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.

An experimental and theoretical investigation of the nonlinear transmission coefficient in a set of Ytterbium-doped silica fibers (YFs) with various concentrations of Yb(3+) ions at continuous-wave 980-nm pumping is reported. An analysis of the obtained experimental data shows that YF transmission coefficient is notably affected by the presence of Yb(3+) - Yb(3+) ion-pairs in the fibers, especially in heavily-doped ones. The last fact is confirmed by the study of the cooperative luminescence and absorption effects in the fibers, where a detailed inspection of their dependence on Yb3+ concentration is presented. The pairs' effect is shown to seriously modify both the nonlinear character of YF transmission coefficient at lambda = 980 nm and Yb(3+) excited-state relaxation. A modeling of the experimental data is performed, which allows to find the coefficients addressing the pairs' effect in each of YFs under study and, as a result, to fit the experimentally measured dependences of YF transmission coefficient on pump power, fiber length, and Yb(3+) concentration.

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

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We report on the first experimental observation of an influence of noise in a pumping diode laser on the appearance of self-induced modulation and self-pulsing in an all-fiber Erbium laser. The diode laser amplitude noise, being a superposition of spectral components, with some of them to resonantly match the frequency of relaxation oscillations in the Erbium fiber laser at the pump rate fixed, acts as a disturbing force, being a source of the modulation effects. This disturbance enforces the relaxation oscillations of the Erbium fiber laser to become non-dumped, which appears as the self-modulation and even self-pulsing of the laser output power.

We report the study of the excited-state absorption (ESA) spectrum of Er3+ in erbium-doped silica fiber (EF) within the telecom band, 1.48–1.59μm. Two experimental techniques, the detection of up-conversion emission following the ESA process and the measurement of the EF nonlinear transmission coefficient, both supplied with a theoretical background, allow obtaining a mutually consistent spectral behavior of the ESA parameter, which is the ratio of ESA to ground-state absorption cross section, at 1.48–1.59μm excitation.