An Optical Path Multiplexing Fiber Sensor for Ultra-Low COD Levels Measurement Using UV-Vis Spectrometry

In this work, we have developed a novel fiber sensor based on optical path multiplexing (OPM) for measuring ultralow chemical oxygen demand (COD) concentration. The cumulative effect brought by the novel OPM structure could amplify sensor’s sensitivity, which significantly improves the detectability in ultraviolet-visible (UV–Vis) spectrometry method. Meanwhile, structural optimization method based on maximum coupling efficiency have been demonstrated. In the UV–Vis absorption spectroscopy experiment, the absorbance of 215 nm wavelength and the area under the absorption spectrum were used to estimate the COD concentration, respectively. The results shown that they all increased linearly with the change of COD concentration, and kept the strictly linear relationship in the ranges of 0–6 mg/L. Their linear correlation coefficients are above 0.9977. Surprisingly, the COD measurement curve represented by the area under absorption spectrum gave a better performance in this study, whose RMSE, LOD, and LOQ were superior to 0.0165, 0.0362, and 0.1207 mg/L, respectively. The good performance indicators revealed accurate measuring ability of the OPM-based fiber sensor for ultralow COD levels. More importantly, the open architecture in OPM-based fiber sensor has shown a good application prospect for in situ water quality assessment.