High-precision measurement method of liquid concentration based on differential laser self-mixing interferometry

This paper proposes a method for measuring liquid concentration based on differential laser self-mixing interference. It measures changes in the liquid's refractive index and uses the refractive index-concentration calibration relationship to determine the concentration. By using a differential detection setup, two self-mixing interference signals with a 180° phase difference are obtained. A translation stage moves a triangular cuvette, introducing a controllable optical path difference, converting changes in the liquid's refractive index into interference fringe changes. Combining SVMD adaptive decomposition, correlation coefficient mode selection, peak extraction, and square wave edge detection algorithms, the interference fringes can be stably extracted and accurately counted, establishing a quantitative relationship between refractive index and concentration. We tested with NaCl solution and glucose solution, and under the experimental conditions, the maximum relative errors in measuring the concentrations of NaCl solution and glucose solution were 2.57% and 4.17%, respectively, which proved that this method can measure concentrations under these specific experimental conditions. This approach is compact, low-cost, robust against interference, and highly accurate, offering what is believed to be a new technical solution for online liquid concentration detection.