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Experiments of ocean surface waves and underwater target detection imaging using a slit Streak Tube Imaging Lidar
Optik 125 (2014) 5199–5201
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Experiments of ocean surface waves and underwater target detection imaging using a slit Streak Tube Imaging Lidar Jian Gao a,∗ , Jianfeng Sun a , Qi Wang a,b a b
National Key Institute of Tunable Laser Technology, Harbin Institute of Technology, Harbin 150001, China Harbin Institute of Technology, Weihai 264200, China
a r t i c l e
i n f o
Article history: Received 25 November 2013 Accepted 5 May 2014 Keywords: Streak tube imaging lidar Imaging Lidar Ocean surface imaging Underwater detection
a b s t r a c t This letter describes a flash Lidar imaging technique that uses a streak tube camera as a receiver to detect short scale surface wave fields for oceanographic, underwater target and coastal measurements. The range and intensity imaging of short scale sea-waves and underwater targets can be supplied by this system through light reflected from the surface of the sea and targets. Through a surface scattering model, more accurate range and intensity information can be extracted. The technique can be applied to surface wave measurements and underwater target detection with high resolution. The feasibility of this technique is demonstrated by applying the technique to data acquired by shipborne Streak Tube Imaging Lidar (STIL) in the Yellow Sea and also the East and South China Seas. © 2014 Elsevier GmbH. All rights reserved.
The ocean surface wave field contains much information related to ocean currents, underwater topography, sea winds and so on [1]. The measurement of sea waves on the ocean surface and underwater targets also plays an important role in Maritime navigation safety as recognized by many developed countries [2]. Streak Tube Imaging Lidar (STIL) has now successfully demonstrated its capacity to uniquely provide valuable high resolution information for land (high-resolution 3D building imaging, land mapping) and coastal applications (underwater vehicle, ship detection, shallowwater bathymetry mapping, high-resolution wind fields, coastal wave fields measurement, underwater target detection) [3]. Two sets of flash Lidars use the streak tube camera as a receiver as shown in Fig. 1. The ability for the measurement of ocean surface waves and underwater targets was developed by the National Key Institute of Tunable Laser Technology, Harbin Institute of Technology. STIL is a promising imaging system with high frame rates and spatial resolution [4,7]. The receiver is a streak tub camera which is an instantaneous optical device. The reflected light from the ocean surface or the underwater target is collected onto a slit in front of the streak tube photocathode by a conventional lens, and the time (range) is resolved by an electrostatic sweep within the streak tube, thus generating a 2-D range-azimuth image on each laser pulse
∗ Corresponding author at: No. 2, Yikuang Street, Nangang Area, Harbin City, Heilongjiang Province, China. Tel.: +86 18545883007; fax: +86 0451 86402920. E-mail address: [email protected] (J. Gao). http://dx.doi.org/10.1016/j.ijleo.2014.05.005 0030-4026/© 2014 Elsevier GmbH. All rights reserved.
[5,6], as shown in Fig. 2. By orienting the fan beam to the sea from the bow of a ship, the along-track dimension is sampled by adjusting the Pulse Repetition Frequency (PRF) of the laser to the forward speed of the ship, thus sweeping out the 3D ocean surface and the intensity imaging in a pushbroom fashion. Fig. 3 shows the STIL system mounted on a ship. The unique feature of this Lidar is its high frame rates and capability for range and intensity imaging with high resolution. This is carried out by using a Nd:YAG laser with a repetition rate tunable up to 100 Hz. Considering the energy of one pulse and the limitation of data acquisition, the laser is operated at 100 Hz (Table 1), yielding an energy of one pulse of 20/50 mJ. Consequently, enough laser shots and a slit streak tube camera can be integrated at each direction during the measurement. During the experiment in the Yellow Sea in 2010, STIL was located on the bow of ship near Xiaoshi Island (37.50◦ N, 122.17◦ E) and measured the sea surface wave field. Shown in Fig. 4(A), there is the 3D imaging of the ocean surface wave field. The color shows the wave height of sea waves (−2 m to 2 m) and the range resolution of 0.05 m. The system also has the ability to detect short scale surface waves (wave height is about 0.2 m) which are caused by underwater vehicles, the fish, sea winds etc. Therefore, we can retrieve wind speed and distinguish whether there is a vehicle underwater through the detection of short scale waves by STIL. Fig. 4(B) shows the intensity imaging of the ocean surface wave field. The gray level shows the intensity of the reflected light by the ocean surface. It can help to distinguish floating objects on the ocean surface, such as foam, oil slicks, sea ice and so on.
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J. Gao et al. / Optik 125 (2014) 5199–5201
Fig. 1. The Streak Tube Imaging Lidar. Fig. 4. The imaging results of ocean surface waves field.
Fig. 2. The operational principle of STIL.
Fig. 5. The imaging result of ship wake flow field. Fig. 3. The STIL imaging system mounted on a ship.
During the experiment in the Bohai Sea in 2011, STIL was located on the ship stern near the city of Xingcheng (40.50◦ N, 120.50◦ E) and measured the ship wake flow. Fig. 5(A) shows the range imaging of the ship wake flow. The color represents the wave height of the wake flow field. Compared with the range imaging, the intensity imaging is more obvious as is shown in Fig. 5(B). Obviously, the foam on the ship wake flow has a high reflectance. It can be helpful to analysis the wake flow field in order to optimize the parameters of the propeller and engine. During the experiments of underwater detection in the Yellow Sea and the East and South China Seas in 2010–2012, STIL was located near the five areas shown on the map presented below
(Fig. 6). For the same target, the effects on the detecting depth are the attenuation coefficient of seawater and the sea state [8]. For a weak sea state, the relationship between the detecting depth and the attenuation coefficient was analyzed (Fig. 7). There is a clean water reservoir in Location A which has the minimum attenuation coefficient. As is shown in Fig. 8(A) and (B) for this location the range and intensity imaging of the target at a depth of 24 m, was achieved when the laser pulse energy was 20 mJ. Fig. 8 (C) and (D) shows the imaging result of targets with
Table 1 Main parameters of the lidar system. Transmitter Wavelength Repetition rate Pulse energy Pulse width Beam vertical divergence Beam horizontal divergence (beam expanded)
532 nm 0–100 Hz 20/50 mJ 8 ns 0.3◦ 45◦
Receiver Field of view Telescope aperture Range resolution Azimuth resolution
45◦ 200 mm 0.05 m 0.1◦
Fig. 6. Map of experiments locations.
J. Gao et al. / Optik 125 (2014) 5199–5201
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Fig. 9. The imaging result of underwater target at location B.
a depth of 5 m. Through the intensity imaging, we can distinguish the characters on the target. Location B is in the South China Sea. The sea water quality is better. And the sea water transparency is 15 m. The maximum detecting depth is 18 m, when the laser pulse energy is 50 mJ. The imaging quality gets worse because of the effects of a rough sea surface caused by sea wind (Fig. 9). Through the intensity imaging, it is not possible to recognize the target but the target can be distinguished in the range imaging. In conclusion, a new flash Lidar based on the streak tube detector was presented for coastal applications (measurements of ocean surface wave field and underwater targets). The Lidar imaging with high frame rates and resolution worked well with both the ocean surface waves and ship wake flow fields. This system can successfully be applied to keep Naval channels safety by detecting underwater obstructions. It also has a great potential for ocean remote, sensing, searching and rescuing on the sea, and also detecting submarines. Fig. 7. The relationship between the detecting depth and attenuation coefficient of sea water.
Acknowledgment This work is funded by National Natural Science Foundation for Young Scholars (60901046). References [1] W. Alpers, Theory of radar imaging of internal waves, Nature 314 (March (6008)) (1985) 245–247. [2] V. Kerbaol, F. Collard, SAR-derived coastal and marine applications: from research to operational products, IEEE J. Ocean Eng. 30 (July (3)) (2005) 472–486. [3] J. Sun, Q. Wang, 4-D image reconstruction for streak tube imaging lidar, Laser Phys. 19 (3) (2009) 502–504. [4] J. Liu, Q. Wang, S. Li, et al., Research on a flash imaging Lidar based on a multiplestreak tube, Laser Phys. 19 (1) (2009) 115–120. [5] W. Jinsong, C. Yuanli, X. Qiang, Imaging by single-slit streak tube laser Lidar, Chin. J. Lasers 35 (4) (2008) 496–500. [6] H. Yang, L. Wu, X. Wang, et al., Signal-to-noise performance analysis of streak tube imaging lidar systems. I. Cascaded model, Appl. Opt. 51 (36) (2012) 8825–8835. [7] S. Li, Q. Wang, J. Liu, Research of range resolution of streak tube imaging system, SPIE 6279 (2007) 1493–1495. [8] D.L. Waldron, L. Mullen, Underwater optical ranging: a hybrid LIDAR-RADAR approach, IEEE (2009).
Fig. 8. The imaging result of underwater target at location A.
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Experiments of ocean surface waves and underwater target detection imaging using a slit Streak Tube Imaging Lidar Jian Gao a,∗ , Jianfeng Sun a , Qi Wang a,b a b
National Key Institute of Tunable Laser Technology, Harbin Institute of Technology, Harbin 150001, China Harbin Institute of Technology, Weihai 264200, China
a r t i c l e
i n f o
Article history: Received 25 November 2013 Accepted 5 May 2014 Keywords: Streak tube imaging lidar Imaging Lidar Ocean surface imaging Underwater detection
a b s t r a c t This letter describes a flash Lidar imaging technique that uses a streak tube camera as a receiver to detect short scale surface wave fields for oceanographic, underwater target and coastal measurements. The range and intensity imaging of short scale sea-waves and underwater targets can be supplied by this system through light reflected from the surface of the sea and targets. Through a surface scattering model, more accurate range and intensity information can be extracted. The technique can be applied to surface wave measurements and underwater target detection with high resolution. The feasibility of this technique is demonstrated by applying the technique to data acquired by shipborne Streak Tube Imaging Lidar (STIL) in the Yellow Sea and also the East and South China Seas. © 2014 Elsevier GmbH. All rights reserved.
The ocean surface wave field contains much information related to ocean currents, underwater topography, sea winds and so on [1]. The measurement of sea waves on the ocean surface and underwater targets also plays an important role in Maritime navigation safety as recognized by many developed countries [2]. Streak Tube Imaging Lidar (STIL) has now successfully demonstrated its capacity to uniquely provide valuable high resolution information for land (high-resolution 3D building imaging, land mapping) and coastal applications (underwater vehicle, ship detection, shallowwater bathymetry mapping, high-resolution wind fields, coastal wave fields measurement, underwater target detection) [3]. Two sets of flash Lidars use the streak tube camera as a receiver as shown in Fig. 1. The ability for the measurement of ocean surface waves and underwater targets was developed by the National Key Institute of Tunable Laser Technology, Harbin Institute of Technology. STIL is a promising imaging system with high frame rates and spatial resolution [4,7]. The receiver is a streak tub camera which is an instantaneous optical device. The reflected light from the ocean surface or the underwater target is collected onto a slit in front of the streak tube photocathode by a conventional lens, and the time (range) is resolved by an electrostatic sweep within the streak tube, thus generating a 2-D range-azimuth image on each laser pulse
∗ Corresponding author at: No. 2, Yikuang Street, Nangang Area, Harbin City, Heilongjiang Province, China. Tel.: +86 18545883007; fax: +86 0451 86402920. E-mail address: [email protected] (J. Gao). http://dx.doi.org/10.1016/j.ijleo.2014.05.005 0030-4026/© 2014 Elsevier GmbH. All rights reserved.
[5,6], as shown in Fig. 2. By orienting the fan beam to the sea from the bow of a ship, the along-track dimension is sampled by adjusting the Pulse Repetition Frequency (PRF) of the laser to the forward speed of the ship, thus sweeping out the 3D ocean surface and the intensity imaging in a pushbroom fashion. Fig. 3 shows the STIL system mounted on a ship. The unique feature of this Lidar is its high frame rates and capability for range and intensity imaging with high resolution. This is carried out by using a Nd:YAG laser with a repetition rate tunable up to 100 Hz. Considering the energy of one pulse and the limitation of data acquisition, the laser is operated at 100 Hz (Table 1), yielding an energy of one pulse of 20/50 mJ. Consequently, enough laser shots and a slit streak tube camera can be integrated at each direction during the measurement. During the experiment in the Yellow Sea in 2010, STIL was located on the bow of ship near Xiaoshi Island (37.50◦ N, 122.17◦ E) and measured the sea surface wave field. Shown in Fig. 4(A), there is the 3D imaging of the ocean surface wave field. The color shows the wave height of sea waves (−2 m to 2 m) and the range resolution of 0.05 m. The system also has the ability to detect short scale surface waves (wave height is about 0.2 m) which are caused by underwater vehicles, the fish, sea winds etc. Therefore, we can retrieve wind speed and distinguish whether there is a vehicle underwater through the detection of short scale waves by STIL. Fig. 4(B) shows the intensity imaging of the ocean surface wave field. The gray level shows the intensity of the reflected light by the ocean surface. It can help to distinguish floating objects on the ocean surface, such as foam, oil slicks, sea ice and so on.
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J. Gao et al. / Optik 125 (2014) 5199–5201
Fig. 1. The Streak Tube Imaging Lidar. Fig. 4. The imaging results of ocean surface waves field.
Fig. 2. The operational principle of STIL.
Fig. 5. The imaging result of ship wake flow field. Fig. 3. The STIL imaging system mounted on a ship.
During the experiment in the Bohai Sea in 2011, STIL was located on the ship stern near the city of Xingcheng (40.50◦ N, 120.50◦ E) and measured the ship wake flow. Fig. 5(A) shows the range imaging of the ship wake flow. The color represents the wave height of the wake flow field. Compared with the range imaging, the intensity imaging is more obvious as is shown in Fig. 5(B). Obviously, the foam on the ship wake flow has a high reflectance. It can be helpful to analysis the wake flow field in order to optimize the parameters of the propeller and engine. During the experiments of underwater detection in the Yellow Sea and the East and South China Seas in 2010–2012, STIL was located near the five areas shown on the map presented below
(Fig. 6). For the same target, the effects on the detecting depth are the attenuation coefficient of seawater and the sea state [8]. For a weak sea state, the relationship between the detecting depth and the attenuation coefficient was analyzed (Fig. 7). There is a clean water reservoir in Location A which has the minimum attenuation coefficient. As is shown in Fig. 8(A) and (B) for this location the range and intensity imaging of the target at a depth of 24 m, was achieved when the laser pulse energy was 20 mJ. Fig. 8 (C) and (D) shows the imaging result of targets with
Table 1 Main parameters of the lidar system. Transmitter Wavelength Repetition rate Pulse energy Pulse width Beam vertical divergence Beam horizontal divergence (beam expanded)
532 nm 0–100 Hz 20/50 mJ 8 ns 0.3◦ 45◦
Receiver Field of view Telescope aperture Range resolution Azimuth resolution
45◦ 200 mm 0.05 m 0.1◦
Fig. 6. Map of experiments locations.
J. Gao et al. / Optik 125 (2014) 5199–5201
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Fig. 9. The imaging result of underwater target at location B.
a depth of 5 m. Through the intensity imaging, we can distinguish the characters on the target. Location B is in the South China Sea. The sea water quality is better. And the sea water transparency is 15 m. The maximum detecting depth is 18 m, when the laser pulse energy is 50 mJ. The imaging quality gets worse because of the effects of a rough sea surface caused by sea wind (Fig. 9). Through the intensity imaging, it is not possible to recognize the target but the target can be distinguished in the range imaging. In conclusion, a new flash Lidar based on the streak tube detector was presented for coastal applications (measurements of ocean surface wave field and underwater targets). The Lidar imaging with high frame rates and resolution worked well with both the ocean surface waves and ship wake flow fields. This system can successfully be applied to keep Naval channels safety by detecting underwater obstructions. It also has a great potential for ocean remote, sensing, searching and rescuing on the sea, and also detecting submarines. Fig. 7. The relationship between the detecting depth and attenuation coefficient of sea water.
Acknowledgment This work is funded by National Natural Science Foundation for Young Scholars (60901046). References [1] W. Alpers, Theory of radar imaging of internal waves, Nature 314 (March (6008)) (1985) 245–247. [2] V. Kerbaol, F. Collard, SAR-derived coastal and marine applications: from research to operational products, IEEE J. Ocean Eng. 30 (July (3)) (2005) 472–486. [3] J. Sun, Q. Wang, 4-D image reconstruction for streak tube imaging lidar, Laser Phys. 19 (3) (2009) 502–504. [4] J. Liu, Q. Wang, S. Li, et al., Research on a flash imaging Lidar based on a multiplestreak tube, Laser Phys. 19 (1) (2009) 115–120. [5] W. Jinsong, C. Yuanli, X. Qiang, Imaging by single-slit streak tube laser Lidar, Chin. J. Lasers 35 (4) (2008) 496–500. [6] H. Yang, L. Wu, X. Wang, et al., Signal-to-noise performance analysis of streak tube imaging lidar systems. I. Cascaded model, Appl. Opt. 51 (36) (2012) 8825–8835. [7] S. Li, Q. Wang, J. Liu, Research of range resolution of streak tube imaging system, SPIE 6279 (2007) 1493–1495. [8] D.L. Waldron, L. Mullen, Underwater optical ranging: a hybrid LIDAR-RADAR approach, IEEE (2009).
Fig. 8. The imaging result of underwater target at location A.