The basic principle of operation of the PEM is the photoelastic effect. A mechanically stressed sample exhibits birefringence proportional to the strain caused by the induced stress. As such, the PEM can be described as a birefringence modulator. Light moves along one axis faster than another because, as a result of resonating the optic, birefringence is induced along the axis that is either compressed or stretched. See Principles of Operation. Fused silica exhibits the phenomenon that light moves faster along the horizontal component when compressed and slower along the horizontal axis (the axis parallel to modulation) when stretched.
