Objective   Spaceborne photon-counting LiDAR plays a vital role in various optical detection applications such as ranging and remote sensing. The spaceborne lidar system transmits pulses and receives the detection echo signal through the satellite platform to detect the target, and then completes the ranging or altimetry task. The traditional linear system extracts the echo signal through light intensity detection, which requires high laser energy and does not make full use of photon information, resulting in low gain of linear detector. In contrast, photon-counting lidar can improve the sensitivity of the system to the response limit of a single photon, and the avalanche photodiode array detector based on Geiger mode is more capable of resolving multi-photon. In practical applications, the laser pulse signal needs to be transmitted back and forth in the space-ground long-distance space. Even though the laser intensity is very high, the number of effective signal photons that can be received is often very small. At this time, if the quantum statistical characteristics of the echo photons arriving at the receiving end of the lidar system are considered, and a reasonable quantum detection threshold is set to make judgments and estimates on the echo signal, it is expected to improve the detection performance of the spaceborne lidar system under specific circumstances.

  Methods   According to the working principle of spaceborne photon-counting LiDAR, a quantum threshold detection model based on quantum statistics theory is established. Advanced photon-number-resolving detection devices are used to filter out photons that fail to reach the minimum detection signal-to-noise ratio, and the signal-to-noise ratio detection formula are reconstructed according to the statistical law of photons. Compared with the classical intensity detection scheme, the minimum detectable signal-to-noise ratio is further reduced. At the same time, the detection probability and false alarm probability of the new quantum threshold detection scheme are analyzed.

  Results and Discussions   The numerical simulation results show that the signal-to-noise ratio of the quantum threshold detection scheme based on photon number-resolving detection is better than that of classical light intensity detection under the condition of few photons arriving (Fig.5). In addition, the quantum threshold detection performance can be further enhanced by using quantum squeezed state (Fig.6). Finally, a simulation experiment of spaceborne photon-counting lidar altimetry is carried out, and the results show that the performance of the quantum threshold detection scheme can obtain a significant gain in detection probability when returning a small number of photon signals (Fig.7).

  Conclusions   Photon-counting lidar can achieve the gain of quantum threshold detection, while other detection schemes based on quantum statistical properties may also provide higher local photon distribution gain, thus improving SNR. Many detection schemes need information about signal strength, and are suitable for applications with known signal strength. While quantum threshold detection does not need to know signal strength, so it is suitable for applications with lidar ranging and measuring unknown prior signal strength. This study shows that in the case of weak signal and strong background noise, PNRD can provide better SNR by thresholding the number of photons rather than directly detecting the intensity, and the detection performance of the system can be further enhanced by using quantum compression emission source. However, the results of photon number resolution under non-ideal conditions will lead to slightly lower SNR, which still needs further study. The combination of quantum compression laser source and quantum threshold detection method can almost always improve SNR in the case of strong noise and weak signal. It can be used not only in navigation ranging and remote sensing detection, but also in any application of weak signal detection under the influence of thermal noise, which also provides a certain reference for the development of new laser radar satellites using quantum characteristics in the future.