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Mid-infrared evanescent wave sensors - a novel approach for subsea monitoring

The sustained health of the marine ecosystems can be seen as a research task of substantial importance, since increasing pollution of the oceans poses a serious threat to the global water resources. Hence, understanding of the physical, chemical and biological parameters determining the condition of the oceans demands persistent monitoring of the aquatic environment. This requires the development of advanced screening instrumentation capable of continuous in situ and real-time monitoring of hazardous pollutants. Remotely operated optical sensor systems able to withstand harsh measurement conditions represent a promising alternative to conventional methods involving extensive sampling procedures. Thus, interest in emerging novel sensor techniques has substantially increased during the last decade. This paper summarizes first achievements towards the development of mid-infrared fibre-optic evanescent wave sensors operating in the spectral range 3-20 µm in seawater. This wavelength region is particularly attractive for optical sensing, since molecule-specific information due to the stimulation of ground vibrational modes of organic substances is provided. First results focusing on the detection of chlorinated hydrocarbons reveal the feasibility of this approach, with detection limits in the low ppb concentration range in artificial seawater. As well as the design of a miniaturized subsea sensor module based on Fourier transform infrared spectroscopy, possibly interfering parameters such as salinity and turbidity are investigated under laboratory conditions.