A carboxy-methyl cellulose coated humidity sensor based on Mach-Zehnder interferometer with waist-enlarged bi-tapers

A fiber-optic Mach-Zehnder interferometer (MZI) humidity sensor is proposed, comprising a pair of waist-enlarged bi-tapers and carboxy-methyl cellulose (CMC) coating. The MZI utilizes intermodal interference between the core mode and cladding modes for the measurement of the effective refractive index (RI) of the CMC film that varies with surrounding humidity, through change in the sensor's interference pattern. The proposed sensor is linearly responsive to relative humidity (RH) within the humidity range from 70% RH to 85% RH, with maximum sensitivity of −0.8578 dB/% RH. The advantages of this sensor are its compact size and a facile fabrication process. More importantly, humidity sensitivity can be improved by changing the thickness of the CMC film, which makes this structure a highly promising for real-time, practical RH monitoring application.