Interrogation methods for fiber Bragg grating sensors

Fiber Bragg Gratings (FBG) have been applied to an ide range of systems both in telecommunication and sensors systems. For sensors the main advantages in using FBG are the ease of multiplexing and the reliability of measurements due to spectral encoding, Although many different ways of analyzing the spectral response of a FBG have been proposed and demonstrated, most of them involve sophisticated electronics and do not take into account practical problems. We describe a simpler robust and lon cost technique in which a reference grating is used as a spectral filter for the sensing grating. In our prototype a 1550 nm pig-tailed LED is used as broad band source. As the system is proposed for strain measurements, special attention is paid to field calibration and long term measurements. Two calibration methods are demonstrated. One uses the digitalized reflection spectra of sensor and filter grating and numerically predicts die sensor output. The other method is based on the application of a controlled opposite strain on the filter grating. Both methods were compared with conventional resistance strain gage and the measuring accuracy is estimated. We also point out applications where FBG strain sensor can have significant advantages over conventional electric counterparts.