Direct in-fiber mode excitation enabled by on-chip spatial field engineering

We introduce an interface-level field-mapping mechanism for direct excitation of higher-order fiber modes by engineering the spatial distribution of optical fields at the chip-fiber interface. Rather than relying on a separate on-chip mode-conversion stage followed by fiber coupling, the proposed approach establishes a correspondence between the on-chip phase-polarization distribution and the target fiber eigenmode at the fiber facet, thereby enabling selective mode generation without post-fiber conversion. Theoretical analysis identifies the field conditions required for specific target modes and predicts an idealized modal-overlap upper bound up to 97.8% under optimized emitter geometry and coupling conditions. To validate the concept, we implement the mechanism on a silicon-on-insulator platform using a grating-emitter array with programmable phase and polarization control. Experimental near-field measurements confirm selective excitation of LP12 and LP21 modes around 1550 nm, in qualitative agreement with the theoretical mode-selection behavior. Although the present proof-of-concept device does not yet reach the idealized overlap limit, the results verify the underlying field-mapping mechanism and provide a compact route toward chip-to-fiber spatial-mode manipulation for integrated mode-division-multiplexing interfaces.