As light can carry information at a higher rate than radio waves (potentially very useful for short-range wireless data transmission), much research has gone into creating visible light communication (VLC) devices, most of them unidirectional. However, high-speed two-way communication is what will be needed. Researchers at Tsinghua University (Shenzhen, China) and National Tsing Hua University, Hsinchu (Taiwan, China) are developing complete high-speed (Gbit/s rates) duplex communication systems based on microLEDs and vertical-cavity surface-emitting laser (VCSEL) arrays. Duplex performance is good, and over a relatively long distance of 2.2 m: using non-return-to-zero on-off keying (NRZ-OOK), the uplinks both reach rates of 2.125 Gbit/s with a bit-error rate (BER) low enough that it falls below the BER required for the forward-error correction used for Ethernet access (and thus is workable for WiFi replacement). To remain eye-safe, the VCSELs operate at low driving currents of 140 and 190 mA.
For the downlink, a blue-emitting indium gallium nitride (InGaN)-based quantum-dot microLED array was driven by an arbitrary waveform generator; a 1 GHz avalanche photodiode (APD) was used for detection. The signal was down-sampled, synchronized, and series-parallel converted; a fast-Fourier transform was then performed and the data streams demodulated and output. For the uplink, light from a near-IR VCSEL array of the type integrated into consumer electronic devices was spread with a lens so that it bounced off the ceiling and then down to an APD-based Ethernet receiver. As the light was diffused, the mobile transmitter/receiver worked well in different positions around the 5 m experimental space. Reference: Z. Wei et al., Opt. Express (2021); https://doi.org/10.1364/oe.412348.
