31.38 Gb/s GaN-based LED array visible light communication system enhanced with V-pit and sidewall quantum well structure

Although the 5G wireless network has made significant advances,it is not enough to accommodate the rapidly rising requirement for broader bandwidth in post-5G and 6G eras.As a result,emerging technologies in higher frequencies including visible light communication(VLC),are becoming a hot topic.In particular,LED-based VLC is foreseen as a key enabler for achieving data rates at the Tb/s level in indoor scenarios using multi-color LED arrays with wavelength division multiplexing(WDM)technology.This paper proposes an optimized multi-color LED array chip for high-speed VLC systems.Its long-wavelength GaN-based LED units are remarkably enhanced by V-pit structure in their efficiency,especially in the“yellow gap”region,and it achieves significant improvement in data rate compared with earlier research.This work investigates the V-pit structure and tries to provide insight by introducing a new equivalent circuit model,which provides an explanation of the simulation and experiment results.In the final test using a laboratory communication system,the data rates of eight channels from short to long wavelength are 3.91 Gb/s,3.77 Gb/s,3.67 Gb/s,4.40 Gb/s,3.78 Gb/s,3.18 Gb/s,4.31 Gb/s,and 4.35 Gb/s(31.38 Gb/s in total),with advanced digital signal processing(DSP)techniques including digital equalization technique and bit-power loading discrete multitone(DMT)modulation format.

Square Geometrical Shaping 128QAM Based Time Domain Hybrid Modulation in Visible Light Communication System

We proposed two kinds of visible light communication(VLC)systems which respectively based on 64QAM/square geometrical shaping(SGS)128QAM time domain hybrid modulation scheme(SGSHY)and 64QAM/128QAM time domain hybrid modulation scheme(REGHY).These two systems can operate around specific forward error correction(FEC)threshold and maximize the achievable information rate(AIR)of the system.The principles of SGSHY and REGHY are proposed in detail,which has very low computation complexity compared with probabilistic shaping.The SGSHY outperforms REGHY at high peak to peak voltage(Vpp).Experimental results show that at high Vpp like 1.4V,which means the system is suffering from high nonlinear distortion,the AIR of SGSHY outperforms that of REGHY by 0.12Gb/s at the 2×10-2 FEC threshold.The AIR of the REGHY is at most 0.36Gb/s higher than that of 64QAM at 0.8V Vpp and 7%FEC threshold,while the(achievable information rate)AIR of SGSHY is at most 0.40Gb/s higher than that of 64QAM at 1.4V Vpp and 20%FEC threshold.

FSK Modulation Scheme for High-Speed Optical Transmission

In this paper, we describe the generation, detection, and performance of frequency-shift keying （FSK） for high-speed optical transmission and label switching. A non-return-to-zero （NRZ） FSK signal is generated by using two continuous-wave （CW） lasers, one Mach-Zehnder modulator （MZM）, and one Mach-Zehnder delay interferometer （MZDI）. An RZ-FSK signal is generated by cascading a dual-arm MZM, which is driven by a sinusoidal voltage at half the bit rate. Demodulation can be achieved on 1 bit rate through one MZDI or an array waveguide grating （AWG） demultiplexer with balanced detection. We perform numerical simulation on two types of frequency modulation schemes using MZM or PM, and we determine the effect of frequency tone spacing （FTS） on the generated FSK signal. In the proposed scheme, a novel frequency modulation format has transmission advantages compared with traditional modulation formats such as RZ and differential phase-shift keying （DPSK）, under varying dispersion management. The performance of an RZ-FSK signal in a 4 x 40 Gb/s WDM transmission system is discussed. We experiment on transparent wavelength conversion based on four-wave mixing （FWM） in a semiconductor optical amplifier （SOA） and in a highly nonlinear dispersion shifted fiber （HNDSF） for a 40 Gb/s RZ-FSK signal. The feasibility of all-optical signal processing of a high-speed RZ-FSK signal is confirmed. We also determine the receiver power penalty for the RZ-FSK signal after a 100 km standard single-mode fiber （SMF） transmission link with matching dispersion compensating fiber （DCF）, under the post-compensation management scheme. Because the frequency modulation format is orthogonal to intensity modulation and vector modulation （polarization shift keying）, it can be used in the context of the combined modulation format to decrease the data rate or enhance the symbol rate. It can also be used in orthogonal label-switching as the modulation format for the payload or the label. As an example, we propose a simple orthogonal optical label switching technique based on 40 Gb/s FSK payload and 2.5 Gb/s intensity modulated （IM） label.

Demonstration of a low‐complexity memory‐polynomial‐aided neural network equalizer for CAP visible‐light communication with superluminescent diode

Visible-light communication(VLC)stands as a promising component of the future communication network by providing high-capacity,low-latency,and high-security wireless communication.Superluminescent diode(SLD)is proposed as a new light emitter in the VLC system due to its properties of droop-free emission,high optical power density,and low speckle-noise.In this paper,we analyze a VLC system based on SLD,demonstrating effective implementation of carrierless amplitude and phase modulation(CAP).We create a low-complexity memory-polynomial-aided neural network(MPANN)to replace the traditional finite impulse response(FIR)post-equalization filters of CAP,leading to significant mitigation of the linear and nonlinear distortion of the VLC channel.The MPANN shows a gain in Q factor of up to 2.7 dB higher than other equalizers,and more than four times lower complexity than a standard deep neural network(DNN),hence,the proposed MPANN opens a pathway for the next generation of robust and efficient neural network equalizers in VLC.We experimentally demonstrate a proof-of-concept 2.95-Gbit/s transmission using MPANN-aided CAP with 16-quadrature amplitude modulation(16-QAM)through a 30-cm channel based on the 442-nm blue SLD emitter.

Polarization-induced photocurrent switching effect in heterojunction photodiodes

The unipolar photocurrent in conventional photodiodes(PDs)based on photovoltaic effect limits the output modes and potential versatility of these devices in photodetection.Bipolar photodiodes with photocurrent switching are emerging as a promising solution for obtaining photoelectric devices with unique and attractive functions,such as optical logic operation.Here,we design an all-solid-state chip-scale ultraviolet(UV)PD based on a hybrid GaN heterojunction with engineered bipolar polarized electric field.By introducing the polarization-induced photocurrent switching effect,the photocurrent direction can be switched in response to the wavelength of incident light at 0 V bias.In particular,the photocurrent direction exhibits negative when the irradiation wavelength is less than 315 nm,but positive when the wavelength is longer than 315 nm.The device shows a responsivity of up to−6.7 mA/W at 300 nm and 5.3 mA/W at 340 nm,respectively.In particular,three special logic gates in response to different dual UV light inputs are demonstrated via a single bipolar PD,which may be beneficial for future multifunctional UV photonic integrated devices and systems.

Demonstration of radar-aided flexible communication in a photonics-based W-band distributed integrated sensing and communication system for 6G

This paper experimentally demonstrates a distributed photonics-based W-band integrated sensing and communication(ISAC) system, in which radar sensing can aid the communication links in alignment and data rate estimation. As a proof-of-concept, the ISAC system locates the users, guides the alignment, and sets a communication link with the estimated highest data rate. A peak net data rate of 68.6 Gbit/s and a target sensing with a less-than-1-cm error and a sub-2-cm resolution have been tested over a 10-km fiber and a 1.15-m free space transmission in the photonics-based W-band ISAC system. The achievable net data rates of the users at different locations estimated by sensing are experimentally verified.

Flexible 2 × 2 multiple access visible light communication system based on an integrated parallel GaN/InGaN micro-photodetector array module

In recent studies, visible light communication(VLC) has been predicted to be a prospective technique in the future 6G communication systems. To suit the trend of exponentially growing connectivity, researchers have intensively studied techniques that enable multiple access(MA) in VLC systems, such as the MIMO system based on LED devices to support potential applications in the Internet of Things(IoT) or edge computing in the nextgeneration access network. However, their transmission rate is limited due to the intrinsic bandwidth of LED.Unfortunately, the majority of visible light laser communication (VLLC) research with beyond 10 Gb/s data rates concentrates on point-to-point links, or using discrete photodetector(PD) devices instead of an integrated array PD. In this paper, we demonstrated an integrated PD array device fabricated with a Si-substrated GaN/InGaN multiple-quantum-well(MQW) structure, which has a 4 × 4 array of 50 μm × 50 μm micro-PD units with a common cathode and anode. This single-integrated array successfully provides access for two different transmitters simultaneously in the experiment, implementing a 2 × 2 MIMO-VLLC link at 405 nm. The highest data rate achieved is 13.2 Gb/s, and the corresponding net data rate(NDR) achieved is 12.27 Gb/s after deducing the FEC overhead, using 2.2 GHz bandwidth and superposed PAM signals. Furthermore, we assess the Huffman-coded coding scheme, which brings a fine-grain adjustment in access capacity and enhances the overall data throughput when the user signal power varies drastically due to distance, weather, or other challenges in the channel condition. As far as we know, this is the first demonstration of multiple visible light laser source access based on a single integrated GaN/InGaN receiver module.

Nanohole array structured GaN-based white LEDs with improved modulation bandwidth via plasmon resonance and non-radiative energy transfer

Commercial white LEDs (WLEDs) are generally limited in modulation bandwidth due to a slow Stokes process,long lifetime of phosphors,and the quantum-confined Stark effect. Here we report what we believe is a novel plasmonic WLED by infiltrating a nanohole LED (H-LED) with quantum dots (QDs) and Ag nanoparticles(NPs) together (M-LED). This decreased distance between quantum wells and QDs would open an extra non-radiative energy transfer channel and thus enhance Stokes transfer efficiency. The presence of Ag NPs enhances the spontaneous emission rate significantly. Compared to an H-LED filled with QDs (QD-LED),the optimized M-LED demonstrates a maximum color rendering index of 91.2,a 43% increase in optical power at 60 m A,and a lowered correlated color temperature. Simultaneously,the M-LED exhibits a data rate of 2.21 Gb/s at low current density of 96 A∕cm2(60 m A),which is 77% higher than that of a QD-LED. This is mainly due to the higher optical power and modulation bandwidth of the M-LED under the influence of plasmon,resulting in a higher data rate and higher signal-to-noise ratio under the forward error correction.We believe the approach reported in this work should contribute to a WLED light source with increased modulation bandwidth for a higher speed visible light communication application.

Challenges and Prospects of Machine Learning in Visible Light Communication

Visible light communication(VLC)is a promising research field in modern wireless communication.VLC has its irreplaceable strength including rich spectrum resources,no electromagnetic disturbance,and high-security guarantee.However,VLC systems suffer from the non-linear effects that exist in almost every part of the system.As a part of artificial intelligence,machine learning(ML)is showing its potential in non-linear mitigating for its natural ability to fit all kinds of transfer functions,which may dramatically push the research in VLC.This paper introduces the application of ML in VLC,describes five recent research of deep learning applications in VLC,and analyses the performance.

Common-anode LED on a Si substrate for beyond 15 Gbit/s underwater visible light communication

Visible light communication based on light-emitting diodes(LEDs) has become a promising candidate by providing high data rates, low latency, and secure communication for underwater environments. In this paper,a self-designed common-anode GaN-based five-primary-color LED(RGBYC LED) on a Si substrate is proposed and fabricated. The design of a common anode is used to mitigate the saturation effect for a low-frequency component. Additionally, compared with commercially available LEDs that suffer from nonlinearity distortion,applying the designed LED can provide much better and broader linearity according to the measurement results.Therefore, the modulation depth and system performance can be further improved to implement a high-speed underwater visible light communication(UVLC) system. There is no nonlinearity compensation algorithm applied due to the good linearity of the proposed LED;thus, the offline digital signal processing is simplified. We experimentally demonstrate 14.81 Gbit/s 64 quadrature amplitude modulation(QAM)-discrete multitone(DMT)and 15.17 Gbit/s bit-loading-DMT transmissions through a 1.2-m-long underwater channel based on the proposed RGBYC LED with an intrasymbol frequency-domain averaging channel estimation and zero-forcing equalization.As far as we know, this is the highest data rate for an LED-based UVLC system.

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;.

Si-substrate vertical-structure InGaN/GaN micro-LED-based photodetector for beyond 10 Gbps visible light communication

Visible light communication(VLC)has emerged as a promising communication method in 6G.However,the development of receiving devices is much slower than that of transmitting devices,limited by materials,structures,and fabrication.In this paper,we propose and fabricate an InGaN/GaN multiple-quantum-well-based vertical-structure micro-LED-based photodetector(μPD)on a Si substrate.A comprehensive comparison of the photoelectrical performance and communication performance of three sizes ofμPDs,10,50,and 100μm,is presented.The peak responsivity of all threeμPDs is achieved at 400 nm,while the passband full-widths at half maxima are 87,72,and 78 nm for 10,50,and 100μmμPDs,respectively.The−20 dB cutoff bandwidth is up to 822 MHz for 50μmμPD.A data rate of 10.14 Gbps is experimentally demonstrated by bit and power loading discrete multitone modulation and the proposed digital pre-equalizer algorithm over 1 m free space utilizing the self-designed 4×450μmμPD array as a receiver and a 450 nm laser diode as a transmitter.This is the first time a more than 10 Gbps VLC system has been achieved utilizing a GaN-based micro-PD,to the best of our knowledge.The investigation fully demonstrates the superiority of Si substrates and vertical structures in InGaN/GaNμPDs and shows its great potential for high-speed VLC links beyond 10 Gbps.

Investigation on performance of special-shaped 8-quadrature amplitude modulation constellations applied in visible light communication

Light-emitting diode(LED)-based visible light communication(VLC) has become a potential candidate for nextgeneration ultra-high-speed indoor wireless communication. In this paper, four special-shaped 8-quadrature amplitude modulation(QAM) constellations are investigated in a single-carrier VLC system. It is numerically verified and experimentally demonstrated that circular(7,1) shows obvious superiority in the performance of the dynamic range of signal voltage peak-to-peak(vpp) value and bit error rate(BER). Next best is rectangular, followed by triangular; circular(4,4) has the worst performance. A data rate of 1.515 Gbit/s is successfully achieved by circular(7,1) employing a red chip LED over 0.5 m indoor free space transmission below a BER threshold of3.8 × 10^(-3). Compared with circular(4,4), the traditional 8-QAM constellation, circular(7,1) provides a wider dynamic range of signal vpp, a higher data rate, and a longer transmission distance. To the best of our knowledge,this is the first investigation into the performance differences of special-shaped 8-QAM constellations in a highspeed, single-carrier VLC system, and the results comprehensively demonstrate that circular(7,1) is the optimal option.

Frontier research of ultra-high-speed ultra-large-capacity and ultra-long-haul optical transmission

极端高速度， ultra-large-capacity 和 ultra-long-haul (3U ) 是光通讯的永久追求。作为光通讯的一个新模式， 3U 传播能极大地支持下一代光因特网和宽带活动通讯网络开发和工艺的进步，因此，它变得国际高技术知识产权竞争的焦点扎根。这篇论文在 3U 传播研究引入科学问题，关键技术和重要成就。

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 5G 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.

Si-substrate LEDs with multiple superlattice interlayers for beyond 24 Gbps visible light communication

High-speed visible light communication(VLC)using light-emitting diodes(LEDs)is a potential complementary technology for beyond-5 G wireless communication networks.The speed of VLC systems significantly depends on the quality of LEDs,and thus various novel LEDs with enhanced VLC performance increasingly emerge.Among them,In Ga N/Ga N-based LEDs on a Si-substrate are a promising LED transmitter that has enabled VLC data rates beyond 10 Gbps.The optimization on the period number of superlattice interlayer(SL),which is a stressrelief epitaxial layer in a Si-substrate LED,has been demonstrated to be an effective method to improve Si-substrate LED’s luminescence properties.However,this method to improve LED’s VLC properties is barely investigated.Hence,we for the first time experimentally studied the impact of SL period number on VLC performance.Accordingly,we designed and fabricated an integrated 4×4 multichromatic Si-substrate wavelength-divisionmultiplexing LED array chip with optimal SL period number.This chip allows up to 24.25 Gbps/1.2 m VLC transmission using eight wavelengths,which is the highest VLC data rate for an In Ga N/Ga N LED-based VLC system to the best of our knowledge.Additionally,a record-breaking data rate of 2.02 Gbps over a 20-m VLC link is achieved using a blue Si-substrate LED with the optimal SL period number.These results validate the effectiveness of Si-substrate LEDs for both high-speed and long-distance VLC and pave the way for Si-substrate LED design specially for high-speed VLC applications.

Experimental verification of performance improvement for a gigabit wavelength division multiplexing visible light communication system utilizing asymmetrically clipped optical orthogonal frequency division multiplexing

Asymmetrically clipped optical orthogonal frequency division multiplexing(ACO-OFDM)has been a promising candidate in visible light communications(VLC)due to its improvement in power efficiency and reduction of nonlinearity based on previous simulation analysis.In this paper,for the first time as far as we know,we experimentally verify that ACO-OFDM would be an efficient scheme to improve the performance of a gigabit wavelength division multiplexing VLC system.Our theoretical investigations reveal that the advantages of ACO-OFDM can be attributed to the reduction of inter-carrier interference caused by signal–signal beating noise.An aggregate data rate of 1.05 Gb∕s is successfully achieved over 30 cm transmission below the 7%forwarderror-correction threshold of 3.8×10−3.The experimental results show that ACO-OFDM can outperform DC-biased optical OFDM by BER performance of 1.5 dB at the same data rate and 4 dB at the same bandwidth,which clearly demonstrates the benefit and feasibility of ACO-OFDM.

240 Gb/s optical transmission based on an ultrafast silicon microring modulator

An ultrafast microring modulator(MRM) is fabricated and presented with V_(π)· L of 0.825 V · cm. A 240 Gb/s PAM-8 signal transmission over 2 km standard single-mode fiber(SSMF) is experimentally demonstrated. PN junction doping concentration is optimized, and the overall performance of the MRM is improved. Optical peaking is introduced to further extend the EO bandwidth from 52 to 110 GHz by detuning the input wavelength. A titanium nitride heater with 0.1 nm/m W tuning efficiency is implemented above the MRM to adjust the resonant wavelength. High bit rate modulations based on the high-performance and compact MRM are carried out. By adopting off-line signal processing in the transmitter and receiver side, 120 Gb/s NRZ, 220 Gb/s PAM-4, and240 Gb/s PAM-8 are measured with the back-to-back bit error ratio(BER) of 5.5 × 10^(-4), 1.5 × 10^(-2), and 1.4 × 10^(-2), respectively. A BER with different received optical power and 2 km SSMF transmission is also investigated. The BER for 220 Gb/s PAM-4 and 240 Gb/s PAM-8 after 2 km SSMF transmission is calculated to be 1.7 × 10^(-2), and 1.5 × 10^(-2), which meet with the threshold of soft-decision forward-error correction,respectively.

Spectrally efficient multi-band visible light communication system based on Nyquist PAM-8 modulation

High-speed multi-user access with high spectral efficiency is one of the key challenges for band-limited visible light communication(VLC) systems. In this paper, we propose a novel scheme for effective multiple-access VLC systems based on multi-band, Nyquist-filtered pulse amplitude modulation(PAM)-8 modulation. Within this scenario, the spectral efficiency can be improved from 1.5 to 2.73 b/s/Hz by implementing an appropriate Nyquist filter to suppress spectral bandwidth. We experimentally demonstrate a multi-band VLC system at 1.2 Gb/s after 1 m indoor free space transmission. The system performances have also been thoroughly investigated for different sub-band numbers, utilizing a rectangular filter in the frequency domain and a Nyquist filter based on square root raised cosine. The results show that the Nyquist-filtered PAM-8 signal can outperform a rectangular filtered signal. The maximum improvement of system capacity is up to 1.67 times for the Nyquist-filtered multi-band system. The results clearly show the advantage and feasibility of multi-band Nyquist PAM for high-speed multiple-access VLC systems.