Multiplexing methods by frequency domain of FBG sensors (OFDR)
FBG sensor is advantageous of being able to have many sensors in one line of fiber optic. To recognize the position of each FBG, there are several methods like WDM (wavelength division multiplex) and TDM (time division multiplex). OFDR (Optical Frequency Domain Reflectometry) is a third option, that can divide FBG by interferometric approach. OFDR makes possible to measure several hundreds of FBG and also to measure with less-than-1mm spatial resolution, both of those are impossible by WDM and TDM.
OFDR technique can be used to simultaneously interrogate several hundreds low-reflectivity FBG strain and/or temperature sensors along a single optical fiber. With OFDR, the optical frequency of a source laser is scanned linearly in time. When the optical signal backscattered from FBGs interferes with the reference-light reflection, which serves as local oscillator, a complex interferogram results. The intensity seen at the photo-detector is a superposition of sinusoids, the amplitude of each sinusoidal component is proportional to the backscattered light intensity at each point along the fiber, and the frequency of each component is proportional to the distance between the reference and the source of the backscattered light from FBG.