Plane-by-plane fs fiber Bragg gratings with low polarization dependence via elliptical beam shaping

Fiber Bragg gratings written with femtosecond lasers via the point-by-point technique typically feature significant polarization dependence of the Bragg wavelength, mainly due to the anisotropic shape of the focused beam. In this work, we propose and experimentally demonstrate a technique to directly inscribe fiber Bragg gratings with a femtosecond laser using a plane-by-plane technique which consists of changing the laser beam shape from a circular to an elliptical profile using a commercial anamorphic prism pair. The gratings fabricated with this technique demonstrate a consistent decrease of polarization dependence of the Bragg wavelength by a factor of ∼4.5, which is due to the reduction of the form birefringence induced by the laser beam using an elliptical input beam.We also provide calculations of the different shapes of the focusing spot for different compression factors, and electromagnetic simulations with birefringence calculations, which support the obtained experimental results. Finally, we demonstrate the possibility of writing tilted gratings by simply rotating the ellipse axis.