This research investigates the visible upconversion luminescence which is induced by
multiphoton absorption of soft glass fiber defects. The study of this phenomenon has thus far been
restricted to standard silica fibers. We observed the emission of green and cyan light as a consequence
of fiber material ionization. We investigate both the commercial ZBLAN step index and in-housemade
tellurite nanostructured graded-index fibers. For the latter, the analysis of the luminescence
signal permits us to determine the core and cladding refractive index difference. Upconversion
luminescence is a powerful tool for characterizing soft glass fibers and a promising platform for
innovative photonic technologies and mid-IR applications.