Performance of Laser Time-Frequency Transfer System in China Space Station

Objective The laboratory module II of the China Space Station (CSS), known as the Mengtian lab experiment module, has been part of the CSS since its launch in October 2022. It carries a Sr optical clock, an H-maser, and a laser-cooled microwave clock, along with a microwave link and a pulsed laser link. The CSS mission's pulsed laser time-frequency transfer (CLT) system is led by the Shanghai Astronomical Observatory (SHAO). In this paper, we aim to present the development and performance evaluation of the CLT system. Methods The CLT payload unit measures 230 mm x 190 mm x 169 mm, with a mass of 6 kg and power consumption of approximately 25 W, subject to fluctuations depending on the operating mode. The onboard hardware includes a laser retro-reflector, a single-photon detection package, and an event timer. The CLT detector utilizes an avalanche photodiode operating in Geiger mode, featuring the K14 SPAD chip with a 100 mu m detection area and a timing precision of 20 ps. The detection optics system is equipped with snowflake attenuators, polytetrafluoroethylene (PTFE) scatterers, pinholes, and an optical filter. For high-precision event timing, an FPGA and the THS788 timing chip are employed, achieving a timing accuracy of 8 ps and supporting a maximum measurement frequency of 20 kHz. To meet the stringent requirements of space-to-ground laser time-frequency transfer for CSS, several technical challenges are addressed. These include enhancements in large-field optical intensity stability detection, compensation for temperature drift-induced delays in the detector, and high-repetition-rate measurements at 10 kHz to improve overall stability. In addition, a real-time calibration channel for compensating delay drift is developed to mitigate the influence of temperature fluctuations and aging effects in the CLT event timer. Results and Discussions The temperature-induced delay drift of the CLT detector is mitigated through optimization of the comparator configuration and bias voltage, including the adjustment of the feedback coefficient. Experimental results demonstrate that, with a turning point at 21 degrees C, the CLT detector achieves temperature drift compensation of 0.14 ps/degrees C when operating above 21 degrees C. The detection optics maintain a 25% relative photon change across varying incident optical angles. Ground-based laboratory tests have confirmed that the CLT payload achieves a timing precision of 23 ps, with an instability of less than 0.5 ps over the course of one day and 0.09 ps over 300 s. Ranging experiments using the CLT laser retro-reflector array (LRA) are conducted by ground-based satellite laser ranging (SLR) systems located in Shanghai, Xi'an, and Beijing. Moreover, dedicated CLT ground stations in Xi'an and Beijing conduct satellite-based CLT measurements. The results indicate that the ranging precision of the Xi'an and Beijing ground stations is approximately 4 mm, with a clock bias measurement precision of 22 ps. Conclusions Our research marks a breakthrough in the engineering development of the pulsed laser time-frequency transfer system. As ground stations connect to high-performance atomic clock signals and sufficient measurement data is collected, the system offers profound insights into the fields of time-frequency metrology, space geodesy, and fundamental physics research. It enables the calibration and validation of microwave systems, a better understanding of clock behavior, comparison of clocks across remote observatories, and testing of Einstein's gravitational redshift effect.