Long-term temperature monitoring by multiplexed fiber Bragg grating sensor system combined with incoherent FMCW optical ranging system

In this paper, we present a long-term multiplexed temperature monitoring system based on fiber Bragg grating (FBG) and the distance division multiplexing technique realized by the incoherent frequency-modulated continuous wave (I-FMCW) optical ranging system. In the I-FMCW optical ranging system, a semiconductor optical amplifier (SOA) is used as the optical intensity modulator. No bias point control is required for an SOA, unlike a LiNbO 3 optical intensity modulator, and therefore, unmanned and automated temperature measurements using nine FBGs are successfully realized over a period of two weeks. The measurement range is over 700 m. The temperature estimated from the Bragg wavelength shift of FBGs is the same as that measured by thermocouples. The results show the effectiveness and usefulness of the proposed system for the long-range, unmanned, and automated structural health monitoring of large-scale and huge-scale structures such as dams, bridges, highways, and buildings.