Fiber Tip Empowered by Axial-Asymmetric Four-Sector Nanostructure for Broad-Angle Light Coupling

Efficient light coupling into the fiber waveguides is important in numerous applications, while the low in-coupling coefficients of commercially available step-index fibers significantly narrow their application domains. It is recently introduced that well-defined periodic nanostructures, such as annulus gratings, allow to substantially boost in-coupling coefficients. In this study, the genetic algorithm is applied for the optimization of trapezoidal polymer gratings. The experimental data showcase multiply enhanced light coupling at preselected incident angles with respect to a bare fiber. As a breakthrough result, the development of an axial-asymmetric four-sector grating, reaching the percentage level of light coupling efficiency in a broad angular range from 0${}^{\circ }$ to 70${}^{\circ }$ that outperforms axial-symmetric gratings by about order of magnitude. Further, the high average light coupling efficiency is verified in the finite angular intervals at multiple wavelengths of 650, 1000, and 1550 nm. This solution highlights the enhancement of algorithm-designed structures axially-asymmetric systems for fiber-based highly efficient light collections, that will further benefit extensive applications such as wide-field endoscopies.