Compact Mach-Zehnder Interferometer for Practical Vernier Effect Sensing System With High Extinction Ratio

Recently, Vernier effect is introduced into fiber optic interferometric sensors to enhance the sensitivity. To achieve a high-quality of the Vernier spectrum, the match of the free spectral range (FSR) and the extinction ratio (ER) in two arms is required. Furthermore, the reference arm should be designed to be different from the sensing arm to avoid crosstalk. In this work, a compact in-line Mach-Zehnder interferometer (MZI) by sandwiching a hollow capillary fiber (HCF) between two sections of no-core fiber (NCF) is proposed for the reference arm in Vernier effect based sensing system. FSR of the in-line MZI can be tuned by modulating the length of the HCF, while the ER can remain high up to ∼20 dB, which is equivalent to that of the polarization-maintaining fiber (PMF) based interferometers. Experimental results show that the MZI exhibits low sensitivity to temperature, strain, refractive index (RI) and torsion. When employed as the reference arm cascaded with PMF Sagnac interferometer (SI), the upper and lower envelope are with high extinction ratio (ER). The proposed MZI is a promising candidate for the reference arm in Vernier effect based sensing system with the merits of compact size, good spectral characteristics and insensitivity to ambient environment.