Integrated fiber optic platform combining FBG and small-period LPG for accurate refractive index sensing and fPSA detection

We present a compact fiber optic sensor operating in reflection mode, cascading a fiber Bragg grating (FBG) and a small-period long period grating (SP-LPG) on a single fiber embedded within a microfluidic chip. The FBG enables real-time temperature compensation, while the SP-LPG offers high sensitivity to refractive index (RI) changes. This dual-grating configuration allows simultaneous detection of temperature and refractive index (RI) variations. After surface functionalization, the sensor achieves sub-nanomolar detection of free prostate-specific antigen (fPSA), with a detection limit of 30 pg/mL. By compensating thermal drift, the device ensures accurate biosensing under dynamic conditions, providing a robust platform for label-free diagnostics and environmental monitoring.