Humidity Sensor Based on an In-Fiber Integrated Mach–Zehnder Interferometer

A relative humidity (RH) sensor based on an integrated Mach-Zehnder interferometer (MZI) consisting of two off-axis twisted deformation points and an air channel passing through the core of a single-mode fiber (SMF) is proposed and experimentally demonstrated. The off-axis deformations are achieved by twisting the SMF under continuous arc discharge. The air channel is created between the two off-axis deformation points by femtosecond laser machining. Because the air channel is exposed to the environment, its refractive index is directly determined by the RH, which affects the optical path difference of the MZI and, in turn, leads to the shifts in the resonance wavelength. A prototype MZI demonstrates an RH sensitivity of approximately -0.144 nm/%RH without the use of any hygroscopic materials. More prominently, an ultrafast response time of about 68 ms is achieved. The proposed MZI structure is therefore demonstrated to be an excellent RH sensor candidate due to its compact size, outstanding performance, ease of fabrication, and all-fiber configuration.