UV-SWIR Achromatic Quarter Wave Retarder for the Multiangle Spectropolarimetric Imager (MSPI)

We describe the design, fabrication, and testing of an achromatic, athermalized quarterwave retarder, a key component for the Multiangle Spectro-Polarimetric Imager (MSPI), which is a candidate for the Aerosol-Cloud-Ecosystem (ACE) mission. The desired wavelength range of the instrument encompasses the UV, visible, near IR, and shortwave IR, extending from 355 nm to 2130 nm, with polarimetric channels in the selected bands longward of 410 nm. Off-the-shelf commercial components are not suitable for the instrument’s quarterwave retarders, and a design for this application is challenging. Our three-element multi-crystal (sapphire:MgF2:quartz) approach achieves targeted retardance of 90°±10° when averaged over any bandpass in the 410-2130 nm range. The athermalization achieves a retardance change of less than 0.1° per 1° C temperature change. A novel manufacturing method realizes tight retardance tolerances even though the crystal birefringence knowledge and plate thickness tolerances are individually inadequate. Testing of the manufactured parts will be performed with Mueller Matrix Imaging Polarimeters (MMIP) at the University of Arizona. Two MSPI camera prototypes currently operate within the UV/Vis/NIR. The ground-based camera (GroundMSPI) operates at the University of Arizona. AirMSPI, the most recent MSPI prototype, operates out of JPL and has flown successful checkout flights on the NASA ER-2. AirMSPI includes an achromatic, athermalized quarterwave retarder optimized for 470, 660, and 865 nm, and demonstrates the effectiveness of the multi-crystal approach that will be extended longward into the SWIR and to shorter wavelengths in the blue. Multispectral polarimetric images of sky, clouds, ground scenes, and man-made objects will be shown over varying angles of solar illumination to illustrate the capabilities of the MSPI imaging approach.