Point‐by‐point femtosecond laser inscription of high‐reflectivity, low‐birefringence fiber Bragg gratings enabled by curvature‐induced astigmatism and beam shaping

Abstract Direct laser writing of fiber Bragg gratings (FBG) in free-standing optical fibers is often avoided due to the cylindrical geometry of the fiber, which distorts laser beam focusing and affects process efficiency. A common approach to mitigate this is immersing the fiber in an index-matching liquid, which adds complexity to the fiber preparation process. Here, we propose an alternative and novel point-by-point (PbP) grating writing approach in free-standing optical fibers that leverages the influence of fiber curvature on focusing conditions to our advantage, allowing for beam shaping, fast alignment, and efficient inscription of high reflectivity FBGs with very low birefringence. We first used ray tracing simulations to accurately assess astigmatic focusing conditions inside optical fiber caused by fiber’s cylindrical shape. Then we implemented an additional slit beam shaping by means of spatial light modulator to achieve nearly circular intensity distribution and refractive index modification in the fiber core cross-section. Moreover, we developed a novel alignment approach that relies on fiber curvature for fast and sub-micrometer accuracy alignment of the laser focus coordinate system to that of the optical fiber. We benchmarked our approach by inscribing a series of first-order PbP FBGs up to 20 mm in length, evaluating grating strength, insertion losses and birefringence under varying focusing conditions and for different values of laser pulse energy. With accurate modeling and experimental implementation, we were also able to demonstrate highly reflective FBGs with birefringence as low as Δ n < 3.3 × 10 − 6 . Our results highlight a promising approach for further development toward versatile, and high-precision FBG inscription technique tailored to the demanding requirements of advanced fiber sensing, high-power lasers, astrophotonics filtering, and quantum photonic systems.