Structure Design And Characteristic Simulation Of Ocean Temperature And Depth Sensor Based On Fiber Bragg Grating

The temperature and pressure parameters are the bases for obtaining the parameters of ocean sound velocity,current velocity,density and heat content,etc.As the basic equipment of ocean exploration,the temperature-depth sensor plays an important role in the fields of ocean economy,ocean ecology and ocean military affairs.Most of the traditional XBT and CTD used for ocean temperature and depth detection are electrical thermometers,in addition to the use of empirical formulas for depth measurement,there are also problems such as low sensitivity,slow response time and poor thermal stability.Fiber Bragg Grating(FBG)has the characteristics of small size,anti-electromagnetic interference,anti-electromagnetic interference and anti-corrosion,and as a good temperature-strain sensitive device,showing high resolution and fast response speed in underwater detection,which can realize high sensitivity dynamic sensing of ocean temperature and depth data.In this paper,extensive research has been made on FBG sensors at home and abroad.Based on the idea of integrating optical fiber grating with other sensitized structures,four temperature and deep sensing structures for XBT and CTD are proposed,the detailed theoretical analysis and simulation for these four structures are carried out.The specific research work and innovation points of this paper are summarized as follows:(1)The temperature-depth sensing structure based on diaphragm and lever is proposed.The secondary pressure sensitivity of diaphragm after sensitization by lever is higher than that of the single diaphragm.Simulation has been made on the materials and size of the structure.The results show that the temperature sensitivity and pressure sensitivity are 31.7 pm/°C and4.21 nm/MPa,respectively,when the FBG is encapsulated in pure copper tube with a radius of 2 mm,the temperature response time is close to the pressure response time,which is less than 30 ms,as a result,the structure is suitable for the rapid temperature depth sensing application in the depth range of 0 ～ 800 m in the ocean.(2)A new temperature-depth sensing structure based on spring is proposed.The structure has been analyzed by finite element method,and proves to improve the pressure sensitivity greatly compared with bare FBG.In the case of steel as the spring material,four springs with average diameter of 4 mm,diameter of 2.5 mm,are used to sensitize FBG,high sensitivity of FBG of 8.96 nm/MPa has been achieved under 1 MPa pressure,suitable for steady-state depth sensing in the range of 0 ～ 400 m.(3)A temperature and depth sensing structure based on diaphragm and liquid filling is proposed.The thermal expansion of the liquid and the compression deformation of the diaphragm are used to sensitize the temperature and depth of the two FBGs.When using a brass diaphragm with a thickness of 1mm and a radius of 10 mm and Ga In Sn liquid,the temperature sensitivity of the structure is 1.065nm/°C,the pressure sensitivity is1.245nm/MPa,and the temperature and depth response time are both within 51 ms,which can be used for rapid sensing in the range of 0～800m in the ocean.(4)A temperature and depth sensing structure based on cascaded diaphragms is proposed.The pressure sensitization of FBG is carried out by means of cascaded diaphragms,which greatly improves the range of the sensing structure.When using three stainless steel diaphragms with a thickness of 3mm and a radius of 20 mm in coaxial parallel cascade,the temperature sensitivity of the sensing structure is 31.7pm/°C,the pressure sensitivity is225.2pm/MPa,and the temperature and depth response times are both ～ 36 ms,suitable for dynamic deep-sea sensing,in the range 0～ 8000 m.Compared with the traditional electrical XBT and CTD sensing structure,the sensing structures designed in this paper not only improve the temperature sensitivity and depth sensitivity,they can also significantly enhance the temperature and depth response speed.The matching between the temperature and depth data acquisition times has been specific addressed in this paper.The design and simulation of the four sensing structures can provide a theoretical basis and options for the research of new optical ocean sensors,and at the same time,have broad application prospects in the field of temperature and depth ocean sensing.