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 $^{\circ}\hbox{C}$, and their polarization properties are monitored. It is shown that the birefringence in Type I-IR gratings inscribed in hydrogen $(\hbox{H}_{2})$-loaded fibers is small $(\sim{\kern-2pt}10^{-6})$ and can be decayed at room temperature, while the birefringence in Type I-IR gratings inscribed in non-$ \hbox{H}_{2}$-loaded fibers is relatively higher $(\sim{ \kern-2pt}10^{ - 5})$ 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 $(\sim{\kern-2pt}10^{ - 4})$ and shows strong temperature variation during annealing.