A method that characterizes two-mode waveguides whose modes cannot be selectively excited (such as buried waveguides) is presented and demonstrated. The theoretical results are presented for N modes, although for the sake of simplicity, only the two-mode case is developed. The values of the optical mode fields are recovered from several images of the waveguide exit face, where both modes interfere with different relative intensity in each image. From these mode fields, both the squared index distribution, except in an additive constant, and the effective index difference can be obtained by inverting the Helmholtz equation. As in the case of the standard monomode intensity method, the relative values of effective indices and index distributions become absolute if the modes are retrieved in a point of known index, for instance, in the substrate. The method can also be applied to multimode guides if only two modes are excited. In fact, an inexpensive setup is proposed to excite the first two modes of a multimode buried waveguide. This waveguide was fabricated by ion exchange in glass and buried by electromigration. The shape of the squared refractive index recovered by the proposed method agrees with that reported in the literature.
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