Optical Spectra and Gain Properties of Ho3+/Pr3+ Co-doped LiYF4 Crystal

The LiYF4 single crystals singly doped Ho3+ and co-doped Ho3+, Pr3+ ions were grown by a modified Bridgman method. The Judd-Ofelt strength parameters (Omega(2), Omega(4), Omega(6)) of Ho3+ were calculated according to the absorption spectra and the Judd-Ofelt theory, by which the radiative transition probabilities (A), fluorescence branching ratios (beta) and radiative lifetime (tau(rad)) were obtained. The radiative lifetimes of 5/6 and 5/7 levels in Ho3+ (1 mol%): LiYF4 are 10.89 and 20.19 ms, respectively, while 9.77 and 18.50 ms in Ho3+/Pr3+ doped crystals. Hence, the tau(rad) of 5/7 level decreases significantly by introduction of Pr3+ into Ho3+:LiYF4 crystal which is beneficial to the emission of 2.9 mu m. The maximum emission cross section of Ho3+:LiYF4 crystal located at 2.05 mu m calculated by McCumber theory is 0.51 x 10(-20) cm(2) which is compared with other crystals. The maximum emission cross section at 2948 nm in Ho3+/Pr3+ co-doped LiYF4 crystal obtained by Fuchtbauer-Ladenburg theory is 0.68 x 10(-20) cm(2), and is larger than the value of 0.53 x 10(-20) cm(2) in Ho3+ singly doped LiYF4 crystal. Based on the absorption and emission cross section spectra, the gain cross section spectra were calculated. In the Ho3+ ions singly doped LiYF4 crystal, the gain cross sections for 2.05 mu m infrared emission becomes positive once the population inversion level reaches 30%. It means that the pump threshold for obtaining 2.05 mu m laser is probably lower which is an advantage for Ho3+-doped LiYF4 2.05 mu m infrared lasers. The calculated gain cross section for 2.9 mu m mid-infrared emission does not become positive until the population inversion level reaches 40% in Ho3+/Pr3+:LiYF4 crystal, but 50% in Ho3+ singly doped LiYF4 crystal, indicating that a low pumping threshold is achieved for the Ho3+:5/6 -> 5/7 laser operation with the introduction of Pr3+ ions. It was also demonstrated that Pr3+ ion can deplete rapidly the lower laser Ho3+:5/7 level and has influence on the Ho3+:5/6 level. The Ho3+/Pr3+:LiYF4 crystal may be a potential media for 2.9 mu m mid-infrared laser.