An underwater target imaging device based on correlation imaging

Abstract

An underwater target imaging device based on correlation imaging includes a transmitting system, a receiving system and a control and computing system placed in a sealed housing; the transmitting system uses laser to irradiate rotating frosted glass to generate pyrophoric light; the receiving system includes a reference arm CCD speckle acquisition module and an object arm gated PMT bucket detection module, and the backscattering is effectively filtered out through the gating function; the speckle image acquired by the reference arm is enhanced by convolution operation, and then the bucket detection value acquired by the object arm and the reference arm speckle are reconstructed and calculated by using a compressed sensing algorithm, and finally the reconstructed DC noise caused by the backscattering of the water body near the target is subtracted. The underwater target is detected by using pyrophoric light correlation imaging, which has a higher transmitting power, and finally the speckle enhancement image denoising is performed in the reconstructed target image, which improves the imaging quality and better applies the pyrophoric light intensity correlation imaging to the detection of underwater targets.

Description

Technical Field

[0001] The present invention relates to the technical field of underwater target detection and imaging, and in particular to an underwater target imaging device based on correlation imaging. Background Art

[0002] Marine science and technology is the primary productive force in the process of marine development and an important means of tapping the marine economy. The detection and tracking of underwater targets are valued by professional researchers because they have both commercial and military value. At present, the detection technology of underwater targets is mainly monopolized by sonar technology. Since water is a better propagation medium for sound waves, and the attenuation of sound waves propagating in liquid media is small, these advantages make sonar technology have a certain technical status. However, sonar technology is also limited by the ocean acoustic environment. The complexity of the ocean acoustic environment will cause the propagation of sound waves to be distorted, especially in offshore areas. Due to factors such as bottom reflection and multipath reflection, the echo signal is complex and difficult to distinguish the target signal.

[0003] In this context, research on new underwater imaging technology and target detection is even more important for the development and utilization of marine resources. Underwater optical imaging has advantages that cannot be ignored by acoustic detection, such as high resolution and relatively little environmental interference. It has been widely used in many fields such as underwater target recognition and underwater tracking. Although the current underwater optical imaging, such as ICCD imaging, has high imaging resolution, it has high requirements on the echo signal strength of the target. Due to the strong attenuation of the water body and the interference of underwater scattered light, the imaging distance is greatly shortened, which seriously affects the ICCD imaging performance. Compared with ICCD imaging devices, the imaging distance of underwater point scanning laser radar is greatly improved, but due to the imaging method based on point scanning, the imaging resolution is not high, and it cannot image moving targets. It can only be used for seabed static target mapping. Correlation imaging is divided into two paths, the object arm and the reference arm, for detection. The object arm does not need to obtain the spatial distribution of the target reflected echo, but only needs to obtain the light intensity fluctuation signal reflected by the target, that is, only the relative fluctuation information of the barrel detection value needs to be obtained, and the image of the target can be obtained by calculation. Therefore, under the same system output power, correlation imaging has a longer detection distance than ICCD imaging, and has better imaging resolution than underwater point scanning laser radar. Laser intensity correlation imaging technology uses active modulated light source illumination to divide the emitted light beam into signal light directed to the target and reference light that does not pass through the target. The echo reflected by the target and the reference light are reconstructed and calculated to obtain the target image. The earliest correlation imaging scheme used entangled two photons as the light source. Through compound counting, the object can be re-displayed on the reference light path that does not pass through the object. Among them, the method of detecting underwater targets using correlation imaging (CN101620273A) uses entangled photon pairs generated by the nonlinear effect of the crystal as the light source to calculate the object arm and the reference arm. This light source is large in size and cannot be operated at high power for a long time, which is not conducive to real scene application.

[0004] In recent years, the development of pseudothermal light sources has enabled the correlation imaging technology to be further applied to practical situations. The underwater imaging device based on computational ghost images (CN106324615A) uses DMD to modulate the light field. The DMD has a low damage threshold and an output power of less than 100w / cm 2 , and there will be a diffraction effect on the DMD surface, which cannot increase the transmission power and thus limits the imaging distance. Summary of the invention

[0005] The purpose of the present invention is to overcome the shortcomings of the above-mentioned prior art and propose an underwater correlation imaging device and a method for suppressing backscattering caused by underwater laser transmission. A blue-green laser is directly irradiated onto a rotating frosted glass to form a randomly fluctuating speckle field. By designing parameters such as the roughness of the frosted glass, the phase and intensity distribution of the speckle field meet the requirements of a pseudothermal light source. Using the frosted glass solution, the laser output power can reach 4Gw / cm 2 , greatly improving the output power, thereby increasing the imaging distance. The use of a PMT barrel detector with a gating function can filter out a large amount of backscattered light to prevent the echo signal from being submerged by noise, greatly improving the imaging quality.

[0006] The technical solution of the present invention is as follows:

[0007] An underwater target imaging device based on correlation imaging is characterized in that it includes a transmitting system, a receiving system and a control and computing system placed in a sealed housing;

[0008] The transmitting system includes a laser light source, a rotating frosted glass, a beam splitter and a transmitting mirror group; the receiving system includes a receiving mirror group, a CCD speckle acquisition module, and a gated PMT barrel detection module;

[0009] The laser light source irradiates the rotating frosted glass to generate a pseudo-thermal light source, which is divided into two beams by a beam splitter, namely an object arm beam and a reference arm beam. The object arm beam irradiates an underwater target after passing through a transmitting lens group, and the echo light is detected by a gated PMT barrel detection module through a receiving lens group; the reference arm beam is detected by a CCD speckle acquisition module to record the fluctuation of the light field;

[0010] The control and computing system comprises:

[0011] A speckle convolution enhancement preprocessing module is used to receive the speckle detected by the CCD speckle acquisition module, perform convolution enhancement on it, and then transmit it to the target image reconstruction module;

[0012] A target image reconstruction module is used to receive the speckle after convolution enhancement, reconstruct the image with the received object arm target data detected by the gated PMT bucket detection module, and transmit it to the denoising module;

[0013] The denoising module is used to receive the reconstructed image and perform denoising processing.

[0014] The reference arm speckle convolution enhancement preprocessing module acquires the i-th reference arm speckle pattern obtained by the reference arm CCD speckle acquisition module. With the enhanced convolution kernel h r (x, y) is convolved to obtain the new enhanced reference arm speckle expression:

[0015] The target image reconstruction module uses the TVAL3 algorithm in the compressed sensing convex optimization algorithm to reconstruct the reference arm speckle data and the object arm bucket detection data, that is, the minimum total variation constraint is introduced into the model, the enhanced Lagrangian method is used to convert the constrained model into an unconstrained objective function, and the alternating direction method is used to solve the objective function, and finally the best approximation of the target image is obtained.

[0016] The denoising module removes the residual water body backscattering reconstruction noise by subtracting a constant matrix from the image obtained by the target image reconstruction module.

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] The underwater pseudo-thermal optical correlation imaging provided by the present invention only needs to obtain the relative fluctuation information of the barrel detection value to form an image. The modulation method of rotating frosted glass is adopted to greatly improve the output power, and it has a longer detection distance than the correlation imaging based on DMD. The gated PMT barrel detection module is used to detect the echo signal of the target, which can filter out most of the backscattering noise. In the process of target image calculation, the present invention takes into account the degradation of speckle propagated to the target surface and the influence of backscattering noise of the water body near the target, and adopts appropriate reference arm speckle enhancement for visibility compensation and denoising algorithm to obtain better image reconstruction quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Other features, objects and advantages of the present invention will become more apparent from the detailed description of non-limiting embodiments made with reference to the following drawings:

[0020] Figure 1 Schematic diagram of an underwater target imaging device based on correlation imaging of the present invention

[0021] Figure 2 Flow chart of the control and computing system in an embodiment of the present invention.

[0022] In the figure: ①: 532nm laser ②: Rotating frosted glass controlled by a motor ③: Light beam splitter ④: Transmitting mirror group ⑤: Receiving mirror group ⑥: Gated PMT barrel detection module ⑦: CCD speckle acquisition module ⑧: Control and calculation system ⑨: Sealed shell. DETAILED DESCRIPTION

[0023] The following is a detailed description of the embodiments of the present invention: This embodiment is implemented on the premise of the technical solution of the present invention, and a detailed implementation method and a specific operation process are given. It should be pointed out that for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention.

[0024] See also Figure 1 , Figure 1 The schematic diagram of the underwater target imaging device based on correlation imaging of the present invention is shown in the figure. As shown in the figure, an underwater target imaging device based on correlation imaging includes a transmitting system, a receiving system and a control and computing system placed in a sealed housing. In this embodiment, the transmitting system includes a 532nm laser, a rotating frosted glass controlled by a motor, a beam splitter and a transmitting mirror group. The receiving system includes a receiving mirror group, a CCD speckle acquisition module, and a gated PMT barrel detection module.

[0025] The function of the emission system is to generate a speckle light field with a light intensity that satisfies a negative index distribution and a phase that satisfies a uniform distribution between 0-2π. The laser generates a 532nm laser with a repetition rate of 2kHz, and the rotating frosted glass is designed by designing the thickness of the frosted glass and the roughness of the surface so that the laser is vertically incident on the frosted glass, generating a randomly fluctuating speckle field. Then, through a beam splitter, the light path is divided into two, one as signal light, which passes through a collimation system and a series of lens-composed emission mirrors and is directly emitted to the target; the other as reference light, which is directly incident on the CCD speckle acquisition module to record the fluctuations of the light field.

[0026] The receiving mirror group is a module composed of a lens group with a certain receiving field of view. The light reflected by the target passes through the receiving mirror group and converges on the gated PMT. The PMT converts the signal light signal into an electrical signal and then inputs it into the computer. The CCD speckle acquisition module records the light intensity fluctuation of the reference arm speckle field and then converts it into an electrical signal and inputs it into the computer.

[0027] The computer includes a signal acquisition card and a timing control card. The timing control card uses the light signal of the laser to give a signal and then controls the data acquisition of the PMT and the acquisition card respectively. The processed reference speckle and the object arm gated PMT barrel detection value are input into the computing system to finally obtain the target image. The control and computing system includes a control system and a computing system, wherein the control system controls the reference arm and the object arm for synchronous acquisition; the computing system is as follows: Figure 2 As shown in the figure, the speckle image collected by the reference arm is enhanced by convolution operation, and then the barrel detection value collected by the object arm and the reference arm speckle are reconstructed by using the compressed sensing algorithm, and finally the reconstructed DC noise caused by the backscattering of the water body near the target is subtracted. The underwater target is detected by using pseudo-thermal light correlation imaging, which has a higher transmission power. The backscattering is filtered out by gated PMT on the device, and finally the speckle enhancement image denoising is performed in the reconstructed target image, which improves the imaging quality and better applies pseudo-thermal light intensity correlation imaging to the detection of underwater targets.

[0028] Specifically, the reference arm speckle convolution enhancement preprocessing module converts the i-th reference arm speckle obtained by the reference arm CCD speckle acquisition module into With enhanced convolution kernel h r (x, y) is convolved to obtain a new reference arm speckle and the convolution kernel h is enhanced r The (x,y) design is as follows:

[0029] In the reference arm speckle, considering that the whole device is placed underwater, if the reference arm speckle is still correlated with the signal received by the underwater object arm according to the speckle field obtained by coherent transmission in the air to restore the target image, due to the scattering and absorption of the water body, the speckle of the reference arm is actually degraded by the water body, so the point spread function of the water body, that is, the underwater convolution kernel h, is used to calculate the r (x,y),

[0030] First, the point spread function represents the normalized transmission power at the transmission distance R and the angle θ from the optical axis. The point spread function f(R,θ) can be expressed by the modulation transfer function The Hanker transform (polar coordinate form of the two-dimensional Fourier transform) is obtained:

[0031]

[0032] Under the condition of small angle approximation, the key to the problem is the relationship between the modulation transfer function and the inherent optical properties of the water scattering coefficient. Here, the inherent optical parameters of the water body are introduced: the volume scattering function. By considering the scattering medium (water) between the light source and the receiver, it is differentiated into an infinitesimal plate and derived:

[0033]

[0034] in Represents the Hankel transform of the volume scattering function of the water body, that is, the scattering term.

[0035] Although the theory is directly applicable to layered media, we restrict ourselves here to homogeneous media to avoid the additional complexity of the distance integral in the above equation. In this case, the scattering term can be written as:

[0036]

[0037] where b is the total scattering coefficient, is the Hankel transform of the scattering phase function s(θ).

[0038] In this case the modulation transfer function can be simplified to:

[0039]

[0040] By choosing the scattering phase function

[0041]

[0042]

[0043] Where θ0 represents the median scattering angle of a single scattering, and θ is related to the coordinate (x, y). Then, the point spread function is obtained by numerical integration of formula (1), i.e., the convolution kernel h r (x, y), the convolution kernel is normalized and then convolved with the reference arm speckle to obtain the new speckle expression:

[0044] Specifically, the target image reconstruction module adopts the TVAL3 algorithm in the compressed sensing convex optimization algorithm. First, the new speckle obtained by the convolution is Rearrange to become row vector a (i) , the reference arm speckle sequence obtained by multiple sampling can be rearranged into a matrix Object arm gated PMT bucket detection value Can be arranged in order into column vectors m is the corresponding maximum number of sampling times, and <·> is the ensemble average of m sampling times. The target reconstruction process can be expressed as the objective function The minimum total variation is introduced into the model, and the enhanced Lagrangian method is used to transform the constrained model into an unconstrained objective function. The alternating direction method is used to solve the objective function, and finally the The best approximation of .

[0045] The denoising module is a supplement to the gated PMT detector to remove the backscattered light of water. The gated PMT barrel detection module must include the backscattered light of the water near the target. After derivation and research, the scattered light returned to the detector by the scatterer near the target at a given distance It is proportional to the emission speckle intensity and can be expressed as (where k is a constant coefficient), we can also Can be arranged in order into column vectors in It represents the sum of the rows of A. The noisy target signal obtained by target reconstruction of the bucket detection value containing scattered noise can be expressed as:

[0046]

[0047] Where e is a unit vector of the same dimension as vector x, y0 is a noise-free bucket detection value vector, and the required original noise-free target vector x0 is:

[0048]

[0049] Finally, the obtained noise-free target vector x0 is rearranged into the target image O GI .

[0050] The underwater imaging device based on pseudothermal correlation imaging provided by the above embodiment of the present invention modulates the emitted speckle field by rotating frosted glass; obtains the signal intensity value corresponding to the target in the detected field of view by adopting a barrel detection method with a gating function; numerically calculates the multiple target reflection intensity values ​​and the reference arm intensity value, calculates the image of the target, and realizes imaging of the target. The underwater imaging device and method based on pseudothermal correlation imaging provided by the above embodiment of the present invention have the characteristics of high detection sensitivity, fast solution speed, and high imaging clarity, and provide a brand-new technical solution for the field of underwater target detection.

[0051] It should be noted that the steps in the method provided by the present invention can be implemented by using corresponding modules, devices, units, etc. in the system. Those skilled in the art can refer to the technical solution of the method to realize the composition of the system, that is, the embodiments in the method can be understood as preferred examples for constructing the system, which will not be elaborated here.

[0052] Those skilled in the art know that, in addition to implementing the system and its various devices provided by the present invention in a purely computer-readable program code, the system and its various devices provided by the present invention can be made to implement the same functions in the form of logic gates, switches, application-specific integrated circuits, programmable logic controllers, and embedded microcontrollers by logically programming the method steps. Therefore, the system and its various devices provided by the present invention can be considered as a hardware component, and the devices for implementing various functions included therein can also be considered as structures within the hardware component; the devices for implementing various functions can also be considered as both software modules for implementing the method and structures within the hardware component.

[0053] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art may make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. An underwater target imaging device based on correlation imaging, characterized in that: It includes a transmitting system, a receiving system and a control and computing system placed in a sealed housing; The transmitting system includes a laser light source, a rotating frosted glass, a beam splitter and a transmitting mirror group; the receiving system includes a receiving mirror group, a CCD speckle acquisition module, and a gated PMT barrel detection module; The laser light source irradiates the rotating frosted glass to generate a pseudo-thermal light source, which is divided into two beams by a beam splitter, namely an object arm beam and a reference arm beam. The object arm beam irradiates an underwater target after passing through a transmitting lens group, and the echo light is detected by a gated PMT barrel detection module through a receiving lens group; the reference arm beam is detected by a CCD speckle acquisition module to record the fluctuation of the light field; The control and computing system comprises: A speckle convolution enhancement preprocessing module is used to receive the speckle detected by the CCD speckle acquisition module, perform convolution enhancement on it, and then transmit it to the target image reconstruction module; A target image reconstruction module is used to receive the speckle after convolution enhancement, reconstruct the image with the received object arm target data detected by the gated PMT bucket detection module, and transmit it to the denoising module; A denoising module, used for receiving the reconstructed image and performing denoising processing; The convolution enhancement is specifically to convert the i-th reference arm speckle detected by the CCD speckle acquisition module With the reference arm underwater convolution kernel h r (x, y) is convolved to obtain the new reference arm speckle after convolution enhancement The image reconstruction specifically uses the TVAL3 algorithm of the compressed sensing convex optimization algorithm to reconstruct the reference arm speckle data and the object arm target data, that is, the minimum total variation constraint is introduced, the enhanced Lagrangian method is used to convert the constrained model into an unconstrained objective function, and the alternating direction method is used to solve the objective function, and finally the best approximation of the target image is obtained; The denoising process is specifically to reconstruct the target of the bucket detection value containing scattered noise to obtain a noisy target signal, which is expressed as: Among them, e is a unit vector of the same dimension as vector x, y0 is the noise-free bucket detection value vector, and the required original noise-free target vector x0 is: Finally, the obtained noise-free target vector x0 is rearranged into the target image O GI .

2. The underwater target imaging device based on correlation imaging according to claim 1, characterized in that: The sealed housing is used to ensure the watertightness and the caliber of the transmitting and receiving of the underwater target imaging device underwater.

3. The underwater target imaging device based on correlation imaging according to claim 1, characterized in that: The transmitting aperture of the transmitting mirror group and the receiving aperture of the receiving mirror group are separated, and the influence of backscattering of the water body is reduced by the angular misalignment between the transmitting angle and the receiving angle.

4. The underwater target imaging device based on correlation imaging according to claim 1, characterized in that: The gated PMT bucket detection module adopts a gated method, calculates the echo time, and uses the gate opening time to filter out most of the water body backscattering, thereby ensuring the detection signal-to-noise ratio.

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