Fiber Humidity Sensor Based on SF-LiBr Composite Film

A novel sensor based on a silk fibroin (SF) and lithium bromide (LiBr) composite film is proposed to sense relative humidity (RH). We fabricate this sensor by coating a tapered single-mode optical fiber (TSOF) with a SF-LiBr composite film. The evanescent field generated in the taper region responds to the surrounding refractive index (RI). The SF-LiBr composite film immobilized on the surface of the taper region is sensitive to RH. As the humidity increases, more water molecules interact with the hydrogen bonds in the SF film. The LiBr enhances the water absorption properties of SF film. Therefore, the RI of SF-LiBr composite film changes according to the humidity of the surrounding ambient, resulting in transmission spectra finally shifts. We can monitor the RH by observing the wavelength shift of the transmission spectrum. The results show that the sensor has a nonlinear response in the range from 29%RH to 99%RH, and the sensitivity can reach −2.3nm/%RH in the range from 29%RH to 39%RH. Furthermore, the sensor possesses excellent stability and remarkable repeatability.