Effect of optical basicity on broadband infrared fluorescence in bismuth-doped alkali metal germanate glasses

Bismuth-doped alkaline metal germanate glasses with infrared fluorescence covering the whole low loss wavelength region were developed. The infrared fluorescence centered around 1.31 μm with the full width at half maximum (FWHM) lager than 200 nm was observed in GeO ₂-R ₂O-Bi ₂O ₃ (R = Li, Na, K) glasses under an 808 nm LD excitation, whereby the fluorescence lifetime is longer than 400 μs. These results indicate that the glass material may have potential applications in broadband optical amplifiers. The intensity of the infrared fluorescence decreases with the increase in the optical basicity of the host glasses. It is suggested that the fluorescence in the near infrared region should be ascribed to the low valence state of bismuth such as Bi ²⁺ or Bi ⁺, because a higher optical basicity favors the higher valence state of the multivalence metal ions.