We present the design and the realization of a compact and robust imaging spectrometer in the mid-infrared spectral range. This camera combines a small static Fourier transform birefringent interferometer and a cooled miniaturized infrared camera in order to build a robust and compact instrument that can be embedded in an unmanned aerial vehicle for hyperspectral imaging applications. This instrument has been tested during a gas detection measurement campaign. First results are presented.

Quantification of gaseous emission fluxes from volcanoes can yield valuable insights on processes occurring in the Earth’s interior as part of hazard monitoring. It is also an important task in the framework of climate change, in order to refine estimates of natural emissions. Passive open-path UltraViolet (UV) scattered observation by UV camera allows the imaging of volcanic plumes and evaluation of sulfur dioxide (SO2) fluxes at high temporal resolution during daytime. Another technique of imaging is now available in the InfraRed (IR) spectral domain. Infrared hyperspectral imagers have the potential to overcome the boundary of daytime sampling of the UV, providing measurements also during the night and giving access simultaneously to additional relevant gas species. In this context the IMAGETNA campaign of measurements took place at Mt Etna (Italy) in June 2015. Three different IR imagers (commercial and under developments) were deployed, together with a Fourier Transform InfraRed spectrometer (FTIR) instrument, a UV camera, a Long Wavelength InfraRed (LWIR) camera and a radiometer. We present preliminary results obtained by the two IR cameras under development, and then the IR hyperspectral imager results, coming from full physics retrieval, are compared to those of the UV camera. The comparison points out an underestimation of the SO2 Slant Column Densities (SCD) of the UV camera by a factor of 3.6. The detailed study of the retrieved SO2 SCD highlights the promising application of IR imaging in volcanology for remotely volcanic plume gas measurements. It also provides a way to investigate uncertainties in the SO2 SCD imaging in the UV and the IR.

Birefringent interferometers are often used for compact static Fourier transform spectrometers. In such devices, several uniaxial birefringent parallel or prismatic plates are stacked, with their optical axes set so that there is an efficient coupling from ordinary to extraordinary and extraordinary to ordinary eigenmodes of two successive plates. Such coupling, aside from few particular cases, is however not perfect, an effect that may adversely affect performance. In order to help the design and the tolerancing of these interferometers, we have developed a numerical modeling, based on the propagation of plane waves inside and through the interface of birefringent media. This tool evaluates the traveled optical path length and the amplitude of the different polarization modes, enabling to predict both the optical path differences on the interferometer outputs and the unwanted coupling strengths and related stray wave amplitudes. The tool behavior is illustrated on Savart and Double-Wollaston interferometers, and compared with experimental characterization of a calcite Double-Wollaston prism. OCIS codes: (260.1440) Birefringence; (120.6200) Spectrometers and spectroscopic instrumentation; (110.4234) Multispectral and hyperspectral imaging; (120.0280) Remote sensing and sensors; (120.4570) Optical design of instruments; (300.6300) Spectroscopy, Fourier transforms; (220.4830) Systems design.