Mach–Zehnder Interferometer Based on Interference of Selective High-Order Core Modes

We report a novel Mach–Zehnder interferometer based on interference of selective high-order core modes in few mode fibers (FMFs). The proposed interferometer is composed of a section of FMF sandwiched between single-mode fibers with small lateral core-offsets. The theoretical and experimental results indicate that the interference patterns are coupled between core modes LP 21 and LP 02 . Because LP 21 and LP 02 both reside in the fiber core, few of the guided light can propagate outside the fiber through the evanescent wave, which makes the sensor insensitive to surrounding refractive index (RI). The experimental results show that the proposed structure has a high strain sensing sensitivity of 3.35 pm/ $\mu \varepsilon $ within the range of 0–1000 $\mu \varepsilon $ at the wavelength of 1513 nm. RI insensitivity makes the device convenient for strain measurement in humid environment.