Solar-Blind Mobile Deep Ultraviolet Optical Communication Utilizing Photomultiplier Tubes

Abstract

Ozone in the atmosphere strongly absorbs deep ultraviolet light with wavelengths between 200 and 280 nm. Therefore, this characteristic is advantageous and promising for unperturbed, non-disturbed information transmission in fields such as secure communications when deep ultraviolet light is employed. However, existing optical communication systems utilizing deep ultraviolet light are characterized by substantial size, which presents significant challenges in terms of local transferability. This paper employs an array of 275 nm deep ultraviolet light-emitting diodes (LEDs) connected in series, paired with photomultiplier tubes (PMTs) as transmitters and receivers. The system is encapsulated with a visual tracking module and mounted on drones and vehicles, achieving mobile duplex real-time communication under sunlight. The communication distance reaches 30 m with a packet loss rate of 1.36%. This work enables rapid and flexible deployment of deep ultraviolet optical communication systems, offering broad application prospects.

1. Introduction

In the era of rapidly advancing communication technologies, spectrum resources are becoming increasingly scarce, and traditional communication methods face numerous challenges such as electromagnetic interference and limited communication capacity [1,2,3,4,5,6,7,8]. Deep ultraviolet (DUV) optical communication, as an emerging technology, has gained prominence in the communications field due to its unique advantages. Utilizing deep ultraviolet light with wavelengths between 200 and 280 nm as the information carrier, it demonstrates significant application potential in scenarios such as indoor/outdoor communication and secure communication. This technology effectively addresses the shortcomings of traditional communication methods, opening new pathways for the advancement of the communications sector [9,10,11,12,13,14,15]. Therefore, in-depth research into the principles, characteristics, and applications of deep ultraviolet communication technology holds significant practical importance for driving innovation in communication technology and meeting growing communication demands [16,17,18,19,20,21,22,23,24,25,26]. Currently, research on deep ultraviolet communication primarily focuses on simplex communication, achieving only signal transmission rather than actual service transmission.

In 2005, solar-insensitive one-way communication was first achieved using deep ultraviolet light-emitting diodes and photodetectors [27]. Kang et al. realized non-line-of-sight communication using deep ultraviolet light at a wavelength of 246 nm [28]. In 2022, high-speed ultraviolet optical communication was achieved by integrating deep ultraviolet LEDs with SiO₂ microcavities, demonstrating a nearly 30% increase in optical output power compared to conventional LEDs without microcavities [29]. In 2023, Yu et al. investigated the electrical and optical properties of AlGaN-based deep ultraviolet micro-LEDs, successfully developing a high-bandwidth solar-blind optical communication system and a mask-free lithography system [30]. In 2024, Wang et al. employed an integrated deep ultraviolet micro-LED array to achieve mixed-signal transmission and communication with a bandwidth reaching 527 MHz [31].

In the current practical application of deep ultraviolet optical communication technology, constrained by factors such as core component integration and system architecture design, existing communication systems generally suffer from bulky size and excessive weight. Moreover, from a communication architecture perspective, most systems can only achieve unidirectional data transmission between fixed nodes—meaning signals can only flow from a single transmitter to a fixed receiver—and cannot support dynamic movement of either transmitter or receiver nodes. This limitation severely restricts the flexibility of deep ultraviolet optical communication in complex scenarios. More critically, the overall lack of modular and lightweight design necessitates cumbersome site setup, equipment debugging, and line connection procedures during emergency tasks. This fails to meet the core requirements of emergency communications for “rapid deployment and instant activation,” limiting its application value in scenarios demanding high timeliness. To address these challenges, this paper focuses on optimizing the design of deep ultraviolet optical communication systems for “miniaturization, mobility, and deployability.” The core breakthrough lies in system integration and carrier adaptation: First, core components—including the light source module, modulation/demodulation module, and signal processing module—were compactly arranged. Specialized housing was fabricated from high-strength, lightweight aluminum alloy. This material not only provides superior mechanical strength to protect internal precision components but also offers excellent thermal dissipation, effectively addressing heat accumulation during operation. Its low density significantly reduces overall weight. The integrated communication system is fully encapsulated within this aluminum alloy housing, forming a modular “deep ultraviolet optical communication unit” with a significantly reduced footprint suitable for mounting. When assembled with a gimbal and camera for alignment, it demonstrates real-time mobile communication under daylight-blind conditions. As illustrated in Figure 1, this technology finds broad application in emergency communications across diverse terrains, including natural disaster scenarios such as earthquake zones, forest fires, and landslides. Real-time communication between drones and ground stations enables rapid transmission of critical information.

2. System Overview

The communication system employs a 275 nm deep ultraviolet LED as the light-emitting device. The manufacturer of deep ultraviolet LED is Zhongke Lu’an Ultraviolet Optoelectronics Technology Co., Ltd., located in Changzhi, China. Deep ultraviolet light-emitting diodes have been widely adopted in optical communications [32]. The photometric characteristics and communication performance of a single LED are shown in Figure 2. We used the 2636B SourceMeter from Keithley Corporation of the United States to characterize the current-voltage (I-V) characteristics and EL spectrum of the deep ultraviolet LED. The LED’s current and power increase significantly with increasing voltage. As shown in Figure 2a, an injection current of 10 mA is achieved at a forward voltage of 5.5 V. When the forward voltage is increased to 6.5 V, a current of 400 mA yields a maximum power output of 2.6 W. The total power consumption of the mobile deep ultraviolet light communication system is 64 W. The emitted light is collected via a 200 μm diameter multimode fiber and then directed into an Ocean Optics HR4000 spectrometer for spectral analysis. Analysis of the spectral characteristics in Figure 2b reveals that the primary emission wavelength of this light source is stable at 275 nm, with a full width at half maximum (FWHM) of 10 nm. Based on these spectral parameters, the optical passband wavelength range of the day-blind filter is determined to be 265–285 nm.

This paper utilized a bias test module to measure the LED’s 3−dB bandwidth, which was characterized using a network analyzer. The manufacturer is Agilent Technologies, an American company headquartered in Santa Clara, California, its model number is PNALN5203C. A DC signal supplied by the RIGOL DP832 programmable DC power supply powered the LED, while the AC signal from the network analyzer was set to 0 dBm to achieve horizontal bias dimming driver transmission power. As shown in Figure 3, the 3−dB bandwidth increases with rising current until it saturates at a certain point, then exhibits a downward trend. The saturation point of the test bandwidth occurred at 25.8 MHz with a bias voltage of 5.5 V.

Figure 4 illustrates the structural composition of the deep ultraviolet light communication system, which adopts a symmetric architecture design with integrated signal transmission and reception capabilities at both ends. The DUV optical communication system primarily consists of a transmitter, a receiver, and a main processing unit (MPU) based on a Xilinx Spartan-6 field-programmable gate array (FPGA). These modules collaborate to achieve signal transmission and processing. At the transmitter end, a camera supporting Transmission Control Protocol (TCP) connects to the FPGA via a standard RJ-45 interface, transmitting video signals to the MPU. The FPGA sequentially applies Reed-Solomon (RS) encoding and on-off keying (OOK) modulation to the input signal, completing interference-resistant encoding and format conversion. The modulated signal is then fed into a drive circuit employing metal-oxide-semiconductor field-effect transistors (MOSFETs), ensuring high operational efficiency and long-term stability. The DUV light emission module comprises nine series-connected deep ultraviolet light-emitting diodes. To address heat dissipation during prolonged operation, an embedded cooling fan is integrated at the module’s rear for efficient thermal management. Experimental results demonstrate that after 12 h of continuous operation, the laser module’s housing temperature remains stably below 37 °C, effectively ensuring signal amplitude stability and transmission reliability. At the receiving end, incident light first passes through a UV filter to eliminate stray light that could interfere with the receiver’s performance, thereby enhancing the signal-to-noise ratio. The receiver employs a Hamamatsu H11900 photomultiplier tube, which exhibits a cathode radiation sensitivity of 70 mA/W at 275 nm, delivering outstanding low-light detection capabilities. After capturing the light signal, the PMT converts it into a photocurrent. This photocurrent is fed into the signal processing unit for multi-stage processing: First, a transimpedance amplifier (TIA) converts the weak photocurrent into an AC amplitude signal. Subsequently, a Bias-Tee circuit lifts the negative level of the signal above zero, eliminating DC offset effects on subsequent processing. Finally, the TLV3501 high-speed comparator restores the processed analog signal to a standard 0–3.3 V digital signal, which is fed back to the FPGA. After decoding and demodulating the digital signal, the FPGA transmits the recovered video stream to display devices via an RJ-45 network interface, enabling real-time visualization of the video signal.

Deep ultraviolet light communication systems can achieve maximum communication rates of up to 20 Mbps. To validate the system’s performance at this rate, a RIGOL DG952 signal generator was used to generate a 20 Mbps test signal, which was then fed into the DUV optical communication system for performance testing. Figure 5a presents waveform comparisons across different signal processing stages at the 20 Mbps data rate, specifically including the transmitter output signal, transimpedance amplifier (TIA) output signal, and the final receiver signal. The transmitter signal is a 20 Mbps pseudo-random binary sequence (PRBS), which possesses ideal digital signal statistical characteristics and effectively simulates random data transmission scenarios in actual communications. Despite the inevitable introduction of environmental noise during signal transmission, the TIA output signal clearly retains the characteristics of high and low levels without significant distortion. After processing by a high-speed comparator, the receiver signal is restored to a digital signal. Its waveform exhibits an inverse correlation with the transmitter PRBS signal, a normal phase shift phenomenon during signal processing that does not affect correct data parsing. The waveform comparison results demonstrate that the DUV optical communication system achieves high-quality communication at a 20 Mbps data rate, highlighting its outstanding interference resistance and stable full-duplex communication characteristics. This directly validates the system’s reliability and feasibility in high-speed transmission scenarios. To further quantify the receiver signal quality, an eye diagram test was conducted on the received signal using a Keysight DSOS604A oscilloscope. The test results are shown in Figure 5b. With an amplitude scale of 200 mV and a time scale of 50 ns, the eye diagram exhibits a clear contour and sufficient eye opening, showing no significant interference or signal degradation. Clear eye diagram characteristics serve as a crucial indicator of receiver signal integrity, further validating the correctness and stability of the DUV optical communication system’s received signals at 20 Mbps. This provides additional experimental support for the system’s high-data-rate communication performance.

3. Experiments and Discussions

To validate the reliability of the day-blind mobile deep ultraviolet optical communication system, we conducted outdoor communication experiments to simulate real-world scenarios. Figure 6a depicts the experimental setup. Under clear skies with a UV index of 5, the deep ultraviolet optical communication system was mounted on a pan-tilt head. Through a visual recognition module, it performed real-time tracking of the mobile platform to achieve optical path alignment. As shown in Figure 6b,c, the two gimbals were mounted on a drone and a ground vehicle, respectively, with a 30 m separation between the two nodes. The drone carried not only the communication system but also a TCP-enabled camera for real-time video transmission and a power supply system for the communication equipment. At the ground vehicle node, a PC was connected to display the real-time video feed. During the experiment, the drone was launched and moved slowly in the same direction as the ground vehicle. Results demonstrated that the optical communication equipment on both nodes achieved real-time alignment. The real-time video feed from the drone node was successfully displayed on the PC at the ground node, with a PLR of 1.36%.

We tested the bidirectional PLRs related to the received optical power at the receiving end, as shown in Figure 7. During testing, we installed a filter with 25% transmittance and an attenuator with 24% transmittance in front of the photomultiplier tube. When the optical power of the receiver is measured within the range of 50~133 μW, the PLR stabilizes below 2%, indicating a wide permissible communication range in practical applications. In the experiment, real-time video communication without frame interruption can be observed when the PLR value is below 2%.

4. Conclusions

This study leverages the high-precision attitude control capabilities of a three-axis gimbal stabilizer, combined with image recognition technology and the low-light detection sensitivity advantages of PMTs, to propose an integrated technical solution merging the image recognition module, three-axis gimbal, and deep ultraviolet optical communication system. It successfully constructs a mobile optical communication device featuring automatic optical path alignment and dynamic target tracking capabilities, effectively meeting the technical requirements for full-duplex DUV optical communication in mobile scenarios. This solution overcomes the optical path alignment bottleneck faced by traditional fixed optical communication systems in mobile environments, providing a practical solution for mobile full-duplex optical communication needs across multiple fields such as search and rescue, geological exploration, and emergency communications. It not only validates the reliability of the technical approach but also offers engineering references and application support for the advancement of mobile optical communication technologies in related domains.