Metasurface-enabled small-satellite polarisation imaging

TL;DR

This paper introduces a lightweight, compact metasurface-based polarisation imaging system for small satellites, enabling full-Stokes polarisation imaging in a single shot, which enhances remote Earth sensing capabilities within size and weight constraints.

Contribution

The paper presents a novel ultra-thin, topology-optimised metasurface design that encodes polarisation states in diffraction orders, allowing full-Stokes polarisation imaging with small satellite optics.

Findings

Design successfully encodes polarisation in five diffraction orders

Enables single-shot full-Stokes polarisation imaging

Maintains high resolution with stitched images over Earth's surface

Abstract

Polarisation imaging is used to distinguish objects and surface characteristics that are otherwise not visible with black-and-white or colour imaging. Full-Stokes polarisation imaging allows complex image processing like water glint filtering, which is particularly useful for remote Earth observations. The relatively low cost of small-satellites makes their use in remote sensing more accessible. However, their size and weight limitations cannot accommodate the bulky conventional optics needed for full-Stokes polarisation imaging. We present the modelling of an ultra-thin topology-optimised diffractive metasurface that encodes polarisation states in five different diffraction orders. Positioning the metasurface in a telescope's pupil plane allows the diffraction orders to be imaged onto a single detector, resulting in the capability to perform single-shot full-Stokes polarisation imaging of the Earth's surface. The five rectangular image swaths are designed to use the full width of the camera, and then each successive frame can be stitched together as the satellite moves over the Earth's surface, restoring the full field of view achievable with any chosen camera without comprising the on-ground resolution. Each set of four out of the five orders enables the reconstruction of the full polarisation state, and their simultaneous reconstructions allow for error monitoring. The lightweight design and compact footprint of the polarisation imaging optical system achievable with a metasurface is a novel approach to increase the functionality of small satellites while working within their weight and volume constraints.