Fabrication of low-damage, high-strength FBG using a weak-reflection femtosecond point-by-point technique

To address the issues of optical fiber damage and consequent mechanical strength degradation caused by increased inscription energy in high-reflectivity femtosecond FBG (fiber Bragg grating), as well as the limitations imposed by accumulated optical transmission loss on wavelength division multiplexing and demodulation accuracy, this study proposes the use of femtosecond laser point-by-point apodization to fabricate weak-reflectivity FBG. This approach aims to reduce fiber damage, enhance FBG mechanical strength, and decrease transmission loss. Firstly, by establishing a damage theory model for femtosecond laser point-by-point inscription, the influence of inscription energy on reflectivity, transmission loss, tensile strength, and cascaded loss was systematically analyzed. On this basis, a femtosecond laser point-by-point apodization inscription system was constructed, producing weak FBGs with low damage and high uniformity. Finally, high signal-to-noise ratio demodulation of the weak FBG array was achieved by increasing the optical source power, thereby completing the reliability verification for the sensing demodulation. Experimental results show that compared to high-reflectivity FBG arrays, the cascaded loss was significantly reduced from 6.8 dB to 0.7 dB, while the tensile strength reached 509 kpsi, achieving 75% of the original fiber's tensile strength. To validate the sensing reliability of the weak FBG array, temperature and strain sensors fabricated using 1% reflectivity FBGs both exhibited a coefficient of determination R² above 0.9996 for linear fitting. Compared to high-reflectivity FBG sensors with 90% reflectivity, the weak-reflectivity FBGs showed no degradation in sensing performance, demonstrating excellent stability and consistency. This research provides theoretical support and a technical pathway for the fabrication and application of weak-reflectivity FBGs, which can be effectively applied in large-scale structural deformation monitoring scenarios, such as in aerospace and aviation.