Absolute thermometry based on Brillouin scattering in gases.

We propose a novel thermometric technique for measuring absolute temperature in gas media based on Brillouin scattering. The method retrieves the temperature from the acoustic velocity of the gas, inferred through the spectral shift experienced by a scattered laser beam during the Brillouin acousto-optic interaction. This approach is inherently contactless, enabling remote sensing applications with high precision. It also exhibits enhanced sensitivity in the cryogenic range and is fully compatible with distributed measurements along recent hollow-core single-mode fibres. This study establishes the theoretical foundations of the technique and provides experimental validation across a range of temperature and pressure conditions. The influence of the gas species on the Brillouin response is analysed, enabling the selection of the optimal gas medium for specific applications. Illustrative distributed measurements demonstrate the strong potential of this technique for cryogenic sensing, where favourable scaling of several parameters leads to significantly improved temperature sensitivity. These results open new avenues for high-accuracy, remote, and minimally invasive thermometric measurements across a wide temperature range, including extreme cryogenic environments.