A novel frequency domain nonlinear compensation method, FD-NC, is proposed for orthogonal frequency division multiplexing (OFDM) based visible light communication (VLC) system. By tackling the memory nonlinear impairments from light emitting diodes (LEDs) in the frequency domain rather than in the time domain, the proposed method has much lower computational complexity than the conventional time domain Volterra nonlinear compensation method (TD-NC). Both theoretical derivation and experimental investigation of the proposed method in OFDM based VLC systems with four types of commercial LEDs are presented. The results of experiments show that the proposed low-complexity FD-NC method with a moderate truncation factor achieves a performance comparable to that of the TD-NC. The application of FD-NC method in the bit-power loading OFDM VLC system is also experimentally demonstrated. Compared with the linear equalization case, at a bit error rate (BER) of 3.8 × 10^-3 (a), the transmission distance of a 960 Mbps VLC system can be extended from 0.7 m to 1.8 m by the FD-NC, and (b) the achievable system capacity can be enhanced by 18.7%~36.5% for transmission distance in the range of 0.5 m~2 m with the FD-NC. The complexity analysis shows that the required number of real-valued multiplications (RNRM) of the FD-NC is independent of linear or nonlinear memory length. The reduction of RNRM achieved by the FD-NC over the TD-NC becomes more profound for a larger nonlinear memory length or a smaller truncation factor.

In this paper, a low-complexity two-level chaotic encryption scheme is introduced and experimentally demonstrated to improve the physical layer security of a 450-nm laser underwater optical wireless communication (UOWC) system using discrete Fourier transform spread discrete multi-tone (DFT-S DMT) modulation. In the first encryption stage, the original bit stream is encrypted with a chaotic sequence based on a one-dimensional Logistic map. In the second encryption stage, the real and imaginary components of the DFT-S symbols are further encrypted with a pair of separate chaotic sequences, which are generated from a two-dimensional Logistic iterative chaotic map with infinite collapse (2D-LICM). The experimental results indicate that the encryption operation has no negative effect on the performance of the proposed UOWC system. For chaotic encryption, the DFT-S DMT gives a better performance than the DMT scheme under different water turbidities. 55-m/4.5-Gbps and 50-m/5-Gbps underwater transmissions are successfully demonstrated by the chaotic encrypted DFT-S DMT scheme. To the best of our knowledge, this is the first time to verify the feasibility of chaotic encryption in a high-speed UOWC system.

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In this paper, a wideband photomultiplier tube (PMT)-based underwater wireless optical communication (UWOC) system is proposed and a comprehensive experimental study of the proposed PMT-based UWOC system is conducted, in which the transmission distance, data rate, and attenuation length (AL) is pushed to 100.6 meters, 3 Gbps, and 6.62, respectively. The receiver sensitivity at 100.6-meter underwater transmission is as low as -40 dBm for the 1.5-Gbps on-off keying (OOK) modulation signal. To the best of our knowledge, this is the first Gbps-class UWOC experimental demonstration in >100-meter transmission that has ever been reported. To further minimize the complexity of channel equalization, a sparsity-aware equalizer with orthogonal matching pursuit is adopted to reduce the number of the filter coefficients by more than 50% while keeping slight performance penalty. Furthermore, the performance of the proposed PMT-based UWOC system in different turbidity waters is investigated, which shows the robustness of the proposed scheme. Thanks to the great sensitivity (approaching the quantum limit) and a relatively larger effective area, benefits of misalignment tolerance contributed by the PMT is verified through a proof-of-concept UWOC experiment.

In this paper, we introduce an energy constraint to improve topology-based inverse design. Current methods typically place the constraints solely on the device geometry and require many optimization iterations to converge to a manufacturable solution. In our approach the energy constraint directs the optimization process to solutions that best contain the optical field inside the waveguide core medium, leading to more robust designs with relatively larger minimum feature size. To validate our method, we optimize two components: a mode converter (MC) and a wavelength demultiplexer. In the MC, the energy constraint leads to nearly binarized structures without applying independent binarization stage. In the demultiplexer, it also reduces the appearance of small features. Furthermore, the proposed constraint improves the robustness to fabrication imperfections as shown in demultiplexer design. With energy constraint optimization, the corresponding spectrum shifts under ±10 nm dimensional variations are reduced by 17% to 30%. The proposed constraint is unique in simultaneously taking both geometry and electric field into account, opening the door to new ideas and insights to further improve the computationally intensive topology-based optimization process of nanophotonic devices.