## 3 Results  
The proposed MFI structure was set into the ring fiber laser cavity. This arrangement consisted of a pump laser diode (QPHOTONICS QDFBLD-980-500); its signal was incorporated into the ring laser cavity by using an optical fiber wavelength division multiplexer (WDM). By using the 980 nm port, the pumped signal was launched to the gain medium (4.5 m of erbium-doped fiber), thus, generating an amplified spontaneous emission (ASE). At the end of the erbium-doped fiber length, a polarization controller (PC) was set using the following configuration: quarter wave plate-half wave plate -quarter wave plate (QWP-HWP- QWP). The purpose of the plates was to increase the system's  
Figure 2: MFI characterization: (**a**) Reflection spectrum of the used MFI obtained using a broadband source at port 1 of an optical fiber circulator (OFC), the questioned MFI at port 2 of the OFC, and the OSA at port 3; (**b**) Spatial frequency analysis of the reflection interference spectrum.  
stability. The ASE spectrum was launched to the MFI using an optical fiber circulator (OFC); at this point, the reflected signal from the MFI was reincorporated into the ring cavity by using port 3 of the OFC. The interference reflection spectrum from the MFI traveled to the optical fiber coupler 90:10. Here, the 10% coupler port, which was connected to an optical spectrum analyzer (OSA, AQ6370 Yokogawa China Co., Ltd., Tokyo, Japan), monitored the output laser. The 90% signal was launched to an optical fiber isolator, which provided a path into the cavity. Lastly, the fiber isolator output and the 1550 nm WDM port were spliced to close the ring cavity. The ring fiber laser cavity is shown in Figure 3.  
The pumping diode was set at 400 mA, achieving the spectrum shown in Figure 4a; it is important to stress that a lower current threshold was observed (70 mA). Four lasing lines constituted the initial laser response. This spectrum had three consecutive lasing emissions, one single line separated from the triple emission by 11.7 nm. The strong relationship between the laser emission and the interference spectrum of the optical filter is well recognized; in our work, this was governed by the MFI. According to the filter spectrum, the region with lower visibility did not have laser emissions (see Figure 4a); moreover, the emissions were centered at the peak interference fringe. The triple-laser emission was made up of peak wavelengths centered at 1527.9949 nm (P1), 1529.4903 nm (P2), and 1531.0256 nm (P3).  
The spacing mode of the initial multi-wavelength spectrum was 1.6 nm; this value corresponds to the free spectral range of the interference spectrum generated by the MFI. The power peaks from the triple-laser emission were: \(-\)26.1580 dBm (P1), \(-\)16.7160 dBm (P2), and \(-\)22.9010 dBm (P3). The side mode suppression ratio (SMSR) values were: 33 dB (P1), 43 dB (P2), and 35 dB (P3). The single-lasing line (P4) had the following characteristics: wavelength centered at 1542.75 nm, power peak of \(-\)19.85 dBm, and SMSR around 41 dB. The stability of the fiber laser was analyzed; the laser spectrum was recorded every 3 min for an h. The centered wavelength and peak power variations are shown in Figure 4b. As can be observed, the proposed fiber laser presents minimal wavelength variations around 0.008 nm for P3 and P4 and maximal for P2 (0.02 nm). The minimal power fluctuations were observed at P2 (0.46 dB), while the maximal intensity fluctuations were presented at P1 (2.5 dB). The initial laser spectrum presented high wavelength stability. It must be noted that the laser results shown above correspond to the room's temperature, which was around 25 \({}^{\circ}\)C, and the MFI air-suspended (1.00 RIU).  
Figure 3: Experimental setup of the ring cavity laser. WDM: wavelength division multiplexer, EDF: erbium-doped fiber, PC: polarization controller, OFC: optical fiber circulator, OC: optical coupler, OSA: optical spectrum analyzer, Of: optical isolator.