Research Of The Key Technologies For Liquid Concentration Measurement Based On Optical Fiber Sensor

Measurement method for liquid concentration is proposed based on the fiber Bragg grating hydroacoustic sensing technology. Some related key issues are studied in this thesis. FBG sensors have the advantages of small size, high sensitivity and large dynamic range, which can fit the requirements of liquid photoacoustic(PA) signal measurement very well.To improve the low optical absorption efficiency of weak absorption solution in the single-beam cylinder liquid PA cells, two liquid multi-pass PA cell structures, zigzag reflection liquid multi-pass PA cell and inner-circle reflection liquid multi-pass PA cell, are designed with the wave equation models established. Proved by calculations and simulations, the design of zigzag reflection PA source improves the optical absorption efficiency effectively, and produces plane acoustic waves which do not attenuate along the transmission distance. The PA signal intensity is hundreds times of that produced by the same-size single beam PA cells. The effects of the thickness of PA cell, the modulation frequency and other parameters are discussed, which provide the theoretical basis for liquid PA cell design.To improve the low sensitivity of the bare FBG underwater acoustic sensors, a novel push-pull elastic diaphragm FBG underwater acoustic probe with hydrostatic compensation is designed. Proved by calculations, simulations and preliminary experiments, the sensitivity of the probe is 1600 times of that of the bare FBG. Also, the push-pull elastic diaphragm FBG underwater acoustic probe has the ability of hydrostatic compensation and isn't sensitive to the temperature change of the environment.For the needs of FBG underwater acoustic signal demodulation, a differential matching FBG self-demodulation method is introduced. The method not only has high resolution of demodulation, but also can compensation the temperature change of environment and eliminate the cross-sensitivity of FBG to stress and temperature. Topology of light path connection of this developed demeodulation method, twice-reflection differential matching FBG self-demodulation structure, has been given.This kind of demodulation method does not need moving parts. What's more, it has the advantages of fast response speed, simple structure, and low price. Sensor multiplexing can be also easily achieved using wavelength division multiplexing, space division multiplexing, time division multiplexing and other multiplexing methods.In the end, preliminary experimental results for liquid concentration measurement are illustrated, which indicate the feasibility if the prioposed idea.