A new model of the process of preparing optical fibers for effective second-harmonic generation by the poling technique is presented. The model is based on the idea that the formation of a second-order susceptibility [chi(2)] structure is a consequence of a nonlinear anisotropic optoelectric hysteresis of doped fibers. Strings of interconnected dopant defects are assumed to be oriented along the fiber, which necessitates the formation of a chi(2) structure with periodically alternating sign.