Advanced modulation formats for underwater visible light communications [Invited]

In this paper, we present a detailed comparison of applying three advanced modulation formats including carrierless amplitude and phase modulation(CAP), orthogonal frequency division multiplexing(OFDM), and discrete Fourier transform spread orthogonal frequency division multiplexing(DFT-S OFDM) in underwater visible light communication(UVLC) systems. Cascaded post-equalization schemes are suggested to compensate the system impairments. For the first time, a two-level post-equalizer is presented to mitigate the nonlinear effect and improve the system performance of UVLC. The first post-equalization is based on a novel recursive least square Volterra. These modulation formats are all experimentally demonstrated with corresponding digital signal processing(DSP) algorithms. The experimental results show that single carrier modulations including CAP and DFT-S OFDM can outperform OFDM. Our experiment results show that up to 3 Gb/s over a 1.2 m underwater visible light transmission can be achieved by using DFT-S OFDM 64 QAM and CAP-64. The measured bit error rate is well under the hard decision-forward error correction(HD-FEC) threshold of 3.8 × 10~(-3).     

Nonlinear adaptive filters for high-speed LED based underwater visible light communication [Invited]

Underwater visible light communication(UVLC) is expected to act as an alternative candidate in nextgeneration underwater 5 G wireless optical communications. To realize high-speed UVLC, the challenge is the absorption, scattering, and turbulence of a water medium and the nonlinear response from imperfect optoelectronic devices that can bring large attenuations and a nonlinearity penalty. Nonlinear adaptive filters are commonly used in optical communication to compensate for nonlinearity. In this paper, we compare a recursive least square(RLS)-based Volterra filter, a least mean square(LMS)-based digital polynomial filter,and an LMS-based Volterra filter in terms of performance and computational complexity in underwater visible light communication. We experimentally demonstrate 2.325 Gb/s transmission through 1.2 m of water with a commercial blue light-emitting diode. Our goal is to assist the readers in refining the motivation, structure,performance, and cost of powerful nonlinear adaptive filters in the context of future underwater visible light communication in order to tap into hitherto unexplored applications and services.     

Metasurfaces enabling structured light manipulation:advances and perspectives [Invited]

Metasurfaces and structured light have rapidly advanced over the past few years, from being paradigms to forming functional devices and tailoring special light beams for wide emerging applications. Here, we focus on harnessing metasurfaces for structured light manipulation. We review recent advances in shaping structured light by metasurfaces on different platforms(metal, silica, silicon, and fiber). Structured light manipulation based on plasmonic metasurfaces, reflection-enhanced plasmonic metasurfaces, metasurfaces on fiber facets, dielectric metasurfaces, and sub-wavelength structures on silicon are presented, showing impressive performance.Future trends, challenges, perspectives, and opportunities are also discussed.     

In-fiber integrated optics: an emerging photonics integration technology [Invited]

In-fiber integrated optics is an attempt to use silica fiber as a substrate, integrating various optical paths or optical components into a single fiber, to build a functional optical device or component, and to realize a micro optical system, achieving various functions. In-fiber integrated optics is expected to be a new branch of photonics integration. This integration technique enables convenient light beams control and manipulation inside in one fiber. It also provides a research platform with micro and nano scale for interaction between light wave and microfluidic materials. In this review, we briefly summarize the main ideas and key technologies of the in-fiber integrated optics by series integration examples.     

High-speed underwater wireless optical communications: from a perspective of advanced modulation formats [Invited]

In this paper, recent advances in underwater wireless optical communication(UWOC) are reviewed for both LED-and LD-based systems, mainly from a perspective of advanced modulation formats. Volterra series-based nonlinear equalizers, which can effectively counteract the nonlinear impairments induced by the UWOC system components, are discussed and experimentally demonstrated. Both the effectiveness and robustness of the proposed Volterra nonlinear equalizer in UWOC systems under different water turbidities are validated.To further approach the Shannon capacity limit of the UWOC system, the probabilistic constellation shaping technique is introduced, which can overcome the inherent gap between a conventional regular quadrature amplitude modulation(QAM) format and the Shannon capacity of the channel. The experimental results have shown a significant system capacity improvement compared to the cases using a regular QAM.     

Dimming control scheme based on hybrid LACO-SCFDM for visible light communications

This paper proposes a hybrid layered asymmetrically clipped optical(HLACO) single-carrier frequency-division multiplexing(SCFDM) scheme for dimmable visible light communication. It designs a signal structure that combines layered asymmetrically clipped optical(LACO)-SCFDM and negative LACO-SCFDM in proportion for improving the inherent weaknesses of orthogonal frequency-division multiplexing(OFDM)-based dimmable schemes and further enhancing the system performance. Compared to the HLACO-OFDM-based dimming scheme, it obtains a lower bit error ratio and enables efficient communication over broader dimming range. Its spectral efficiency realizes 2.875 bit·s-1·Hz-1 within the dimming range of 30%–70%, and the attainable average spectral efficiency gains exceed at least 19.21% compared to other traditional dimmable schemes.     

Optimization design of RIS-assisted high-capacity visible light communications based on HDMA

In this paper, a new non-orthogonal multiple access (NOMA) scheme is proposed for the reconfigurable intelligent surface (RIS) assisted high-capacity visible light communication (VLC) system, which is named hybrid domain multiple access (HDMA). HDMA enjoys the benefit of hybrid-domain signals, including the power domain, code domain, and frequency domain, where the message passing algorithm (MPA) and successive interference cancellation (SIC) detectors are jointly used at the HDMA receiver. Furthermore, to achieve a higher communication capacity for the VLC system, we proposed an optimization model by jointly optimizing the power allocation ratio and RIS reflection units. The simulation results verified the proposed scheme. By comparing the system capacity of different RIS allocation schemes and multiple access methods, the VLC system based on HDMA proposed in this paper can significantly improve its communication capacity.     

Recent advance in hollow-core fiber high-temperature and high-pressure sensing technology [Invited]

The pure-silica hollow-core fiber(HCF) has excellent thermostabilities that can benefit a lot of high-temperature sensing applications. The air-core microstructure of the HCF provides an inherent gas container, which can be a good candidate for gas or gas pressure sensing. This paper reviews our continuous efforts to design, fabricate, and characterize the hightemperature and high-pressure sensors with HCFs, aiming at improving the sensing performances such as dynamic range,sensitivity, and linearity. With the breakthrough advances in novel anti-resonant HCFs, sensing of high temperature and high pressure with HCFs will continuously progress and find increasing applications.     

Visible light positioning: moving from 2D planes to 3D spaces [Invited]

The global navigation satellite system(GNSS) is a well-established outdoor positioning system with industry-wide impact due to the multifaceted applications of navigation, tracking, and automation. At large, however, is the indoor equivalent. One hierarchy of solutions, visible light positioning(VLP) with its promise of centimeter-scale accuracy and widespread coverage indoors, has emerged as a viable, easy to configure, and inexpensive candidate. We investigate how the state-of-the-art VLP systems fare against two hard barriers in indoor positioning: the need for high accuracy and the need to position in the threedimensions(3D). We find that although most schemes claim centimeter-level accuracy for some proposed space or plane, those accuracies do not translate into a realistic 3D space due to diminishing field-of-view in 3D and assumptions made on the operating space. We do find two favorable solutions in ray–surface positioning and gain differentials. Both schemes show good positioning errors, low-cost potential, and single-luminaire positioning functionality.     

Recent achievements on underwater optical wireless communication [Invited]

The growing number of underwater activities is giving momentum to the development of new technologies, such as buoys, remotely operated vehicles, and autonomous underwater vehicles. The data collected by these vehicles need to be transmitted to a high-speed central unit. Clearly, wired solutions are not suitable, since they strongly impact the mobility. In this scenario, a promising solution is offered by underwater optical wireless communication(UOWC) technology, which can achieve both high-speed and wireless operation. Here, we provide a comprehensive survey on the challenges, the experimental realizations, and the state of the art in UOWC researches.     

Independent reflecting element interaction characterization for indoor visible light communication based on new generation lighting

Indoor visible light communication （VLC） based on next generation environmentally friendly lighting is important in energy conservation. However, at present, the efficient characterization of VLC channels, including sophisticated reflection, has not yet been proposed. In this letter, we present a fast and comparably accurate channel characterization algorithm called independent reflecting element interaction characterization （IREIC）, which can be used to describe optical power, illuminance, and impulse response.     

Integrated optical delay lines:a review and perspective[Invited]

Optical delay lines(ODLs) are one of the key enabling components in photonic integrated circuits and systems.They are widely used in time-division multiplexing, optical signal synchronization and buffering, microwave signal processing, beam forming and steering, etc. The development of integrated photonics pushes forward the miniaturization of ODLs, offering improved performances in terms of stability, tuning speed, and power consumption. The integrated ODLs can be implemented using various structures, such as single or coupled resonators, gratings, photonic crystals, multi-path switchable structures, and recirculating loop structures.The delay tuning in ODLs is enabled by either changing the group refractive index of the waveguide or changing the length of the optical path. This paper reviews the recent development of integrated ODLs with a focus on their abundant applications and flexible implementations. The challenges and potentials of each type of ODLs are pointed out.     

Intelligent algorithms: new avenues for designing nanophotonic devices [Invited]

The research on nanophotonic devices has made great progress during the past decades. It is the unremitting pursuit of researchers that realize various device functions to meet practical applications. However, most of the traditional methods rely on human experience and physical inspiration for structural design and parameter optimization, which usually require a lot of resources, and the performance of the designed device is limited. Intelligent algorithms, which are composed of rich optimized algorithms, show a vigorous development trend in the field of nanophotonic devices in recent years. The design of nanophotonic devices by intelligent algorithms can break the restrictions of traditional methods and predict novel configurations, which is universal and efficient for different materials, different structures, different modes, different wavelengths, etc. In this review, intelligent algorithms for designing nanophotonic devices are introduced from their concepts to their applications, including deep learning methods, the gradient-based inverse design method, swarm intelligence algorithms, individual inspired algorithms, and some other algorithms. The design principle based on intelligent algorithms and the design of typical new nanophotonic devices are reviewed. Intelligent algorithms can play an important role in designing complex functions and improving the performances of nanophotonic devices, which provide new avenues for the realization of photonic chips.     

Effects of air bubbles on underwater optical wireless communication [Invited]

The received signal intensity fluctuation and communication performance of an underwater optical wireless communication(UOWC) system under the air bubble effects are experimentally investigated. For different bubble density and size, lognormal, gamma, Weibull, and generalized extreme value distributions are tested to fit the fluctuation of the signal intensity at the receiving end. The best fitting distribution is found to vary with bubble parameters. The communication system performance with on–off keying and pulse position modulation is further studied.     

Hybrid mono-crystalline silicon and lithium niobate thin films [Invited]

The heterogeneous integration of silicon thin film and lithium niobate(LN) thin film combines both the advantages of the excellent electronics properties and mature micro-processing technology of Si and the excellent optical properties of LN,comprising a potentially promising material platform for photonic integrated circuits. Based on ion-implantation and wafer-bonding technologies, a 3 inch wafer-scale hybrid mono-crystalline Si/LN thin film was fabricated. A high-resolution transmission electron microscope was used to investigate the crystal-lattice arrangement of each layer and the interfaces. Only the H-atom-concentration distribution was investigated using secondary-ion mass spectroscopy. Highresolution X-ray-diffraction ω–2θ scanning was used to study the lattice properties of the Si/LN thin films. Raman measurements were performed to investigate the bulk Si and the Si thin films. Si strip-loaded straight waveguides were fabricated, and the optical propagation loss of a 5-μm-width waveguide was 6 d B/cm for the quasi-TE mode at1550 nm. The characterization results provide useful information regarding this hybrid material.     

Broadband and lossless lithium niobate valley photonic crystal waveguide [Invited]

We investigate the influences of structure parameters and interface shapes on the bandwidth of the edge state of lithium niobate valley photonic crystals. By increasing the size difference of two air holes in the same unit cell, we find that the bandwidth of the lossless nontrivial edge state possesses a peak value of 0.0201(a/λ), which can be used to construct broadband valley photonic crystal waveguides. Mode field distributions verify that the waveguide is robust against sharp bends and exhibits chirality. When the unit cell is arranged in a bearded interface with the top and bottom components showing negative and positive valley Chern numbers, respectively, we find that the lithium niobate valley photonic crystal is more likely to exhibit a lossless edge state, which is difficult to be realized in valley waveguides with low refractive index materials. This work can provide guidance on the design of the high-performance topological waveguide.     

Deflection angle switching with a metasurface based on phase-change nanorods [Invited]

We propose the active metasurface using phase-change material Ge_2Sb_2Te_5(GST), which has two distinct phases so called amorphous and crystalline phases, for an ultrathin light path switching device. By arranging multiple anisotropic GST nanorods, the gradient metasurface, which has opposite directions of phase gradients at the two distinct phases of GST, is demonstrated theoretically and numerically. As a result, in the case of normal incidence of circularly polarized light at the wavelength of 1650 nm, the cross-polarized light deflects to-55.6° at the amorphous phase and +55.6° at the crystalline phase with the signal-to-noise ratio above 10 dB.     

Underwater wireless optical communication:why,what, and how? [Invited]

Foreseeing the proliferation of underwater vehicles and sensors, underwater wireless optical communication(UWOC) is a key enabler for ocean exploration, with strong competitiveness in short-range bandwidth-intensive applications. We provide a tutorial on the basic concepts and essential features of UWOC, as well as an overview of work being conducted in this field. Research challenges, arising from the characteristics of underwater channels, and possible roadmaps are discussed in detail. This review is expected to be of great use for the link designers of this field.     

Laser printing based on curvature-driven shape transition of aluminum nanodiscs [Invited]

Plasmonic structural colors have plenty of advantages over traditional colors based on colorants. The pulsed laser provides an important method generating plasmonic structural colors with high efficiency and low cost. Here, we present plasmonic color printing Al nanodisc structures through curvature-driven shape transition. We systematically study the mechanism of morphologic evolution of the Al nanodisc below the thermal melting threshold. A multi-pulse-induced accumulated photothermal effect and subsequent curvature-driven surface atom diffusion model are adopted to explain the controllable shape transition. The shape transition and corresponding plasmonic resonances of the nanodisc can be independently and precisely modulated by controlled irradiations. This method opens new ways towards high-fidelity color prints in a highly efficient and facile laser writing fashion.     

Mode division multiplexing: from photonic integration to optical fiber transmission [Invited]

To overcome the capacity crunch of optical communications based on the traditional single-mode fiber(SMF), different modes in a few-mode fiber(FMF) can be employed for mode division multiplexing(MDM). MDM can also be extended to photonic integration for obtaining improved density and efficiency, as well as interconnection capacity. Therefore, MDM becomes the most promising method for maintaining the trend of "Moore's law" in photonic integration and optical fiber transmission. In this tutorial, we provide a review of MDM works and cutting-edge progresses from photonic integration to optical fiber transmission, including our recent works of MDM low-noise amplification, FMF fiber design, MDM Si photonic devices, and so on. Research and application challenges of MDM for optical communications regarding long-haul transmission and short reach interconnection are discussed as well. The content is expected to be of important value for both academic researchers and industrial engineers during the development of next-generation optical communication systems,from photonic chips to fiber links.