Results and Discussions The designed fiber SPR sensor with silver film and high-purity indium film structures has an excellent performance in salinity detection. Fig. 8 shows the resonance spectrum of the structure coated with only a single silver film. As the refractive index of the external environment increases, the resonance peak shifts to the long wavelength direction, and the sensor shows a small amount of wavelength shift. After calculation, the SPR sensor with a single silver film structure has a refractive index sensitivity of 3450.74 nm/RIU. For the SPR sensor with a silver film structure in Fig. 9, its resonance peak shifts significantly to the long wavelength direction as the refractive index of the solution increases. The linear relationship between the resonance wavelength and the refractive index of the solution is positive, and the sensitivity of the sensor is as high as 4989.34 nm/RIU, which is converted into a mass fraction sensitivity of 9.1 nm/% by Eq. 7. This is because high-purity indium has a higher refractive index, and the presence of high-refractive-index materials can increase the penetration depth of evanescent waves, and the evanescent field is enhanced, which results in an increase in the interaction between evanescent waves and surface plasmon waves. The plasmonic effect of the silver film is enhanced, which in turn causes an increase in wavelength drift and improves the sensitivity of the sensor. Compared with that of the SPR sensor coated with only a single silver film structure, the sensitivity is improved by about 44%. Compared with that of the traditional fiber SPR sensor, the sensitivity is improved by about 126%.

As people tend to pay much attention to the ecological environment, the detection of liquid salinity in hydrological environments has gradually become an indispensable part of research fields such as agricultural planting, aquaculture, and monitoring of the marine environment. Surface plasmon resonance (SPR) technology has unique advantages in liquid salinity detection due to its fast response speed and high precision. The optical fiber SPR sensor combines optical fiber and plasma technology to overcome the limitations of prism-type SPR sensors. It not only has the advantages of simple structure and water resistance of optical fiber sensors but also has a wide detection range and high detection sensitivity of surface plasmon technology and convenient operation. However, traditional optical fiber SPR sensors based on multi-mode or single-mode single metal film structures generally face many problems. Specifically, the metal film is easy to fall off, and the detection sensitivity is low. In addition, the stability is poor. Indium has excellent electrical properties and positive flexibility. Therefore, it is an important optoelectronic material and is easy to form a firm coated on a metal surface. Furthermore, it is not easy to react with alkaline solutions and has good corrosion resistance. Therefore, this paper uses the strong coupling characteristics of a three-core fiber and the principle of SPR to design a tapered fiber SPR sensor based on a silver film and a high-purity indium film, which can improve the sensitivity and corrosion resistance of the sensor.

This thesis is based on the theory of an optical fiber SPR sensor, and the propagation law of light in a tapered fiber is studied. The total reflection coefficient of the sensor is obtained by using the Fresnel formula, and the influence of the diameter of the cone waist region, the length of the sensing region, and the thickness of the metal film on the sensor is analyzed by numerical calculation, and the optimal parameters are determined. The sensor is fabricated according to the numerical values determined by the simulation, and the optical fiber is tapered by a hydrogen flame fusion taper. The silver film and the high-purity indium film are prepared by magnetron sputtering and molecular vapor deposition, respectively. First, the optical fiber SPR sensor with a single silver film structure is studied, and the change in its resonance peak is observed by dropping salt solutions of different mass fractions. Then, under the same experimental conditions, the sensing performance of the optical fiber SPR sensor with silver film and high-purity indium film structures is studied. After comparing the two sets of experimental data, a conclusion is drawn.

In this paper, an Ag/In structure SPR sensor based on a three-core fiber is designed. The multi-core fiber SPR sensing mechanism is theoretically analyzed, and the system structure parameters are determined. In addition, two fiber SPR sensors with only a single silver film structure and a silver film plus high-purity indium structure are experimentally studied. By comparing the experimental results, the resonance peak of the Ag/In structure SPR sensor shows a larger wavelength shift, and its refractive index sensitivity is about 44% higher than that of the Ag structure fiber SPR sensor in a mass fraction measurement range of 1.4%-3.6%, which can realize salinity measurement in a full depth range of 0--5000 m. Therefore, the introduction of high-purity indium outside the silver film can improve the sensitivity and stability of the sensor, and the sensor can be applied in fields such as environmental monitoring, biomolecular measurement, and climate prediction.