Objective Owing to the large loss of the laser during the transmission process, the echo signal light is weak when the target is faint; Therefore, the requirements for the single pulse energy of the laser and the aperture of the telescope are extremely high. Photon counting detection has the advantage of high sensitivity, and can detect weak echo signals. By combining the two technologies of single-photon detection and time-correlated single-photon counting (TCSPC), laser energy consumption and telescope aperture size can be significantly reduced when detecting faint targets. Photon counting ranging lidar has great application potential in faint target detection and laser remote sensing, etc. Although detection method using solid-state dense beam splitting laser illumination can effectively improve detection timeliness compared to point-by-point scanning methods and ensure high detection spatial resolution, laser energy loss is significant. In order to ensure the efficient detection of targets and reduce the consumption of laser energy caused by dense beam splitting, a detection method combining rotary scanning and push scanning was proposed. In order to better understand the influencing factors of photon counting ranging, the relationship between the above two and photon counts was explored.
Methods A single-photon ranging system was built. After illuminating the target by laser beam splitting method, which divides the laser into three beams using optical fibers, a single-photon array detector is used to collect signal photons from different target points in parallel (Fig.2). After data collection is completed, the optical fiber bracket is rotated 30° to simulate rotational scanning, and signal-photon counting is collected again. After rotation, an electric displacement table is used to move the fiber optic bracket to simulate scanning, and signal-photon counting data is collected. This process is repeated to collect a total of 18 target point data. 50 sets of data are collected repetitively at each target point, using the standard deviation of the measured distances as the ranging accuracy of the LiDAR system; The root mean square error (RMSE) between the measured distances and the true distance is used as the ranging accuracy. Taking the laser echo photon data collected by a single pixel of an array detector as an example, the relationship between ranging accuracy and photon counting is explored. After illuminating the target, a single-photon detector is used to collect signal-photon data, changing the acquisition time of the detector to change the number of detection photons. Under each detection photon condition, 30 sets of data are repeatedly collected, and then the ranging accuracy and precision under different photon counting conditions are calculated. To further investigate the impact of target position on ranging accuracy and accuracy, after each acquisition, the target is moved back 3 cm to change the target position, and then the detector acquisition time is changed to change the number of detected photons. Under the conditions of each target position and photon count, 30 sets of photon data are repeatedly collected, and the ranging accuracy at different positions are calculated under different photon counts. Based on the measured distance values of 18 target points, the three-dimensional features of the target are restored using interpolation method. And the 3D image of the target recovered from rotating scanning detection is compared with the 3D image recovered from without rotating scanning to demonstrate the feasibility of using rotating scanning for high spatial resolution detection.
Results and Discussions The ranging precision and accuracy at the pixels corresponding to the target points are calculated for the system built in the experiment. Ranging precision and accuracy decrease with the increase of the number of photons and gradually tend towards a constant (Fig.3). Their relationship with photon counts is independent of the target position (Fig.4). The depth information measured after rotary scanning increased by 33% compared to the depth information measured without rotary scanning (Fig.6). It can be concluded that the method with rotating scanning can effectively improve the spatial resolution of the detected target. Meanwhile, compared to the method without rotating scanning, the detection method based on rotating scanning reduces the laser energy loss under the condition that the total number and distribution of detected target points are the same.
Conclusions A new high-efficiency photon counting ranging lidar is proposed, which plays a certain role in promoting the high spatial resolution detection of target depth features. The laser beam is splitted to detect multiple target points, and push scanning is used to expand the detection area; During the push scanning process, the laser beam group is quickly rotated to measure the distance of more dense scanning points within the same time. While improving the timeliness of detection, it also improves the spatial resolution of detection, making the depth characteristics of the target more obvious in space. Meanwhile, compared to pure solid-state laser beam splitting illumination methods, the total laser energy consumption is reduced. And the distance information of the target increased compared to the distances obtained without rotary scanning. The ranging accuracy and accuracy of the system were measured, and the ranging accuracy was better than 1.48 cm and the accuracy was better than 2.78 cm. And both values are negatively correlated with the number of photons until they tend to remain unchanged, which is independent of the target position. The proposed method has advantages such as high timeliness, low total energy, and high detection spatial resolution, and has application potential in distance detection of targets with spatial two-dimensional features.
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