Chirped Fiber Bragg Grating-Based Cavity Sensor for Simultaneous Temperature and Refractive Index Sensing

In this work, a compact dual-parameter fiber optic sensor for temperature, refractive index, and label-free biomarker-related measurements is presented. The sensor is based on a chirped fiber Bragg grating cleaved inside the grating region, leaving residual grating lengths of 3 mm, 4 mm, and 5 mm. After cleaving, the reflected spectrum becomes narrower and forms clear sidelobes, allowing the main-band wavelength shift to be used for temperature monitoring, while the sidelobe intensity variation is used for refractive index detection. Temperature calibration from 25 °C to 80 °C showed approximately linear responses with sensitivities close to 10 pm/°C. Refractive index calibration in 10-15% sucrose solutions showed a monotonic decrease in sidelobe intensity with increasing refractive index, with sensitivities reaching about -220 dB/RIU for selected valley features and around -200 dB/RIU in the matrix analysis. An extended calibration from 0% to 35% sucrose confirmed that the RI-dependent response was maintained over a broader range, although with increased nonlinearity. Cross-sensitivity coefficients were incorporated into 2 by 2 sensitivity matrices and their inverses for simultaneous parameter reconstruction. Additional measurements under combined temperature and sucrose concentration changes demonstrated temperature reconstruction with RMSE values of 0.87-2.69 °C, depending on the sensor and experimental conditions. Finally, as a proof-of-concept experiment, the cleaved-CFBG sensors were biofunctionalized with anti-VEGF antibodies and tested for VEGF detection from 10^-15 M to 10^-10 M. Overall, the proposed design is simple, compact, compatible with standard interrogation systems, and shows promise as a thermo-refractometric platform for biomedical biosensing applications.