Although visible light communication has become a research hotspot, its development continues to focus on how to improve the transmission rate, transmission distance, and equipment utilization. The development of metal organic chemical vapor deposition technology leads to micro-light-emitting diodes (micro-LEDs) and other high-performance Ⅲ‑nitride devices. Compared with the white commercial LEDs, the micro-LED has the advantage of high modulation bandwidth, high brightness, and low coherence in the visible light communication (VLC). A variety of optical wireless transmissions using multiple quantum well LEDs or photodetectors has recently been reported. Considering that miniature high-speed visible light communication using LEDs is a potential complementary technology for dual-functional wireless communication network towards 6G, we propose that GaN-based multiple quantum well (MQW) diodes on the silicon substrate can simultaneously emit and detect light, which in practice can perform transmitting devices and receiving devices simultaneously in the VLC.

Based on the schematic of the cross-sectional structure of the InGaN/AlGaN diodes, we design and fabricate a Si-substrate micro-ring light-emitting diode (MR-LED) using a standard semiconductor process. We begin by evaluating the photoelectrical performance and communication performance of MR-LED. The optoelectronic characteristics of the MR-LED including I-V relation, electroluminescence, and the response curve of the LED are measured by an Agilent Instrument B1500A source meter and an Oriel Instrument IQE-200 B quantum efficiency system. Subsequently, for characterizing the communication performance, we propose out-of-plane visible light communication where a Hamamatsu C12702-11 photodiode module detects spatial modulated light emission by MR-LED, and the MR-LED pluses its light in coded pseudorandom binary sequence signals or carries image information. The photogenerated electron-hole pairs lead to an induced photocurrent when we employ a 375 nm and 20 mW laser beam to illuminate the MR-LED. We extract the signals detected by MR-LED. When the diode is turned on with external illumination, the measured current is a summation of the driving current and photocurrent. In this situation, the diode simultaneously emits and detects light. When appropriately biased and illuminated, the induced photocurrent is distinguishable from the driving current. We can then extract the photocurrent signal for analysis and implement a spatial full-duplex communication system.

According to the photoelectrical performance of MR-LED, the turn-on voltage of the diode is 2.8 V, and the dominant EL peak is measured at approximately 379.4 nm and an injection current of 5 mA. The overlap area between the luminescence spectrum and the detection spectrum of the MR-LED is 20 nm, which proves that the communication system of simultaneous light transmission and light reception is feasible from an optical point of view (Fig. 4). The MR-LED is observed to provide a -3 dB frequency response exceeding 66.8 MHz, and thus is suitable for high-speed VLC. The external photodiodes detect the spatial light emission to convert the photos back into electrons at a rate of 150 Mbit/s. The KEYSIGHT DSOS604A digital storage oscilloscope shows resolved eye diagrams at the rate of 150 Mbit/s (Fig. 5). In optical wireless image transmission systems, MR-LED emits signals carrying the image information. The signal received by the photodetector is amplified and then restored in MATLAB, and an eye diagram is displayed on the oscilloscope (Fig. 6). As a receiver, the MR-LED based on negative voltage of -2 ,-4,and -6 V detects the 375 nm laser modulated light signal. The received signal amplitude is around 150, 280, and 350 mV respectively. Therefore, the higher negative bias voltage loaded on the MR-LED leads to better detection performance of the MR-LED. When biased at 4.15 V, the diode as a receiver operating in the simultaneous emission-detection mode can still receive different frequency laser signals. As the frequency of the external light signal increases, the amplitude of the received signal is distorted when the MR-LED is emitting light. The amplitude of the received signal increases from 38.8 mV to 110.8 mV as the V_bias rises from 3.85 V to 4.15 V (Fig. 7). Above the turn-on voltage of 2.8 V, the increase in the biased voltage slightly influences the amplitude of the received signals. The results show that the MQW-diode can sense light in either the detector or emitter mode, indicating the possibility of spatial full-duplex communication using visible light.

We propose, fabricate, and characterize GaN-based MQW diodes with micro-ring geometry. Due to the spectral overlap between the emission and absorption spectra, a multifunctional MR-LED allows light emission and detection simultaneously. As a transmitter, the MR-LED demonstrates out-of-plane data transmission at 150 Mbit/s using on-off keying modulation. The optical wireless transmission of image data is also implemented by software processing. As a receiver, whether illuminated or not, the MR-LED can detect free-space optical signals under different bias voltages. The realization of space full-duplex communication shows that the multi-functional MR-LED can reduce material costs and processing costs in a miniature high-speed VLC system.