Concept and demonstration for a method of integrating depolarization in a slit homogenizer for spectrometry applications

Abstract

The accuracy of satellite remote sensing of trace gases by imaging spectrometers depends highly on the uniformity of the instrument spectral response function. Studies have shown that scene inhomogeneity across the spectrometer’s slit width (spectral direction) can cause errors in the measured spectral radiances, leading to trace gas retrieval inaccuracy. One mitigation approach recent imaging spectrometers use is a slit homogenizer to redistribute scene radiance within the slit. This on-board hardware device functions like a slab waveguide, with rays making multiple bounces between narrowly spaced, highly reflective, plane-parallel mirrors. This presents a challenge as any difference in s- and p-polarization reflectance for the mirror surfaces tends to multiply with each bounce, producing a net linear polarization sensitivity (LPS) in the system’s throughput which also results in retrieval error. Our solution is a slit homogenizer design that mitigates for LPS by employing total internal reflection (TIR) and a birefringent internal medium. TIR ensures high and equal reflectance while a birefringent material such as sapphire, with an appropriately oriented optic axis, provides high-order retardance between bounces. This introduces polarization scrambling in the manner of a Lyot-depolarizer. We provide a basic analysis of the device’s geometrical optics, detailing the crystalline optic axis orientation for the device cut from a sapphire boule and readily-available R-cut material. Preliminary lab testing was performed on three mirror-pair and two sapphire plate homogenizers at multiple visible wavelengths. The results show that our sapphire plate slit homogenizers decrease LPS by at least an order of magnitude compared to the mirror-based ones.