Main Factors for Optimization of PM-FBG Inscribed by Femtosecond Laser Point-by-Point Technology

A polarization-maintaining fiber Bragg grating (PM-FBG) is a crucial component of fiber optic sensors, and the inscription method associated with it has garnered significant interest. Compared to alternative technologies, the femtosecond laser point-by-point inscription method offers greater flexibility. However, it is still unclear how the controllable factors of the femtosecond laser point-by-point inscription process relate to the spectral characteristics of PM-FBG. The main factors that affect the spectral properties of PM-FBG inscribed via the femtosecond laser point-by-point method, including the pulse energy, grating length, fiber type, and grating apodization, are investigated experimentally and theoretically in this paper. An experimental investigation was conducted to examine the relationship between the pulse energy and side lobe suppression ratio (SLSR), and the optimal pulse energy was approximately 30 nJ to obtain the highest SLSR. The theoretical model of the full width at half maximum (FWHM) is presented, and it is shown that the FWHM tends to saturate with grating lengths greater than 8 mm. There is a linear relationship between the distance of the two reflection peaks and the birefringence of the polarization-maintaining fiber (PMF), regardless of the birefringence induced by the inscription process. The oblique apodization method can decrease the SLSR of the two reflection peaks, but it also broadens their FWHM. This study provides compelling evidence to support the improvement in the spectral performance of PM-FBGs inscribed via the point-by-point femtosecond laser inscription method.