The space-based gravitational wave detection mission needs to realize the construction of ultra-long distance (3×10⁶ km) laser link between two satellites through the laser acquisition and pointing system, and realize the capture precision of 1μrad and the pointing jitter control of 10nrad/Hz at 1 mHz?1 Hz. The laser power becomes very small after being transmitted over millions of kilometers. Ultra-long distance, ultra-low power and high precision requirements make the laser link construction process of Taiji program a great challenge. In order to realize the networking of three satellites, the whole laser link construction process needs to establish three bidirectional laser links in turn. Space-based gravitational wave detection proposes a scheme to gradually build a bidirectional laser link by using three-level detectors of star tracker (STR), complementary metal oxide semiconductor (CMOS) capture camera and quadrant photodiode (QPD), and finally realize the ultra-stable laser pointing jitter control through the high-precision attitude information measured by the differential wavefront sensing (DWS) technology. At present, the scheme is in the stage of theoretical demonstration. In order to test the scheme, we adopt the laboratory’s laser acquisition and pointing integrated optical system and a card based on a ZYNQ chip, and expect to realize autonomous control of the whole laser link construction process.

We use ground optical experiment to simulate the actual laser link construction progress between two satellites under the laboratory conditions. The whole experiment includes two optical benches which are actually realistic restoration of Taiji program acquisition and pointing light path (Fig. 3), and they are respectively mounted on two hexapod displacement tables to simulate the adjustment of satellite attitude. To fully simulate the whole progress of the actual laser link construction, the ground optical experiment is divided into four stages: initialization, coarse acquisition, fine acquisition and laser pointing. According to the scheme of laser link construction, we design the whole progress flow control, spiral scanning control, spot centroid location and closed-loop control by Verilog, and verify every module separately. And then we use ZYNQ card to control the whole experiment.

The experimental results show that the bidirectional laser link is successfully constructed in the atmospheric environment. Finally, corresponding to the wavefront of the actual system’s telescope, the acquisition precision reaches 0.07μrad and the control accuracy of the final laser pointing control process reaches 9.7nrad/Hz in the sensitive frequency band of the Taiji program, which can meet the task requirements. The main influencing factors of the spot centroid control effect in the fine acquisition stage are the calibration error during the DWS calibration and the position error caused by the long-term drift of the manual four-axis displacement table. The internal stress change of the manual four-axis displacement table results in a large difference between the actual displacement table position and the position calibrated before the experiment in the pitch and yaw directions, which causes the position of the spot centroid during the experiment to deviate from the zero point of the hexapod displacement table. And the main factor affecting the control accuracy during the laser pointing stage is the background noise. By collecting the DWS data without controlling the hexapod displacement table when the platforms are aligned, it is found that the laser pointing precision is close to the background noise level (Fig. 13). Therefore, the background noise is the main noise source, which includes atmospheric fluctuation, temperature shift and environmental vibration.

This paper describes the basic composition of the control system in detail for the ground simulation experiment of building the Taiji program inter-satellite laser link, and completes the overall design and board level implementation of the entire acquisition and pointing integrated experimental control system. In this paper, the receiver and scanner are tested at different starting locations, and the data in the final laser pointing control stage are analyzed. It can be found that the experiment can complete the bidirectional laser link construction process for the platforms of receiver and scanner at different starting positions, and the control effect of the final pointing stage meets the requirements of the Taiji program. The experiment successfully verifies the feasibility of the laser link construction scheme, which meets the acquisition accuracy and the pointing jitter control accuracy in front of the telescope. It provides an experimental support for further ground simulation experiments. It also provides a basis for the realization of on-board control system of the laser link construction of Taiji program in the next step, and plays an important role in transferring the scheme from theoretical demonstration to engineering implementation.