Photonics Research

Visible light communication is considered as an indispensable part of the future 6G network because of its extremely rich available spectrum resources (400-800THz). It is expected that by 2030, data traffic on communications networks will surge to 5000 EB per month. At the same time, the development of the Internet of Things system also brings an increasing number of access requirements. The multi-access function will bring convenience to inter-satellite, underwater, terrestrial and indoor networks, enabling applications such as multi-access edge computing and underwater UAV networks. However, most of the current visible light transmission experiments are limited to point-to-point transmission scenarios. And in the multiple access experiment, the devices used are not integrated devices. The volume and cost limit the application of this kind of system. Compared with the discrete devices, the integrated receiver array is more conducive to the miniaturization of the system. At the same time, visible light receivers usually use silicon-based detection units, which are generally more suitable for receiving long-wavelength visible and infrared signals. For underwater visible light channel, short wavelength visible light has better transmittivity, so a material that is more sensitive to short wavelength is needed to make a light detector.


Fig. 1. (a) The SEM images of the proposed micro-PD array (b) The detailed SEM image of two detectors (c) The optical microscopic image of the PD array chip. All cathodes on this chip are connected into the common cathode located at the center of the chip (d) The intersection of a single detector unit, showing its layer structure.


InGaN/GaN materials have tunable wide bandgap structure, high electron mobility, and stronger environmental durability than silicon-based materials. Based on this characteristic, an integrated array receiver based on InGaN/GaN material is designed in this study. Relevant research results were recently published in Photonics Research, Volume 12, No. 4, 2024. [ Zengyi Xu, Xianhao Lin, Zhiteng Luo, Qianying Lin, Jianli Zhang, Guangxu Wang, Xiaolan Wang, Fengyi Jiang, Ziwei Li, Jianyang Shi, Junwen Zhang, Chao Shen, Nan Chi. Flexible 2 × 2 multiple access visible light communication system based on an integrated parallel GaN/InGaN micro-photodetector array module[J]. Photonics Research, 2024, 12(4): 793 ]


As shown in FIG. 1, the receiver possesses a 4X4 array structure. The side length of the receiving unit is 50um. From top to bottom in each cell are N-type GaN, superlattice layer, low-temperature GaN, multi-quantum-well layer, P-type GaN and silver reflector layer. The bottom layer is a layer of conductive substrate material.


The specific way in which the receiver works is shown in FIG. 2. When two independent optical signals are illuminated on the two electrodes, photogenerated charge carriers will be generated respectively. The charge carriers move toward the metal electrode as the cathode and the conductive substrate as the anode, respectively. The co-electrode design combines two carriers into one. The original signal before superposing is a scalar signal, and the superposed signal becomes a vector signal in the complex plane. The signal is then amplified by an external amplifier.


Fig. 2. 3D schematics of the array device and how the independently received signals are superposed via the common cathode and anode structure.


The principle of the communication system is shown in FIG. 3. The arbitrary waveform generator in the figure produces two independent signals. The signal itself passes through a pair of filters with orthogonal impulse response to complete the shape filtering, so as to ensure the orthogonality between the signals. The signal is amplified by an electrical amplifier and coupled with a DC offset signal, so as to drive the 405nm laser to emit a high-speed optical signal. The optical path system collimates the laser through a lens, then converge the beams, and finally focuses the light on the receiver array through an objective lens.


The digital signal processing of the system is shown in FIG. 4. The original signal in the figure can be mapped into a regular set of symbols with equal probability. Or use Huffman coding to map the original signal into a set of symbols with a distribution close to Maxwell-Boltzmann distribution, which is often used in probabilistic shaping technology. The two signals are a two-dimensional vector signal after receiving. Then, the two signals are separated by matched filtering, down-sampling, post-equalization and Huffman decoding. Finally, the data of the two signals are solved, and the final bit error rate of the system is calculated. Since there are several Huffman codebooks applicable, it provides flexible modulation orders between integer orders of conventional QAM, refining the granularity of system rate regulation.


Fig. 3. The setup schematics for the VLLC communication system with superposition modulation. The signals for in-phase and quadrature channels are independently generated from the same AWG and transmitted in two identical optical channels. The original scalar symbols finally form a 2D vectorial symbol at the receiver side.


The results of the experiment are shown in FIG. 5. The highest communication rate of the system can reach 13.2Gb/s. After subtracting the redundancy required for forward error correction, the net transmission rate is 12.27Gb/s. In order to demonstrate the signal fluctuation in the actual transmission process, we set the signal power of channel 1 unchanged, but change the signal Vpp of channel 2, and measure the net transmission rate of the system at this time. In the experiment, it is found that the performance of the system decreases with the deterioration of the signal power imbalance. However, Huffman coding makes better use of the signal-to-noise ratio of the system because of its shaping gain, and can significantly improve the transmission rate under the same channel conditions. In FIG. 6 (e), Huffman coding improves the system tolerance to signal Vpp ranging from 34.4% to 157% at the cost of 5% to 12.5% communication rate


Fig. 4. The DSP process for the communication system adopting superposition modulation enhanced by Huffman coding. By replacing conventional, integral-ordered QAM modulation with probabilistically shaped symbols, this system presents stronger flexibility and shaping gain in power utilization.


"Highly integrated PD array devices can provide high-speed multiple access services for future optical wireless networks," said the research team. "It contributes to pave the way for high-speed visible light communication in the future 6G communication era. With the help of the coding strategy, the proposed system can achieve fine-grained rate regulation, as well as higher tolerance to signal power fluctuations and imbalance. To the best of our knowledge, this is the first demonstration of multiple visible laser source access based on a single integrated GaN/InGaN receiver module."


Fig. 5. The comparison between the system performance with or without Huffman-coding. The result shows that for both the noise-dominated and nonlinearity-dominated channel impairment region, Huffman-coding effectively alleviates the channel effect and improves the spectrum efficiency (SE).