Title: Technology Advances in Photoelastic Modulator-Based Polarimetric Imaging
Presenting Author: David J Diner
Organization: Jet Propulsion Laboratory

Abstract:
Our Multiangle SpectroPolarimetric Imager (MSPI) technology development is aimed at advancing our understanding of the climate and air quality impacts of airborne particles. Measurement requirements established for the Decadal Survey ACE mission include  0.005 uncertainty in degree of linear polarization (DOLP) in multiple spectral bands. Our approach to achieving this objective is rapid rotation of the plane of linear polarization using photoelastic modulators (PEMs) along with synchronous detection of the modulated signal. Two PEMs with slightly different frequencies are employed in series to generate a beat signal, and a custom readout integrated circuit provides synchronous detection. Our first generation airborne instrument, AirMSPI, contains an ultraviolet-visible-near infrared pushbroom camera that measures polarization at 470, 660, and 865 nm. AirMSPI has participated in several NASA field campaigns since it became operational in 2010. Our second generation AirMSPI-2 instrument extends the approach into the shortwave infrared. The dual PEM approach is very effective, but has the following limitations: noise sensitivity of the beat signal limits the polarimetric measurements to the visible, near-IR, and SWIR; measurement of the Stokes parameters Q and U requires separate detector rows; and practical considerations limit the polarimetry to a subset of all spectral bands in the camera. These limitations can be overcome by demodulating the signal from just a single PEM. We are now exploring UV-MWIR polarimetry, capitalizing on the gain and photon counting capabilities of avalanche photodiode detectors (APDs). This concept offers the means to recover the Stokes parameters in all spectral bands, and provides simultaneous measurements of intensity, Q, and U in each pixel.