The Study On All-fiber Ocean Salinity Sensing Technology

The ocean is an important space and resource treasure for human survival and development.Accurate detection and continuous observation of the ocean will help human better understand and develop the ocean.Among many marine hydrological elements,the measurement of seawater salinity is particularly important.In this paper,an all-fiber ocean salinity sensing technology is studied.Based on active fiber intracavity sensing technology,a reflective fiber multimode interference structure is used to measure the salinity of seawater indirectly by measuring the refractive index of seawater.And then high-precision measurement of seawater salinity is achieved.The main work of this paper is summarized as follows:(1)The research background of ocean salinity sensor,the definition and development of ocean salinity,and the development trend of ocean salinity detection technology based on electrode conductivity sensor are introduced.According to the current technical solutions for the refractive index measurement by optical fiber sensing,the potential application of the fiber sensor based on multimode interference structure in the field of ocean salinity measurement is expounded.(2)Sensing characteristics of discrete passive ocean salinity sensing system based on no-core fiber multimode interference structure are studied.According to the theory of optical fiber sensing based on multimode interference structure,the refractive index and temperature sensing characteristics of multimode interference structure based on no-core fiber are simulated and then the characteristics are analyzed and demonstrated through experiments.The multimode interference structure based on no-core fiber has a refractive index sensitivity of 125.23 nm/RIU and a temperature sensitivity of 6.11 pm/°C.The optical fiber sensing structure has the characteristics of high refractive index sensitivity and low temperature cross-sensitivity.The salinity detection limit of the discrete passive sensing system is 2.45‰.(3)Sensing characteristics of active intracavity modulation ocean salinity sensing system based on no-core fiber multimode interference structure are studied.The fiber multimode interference structure is combined with active intracavity sensing technology to optimize the ocean salinity detection limit.The temperature selfcompensated active intracavity ocean salinity sensing system and the reflective active intracavity ocean salinity sensing system are designed.The experimental study of refractive index characteristic,temperature characteristic and output stability of these two sensing systems are respectively carried out.The advantages of the reflective active intracavity sensing system in engineering application are analyzed.The salinity detection limit of the system is calculated to be 0.0042 ‰.(4)The prototype of the all-fiber ocean salinity sensing system is designed and its performance is tested.The reflective active intracavity ocean salinity sensing system is engineered,including the layout design of the prototype,the packaging design of the sensing probe,the design of the fiber sealed mechanical part and the design of the data acquisition module based on LabVIEW.The all-fiber ocean salinity sensing system prototype is tested for performance.