Multiplexing low-coherence interferometer sensors: laboratory tests and design for integration within RC structures

One of the advantages of fiber optics with respect to traditional electrical gauges is they can act both as sensors and as a pathway for signals produced by other sensors. This feature allows adoption of a simple sensor system architecture, even when arrays of many sensors are needed. In this paper, we present the development and laboratory validation of in-line multiplexing for the low-coherence interferometric SOFO standard deformation sensor. A standard SOFO, as developed, produced and commercialized by Smartec SA, employs total reflectors at the end of the measurement and the reference fibers, allowing measurement of the strain over a single field. In the solution presented, broadband Fiber Bragg Gratings (FBGs) are employed as partial reflectors to obtain in-line multiplexing. These FBGs, presenting a 5% reflectivity in their reflection spectra, are produced using a chirped phase mask. An experiment was carried out on a 3-field sensor, to investigate effectiveness, resolution and temperature sensitivity. Outcomes show clear fringe visibility for each pair of gratings and a resolution of about 2.5 μm RMS, of the same order as the single field sensor. Issues regarding the maximum number of measurement fields on a single line and deformation range limits are discussed.