We show that the Maker fringe analysis previously used to characterize the nonlinearity of thermally poled silica does not yield a unique nonlinear profile d33(z). This problem can be solved by using this method in conjunction with another independent measurement technique in order to first determine the general shape of the nonlinear profile (e.g. whether it exhibits one or multiple peaks). We demonstrate the validity of this approach by characterizing the same poled silica sample first by the Maker fringe analysis, then by imaging the second-harmonic intensity distribution generated by its nonlinear region. The latter indicates that the second-harmonic intensity exhibits a single-peaked distribution, and the former that the d33(z) profile is closely modeled by a buried depth of 5 micrometers , a 1/e width of 10 micrometers , and a peak d33 of 0.44 pm/V.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.