Energy transfer up-conversion and excited state absorption of laser radiation in Nd:YLF laser crystals

Experiments at room temperature were carried out to study the higher order processes that produced the yellow and blue fluorescence observed in Nd:YLF laser crystals, which were pumped by lasers with wavelengths between 785 nm and 811 nm, and with the pump power density ranging from 10 to 30 kW/cm², or the stored energy density, 15-45 J/cm³. The analysis of the experimental results provided an explanation for the performance degradation of a Q-switched laser with increasing pump intensity. These higher order processes were energy transfer up-conversion and excited state absorption of both pump and laser radiation. A model was also developed based upon rate equations to describe these higher order processes. This model allowed us to discriminate between these processes and to determine which one played the most important role. The experimental results agreed well with the predictions of the model. It is concluded that the energy transfer up-conversion process is the dominant mechanism for the population of the ⁴G_7/2 multiplet. It is also concluded that excited state absorption of the 1.05 μm or 1.3 μm radiation from the ⁴G_7/2 multiplet play a significant role in populating the ²P_3/2, ² D(1)_5/2, and ²P_½ manifolds, and in the performance of Nd:YLF lasers operating in the pulsed mode. The effect of excited state absorption of the pump radiation from the ⁴F_½ multiplet is much less important