CN113552069A - Laser ultrasonic underwater target detection method and system based on interferometric synthetic aperture - Google Patents
Laser ultrasonic underwater target detection method and system based on interferometric synthetic aperture Download PDFInfo
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Abstract
The invention relates to a laser ultrasonic underwater target detection method and a system based on an interference synthetic aperture, which is characterized in that a laser control unit of an industrial personal computer is used for exciting a pulse laser, emitting a pulse laser beam, a single narrow pulse forms two independent light beams with different paths after being acted by a beam splitter, then the two independent light beams are respectively reflected by a prism to complete light path deflection, so that the two homologous pulse lasers are emitted into water at different moments and generate a photoacoustic effect, ultrasonic waves are generated in liquid and are radiated and propagated, a sound field signal acted with a target object is received by an optical fiber hydrophone, then the two groups of signals are collected and sent to an A/D converter for signal conversion after amplification and filtering denoising treatment, finally the demodulation of the signals is completed, the time domain and frequency domain conditions of the underwater target object are displayed by an oscilloscope, and the data are provided to the industrial personal computer to complete the processing analysis of the interference synthetic aperture, and further realizing the contour detection and depth positioning measurement and calculation of the underwater target object.
Description
The technical field is as follows:
The invention relates to the field of underwater target detection, in particular to a laser ultrasonic underwater target detection method and system based on interferometric synthetic aperture.
Background art:
the precise positioning of underwater targets and the accurate surveying of submarine topography are technical difficulties in the fields of marine military and underwater industry all the time, and with the formal opening of the Beidou No. three global satellite navigation system, China is greatly promoted in the aspect of rapid real-time positioning of targets, but is still required to be promoted in the aspect of underwater precise detection. For depth measurement of liquid media and distance measurement of underwater objects, a depth measuring hammer or a microwave mode is generally used for measuring and calculating in lakes with small water surface disturbance, and the obtained data is more accurate due to little interference of environmental factors; for turbulent river and sea, an echo detector and a multi-beam detection system are generally adopted for measurement, and the result is seriously deviated due to the influence of unstable factors such as ocean current surge, underwater organisms and the like. The method has important influence on the fields of building modern engineering of submarine tunnels, analyzing the motion rule of ocean terrain to early warn earthquake, underwater anti-submergence in military, sea area division and the like. Therefore, there is a need to develop a flexible, reliable and accurate underwater target contour detection and ranging system.
The invention content is as follows:
the invention provides a laser ultrasonic underwater target contour detection method and system based on interferometric synthetic aperture, which overcome the problem that the data acquired by the existing underwater target detection equipment and method is easily influenced by water environment and can not be accurately surveyed.
The technical scheme of the invention is as follows:
a laser ultrasonic underwater target detection method based on interference synthetic aperture is characterized in that a laser control unit of an industrial personal computer is used for exciting a pulse laser, emitting a pulse laser beam, a single narrow pulse forms two independent light beams with different paths after being acted by a beam splitter, then the two independent light beams are reflected through prisms respectively to complete light path deflection, so that two homologous pulse lasers emit into water at different moments and generate a photoacoustic effect, ultrasonic waves are generated in liquid and are radiated to propagate, a sound field signal acted with a target object is received through an optical fiber hydrophone, then the two groups of signals are amplified, filtered and denoised, collected and sent to an A/D converter for signal conversion, finally the demodulation of the signals is completed, the time domain and frequency domain conditions of the sound field of the underwater target object are displayed through an oscilloscope, and data are provided for the industrial personal computer to complete the processing analysis of the interference synthetic aperture, and further realizing the contour detection and depth positioning measurement and calculation of the underwater target object.
The detection method comprises the following steps:
s1: outputting stable pulse laser beam with large energy;
s2: the positions and angles of a beam splitter and two groups of prisms in a laser scanning control module are accurately adjusted, so that the amplitude attenuation of light beam energy caused by transmission and deflection is reduced to the minimum, meanwhile, the incident direction of two beams of homologous laser is enabled to face to the area where an underwater target is located, the pulse laser in the air is irradiated to a layered interface, ultrasonic waves are excited in water and continuously diffused to the deep part;
s3: the method comprises the following steps that an optical fiber hydrophone arranged near an underwater target finishes collecting sound field signals of a region where the target is located, underwater weak signals are amplified through a preposed signal amplifier, then primary noise reduction processing is carried out through a filter, processed signal data are sent to a data acquisition card, and then the signals are converted into required signal types through an A/D converter;
s4: demodulating signals acquired by the optical fiber hydrophone, transmitting the obtained waveform condition of the ultrasonic amplitude to an oscilloscope for displaying, obtaining the geometric distance between two groups of incidence points and an underwater target object through the underwater transit time and the propagation speed of the ultrasonic signals, and simultaneously obtaining the phase information of the ultrasonic interference synthetic aperture signals; wherein, the two groups of signals obtained are respectively:
s1(R)＝u1(R)exp((iφR))
s2(R)＝u1(R+ΔR)exp(iφ(R+ΔR))
The phase information of the two groups of signals consists of phases determined by the propagation paths and random phases generated by different scattering characteristics of underwater targets, and the phase information comprises the following components:
wherein arg { u1} and arg { u2The two groups of random phases at the same point have basically the same contribution, so that the two groups of random phases can be mutually offset when multiplied by complex conjugate;
s5: carrying out preliminary coarse matching on ultrasonic synthetic aperture phase images formed by two groups of complex matrixes, carrying out complex conjugate multiplication on the two images after pre-filtering and data precise matching so as to generate interference fringe images, extracting a phase difference relation, and improving the distribution condition of dense interference fringes through flattening operation; wherein the two images are complex conjugate multiplied:
obtaining an interference fringe pattern in which random phases in the interference pattern have been cancelled, only the phase difference due to the propagation path:
in the processing process, only the main value of the interference phase can be obtained, and the problem of fuzzy whole cycle exists, so the unwrapping operation is needed;
s6: carrying out self-adaptive filtering processing and interference calculation on the obtained interference fringe image to obtain a coherence coefficient; wherein, the coherence coefficient γ ∈ (0, 1) is estimated by truncating a plurality of complex numbers of a plurality of local windows in the interference fringe image, and the closer to 1, the smaller the interference phase is interfered by noise:
S7: phase unwrapping is carried out on the interference image by using a least square method, the phase of the whole cycle is determined, and the obtained absolute phase difference psimSo as to calculate the accurate slope distance difference Delta R, and the formula H is H-R2cosθ2And determining the specific depth elevation of the underwater target.
S7: phase unwrapping is carried out on the interference image by using a least square method, the phase of the whole cycle is determined, and the extracted absolute phase difference psimObtaining the specific relation between the phase difference and the incident point distance
So as to calculate the accurate slope distance difference Delta R, and the formula H is H-R2cosθ2The specific location height of the underwater target can be determined.
In step S5, when the obtained interference pattern generates a lot of dense fringes due to the flat profile of the underwater target, so that the phase pattern is distorted, the phase pattern can be determined whether to perform the flattening process by measuring and calculating the image energy, so as to eliminate the influence thereof.
In step S5, the obtained fringe pattern corresponds to the shape of the profile of the underwater target, where the most dense position is the highest peak of the target profile, and the most sparse position is the deepest position of the target profile.
The present invention performs the unwrapping operation on the wrapping phase by using the least square method of the minimum norm method in step S7, and the basic idea is to minimize the square sum of partial differential derivative differences between the wrapping phase and the unwrapping phase in the discrete form:
wherein ,φi,j(i-0, 1,2 … M-1; j-0, 1,2 … N-1) is a unwrapping phase function having a value range [ -pi, pi),
and
for the winding phase difference of the interference image element (i, j) in the x direction and the y direction, the specific steps of phase unwrapping are introduced by the least square method based on FFT:
step 7-1: calculating rho on the pixel within the range of i being more than or equal to 0 and less than or equal to M and j being more than or equal to 0 and less than or equal to N of the interference patterni,jValue of
Step 7-2: for rhoi,jEach row is mirror-symmetrical to obtain
Then, the two-dimensional Fourier transform is carried out on the obtained FnReplacing rho by this functioni,jAfter the operation is completed for each row, repeating the above process for all columns to obtain Pm,n；
And 7-3: calculating phim,n，
wherein ,Pm,nIs rho after mirror symmetry processingi,jA two-dimensional fourier transform form of (a);
and 7-4: for phim,nPerforming inverse Fourier transform to obtain a unwrapping function phii,jLeast squares estimation of
ψm(ii) a Finally, in step S7
Calculating an accurate slope distance difference delta R, and obtaining the following through a trigonometric function:
according to geometric relationship have
The available target depth:
in the formula, H is the distance between a target to be detected and the water bottom, H is the total depth of the water body, B is the distance between a laser incidence point A1 and A2, R1 is the distance between a laser incidence point A1 and the target, R2 is the distance between a laser incidence point A2 and the target, delta R is the difference between the oblique distances of R1 and R2, and theta is the inclination distance of theta 2Is the angle of refraction of incident point a 2.
Another objective of the present invention is to provide a laser ultrasonic underwater target detection system device for implementing the above method, which includes a laser control and excitation module, a data receiving module, and a post-processing module, and is characterized in that: the laser control and excitation module is electrically connected and controlled by an industrial personal computer, realizes the excitation and deflection scanning functions of laser in the air, is connected with the data receiving module to complete data acquisition and signal type conversion, and transmits the data to the post-processing module, and the data obtained by connecting the post-processing module is analyzed, and the laser control and excitation module is composed of Nd: the YAG pulse laser device comprises a YAG pulse laser device, a beam splitter and a reflecting prism, wherein the data receiving module comprises an optical fiber hydrophone, a preamplifier, a filter, a data acquisition card and an A/D converter, the post-processing module comprises a demodulation system and an oscilloscope, the pulse laser device controls relevant parameters such as light beam triggering time, frequency and energy and the like by an industrial personal computer, the laser forms two light beams of different lines through the beam splitter, and the two laser beams are obliquely incident to the water surface after being reflected by the prism at certain angles at different moments; the high-sensitivity optical fiber hydrophone is used for modulating and converting the received signal of the underwater target into an electric signal; the signal amplification and filtering device amplifies the received electric signal by an amplifying circuit and performs signal filtering and noise reduction; the data acquisition card and the A/D converter are used for acquiring data and converting the obtained analog signals into digital signals; the demodulation system is used for demodulating and extracting the acoustic signals picked up by the optical fiber hydrophone; the oscilloscope is used for observing and analyzing the obtained information such as the time domain waveform, the frequency spectrum and the like of the acoustic signal.
The pulse laser is electrically connected with the industrial personal computer, the beam splitter is arranged at the running track penetration position of the pulse laser, a light beam deflection control unit formed by two groups of reflecting prisms is arranged on the way of a light-emitting path of the beam splitter, the distance between the prisms is multiple times of half wavelength of the laser, the hydrophone is arranged near an underwater target, the output end of the hydrophone is sequentially connected with a preamplifier and a filter, a data acquisition unit and an A/D converter are sequentially and electrically connected behind the amplifying and filtering device, and the data acquisition module is connected with a post-processing module formed by a demodulation system and an oscilloscope.
Furthermore, the laser used for generating the pulse laser is preferably an Nd: YAG laser with the wavelength of 1064nm and the pulse width of 5-10ns, and the water absorption of the wavelength is strong, so that high ultrasonic energy can be generated in water.
Furthermore, the laser control unit composed of the spectroscope and the prism is used for deflecting and adjusting the laser transmission direction, so that different beam splitting lasers generated by the homologous laser are emitted into water at a certain incident angle, and laser sounding in liquid is realized.
Furthermore, the prism distance is set to be several times of the half wavelength of the laser, so that constructive interference occurs between the two groups of prisms, and the measurement accuracy is further improved.
Different from the prior art, the invention has the advantages that: (1) the positioning and measuring precision of the slant range difference data obtained by the interference phase mode is higher, and the data obtained by the method has higher reliability compared with the data obtained by directly using the time-speed relation of the ultrasonic echo signals. (2) The advantages of high peak power, narrow pulse width, small divergence and the like of pulse laser are used as an ultrasonic excitation source in the air, the characteristics of low attenuation degree, good directivity and the like of broadband ultrasonic excited by an underwater channel are combined, and the detection of an underwater target object is realized by utilizing the photoacoustic effect. (3) Different from the traditional depth measurement mode of ship-borne towed sonar, when meeting the problem that sonar hoisting is difficult to realize in reef or algae dense areas, the invention can complete ultrasonic excitation in a laser triggering mode through an aerial vehicle-mounted or small ship, and has the characteristics of flexible detection means, large detection range and the like. (4) The scheme is based on the interferometric synthetic aperture technology, the interferometric fringe pattern corresponds to the shape of the profile of an underwater target, the most dense position is the highest peak position of the target profile, the most sparse position is the deepest position of the target profile, and the method has the advantages of improving the signal-to-noise ratio and enhancing the detection precision. (5) The interference synthetic aperture detection method is subjected to iterative computation processing at the same measuring and calculating position, so that the precision can be improved; and changing the position of the laser incident point for exciting the underwater sound field by deflecting the light beam, scheduling the receiving position of the optical fiber hydrophone, repeating the data acquisition process, and processing to obtain the contour imaging condition of the underwater target.
Description of the drawings:
FIG. 1 is a schematic diagram of modules constituting the system
FIG. 2 is a schematic diagram of the system
FIG. 3 is a schematic diagram of a model for calculating interferometric synthetic aperture of the system
FIG. 4 is a flow chart of the interferometric synthetic aperture method of the present invention
FIG. 5 is a diagram showing a simulated underwater sound field propagation distribution of dual-source laser induced sound
FIG. 6 is a diagram showing interference fringes obtained in the example
The specific implementation mode is as follows:
in order to make the technical solutions and features of the present invention more intuitive and understandable, the following detailed descriptions are provided in conjunction with the accompanying drawings and the detailed description:
example 1:
in one embodiment of the present invention, as shown in fig. 1 and 2, a laser ultrasonic underwater target detection system is provided, which comprises a laser control and excitation module, a data receiving module and a post-processing module. The laser control and excitation module is electrically connected and controlled by an industrial personal computer, laser excitation and deflection scanning are realized in the air, the laser control and excitation module is connected with the data receiving module to complete data acquisition and signal type conversion and send the data to the post-processing module, and the data obtained by the post-processing module is connected for analysis. The laser control and excitation module is composed of Nd: YAG pulse laser, beam splitter and prism, the data receiving module includes fiber hydrophone, preamplifier, filter, data acquisition card and A/D converter, and the post-processing module includes demodulation system and oscilloscope.
The specific scheme and the connection relation are as follows:
the pulse laser is controlled by an industrial personal computer to trigger relevant parameters such as time, frequency and energy of light beam triggering, a single beam of laser forms two light beams of different lines through a spectroscope, and two beams of laser are obliquely incident to the water surface after being reflected by a prism at a certain angle at different moments.
The high-sensitivity optical fiber hydrophone is used for modulating and converting the received signals of the underwater target into electric signals.
The signal amplifying and filtering device amplifies the received electric signals through an amplifying circuit and performs signal filtering and noise reduction processing.
The data acquisition card and the A/D converter are used for acquiring data and converting the obtained analog signals into digital signals.
The demodulation system is used for demodulating and extracting the acoustic signals picked up by the optical fiber hydrophone.
The oscilloscope is used for observing and analyzing the obtained information such as the time domain waveform, the frequency spectrum and the like of the acoustic signal.
The pulse laser is electrically connected with an industrial personal computer, the spectroscope is arranged at the running track penetrating position of the pulse laser, a light beam deflection control unit formed by two groups of prisms is positioned on the way of the light-emitting path of the spectroscope, the prism distance is multiple times of half wavelength of the laser, the hydrophone is positioned near an underwater target, the output end of the hydrophone is sequentially connected with a preamplifier and a filter, a data acquisition unit and an A/D converter are sequentially and electrically connected behind the amplification and filtering device, and the data acquisition module is connected with a post-processing module formed by a demodulation system and an oscilloscope.
Fig. 3 shows a detection method based on interferometric synthetic aperture, which includes the following steps:
s1: the laser excitation unit is adjusted to output a stable, high-energy laser beam.
S2: the positions and angles of a beam splitter and two groups of prisms in the laser scanning control module are accurately adjusted, amplitude attenuation of light beam energy caused by transmission and deflection is reduced to the minimum, meanwhile, the incident direction of two beams of homologous laser faces to an area where an underwater target is located, the pulse laser in the air is irradiated to a layered interface, ultrasonic waves are excited in water and continuously diffused to the deep position.
S3: the method comprises the steps that an optical fiber hydrophone placed near an underwater target finishes collection of sound field signals of a region where the target is located, underwater weak signals are amplified through a preposed signal amplifier, then primary noise reduction processing is carried out on the underwater weak signals through a filter, processed signal data are sent to a data acquisition card, and then the signals are converted into required signal types through an A/D converter.
S4: and a demodulation system in the post-processing module demodulates the signal acquired by the optical fiber hydrophone and transmits the obtained waveform condition of the ultrasonic amplitude to an oscilloscope for display. And acquiring the geometrical distances from the two groups of incidence points to the underwater target object through the underwater transit time and the propagation speed of the ultrasonic signal, and simultaneously acquiring the phase information of the ultrasonic interference synthetic aperture signal.
S5: the method comprises the steps of carrying out preliminary coarse matching on ultrasonic synthetic aperture phase images formed by two groups of complex matrixes, carrying out complex conjugate multiplication on the two images after pre-filtering and data precise matching, further generating interference fringe images, extracting phase difference relation, and improving the distribution condition of dense interference fringes through flattening operation.
S6: and (4) carrying out self-adaptive filtering processing and interference calculation, and evaluating the quality of the interference fringes by using a coherence coefficient gamma obtained by an estimation algorithm so as to confirm whether to carry out the next step.
S7: phase unwrapping is carried out on the interference image by using a least square method, the phase of the whole cycle is determined, and the extracted absolute phase difference psimObtaining the specific relation between the phase difference and the incident point distance
So as to calculate the accurate slope distance difference Delta R, and the formula H is H-R2cosθ2The specific location height of the underwater target can be determined.
Further, the two sets of signals obtained in step S4 are:
s1(R)＝u1(R)exp((iφR))
s2(R)＝u1(R+ΔR)exp(iφ(R+ΔR))
the phase information of the two groups of signals consists of phases determined by the propagation paths and random phases generated by different scattering characteristics of underwater targets, and the phase information comprises the following components:
wherein arg { u1} and arg { u2The two groups of random phase contributions at the same point are basically the same, so that the two groups of random phases can be mutually counteracted when multiplied by complex conjugate.
Further, the two images are complex conjugate multiplied in step S5:
obtaining an interference fringe pattern in which random phases in the interference pattern have been cancelled, only the phase difference due to the propagation path:
in the process, only the main value of the interference phase can be obtained, and the whole cycle is fuzzy, so that the unwrapping operation is required.
Further, in step S5, when the obtained interference pattern generates a large number of dense fringes due to the flat profile of the underwater target, so as to distort the phase diagram, it is determined whether to perform the deplature process by measuring and calculating the image energy, so as to eliminate the influence thereof.
Further, in step S5, the obtained fringe pattern corresponds to the shape of the underwater target profile, where the most dense position is the highest peak of the target profile, and the most sparse position is the deepest position of the target profile.
Further, in step S6, a coherence coefficient γ ∈ (0, 1) is estimated by truncating a plurality of complex numbers of a plurality of local windows in the interference fringe image, and a value closer to 1 indicates that the interference phase is less interfered by noise:
further, in step S7, the winding phase is unwrapped by the least square method of the minimum norm method, and the basic idea is to minimize the square sum of partial differential derivative differences between the discrete winding phase and the unwrapped phase:
wherein ,φi,j(i-0, 1,2 … M-1; j-0, 1,2 … N-1) is a unwrapping phase function having a value range [ -pi, pi),
and
the phase difference of the winding of the interference image element (i, j) in the x direction and the y direction is disclosed.
The specific steps of phase unwrapping are described below in terms of a least squares FFT-based approach:
(1) calculating rho on the pixel within the range of i being more than or equal to 0 and less than or equal to M and j being more than or equal to 0 and less than or equal to N of the interference patterni,jValue of
(2) For rhoi,jEach row is mirror-symmetrical to obtain
Then, the two-dimensional Fourier transform is carried out on the obtained FnReplacing rho by this functioni,jAfter the operation is completed for each row, repeating the above process for all columns to obtain Pm,n。
(3) Calculating phim,n，
wherein ,Pm,nIs rho after mirror symmetry processingi,jIn the form of a two-dimensional fourier transform.
(4) For phim,nPerforming inverse Fourier transform to obtain a unwrapping function phii,jIs estimated by the least squares ofm。
Finally, in step S7, in conjunction with FIG. 3, the method includes
Calculating an accurate slope distance difference delta R, and obtaining the following through a trigonometric function:
according to geometric relationship have
The available target depth:
in the formula, H is the distance between a target to be detected and the water bottom, H is the total depth of the water body, B is the distance between a laser incidence point A1 and A2, R1 is the distance between a laser incidence point A1 and the target, R2 is the distance between a laser incidence point A2 and the target, delta R is the difference between the oblique distances of R1 and R2, and theta is the inclination distance of theta 2Is the angle of refraction of incident point a 2.
Claims (7)
1. A laser ultrasonic underwater target detection method based on interference synthetic aperture is characterized in that a laser control unit of an industrial personal computer is used for exciting a pulse laser, emitting a pulse laser beam, a single narrow pulse forms two independent light beams with different paths after being acted by a beam splitter, then the two independent light beams are reflected through prisms respectively to complete light path deflection, so that two homologous pulse lasers emit into water at different moments and generate a photoacoustic effect, ultrasonic waves are generated in liquid and are radiated to propagate, a sound field signal acted with a target object is received through an optical fiber hydrophone, then the two groups of signals are amplified, filtered and denoised, collected and sent to an A/D converter for signal conversion, finally the demodulation of the signals is completed, the time domain and frequency domain conditions of the sound field of the underwater target object are displayed through an oscilloscope, and data are provided for the industrial personal computer to complete the processing analysis of the interference synthetic aperture, and further realizing the contour detection and depth positioning measurement and calculation of the underwater target object.
2. The interferometric synthetic aperture based laser ultrasonic underwater target detection method of claim 1, which comprises the following steps:
S1: outputting stable pulse laser beam with large energy;
s2: the positions and angles of a beam splitter and two groups of prisms in a laser scanning control module are accurately adjusted, so that the amplitude attenuation of light beam energy caused by transmission and deflection is reduced to the minimum, meanwhile, the incident direction of two beams of homologous laser is enabled to face to the area where an underwater target is located, the pulse laser in the air is irradiated to a layered interface, ultrasonic waves are excited in water and continuously diffused to the deep part;
s3: the method comprises the following steps that an optical fiber hydrophone arranged near an underwater target finishes collecting sound field signals of a region where the target is located, underwater weak signals are amplified through a preposed signal amplifier, then primary noise reduction processing is carried out through a filter, processed signal data are sent to a data acquisition card, and then the signals are converted into required signal types through an A/D converter;
s4: demodulating signals acquired by the optical fiber hydrophone, transmitting the obtained waveform condition of the ultrasonic amplitude to an oscilloscope for displaying, obtaining the geometric distance between two groups of incidence points and an underwater target object through the underwater transit time and the propagation speed of the ultrasonic signals, and simultaneously obtaining the phase information of the ultrasonic interference synthetic aperture signals; wherein, the two groups of signals obtained are respectively:
s1(R)＝u1(R)exp((iφR))
s2(R)＝u1(R+ΔR)exp(iφ(R+ΔR))
The phase information of the two groups of signals consists of phases determined by the propagation paths and random phases generated by different scattering characteristics of underwater targets, and the phase information comprises the following components:
wherein arg { u1} and arg { u2The two groups of random phases at the same point have basically the same contribution, so that the two groups of random phases can be mutually offset when multiplied by complex conjugate;
s5: carrying out preliminary coarse matching on ultrasonic synthetic aperture phase images formed by two groups of complex matrixes, carrying out complex conjugate multiplication on the two images after pre-filtering and data precise matching so as to generate interference fringe images, extracting a phase difference relation, and improving the distribution condition of dense interference fringes through flattening operation; wherein the two images are complex conjugate multiplied:
obtaining an interference fringe pattern in which random phases in the interference pattern have been cancelled, only the phase difference due to the propagation path:
in the processing process, only the main value of the interference phase can be obtained, and the problem of fuzzy whole cycle exists, so the unwrapping operation is needed;
s6: carrying out self-adaptive filtering processing and interference calculation on the obtained interference fringe image to obtain a coherence coefficient; wherein, the coherence coefficient γ ∈ (0, 1) is estimated by truncating a plurality of complex numbers of a plurality of local windows in the interference fringe image, and the closer to 1, the smaller the interference phase is interfered by noise:
S7: phase unwrapping is carried out on the interference image by using a least square method, the phase of the whole cycle is determined, and the obtained absolute phase difference psimSo as to calculate the accurate slope distance difference Delta R, and the formula H is H-R2cosθ2And determining the specific depth elevation of the underwater target.
3. The method for detecting the underwater target of claim 1, wherein in step S5, when the obtained interference pattern has a lot of dense fringes due to the flat profile of the underwater target, so as to distort the phase diagram, the image energy is evaluated to determine whether to perform the deplature process to eliminate the influence; in step S5, the obtained interference fringe pattern corresponds to the shape of the profile of the underwater target, where the most dense position is the highest peak position of the target profile, and the most sparse position is the deepest position of the target profile.
S7: phase unwrapping is carried out on the interference image by using a least square method, the phase of the whole cycle is determined, and the extracted absolute phase difference psimObtaining the specific relation between the phase difference and the incident point distance
So as to calculate the accurate slope distance difference Delta R, and the formula H is H-R2cosθ2The specific location height of the underwater target can be determined.
4. The method for detecting the underwater target of the laser ultrasonic based on the interferometric synthetic aperture as claimed in claim 1, wherein the winding phase is unwrapped by the least square method in the minimum norm method in step S7, and the basic idea is to minimize the square sum of partial differential derivative differences between the discrete winding phase and the unwrapped phase:
wherein ,φi,j(i-0, 1,2 … M-1; j-0, 1,2 … N-1) is a unwrapping phase function having a value range [ -pi, pi),
and
for the winding phase difference of the interference image element (i, j) in the x direction and the y direction, the specific steps of phase unwrapping are introduced by the least square method based on FFT:
step 7-1: calculating rho on the pixel within the range of i being more than or equal to 0 and less than or equal to M and j being more than or equal to 0 and less than or equal to N of the interference patterni,jValue of
Step 7-2: for rhoi,jEach row is mirror-symmetrical to obtain
Then, the two-dimensional Fourier transform is carried out on the obtained FnReplacing rho by this functioni,jAfter the operation is completed for each row, repeating the above process for all columns to obtain Pm,n；
And 7-3: calculating phim,n，
wherein ,Pm,nIs rho after mirror symmetry processingi,jA two-dimensional fourier transform form of (a);
and 7-4: for phim,nPerforming inverse Fourier transform to obtain a unwrapping function phii,jLeast squares estimation of
ψm(ii) a Finally, in step S7
Calculating an accurate slope distance difference delta R, and obtaining the following through a trigonometric function:
according to geometric relationship have
The available target depth:
wherein H is the distance between the target to be measured and the water bottom, and H is waterThe total depth of the body, B is the distance between the laser incidence points A1 and A2, R1 is the distance from the laser incidence point A1 to the target, R2 is the distance from the laser incidence point A2 to the target, Delta R is the difference between the oblique distances of R1 and R2, and theta is 2Is the angle of refraction of incident point a 2.
5. A system for realizing the method for detecting the underwater target of the laser ultrasonic based on the interferometric synthetic aperture according to any one of the claims 1 to 4, which comprises a laser control and excitation module, a data receiving module and a post-processing module, and is characterized in that the laser control and excitation module is electrically connected and controlled by an industrial personal computer, the functions of laser excitation and deflection scanning are realized in the air, the data receiving module is connected to complete data acquisition and signal type conversion, the data are transmitted to the post-processing module, the data obtained by connecting the post-processing module are analyzed, and the laser control and excitation module is composed of Nd: the YAG pulse laser device comprises a YAG pulse laser device, a beam splitter and a reflecting prism, the data receiving module comprises an optical fiber hydrophone, a preamplifier, a filter, a data acquisition card and an A/D converter, the post-processing module comprises a demodulation system and an oscilloscope, wherein the pulse laser device controls working parameters by an industrial personal computer, laser forms light beams of two different lines through the beam splitter, and two beams of laser are reflected by the prism at set angles at different moments and then obliquely enter the water surface; the high-sensitivity optical fiber hydrophone is used for modulating and converting the received signal of the underwater target into an electric signal; the signal amplification and filtering device amplifies the received electric signal by an amplifying circuit and performs signal filtering and noise reduction; the data acquisition card and the A/D converter are used for acquiring data and converting the obtained analog signals into digital signals; the demodulation system is used for demodulating and extracting the acoustic signals picked up by the optical fiber hydrophone; the oscilloscope is used for observing and analyzing the obtained acoustic signals; the pulse laser is electrically connected with the industrial personal computer, the beam splitter is arranged at the running track penetration position of the pulse laser, a light beam deflection control unit formed by two groups of reflecting prisms is arranged on the way of a light-emitting path of the beam splitter, the distance between the prisms is multiple times of half wavelength of the laser, the hydrophone is arranged near an underwater target, the output end of the hydrophone is sequentially connected with a preamplifier and a filter, a data acquisition unit and an A/D converter are sequentially and electrically connected behind the amplifying and filtering device, and the data acquisition module is connected with a post-processing module formed by a demodulation system and an oscilloscope.
6. The system for realizing the laser ultrasonic underwater target detection method based on the interferometric synthetic aperture as claimed in claim 5, wherein a laser for generating the pulse laser is a Nd: YAG laser with a wavelength of 1064nm and a pulse width of 5-10 ns.
7. The system for realizing the laser ultrasonic underwater target detection method based on the interferometric synthetic aperture according to claim 5, characterized in that a laser control unit composed of a spectroscope and a prism is used for deflecting and adjusting the laser transmission direction, so that different split laser beams generated by a homologous laser are emitted into water at a set incident angle to realize laser sounding in liquid, and the prism distance is set to be a plurality of times of half wavelength of the laser, in order to generate constructive interference between two groups of prisms and further improve the measurement accuracy.
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