When an orbital-angular-momentum (OAM) beam propagates through the dynamic air-water interface, the aerosol above the water and the water surface curvature could induce various degradations (e.g., wavefront distortion, beam wandering, scattering, and absorption). Such time-varying degradations could affect the received intensity and phase profiles of the OAM beams, resulting in dynamic modal power loss and modal power coupling. We experimentally investigate the degradation for a single OAM beam under dynamic aerosol, dynamic curvature, and their comprehensive effects. Our results show the following: (i) with the increase of the aerosol strength (characterized by the attenuation coefficient) from ∼0 to ∼0.7-1.3 dB/cm over ∼7 cm, the power coupling ratio from OAM -1 to +2 increases by 4 dB, which might be due to the amplitude and phase distortion caused by spatially dependent scattering and absorption. (ii) With the increase of the curvature strength (characterized by the variance of curvature slope over time) from ∼0 to ∼2 × 10-5 rad2, the power coupling ratio from OAM -1 to +2 increases by 11 dB. This could be caused by both the wavefront distortion and the beam wandering. (iii) Under the comprehensive effect of aerosol (∼0.1-0.6 dB/cm) and curvature (∼6 × 10-7 rad2), there is an up to 2 dB higher modal power loss as compared with the single-effect cases. (iv) The received power on OAM -1 fluctuates in a range of ∼6 dB within a 220 ms measurement time under aerosol (∼0.1-0.6 dB/cm) and curvature (∼6 × 10-7 rad2) effects due to the dynamic degradations. We also demonstrate an OAM -1 and +2 multiplexed 2-Gbit/s on-off-keying link under dynamic aerosol and curvature effects. The results show a power penalty of ∼3 dB for the bit-error-rate at the 7% forward-error-correction limit under the comprehensive effect of aerosol (∼0.1-0.6 dB/cm) and curvature (∼6 × 10-7 rad2), compared with the no-effect case.
- Air-water interface
- free-space optical communication
- mode division multiplexing
- orbital angular momentum mode