Title: Advances in Photoelastic Modulator-Based Polarimetric Imaging
Presenting Author: Russell Chipman
Organization: University of Arizona
Co-Author(s): Bruce Hancock, Ab Davis, Jeff Beck, David Diner

Abstract:
Under the ESTO Instrument Incubator Program (IIP), we have built three high-accuracy polarimeters for the study of aerosol and cloud microphysics. The Ground-based and first-generation Airborne Multiangle SpectroPolarimetric Imagers (GroundMSPI, AirMSPI-1) operate in the ultraviolet, visible, and near-infrared, while the second-generation airborne sensor, AirMSPI-2, extends the measurements into the shortwave infrared. Key technologies, including a variable retarder (consisting of dual photoelastic modulators and broadband achromatic quarter waveplates), and specialized focal planes (containing high-speed readout integrated circuits and miniaturized spectropolarimetric stripe filters) were developed and matured to Technology Readiness Level 6. A satellite instrument based on these technologies was recently selected for flight as part of NASA's Earth Venture Instrument (EVI-3) program. The Multi-Angle Imager for Aerosols (MAIA) is designed to target major cities around the world for the purpose of associating different types of airborne particles with adverse health impacts. MAIA will use the power of multiangle, multispectral, and polarimetric imaging to determine the abundance and properties of near-surface aerosols. Birth, death, and hospital records will be used to establish linkages to disease. Further development of PEM-based polarimetry is currently in progress under ESTO's Advanced Component Technology (ACT) program. While the MSPI/MAIA approach uses two 42 kHz photoelastic modulators (PEMs) with a difference frequency of 25 Hz to generate a low-frequency beat modulation, the approach being studied in the laboratory under the ACT program uses a single PEM coupled with avalanche photodiodes to demodulate the high-frequency signals over a broad spectral range. In contrast to the dual-PEM approach, which requires two detector channels to measure the Stokes parameters I, Q, and U, the single-PEM system only requires a single detector channel to obtain all three variables. Another key benefit of the single-PEM approach is reduced sensitivity to random noise.