Investigations on the NIR Fluorescence Band Modulation of Bi-Doped Silica-Based Glasses and Fibers

Bi-doped glasses and fibers have been widely applied in solid-state and fiber lasers. However, the mechanism underlying near-infrared (NIR) luminescence remains unclear, and Bi-related luminescence centers (BLCs) are prone to alteration during fiber fabrication, making it challenging to achieve high-performance Bi-doped glass fibers. In this work, Bi-, Bi-Al-, and Bi-Ge-doped silica glasses were investigated to elucidate the origin of NIR luminescence. Two broad NIR fluorescence bands were observed in silica glasses, originating from distinct BLCs. The longer-wavelength fluorescence band at 1423 nm, demonstrating sensitivity to Bi doping concentration and homogeneity, is attributed to Bi clusters (aggregates of Bi+ ions), whereas the shorter-wavelength emission, independent of Bi concentration, originates from isolated Bi+ ions. A vacuum-assisted melting-in-tube method with a single-step heating process was employed to fabricate Bi-doped silica-based glasses and fibers. The fluorescence bands of the fibers remained consistent with those of the precursor glasses, indicating no new BLCs were formed during fiber fabrication. The modulation of fluorescence bands was primarily governed by Bi cluster formation. Suppressing Bi clustering through co-doping with Al/Ge or optimizing fabrication conditions offers an effective route to tailor the fluorescence properties of Bi-doped glasses and fibers.