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

摘要

The processes of excitation, nonradiative energy transfer, and relaxation of excited states relevant to the laser performance of YVO4:Yb, Ho and YVO4: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 YVO4:Ho crystal provides an efficient means of population buildup on the 5/7 level via a Yb3+-Ho3+ 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 YVO4 matrix. The YVO4:Tm system is better suited for laser diode pumping thanks to a relatively intense pump band associated with the H-3(6)-H-3(4) transition around 800 nm. In the limit of a low Tm3+ concentration, the quantum efficiency of the H-3(4) level is sufficiently high for the consideration of laser operation associated with the H-3(3)-F-3(4) transition. At a higher thulium concentration, the H-3(4) excitation is transferred to the F-3(4) level by a cross relaxation process and the laser operation associated with the F-3(4)-H-3(6) transition near 1800 nm is feasible, but the desired efficiency of this process sets a lower limit on the thulium concentration. In the YVO4:Yb, Tin system, an alternative means of optical pumping may be used, consisting of excitation of ytterbium ions followed by Yb3+-Tm3+ energy transfer. In the assessment of the efficiency of energy transfer from ytterbium ions to the upper laser level 5/7 (HO3+) or F-3(4) (Tm3+), additional active losses associated with up-conversion phenomena should be accounted for, however.