The refractive index profile of highly-oriented fibres

A new method is suggested for measuring the refractive index profile of homogeneous highly-oriented fibres. This method depends on the principle of the variable wavelength interferometric technique. A numerical fitting is used to determine the fringe shift profile at a certain wavelength. The refraction of the light beam by the fibre is taken into consideration in the mathematical treatment. Poly(ethylene terephthalate) and poly(ethylene 2,6-naphthalene-dicarboxylate) fibres are used as examples of these highly-oriented fibres. The diffraction of a He-Ne laser beam is used to determine the diameter and the cross sectional shape of these fibres. Also, a two-beam interference microscope (Pluta microscope) is used for measuring the refractive index profile of these fibres, at the same wavelength, for light vibrating perpendicular to the fibre axes to confirm the results of the new method. Microinterferograms are given for illustration.