As the large-scale, commercial use of 5G networks has grown, so has the need for broader bandwidth to accommodate high-speed communications. To meet the resource needs of the post-5G and 6G eras, researchers at Fudan University are exploring visible light communication (VLC) to achieve data rates at the terabit-per-second level. The researchers use multicolor LED arrays with wavelength division multiplexing (WDM) technology. They enhance the efficiency of the long-wavelength GaN-based LED units, especially in the green and yellow wavelengths, by using a V-pit structure. According to the Fudan team, the GaN-based LED array system for VLC demonstrates significantly improved data rates compared with earlier research. (a): In the 8-wavelength, 4×4 LED array chip image, the colors of each dash line box represent the color of the LED unit. (b): The scanning electron microscope image of the V-pit structure. (c): The vertical profile of the V-pit structure. (d): Layers of the red LED units (660 and 620 nm). (e): Layers of the GaN-based LED units (wavelengths from 570 to 450 nm). Courtesy of OES. To build the system, the researchers used a silicon (Si) substrate, GaN-based LED with a 3D-structured quantum well. In the active layer of the LED, the 3D structure is open to the p-type GaN layer. The 3D structure is a V-shaped pit, or V-pit, with a hexagonal profile. To lengthen spontaneous emission wavelengths in GaN-based LEDs, it is usually necessary to add a high indium (In) component in the quantum well. This can lead to a mismatch between the GaN and InN lattices. The V-pit structure of the system helps to screen the dislocations caused by mismatches between GaN and InN lattices in the GaN-based LEDs. This improves the quantum well quality and the optical efficiency of the GaN-based LEDs with long wavelengths — for example, with wavelengths in the green and yellow bands. The multicolor LED array contains eight different LED units. Up to eight independent channels for WDM can be used at the same time. The researchers wrote advanced digital signal processing technology programs for the system, including a discrete multitone modulation/demodulation program, a digital pre-equalization technique, and a software post-equalizer based on a neural network. (a): The equivalent circuit for fitting both LEDs with and without V-pits (with tiny V-pits). The branch in the dash yellow box is dedicated to representing the extra current introduced by the V-pit area. The other branch, in the intrinsic LED part, represents the flat quantum well region. The fitting result using the proposed equivalent circuit, (b) for the sample without V-pits and (c) for the sample with V-pits. Courtesy of OES. The researchers investigated the function of the V-pit in the GaN-based LEDs through a physical model simulation and through equivalent circuit modeling of the V-pit device. They found that during a model simulation, the V-pit strongly enhanced the density of the current in its vicinity. Several carriers flooded into the V-pit and were horizontally transported in the quantum well to the neighboring flat area. Based on this phenomenon, the researchers added a special branch, representing quantum wells near the V-pit, in the LED equivalent model. This new circuit model fit the response curve of the device. The model showed that the V-shaped pit effectively reduced the series resistance of the device and enhanced the device’s response to high-frequency signals. These results indicate that the V-pit can enable the GaN-based LED system to achieve higher electro-optic conversion efficiency and broader bandwidth. The researchers concluded that, in theory, the V-pit structure has a positive effect on the communication performance of the LED system. They verified the theory in experiments with a real communication system in the laboratory. In a test using a laboratory communication system, the data rates of the eight channels of the system, going from shortest to longest wavelength, were 3.91, 3.77, 3.67, 4.40 , 3.79, 3.18, 4.31, and 4.35 Gbit/s — totaling a record-breaking 31.38 Gbit/s single-chip communication rate — with the use of advanced digital signal processing techniques. Summary of the communication rate. (a): The spectrum efficiency and modulation bandwidth of the proposed 8-wavelength WDM system. (b): The bit error rate of each channel; all the bit error rates are lower than the 7% hard decision-forward-error-correction threshold. (c): Comparison in data rate with the original design. The total data rate of the size-improved design device is 31.38 Gbit/s. Courtesy of OES. The system’s large channel capacity suggests that a multicolor, V-pit-enhanced, GaN-based LED array is a promising technology for 6G. The equivalent circuit model and simulation suggest that V-pits significantly enhance the performance and electro-optic characteristics of the device, as well as its response to high-frequency signals. The researchers said that the biggest improvement attributed to the use of V-pits was in the green and yellow spectrum. This finding suggests that the “yellow gap” problem in VLC could be resolved by using the V-pit structure, resulting in a competitive communication rate in the green and yellow wavelengths. Visible light offers several advantages for broadband communications. It uses the ultrawide spectrum from 400 to 800 THz, which requires no licensing and is secure and environmentally friendly, with no electromagnetic radiation. With the help of commercially available LED technology, VLC systems can be integrated with lighting systems. In the future, a more accurate modeling method could improve the V-pit-enhanced, GaN-based LED, further increasing the channel capacity to satisfy the ultrahigh communication capacity requirement of post-5G and 6G communication networks. The research was published in Opto-Electronic Science (www.doi.org/10.29026/oes.2023.230005).
