Fig. 1. Framework of demonstrated 50 Gb/s PAM4 UWOC systems across the water–air–water interface.

Fig. 2. Reflective SLM with an electrical controller for developing a water–air–water interface and mitigating the performance degradation due to laser beam misalignment.

Fig. 3. Optical spectrum of VCSEL2 with (a) free-running state, and (b) light injection and optoelectronic feedback techniques locked at 671.2 nm.

Fig. 4. Modulation responses of VCSEL with free-running (under various operational currents), light injection, and light injection and optoelectronic feedback.

Fig. 5. BER performance of the 50 Gb/s PAM4 signal in the first 5 minutes for the states with or without a Keplerian beam expander or an SLM electrical controller.

Fig. 6. BER performances of 50 Gb/s PAM4 signal in the first hour for the scenarios with or without a Keplerian beam expander or an SLM electrical controller.