Low Polarization Optical System Design

Anna-Britt Mahler

Polarization Laboratory Group

College of Optical Sciences

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

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

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

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

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

MSPI mechanical assembly

Removeable mountfor mirror 2 adjustment

Entrance aperturebaffles

Focal planearray mount

Mirror 3fixed

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

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

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

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

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

Next step

Design new coatings with revised refractive indices

Understand system polarization by raytracing Measure next round of samples from vendor

Acknowledgements

This research is supported by

NASA – Jet Propulsion Laboratory