Fiber Bragg Grating Sensor For Salinity Measurement

The importance of the salinity measurement is stated from the angles of its application in marine resource development, agriculture, ecology and environment protecting, oceanography and climate predictions, exploration, industry, military application, and so on. Nowadays, me-thods such as electric conductivity, density measuring, diffraction index measuring, microwa-ves applied to salinity measurement have their own disadvantages. Therefore, this paper is co-mmitted to the development of a type of optical salinity sensor to overcome the shortcomings.The advantages and disadvantages of various salinity measurement are analyzed. We ma-ke evaluations on different kinds of preparation methods of optical fiber grating sensors. Also, Phase mask method is used when a Fiber Bragg grating is written by a193nm excimer laser. This paper chooses polyimide as the moisture sensitive film, and introduce the physical and chemical properties and potential application of polyimide.The strain and temperature sensing models of fiber Bragg grating are presented. This paper researches the diffusion process of the water in the coating material of the FBG and the sensing mechanisms of polyimide-coated FBG salinity sensor. When we reduce or increase the NaCl concentrations of the solution, polyimide can absorb water and swell or emit water and shrink, which is converted to a mechanical response in the form of a stress change. We can derive the equation about the sensitivity of the sensor system from the theory of elasticity. We can impr-ove the sensitivity by choosing the materials with high coefficient of expansion, increasing the thickness of films or decreasing the radius of fiber.The experimental equipments and method of etching fiber Bragg grating by hydrofluoric acid are referred. The experiment of coating of polyimide film is also introduced. First, do pr-etreatment to the fiber with silane coupling agent. Second, coat the fiber by dip coating proce-ss and then put it into the drying oven to heat and solidify it.A type of optical salinity sensor with a fiber Bragg grating (FBG) by etching the fiber clad-ding and coated with polyimide is demonstrated. In the case of no loss of sensor response time, the method achieves FBG salinity sensor’s better performance by etching the fiber cladding of the FBG. The experimental results showed that the salinity sensitivity of the fiber Bragg grating sensor is0.0348nm/(mol/L), which increases2.2times.