Population of lasing erbium levels in YLF : Er3+ crystals under upconversion cw LD pumping

Formation of inverse population of working levels of 3 μm and green laser transitions in LiY1−xErxF4 (x=0.003–1) crystals under IR cw InGaAs laser-diode pumping is studied. Populations of studied levels were experimentally controlled by luminescence spectra. Dependencies of steady-state population on the dopant concentration and pump power density are calculated from a system of rate balance equations for 7 lowest excited erbium states. The energy-transfer mechanisms are determined and the predominant mechanisms elucidated. Microparameters and concentration dependence of the energy-transfer rates and non-linear coupling are obtained from theoretical and experimental rates of intra- and intercenter relaxation. Good agreement between the theory and experiment is obtained.