The Taiji program consists of three satellites that form an equilateral triangle with a side length of 3 million kilometer. The main scientific goal is to detect gravitational wave sources, such as the merger of medium-mass black holes and the rotation of medium-mass black hole binaries. The Taiji program uses laser interferometry to measure small shifts between stars caused by gravitational waves. Limited by satellite loads, laser interferometry systems must be highly integrated with the measurement systems. First, the laser communication link transmits data from the two satellites to the main spacecraft. After preprocessing is completed, the scientific data are transmitted to the ground station. The main requirement of the Taiji program laser communication is real-time communication, with a bit error rate of less than 10^-6 and a rate of more than 15 kb/s. To meet the needs of the Taiji program inter-satellite laser communication, a communication scheme and system parameter design based on a phase meter system are proposed in this paper. An experimental verification system is planned to be set up under laboratory conditions to verify the rationality of the designed parameters and implementation of the scheme.

The validity of the proposed method was verified under laboratory conditions by setting up a ground electronic simulation system. To more closely simulate the actual transmission process of inter-satellite laser communication links, a ground optical verification system was built in this study. To fully simulate the actual situation of inter-satellite laser communication, the ground optical verification system was divided into three parts: spread-spectrum modulation, link transmission, and phase demodulation. Modulation and demodulation were completed on K7-FPGA (field programmable gate array, FPGA). Link transmission was performed using a laser with a wavelength of 1064 nm. In this experiment, the communication codes and the pseudo-random noise (PRN) code were modulated to the laser phase using a direct sequence spread spectrum at the transmitting end. This information was sent to the receiving end through a laser link. At the receiving end, a phase-locked loop (PLL) was used for carrier synchronization, and a delay‐locked loop (DLL) for code synchronization, completing the communication function.

The demodulation performance is evaluated in this study by measuring the error rate of the mixed code parsing. The mixed codes are transmitted in three experiments, and the correct and incorrect numbers are counted. The test results indicate that the average error probabilities of the electronic and optical systems are 0.20% and 1.3%, respectively. This is mainly because the filter has a wide transition band. If the sampling decision is made in the transition zone, phase ambiguity can easily occur. This phenomenon leads to sampling decision errors that affect the demodulation performance. In addition, the background noise caused by the components and environment of the system is the main cause of mixed code errors, which are widely found in optical and electronic systems. Noise in an optical system is significantly higher than that in an electronic system. Therefore, the error rate of mixed code analysis in an optical verification system is much higher than that in an electronic system. The communication bit error rate was then measured to evaluate the communication performance of the entire system. The test results show that when 10⁶ codes were transmitted by the electronic simulation and optical verification systems, the number of correct codes received by the three tests is 10⁶. The experimental results show that a ground verification system based on the requirements of the Taiji program inter-satellite laser communication can be effectively integrated with a phase meter system. The parameters of the communication system designed in this study are verified to be reasonable. Under the condition that the communication bit rate is 19.5 kb/s, the bit error rate of the communication system is within 10^-6, which can meet the requirements of the Taiji program.

Based on the current phase meter system, an inter-satellite laser communication scheme is designed in this study using system parameters according to the requirements of the Taiji program. In this paper, a scheme for inter-satellite laser communication is described in detail and the reasons for the selection of various system parameters are analyzed. In addition, the validity of the modulation and demodulation system based on the FPGA are verified using an electronic simulation system. Then, the rationality of the laser communication system parameters and scheme is verified using an optical system. The communication error rate is tested in this study. The experimental results show that the bit error rate of the communication system is within 10^-6 under the premise of meeting the communication rate, which satisfies the requirements of laser communication in the Taiji program. The conclusions obtained in this study lay a solid technical foundation for future laser communication parameters and scheme designs within the parameters of the Taiji program.