Space laser communication uses lasers as an information carrier. Compared to traditional microwave communication, it offers significant advantages, including high bandwidth, fast speed, high directional accuracy, strong security, and lightweight equipment. However, over deep space distances of hundreds of thousands of kilometers, laser communication has long faced three major technical challenges, often referred to as the "three mountains": extreme difficulty in precise beam alignment, severe signal attenuation, and difficulty in increasing transmission rates.
Led by the Space Application Engineering and Technology Center of the Chinese Academy of Sciences, and in collaboration with the Zhejiang Lab, Yunnan Observatories, and the Shanghai Institute of Microsystem and Information Technology, the research team spent five years overcoming these three major hurdles.
First, solving the "alignment challenge." Earth-Moon communication is often compared to "threading a needle at a distance of 400,000 kilometers." Even a microscopic angular deviation at the transmitting end can result in a positional error of several kilometers upon reaching the target. To address this, the team innovated a two-way acquisition and tracking solution designed for ultra-long distances and extremely weak signals. By comprehensively calculating and calibrating satellite orbits, telescope installation and deformation errors, atmospheric refraction, and laser time-of-flight, they ensured that the satellite and the ground telescope maintained high-precision alignment while in motion.
Second, overcoming the "weak signal" issue. After traveling 400,000 kilometers, the laser signal reaching the ground is reduced to just a few photons and is highly susceptible to interference from background noise such as moonlight, starlight, and city lights. To tackle this extreme challenge, the team relied on independently developed high-speed superconducting single-photon detection technology and a series of complex, high-sensitivity communication signal processing algorithms. This allowed them to successfully extract the valid signal from massive amounts of noise-equivalent to accurately hearing a pin drop in a bustling, noisy city from thousands of miles away.
Finally, breaking the "slow communication" barrier. To achieve high data rates under extremely low signal-to-noise ratio conditions, the team developed high-bandwidth, highly efficient communication processing technology capable of picosecond-level time recognition. They adopted high-order pulse position modulation and highly reliable signal processing schemes, supplemented by strong error-correction coding strategies to combat noise. This test initially achieved an uplink speed of 1.25 Mbps (megabits per second) and a downlink speed of 100 Mbps. For example, downloading an 8K high-definition lunar surface image took about 4 to 5 minutes using a traditional 5 Mbps microwave link. With the new 100 Mbps-class laser communication, the transmission time was drastically reduced to just 12 seconds, an efficiency increase of nearly 20 times.
Currently, this "information superhighway" between the Earth and the Moon has been fully established. As global lunar exploration and crewed lunar landing missions accelerate, cislunar space is about to experience a "data explosion." This newly established two-way high-speed laser link will not only provide a stable and high-speed new method of information transmission for future missions, supporting real-time return of high-definition lunar video and massive amounts of scientific data, but will also strongly support China in achieving more original breakthroughs in space science and applications.
