Fiber Bragg Grating Refractive Index Sensors With a Circular Refractive Index Modulation Region via Femtosecond Laser Phase Mask and Fiber Rotation

This study presents a novel fiber Bragg grating (FBG) refractive index (RI) sensor featuring a circular refractive index modulation (RIM) region, fabricated by integrating femtosecond laser phase mask inscription with in-situ fiber rotation. This method enables a symmetrically enlarged RIM cross-section, which was theoretically and experimentally proven to enhance the core mode's response to the surrounding RI (SRI). The circular geometry effectively suppresses birefringence, yielding high reflectivity and a stable side-mode suppression ratio. Utilizing a single FBG pair, we demonstrated simultaneous SRI and temperature measurement. For a sensor with a 5-μm RIM and a 15-μm cladding diameter, a high RI sensitivity of 10.96 nm/RIU was achieved within an SRI range of 1.333 to 1.413. Over a temperature range of 25 °C to 75 °C, it maintained a temperature sensitivity of 9.9 pm/°C and a temperature-compensated RI sensitivity of 9.51 nm/RIU. Benefiting from the synergistic effect of the circular RIM and reduced cladding, our sensor offers a streamlined fabrication process—superior to time-consuming direct writing—coupled with high sensitivity and robustness, making it highly suitable for practical sensing applications.