A simple method based on the sol-gel technology has been developed to coat passive microspheres with an active coating. The microspheres were prepared by fusion of a standard telecom fiber with a dimension of about 200 μm and 400 μm and have been respectively dipped in a 70SiO2-30HfO2 sol activated by 1 mol% and 0.1 mol% of erbium ions.Here we first report about the luminescence properties of a silica-hafnia coating doped with erbium ions and then whispering gallery mode spectra were analysed for different sphere diameters, thickness of coating and erbium concentration. The thickness of the coating has been chosen in order to support at least one whispering gallery mode at 1.5 μm. A simple method based on the sol-gel technology has been developed to coat passive microspheres with an active coating. The microspheres were prepared by fusion of a standard telecom fiber with a dimension of about 200 μm and 400 μm and have been respectively dipped in a 70SiO2-30HfO2 sol activated by 1 mol% and 0.1 mol% of erbium ions.Here we first report about the luminescence properties of a silica-hafnia coating doped with erbium ions and then whispering gallery mode spectra were analysed for different sphere diameters, thickness of coating and erbium concentration. The thickness of the coating has been chosen in order to support at least one whispering gallery mode at 1.5 μm.