# Reflective Fiber Surface Plasmon Resonance Sensor for High-Sensitive Mercury Ion Detection
## 2 Simulation and Experiment  
### Preparation of Optical Fiber SPR Sensing Probe  
As shown in Figure 1, the reflective fiber SPR Hg\({}^{2+}\) sensor is made of an SMF (with a core diameter of 8.2 \(\upmu\)m, cladding diameter of 125 \(\upmu\)m, and length of 1 cm) and an MMF (with a core diameter of 62.5 \(\upmu\)m and cladding diameter of 125 \(\upmu\)m). The difference in the core diameters between the MMF and the SMF makes the light leak into the cladding of the SMF. When the leaked light is reflected at the boundary of the cladding region and the surrounding environment under conditions of total internal reflection, the light wave induces the optical evanescent wave on the surface of the SMF. A gold thin film for exciting SPR is coated on the surface of the SMF, whose coating layer is removed. Further, the surface of the gold film of the SMF portion is coated with an Hg\({}^{2+}\) sensitive film. When light enters the SMF from the MMF, a portion of the light enters the cladding of the SMF, producing a large number of cladding modes. The coupling mode and surface plasmon wave couples in accordance with the phase matching conditions, resulting in an SPR spectrum. It is well known that the SPR spectrum depends on the surrounding refractive index, incident angle of light (which is a fixed value for this proposed sensor), and thickness and length of the Au film. By using the simulation model proposed in [22], Figure 2 shows the simulation results of SPR spectra along with the variations of the gold film length, gold film thickness, and refractive index around the gold film. It can be seen from Figure 2a that the length of the gold film has no influence on the wavelength position of the resonance valley, but has a large influence on the shape of the resonance spectrum. On the other hand, the thickness of the gold film influences both the wavelength position and the shape of the resonance spectrum (see Figure 2b), and even makes the SPR valley disappear. When the length and thickness of the gold film are set as 10 mm and 40 nm (the optimum parameters), a change of the SPR spectra along with the refractive index variation can be obtained as shown in Figure 2c. As seen, the valley of the SPR spectrum generates an obvious red shift with the increase of the refractive index. Since the Hg\({}^{2+}\) sensitive film is coated on the surface of the metal film,  
Figure 1: Schematic diagram of the sensing principles. the refractive index around the metal film changes with the variation of the Hg\({}^{2+}\) concentration due to adsorption of Hg\({}^{2+}\), which will then affect the SPR resonance wavelength and intensity. The optical signal carrying the resonant wavelength is reflected back to the MMF, and the SPR wavelength in the output spectrum shifts. Therefore, the change in the resonant wavelength of the output spectrum can reflect the information of the Hg\({}^{2+}\) concentration in the measured solution. In this paper, Au film was coated on the surface of the fiber by using a JS-1600 ion sputtering apparatus. From the above simulation analyses, we know that the SPR spectrum depends on the length and thickness of the gold film. For our fabrication experiment of the sensing head, the length of the gold film can be controlled by adjusting the length of the SMF. However, the thickness of the gold film depends on both the coating current and the coating time of the ion sputtering. Besides, according to the simulation results of the SPR spectrum and our practical experiences, the thickness of the lateral Au film should be around 40 nm to effectively excite SPR while the thickness of the terminal Au film should be larger than 80 nm to ensure efficient reflectivity on the fiber end. To meet this requirement, we placed the SMF vertically in the JS-1600 ion sputtering apparatus--namely, the end face of the SMF without coating faces the Au target--to ensure that the thickness of the terminal Au film is larger than that of the lateral Au film.