# Refractive Index Sensing through Surface Plasmon Resonance in Light-Diffusing Fibers
## 3 Results and Discussion
Finally, Figure 5 shows the experimentally obtained resonance wavelength versus the SRI, together with the results of a quadratic fitting. From Figure 5, we see that the sensitivity is not constant, increasing from \(\approx\)1500 to \(\approx\)4000 nm/RIU in the analyzed range. Several comparative analyses between different fiber-optic-based SPR sensor configurations (D-shaped fibers, tapered fibers, single-mode fibers, U-shaped fibers, TFBGs fibers, heterocore fibers, etc.) can be found in excellent review papers [4; 5; 24]. Here, we just compare the performance of our sensor with those of a similar SPR sensor configuration based on an HCS optical fiber (FT300 EMT from Thorlabs) [19]. In that paper, a very similar setup based on a tungsten halogen light source and a spectrophotometer was used to characterize the sensor. The HCS sensor presented in Reference [19] had a length of 2 cm, a core and cladding diameter of 300 and 330 \(\upmu\)m, respectively, and was coated with a thin gold film. We compare in Table 1 the performance of our LDF-based SPR sensor with those of the HCS sensor as extracted from Reference [19] for two refractive indices of interest (1.367 and 1.384). These values have been chosen as they are typical of biosensors based on bioreceptors with analytes in water solution (\(\approx\)1.36), and of chemical sensors based on thin films of polymer receptors with compounds in water solution (\(\approx\)1.38). Furthermore, the \(\Delta\lambda_{\text{SPR}}\) parameter appearing in Equation (4) has been chosen in relation to \(\Delta n_{sm}=0.015\), while the \(\Delta\lambda\) was taken at the middle level between the maximum and minimum power. Table 1 shows that the sensitivity of the LDF sensor is better than the HCS sensor; vice versa, the SNR is worse. We emphasize that the HCS sensor chosen for comparison had a core diameter (300 \(\upmu\)m) larger than our LDF (170 \(\upmu\)m), and that, in general, the performance of SPR sensors based \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sensor Configuration** & **\(\mathbf{n_{sm}}\)** & \begin{tabular}{c} **Resonance** \\ **Wavelength (nm)** \\ \end{tabular} & \
\begin{tabular}{c} **Sensitivity** \\ **[nm/RIU]** \\ \end{tabular} &
\begin{tabular}{c} **SNR** \\ **(\(\mathbf{\Delta n_{sm}}=\mathbf{0.015}\) )** \\ \end{tabular} \\ \hline SPR sensor based on multimode HCS optical fiber [19] & 1.367 & 647 & 1592 & 0.404 \\ SPR based on LDF & 1.367 & 701 & 2831 & 0.254 \\ SPR sensor based on multimode HCS optical fiber [19] & 1.384 & 715 & 2043 & 0.924 \\ SPR based on LDF & 1.384 & 755 & 3554 & 0.276 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of performance of two different SPR multimode optical fiber sensors for two refractive indices (1.367 and 1.384). Figure 5: SPR wavelength as a function of the SRI. The circles are extracted from experimental data, while the continuous line is the quadratic fitting curve. on multimode fibers improve with the diameter of the fiber [20]. We argue that the LDF sensor has a higher sensitivity compared to the HCS due to a higher number of modes internally excited by the nanovoids. Similarly, the increased modal content may also explain the higher FWHM of the SPR response. However, a more extensive analysis of the modal distribution inside the LDF must be carried out in order to fully explain the obtained results.