Interferometric gas sensor

A detector system for spatially resolved detection of a gas substance in an area, the detector system comprising:

• a telescope providing an optical beam corresponding to the area;
• at least one detector comprising an image sensor;
• a band filter arranged in a beam path of the optical beam before the at least one detector, the band filter being arranged for transferring a wavelength spectrum of interest of the optical beam including an absorption wavelength corresponding to the gas substance;
• a polarizing beam splitter;
• and
• at least one interferometric stage, wherein each interferometric stage of the at least one interferometric stage comprises a retarder for creating an optical path difference for measuring absorption dips due to presence of a gas of interest in the area,
• wherein the telescope is arranged to image the area onto the at least one detector,
• wherein the polarizing beam splitter is arranged to produce polarized beams, of the optical beam, of mutual orthogonal polarization in respective branches, wherein each respective branch comprises a mutually orthogonally polarized one of the polarized beams;
• wherein the at least one detector is provided in at least one of the respective branches, and
• wherein the at least one interferometric stage is arranged for receiving one of the polarized beams to produce an interferometric image on the at least one detector;
• wherein the retarder comprises multiple birefringent media having mutually different birefringency, and
• wherein each one of the multiple birefringent media has an optical axis that is not aligned with a received one of the polarized beams in order to create polarized beam components traveling along the respective optical axes of the birefringent media having the optical path difference,
• wherein the multiple birefringent media are arranged with optical axes relative to each other so that:
• at least one increases an optical path difference between the polarized beam components, and
• at least one decreases the optical path difference between the polarized beam components;
• wherein thicknesses of the multiple birefringent media are tuned to minimize a focal shift between the polarized beam components.