Chloride ion optical sensing using a long period grating pair

In this work, an investigation into the use of a compact non-coated long period grating (LPG) pair applied to chloride ion monitoring in known aqueous solutions over the few parts per million (ppm) region is presented. The LPG pair used in this work (LPGP) comprised two 2.5 dB single LPGs, with a physical centre-to-centre separation of around 30 mm, fabricated using an excimer ultraviolet laser via a 250 μm amplitude mask. The basis of the measurement using the LPGP was refractive index monitoring, with the sensor being calibrated using solutions of known refractive indices (RIs). Tests were carried out on aqueous solutions, containing sodium chloride (NaCl) at various concentrations ranging from 0 to 23.2 g/100 ml. Cross comparison with the response of a single LPG (LPG1) under similar testing conditions was also carried out. The results have shown that through monitoring the induced wavelength shift, the LPGP could measure a RI change of ±0.003, reaching a precision of ∼6 times better than what was achieved with a single LPG. In addition, it was shown that the LPGP can provide additional information, through the induced spectral variations of the interference fringes created, as a result of the small RI changes arising from the changes in the NaCl concentration. The spectral analysis was carried out by applying a Fast Fourier Transform to the spectra under study and small quantities of chloride ions (10 ppm) can thus be detected using the LPGP sensor. The work has shown the potential for applications in early stage corrosion monitoring (and indeed prevention), for example for marine or transport structures, due to chloride ion presence and ultimate ingress.