Light fidelity (LiFi), which is emerging as a compelling technology paradigm shifting the common means of high-capacity wireless communication technologies, requires wearable and full-duplex compact design because of its great significance in smart wearables as well as the ‘Internet of Things’. However, the construction of the key component of wearable full-duplex LiFi, light-emitting/detecting bifunctional fibres, is still challenging because of the conflicting process between carrier separation and recombination, as well as the highly dynamic film-forming process. Here, we demonstrate light-emitting/detecting bifunctional fibres enabled by perovskite QDs with hybrid components. The hybrid perovskite inks endow fibres with super-smooth QD films. This, combined with the small exciton binding energy and high carrier mobility of perovskite QDs, enables successful integration of electroluminescence and photodetection into monofilaments. The bifunctional fibres possess the narrowest electroluminescence full width at half maximum of ~19 nm and, more importantly, the capability for simultaneously transmitting and receiving information. The successful fabrication of narrow emission full-duplex LiFi fibres paves the way for the fabrication and integration of low crosstalk interoperable smart wearables.

The characteristics of monolithically integrated light-emitting diodes (LEDs), photodetectors (PDs), and waveguides on a GaN-on-Si wafer are investigated. The InGaN/GaN multi-quantum wells, which are responsible for blue light emission in LEDs, are also used for photodetection in PDs. Despite the Stokes shift, a spectral overlap of $\sim 25\mathrm{nm}$ between the emission and absorption spectra provides the PDs with sufficient photosensitivity to signals from the emitter while remaining insensitive to ambient lighting. Optical interconnects in the form of linear or bent suspended waveguides bridging the LEDs and PDs are formed by selective detachment of etched GaN mesas from the Si substrate. Additionally, the PDs can be detached from the substrate and remounted on an elevated platform, owing to the flexibility of the thin-film waveguide. The $150\mathrm{\mu m}\times 150\mathrm{\mu m}$ LEDs and PDs exhibit rapid response on nanosecond time scales, which is attributed to fast radiative recombinations as well as minimized resistive-capacitive (RC) delays, enabling transmission of pseudorandom binary sequence (PRBS) data signals at rates of 250 Mb/s with an opening in the eye diagram. Together with multichannel transmission free of crosstalk, the ability of the planar and three-dimensional monolithic photonic systems to handle visible-light communication (VLC) applications is demonstrated.

A monolithic multicomponent system is proposed and implemented on a III-nitride-on-silicon platform, whereby two multiple-quantum-well diodes (MQW-diodes) are interconnected by a suspended waveguide. Both MQW-diodes have an identical low-In-content InGaN/Al_0.10Ga_0.90N MQW structure and are produced by the same fabrication process flow. When appropriately biased, both MQW-diodes operate under a simultaneous emission-detection mode and function as a transmitter and a receiver at the same time, forming an in-plane full-duplex light communication system. Real-time full-duplex audio communication is experimentally demonstrated using the monolithic multicomponent system in combination with an external circuit.

A preliminary on-chip integration of GaN-based laser, modulator, and photodetector grown on Si is reported. The modulator is integrated into the laser and shares the same InGaN quantum well active region with the laser and the photodetector. By varying the applied voltage to the modulator, the absorption of the modulator can be adjusted due to the changed band bending of the InGaN quantum well active region, and hence the threshold current and the light output power of the laser can be tuned. The photodetector can effectively detect the output power of the laser tuned by the applied voltage to the modulator, which opens up a new way for GaN-based on-chip photonic integration on Si.

Underwater wireless optical communication (UWOC) has been widely considered a supplement to traditional underwater acoustic communication. A real-time UWOC video delivery system was developed in a laboratory water tank based on a field-programmable gate array (FPGA) with binary frequency shift keying (2FSK) modulation. The system achieved full-duplex communication by using the transmission control protocol (TCP) and forward error correction (FEC). A high-power 445 nm light-emitting diode (LED) array was adopted to enhance the transmitted optical power and increase the transmission link distance. We present an underwater optical channel model that considers the effects of both geometry and channel loss, especially considering the impact of the refractive index of the optical medium and the non-line-of-sight (NLOS) links formed by water surface reflection. MATLAB was used to simulate this channel model and predict the received optical power distribution on the receiving plane. Additionally, we propose improved calculation methods for the consumed electrical power and transmitted optical power of the LED array. We also investigate the relationship between the optimum avalanche gain of an avalanche photodiode (APD) and the signal-to-noise ratio (SNR). This full-duplex system achieved a 1 Mbps data transmission rate at an SNR of 10.1 dB and a distance of 10 m for an underwater link. In addition, when the optical power of the LED array is enhanced, the link range is predicted to be 14.5 m with an attenuation coefficient of 0.056 /m.

Underwater wireless optical communication (UWOC) is a promising technology that can be a candidate to improve the communication capacity and speed in aquatic media. The aim of this study is to examine the performance of a silicon photomultiplier (SiPM) array-based multiple-input multiple-output (MIMO) UWOC system. A SiPM is a modern solid-state photodetector with extremely high sensitivity up to the single-photon level or a photon-counting ability, which helps in detecting extremely weak light signals after long-distance underwater channel attenuation. We clarify the basic characteristics and photon-counting detection mode of a SiPM. In particular, the photocount of a SiPM is approximated by a Gaussian distribution, and theoretical analysis shows that only 13.3 photons need to be detected during “1” symbol period to achieve a bit error rate of 10⁻³ in an ambient light environment. Moreover, a SiPM also has a better analog mode detection ability than an avalanche photodiode (APD) and realizes 2 Mbps analog communication owing to its unique array structure and high photon detection efficiency. Furthermore, MIMO, i.e., spatial diversity, is applied as an effective method to relax the link alignment, improve the system performance, and alleviate the effect of optical turbulence. In our experiment, with a photon-counting 6×3 MIMO scheme, an energy per bit of 7.38×10⁻⁹ J/bit is achieved at a scintillation index of 4.66×10⁻³ in a 10 m water tank with 1 Mbps on-off-keying (OOK) modulation. To the best of our knowledge, this is the first study on a MIMO-UWOC system based on the photon-counting mode of a SiPM array. This UWOC system combines the advantages of SiPMs and the MIMO scheme and has the potential to realize long-distance UWOC under optical turbulence.