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This paper reports on successful preparation of α-NaYF 4 single crystals co-doped with ∼1.9 mol% Tm 3+ and various concentrations (3.85 mol%, 7.69 mol%, 11.54 mol%, 15.38 mol%) of Yb 3+ by using a flux-Bridgman method. The fluorescence decay curve was measured to investigate the luminescent properties of the Yb 3+ /Tm 3+ co-doped α-NaYF 4 , and the energy transfer process from Yb 3+ to Tm 3+ ; the J-O intensity parameters of Tm 3+ were further calculated and analyzed according to the absorption spectra. Results show that, an intense 1.8 μm emission was achieved with Yb 3+ as sensitizer for Tm 3+ in the α-NaYF 4 single crystal under the excitation of 980 nm LD (Laser Diode) because of the strong energy transfer from Yb 3+ to Tm 3+ . The maximum emission intensity at 1.8 μm is obtained at about 15.38 mol% doping concentration of Yb 3+ when the concentration of Tm 3+ ions is fixed at ∼1.90 mol% in the current research. Moreover, the calculated maximum value of emission cross section at 1.8 μm is 1.63 × 10 −20 cm 2 for 3.85 mol% Yb 3+ /1.9 mol% Tm 3+ sample, and the obtained energy transfer rate ( W ET ) and energy transfer efficiency ( η ) are 1543 s −1 and 83.8%, respectively. Our analysis of the fluorescence dynamics indicates that electric dipole-dipole interaction is dominant for the energy transfer from Yb 3+ ions to Tm 3+ ions by using Inokuti-Hirayama’s model. [ABSTRACT FROM AUTHOR]