## 1 Introduction  
In recent years rare-earth-doped glasses have been subject of intense research as host materials because of their significant optical properties, which make them adequate as infrared and upconversion lasers, optical amplifiers, and active photonic devices [1, 2, 3, 4, 5, 6, 7, 8, 9]. In particular, silicate and aluminosilicate glasses present excellent thermal and mechanical properties, and corrosion resistance to be used in practical applications [10, 11, 12, 13, 14, 15, 16, 17]. In addition, their maximum phonon energy (\(\sim\)1050 cm\({}^{-1}\)) is much lower than that of phosphate and borate glasses, \(\sim\)1300 cm\({}^{-1}\) and \(\sim\)1350 cm\({}^{-1}\), respectively, so that quantum efficiency is less influenced by multiphonon relaxation processes [18, 19, 16, 17].  
Er\({}^{3+}\) ions are among the most interesting active centers to be studied because of its potential applications in the field of infrared optical amplification related to the radiative efficiency of the \({}^{4}\)I\({}_{13/2}\)\(\rightarrow\)\({}^{4}\)I\({}_{15/2}\) emission at around 1.55 \(\upmu\)m [1, 2, 20]. Furthermore, the rich energy level structure of this rare-earth allows the excitation of the \({}^{2}\)H\({}_{11/2}\)\(\rightarrow\)\({}^{4}\)I\({}_{15/2}\), \({}^{4}\)S\({}_{3/2}\)\(\rightarrow\)\({}^{4}\)I\({}_{15/2}\), and \({}^{4}\)F\({}_{9/2}\)\(\rightarrow\)\({}^{4}\)I\({}_{15/2}\) upconversion emission bands centered at around 530, 550, and 665 nm using wavelengths in the near infrared spectral regions [1, 2, 22]. NIR-to-visible energy conversion mechanism involves the conversion of low-excitation-energy photons into high-energy emitted light in the visible range through non-linear anti-Stokes processes. In addition, these ions can also be used as local ordering probe because of the close relation between their spectroscopic properties and the local structure and bonding at the ion site [23, 24, 25, 26, 27].  
In a previous work, we reported on how to control the optical transmission of aluminosilicate glasses manufactured departing from commercial glass-ceramics by means of the Laser Floating Zone (LFZ) technique [28]. This technique utilizes an infrared laser source to create a molten zone in the material from which, by controlling the solidification rates, a new material with controlled microstructure is produced. Surrounding medium during the fabrication process, in terms of oxidizing or reducing atmosphere, can also be controlled. We reported that when the fabrication of these aluminosilicate glasses took place in an oxidizing atmosphere, Ti\({}^{3+}\) centers contained in the glass turned into Ti\({}^{4+}\) ions, giving rise to colorless glasses, the transmittance of which ranged 80% in the visible spectral range. Therefore, it was possible to tailor the resulting optical transmission. In this work we have taken advantage of this feature to fabricate erbium-doped glasses in oxidizing atmosphere to give rise to optical active glasses of high optical transmission. Spectroscopic and NIR-to-visible upconversion properties have been studied and the possible excitation mechanisms responsible for this upconversion luminescence were discussed.