Large-Diameter Diaphragm Fabry–Pérot Interferometer for High-Sensitivity Temperature Sensing Using a Hermetically Sealed Tunable Medium: Up to 190 nm/K

This paper presents a proof-of-concept investigation into a novel hermetically sealed tunable-medium Extrinsic Fabry–Pérot Interferometer (EFPI) temperature sensor architecture. A series of tuneable-sensitivity EFPI temperature sensors is demonstrated, comprising a large-diameter fused silica diaphragm with a 800 m diameter, significantly exceeding conventional designs (typically ∼125 m), with polished diaphragm thicknesses ranging from 28 to 49 m, housed in hermetically sealed rigid melting point capillaries with a 1.8 mm internal diameter. By exploiting thermally induced pressure differentials generated by a tunable Krytox GPL 105 oil/air fill fraction within the sealed rigid cavity, the sensors demonstrate a continuously tuneable sensitivity design space spanning 0.45 to 190 nm/K. An exact nonlinear thermal pressure model is derived and validated, replacing the linearised approximation which is shown to be inapplicable at fill fractions approaching unity. The low-sensitivity configuration (0.45 nm/K) was characterised at the National Standards Authority of Ireland (NSAI) National Metrology Laboratory against ITS-90 fixed points: the Triple Point of Water (273.16 K) and the Gallium Fixed Point (302.9146 K), with traceability to the International Temperature Scale of 1990 (ITS-90), yielding an instrument-limited resolution of <1.1 mK, consistent with the metrological validation environment. The high-sensitivity configurations (21 and 190 nm/K) were characterised on a laboratory bench, achieving instrument-limited theoretical resolutions of <24 K and <2.6K respectively, pending future metrological validation. The 190 nm/K sensitivity represents an improvement of approximately 21.7× over the closest directly comparable prior Citationutilised fusion splicing and manual polishing. Future development priorities include metrological validation of the high-sensitivity configurations, long-term stability characterisation, thermal cycling, and progression towards an all-glass hermetically sealed construction.