Fiber-optic Microfluidic Platform for Refractive Index Measurement of Liquids and Suspensions

We present a microfluidic platform based on a fiber-optic three-way Mach-Zehnder interferometer (MZI), aimed to measurement of the refractive index (RI) of liquids and characterization of suspended glass particles (cylinders) simultaneously. The measurement principle is based on low coherence interferometry, where the maximum position of the interferogram Gaussian envelope depends on the optical path difference (OPD) between the measuring and the reference arm of the MZI. An algorithm was developed for calculating the refractive index of liquids and glass particles, as well as for finding the particle diameter from the raw photodetector signals. The physical particle diameter is calculated from the measured particle transit time while passing through the test beam. We found very good agreement between the experimental results and the literature data on the examined liquids refractive index and dimensions and refractive index of suspended particles. The accuracy of the refractive index measurement was about 1 %, mainly determined by the accuracy of position reading of the mechanical scanner. The minimal sample volume can be as small as 1 µl is capable of measuring the refractive index of different liquids and gases and their suspensions simultaneously. The proposed method is attractive for label-free biological, biochemical and chemical sensing because of its high sensitivity and accuracy and self-calibrating feature.