# Advanced Progress of Optical Wireless Technologies for Power Industry: An Overview
## 3 Wireless Power Transfer of Power Industry Optical Wireless Communications
### Optical Wireless Power Transfer  
Recently, WPT technology has received extensive attention and has rapidly developed from a theory to commercial products [91]. The existing WPT technology can be roughly divided into transmission based on non-radiative coupling and transmission based on radiated radio frequency [92]. Transmission technologies based on non-radiative coupling include inductive coupling [93], magnetic resonance coupling [91], and capacitive coupling [94]. The technology based on non-radiative couplers has high transmission efficiency and has been diffusely applied in people's household appliances, for instance wireless chargers for smart phones. However, the effective transmission distance of non-radiative coupling technology is less than a few meters. Although RF based transmission technology could transmit power over longer distances, the transmission efficiency is inversely proportional to the square of the distance [95]. Although the transmission technology based on RF-beam forming has been proposed to increase the transmission distance [96], there is still a limit to the effective transmission distance. Furthermore, it can cause electromagnetic interference (EMI) to other electronic devices. To make the transmission distance longer, optical wireless power transfer (OWPT) is a feasible solution. As a wireless energy transmission technology that uses light as an energy transmitter, in OWPT, the transmitting terminal converts the electrical energy into optical power, and the receiving terminal uses an optical power receiver to receive, for instance a solar cell, to convert the received optical power into electrical energy. Different from that of the RF beams, the size and the power of a laser beam could maintain over longer distance. Thus, the transmission efficiency of a single-wavelength laser does not decrease with the increase of distance. Moreover, it does not create EMI on existing radio communications. The paper [97] explains that the expected DC to DC transmission efficiency of WPT technology is a function of distance. Although OWPT is less efficient over short distances than other technologies, it is expected to be more efficient over long distances. So far, there have been many studies on OWPT [90; 98; 99; 100; 101; 102]. Most of them are based on a laser emitter and a solar cell receiver, while some OWPT uses a LED transmitter that has been proven [98; 99]. Although OWPT is a solution suitable for long-distance wireless power transmission, research on OWPT is still rare. In the experiment of [103], similar to other traditional OWPT work, authors make a LD as an optical power transmitter. However, they proved that both a PD and a solar cell could be used to receive light power. After that, the PD and solar cells as the performance of the optical power receiver also were compared. Figure 25 illustrates the construction of OWPT. At the transmitter, as the energy is usually electrical energy, a light source (such as laser, LED, etc.) is needed to convert the electrical energy into optical power. When transferred to the light source of electricity for P\({}_{\text{ET}}\), and light source the light output power for P\({}_{\text{O}\text{I}}\), the electric-to-optic (E/O) power conversion ratio of the optical source is C\({}_{\text{E/O}}\). In [104], [PERSON] et al. demonstrated underwater OWPT using LD as power transmitter through experiments. They studied the properties of solar cells and PD as power receivers. They also optimized LD, PD, and solar cells for maximum transmission efficiency. The maximum transmission efficiency of back-to-back OWPT measured with the PD receiver was 4.3%. They then demonstrated OWPT in tap water and seawater. The result shows that the attenuation in sea water is 3 dB/m. The application of OWPT in indoor environment was described in [104; 105]. In these studies, ambient light was determined to be negligible, so the concept of OWPT to SCs was investigated under dark conditions.