Fiber optic Mach-Zehnder interferometer for high-precision liquid refractive index measurement

Abstract

High-precision refractive index measurement of liquid is widely demanded in chemical analysis, biochemical detection and environmental monitoring. A hybrid fiber-optic Mach-Zehnder interferometer (MZI) refractive index measurement system combining fiber transmission and free-space interference is proposed in this paper. Taking NaCl aqueous solution as the test sample, the quantitative relationship between solution concentration, refractive index and interference phase shift is theoretically derived based on wave optics. The beam propagation and interference field characteristics are simulated by VirtualLab and MATLAB, and the Fourier Transform Method (FTM) is adopted to demodulate interference fringe images for extracting phase variation. A 650 nm semiconductor laser, single-mode fiber and CCD camera are used to build the experimental platform. Experimental results show that the cumulative phase change presents an excellent linear correlation with NaCl solution concentration with a determination coefficientܴ R2 = 0.975. The measured sensitivity is 815.4 rad/(g/100mL), and the relative deviation from the theoretical value of 840.98 rad/(g/100mL) is only 3.05%. The system achieves a good balance between structural compactness, anti-interference ability and measurement accuracy. Error sources are analyzed quantitatively, and the combined standard uncertainty of the system is calculated as 181 interference fringes. The proposed scheme has favorable application potential for on-line detection of liquid refractive index and concentration.