Pressure, temperature and refractive index determination of fluids using a single fibre optic point sensor

•A miniature all-silica multi-parameter fibre optic sensor to simultaneously measure pressure, temperature and refractive index (RI) is described.•The sensor consists of a Fibre Bragg Grating (FGB) as well as an Extrinsic Fabry-Perot Interferometer (EFPI) air and glass cavity.•The sensor tested shows a linear response of 0.58/RIU for the fringe visibility in the RI range from 1.32 to 1.365 with a standard deviation at atmospheric pressure of 1.77·10−3 and in the temperature range from 20°C to 80°C.•Simulations verify that the cross-sensitivity of the RI measurement to applied pressure and temperature variations can be eliminated.•The multi-parameter fibre optic sensor can be deployed in harsh environments, e.g. chemical process monitoring, downhole geothermal or oil applications. The accurate determination of physico-chemical fluid parameters is of major importance in many process engineering applications. Not only the monitoring of the phase distribution in piping systems but also the detection of chemical concentrations in liquids is of interest for an efficient, safe and reliable operation of laboratory as well as industry scale applications. In this paper, we present a miniature all-silica fibre optic sensor capable of measuring pressure, temperature and refractive index of a fluid at a single point simultaneously. In future, such a sensor can be utilized in applications, where these quantities and, hence, phase composition or chemical concentration have to be monitored possibly under harsh environments such as geothermal or oil drilling wells.