# A Comprehensive Review of Micro UAV Charging Techniques
## 5 Distributed Laser Charging (DLC) for UAVs  
As can be seen by the transformation of electronic devices into wearable devices, desktops into tablets and laptops, and desk phones into cell phones, mobility has become a major issue for these devices. Internet of things (IoT) and mobile devices like smartphones and sensors, and emerging commercial technologies such as UAVs, are powered  
Figure 11: Spectral response of PV materials [90]. by batteries with limited operation times. IoT sensors, especially those deployed in harsh environment such as underwater and in volcanoes, are difficult to charge. UAV battery swapping also causes disruptions in continuous missions. Meanwhile, the charging of mobile devices such as smartphones, laptops, tablets and wearables involve the difficulty of finding a power socket and carrying a power cord to recharge any device also incurs disruption. Therefore, the concept of charging mobile devices, anytime, anywhere has gained substantial research interest. A promising solution has emerged as wireless charging of UAVs, which has the capability to replenish the battery over the ether. Wireless charging has several advantages in terms of on-demand availability, usage flexibility, product durability and user convenience [96]. To solve the problem of distance and power for IoT devices and UAVs, and support the paradigm of wireless charging, the distributed laser charging (DLC) system [97] has appeared as an emerging wireless charging technique. Distributed laser charging, also known as spatially distributed laser cavity resonance, is a practical light-based charging technique which has recently been commercialized. According to the relevant patent cooperation treaty (PCT) patents (US 9,905,988 B2 and US 9,225,140 B2) [98], we can summarize this technology as follows: retroreflectors in the receiver part-reflect the incident beam from the transmitter and also deflect the transmitted light beam path back to the transmitter. It also allows tunable beam parameters. The power flow due to resonance is stopped when line-of-sight (LOS) between transceivers is blocked. Once this LOS path is restored then optical power transmission is possible at longer distances. Besides laser charging, the DLC method guarantees self-alignment to charge UAVs and IoT devices without any special tracking and positioning, as long as transceivers are in line of sight with each other. In addition, WPT of DLC can be stopped after blocking LOS by any intermediate object. A small DLC receiver can be easily mounted in a smartphone or sensor while the transmitter can be installed on a room ceiling like a light bulb. Moreover, a single DLC transmitter is capable of charging multiple devices simultaneously. DLC provides secure charging for UAV and mobile devices with a similar connection such as Wi-Fi. As DLC uses PV cells rather than a collecting lens, it is a practical and cost-efficient solution for UAVs. As equipping UAVs with heavy batteries puts a burden on the UAV, so DLC techniques can be a lightweight solution long to extending the flight time of UAVs. In [99] the authors investigated relationships between energy, power and battery dynamics for a DLC-based quadrotor UAV. Figure 12 presents DLC mechanism, applications and features, etc. It outlines a brilliant idea to charge mobile electronic devices anywhere and anytime. In Figure 12a,b, a DLC transmitter 1 is attached with an LED array to a DLC-equipped light bulb. This DLC transmitter can be easily installed on the ceiling to charge IoT and mobile devices located wihin its range. In the second scenario, a DLC transmitter is attached to a drone which is used to charge IoT devices such as tablets and mobile phones. Meanwhile, the drone is also equipped with a DLC receiver. Thus, it can be charged by DLC transmitter 2. It is worth noting that the efficiency of a DLC system is highly influenced by several factors, e.g., electricity-to-laser and laser-to-electricity conversion efficiency, laser transmission attenuation and laser wavelength. [PERSON] et al.