Group and phase birefringence dispersion of pure and doped lithium niobate crystals obtained by analysis of interference pattern observed behind a plane polariscope

摘要

A sample of lithium niobate crystal with known thickness is set on a vertical rotary stage, placed in between two crossed plane polarizers and illuminated by a collimated beam of white light. The sample is rotated to an appropriate position, and interference fringes observed behind an analyzer are recorded by a fiber-optic spectrometer. The recorded channeled spectrum is then analyzed, and from measured positions of interference minima (dark fringes) and known crystal's thickness, the group birefringence of the crystal sample is obtained as a function of wavelength. The fringe order versus position of the fringe within the spectrum is fitted by appropriate dispersion function, and as the result phase birefringence as a function of wavelength is found. The measurement is performed for a sample declared as undoped, a sample doped with 0.025 wt. % of Fe, and a sample doped with 0.025 wt. % of Fe and 0.075 wt. % of Mn in the wavelength ranges (470 to 780) nm and (900 to 1700) nm. A good agreement between group birefringence dispersion obtained from positions of interference minima and known sample's thickness and that calculated from phase birefringence dispersion given by Sellmeier dispersion model (used for undoped sample) and Cauchy model (used for doped samples) is found.