## 3 MZFI Performance  
The spectral response of the MZFI under controlled curvature conditions was measured using amplified spontaneous emission (ASE) from the erbium-doped fiber as the input light source before implementing the laser cavity. Figure 3 shows the baseline ASE spectrum (black line) and reshaped ASE (red line) obtained with the MZFI maintained straight (no curvature added). The reshaped ASE spectrum exhibits a weak comb-like shape from 1540 nm to 1560 nm. This phenomenon is due to the weak higher-order coupling modes resulting from the geometrical dimensions of the MZFI and the adiabaticity criteria (quasi-abrupt tapers) in the fabrication process [24]. The adiabaticity criteria define the gradual transition of fiber diameter in tapered structures, light mode propagation and coupling to higher-order modes. Quasi-abrupt tapers feature a slow tapering transition length that allows for weak higher-order coupling modes without significant energy loss. Additionally, waist length, waist diameter, and separation taper length impact the comb-like spectrum's fringe visibility, spectral positioning, and free spectral range, necessitating a trade-off to achieve optimal light transmission and spectral characteristics. As the applied curvature increases, the stress induces a modification in the refractive index profile  
Figure 3: ASE spectrum as the light source from the EDF (black line) and the modified spectrum after passing through the MZFI (red line). The laser oscillation at 1563.07 nm is shown by the blue line. Figure 2: (**a**) MZFI structure formed with a pair of tapered sections fabricated on SMF-28, (**b**) displacement mechanism to used induce curvature in the MZFI. via the elasto-optic effect, which reconfigures the spectral characteristics of the MZFI [25]. Consequently, according to [25], increased fringe visibility is expected to result in greater interference peaks and more defined comb shapes, highly desirable characteristics in multi-wavelength fiber lasers. The measured insertion loss of the MZFI was about \(\sim\)1 dB. By closing the ring laser cavity and increasing the pump power to 40.5 mW, a single laser wavelength was stabilized at 1563.705 nm, reaching an OSNR of approximately 45 dB, as depicted by the blue line in Figure 3.  
The controlled curvature was applied by displacing the translation stage in steps of approximately 1.27 mm. The curvature is inversely related to the bending radius and can be calculated using the expression [26]: \(C=\frac{1}{R}=\sqrt{\frac{24x}{\left(L_{0}-x\right)^{3}}}\), where \(L_{0}\) represents the initial distance between the two mounts, \(x\) denotes the linear displacement of the translation stage, \(R\) is the bending radius, and \(C\) is the curvature in units of m\({}^{-1}\). Figure 4a depicts the modified ASE spectral transmission of the MZFI under different applied curvature levels (from 0 m\({}^{-1}\) to 2.93 m\({}^{-1}\)). As can be observed, a more defined comb-like spectral modulation is achieved by increasing the curvature, which strengthens the depth of the attenuation bands (increasing fringe visibility) and furthermore induces a wavelength shift. This detailed spectral response is shown in the inset of Figure 4b between the wavelengths of 1541 nm and 1560 nm. The same behavior occurs in all ASE spectra, but it is less apparent due to the peak shape between 1520 nm and 1540 nm and the marked intensity decrease beyond 1560 nm, resulting from the unique fluorescence emission of erbium-doped fiber (EDF). The measured free spectral range (FSR) is about 5 nm (inset Figure 4b) and remains relatively stable due to fiber curvature. In this context, the FSR mainly depends on the separation length between the tapers; a longer distance L results in a narrower FSR. It is crucial to characterize aspects such as the wavelength shift and fringe visibility in response to applied curvature. A blue shift is observed as the curvature is applied by selecting and monitoring the peak at 1555.8 nm (corresponding to 0 m\({}^{-1}\)).