Highly sensitive birefringence measurement in THz frequency region and its application to stress measurement

We have constructed a terahertz time-domain spectroscopy (THz-TDS) system which is able to detect a birefringence as small as An ∼ 5 × 10⁻⁴. This polarization sensitive measurement system is applied to detect the structural orientation of various fibrous materials such as polymers, papers, textiles, etc. It is also applied to the stress induced birefringence measurements for polymers, from which the stress-optic coefficients in THz region are obtained. The stress birefringence measurement with THz radiation is useful for optically opaque materials for which the ordinary photoelastic stress measurement is not possible.