Assessment of laser potential of YVO4:Yb, Ho and YVO4:Yb, Tm

The processes of excitation, nonradiative energy transfer, and relaxation of excited states relevant to the laser performance of YVO 4 :Yb, Ho and YVO 4 :Yb, Tm crystals are investigated. The main shortcoming of holmium-doped laser materials is a lack of pump bands corresponding to the emission wavelengths of powerful diode lasers. The incorporation of ytterbium ions into the YVO 4 :Ho crystal provides an efficient means of population buildup on the 5 I 7 level via a Yb 3+ -Ho 3+ energy transfer. This transfer process has to be phonon-assisted; nevertheless, its efficiency is quite high owing to the extended spectrum of lattice vibrations of the YVO 4 matrix. The YVO 4 :Tm system is better suited for laser diode pumping thanks to a relatively intense pump band associated with the 3 H 6 - 3 H 4 transition around 800 nm. In the limit of a low Tm 3+ concentration, the quantum efficiency of the 3H 4 level is sufficiently high for the consideration of laser operation associated with the 3 H 4 - 3 F 4 transition. At a higher thulium concentration, the 3 H 4 excitation is transferred to the 3 F 4 level by a cross relaxation process and the laser operation associated with the 3 F 4 - 3 H 6 transition near 1800 nm is feasible, but the desired efficiency of this process sets a lower limit on the thulium concentration. In the YVO 4 :Yb, Tm system, an alternative means of optical pumping may be used, consisting of excitation of ytterbium ions followed by Yb 3+ -Tm 3+ energy transfer. In the assessment of the efficiency of energy transfer from ytterbium ions to the upper laser level 5 I 7 (Ho 3+ ) or 3 F 4 (Tm 3+ ), additional active losses associated with up-conversion phenomena should be accounted for, however.