Simplified system based on photoelastic modulation technique for low-level birefringence measurement

In this article, the authors present a new low-level birefringence detection (LLBD) system based on the use of a photoelastic modulator. This system uses a single modulator so that only one detector is required to capture all the frequency components in the optical beam. The design removes gain errors associated with the conventional dual-beam approach. In addition, we also incorporate a complete analytical signal-processing algorithm to enhance measurement accuracy by eliminating the requirement of small phase angle approximation. To demonstrate the operation of our LLBD system, we performed birefringence measurement on a silicon wafer to characterize the distribution of its internal stress. Results obtained from our experiments indicate that the instrument has high sensitivity and good repeatability. The birefringence measurement sensitivity of our system was found to be about 0.03degrees (or 0.096 nm at 1152 nm). Several samples with different levels of birefringence were studied using our LLBD system. (C) 2003 American Institute of Physics.