The refractive index is one of the key parameters in non-invasive and label-free sensing applications. The past decade has witnessed various miniature optofluidic devices that offer several analytical functions with minuscule samples (picoliter or nanoliter) through the fusion of optics and microfluidic sciences. However, the realization of a compact, wide-range, and less-expensive refractometer is still a great challenge.

In this paper, the authors have proposed a novel, to the best of our knowledge, self-mixing optical feedback interferometry (SM-OFI)-based refractometer that correlates the refractive index of a flowing liquid to the induced Doppler frequency shift.

The proposed method was experimentally tested on the saline water and benzyl chloride and found close agreement with the literature results. The refractive indices of the saline water and benzyl chloride were measured to be 1.3346 and 1.54079, respectively, with a standard deviation of the order of 10-5.

The induced Doppler shift was linearly increased with the concentration of the liquid during the concentration profiling. Hence, the proposed method was also capable to profile the liquid concentration.

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The well-known compatibility and cost-effectiveness of the SM-OFI setup also support the proposed method for miniature applications. The compactness and the portability of the experimental setup also make it compatible in areas of application where the size of the analytical section is a decisive parameter, such as biosensors, particle manipulators, chemically active devices, lab-on-chip instruments, etc.