The Study And Application Of Multi-Parameter Sensing Technology Using Special Fiber Gratings

Fiber Grating sensor is one of the sensors that have been developing quickly in recent years. Fiber grating sensors have many special advantages, such as high sensitivity, electromagnetic interference resistance, corrosion resistance, large dynamic measurement range, small size, easily multiplexing. These advantages make fiber grating sensor to be widely studied and applied. Currently, a single fiber grating sensor head is always used for sensing a single parameter, such as temperature, strain, magnetic field intensity. While multiple fiber grating sensor heads are required to measure multi-parameters and optical spectrum analyzer is required to demodulate the wavelength signals, which increases the cost and complexity of the system. Therefore, it is important to investigate the multi-parameter sensing measurement performance of a single fiber grating, especially for the theoretical design and experimental investigation of some all-fiber (multi-parameter) sensor systems.This paper conducts a detail study on the multi-parameter sensing performances of three kinds of special fiber gratings and on the applications of these fiber gratings. The main content of the paper is as follow:(1) The temperature and concentration sensing characteristics of a long period fiber grating have been studied theoretically and experimentally. The temperature and concentration of solution have been measured real-time by a long period fiber grating, according to different temperature and concentration response coefficients of the applied two loss peaks of the long period fiber grating.(2) Two all-fiber sensor systems have been investigated to conduct temperature and concentration sensing experiments by using a long period fiber grating and a fiber Bragg grating, which wavelength-matched with each other, according to the linear filtering principle of the long period fiber grating loss peak. In the all-fiber temperature sensor, a fiber Bragg grating has been used as a sensor head and a wavelength-matched long period grating has been used to convert wavelength into intensity for interrogation. A temperature resolution of 0.013℃has been realized. In the all-fiber concentration sensor, a long period fiber grating has been used as sensor head and a wavelength-matched fiber Bragg grating has been used as an interrogator. A concentration resolution of 0.104g/L for NaCl solution has been achieved.(3) The temperature and stress sensing characteristics of a polarization maintaining fiber grating have been studied theoretically and experimentally. A polarization maintaining fiber grating is embedded in GFRP in the experiment. The temperature and the internal stress sensing experiments have been accomplished. After embedded, stress sensitivities are 197pm/MPa (x-axis) and 200pm/MPa(y-axis). Meanwhile, temperature sensitivities of two reflection peaks increase by 53.6%(x-axis) and 57.9%(y-axis). Furthermore, a system for temperature and strain monitoring simultaneously with a single PMFG has been designed and discussed according to the experimental results.(4) The theoretical knowledge of a chirped fiber Bragg grating is described. A chirped fiber Bragg grating is embedded in GFRP. The temperature and stress sensing performances of the CFBG have been investigated. Experiment results show that the stress sensitivity is 537pm/MPa and the temperature sensitivity increase by 143.80% after the CFBG being embedded into the CFRP. An all-fiber multi-parameter sensor system has been design for temperature and strain monitoring simultaneously according to the experiment result. Based on a single chirped fiber Bragg grating sensor head, the system is suitable for long-distance, non-destructive on-line monitoring of the major health parameters (temperature and strain) of a composite structure.