Detector

Measurements oflow levels of strain birefringence in fused silica glass have been made using a system based on a photoelastic modulator (PEM). Measurements of sample net retardation have been made with a resolution of 0.1 nanometers. Measured values of a strain birefringence constant for fused silica are in good agreement with estab-lished data. 1. EXPERIMENTAL SETUP Laser light source Kemp ' has described a method for determination ofthe net retardation of a birefringent sample using a photoelastic modulator. Figure 1 shows the basic experimental setup. A Polarizer, ÷45° verticaloptical bench has been suggested since this greatly simplifies the support of the sample. I I PEM, 0° For this method, it is assumed that the retardation axes of the sample are known. For Sample, 0° samples with initially unknown axes, the axes can be found experimentally by rotation of the sample until a maximum signal is observed. Other more complex techniques are men- Polarizer,-45° tioned at the end of the paper