Abstract – Optical amplifier usage in the fiber optics transamission systems significantly improves the transmission distance. But it also influence the signal quality. This article describes a study on optical intensity characteristics and its impact to optical signal propagation in the erbium doped fiber (EDF) used in the doped fiber amplifiers. First of all the far field optical power distribution measurements are performed at the output of Er3+ doped fiber using transverse shift measurement method. Based on the experimental results, the fiber effective area Aeff has been calculated. Following from the effective area results comparative analysis and estimation of the optical intensity results is given for EDF at different wavelengths. It was found that in the wavelength range of 1465-1575nm optical intensity changes from 66.94kW/cm2 at ė=1575nm to 70.18kW/cm2 at ė=1470nm. But the nonlinear refractive index change in the erbium doped amplifier amplification range is found to be 1.04 times.