High-precision group refractive index measurement of static and mechanically perturbed liquids based on femtosecond frequency-comb dispersive interferometry

Abstract

A femtosecond optical-frequency-comb-based dispersive interferometric method is proposed for high-precision bulk-liquid group refractive index measurement. Broadband spectral interferograms are transformed into the delay domain, where the target side peak is selected by window filtering. The single-pass-equivalent optical path difference is then extracted from the unwrapped spectral phase slope and converted into the liquid group refractive index using a calibrated quartz sample cell. The method was experimentally evaluated using a commercial ethanol–water mixture and aqueous NaCl solutions with concentrations from 2.5% to 15%. For the ethanol–water mixture, repeated measurements yielded a mean group refractive index of 1.354 61 with a standard deviation of 1.39 × 10−4 RIU. For the NaCl solutions, the measured group refractive index increased from 1.374 70 to 1.388 92, with a fitted sensitivity of 1.106 17 × 10−3 RIU/% and R2 = 0.9902. The combined standard uncertainty was 4.56 × 10−5 RIU for the ethanol–water mixture and 1.46 × 10−5–5.99 × 10−5 RIU for the NaCl solutions. A preliminary vibration test from 180 to 260 Hz showed that the target delay-domain side peak remained distinguishable, indicating that OPD extraction remained feasible under low-frequency mechanical perturbation. The proposed method provides a non-scanning route for precise liquid group-index measurement and concentration analysis.