DAS quazi-distributed system combined with temperature measuring on addressed fiber Bragg gratings

Abstract

The work presents a concept of microwave-photonic sensor system (MWPSS) for real-time distributed acoustic sensing (dynamic mode) and temperature (quazi-static mode) measurements in underground cable power transmission lines with embedded fiber optic sensor cable. The proposed MWPSS is based on the addressed fiber Bragg structures (AFBSs), consisting from two symmetrical ultra-narrow fiber Bragg gratings (2λ-FBG) each, written in this sensor cable. AFBSs have common middle wavelength, and its ultra-narrow FBGs have equal bandwidths and unique address frequency spacing between them, thus realizing the microwave-photonic interrogation method by means of beating signal detection on address frequencies. Continuous broadband laser radiation from a laser source, passing through a pass band frequency filter, generates laser radiation with a width equivalent to the measurement range. The radiation goes to AFBSs array and reflected from it passes through an inclined filter – trapezoidal optical filter. Then we got only reflected address components of AFBS and their beating at the output of photodetectors. Therefore, as we know addressed frequencies, we can realize multiplicative optical response analysis allows to define the middle Bragg wavelengths shift of each AFBSs in array and to correlate it with the value of the influencing physical fields (acoustic vibration and temperature). These values, got by interrogator, are output parameters from MWPSS. This analysis are realized in radiofrequency range, so the accuracy as for quazi-static and dynamic modes of measurements is better than for systems with optical interrogators.