The Er doped fused silica coverslips,thermally poled in vacuum,were etched in 40 % HF transversely to the poling direction,and the resulted ridge channel ridge shaped poled region was observed by AFM.This kind of etched shape results form the different etched rates of the positive and negative charge layers corresponding to the channel and ridge respectively.The negative positive negative space charge distribution in the poled region can be proved by the multiple charge model.For the first time we made use of the two Gauss model to fit the electrical field in the poled region and a theoretical best fit Maker fringe was obtained.