Reversible photo-darkening and resonant photo-bleaching of Ytterbium-doped silica fiber at in-core 977-nm and 543-nm irradiation

We report experimental results showing novel features in photo-darkening and photo-bleaching of a heavily-doped Ytterbium silica fiber exposed to in-core 977-nm and 543-nm irradiation. It is shown that pumping of the fiber at the resonant 977-nm wavelength leads to an increase of absorption in the spectrally wide range (400–1000 nm, photo-darkening) and a simultaneous decrease of the resonant (Yb 3+ ) absorption coefficient near ∼ 1 μm (resonant photo-bleaching). Such a character of the Ytterbium fiber spectra transformations allows us to propose that they are a signature of the 977-nm light-induced Yb 3+ → Yb 2+ conversion process, with the spectrally wide absorption, or photo-darkening, to arise owing to the Yb 2+ centers formation and the resonant photo-bleaching to stem from the corresponding decrease of Yb 3+ ions concentration. It is as well demonstrated that an exposure of the Ytterbium fiber, where the aforementioned transformations have occurred, to the 543-nm light results in a partial return of the initial fiber properties – bleaching of the spectrally wide excess loss in the visible and increasing of the resonant 977-nm (Yb 3+ ) absorption peak. Therefore, we reveal a reverse process, Yb 2+ → Yb 3+ , possibly taking place in the last case.