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SummaryA simple and reliable method to excite selectively different annular zones of a multimode graded index fiber has been implemented. This technique, exploring concentric layers of the fiber core with different chemical compositions, can find application in differential measurements of material properties. On the basis of a ray optics picture of propagation, a light beam having angular and spatial dimensions much smaller than the relevant dimensions of the fiber was employed for selective excitation.Different annular configurations have been obtained varying the position and angle of incidence on the fiber end. Results obtained by an apparatus consisting of standard components only on low loss communication fibers are reported for different refractive index profiles and lengths up to 100 meters.

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