Review of the formula of thermal focal length in side-pumped laser rods

► The expressions of focal length in side-pumped rod lasers were studied. ► Overestimating the material's thermal conduction and without considering the nonuniform heat deposit lead to underestimation of thermal lens. ► Temperature-dependent variation of the refractive index constitutes the major contributions to thermal lensing in Nd:YAG rods. ► Temperature- and stress- dependent variation of the refractive index make the major contributions to thermal lensing in Nd:Glass rods.