High-Accuracy CaAlg-Coated D-Shaped Optical Fiber Humidity Sensor Using Multipeak Wavelength Shift Averaging Method

A calcium alginate (CaAlg)-coated D-shaped single mode fiber humidity sensor is proposed in this paper. The sensor is fabricated by grinding single mode fiber and wrapping CaAlg with rolling stress. The D-shaped optical fiber sensor utilizes outward cladding mode to enhance sensing sensitivity to external refractive index changes. To visualize the spectrum response of D-shaped optical fiber humidity sensor, the periodic resonance spectrum is generated by tapered fiber connected to a broadband light source. External humidity changes are detected by observing transmission spectrum wavelength shift. The multi-peak wavelength shift averaging method is used to improve the humidity measurement linear accuracy. Results show that the relative humidity (RH) sensitivity is 0.0269 nm/%RH in 10-99%RH range, with temperature insensitive. This study provides an economical method for achieving accurate humidity detection using relatively low resolution commercial optical spectrometers. With high humidity sensing sensitivity, measurement precision, and temperature insensitivity, the proposed sensor has potential applications in practical humidity measurement.