The influence of Thulium impurity on photobleaching in a photodarkened ytterbium-doped silica double-cladding fiber

We report on the photobleaching effect on photodarkening in an ytterbium (Yb)-doped silica double-cladding fiber by pumping it with 793 nm, 808 nm and 1080 nm lasers. Enlightened by the different recovery processes under 793 nm, 808nm and 1080 nm pumps, it is suggested that the thulium (Tm) plays important roles in the photobleaching of photodarkening in a Yb-doped silica double-cladding fiber. Based on the related reports, a new idea is proposed, stating that the photodarkening and bleaching phenomena in the Yb-doped fiber could be analyzed from the concept of an e⁻ and h⁺ equilibrium. The photodarkening is due to the electron, ionized by the ultraviolet photon. The bleaching is due to the offset of e⁻ and h⁺. The Tm ions, existing as a critical factor for photodarkening pumped by its absorbent wavelength, could accelerate the offset of e⁻ and h⁺. Based on this, the photodarkening phenomenon in Yb-doped fiber lasers could be suppressed by pumping at the wavelengths located at the absorption peaks of the Tm. (letter)