Underwater target three-dimensional correlated imaging method based on vortex light spatial filtering

Abstract

The invention discloses an underwater target three-dimensional correlated imaging method based on vortex light spatial filtering, and the method comprises the steps: (1) transmitting pulse laser, and enabling the pulse laser to irradiate an underwater target after passing through a beam expander and a transmitting lens group; (2) speckle code modulation is carried out on emitted or received light intensity distribution through a digital micromirror array DMD; (3) filtering the light modulated by the DMD through a spiral phase plate and a mask plate with a transparent ring to filter scattered light and background light; (4) obtaining the fluctuation change of the filtered light intensity by adopting a photomultiplier and carrying out synchronous sampling to obtain the echo light intensity of targets at different distances; and (5) performing reconstruction calculation according to the echo light intensity of the targets at different distances and the speckle coding modulation mode of the DMD to obtain target images of slices at different distances, namely three-dimensional imaging. According to the invention, water scattering and background light interference can be effectively filtered out, and high-signal-to-noise-ratio and high-resolution three-dimensional imaging of an underwater target is realized.

Description

Technical Field

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

[0002] Underwater laser imaging is of great significance in the fields of marine safety and underwater rescue. In underwater laser imaging, the scattering of water makes the signal light reflected by the target submerged in the scattered light noise, which makes the signal-to-noise ratio worse and makes imaging impossible.

[0003] The laser imaging of the traditional underwater scanning system uses a thinner light beam for point-by-point scanning (such as CN106643671A, CN112556659A), so the sampling frequency requirement is high and the imaging resolution is low. It is mainly used for scanning underwater terrain.

[0004] In underwater range-gated imaging (such as CN106066172A and CN105699984A), the detector has low sensitivity, short detection distance, and slow sampling speed. At the same time, it is affected by water scattering and background light, which degrades the imaging results.

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

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

[0007] A method for three-dimensional correlation imaging of underwater targets based on vortex light spatial filtering, comprising: (1) Emitting pulsed laser to illuminate underwater targets 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; (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 image at different distance slices, that is, three-dimensional imaging.

[0008] Furthermore, in step (1), a 532 nm pulsed laser is used.

[0009] Optionally, 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.

[0010] Optionally, in step (2), speckle coding modulation is performed on the received light intensity distribution by using 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.

[0011] Furthermore, the specific process of step (3) is as follows: The DMD modulated light passes through the spiral phase plate, where the coherent channel light reflected by the target becomes vortex light concentrated in the annular area, while the water scattered light and the 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, thereby filtering out the water scattered light and background light.

[0012] Furthermore, 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.

[0013] Furthermore, in step (5), 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 mask.

[0014] Speckle Coded Modulation Mode That is, the 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 is the spatial two-dimensional coordinate position on the matrix or 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, Fourier speckle, etc.

[0015] Furthermore, the distance Echo time The corresponding relationship is: ,in, Represents the speed of light.

[0016] Compared with the prior art, the present invention has the following beneficial effects: The present invention can solve the problem of imaging range 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 higher detection signal-to-noise ratio and imaging quality; at the same time, the method can suppress the time broadening and delay of the echo pulse, reduce the error of underwater pulse laser ranging, and improve the distance resolution of three-dimensional imaging of underwater targets. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a flow chart of a method for three-dimensional correlation imaging of underwater targets based on vortex light spatial filtering according to an embodiment of the present invention.

[0018] Figure 2 Schematic diagram of a first imaging system in an embodiment of the present invention.

[0019] Figure 3 FIG. 4 is a schematic diagram of a second imaging system in an embodiment of the present invention. DETAILED DESCRIPTION

[0020] The present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be pointed out that the embodiments described below are intended to facilitate the understanding of the present invention and do not have any limiting effect on the present invention.

[0021] like Figure 1 As shown, a method for three-dimensional correlation imaging of underwater targets based on vortex light spatial filtering includes the following steps: S01, uses 532nm pulsed laser to illuminate underwater targets through beam expansion and transmitting mirrors.

[0022] S02, performing speckle coding modulation on the intensity distribution of the emitted or received light through a digital micromirror array (DMD).

[0023] S03, filtering out scattered light and background light through a spiral phase plate and a mask with a transparent ring.

[0024] The DMD modulated light 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 scattered light and incoherent background light are mainly concentrated in the central area with a 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 to complete the filtering of the scattered light and background light.

[0025] S04, a photomultiplier tube is used to synchronously sample the filtered light intensity fluctuations.

[0026] An industrial computer is used to control the timing synchronization controller to synchronously trigger the digital micromirror array DMD, photomultiplier tube PMT, high-speed data acquisition card and pulse laser. The photomultiplier tube is used to convert the fluctuation of light intensity at different distances after filtering into electrical signals. Finally, a high-speed data acquisition card is used to convert the detected photoelectric signals into digital signals for collection.

[0027] 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 slices at different distances of the target.

[0028] The three-dimensional image reconstruction calculation uses the target echo intensity at different distances after filtering out scattered light and background light and the speckle coding modulation of the digital micromirror array DMD to reconstruct the target image at different distance slices, where the total light intensity after filtering is expressed as: ; in, is the total intensity of the echo before the vortex light spatial filtering, is the noise light filtered out by the spiral phase plate and mask. The target image at different echo times The light intensity ( ), different distances Slice target image association reconstruction result It is expressed as: ; ; in, 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.

[0029] In the embodiments of the present invention, two imaging systems are constructed for the above-mentioned underwater target three-dimensional correlation imaging method.

[0030] like Figure 2 As shown, the whole system includes a laser emission module, a filtering module, a synchronous modulation receiving module and an image reconstruction calculation module, wherein: The laser emission module comprises: a pulse laser 1 with a central wavelength of 532 nm, a beam expander 2, and a transmitting mirror group 3. The laser emission module is used to illuminate the area where the underwater target 4 is located.

[0031] The synchronous modulation receiving module includes: a receiving telescope 5, a digital micromirror array 6 (DMD), a converging lens 9, a photomultiplier tube 10, a timing synchronization controller and a high-speed data acquisition card, and the timing synchronization controller and the high-speed data acquisition card are integrated in an industrial computer 11.

[0032] The filtering module comprises: a spiral phase plate 7 and a mask plate 8 with a transparent ring.

[0033] The image reconstruction calculation module uses the industrial computer 11 to reconstruct the target light intensity at different distances and the speckle coding modulation of the DMD to obtain the target images of slices at different distances to obtain the target three-dimensional image.

[0034] Figure 2 In the process, the digital micromirror array 6 (DMD) performs speckle coding modulation on the received light intensity distribution, as follows: The pulsed laser 1 emits a pulsed laser with a central wavelength of 532nm, which is directly irradiated onto the underwater target 4 through the beam expander 2 and the transmitting 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 is converted into vortex light concentrated in the annular area, and the scattered light and the incoherent background light are mainly concentrated in the central area in a quasi-Gaussian distribution, and then the mask plate 8 with a transparent ring is used to make only the annular area formed by the channel light pass through the mask plate to complete the filtering of the scattered light and the background light. Then, the photomultiplier tube 10 is used to convert the fluctuation of the light intensity reflected by the target at different distances after filtering into an electrical signal, and finally the high-speed data acquisition card is used to convert the detected photoelectric signal into a digital signal for collection. The image reconstruction calculation module is used to perform image three-dimensional reconstruction calculation on the speckle coding modulation change of the digital micromirror array 6 and the echo light intensity change at different distances collected by the photomultiplier tube 10 and the high-speed data acquisition card to obtain a three-dimensional image of the target.

[0035] like Figure 3 As shown, the digital micromirror array 6 (DMD) performs speckle coding modulation on the emitted light intensity distribution, as follows: The pulsed laser 1 emits a pulsed laser with a central wavelength of 532nm, which passes through a beam expander 2 and a transmitting mirror group 3 and is irradiated onto a digital micromirror array 6 (DMD). The digital micromirror array 6 encodes the emitted light spot according to the speckle coding modulation mode output by the industrial control computer 11. The coded light spot is used to illuminate the underwater target 4, and the target reflected light, water scattered light and background light in the field of view are received through the receiving telescope 5; the received echo light intensity is then passed through the spiral phase plate 7, where the coherent channel light reflected by the target is converted into vortex light concentrated in the annular area, and the scattered light and incoherent background light are mainly concentrated in the central area in a Gaussian-like distribution. The mask 8 with a transparent ring allows only the annular area formed by the channel light to pass through the mask to complete the filtering of the scattered light and background light. Then, a photomultiplier tube 10 is used to convert the fluctuation of the light intensity reflected by the target at different distances after filtering into an electrical signal, and finally a high-speed data acquisition card is used to convert the detected photoelectric signal into a digital signal for collection. The image reconstruction calculation module performs three-dimensional image reconstruction calculation on the speckle coding modulation changes of the digital micromirror array 6 and the echo intensity changes at different distances collected by the photomultiplier tube 10 and the high-speed data acquisition card to obtain a three-dimensional image of the target.

[0036] The embodiments described above provide a detailed description of 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 intended to limit the present invention. Any modifications, supplements and equivalent substitutions made within the scope of the principles of the present invention should be included in 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; (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 image at different distance slices, that is, three-dimensional imaging.

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 (3) is as follows: The DMD modulated light passes through the spiral phase plate, where the coherent channel light reflected by the target becomes vortex light concentrated in the annular area, while the water scattered light and the 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, thereby filtering out the water scattered light and background light.

6. 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.

7. The method for three-dimensional correlation imaging of underwater targets based on vortex light spatial filtering according to claim 1, characterized in that: In step (5), 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 mask.

8. The method for three-dimensional correlation imaging of underwater targets based on vortex light spatial filtering according to claim 7 is 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.

9. The method for three-dimensional correlation imaging of underwater targets based on vortex light spatial filtering according to claim 7, characterized in that: distance Echo time The corresponding relationship is: ,in, Represents the speed of light.

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