Quenching of erbium and ytterbium luminescence by the random walk of H2 and D2 molecules in the silica glass of active optical fibers

Photoluminescence of Er3 + (transition 4I13/2 → 4I15/2, emission wavelength λe ~ 1550 nm) and Yb3 + (2F5/2 → 2F7/2, λe ~ 1040 nm) ions incorporated in the silica of optical fibers immersed in a H2 and/or D2 environment at a pressure of up to 120 bar at temperatures of 80-873 K is investigated. The spectra and decay kinetics of the steady state luminescence excited by a 975-nm laser diode are recorded. It is shown that the thermal motion of interstitial H2 and/or D2 molecules quickens the decay kinetics and decreases the luminescence intensity. The effect becomes noticeable at temperatures greater than ~ 200 K and significantly increases with a further temperature increase in H2 and/or D2 environments. The luminescence decay curves increasingly differ from single exponents with the increase of temperature and become best fitted by stretched exponential dependencies. This effect is reversible at temperatures of up to 800 K. We explain the revealed quenching effect of excited f-electron in terms of the rare earth ions due to the appearance of an additional relaxation channel involved in the excitation of vibrational degrees of freedom of diatomic molecules experiencing random walks inside the glass network. This study found that the closer the electron excitation energy of a rare earth ion to an energy gap in the vibration spectrum of a molecule, the greater the deactivation effect. The quenching rate significantly decreases when the molecules' rotational degrees of freedom are frozen out.