Miniature fiber-optic Fabry-Perot refractive index sensor for gas sensing with a resolution of 5x10^-9 RIU

This paper presents a micro-machined, high-resolution refractive index sensor suitable for monitoring of small changes in the composition of gases. Experimentally demonstrated measurement resolution, induced by gas composition variation, proved to be in the range of 5x10-9 of a Refractive Index Unit (RIU). The proposed all-silica, all-fiber sensor consists of an open-path Fabry-Perot micro-cavity that includes an in-fiber collimation and temperature-sensing segment. It is shown that a sensor's resolution depends strongly on the signal interrogator's properties and that, for a given interrogator, there is an optimum Fabry-Perot cavity length that yields the highest system resolution. Furthermore, high-resolution pressure and in situ temperature compositions of measurement results are required to obtain an unambiguous correlation between the gas composition and measured Refractive Index within the presented resolution range.