Sensor head development for mid-infrared fibre-optic underwater sensors

In the process of developing a mid-infrared fibre-optic sensor for operation in the marine environment, a number of parameters influencing the sensor performance have to be taken into consideration. Optical engineering and spectroscopy are the key sciences when designing an optical sensor. Other disciplines strongly influencing the design considerations for this application are marine engineering and hydrodynamics. The development and performance of a sensor head for application in the marine environment is discussed in this paper. Following optical, chemical, hydrodynamic and stability considerations, a U-shaped sensor head was found to be the most suitable sensor geometry. A prototype of such a sensor head has been built, attached to a specially designed underwater FT-IR spectrometer and subjected to simulated real-world flume-tank tests under controlled conditions. In combination with a compatible IR spectrometer, this sensor head allowed the qualitative and quantitative detection of a range of environmentally relevant analytes in water down to ppb concentration levels. Furthermore, it could be demonstrated that the sensitivity can be increased significantly by using tapered fibres for the sensor head.