A Deep Dynamic Focusing Acoustic Lens Device and Method Based on Cascade Acoustic Holography

Abstract

The present invention discloses a depth dynamic focusing acoustic lens device and method based on cascaded acoustic holography. It includes a dynamic focusing acoustic lens cascaded structure mainly composed of two acoustic holographic phase plates cascaded axially in a liquid-filled cavity. One acoustic holographic phase plate remains fixed, and the other acoustic holographic phase plate moves freely. The dynamic movement of the focus along the depth is achieved by moving axially and rotating circumferentially. Both phase holographic plates are composed of a tightly arrayed plurality of microstructural units with different thicknesses. The two sides are respectively concave-convex and flat, and both the concave-convex planes face the inside of the cavity. The present invention can simply and conveniently achieve dynamic acoustic energy focusing at multiple positions, thereby realizing detection and imaging on different planes of the object to be measured, greatly improving the detection efficiency. When in use, it can be directly and closely attached to the surface of a conventional ultrasonic transducer and the component to be measured, and different plane target scanning and medical treatment can be achieved through simple movement and rotation between the holographic plate components in the structure.

Description

Technical Field

[0001] The present invention relates to an ultrasonic acoustic lens technology, and specifically relates to a depth dynamic focusing acoustic lens device and method based on cascaded acoustic holography. Background Art

[0002] Biomedical ultrasound is an important new discipline that uses ultrasound as a carrier, relies on engineering techniques such as signal processing technology and electronic science, and utilizes the biological effects of ultrasound to achieve disease diagnosis and treatment. Based on the ultrasonic wave effect, medical ultrasound imaging-diagnosis techniques such as B-ultrasound, ultrasound guidance, and ultrasound contrast have been developed. Using a medical ultrasound probe based on the pulse-echo method, acoustic parameters such as the acoustic impedance and acoustic attenuation of biological tissues can be characterized. Compared with industrial non-destructive testing, the complexity of biological tissues makes their echo signals carry more abundant information. For example, based on the elastic coefficient extracted from the echo signal, the detection of parts such as the prostate and breast can be realized; based on the harmonic components, the nonlinear characteristics of biological tissues can be obtained, and the pathological diagnosis of the endocardium, liver cirrhosis lesions, etc. can be realized. At present, the application of medical ultrasound equipment has also received more and more attention. The characteristics of non-invasive, real-time, repeatable, and wide range of applicable objects of ultrasound detection make it an indispensable means for preoperative and intraoperative clinical detection, and thus it has become one of the five major techniques for medical imaging examinations.

[0003] Meanwhile, ultrasound itself carries huge energy. Existing medical research has shown that the low-dose thermal effect generated by ultrasound can promote the metabolism of human functions, improve blood circulation, and accelerate wound healing. Currently, the incidence of tumors at home and abroad is increasing year by year. For the treatment of tumors, in addition to traditional surgery, new diagnosis and treatment technologies are emerging in an endless stream. In addition to laparoscopic surgery, minimally invasive treatment has also achieved very good results in clinical applications. However, minimally invasive treatment based on technologies such as radiofrequency, microwave, or cryosurgery still belongs to invasive methods and may cause bleeding or metastasis of punctured tumors. Therefore, invasive high-intensity focused ultrasound (HIFU) ablation treatment has received more and more attention in the past 20 years. As the ultrasound excitation power increases, a high-precision sound beam will propagate in human tissues and focus on the target area. The huge energy carried will cause rapid temperature rise in the local area, and the resulting high-temperature thermal effect can cause the tissue to rapidly heat up and then lead to irreversible coagulative necrosis. Since Lynn proposed the idea of non-invasive surgery on the body from outside the body and conducted damage experiments on animal brain cells and achieved obvious results in 1942, medical treatment based on HIFU ultrasound has gradually developed. However, most of the existing ultrasonic medical diagnosis and treatment technologies achieve the focusing of sound energy in the target area through an arc-shaped multi-element probe. It is difficult to perfectly fit when contacting the human epidermis, which will lead to insufficient sound energy coupling and reduce the utilization rate of sound energy. At the same time, the focusing of most existing medical focusing probes is fixed. Although horizontal scanning in the same plane can be achieved by moving the transducer in the plane, it is difficult to achieve the focusing of sound energy in multiple planes based on a single focusing transducer. In addition, characteristics such as multi-elements and arc shapes have led to high prices and long preparation periods for existing ultrasonic medical probes.

[0004] Acoustic holography technology is the extension and development of computer holography technology in the field of acoustics. It records the phase or amplitude information of the target holographic sound field at the positions of acoustic holographic pixel units arranged in a two-dimensional space through calculation and realizes the reconstruction of the target sound field under specific incident conditions. Currently, researchers have processed and prepared polymer metasurfaces with variable thickness, called acoustic holographic phase plates, through 3D printing technology to freely control and reconstruct any complex sound field. However, due to its specific acoustic response characteristics, the 3D-printed acoustic holographic phase plate can only realize a single holographic sound field.

[0005] Therefore, the existing technology lacks holographic phase plates and device methods based on simple and compact structures and high resolutions. Summary of the Invention

[0006] In order to overcome the deficiencies of the above-mentioned existing technologies, aiming at the deficiencies of the existing focused acoustic lens such as complex structure, large thickness, and only having one scanning plane, and at the same time considering the problems such as the existing acoustic holographic phase plate can only achieve a single target sound field, the present invention provides a depth dynamic focusing acoustic lens device and method based on cascaded acoustic holography.

[0007] The realization of depth-adjustable dynamic focusing in the present invention can not only greatly improve the reconstruction freedom degree of the acoustic holographic phase plate, but also effectively improve the target scanning imaging efficiency in practical applications to achieve multi-plane tomography imaging, or realize medical diagnosis and treatment applications at different depths, etc.

[0008] When the device of the present invention is perfectly attached to a conventional transducer and the component to be measured, a focusing sound field that changes dynamically along the depth can be realized in the component to be measured. During use, the two side surfaces of the dynamic focusing acoustic lens device can be directly attached to the surface of a conventional ultrasonic transducer and the component to be measured. Without water immersion, multi-plane scanning imaging of the target can be achieved through the assembled ultrasonic transducer - planar multi-focus acoustic lens device, greatly improving the resolution and efficiency of target imaging.

[0009] The technical solution adopted by the present invention is as follows:

[0010] I. A depth dynamic focusing holographic lens device based on a cascaded acoustic holographic lens:

[0011] It includes an ultrasonic transducer for emitting ultrasonic waves;

[0012] It includes a dynamic focusing acoustic lens cascaded structure with two acoustical holographic phase plates with relative position changes, arranged in front of the probe of the ultrasonic transducer, for performing depth dynamic focusing adjustment on the ultrasonic waves emitted by the ultrasonic transducer, and then focusing on the object to be measured;

[0013] It includes a connecting piece for optically coaxially connecting the ultrasonic transducer and the dynamic focusing acoustic lens cascaded structure.

[0014] The object to be measured can usually be mechanical structure defect detection and human tissue imaging.

[0015] The ultrasonic transducer is a conventional ultrasonic transducer.

[0016] The connecting piece adopts a double-opening flange structure. Both ends of the double-opening flange structure are provided with coaxially penetrating openings, and both ends of the openings are threaded ports.

[0017] The dynamic focusing acoustic lens cascaded structure includes two acoustical holographic phase plates with a changing relative distance between the phase surfaces, and a liquid for sound transmission is arranged between the acoustical holographic phase plates;

[0018] The specific structure of the cascaded dynamic focusing acoustic lens includes a first acoustic holographic phase plate, a liquid-filled cavity, a second acoustic holographic phase plate, and a sealing bottom plate. One end of the liquid-filled cavity is fixedly installed with the first acoustic holographic phase plate, and the other end of the liquid-filled cavity is fixedly installed with the sealing bottom plate. A second acoustic holographic phase plate is movably installed inside the other end of the liquid-filled cavity near the sealing bottom plate. The second acoustic holographic phase plate can axially move or rotate around the center relative to the first acoustic holographic phase plate. The liquid-filled cavity between the first acoustic holographic phase plate and the sealing bottom plate is filled with the liquid.

[0019] The second acoustic holographic phase plate is connected to the inner wall of the liquid-filled cavity through an axial movement guide rail, a circumferential movement guide rail structure; both the axial movement guide rail and the circumferential movement guide rail include a gear, a motor, and a rack. The rack is fixedly installed on the inner wall surface of the liquid-filled cavity, the motor is fixedly installed on the second acoustic holographic phase plate, the output shaft of the motor is coaxially connected to the gear, and the gear meshes with the rack.

[0020] Specifically, the axial movement guide rail includes an axial gear, a motor, and a linear axial rack. The axial rack is fixedly installed on the inner wall surface of the liquid-filled cavity and is arranged along the axial direction. The motor is fixedly installed on the back surface of the second acoustic holographic phase plate, the output shaft of the motor is coaxially connected to the axial gear, and the axial gear meshes with the axial rack.

[0021] The circumferential movement guide rail includes a circumferential gear, a motor, and an annular circumferential rack. The annular circumferential rack is fixedly installed on the inner wall surface of the liquid-filled cavity and is arranged along the circumferential direction. The motor is fixedly installed on the back surface of the second acoustic holographic phase plate, the output shaft of the motor is coaxially connected to the circumferential gear, and the circumferential gear meshes with the circumferential rack.

[0022] A pair of axial movement guide rails and circumferential guide rails are machined on the inner side wall of the liquid-filled cavity. The second acoustic holographic phase plate is directly fixed on the above-mentioned axial movement guide rail and circumferential guide rail as a movable structure in the cascaded holographic system. The other side of the liquid-filled cavity is encapsulated by a polymer flat plate, and finally deionized water can be injected into the liquid-filled cavity structure as a liquid background medium.

[0023] Wherein, the thickness of the encapsulating flat plate at the bottom of the liquid-filled cavity h d and the thickness of the side wall h m are both evenly distributed. And to reduce the attenuation and reflection of the incident sound wave from the liquid cavity structure, while maintaining the stiffness, h d the thinner the better.

[0024] For the convenience of assembly and subsequent rotation of the second acoustic holographic phase plate described below, the first acoustic holographic phase plate, the second acoustic holographic phase plate, and the liquid-filled cavity are all cylindrical structures. Moreover, the liquid-filled cavity is axisymmetric, while the first acoustic holographic phase plate and the second acoustic holographic phase plate that make up the cascaded acoustic holographic system are also non-axisymmetric as a whole.

[0025] The lower surface of the dynamic focusing acoustic lens device is directly attached to the surface of the component to be measured using a coupling agent, and the assembled transducer-focusing acoustic lens device is moved along the surface of the component to achieve imaging detection of the object to be measured.

[0026] The diameter of the second acoustic holographic phase plate D 2 should be smaller than the inner diameter of the liquid-filled cavity D 0 , and it is connected and moved along the axial and circumferential slide rails inside the liquid-filled cavity at four angles in the circumferential direction to realize the moving cascaded system under different intervals and the rotating cascaded system under different relative rotation angles.

[0027] The moving cascaded system: The position of the first acoustic holographic phase plate is kept fixed, while the second acoustic holographic phase plate moves along the axial moving guide rail. After the second acoustic holographic phase plate is controlled by the control system to move a specific distance along the axis and then kept fixed, a specific focused sound field can be realized on different target planes of the object to be measured.

[0028] The rotating cascaded system: The position of the first acoustic holographic phase plate is kept fixed, while the second acoustic holographic phase plate is fixed in a specific axial plane and rotates along the circumferential moving guide rail in this plane. After the second acoustic holographic phase plate is controlled by the control system to rotate a specific distance along the circumference and then kept fixed, a specific focused sound field can be realized on different target planes of the object to be measured.

[0029] Both the first acoustic holographic phase plate and the second acoustic holographic phase plate are polymer flat plate structures with one side surface flat and the other side surface uneven with phase distribution. The first acoustic holographic phase plate and the second acoustic holographic phase plate are arranged at intervals along the optical axis, and their uneven surfaces face each other and their flat surfaces face outward.

[0030] The uneven surfaces of the first acoustic holographic phase plate and the second acoustic holographic phase plate are locally uneven or completely uneven; the locally uneven areas are square or circular.

[0031] The first acoustic holographic phase plate and the second acoustic holographic phase plate are made of a high molecular polymer material with an acoustic attenuation coefficient less than 10%. The high molecular polymer material can usually be a photosensitive resin material and cannot be rubber, glass, etc.

[0032] The side walls of the sealed bottom plate and the liquid filling cavity are made of the same polymer material as the above-mentioned acoustic holographic phase plate.

[0033] The optimized phase distribution on the uneven surfaces of the first acoustic holographic phase plate and the second acoustic holographic phase plate is obtained by designing and setting with the improved multi-layer iterative angular spectrum method of the present invention. At the same time, the improved iterative design of the phase distribution on the planes of the first acoustic holographic phase plate and the second acoustic holographic phase plate is carried out, achieving further innovation; the thickness of the uneven surfaces on the first acoustic holographic phase plate and the second acoustic holographic phase plate is then set according to the optimized phase distribution of the uneven surfaces.

[0034] The multi-layer iterative angular spectrum method designed by the present invention is divided into two parts according to the left and right sides of the plane of the second acoustic holographic phase plate. The side of the acoustic holographic metasurface close to the ultrasonic probe is called the incident side, and the side close to the target plane is called the transmission side. The calculation process includes forward and backward sound field propagation models. By comparing and optimizing with the sound field of the target plane, the phase distributions of the first acoustic holographic phase plate and the second acoustic holographic phase plate can be calculated simultaneously.

[0035] The phase distribution on the uneven surfaces of the first acoustic holographic phase plate and the second acoustic holographic phase plate is obtained by processing with the multi-layer iterative angular spectrum method of the following process;

[0036] First, before the start of iterative optimization, the phase distributions on the uneven surfaces of the first acoustic holographic phase plate and the second acoustic holographic phase plate are respectively used as the first phase distribution φ 1 ’ and the second phase distribution φ 2 ’ , and the first phase distribution φ 1 ’ and the second phase distribution φ 2 ’ are randomly set to arbitrary values, and are processed according to the following forward propagation process and backward optimization process that are continuously alternately iterated:

[0037] 1) Forward propagation process:

[0038] First, according to the first phase distribution φ 1 ’ , the sound field z =0 on the transmission side of the plane where the first acoustic holographic phase plate is located P 0 ( x , y , z =0):

[0039] P 0 ( x , y , z =0)= e ^(j φ 1 ’ )

[0040] where \(e\) represents the natural constant, \(j\) represents the imaginary unit, x , y , z represent the three-dimensional coordinates of specific positions respectively, and \(\hat{}\) represents the power;

[0041] Next, calculate the acoustic field of the ultrasonic wave propagating in the positive z-axis direction from the transmission side of the plane where the first acoustic holographic phase plate is located z =0 to the plane where the second acoustic holographic phase plate is located z = l i ’ The acoustic field on the incident side is P i ’ ( x , y , z = l i ’ ), and then calculate the acoustic field distribution on the incident side of the plane where the second acoustic holographic phase plate is located z = l i ’ The acoustic field distribution P i ’ ( x , y , z = l i ’ ) passing through the second acoustic holographic phase plate with the second phase distribution of φ 2 ’ The acoustic field distribution on the transmission side after the plate is:

[0042] P i ’’ ( x , y , z = l i ’ )= P i ’ ( x ,y , z = l i ’ )* e ^(j φ 2 ’ )

[0043] Similarly, the forward angular spectrum method is used to calculate the sound field distribution on the transmission side of the second acoustic holographic phase plate. P i ’’ ( x , y , z = l i ’ ) and propagates forward to the target plane. z = l i The sound field distribution is P i ’’’ ( x, y , z = l i );

[0044] Then, based on the sound field distribution obtained by forward propagation on the target plane z = l i the sound field distribution P i ’’’ ( x, y , z = l i ) is optimized in the following way on the target plane z = l i the sound field distribution on Q i ’’’ ( x, y , z = l i ):

[0045] Specifically, according to the sound field distribution P i ’’’ ( x, y , z = l i ) its phase part is obtained: φ i = angle ( P i ’’’( x, y , z = l i ) ), where angle () represents the phase extraction function, and then the amplitude part of the sound field distribution P i ’’’ ( x, y , z = l i ) is replaced with a preset target amplitude distribution A i , so as to obtain the optimized sound field distribution on the target plane z = l i Q i ’’’ ( x, y , z = l i ). Q i ’’’ ( x, y , z = l i ) = A i *e^(j φ i );

[0046] 2) Reverse optimization process:

[0047] First, for the optimized sound field distribution on the target plane z = l i Q i ’’’ ( x, y , z = l i ), at this time, the reverse angular spectrum method is used to calculate its reverse propagation to the plane where the second acoustic holographic phase plate is located z = l i ’ The sound field distribution on the transmission side is Q i ’’ ( x , y , z = l i ’ );

[0048] ​​Then, according to the plane where the second acoustic holographic phase plate is obtained during the forward propagation process of this round z = l i ’ The sound field distribution on the incident side P i ’ ( x , y , z = l i ’ ) and the plane where the second acoustic holographic phase plate is obtained during the backward propagation process z = l i ’ The sound field distribution on the transmission side Q i ’’ ( x , y , z = l i ’ ), the optimized second phase distribution is obtained according to the following processing φ 2:

[0049] φ 2= angle (∑( Q i ’’ ( x , y , z = l i ’ )*conj( P i ’ ( x , y , z = l i ’ ))))

[0050] Among them, conj ( ) represents the conjugate function, angle ( ) represents the phase extraction function; ∑( ) is the summation function; through this operation, all the situations corresponding to the target sound fields can be integrated onto the same second acoustic holographic phase plate.

[0051] At this time, the plane where the second acoustic holographic phase plate is located z = l i ’ The sound field distribution on the transmission side Qi ’’ ( x , y , z = l i ’ ) The second acoustic holographic phase plate with the second phase distribution optimized by reverse transmission is φ The second acoustic holographic phase plate with 2, and the sound field distribution on the incident side of the second acoustic holographic phase plate is obtained Q i ’ ( x , y , z = l i ’ ) is:

[0052] Q i ’ ( x , y , z = l i ’ )= Q i ’’ ( x , y , z = l i ’ )*conj( jφ 2)

[0053] Then, continue to calculate the sound field distribution on the incident side of the plane where the second acoustic holographic phase plate is located by using the reverse angular spectrum method z = l i ’ The sound field distribution on the incident side Q i ’ ( x , y , z = l i ’ ) Propagate backward to the plane of the first acoustic holographic phase plate z = 0 The sound field distribution on the transmission side Q i ( x, y , z = 0);

[0054] Finally, from the plane where the first acoustic holographic phase plate is located z = 0 The sound field distribution on the transmission side Q i (x, y , z = 0) Extract and obtain the optimized first phase distribution:

[0055] φ 1 = angle (∑ Q i ( x, y , z = 0))

[0056] 4) The optimized first phase distribution obtained in step 1) φ 1 and the optimized second phase distribution φ 2 are substituted back into step 1) and respectively replace the first phase distribution φ 2 ’ and the second phase distribution φ 2 ’ , and the above steps 1) to 3) are repeated multiple times to finally obtain the optimized first phase distribution φ 1 and the optimized second phase distribution φ 2 as the phase distributions on the first acoustic holographic phase plate and the second acoustic holographic phase plate.

[0057] The uneven surfaces in the first acoustic holographic phase plate and the second acoustic holographic phase plate are composed of a close array of multiple microstructural units. The width of each microstructural unit is constant, the thickness of each microstructural unit is constant everywhere within itself, and the thicknesses of the individual microstructural units are different;

[0058] The thicknesses of the uneven microstructural units on the first acoustic holographic phase plate and the second acoustic holographic phase plate are obtained by processing according to the following process based on the optimized phase distributions of the first and second acoustic holographic phase plates:

[0059] The optimized phase distributions of the first and second acoustic holographic phase plates are extracted at equal intervals w to obtain the phase values corresponding to the m th microstructural thickness unit φ m , w denotes the periodic interval of the microstructural units, and then the thickness dimension of each microstructural unit is calculated according to the following formula h um :

[0060] h um = ( φ - k w H) / ( k u -k w )

[0061] Among them, k u and k w are the acoustic holographic phase plate materials of the first and second acoustic holographic phase plates and the wave numbers of the liquid inside the liquid-filled cavity, respectively. H is a fixed distance preset for the transmission plane from the incident plane. φ represents the phase of the microstructure thickness unit.

[0062] In the present invention, both the first acoustic holographic phase plate and the second acoustic holographic phase plate are set as an array polymer structure of microstructure units with different thicknesses, and the thickness of each thickness microstructure unit h m is related to the operating frequency f of the planar multi-focus acoustic lens and the two-dimensional phase space distribution. During design, the thickness dimensions of the thickness microstructure units on the planes of the first acoustic holographic phase plate and the second acoustic holographic phase plate are obtained by processing according to the multi-layer iterative angular spectrum method h mi , and the width dimension of the thickness microstructure unit is kept w unchanged during the design process.

[0063] In specific implementation, the positions and size resolutions of the foci in the multi-focus focusing sound field realized by the planar multi-focus acoustic lens are designed according to the target plane and the detection accuracy respectively. For the target plane being l i , and the focal spot size being σ for the target sound field, at this time, the amplitude distribution of the focusing sound field on the plane z = l i is expressed as:

[0064] A ( x, y , z = l i ) = e^-(( x - x 0) 2 + ( y - y 0) 2 / σ 2 )

[0065] Among them, l i is the focusing focal length, x 0 and y 0 are the coordinates of the acoustic energy focusing point on the focal plane l i , and ^ represents the power.

[0066] For a dynamic focusing sound field with variable axial focal length, there is: x 0 = 0, y 0 = 0, and the focal lengths and size resolutions of each focus are preset to be l i and σ i , so each focal plane z = l i corresponds to a target sound field distribution A i ( x, y , z = l i ) = e^-( x 2 + y 2 / σ i 2 ).

[0067] Then, according to the target depth focusing sound field distribution, a moving cascade system and a rotating cascade system are designed respectively. In particular, for the moving cascade system, the distances between the second acoustic holographic phase plate and the first acoustic holographic phase plate are preset to be z = l i ’ ; for the rotating cascade system, the distance between the second acoustic holographic phase plate and the first acoustic holographic phase plate is preset to be z = l 1 ’ , and the rotation angles of the second acoustic holographic phase plate relative to the first acoustic holographic phase plate are θ i ’ .

[0068] During implementation, the above preset conditions (the distance and rotation angle between the two holographic plates) and the target sound field are brought into the multi-layer iterative angular spectrum method of the present invention for processing.

[0069] One side of the surface of the prepared first acoustic holographic phase plate and the second acoustic holographic phase plate is an uneven surface, and the other side is a flat surface. When in use, the two acoustic holographic phase plates are placed with the uneven surfaces facing each other. At this time, no water immersion is required. Just directly attach the flat surface of the first acoustic holographic phase plate to the surface of the ultrasonic transducer matching layer, and the plane incident wave excited by the ultrasonic transducer can be modulated and propagated to the liquid filling cavity. After being modulated by the second acoustic holographic phase plate, it is coupled to the object to be measured through the flat bottom of the liquid filling cavity, and then a dynamically changing focusing sound field distribution can be directly realized in the object to be measured for multi-plane defect detection imaging.

[0070] When the dynamic focusing acoustic lens is used for detecting and imaging a measured object, it is assembled with an ultrasonic transducer using a double-headed flange structure to form a transducer-acoustic lens device. Then, after applying a layer of ultrasonic coupling agent under the dynamic focusing acoustic lens, it is directly pressed on the surface of the measured component to generate a focusing sound field with a depth-dynamic change in the measured component. Then, based on the acoustic reciprocity theorem, after collecting and phase-demodulating the defect echo signal, enhanced reception of the echo signal can be achieved. Therefore, the dynamic focusing acoustic lens with multiple axial foci can realize dynamic detection and imaging of defects at different depths, has the application potential of tomography imaging, and greatly improves the efficiency of ultrasonic detection.

[0071] The device of the present invention can achieve dynamic switching between focusing sound fields with different focal lengths by designing cascaded focusing acoustic holography of the first acoustic holographic phase plate and the second acoustic holographic phase plate, improving the degree of freedom of sound field modulation.

[0072] Considering the size and processing accuracy of the metasurface, the operating frequency of the flat multi-focus acoustic lens can be selected from 100 to 3000 kHz. When the operating frequency changes, the unit thickness of the structures of the first acoustic holographic phase plate and the second acoustic holographic phase plate needs to be redesigned. h um The design is simple and efficient.

[0073] In addition, the first acoustic holographic phase plate, the second acoustic holographic phase plate, the liquid-filled cavity, and the bottom plate are all prepared by 3D printing and use the same resin material. The acoustic impedance of the resin material is relatively matched with the impedance of the matching layer of the piezoelectric transducer and the background water medium, so the acoustic energy transmittance is high, greatly improving the utilization rate of acoustic energy.

[0074] II. A depth-dynamic focusing holographic lens modulation method for a depth-dynamic focusing holographic lens device:

[0075] The ultrasonic transducer emits ultrasonic waves that are focused on the target plane through the cascaded structure of the dynamic focusing acoustic lens. By changing the relative distance and relative rotation angle between the first acoustic holographic phase plate and the second acoustic holographic phase plate, the focusing sound field distribution of the overall cascaded structure of the dynamic focusing acoustic lens can be changed, and the focal length of the focused target plane can be modulated.

[0076] In the present invention, the target plane is the plane where the target sound field is located, that is, the focal plane or the focusing plane.

[0077] In a dynamic focusing acoustic lens device based on cascaded acoustic holography of the present invention, it includes a first acoustic holographic phase plate and a second acoustic holographic phase plate prepared by 3D printing technology. The two together form a moving / rotating cascaded acoustic holographic system, and a liquid-filled cavity for fixing the above cascaded acoustic holographic system. After combination, a dynamic focusing acoustic lens device is finally formed.

[0078] Both sides of the dynamic focusing acoustic lens device are flat surfaces. During actual operation, the upper and lower flat surfaces of the dynamic focusing acoustic lens device can be directly attached to the surface of a conventional ultrasonic transducer and the surface of the object to be measured respectively. Among them, after directly assembling the dynamic focusing acoustic lens device with the conventional ultrasonic transducer using a double-opening flange structure, a focused sound field with a specific focal length can be generated in the object to be measured. Then, by changing the distance and rotation angle of the second acoustic holographic phase plate relative to the first acoustic holographic phase plate in the cascaded acoustic holographic device, the axial focal position can be dynamically changed. By moving the assembled transducer-acoustic lens device along the surface of the object to be measured, scanning imaging on different planes of the object can be achieved, greatly improving the imaging range and freedom.

[0079] In the present invention, two acoustic holographic phase plates are cascaded axially and then encapsulated in a liquid-filled cavity structure to form a dynamic focusing acoustic lens with flat surfaces on both sides. When in use, the upper and lower surfaces of the dynamic focusing acoustic lens device can be closely attached to the surface of a conventional ultrasonic transducer and the surface of the component to be measured respectively. During the process, first, the mechanical vibration excited by the ultrasonic transducer is coupled into the dynamic focusing acoustic lens. After wavefront phase modulation by the cascaded acoustic holographic device, it is then radiated to the component to be measured below through the coupling interface on the lower surface of the dynamic focusing acoustic lens. Finally, an arbitrary target focused sound field can be achieved in the component to be measured. Then, by changing the distance and relative rotation angle between the cascaded acoustic holographic devices, acoustic energy focusing at different depths can be achieved. Therefore, through the above planar multi-focus acoustic lens device, the tester can simply and conveniently achieve dynamic acoustic energy focusing of the plane wave excited by a conventional piezoelectric probe at multiple positions, and then achieve detection and imaging on different planes of the object to be measured, greatly improving the detection efficiency. When in use, it can be directly and closely attached to the surface of a conventional ultrasonic transducer and the component to be measured, and different plane target scanning and medical treatment can be achieved through simple movement between the holographic plate components in the structure.

[0080] The planar multi-focus acoustic lens device provided by the present invention has the following beneficial effects:

[0081] (1) The present invention utilizes a cascaded acoustic holographic system. By quickly moving / rotating the second acoustic holographic phase plate, a focused sound field with a dynamically moving focus point along the z-axis direction can be achieved, thereby simply and efficiently realizing multi-plane scanning imaging of the target object. During use, by assembling the first acoustic holographic phase plate and the second acoustic holographic phase plate in the liquid-filled cavity, a planar focused acoustic lens with planar surfaces on both sides can be prepared. During detection, its flat surfaces on both sides can be directly and perfectly attached to the surface of a conventional ultrasonic transducer and the component to be measured, thereby enabling in-situ detection of the object to be measured without water immersion.

[0082] (2) The planar polymer acoustic lens used in the present invention is convenient to process and inexpensive, and can be rapidly prepared by 3D printing. At the same time, the preparation material can be selected to have an impedance match with the matching layer material of the ultrasonic transducer, greatly improving the transmission rate and conversion rate of the sound field. In addition, by directly replacing the first acoustic holographic phase plate and the second acoustic holographic phase plate, multiple sets of focus-moving focused acoustic lenses can be realized, with high operability and reliability.

[0083] The present invention can modulate the plane wave incident on the probe into a sound field focused at different depths and be used to generate a focused sound field with dynamic changes along the depth under the skin of the medical treatment object, realizing high-resolution tomography imaging and ablation treatment at different depths, etc. Moreover, the acoustic lens device is completely planar, has a relatively good match with the acoustic impedance of water, has a high acoustic energy coupling efficiency, a simple structure, and a low price. Description of the Drawings

[0084] Figure 1 is the detection principle diagram based on the cascaded dynamic focusing acoustic lens;

[0085] Figure 2 is the schematic diagram of the device for moving and rotating the cascaded acoustic holographic phase holographic plate;

[0086] Figure 3 is the partial sectional view of the dynamic focusing acoustic lens based on cascaded acoustic holography in Embodiment 1 of the present invention;

[0087] Figure 4 is Figure 3 the A-A sectional view of;

[0088] Figure 5 is the detailed view of the liquid-filled cavity and the axial and circumferential moving guide rails inside the dynamic focusing acoustic lens 3 based on cascaded acoustic holography in Embodiment 1 of the present invention;

[0089] Figure 6 is the schematic diagram of the fixed phase holographic plate and the moving phase holographic plate of the dynamic focusing acoustic lens 3 based on cascaded acoustic holography in Embodiment 1 of the present invention;

[0090] Figure 7It is a fixed acoustic holographic phase plate, a movable acoustic holographic phase plate and their dimensional diagrams. Among them, (a) represents the side view of the first acoustic holographic phase plate, (b) represents the side view of the second acoustic holographic phase plate, (c) represents the cross-sectional view of the first acoustic holographic phase plate, and (d) represents the cross-sectional view of the second acoustic holographic phase plate;

[0091] Figure 8 It is the design schematic diagrams of the first and second acoustic holographic phase plates in Embodiments 1 and 2 of the present invention. Among them, (a) represents the design schematic diagrams of the first and second acoustic holographic phase plates under the movable cascade system, and (b) represents the design schematic diagrams of the first and second acoustic holographic phase plates under the rotating cascade system;

[0092] Figure 9 It is the phase distribution design flowcharts of the first acoustic holographic phase plate and the second acoustic holographic phase plate in the movable and rotating cascade systems of Embodiments 1 and 2 of the present invention;

[0093] Figure 10 It is the focused sound field distribution diagrams with different focal lengths achieved in Embodiment 1 of the present invention. At this time, when the second acoustic holographic phase plate moves to different distances, it corresponds to the focused sound fields with different focal lengths. Among them, (a) represents the first optimized phase distribution diagram and the second optimized phase distribution diagram, (b) represents the sound field propagation diagram after the incident ultrasound is modulated by the first phase acoustic holographic phase plate, (c) represents the amplitude-phase distribution diagram on the transmission side when the second acoustic holographic phase plate is in different planes, (d) represents the sound field propagation diagram from the transmission side of the second acoustic holographic phase plate to the target plane; (e) represents the sound field amplitude distribution diagrams on different target planes.

[0094] Figure 11 It is the focused sound field distribution diagrams with different focal lengths achieved in Embodiment 2 of the present invention. At this time, when the second acoustic holographic phase plate rotates to different azimuth angles, it will correspond to the focused sound fields with different focal lengths. Among them, (a) represents the first optimized phase distribution diagram and the second optimized phase distribution diagram, (b) represents the sound field propagation diagram after the incident ultrasound is modulated by the first phase acoustic holographic phase plate, (c) represents the amplitude-phase distribution diagram on the transmission side when the second acoustic holographic phase plate rotates to different angles, (d) represents the sound field propagation diagram from the transmission side of the second acoustic holographic phase plate to the target plane; (e) represents the sound field amplitude distribution diagrams on different target planes.

[0095] Explanation of reference numerals: ultrasonic transducer 1, double-opening flange structure 2, dynamic focusing acoustic lens cascade structure 3, first acoustic holographic phase plate 31, liquid-filled cavity 32, second acoustic holographic phase plate 33, sealed bottom plate 34, axial moving guide rail 35, circumferential moving guide rail 36. Detailed implementation manners

[0096] To enable those skilled in the art to better understand the technical solutions of the present invention and be able to implement them, the present invention will be described in detail below in conjunction with the accompanying drawings and specific embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the scope of protection of the present invention.

[0097] In the description of the present invention, it should be understood that the orientation or positional relationship indicated by terms such as "middle", "upper", "lower", "left", "right", "lateral", "longitudinal", "horizontal", "vertical", "axial", "mirror image", "length", "width", "thickness", etc. is based on the orientation or positional relationship shown in the accompanying drawings. It is only for the convenience of describing the technical solutions of the present invention and simplifying the description, rather than indicating or implying that the device or equipment referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation to the present invention. In addition, the terms "comprising" and "having" and any variations thereof are intended to cover non-exclusive inclusion. In the description of the present invention, unless otherwise specified, the meaning of "a plurality" is two or more, which will not be elaborated here.

[0098] As Figure 1 and Figure 2 shown, the cascaded depth dynamic focusing acoustic lens of the present invention is composed of two phase holographic acoustic lens structures cascaded axially. One of the phase holographic acoustic lenses remains fixed, while the other phase holographic acoustic lens can move axially and rotate circumferentially. The two phase holographic acoustic lenses are fixed axially in a liquid cavity and the relative position is changed by moving through the internal guide rail. Both of the two phase holographic plates are polymer structures and are composed of a closely arranged array of multiple microstructural units. The thicknesses of the individual microstructural units are different, forming a structure with one side concave-convex and the other side flat. At this time, the concave-convex plane of the fixed phase plate faces the inside of the cavity, so that both the upper and lower surfaces of the cascaded acoustic lens structure are flat. In actual use, the cascaded depth dynamic focusing acoustic lens can be directly connected to a commercial flat probe through a flange connector and used. At this time, the flat surface of the fixed phase holographic lens in the cascaded depth dynamic focusing acoustic lens is directly coupled with the matching layer of the ultrasonic transducer, coupling the mechanical vibration signal excited by the ultrasonic transducer into the cascaded dynamic focusing acoustic lens, and performing phase modulation on it therein. Then, through the other flat surface of the cascaded depth dynamic focusing acoustic lens, it is perfectly attached to the surface of the object to be measured, and the modulated ultrasonic signal can be coupled into the object to be measured, realizing a dynamically variable focusing sound field along the depth direction.

[0099] The specific dynamic focusing acoustic lens device is designed as Figures 3 - 4 shown.

[0100] It includes an ultrasonic transducer 1 for emitting ultrasonic waves;

[0101] A cascaded structure 3 of a dynamic focusing acoustic lens including an acoustic holographic phase plate with a relative change in the vertical position of two surfaces is arranged in front of the probe of the ultrasonic transducer 1 and is used to perform depth dynamic focusing adjustment on the ultrasonic waves emitted by the ultrasonic transducer 1 and then focus them on the object to be measured.

[0102] It includes a connecting piece for optically coaxially connecting the ultrasonic transducer 1 and the cascaded structure 3 of the dynamic focusing acoustic lens.

[0103] The connecting piece adopts a double-opening flange structure 2. Both ends of the double-opening flange structure 2 are provided with coaxially penetrating openings, and both ends of the openings are threaded, so that both ends of the openings are respectively connected to the ultrasonic transducer 1 and the cascaded structure 3 of the dynamic focusing acoustic lens through threads, thereby coaxially connecting the ultrasonic transducer 1 and the cascaded structure 3 of the dynamic focusing acoustic lens. The dynamic focusing acoustic lens 3 can be tightened by a spiral method through the through hole below the double-opening flange structure 2 through the side thread and positioned by a limiting platform inside the double-opening flange structure 2.

[0104] The cascaded structure 3 of the dynamic focusing acoustic lens includes two acoustic holographic phase plates with relative changes in distance and angle between the phase surfaces, and a liquid for acoustic transmission is arranged between the acoustic holographic phase plates, so that there is always liquid covering between the acoustic holographic phase plates when the relative distance between the phase surfaces changes. The acoustic holographic phase plate is actually a phase holographic acoustic lens.

[0105] More specifically, the cascaded structure 3 of the dynamic focusing acoustic lens specifically includes a first acoustic holographic phase plate 31, a liquid filling cavity 32, a second acoustic holographic phase plate 33, and a sealing bottom plate 34. One end of the liquid filling cavity 32 is fixedly installed with the first acoustic holographic phase plate 31, the other end of the liquid filling cavity 32 is fixedly installed with the sealing bottom plate 34, and the second acoustic holographic phase plate 33 is movably installed in the other end of the liquid filling cavity 32 close to the sealing bottom plate 34. The second acoustic holographic phase plate 33 can axially move or rotate around the center relative to the fixed first acoustic holographic phase plate 31, and the liquid filling cavity 32 between the first acoustic holographic phase plate 31 and the sealing bottom plate 34 is filled with liquid. The liquid can be water.

[0106] Among them, the first acoustic holographic phase plate 31 is relatively fixed and sealed to one end of the liquid filling cavity 32, the sealing bottom plate 34 is relatively fixed and sealed to the other end of the liquid filling cavity 32, and the second acoustic holographic phase plate 33 is relatively movable with respect to the liquid filling cavity 32.

[0107] As Figure 5 shown, the second acoustic holographic phase plate 33 is connected to the inner wall of the liquid filling cavity 32 through an axial movement guide rail 35 and a circumferential movement guide rail 36 structure. As Figure 6As shown, the axial movement guide rail 35 and the circumferential movement guide rail 36 are respectively used for the axial movement or rotation around the center of the second acoustic holographic phase plate 33 relative to the first acoustic holographic phase plate 31; both the axial movement guide rail 35 and the circumferential movement guide rail 36 include a gear, a motor and a rack. The rack is fixedly installed on the inner wall surface of the liquid filling cavity 32, the motor is fixedly installed on the second acoustic holographic phase plate 33, the output shaft of the motor is coaxially connected to the gear, and the gear meshes with the rack. When the motor operates, it drives the gear to rotate. Since the rack is fixed, it further drives the whole composed of the motor, the gear and the second acoustic holographic phase plate 33 to move along the rack.

[0108] Specifically, the axial movement guide rail 35 includes an axial gear, a motor and a linear axial rack. The axial rack is fixedly installed on the inner wall surface of the liquid filling cavity 32 and is arranged along the axial direction. The motor is fixedly installed on the back surface of the second acoustic holographic phase plate 33, the output shaft of the motor is coaxially connected to the axial gear, and the axial gear meshes with the axial rack.

[0109] The circumferential movement guide rail 36 includes a circumferential gear, a motor and an annular circumferential rack. The annular circumferential rack is fixedly installed on the inner wall surface of the liquid filling cavity 32 and is arranged along the circumferential direction. The motor is fixedly installed on the back surface of the second acoustic holographic phase plate 33, the output shaft of the motor is coaxially connected to the circumferential gear, and the circumferential gear meshes with the circumferential rack.

[0110] In specific implementation, it includes an ultrasonic transducer 1, a double-opening flange structure, a structurally fixed first acoustic holographic phase plate, a movable second acoustic holographic phase plate, and a fixed polymer sealing bottom plate 34 arranged in sequence from top to bottom.

[0111] It should be noted that in order to ensure sufficient contact and coupling between the ultrasonic transducer 1 and the dynamic focusing acoustic lens 3, two small holes are processed at the upper opening of the double-opening flange structure 2 for water inlet and outlet respectively, so that the interface where the ultrasonic transducer 1 contacts the dynamic focusing acoustic lens is in a wet state. Among them, the ultrasonic transducer 1, the double-opening flange structure 2, and the dynamic focusing acoustic lens 3 are all symmetric with respect to the axis of the ultrasonic transducer 1. The direction perpendicular to the upper and lower surfaces of the dynamic focusing acoustic lens 3 is denoted as the z direction, and the plane parallel to the upper and lower surfaces of the dynamic focusing acoustic lens 3 is denoted as xoy plane.

[0112] In the present invention, the moving / rotating cascaded acoustic holographic system composed of the first acoustic holographic phase plate 31 and the second acoustic holographic phase plate 33 arranged in parallel along the axis is the core component of the invention. There is relative movement and rotation between the two. During the working process, the first acoustic holographic phase plate 31 is kept fixed at one end face of the liquid filling cavity 32 unchanged, and the second acoustic holographic phase plate 33 is fixed on the axial movement guide rail 35 and the circumferential movement guide rail 36 of the liquid filling cavity 32, and can move or rotate along the guide rail. Its movement and rotation are both controlled by an external control system.

[0113] Both the first acoustic holographic phase plate 31 and the second acoustic holographic phase plate 33 are polymer flat structures with a flat surface on one side and an uneven surface with a phase distribution on the other side. The first acoustic holographic phase plate 31 and the second acoustic holographic phase plate 33 are arranged at intervals along the optical axis, and their uneven surfaces face each other, while their respective flat surfaces face outward.

[0114] The uneven surfaces of the first acoustic holographic phase plate 31 and the second acoustic holographic phase plate 33 can be locally uneven or completely uneven; if they are locally uneven, the locally uneven area is square or circular.

[0115] If both the first acoustic holographic phase plate 31 and the second acoustic holographic phase plate 33 are set with local unevenness, the sizes of their locally uneven areas are the same.

[0116] For the first acoustic holographic phase plate 31 and the second acoustic holographic phase plate 33, a polymer material with an acoustic attenuation coefficient less than 0.2 is used. The polymer material can usually be the resin used in conventional 3D printing and cannot be rubber, etc. The main body parts of the first acoustic holographic phase plate 31 and the second acoustic holographic phase plate 33 are made of the same material as the uneven part and are integrally formed.

[0117] For the sealing bottom plate 34, a polymer material consistent with the liquid filling cavity is used.

[0118] In a specific implementation, the sealing bottom plate 34 can be made of the same material as the first acoustic holographic phase plate 31 and the second acoustic holographic phase plate 33.

[0119] One end of the liquid filling cavity 32 in contact with the conventional ultrasonic transducer 1 is sealed and installed by the first acoustic holographic phase plate fixed on the structure, while the other end is sealed and installed by a polymer sealing bottom plate 34, which can be in direct contact coupling with the measured target through a coupling agent.

[0120] The fixed liquid filling cavity 32 is directly fixed to the conventional ultrasonic transducer 1 through the double - opening flange structure 2 and is in close contact coupling with the matching layer of the ultrasonic transducer 1. The movable second acoustic holographic phase plate 33 can move or rotate in the liquid of the liquid filling cavity 32, and the space vacated after the movement or rotation of the second acoustic holographic phase plate 33 is occupied by the liquid.

[0121] The second acoustic holographic phase plate 33 realizes axial movement along the cascaded acoustic holographic dynamic focusing lens device or circumferential rotation around the cascaded acoustic holographic dynamic focusing lens device through the axial movement guide rail 35 and circumferential movement guide rail 36 structures on the liquid filling cavity 32, so as to realize different focused sound fields in the measured object.

[0122] The optimized phase distributions on the surfaces with unevennesses of the first acoustic holographic phase plate 31 and the second acoustic holographic phase plate 33 are obtained by designing and setting using the improved multi-layer iterative angular spectrum method of the present invention. At the same time, the optimized iterative algorithm optimizes the phase distributions on the planes of the first acoustic holographic phase plate and the second acoustic holographic phase plate, achieving further innovation; the thicknesses of the surfaces with unevennesses on the first acoustic holographic phase plate 31 and the second acoustic holographic phase plate 33 are then set according to the optimized phase distributions on the surfaces with unevennesses.

[0123] In the present invention, based on the known target focal length, the relative distance or relative rotation angle between the first acoustic holographic phase plate and the second acoustic holographic phase plate, first, the conventional iterative angular spectrum method is improved to form a multi-layer iterative angular spectrum method. The phase distributions on the planes of the first acoustic holographic phase plate and the second acoustic holographic phase plate are iteratively designed by the multi-layer iterative angular spectrum method, then the surface thicknesses at different pixel point positions are designed, and finally, the first acoustic holographic phase plate and the second acoustic holographic phase plate can be prepared by additive manufacturing.

[0124] The phase distributions on the surfaces with unevennesses of the first acoustic holographic phase plate 31 and the second acoustic holographic phase plate 33 are obtained by processing using the multi-layer iterative angular spectrum method of the following process;

[0125] z = 0 represents the plane where the first acoustic holographic phase plate 31 emits ultrasonic waves, z = l i ’ represents the plane perpendicular to the optical axis at the second acoustic holographic phase plate 33, z = l i represents the target plane. And the sides of the acoustic holographic phase plate close to and far from the ultrasonic transducer are respectively set as the incident side and the transmission side.

[0126] First, before the iterative optimization starts, the relative distance between the two acoustic holographic phase plates and the target sound field distribution are known in advance. The phase distributions on the uneven surfaces of the first acoustic holographic phase plate 31 and the second acoustic holographic phase plate 33 are respectively used as the first phase distribution φ 1' and the second phase distribution φ 2'. The first phase distribution φ 1' and the second phase distribution φ 2' are randomly set to arbitrary values. The entire iterative optimization process is divided into a forward propagation process and a backward optimization process that are continuously alternately iterated. It is processed according to the following forward propagation process and backward optimization process that are continuously alternately iterated:

[0127] 1) Forward propagation process:

[0128] First, calculate the sound field distribution on the transmission side of the first acoustic holographic phase plate when the ultrasonic wave emitted by the ultrasonic transducer 1 propagates forward. P 0 ( x , y , z = 0) is:

[0129] P 0 ( x , y , z = 0) = e ^(j φ 1')

[0130] Among them, e represents the natural constant, j represents the imaginary number, x , y , z respectively represent the three-dimensional coordinates of the specific position, and ^ represents the power;

[0131] Then, calculate the sound field on the transmission side of the first acoustic holographic phase plate through the forward angular spectrum method P 0 ( x , y , z = 0) propagates along the positive z-axis l i ’ to the sound field distribution on the incident side of the plane of the second acoustic holographic phase plate at a distance P i ’ ( x , y , z = l i ’ ).

[0132] Next, when the incident-side sound field P i ’ ( x , y , z = l i ’ ) propagates through the second acoustic holographic phase plate with a phase distribution of φ 2 ’ , the sound field distribution on its transmission side P i ’’ ( x , y , z = l i’ ) can be further expressed as:

[0133] P i ’’ ( x , y , z = l i ’ ) = P i ’ ( x , y , z = l i ’ ) * e ^(j φ 2 ’ )

[0134] Next, the sound field on the transmission side of the second acoustic holographic phase plate is calculated using the conventional forward angular spectrum method P i ’’ ( x , y , z = l i ’ ) propagates along the positive z-axis to the target plane z = l i of the sound field distribution P i ’’’ ( x, y , z = l i );

[0135] Finally, after the above process is completed, according to the sound field distribution on the target plane z = l i of the sound field distribution P i ’’’ ( x, y , z = l i ) is processed according to the following formula to obtain the sound field distribution on the target plane z = l i :

[0136] First, according to the sound field distribution P i ’’’ (x, y , z = l i ) Obtain its phase distribution: φ i = angle ( P i ’’’ ( x, y , z = l i ))), where angle () represents the phase function, that is, the phase distribution in the sound field distribution is retained, and the amplitude part in the sound field distribution on the target plane z = l i is replaced by the amplitude distribution in the target sound field A i , so as to obtain the optimized sound field distribution propagating forward to the target plane z = l i of Q i ’’’ ( x, y , z = l i ):

[0137] Q i ’’’ ( x, y , z = l i ) = A i *e^(j φ i ( x, y , z = l i ))

[0138] Among them, φ i ( x, y , z = l i ) represents the phase distribution after optimization on the target plane.

[0139] 2) Reverse optimization process:

[0140] For the target plane z = l i optimized sound field Q i’’’ ( x, y , z = l i ), using the reverse angular spectrum method, it propagates from the target plane z = l i along the negative z-axis of the optical axis to the plane of the second acoustic holographic phase plate z = l i ’ The sound field distribution on the transmission side is Q i ’’ ( x , y , z = l i ’ ).

[0141] 3) When the plane where the second acoustic holographic phase plate is located z = l i ’ The sound field distribution in front of the transmission side Q i ’’ ( x , y , z = l i ’ ) propagates backward to the second acoustic holographic phase plate with the phase distribution to be optimized. According to the following formula, based on the sound field distribution on the incident side of the second acoustic holographic phase plate obtained in the forward propagation process of step 1) P i ’ ( x , y , z = l i ’ ) and the sound field distribution on the transmission side of the second acoustic holographic phase plate obtained in the backward propagation process of step 2) P i ’ ( x , y , z = l i ’ ), jointly processed to obtain the optimized second phase distribution φ 2:

[0142] φ 2 = angle(∑( Q i’’ ( x , y , z = l i ’ )*conj( P i ’ ( x , y , z = l i ’ )))),

[0143] where conj() represents the conjugate function, angle() represents the phase extraction function, and ∑() is the summation function.

[0144] Next, based on the optimized phase distribution of the second acoustic holographic phase plate, φ 2 the acoustic field distribution on the plane of the second acoustic holographic phase plate can be calculated according to the following formula z = l i ’ The acoustic field distribution on the incident side Q i ’ ( x , y , z = l i ’ ) is:

[0145] Q i ’ ( x , y , z = l i ’ )= Q i ’’ ( x , y , z = l i ’ )*conj(i φ 2)

[0146] Then, the acoustic field distribution Q i ’ ( x , y , z = l i’ ) Calculate to obtain the backpropagation to the plane of the first acoustic holographic phase plate z = 0 The sound field distribution on the transmission side is:

[0147] Q i ( x, y , z = 0)

[0148] Finally, according to the sound field distribution Q i ( x, y , z = 0) Obtain the optimized first phase distribution according to the following formula:

[0149] φ 1 = angle(∑ Q i ( x, y , z = 0))

[0150] 4) The optimized first phase distribution obtained in step 1) φ 1 and the optimized second phase distribution φ 2 are returned to step 1) and substituted, and the above steps 1) to 3) are repeated multiple times. Specifically, it can be implemented through approximately 60 forward and backward propagation iterations to finally obtain the optimized first phase distribution φ 1 and the optimized second phase distribution φ 2 as the optimized phase distributions on the first acoustic holographic phase plate and the second acoustic holographic phase plate.

[0151] In the case of multiple target sound fields, the sound field distributions Q i ’’ (x, y, z = l i ’ ) and Q i (x, y, z = 0) are various. Therefore, through the summation operation ∑(), the phase distributions of each target sound field on the above two planes are integrated into the finally optimized first phase distribution φ1 and second phase distribution φ2.

[0152] If there are multiple target sound field situations, at this time, the phase distribution situations of all i cases are summed through the phase summation method ∑(), and the phase information corresponding to different target sound field situations is integrated onto the first and second acoustic holographic phase holographic plates. Furthermore, in the subsequent experimental process, by moving and rotating the second acoustic holographic phase plate, multiple target sound field situations can be reconstructed.

[0153] The uneven surfaces in the first acoustic holographic phase plate and the second acoustic holographic phase plate are composed of a closely arranged array of multiple microstructural units. Each microstructural unit has a constant thickness, and the thicknesses of the individual microstructural units are different, thus forming the uneven surface structure. In a specific implementation, each microstructural unit is square, with the same side length dimension pre-set, that is, the periodic interval of the microstructural units is w .

[0154] Next, design the thickness dimensions of the uneven surfaces on the planes of the first acoustic holographic phase plate and the second acoustic holographic phase plate. The thicknesses of the surfaces with unevenness on the first acoustic holographic phase plate 31 and the second acoustic holographic phase plate 33 are obtained through the following process according to the optimized phase distribution of the surfaces with unevenness:

[0155] The optimized phase distribution of the first acoustic holographic phase plate / second acoustic holographic phase plate extracts phase values at equal intervals w to obtain the phase corresponding to the m th microstructural thickness unit φ m , w where w represents the periodic interval of the microstructural units, and then the thickness dimension of each microstructural unit is calculated according to the following formula h um :

[0156] h um = ( φ - k w H) / ( k u -k w )

[0157] where k u and k w are the wave numbers of the sound waves of the liquid filled in the liquid filled cavity 32 of the acoustic holographic phase plate materials of the first and second acoustic holographic phase plates respectively, φ represents the phase of the microstructural thickness unit. H is the measurement plane on the transmission side. Finally, the thickness dimensions of the microstructural units at various positions of the first acoustic holographic phase plate and the second acoustic holographic phase plate are obtained h um ( x , y ).

[0158] In specific implementation, the ultrasonic transducer 1 emits ultrasonic waves, which are focused onto the plane of the target object through the cascaded structure of the dynamic focusing acoustic lens 3. By changing the relative distance and relative rotation angle between the first acoustic holographic phase plate 31 and the second acoustic holographic phase plate 33, the focusing sound field distribution of the overall dynamic focusing acoustic lens cascaded structure 3 based on cascaded acoustic holography is changed, and the focal length of the focused target plane is modulated, greatly improving the sound field modulation freedom and application potential of the dynamic focusing acoustic lens device.

[0159] As Figure 7 shown in (a) and (b) of D , the side views of the first and second acoustic holographic phase plates provided by the present invention are shown. It can be seen that both the first acoustic holographic phase plate 31 and the second acoustic holographic phase plate 33 are composed of an array of microstructure units with different thicknesses, and both show concave-convex changes on one surface and a flat surface on the other side. Among them, the diameters of the first acoustic holographic phase plate 31 and the second acoustic holographic phase plate 33 Figure 7 are equal. w . It should be noted that the thicknesses of the first acoustic holographic phase plate 31 and the second acoustic holographic phase plate 33 h m are designed based on the incident frequency f and the first and second phase distributions of this plane. When changing the target sound field distribution, only the thicknesses of the microstructure units on the first acoustic holographic phase plate 31 and the second acoustic holographic phase plate 33 need to be redesigned h m to reproduce the target focusing sound field.

[0160] Next, a detailed description is given of the realization of the depth dynamic focusing sound field of the dynamic focusing acoustic lens 3 in the embodiment of the present invention. Assume that the working frequency f at this time is 2.5 MHz, the liquid inside the liquid filling cavity 32 and the background medium are water (the sound speed c 0 is 1500 m / s, the wavelength λ = c 0 / f is 0.6 mm), and at the same time, the sound speed and wave number of the liquid filling cavity prepared by 3D printing are respectively c u = 2650 m / s and k u = 2πf / c w = 5.9×10 3 .

[0161] At this time, the side wall thickness and the end face flat plate thickness of the liquid filling cavity 32 are set to be h s = 2 mm, and the inner diameter and outer diameter dimensions are respectively D 1 = 72 mm and D 2 = 76 mm, and the diameters of the first acoustic holographic phase plate 31 and the second acoustic holographic phase plate 32 are D= 50 mm. Then, based on the optimized first and second phase distributions φ 1' and φ 2', the thickness dimensions of the first acoustic holographic phase plate 31 and the second acoustic holographic phase plate 32 are designed h u .

[0162] Example 1 is as follows:

[0163] For a dynamic focusing acoustic lens based on a moving cascaded acoustic holographic system, it includes the following steps:

[0164] S1: Based on Figure 8 (a) of, the focal lengths of three depth focusing sound fields are set to be respectively l 1 = 30 mm, l 2 = 40 mm and l 3 = 50 mm, and the size resolutions of the three focal points are all σ = 0.8 λ . Therefore, the sound field amplitudes on the three target planes at this time are: A 10 ( x , y , z=l 1) = A 20 ( x , y , z=l 2) = A 30 ( x , y , z=l 3) = e^-( x 2 + y 2 / σ 2 );At the same time, it is set that the first acoustic holographic phase plate is located at z= 0 plane, and the second acoustic holographic phase plate can be axially moved to z=l 1 ’ = 12 mm, l 2 ’ = 14 mm andl 3 ’ = 16 mm.

[0165] Next, as Figure 9 shown, design the phase distributions on the plane of the first acoustic holographic phase plate 31 and the plane of the second acoustic holographic phase plate 32 based on the multi-layer iterative angular spectrum method of the present invention z= 0 and z=l i ’ : φ 1' and φ 1':

[0166] First, calculate the sound field on the transmission side of the plane of the first acoustic holographic phase plate z= 0 propagating along the positive z-axis to the three target planes. The calculation process is as follows: Calculate the spectral distribution on the transmission side of the plane of the first acoustic holographic phase plate P 0 ( x , y , z = 0) = e ^( iφ 1 ). Propagate the sound field on the transmission side of the plane of the first acoustic holographic phase plate z= 0 forward P 0 ( k x , k y , z= 0) respectively for l 1 ’ , l 2 ’ , and l 3 ’ distances, and then calculate the sound field distributions on the incident sides of the second acoustic holographic phase plate P 1 ’ ( x , y , z = l 1 ’ ), P 2 ’ ( x , y , z = l 2 ’ ) and P 3 ’ ( x , y ,z = l 3 ’ )。Next, calculate the sound field distribution of the sound field transmitted through the second acoustic holographic phase plate 33 on the transmission side as P i ’’ ( x , y , z = l i ’ )= P i ’ ( x , y , z = l i ’ )* e ^( iφ 2 )。Based on the forward angular spectrum method, further obtain the P 1 ’’ ( x , y , z = l 1 ’ ), P 2 ’’ ( x , y , z = l 2 ’ ) and P 3 ’’ ( x , y , z = l 3 ’ ) propagate along the positive z-axis, and calculate the sound field distributions on the target focusing plane z = l 1 , l 2 , l 3 respectively P 1 ’’’ ( x , y , z = l 1 ), P2 ’’’ ( x , y , z = l 2 ), P 3 ’’’ ( x , y , z = l 3 )。Respectively retain the phase distributions of the three planes φ 01 = arg ( P 1 ’’’ ( x , y , z = l 1 )), φ 02 = arg ( P 2 ’’’ ( x , y , z = l 2 )), φ 03 = arg ( P 3 ’’’ ( x , y , z = l 3 )),and replace the sound field amplitude with A 1, A 2, A 3, the sound field distribution of the target plane can be obtained as Q 1 ’’’ ( x , y , z=l 1)= A 1*e^( jφ 01 ), Q 2 ’’’ ( x , y , z=l 1)= A 2*e^( jφ 02 ), Q3 ’’’ ( x , y , z=l 1)= A 3*e^( iφ 03 )。

[0167] Next, propagate the optimized sound field distribution Q i ’’’ ( x , y , z=l i ) in the reverse direction along the z-axis to the first acoustic holographic phase plane and the second acoustic holographic phase plane. The calculation process is as follows: For the sound field z = l 1 , l 2 , l 3 on the target plane Q 1 ’’’ ( x , y , z=l 1), Q 2 ’’’ ( x , y , z=l 2), Q 3 ’’’ ( x , y , z=l 3) Propagate them respectively in the reverse direction ( l i -l i ’ ) to the transmission side of the second acoustic holographic phase plate z = l 1 ’ , l 2 ’ , l 2 ’ , and obtain the sound field distribution Q 1 ’’ ( x , y , z = l 1 ’ ), Q2 ’’ ( x , y , z = l 2 ’ ) and Q 3 ’’ ( x , y , z = l 3 ’ ). Then, based on the sound field distributions on the incident side and the reflection side of the second acoustic holographic phase plate in the forward propagation and the backward propagation P i ’ ( x , y , z = l i ’ ) and Q i ’’ ( k x , k y , z = l i ’ ), update the phase of the second acoustic holographic phase plate to φ 2 ’ = angle( P 1 ’ ( x, y , z = l 1 ’ ) * conj( Q 1 ’’ ( x, y , z = l 1 ’ )) + P 2 ’ ( x, y , z = l 2 ’ ) * conj( Q 2 ’’ ( x, y , z= l 2 ’ ))+ P 3 ’ ( x, y , z = l 3 ’ )*conj( Q 3 ’’ ( x, y , z = l 3 ’ )))。Then, based on φ 2 ’ and three transmission-side sound fields Q 1 ’’ ( x, y , z = l 1 ’ ), Q 2 ’’ ( x, y , z = l 2 ’ ), Q 3 ’’ ( x, y , z = l 3 ’ ), the incident-side sound field of the second acoustic holographic phase plate is obtained Q i ’ ( x, y , z = l i ’ )= Q i ’’ ( x, y , z = l i ’ )*conj( e ^( iφ 2 )),The Q 1 ’ , Q 2’ , Q 3 ’ Continue separately z = l i ’ Propagate to the transmission side of the first acoustic holographic phase plate z = 0 plane to obtain the sound field distribution Q 1 ( x , y , z = 0), Q 1 ( x , y , z = 0) and Q 1 ( x , y , z = 0), then update the phase distribution of the first acoustic holographic phase plate to φ 1 ’ = angle( Q 1 ( x , y , z = 0) + Q 2 ( x , y , z = 0) + Q 3 ( x , y , z = 0)).

[0168] According to the above forward and backward propagation models for z= 0 and z = l i ’ the phase distribution on the plane φ 1 ’ and φ 2 ’ perform repeated optimization iterations. After about 60 times, convergence can be achieved, and the finally obtained phase distribution φ 1 ’ and φ 2 ’ are as shown in (a) of Figure 10 .

[0169] S2: Then, the thickness of the array elements of the first acoustic holographic phase plate and the second acoustic holographic phase plate h um can be calculated by the following formula: h um =( φ - k w H) / ( k u -k w ), where H = 3λ.

[0170] At this time, the sound field after the incident ultrasound is modulated by the first acoustic holographic phase plate is as shown in Figure 10 (b) of. When the first acoustic holographic phase plate 31 is assembled with a conventional ultrasonic transducer, the original plane wave sound field will be modulated by the first acoustic holographic phase plate 31, and different sound field distributions will be shown on three planes of 10 mm, 12 mm, and 14 mm. The corresponding amplitude and phase distributions are respectively as shown in Figure 10 (c) of (the left column is the amplitude distribution after modulation, and the right column is the phase distribution after modulation). After further modulation by the second acoustic holographic phase plate 33, the transmitted sound field will be sound energy focused at the planes of 30 mm, 40 mm, and 50 mm respectively. The sound field propagating from the transmission side of the second acoustic holographic phase plate to the target plane is as shown in Figure 10 (d) of, and the sound fields of different target planes are as shown in Figure 10 (e) of.

[0171] Example 2 is as follows:

[0172] For a dynamic focusing acoustic lens based on a rotating cascade acoustic holographic system, it includes the following steps: S1: Based on Figure 8 (b) of, first set the focal lengths of the depth focusing sound fields to be l 1 = 30 mm, l 2 = 40 mm, and l 3 = 50 mm respectively, and the lateral resolution of all three focal points is σ = 0.8 λ . Therefore, the sound field amplitudes on the three target planes at this time are: A 10 ( x , y , z=l 1) = A 20 ( x , y , z=l 2) = A 30 ( x , y ,z=l 3)= A e^-( x 2 + y 2 / σ 2 ), where A = 1; At the same time, it is set that the first acoustic holographic phase plate is located in the z= 0 plane, while the second acoustic holographic phase plate is located in the z=l 1 ’ = 12 mm plane, but they will rotate 0°, 90° and 180° respectively around the axis.

[0173] Next, as Figure 9 shown, design the phase distributions on the plane z= 0 of the first acoustic holographic phase plate 31 and the plane z=l 1 ’ of the second acoustic holographic phase plate 33 φ 1 ’ and φ 2 ’ :

[0174] First, calculate the sound field propagating along the positive z-axis to the target plane. The calculation process is as follows: Calculate the sound field on the transmission side of the first acoustic holographic phase plate 31 P 0 ( x , y , z = 0) = e^-( jφ 1 ’ ) propagating forward l 1 ’ distance, and the sound field distribution on the incident side of the second acoustic holographic phase plate 33 plane P 1 ’ ( x , y , z = l 1 ’ ). Then, calculate the sound field distributions on the transmission sides of the second acoustic holographic phase plates 33 respectively as P 1 ’’ ( x , y , z = l 1 ’ ) = P 1 ’ ( x , y , z = l 1’ ) e ^( jφ 2 ’ ), P 2 ’’ ( x , y , z = l 1 ’ )= P 1 ’ ( x , y , z = l 1 ’ ) e ^( j ( φ 2 ’ ) T ), P 3 ’’ ( x , y , z = l 1 ’ )= P 1 ’ ( x , y , z = l 1 ’ ) e ^( j ( φ 2 ’ ) R ). Wherein, 'T' and 'R' represent the transposition and upside-down flipping of the matrix respectively. Further, the above sound field is propagated forward ( l i - l 1 ’ ) distance to the target focal plane, and obtain the target focal plane z = l 1 , l 2 , l 3 Sound field distribution P 1 ’’’ ( x , y , z = l 1 ), P 2 ’’’ (x , y , z = l 2 ), P 3 ’’’ ( x , y , z = l 3 )。Respectively retain the phase distributions of the three planes φ 01 = arg ( P 1 ’’’ ( x , y , z = l 1 )), φ 02 = arg ( P 2 ’’’ ( x , y , z = l 2 )), φ 03 = arg ( P 3 ’’’ ( x , y , z = l 3 )),and replace the sound field amplitude with A 1, A 2, A 3, the sound field distribution of the target plane can be obtained as Q 1 ’’’ ( x , y , z=l 1)= A 1*e^( jφ 01 ), Q 2 ’’’ ( x , y , z=l 1)= A 2*e^( jφ 02 ), Q 3 ’’’ (x , y , z=l 1)= A 3*e^( iφ 03 )。

[0175] Next, the optimized target plane sound field distribution Q i ’’’ ( x , y , z=l i ) is propagated in the reverse direction along the z-axis to the plane of the first acoustic holographic phase plate. The calculation process is as follows: The target plane sound field Q 1 ’’’ ( x , y , z=l 1), Q 2 ’’’ ( x , y , z=l 2), Q 3 ’’’ ( x , y , z= l 3) are respectively propagated in the reverse direction ( l i -l 1 ’ ) and then the sound field distribution on the plane of the second acoustic holographic phase plate is obtained as z = l 1 ’ The sound field distribution on the transmission side is Q 1 ’’ ( x , y , z = l 1 ’ ), Q 2 ’’ ( x , y , z = l 1 ’ ), Q 3 ’’ ( x , y , z = l 1 ’ )。Next, based on the sound field distribution on the incident side of the second acoustic holographic phase plate in the forward propagation P 1’ ( x , y , z = l 1 ’ ) and the sound field distribution on the transmission side in backpropagation Q i ’’ ( k x , k y , z = l i ’ ), update the phase of the second acoustic holographic phase plate to φ 2 ’ = angle( P 1 ’’ ( x, y , z = l 1 ’ ) * conj( Q 1 ’’ ( x, y , z = l 1 ’ )) + P 1 ’’ ( x, y , z = l 1 ’ ) * conj( Q 2 ’’ ( x, y , z = l 1 ’ )) + P 1 ’’ ( x, y , z = l 1 ’ ) * conj( Q 3 ’’ ( x, y , z = l 1 ’ ))). Then, based on φ 2 ’ and the sound field on the transmission side Q 1 ’’ ( x, y , z = l 1’ ) Q 2 ’’ ( x, y , z = l 1 ’ ) Q 3 ’’ ( x, y , z = l 1 ’ )), calculate the sound field on the incident side of the second acoustic holographic phase plate Q 1 ’ ( x, y , z = l 1 ’ )= Q 1 ’’ ( x, y , z = l 1 ’ )*conj( e ^( iφ 2 ’ )), Q 2 ’ ( x, y , z = l 1 ’ )= Q 2 ’’ ( x, y , z = l 1 ’ )*conj( e ^( i ( φ 2 ’ ) T )), Q 3 ’ ( x, y , z = l 1 ’ )= Q 3 ’’ ( x, y , z = l 1 ’ )*conj( e ^( i ( φ2 ’ ) R ) and continue to propagate along the negative z-axis to the transmission side of the first acoustic holographic phase plate z = 0 plane to obtain its sound field distribution Q 1 ( x , y , z = 0), Q 1 ( x , y , z = 0) and Q 1 ( x , y , z = 0), and update the phase distribution of the first acoustic holographic phase plate to φ 1 ’ = angle( Q 1 ( x , y , z = 0) + Q 2 ( x , y , z = 0) + Q 3 ( x , y , z = 0))).

[0176] According to the above forward and backward propagation models for z= 0 and z = l i ’ the phase distribution on the plane φ 1 ’ and φ 2 ’ are repeatedly optimized and iterated. After about 60 times, convergence can be achieved, and the finally obtained phase distributions φ 1 ’ and φ 2 ’ are as shown in (a) of Figure 11 .

[0177] S2: Then, based on the above phase distribution, the array element thicknesses of the first acoustic holographic phase plate and the second acoustic holographic phase plate hui It can be calculated by the following formula: h um =( φ - k w H) / ( k u -k w ), where H = 3λ.

[0178] As shown in (a) of Figure 11 , the phase distributions of the first acoustic holographic phase plate 31 and the second acoustic holographic phase plate 33 are respectively shown. First, when the first acoustic holographic phase plate 31 is assembled with a conventional ultrasonic transducer, the original plane wave sound field will be modulated by the first acoustic holographic phase plate 31. At this time, the sound field after the incident ultrasound is modulated by the first acoustic holographic phase plate is as shown in (b) of Figure 11 . The modulated sound wave continues to propagate forward, and then after being coupled with the second acoustic holographic phase plate 33 rotated by 0°, 90° and 180°, the sound field at z = 12 mm will be remodulated, and the amplitude and phase distributions are respectively shown in (c) of Figure 11 (the left column is the modulated amplitude distribution, and the right column is the modulated phase distribution). The modulated sound wave continues to propagate forward. At this time, the sound field transmitted from the transmission side of the second acoustic holographic phase plate to the target plane is as shown in (d) of Figure 11 . After being modulated by the second acoustic holographic phase plate 33, sound energy will be focused at the planes of 30 mm, 40 mm and 50 mm respectively. At this time, the sound fields of different target planes are as shown in (e) of Figure 11 .

[0179] In the above embodiments, the descriptions of each embodiment have their own emphases. For parts not detailed in a certain embodiment, reference can be made to the relevant descriptions of other embodiments.

[0180] The embodiments of the present application have been introduced in detail above. Specific examples are used in this article to elaborate on the principle and implementation manner of the present invention. The descriptions of the above embodiments are only used to help understand the method and its core idea of the present invention; at the same time, for those of ordinary skill in the art, according to the idea of the present invention, there will be changes in the specific implementation methods and application scopes. In summary, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A depth dynamic focusing acoustic lens device based on cascaded acoustic holography, characterized in that: It includes an ultrasonic transducer (1) for emitting ultrasonic waves; It includes a dynamic focusing acoustic lens cascaded structure (3) with two acoustic holographic phase plates with relative position changes, arranged in front of the probe of the ultrasonic transducer (1) for performing depth dynamic focusing adjustment on the ultrasonic waves emitted by the ultrasonic transducer (1), and then focusing on the object to be measured; It includes a connecting piece for optically coaxially connecting the ultrasonic transducer (1) and the dynamic focusing acoustic lens cascaded structure (3); The dynamic focusing acoustic lens cascaded structure (3) includes two acoustic holographic phase plates with a changing relative distance between the phase surfaces, and a liquid for sound transmission is arranged between the acoustic holographic phase plates; the dynamic focusing acoustic lens cascaded structure (3) specifically includes a first acoustic holographic phase plate (31), a liquid filling cavity (32), a second acoustic holographic phase plate (33) and a sealing bottom plate (34). A first acoustic holographic phase plate (31) is fixedly installed at one end of the liquid filling cavity (32), a sealing bottom plate (34) is fixedly installed at the other end of the liquid filling cavity (32), and a second acoustic holographic phase plate (33) is movably installed in the other end of the liquid filling cavity (32) close to the sealing bottom plate (34). The second acoustic holographic phase plate (33) can axially move or rotate around the center relative to the first acoustic holographic phase plate (31), and the liquid filling cavity (32) between the first acoustic holographic phase plate (31) and the sealing bottom plate (34) is filled with the liquid; The optimized phase distribution on the uneven surfaces of the first acoustic holographic phase plate (31) and the second acoustic holographic phase plate (33) is obtained by setting with the multi-layer iterative angular spectrum method of the following process; First, before the iterative optimization starts, the phase distributions on the uneven surfaces of the first acoustic holographic phase plate (31) and the second acoustic holographic phase plate (33) are respectively used as the first phase distribution φ 1 ’ and the second phase distribution φ 2 ’ , and the first phase distribution φ 1 ’ and the second phase distribution φ 2 ’ are randomly set to arbitrary values, and the following forward propagation process and reverse optimization process that are continuously alternately iterated are performed: 1) Forward propagation process: First, according to the first phase distribution φ 1 ’ , obtain the sound field on the transmission side of the plane where the first acoustic holographic phase plate is located z = 0 P 0 ( x , y , z = 0): P 0 ( x , y , z =0)= e ^(j φ 1 ’ ) where e represents the natural constant and j represents the imaginary number, x , y , z represent the three-dimensional coordinates of specific positions respectively, and ^ represents the power; Next, calculate the acoustic field distribution of the ultrasonic wave propagating along the positive z-axis from the transmission side of the first acoustic holographic phase plate plane z =0 to the plane where the second acoustic holographic phase plate is located z = l i ’ The acoustic field on the incident side is P i ’ ( x , y , z = l i ’ ), and then calculate the plane where the second acoustic holographic phase plate is located z = l i ’ The acoustic field distribution on the incident side P i ’ ( x , y , z = l i ’ ) The transmission side acoustic field distribution after passing through the second acoustic holographic phase plate with the second phase distribution of φ 2 ’ is: P i ’’ ( x , y , z = l i ’ )= P i ’ ( x , y , z = l i ’ )* e ^(j φ 2 ’ ) Similarly, the forward angular spectrum method is used to calculate the sound field distribution on the transmission side of the second acoustic holographic phase plate. P i ’’ ( x , y , z = l i ’ ) Propagate forward to the target plane z = l i The sound field distribution of P i ’’’ ( x,y , z = l i ); Then, according to the target plane obtained by forward propagation z = l i of the sound field distribution P i ’’’ ( x,y , z = l i ) optimize the target plane in the following manner z = l i of the sound field distribution on Q i ’’’ ( x,y , z = l i ): Specifically, according to the sound field distribution P i ’’’ ( x,y , z = l i ) to obtain its phase part: φ i = angle ( P i ’’’ ( x,y , z = l i ) ), where angle () represents the phase extraction function, and then the amplitude part of the sound field distribution P i ’’’ ( x,y , z = l i ) is replaced with a preset target amplitude distribution A i , so as to obtain the optimized sound field distribution in the target plane z = l i in this round of forward propagation Q i ’’’ ( x,y , z = l i ); 2) Reverse optimization process: For the target plane z = l i Optimized sound field distribution Q i ’’’ ( x,y , z = l i ), at this time, the reverse angular spectrum method is used to calculate its reverse propagation to the plane where the second acoustic holographic phase plate is located z = l i ’ The sound field distribution on the transmission side is Q i ’’ ( x , y , z = l i ’ ); 3) According to the plane where the second acoustic holographic phase plate is obtained during this round of forward propagation z = l i ’ The sound field distribution on the incident side P i ’ ( x , y , z = l i ’ ) and the plane where the second acoustic holographic phase plate is obtained during the backward propagation z = l i ’ The sound field distribution on the transmission side Q i ’’ ( x , y , z = l i ’ ), the optimized second phase distribution is obtained according to the following processing φ 2: φ 2= angle (∑( Q i ’’ ( x , y , z = l i ’ )*conj( P i ’ ( x , y , z = l i ’ )))) Among them, conj () represents the conjugate function, angle () represents the phase extraction function; ∑() is the summation function; At this time, the plane where the second acoustic holographic phase plate is located z = l i ’ The sound field distribution on the transmission side Q i ’’ ( x , y , z = l i ’ ) is reversely transmitted through to optimize the second phase distribution to be φ The second acoustic holographic phase plate with 2, and the sound field distribution on the incident side of the second acoustic holographic phase plate is obtained Q i ’ ( x , y , z = l i ’ ) is as follows: Q i ’ ( x , y , z = l i ’ )= Q i ’’ ( x , y , z = l i ’ )*conj( jφ 2) Next, continue to calculate the plane where the second acoustic holographic phase plate is located using the reverse angular spectrum method z = l i ’ The sound field distribution on the incident side Q i ’ ( x , y , z = l i ’ ) Propagate backward to the plane of the first acoustic holographic phase plate z = 0 The sound field distribution on the transmission side Q i ( x,y , z = 0); Finally, from the plane where the first acoustic holographic phase plate is located z = 0 sound field distribution on the transmission side Q i ( x,y , z = 0) is extracted to obtain the optimized first phase distribution: φ 1= angle (∑ Q i ( x,y , z =0)) 4) Substitute the optimized first phase distribution φ 1 obtained in step 1) and the optimized second phase distribution φ 2 back into step 1), and repeat the above steps 1) - 3) multiple times to finally obtain the optimized first phase distribution φ 1 and the optimized second phase distribution φ 2 as the phase distributions on the first acoustic holographic phase plate and the second acoustic holographic phase plate.

2. The depth dynamic focusing acoustic lens device based on cascaded acoustic holography according to claim 1, characterized in that: The connecting piece adopts a double-opening flange structure (2), and both ends of the double-opening flange structure (2) are provided with coaxially penetrating openings, and both ends of the openings are threaded ports.

3. The depth dynamic focusing acoustic lens device based on cascaded acoustic holography according to claim 1, characterized in that: The second acoustic holographic phase plate (33) is connected to the inner wall of the liquid filling cavity (32) through an axial movement guide rail (35) and a circumferential movement guide rail (36) structure; both the axial movement guide rail (35) and the circumferential movement guide rail (36) include gears, motors and racks. The racks are fixedly installed on the inner wall surface of the liquid filling cavity (32), the motors are fixedly installed on the second acoustic holographic phase plate (33), the output shafts of the motors are coaxially connected to the gears, and the gears are engaged with the racks.

4. A depth dynamic focusing acoustic lens device based on cascaded acoustic holography according to claim 1, characterized in that: Both the first acoustic holographic phase plate (31) and the second acoustic holographic phase plate (33) are polymer flat plate structures with a flat surface on one side and an uneven surface with a phase distribution on the other side. The first acoustic holographic phase plate (31) and the second acoustic holographic phase plate (33) are arranged at intervals along the optical axis, and their uneven surfaces are arranged relatively and their flat surfaces are arranged outward.

5. The depth dynamic focusing acoustic lens device based on cascaded acoustic holography according to claim 1, wherein: The first acoustic holographic phase plate (31) and the second acoustic holographic phase plate (33) are made of a polymer material with an acoustic attenuation coefficient less than 10%. The sealing bottom plate (34) and the side wall of the liquid filling cavity are made of the same polymer material as the above-mentioned acoustic holographic phase plates.

6. The depth dynamic focusing acoustic lens device based on cascaded acoustic holography according to claim 1, wherein: The thickness of the surface with unevenness on the first acoustic holographic phase plate (31) and the second acoustic holographic phase plate (33) is set according to the optimized phase distribution of the surface with unevenness.

7. The depth dynamic focusing acoustic lens device based on cascaded acoustic holography according to claim 6, characterized in that: The uneven surfaces in the first acoustic holographic phase plate and the second acoustic holographic phase plate are composed of a closely arranged array of multiple microstructural units. The thickness of each microstructural unit is constant everywhere, and the thicknesses of the individual microstructural units are different. The thicknesses of the uneven microstructural units on the first acoustic holographic phase plate (31) and the second acoustic holographic phase plate (33) are obtained through the following process according to the optimized phase distribution of the first and second acoustic holographic phase plates: The optimized phase distributions of the first and second acoustic holographic phase plates are equally spaced w The phase values are extracted to obtain the phase corresponding to the m th microstructure thickness unit φ m , w denotes the periodic interval of the microstructure units, and then the thickness dimension of each microstructure unit is calculated according to the following formula h um : h um =( φ - k w H) / ( k u -k w ) Among them, k u and k w are the wave numbers of the acoustic holographic phase plate material and the liquid inside the liquid-filled cavity (32), respectively, H is a fixed distance preset for the transmission plane from the incident plane, φ represents the phase of the microstructure thickness unit.

8. A depth dynamic focusing holographic lens modulation method using the depth dynamic focusing acoustic lens device according to any one of claims 1-7, characterized in that: The ultrasonic transducer (1) emits ultrasonic waves, which are focused on the target plane through the dynamic focusing acoustic lens cascade structure (3). By changing the relative distance and relative rotation angle between the first acoustic holographic phase plate (31) and the second acoustic holographic phase plate (33), the focusing sound field distribution of the overall dynamic focusing acoustic lens cascade structure (3) is changed, and the focal length of the focused target plane is modulated.

Images