  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 optical glass 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.
