Measurement of axially nonsymmetrical refractive-index distribution of-a single-mode fiber by a multidirectional scattering-pattern method

A high spatial resolution, typically
0.2 \sim 0.3 \mu
m, has previously been achieved by the "scattering-pattern method" proposed by one of the authors for measuring refractive-index profiles of optical fibers. This method, therefore, is especially suitable for measuring the index profiles of single-mode fibers. Described in this paper is the modification of this technique for measuring nonsymmetrical index distributions in single-mode fibers. In the modified method, a two-dimensional index distribution in an axially nonsymmetrical fiber is computed from a set of its forward scattering patterns for different directions of incident light. The distribution to be measured is somewhat restricted, i.e., it must have point symmetry. However, the point symmetry includes double symmetry which holds in most of polarization-maintaining fibers. The spatial resolution is as high as the scattering-pattern method for the axially symmetrical case; in the present experiment the resolution limit is 0.28 μm. Results of computer simulation and measurement are shown.