Quasi-distributed frequency-modulated continuous-wave reflectometric optical fiber strain sensor

Abstract

We describe a quasi-distributed optical fiber strain sensor, whose principle of operation is based on the coherent frequency- modulated continuous-wave (FMCW) reflectometry technique. The sensing system is basically composed of a laser diode, which is frequency swept by a triangle waveform injection current and an unbalanced two- beam interferometer. Its test arm consists of a number of single-mode fibers, which act as sensing fibers, with a mirror at the far end and mechanical splices as the connectors as well as the reflectors. Theoretical analysis shows that the measuring resolution of the strain is inversely proportional to the length of the sensing fiber. To measure the strain variation of each sensing fiber, the phase shifts of the beat signals are demodulated by a heterodyne signal processing system. The requirement of a high precision temperature control of light source can be eliminated in this way, since temperature effect is insignificant to the phase demodulation technique. To calibrate and verify the linearity, feasibility, and accuracy of this system, tensile and bending tests were carried out and strain gauge was employed as the benchmarking tool over the experimental results. It was observed that this system is feasible and its accuracy is excellent.