1. Low Polarization Optical System Design Anna-Britt Mahler Polarization Laboratory Group College of Optical Sciences

2. 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

3. 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

4. What is meant by low polarization? Low diattenuation: Polarization-dependent transmission or reflection Low retardance: Difference in phase shift for different eigenpolarizations Reflectance Absolute Phase on Reflection

5. 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

6. MSPI camera design Spherical primary mirror Ellipsoidal secondary mirror Ellipsoidal tertiary mirror Telecentric In image space Goal is to measure very low degrees of linear polarization in order to better characterize atmospheric aerosols 3 - mirror off-axis telescope

7. MSPI mechanical assembly Removeable mount for mirror 2 adjustment Entrance aperture baffles Focal plane array mount Mirror 3 fixed

8. Low diattenuation coating modeling Low diattenuation coating Diattenuation compensating coatings RsRs RpRp (  m) 1 2 3 Combination Measurement at 14 degrees

9. 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

10. Diattenuation pupil map* On-axis field has a max D of 0.19% Off-axis field has a max D of 0.33% * Obtained with Code V POLDSP macro

11. 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

12. 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 60  Reflectance Modeled Measured Optimized 60  Diattenuation Modeled Measured Optimized

13. Next step Design new coatings with revised refractive indices Understand system polarization by raytracing Measure next round of samples from vendor

14. Acknowledgements This research is supported by NASA – Jet Propulsion Laboratory