After more than a year of orbital testing, China has achieved a significant breakthrough in testing laser communication between the Earth and the Moon. Researchers successfully established a bidirectional laser communication link spanning approximately 400,000 kilometers between these celestial bodies, marking the first instance of such a high-speed laser connection between them.
This success represents a substantial step forward for Chinese space laser communications, extending their application from near-Earth orbit to the lunar environment. Laser communication transmits information using laser beams and offers several advantages over traditional microwave communication, including higher bandwidth, faster data transfer rates, improved pointing accuracy, and enhanced security. Furthermore, the equipment is more compact and lightweight, enabling flexible high-speed bidirectional transmission and reception.
However, increasing the transmission distance presents serious technical challenges. Yang Lei, a researcher from the Technological and Engineering Center for Space Utilization at the Chinese Academy of Sciences (CAS) and head of the laser communication experiment team, noted that 'deep space communication can cover hundreds of thousands of kilometers, which imposes extremely stringent requirements on beam pointing accuracy. When a laser signal travels such a vast distance to Earth, it attenuates significantly, becoming extremely weak and difficult to detect. At the same time, high-speed transmission technologies face limitations, making it challenging to achieve even higher data rates.'
The primary obstacle was ensuring precise aiming over ultra-long distances. The communication between the Earth and the Moon was likened to 'threading a needle across thousands of miles': an extremely narrow beam of light must be directed at a fast-moving target at a distance of about 400,000 kilometers. Even a minimal angular deviation on the transmitting side could result in a positioning error of several kilometers by the time the beam reaches the intended receiver.
To solve this problem, the research group developed an innovative approach to bidirectional acquisition and tracking designed for ultra-long distances and extremely weak signals. By systematically calculating and calibrating a range of factors—including satellite orbits, mounting errors, telescope deformations, atmospheric refraction, and laser propagation time—the team managed to ensure precise alignment of both orbital satellites and ground telescopes during movement, guaranteeing that the laser beam accurately reached its target.
After solving the pointing issue, the team encountered another hurdle: detecting and identifying extremely weak signals. By the time the laser traveled about 400,000 kilometers to Earth, its signal had attenuated to just a few photons, leaving the ground telescope with very little signal to register. Simultaneously, interference from moonlight, starlight, and city lights further complicated the separation of the useful signal—it was akin to trying to hear a pin drop in a noisy city thousands of miles away.
To combat this, the team implemented ultra-sensitive detectors capable of registering individual photons and developed complex signal processing algorithms. These technologies allowed the team to filter out huge amounts of noise and accurately extract the useful signal.
Data transfer speed also required compliance. After achieving stable signal reception, the team continued to improve data processing efficiency. Initially, the experiment reached an uplink speed of 1.25 Mbps and a downlink speed of 100 Mbps. Yang explained: 'Take an 8K resolution image of the lunar surface. Transmitting it via a conventional microwave link at 5 Mbps would take four or five minutes. With laser communication at 100 Mbps, it takes only about 12 seconds.'
The experiment was led by the Technological and Engineering Center for Space Utilization, with participation from Zhejiang Lab, Yunnan Observatories under CAS, and the Shanghai Institute of Micro-systems and Information Technology under CAS. Yang concluded: 'Now, the Earth-Moon 'information superhighway' has been launched. In the future, we will be able to obtain more scientifically original datasets.' He added that the capabilities of high-speed information transfer provided by the developed technologies will also become a new means of high-speed data transmission for China's crewed lunar missions, the development of a lunar research station, and deep space exploration, contributing to more original scientific results in Chinese space science and applications.