Birefringence Simulation of Annealed Ingot of Calcium Fluoride Single Crystal

We developed a method for simulating birefringence of an annealed ingot of calcium fluoride single crystal caused by the residual stress after annealing process. The method comprises the heat conduction analysis that provides the temperature distribution during the ingot annealing, the elastic thermal stress analysis using the assumption of the stress-free temperature that provides the residual stress after annealing, and the birefringence analysis of an annealed ingot induced by the residual stress. The finite element method was applied to the heat conduction analysis and the elastic thermal stress analysis. In these analyses, the temperature dependence of material properties and the crystal anisotropy were taken into account. In the birefringence analysis, the photoelastic effect gives the change of refractive indices, from which the optical path difference in the annealed ingot is calculated by Jones calculus. An approximate method for calculating the optical path difference using the average stress along the wave normal is also proposed and the relation between the Jones calculus and the approximate method is discussed. It is found that the result of the approximate method agrees very well with that of the Jones calculus. The distribution of the optical path difference in the annealed ingot obtained from the present calculation agrees reasonably well with that of the experiment. Its calculated value also agrees reasonably well with that of the experiment, when a stress-free temperature is adequately selected.