Research On The Reflective Fiber Optic SPR Sensor For Simultaneous Measurement Of Seawater Salinity And Temperature

Seawater salinity and temperature are the basic parameters of the ocean state equation,which affect the physical and chemical properties of seawater and the economic benefits of the marine industry.Therefore,the measurement of seawater salinity and temperature is of great significance.The traditional measurement method of the seawater salinity and temperature usually relies on the combination of physics and chemistry.It has many limitations and cannot meet the needs of contemporary detection.In recent years,the fiber optic sensor has the advantages of high sensitivity,rapid response,and immunity to electromagnetic interference,which has a wide application prospect.Among them,Surface Plasmon Resonance(SPR)sensor is one of the most promising fiber optic sensor.This kind of sensor has various structures,high sensitivity and good multi-parameter performance,which has attracted extensive attention from scholars.At present,fiber optic SPR sensors have achieved high performance measurement with single parameter,which have certain applications in the field of multi-parameter measurement at the same time.Focusing on the sensing characteristics of fiber optic SPR sensors,from transmissive to reflective,tandem to parallel,a variety of reflective twoparameter fiber SPR sensors were designed to achieve simultaneous measurement of seawater salinity and temperature.The main research work includes the following aspects:(1)The SPR sensing mechanism model of the three-layer medium was analyzed and simplified,and then the sensor mechanism suitable for the fiber SPR sensor was obtained.The performance of the fiber optic SPR sensor was simulated by Comsol and Matlab software.The effects of three parameters:metal film thickness,metal film type and sensing region length on sensor performance were analyzed.The simulation results showed that the optimum film thickness of the metal film was 40 nm,and the optimum length of the sensing region was 8-12 mm.At the same time,the sensitivity of the silver-based fiber optic SPR sensor reached 2700nm/RIU,and the sensitivity of the gold-based fiber optic SPR sensor was 2107nm/RIU.In practice,the appropriate parameters are selected considering the actual characteristics of the sensor.The simulation results provide theoretical support for the preparation of the sensor following.(2)Three tandem-type fiber optic SPR sensors for the simultaneous measurement of seawater salinity and temperature were designed and optimized.Then a complete seawater salinity and temperature detection system was established.The silver-based multimode fiber-single mode fiber-multimode fiber(MSM)SPR sensor was prepared firstly,which demonstrated that the structure can simultaneously measure seawater salinity and temperature with the sensitivity of 0.484 nm/‰(for salinity)and-1.36nm/℃(for temperature),respectively.Then,in order to improve the stability of the sensor and obtain remote measurement capability,a reflective MSSPR sensor based on gold film was proposed.The sensor sensitivity was 0.31nm/‰ and2.02nm/℃,respectively.Finally,to further improve the mechanical strength of the sensor and optimize the output spectrum of the sensor,a reflective fiber optic SPR sensor based on 600μm plastic-cladding fiber was prepared with sensitivity of 0.38nm/‰ and-1.87nm/℃ respectively.The sensor had practical application potential.(3)In order to solve the problem of sensitive film coatin g and sensor packaging,a brass tubepackaged fiber optic SPR temperature sensor with the measurement sensitivity of-1.87 nm/℃was proposed.On the basis of this,a transmissive sensor based on Hollow Core Fiber(HCF)was designed.This new type of sensor was the first time to measure the seawater salinity and temperature in the inner and outer channels of the sensor.The package was simple and the sensor measurem ent sensitivity was 0.3769nm/‰ and-0.956nm/℃,respectively.Finally,in order to further optimize the sensor,the preparation of reflective fiber optic SPR sensor based on HCF was explored,and the salinity measurement(0.357nm/‰)was realized.At the same time,the difficulties in the preparation of reflective thermo-salt dual-parameter sensors were pointed out.In this paper,the sensors for simultaneous measurement of fiber SPR seawater salinity and temperature were studied theoretically and experimentally.Two kinds of sensors were proposed to realize dual-parameter measurement,and the performance of the prepared sensor was gradually improved.In addition,the hollow-core characteristics of HCF were used for the first time to achieve internal and external dual-channel measurement of seawater salinity and temperature.It had innovative and large practical application value,and promoted the application of optical fiber SPR sensor in seawater salinity and temperature measurement.