Femtosecond laser multi-layer composite apodization technique to expand the bandwidth of fiber Bragg gratings

We propose and demonstrate a multi-layer composite apodization technique using a femtosecond laser to expand the bandwidth of fiber Bragg gratings (FBGs) with maintained reflectivity and a good side lobe suppression ratio (SLSR). A theory of multi-layer composite apodization incorporating two apodization functions is developed. The calculation results confirm the expanding effect of bandwidth and a gentler group delay characteristic by the proposed technique. The composite apodization is achieved using amplitude modulation by a spatial light modulator and variation of the inscribing path by programming simultaneously. Multi-layer line-by-line inscription is employed to achieve approximate plane modulation. The repeated results reveal that the FBG inscribed using the proposed technique exhibits a wider bandwidth without severe degradation in the SLSR while maintaining similar reflectivity, which is approximately 17 times wider than that inscribed by point-by-point conventional apodization. The results of the calculation for the influence on the spectrum with different inscribing layers in the radial direction are in good agreement with the experimental findings, demonstrating that the composite AFBG at 4 layers achieves an optimal balance between bandwidth and SLSR. The length of the FBG with the widest bandwidth is only 0.6 mm. The technique has potential in compact optical devices.