The refractive index profile of highly-oriented fibres

A new method is suggested for measuring the refractive index profile of hom ogeneous highly-oriented fibres. This method depends on the principle of th e variable wavelength interferometric technique. A numerical fitting is use d to determine the fringe shift profile at a certain wavelength. The refrac tion of the light beam by the fibre is taken into consideration in the math ematical treatment. Poly(ethylene terephthalate) and poly(ethylene 2,6-naph thalene-dicarboxylate) fibres are used as examples of these highly-oriented fibres. The diffraction of a He-Ne laser beam is used to determine the dia meter and the cross sectional shape of these fibres. Also, a two-beam inter ference 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. M icrointerferograms are given for illustration.