ИСТИНА

The problem of photodarkening of Yb-doped glass fibers intended for high peak power amplification receives substantial attention with different models being forwarded to account for the observed induced losses. Involvement of the charge-transfer state (CTS) excited by the cooperative excitation of Yb-ions resulting in the formation of Yb2+ ions and oxygen hole centers in the process of CTS relaxation was suggested in [1,2] . However, charge-transfer transitions, which are well-studied for Yb-doped crystalline matrices, were hardly studied for glass materials. We attempt to bridge this gape presenting the results of time-resolved luminescent spectroscopy of glass samples comparing them with those for YAG-Yb and Y2O3-Yb single crystals. The fiber core preform samples (transverse slices) were fabricated by MCVD method with two compositions: Yb2O3/P2O5/SiO2 and Yb2O3/Al2O3/SiO2. A strong absorption in the spectral range 5 to 7 eV was observed for both types of the samples. It was proposed to be associated with CT transitions. Excitation spectra of intraconfigurational 2F5/2 → 2F7/2 transitions resulting in the well-known infrared Yb3+ luminescence have pronounced maxima in the same region. In case of Yb-doped crystals these transitions represent the final stage of the radiative relaxation of the CTS with the previous one being CTS → 2F5/2 transition. A more complicated situation in case of glasses is discussed. Some of the CT-excited Yb3+ ions (not more than several percent) do not relax back to the ground state radiatively, but “trap” electrons from the nearest oxygen atoms forming divalent ytterbium (Yb2+ ). A weak and broad visible luminescence near 2-2.5 eV may have a contribution of photoinduced Yb2+ [3]. References [1] A.A. Rybaltovsky, K.K. Bobkov, V.V. Velmiskin, A.A. Umnikov, I.A. Shestakova, A.N. Guryanov, M.E. Likhachev, M.M. Bubnov, E.M. Dianov, Proc. of SPIE Photonics West 2014, 8961 (2014) 896141. [2] A.A. Rybaltovsky, A.A. Umnikov, K.K. Bobkov, D.S. Lipatov, A.N. Romanov, M.E. Likhachev, V.B. Sulimov, A.N. Gur’yanov, M.M. Bubnov, E.M. Dianov, Quantum Electronics, 43 (2013) 1037. [3] S. Rydberg, M. Engholm, Optics Express, 21 (2013) 6681