Miniature fiber optic SPR high sensitivity humidity sensor based on coated polyvinyl alcohol film

• A highly sensitive SPR humidity sensor based on a miniature tapered fiber with PVA coating is demonstrated. • The PVA is applied to the surface of the gold (Au) film of a miniature tapered fiber SPR sensor. • The sensitivity of the proposed humidity sensor is 1.542 nm/%RH. • The cross-sensitivity of the sensor's temperature and humidity is relatively small. For the measurement of relative humidity, this study employed Surface Plasmon Resonance (SPR) technology, a sensor with a micro-conical fiber structure (sensing length is only 375 μm) was combined with Polyvinyl Alcohol (PVA) to create a humidity sensor that is highly sensitive, compact, dense, and exhibits good linearity. The humidity sensor is based on a gold film (thickness of about 40 nm) on the surface of the miniature fiber optic structure, and by applying PVA coating, the SPR humidity sensor shows resonance peaks in the transmission spectrum in ambient air, and as the number of layers of PVA coating (thickness) is added, the resonance peaks' positions are red-shifted. The experimental findings demonstrated that this SPR humidity sensor's sensitivity was 0.162 nm/%RH at an initial resonance wavelength of about 550 nm (one layer) and about 1.542 nm/%RH at an initial resonance wavelength of 656 nm (five layers) in the range of 46 %RH to 93 %RH. The wavelength and the relative humidity had a good linear connection. Temperature tests revealed the sensor has a −0.152 nm/°C temperature sensitivity between 20 °C and 50 °C and has a high linearity (0.999). Therefore, the sensor has potential applications in humidity measurement.