High-resolution liquid refractive index sensor based on side-polished d-shaped fiber and phase-sensitive optical frequency domain reflectometry (φ-OFDR)

A compact high-resolution distributed liquid refractive index (RI) sensor based on the side-polished D-shaped fiber (DSF) that needs no post-fabrication modifications is presented. Since phase demodulation can offer better sensing resolution than most traditional amplitude cross-correlation algorithms, for what we believe to be the first time, we propose and experimentally investigate the integration of phase-based optical frequency domain reflectometry (φ-OFDR) with a DSF in its bare state and as fabricated for liquid RI monitoring. The principle of refractive index measurement by phase tracking in φ-OFDR is described with an explanation of the data processing algorithm. A new polarization control method has been developed to maintain the input polarization state along the fiber to minimize polarization-induced measurement instabilities originating from the geometry of the sensor head. The system reaches a spatial resolution of less than 0.78 mm which allows accurate detection of refractive index variations in liquids. The RI sensitivity of the sensor was established using different concentrations of glycerol in distilled water solutions. By analyzing phase shifts in response to different liquid samples, we successfully measured the RI in a range as wide as 1.333 to 1.431. The RI measurement sensitivity was determined to be 0.1547 RIU^-1 (162 nm/RIU) with a linear correlation (R² = 0.9829) between phase shifts in response to different RI solutions, demonstrating the suitability of the approach for practical applications in biochemical and environmental monitoring. Furthermore, the fine spatial resolution of our system lays the foundation for future multiplexed sensing applications.