In-Fiber Acousto-Optics for the Broadband Measurement of the UV-Induced Refractive Index Change in Photosensitive Fibers

In fiber acousto-optics has been demonstrated to be a versatile, highly sensitive technique that allows characterizing different parameters of singlemode or few-mode fibers, such as dispersion curves, radius or core refractive index, in a broadband wavelength range. The working principle of the technique relies in the fact that a variation in a parameter of the fiber leads to a shift in the optical wavelength that fulfills the acousto-optic phase matching condition. Thus, by measuring this wavelength shift it is possible to evaluate the change in the parameter under study. The technique shows a low detection limit: for example, it is as low as 10-8 for the core refractive index, in singlemode fibers. In addition, short sections of fiber (~20 cm) are required to perform the measurements.To fabricate gratings in photosensitive fibers, the core refractive index is increased, typically, in 10-5 – 10-4. In this work we characterize the refractive index change induced by UV radiation in hydrogenated, telecom-fiber (SMF28 from Corning), by means of the acousto-optic tool. Broadband characterization is performed in one go, using only one section of irradiated fiber. We characterized the refractive index variation as a function of the UV energy delivered to the fiber, in the range 1 – 2 μm.