Remote Fiber - Optic Strain Sensing On Composites With Simultaneous Interferometric and Polarimetric Readout

Abstract

In preparation of fiber - optic strain measurements on an aircraft wing the performance of an interferometric strain gauge is investigated. A double - polarization Michelson-interferometer is employed for remote sensing of the bending induced surface strain (up to 2500 με) of plates made from Carbon fiber composites (CFRP). Figure 1 shows the arrangement of a surface adhered fiber - optic strain gauge (OSG) with reference (R) and sensing arm (S)) of equal length fixed nearby a conventional resistive gauge (ESG). The fiber jacket is removed in the sensing region. 1 = distance between the supports = 65 mm, L = lenth of sensitive fiber section = 44.4 mm ≥ Lfil,2 = adhesive covered sections with different lengths for different experiments i, 2h = thickness of plate = 1 mm. The double-polarization method is employed for eliminating the ambiguity in fringe counting [1]. Fig. 2 shows a schematic of the experimental setup. A linearly polarized wave from a 1 mW HeNe laser (L) is guided to the interferometer via 5 m of cabled polarization maintaining input fiber which is connected to the input arm of a conventional 3 dB directional coupler via a NTT-FC single mode connector. A polarizing beam splitter (PB) splits the collimated (G) output wave into two orthogonally polarized interference signals (horizontal H, vertical V) with different phase offset (φH, ΦV) which are guided to photodiodes via multimode fibers (MF):