Bragg-grating interrogation scheme using spectral filtering and amplitude-to-phase optical conversion

In recent years fiber Bragg gratings (FBGs) have proved themselves as the most appropriate sensors for applications in smart structures, civil engineering or harsh environments. Among fiber optic sensors, FBGs emerge as the most successful due to the fact that sensing information is encoded in an absolute parameter, namely the resonant wavelength. Nevertheless, the need to convert this absolute parameter into an optical intensity modulation, more suited to be handled using low cost instrumentation, also imposes additional constrains to their practical implementation. In this paper a double conversion interrogation technique is proposed, which provides a robust low-cost system for FBG sensor demodulation. This FBG interrogation technique uses all-fiber low-cost devices. A long period grating has been employed as spectral filter for wavelength-to-amplitude conversion, providing a high measuring range (>2 nm). By dividing the signal before filtering, adding a delay line and recombining after filtering, power referencing is achieved in the optical domain thus employing only one detector. Insensitivity to power variations as high as 75% of the total optical power has been proven.