Single-mode sapphire photonic crystal fiber Bragg grating

A single-mode sapphire photonic crystal fiber (S-PCF) fabricated via femtosecond laser direct writing is demonstrated, with a fiber Bragg grating inscribed in the core. A two-step cladding strategy is proposed: waveguide arrays are written point-by-point along the fiber axis in a hexagonal lattice to form an artificial photonic crystal cladding. The lower half of the lattice is first inscribed, followed by embedding the FBG in the core and then completing the upper half to form a full PCF. This approach enables flexible geometry design, refractive index modulation, and precise modal control. Experiments show single-mode guidance under different lattice pitches, cladding layers, and index modulation. The fabricated sapphire PCF-FBG exhibits effective confinement with a near-Gaussian mode distribution, 0.20 nm bandwidth, 14.45 dB SNR, and a reflectivity of 48.5%. It also demonstrates stable high-temperature performance from 20 to 1500 °C with enhanced temperature sensitivity.