Active imaging over long ranges is of considerable interest in a wide range of applications including remote sensing and target recognition. Single-photon light detection and ranging (LiDAR) presents single-photon sensitivity and picosecond time resolution, which is desirable for long-range imaging. Important progress has been made in the field, and 3D imaging at up to10-km range has been reported. However, further expanding the imaging range presents enormous challenges, because only weak echo photons return and are mixed with strong noise.

The interest is to significantly push the imaging range. Recently, Z.-P. Li et al. from the University of Science and Technology of China demonstrated single-photon 3D imaging at a record-breaking range of up to 45-km ranges in urban environments, with a signal level of ~1 photon per pixel. The related research results are published in Photonics Research, Vol. 8, Issue 9, 2020 (Zheng-Ping Li, Xin Huang, Yuan Cao, et al. Single-photon computational 3D imaging at 45 km[J]. Photonics Research, 2020, 8(9): 09001532)

The researchers developed an advanced technique based on both hardware and software implementations that are specifically designed for long-range single-photon LiDAR. In hardware, they constructed a high-efficiency, low-noise coaxial-scanning system, facilitating the capability to both efficiently collect the weak echo photons and highly suppress the background noise. In software, they developed a computational imaging algorithm, which can achieve super-resolution ability and high photon efficiency at the presence of low photon detections.

The system represents a significant milestone towards low-power and high-resolution LiDAR over extra-long ranges. The results may not only lead to new imaging systems for remote sensing and surveillance, but also have important implications for a fundamental problem of the ultimate sensitivity limit in imaging science.

Long-range single-photon imaging over 45 km in urban environments at the city of Shanghai, China