A fibre-optic mode-filtered light sensor for general and fast chemical assay

A simple and fast-response fibre-optic chemical sensor based on mode-filtered light detection (MFLD) has been successfully developed. The sensor was constructed by inserting an unmodified fibre core into a silica capillary tubing; a charge-coupled device which acted as a multi-channel detector was positioned alongside the capillary to detect the emanated mode-filtered light. An interesting finding was observed: there was an increase in the signal upon the decrease in the sample refractive index when an unclad optical fibre was employed, which was different from the results of a polymer-clad fibre reported previously. This phenomenon of opposite signal trend can clearly be interpreted by applying a mathematical derivation based on light propagation in the optical fibre. The derived mathematical model correlates well with the experimental results. It also provides a good theoretical foundation for the future development of MFLD-based analyser in conjunction with liquid chromatographic separation and assay. The proposed MFLD sensor was successfully applied to determine acetic acid with a linear response in the range 0–90 v/v % and a correlation coefficient of 0.9959. The sensor has the advantages of high S/N ratio and very fast response time. It offers the potential for use as a general sensor in food and chemical industries.