low polarization optical system design anna-britt mahler polarization laboratory group college of...
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Low Polarization Optical System Design
Anna-Britt Mahler
Polarization Laboratory Group
College of Optical Sciences
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Why do we care about polarization?
Systems sensitive to instrumental polarization include…
Interferometers Polarimeters and ellipsometers Systems with large angles of incidence Remote sensing radiometers and spectrometers Liquid crystal displays and projectors Monochromators Fold mirrors
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Examples of polarization-sensitive space-based satellite systems MODIS (MODerate Resolution Imaging
Spectroradiometer) Designed for diattenuation < 2% for all reflective solar
bands POLDER (Polarization and Directionality of Earth’s
Reflectances) Designed for diattenuation < 2%
MSPI (Multiangle SpectroPolarimetric Imager) Designed for diattenuation < 0.1% on the ground and
diattenuation < 0. 25% in space
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What is meant by low polarization? Low diattenuation:
Polarization-dependent transmission or reflection
Low retardance: Difference in phase shift for different eigenpolarizations
s p
s p
R R
R R
D
20 40 60 80Angle of Incidence
25
50
75
100
125
150
175
esahP
s
pLinear Retardance
75 50 25 0 25 50 75Angle of Incidence
0.825
0.85
0.875
0.9
0.925
0.95
0.975
1
%ecnatcelfeR
s
p
Linear Diattenuation
Reflectance Absolute Phase on Reflection
s p
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Low polarization design
Polarization is introduced by Surface geometry Materials
Model elements and coatings Polarization introduced by individual elements End-to-end optical system analysis
Measure elements and coatings during the design phase
Obtain samples from vendors Mueller matrix imaging polarimeter
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MSPI camera design
Spherical primary mirror
Ellipsoidal secondary mirror
Ellipsoidal tertiary mirror
TelecentricIn imagespace
Goal is to measure very low degrees of linear polarization in order to better characterize atmospheric aerosols
3 - mirror off-axis telescope
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MSPI mechanical assembly
Removeable mountfor mirror 2 adjustment
Entrance aperturebaffles
Focal planearray mount
Mirror 3fixed
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Low diattenuation coating modeling Low diattenuation coating Diattenuation compensating
coatings
0.45 0.5 0.55 0.6 0.65 0.7
0.58
0.62
0.64
0.66
0.68
Rp
0.45 0.5 0.55 0.6 0.65 0.7
-0.002
0.002
0.004
0.006
Diattenuation
Rs Rp
(m)
(m)
0.5 0.75 1.25 1.5 1.75 2
-0.015
-0.01
-0.005
0.005
0.01Diattenuation
1
2
3 Combination
Measurement at 14 degrees
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End-to-end polarization aberration analysis
Model considers a few chief rays experienced by various fields
Real raytracing allows accurate polarization aberration characterization across the pupil
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Diattenuation pupil map*
On-axis field has a max D of 0.19%
20:12:14
aspc29k
Diattenuation Pupil Map
AM 30-Jan-07
Field 1Zoom Pos 1Wvl num 1
0.0053 Diattenua
-1 0 1
X-AXIS
-1
0
1
Y-AXIS
20:12:18
aspc29k
Diattenuation Pupil Map
AM 30-Jan-07
Field 2Zoom Pos 1Wvl num 1
0.19 Diattenua
-1 0 1
X-AXIS
-1
0
1
Y-AXIS
Off-axis field has a max D of 0.33%
* Obtained with Code V POLDSP macro
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Low diattenuation coating measurements Obtained coating
samples from vendor Mueller matrix imaging
polarimeter absolute reflectance polarization properties
Mueller matrix image of low-polarization microscope objectives NA = 0.55 D < 8% at edge of pupil
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Low diattenuation coating measurements Compared model to
measurement Optimized the modeled
refractive indices (n and k) and thicknesses to match measured
Currently optimizing coatings for low diattenuation using new indices and thicknesses
0.45 0.5 0.55 0.6 0.65 0.7
0.65
0.7
0.75
0.8
0.85
Measured vs Model Data60 Reflectance
Modeled
Measured
Optimized
0.45 0.5 0.55 0.6 0.65 0.7
-0.25
-0.2
-0.15
-0.1
-0.05
0.05
Measured vs Model Data60 DiattenuationModeled
Measured
Optimized
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Next step
Design new coatings with revised refractive indices
Understand system polarization by raytracing Measure next round of samples from vendor
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Acknowledgements
This research is supported by
NASA – Jet Propulsion Laboratory