Author: WANG Chengmiao |
Recently, the liquid crystal adaptive optics research team from Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP), Chinese Academy of Sciences (CAS) has built a high-density integrated beam array generation system using Liquid Crystal Optical Phased Array (LCOPA). The number of sub-beams in the integrated beam array is 40×40, and the average pointing accuracy is less than 20μrad. This study is published on Optics Communications.
In the field of lidar, single beam scanning is gradually difficult to meet the scanning speed requirements of target detection in large field of view. Therefore, multi-beam parallel scanning has become the first choice to greatly improve the scanning speed. In order to ensure the accuracy of detecting target details, the pointing accuracy of sub-beams in the integrated beam array must be considered at the same time.
Firstly, the initial beam splitting phase calculated by Gerchberg-Saxton (GS) algorithm is loaded into LCOPA, making a single incident plane wave diffract into 40×40 sub beams in the far field. However, due to the imperfection of GS algorithm, the sub-beams are often accompanied by side lobes and have uneven intensity, which affects the precise pointing control of the sub-beam centroids. In order to solve this problem, the preset spot size is limited to the diffraction limit, which is much smaller than that of the light source image. The actual complex amplitude is the convolution of the preset complex amplitude and the light source image, that is, the reproduction of the light source image at each grid point. In this way, the spot morphology can be unified and the pointing angle accuracy corresponding to the centroid position can be guaranteed.
Fig.1 Schematic diagram of the actual output array when the preset sub-beam reaches the diffraction limit (Image by WANG).
Aiming at the problem of zero-order light leakage interfering with sub-beams and cannot be eliminated by conventional tilt method, a light leakage avoidance method based on diffraction order splicing is proposed, in which the upper half of the main diffraction order is moved up and the lower half of the upper diffraction order is moved down, so that the two parts are stitched together into a complete beam array at the cut-off frequency and the leakage spot at the zero point of spatial frequency can be avoided.
Fig.2 Flow diagram of the diffraction order splicing (Image by WANG).
Finally, the cycle correction for intensity and pointing position is carried out, which decrease the average pointing error to less than 20μrad and further homogenized the intensity distribution. By loading this high quality beam array on the existing beam scanning control system, the scanning frame rate can be increased by several orders of magnitude, while the scanning accuracy at the level of 10μrad can be taken into account, which provides more favorable support for the space target detection in a large field of view.
WANG Qidong
Changchun Institute of Optics, Fine Mechanics and Physics
E-mail: qdwang@ciomp.ac.cn
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