In this special issue, the characteristics of systems and devices, necessary technologies, and the latest trends were explained for optical wireless power transmission and optical fiber power transmission that use laser light energy. Although the principle and characteristics of power supply by laser light can be easily understood, only very limited efforts have been made so far. This is a field that has recently attracted attention, and this special issue is effective in understanding the latest trends in this area for practical use in near future.

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Since most communications are carried out wirelessly, if power supplies were also to become wireless, the impact on equipment and society would be significant. Compared to conventional wireless power transmission methods, optical wireless power transmission (OWPT) has the following advantages: compactness, long-distance power transmission, and no electromagnetic interference. Therefore, such transmission is expected to be applied to implantable devices in the body, many small IoT terminals and information devices, home appliances, such mobilities as drones, robots and electric vehicles, underwater applications, and space applications. On the other hand, although it can be configured with mature devices like laser light sources and solar cells, this field is just beginning to attract attention, and research and development is in its initial, basic stages. We describe OWPT’s advantages and challenges and the research status of several early systems, including the author’s results.

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In order to realize FSOC (free space optical communication) or OWPT (optical wireless power transfer) against freely moving objects, technology for accurately controlling the direction of the light beam according to the position of the target is vital. Authors have developed a novel beam direction control technique utilizing image recognition technology. This paper presents the characteristics of this technique and experimental results when applied to FSOC, and then discusses its application to OWPT.

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Hybrid optical wireless power and data transmission system have been designed. Two wavelength lasers are used to transmit power and data simultaneously. We demonstrated 1.4 W power and 3 GHz bandwidth for meter-range transmission.

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Optical Wireless Power Transmission (OWPT) has attracted attention as a new wireless power supply method. We investigated a middle-distance (~100 m) OWPT using GaInP solar cells, which are suitable for OWPT in the visible wavelength range. We obtained light-reaching rates of 95.3% and 94.5% at 10-m and 20-m distances. We also achieved photo-electric converter efficiency of 44.2% with GaInP solar cells and 41.8% system efficiency from laser power into solar output power, leading to a visible OWPT system.

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Optical wireless power transmission can be expected to cover long distances. However, in long-distance outdoor propagation, power loss is a problem because scintillation and beam wander occur due to atmospheric fluctuations. In this paper, we use Laguerre Gaussian (LG) beam as the carrier wave and apply a wavefront correction filter to improve the received intensity efficiency.

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In future wireless communication networks, a huge number of remote antenna units will be required for mobile communication and IoT to provide various kinds of communication services. In such networks, radio-over-fiber will be an indispensable technology, and then, many optical fibers will be installed. Power-over-fiber, which delivers the power required for driving remote antenna units into optical fibers, is an attractive technique that simultaneously transmits data signals using a single optical fiber, provides cost-effective installation, operation, and maintenance, and achieves the power savings in the entire networks. This paper introduces recent trends and our works using double-clad fibers for remote antenna units, and discusses the future prospects of power-over-fiber technologies.

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We developed an electrically insulated remote antenna that protects radio equipment from lightning strikes. Power over fiber (PoF) and radio over fiber (RoF) techniques are used in our system to transmit power and radio signals over optical fibers. This paper introduces our developed system and results of its impulse tests. The proposed system can remove waveguides or coaxial cables, both of which are main paths of a surge current from an antenna to a transceiver. We developed a 6.5-GHz band radio frequency (RF) amplifier module with 500-mW RF output power using a high power PoF, which can successfully supply approximately 20 W of electric power. Through 18-kA impulse tests that simulate a direct lightning hit to an antenna or an antenna tower, we showed that our developed remote antenna had high lightning resistance. This technique can be applied to various distributed remote antennas.

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Optical wireless power transmission (OWPT) offers many advantages over other methods. Among them, an optical method is the only approach for remote WPT underwater due to the absorption loss of water. In this study, using a fly-eye lens system, which is effective for efficient solar cell output, a 450 nm wavelength laser beam with an output of 17.5 W was irradiated onto a GaAs solar cell through a 90 cm-long water tank, and a maximum output of 2.25 W was obtained. An experimental system corresponding to long-distance underwater optical wireless power transmission was constructed using multilayer reflectors, and a solar cell output power of 0.81 W was obtained at a laser output power of 17.5 W and an underwater propagation distance of 9.9 m.

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Electric vehicle (EV) are becoming increasingly popular as a countermeasure against global warming, however there are many issues regarding the power supply in current EVs using a wired power supply. Dynamic charging using optical wireless power transmission (OWPT) is attractive method solving issues. This paper analyzed CO2 emissions of OWPT based EV (OWPT-EV) and current EV for lifecycle of cars. The results showed that OWPT-EV can suppress CO2 emissions in the case of short driving distance even at relatively low power transmission efficiency of less than 25%, and CO2 emissions can be suppressed in many usage cases at efficiencies of 36% or higher.

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Optical wireless transmission has attracted interest for applications, such as space solar power system and power transmission to a moving object. The 1.06-μm wavelength range affords higher transmissivity and a higher laser power than other wavelengths for long-distance atmospheric energy transmission. This wavelength range also supports optical fiber power transmission. In this study, we fabricated an InGaAsP photovoltaic device structure by using metal-organic vapor-phase epitaxy and investigated the electrode shape dependence of the device characteristics under laser irradiation. We measured the electrode width dependence of the current–voltage characteristics under 1.06-μm laser irradiation and compared a meshshaped electrode with a mesh-shaped electrode having an outer ring electrode. We found that having a large mesh electrode width and the use of an outer ring electrode effectively increased the fill factor. The maximum power conversion efficiency of a fabricated device without an anti-reflection coating was 20.3%.

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We have developed a highly stable noise-like pulse (NLP) laser by using nonlinear polarization rotation. We have constructed a Fabry-Perot resonator that includes a mechanism for the passive cancellation of a change in polarization with a Faraday rotator mirror (FRM). The FRM rotates the polarization state of input lights by 90 degrees and reflects the lights, so this effect cancels out the polarization fluctuation caused by light travelling back and forth through a fiber. In this experiment, the output pulse parameters were a pulse width of 18.92 ns (FWHM), a spectrum bandwidth of 70.04 nm (FWHM) and an autocorrelation waveform of 0.57 ps (FWHM). In addition, the resonator could generate NLPs even if the polarization state in the resonator changed by a polarization controller (PC). From these, we succeeded in developing the high stability NLP fiber laser constructed non-PM fiber.

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