Ultrasensitive gas-pressure sensor with all-fiber parallel Fabry–Perot interferometers to obtain the dual harmonic Vernier effect

This paper proposes and demonstrates an ultrasensitive parallel all-fiber sensor based on the dual harmonic Vernier effect (DHVE) for gas-pressure measurements. The sensor comprises two Fabry–Perot interferometers (FPIs) arranged in parallel: one with a closed cavity formed by a SiO₂ film and the other with an open cavity utilizing a hollow-core fiber. Unlike previous implementations of the Vernier effect, the DHVE combines the dual and harmonic Vernier effects. For this theory, detailed theoretical derivations are conducted, followed by comprehensive simulations and experimental validations. The two interferometers exhibit opposite wavelength drifts in response to the same gas pressure change, creating a specific optical path difference that generates a high-order Vernier effect. The fabricated sensor based on the DHVE achieves a sensitivity of −2035.3 nm/MPa. Furthermore, we experimentally investigate the influence of harmonic order and detuning ratio on the sensitivity of the DHVE and the relation of the output behavior of the DHVE with the wavelength direction. Notably, the DHVE-based sensor enhances fabrication tolerance and, owing to its all-fiber design, exhibits excellent stability and low temperature sensitivity, with a measured sensitivity of 127.6 pm/°C.