Development of stripped-cladding optical fiber sensors for continuous monitoring: II: Referencing method for spectral sensing of environmental corrosion

Corrosion affects a variety of metallic structures. Methodologies for sensing corrosion have involved indirect measurements of environment corrosiveness in which the degradation of test probes or components is evaluated. Optical fiber corrosion transducers consisting of metal coated glass waveguide have been proposed in the past. Here, we propose a multi-wavelength optical fiber corrosion sensor. As the metal (nickel) corrodes and separates from the fiber, the light transmission of the waveguide changes due to a large difference in the refractive indices of nickel and air. In order to create a sensor capable of long-term measurements, a self-referencing method was developed that is based on observations of spectral signals ratios. The proposed approach differs from other procedures in that the optical signal shows a frequency dependence as the waveguide coating corrodes. The sensor requires only relatively inexpensive light sources and spectral detectors. A first-principles algorithm was used to model laboratory measurements. Good qualitative agreement between theoretical calculations and experimental results, suggests that theoretical models can be used to quantitatively interpret spectral signals in terms of cumulative corrosion. The sensor design is well suited for autonomous operation.