Sensitivity-Enhanced Moisture Sensor Based on θ-Shape Bending Fiber Coated With Copper-Polyvinyl Alcohol Thin Film

This paper presents a compact and easy fabrication of a moisture sensing probe based on macro-bending loss induces in a
$\theta $
-shaped Mach-Zehnder interferometer (MZI) for the first time. The interferometer configuration was simply fabricated by bending a segment of single-mode fiber (SMF) into two elliptical loops divided by a fiber bridge to form a shape of
$\theta $
symbol without any splicing. The sensing mechanism is reliant on the optical path difference among the core and cladding modes in the
$\theta $
-shaped configuration induces by appropriately adjusting the SMF bending diameters, which causes the resonance wavelength to shift towards relative humidity (RH) variation. Two sensing probes based on
$\theta $
-shaped bend structure with an optimized bending diameter of 1.5 cm were fabricated to investigate the effect of Copper-Polyvinyl Alcohol (Cu/PVA) coatings, which served as a moisture-sensitive layer on the sensor sensitivity. The coated macro-bend structure exhibits an excellent sensitivity of −1.469 nm/RH% with a polynomial fitting coefficient of 99.645% in the 55%-97% relative humidity (RH) range, which is 1.4 times higher than that of an uncoated structure. This result presents a feasible and practical method to construct excellent sensitive optical fiber moisture sensors based-bending loss and Cu/PVA nanostructured coatings.