Demonstration of low-crosstalk spiral fractional orbital angular momentum multiplexing for free-space optical communication.

Fractional-order orbital angular momentum (OAM) modes offer continuously tunable topological charges for higher-density space division multiplexing, yet remain limited by severe modal crosstalk (XT). Spiral fractional OAM (SF-OAM) is a distinct class featuring a spiral intensity distribution and a radial phase discontinuity, with a controllable auto-focusing property. By combining conjugate phase demodulation with optimal spatial focusing, we propose and experimentally demonstrate low-XT free-space optical communication based on SF-OAM multiplexing. The dual-isolation mechanism enables mode isolation exceeding 12 dB, improving more than 8 dB over the theoretical XT floor of conventional fractional OAM. Proof-of-concept transmission validates the scheme with minimal signal degradation, establishing SF-OAM as a viable pathway to high-density, hybrid-order OAM communication.