Femtosecond inscription of phase-shifted gratings by overlaid fiber Bragg gratings.

Two slightly shifted gratings are inscribed, one over the other, in an SMF fiber with a femtosecond laser and a phase mask. The transmission spectrum of the complex structure is similar to that of a phase-shifted grating; yet, the fabrication process is fast and simple compared to standard methods. High-quality semi-phase-shifted gratings with -24  dB transmission loss and <100  pm transmission bandwidth are presented. Their application as highly narrow micro-resonators and notch filters seems feasible.