Measurement of the internal electric field in periodically poled congruent lithium niobate crystals by far-field diffraction.

A simple nondestructive diffraction method is introduced to estimate the internal electric field present in the congruent lithium niobate crystal. Due to the presence of an unrelaxed internal field, the as-poled sample acts as an index grating and diffracts the incident light beam. By analyzing the diffraction patterns, we calculated the refractive index difference ($\Delta n = 1.