Absolute thermometry based on Brillouin scattering in gases

Reliable temperature measurement is indispensable in science, engineering, and environmental monitoring. However, many traditional sensors, including contact probes, pyrometers, or fibre optic devices, face challenges in cryogenic temperatures or situations where contact must be minimised. Existing optical-fibre sensors encounter reduced sensitivity and require complex calibration, particularly below 100 K, making precise measurements extremely challenging.

 

In a breakthrough published in Light: Science & Applications, researchers from EPFL (Switzerland) and Universidad Técnica Federico Santa María (Chile) demonstrate a novel thermometric method based on stimulated Brillouin scattering (SBS) in gases, using hollow-core optical fibres (HCFs) as light–gas interaction platforms. Unlike silica fibres, whose Brillouin properties depend on dopants, residual stress, and temperature-dependent behaviour, gases offer predictable, calibration-free thermodynamic properties, enabling direct retrieval of temperature from the Brillouin frequency shift (BFS).

 

The method exploits the fact that the BFS in gases depends solely on the acoustic velocity, which is governed by fundamental gas laws. This makes the technique inherently absolute, requiring no reference measurement, no pre-calibration, and no material-dependent correction factors. The BFS was shown to follow theoretical predictions extremely well for gases including nitrogen, neon, and argon, even near liquefaction temperatures where non-ideal gas behaviour emerges.

 

The team also identifies optimal gases for different situations, including ambient temperature and cryogenic thermometry. While argon is the best medium for room temperature measurements, neon is particularly suitable for temperatures near 77 K, and helium enables measurements approaching 4 K, although its permeability through silica requires metallic encapsulation. It is worth mentioning that the sensitivity of gas-based Brillouin thermometry increases sharply as temperature decreases, outperforming silica-fibre sensors, whose sensitivity drops to nearly zero around 80 K.

 

A major milestone of the study is the first demonstration of distributed Brillouin temperature sensing in gases at cryogenic temperatures. Using a time-domain Brillouin echoes technique along an 18.5-m gas-filled HCF, the researchers reconstructed spatial temperature profiles with sub-kelvin accuracy and spatial resolutions down to 45 cm. The technique directly mapped BFS to temperature using gas-law models, entirely without empirical calibration, highlighting a decisive advantage over existing fibre-optic Brillouin sensors. Experiments achieved temperature uncertainties as low as 0.050 K in cryogenic distributed sensing.

 

Beyond fibre-integrated sensing, the authors emphasize that free-space Brillouin scattering could extend this method to fully contactless thermometry, ideal for delicate cryogenic systems where minimal thermal perturbation is essential. Since each scattered photon generates only a single phonon, the heat deposited in the Brillouin process is estimated to remain below 1 nW, far lower than in existing thermometry techniques. This opens new possibilities for precision measurements in superconductivity, quantum technologies, cryo-biology, and space instrumentation.

 

“The inherent predictability of gas thermodynamics makes Brillouin scattering an ideal foundation for a new class of absolute temperature sensors,” the scientists note. This innovation opens new pathways for high-accuracy, minimally invasive, and remote thermometry across fields requiring precise thermal monitoring - from fundamental physics to advanced cryogenics.

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