Femtosecond laser parallel plane-by-plane inscribed fiber Bragg grating based on superimposed blazed grating phase masks

We propose and demonstrate what we believe to be a new method for femtosecond laser multi-foci parallel plane-by-plane inscription of fiber Bragg grating. The method involves diffracting a femtosecond laser into multiple beams by superimposing multiple blazed grating phase masks with different parameters on the spatial light modulator. This method enables the production of multiple foci with controllable positions and single-pulse energies within the fiber core. Since the blazed grating phase masks do not disrupt the phase of the diffracted beams, combined with diaphragm beam shaping, each focus can induce a refractive index modulation plane of the same size on the core through the coating. Based on this method, we fabricated a high-reflectivity fiber Bragg grating in 20/400 µm double-clad passive fiber, achieving reflectivity greater than 99.9% and insertion loss less than 0.08 dB. The fabrication process involves superimposing two blazed grating phase masks, which generate two foci separated by 13.58 µm and aligned along the radial direction of the fiber. These two foci enable the concurrent fabrication of two connected refractive index modulation planes, which are then stacked into a single FBG period through a single scan. Compared to the conventional plane-by-plane inscribed fiber Bragg grating, this method can significantly reduce fabrication time.