Enhanced sensing with quasi-degenerate mode in fiber Bragg gratings ring cavities

In this paper, we report the model and the experimental demonstration of a new optical resonator formed by inserting a Fiber Bragg Grating (FBG) in a closed fiber loop. The spectral characteristics of the ring depend on the reflectivity of the FBG. When the FBG reflectivity tends to zero, the overall resonant system exhibits the usual degenerate mode conditions. On the other hand, when the reflection coefficient FBG is not vanishing, the spectral features of the resonator display split resonant modes associated to the degeneracy removal of two counterpropagating modes that resonate within the cavity. The splitting magnitude is maximal in the region of maximum reflectivity of the FBG. Moreover, it varies by acting on the FBG physical parameters (e.g. strain, temperature, group index). By considering the spectral region where the splitting magnitude variation is almost maximal, i.e. quasi degenerate mode condition, we prove that the device can be used also as a slow light sensor by monitoring the power change associated with the applied strain showing, in this case, a sensitivity of 3.6x10^(6)x eps^(-1). To the best of our knowledge, this represents a factor 20 improvement over the best slow-light FBG sensors.