## 3 Schematic Configuration and Experimental Setup  
The following is a diagram of the experimental setup used for the multi-wavelength thin-core erbium-doped fiber laser (MTEFL), as presented in Figure 4a. The laser operates within a 9-meter-long cavity and utilizes a 980 nm pumping laser diode (Bookham LC94Z174-20R; Bookham, Inc., San Jose, CA, USA) together with an LDC that provides  
Figure 3: (**a**) Interference spectra generated in the optical cavity of each MZI filter, (**b**) spatial frequency of the transmission spectrum generated by every filter. a power output of 135 mW. In the setup, a 980/1550 Wavelength Division Multiplexer (WDM) is used, along with an Optical Isolator (ISO), a 1-meter-long erbium-doped fiber (EDF) (Thorlabs ER80-8/125; Thorlabs, Inc., Newton, NJ, USA), and an Optical Coupler (OC). Within the OC, 1% of the pumping power is directed toward the Optical Spectrum Analyzer (OSA; Yokogawa AQ6370D; resolution 0.02 nm; Yokogawa Electric Corporation, Musashino, Tokyo, Japan), while the remaining 99% is retained within the ring array to facilitate lasing. Additionally, a PC is incorporated into the setup. The MZI filter is positioned at the center of a 15-gauge steel wire that measures 350 mm in length and 1.44 mm in diameter. Small pieces of adhesive tape are used to secure the MZI to a metric rod, preventing any potential rotation that could affect the optimal functioning of the filter. Two stationary bases support the optical fiber (OF). One of the bases features a one-axis motorized translation stage (MT1-Z8), which utilizes a Z812B motor and a Kinesis KDC101 motor driver. The motor driver has a resolution of 0.1 mm per step, allowing for the adjustment of the curvature of the filter, as shown in Figure 4b. The tuning and multi-line switching (multi-wavelength) are achieved by adjusting multiple wavelengths through the movements of the left motor. The lineal initial displacement is 14.70 mm to 224.70 mm, with an increment of 0.10 mm. When the curvature sensor is bent, the dip wavelength in the interference spectrum can be given by [27]:  
\[\lambda_{m}=\frac{2\Delta n_{eff}L}{2m+1}+\frac{kLb}{2m+1}\times\frac{1}{R} \tag{4}\]  
where \(k=-0.1649\times 10^{-9}\)\(\mu\epsilon^{-1}\) is the strain refractive index coefficient, and \(b\) is the distance from the cladding to the core. At each linear increment, \"x\" is measured and estimated by the curvature expression [28]:  
\[\mathrm{C}=\frac{1}{R}=\sqrt{24\mathrm{x}/\left(\mathrm{L}_{0}/2\right)^{3}} \tag{5}\]  
where \"R\" is the curvature radio, \"x\" is the movement distance of the moveable fiber end, and \"L\({}_{0}\) = 210 mm\" is the distance between the edges of the two bases. The curvature range is from 2.3452 m\({}^{-1}\) to 6.0495 m\({}^{-1}\). The curvature radius is adjusted over the MZI, generating single-, double-, triple- and quadruple-line emission. We used 147 samples in a  
Figure 4: Diagram of the MTEFL ring array: (**a**) illustrates the configuration scheme, and (**b**) depicts the process for inducing curvature in the MZI filter to generate wavelength-switchable tunable emission lines. total sweep of the first and last emission lines of the MTEFL (Figure 5) at room temperature (25 \({}^{\circ}\)C). In state-a, we obtained a single tunable lasing line ranging from 1556.63 nm up to 1564.25 nm, with a wavelength-tunable range of 7.62 nm into a curvature range from 2.3452 m\({}^{-1}\) to 4.9323 m\({}^{-1}\). In state-b, the curvature radius is 4.9537 m\({}^{-1}\), generating a signal with quadruple lines. In state-c, we obtained two different signals with triple lasing lines, with a curvature of 4.9750 m\({}^{-1}\) and 4.9962 m\({}^{-1}\). In state-d, we could tune the dual narrow lasing line signal from 1564.56 nm to 1561.34 nm, with a wavelength-tunable range of 3.22 nm into a curvature range from 5.0173 m\({}^{-1}\) to 5.2843 m\({}^{-1}\). Lastly, in state-e, we could tune dual lasing lines, with one ranging from 1585.69 nm to 1572.53 nm and the second line from 1576.89 nm to 1563.68 nm.