Imaging conoscope for investigation of optical inhomogeneity in large boules of uniaxial crystals

The name “conoscope” in Greek suggests that this tool should be used for observing interference patterns of birefringent crystals in the convergent beam of light. As such, the conoscope has been frequently used in laboratories for quick and usually qualitative estimation of optical inhomogeneity of crystals. In this paper we have described details of the computer-controlled imaging conoscope used for quantitative investigation of optical inhomogeneity in uniaxial crystals. To the best of our knowledge, this is the first conoscope used in an automated arrangement. Its working is based on equations derived for a plane–parallel uniaxial crystal plate cut out obliquely to the optical axis, which is next applied for two specific plate orientations practically investigated, i.e. for plates cut out perpendicularly and parallelly to this axis. It was found that these equations are more accurate than those published by other investigators. A practical investigation of a LiNbO3 crystal pulled by the Czochralski method from a congruent melt has been presented. Two birefringence inhomogeneity maps acquired for the above-mentioned two specific orientations in this crystal were used for eliminating its inhomogeneous areas from further use in optics. A theoretical error analysis carried out also in this work has shown that the optical inhomogeneity could be detected with a relative error usually not exceeding a small fraction of a percent.