Fiber in-line Mach–Zehnder interferometer based on near-elliptical core photonic crystal fiber for temperature and strain sensing

A fiber in-line Mach-Zehnder interferometer is fabricated by selectively filling liquid into one air hole of the innermost layer of a photonic crystal fiber (PCF). The refractive index of the liquid is so close to that of the background silica in the wavelength range of 1300-1600 nm that the two-mode PCF evolves into multimode PCF with an elliptically shaped core. Due to the different propagation constants, interference can occur between the fundamental mode and higher-order modes of the liquid-filled PCF. Such a device is applied in temperature and strain measurements with high sensitivities of 16.49 nm/°C and -14.595 nm/N, respectively.