The photo-elastic fringe pattern measurement technique is a nondestructive method widely used to measure the mechanical
stress distribution inside solid materials. The mechanical stress distribution is evaluated by generating the photo-elastic fringe
pattern and counting the number of isochromatic fringes (dark lines). If the amount of the mechanical stress is very small,
however, and the corresponding birefringence is smaller than t radians, no fringe lines are produced. This method is,
therefore, not suitable when the amount of the mechanical stress is small. Furthermore, ii 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
7r radians by using an electro-optical phase modulator (Pockels cell) and a two-dimensional lock-in amplifier.1 This system
can also measure the azimuth angle simultaneously without rotating the sample.
Here we report the principle of this two-dimensional measurement system and give the results of measuring the 2-D
birefringence distribution in PMMA fixed beam.
|