KEYWORDS: Target detection, Signal to noise ratio, Scattering, Signal detection, Pulsed laser operation, Laser energy, Laser scattering, Light scattering, Monte Carlo methods, Backscatter
The characteristic of an incident light beam scattered from underwater bubble cluster was investigated and a Monte Carlo
model was employed to simulate the backscattered signal from bubbly wake by laser pulses. An optical ship wake
detecting experimental system was designed and built, in which there was a frequency doubling and Q-switched YAG
solid state laser adopted in the transmitter as the light source, and a telescope system with large limiting aperture and
small field-of-view(FOV) in the receiver. A photomultiplier tube was employed as the detector while simulated bubbly
ship as the detecting targets. The optimization of the system's parameters was studied in lab, the results showed that The
influence of laser pulse energy and the FOV size of receiving optical system on the detecting effect was studied by
experiments. The results showed that for near targets with small optical depth(ι ⪅1.4), the optimized Signal-to-Noise
Ratio(SNR) is obtained using a laser with single pulse energy of 60mJ~4;84mJ while for far targets with large optical
depth(ι⪆2.0), the optimized SNR is obtained using a laser with single pulse energy of 108mJ or above. Good detecting
effect is achieved using a receiving optical system with the FOV of 4°.
The traditional methods to detect underwater objects are based on the acoustic detecting techniques. However, since the
transmutation of the sound wave in ocean is often influenced by the oceanic boundary conditions and ambient factors
such as salinity, temperature and pressure of the sea water, acoustic detecting techniques will cause large error in
orientation and direction, which makes it difficult to capture and discriminate small objects. Instead, the photoelectric
detection technology for underwater objects has the advantages of precise direction and orientation due to the high
propagation velocity of light in water, consequently, the influence of ambient factors can be negligible. In this paper, we
described the theory of underwater photoelectric detection and the system structure. Two kinds of experimental system
are set up. One is imaging photoelectric detecting system; the other is non-imaging photoelectric detecting system. The
underwater photoelectric detecting systems are made up of a short pulsed laser, a received optical system, narrowband
optical filters, high-speed photoelectric detectors, a high-speed data acquisition and processing system etc. Both of the
imaging and non-imaging photoelectric systems are employed to detect the reflection of target plane and simulated ship
wakes. The key factors that affect the detection performance are analyzed in the paper. The experimental results show
that for the imaging system, utilizing range-gated techniques can decrease the interference from background optical
noise, while for the non-imaging system, using combining filters can suppress the backscattered optical noise from water.
The conclusion is that both imaging and non-imaging photoelectric detection system are suitable for detecting
underwater objects or their wakes.
A ship wake in the ocean contains lots of microsize air bubbles with diameter of 10μm~200μm. When a beam of light
propagates through the ship wake, it leads to attenuation of the light energy and change of its spatial distribution due to
absorption and scattering caused by ocean water and bubble mass together, but they don't share the same scattering
mechanism, leaving absorption out of account. Scattering from micro bubbles can usually be explained by Mie scattering
theory. In this paper, a method based on optical scattering theory is proposed for detecting the target of ship wake in the
ocean, and an experimental system is built up according to the Lambert-Beer law. In the experiment, a semiconductor
laser with wavelength about 532nm was employed as the light source, an experimental device which modeled the target
of a ship wake to generate bubble mass in water, and a high-speed PIN photodiode as the detector. Besides, the optical
receiving system included a set of optical lens and apertures, which were employed to limit the field of view(FOV) of the
receiver detector. After measurement of the small angle forward scattering rays scattered from bubble mass with a small
FOV, the target signals was got and processed. Then adjustments were made to the optical receiving system and
experiments were implemented with varying the FOV to find out the influence of FOV on target signal SNR and obtain
the optimum FOV. The results showed that adjusting the FOV according to different microbubble size density could
improve SNR.
Unlike optical detection of air targets, detecting underwater targets by optical technique is more complicated because of
strong backscattering and absorption attenuation caused by water itself while rays propagating underwater. If a pulsed
laser is employed as the active illuminator of an underwater detection system, it's possible to decrease the effect of
backscattering light from water to a large degree and improve detection distance of the system consequently. In this
paper, a set of underwater optical detection system is designed. This system can be used to detect underwater targets or
micro bubbles in water. In order to optimize the system's performance, different photoelectric detectors are used in the
system and contrastive experiments are implemented. A high-speed PMT and a high-speed PIN photodiode are
employed without adjusting other system components. Accordingly, optical receiving systems and amplification circuits
were designed, and a series of contrastive experiments were implemented. In these experiments, a board and micro
bubbles mass works as targets which are placed in changing distance to detection system. By analyzing of the
experimental results, it is concluded that both of the PMT and PIN detector can detect the reflected signal of board and
micro bubbles effectively in a certain range. And they can be used in different applications because of their different
characteristics.
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