Xu and colleagues report on laboratory tests in which they employed an array of commercially available solar cells to develop an optimal lens-free system for high-speed optical detection underwater in the Optica Publishing Group journal Optics Letters. Photodiodes, which have traditionally been utilized as detectors in wireless optical communication, have a much smaller detecting area than solar cells.
“We exhibited the highest bandwidth ever attained for a commercial silicon solar panel-based optical communication system with a large detection area, to the best of our knowledge,” Xu added. “With this type of system, data sharing and power generation might be combined into one device.”
Optimizing solar cells for communication
Light-based underwater wireless communication is faster, has less latency, and uses less power than radio or acoustic waves. Most long-distance high-speed optical systems, on the other hand, are not suitable for use underwater because they necessitate precise alignment between the transmitter and the receiver.
Solar cells can be used as detectors in an underwater wireless communication system because they detect light from a wide area and convert it to an electrical signal. This reduces the need for transmitter-receiver alignment. Solar cells, on the other hand, are designed for energy harvesting rather than communication, making high bandwidth difficult to obtain.
The researchers tested the new design, which used a 3×3 solar array to create a detection area of 3.4 × 3.4 centimeters, in a 7-meter-long water tank that emulated an underwater channel. Mirrors were used to extend the path length of the optical signal, creating a transmission distance of 35 meters. The system showed reliable stability, low power consumption, and high performance. As the size of the solar array increases from 1×1 to 3×3, the −20-dB bandwidth increases from 4.4 MHz to 24.2 MHz.
Even though a simple modulation scheme was used, the new system exhibited 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. Applying a reverse bias voltage of 90 V boosted the bandwidth further, allowing them to achieve a −20-dB bandwidth of 63.4 MHz. This bandwidth enabled a 35-m/150-Mbps underwater wireless optical link using the simplest form of amplitude-shift keying modulation.
“The proposed technique is fairly cost-effective because solar cells are mass-produced,” Xu explained. “This form of detector could be employed in visible light communication, a type of wireless communication that employs visible light from LEDs and other sources to transfer data over long distances,” according to the researchers.