All-fiber hybrid cavity for sensing applications

By now, fiber Bragg gratings (FBGs) represent a well assessed technology in both communications and sensing fields. In particular, thanks to their small signal bandwidth and wavelength encoded information, they allow simple measurements in reflection for sensing purposes and also easy multiplexing capability in realizing sensors arrays. Unfortunately, to make FBGs sensitive to surrounding-medium refractive index (SRI), hosting fiber structuring is needed. In last years, also tilted FBGs (TFBGs) - intrinsically SRI sensitive structures - have been proposed as promising technological platform for several sensing applications. However, complex spectral features combined with the difficulty to be configured as quasi distributed or multi-point sensors network limit the practical exploitation of this assessed and mature technology. It would be extremely useful to merge the peculiar spectral characteristics of both grating types. To address this issue, here, we propose a hybrid cavity involving two unbalanced uniform FBGs written at both sides of a TFBG to form an all-fiber interferometer. The proposed configuration provides a wavelength gated reflection signal with interference fringes depending on the cavity features modulated by spectral dips associated to the wavelength dependent optical losses due to cladding mode coupling occurring along the TFBG. Such a structure preserves the advantages of uniform FBGs in terms of interrogation methods and allows the possibility to contemporarily measure multiple parameters.