Demodulation of Temperature and Salinity With Variable Sensitivity Matrix Based on Developed Refractive Index of Seawater in Infrared Band

Abstract

Two-parameter demodulation based on variable sensitivity matrix (VSM) for temperature and salinity (TS) measurement in seawater is proposed based on the developed refractive index (RI) expression of seawater in the infrared band of 1200–1650 nm. The RI expression of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${n}\,{(}{S},{T}, \lambda $ </tex-math></inline-formula> ) is obtained from a great deal of RI data measured by microfiber Mach–Zehnder interferometer (MMZI) and fit by different methods. The accuracy of the developed <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${n}\,{(}{S},{T},\lambda $ </tex-math></inline-formula> ) is verified experimentally, and a demodulation error of about 2.5% is reached by the VSM method, which is at least twice lower than the traditional sensitivity matrix used. The VSM and RI expression proposed here will be useful for theoretical or experimental research on optical fiber sensors operated in an infrared band, and they can also be expanded to other sensors based on RI change of seawater.