Stress birefringence is also known as the photoelastic effect. Under the action of pressure or tension, the refractive index of transparent isotropic media will change, thus showing optical anisotropy. If the medium is originally an anisotropic crystal, the external force will cause it to produce an additional birefringence. If the stress is non-uniform on the crystal, the birefringence will not be uniform. Thus different points on the light wave passing through it produce different phase differences. Using the stress birefringence effect, the phase difference of optical materials can be tested and the stress distribution of various mechanical structures can be observed.
In various types of optical systems. including optical lithography, high-energy lasers, LCD projectors, and telecommunications, stress birefringence is a problem that needs to be dealt with. For example, under the influence of external mechanical action, the changing stress distribution in the lens element presents a generalized three-way stress state. And the optical properties become anisotropic and non-uniform, which will cause wavefront aberration or polarization error in the optical system.
The ideal optical glass is isotropic. But during the annealing process, internal stress will be generated due to the inconsistent temperature inside and outside the glass, or the inconsistent temperature in the annealing furnace. The existence of internal stress in optical glass destroys the isotropy and produces birefringence. That means when a beam of light passes through the glass with internal stress, two beams of light with different propagation speeds will be generated. Stress birefringence is measured by the optical path difference per unit length (nm/cm).
Fig1 direction of stress birefringence in round glass plates
According to international standards, there are two main types of stress birefringence in glass, which exist in the middle and the edge of the glass respectively. The former is described as the optical path difference on the unit length in the middle of the longest side. The latter is expressed as the largest optical path difference on the unit length 5% from the edge of the glass . When proceeding measurement, the beam is required to be perpendicularly incident upon the sample surface. The measurement points and beam incident directions in the middle and edge are shown as A, B points and I, II directions in Figure 1 (I is the beam direction for measuring the stress in the middle; II is the beam direction for measuring the edge stress).
According to the stress distribution law after glass annealing, each measurement point in the middle and edge above generally has only one principal stress. And the stress direction is parallel to the glass surface. In this way, the measurement beam should be perpendicularly incident upon the surface, like I and II directions in Figure 1. If the optical path difference per unit thickness is used to measure the quality of glass after annealing, the annealed glass blank is only allowed to be ground or polished on the surface, and not allowed to be cut. Because the stress distribution and the magnitude of the stress will change after cutting.
An optical measurement equipment that uses stress birefringence is called a Birefringence Measuring System. It is widely used in the measurement of material mechanics. For some complex-shaped parts in the mechanical structure, the stress distribution under different loads is very complex. We can produce a corresponding model with transparent materials, and apply mechanical force to the model according to the actual force in use. Using the polarized light interference device, the stress distribution can be analyzed.
