In addition to piezoelectric effect, the photoelectric effect of LN crystal is very rich, among which the electro-optical effect and nonlinear optical effect have outstanding performance and are most widely used. Moreover, LN crystal can be used to prepare high-quality optical waveguide by proton exchange or titanium diffusion, and also can be used to prepare periodic polarization crystal by polarization reversal. Therefore, LN crystal has many applications in E-O modulator (as shown in the figure), phase modulator, integrated optical switch, E-O Q-switch, E-O deflectors, high voltage sensors, wavefront detection, optical parametric oscillators and ferroelectric superlattices etc.. In addition, the LN crystal-based applications of birefringent wedge angle plates, holographic optical devices, infrared pyroelectric detectors and erbium-doped waveguide lasers have also been reported.

Unlike piezoelectric applications, these applications involving optical transmission require different performance for LN crystals. Firstly, the propagation of light wave, with wavelength from hundreds of nanometers to a few microns, requires the crystal not only to have excellent optical uniformity but also to be strictly controllable of the crystal defects with size comparable to the wavelength. Secondly, it is usually necessary for the optical application to control the phase and polarization parameters of the light wave propagating in the crystal. These parameters are directly related to the size and distribution of the refractive index of the crystal, so it is necessary to eliminate the internal and external stress of the crystal as much as possible. LN crystals that meet the requirements of optical applications are often called “optical grade LN crystals”.

Z-axis and X-axis are mainly adopted for the growth of optical grade LN crystal. For LN crystal, Z-axis has the highest geometric symmetry which is consistent with the symmetry of thermal field. Therefore Z-axis is conducive to the growth of high-quality LN crystal which is suitable to be cut into squares or special-shaped blocks. Ferroelectric superlattice devices are also made from Z-axis LN wafers. X-axis LN crystal is mainly used to prepare X-cut LN wafer, so as to be compatible with cutting, chamfering, grinding, polishing, photoliography and other subsequent processing technologies developed by the semiconductor process. X-axis LN crystal is mainly used in most E-O modulators, phase modulators, birefringent wedge slices, waveguide lasers and so on.

High-quality LN crystal (LN Pockels cell) developed by WISOPTIC.