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Research on streak tube imaging lidar based on photocathode range gating method
Optics Communications 432 (2019) 79–83
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Research on streak tube imaging lidar based on photocathode range gating method Zihao Cui a,b ,∗, Zhaoshuo Tian a,b , Yanchao Zhang a,b , Zongjie Bi a , Gang Yang a , Erdan Gu b a b
Harbin Institute of Technology at Weihai, Weihai 264209, China Shandong Institute of Shipbuilding Technology, Weihai, China
ARTICLE Keywords: STIL 3D imaging Range gating Photocathode
INFO
ABSTRACT In this paper, a streak tube laser lidar(STIL) based on cathode gating (CG) method is introduced,range gating of laser echo is realized by triggering the photocathode voltage of the streak tube. A Nd:YAG pulse laser is used as the light source, with a wavelength of 532 nm and a maximum repetition rate of 10 Hz,the raw image was collected by CCD with a resolution of 640×480. Imaging experiments of a target with a stand-off distance of 2 km are carried out by using the developed STIL system, the intensifier gating (IG) method and the CG method were used respectively. The experimental results show that, compared with IG mode, the CG mode with the same parameters has the advantages of strong detection ability and high signal-to-noise ratio(SNR). It is proved that CG mode is more suitable for the range gating of the STIL, and the SNR of the raw image is raised from 4.2 dB in non-gating mode to 16.8 dB in CG mode which effectively improves the image definition.
1. Introduction STIL has become an efficient method for 3D data acquisition because of its advantages of high resolution [1,2], large field of view, high frame frequency [3,4]. In recent years, it has attracted more and more attention. It has important applications in many fields, such as topographic mapping, underwater obstacle detection, power line survey and the short scale sea-wave imaging [5–7]. However, scattering and absorption caused by transmission medium affect the detection ability of the lidar system [8,9], how to suppress backscattering and background interference effectively is the key problem of laser active imaging [10,11]. Range gating is one of the most effective methods to improve the SNR of the lidar system [12,13]. IG is a widely used method in the lidar system [14,15] and this method realizes the range gating of laser signal by triggering the image intensifier to improve the SNR of the lidar system. But when the method is applied to a STIL system, there are two shortcomings: firstly, because the range gate width is smaller than the afterglow time of the fluorescent screen of the streak tube, the intensifier cannot receive all the light intensity of the target signal, which leads to the attenuation of the detection signal intensity; secondly, in the process of range gating, the detection signal is superimposed by the interference afterglow outside the range area and the target echo inside the range area, which results in a decrease in the SNR of the raw image. In CG mode, the laser signal is range gated by triggering the photocathode. Only photoelectrons excited by the echo signal in the range area are allowed to enter the STIL system, and all screen image information comes from the range area, by which
the interference signals outside the range area are filtered effectively. At the same time, the intensifier works in continuous mode, and the intensity of the raw image on the CCD can be effectively improved by making full use of the afterglow intensity of the fluorescent screen of the streak tube. In this paper, a dual-gated mode STIL system is developed based on the existing research foundation [16]. The imaging experiments of a target 2 km away was carried out by using the system in different gating modes. The experiments use the same lidar system, in which the streak tube, the receiving lens, the optical system and the scanning system are all exactly the same. The experimental results show that CG mode is more suitable for STIL range gating system. At the same time, compared with the non-gating mode, the raw image obtained by CG mode has higher SNR, which further improves the imaging ability of STIL system in strong backscattering environment. And the superiority of this method is proved. 2. System structure The developed STIL system is mainly composed of 4 parts, emission subsystem, receiving subsystem, control subsystem and data processing subsystem. The schematic of the system is shown in Fig. 1. The emission subsystem uses a Nd:YAG pulse laser with a wavelength of 532 nm, a pulse width of 8 ns, a pulse energy of 100 mJ and a maximum repetition rate of 10 Hz. The divergence angle adjustment is carried out by using the beam expanding mirror, and the laser beam is transformed into a
∗ Corresponding author at: Harbin Institute of Technology at Weihai, Weihai 264209, China. E-mail address: [email protected] (Z. Cui).
https://doi.org/10.1016/j.optcom.2018.09.041 Received 23 May 2018; Received in revised form 24 August 2018; Accepted 17 September 2018 Available online xxxx 0030-4018/© 2018 Elsevier B.V. All rights reserved.
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Optics Communications 432 (2019) 79–83
Fig. 1. Schematic of the STIL system based on a 532 nm wavelength illumination.
in continuous state. In CG mode, the signal is transmitted to the streak tube power supply, and the photocathode voltage is triggered. 3. Gating principle The operating process of streak tube sweep voltage and the trigger timing of main equipment are shown in Fig. 2. During the working process, the sweep voltage between the deflecting plates will experience two processes: rising edge and falling edge. The display area of the screen depends on the voltage between the deflection plates which composed of offset voltage and sweep voltage. Since the linearity of the falling edge is significantly higher than that of the rising edge, we use the falling edge as the deflection voltage and the range resolution of the streak tube mainly depends on the slope of the falling edges. The rising edge of the sweep voltage lasts much longer, and has the same deflection ability of the falling edge. During the rising time, light signal unwanted can also be detected and displayed on the fluorescent screen. At the same time, since the minimum exposure time of CCD is longer than the duration of the falling edge and the rising edge, the interference signal during the rising edge will also be collected by CCD, resulting in the reduction of the SNR of the system. In order to improve SNR of the system, a range gating system is designed to reduce the interference in the rising edge of the sweep voltage. We have designed two modes in the range gating system, one is IG mode and the other is CG mode. The timing control of the STIL system is accomplished by the signal generator card controlled by computer. The gating process is shown in Fig. 3. The laser signal entering the system mainly includes background interference, jamming target echo and target signal. In Fig. 3, three kinds of signals are labeled by 1, 2 and 3, in which 1 and 2 are interference signal, 3 is an effective signal. In sweep condition, static images can be deflected from the screen area by adjusting the offset voltage between deflecting plates, which mainly includes 1 and 2 types signal. Gating process in IG mode is shown in Fig. 3(a), the photocathode voltage of the streak tube is constant, the photo-excited electron can enter the streak tube at any time. In order to reduce the noise interference, the image intensifier is controlled by trigger signal. The image inside the range gate is enhanced, and then collected by CCD. The enhanced image contains not only the target echo information, but also the afterglow of the interference signals, which reduce the SNR of the system. At the same time, due to the gate width limit, the target signal is only partially collected, which reduces the intensity of the raw image and affects the detection ability of the system. Compared with IG mode, streak tube photocathode is controlled by trigger signal in CG mode, as shown in Fig. 3(b), and the image intensifier works in continuous mode. In CG mode, only the signal in the range gating area can enter the streak tube, the background interference and the noise caused by the rising edge are removed effectively while making full use of the afterglow intensity of the signal. Therefore, the raw image obtained by the system will get a higher SNR.
Fig. 2. Operating process of streak tube sweep voltage and the trigger timing of main equipment.
linear beam by a cylindrical lens. The receiving subsystem is mainly composed of receiving lens, streak tube, light cone, image intensifier and CCD. Cassegrain lens is used as a receiving lens in the receiving subsystem with a focal length of 800 mm and a view angle of 3◦ . The received light passes through a narrow bandpass filter with a center wavelength of 532 nm and a bandwidth of 3 nm then focused on the photocathode of the streak tube. The laser echo signal is converted into photoelectrons by the photocathode of streak tube. The electrons are accelerated by the electric field between the photocathode and the grid, and then go into the deflection electric field. The raw image of the target will be obtained after the electron bombardment on the screen. The streak tube uses a P43 fluorescent screen, the afterglow time is 1.2 ms, the raw image is coupled to the image intensifier by a light cone, and the enhanced image is collected by CCD. The system does not adopt slit at the photocathode, therefore the system can quickly obtain the 2D image of the target without changing the hardware. By determining the relative position of laser beam and the target, the time spent on searching and matching can be reduced. The image data processing subsystem is based on the LabVIEW platform on computer, which can realize the processing of the raw image in real-time. And the intensity image, range image and 3D image can be displayed in real time synchronized. The control subsystem is mainly composed of computer, signal generator card, precision delay device and power supply. The pulse frequency of the laser and the exposure time of the CCD are adjusted by the signal generating card which controlled by computer. A photodetector is used to monitor the time of laser emission and its output signal is also used as the time reference of the control circuit. After a precision delay with a minimum step length of 6 ns, the signal is sent to the gating device as the trigger signal. The power supply of the streak tube has two working states, static and sweep, at the same time, there are also two working modes of cathode voltage, continuous and triggered. In IG mode, image intensifier is the gating device and the photocathode works 80
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Optics Communications 432 (2019) 79–83
Fig. 3. Working status chart of photocathode and image intensifier in different gating modes. (a) Photocathode works in continuous mode and intensifier was triggered in IG mode, with enhanced image including target echo and disturb afterglow; (b) Photocathode was triggered and image intensifier works in continuous mode in CG mode.
Fig. 4. Experimental setup and the picture of the target. (a) Experimental setup of the STIL system by which two gating modes can be realized; (b) Close range picture of the target.
4. Experimental results and analysis
intensifier can work in the continuous mode. So the afterglow energy of the signal is all collected and amplified, the signal intensity is obviously increased. For the above reasons, we adopted CG mode to conduct range gating for the lidar system. Comparison experiments were carried out to compare the SNR of cathode gating mode and non-gating mode. The streak tube is in sweep condition and the image intensifier is in continuous mode. The experiment was carried out in the daytime, and the imaging results are shown in Figs. 7 and 8. Under daylight condition, the background light interference is strong. Although a narrow bandwidth filter is used in the system, the raw image tends to be saturated under static condition. Fig. 7(a) is the raw image obtained after removing the background light from the screen with sweep voltage in the non-gating mode. The laser signal expands along the horizontal axis, reflecting the 3D information of the target. Affected by the rising edge of sweep voltage, the interference signal is strong in the raw image. Fig. 7(b) is the raw image obtained in CG mode. Compared with Fig. 7(a) the interference signals caused by the rising edge are filtered out, the image noise is significantly reduced and the image clarity is improved. The intensity curve of the raw image can be obtained by vertically summing the pixel intensity of the raw image. The sum of the intensity values of different transverse coordinates is expressed as:
The developed STIL system was composed by two layers as shown in Fig. 4(a). The lower layer is composed of a laser, a streak tube power supply, a control module and an output optical subsystem. The upper layer is composed of the receiving optics subsystem, the streak tube, the image intensifier and the CCD. An experiment was carried out to image an outdoor target by this system, with a laser pulse frequency of 10 Hz, and a CCD exposure time of 1 ms. the target is a tall building with a distance of 2 km, the photo of the building was shown in Fig. 4(b). The raw images obtained by the lidar system with a delay time of 15 μs are shown in Fig. 5. Fig. 5(a) is the raw image in IG mode, and Fig. 5(b) is the raw image in CG mode, both images use a same range gate with a width of 5 μs. In IG mode, the system realizes the range gating by trigger image intensifier and intercepting streak tube fluorescent screen image. Limited by gate width, only part of the signal is collected and amplified by image intensifier, which reduces the intensity of the signal. Imaging experiments were carried out with different gate widths in IG mode, and the experimental results are shown in Fig. 6. The results show that IG mode has a certain impact on signal intensity, which affects the further improvement of SNR. With the increase of gate width, the intensity of raw image in IG mode will be improved. But as the width of the gate increases, the filtering ability of the system to the noise caused by the rising edge will becomes weaker. In CG mode, the range gating is accomplished by the streak tube photocathode, the interference caused by afterglow and the interference caused by rising edge are effectively avoided. Simultaneously, because all the image information comes from the range area, the image
𝑉𝑖 =
𝑛 ∑
𝐼𝑖𝑗
(4.1)
𝑗=1
where: 𝑉𝑖 is the sum of all pixel intensity values with a horizontal coordinate of i, 𝐼𝑖𝑗 is the strength value of the ij pixel. Then the SNR 81
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Optics Communications 432 (2019) 79–83
Fig. 5. Comparison of raw images in different gating modes. (a) The raw image in IG mode; (b) The raw image in CG mode with a same delay time and a same gate width of (a).
Fig. 6. Raw images with different gating width in IG mode. (a) Image with a gate width of 5 μs; (b) Image with a gate width of 10 μs; (c) Image with a gate width of 20 μs; (d) Image with a gate width of 30 μs.
Fig. 7. Imaging results of comparison experiment when the streak tube works in the sweep condition (a) The raw image in non-gating mode; (b) The raw image in CG mode; (c) The intensity accumulation figure of the raw image in non-gating mode; (d) The intensity accumulation figure of the raw image in CG mode.
of the raw image can be expressed as: 𝑆𝑁𝑅 = 20 lg
𝑉𝑠 𝑉𝑛
the target signal and the noise are similar, it is difficult to distinguish the boundary and surface characteristics of the target in the intensity image. Fig. 8(b) is the intensity image obtained in the CG mode. The image clarity was significantly improved and background noise was effectively filtered out. The image obtained by CG mode has a higher SNR, which provides a good foundation for image processing.
(4.2)
where 𝑉𝑠 is the signal strength, 𝑉𝑛 is the noise intensity. Fig. 7(c) was the intensity curve obtained by non-gating mode with a SNR of 4.2 dB and Fig. 7(d) was the intensity curve obtained by CG mode with a SNR of 16.8 dB. The image of the target was reconstructed by 600 frames, the intensity images of the target are shown in Fig. 8. Fig. 8(a) is the intensity image under non-gating mode. Due to the low SNR of the raw image, the information at the bottom of the target is lost, which affects the integrity of the image. Because the intensity of
5. Conclusion In this paper, a range gated STIL system based on photocathode gating is introduced, and a target detection experiment with a distance of 2 km is carried out by using the system. The experimental results show that, IG mode has a certain impact on signal intensity, under the same 82
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Optics Communications 432 (2019) 79–83
(a)
(b)
Fig. 8. Intensity image obtained by the lidar system in sweep condition (a) Intensity image under non-gating mode; (b) Intensity image under CG mode.
experimental condition, CG mode is more suitable for STIL range gating system. Range gating can effectively reduce the interference caused by the rising edge of sweep voltage. Compared with non-gating mode, CG mode can improve imaging quality effectively, and the SNR of the raw image is improved from 4.2 dB to 16.8 dB.
[7] J. Gao, J. Sun, Q. Wang, et al., 4-D imaging of the short scale ocean waves using a slit streak tube imaging Lidar, Optik 136 (2017) 136–143. [8] V.A. Banakh, I.A. Razenkov, Lidar measurements of atmospheric backscattering amplification, Opt. Spectrosc. 120 (2) (2016) 326–334. [9] J. Spinhirne, Lidar aerosol and cloud backscatter at 0.53, 1.06 and 1.54 microns, in: 29th AIAA Aerospace Sciences Meeting. 29th AIAA Aerospace Sciences Meeting, 2013. [10] J. Gao, J.F. Sun, Q. Wang, Research of effect of underwater backscattering on the detection of sea wave using a Slit Streak Tube Imaging Lidar, in: International Conference on Optoelectronics and Microelectronics, IEEE, 2013, pp. 151–154. [11] W. Jin, F. Cao, X. Wang, et al., Range-gated underwater laser imaging system based on intensified gate imaging technology, SPIE Int. Soc. Opt. Eng. 6621 (2008) 66210L-66210L-6. [12] Y. Shi, W. Jin, L. Li, The integrated platform of controlling and digital video processing for underwater range-gated laser imaging system, in: Selected Papers From Conferences of the Photoelectronic Technology Committee of the Chinese Society of Astronautics, International Society for Optics and Photonics, 2015, p. 95221R. [13] M. Chen, Z. He, F. Ao, Study and implementation for range-gated underwater laser imaging system, SPIE Int. Soc. Opt. Eng. 6625 (2008) 54. [14] W. Liang, H. Guo, W. Zhang, Simulation of signal-to-noise ratio for the laser rangegated imaging system, in: Applied Optics and Photonics China, International Society for Optics and Photonics, 2015, pp. 655–657. [15] W. Ge, Analysis on the influences of range-gated underwater laser imaging system parameters on the signal to noise ratio, in: International Symposium on Photoelectronic Detection & Imaging, 2013, p. 89050K. [16] Z. Tian, Z. Cui, L. Zhang, et al., Control and image processing for streak tube imaging lidar based on VB and, MATLAB, Chin. Opt. Lett. 12 (6) (2014) 67–70.
Acknowledgments The project is supported by the National Natural Science Foundation of China (61605033), (61505043); the Natural Science Foundation of Shandong Province, China (ZR2016FQ24); and Fundamental Research Funds for the Central Universities, China (Grant No. HIT.NSRIF.201719). References [1] K. Kinoshita, Y. Inagaki, Y. Ishihara, et al., Femtosecond synchroscan streak tube, Japan. J. Appl. Phys. 41 (41) (2002) 389–392. [2] G. Ye, R. Fan, Z. Chen, et al., Range accuracy analysis of streak tube imaging lidar systems, Opt. Commun. 360 (2016) 7–14. [3] C. Ma, S. Han, The research on the reconstruction of intensity image based on streak tube imaging lidar, Proc. SPIE Int. Soc. Opt. Eng. 7850 (1) (2010) 2280–2283. [4] J. Sun, Q. Wang, 4-D image reconstruction for streak tube imaging lidar, Laser Phys. 19 (3) (2009) 502–504. [5] J.W. Mclean, High-resolution 3D underwater imaging, Proc. SPIE Int. Soc. Opt. Eng. 3761 (1999) 10–19. [6] A.J. Nevis, J.S. Taylor, Advantages of three-dimensional electro-optic imaging sensors, SPIE Int. Soc. Opt. Eng. 5089 (2003).
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Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Research on streak tube imaging lidar based on photocathode range gating method Zihao Cui a,b ,∗, Zhaoshuo Tian a,b , Yanchao Zhang a,b , Zongjie Bi a , Gang Yang a , Erdan Gu b a b
Harbin Institute of Technology at Weihai, Weihai 264209, China Shandong Institute of Shipbuilding Technology, Weihai, China
ARTICLE Keywords: STIL 3D imaging Range gating Photocathode
INFO
ABSTRACT In this paper, a streak tube laser lidar(STIL) based on cathode gating (CG) method is introduced,range gating of laser echo is realized by triggering the photocathode voltage of the streak tube. A Nd:YAG pulse laser is used as the light source, with a wavelength of 532 nm and a maximum repetition rate of 10 Hz,the raw image was collected by CCD with a resolution of 640×480. Imaging experiments of a target with a stand-off distance of 2 km are carried out by using the developed STIL system, the intensifier gating (IG) method and the CG method were used respectively. The experimental results show that, compared with IG mode, the CG mode with the same parameters has the advantages of strong detection ability and high signal-to-noise ratio(SNR). It is proved that CG mode is more suitable for the range gating of the STIL, and the SNR of the raw image is raised from 4.2 dB in non-gating mode to 16.8 dB in CG mode which effectively improves the image definition.
1. Introduction STIL has become an efficient method for 3D data acquisition because of its advantages of high resolution [1,2], large field of view, high frame frequency [3,4]. In recent years, it has attracted more and more attention. It has important applications in many fields, such as topographic mapping, underwater obstacle detection, power line survey and the short scale sea-wave imaging [5–7]. However, scattering and absorption caused by transmission medium affect the detection ability of the lidar system [8,9], how to suppress backscattering and background interference effectively is the key problem of laser active imaging [10,11]. Range gating is one of the most effective methods to improve the SNR of the lidar system [12,13]. IG is a widely used method in the lidar system [14,15] and this method realizes the range gating of laser signal by triggering the image intensifier to improve the SNR of the lidar system. But when the method is applied to a STIL system, there are two shortcomings: firstly, because the range gate width is smaller than the afterglow time of the fluorescent screen of the streak tube, the intensifier cannot receive all the light intensity of the target signal, which leads to the attenuation of the detection signal intensity; secondly, in the process of range gating, the detection signal is superimposed by the interference afterglow outside the range area and the target echo inside the range area, which results in a decrease in the SNR of the raw image. In CG mode, the laser signal is range gated by triggering the photocathode. Only photoelectrons excited by the echo signal in the range area are allowed to enter the STIL system, and all screen image information comes from the range area, by which
the interference signals outside the range area are filtered effectively. At the same time, the intensifier works in continuous mode, and the intensity of the raw image on the CCD can be effectively improved by making full use of the afterglow intensity of the fluorescent screen of the streak tube. In this paper, a dual-gated mode STIL system is developed based on the existing research foundation [16]. The imaging experiments of a target 2 km away was carried out by using the system in different gating modes. The experiments use the same lidar system, in which the streak tube, the receiving lens, the optical system and the scanning system are all exactly the same. The experimental results show that CG mode is more suitable for STIL range gating system. At the same time, compared with the non-gating mode, the raw image obtained by CG mode has higher SNR, which further improves the imaging ability of STIL system in strong backscattering environment. And the superiority of this method is proved. 2. System structure The developed STIL system is mainly composed of 4 parts, emission subsystem, receiving subsystem, control subsystem and data processing subsystem. The schematic of the system is shown in Fig. 1. The emission subsystem uses a Nd:YAG pulse laser with a wavelength of 532 nm, a pulse width of 8 ns, a pulse energy of 100 mJ and a maximum repetition rate of 10 Hz. The divergence angle adjustment is carried out by using the beam expanding mirror, and the laser beam is transformed into a
∗ Corresponding author at: Harbin Institute of Technology at Weihai, Weihai 264209, China. E-mail address: [email protected] (Z. Cui).
https://doi.org/10.1016/j.optcom.2018.09.041 Received 23 May 2018; Received in revised form 24 August 2018; Accepted 17 September 2018 Available online xxxx 0030-4018/© 2018 Elsevier B.V. All rights reserved.
Z. Cui et al.
Optics Communications 432 (2019) 79–83
Fig. 1. Schematic of the STIL system based on a 532 nm wavelength illumination.
in continuous state. In CG mode, the signal is transmitted to the streak tube power supply, and the photocathode voltage is triggered. 3. Gating principle The operating process of streak tube sweep voltage and the trigger timing of main equipment are shown in Fig. 2. During the working process, the sweep voltage between the deflecting plates will experience two processes: rising edge and falling edge. The display area of the screen depends on the voltage between the deflection plates which composed of offset voltage and sweep voltage. Since the linearity of the falling edge is significantly higher than that of the rising edge, we use the falling edge as the deflection voltage and the range resolution of the streak tube mainly depends on the slope of the falling edges. The rising edge of the sweep voltage lasts much longer, and has the same deflection ability of the falling edge. During the rising time, light signal unwanted can also be detected and displayed on the fluorescent screen. At the same time, since the minimum exposure time of CCD is longer than the duration of the falling edge and the rising edge, the interference signal during the rising edge will also be collected by CCD, resulting in the reduction of the SNR of the system. In order to improve SNR of the system, a range gating system is designed to reduce the interference in the rising edge of the sweep voltage. We have designed two modes in the range gating system, one is IG mode and the other is CG mode. The timing control of the STIL system is accomplished by the signal generator card controlled by computer. The gating process is shown in Fig. 3. The laser signal entering the system mainly includes background interference, jamming target echo and target signal. In Fig. 3, three kinds of signals are labeled by 1, 2 and 3, in which 1 and 2 are interference signal, 3 is an effective signal. In sweep condition, static images can be deflected from the screen area by adjusting the offset voltage between deflecting plates, which mainly includes 1 and 2 types signal. Gating process in IG mode is shown in Fig. 3(a), the photocathode voltage of the streak tube is constant, the photo-excited electron can enter the streak tube at any time. In order to reduce the noise interference, the image intensifier is controlled by trigger signal. The image inside the range gate is enhanced, and then collected by CCD. The enhanced image contains not only the target echo information, but also the afterglow of the interference signals, which reduce the SNR of the system. At the same time, due to the gate width limit, the target signal is only partially collected, which reduces the intensity of the raw image and affects the detection ability of the system. Compared with IG mode, streak tube photocathode is controlled by trigger signal in CG mode, as shown in Fig. 3(b), and the image intensifier works in continuous mode. In CG mode, only the signal in the range gating area can enter the streak tube, the background interference and the noise caused by the rising edge are removed effectively while making full use of the afterglow intensity of the signal. Therefore, the raw image obtained by the system will get a higher SNR.
Fig. 2. Operating process of streak tube sweep voltage and the trigger timing of main equipment.
linear beam by a cylindrical lens. The receiving subsystem is mainly composed of receiving lens, streak tube, light cone, image intensifier and CCD. Cassegrain lens is used as a receiving lens in the receiving subsystem with a focal length of 800 mm and a view angle of 3◦ . The received light passes through a narrow bandpass filter with a center wavelength of 532 nm and a bandwidth of 3 nm then focused on the photocathode of the streak tube. The laser echo signal is converted into photoelectrons by the photocathode of streak tube. The electrons are accelerated by the electric field between the photocathode and the grid, and then go into the deflection electric field. The raw image of the target will be obtained after the electron bombardment on the screen. The streak tube uses a P43 fluorescent screen, the afterglow time is 1.2 ms, the raw image is coupled to the image intensifier by a light cone, and the enhanced image is collected by CCD. The system does not adopt slit at the photocathode, therefore the system can quickly obtain the 2D image of the target without changing the hardware. By determining the relative position of laser beam and the target, the time spent on searching and matching can be reduced. The image data processing subsystem is based on the LabVIEW platform on computer, which can realize the processing of the raw image in real-time. And the intensity image, range image and 3D image can be displayed in real time synchronized. The control subsystem is mainly composed of computer, signal generator card, precision delay device and power supply. The pulse frequency of the laser and the exposure time of the CCD are adjusted by the signal generating card which controlled by computer. A photodetector is used to monitor the time of laser emission and its output signal is also used as the time reference of the control circuit. After a precision delay with a minimum step length of 6 ns, the signal is sent to the gating device as the trigger signal. The power supply of the streak tube has two working states, static and sweep, at the same time, there are also two working modes of cathode voltage, continuous and triggered. In IG mode, image intensifier is the gating device and the photocathode works 80
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Optics Communications 432 (2019) 79–83
Fig. 3. Working status chart of photocathode and image intensifier in different gating modes. (a) Photocathode works in continuous mode and intensifier was triggered in IG mode, with enhanced image including target echo and disturb afterglow; (b) Photocathode was triggered and image intensifier works in continuous mode in CG mode.
Fig. 4. Experimental setup and the picture of the target. (a) Experimental setup of the STIL system by which two gating modes can be realized; (b) Close range picture of the target.
4. Experimental results and analysis
intensifier can work in the continuous mode. So the afterglow energy of the signal is all collected and amplified, the signal intensity is obviously increased. For the above reasons, we adopted CG mode to conduct range gating for the lidar system. Comparison experiments were carried out to compare the SNR of cathode gating mode and non-gating mode. The streak tube is in sweep condition and the image intensifier is in continuous mode. The experiment was carried out in the daytime, and the imaging results are shown in Figs. 7 and 8. Under daylight condition, the background light interference is strong. Although a narrow bandwidth filter is used in the system, the raw image tends to be saturated under static condition. Fig. 7(a) is the raw image obtained after removing the background light from the screen with sweep voltage in the non-gating mode. The laser signal expands along the horizontal axis, reflecting the 3D information of the target. Affected by the rising edge of sweep voltage, the interference signal is strong in the raw image. Fig. 7(b) is the raw image obtained in CG mode. Compared with Fig. 7(a) the interference signals caused by the rising edge are filtered out, the image noise is significantly reduced and the image clarity is improved. The intensity curve of the raw image can be obtained by vertically summing the pixel intensity of the raw image. The sum of the intensity values of different transverse coordinates is expressed as:
The developed STIL system was composed by two layers as shown in Fig. 4(a). The lower layer is composed of a laser, a streak tube power supply, a control module and an output optical subsystem. The upper layer is composed of the receiving optics subsystem, the streak tube, the image intensifier and the CCD. An experiment was carried out to image an outdoor target by this system, with a laser pulse frequency of 10 Hz, and a CCD exposure time of 1 ms. the target is a tall building with a distance of 2 km, the photo of the building was shown in Fig. 4(b). The raw images obtained by the lidar system with a delay time of 15 μs are shown in Fig. 5. Fig. 5(a) is the raw image in IG mode, and Fig. 5(b) is the raw image in CG mode, both images use a same range gate with a width of 5 μs. In IG mode, the system realizes the range gating by trigger image intensifier and intercepting streak tube fluorescent screen image. Limited by gate width, only part of the signal is collected and amplified by image intensifier, which reduces the intensity of the signal. Imaging experiments were carried out with different gate widths in IG mode, and the experimental results are shown in Fig. 6. The results show that IG mode has a certain impact on signal intensity, which affects the further improvement of SNR. With the increase of gate width, the intensity of raw image in IG mode will be improved. But as the width of the gate increases, the filtering ability of the system to the noise caused by the rising edge will becomes weaker. In CG mode, the range gating is accomplished by the streak tube photocathode, the interference caused by afterglow and the interference caused by rising edge are effectively avoided. Simultaneously, because all the image information comes from the range area, the image
𝑉𝑖 =
𝑛 ∑
𝐼𝑖𝑗
(4.1)
𝑗=1
where: 𝑉𝑖 is the sum of all pixel intensity values with a horizontal coordinate of i, 𝐼𝑖𝑗 is the strength value of the ij pixel. Then the SNR 81
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Fig. 5. Comparison of raw images in different gating modes. (a) The raw image in IG mode; (b) The raw image in CG mode with a same delay time and a same gate width of (a).
Fig. 6. Raw images with different gating width in IG mode. (a) Image with a gate width of 5 μs; (b) Image with a gate width of 10 μs; (c) Image with a gate width of 20 μs; (d) Image with a gate width of 30 μs.
Fig. 7. Imaging results of comparison experiment when the streak tube works in the sweep condition (a) The raw image in non-gating mode; (b) The raw image in CG mode; (c) The intensity accumulation figure of the raw image in non-gating mode; (d) The intensity accumulation figure of the raw image in CG mode.
of the raw image can be expressed as: 𝑆𝑁𝑅 = 20 lg
𝑉𝑠 𝑉𝑛
the target signal and the noise are similar, it is difficult to distinguish the boundary and surface characteristics of the target in the intensity image. Fig. 8(b) is the intensity image obtained in the CG mode. The image clarity was significantly improved and background noise was effectively filtered out. The image obtained by CG mode has a higher SNR, which provides a good foundation for image processing.
(4.2)
where 𝑉𝑠 is the signal strength, 𝑉𝑛 is the noise intensity. Fig. 7(c) was the intensity curve obtained by non-gating mode with a SNR of 4.2 dB and Fig. 7(d) was the intensity curve obtained by CG mode with a SNR of 16.8 dB. The image of the target was reconstructed by 600 frames, the intensity images of the target are shown in Fig. 8. Fig. 8(a) is the intensity image under non-gating mode. Due to the low SNR of the raw image, the information at the bottom of the target is lost, which affects the integrity of the image. Because the intensity of
5. Conclusion In this paper, a range gated STIL system based on photocathode gating is introduced, and a target detection experiment with a distance of 2 km is carried out by using the system. The experimental results show that, IG mode has a certain impact on signal intensity, under the same 82
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(a)
(b)
Fig. 8. Intensity image obtained by the lidar system in sweep condition (a) Intensity image under non-gating mode; (b) Intensity image under CG mode.
experimental condition, CG mode is more suitable for STIL range gating system. Range gating can effectively reduce the interference caused by the rising edge of sweep voltage. Compared with non-gating mode, CG mode can improve imaging quality effectively, and the SNR of the raw image is improved from 4.2 dB to 16.8 dB.
[7] J. Gao, J. Sun, Q. Wang, et al., 4-D imaging of the short scale ocean waves using a slit streak tube imaging Lidar, Optik 136 (2017) 136–143. [8] V.A. Banakh, I.A. Razenkov, Lidar measurements of atmospheric backscattering amplification, Opt. Spectrosc. 120 (2) (2016) 326–334. [9] J. Spinhirne, Lidar aerosol and cloud backscatter at 0.53, 1.06 and 1.54 microns, in: 29th AIAA Aerospace Sciences Meeting. 29th AIAA Aerospace Sciences Meeting, 2013. [10] J. Gao, J.F. Sun, Q. Wang, Research of effect of underwater backscattering on the detection of sea wave using a Slit Streak Tube Imaging Lidar, in: International Conference on Optoelectronics and Microelectronics, IEEE, 2013, pp. 151–154. [11] W. Jin, F. Cao, X. Wang, et al., Range-gated underwater laser imaging system based on intensified gate imaging technology, SPIE Int. Soc. Opt. Eng. 6621 (2008) 66210L-66210L-6. [12] Y. Shi, W. Jin, L. Li, The integrated platform of controlling and digital video processing for underwater range-gated laser imaging system, in: Selected Papers From Conferences of the Photoelectronic Technology Committee of the Chinese Society of Astronautics, International Society for Optics and Photonics, 2015, p. 95221R. [13] M. Chen, Z. He, F. Ao, Study and implementation for range-gated underwater laser imaging system, SPIE Int. Soc. Opt. Eng. 6625 (2008) 54. [14] W. Liang, H. Guo, W. Zhang, Simulation of signal-to-noise ratio for the laser rangegated imaging system, in: Applied Optics and Photonics China, International Society for Optics and Photonics, 2015, pp. 655–657. [15] W. Ge, Analysis on the influences of range-gated underwater laser imaging system parameters on the signal to noise ratio, in: International Symposium on Photoelectronic Detection & Imaging, 2013, p. 89050K. [16] Z. Tian, Z. Cui, L. Zhang, et al., Control and image processing for streak tube imaging lidar based on VB and, MATLAB, Chin. Opt. Lett. 12 (6) (2014) 67–70.
Acknowledgments The project is supported by the National Natural Science Foundation of China (61605033), (61505043); the Natural Science Foundation of Shandong Province, China (ZR2016FQ24); and Fundamental Research Funds for the Central Universities, China (Grant No. HIT.NSRIF.201719). References [1] K. Kinoshita, Y. Inagaki, Y. Ishihara, et al., Femtosecond synchroscan streak tube, Japan. J. Appl. Phys. 41 (41) (2002) 389–392. [2] G. Ye, R. Fan, Z. Chen, et al., Range accuracy analysis of streak tube imaging lidar systems, Opt. Commun. 360 (2016) 7–14. [3] C. Ma, S. Han, The research on the reconstruction of intensity image based on streak tube imaging lidar, Proc. SPIE Int. Soc. Opt. Eng. 7850 (1) (2010) 2280–2283. [4] J. Sun, Q. Wang, 4-D image reconstruction for streak tube imaging lidar, Laser Phys. 19 (3) (2009) 502–504. [5] J.W. Mclean, High-resolution 3D underwater imaging, Proc. SPIE Int. Soc. Opt. Eng. 3761 (1999) 10–19. [6] A.J. Nevis, J.S. Taylor, Advantages of three-dimensional electro-optic imaging sensors, SPIE Int. Soc. Opt. Eng. 5089 (2003).
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