Upconversion-pumped population kinetics for4I13/2 and4I11/2 laser states of Er3+ ion in several host crystals
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- Pollack, S.A. & Chang, D.B. Opt Quant Electron (1990) 22: S75. doi:10.1007/BF02089002
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Population kinetics of the upper4I11/2 and lower4I13/2 laser states of the Er3+ ion were studied experimentally and theoretically in (Er)BaY2F8, (Er)YLF, (Er)YSGG, (Er)CaF2 and (Er)YALO. Fluorescence from these states to the4I15/2 ground state was excited through upconversion simultaneously with the4I11/2 →4I13/2 lasing 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 τ1 is shortened and that of the upper state τ2 increased 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)BaY2F8 spectral 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 short-pulse mode.