Design Of Seawater Salinity Detection System Based On FPGA And Fiber Grating

The ocean is an important part of the earth's environment and contains the most abundant ecological and mineral resources on earth.Exploring the ocean,developing the ocean,and protecting the ocean have always been the top priorities of my country's implementation of the strategic goal of "ocean power." The stability of marine ecology maintains the development and prosperity of the offshore economy.Seawater as the basic composition of the ocean as a complex research object is the basis for exploring marine science,and seawater salinity,which is an important parameter of seawater,is also an important factor affecting ocean currents and marine ecology.The exploration and analysis of seawater salinity in various sea areas is not only a basic requirement for marine surveying and mapping,but also an important assessment of marine ecology.With the advancement of sensing technology,seawater salinity measurement methods such as conductivity method and remote sensing method have been slightly insufficient in terms of measurement accuracy and accuracy after the latest TEOS-10 salinity standard.The optical method for measuring seawater salinity is based on the measurement of the refractive index of the substance.It is also the best method to establish the relationship between salinity and density under the current standard,which has aroused widespread concern.The accuracy of optical salinity measurement methods is relatively high,but its precise optical path structure and complex demodulation scheme make it unable to be used as a field detection device.In response to this problem,this paper uses fiber grating to detect seawater salinity.The grating has the advantages of electrical insulation,corrosion resistance,high sensitivity,and so on.It is one of the ideal devices to achieve seawater salinity detection.Under the latest standards,in order to obtain continuous and accurate seawater salinity data in the ocean,a grating salinity measurement system design based on the change in the resonant wavelength of the fiber grating due to the difference in refractive index was developed.The cladding of the grating is processed by HF etching,and a seawater salinity sensor with higher sensitivity is prepared,so that both the resonance wavelength offset and the peak loss have a higher responsiveness to small changes in salinity.The sensor and demodulation system is constructed by cascading LPFG and FBG,and the light intensity signal detected by the photodetector is related to the salinity by using the characteristics of FBG narrow-band filtering.By designing an FPGA high-speed data acquisition terminal,the basic functions of salinity acquisition and analysis are realized,which provides a new solution for real-time online monitoring of seawater salinity.The research content of this topic mainly includes the following three aspects:1.In response to the problem of insufficient sensitivity of traditional LPFG to measure seawater salinity,an LPFG grid corrosion test was carried out.Through the exploration of the HF corrosion rate,the configured HF grating etching liquid was used to produce a sensor with greater resonance loss and higher sensitivity.The sensitivity of the probe is increased by 90 nm/RIU.2.In order to overcome the problem that LPFG wavelength demodulation depends on the spectrum analyzer and cannot be used in the field,an FPGA highspeed data acquisition terminal based on FBG narrowband filter demodulation technology is designed to achieve continuous and high-precision data acquisition.The relationship between salinity and light intensity,and verified the detection accuracy of the demodulation system.3.Using MAX410 to modulate the output signal of the photodetector,a DAS system with AD7606 B as the core is developed and designed,with a voltage detection accuracy of up to 38.14 ?V.In addition,the host computer software with data transmission and storage functions was developed to realize the automatic monitoring of seawater salinity by the system.