Test MethodOnline double-arm of a multiple-beam Fizeau system: I. Optical setup for simultaneous recording of two interferometric patterns in the same frame

An online optical setup for simultaneous recording of two interferometric patterns with two different colours in the same frame on the basis of a multiple-beam Fizeau system is designed. This setup allows measuring of refractive indices (n┴ and n║) for the two directions of the light vibration, for one sample of the fibre, simultaneously from a single captured frame. These simultaneous measurements enable one to obtain accurate determination of the birefringence (Δn=n‖−n┴) for the tested sample. Software developed for the digital processing of the patterns is used for automatic digital fringe analysis. The novel optical setup is used for the online investigation of PP fibres under creep deformation for duration t = 1000 s, followed by a relaxation process for duration t = 4000 s. Optical parameters for the material of the tested fibre are studied interferometrically using the novel setup. Microinterferograms and graphs are given for illustration.