Linearized Temperature-Compensated Hydrogen Sensing With Partially Pd-Alloy-Coated π-FBGs

Intraspectral referencing comprises a single-sensor solution, where simultaneous measurements of hydrogen concentration and temperature are achieved within a single, partially palladium-coated <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pi $ </tex-math></inline-formula>-phase-shifted fiber Bragg grating (FBG). Highly localized, hydrogen-induced strain sections provoke the notch and the envelope of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pi $ </tex-math></inline-formula>-FBG spectrum to yield different hydrogen responses, while temperature affects both features almost identically. The precise temperature compensation in sub-Kelvin ranges was used to characterize the functional layer in the range of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$20~^{\circ }$ </tex-math></inline-formula>C–<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$50~^{\circ }$ </tex-math></inline-formula>C and at 250–20 000 ppm H2 in N2. Upon initial hydrogen exposure after fabrication, a gradually decreasing sensitivity drift was observed, which relaxed and stabilized after sufficiently long initialization with hydrogen. The desorption consisted of at least two processes, a fast desorption when large amounts of hydrogen are present in the nanofilm, followed by a notably slow desorption (~5 h) until the baseline was recovered. A zero-point referenced calibration scheme of the PTFE-capped Pd91:Ni09 fiber optic hydrogen sensor revealed a strong nonlinear response, especially at low concentrations <1000 ppm. This required linearization, which led to effective decoupling of the temperature cross-sensitivity through iterative data processing. Decoupled concentrations and temperatures show a lower detection limit of 250 ppm, a significant enhancement in baseline stability, and time constants of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${t}_{{90}} =19$ </tex-math></inline-formula> s for ab- and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${t}_{{10}} =18$ </tex-math></inline-formula> s for desorption. This work paves the way for both practical applications of fiber optic H2 sensors and fundamental research in Pd nanofilm engineering.