We report measurements of the holographic diffraction efficiency (HDE) of single photorefractive gratings fixed by an externally applied voltage pulse in Sr0.75Ba0.25Nb2O6. The polarization grating HDE and decay rate are strongly dependent on the grating spacing and also depend on the intensity of the light beams during the recording and erasing/reconstruction process. Knowledge of the dependence of the fixing process on spatial frequency may be useful for the creation of physical models of local polarization switching in ferroelectrics.