Characterization of twin Bragg gratings for sensor application

Twin gratings are important devices for sensor application. They are formed by two gratings inscribed in cascade in a single fiber. We present here a detailed study of twin Bragg gratings. We first show the evolution of twin Bragg gratings spectra versus the spacing between the gratings and their length. Owing to the coupled mode theory, we explain the particular shape of the various spectra. Then we demonstrate the influence of the reflectivity of each Fiber Bragg Grating on the global spectral response of the two cascaded FBGs. All the gratings we have inscribed are simulated using the Transfer Matrix Method in order to retrieve their parameters (grating length, period, index modulation and distance between gratings)from experimental reflection spectra. Finally we characterize these FBGs for temperature sensing. We discuss the design suitable to sensor applications and describe the interrogation technique used in this case. We use a digital algorithm based on Fast Fourier Transform. It offers precise, fast and absolute measurements.