Simple wavelength-to-phase mapping FBG's interrogation method

Abstract

In this work, an interrogation method for FBG sensors is presented based on a wavelength-to-time transformation procedure. The proposed scheme uses an externally modulated broadband source, an optical circulator and three Bragg gratings fixed in three different fiber branches, one acting as sensor (FBGS) and others two (FBG1 and FBG2) acting as wavelength calibration devices. Spectral positions of the reflectivity maximum of FBG1,2 are slightly different and fixed while FBGS spectral position lies in the reflection spectral range of FBG1,2. Moreover, an optical time delay between the reflections of both calibration gratings is introduced including a path length greater in one of the branches. Reflections from both gratings are added by employing a 50/50 coupler. By this set-up, a phase sensible detection technique is generated allowing to reach a sensitivity of up to 660°/nm in the performed experiments. A theoretical analysis of the device performance was carried out showing good agreement with the experimental results. This system combines low cost and high flexibility to be customized in a particular application.