**4. Conclusions**

This paper shows a method of fabrication of active POFs starting from only core PMMA fibers, by immersing them in a solution of methanol and the organic dye rhodamine B for an appropriate period of time at a suitable temperature. We demonstrate that the penetration behavior gradually changes from the ideal diffusion category known as Case II to the one known as Case I or Fickian diffusion as the temperature is increased from low values (10 ◦C) to high ones (50 ◦C). Although the front penetrates more slowly at lower temperatures, the corresponding linear-attenuation coefficients are lower, due to the absence of hole formation in the core material if, and only if, the temperature

of the doping process is lower than the corresponding glass-transition temperature. Moreover, we also measure and analyze the spectral characteristics and the intensity of the emission of doped fibers as functions of both the propagation distance and the excitation time. The corresponding results are in good agreemen<sup>t</sup> with those reported for similar active POFs doped by means of other fabrication techniques, such as the gel-polymerization technique. Furthermore, our fibers present a higher photostability, owing to the lower probability of formation of dopant aggregates when using our doping technique. This improvement, together with the rest of optical features and ease of fabrication, could be very interesting in the design process of all-optical devices based on active POFs, such as switches, lasers and amplifiers, optical sensors and solar concentrators.

**Author Contributions:** I.A. and J.Z. conceived and designed the experiments; E.A. and G.D. fabricated the PMMA optical fibers; I.A., M.A.I. and M.A. performed the experiments; I.A., J.Z., and J.A. analyzed the data; all authors contributed to the scientific discussion and wrote the paper.

**Funding:** This research was funded by European Regional Development Fund (ERDF), by Ministerio de Economía y Competitividad (MINECO) (TEC2015-638263-C03-1-R) and by Eusko Jaurlaritza (ELKARTEK KK-2016/0030, ELKARTEK KK-2016/0059, ELKARTEK KK-2017/00033, ELKARTEK KK-2017/00089, IT933-16). The work of Mikel Azkune was supported in part by a research fellowship from the Universidad del País Vasco/Euskal Herriko Unibertsitatea (UPV/EHU), Vicerrectorado de Euskera y Formación Continua, while working on a Ph.D. degree.

**Acknowledgments:** We would like to thank Ester Zuza for her insightful and constructive comments during the development of this work.

**Conflicts of Interest:** The authors declare no conflict of interest.
