A method for underwater target identification based on laser complex amplitude measurement in wake area

Abstract

The present invention provides an underwater target identification method based on the laser complex amplitude measurement in the wake area. The present invention uses wake turbulence as a new detection parameter, introduces the Hartmann-Shack wavefront sensing technology to measure the complex amplitude information of the laser transmitted in the wake turbulence, and realizes wake identification by acquiring the power spectrum characteristics of the laser phase fluctuation. The present invention makes unified use of the intensity information and phase information, spatial characteristics and temporal characteristics of the laser after transmission in the wake, and gives full play to the advantages of the long duration and long propagation distance of the internal waves and turbulence in the hydrodynamic wake, breaking through the limitations of the traditional method of using the bubble curtain as a single detection parameter, such as short identification distance and large noise, and realizing the breakthrough of laser wake identification technology from traditional "intensity" identification to more accurate "complex amplitude" identification, and accurately grasping the approximate relationship between the wake physical parameters and the motion state of the underwater target.

Description

Technical Field

[0001] The present invention provides a new method for optical detection of underwater vehicles, and specifically relates to an underwater target identification method based on laser complex amplitude measurement in the wake region, which detects underwater vehicles based on the complex amplitude parameters of laser transmitted in the wake of the underwater vehicle. Background Art

[0002] With the development of acoustic noise reduction technology, surface acoustic wave noise reduction technology can make the reflected sound waves of underwater vehicles close to the background noise of the ocean itself, which has brought great difficulties to the traditional method of sonar submarine detection, and new detection methods are urgently needed. Researchers have analyzed and found that when a ship is sailing in the water, the rotational cavitation of the propeller and the breaking of the waves on the sea surface will cause a wake containing a large number of bubbles and turbulence to form at the stern. Since the wake is inevitable and has significantly different physical properties from the surrounding seawater, underwater vehicle detection based on the wake can reduce the effectiveness or even ineffectiveness of traditional acoustic interference and stealth methods, which is of great significance for ensuring marine safety. Although the current wake detection is dominated by acoustic detection, the accuracy of acoustic wave detection is low and it is easily affected by acoustic interference. Compared with acoustic wave detection, optical detection has the advantages of short wavelength, fast propagation speed, good directionality, and immunity to acoustic interference. It is a more promising means of underwater vehicle detection.

[0003] At present, the optical detection methods of wake can be mainly classified into three categories: continuous laser scattering intensity detection, pulse echo detection and polarization detection. All three methods take the bubbles in the wake as the detection target and detect the wake by detecting the scattered light intensity when the laser propagates in the wake area. However, relying on bubble scattered light signals to achieve wake identification inevitably faces many difficulties in principle: first, the size of bubbles in the wake is mostly millimeter-level, resulting in weak reflected signals and significant interference from water noise, which seriously affects the detection accuracy and detection distance of the instrument; second, for underwater vehicles, if the diving depth is deep, the time for bubbles to exist and dissipate in the water will be very short, so the underwater vehicle can hide bubbles by changing the speed and navigation depth, thereby reducing the probability of the instrument accurately detecting the underwater vehicle; finally, because the scattering characteristics of bubbles to incident light are related to the particle size and relative refractive index of the bubbles, using bubble curtains as detection parameters requires in-depth research on the scattering characteristics of bubble groups. However, this problem involves a large number of bubbles of different scales and is very complicated. At present, a reliable mathematical model and a complete theoretical system have not been established, resulting in the difficulty of exploring the mechanism of wake optical technology. Therefore, using bubbles as detection targets has led to short optical wake detection distance, low sensitivity, low signal-to-noise ratio, and difficulty in real application, and it is urgent to make a breakthrough in the exploration mechanism.

[0004] Fortunately, in real seawater with density stratification, the wake characteristics include not only bubble curtains but also turbulence. The thermodynamic state parameters of the real ocean, such as density, temperature, and salinity, vary significantly with depth, and often present a macroscopic hierarchical structure in the vertical direction. The jet generated by the propeller of the underwater vehicle will destroy the hierarchical structure of the seawater, thereby causing internal seawater oscillations to generate turbulence. Compared with the bubble curtain, turbulence has the following advantages: 1) Compared with bubbles, the turbulence scale is large. Related studies show that the turbulence and other disturbances generated by the movement of large underwater submersible bodies can still form a wake wave height of 12 to 55 cm about 1 km behind the boat, which is significantly superior to millimeter-level bubbles. Using this as a detection parameter is expected to improve the signal-to-noise ratio of the wake signal; 2) Compared with bubbles, turbulent wakes last longer, propagate farther, and are difficult to eliminate. It has been reported that under sea conditions of level 3 to 4, submarine turbulence can persist on the sea surface for more than 1 to 1.5 hours. This provides a larger time window for wake detection, which helps to improve the success rate of detection.

[0005] Our analysis found that the turbulence in the wake will change the refractive index of the water medium, causing the laser to change during transmission, resulting in irregular refraction, which changes the wavefront characteristics of the laser after transmission inside and outside the wake, causing the following Figure 2 The different wavefront aberrations of different orders are shown. According to the Huygens-Fresnel principle, the wavefront function uniquely determines the light field, and reproducing the wavefront will reproduce all the observation results. Therefore, based on the wavefront characteristics of the laser in the wake area, the wake turbulence encountered during the laser transmission process can be reproduced to realize the identification of the wake.

[0006] The wavefront characteristics describe the spatial distribution of the light field when the laser is transmitted in turbulence, which can be expressed by the complex amplitude parameter:

[0007]

[0008] Where P represents the point (x, y, z), a(P) reflects the spatial distribution of the amplitude, Reflects the spatial distribution of the phase. It can be seen from formula (1-1) that the complete information carried by the wake reflected light signal includes amplitude and phase. However, the traditional bubble detection method only uses the intensity signal and fails to realize the joint detection of amplitude and phase. Therefore, the present invention proposes to use the complex amplitude of the transmitted laser to realize wake identification. The superiority of this parameter is reflected in: the complex amplitude combines two parameters of different properties in the light field, which is a natural coupling of the intensity and phase of the light signal. It is essentially a multi-parameter target feature, which can provide more information and new observation angles, and is of great significance for improving the signal-to-noise ratio, detection accuracy and anti-interference performance of wake detection.

[0009] Hartmann-Shack wavefront sensing technology (HS) is a simple and efficient technology for measuring the complex amplitude information of a beam. Figure 3 As shown. The microlens array divides the incident light beam to be measured into sub-apertures and converges them on the photodetector to form a spot array image. After the image is collected by the photodetector placed on the focal plane of the microlens, the wavefront restoration algorithm can reconstruct the spatial distribution of the wavefront distortion to be measured within the full aperture by calculating the centroid position offset data of each sub-spot.

[0010] This technology has been widely used in the fields of optical detection, laser beam purification, laser communication, astronomical observation, etc. Therefore, the present invention selects HS technology to detect the complex amplitude of the laser transmitted in the wake, extract its spatial spectrum information, and extract the time spectrum of the wavefront changing with time from the spatial spectrum, so as to unify the intensity information and phase information, spatial characteristics and time characteristics of the laser after transmission in the wake, and realize the breakthrough of laser wake detection technology from traditional "intensity" identification to more accurate "complex amplitude" identification. Summary of the invention

[0011] The technical problem to be solved by the present invention is that the current optical wake detection using bubbles as the detection target results in a short distance, low sensitivity, low signal-to-noise ratio, and difficulty in actual application, and a breakthrough in the exploration mechanism is urgently needed.

[0012] The technical solution adopted by the present invention to solve its technical problem is: the present invention proposes to select the Hartmann technology in the wake to detect the complex amplitude of the laser transmitted in the wake, extract its spatial spectrum information, and extract the time spectrum of the wavefront changing with time from the spatial spectrum, so as to unify the intensity information and phase information, spatial characteristics and time characteristics of the laser after transmission in the wake, and realize the breakthrough of laser wake detection technology from traditional "intensity" identification to more accurate "complex amplitude" identification.

[0013] The specific technical solutions are as follows:

[0014] A method for underwater target recognition based on laser complex amplitude measurement in the wake region, such as Figure 1 As shown, the method comprises the following steps:

[0015] Step (1), the laser radar emits laser to the sea surface;

[0016] Step (2), using a Hartmann-Shack sensor to receive the laser reflected from the sea surface: the reflected light is received by a photodetector after passing through a microlens array to form a spot image, and based on the image, a Zernike mode wavefront restoration method is used to obtain wavefront information and derive the complex amplitude, thereby obtaining the spatial spectrum of the complex amplitude after the laser is transmitted in the wake area;

[0017] Step (3), analyzing the variation of the complex amplitude of the laser after it is transmitted in the wake region with time, and summarizing the time spectrum;

[0018] Step (4) realizes wake identification by establishing an approximate correspondence between the spatiotemporal characteristics of the complex amplitude of the transmitted laser and the wake distribution.

[0019] Furthermore, the Hartmann-Shack sensor is used to detect the complex amplitude of the transmitted laser in the wake, so that the spatial spectrum information of the transmitted laser can be extracted. By summarizing the time law, the time spectrum of the complex amplitude of the transmitted laser in the wake that changes with time can be extracted, thereby achieving the simultaneous acquisition of the spatiotemporal characteristics of the wake.

[0020] Furthermore, before and after the laser wave is restored based on the Zernike mode wavefront restoration method, the power spectral density curve of each order is used to represent the changes in the spatiotemporal characteristics of each order of aberrations. The power spectrum curve is fitted using the nonlinear least squares method to extract the parameters of each order of power spectrum curve containing the laser transmission characteristics in the wake, so as to achieve the purpose of mastering the modulation characteristics of the wake turbulence on the transmitted laser, and use this as the basis for identifying the wake.

[0021] Compared with the prior art, the present invention has the following advantages: the present invention proposes a new wake detection parameter, that is, the complex amplitude information of the transmitted laser is a natural coupling of intensity and phase, which has obvious advantages over wake bubbles; the HS technology to be adopted in the present invention can accurately and in real time grasp the complex amplitude changes during laser transmission. The present invention can derive the modulation effect of the underwater wake on the spatiotemporal characteristics of the laser, and use this as the basis for identification to achieve the best observation of the wake of the underwater vehicle and the inversion of its motion state. The present invention comprehensively utilizes the phase and intensity characteristics of the laser when it is transmitted in the wake area, and provides a new multi-parameter wake identification method, which helps to improve the detection accuracy, range and anti-scattering noise capability of optical wake detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 A schematic diagram of the system structure of the present invention;

[0023] Figure 2 This is a schematic diagram of the installation and use of the present invention on a drone;

[0024] Figure 3 It is a power spectrum density curve of a certain order aberration change;

[0025] Figure 4 This is the power spectral density curve fitting result diagram. DETAILED DESCRIPTION

[0026] The specific embodiments of the present invention are as follows Figure 1 , 2 As shown, the whole set of equipment consists of a set of laser radar, Hartmann sensor and corresponding processor. The underwater target identification method based on the laser complex amplitude measurement in the wake area of ​​the present invention comprises the following steps:

[0027] Step (1), the laser radar emits laser to the sea surface;

[0028] Step (2), using a Hartmann-Shack sensor to receive the light signal reflected by the sea surface waves of the wake. The laser reflected from the sea surface is received by a photodetector after passing through a microlens array to form a spot image. Through this image, the wavefront is obtained using the Zernike mode wavefront restoration method, and the spatial spectrum of the complex amplitude after the laser is transmitted in the wake area is further derived. In this use example, by detecting the slope of each sub-aperture wavefront, a multivariate linear equation system can be established based on the relationship between the slope and the two-dimensional surface to obtain the coefficients of each order basis function, restore the distorted wavefront, and obtain the spatial characteristics of the transmitted laser;

[0029] Step (3), analyze the variation of the complex amplitude of the laser after it is transmitted in the wake region with time, and summarize the time spectrum. In this use case, the statistical characteristics of the sub-aperture wavefront slope in the time domain reflect the variation of the wavefront aberration in this specific time period. Figure 3 As shown, the change of the aberration's temporal and spatial characteristics can be represented by the power spectral density curve of each order:

[0030] Step (4) is to achieve wake identification by establishing an approximate correspondence between the spatiotemporal characteristics of the complex amplitude of the transmitted laser and the wake distribution. The transmission characteristics of the laser in the wake can be constructed by analyzing each order of the power spectrum density curve. In this use case, the autoregressive model parameters are obtained by fitting the power spectrum density curve using the nonlinear least squares method. The power spectrum fitting results based on the obtained autoregressive model parameters are plotted together with the actual power spectrum curve. Figure 4 It can be seen that the fitting degree is good, which proves that this method can well realize the extraction of the characteristics of the complex amplitude power spectrum density curve of the transmitted laser, such as Figure 4 This step realizes the extraction of the spatiotemporal variation characteristics of the complex amplitude of the transmitted laser.

Claims

1. A method for underwater target identification based on laser complex amplitude measurement in the wake region, characterized in that: The method comprises the following steps: Step (1), the laser radar emits laser to the sea surface; Step (2), using a Hartmann-Shack sensor to receive the laser reflected from the sea surface: the reflected light is received by a photodetector after passing through a microlens array to form a light spot image; based on the image, a Zernike mode wavefront restoration method is used to obtain wavefront information to derive complex amplitude information, thereby obtaining a spatial spectrum of the complex amplitude after the laser is transmitted in the wake region; Step (3), analyzing the variation of the complex amplitude of the laser with time after the laser is transmitted in the wake region, and obtaining the time spectrum of the complex amplitude of the laser; Step (4), by establishing an approximate correspondence between the time spectrum and spatial spectrum of the complex amplitude of the transmitted laser and the wake distribution, wake identification is achieved; In step (4), the power spectrum density curve of each order is used to represent the change of the spatiotemporal characteristics of the complex amplitude, and the power spectrum curve is fitted using the nonlinear least squares method; the parameters of each order power spectrum curve when the laser is transmitted in the wake are extracted by combining the autoregressive model and the least squares method, so as to understand the modulation characteristics of the wake turbulence on the transmitted laser, which is used as the basis for identifying the wake.

2. The underwater target identification method based on the wake area laser complex amplitude measurement according to claim 1 is characterized by: In steps (2) and (3), a Hartmann-Shack sensor is used to detect the wavefront information of the laser transmitted in the wake, thereby achieving simultaneous extraction of the spatial spectrum and the temporal spectrum of the complex amplitude of the laser transmitted in the wake.

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