O-MEMS - Stanford EE - Stanford University

STANFORD

OPTICAL MICRO-ELECTROMECHANICAL SYSTEMS (O-MEMS) J. S. Harris, Jr. Stanford University

O-MEMS APS Short Course 3/11/01

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Outline STANFORD

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Overview Advantages l Fundamentals l

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Mechanical O-MEMS l l l

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Semiconductor Optical Device O-MEMS l l l

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Optical Communications Network Switches Optical Bench Displays l Torsional Mirror l Grating Light Valve Tunable Laser Tunable Detector Modeling

MEMS Systems Examples Summary


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What is O-MEMS STANFORD

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Optics l l l l

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Reflective Refractive Diffractive Waveguiding

Semiconductor Devices l l

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A Marriage of Three Technologies

Optoelectronic III-V Devices Si-CMOS Processing and Control Electronics

Semiconductor Based Micromachining l l l l

Lithography Deposition Epitaxy Etching


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The Past Millennium STANFORD

Palomar Telescope

Tunable Laser

Optical Table


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The New Millenium STANFORD

NASA Space Telescope

Monolithic Tunable Laser 4.3 m Active Laser Stripe

Semiconductor Edge-Emitting Laser

Pre-aligned Micro-Fresnel Lens

Optical Table 3D Structures Pre-aligned to the Lens


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Integration System on a chip STANFORD

Laser-to-fiber coupling Micropositioners for mirrors and gratings High-resolution raster scanner


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The Major Driving Force: Communications Link Capacity STANFORD


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Advantages of O-MEMS “A Marriage of Technologies” STANFORD

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Small force required - Photons have zero mass Very small displacements required - /4 Precision displacement possible Based upon compatible semiconductor processing l l

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Mass, low cost production Added functionality thru integration

Reduced size, weight, and cost Physically robust Reduced physical alignment - Photolithography High resonant frequencies Speed of light (no RC limitation)


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O-MEMS: an Enabling Technology STANFORD

Communications: Fiber Switches. Femtosecond Lasers, Pulse Shaping, Modulators, Tunable Optical Devices l Optical Interconnects l Optical Data Storage l Displays: Projection, Head Mounted - Virtual Reality l

Repro-graphics: Printing, Scanners l Adaptive Optics l Optical Transducers and Sensors l Optical Spectroscopy and Instrumentation l


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Fundamental Optical Functions STANFORD

Reflection

Refraction

Diffraction

Guiding


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Active Device Functions STANFORD

Generation Laser

Modulation Mach-Zehnder


Detection Photodiode

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O-MEMS Active Devices STANFORD

Generation VCSELs

Modulation QCSE Modulator

Detection Photodiode

All Active Devices are Surface Normal O-MEMS APS Short Course 3/11/01

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Technologically Available Optoelectronic Materials STANFORD

AlN 6.0

Bandgap energy (eV)

4.0

(.21 µm)

(.31 µm)

ZnS

GaN

3.0

Visible Spectrum

(.41 µm)

α–SiC

2.0

(.62 µm)

1.0

(1.24 µm)

MgSe ZnSe AlP

CdS

GaP

InN

CdSeAlSb GaAs

Si

GaSb

Ge

3.4

5.4

InSb

InAs

(Ge)

3.2

CdTe

InP

BULK

SLE

3.0

ZnTe

AlAs

5.6

5.8

6.0

6.2

6.4

Lattice Constant (Å) O-MEMS APS Short Course 3/11/01

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Optical Communications Material Choices STANFORD

2.5 AlAs

Bandgap (eV)

2

InxAl1-xAs

AlxGa1-xAs 1.5 GaAs

InP InxGa1-xAs

1

0.5

980nm InxGa1-xAsyP1-y

1300nm 1550nm

GaNyAs1-y GaxIn1-xNyAs1-y on GaAs

InAs InNyAs1-y

0 5.4

5.5

5.6

5.7

5.8

5.9

6

6.1

6.2

Lattice Parameter (Å)

●

GaNAs and GaInNAs have large bandgap bowing ● Long wavelength material lattice matched to GaAs


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Surface Micromachining STANFORD

PolySi Nitride Oxide

Slider

Hinge

V-groove for alignment

Mirror


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Principle of Operation STANFORD

l

Coupled Cavity: Air Gap + Semiconductor Cavity ∆d d0

d

V F=

l l

l l

∂W ∂ 1 2 0AV = CV = ∂z ∂z 2 2d 2

2

Electrostatic Force from applied voltage Balance between electrostatic and elastic restoring force: d ~ V2/d2 Effective cavity length: / 0 = d/d0 Light emission restricted to cavity modes


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Design Issues: Mechanical Actuation Limit STANFORD

Force

V =V 3 crit V 4 V Electrostatic 2 V 1 Force (dotted) Increasing Voltage

0

Elastic restoring force (solid)

Membrane Displacement

d

d0

l

Nonlinear nature of Electrostatic Force

l

Stable solutions: displacement less than ~Lg0 / 3


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MIRRORS STANFORD

Texas Instrument’s DMD NASA's Next Generation Space Telescope (2008) with 4M micromirrors by Sandia NL Lucent’s Optical X-Connect


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Electrostatic-combdrive STANFORD

Folded beams (movable comb suspension)

x

Ground Comb Anchors plate drive

Electric field distribution in comb-finger gaps


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actuators – Electrostatic Combdrives STANFORD

Springs Fiber grooves or channels

manipulator comb drives

Bryant Hichwa etal, OCLI/JDS Uniphase, “A Unique Latching 2x2 MEMS Fiber Optics Switch”, Optical MEMS 2000, Kauai, August 21-24 th, 2000.


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Wavelength Switching Speed STANFORD Wafer 3976R2.1: 20 m Square Membrane

l -10V DC / 5

Bias conditions:

0V DC

l l

10 µs

Intensity (a.u.)

5 µs

4 µs 3 µs

l

2 µs

Constant 10mA Diode Current Membrane driven by square pulse, 500ns to 10 s variable pulse width, 10V Amplitude, 20% duty cycle

Time-averaged Spectrum:

1 µs

l

90% of average

shift in 1 s

500 ns

950 955 960 965 970 975 980 985 990

Wavelength (nm)


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Micromirror Reliability “Off” position

1 0.5 0 -0.5 -1 0

10

20

30

40

50

60

measurement #

70 Change in Res. Frequency

Angle (degrees)

STANFORD

x 10-3

80 2.00% 1.50% 1.00% 0.50% 0.00% -0.50%


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1.E+04

1.E+06 1.E+08 # of Cycles

1.E+10 JSH-24

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Outline STANFORD

l

Overview l l

Advantages Fundamentals

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Mechanical O-MEMS

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Optical Communications Network Switches Optical Bench l Displays l Torsional Mirror l Grating Light Valve Semiconductor Optical Device O-MEMS l l

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Tunable Laser Tunable Detector Modeling



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WDM Crossbar Switch STANFORD

Input Ports

•• •

•••

1 2

λ1OXC λ2OXC

OXC λM

N

Output Ports 1 2 •••

Optical DMUX

Optical MUX

N

Architecture of WDM Switch The optical input signals are demultiplexed, and each wavelength is routed to an independent NxN spatial cross-connect O-MEMS APS Short Course 3/11/01

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O-MEMS Modulation Means STANFORD

Properties of Light: l Intensity, Wavelength, Polarization, and Phase Each of these can be modulated, although intensity is the most common mode

Modulation Means: Linear Motion l Deflection l Reflection l Diffraction l Interference l


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2 x 2 fiber-optic switch STANFORD

§

Out 2

§

Out 1 §

In 1

In 2

§ § §

Compact design One-mask fabrication using DRIE on SOI Integration of fibers, lenses, and micromirrors 2 by 1 operation By-pass switch AT&T, JDSU.......


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1 x 2 Matrix Switch STANFORD


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MEMS-based OXC switches STANFORD

2X2 Fiber-optic switch

§

§ § §

2

2

Some issues: §

1

8x8 Fiber-optic switch

1

Switch scalability with minimum insertion loss Large device count Non-standard fabrication processes Actuator reliability problems

C. Marxer, N.F. de Rooij, Jrnl of Lightwave Tech., Vol. 17, No. 1, Jan 1999 L.Y. Lin, E.L. Goldstein, R.W. Tkach, Jrnl of selected topics in Quantum Electronics, Vol. 5, No. 1, Jan 1999


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NxN Matrix OXC STANFORD

§

§ § § § §

Simple 1 by 2 cross-points Digital mirrors N2 scaling Large motion Reliability?? OMM, Onix, AT&T, Agilent.....


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Beam Steering Optical Switch STANFORD

§ § s = N ?k 2 l § Output § fiber array

Analog mirrors 2N scaling Accuracy? Lucent, C-speed, Xros(NT),.....

Input fiber array


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2x2 Beam Steering Switch* STANFORD

Input Mirror Array

M1

Input A

M2

Input B

Output A

M4 Output Mirror Array

*P.M. Hagelin, U. Krishnamoorthy, et. al, paper published in § § § § § §


Output B

M3

Photonics Technology Letters, July 2000.

Silicon micromachined v-grooves 1.55 m diode laser source Comb-drive actuated mirrors Bulk optics for optical field scaling -4.2 dB measured insertion loss -54 dB measured cross-talk

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Switching Characteristics Transmitted Optical Power at 1550 nm [dB]

STANFORD

0 -20 -40 Output A Output B

-60

M1

M1 M2 B A

M4

M1

M2

M3

M4 M3

B

M4 M3

A

M1: 21.65V to 0V M1: 0V M3: 25.52V M3: 25.52Vto 0V

M1

M2 B A

M1: 0 V M3: 0Vto 25.52V


M2 B A

M4 M3

M1: 0V to 22.24V M3: 25.52V JSH-34

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DMD Light Switches Mirror –10 deg Mirror +10 deg

Hinge

CMOS Substrate

Yoke Landing Tip

DMD Optical Switching Principle

1

SEM Photomicrographs of DMD Chips

(a)

(b)

(c)

(d)

Grating Light Modulator STANFORD

Beams up, reflection Individual ribbons are from 1 to 2 µm wide and Top electrode from 25 to 100 µm long. Silicon Nitride

Beams down, diffraction

Silicon Substrate Silicon Dioxide

Substrate electrode

Cross section O-MEMS APS Short Course 3/11/01

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2

Grating Light Valve Projector STANFORD


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Micromachined Free-Space Micro-Optical Bench (FS-MOB) Active Laser Stripe

Semiconductor Edge-Emitting Laser Pre-aligned Micro-Fresnel Lens

3D Structures Pre-aligned to the Lens

Active Opto Devices

Micro-Actuators

Diffractive Optical Elements

Micromachined FS-MOB Refractive Optical Elements

M. C. Wu

Pre-Aligned and Monolithically Fabricated

Micro-Positioners

Integrated Photonic Laboratory

3

Out-of-Plane micro-Fresnel lens l

Low cost, batch processing technique to fabricate free-space micro-optical system.

l

All optical elements fabricated at the same time by photolithography.

→The optical system can be pre-aligned. l

Greatly reduce the size, weight, and volume of free-space micro-optical systems

Fabrication Processes :

PSG-2

Poly-1

Poly-2

PSG-1

Si or GaAs • Ref: Lin, Lee, Pister, and Wu, Electronics Letters, v.30, p.448, March 1994. M. C. Wu


Micro-Fresnel Lens with Integrated Scratch Drive Actuators (SDA) Micro-Fresnel Lens

Translation Stage

Scratch Drive Actuators (SDA)

Spring

M. C. Wu


4

Outline STANFORD

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Overview l l

l


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Advantages Fundamentals Optical Communications Network Switches Optical Bench Displays l Torsional Mirror l Grating Light Valve

Semiconductor Optical Device O-MEMS Tunable Laser l Tunable Detector l Modeling l

l l



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Need for Tunable Devices STANFORD

• Wavelength division multiplexing (WDM):

• •

•

• tunable vertical cavity lasers • tunable photodiodes • tunable phototransistors Optical Routing and Switching: • Beam steering Spectroscopy: • Gas sensing • Laser spectroscopy • Cavity ring-down spectroscopy • Intracavity laser absorption spectroscopy Adaptive Optics: • Tunable mirror arrays


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Ar Ion Pumped Ti:Sapphire Tunable Laser STANFORD


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Laser Background STANFORD

Mirror 1

Mirror 2

l

Laser: Gain + Feedback

L

Circ Wave

Inc

Gain Medium α(ω), g(ω)

(Mirrors)

Trans

Refl r1

Pump

r2

l

Oscillation/Lasing l

Gain > Loss (internal + mirror)

l

Resonance condition 2ωnL/c = 2πm or nL = mλ/2


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Laser Background STANFORD

∆ωcavity

∆ωax

l

Resonance c m nL

I circ I inc l

Axial Mode Spacing axial

ω l

, where m = 1, 2, ...

c nL

Cavity Finesse F

g rt 1 g rt

axial cavity


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Semiconductor Lasers: In-Plane vs. Vertical-Cavity STANFORD

In-Plane Edge-Emitting Laser

Vertical-Cavity Surface-Emitting Laser Top DBR

p AlGaAs cladding QW active region n AlGaAs cladding

GaAs substrate

GaAs substrate

p AlGaAs cladding + QW Active Region + n AlGaAs cladding Bottom DBR

l

Cavity length ~ 100-400 µm

l

Cavity length ( ) ~ 0.25 µm

l

High gain (2gL), low reflectance mirror is sufficient for lasing

l

Low gain (2gL), high reflectance mirror (> 99%) needed for lasing

l

Mirrors usually formed by cleaving ~ 30% reflectance

l

l

Small emission aperature produces highly astigmatic beam

Mirrors are made of 1/4 alternating low and high index materials such as: AlAs/GaAs

l

Circular beam


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Why Vertical Cavity Devices? STANFORD

Short Cavity

Cavity Mode

∆λaxial ~ a few Å

λ

gain/transmission

gain/transmission

Long Cavity

Cavity Mode

∆λaxial ~ 100 nm

λ

• Axial mode spacing is inversely proportional to cavity length: ∆ωax = πc/nL

• Wide axial mode spacing in vertical cavities possibility for broad and continuous wavelength tuning (∆λ < ∆λaxial, ~100 nm or ~10%)


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Wavelength Tuning in VCSELs STANFORD

R

I Mirror 1

r1

Ladj

Top Distributed Bragg Reflector QW active region λ Cavity Spacer

Pump

Circulating Wave Gain α(ω), g(ω)

Bottom DBR

GaAs substrate Conventional VCSEL nL = mλ/2

r2 T

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Mirror 2

Tunable VCSEL nL + Ladj = mλ/2


L Lcavity

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Hybrid Dielectric DBR Tunable VCSEL Structure STANFORD

Membrane Contact Pad

Spacer Layer Deformable Membrane Top Mirror

p-contact p-Contact Layer Quantum Well Active Region & Cavity Spacer

Air Gap

n-Distributed Bragg Reflector Anti-Reflective Coating

p-Contact Layer O-MEMS APS Short Course 3/11/01

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Dielectric Mirror Deposition & Current Aperture Formation STANFORD

l

Grow epitaxial layers using MBE

l

Deposit backside anti-reflective coating & ohmic contact Deposit Si3N4 mechanical layer & Si3N4/SiO2 dielectric DBR Pattern/Etch to high Al content AlGaAs current confinement layer Wet Thermal oxidation of AlGaAs to form current aperture

l

l

l


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Top Mirror Formation STANFORD

l

l

l

Pattern/Etch dielectric DBR to form central reflector region Evaporate/Liftoff Ti-Au adhesion layer

Evaporate/Liftoff Au mirror layer


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Intracavity Contact STANFORD

l

l

Plasma etch mechanical nitride layer to form membrane Recess etch Al0.85Ga0.15 As sacrificial layer

l

Wet Etch sacrificial layer for intracavity contact

l

Evaporate/Liftoff Ti-Au intracavity contact


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Membrane Release STANFORD

l

Pattern photoresist for membrane release

l

Wet Etch membrane release

l

Plasma removal of photoresist


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SEM Images of Tunable Structure STANFORD

• 20-40 µm diameter membrane central reflector

• Shadow indicates the full releasing of the membrane. 4.3 m

• 85 X 5 µm membrane legs Oxidized current aperture


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Semiconductor Coupled Cavity: 1.5 Pair Dielectric DBR STANFORD

Air Gap ~3/4 λ + δ GaAs 2λ Cavity Oxidized AlAs current aperture 2-60Å InGaAs QWs

Power (µW)

1500Å Au 1.5 pairs dielectric mirror λ/4 GaAs

22.5 Period GaAs/AlAs λ/4 DBR

5

5

4

4

3

3

2

2

1

1

0

Voltage (V)

Structure

0

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 Current (mA)

l

SiO2/Si3N4/SiO2 stress-matched 1.5 pair dielectric DBR

l

99.91% calculated top mirror reflectivity

l

Typical device Ith ~ 0.35-0.7mA, Quantum efficiency ~ 6.5%


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Semiconductor Coupled Cavity: 1.5 Pair Dielectric DBR STANFORD

0.5 RT 0.5mA CW

965

17.5V

0.3

Wavelength (nm)

Intensity (a.u.)

0.4

970

13.4V 9.4V 0 V 17.2V 15.4V 11.4V 5.4V 7.4V 16.4V

17.6V

17.7V

0.2 0.1

960

955 950

17.8V

0

9500

9550 9600 9650 Wavelength (Å)

9700

945

0

4

8 12 Voltage (V)

l

19.1 nm continuous tuning for 17.8 V membrane bias

l

24 dB mode suppression ratio


16

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VCSEL Wavelength Tuning Transient Response STANFORD

Wavelength switching from 977.6 nm to 977.0 nm < 1 sec rise time, ~ 2 sec settling time

l l

2 0 -2

Voltage (V)

Signal (a.u.)

Tuning Voltage

-4 -6

977.6 nm

-8 -10 -12

977.0 nm

-14 -4

-2

0 2 Time (µsec)


4

6 JSH-61

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Resonant Cavity Photodetection STANFORD

h

QW absorbing layers Circulating wave Bottom DBR (R2)

Quantum Efficiency ( )

Top DBR (R1)

p contact

1

0.8

R 1 = 0.8 R 2 = 0.99

R 1 = 0.4 0.6

0.4

R 1 = 0.8

0.2 R 2 = 0.9

n contact 0 0.01

l

0.1

1

10

Absorption ( d)

Quantum Efficiency max

R 1 = 0.2

 (1+ R2 e − d )  − d  = ) − d 2 (1− R1 )(1− e  e ) (1− R R   1 2 (Kishino, 1991)


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Tunable PIN Photodiode STANFORD

5 10 -6

• Reverse Bias The Tunable Laser (PIN diode) • Narrow Linewidth (below 4nm) • Tuning Range: 28.5 nm

Normalized Photoresponse (A.U.)

1.5 Volt 7.5 Volt 11.5 Volt

4 10 -6

16.5 Volt

3 10 -6

2 10 -6

1 10 -6

0 920

930

940

950

960

970

980

990

Wavelength (nm)


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Mirror Curvature STANFORD

MUMPS Poly2

l

2-D Interferometry

l

Optical far-field measurements

Static deformation 1.2 µm


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New AlOx/GaAs Top Membrane Structure STANFORD

GaAs/AlOx DBR Si3N4/Gold/Ti/GaAs Supporting Legs Silicon nitride Gold

n+ GaAs Substrate Tuning Contact

GaAs

DBR

Air Gap

• AlOx/GaAs: higher index contrast ratio, fewer DBR mirror pairs • Independent leg thickness enables stress and deflection optimization. O-MEMS APS Short Course 3/11/01

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One-Dimensional Model of Movable Membrane STANFORD

• One-Dimensional Model: Four springs with Hook’s constant keff • •

attached to central reflector The balance of forces: Electrostatic force contributed from central plate and four legs is equal to the effective spring force Describe the system accurately without loss of physical meaning


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Optical Properties of the cavity STANFORD

• Modeling of O-MEMS

• • •

devices is virtually nonexistant: serious design limitation Surface deformation scatters light out of the cavity Cavity loss increases linewidth Two top membrane designs compared using Fox & Li method


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Optical Field Distribution STANFORD

• Fox & Li method used to

•

Normalized Field Pattern (A.U.)

•

simulate a plane wave bouncing between two mirrors. Examples • Tilted but flat surface • Symmetrically bent surface Result: • Tilted: the mode shifts • Symmetrical: the mode

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1

-10

-5

0

5

10

Radial Distance (mm)

intensity decreases in center, but increases at edges--leads to multi-mode lasing O-MEMS APS Short Course 3/11/01

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Outline STANFORD

l

Overview l l

l


l

Advantages Fundamentals Optical Communications Network Switches Optical Bench Displays l Torsional Mirror l Grating Light Valve

Semiconductor Optical Device O-MEMS l l l

Tunable Laser Tunable Detector Modeling

MEMS Systems Examples l Summary l


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2D Scanning Microlens with Integrated Microactuators and Hybrid-Integrated VCSEL Integrated 3D Optics

“Raised” Microlens

Actuator Emitting Spot

VCSEL

Si Free-Space Micro-Optical Bench

M. C. Wu

Hybrid-Integrated Vertical Cavity Surface-Emitting Laser


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Integrated Michelson Interferometer Measure displacement and Movement direction of external Object with high resolution

Integrated Interferometer

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Displacement Sensor Performance

System resolution < 20nm

Micro Bio-Assay System STANFORD


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Micro Bio-Assay System STANFORD

Micro Fluidic Chamber

Micro Lens Array


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Cavity Ring Down Spectroscopy STANFORD

Absorption spectroscopy for detection of low levels of species

Tunable Laser

Coupling Optics Sample

Detector

Ring-down Cavity Iout

B A

A B

λ

Time O-MEMS APS Short Course 3/11/01

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O-MEMS is an Enabling Technology for the New Millennium STANFORD

Communications: Fiber Switches. Femtosecond Lasers, Pulse Shaping, Modulators, Tunable Optical Devices l Optical Interconnects l Optical Data Storage l Displays: Projection, Head Mounted - Virtual Reality l

Repro-graphics: Printing, Scanners l Adaptive Optics l Optical Transducers and Sensors l Optical Spectroscopy and Instrumentation l


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Where To Find Information? STANFORD

O-MEMS WWW Web Sites l

l

l

l

l

l

l

l

http://snowmass.stanford.edu/ Tunable Filters, Photodetectors and Lasers: http://www.htc.honeywell.com/cap/sensors/sensors.html fiber-optic and sensors http://www.sandia.gov/LabNews/LN09-15-95/microengine.html Microengines http://www.janet.ucla.edu/dmsr.index.html Micromachined optical bench http://eto.sysplan.com/ETO/MEMS/Prog_Summaries/steering.html DARPA ETO Electromagnetic/Optical Beam Steering program http://www.ti.com/dlp/docs/papers/mems/0memsab.htm TI's Digital Mirror Devices http://www1.psi.ch/www_f3b_hn/home.html Interferometer http://dmtwww.epfl.ch/ims/www.html MEMS WWW sites around the world


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