Femtosecond Laser-Inscribed High-Order Bragg Gratings in Large-Diameter Sapphire Fibers for High-Temperature and Strain Sensing

Aiming for structural health monitoring applications in harsh environments, a high-order sapphire fiber Bragg grating (HO-SFBG) written with a femtosecond (fs) laser in a large-diameter sapphire fiber is demonstrated. The radial refractive index modulation area induced by scanning exposure is greater than 60% of the fiber cross-section area. The large mode overlapping area leads to a transmission resonance strength of approximately 3.0 dB. As demonstrated by numerical simulations of mode excitation and multimode resonance, each mode resonance is significantly superposed when the coupling coefficient is above 0.0005. A study of the high temperature and strain sensing characteristics of the HO-SFBG shows that each high-order resonance has different response characteristics. This type of large-diameter SFBG has important application potential for high-temperature smart materials with embedded fiber sensors.