Abstract: The present invention relates to a method for the periodic domain inversion of ferroelectric flux-grown crystals that are low conductive or made low conductive. The invention also relates to an arrangement for monitoring the domain inversion of crystals while using a laser. The invention also relates to the use of crystals in applications of periodic domain inverted crystals, partly for generating light (electromagnetic radiation) at new wavelengths by non-linear optical frequency mixing (frequency doubling, difference frequency generation, summation frequency generation, optical parametric oscillation, etc.) and partly for electro-optical applications, such as light beam modulation, and partly for acoustic applications. such as the generation of acoustic waves from electric voltages applied across the crystal.