Characterization of the Birefringence in Fiber Bragg Gratings Fabricated With an Ultrafast-Infrared Laser

The changes of birefringence in Type I-infrared (Type I-IR) and Type II-IR fiber Bragg gratings induced by an ultrafast-IR laser in SMF-28 fibers are examined after and/or during grating inscription. The gratings are then annealed at increased temperatures up to 800 degC, and their polarization properties are monitored. It is shown that the birefringence in Type I-IR gratings inscribed in hydrogen (H₂)-loaded fibers is small (~10^-6) and can be decayed at room temperature, while the birefringence in Type I-IR gratings inscribed in non-H₂-loaded fibers is relatively higher (~10^-5) and shows strong dependence on the polarization of the IR laser beam. It has the same annealing resistance as the induced index. For Type II-IR gratings, the birefringence is an order of magnitude higher than in Type I-IR gratings (~10^-4) and shows strong temperature variation during annealing