The concentration dependent Yb³⁺: phosphate (P₂O₅ + K₂O + MgO + Al₂O₃ + Yb₂O₃) glasses have been prepared and characterized their fluorescence and laser properties. The stimulated emission cross-section and laser performance parameters were determined from the measured absorption spectra using the method of reciprocity. The refractive index, absorption and emission cross-sections and fluorescence lifetimes varied with Yb³⁺ ion concentration. The higher emission cross-section was found to be 1.01 × 10-20 cm² at the extraction wavelength of 1005 nm. The fluorescence lifetime of ²F_5/2 level decreases from 1.04 ms to 0.28 ms with increase of Yb₂O3 concentration from 0.05 to 6.0 mol %. The gain cross-section spectra can be obtained from the measured absorption and emission cross-sections with different population levels. The values of emission cross-section, fluorescence lifetime, minimum pulse duration, pump power, extraction efficiency and gain coefficients suggest that these glasses can be used as a laser gain media for the generation of ultrashort pulse and high power laser applications.