A novel polyvinyl alcohol and hypromellose gap-coated humidity sensor based on a Mach-Zehnder interferometer with off-axis spiral deformation

A novel humidity sensor based on a Mach-Zehnder interferometer (MZI) in normal single-mode fiber (SMF) was developed and investigated. The MZI is composed of a polyvinyl alcohol coated gap that serves as a sensitive beam splitter at the input end of the MZI and an off-axis spiral deformation point that acts as a beam combiner at the output end of the MZI. The coated gap is formed between the end faces of 2 SMFs, where the humidity-sensitive polyvinyl alcohol and hypromellose film is deposited on the end face of the input SMF. The off-axis spiral deformation is achieved by twisting the lead-out fiber under continuous arc discharging with the appropriate off-axis radius. When the humidity of the external environment changes, both the refractive index and the thickness of the hygroscopic film changes, so the coupling ratio of the propagation light changes accordingly, resulting in an intensity change in the output signal of the interferometer. The interferometer was applied for high-sensitive humidity sensing in the range of 20-80%, with a sensitivity of 0.21 dB/%RH and a fast rise time of 0.795 s. Thanks to its low cost, simple preparation method, and short manufacturing time, the humidity sensor proposed in this paper can be used in a wide range of applications in the fields of structural health testing, medical treatment, industrial production, and environmental monitoring.