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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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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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Ion ExchangeIon Exchange
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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Ion ExchangeIon Exchange
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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Ion ExchangeIon Exchange
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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Ion ExchangeIon Exchange
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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Ion ExchangeIon Exchange
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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Ion ExchangeIon Exchange
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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Ion ExchangeIon Exchange
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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Ion ExchangeIon Exchange
Process ModelingProcess ModelingBoundary Conditions
AgC
0 AgC
0 AgC
0 AgC
0
y
C Ag
(burial)0
exchange)(1 AgC
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Ion ExchangeIon Exchange
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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Ion ExchangeIon Exchange
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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Ion ExchangeIon Exchange
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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Ion ExchangeIon Exchange
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
Optical ModelingOptical Modeling
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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Optical ModelingOptical Modeling
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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Parameter ExtractionParameter Extraction
•• 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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Parameter ExtractionParameter Extraction
•• 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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Parameter ExtractionParameter Extraction
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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Parameter ExtractionParameter Extraction
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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ModelModel Validation Validation
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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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Model ValidationModel Validation
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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Model ValidationModel Validation
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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Multimode Interference DevicesMultimode Interference Devices
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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Multimode Interference DevicesMultimode Interference Devices
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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Multimode Interference DevicesMultimode Interference Devices
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