Nanoprinted microstructure-assisted light incoupling into high-numerical aperture multimode fibers.

The coupling of light into optical fibers is limited by the numerical aperture (NA). Here, we show that large-area polymer axial-symmetric microstructures printed on silica multimode fibers improve their incoupling performance by two to three orders of magnitude beyond the numerical aperture limit. A ray-optical mathematical model describing the impact of the grating-assisted light coupling complements the experimental investigation. This study clearly demonstrates the improvement of incoupling performance by nanoprinting microstructures on fibers, opening new horizons, to the best of our knowledge, for multimode fiber applications in life sciences, quantum technologies, and "lab-on-fiber" devices.