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==== Laser Power Transfer ====

LPT uses a focused, coherent  beam of light towards a distant PV receiver that converts that light into electricity. It consists of three main subsystems:
 
  * The laser emission subsystem
  * Ordered List ItemTransmission
  * The laser receiving subsystem

Using those three subsystems, a general efficiency equation can be made:
η_Total=η_Laser*η_Transmission*η_Receiver

The total efficiency depends on the efficiency of the electro-optical conversion of the laser, the efficiency of the transmission, and the photoelectric conversion at the receiver side. This means that the efficiency of an LPT system can be improved by optimizing the laser emission, transmission, and receiver. Typically, the total efficiency lies between 5-35%. It is clear that LPT systems still need some optimisation to increase that efficiency. The above-mentioned subsystems will now be discussed in further detail.

The laser emission subsystem is responsible for converting the electrical energy into coherent light. It consists of three parts: a power supply, a laser diode, and beam-shaping optics. The power supply provides a stable DC current, ensuring a consistent laser output. The laser diode converts the electrical energy into coherent light. To achieve maximum electro-optical conversion efficiency, it is crucial to use lasers with high performance. High-power semiconductor lasers are widely used today because of their compact size, low cost, and simplicity. The problem is that these high-power semiconductor lasers often produce beams that are not evenly distributed and spread out quickly. This is making them less suitable for direct long-distance wireless power transfer. To overcome this, the laser beam must be processed through collimation , homogenization  and other beam-shaping techniques to improve the uniformity and directionality. Those techniques are part of the beam-shaping optics. The main goals of the beam-shaping optics are to minimize losses, maximize power density ,and ensure a safe and efficient transmission.

The efficiency of laser transmission is affected by multiple factors. The main factors are laser absorption and reflectivity of the medium. Modern components can now achieve a reflectivity rate exceeding 99.9%, which minimises losses within the internal optical path. The relation between a laser’s wavelength and how much a medium absorbs it, depends on factors like environment and transmission conditions. To reduce energy loss, it is crucial to select the laser’s operating wavelength based on the specific application and medium characteristics. Additionally, the absorption coefficient is not constant. It changes along the transmission path. In short-range scenarios, when there are no obstructions, the losses are minimal. However, the presence of obstacles or longer distances introduces refraction and beam weakening. This can lead to  challenges for maintaining beam focus and energy density.

For ground-based LPT, selecting the laser wavelengths within the atmospheric transmission window (typically between 780 nm and 1100 nm) and operating under favourable weather conditions are important to maximise the transmission efficiency. In underwater applications, blue-green lasers in the 450–560 nm range are preferred because of their excellent ability to travel through water.
The purpose of the laser receiver subsystem is to receive the optical laser energy and convert it back to usable electrical energy. During the conversion of laser energy to electrical power, PV cells generate heat, which, if not properly managed, can degrade their performance and lifespan. Thermal management systems are essential to dissipate this heat and maintain optimal operating temperatures [38]. In this process, the conversion efficiency will directly affect the power delivery of the LPT system. It consists of a PV cell or an array of PV cells. To achieve the highest possible efficiency, the receiver must be precisely aligned with the laser beam and specifically adjusted to match the wavelength of the incoming laser. For example, gallium arsenide (GaAs) and indium gallium phosphide (InGaP) PV cells are commonly used because of their high conversion efficiencies with lasers in the near-infrared range.
To allow only the laser beam to pass through and block other types of extra light, such as ambient or artificial light, optical filters are used. This improves the overall efficiency and durability of the receiver. One of the most common types is the bandpass filter, which allows a specific range of wavelengths to pass through while blocking all others. This makes it ideal for isolating the laser light from background noise. Another frequently used filter is the dichroic filter, also known as an interference filter. It reflects unwanted wavelengths while the desired ones can pass through it. This ensures a high precision and low loss. This is especially useful in systems where maintaining a clean laser signal is critical. Notch filters are designed to block a very narrow range of wavelengths while passing the rest, which can be useful in systems with multiple lasers or potential sources of interference.

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