Research On Interferometric Fiber Optic Sensor Based On FMF Temperature And Salinity Measurement

The development of salinity detection technology is closely related to the survival and development of human society.Accurate measurement of salinity and its temperature has an important impact on food industry,marine ecosystem,aquaculture,industrial water and sewage.At present,the detection technologies of salinity include conductivity method,microwave remote sensing method,and optical fiber sensing technology.Mach-Zehnder interferometer(MZI)in optical fiber sensing technology has shown the advantages of anti-electromagnetic interference,corrosion resistance,small size,high accuracy,and has become a new research direction in salinity measurement.In this thesis,based on the MZI interferometer,three kinds of salinity sensors are designed using few-mode fiber(FMF),and the research work is carried out from two directions of online MZI and curved MZI.1.A few-mode fiber temperature salinity sensor based on no-core fiber(NCF)coupling is designed and fabricated.The structure consists of two sections of NCF and one section of FMF,where the length of the NCF is 2mm.The light energy distribution diagram is simulated by Rsoft software,and FMFs with different lengths are made.By comparing the interference spectrum and the spatial spectrum diagram,the most suitable FMF length is selected.The experimental results show that when the temperature increases from 5°C to 30°C,the highest temperature sensitivity is 0.155nm/°C;when the salinity increases from 0 % to 20 %,the highest salinity sensitivity is-0.199 nm/%.The sensor can achieve simultaneous measurement of temperature and salinity,and has good stability and linearity.2.A few-mode fiber temperature salinity sensor based on eccentric structure is designed and fabricated.The structure consists of two sections of single-mode fiber and a section of FMF,and the eccentric structure is formed by eccentric fusion at the fusion point of SMF and FMF.The experimental results show that when the temperature increases from 5℃ to 30℃,the highest temperature sensitivity is 0.150 nm/℃;when the salinity increases from 0 % to 20 %,the highest salinity sensitivity is-0.270 nm/%.The sensor is simple to make,has higher sensitivity,and has a good linear relationship.3.A temperature and salinity sensor based on U-shaped few-mode fiber is designed and fabricated.The structure consists of two sections of SMF and one section of FMF,and the FMF is formed into a U-shaped fiber optic sensor by means of flame heating.The experimental results show that in the salinity range of 0 % to 10 %,the sensitivity reaches 1.847 nm/%;in the temperature range of 5°C to 30°C,it has the characteristics of temperature insensitivity.The sensor can avoid the influence of temperature on salinity measurement,and has the characteristics of easy manufacture,simple structure and high sensitivity,and has a good application prospect in the field of seawater temperature and salinity measurement.To sum up,three novel sensing structures are designed and fabricated in this thesis.The first one uses special optical fiber as a coupler to realize simultaneous measurement of temperature and salinity;the second one uses microstructure as a coupler to make the sensor fabricated.It is simpler and has improved salinity sensitivity;the third structure makes a novel sensor by bending the optical fiber,and greatly improves the salinity sensitivity,as well as being insensitive to temperature,avoiding the effect of temperature on salinity measure the impact.