By integrating the laser frequency-modulated continuous-wave (FMCW) technology, cross-correlation demodulation (DM), and Fabry-Perot (F-P) interferometry, we demonstrate a high-precision optofluidic refractive index (RI) sensing system. A 1312 nm distributed feedback (DFB) laser is current-modulated to generate a continuously swept wavelength source, whose spectral stability is improved through real-time calibration using a hydrogen fluoride (HF) molecular absorption line. An F-P RI sensing probe is theoretically designed and experimentally fabricated, enabling high-resolution demodulation through cross-correlation algorithms, while a counting-based scheme extends the measurement range. In experiments with sodium chloride solutions of varying concentrations, the system achieves wavelength resolution of 0.024 pm and an RI detection limit of 3.2×10⁻⁸ RIU. The platform’s capability for real-time biochemical monitoring is demonstrated via the photochemical reaction of chlorophyll in ethanol. Under 405 nm laser irradiation, the solution exhibits a light-induced RI reduction, primarily attributed to photothermal effects. This optofluidic RI sensor combines the ultra-high precision, broad dynamic range, and robust stability, offering significant potential for liquid refractometry and kinetic studies of chemical and photochemical reactions.

A metal-clad planar polymer waveguide refractive index sensor based on epoxy (EPO) polymer materials by using light intensity interrogation at 850 nm is designed. The polymethyl methacrylate (PMMA) material is deployed as the low refractive index (RI) buffer layer in order to better couple the optical guided mode and the surface plasmon polaritons (SPP) mode for working in water environment. The effects of the gold film thickness, PMMA buffer layer thickness, waveguide layer thickness, waveguide width, and gold length on the sensor sensing characteristics have been comprehensively studied. Simulation results demonstrate that the normalized transmission increases quasi-linearly with the increment of RI of the analyte from 1.33 to 1.46. The sensitivity is 491.5 dB/RIU, corresponding to a high RI resolution of 2.6×10⁻⁹ RIU. The designed SPP-based optical waveguide sensor is low-cost, wide-range, and high-precision, and has a broad application prospect in biochemical sensing with merits of miniaturization, flexibility, and multiplexing.

A waveguide coupled surface plasmon sensor for detection of liquid with high refractive index (RI) is designed based on polymer materials. The effects of variation of the thickness of the Au film, polymethyl methacrylate (PMMA) buffer, and waveguide layer on the sensing performance of the waveguide are comprehensively investigated by using the finite difference method. Numerical simulations show that a thinner gold film gives rise to a more sensitive structure, while the variation of the thickness of the PMMA buffer and waveguide layer has a little effect on the sensitivity. For liquid with high RI, the sensitivity of the sensor increases significantly. When RI of liquid to be measured increases from 1.45 to 1.52, the sensitivity is as high as 4518.14nm/RIU, and a high figure of merit of 114.07 is obtained. The waveguide coupled surface plasmon RI sensor shows potential applications in the fields of environment, industry, and agriculture sensing with the merits of compact size, low cost, and high integration density.