Refractive Index Measurement of Liquids by Double-Beam Laser Frequency-Shift Feedback

The liquid refractive index measurement has become an important process in many research and industrial applications. However, the measurement results using traditional methods cannot be calibrated, and the corresponding accuracy is limited, since those methods cannot trace to laser wavelength. This letter describes a novel method to measure the liquid refractive index based on the double-beam laser frequency-shift feedback. The two beams are incident on the liquid surface and the mirror under liquid, respectively. Utilizing the high sensitivity of the frequency-shift feedback of solid lasers and high stability of heterodyne interference, the height variation of the liquid surface and the depth variation of the mirror under liquid are measured. Addressing these two variations, the traceable measurement of liquid refractive index is realized. The measurement results of five different liquid samples and NaCl solution with various concentrations are presented, which proved the system has high repeatability of better than 0.00005.