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.
In the first part of this work (see preceding paper) the theory of the method lying upon a novel concept of birefringence, azimuths, and transmission mapping in large area (up to 6 in. diameter) wafers has been presented. The arrangement consisting of two HR-type linear polarizers rotated simultaneously by a stepper motor versus an immobile wafer and using a video frame grabber (VFG)/TV camera detecting system is capable of collecting data and plotting the three maps within a fraction of a minute. A detailed error analysis presented in the preceding paper has shown that in usual circumstances the VFG with 256 grey levels enables determination of birefringence with an error not greater than approximately 5×10−7, whereas errors of the azimuths and transmission are fractions of a degree and of a percent, respectively. In this part of the work the arrangement constructed is fully described and a set of polariscopic images and measured maps are presented for an exemplary 4 in. GaAs wafer.
Effects of thermal deformations of optical elements may be visualised and measured by interferometric methods. They correspond to the deformation of heated optical surface and/or deformation of the wave front of the light beam propagating through optical elements. In one of the experiments the optical effects for a single optical element heated by CO2 laser were measured. Single element was combined with the second optical element for spherical aberration correction. The measurement stand used was composed of five modules: interferometers in Fizeau or Twyman -Green configuration (He-Ne, aperture 100 mm), CO2 laser with the device for changing and measuring the power of the laser beam, the substrate module, thermovision camera AGEMA 470 for temperature measuring and the module for control, registration and analysis of interferometric images. The results of measurements provide the form of the wave front shape in combination with the thermal gradient of the surface. The experimental results are compared with computer simulated effects of the optical path difference and the temperature distribution calculated by the finite element method.
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