Abstract: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 propertiesbleaching 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.

Dependences of spectral positions of laser line λg on the left (blue) and right (red) bounds of the SLLS regime

We report a study of the attenuation spectra transformations for a series of Bismuth (Bi) doped silica fibers with various contents of emission-active Bi centers, which arise as the result of irradiation by a beam of high-energy electrons. The experimental data reveal a substantial decrease of concentration of the Bi centers, associated with the presence of Germanium in silica glass, at increasing the irradiation dose (the resonant-absorption bleaching effect in germano-silicate fiber). In contrast, the spectral changes that appear in Bi doped alumino-silicate fiber have through irradiation a completely different character, viz., weak growth of the resonant-absorption peaks ascribed to the Bi centers, associated with the presence of Aluminum in silica glass. These results demonstrating high susceptibility of Bi centers to electron irradiation while opposite routes of the irradiation-induced spectral changes in Bi doped germanate and aluminate fibers seem to be of worth notice for understanding the nature of these centers.

A modeling of an all-fiber self-Q-switched Erbium laser is developed, implying that the mechanism of self-Q-switching is the power-dependent thermo-induced lensing in Erbium fiber that stems from the excited-state absorption at the laser wavelength.

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We report an experimental and theoretical investigation of the nonlinear transmission coefficient of a heavily doped (2300 ppm) Erbium silica fiber at continuous-wave pumping at the wavelength 1560 nm. It is shown that the fiber transmission is essentially deteriorated by the nonlinear losses, which are caused by the excited-state absorption (ESA) and Erbium ion pairs (IP) presented in the fiber. These phenomena inevitably result in worsening of the amplifying and lasing potential of the heavily doped Erbium fiber. We demonstrate the latter on the example of an Erbium fiber laser (wavelength, lambda= 1560 nm) under IR (wavelength, lambda= 978 nm) pumping, where the heavily doped Erbium fiber is used as an active medium. The developed theory, addressing both the nonlinear transmission coefficient of the fiber at the 1560-nm pumping and the generation characteristics of the Erbium fiber laser, takes into account the additional losses and non-radiative relaxation factors stemming from the ESA- and IP-effects and allows getting a good agreement between the modeling and experimental results.

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