Mode division multiplexing using Laguerre–Gaussian beams with VSB-CSRZ modulation for high-capacity underwater optical wireless communication

Underwater optical wireless communication (UOWC) is a promising technology for high-speed data transmission underwater due to its large bandwidth, low latency, and high capacity. In this work, a high-capacity UOWC system based on mode division multiplexing (MDM) and vestigial sideband carrier-suppressed return-to-zero (VSB-CS-RZ) modulation is presented. Four orthogonal Laguerre–Gaussian modes, i.e., LG 0 0 LG₀⁰ , LG 13 0 LG₁₃⁰ , LG 40 0 LG₄₀⁰ , and LG 80 0 , LG₈₀⁰, are employed, each carrying an independent 20 Gbps data stream, resulting in a total transmission capacity of 80 Gbps at 532 nm. The effect of water turbidity is investigated for the transmission performance in five Jerlov water types (JI, JIA, JIB, JII, and JIII). The system performance is evaluated in terms of Q-factor, log(BER), and eye diagrams. The results show similar transmission characteristics for all the four modes in clear ocean water, confirming their effectiveness for multiplexed UOWC transmission. The maximum transmission distances are 75, 60, 31.5, 14.5, and 5.1 m for JI, JIA, JIB, JII, and JIII waters, respectively, at forward error correction threshold of log(BER) = −6. Moreover, a reduction in the beam divergence angle from 1 to 0.8 mrad significantly improves the signal quality and transmission range, confirming the suitability of the proposed architecture for high-speed and spectrally efficient underwater communication links with reliable mode separation and efficient optical bandwidth utilization.