The birefringence effect caused by the internal stress of optical components will affect the polarization state of light, which can not be tolerated in micro-lithography, laser optics and astronomy. In general, the requirements for accurate measurement of small stress birefringence are very demanding. The image polarization measuring instrument, which can give the spatial distribution and direction of stress birefringence at the same time, has solved this problem well.
Under less stringent conditions, optical glass can generally be considered to be homogeneous, with the refractive index being equal everywhere in all directions. However, the stress caused by the material or the production process will deform the structure of the material, resulting in local density differences along the axial direction. The propagation speed of light in a medium is related to the density of the material. The change of the local density leads to the difference of the speed of light propagating in the medium and the change of the refractive index related to the direction. The birefringence phenomenon of the medium under the action of stress is the so-called stress birefringence (SBR).
In addition to optically isotropic materials, there are also many naturally occurring optically anisotropic materials, also known as birefringent materials, such as calcite and quartz crystals. For these materials, changes in the refractive index ratio can also be seen under mechanical stress, which can be so large as to cause damage to the crystalline material. Even small changes in the local refractive index can have a negative impact on the imaging quality of the optical element, thereby affecting its functionality. In addition, birefringence changes the polarization state of the transmitted light, which is detrimental in applications such as metrology. Therefore, it is very important to accurately determine the stress birefringence and its spatial distribution in the manufacture of optical materials and components.
