A Three-Dimensional Correlation Imaging Method for Underwater Targets Based on Vortex Light Spatial Filtering

Abstract

The present invention discloses a three-dimensional correlation imaging method for underwater targets based on vortex light spatial filtering, comprising: (1) emitting pulsed laser, which is irradiated onto the underwater target after passing through a beam expander and a transmitting mirror group; (2) performing speckle encoding modulation on the light intensity distribution of the emitted or received light through a digital micromirror device (DMD); (3) filtering the light modulated by the DMD through a spiral phase plate and a mask plate with a transparent ring to achieve the filtering of scattered light and background light; (4) using a photomultiplier tube to obtain the filtered light intensity fluctuation changes and performing synchronous sampling to obtain the echo light intensities of targets at different distances; (5) performing reconstruction calculation according to the echo light intensities of targets at different distances and the speckle encoding modulation mode of the DMD to obtain the target images of different distance slices, namely three-dimensional imaging. By using the present invention, the interference of water body scattering and background light can be effectively filtered, and high signal-to-noise ratio and high-resolution three-dimensional imaging of underwater targets can be achieved.

Description

Technical Field

[0001] The present invention belongs to the field of underwater optical imaging, and in particular relates to a three-dimensional correlation imaging method for underwater targets based on vortex light spatial filtering. Background Art

[0002] Underwater laser imaging is of great significance in the fields of marine security, underwater rescue, etc. In underwater laser imaging, the scattering of water makes the signal light reflected by the target submerged in the scattered light noise, resulting in a poor signal-to-noise ratio and inability to image.

[0003] Traditional laser imaging with an underwater scanning system uses a relatively thin beam for point-by-point scanning (such as CN106643671A, CN112556659A). Therefore, it requires a high sampling frequency and has a low imaging resolution, mainly used for scanning the underwater terrain.

[0004] Underwater range-gated imaging (such as CN106066172A, CN105699984A) has a low detector sensitivity, a short detection range, and a slow sampling speed. At the same time, affected by water body scattering and background light, the imaging result degrades.

[0005] Therefore, there is an urgent need to design an optical imaging method for underwater targets with high resolution and high sensitivity. Summary of the Invention

[0006] The present invention provides a three-dimensional correlation imaging method for underwater targets based on vortex light spatial filtering, which can effectively filter out the interference of water body scattering and background light and achieve high signal-to-noise ratio and high-resolution three-dimensional imaging of underwater targets.

[0007] A three-dimensional correlation imaging method for underwater targets based on vortex light spatial filtering includes:

[0008] (1) Emitting pulsed laser, which is irradiated onto the underwater target through a beam expander and a transmitting mirror group;

[0009] (2) Using a digital micromirror device (DMD) to perform speckle encoding modulation on the light intensity distribution of the transmitted or received light;

[0010] (3) The light modulated by the DMD is filtered through a spiral phase plate and a mask plate with a transparent ring to achieve the filtering of scattered light and background light;

[0011] (4) Using a photomultiplier tube to obtain the light intensity fluctuation change after filtering and perform synchronous sampling to obtain the echo light intensity of targets at different distances;

[0012] (5) According to the echo light intensity of targets at different distances and the speckle encoding modulation mode of the DMD, perform reconstruction calculation to obtain the target images of different distance slices, that is, three-dimensional imaging.

[0013] Further, in step (1), a pulsed laser of 532 nm is used.

[0014] Optionally, in step (2), the digital micromirror device (DMD) is used to perform speckle coding modulation on the emitted light intensity distribution. Specifically:

[0015] After passing through the beam expander and the emission mirror group, the pulsed laser is first irradiated onto the digital micromirror device (DMD). The DMD performs speckle coding on the laser spot and then irradiates the underwater target. Then, the received echo light intensity in the field of view is received through the receiving telescope.

[0016] Optionally, in step (2), the digital micromirror device (DMD) is used to perform speckle coding modulation on the received light intensity distribution. Specifically:

[0017] After passing through the beam expander and the emission mirror group, the pulsed laser is directly irradiated onto the underwater target. Then, the light intensity distribution in the field of view is received through the receiving telescope and imaged onto the digital micromirror device (DMD) to achieve speckle coding modulation of the received field of view.

[0018] Further, the specific process of step (3) is as follows:

[0019] The light modulated by the DMD passes through the spiral phase plate. Among them, the coherent channel light reflected by the target becomes vortex light and is concentrated in the annular region, while the water body scattered light and the incoherent background light are concentrated in the central region in a quasi-Gaussian distribution. Then, only the annular region formed by the channel light is allowed to pass through the mask plate with a transparent ring to complete the filtering of the water body scattered light and the background light.

[0020] Further, the specific process of step (4) is as follows:

[0021] The photomultiplier tube is used to convert the light intensity fluctuations at different distances after filtering into electrical signals, and then the high-speed data acquisition card is used to convert the detected electrical signals into digital signals for acquisition to obtain the echo light intensity of the targets at different distances.

[0022] Further, in step (5), the target images at different distances correspond to the light intensities at different echo times The associated reconstruction results of the target images of different distance slices are expressed as:

[0023] ;

[0024] ;

[0025] wherein, represents the ensemble average, is the time or distance The echo intensity of the target at is the speckle coding modulation mode of the digital micromirror device (DMD) at time ; and is the total filtered light intensity, expressed as:

[0026] ;

[0027] In the formula, is the total echo light intensity before filtering, is the noise light filtered by the spiral phase plate and the mask plate.

[0028] The speckle coding modulation mode is the value of the point on the two-dimensional speckle coding modulation matrix containing only 0 and 1, and the expression of the numerical values 0 and 1 is realized by the deflection of the micromirrors on the digital micromirror device (DMD). and are the spatial two-dimensional coordinate positions on the matrix or the DMD target surface. Specifically, it includes, but is not limited to, various speckle coding modulation modes such as Hadamard speckle, Bernoulli speckle, Gaussian random speckle, and Fourier speckle.

[0029] Furthermore, the corresponding relationship between the distance and the echo time is: , where represents the speed of light.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] The present invention can solve the problem of the imaging distance blind area at the backscattering peak in range-gated or time-gated imaging and the problem of image resolution degradation caused by forward scattering of water bodies, and can obtain a higher detection signal-to-noise ratio and imaging quality; at the same time, this method can suppress the time broadening and delay of the echo pulse, reduce the error of underwater pulsed laser ranging, and improve the distance resolution of underwater target three-dimensional imaging. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 is a flowchart of a three-dimensional correlation imaging method for underwater targets based on vortex light spatial filtering according to an embodiment of the present invention.

[0033] Figure 2 is a schematic diagram of the first imaging system according to an embodiment of the present invention.

[0034] Figure 3 is a schematic diagram of the second imaging system according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0035] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be noted that the following embodiments are intended to facilitate the understanding of the present invention and do not limit it in any way.

[0036] As Figure 1 shown, a three-dimensional correlation imaging method for underwater targets based on vortex light spatial filtering includes the following steps:

[0037] S01, irradiating an underwater target with a pulsed laser of 532 nm through a beam expander and a transmitting mirror.

[0038] S02, performing speckle coding modulation on the light intensity distribution of the transmitted or received light through a digital micromirror device (DMD).

[0039] S03, filtering out scattered light and background light through a spiral phase plate and a mask plate with a transparent ring.

[0040] The light modulated by the DMD passes through the spiral phase plate. Among them, the coherent channel light reflected by the target becomes vortex light and is concentrated in the annular region, and the scattered light and incoherent background light are mainly concentrated in the central region in a quasi-Gaussian distribution. Then, only the annular region formed by the channel light is allowed to pass through the mask plate with a transparent ring to complete the filtering of scattered light and background light.

[0041] S04, synchronously sampling the filtered light intensity fluctuations with a photomultiplier tube.

[0042] An industrial control computer is used to control the timing synchronization controller to synchronously trigger the digital micromirror device (DMD), the photomultiplier tube (PMT), the high-speed data acquisition card, and the pulsed laser. The photomultiplier tube converts the filtered light intensity fluctuations at different distances into electrical signals, and finally the high-speed data acquisition card converts the detected electrical signals into digital signals for acquisition.

[0043] S05, performing reconstruction calculation according to the light intensity of the target at different distances and the speckle coding modulation mode of the DMD to obtain three-dimensional images of different distance slices of the target.

[0044] The three-dimensional image reconstruction calculation is performed by reconstructing the backscattered light intensity of the target at different distances after filtering out scattered light and background light and the speckle coding modulation of the digital micromirror device (DMD) to obtain the target images of different distance slices. The total filtered light intensity is expressed as:

[0045] ;

[0046] Among them, is the total echo light intensity before vortex light spatial filtering, is the noise light filtered out by the spiral phase plate and the mask plate. At different distances The target images at [[]] correspond to different echo times of the light intensity ( ), different distances The correlation reconstruction results of the target images of different slices are expressed as:

[0047] ;

[0048] ;

[0049] where represents the ensemble average, is the echo intensity of the target at time or distance [[]], is the speckle coding modulation mode of the digital micromirror device DMD at time is the width of the time slice.

[0050] In the embodiments of the present invention, two imaging systems are built for the above-mentioned three-dimensional correlation imaging method of underwater targets.

[0051] As Figure 2 shown, the whole system includes a laser emission module, a filtering module, a synchronous modulation receiving module, and an image reconstruction calculation module, where:

[0052] The laser emission module includes: a pulsed laser 1 with a central wavelength of 532 nm, a beam expander 2, and a transmitting mirror group 3, and irradiates the area where the underwater target 4 is located through the laser emission module.

[0053] The synchronous modulation receiving module includes: a receiving telescope 5, a digital micromirror device 6 (DMD), a converging lens 9, a photomultiplier tube 10, a timing synchronization controller, and a high-speed data acquisition card. The timing synchronization controller and the high-speed data acquisition card are both integrated in the industrial control computer 11.

[0054] The filtering module includes: a spiral phase plate 7 and a mask plate 8 with a transparent ring.

[0055] The image reconstruction calculation module uses the industrial control computer 11 to perform reconstruction calculations on the light intensity of targets at different distances and the speckle coding modulation of the DMD to obtain the target images of different distance slices and obtain the target three-dimensional image.

[0056] Figure 2 In

[0057] The pulsed laser with a central wavelength of 532 nm emitted by the pulsed laser 1 is directly irradiated onto the underwater target 4 through the beam expander 2 and the emission mirror group 3; the light intensity distribution received in the field of view is imaged onto the digital micromirror array 6 (DMD) through the receiving telescope 5 to realize the speckle coding modulation of the receiving field of view; the light modulated by the digital micromirror array 6 passes through the spiral phase plate 7, where the coherent channel light reflected by the target becomes vortex light concentrated in the annular region, and the scattered light and the incoherent background light are mainly concentrated in the central region in a quasi-Gaussian distribution. Then, only the annular region formed by the channel light is transmitted through the mask plate 8 with a transparent ring to complete the filtering of the scattered light and the background light. Then, the photomultiplier tube 10 is used to convert the light intensity fluctuations of the target reflected at different distances after filtering into electrical signals, and finally, the high-speed data acquisition card is used to convert the detected electrical signals into digital signals for acquisition. The image three-dimensional reconstruction calculation module performs image three-dimensional reconstruction calculation on the speckle coding modulation change of the digital micromirror array 6 and the light intensity change of the echo light at different distances collected by the photomultiplier tube 10 and the high-speed data acquisition card to obtain the three-dimensional image of the target.

[0058] As Figure 3 shown, the digital micromirror array 6 (DMD) performs speckle coding modulation on the emitted light intensity distribution, specifically as follows:

[0059] The pulsed laser with a central wavelength of 532 nm emitted by the pulsed laser 1 passes through the beam expander 2 and the emission mirror group 3 and irradiates onto the digital micromirror array 6 (DMD). The digital micromirror array 6 encodes the emitted light spot according to the speckle coding modulation pattern output by the industrial control computer 11. The underwater target 4 is irradiated with the encoded light spot, and the target reflected light, water body scattered light, and background light in the field of view are received through the receiving telescope 5; then, the received echo light intensity passes through the spiral phase plate 7, where the coherent channel light reflected by the target becomes vortex light concentrated in the annular region, and the scattered light and the incoherent background light are mainly concentrated in the central region in a quasi-Gaussian distribution. Only the annular region formed by the channel light is transmitted through the mask plate 8 with a transparent ring to complete the filtering of the scattered light and the background light. Then, the photomultiplier tube 10 is used to convert the light intensity fluctuations of the target reflected at different distances after filtering into electrical signals, and finally, the high-speed data acquisition card is used to convert the detected electrical signals into digital signals for acquisition. The image three-dimensional reconstruction calculation module performs image three-dimensional reconstruction calculation on the speckle coding modulation change of the digital micromirror array 6 and the light intensity change of the echo light at different distances collected by the photomultiplier tube 10 and the high-speed data acquisition card to obtain the three-dimensional image of the target.

[0060] The above-described embodiments have detailed the technical solutions and beneficial effects of the present invention. It should be understood that the above are only specific embodiments of the present invention and are not used to limit the present invention. Any modifications, supplements, and equivalent replacements made within the scope of the principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for three-dimensional correlation imaging of underwater targets based on vortex optical spatial filtering, characterized in that: include: (1) Emitting pulsed laser, which irradiates underwater targets after passing through a beam expander and a transmitting mirror group; (2) Speckle coding modulation of the transmitted or received light intensity distribution through a digital micromirror array (DMD); (3) The light modulated by the DMD is filtered through a spiral phase plate and a mask with a transparent ring to filter out scattered light and background light. The specific process is as follows: The light modulated by the DMD passes through the spiral phase plate, where the coherent channel light reflected by the target becomes vortex light concentrated in the annular area, and the water scattered light and incoherent background light are concentrated in the central area in a quasi-Gaussian distribution; then, a mask with a transparent ring is used to allow only the annular area formed by the channel light to pass through the mask, thus completing the filtering of the water scattered light and background light; (4) Use a photomultiplier tube to obtain the filtered light intensity fluctuations and perform synchronous sampling to obtain the echo intensity of targets at different distances; (5) Reconstruction calculation is performed based on the echo intensity of the target at different distances and the speckle coding modulation mode of the DMD to obtain the target images of slices at different distances, that is, three-dimensional imaging; Different distances The target image at different echo times Light intensity at different distances Slice target image association reconstruction result It is expressed as: ； ； In the formula, represents the ensemble average, for Time or distance The echo strength of the target at for The speckle coding modulation mode of the digital micromirror array DMD at the moment, is the width of the time slice; is the total light intensity after filtering, expressed as: ； In the formula, is the total intensity of the echo before filtering, is the noise light filtered out by the spiral phase plate and the mask; Echo time The corresponding relationship is: ,in, Represents the speed of light.

2. The underwater target three-dimensional correlation imaging method based on vortex light spatial filtering according to claim 1 is characterized in that: In step (1), a 532 nm pulsed laser is used.

3. The underwater target three-dimensional correlation imaging method based on vortex light spatial filtering according to claim 1 is characterized in that: In step (2), the emitted light intensity distribution is subjected to speckle coding modulation by a digital micromirror array (DMD), specifically: After passing through the beam expander and the transmitting mirror group, the pulse laser is first irradiated onto the digital micromirror array (DMD). The DMD performs speckle encoding on the spot of the pulse laser before irradiating the underwater target. The echo light intensity in the field of view is then received through the receiving telescope.

4. The underwater target three-dimensional correlation imaging method based on vortex light spatial filtering according to claim 1 is characterized in that: In step (2), the received light intensity distribution is subjected to speckle coding modulation by a digital micromirror array (DMD), specifically: The pulsed laser passes through the beam expander and the transmitting mirror group and directly irradiates the underwater target; then the light intensity distribution in the field of view is received by the receiving telescope and imaged onto the digital micromirror array DMD, realizing speckle coding modulation of the receiving field of view.

5. The method for three-dimensional correlation imaging of underwater targets based on vortex light spatial filtering according to claim 1, characterized in that: The specific process of step (4) is as follows: A photomultiplier tube is used to convert the filtered light intensity fluctuations at different distances into electrical signals, and then a high-speed data acquisition card is used to convert the detected electrical signals into digital signals for collection to obtain the echo light intensity of targets at different distances.

6. The method for three-dimensional correlation imaging of underwater targets based on vortex light spatial filtering according to claim 1, characterized in that: Speckle Coded Modulation Mode is a point on the two-dimensional speckle coded modulation matrix containing only 0 and 1 The numerical value of 0 and 1 is expressed by the deflection of the micromirrors on the digital micromirror array DMD. and It is the spatial two-dimensional coordinate position on the matrix or DMD target surface.

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