Investigation of the 2-D modal coupling of a Laguerre Gaussian beam through the dynamic air–water interface

When a Laguerre Gaussian (LG) beam propagates through the dynamic air–water interface, the aerosol above the water and the water surface curvature could induce various degradations, resulting in modal power coupling. Such degradations include (i) beam distortion, which affects the spatial amplitude and phase of the beam, and (ii) beam wandering, which induces misalignment between the receiver and the beam. Our results for a transmitted LG₁₁ beam show that: (i) with the increase of the sine-shape water curvature frequency from 4 to 8 Hz, the modal coupling to adjacent modes increases from ∼−11 to ∼−7 dB; (ii) with the increase of aerosol attenuation from 1–2 to 3–5 dB, the modal coupling increases from ∼−12 to ∼−8 dB; (iii) the combination of curvature and aerosol effect could induce stronger modal coupling compared to the single-effect cases. Furthermore, we study the contributions of beam distortion and beam wandering to modal coupling. We find that: (i) there is ∼−8 dB modal coupling from LG₁₁ to adjacent modes under the simulated beam-wandering-only case (the beam wandering corresponds to the one under 8 Hz curvature); (ii) the higher modal coupling for the experimental result under the 8 Hz curvature (∼−7 dB) might be due to beam distortion. Additionally, we demonstrate a 1-Gbit/s on-off-keying (OOK) link carried by a single LG₁₁ beam under aerosol and curvature effects.