An unconventional optical diffraction technique is exploited to characterize periodically poled lithium niobate crystals grown by the off-center Czochralski technique. The method, based on maps of the diffracted intensity in the reciprocal space, pointed out that a modulation in the refractive index occurs with a periodicity close to that measured by the chemical etching procedure. In this case, therefore, the linear optical properties of domains with opposite polarization are different.