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Diffraction of lightWhen edges are thick and holes are small, scalar diffraction theoryis no longer valid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