Focal-plane wavefront sensing with narrowband light using a short multi-mode fiber

We propose a focal-plane wavefront sensor (FPWFS) based on a short multimode fiber (MMF). By coupling the aberrated focal-plane field into an MMF of length ≲1 cm, we preserve modal interference over a 10 nm bandwidth at near-infrared wavelengths, but broader bandwidths could be achieved by appropriately tuning the fiber length. The resulting output intensity pattern encodes pupil phase information, enabling wavefront recovery via a neural network. Our approach resolves the inherent sign ambiguity of even pupil-phase aberrations and operates on millisecond timescales using readily available computing hardware, suitable for real-time adaptive optics. Unlike traditional pupil-plane sensors, the proposed FPWFS shares the optical path with the science beam, eliminating non-common-path aberrations by enabling simultaneous wavefront and focal-plane intensity retrieval. Its simplicity, compactness, and low cost make it an attractive candidate for free-space optical communication and astronomical instrumentation.