Ultrasensitive Refractive Index Sensor Based on Mach–Zehnder Interferometer and a 40μm Fiber

An ultra-high sensitivity refractive index (RI) sensor, based on an in-line fiber Mach-Zehnder interferometer (MZI) and utilizing a special 40 μm fiber which has been specifically designed for the purpose, supporting only the LP₀₁ and LP₁₁ modes propagating in the fiber, has been proposed and numerically demonstrated in this work. The all-fiber MZI, based on LP₀₁ and LP₁₁ mode interference, was fabricated by utilizing a special design with two gradually tapered joints at both sides of this multi-clad thin diameter fiber (TDF). When the TDF diameters varied from 40 μm to 5 μm, a sudden change in the direction of the wavelength shift in the transmission spectrum of the TDF-MZI devices was observed both in liquid RI and air relative humidity (RH) monitoring experiments, as the surrounding RI or RH was seen to increase monotonically. Ultra-high sensitivity values of 5942.8nm/RIU (1.335-1.342) and 21292.2nm/RIU (1.4000-1.4025) were obtained in the experiments carried out, through the use of a tapered TDF-MZI device with TDF diameters of ~5 μm and ~9 μm, respectively. The maximum RH sensitivity of 1.084nm/%RH was obtained by the use of the TDF-MZI device with a TDF diameter of ~12 μm. A compact all-fiber TDF-MZI sensor was thus created with an overall sensing length of <; 4mm, showing the advantages of high sensitivity, low loss, and flexibility in the tunable monitoring direction of the wavelength shift. This design is well suited to various applications, where the high sensitivity RI and RH measurements are required at precise locations.