Research On Seawater Tide Level Sensing Technology Based On Fiber Bragg Grating

Seawater tide level monitoring is an important component of the field of marine environmental monitoring,and tide level data is of great significance for marine ecology,marine economy,and marine military.The pressure type tide gauge uses a pressure sensor to measure changes in seawater pressure to obtain information on seawater tide level.The system is simple to install and widely used.At present,most pressure type tide gauges use piezoresistive sensors to measure pressure,which has problems such as zero drift,water leakage,and high maintenance costs in the later stage.In view of this,this thesis designs a pressure and temperature sensor based on Fiber Bragg Grating（FBG）for seawater tide level monitoring,which has advantages such as good long-term stability,underwater passive,low cost,and simple deployment.The main work of the thesis is summarized as follows.Firstly,the mainstream tide level monitoring methods at home and abroad were introduced,and the advantages and disadvantages of different monitoring methods were compared and analyzed;The current research status of Fiber Bragg Grating pressure sensing technology is introduced.Then,the coupling mode theory of Fiber Bragg Grating was elaborated,and the principles of FBG temperature and pressure sensing and series temperature compensation were introduced in detail;The finite element method was used to simulate and analyze the pressure characteristics of the elastic diaphragm.Under a pressure of 1MPa,the sensor simulation and theoretical pressure sensitivities were-1.1573 nm/MPa and-1.2082 nm/MPa,respectively.The simulation results were consistent with the theoretical derivation,verifying the rationality of the sensor elastic diaphragm design.Next,the grating pairs used for the sensor are engraved using femtosecond laser point by point method,with FBG1 as the pressure grating and FBG2as the temperature compensation grating;Designed elastic diaphragm,pressure shell,and temperature plate structures,and used finite element method to simulate and analyze the compressive and thermal conductivity performance of the pressure shell,verifying the reliability of the pressure shell structure design;Propose wave attenuation and atmospheric compensation methods to optimize the tidal level measurement performance of the sensor.On this basis,pressure and temperature calibration experiments were conducted.The pressure sensitivity of the sensor was-6.12 nm/MPa（in the range of 0～0.3 MPa）,the temperature sensitivity of FBG1 was about 29 pm/℃,and the temperature sensitivity of FBG2 was about 17 pm/℃.The linearity of temperature and pressure calibration results was good（>99%）;In the study of pressure and temperature stability,the sensor pressure measurement error is less than 5×10^-4 MPa,FBG1 temperature measurement error less than 0.2℃,FBG2 temperature measurement error less than 0.13℃,sensor pressure and temperature stability is good;In the study of pressure repeatability,it was found that with the increase of usage time,the pressure measurement error of the sensor gradually decreases,indicating good pressure measurement repeatability of the sensor;An indoor simulation experiment was conducted,in which the sensor can reflect real-time changes in water level,meeting the design requirements.Finally,the fiber optic tide sensor was placed at the Luhaifeng Ocean Farm for actual measurement,and compared with the data of the float type tide gauge and the tide website.The comparison result is that the fiber grating tide sensor can better reflect the tide level cycle,tide height and tide time,which verifies the feasibility of using the fiber grating tide sensor for tide level monitoring.