Optical method to determine the composition of lithium niobate crystals by digital holography

Because the physical properties of lithium niobate (LiNbO₃) strongly depend on composition, accurate and convenient methods for the determination of the composition are of great significance. Although several optical methods, including the measurement of UV absorption edge, the birefringence, and the second-harmonic generation, have been proved to be convenient for an accurate and fast standard determination of composition in LiNbO₃ single crystals, their research and commercial applications are limited by the doping component and the complex nonlinear relationships. Based on preliminary work, a novel optical method to determine the composition of LiNbO₃ crystals by digital holography is proposed. This method is based on the static internal field, which is obtained by means of the three-dimensional (3D) static measurement of the phase difference between antiparallel poling states without applying external voltage by digital holography. In order to investigate the influences of composition and doping on the static internal field in LiNbO₃ crystals, the measured static internal fields from various LiNbO₃ samples with different stoichiometry, doping type, and doping level are summarized and compared. Excluding the influence of dopant, the composition has been proved to be a unique key influencing factor on the static internal field in LiNbO₃ crystals. A systematic measurement based on the static internal field from various sources with compositions ranging from 48.5 to 49.9 mol.% (Li/[Li+Nb] ratio) has been carried out. The approximate linear fit between the static internal field and composition can provide an easy, reliable, and sensitive determination of the composition in undoped and doped LiNbO₃ samples.