The challenge for next-generation underwater optical wireless communication systems is to develop optical transceivers that can operate with low power consumption by maximizing the transmission capacity according to the transmission distance between transmitters and receivers. This study proposes an underwater wireless optical communication (UWOC) system using an optical transceiver with an optimum transmission rate for the deep sea with near-pure water properties. As a method for actualizing an optical transceiver with an optimum transmission rate in a UWOC system, time-domain hybrid pulse amplitude modulation (PAM) (TDHP) using a transmission rate and distance-adaptive intensity modulation/direct detection optical transceiver is considered. In the TDHP method, variable transmission capacity is actualized while changing the generation ratio of two intensity-modulated signals with different noise immunities in the time domain. Three different color laser diodes (LDs), red, blue, and green are used in an underwater channel transmission transceiver that comprises the LD and a photodiode. The maximum transmission distance while changing the incidence of PAM 2 and PAM 4 signals that calibrate the TDHP in a pure transmission line and how the maximum transmission distance changes when the optical transmitter/receiver spatial optical system is altered from the optimum conditions are clarified based on numerical calculation and simulation. To the best knowledge of the authors, there is no other research on data-rate and distance adaptive UWOC system that applies the TDHP signal with power optimization between two modulation formats.

Optical wireless communication is a promising technology for underwater broadband access networks, which are particularly important for high-resolution environmental monitoring applications. This paper focuses on a deep-sea monitoring system, where an underwater optical wireless network is deployed on the seafloor. We model such an optical wireless network as a general queueing network and formulate an optimal relay placement problem, whose objective is to maximize the stability region of the whole system, i.e., the supremum of the traffic volume that the network is capable of accommodating. The formulated optimization problem is further shown to be non-convex, so that its global optimization is non-trivial. In this paper, we develop a global optimization method for this problem and we provide an efficient algorithm to compute an optimal solution. Through numerical evaluations, we show that a significant performance gain can be obtained by using the derived optimal solution.

We present a power-efficient and reduced-complexity all-optical approach to generate high-bit rate superchannels, based on joint time-frequency multiplexing. Both optical orthogonal frequency division multiplexing (OFDM) and Nyquist-optical time division multiplexing (N-OTDM) signals are generated and processed using a wavelength-selective switch (WSS). We use the fractional Fourier transform (FrFT) to introduce the largest degree of flexibility in optical networking, and we experimentally demonstrate that the transmitted signal can be received either in parallel, as OFDM subcarriers, or in series, as an N-OTDM signal. Compared to conventional optical OFDM, we achieve peak to average power ratio (PAPR) reduction; compared to N-OTDM, we introduce flexible chromatic dispersion (CD) management.

A disaster-resistant, point-to-multipoint network configuration that connects distributed optical terminals to a centralized optical terminal, through an exclusive fiber link, is required in edge computing strategies such as 5G, IoT applications, ultrareliable and low latency applications, high-performance content delivery, artificial intelligence applications, and hyperconverged infrastructure. Therefore, we introduce a shared link orientation that fully connects the distributed optical terminal side to the centralized optical terminal side represented by the conventional optical access network. We also propose a method that switches a fully-coupled or half-split network to the centralized optical terminal on the optical switch aggregation station side. We report a verification experiment on the proposed method to confirm the operation of wavelength conversion/optical path switching when a link failure occurs, achieving < 2 dB penalty with small back reflection in wavelength collision-free secondary link transmissions with primary link failure. The unique point of this research is that the centralized optical terminal can easily change from the link-occupancy-oriented network to the link-sharing-oriented network. This work will lead to development of a novel disaster-resistant network configuration for wavelength division multiplexing-based coherent passive optical network systems.

A block-ciphered (M-ary) optical code division multiplexing (OCDM) can provide larger security than a conventional OCDM system based on bit ciphering. We propose a polarization multiplexed (POL-MUX) M-ary OCDM system and demonstrated 2.5 Gbps, POL-MUX 256 ( = 16X16)-ary OCDM transmission using a single multi-port optical encoder/decoder (E/D). We show that this architecture reduces the number of required optical codes and enhances the system confidentiality.

A PAM4-OCDM system with optical multi-/demultiplexing and electrical pre-/post-processing is proposed for short-reach applications. We experimentally demonstrate, for the first time, a power-efficient 4 OC x 10 GSymbol/s PAM4-OCDM system. The four PSK-OCs are simultaneously generated using a single light source and a passive multiport optical encoder and received by a single optical decoder and cascaded DSP. The effectiveness of the electrical-domain amplitude level pre-tuning and post-equalizer are demonstrated, considering different values of shot and beat noises.