In-Fiber Mach–Zehnder Interferometer Based on Waist-Enlarged Taper and Core-Mismatching for Strain Sensing

An in-fiber Mach–Zehnder interferometer for strain measurement is proposed and experimentally demonstrated. The sensor consists of a taper followed by a short section of a multi-mode fiber (MMF) and a dispersion compensating fiber (DCF), which is sandwiched between two single mode fibers (SMFs). The taper is used as a fiber coupler to excite cladding modes in the SMF, and these cladding modes transmit within the MMF and the DCF. The core mode and the cladding modes interfere in the DCF–SMF fusion point to form intermodal interference. A well-defined interference spectrum is obtained in the experiment. Selected interference dips are used to measure the strain changes. The experimental results show that this device is sensitive to strain with the wavelength-referenced sensitivity of 2.6 pm/με and the power-referenced sensitivity of 0.0027 dB/με, respectively.