Femtosecond Laser Inscribed Phase-Shifted Superstructure FBG and Its Application to Differential-Type High Refractive Index Sensor

We demonstrate a phase-shifted superstructure fiber Bragg grating (PSSFBG) fabricated using a femtosecond laser-based point-by-point direct writing technique. The proposed PSSFBG simultaneously generates multiple pairs of equivalent phase-shifted resonance peaks that are mirror images of each other. The magnitude of the inserted phases can be precisely controlled due to a three-orders-of-magnitude reduction in fabrication precision requirements. Specifically, PSSFBGs with phase shifts ranging from 0 to 2π at fine intervals of 0.125π have been successfully implemented—a task challenging to achieve with high yield rates using previous methods. The proposed PSSFBG not only fulfills nearly all applications of ordinary phase-shifted FBGs but also enables specialized applications such as dual-wavelength switches, odd-even channel filters, and phase-demodulated FBG sensors. As an application example, we developed a differential-type high refractive index (RI) sensor based on the PSSFBG design, enhanced by micro-groove processing in the phase insertion region. This sensor demonstrates remarkable effectiveness in measuring RI values higher than those of the fiber itself, exhibiting exceptional linear sensitivity of 544 dB/RIU, which is competitive within the family of power-interrogated fiber-optic RI sensors. Furthermore, it features high precision and effectively mitigates the impact of power perturbations from the light source and optical path.