Femtosecond Laser Inscribed Small-Period Long-Period Fiber Gratings With Dual-Parameter Sensing

A small-period long-period grating (LPG) with a period of $40\mu \text{m}$ fabricated in a single-mode fiber by femtosecond laser direct writing has been demonstrated. A series of high-order Bragg resonant peaks of Bragg grating and attenuation bands of LPG can be observed simultaneously in the broadband spectrum range that exhibited different responses to surrounding refractive index, temperature, and axial strain. Meanwhile, we provided a sensitivity matrix to correct the errors caused by temperature in measurements of refractive index and strain to achieve a dual-parameter fiber sensor at high temperature.