Amplitude interference immune pH sensing devices based on white light interferometry

We introduce here a fiber optic pH sensor based on white light interferometry which overcomes the major drawbacks of many optical fiber pH sensors such as their dependence to optical power variations or leaching of the indicators as well as the dependence of large coherence light sources. Here, communications optical fibers hold the basic sensing structure, which is fabricated using the Layer-by-Layer self electrostatic self assembly method and consists on the alternate deposition of the polymers poly(allylamine hydrochloride) and poly(acrylic acid) as the cationic and anionic monolayers respectively up to 50 bilayers. These nano-coatings operate as a Fabry-Perot Interferometric cavity of a few hundreds of nanometers suitable to be excited with a halogen white light source (FWHM ∼300nm). In fact, the FPI-cavity acts as an intensity independent pH detector by monitoring the changes in the spectra waveform originated due to the changes in thickness of the coatings. Here, several measurements were performed with the fabricated sensors showing their robustness against changes of the optical power and sensitivity between 0.051 and 0.0187 pH units for the best and worst case respectively.