**Abstract:**
Based on the optical properties of blue-green light propagation underwater, an underwater high-speed laser communication system has been designed. The system utilizes the STM32F407 microcontroller and the AD9660 laser driver chip to create the optical transmitter. The optical receiver includes a photodetector, signal conditioning circuit, automatic gain control (AGC) circuit, and demodulation circuit, enabling 10/100 M Ethernet data communication. The driving signal modulation method employs Digital Pulse Interval Modulation (DPIM), while a PIN photodiode is used as the optical receiver to convert optical signals into electrical signals. After signal conditioning, the demodulated data is collected via the demodulation circuit, and the control unit restores the original signal. Experimental results demonstrate that the system achieves high-speed, low-error-rate communication over a 50-meter underwater point-to-point distance at 1 Mb/s, offering a novel technical solution for data acquisition and networking among underwater devices.
**Keywords:** laser communication; laser drive; photoelectric detection; underwater network
**CLC number:** TN929.1
**Document identification code:** A
**DOI:** 10.16157/j.issn.0258-7998.170371
**Chinese citation format:** Zhang Jun, Cai Wenyu, Wen Duanqiang. Research on underwater high-speed laser communication system and networking technology[J]. Application of Electronic Technique, 2017, 43(9): 53-56, 60.
**English reference format:** Zhang Jun, Cai Wenyu, Wen Duanqian. Research on technology of underwater high speed laser communication system and networking [J]. Application of Electronic Technique, 2017, 43 (9): 53-56, 60.
**0 Preface**
Underwater acoustic technology remains one of the most mature methods for underwater communication. However, sound waves in water have a very low transmission rate—less than one-twentieth of the speed of light—and consume significant power, making it unsuitable for use on small or energy-constrained underwater robots. Optical communication technology offers advantages over traditional acoustic systems by overcoming issues such as limited bandwidth, environmental interference, and high transmission latency. This makes it ideal for applications requiring high-speed, medium-range communication. Underwater optical communication can be categorized into LED-based and laser-based systems. While LEDs have limitations such as wide divergence angles and short transmission distances, lasers are more suitable due to their higher efficiency and better performance in water. Additionally, reliable optical communication methods are essential for underwater sensor networks, where they provide a fast, stable, and cost-effective way for node-to-node communication. In long-distance scenarios, optical communication can complement existing methods, enabling more effective wireless networking of multi-sensor systems.
**1 System Working Principle**
This paper presents a wireless communication scheme for medium-to-short range underwater environments using laser communication. The overall system architecture consists of a power supply, control unit, transmitter, and receiver. The control unit manages peripheral modules and facilitates data exchange with an external PC via Ethernet. The transmitter modulates the laser with an excitation signal to generate an optical signal, while the receiver captures incoming optical signals, converts them into electrical signals, processes them, and adjusts the gain automatically to minimize the impact of signal strength variations.
**2 System Hardware Design**
**2.1 Transmitter Circuit Design**
The laser is a sensitive device, and its stability depends heavily on the input current. Fluctuations or large ripples can damage the laser or reduce its lifespan. Therefore, a current source driver is necessary instead of a voltage source. This system uses the AD9660 from Analog Devices as the laser driver, which supports high modulation frequencies, low rise/fall times, and efficient optical power control.
**2.2 Receiver Circuit Design**
The receiver includes a photoelectric conversion circuit, a low-pass filter, and an automatic gain control (AGC) circuit. The photoelectric conversion circuit converts the weak current from the photodiode into a voltage signal. The low-pass filter helps remove noise while amplifying the signal, and the AGC ensures consistent signal levels.
**2.2.1 Photoelectric Conversion Circuit**
A transimpedance amplifier (TIA) is used to convert the photodiode’s current output into a voltage signal. The TIA must have high input impedance, wide bandwidth, and low noise. This system uses the TI OPA657 as the core component.
**2.2.2 Automatic Gain Control Circuit**
The AGC circuit uses a voltage-controlled amplifier (VCA810) and an operational amplifier (OPA820) to adjust the signal amplitude dynamically. The microcontroller controls the D/A output to maintain a stable Vpp of around 1 V.
**2.3 Demodulation Circuit Design**
The demodulation circuit includes a high-speed A/D converter (ADS830E) and a FIFO buffer (IDT7204). The A/D chip samples the analog signal at 30 MHz, and the input is biased to 2 V using a resistor divider. The signal is then processed through a voltage follower before being converted.
**3 System Software Design**
**3.1 Overall Software Design**
The embedded software handles system initialization, LwIP protocol stack setup, peripheral module control, and data processing. The laser driver program controls the AD9660, while the AGC program adjusts the gain dynamically. The demodulation program reads data from the FIFO buffer, ensuring no data loss. The Ethernet program manages data transfer between the underwater system and the server.
**3.2 Laser Driver Code**
The laser driver code is based on the timing requirements of the AD9660. It involves initializing the GPIO ports, setting up the bias and modulation loops, and loading the modulation signal onto the laser diode. The process includes enabling the chip, establishing the bias loop, activating the modulation loop, and disabling the chip when not in use.
**3.3 Ethernet Software Design**
The Ethernet protocol stack (LwIP) is configured to support the DP83848 PHY chip. After hardware drivers are set up, the LwIP kernel initializes, and the application functions run to handle data transmission and reception.
**3.4 Network Protocol Design**
The protocol defines three packet types: data, control, and status. Each packet contains a primary and secondary node ID, packet type, and data segment. Node numbers help identify and organize data from different sensors. The primary node represents the area, and the secondary node represents the specific sensor connected to it.
**4 Comprehensive Debugging**
Testing was conducted both on land and underwater, covering distances from 1 m to 40 m. Bit error rates were measured at 2 mm intervals. Results show that bit error rates increase with distance and that terrestrial conditions outperform underwater ones under similar conditions. Higher transmission rates also lead to higher error rates.
**5 Summary**
This paper presents a high-speed blue-green laser communication system for underwater environments, offering a new networking solution for underwater sensor observation networks. The system features a compact design, low power consumption, and high data rate, addressing the limitations of traditional acoustic communication and enabling effective underwater sensor networking.
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