The ferroelectric domain structures of periodically poled KTiOPO4 and two‐dimensional short range ordered poled LiNbO3 crystals are determined non‐invasively by interferometric measurements of the electro‐optically induced phase retardation. Owing to the sign reversal of the electro‐optical coefficients upon domain inversion, a π phase shift is observed for the inverted domains. The microscopic setup provides diffraction‐limited spatial resolution allowing us to reveal the nonlinear and electro‐optical modulation patterns in ferroelectric crystals in a non‐destructive manner and to determine the poling period, duty cycle and short‐range order as well as detect local defects in the domain structure. Conversely, knowing the ferroelectric domain structure, one can use electro‐optical microscopy so as to infer the distribution of the electric field therein.
