Inscription of phase-shifted gratings with ultrashort pulses using defocusing and phase-mask shifting

Two slightly shifted wavelength gratings are inscribed one over the other in a single mode fiber by shifting the phasemask between two positions. The inscription setup includes a NIR femtosecond laser, a phase-mask, a defocusing spherical lens and a cylindrical focusing lens. A first fiber Bragg grating (FBG) is inscribed, while the second overlapping FBG is inscribed only after a slight shift of the phase-mask, enabling a slight wavelength shift. The transmission spectrum of the complex structure is like that of a phase shifted grating, while the inscription process is very fast and simple compared to other standard methods. A high-quality phase shifted grating with two −20 dB transmission dips, a 15 dB transmission peak with a 30 pm transmission bandwidth at 3 dB is achieved. It is also observed that this phase shifted grating structure is birefringent.