  Intensified underwater activities have driven the escalating demand for
reliable, flexible, and high data-rate underwater communication links. Optical
wireless communication (OWC) emerges as the most promising technology for
short- to medium-range communication, facilitating the real-time high-speed
transmission of information from undersea to an aerial vehicle which can
subsequently relay the information to a terrestrial station. However,
establishing a robust water-air link confronts two primary challenges: (i) beam
wandering due to the time-varying refraction when the light beam passes through
the undulating ocean surface and (ii) the drone's movement when it hovers above
the ocean surface. This paper experimentally demonstrated a real-time
imaged-based beam tracking system to mitigate beam misalignment due to dynamic
waves and receiver movement over a 0.14-m underwater and 1.83-m free-space OWC
channel. Experimental results evince a notable reduction in the standard
deviation of the received light spot offset from the receiver. Moreover, the
tracking system can proficiently accommodate receiver velocities of up to 150
cm/s while maintaining a paltry packet loss rate (PLR) below 10%. By addressing
the combined effects of dynamic waves and moving receivers, the proposed beam
tracking system successfully enables a 70% reduction in PLR and an order of
magnitude decrease in bit error rate (BER). This results in a substantial
17-fold surge in maximum throughput, from 50 Mbps to 850 Mbps. The experimental
results validate the feasibility and effectiveness of the beam tracking system
for vanquishing the detrimental effects in the complex water-air OWC (WA-OWC)
channel and supporting high-speed data transmission.
