Some characteristics of population inversion of the 4I13/2 level of erbium ions in ytterbium–erbium glasses

A comparative investigation is made of phosphate, germanate, and lithium silicate glasses activated with Yb³⁺ and Er³⁺ ions in order to determine their potential usefulness for the development of laser-pumped (λ = 1.06 μ) amplifiers for short (10^–8 sec) pulses at the λ = 1.54 μ wavelength. Some features of the spectral and luminescence charactenstics and the kinetics of energy transfer in these glasses are studied. It is shown that the increase in the population inversion of the ⁴I_13/2 level of Er³⁺ ions in lithium silicate and germanate glasses is considerably slower than the increase in the absorbed pump energy. Possible mechanisms for this anomalous behavior are discussed. It is shown theoretically and experimentally that the main reason for this is reverse transfer of excitation energy from Er³⁺ to Yb³⁺ ions, which is almost absent in phosphate glasses.