Interferometric Studies on Polymer Fibres. I. Application of Multiple-Beam Fizeau Method for the Characterisation of Homogeneous Polymer Fibres

Abstract

Multiple-beam Fizeau fringes with a modified method are described for the measurement of refractive indices and birefringence of fibres. This method is used to overcome difficulties in measuring refractive indices of immersion liquids as well as the correcting for dispersion and temperature. Two fibres are used for each measurement. The first fibre is of known mean refractive index and dispersion properties. The other fibre is of unknown values of refractive indices. A mathematical formula is derived for the modified method in case of the shape of multiple-beam interference Fizeau fringes crossing a homogeneous cylindrical fibre, as well as an equation to determine the refractive indices and birefringence for fibre having irregular cross-section. This method with microstrain device is applied to determine the optical properties of “Terylene” polyester fibre dynamically with different draw ratios. Also stretched polyethylene fibre is studied using the modified method. The calculated refractive indices using this method are compared with those obtained using the normal method. Microinterferograms are given for illustration.