Imaging trace gas concentrations with a compact snapshot device that converts their spectral features into a polarization signal

While the concentrations of trace gases in Earth’s atmosphere were roughly constant for thousands of years, human activities such as the burning of fossil fuels have caused significant changes ever since the industrial revolution. The remote sensing of trace gases has proved to be a useful technique for mapping and monitoring on the global scale. We introduce a novel snapshot imaging instrument concept for measuring trace gases that is compact and lightweight compared to current state-of-the-art instruments. It uses polarization optics to create a spectral filter that turns the molecular absorption strength of a gas into a linear polarization signal. By measuring this signal with a polarization camera based on a beamsplitter or a micropolarizer array, the linear fractional polarization is retrieved instead of a full spectrum for every spatial resolution element which greatly reduces the sampling requirements for such an instrument. The combination of a linear polarizer, multiple-order retarder and quarter-wave plate make it possible to measure the complete fractional linear polarization, which is linearly related to the absorption strength of the trace gas in the instrument’s field of view. While only 2 channels (in/out of phase) are necessary to get a valid gas measurement, a polarization camera based on a pixelated micropolarizer array measures in quadrature, making this instrument robust in case of spectral filter instability issues. In this study, we present simulations and performance optimizations of the instrument concept as well as plans for laboratory measurements that demonstrate its functionality. Simulations based on NO₂ at 400 - 500 nm demonstrate proof of principle, and constitute the basis of an optimization of the spectral filter consisting of a custom design for measuring NO₂ in a gas cell by including custom band-pass filter and multiple-order retarder. We are constructing the first prototype called GasCam using off-the-shelf components for laboratory measurements. Both the simulations and laboratory measurements are fundamental steps to the realization of a ground- and/or space-based instrument network in the future.