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Abstract
The integration of wireless light communication into a wireless fidelity (Wi-Fi) module and gateway enables real-time integrated communication networks that satisfy practical application demands. In particular, wireless green light communication tools can operate underwater and in free-space environments. Here, we design, fabricate, and characterize a full-duplex light communication system using green laser diodes (LDs). Operating within the transmission control protocol/internet protocol (TCP/IP), full-duplex wireless data transmission is confirmed in underwater and free-space environments at a communication rate of 10 Mbps. Through connections to a Wi-Fi module and gateway, the system is accessed by the network via the TCP/IPv4 internet scheme, and real-time video transmission is demonstrated.
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1. Introduction
Most existing wireless communication systems use radio frequency (RF) technology to transmit information. However, the growing demand for wireless data has led to the recognition that the RF spectrum is insufficient for meeting the future needs of wireless communications; therefore, the optical spectrum should be considered for wireless communications [1–8]. Moreover, the coexistence and sharing of wireless optical communication technologies, namely cellular and Wi-Fi, should be considered possible options for spectrum management and spectrum congestion mitigation in future wireless communication networks [9]. Wireless optical communication technologies are emerging technologies that provide abundant spectrum resources without licensing [10,11] to provide high-speed and reliable information services for indoor or outdoor environments, including free space and water transmissions [12–14]. Considering all types of visible light communication (VLC) technologies, Laser wireless optical communication using LDs as communication devices has several significant advantages over other kinds of visible light wireless communication technologies in terms of bandwidth, transmission distance, and anti-interference performance [15–19], especially in applications requiring high speed, long distance, and security. Lasers, especially pulsed lasers, are highly promising light sources for achieving transmission across extremely long distances.
Underwater wireless communication technology is a new field of great interest to the military, industrial, and scientific communities, which plays an essential role in tactical surveillance, pollution monitoring, oil control and maintenance, offshore exploration, climate change monitoring, and oceanographic research [20]. The demand for full-duplex optical networks applied in aquatic equipment is increasing rapidly. Regarding the band selection of the light source, the blue-green spectral band (450∼550 nm) is the only low-loss window area of seawater [21,22], and the greater the water quality is, the better the performance. Class I water quality, transmission distance 400 m, channel loss ≤ 85 dB. Deep-sea ocean water quality can be up to class I, blue and green laser has important application significance. To make our laser communication system adaptable to underwater communication and realize long-distance communication in the atmosphere, we used a green laser as the transmitter device of the system.
In recent years, a lot of research has been conducted on VLC, including ultra-high-speed modulation methods, circuit design, and practical-oriented communication systems. In 2013, Yeh et al. demonstrated a real-time white-light phosphor-LED VLC system with a total throughput of 37 Mbps using pre-equalization at a free-space transmission length of 1.5 m [23]. In 2014, Li et al. demonstrated a VLC system achieving 550 Mbps real-time rate over a distance of 60 cm based on commercial phosphorescent white LEDs. They used pre-emphasis and post-equalization circuits to extend the 3 dB bandwidth of the optical communication link from 3 to 233 MHz [24]. The white light LED communication system is low cost and low energy consumption, but it cannot solve the challenge of long-distance duplex wireless optical communication. In 2018, Li et al. developed two pairs of 100 Mbps transceivers based on 520 nm LDs and PD modules and experimentally verified a 100 Mbps full-duplex underwater wireless optical communication system. Moreover, the impact of different types of seawater on the performance of wireless optical channels is experimentally investigated [25]. However, the compatibility of VLC with Wi-Fi is not explored.
Most of the current VLC systems have the problem of short communication distances and are not designed with specialized optical structures and shells. We used a laser as the transmitter device and designed a sophisticated optical structure and system package shell to enable the system to realize long-distance communication. We also verified the heterogeneous integration of the laser communication system with the backbone network. In this paper, we present a full-duplex wireless light communication system using 520 nm LDs, high-sensitivity avalanche photodiodes (APDs), as well as a set of lenses, attenuators, and filters to achieve high-speed and reliable communication. We conducted validation experiments in the atmosphere and underwater at a transmission rate of 10 Mbps and experimentally demonstrated real-time video transmission over a TCP/IP connection. We established a wireless light communication network by integrating a Wi-Fi module into the system, such that all users within 45 m2 of the system can access the network and communicate data thereon.
2. Experiments and discussions
The optoelectronic characteristics and communication performance of the LD are shown in Fig. 1. We characterized the current-voltage (I-V) characteristics of the green LD using a Keithley 2636B SourceMeter. As the voltage increases, the current and power of the LD clearly increase. As shown in Fig. 1(a), at a forward voltage of 5 V, the injection current was measured to be 10 mA. When the forward voltage was increased to 7.3 V with a current of 98 mA, the maximum electrical power obtained was 0.7 W.
Fig. 1. Photoelectric characterization of the green LD. (a) The current-voltage curve of the LD. (b) EL spectra of the LD. (c) 3-dB bandwidth of the LD.
We characterized the electroluminescence (EL) spectra of the LD using an Ocean Optics HR4000 spectrometer and a Keithley 2636B SourceMeter. A 200-µm diameter multimode fiber was used to collect the light emitted from the laser and transmit the data to the HR4000 spectrometer. As shown in Fig. 1(b), as the injection current increases from 35 mA to 55 mA, the luminous intensity clearly increases, while the central spectral peak redshifts from 518.3 nm by 1.2 nm to 519.5 nm. The half-wavelength of the spectrum is approximately constant at 1 nm, which is much narrower than that of other light-emitting devices, such as LEDs. This feature enables the positioning of a narrowband filter in front of the system’s receiving end to significantly improve the system’s immunity to noise sources contributing to other background light.
The verification communication link comprises an Agilent 33522A arbitrary waveform generator (AWG) and a high-speed positive-negative photodiode (PIN) to characterize the communication performance of the LD. In this test, the LD is powered at 5.5 V and modulated with a bandwidth of 40 Mbps. Two identical pseudorandom bit sequence (PRBS) signals with data rates of 40 Mbps are generated from the AWG; one is transmitted by the LD optical path and received by the PIN, whereas the other is used as a trace to be displayed on an oscilloscope (Keysight, DSOS604A) for synchronization with the received signal at the output of the PIN. As shown in Fig. 1(c), the received signal accurately reflects the characteristics of the original PRBS signal, indicating that the LD has stable transmission performance at a transmission rate of 40 Mbps.
In this paper, an Agilent Technologies PNA-LN5203C network analyzer is used to test the 3-dB bandwidth of the LD. Figure 1(c) shows the 3-dB bandwidth measurement based on the frequency response of the system. We used a bias-tee module to drive the transmitter in this measurement and plotted a curve from 5.7 to 6.6 V. The AC signal is set to the power level of 0 dBm given by the Agilent Technologies PNA-LN5203C network analyzer, and a RIGOL DP832 programmable DC power supply provides the direct current (DC) offset voltage. While the offset voltage increases from 5.7 to 6.6 V, the 3-dB bandwidth increases from 10 MHz to 20.3 MHz, then drops to 17.4 MHz. When the transmitter in our system is biased at 6 V, the transmitter exhibits the 3-dB bandwidth of 20.3 MHz.
Figure 2 shows our basic concept for building the full-duplex laser wireless communication system. The system uses a green laser as a message carrier. The information carried by the laser is encoded by an RS(255, 239) encoder and then efficiently modulated into a green laser pulse sequence for transmission. The RS encoder in this work realizes 8 bytes of error correction for every 239 bytes of data. Therefore, as long as the received signal amplitude reaches the threshold of the hysteresis comparator, the data packets can be free of error. If the RS encoder fails to correct for several times, it indicates the excess noise or disabled transmission range, thus a full-duplex communication cannot be established. High-sensitivity photoelectric detection detects the optical signal at the receiving end, and the information is restored by demodulation and decoding.
Figure 3 shows a schematic diagram of the full-duplex laser wireless communication system. Full-duplex light communication is achieved with a data rate of 10 Mbps under the TCP/IP scheme. The system uses a webcam as the message source. In the transmitter part, we use a Bias-Tee module (Mini-Circuits, ZFBT-4R2GW+) to drive a 520 nm green TO56 LD (OSRAM, PLP 520). The radio frequency (RF) of the Bias-Tee module is provided synchronously by transistor-transistor logic (TTL) signals modulated by an on-off keying (OOK) modem in the central processing unit (Xilinx Spartan 6) of the field-programmable gate array (FPGA), and a 6 V-powered LM2596 external module supplies DC signals. At the receiver end, we used a Hamamatsu S14124-20 APD with a quantum efficiency of 78% at 520 nm as the core device to construct a receiver consisting of a neutral density (ND) filter, an optical lens, a bandpass filter (HNIF-010-520-D25), and a signal processing circuit. The green laser light is recorded by the high-voltage APD through the ND filter, lens, and bandpass filter, generating a µA-level photocurrent, which is received and sensed at a wavelength of 520 nm and a voltage of 365 V generated by the voltage boosting module. The transmitter and receiver side circuits are shown at the bottom of Fig. 3. Signal processing at the receiving end is performed mainly by a trans-impedance amplifier (TIA, Texas Instruments, OPA2846), an operational amplifier (OPA, Texas Instruments, OPA2846), and a hysteresis comparator (CMP, Texas Instruments, TLV3501-SOT), which work together to recover the input signals and ultimately display real-time video on Personal Computer 1.
The system is accessed by the network via a wireless module based on the TCP/IPv4 internet scheme. As shown in Fig. 4, PCs connected to both ends of the system can realize bidirectional information exchange, transmitting files in .txt, .jpg, and .mp4 formats. We integrated the wireless module into the transceiver such that the video information from the webcam could be broadcast through the wireless module and that other PCs under the same gateway could simultaneously access the full-duplex wireless laser communication network and produce real-time video. After the laser communication link is formed, the PCs switch between the full-duplex laser wireless communication network and the backbone network to upload or download files.
Figure 5 shows perspective and exploded views of the system’s internal structure. The transceiver contains a separate transmitter unit and a receiver operating with a set of front optics. To prevent other bands of light from causing unwanted photocurrent interference within the spectral bandwidth response of the APD, a bandpass narrowband filter with a diameter of 25.4 mm is inserted at the front of the APD to suppress the background light noise, and this APD is implemented as a deep notch structure. The transmission band of the filter is 510∼530 nm, and the average transmittance is above 50%. The addition of the filter gives the system a robust anti-interference capability. Figure 5(b) shows the internal implementation details of the system, where each component is clearly shown and labeled.
Fig. 5. Construction of full-duplex wireless laser communication system. (a) Perspective view of the system. (b) Exploded view of the system.
The maximum transmission rate of the system is 10 Mbps. As shown in Fig. 6(a), we use the 10 Mbps PRBS signal generated by the AWG to access the system instead of the FPGA signal at the transmitter end, and the output signal after judgment at the receiving end is exactly inverse to the original signal. Finally, the CMP digitizes the input and restores it to a 0-3.3 V signal. The analog signal after amplifying the trans-impedance amplifier signal at the link level is captured. An eye pattern of this signal is generated by an oscilloscope (Keysight, DSOS604A), as shown in Fig. 6(b), where we can observe a clear eye pattern confirming the correctness of the signal at the receiving end below Fig. 6(a). The width and amplitude of the eye pattern are 100 ns and 200 mV, respectively, as shown in Fig. 6(b).
Fig. 6. Maximum system transfer rate. (a) 10 Mbps PRBS flowing at different points in the system. (b) eye pattern accumulated from signals before the TIA.
We measured the trend of bit error rate (BER) with optical power using an Agilent Technologies E8403A parallel bit error ratio tester and a THORLABS PM100D optical power meter. We established a unidirectional transmission link to measure the in relation to the received optical power of the APD, which is regulated by a neutral density (ND) filter. As shown in Fig. 7, the BER is greater than 1E-3 when the optical power in front of the APD is less than 50 µW, at which time severe errors occur. BER below 1E-3 can be corrected by RS coding.
Fig. 8. System free-space communication experiment. (a) Demonstration of system free-space communication. (b) ND value in front of the APD: 2000.
As shown in Fig. 8, with the APD voltage tuned to 365 V, the LD direct current voltage at 6 V, and the ND value of the front attenuation slice at the receiver end set to 2000, the system completes full-duplex video communication in a free-space environment across a distance of 120 m. Visualization 1 shows an experiment with the detailed architecture of this network. The data recorded by the network camera is transmitted to the opposite side via the laser communication system on the first side of the system, and the corresponding real-time video is displayed on the computer on the second, opposite side.
As shown in Fig. 9, we also placed the laser communication link underwater by setting the system on both sides of a water tank 1.5 m in length. The webcams and the PCs used in the experiment were connected by the same gateway. As shown in Visualization 2, after establishing the communication link, the videos from both webcams are synchronously displayed on PC 2, which is connected to the wireless module of the switch to access the full-duplex laser wireless communication network via Wi-Fi. Finally, the system was tested via TCP/IP-based transmission experiments to verify the underwater feasibility of the full-duplex laser wireless communication system. A video captured by webcam 2 was displayed in real time on PC 1.
We used a THORLABS PM100D optical power meter to measure the optical power loss rate of the laser in the atmosphere and water to be 0.044 dB/m and 0.8 dB/m, respectively. The transmitted optical power of the LD is 21.5 mW at 6 V. According to Fig. 7, we take 50 µW as the minimum received optical power for normal communication of the system, and then the maximum communication distances of the system in the atmosphere and underwater are estimated to be 33 m and 598 m, respectively. In practice, whether the receiver and transmitter of the laser communication system can be aimed or not will greatly affect the quality of communication.
3. Conclusion
In summary, we used 520 nm green LDs to establish a wireless light communication system that achieves full-duplex data transmission with a communication rate of 10 Mbit/s in both free-space and underwater environments. In addition, we integrated a Wi-Fi module into the transceiver to construct an integrated communication network. Using this network, video information from the webcam was wirelessly broadcasted to a PC over the system modules. This work provides an integrated laser transceiver solution that can present future opportunities for long-range, high-rate communications in free space, while underwater, and across other challenging transmission media in critical data and Internet of Things applications.
Funding
National Key Research and Development Program of China (2022YFE0112000); National Natural Science Foundation of China (U21A20495); 111 Project (D17018).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are available from the authors upon reasonable request.
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