Index profile of multilayer fibers of an elliptical cross-section.

A mathematical formula is derived from the multiple-beam Fizeau fringers crossing an elliptical multilayer fiber. A formula for the two-beam interference fringes crossing this fiber is also given. A model that divides the elliptical fiber cross-section into elliptical zones of constant refractive index is assumed. The index profile of the poly(aryl-ether-ether-ketone) fiber is calculated by using the mathematical formulas. Microinterferograms of both the multiple-beam Fizeau fringes and the totally duplicated image of the fiber from a two-beam intereference microscope are used for the determination of the refractiveindex profile of the fiber. The cross-sectional shape is determined from the diffraction pattern of a He-Ne laser beam. Microinterferograms are given for illustration.