A fiber-optic polarimetric strain sensor of sensing length with three-wavelength passive quadrature digital phase demodulation is investigated. The demodulation unit uses a superluminescent diode light source with narrow-band interference filters in front of the photodiodes and real-time processing of the interference intensities by an arctan-phase-stepping algorithm. Quasi-static strain sensing is performed during slow periodic compression of a composite reinforced plastic rod with a sensor glued to its surface. The measured displacement sensitivity of , with a resistive strain gauge as a reference, agrees well with the value of previously determined by fringe-distance measurement [Bock et al., Pure Appl. Opt. 5, 125 (1996); ]. Despite a coherence-limited fringe contrast of only a few percent, a linearity of the phase–strain characteristic of the order of 1% and a strain resolution of are demonstrated.
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