Bismuth-doped glasses have recently received significant interest as potential material for ultrabroadband optical amplification in the telecommunication spectral bands, as well as as gain material for fiber lasers. However, the nature of the active centers that are responsible for the observed near-infrared (NIR) luminescence is still highly debated. In order to probe the mechanism that leads to NIR emission in bismuth-containing glasses, femtosecond (fs) laser irradiation was used. It is shown that local absorption properties in the visible spectral range can be altered in initially transparent bismuthate glasses after fs laser irradiation. Induced absorption centers exhibit the well-known broadband optical emission peaking at similar to 1250 nm when excited with a 785 nm diode laser. Absorption and emission intensities increase with increasing average pulse energy. These observations are interpreted as a photo-induced reduction reaction of Bi3+ to Bi+ species, while the previously discussed formation of Bi-clusters by ion diffusion is excluded due to the very short interaction time that results from the use of fs laser. Bi+ species are, therefore, proposed as the major origin of NIR emission from Bi-doped glasses.