The three-dimensional (3D) lidar technique has been widely used in a variety of fields, such as city construction, topographic mapping, underwater detection, and robotics[
Chinese Optics Letters, Volume. 16, Issue 4, 041101(2018)
Depth resolution improvement of streak tube imaging lidar system using three laser beams
The work proposes a three-laser-beam streak tube imaging lidar system. Besides the main measuring laser beam, the second beam is used to decrease the error of time synchronization. The third beam has
The three-dimensional (3D) lidar technique has been widely used in a variety of fields, such as city construction, topographic mapping, underwater detection, and robotics[
A typical schematic of the STIL system is shown in Fig.
Figure 1.Schematic of the STIL system. (a) Schematic of the data collection process. (b) The work principle of the streak array detector. (c) The streak image on the CCD.
Depth resolution is one of the most critical issues of a lidar system[
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Figure 2.Optical emission part of the three-laser-beam STIL system.
In essence, streak tube imaging is a process of measuring time-of-flight (TOF) of a laser pulse, so the timing control is crucial to the depth resolution. In a typical STIL system, the timing of the laser source, sweep voltages, MCP, and CCD are triggered by a timing control unit. Usually, timing jitters exist in the units mentioned above. The time gates of MCP and CCD are adopted to promote the signal-to-noise ratio (SNR), and the time jitters of their delays do not influence the depth resolution of the STIL system seriously if the signal can go through the time gates. However, the time jitters of the timing control unit, laser source, and sweep voltages influence the depth resolution significantly. According to the error transfer formula, the error caused by time jitter is given by
As shown in Fig.
A simple experiment without a scan is carried out to prove that Beam B is beneficial for depth resolution. In the experiment, the target is a simple plane, the timing control unit is a digital delay generator (DG645, Stanford Research Systems Inc.) with the maximum jitter of 25 ps, the maximum jitter of laser source trigger to output is 400 ps, and the jitter of the sweep voltage is
Figure 3.Streak image of a simple plane.
As shown in Fig.
In general, the shape of the streak signal on the time-resolved axis can be approximated using a Gaussian function[
For the
In general, we obtain the gray values and calculate the centroid on the time-resolved axis as the coordinate of the signal[
Because of the pulse width of the laser, the width of the laser beam, the resolution of the optical receiving system, and the dispersion of streak tube and MCP, the streak signal has a size of several pixels on the CCD. The distance of the target is obtained by calculating the centroid of the streak signal on the time-resolved axis. Because the pixel is the minimum measurement unit of a CCD, and it cannot be separated, we cannot get the accuracy distance by calculating the centroid with a limited number of discrete gray values. There is an error between the calculated value and theoretical value, as shown in Fig.
Figure 4.Error between the calculated value and theoretical value.
The error
To decrease the error caused by discrete sampling by the CCD, Beam C is utilized, as shown in Fig.
In the simulation,
Figure 5.Distance error with the theoretical distance.
To validate the feasibility of the three-laser-beam STIL system, an indoor 3D imaging experiment using the scheme is carried out, as shown in Fig.
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Figure 6.(a) Schematic of the three-laser-beam STIL system. (b) The streak signal of Beam A, Beam B, and Beam C on the CCD.
The target is a model made by a 3D printer with the printing accuracy of 0.1 mm. There are some letters (“H”, “I”, “T”) and a punctuation mark (“!”) on a base plate, and their heights are 2, 5, 10, and 20 mm, respectively, as shown in Fig.
Figure 7.(a) Photograph of the model. (b) Size of the base. The unit is millimeters. (c) Heights of “H”, “I”, “T”, and “!”. They are 2, 5, 10, and 20 mm, respectively.
Using the three-laser-beam STIL system, the 3D image of the model is obtained. We present three processing results of the 3D imaging to compare the imaging quality obtained by one beam (Beam A), two beams (Beam A and Beam B), and three beams (Beam A, Beam B, and Beam C). Furthermore, the cutaway drawings of the three results are presented to study the quality intuitively. The results are shown in Fig.
Figure 8.Results of the 3D imaging experiment. Different colors are used to represent distances. (a) The image obtained using only Beam A, (c) the image obtained using Beam A and Beam B, and (e) the image obtained using Beam A, Beam B, and Beam C. (b), (d), and (f) are the cutaway drawings of (a), (c), and (e) near the blue line separately.
As shown in Fig.
The distance of the base plate of the model is
In conclusion, this Letter proposes a three-laser-beam STIL system to improve depth resolution. The scheme can effectively decrease the timing jitter and errors caused by discrete sampling. The experimental results show that when the time bin size is 0.066 ns (i.e., corresponding to distance of 9.9 mm), the depth resolution is better than 2 mm, which corresponds to
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Zhaodong Chen, Rongwei Fan, Guangchao Ye, Tong Luo, Jiayu Guan, Zhigang Zhou, Deying Chen, "Depth resolution improvement of streak tube imaging lidar system using three laser beams," Chin. Opt. Lett. 16, 041101 (2018)
Category: Imaging Systems
Received: Jan. 17, 2018
Accepted: Feb. 9, 2018
Published Online: Jul. 12, 2018
The Author Email: Deying Chen (deyingchenhit@163.com)