Temperature-compensated fiber optic hydrogen sensing with an in-fiber Fabry-Perot etalon and a Pd:Ni-coated cavity

This study presents an in-situ temperature-decoupled sensor platform utilizing an in-fiber Fabry-Perot (FP) etalon made of two fiber Bragg gratings (FBGs) for simultaneous hydrogen concentration and temperature sensing. The platform’s key feature is a palladium-alloy-coated fiber section between the FBGs, while the FBGs themselves remain without coating. This design shifts the FP transmission fringes due to hydrogen-induced strain and temperature. In contrast, the envelope function induced by the FBG spectra shifts only with temperature, enabling intra-spectral referencing of the fringe wavelength to the envelope vertex wavelength and thereby compensating for temperature effects. This work discusses a fully zero-point-referenced sensor calibration for simultaneous hydrogen and temperature monitoring. Hydrogen calibration demonstrated precise measurement of concentrations ranging from 500 to 20,000 ppm(H 2 ) in a temperature range of 20 ∘ C to 50 ∘ C. Utilizing an iterative matrix algorithm, the non-linear hydrogen response was linearized and temperature cross-sensitivity was almost completely decoupled, leading to the representation of hydrogen concentrations with standard errors of the mean of 4.3% at repeated 1,000 ppm cycles, 2.8% at repeated 10,000 ppm cycles, and temperature measurement uncertainties of ±0.2 ∘ C. This proof-of-concept demonstrates the detection of low hydrogen quantities without distraction from temperature and thereby opens the path for fiber optic hydrogen sensing applications outside laboratories.