In-Fiber Mach–Zehnder Interferometer Based on Up-Taper Fiber Structure With Er3+ Doped Fiber Ring Laser

Abstract

A robust optical fiber modal Mach–Zehnder (MZ) interferometer is proposed, which is embedded in a fiber ring laser system as a tunable filter to improve the detection limit (DL) due to its narrow full width half maximum (FWHM) and high signal-to-noise radio (SNR). The sensing element is composed of two cascaded up-taper joints fabricated by shoving fusion. After filtering, the laser spectrum possesses the sensing characteristics that the interferometer possessed. Temperature and refractive index (RI) are investigated as measurands in experiments and the response sensitivities are 92.1 pm/°C and 44.8 nm/RIU, respectively. Accordingly, the DLs for these two measurands are calculated as 0.19 °C and $3.86 \times {10^{ - 4}}$ (RIU). Moreover, the axis strain response is demonstrated to be much lower than many modal interferometers even under the help of fiber ring laser. Thus, the property contributes to the resistance of the outer force and disturbance. Compared to the simple sensing device, the proposed fiber ring laser sensor significantly improves the DL, which is one of the most crucial parameters. It is the first time to combine the fiber ring laser device with the up-taper MZ interferometer as far as we known. It is a feasible balanced way to improve the sensing capability without the expense of mechanical strength of sensing element.