Evanescent-field fiber sensor for the water content in lubricating oils with sensitivity increase by dielectrophoresis

A fiber-optical evanescent-field absorption sensor for the in situ determination of the water content in lubricating oils is described. A sapphire fiber, combined with a thermal emitter, an optical filter, and a PbS photodetector is used to measure the absorbance in the evanescent field surrounding the cladding-less fiber in the spectral range of water absorption at about 2.95 μm. To improve the sensor sensitivity dielectrophoresis is applied to increase the concentration of water droplets in the region of the evanescent field. With a suitable arrangement of the electrodes the sensitivity is increased by a factor of 1.7 on average compared to an identical sensor operated without dielectrophoresis.