Apodized point-by-point fiber-Bragg gratings

Fiber-Bragg gratings (FBGs) inscribed with a femtosecond laser have proven advantages for a variety of applications because they exhibit excellent stability at elevated temperatures [1] and under intense optical pumping [2], and can be directly inscribed into virtually any transparent fiber material [3]. Applications in high-power and monolithic fiber lasers and high-temperature sensing have been demonstrated [2,4]. The point-by-point (PbP) technique for fabricating FBGs with a femtosecond laser is uniquely flexible as it produces gratings which consist of discrete refractive index modifications with sub- to few-micron dimensions, yet with high refractive index contrast. By controlling the transverse and longitudinal position of each modification, the local phase and coupling strength of the grating may be precisely tailored, enabling complex grating profiles to be implemented without any need for a phase mask. Despite this inherent flexibility, only uniform FBGs had been demonstrated with the PbP technique until our recent report of a suite of non-uniform PbP FBGs, including linearly-chirped, phase-shifted, sampled and superstructure gratings [5].