A low cost fiber-optic humidity sensor based on silica sol–gel film

A highly sensitive low cost fiber-optic humidity sensor based on evanescent wave scattering and evanescent wave absorption has been developed. The sensor is fabricated using a U-bend optical fiber which is coated with silica film using sol–gel process and doped with Methylene Blue. We investigated the sensing property of the sensor in different aspects and found that the normalized absorbance of the sensor at wavelength 540 nm is logarithmic linear with the relative humidity (RH%) in the range between 1.1% and 70% at room temperature (291 K). In RH between 1.1% and 4.1%, the sensor shows good linear relationship with a sensitivity of 0.0087 a.u.%−1 (0.087 dB%−1) and limit of detection (LOD) of 0.062%. We also found that the sensitivity rely more on the length of sensing region than the bend radius ratio. The response and recovery time of the sensor is 20 s–3 min depending on the RH variation. The sensor also exhibits good reversibility and seems unaffected by a series of tested non-air gases. The newly designed sensor is suitable to be a supplement of the conventional fiber-optic based humidity sensor, particularly in low RH environment applications.