Measurement of UV-induced losses and thermal effects in photosensitive fibers using whispering gallery modes

Summary form only given. When a photosensitive (PS) fiber is exposed to UV-irradiation, a permanent refractive index change is induced in the core. As a result, according to Kramers-Kronig relations, the absorption coefficient (α) is also increased. This increment of the absorption can lead to a significant heating of the fiber when it is illuminated by a moderate optical power. Thermal effects may produce spectral changes in some fiber devices, as for example Long Period Gratings (LPGs) or Fiber Bragg Gratings (FBGs) [1]. In this paper, in order to measure the temperature variations induced by an optical signal, we employ a technique based on the analysis of whispering galley mode (WGM) resonances shift with temperature. An increment of temperature in the PS fiber produces a spectral shift of the resonances. The technique provides the possibility of measuring the thermal variations along the fiber, with good temperature and axial resolution (~0.06 oC and tens of microns, respectively, are easily achievable). Fig.1(a) shows the thermal profile of a 5 mm long UV-irradiated section of a boron codoped photosensitive fiber (PS980, Fibercore), along its length. The resonances are excited at different points of the PS fiber, which plays the role of a microresonator, with the assistance of an auxiliary 2 μm taper. The setup used for the measurements is similar to that reported in [1]. Fig.1(b) shows the different heating for an irradiated and non-irradiated section of a fiber.