An optically active device comprising an optical fiber (10), a light source (12) and an optical device (13) is disclosed. The optical fiber (10) comprises a core made of an optical functioning glass doped with Pr 3+ as an active ion and transmits light at a first wavelength. The light source (12) generates excitation light at a second wavelength shorter than the first wavelength. The optical device (13) directs the excitation light from the light source (12) into the optical fiber (10). Then, Pr 3+ in the core of the optical fiber (10) is stimulated to emit light at the second wavelength. As a result, an optical function such as optical amplification can be effected at the second wavelength.