High-sensitivity relative humidity fiber-optic sensor based on an internal–external Fabry–Perot cavity Vernier effect

This study experimentally demonstrates a high-sensitivity fiber-optic relative humidity (RH) sensor based on sensitivity amplification and a reduction mechanism, employing an internal–external Fabry–Perot cavity (IEFPC) Vernier effect and a chitosan film as a Fabry–Perot (FP)-sensing cavity. The proposed sensor is constructed using cascaded FP interferometers comprised of an air cavity formed by a hollow-core fiber (HCF), a chitosan cavity, and an air–chitosan hybrid cavity. The chitosan cavity is fabricated by dipping the HCF into a chitosan solution to form a thin chitosan film. Thus, the thickness of the chitosan film could be controlled precisely based on dipping time and capillary effect. As the optical path lengths of an air–chitosan hybrid cavity and an air cavity are similar, the IEFPC Vernier effect is generated, amplifying the air–chitosan hybrid cavity’s low sensitivity to the chitosan cavity’s high sensitivity. The experimental results agree with the theoretical analysis, supporting the fact that the sensor’s sensitivity is related only to the thickness of the chitosan film. The sensitivity of the sensor reaches up to 7.15 nm/% RH, ranging 40%–92% RH at 25°C. Fabrication of the proposed sensor is cost-effective. The proposed sensor also exhibits superior stability performance, a low-temperature cross-sensitivity of 0.0068% RH/°C, and repeatable fabrication. The proposed IEFPC Vernier effect model functions well for cascaded cavities, which plays a guiding role in the sensitivity improvement of such a structure within a fiber-optic sensing context.