A multiple quantum well (MQW) Ⅲ-nitride diode is a chip-based structure that simultaneously emits and absorbs light across a range of wavelengths, with its emission and absorption spectra partially overlapping. This characteristic allows the diode to function as both a transmitter and a receiver^[1−5]. Therefore, only two identical MQW diodes are needed to set up a wireless communication link since they can modulate a carrier wave to optically encode information and detect modulated light to recover information at the other end^[6−9]. Based on this phenomenon, integrating multiple optical transmitters, optical receivers and other related structures on the same chip to build a miniature integrated programmable photonic processor is an important step^[10−13]. Zheng et al. reported the monolithic integration of enhancement-mode n-channel and p-channel GaN field-effect transistors and the fabrication of GaN-based complementary logic integrated circuits^[14]. In our previous works, asymmetric optical links were demonstrated using monolithic MQW Ⅲ-nitride diodes, where one MQW diode functions as a transmitter to emit light and another monolithically integrated MQW diode which serves as a receiver to absorb the reflected light. We also reported an in-plane full-duplex light communication system, whereby two MQW diodes are interconnected by a suspended waveguide^{[15, 16]}. Zhang et al. monolithically integrated Ⅲ-nitride transmitter, modulator, waveguide, beam splitter, receiver, and monitor into a single chip to establish an in-plane light communication system^[17]. Optical wireless communication using light-emitting diodes (LEDs) is expected to play an important role in the construction of new all-optical communication networks, as an important part of future 6G all-scene communication coverage, in the future^{[18, 19]}. Optical wireless communication technology enables the communication range to be extended from free space to underwater, deep space, mountainous jungle and other environments and can be used as a means of emergency communication in complex environments^[20−24]. For example, blue LEDs and green LEDs are used in underwater environments for high-speed optical wireless communication^{[25, 26]}. Yang et al. reported an underwater optical communication system. The system achieved data transmission at 240 Mbps over a 50-m underwater distance using a PMT as the receiver^[27]. Recently, in association with wavelength division multiplexing technology, Zhang et al. combined with blue−green laser tubes to achieve underwater optical communication with a transmission distance of 20 m^[28].

As shown in Fig. 1, we propose a mobile blue-light underwater bidirectional wireless light communication system, which uses two Cree XRE-Q5 blue MQW Ⅲ-nitride diodes with commercial optical systems. The spectral overlap between the electroluminescence (EL) and responsivity spectra of MQW diodes plays a key role in the time-division multiplexing (TDM) scheme. By integrating a programmable circuit, the functionalities of two identical blue MQW diodes can be automatically changed so that they act as an optical transmitter or receiver, which can enable TDM transmission over a single optical path. The communication system, an image identification module and a gimbal stabilizer are combined to dynamically maintain the optical channel alignment for mobile blue-light TDM communication. Real-time underwater mobile TDM audio communication through a 1.6-m-long water tank is demonstrated using blue light, which provides a simple and cost-effective approach toward underwater mobile light communication.

Fig. 2(a) shows an optical microscope image of a Cree XRE-Q5 blue MQW Ⅲ-nitride diode. The diode has a square size of 948 × 948 μm² and is attached to a printed circuit board (PCB) solder pad. The light optical power (LOP) and current−voltage (I−V) plots when the MQW diode operates in light-emitting mode are summarized in Fig. 2(b), which are separately characterized by a PM 100D optical power monitor and a Keithley 2636B system. The inset shows optical images of the diode biased at 0 and 2.43 V. The EL spectra were measured by using a 200-μm-diameter multimode optical fiber to collect the emitted light and send it to an Ocean Optics HR-4000 spectrometer. The responsivity spectra were measured with an Oriel IQE-200B. As illustrated in Fig. 2(c), the EL and responsivity spectra exhibit an overlapping region of approximately 54 nm, suggesting that the MQW diode can detect and modulate shorter wavelength photons emitted by a diode with the same MQW structure^[29]. As the injection current increases, the junction temperature rises and band renormalization occurs in the quantum well, resulting in a change in the band gap corresponding to the maximum recombination rate. Therefore, the peak wavelength changes with the increase in current. As the current increases from 0.1 to 1 A, the peak wavelength shifts from 463.8 to 458.6 nm. Concurrently, the full width at half maximum (FWHM) increases from 18.56 to 26.84 nm, whereas the additive value of the FWHM gradually decreases. Two identical MQW diodes are separately used as a transmitter and a receiver to form a wireless light communication system. Fig. 2(d) shows the change in the photocurrent of the MQW receiver versus the injection current of the MQW transmitter. With increasing injection current of the MQW transmitter, the light intensity increases, and thus, the photocurrent generated at the MQW receiver end increases.

By integrating a programmable circuit, the wireless light communication system using two identical MQW diodes can be operated under the TDM scheme. As shown in Fig. 3, MQW diode A is used as an optical transmitter to pulse its emitted light with audio signals from a phone. The Bluetooth circuit receives the electrical signal from phone A and sends it to the transmitter circuit for amplification. The amplified signals are loaded onto MQW diode A after the stm32 module and converted into optical signals. At the other end, MQW diode B, which acts as a receiver, detects the encoded light and converts optical signals into electrical signals, and the sound is finally played via an audio player. In timeslot 2, diode A and diode B are defined as the receiver and transmitter, respectively, leading to TDM communication with a single optical channel. The switching interval is 5 s. The data can only be transmitted in one direction during the switching interval because this system is a half-duplex communication system over a single optical path using a TDM scheme.

Both the light output power and the 3-dB bandwidth depend on the offset voltage of the transmitter. With increasing offset voltage, as shown in Fig. 4(a), the light outpower power increases, which leads to an increase in the photocurrent at the receiver end. Moreover, the 3-dB bandwidth is improved from 3.61 to 4.26 MHz as the offset voltage increases from 3.3 to 4.3 V, as shown in Fig. 4(b). The response power of the MQW transmitter does not increase all the time. It is related to the characteristics of the device itself, and may be caused by the heat dissipation. Fig. 4(c) shows the 3-dB bandwidth of the MQW receiver. When the bias voltage varies from −2 to 2 V, the bandwidth is maintained around 350 kHz.

Fig. 5 shows a schematic diagram of the transmitter and receiver circuits in a single time slot. The transmitter circuit is powered by a 12 V differential power supply, which can increase the transmit power. The receiver circuit is powered by a 15 V direct current, which can avoid crosstalk in the circuit and improve the signal stability. The bias-tee circuit consists of a 20 mH inductor and a 220 μF inductor and mixes the amplified signal with the bias voltage. When the MQW diode acts as a transmitter, the bias voltage is 4 V. At the transmitter end, the input signals V_i are amplified by the amplifier and mixed with the 4 V bias voltage of the bias-tee circuit to produce V_t. Then these signals are loaded onto the transmitter, which pulses the emitted light with electrical signals. After the light propagates through the air−water channel, the receiver senses the modulated light and transcribes optical signals into electrical signals V_r. When the MQW diode operates as a receiver, the bias voltage is 0 V. The electrical signals V_r are amplified and filtered by the receiving circuit to generate the final output signals V_o.

Pseudorandom binary sequence (PRBS) signals are generated at a rate of 100 kbps by using a RIGOL DG952 signal generator. Fig. 6(a) compares the PRBS signals V_i with V_t at the transmitter end. V_t is amplified to encode the emitted light. PRBS signals V_r along with V_o at the receiver end are shown in Fig. 6(b). The V_o signal amplitude is amplified. The received eye diagram is illustrated in Fig. 6(c), which shows clear and open patterns. The eye diagram is 5.2 μs per cell transversely and 1 V per cell longitudinally. Fig. 6(d) shows the waveform of each node when audio signals are used for communication.

Light alignment is a crucial issue for mobile light communication. In our case, an image identification module integrated with a gimbal stabilizer is used to automatically detect the locations of moving targets to establish mobile blue-light TDM communication. Fig. 7(a) shows an exploded view of the mobile blue-light TDM communication system. All the modules are packaged in a 14 × 12.5 × 12.5 cm³ box. The motion status of the vehicle is remotely controlled; thus, light alignment is dynamically maintained by integrating the image identification module with a gimbal stabilizer. As shown in Fig. 7(b), the communication module, image identification module and gimbal stabilizer are housed together and mounted on a moving vehicle along with a power source. Each side of the system is equipped with a 230 Wh portable power supply, which can be used to supply power to each module through different DC power adapters. Image identification is realized by a camera, an image transmission module (ITM) and a terminal, in which the camera is used to capture images, and the ITM synchronizes the images to the terminal. Cell phones or computers can be used as terminals to accomplish target setting. The ITM then provides feedback to the gimbal stabilizer, which performs a three-axis rotation so that the target is always at the center of the camera image. Therefore, the optical path alignment between the two light communication ends is dynamically maintained to realize mobile wireless light communication. The communication beam covers the receiver to carry out effective communication with an angle tolerance of ±15°, and the maximum rotation speed of the three-axis gimbal stabilizer is 93.5° s⁻¹.

Figs. 8(a) and 8(b) show mobile underwater TDM audio communication using blue light, which is experimentally demonstrated in Supplementary material. Real-time audio communication between mobile nodes is realized under codirectional and opposite motions. The light attenuation by the water in the tank is 1.74 dB/m, and the water tank glass thickness is 12 mm. During the first time slot, the MQW diode acting as the transmitter pulses its emitted light with audio signals. The encoded light passes through the water tank and illuminates another MQW diode at the receiver end. The diode converts the modulated light information into electrical signals. After the signals are filtered and amplified by the circuits, the audio player plays the sound. After the switching interval, the functionalities of the two blue MQW diodes are automatically changed, and the diodes separately operate under light-detecting and light-emitting modes. Therefore, the communication channel is the same, but the light propagation direction is different, leading to the TDM scheme. In reality, the effective throughput and communication coverage of the developed communication system depend on the used MQW diodes and their optical systems. By combining high power MQW diodes with advanced optical systems, the communication coverage of the developed system can be greatly improved, and the communication distance between transmitter and receiver can be tuned.

By utilizing their emission-detection spectral overlap, two identical blue MQW diodes can separately act as a transmitter and a receiver to establish a wireless light communication system via a single optical link. An image identification module integrated with a gimbal stabilizer is used to address the light alignment issue, with which mobile TDM communication and integration of mobile nodes into a practical network can be achieved. Real-time mobile underwater TDM audio communication through a 1.6-m-long water tank is experimentally demonstrated. This work provides a simple and cost-effective mobile light communication system sharing a single optical channel.

Supplementary materials to this article can be found online at https://doi.org/10.1088/1674-4926/24080022.