In spite of their ubiquitous applications, the characterization of glass fibers by means
of all-optical techniques is still facing some limitations. Recently, X-ray absorption has been
proposed as a method for visualizing the inner structure of both standard and microstructure optical
fibers. Here, we exploit X-ray absorption as nondestructive technique for the characterization
of glass optical fibers. Starting from absorption contrast X-ray computed micro-tomography
measurements, we obtain information about the spatial profile of the fiber refractive index at
optical frequencies. We confirm the validity of our approach by comparing its results with
complementary characterization techniques, based on electron spectroscopy or multiphoton
microscopy