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The samples of Er3+/Yb3+ co-doped phosphate glasses with different Er3+ doped concentrations were prepared by
high-temperature melting method. By calculating the electronic polarizabilities and optical basicity of the samples, we
find, with the increasing of Er3+ ions concentration, the polarizability and optical basicity of oxide ions become large,
which indicates that the covalence of the glasses becomes small. Based on Judd-Ofelt (J-O) theory, the J-O intensity
parameters Ωλ (λ=2,4,6), spontaneous radiative lifetime, and fluorescence branching ratio were calculated. With the
increasing of Er3+ concentration, the Ω2 values are decrease gradually, which also proves that the covalence of the glasses
becomes small. The ratios of Ω4/Ω6 are within the range of 7.51-9.84, which are larger. Thereby, our glasses are suitable
laser materials. We also studied the upconversion (UC) emission of the samples. The strong 657 nm red emission, 546
nm and 523 nm green emissions were observed under 975 nm laser diode (LD) excitation. The UC emission intensities
depend on the ratio of Er3+/Yb3+. When the ratio is 0.125, the emissions are strongest. The emission cross sections of UC
emissions were analyzed using McCumber theory and Füchtbauer Ladenburg (FL) method. The emission cross section
of UC red emission is larger (about 0.5×10-20cm2),which proves our samples are better red light materials.
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