Optical element made from fluoride single crystal and method of manufacturing the optical element

摘要

The present invention provides a method for calculating the birefringence, of an optical element and selecting the direction of minimum birefringence in the optical element. A material for manufacturing the optical element is a fluoride single crystal with a birefringence value that is reduced compared to conventional materials. The fluoride single crystal is cut from a fluoride single crystal ingot obtained by crystal growth so that the ä111ü crystal planes are two parallel planes, after which the optical performance is improved by subjecting this material to a heat treatment. The birefringence of the optical element is calculated by converting known piezo-optical constants in a specified three-dimensional orthogonal coordinate system for the optical material into piezo-optical constants in an arbitrary three-dimensional orthogonal coordinate system. The amount of change in the refractive index DELTA n1 of the optical material in a first direction along the direction of one coordinate axis of the arbitrary three-dimensional orthogonal coordinate system. and the amount of change in the refractive index DELTA n2 of the optical material in a second direction which is perpendicular to the first direction are calculated using a uniaxial stress that is applied to the optical material along the first direction and the piezo-optical constants in the arbitrary three-dimensional coordinate system. The amount of birefringence as seen from a third direction perpendicular to the first direction arid the second direction is determined in the arbitrary three-dimensional orthogonal coordinate system by determining the difference between the amount of change in the refractive index DELTA n1 and the amount of change in the refractive index DELTA n2.