Research team leader Jing Xu from Zhejiang University in China said, “There is a critical need for efficient underwater communication to meet the increasing demands of underwater data exchange in worldwide ocean protection activities,” Examples include coral reef conservation efforts that require data links to transmit information from divers, submersibles and underwater sensors to surface ships that support their work.
An array of commercially available solar cells was used to create an optimized lens-free system for high-speed optical detection underwater, as reported by Xu and colleagues in the Optica Publishing Group journal Optics Letters. In wireless optical communication, solar cells offer a much larger detection area than photodiodes, which have traditionally been used as detectors.
A large detection area commercial silicon solar panel-based optical communication system was demonstrated to the best of Xu’s knowledge, with the highest bandwidth ever achieved. With a single device, this type of system could be used for both data exchange and power generation.
The speed, latency, and power consumption of light-based underwater wireless communication are all superior to those of radio or acoustic waves. However, most long-distance high-speed optical systems cannot be implemented underwater because of the strict alignment required between the transmitter and receiver that detects the incoming light signal.
In an underwater wireless communication system, the alignment of the transmitter and receiver is simplified by using solar cells as detectors, which can detect light from a wide area and convert it to an electrical signal. Due to the fact that solar cells are designed more for energy harvesting than communication, achieving high bandwidth has proven to be difficult.
Achieving high-speed links using off-the-shelf silicon solar cells has required complex modulation schemes and algorithms, which require intensive computing resources that use extra power and create a high processing latency,” said Xu. In order to improve the performance of our solar cell-based detector, we used modeling and simulation to optimize the peripheral circuit.
Engineers put the new design through its paces in a 7-meter-long water tank that simulated an underwater channel to see how well it worked. The optical signal’s pathlength was extended by 35 meters through the use of mirrors. The system proved to be stable, efficient, and effective. The?20-dB bandwidth increases from 4.4 MHz to 24.2 MHz as the size of the solar array grows from 1×1 to 3×3.
While the modulation scheme was simple, the new system showed a much higher detection bandwidth — which leads to a higher data rate — than has been reported in other studies using commercial silicon solar cells with a large detection area as detectors. In order to increase the bandwidth even further, they applied a reverse bias voltage of 90 V, resulting in 63.4 MHz of bandwidth. These resources made it possible to implement an amplified-shift keying modulated underwater wireless optical link with a range of 35 meters and a data rate of 150 megabits per second (Mbps).
He said, “The proposed scheme is cost-effective because solar cells are mass produced. Detectors of this type could also be used in visible light communication, a form of wireless communication that uses visible light from LEDs and other sources to transmit data over long distances. “
The researchers plan to conduct further testing of the system’s performance with weak optical signals in order to further optimize it for underwater communication. With movement and mud, this will show how effective it is. Key parameters like number of solar cells in array and required reverse bias voltage are also being fine-tuned to make it more practical.
