Objective With the development of the detection system, the photon-counting imaging lidar based on single-photon detection technology has greatly improved the detection sensitivity of the echo optical signal, effectively reduced the demand of the system for laser power, and made it possible for miniaturized, long-distance, high-resolution, and high-precision laser 3D imaging equipment, which is widely used in the field of long-distance ranging and imaging and has become a research hotspot.Since the number of echo photons in long-distance laser detection is only the order of single photons, the detection performance of the detector is highly required. So at present, most photon-counting lidar remote imaging generally adopts the method of area array staring detection or unit micro-mirror scanning to accumulate the fixed target for a long time to improve the signal-to-noise ratio, which is not conducive to real-time dynamic measurement of large-scale and large-range targets.Although there are many schemes using SPAD (Single-photon Avalanche Diode) to carry out ranging and imaging experiments at home and abroad, Si-based SPAD in the visible band is mainly used. Compared with InGaAs/InP SPAD, the dark count, detection efficiency, afterpulse probability, dead time and other indicators are not ideal. In order to achieve higher single-photon detection performance in near-infrared band, InGaAs/InP SPAD mainly adopts gated quenching mode, which is more suitable for the situation where the target distance is known, while not suitable for the situation where the target range is large, and the relevant research on the range measurement and imaging experiment using the active quenching mode InGaAs/InP SPAD of the free-running system is less, which is only at the stage of principle prototype. Therefore, this study proposes a linear array imaging lidar scheme based on InGaAs/InP single-photon detector.
Methods Aiming at the working requirements of the eye safety band, based on the free-running mode InGaAs/InP SPAD, a set of remote linear array photon-counting lidar scanning imaging prototype system with multiple transceivers is designed, 128 units of InGaAs/InP SPAD are spliced into a linear array arrangement, the working band of the system is 1 550 nm, the laser repetition frequency is 20 kHz and the laser scanning imaging in the horizontal 200° range is realized through scanning in 2 seconds (Fig.3, Tab.5).At the same time, the factors affecting the detection probability of the detector in the sunlight background are analyzed, and the optimal working point of the system is obtained by combining with the active quenching circuit design (Fig.1) and the adjustment of the working temperature and bias voltage. Point cloud filtering and afterpulse preprocessing algorithms are used to reduce the original data rate of a single receiving channel and characterize isolated targets in the scanning field of view (Fig.2).
Results and Discussions By analyzing the characteristics of afterpulse and noise, it can be seen that the background noise signal, including the background noise and the dark count signal, is randomly distributed in the whole space. With the increase of the number of echo statistics per unit ranging period, the background noise will increase. Behind the light count signal, there are two levels of obvious afterpulse signal, and the afterpulse caused by the dark count will also lead to the increase of the background noise (Fig.5). After the processing of the point cloud filtering and afterpulse preprocessing algorithm, the original data rate of a single receiving channel is reduced from 200 kbps to less than 1 kbps, and the obvious afterpulse signal behind the target point cloud is removed. Compared with recording single echo, recording four echoes in a single ranging cycle can increase the effective data volume by about 5%. After the processing of the imaging algorithm, the system successfully realizes three-dimensional imaging of multi-range targets under daylight conditions, the maximum detection distance is more than 3 km, and the imaging targets are clear (Fig.7).
Conclusions In this study, a long-distance imaging lidar system based on photon-counting detection technology is designed, and the detection performance of single-photon detector is studied. After that, the noise and after-pulse characteristics, imaging clarity, and other indicators of the lidar are verified by fixed-point ranging experiments and scanning imaging experiments. The experimental results show that the system can successfully detect multi-range target information, and can detect long-distance targets of more than 3 km. The final reconstructed image is clear, and the noise suppression effect is perfect. The system is suitable for three-dimensional point cloud imaging of long-distance targets. However, due to the significant after-pulse effect of InGaAs/InP SPAD, the increase of dark count will seriously affect the detection performance. The afterpulse and background noise can be significantly filtered by point cloud filtering and afterpulse preprocessing algorithm, and the point cloud data can be compressed for subsequent processing.
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