Population kinetics of the upper4I11/2and lower4I13/2laser states of the Er3+ion were studied experimentally and theoretically in (Er)BaY2F8, (Er)YLF, (Er)YSGG, (Er)CaF2and (Er)YALO. Fluorescence from these states to the4I15/2ground state was excited through upconversion simultaneously with the4I11/2→4I13/2lasing using 1.53 µm radiation from an erbium : glass laser for optical pumping. Lifetimes of both states are altered during lasing by co-operative energy transfer processes: the lifetime of the lower state τ1is shortened and that of the upper state τ2increased with the resultant ratio τ2/τ1>1. After lasing the lifetime ratio returns to the ‘normal’ value τ2/τ1<1; that is, one obtained under weak ultraviolet excitation. Kinetic rate equations for the population density functions for both laser states were set up and solved by approximation in three time domains. It was assumed that only one co-operative energy transfer process operates in the laser crystals and determines the population inversion kinetics. Consistency relationships for comparison of the theoretical results with the experiment were developed. Only (Er)BaY2F8spectral features showed close agreement with theory, resulting in a high score of 94 for the overall correlation in the consistency test, whereas all other crystals scored<50. As a result of this high correlation, a close match between theoretical and experimental population decay curves was shown for (Er)BaY2F8. Most probably, more than one energy transfer process shapes the decay curves and determines the population inversion kinetics for the other laser crystals. (Er)YALO showed little lifetime change for the laser states, apparently due to inefficient co-operative energy transfer processes. As a result it probably lased in a self-terminating shor
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