An optical member for a synthetic quartz glass

Is controlled fast axis direction of the optical axis direction, together with the off-axis birefringence is reduced, the birefringence of the optical axis direction size of synthetic quartz glass for an optical member which is controlled to an arbitrary value provide. The following optical wavelength 250nm and an optical member for a synthetic quartz glass used in the optical device as a light source, with respect to the principal optical axis of the optical member, the optically effective region of a plane perpendicular the principal optical axis direction is measured from the direction parallel to define the average value of the values BRcos2shita There, an optical member for the synthetic quartz glass. However, the numerical value of the BR, the theta