## 4 Experimental Investigation of MTEFL Operation
Among these peaks, \(\lambda_{3}\) exhibited the highest power, with the power differences between each lasing emission line being less than 10.89 dB, as shown in Figure 7b. In state-c, the curvature was incremented to 4.9750 m\({}^{-1}\), resulting in the generation of triple-emission lasing lines, as shown in Figure 8a. Specifically, these triple-emission lines produced maximum peaks at \(\lambda_{\rm IA}=1558.62\) nm, \(\lambda_{\rm IB}=1564.22\) nm, and \(\lambda_{\rm IC}=1564.53\) nm. The SMSR for these peaks was measured at 27.5 dB, 30.44 dB, and 24.71 dB, with 3 dB linewidths of 0.03 nm, 0.04 nm, and 0.02 nm, respectively. The adjacent peaks were separated by 5.60 nm and 0.31 nm. Among these wavelengths, \(\lambda_{\rm IB}\) exhibited the highest power, with the peak power difference between each lasing line being less than 5.73 dB, as depicted in Figure 8b. An MZI with a thin-core has the advantages mentioned above [10; 11; 20]. But, as EDF has a gain medium [30], the system has a fluctuation that can be seen when increasing the  
Figure 6: (State-a) single-emission signal. (**a**) Tuning lines, (**b**) emission samples of curvature and SNR, (**c**) SMSR and SNR of the most significant peaks and their comparison, (**d**) the polynomial fit between the curvature data and its wavelength shift. curvature (4.9537 m\({}^{-1}\) to 4.9962 m\({}^{-1}\)) as in the b- and c-states. From this curvature range, the laser has a switch behavior, and we believe that the high-order modes generated in the cladding influence the emission spectrum of the filter, and, therefore, the wavelength lasing oscillations compete significantly with the overall homogeneity of the gain medium's laser spectrum, as reported by [PERSON] et al. [31]; therefore, the spacing between the wavelengths observed in Figure 7 (0.90 nm, 0.78 nm) and Figure 8 (0.31 nm and 0.58 nm) is generated. When the curvature was increased to 4.9962 m\({}^{-1}\), a second triple-emission lasing line was obtained, as illustrated in Figure 8c. This new triple-emission line produced maximum peaks at \(\lambda_{2\text{A}}\) = 1558.27 nm, \(\lambda_{2\text{B}}\) = 1558.85 nm, and \(\lambda_{2\text{C}}\) = 1564.81 nm. The associated signal had SMSR values of 29.15 dB, 31.16 dB, and 36.83 dB, with 3 dB linewidths of 0.02 nm, 0.04 nm, and 0.02 nm, respectively. The adjacent peaks were separated by 0.58 nm and 5.96 nm. Among these lines, \(\lambda_{2\text{C}}\) exhibited the highest power, and the peak power difference between each lasing line was less than 8.51 dB, as shown in Figure 8d. Figure 8: (State-c) double switch of triple-emission signals: (**a**) SMSR on the first switch, (**b**) separation and power of peaks, (**c**) SMSR of the second switch, (**d**) separation and the power of peaks. Figure 7: Switch with quad-emission line. (**a**) SMSR on each peak, (**b**) power difference between the peaks and their separation. In state-d, when the curvature is applied to the filter that goes from 5.0173 m\({}^{-1}\) to 5.2843 m\({}^{-1}\), a tunable narrow-emission dual-wavelength line is achieved, with a tuning range of 2.24 nm and a wavelength interval of approximately 0.16 nm (see Figure 9a). Throughout the tuning process, a total of 14 samples were analyzed, labeled as \(\lambda_{1}\), \(\lambda_{2}\),..., \(\lambda_{14}\). The initial \(\lambda_{1}\) (represented by the blue line) features a dual wavelength at \(\lambda_{1A}\) = 1563.71 nm and \(\lambda_{1B}\) = 1564.56 nm, with an SMSR of 39.12 dB and 3 dB linewidth of 36.59 dB, corresponding to 0.97 and 0.41 nm, respectively. The \(\lambda_{1A}\) showed the highest power, with the peak power difference being less than 2.53 dB. The final dual-wavelength sample, \(\lambda_{14}\) (represented by the orange line), has wavelengths \(\lambda_{14A}\) = 1561.34 nm and \(\lambda_{14B}\) = 1562.27 nm, with an SMSR of 33.75 dB and a linewidth of 0.98 nm and 0.41 nm for each, respectively. The \(\lambda_{14B}\) wavelength also exhibited the highest power, with the peak power difference being less than 1.45 dB.