Demonstration of LWIR channeled spectro-polarimeter (Conference Presentation)

Abstract

A compact long wave infrared (LWIR) channeled spectro-polarimeter (IRCSP) has been developed for integration into the ESTO-funded Submm-Wave and LWIR Polarimeters (SWIRP) project to measure the microphysical properties of cloud ice. The IRCSP rotates incident linearly polarized light using the combination of a quarter waveplate with a fast axis at 45◦ and a thick birefringent crystal; the output polarization state’s orientation is then a function of wavelength. To modulate and then measure the rotated light, a subsequent wiregrid linear polarizer tilted at 20◦ generates two output paths with opposite polarities in reflection and transmission to enable joint radiometric and polarimetric measurement and correct for atmospheric attenuation. The two symmetric optical paths following the linear polarizer each consist of a diffraction grating and uncooled microbolometer to simultaneously measure the resulting intensity fringes. Angle and degree of linear polarization (AOLP, DOLP) are retrieved across 8.5-12.5 µm with 1 µm resolution using Fourier decomposition of the modulated spectrum. The IRCSP will not only measure H-V variance but will produce the first full linear Stokes measurements (I, Q, and U) of upper-tropospheric cirrus clouds in the LWIR. Following thermal and polarimetric calibration, the polarimeter is expected to achieve 0.5% DOLP accuracy over 90% of the spectral band.