Energy parameters of lasers utilizing erbium glasses sensitized with ytterbium and chromium

An experimental investigation was made of the effect of sensitizing ytterbium- and erbium-activated lead barium phosphate glasses with Cr$^{3+}$ ions on the energy parameters of lasing due to the $^4I_{13/2}$–$^4I_{15/2}$ transition in Er$^{3+}$ ions ($\lambda_1=1.54$ $\mu$). It was found that substantial sensitization was achieved in phosphate glasses for only low concentrations of Cr$^{3+}$ ions ($\lesssim0.07$ wt.%) so that the efficiency of flashlamp-pumped erbium lasers could be improved by a factor of 1.5–4. The optimal conditions for achieving the best energy parameters of these lasers under free-lasing conditions were determined allowing for the spectral and energy distributions of the flashlamp radiation in the absorption range of the coactivator ions. By implementing these conditions using active elements 6 mm in diameter and 85 mm long, containing 17 wt.% Yb$_2$O$_3$, 0.25 wt.% Er$_2$O$_3$, and 0.07 wt.% Cr$_2$O$_3$, it was possible to achieve an efficiency of 1.2% for an electrical pump energy of 1 kJ. This was the highest efficiency achieved so far for erbium lasers. Measurements were made of the efficiency of transfer of the excitation energy from Cr$^{3+}$ ions to Yb$^{3+}$ ions at high levels of excitation of the medium.