Objective  The emission lens of the laser wireless power transmission system is mostly a collimated lens, which is designed using the optical fiber collimation principle and the non-focus mode of the optical design software. The end face of the optical fiber is placed at the focal plane of the lens, and the beam on and off the axis is emitted externally in the form of parallel beam. Because the end face of the optical fiber has a object height, there is a geometric divergence angle between the on-axis beam and the off-axis beam. For the collimated lens with the image plane at infinity, the off-axis beam and the on-axis beam present a staggered superposition state on the illuminated surface at a relatively close distance. Even if the illuminance distribution of the rectangular fiber core is uniform, the light spot on the receiving surface of the power transmission still presents a Gaussian distribution that gradually weakens from the center to the periphery, and the light spot boundary is not clear, which reduces the power transmission efficiency of the laser wireless power transmission system. In order to improve the optical power transmission efficiency of the laser wireless power transmission system and avoid the blurring of the light spot boundary and the poor illumination uniformity at the receiving surface at a distance of hundreds of meters caused by the use of a collimated lens, the development of a focusing transmission optical system based on the conjugate imaging principle was carried out.

  Methods  Firstly, the design principles of collimation method and conjugate imaging method are analyzed theoretically. Then, aiming at the 808 nm semiconductor laser light source output by optical fiber, a transmitting optical system with a focal length of 550 mm and an aperture of 260 mm is designed using conjugate imaging method (Fig.2). Focusing design is realized through the movement of optical fiber end face. The movement of optical fiber end face under different focusing distances is analyzed (Fig.4). Compared with the design results of collimation method after focusing, the wave aberration at 200 m-1 km is smaller (Fig.5). Lighttools software is used to simulate and compare the illumination spot before and after focusing.

  Results and Discussions  The simulation results show that by adding a focusing mechanism to the transmission optical system designed based on the conjugate imaging principle, clear light spot boundaries can be obtained at different distances (Fig.6). The structure of the laser emission optical system is designed. The focusing structure rotates 360° and the end face of the optical fiber moves 2 mm, which meets the requirement of 1.19 mm of total end face movement of the optical fiber in the range of 200 m-1 km. The laser emission optical system is processed. The test optical path is built with ZYGO interferometer, standard lens and plane reflector. The wave aberration RMS of the laser emission optical system when focusing to infinity is 0.092λ(λ= 632.8 nm) (Fig.9). The results show that the laser wireless power transmission system can obtain a clearer and more uniform illumination spot by using the focusing emission optical system designed based on the conjugate imaging principle.

  Conclusions  A focusing laser emission optical system is developed, which can be used for laser wireless power transmission at different distances. Through theoretical analysis of the collimation method and the conjugate imaging principle, the design method of the laser emission lens is determined in the case of non-infinite distance. The optical system design is carried out. The relationship between the focusing movement and the energy transfer distance is analyzed. The changes of the light spot before and after focusing at different distances are simulated and compared. Finally, the equipment development is completed, and the optical performance test is carried out to meet the design requirements.