Recently, wireless power charging has attracted considerable attention and has been rapidly deployed in real-life applications. For example, underwater robots working for a long time, it could be efficient to charge the batteries wirelessly without replacing them. However, the radio frequency (RF)-based wireless power transfer is not efficient in the water, especially in the long distance over a meter, because the absorption of RF waves is severe in the water, whereas that of visible light is relatively low. So, the researchers studied a new optical wireless power charging technology using laser for charging an underwater robot wirelessly in a far distance over a meter.
In this study, the research group experimentally demonstrated an underwater optical wireless power transfer using laser diode (LD) transmitter and two types of optical power receivers: a solar cell and a photodiode (PD). A mathematical expression for the transfer efficiency of optical wireless power transfer was also derived. The maximum electric-to-optic (E/O) conversion efficiency of the LD was 47.4 % with the optimization of the operating voltage. The maximum O/E conversion efficiencies of the PD and the solar cell were 10.4 % and 8.7 %, respectively, with the optimization of the load resistance and incident optical power. However, the optimum points of the LD and the receivers are different, so the researchers calculated the transfer efficiency by multiplying the efficiencies of the LD and the receivers at each power level. Our experimental results show a total transfer efficiency of 4.3 % in the near distance. They also showed an attenuation of 3 dB/m in the seawater, which means that the transfer efficiency drops to a half after each one meter. The results have been published in Chinese Optics Letters, Vol 16, Issue 8, 2018 (S. -M. Kim et al., Experimental demonstration of underwater optical wireless power transfer using a laser diode).
"It is a very valuable study that has experimentally verified the possibility of underwater optical wireless power transfer technology," said Dr. Sanghwa Yoo from Electronics and Telecommunications Research Institute (ETRI), "We are already expecting many new studies to begin with this paper."
Now, the researchers can deliver power wirelessly with a transfer efficiency of 2 % up to 1 m in the seawater. Although currently the transfer efficiency seems not to be enough for real applications, the researchers expect that the transfer efficiency can be enhanced up to more than 20 % after further researches. The researchers believe that optical wireless power charging technology will be used for long-distance and underwater wireless power charging and our report will be a first step.
该课题组使用激光二极管发射器和两种类型的光功率接收器——太阳能电池和光电二极管实现了水下光学无线电力传输。还导出了传输效率的数学表达式。通过优化工作电压，激光二极管的最大电光转换效率为47.4 ％。通过优化负载电阻和入射光功率，光电二极管和太阳能电池的最大电光转换效率分别为10.4 ％和8.7 ％。然而，激光二极管和接收器的最佳效率点是不同的，因此课题组将每个功率水平的激光二极管和接收器的效率相乘来计算传输效率。实验结果表明，使用激光二极管发射器和光电二极管接收器的近距离总传输效率为4.3 %，其在海水中的衰减为3 dB/m，这表示传输效率在激光每前进一米后下降一半。该结果发表于Chinese Optics Letters 2018年第16卷第8期（S.-M. Kim et al., Experimental demonstration of underwater optical wireless power transfer using a laser diode)。
目前，该课题组研制的系统在海水中无线传输电力的效率在距离一米处达到2 ％。 虽然目前传输效率尚不足以满足实际应用，希望经过进一步的研究，可将传输效率提高至20 %以上。作者相信光学无线充电技术将成功实现长距离水下无线充电，该研究将是迈向成功的第一步。