The Institute of Mechanics has made progress in the construction of space gravit

Taiji plans to detect space gravitational waves in the form of satellite formation, and the construction of inter satellite laser link is one of the key links. Compared with the traditional inter satellite laser link construction tasks applied to inter satellite laser communication, gravity field measurement and other fields, the Taiji Plan needs to apply limited on-board resources to achieve 3 million km ultra long distance laser capture and 1 nrad/Hz1/2 level ultra high precision pointing, so its implementation is much more difficult.

Therefore, a three-stage acquisition detection scheme is proposed to suppress the laser pointing error step by step through star sensor (STR), CMOS capture camera and four quadrant detector (QPD). At present, the research on this scheme is still at the stage of simulation and methodology demonstration of key technical principles, without fully considering the coupling relationship between system parameters and core detection technologies in each stage. It is urgent to further verify the feasibility of the main technical indicators of the laser link scheme through the whole process ground simulation experiment.

In response to the above problems, the Gravitational Wave Experiment Center of the Institute of Mechanics and Dr. Gao Ruihong, a core member of the Tai Chi team of the Hangzhou Institute of Higher Studies of the University of Science and Technology of China, carried out the research on the ground verification technology of building ultra-high precision inter satellite laser links for the Tai Chi program, and designed and built an integrated ground simulation experiment system for laser capture, tracking and pointing (as shown in Figure 1). On the basis of the complete restoration of the optical system and implementation process of the acquisition and tracking scheme, the system fully considers the simulation of the actual operation of the laser far field wavefront, Gaussian flat topped beam reception, weak received light intensity and other space conditions. The system uses a small aperture aperture combined with large divergence angle outgoing light. According to reasonable parameter design and device selection, it realizes the simulation of double ended approximate Fraunhofer diffraction and the reception of Gaussian flat topped beam in the laboratory near field transmission.

Figure 1 Physical picture of the acquisition, tracking and pointing integrated ground simulation experiment system.

CMOS and QPD two-level detectors are placed on the optical platform, and the whole process of acquisition tracking and aiming can be automatically simulated using the self-developed host computer software. In the simulation experiment, DWS signal is used to monitor the change of laser pointing angle in real time. The experimental data shown in Figure 2 shows the full process of pointing angle change from initial pointing to scanning open-loop capture to closed-loop capture to precise pointing, realizing the gradual suppression of the pointing deviation of hundreds of micro radians at the initial time.

Fig. 2 Yaw direction angle change in the whole process of acquisition tracking simulation experiment.

In the aspect of laser capture and detection technology, an improved centroid algorithm is proposed and adopted for the first time to achieve sub-pixel spot center positioning accuracy in the case of 100 picowatt level weak light. The conjugate imaging system is designed before QPD to reduce the influence of beam walk on the nonlinear error of DWS technology and improve the angle measurement accuracy in the precision pointing phase. At the QPD detector, the results of laser capture and laser precision pointing are shown in Figure 3. The telescope corresponding to the actual 400 times magnification can meet the requirements of the Taiji Plan, fully verifying the feasibility of the proposed scheme.

Fig. 3 (a) Schematic diagram of angle residual error after laser capture. (b) Amplitude spectral density curve of residual pointing jitter in laser precision pointing phase.

In conclusion, from the perspective of physical experiments, the research work designed and built a ground simulation experiment system for building intersatellite laser links. On the one hand, it provides a simulation experiment platform for the corresponding key technology research, verifies the coupling relationship between the key technologies, proposes the improvement strategy in methodology and guides the device parameter selection; On the other hand, the feasibility of the whole scheme is fully verified, providing complete theoretical verification and technical support for the future scheme to be transferred to the engineering realization stage. Relevant research results have recently been published in the international top optical journal Optics and Lasers in Engineering.

Source: Institute of Mechanics, Chinese Academy of Sciences