Fibre optic microsensor for refractive index and salinity based on SPR

A new refractive index microsensor based on surface plasmon resonance (SPR) for fine scale measurements in aquatic environments is presented. The local light conditions in marine sediments determine the activity of photosynthetic organisms. The light field can be investigated by scalar irradiance microprobes at a spatial resolution better than 100 mu m but the refraction index still has to be assumed constant or measured by techniques with lower spatial resolution. it is well known from other microsensor measurements that the microenvironment strongly determines the metabolism of the organisms. Therefore pie developed the new sensor to access the fine scale distribution of refraction index. A second parameter of interest is the salinity which can be assumed not to be constant in some biofilms. As the salinity is usually measured with a refractometer, we checked if we could measure salinity independent of the ambient optical conditions in the sediment with the new microsensor. The microsensor is formed by a multimode silica fibre where the plastic jacket and fire cladding has been removed. The tip is then tapered and a gold layer is deposited. Due to the tapered geometry the diameter is decreased to achieve a higher spatial resolution for profiling applications. SPR is excited at the fibre tip by coupling polychromatic light into the fibre. The reflected light is measured with a spectrometer. A refractive index change of the fibre tips surrounding area causes a wavelength shift of the spectral intensity distribution. The effects of different tip geometries have been characterised and results are presented. An appropriate measuring system is proposed.