Measurement of birefringence distribution using optical-phase modulation and a two-dimensional lock-in amplifier system

Abstract

The photoelastic fringe pattern measurement technique is a nondestmctive method and is widely used to measure the mechanical-stress distribution inside solid materials. This distribution is evaluated by generating a photoelastic fringe pattern and counting the number of isocbromatic fringes (dark lines). If the amount of the mechanical-stress is very small, however, and the corresponding birefringence is smaller than π radians, no fringe lines are produced. This method is therefore not suitable when the amount of the mechanical stress is small. Furthermore, if the azimuth angle of the mechanical-stress is to be measured, the sample needs to be rotated. We therefore developed a highly sensitive measurement system suitable for measuring birefringence distributions smaller than π radians by using an electro-optical phase modulator (Pockets cell) and a two-dimensional lock-in amplifier system. This system can also at the same time measure the azimuth angle without rotating the sample. Here we report the principle of this two-dimensional measurement system and give the results of measuring the 2-D bireffingence distribution in a glass plate.