Spectrum of stimulated emission due to self-saturating transitions in high-concentration media

A study was made of various cases of a population inversion between the ⁴I_11/2 and ⁴I_13/2 levels in Y₃Al₅O₁₂:Er³⁺ crystals. It is found that when nonradiative transitions from the upper levels to the active levels by the mechanism of multiphonon energy transfer are more probable, laser action occurs due to transitions between the Stark components of the ⁴I_11/2 and ⁴I_13/2 levels with the highest oscillator strengths. If nonradiative transitions due to ion-ion interaction are more probable, a "red shift" is observed in the lasing spectra.