A new method of birefringence measurements using a Faraday modulator. Application to measurements of stress-optical coefficients

摘要

The determination of the stresses in transparent thin plates (or films) is important in micro-electronic and optoelectronic technologies. Several polarimetric optical methods for measuring birefringence and hence stresses have recently been described in scientific papers. They use an image-analysis technique (whole-field analysis), a photoelastic modulator or a rotating polarizer (point-by-point analysis). We present a new method using a polarimetric device with Faraday modulation of the light beam. It allows one to determine with high precision (to within some 0.05°) at each point of the sample the birefringence (residual or induced) which is characterized by the direction α and a phase shift △ introduced between optical waves polarized along orthogonal linear eigenpolarizations. This method can easily be automated. Two examples are given, namely measuring the birefringence of a ferrofluid plate subjected to a transverse magnetic field and determination of photoelastic coefficients of various glasses.