A chitosan-coated humidity sensor based on Mach-Zehnder interferometer with waist-enlarged fusion bitapers

•The proposed sensor has high humidity sensitivity of −119.6 pm/RH and resolution of ∼0.17 RH within the RH range from 10% to 90%.•The proposed sensor has advantages of low-cost, robustness, and low temperature sensitivity.•It is very suitable for use as remote single-point monitoring biosensors or disposable low-cost fiber-optic bioprobes.•The chitosan coat has good biocompatibility for future in vivo biomedical applications such as immunosensing and DNA hybridization detections.

A novel humidity sensor, which adopts a Mach-Zehnder interferometer (MZI) in normal single mode fiber (SMF) modified by the deposition of chitosan (a moisture-sensitive natural polymer) on the cladding, is proposed and experimentally demonstrated. It is fabricated by the fusion splicing of a segment between the two SMF with waist-enlarged fusion bitapers. This all-fiber MZI based on SMF incorporates intermodal interference between the core mode and the cladding mode. Due to the fact that it is sensitive to external refractive index and that the RI of the chitosan multi-layer film coat depends on the environmental humidity, the SMF-MZI with a chitosan coating layer of nanometer thickness is employed in humidity measuring. The sensitivity of ∼119.6 pm/RH (relative humidity unit) is achieved within the range from 10% to 90% on the experimental level. Moreover, the chitosan coat has good biocompatibility for in vivo biomedical applications like immunosensing and DNA hybridization detection in the near future.