The effects of ray bending due to graded refractive-index profiles have been studied in relation to the measurement of optical-fiber refractive-index profiles by microscopic interferometric techniques. In particular, the wavefront curvature produced by a simple parabolic profile has been calculated analytically. It is concluded that profile measurements by currently used methods require samples for which the fiber thickness (length) is much less than the effective focal length of the fiber. For parabolic profile fibers with diameter d ≳ 100 µm, this poses no problem; however, for small-core versions of this fiber, sample thicknesses of 10–15 µm may be required and an alternate measurement technique is described. Interferograms made on a Leitz transmitted-light interference microscope are included in illustrations.
J. Stone and C. A. Burrus, "Focusing Effects in Interferometric Analysis of Graded-index Optical Fibers," Appl. Opt. 14, 151-155 (1975)