Visible light communication (VLC) has been dubbed an important ancillary technology to wireless communication. Light-emitting diode (LED) solid-state lighting technology offers low power consumption and cost, small size, and a long lifespan. Plus, it’s environmentally friendly. Because of this, LED-based visible light communication (VLC) technology has attracted worldwide research, and VLC technology has rapidly developed over the past decade.
The flickering of LEDs cannot be identified by the naked eye, thanks to the VLC system’s high-frequency signal. So, by adding relatively inexpensive front-end components, VLC can be easily implemented in existing lighting infrastructures to achieve data communications with speeds in the Gbps range.
A research team has presented an overview of micro light-emitting diodes (μLEDs) for VLC. The group discusses methods to improve the modulation bandwidth in terms of epitaxy optimization, crystal orientation, and active region structure.
Here is a Tech Briefs interview, edited for length and clarity, with Xiamen University’s Tingwei Lu, author of the research.
Tech Briefs: What inspired your research?
Lu: In recent years, as the research on micro-LEDs has become more intensive, researchers have also discovered that μLEDs are also advantageous in high-speed visible light communication (VLC) systems. Nan Chi’s group and Pengfei Tian’s group from Fudan University, Bin Liu’s group from Nanjing University, and Dawson’s group from the University of Strathclyde have made outstanding contributions to this research topic, and we have been inspired by their publications. In recent years, our group has also conducted relevant research on μLEDs for high-speed VLC.
Currently, μLED-based high-speed VLC systems have received wide attention from academia and industry because of their development prospects and technical advantages. Therefore, we collated the high-speed VLC systems based on μLED devices in recent years and completed this review article, hoping to help promote the diversification of μLED applications.
Tech Briefs: What were the biggest technical challenges you faced?
Lu: Among the current research on μLED devices for high-speed VLC, blue μLED devices are the most mature and can already achieve modulation bandwidths of more than a GHz. However, RGB-μLEDs, which can satisfy both display and communication requirements, are more promising for applications in VLC compared to monochromatic LEDs.
Currently, long wavelength μLED devices are the main constraint for manufacturing white μLEDs. The "green gap" limits the performance of the green µLED. Green μLEDs have a high proportion of indium, which requires a relatively low temperature when fabricating, which leads to strong quantum confinement Stark effect (QCSEs) and a reduced radiative recombination efficiency. For long wavelength micro-LED devices, our group has conducted relevant research. In 2020, we fabricated high modulation bandwidth green semipolar micro-LED devices using optimized external research structures and chip processes, achieving the highest modulation bandwidth of 756 MHz.
Compared to green μLEDs, the challenges for red μLEDs are more daunting. Red µLEDs are mainly fabricated using the AlGaInP material system, which leads to long carrier diffusion length and enhanced non-radiation recombination. Therefore, the efficiency degradation of red µLEDs is more severe, with the size of devices decreasing to a few microns. Therefore, fabricating red μLED devices with high efficiency and high bandwidth is still the biggest technical challenge and research hotspot.
Tech Briefs: Can you explain in simple terms how the technology works?
Lu: VLC technology uses visible light in the 400~780 nm band to complete wireless communication. The transmitter in a VLC system is composed of a visible-light-emitting device such as an LED or LD to realize data transmission. LEDs with diffusers, energy-saving properties, and environmental protection are good light sources for providing uniform illumination. Flickering from LEDs cannot be captured directly by the human eye, owing to the high frequency of the signal in the VLC system. Thus, by adding inexpensive front-end components, VLC can be easily implemented into existing lighting infrastructures to achieve data communications with speeds of up to Gbps.
The receiver in a VLC system consists of a photodetector (PD) and signal-processing circuitry. After the PD receives the optical signal and restores it to a current signal, it generates a voltage signal and completes the digital-to-analog conversion after the signal passes through the signal-processing circuit. The electrical signal is then amplified, equalized, and demodulated to extract the valid data.
Although LEDs have attracted considerable interest in high-speed VLC applications, many factors limit the modulation bandwidth of conventional LEDs. Recently, research has gradually focused on the application of µLEDs with an active area of less than 100 μm in VLC. Compared with conventional LEDs, µLEDs have a higher modulation bandwidth, owing to their smaller size, high injection-current density, and lower RC time constant, which greatly improves the transfer rate of VLC systems.
Tech Briefs: What’s the next step with regards to your research/testing?
Lu: In future research, we plan to further study RGB-μLED and μLED-based white VLC systems. Moreover, the fabrication of long-wavelength μLED devices will be the focus of our research. In 2022, our group combined superlattice structure, atomic layer deposition passivation, and distributed Bragg reflectors to fabricate high-efficiency InGaN red micro-LEDs for VLC. In the future, we hope to continue research in this direction.
In addition to RGB-μLED, the white-light VLC system that can be used as a full-color display backlight and realize data transmission is also included in our future research plans. In recent work, we have improved the stability and response speed of the color conversion layer, and that, combined with a high-speed semi-polar blue μLED as the excitation light source to achieve a white LED VLC system with high stability and high transmission rate, lays a foundation for future research. Hence, further optimization of the performance of the color conversion layer and of the excitation light source are also future research directions.
Tech Briefs: Do you have any advice for engineers aiming to bring their ideas to fruition?
Lu: Our team believes that if you want to put your ideas into practice, you need to gain enough experience, and then gradually realize it. Our group is committed to the research of semiconductor optoelectronic devices and has explored μLED preparation processes for a long time. Since most of the μLEDs currently are optimized for display applications, the initial research direction of our group was a full-color display based on μLEDs. Then, as the device manufacturing process gradually matured, we also began to use μLEDs for high-speed communication.
Fortunately, thanks to the accumulation of previous work, we have achieved good results. Therefore, we believe that, in the research direction of semiconductor optoelectronic devices, breakthroughs can only be made by being down-to-earth and accumulating experience continuously.