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

Abstract

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> 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 <formula formulatype="inline"><tex>$^{\circ}\hbox{C}$</tex></formula>, and their polarization properties are monitored. It is shown that the birefringence in Type I-IR gratings inscribed in hydrogen <formula formulatype="inline"><tex>$(\hbox{H}_{2})$</tex></formula>-loaded fibers is small <formula formulatype="inline"><tex>$(\sim{\kern-2pt}10^{-6})$</tex></formula> and can be decayed at room temperature, while the birefringence in Type I-IR gratings inscribed in non-<formula formulatype="inline"><tex>$ \hbox{H}_{2}$</tex></formula>-loaded fibers is relatively higher <formula formulatype="inline"><tex>$(\sim{ \kern-2pt}10^{ - 5})$</tex></formula> and shows strong dependence on the polarization of the IR laser beam. It has the same annealing resistance as the induced index. For Type II-IR gratings, the birefringence is an order of magnitude higher than in Type I-IR gratings <formula formulatype="inline"><tex>$(\sim{\kern-2pt}10^{ - 4})$</tex> </formula> and shows strong temperature variation during annealing. </para>