Optimized configurations for mid-infrared fibre optic sensors in the marine environment

As part of a European research project in the field of marine science and technology, efforts have been made to develop a portable sensor system for chlorinated hydrocarbons in seawater. This novel analytical tool for marine real-time, in-situ pollution monitoring consists of a robust, miniaturized FT-IR spectrometer in a sealed aluminum pressure vessel and a suitable fibre-optic sensor head attached to one of the container end plates. The signal generation is based on fibre evanescent wave spectroscopy, an application of the internal reflection spectroscopy principle. The sensor head is coated with a hydrophobic polymer to enrich hydrophobic analytes from the seawater matrix and to protect the fibres from corrosion by aggressive seawater constituents. This real-world application imposes a number of restrictions on the system, originating from both, engineering considerations and physico-chemical limitations. Various sensor layouts, e.g. a fibre coil, have been developed and tested in order to find a sensor head geometry with optimal sensitivity and operating stability under these harsh conditions.