Direct Laser Writing of Bragg Gratings in Silica Glass by Helical Motion of a Lenticular Beam Waist

High quality uniform Bragg gratings of the third order have been inscribed in a single-mode silica-based fiber by a helical motion of a lenticular beam waist of the femtosecond laser. The technique developed paves the way for fabrication of helical fiber Bragg gratings able to change the orbital angular momentum of light.