWaveguide Laser

Erbium/Ytterbium

codoped glass

Ion exchanged waveguide

Undoped glass

Single mode waveguide

Multimode section (100 µm)

Dielectric mirror (R>99%)

10 mm 30 mm 10 mm

Surface Relief Bragg

grating (R = 72 %)

965 nm

pump


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Output Power vs. Pump PowerOutput Power vs. Pump Power

0

10

20

30

40

50

60

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Pump power (W)

O u t p u t p o w e r ( m

W ) Wavelength = 1538 nm

Slope = 4.9 %

Threshold = 280 mW

Output power = 54 mW


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Outlook Outlook

•• New device innovationsNew device innovations•• ModeMode--locked waveguide laserslocked waveguide lasers

•• MMMM--pumped multipumped multi--λλ laser arrayslaser arrays•• Tunable dispersion compensatorsTunable dispersion compensators

•• All All--optical header recognition chipoptical header recognition chip

•• Exotic host glassesExotic host glasses•• Integration with other materialsIntegration with other materials


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ConclusionConclusion

•• IonIon--Exchange in GlassExchange in Glass

•• A Proven Low A Proven Low--Cost Integrated OpticsCost Integrated OpticsTechnologyTechnology

•• Offers Unique Advantages for VariousOffers Unique Advantages for Various

Passive and Active Waveguide DevicesPassive and Active Waveguide Devices

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