Optofluidic Fabry-Perot interferometric sensor for the real-time measurement of refractive index

In this work, a microfluidic system combined with a fibre-optic extrinsic Fabry-Perot interferometer is proposed to measure refractive index continuously and in real time. A microfluidic platform was designed and created for this purpose through 3D printing. The Fabry-Perot cavity is an integral part of the microfluidic chip and is perpendicular to the sample flow. The light is conducted through a single mode optical fibre and the refractive index measurements were based on the optical power and wavelength shift of the reflected spectra. The developed optofluidic setup was characterised using different concentrations of glucose solutions. A sensitivity of 1102 nm/RIU was obtained when using the wavelength shift, however, when the same solution was analysed over time, the signal for wavelength shift measurements was found to be unstable. The optical power shift was correlated with the refractive index and a sensitivity of -79.6 dB/RIU was obtained, with a good linearity (r2 = 0.996). Good results were verified in terms of stability with a maximum standard deviation of 0.028 dB and a sensor resolution of 4.3×10−4 RIU. This sensor has a great potential for applications in which refractive index real-time measurements are required, such as food and beverages industry process control.