Fig. 1. Luminescence spectra of typical Bi-doped tantalum germanate glass (1) before and (2) after being heat treated at 750°C for 1 h upon the excitation of the 808 nm laser diode (LD). Inset shows the change from the original reddish brown transparent sample to the devitrified opaque sample.

Fig. 2. (Color online) Luminescence spectra of gallogermanate glasses doped with different contents (1% to 10%, as labeled) of Bi upon the excitation of the 808 nm LD. Inset shows that the new developed glass keeps the reddish brown transparency though being heated at 600°C for 1 h.

Fig. 3. (Color online) (a) Luminescence (λex=808  nm) and (b) absorption spectra of 5 mol% Bi-doped gallogermanate glasses melted at different temperatures (1250°C–1500°C).

Fig. 4. Dependence of emission intensity on temperature during the heating and cooling yoyo experiments.

Fig. 5. (Color online) Emission spectra of Bi-doped gallogermanate glass (blue line) and fiber (green line), λex=808  nm. Inset shows the processes from bulk glass to fiber. Last photo in the inset is the cross section of the fiber.

Fig. 6. (Color online) Emission spectra of Bi-doped gallogermanate glass fiber excited by (a) 808 nm LD and (b) 980 nm LD with different incident powers.

Fig. 7. (Color online) ASE spectra of Bi-doped gallogermanate glass fibers with different lengths excited by the 980 nm fiber laser with 450 mW output power. Bi-doped silica glass fiber with 2 m length shows no obvious ASE in the spectral range.