The paper deals with diffusion processes of the core dopant during thermal connection (splicing in electric arc) of telecommunication fibers whose parameters vary considerably. A method for defining values of diffusion coefficients GeO2 in SiO2 in splicing temperature has been presented. Better consistence of the dopant Gaussian and diffusion distribution has been found the longer the diffusion time-that is, splicing time. Theoretical analysis of thermally diffused expanded core (TEC) transit area transmission properties was performed which revealed that it is possible to obtain splice loss of thermal connections (splice) of fibers with different parameters, such as of the type G.652 and G.655, below 0.08 dB, using classical connection in electric arc. Whereas, using connection methods that increase the length of TEC, for example, connection by means of gas micro-burners, zero attenuation of such a connection can be accomplished.