The Taiji Program, led by the Chinese Academy of Sciences, is a space-based gravitational wave detection mission designed to detect gravitational wave signals in the frequency band of 0.1 MHz to 1 Hz in deep space. The main auxiliary functions of the laser link in the Taiji Program are intersatellite laser communication, ranging, and clock noise transfer. To integrate these functions within the same laser interferometric link used for gravitational wave detection, the mission proposes to implement sideband frequency doubling for clock noise transfer to suppress clock noise. Furthermore, pseudorandom code spread spectrum modulation is employed to achieve intersatellite laser communication. At present, research on the auxiliary functions of laser links for space gravitational wave detection remains limited. These functions often operate independently in separate systems, and no previous studies have tested the coupling of these auxiliary functions within a single laser link. This study aims to fill this gap by analyzing and designing parameters for the coupled modulation of clock signals and pseudorandom codes through simulation and experimental methods. The study aims to validate the feasibility and performance of the dual-modulation scheme within a single laser link, achieving functional integration of the interferometric auxiliary functions in the Taiji Program.

This study examined the requirements, principles, and methods for using a laser link to achieve intersatellite clock noise transfer and laser communication. It discussed the coupling relationship between the two functions to establish parameter planning. To validate the concept, a ground-based optical experiment was set up to simulate the laser interferometric link between the two satellites. The setup involved two independent signal generators acting as clock sources for the system clocks of the satellites. A power combiner was used to couple the communication information and the clock signal. The combined signal was then modulated into two laser carriers using an electro-optic modulator for interference testing. The phase information was extracted, and the data were demodulated through phase-locked and delay loops. Finally, the suppression results of the clock noise and the communication bit error rate were determined through data post-processing using MATLAB.

The experimental and simulation results reveal that simultaneously modulating the clock signal and pseudorandom code in the laser link introduces a measurable impact on the main detection signal. Specifically, as the modulation depths of both signals increase, the signal-to-noise ratio of the main detection signal decreases. Furthermore, a coupling effect was observed between the clock noise sidebands and the sidelobes of the pseudorandom code. When the modulation depth of the pseudorandom code is relatively high, the signal-to-noise ratio of the sidebands becomes constrained and decreases. In extreme cases, sidebands may be overshadowed by sidelobes, which renders effective clock noise extraction impossible. Conversely, if the modulation depth of the pseudorandom code is relatively low, environmental noise significantly affects the performance, greatly increasing the communication bit error rate. Through experimental analysis and parameter optimization, the clock noise suppression effect achieved surpasses 2π×10^-5 rad/Hz^1/2 in the frequency bands of 0.1?1.0 Hz and 0.1?0.5 mHz, meeting the requirements of the Taiji Pathfinder. This validates the effectiveness of the clock noise transfer scheme by simultaneously coupling the pseudorandom code and the clock signal modulation in the laser link. At the same time, the communication bit error rate is successfully maintained below 10^-6 (Fig. 19).

The auxiliary laser interferometry functions for gravitational wave detection in the Taiji Program mainly include clock noise transfer and intersatellite laser communication ranging. To meet the system design requirements that the sideband laser power in the laser link system does not exceed 10% of the total laser power and that the pseudorandom code modulation power remains below 1%, this study employs principle analysis and simulation fitting to design an optimized parameter structure for these functions. It also evaluates their integration within the same laser link. The influence of pseudorandom code modulation depth on the clock noise transfer was analyzed, leading targeted improvements. These enhancements enable clock noise suppression to reach a level better than 2π×10^-5 rad/Hz^1/2 in the frequency bands of 0.1?1.0 Hz and 0.1?0.5 mHz. This performance is consistent with the suppression effect achieved using the sideband frequency doubling method. By reducing the pseudorandom code modulation depth to improve the signal-to-noise ratios of the main and sideband signals, the communication bit error rate is successfully maintained below 10^-6. Overall, this work demonstrates the successful integration of auxiliary laser link functions, validates the effectiveness of the proposed two schemes, and lays experimental and theoretical groundwork for the comprehensive integration of interferometric auxiliary functions.