Determination of geometry-induced positional distortion of ultrafast laser-inscribed circuits in a cylindrical optical fiber

Positional distortion is a defocusing phenomenon in ultrafast laser inscription of fiber optic circuits induced by the cylindrical geometry of an optical fiber. In this Letter, a study on the positional distortion of ultrafast laser processing assisted by tightly focusing optics has been conducted. Attention has been paid to the effect of numerical aperture (NA) of the focusing optics and location of the laser-writing plane. The occurrence of convex positional distortion that decreased with the NA was observed in an array of laser-inscribed optical tracks when scanning across the fiber. It exhibited a maximum distortion of 28.9 and 23.8 μm in the center plane of the fiber for the 0.42-NA and 0.85-NA dry objective lenses, respectively, but only a negligible positional distortion in the track array written in an off-center plane.