Single-photon imaging goes long ranges
Long-range single-photon imaging over 45 km in urban environments at the city of Shanghai, China
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.
激光雷达研究新的里程碑：远距离单光子成像
在中国上海市区进行的45 km远距离单光子成像实验示意图
远距离主动成像在遥感和目标识别等领域有着广阔的应用前景，受到了研究人员的广泛关注。单光子激光雷达作为一种具有单光子级探测灵敏度和皮秒级时间分辨率的新兴雷达，是实现远距离主动成像的理想方案。
近年来，有关单光子激光雷达的研究不断取得重要进展，特别是在成像距离方面，单光子雷达的三维成像距离已被拓展至10 km。然而，随着成像距离的不断增加，回波光子数将愈发稀少且混有大量的噪声，因此，进一步拓展成像距离面临着巨大的挑战。
近日，中国科学技术大学的徐飞虎教授、潘建伟教授课题组在城市环境中通过平均每个像素点探测约一个信号光子，实现了距离达45km的单光子三维成像，创下了新的成像距离纪录。该远距离单光子成像雷达系统在硬件端和软件端均发展了适用于远距离成像的先进技术。研究成果发表在Photonics Research 2020年第9期上（Zheng-Ping Li, Xin Huang, Yuan Cao, et al. Single-photon computational 3D imaging at 45 km[J]. Photonics Research, 2020, 8(9): 09001532）。
在硬件方面，研究人员搭建了一套高效率、低噪声的同轴扫描收发系统，既能对微弱的回波光子信号进行收集，又能高度抑制背景噪声。在软件上，研究人员开发了一套适配于远距离的计算成像算法。在光子信号极少的情形下，该算法仍具有极高的光子效率和超分辨能力。
该远距离单光子成像系统对于面向低功耗和高分辨率的超远距离激光雷达研究具有里程碑式的重要意义。该成果一方面可能引领遥感监测等领域有关新式成像系统的研究；同时，其对于成像科学中的一个基本问题——终极灵敏度极限的讨论有着重要的价值。