Er3+-doped GeGaSbS glasses for mid-IR fibre laser application: Synthesis and rare earth spectroscopy

With an infrared transparency extended to 10 µm, low multiphonon relaxation rates and suitable rare earth solubility, sulphide glasses in the Ge-Ga-Sb-S system allow radiative emission from rare earth ions in the mid-IR range. The Er3+ ion, widely studied in glass fibres for optical amplification at 1.5 µm, presents an interesting transition for mid-IR applications around 4.5 µm (4I9/2→ 4I11/2). Thus, the aim of this work is to evaluate the Er3+-doped Ge20Ga5Sb10S65 glass as a potential fibre laser source operating in the 3-5 µm mid-IR spectral region. For that purpose, absorption and emission spectra were recorded from visible to mid-IR and the radiative lifetimes of the involved excited levels (4I9/2, 4I11/2 and 4I13/2) were determined. Experimental results were compared with those obtained from a Judd-Ofelt analysis based on the absorption cross-sections of all observable transitions. The 4I9/2 radiative quantum efficiency was estimated to be 64% and the emission cross-section at 4.6 µm was found equal to 2.85 X 10-21 cm2. Core only and core/clad Er3+-doped Ge20Ga5Sb10S65 glass fibres were successfully drawn with minimum optical losses of about 1.5 and 10 dB/m at 5.2 and 3.5 µm, respectively. Fluorescence signals in the mid-IR (at 2.7 and 4.6 µm) were clearly observed with both fibres.