## 4 Tunable and Switchable Performance  
Following the evaluation of the filter performance, the ring cavity was closed and the pump power was adjusted to 50.6 mW; single-wavelength laser oscillation at 1563.705 nm was achieved (refer to Figure 3). The fiber laser performance was evaluated by increasing the curvature level of the MZFI from 0 m\({}^{-1}\) to 2.79 m\({}^{-1}\). During the process, distinct output laser characteristics were observed, including tunable single-wavelength, dual-switchable, and switchable multi-wavelength emissions with up to six wavelengths. Figure 6 displays the tunable single emission ranging from 1563.705 nm to 1558.05 nm when the curvature is gradually increased from 0 m\({}^{-1}\) to 1.97 m\({}^{-1}\), resulting in a maximum tunable range of 5.65 nm. The measured OSNR is about 55 dB for most of the lasing wavelengths, while the amplitude power difference of each wavelength is less than 1.6 dB. In contrast, four sets of switchable dual-wavelength emissions were generated between 1543.21 nm and 1563.66 nm for curvatures of 1.53 m\({}^{-1}\), 2.33 m\({}^{-1}\), 2.5 m\({}^{-1}\), and 2.75 m\({}^{-1}\), as depicted in Figure 7. The wavelength separations were as follows: 4.59 nm for 1.53 m\({}^{-1}\) (1559.07 nm and 1563.66 nm, Figure 7a), 6.07 nm for 2.33 m\({}^{-1}\) (1543.21 nm and 1549.287 nm, Figure 7b), 5.05 nm for 2.5 m\({}^{-1}\) (1554.61 nm and 1559.667 nm, Figure 7c), and 5.01 nm for 2.75 m\({}^{-1}\) (1556.8 nm and 1561.877 nm, Figure 7d). These lasing wavelengths demonstrated an optical signal-to-noise ratio (OSNR) above 40 dB. The curvature was carefully increased to 2.363 m\({}^{-1}\) to induce switchable multi-wavelength laser emission. This adjustment caused the lasing line to exhibit a slight blue shift, resulting in a single wavelength centered at 1562.22 nm with an amplitude of 38.4 dB, as illustrated in Figure 8a. From the figure, the optical spectrum in the range of 1558 nm to 1567 nm exhibits a slight comb-like pattern with a flattened profile at the upper portion. This effect, resulting from the MZFI being subjected to curvature, leads to a flattened gain profile of the erbium-doped fiber, thereby promoting mode competition  
Figure 5: The wavelength shifts as a function of curvature (0 m\({}^{-1}\) to 2.93 m\({}^{-1}\)) and the variation of the measured fringe visibility. and allowing multiple longitudinal modes to oscillate simultaneously. These adjacent modes, observable in both directions relative to the actual laser emission, can be further amplified to generate more laser lines by precisely controlling the polarization states and the curvature applied to the MZFI. By maintaining the current curvature and adjusting the polarization state via P.C., switchable multi-wavelength laser emission was realized, resulting in the sequential production of laser lines. Dual-wavelength (1562.22-1562.875 nm), triple-wavelength (1560.175-1562.22-1562.88 nm) and quadruple-wavelength (1560.85-1561.51-1562.26-1562.86 nm)  
Figure 6: Tunable single laser wavelength between 1563.705 nm to 1558.05 nm for the curvatures from 0 m\({}^{-1}\) to 2.79 m\({}^{-1}\). Figure 7: **(a)**. Switchable dual-wavelength emissions generated at curvatures of 1.53 m\({}^{-1}\) with 1559.07 nm and 1563.66 nm, **(b)** 2.33 m\({}^{-1}\) with 1543.21 nm and 1549.287 nm, **(c)** 2.5 m\({}^{-1}\) with 1554.61 nm and 1559.667 nm, **(d)** 2.75 m\({}^{-1}\) with 1556.8 nm and 1561.877 nm. emissions were obtained, as shown in Figure 8b,c and Figure 9a, with an OSNR of 35.7 dB, 35 dB, 34.9 dB, respectively. To excite and amplify additional adjacent modes beyond the quadruple-wavelength laser emission, the curvature was slightly adjusted to 2.417 m\({}^{-1}\). This adjustment resulted in a slight gain flattening across the spectral range of the multi-wavelength emission.