LWIR Spectro-Polarimeter for Cloud-Induced Polarization Measurements

A long wave infrared (LWIR) channeled spectro-polarimeter (IRCSP) is in development for integration into the ESTO funded Submm-Wave and LWIR Polarimeters (SWIRP) project to measure the microphysical properties of cloud ice. The IRCSP channels will extend the sensitivity of submm-wave sensors to measure upper-tropospheric cirrus that have low ice water content (IWC) with a preferred crystal shape and orientation. The channeled spectro-polarimeter concept utilizes a quarter wave plate, high order birefringent crystal, linear polarizer, and diffraction grating to encode wavelength dependent polarimetric information as an intensity fringes at the detector. A Mueller matrix model in conjunction with Fourier decomposition is then implemented in a data reduction procedure to retrieve the angle and degree of linear polarization (AOLP, DOLP) as a function of wavelength. The conical scanning configuration of SWIRP preserves both the horizonal (H) and vertical (V) polarization information across 8.5-12.5 microns with 1 µm resolution. Since both the modulated H and V channels are measured at the detector, the system is able to measure the full set of linear stokes parameters (I,Q,U) and correct for atmospheric attenuation. Recent advances in uncooled microbolometer thermal imagers has enabled a compact instrument configuration with a length of less than 10 cm, as well as excellent thermal resolution with a camera noise equivalent temperature difference (NEDT) of < 40 mK. The IRCSP and data reduction procedure produces a 0.15% accuracy in the DOLP measurement. Following thermal calibration and tolerancing, the polarimeter is expected to achieve 0.5% DOLP accuracy over 90% of the spectral band. In conjunction with the proposed submm-wave polarimeter, the LWIR polarimeter will provide the critical information needed for better understanding cloud ice microphysics and their interactions with radiation, dynamics and precipitation, as highlighted in the 2017 Earth Science Decadal Survey.