## 3 Results
### Refractive Index Response  
The fiber laser response was analyzed when liquid refractive index variations were applied around the MFI. We set an initial refractive index close to 1.332 RIU (distilled water), obtaining the laser spectra shown in Figure 5. The P1, P2, and P4 laser peaks underwent insignificant wavelength variations (0.4 nm), and a new laser peak was obtained, which had the following characteristics: centered wavelength at 1543.79 nm (P5), a peak power of \(-\)21.25 dBm, and SMSR exceeding 30 dB. Two dual-wavelength emissions comprised this quadruple-laser spectrum, both of them separated 13.2 nm; thus, each dual emission had a mode offset of 1.6 nm. This spectrum was analyzed during an hour, evaluating the wavelength and peak power fluctuations (Figure 5b). According to the data, the P1 and P3 lasing modes had power fluctuations exceeding 7 dB. However, the P2 and P5 peaks exhibited lower power fluctuations. Additionally, the quadruple-laser spectrum had minimal wavelength variations (<0.1 nm). The power instabilities observed can be adjusted by using the PC. Other techniques, such as nonlinear optical loop mirror and nonlinear fiber optics can also be implemented to reduce these variations [42; 43]. However, these techniques will modify the fiber laser response. Figure 4: Laser emission: (**a**) Quadruple-wavelength laser emission output for the initial ring cavity configuration (black line). Unequivocal correspondence between the laser emission (black line) and the MFI reflection spectrum (red line); (**b**) One-hour laser emission wavelength stability assessment (red) and power (blue). Figure 5: Refractive index response: (**a**) Quadruple-wavelength laser emission output for distilled water immersion (RIU 1.332); (**b**) Stability analysis for refractive index surrounding alteration (wavelength (red) and power (blue)). Different water-glycerin mixtures were used to increase the liquid refractive index around the MFI; their values were estimated using a Refracto 30 GS refractometer. The refractive index ranged from 1.3323 to 1.3799 RIU; the minimal variation between each sample was 0.003 RIU. When the liquid refractive index was increased around the MFI, the laser response was consistently altered across the wavelength range shown in Figure 6a. It is important to stress that, typically, multi-wavelength fiber laser arrangement exhibit switched lasing mode when any physical effect perturbs the optical fiber filter. However, our experimental results showed that the entire spectrum displayed a constant wavelength shift to a longer wavelength when linear refractive index increments were applied. By considering the total wavelength shifting, the following tuning parameters can be estimated: 15.71 nm/RIU (P1) 14.33 nm/RIU (P2), 16.13 nm/RIU (P4), and 13.77 nm/RIU (P5) (Figure 6b). Moreover, the following high linear wavelength shifting responses were presented: R\({}^{2}\) = 0.97609 (P1), R\({}^{2}\) = 0.98706 (P2) R\({}^{2}\) = 0.98426 (P3), and R\({}^{2}\) = 0.8756 (P4) (Figure 6b). It was also observed that the peaks had different dynamic ranges; for instance, the P5 laser peak disappeared for refractive index variations exceeding 1.3476, while, P2 and P4 were present in almost all the refractive index ranges. The dynamic range variation is strongly related to the refractive index interference pattern response of the MFI. Here, when the refractive index was applied, the interference pattern underwent a wavelength shift, as well as a visibility decrement [41]; as a result, some peaks start to disappear as the visibility is reduced. These responses were strongly related to the frequencies generated by the MFI (Figure 2b), some of these frequencies disappeared as the refractive index increased [41]; hence, the lasing modes obtained had different dynamic ranges. The P2 peak was selected for the loop hysteresis analysis because of its minimal power and wavelength fluctuations (Figure 7).