Rare-earth (RE) doped optical fibers are extensively used in lasers and optical amplifier devices. These key applications rely on the qualities of silica glass: mechanical and chemical stability, high optical damage threshold, low cost, etc. However, silica glass has certain characteristics which may make it less efficient compared to other types of glass, particularly in some potential applications using RE ions: high phonon energy, low solubility of RE ions, etc. To overcome these limitations, one recent strategy consists of developing a fabrication method which triggers RE encapsulation in phase-separated nanoparticles. The development of this family of optical fibers was driven by this requirement: the particles must be as small as possible to avoid light scattering. However, recent studies discussed in this article tend to disapprove this doxa. First, we present the fabrication process of such fibers, emphasizing the drawing step as a process to control the shape and size of the nanoparticles. Then, we discuss on the characterization of the composition of these nanoparticles at the nm-scale. To reach this goal, we took advantage of a recent technology: Atom Probe Tomography. These results will be compared with molecular dynamics simulations. We demonstrate that the phase-separated nanoparticle composition and therefore the chemical environment of the encapsulated RE ions is nanoparticles size dependent. As a consequence, the smallest nanoparticles, promoted by the doxa, would offer limited alteration of the luminescent properties. Finally, light scattering is not only an issue but is also an opportunity to develop new temperature, strain, refractive index multiplexed optical fiber sensors.