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The Optics ofGrating LightValvesFYS4230

Hkon Sagberg, SINTEF

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The Optics of Grating Light Valves

Introduction to Grating Light Valves

Crash course in optical diffraction theory

The grating light valve (GLV) display

Grating light valves for other applications

Spectroscopy

Displacement sensing

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Grating lightvalves (GLV)

Deformable MirrorDevice (DMD)

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Two methods for steering light

Deflection Diffraction

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The grating light valve (GLV)

Grating Light Valve

Electrostatically deflect ribbonsDistance to wafer /4Light is reflected or diffractedDiffracted light is projected to screen

Possible to use pull-in or pull-controlPossible to have one row of ribbon pixels only

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Diffraction from a grating light valve

d

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GLV used in a display device

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Diffraction of LightMaxwells equations may be reduced to thescalar wave equation.

U may be any component ofvectors H and E

Assumptions: Linear, isotropic, homegeneous and non-dispersive

2

2

2

22

tU

cnU

=

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Diffraction of light

Example solutions of the wave equation arethe plane wave and spherical wave:

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Diffraction of lightLight bends or diffracts around edges. Small apertures give large diffraction angles

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Diffraction of lightLight bends or diffracts around edges. Small apertures give large diffraction angles

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Diffraction of lightLight bends or diffracts around edges. Small apertures give large diffraction angles

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Diffraction of lightHere, edge effects are small

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Diffraction of lightWhen edges are thick and holes are small, scalar diffraction theoryis no longer valid

???????

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The Huygens-Fresnel principle says:The light disturbance at a point P arises from thesuperposition of secondary waves that proceed from a surface situated between this point and the lightsource.

Christiaan Huygens Augustin-JeanFresnel

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Diffraction of Light

A mathematical representation of the Huygens-Fresnel principle:

Spherical wave

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Diffraction of Light

Far away from the aperture, and for small diffraction angles we may use the Fraunhoferapproximation.

= S xxikxUCU d)exp()()( x

S

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Diffraction of Light

Far away from the aperture, and for small diffraction angles we may use the Fraunhoferapproximation.

= S xxikxUCU d)exp()()( x

S

Fourier transform!

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Diffraction of Light

22

2sin

2

=

=

k

ak

ak

I

We may now calculate the diffraction from a slit:

a Minima for:

am =

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Babinets principleThe diffracted field from an aperture is the same as from an obscuration of the same sizeand shape

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Diffraction from a grating light valve

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The grating equation

d

dm =sin

Phase difference = an integer numberof wavelengths

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Diffraction from a grating light valveh

d

)(sin4 22

2

00

khUU

II

==

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Diffraction from a grating light valve

Angle given by the grating equation

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Diffraction from a grating light valve

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How to design a GLV display device

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Different pixels for different wavelengths

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How to design a GLV display device

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Grating light valves

Can you spot thedifference betweenthe top and bottomGLV?

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The clamped-clamped beam withdistributed load

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1-0.35

-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

0

dxxkhLI

I= )]([sin

4 22

0

h(x)

Must integratediffraction efficiencyover the length ofthe beam:

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How to design a GLV display device:

High optical throughput requires large diffraction angle and short grating periodsThe shorter the period the more light is lost in the gaps between the ribbonsClamped-clamped beams/ribbons result in low fill factor

Only a fraction of the beam is optically useful

Different wavelengths require different modulation heights

Challenges

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Grating light valves for spectroscopy

PolychromatorSINTEF CDOE

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Polychromix/Senturia

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Polychromix (Senturia)

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SINTEF CDOE

Start with BSOI wafer with /4 (500nm) thickburied oxide layerReactive Ion Etch (RIE) of the diffraction gratingAluminum sputtering and annealingAluminum etch and DRIE (Bosch process) ofdevice layerOxide etch (release) using vapor phase HF

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SEM images of the fabricated device

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This animation shows the principle of a first-order grating light valve. Only a narrow exit angle is considered.

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Our aim is to measure gas concentration using a micromechanical infrared filter

OpticalMEMS device

The figure shows an old, established infraredsensing method for measuring gas concentration

Image taken from Moseley et al, Techniques and mechanisms

in gas sensing

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2150 2200 2250 2300 2350 2400 2450 2500 2550

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1000 ppm karbondiosyd og vann, 50 cm

Tran

smis

jon

Blgetall [cm-1]

Infrared transmittance spectrum of CO2

Reference measurements must be made in one or more wavelength bands outside the absorbing region

Example: Carbon dioxide absorptionSingle detectorAlternating filter

Time

Det

ecto

rsig

nal

Abs. meas.

Ref. meas.

No Gas present

Gas present

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Design concept: Optical filteringwith Modulated diffraction gratings

Shine white light (broadband IR) onto a diffraction gratingCollect the light diffracted into a chosen angleChange the color and intensity of thelight by electromechanicalmodifications of the grating shape

Design objective:Electromechanically simple No position feedbackNo calibrationNo driftRobustLow-cost

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When used with wider grating elements, for diffraction order M=6, theneighboring diffraction orders come closer...

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Actuation characteristic

Bistable operation withsnap-down at 4.2VHysteresis due to pull-in effect

Deflection vs. voltage

-500

-400

-300

-200

-100

00 1 2 3 4 5

Voltage (V)

Defle

ctio

n (n

m)

Decreasing VIncreasing V

Idle (0V)

Fully actuated (5V)

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0V

5V

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Infrared spectral characterization

MOEMS Device IR

SourceFiber to FTIR spectrometer

Actuation voltage

1.7 1.8 1.9 2 2.1 2.2 2.3 03

3.33.5

3.73.9

4.14.3

4.54.7

10.120.1

Actuation voltage in V

Wavelength in m

Ligh

t int

ensi

ty o

n de

tect

or

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Grating light valves for displacementsensing (F.L. Degertekin)

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Grating light valves for displacementsensing (F.L. Degertekin)

The Optics of Grating Light ValvesTwo methods for steering lightThe grating light valve (GLV)Diffraction from a grating light valveGLV used in a display deviceDiffraction of LightDiffraction of lightDiffraction of lightDiffraction of lightDiffraction of lightDiffraction of lightThe Huygens-Fresnel principle says:The light disturbance at a point P arises from the superposition of secondary waves that Diffraction of LightDiffraction of LightDiffraction of LightDiffraction of LightBabinets principleDiffraction from a grating light valveThe grating equationDiffraction from a grating light valveDiffraction from a grating light valveDiffraction from a grating light valveHow to design a GLV display deviceDifferent pixels for different wavelengthsHow to design a GLV display deviceGrating light valvesThe clamped-clamped beam with distributed loadHow to design a GLV display device:Grating light valves for spectroscopyPolychromix/SenturiaPolychromix (Senturia)SINTEF CDOESEM images of the fabricated deviceOur aim is to measure gas concentration using a micromechanical infrared filterReference measurements must be made in one or more wavelength bands outside the absorbing regionDesign concept: Optical filteringwith Modulated diffraction gratings Actuation characteristicInfrared spectral characterizationGrating light valves for displacement sensing (F.L. Degertekin)Grating light valves for displacement sensing (F.L. Degertekin)