One of limiting factors for data transfer capacity in fiber optics transmission systems is optical fiber nonlinear response to high intensity optical radiation. The main reason for optical signal high intensity is optical fiber core size. For single mode fiber it is typically around 10µm in diameter. In addition optical field in a telecommunications single mode fiber is not evenly distributed or even fully contained within the core. It is larger at the fiber central axis than near the core-cladding interface and extends even into the cladding depending on the actual refractive index profile. The real optical fiber refractive index profile is dependent on the fiber type and the manufacturing process and therefore not always can be accurately determined. The effective area parameter Aeff has been defined for the purposes of calculating nonlinear effects. It is a single value, based on the modal field distribution, and can be used to calculate a value of optical intensity.