FMCW reflectometric optical fiber strain sensor

This paper describes 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 ty 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 the sensing fibers, with a min-or 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 gauge length of the sensing fiber. To measure the stain 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 phase demodulation technique. To calibrate and verify the feasibility and accuracy of this sensing system, tensile and bending tests were carried out and their results were calculated and compared with the readings from strain gauge. It was observed that this system is feasible and its accuracy is satisfactory.