Research On Liquid Concentration Measurement System Based On Fiber Bragg Grating

Concentration is one of important parameters to denote the characteristics of liquid solution, it is widely applied to liquid concentration monitoring and controlling in many fields such as paper making, chemical industry, sugar manufacture, foodstuff, medicine and environmental monitoring. It is a main technology means to guarantee and improve the product quality. The study on new method for liquid concentration measurement is of important significance. At present the application of optical fiber grating has more and more attention in the field of concentration sensing. Fiber Bragg grating (FBG) has the advantages of anti-electromagnetic interference, high insulation, corrosion resistance, small volume, light weight, high sensitivity, wavelength absolute encode, easy to connect with optical fibers and ease of multiplexing network. So it has great research significance and potential applications to design a liquid concentration sensor based on the sensitive element of FBG.This paper starts with the coupled-mode theory, and establishes the FBG model for liquid concentration sensor. The detailed theoretical analysis and experimental researches on the concentration sensing characteristics are carried out, which are aimed at fiber Bragg grating three-layer structure model and two-layer structure model. At the same time, the corresponding relationships between concentration and Bragg wavelength of sucrose, ethanol and Nacl the three different kinds of solution are achieved respectively through the experiment, which verifies the correctness of the proposed model.Secondly, a novel package structure of fiber Bragg grating concentration sensor is designed, and a method of the double FBG single-ended photolithography is proposed, which has resolved the problem of FBG temperature and strain cross-sensitization. In order to reduce the stress influence on the optical fiber grating, a cylindrical package structure with semicircular baffle is devised, and exerts a certain amount of prestress on the sensitive part of the fiber. This structure can not only protect the grating but also can remove the influence of strain factor. The sensor performance has been tested by the test experimental platform, and through experiment verifies the feasibility of the FBG liquid concentration measurement scheme.Finally, the FBG concentration sensor of remote measurement and control system is built, and the communication program of upper computer software is written. The system can realize historical data record of the liquid concentration and update the change curve. So this test system provides experimental foundation for the solution concentration real-time online monitoring in areas such as biological and chemical fields.