Distortion-free femtosecond laser inscription in free-standing optical fiber.

In this paper, we report the first experimental demonstration, to the best of our knowledge, of low-energy distortion-free femtosecond laser inscription in a free-standing fiber without the need for oil immersion. The methodology proposed features radial-plane beam shaping to eliminate the focal-point distortion induced by the cylindrical geometry of the fiber, thus achieving optimal focusing and spot-like inscription within the fiber. Based on the proposed methodology, a strong, first-order fiber Bragg grating is achieved for the first time, to the best of our knowledge, in a coated free-standing single-mode fiber by using the point-by-point inscription technique with only 50 nJ laser pulse energy.