Characterization of the birefringence in fiber Bragg gratings fabricated with an ultrafast-infrared laser

The changes of birefringence in Type I-infrared (Type MR) 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 degrees C, and their polarization properties are monitored. It is shown that the birefringence in Type I-IR gratings inscribed in hydrogen (H-2)-loaded fibers is small (similar to 10(-6)) and can be decayed at room temperature, while the birefringence in Type I-IR gratings inscribed in non-H-2-loaded fibers is relatively higher (similar to 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 (similar to 10(-4)) and shows strong temperature variation during annealing.