## 4 Discussion  
Based on the results, we identified two simple methods to tune multi-wavelength laser spectra generated by a modal fiber-optic interferometer. Most of the features observed in the laser response depend on the modal interferometer; this one was fabricated using a cost-effective technique involving inexpensive fibers (conventional single-mode fiber and microfiber). In this technique, the fiber structure acts as a fiber filter into the erbium ring laser cavity. The initial laser spectrum is comprised of four lasing lines (a triple consecutive laser emission and one single-lasing line), where lower wavelengths drift (0.008 nm) and minimal power variations of 0.46 dB were observed. When the modal interferometer is immersed in distilled water (initial refractive index value), we observed four lasing modes: two dual laser emissions separated 13.2 nm, with a spacing mode of 1.6 nm, and wavelengths variations lower than 0.1 nm. When the refractive index is increased, the entire quad-laser spectrum is tuned, resulting in a tunable resolution of 0.05 nm. According to the hysteresis analysis, this laser offers minimal wavelength variation when it is operated in a forward and backward refractive index direction. Besides, according to the results the refractive index variations generate a 2 nm tuned range, this range can be compared with other works [(33; 45; 46)]. Here, it is important to stress that despite the higher tuning range previously reported [(27; 29; 32; 33; 45; 46; 47; 48; 49)], some works present irregular spectra, strong relation between the number lasing modes and the tuned step, lower resolution, and higher wavelength drift. The temperature response shows tunable multi-wavelength responses for different multi-wavelength laser spectra; the resolution is similar to that observed in refractive index analysis. However, temperature increments must exceed 90 \({}^{\circ}\)C; minimal temperature variations  
Figure 9: Temperature data analysis for peaks P0 to P6. Temperature-wavelength response for peaks P2, P3, and P6.  
will result in lower resolutions. According to the results and considering the high sensitivity of the interferometers [44], the proposed scheme can also be an alternative method to detect temperature and refractive indexes using a signal acquisition system, which, based on a near-infrared photodetector, can reduce the cost for sensing applications. Moreover, the fiber laser arrangement can be used for remote sensing applications. Despite the power instabilities observed, the laser offers better performance than some works [14; 18]. The laser presented in this work is an alternative, cost-effective setup for tuning multi-wavelength spectra using simple and easy-to-control techniques.  
Conceptualization, Y.L.-D. and D.J.-V.; Methodology, D.J.-V. and J.M.S.-H.; Validation, J.M.E.-A., and J.C.H.-G.; Formal Analysis, D.F.G.-M., D.J.-V., Y.L.-D. and L.A.H.-P.; Investigation, Y.L.-D.; Resources, J.M.E.-A., and R.R.-L.; Data Curation, Y.L.-D., E.G.-A.; Writing-Original Draft Preparation, D.F.G.-M., D.J.-V., Writing-Review & Editing, D.J.-V.; Visualization, D.J.-V., J.M.S.-H., Y.L.-D. and L.A.H.-P.; Supervision, J.M.E.-A., and R.R.-L.; Project Administration, J.M.E.-A., and R.R.-L.; Funding Acquisition, J.M.E.-A., and D.J.V.  
This research was funded by CONACYT, grant number [577494/307127], and the Direccion de Apoyo a la Investigacion y al Posgrado [UGTO-DAIP CIIC 283/2018].  
We would like to thank to the National Lab UG for the micrographs.  
The authors declare no conflict of interest.