TFBG Side Modes and Fresnel Reflection-Based Sensing System for Solution Concentration Measurement

Solution concentration (SC) measurement is of great significance in various fields. Various optical fiber SC sensors have been developed accordingly, in which the Fresnel refraction-based optical fiber sensor has the advantages of avoiding damage to sensing units, reusability, and low maintenance cost. In this study, a tilted fiber Bragg grating (TFBG) side modes and Fresnel reflection-based sensing system for SC measurement is proposed. Theoretically and experimentally, the nonuniformity of the grating period causing the forming of irregular TFBG side mode fringes is simulated and proved, and the feasibility of the dip intensity of the TFBG side mode resonance to quantitatively characterize the SC is demonstrated. As a result, a maximum concentration sensitivity of −0.471 dB/% at the glycerol SC (GSC) of 95.83% is achieved by increasing outpower of the light source and using an optical circulator. In addition, a method for simultaneous measurement of solution temperature and refractive index (RI) by using one side mode resonance signal is proposed and proved, which can significantly reduce the response time and improve the accuracy of concentration measurement.