Abstract

Despite of a longer lifetime of the lower laser level ⁴I_11/2 as compared to the upper laser level ⁴I_13/2 efficient room temperature laser emission between these levels has been obtained in concentrated Er³⁺ systems; this is caused by an up-conversion process from the ⁴I_13/2 level which recirculates its excitation to the upper level ⁴I_9/2 from which the major part decays to the emitting level. Since the up-conversion process takes place essentially inside the n.n. pairs of excited Er ions, a high concentration (usually above 5%) is necessary to on-set the migration within the ⁴I_13/2 levels making the up-conversion effective. However, at these concentrations other energy transfer processes could also take place, such as an up-conversion from the emitting level ⁴I_11/2 and various cross-relaxation processes which could compete with the radiative and non-radiative de-excitation. Thus the energy transfer processes lead to a tremendous recirculation of excitation inside the system of Er³⁺ ions and make the three micron emission possible, regardless of the pump level. This emission is usually modelled numerically by solving the system of rate equation for levels up to the ⁴S_3/2. However, it was shown that in case of garnet crystals, characterised by strong multiphonon processes, an analytical solution could be also obtained.

© 1996 IEEE