Optical fibre sensor for fuel cell and other fluid concentration measurement

This work describes the development of a fibre-optic fluid concentration sensor that is based on evanescent field absorption. In addition, a second optical sensor principle is investigated by means of the development of a novel type of low insertion loss fibre-cavity. The absorption sensor probe is evaluated on several widely applied liquid mixtures, i.e., the aqueous solutions of acetone, ethanol, methanol, 1-propanol and 2-propanol. The need to efficiently monitor these fluids emerges from the ever increasing reseach activity in liquid feed fuel cells which operate on, amongst others, the four latter alcohol/water mixtures. A comprehensive investigation of the influence of different fibre parameters and geometries (e.g. fibre core radius, numerical aperture and bend radius) on the sensitivity of various sensor probes is carried out theoretically as well as experimentally. Broadband measurements in the visible wavelength area are carried out in order to select the most suitable wavelength for further experiments with a low cost LED-based hardware setup. It will be shown that the novel meander-shaped sensor probe exceeds the sensitivity of a standard U-bend sensor almost 20-fold. Fibre-cavities offer the possibility to operate absorption based optical fibre sensors in a particular configuration, the so called ringdown scheme. Current approaches suffer from high insertion loss and costly as well as stationary equipment. The cavity proposed in this investigation shows how those drawbacks can be overcome. It is treated separately in order to estimate the gain in sensitivity it can provide, independent of the type of sensor that is inserted into the cavity. The novel design reduces the insertion loss of common passive fibre-cavity designs, i.e. close to 100%, to normal fibre coupling loss. The aim of this work is to provide a fundamental investigation into a highly sensitive refractive index sensor solution that is applicable to a wide range of liquid mixtures rather than being specifically designed for a single purpose. The application range could be further extended to areas where existing sensors are based on the same elementary optical sensor principle such as pH monitoring. It will be shown that the proposed sensor solution combines high sensitivity with small sensor dimensions, a fast response time, low production and operational cost and ease of operation. Hence, both approaches the evanescent field absorption sensor as well as the fibre-cavity design show significant improvement over currently applied techniques.