Comparative spectral analysis of tapered optical fiber Mach–Zehnder interferometer as chloride ion concentration sensor

This paper presents the comparative spectral analysis of a tapered optical fiber Mach–Zehnder interferometer (MZI) as a chloride ion concentration sensor by analyzing transmission loss, wavelength shift, average interference fringe spacing, average amplitude, and three ranges of integral areas. An 8-cm optical fiber MZI sensor was fabricated and fusion spliced using a tapering process. We measured chloride ions in samples of sodium chloride solutions with different weight concentrations ranging from 0.015% to 25% and analyzed those interference spectra regarding normalized intensity, wavelength shift, average interference fringe spacing, average amplitude, and three ranges of the integral areas (1520-1580 nm, 1540-1600 nm, and 1520-1620 nm). Results show that the optical fiber MZI sensor exhibited a linear increase in the normalized intensity as well as average interference fringe spacing when the concentration increased. The optical fiber MZI sensor also displayed a linear decrease in the wavelength shift, average amplitude, and three ranges of the integral areas when the concentration increased. Compared with traditional sensing parameters such as normalized intensity or wavelength shift, two integral areas (areas between 1520-1580 nm or 1520-1620 nm) which possessed linear increase with adjusted R-squared (0.9286 and 0.91788, respectively), when the concentration increased, are recommend as potential sensing parameters.