The development of new active optical fibre devices requires materials with « augmented » intrinsic properties, though using luminescent ion (LI) –doped silica as host glass. Optical fibers containing dielectric nanoparticles (DNP) grown by phase separation are proposed. DNP would optimally fully encapsulate LI to produce « engineered » spectroscopic properties. To determine the composition of DNP, Secondary Ion Mass Spectrometry imaging at high spatial resolution is employed. Through the use of this technique, we demonstrate without ambiguity the partition of Mg, P and Er in DNP. By increasing Mg concentration, Si concentration in DNP decreases, which explains the spectroscopic behaviour of Er.