Direct Experimental Determination of the Optimum Chromium Concentration in Continuous-Wave Cr$^{2 + }$:ZnSe Lasers

We employed several experimental techniques to measure the concentration dependence of the important laser parameters, and directly determine the optimum ion concentration for continuous-wave (CW) operation in room temperature Cr
²⁺
:ZnSe lasers. By using diffusion doping, 40 polycrystalline Cr
²⁺
:ZnSe samples with ion concentrations in the range of 0.8 times 10
¹⁸
to 66 times 10
¹⁸
ions/cm
³
were prepared and used in this paper. Based on the spectroscopic measurements, empirical formulae showing the concentration dependence of the passive laser losses, fluorescence lifetime, and the fluorescence efficiency were obtained. By using the fluorescence efficiency data, the optimum chromium concentration, which maximizes the 2400-nm fluorescence intensity at a fixed excitation power, was determined to be 6 times 10
¹⁸
ions/cm
³
. The dependence of the optimum concentration on sample length was further discussed. The CW power performance of the samples was also evaluated. At an incident pump power of 2.1 W, the optimum concentration for lasing was determined to be 8.5 times 10
¹⁸
ions/cm
³
that was in good agreement with the fluorescence measurements. The predictions of the fluorescence analysis and laser power measurements were in good agreement at low chromium concentrations. The observed discrepancy at higher doping levels was attributed to thermal loading.