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We present a novel approach which could be useful for real-time all-fiber measurements of magnetic fields applied either longitudinally or transversally. We used a microstructured optical fiber, whose suspended core is coated with a nanocomposite magneto-optic thin film. It is obtained by doping a sol-gel solution with cobalt ferrite nanoparticles based on an innovative microfluidic deposition process. When any magnetic field is applied, several effects are induced by the appearance of off-diagonal elements in the nanocomposite material permittivity tensor. It results in a variation of the emerging light polarization state measured for the C-band telecom spectral range, at 1550 nm. One of the best-known magneto-optic effects is the Faraday rotation, observed with longitudinal magnetic fields. Therefore, our functionalization greatly exalts this effect already present in commercial fibers due to the Verdet constant of silica. A 300mT and 1 cm long longitudinal magnetic field creates up to 15° of rotation compared to 0.1° for a classical optical fiber, while for a transverse magnetic field, conventional optical fibers are almost totally insensitive to it. However, in our configuration different off-diagonal elements of permittivity tensor will be activated causing non-reciprocal variation of the propagation constant of the eigenmodes as well as another process leading to a reciprocal magneto-refractive effect. It results in an additional polarization variation of several degrees per centimeter in transversal magnetic field configuration. The interest of such a highly sensitive functionalized optical fiber, is found where conventional magnetic probes cannot be used. Above all, the optical fibers are relatively unaffected by radiations, unlike conventional electronic sensors, so it is conceivable to use them in these exclusion zones where the sensitive part is deported from the electronic measurement system.
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