Atmospheric turbulence limits the accuracy of data return and position adjustment over long distances, resulting in significant uncertainty in the long-axis beam tracking phase. With two-dimensional (2D) reflectors for beam tracking, the receiver does not need to transmit data back to the transmitter, but instead carries out short-axis tracking from the 2D reflector to the receiving antenna, which is convenient and less affected by the environment. At long distances, the laser beam passes from the transmitter to the 2D reflector with a significant beam expansion effect. In a 2D mirror-assisted acquisition pointing and tracking (APT) system, the difference from the conventional long-axis follow-through at the transceiver end is the need for short-axis follow-through from the 2D mirror to the receiver end. In the alignment control stage, the spot control needs to be maintained within the effective range of the detector. The detector target surface at the receiving end detects the offset information of the reflected light spot, and controls the servo mechanism through the tracking controller to adjust the optical axis of the reflected beam from the 2D mirror, so that the reflected beam pointing is adjusted. The diffused spot can completely cover the entire 2D mirror, but the laser beam reflected by a single 2D mirror is limited by the size of the mirror, and can only be reflected for the part of the beam that is diffused over a long distance. To improve alignment efficiency and optical energy utilization, we conduct a study on the employment of dual mirrors to control beam alignment for wireless optical communications.

In the alignment control of the 2D reflectors to the receiving antenna, a single-detector multi-actuator tracking control strategy is proposed.We use multiple 2D mirrors for beam tracking control. Multiple 2D mirrors are equivalent to one large mirror that can increase the reflection area. To distinguish the double-reflected light spot on the detection surface, a filter disc is inserted in the optical path between the reflector and the receiving end, and the filter, rotation is achieved by rotating the corresponding discs through motor control. When the attenuation filters on the two optical paths rotate at different speeds, the two beams of light at consecutive times will appear as two beams of light and dark alternating with different frequencies on the detecting surface, thus distinguishing the two light spots. We design and study a double-reflected spot identification and tracking control method for double-spot detection in the case of overlapping reflected spots. For the overlapping double-spot images with offset, a multi-spot/overlapping spot center extraction method is designed, and the binarised image after edge extraction is adopted to judge the overlapping spots by the shape factor. Then the spot image is segmented by quadratic overlapping spot segmentation based on least squares ellipse fitting, and segmentation is conducted for three cases of no overlapping, less overlapping, and more overlapping.

A system architecture employing multiple reflectors for beam alignment is proposed for wireless optical communication APT system, and a reflective spot alignment control method based on two 2D reflectors is investigated (Fig.1). The structure is simple and low-cost compared to the multi-transmitter-multi-receiver wireless optical communication system. By introducing a filter carousel module and applying different frequency perturbations to the reflected beams separately, a method for the discrimination and tracking control of double-reflected spots is designed and investigated for the double-spot detection in the case of overlapping reflected spots (Fig.2). The images of the two reflected beams received on the detection surface will differ depending on the rotation period of the filter disc (Fig.4). The higher the rotational speed of the discs, the smaller the rotational period, the more times the reflected beams are captured per unit of time, and the greater the total light intensity value on the detection surface at the corresponding moment. When the two discs are controlled to rotate at different speeds, there will be a difference in the number of bright spots captured on the detection surface per unit of time. Therefore, by observing the number of bright spots captured on the detection surface per unit of time, it is possible to achieve the purpose of distinguishing between two spots within the field of view of the camera at the receiving end.

After many statistical calculations, the standard deviation of the positioning of the spot center position does not exceed 0.2 pixel when there is no overlap of the spots, and the standard deviation of the positioning of the spot center position stays below 0.5 pixel when there is overlap of the spots. The smaller the degree of overlapping R_overlap of the two spot images, the better the stability, and the stability of the non-overlapping spot is better than that of the overlapping spots as a whole, so the proposed algorithm has good effect on the separation of overlapping spots.