Enhanced numerical aperture in a multimode fibre via a distal Fresnel zone plate for high-resolution imaging

In this work, we present the first full transmission-matrix (TM) characterization of a multimode fibre (MMF) with a distal Fresnel zone plate (FZP). We experimentally demonstrate that the composite MMF–FZP system achieves a numerical aperture (NA) that exceeds the intrinsic numerical aperture of the bare fibre and map the resulting NA spatially across the focal plane. We show that a 100 μm FZP attached to the distal facet of a 105 μm step index MMF at wavelength 660 nm can achieve a mean focal spot of 1.2 μm (compared with 1.5 μm for a bare fibre), in a focal plane 170 μm from the facet over a 40 μm x 40 μm lateral area.