Highly Sensitive and Ultra-Wide Detection Range Refractive Index Sensor Around a Singular Point in Mach–Zehnder Interference

Refractive index (RI) optical fiber sensors are often used in biomedical and petrochemical fields. However, existing optical fiber sensors can only detect liquids with low RI, and cannot meet the detection needs of high RI liquids such as oil, hydrocarbons, and benzene. Due to the limitation of core RI, the detection range of fiber RI sensor based on surface plasmon resonance (SPR) effect, fiber Bragg grating (FBG), Fabry-Pérot interferometer (FPI), and optical waveguide principle is limited to 1.45. It is essential to develop a highly sensitive sensor that can detect liquids with ultra-wide RI. In this article, an RI fiber sensor based on Mach-Zehnder interferometer (MZI) with lateral offset composite structure is proposed and experimentally verified. MZI was fabricated through precise cutting and lateral offset splicing between common single-mode fiber (SMF) and multi-mode fiber (MMF). High sensitivity and ultra-wide RI range of 1.33–1.50 near MZI singularities are realized. To our best knowledge, this is the first report of continuous detection of an ultra-wide RI range from 1.33 to 1.50 in experiments using fiber optic sensors. In this range, the transmission spectrum is first blue shifted and then red shifted. There is a highly amplified sensitivity peak at the singular point of the curve. The sensor has the advantages of wide RI detection range, good recovery characteristics, high stability, and low production cost, making it a strong competitor in the field of wide range and high sensitivity RI sensing.