阅读说明：本技术 一种基于梯度折射率调控的三维多功能声学透镜及其实现方法 (Three-dimensional multifunctional acoustic lens based on gradient refractive index regulation and control and implementation method thereof ) 是由 曹暾 廉盟 段麟秋 王嘉园 陈�峰 贾婧媛 苏莹 于 2021-07-01 设计创作，主要内容包括：一种基于梯度折射率调控的三维多功能声学透镜及其实现方法,属于声学测控领域。曲面声学透镜包括按照梯度变化规律排布的10个温敏水凝胶片段II,每两层温敏水凝胶片段I之间添加一个用于阻隔的热量传递的橡胶薄膜II,水凝胶片段I是由琼脂糖溶液注入3D打印的模具中凝固而成。实现过程中,进行实验平台搭建与测试：建立测试坐标系,将曲面声学透镜、超声换能器、两个水听器放置在水箱中,通过两个水听器A同时测量到瞬时声压分布状态。本发明设计依靠材料折射率梯度变化的曲面声学透镜,建立温度与声学折射率的映射关系,能够实现声学折射率的准确可控；通过实验平台的搭建与声压测试,获得相应的声压曲线,验证曲面声学透镜作用,方法可靠性高。(A three-dimensional multifunctional acoustic lens based on gradient refractive index regulation and a realization method thereof belong to the field of acoustic measurement and control. The curved surface acoustic lens comprises 10 temperature-sensitive hydrogel segments II which are arranged according to a gradient change rule, a rubber film II for blocking heat transfer is added between every two layers of the temperature-sensitive hydrogel segments I, and the hydrogel segments I are formed by injecting agarose solution into a 3D printing mold and solidifying. In the implementation process, the experimental platform is built and tested: and establishing a test coordinate system, placing the curved acoustic lens, the ultrasonic transducer and the two hydrophones in a water tank, and simultaneously measuring the instantaneous sound pressure distribution state through the two hydrophones A. The invention designs the curved surface acoustic lens which depends on the gradient change of the material refractive index, establishes the mapping relation between the temperature and the acoustic refractive index, and can realize the accurate control of the acoustic refractive index; through the establishment of an experimental platform and the sound pressure test, a corresponding sound pressure curve is obtained, the effect of the curved surface acoustic lens is verified, and the method is high in reliability.)

1. The three-dimensional multifunctional acoustic lens based on gradient refractive index regulation is characterized in that the gradient refractive index curved surface acoustic lens comprises 10 temperature-sensitive hydrogel segments I and 9 rubber films II, the 10 temperature-sensitive hydrogel segments I with different acoustic refractive indexes are arranged according to a gradient change rule, one rubber film II used for blocking heat transfer is added between every two layers of the temperature-sensitive hydrogel segments I, and low-temperature-resistant glue is adhered together to form the curved surface acoustic lens.

2. The three-dimensional multifunctional acoustic lens based on gradient refractive index regulation and control as claimed in claim 1, wherein the temperature-sensitive hydrogel segment I is formed by injecting agarose solution with 65% of water content into a 3D printing mold for solidification, the refractive index of the temperature-sensitive hydrogel segment I is related to temperature, and the refractive index of the temperature-sensitive hydrogel segment I can be controlled through physical cooling.

3. A method for realizing a gradient-index based three-dimensional multifunctional curved surface acoustic lens as claimed in claim 1 or 2, which comprises the following steps:

firstly, designing a multifunctional curved surface acoustic lens

The curved surface acoustic lens is mainly formed by arranging 10 temperature-sensitive hydrogel segments I and 9 rubber films II according to a rule, and the acoustic refractive indexes of the 10 temperature-sensitive hydrogel segments I are in gradient change along the central axial direction of the lens; the temperature-sensitive hydrogel segment I comprises a square bottom plate (1) and9 annular gradient curved surface shells (2-10), a round hole is arranged in the middle of the bottom plate (1), and the radius rho of the hole is₀(ii) a The curved surface shells (2-10) are respectively gradient circular ring thin layers with different sizes, the radius is gradually reduced along the axial direction of the lens, the height of each layer is the same, and the wall thickness of 9 circular rings is the same; the thickness of the bottom plate (1) is the same as that of the curved surface shell (2-10); a layer of rubber film II for heat insulation is added between the adjacent temperature-sensitive hydrogel segments I, and the thickness of the rubber film II is smaller than that of the circular ring;

second, lens material selection and preparation

2.1) preparing a curved acoustic lens by adopting a temperature-sensitive hydrogel material, wherein the relationship between the acoustic refractive index and the sound velocity is as follows:

wherein, c₀The acoustic velocity of water is obtained, v is the acoustic velocity of the temperature-sensitive hydrogel material, and n is the acoustic refractive index of the temperature-sensitive hydrogel relative to water;

2.2) when the temperature is reduced, the free water in the temperature-sensitive hydrogel is frozen to cause the sound velocity to be increased, so that the sound velocity of the material is changed, the acoustic refractive index is promoted to be changed, and vice versa; firstly, pre-cooling temperature-sensitive hydrogel with the water content of 65 percent to-30 ℃ by using liquid nitrogen; then, sticking a temperature sensor on the side wall of the temperature-sensitive hydrogel material; and finally, stopping liquid nitrogen injection, measuring the sound velocity of the temperature-sensitive hydrogel material in the temperature rise process in real time at the room temperature of 20 ℃, and fitting the ultrasonic sound velocity curve by adopting a least square method to obtain accurate material parameters, wherein the fitting formula is as follows:

v＝0.6937t²-12.782t+2054.1 (2)

wherein t is temperature, and v is the sound velocity of the temperature-sensitive hydrogel material;

2.3) according to the formulas (1) and (2), the corresponding relation between the temperature of the surface of the temperature-sensitive hydrogel fragment I and the acoustic refractive index of the temperature-sensitive hydrogel fragment I can be obtained;

2.4) assembling a curved acoustic lens: manufacturing a layering mold by utilizing 3D printing, wherein the layering mold is used for fixing the shape of the temperature-sensitive hydrogel; injecting a homogeneous hydrogel solution with the water content of 65% into a 3D printing mold for cooling in a room temperature environment; stripping the hydrogel solution from the mold after the hydrogel solution is in a gel state to obtain different temperature-sensitive hydrogel segments I;

the circuit is used for providing bias voltage to accurately control the temperature and the acoustic refractive index of different temperature-sensitive hydrogel segments I of the curved acoustic lens: firstly, carrying out cooling treatment on temperature-sensitive hydrogel segments I, sticking a plurality of conductive electrode plates on the inner surface of each temperature-sensitive hydrogel segment I, and putting the temperature-sensitive hydrogel segments I in an environment with the constant temperature of-35 ℃; secondly, conducting a conductive electrode plate, heating the temperature-sensitive hydrogel segment I when current passes through the conductive electrode plate, and generating Joule heat to cause the temperature-sensitive hydrogel material to change the state of combined water in the structure of the temperature-sensitive hydrogel material and change the sound velocity of the temperature-sensitive hydrogel segment I material; finally, according to the corresponding relation between the temperature of the temperature-sensitive hydrogel segment I determined in the step 2.3) and the acoustic refractive index thereof, the sound velocity of each temperature-sensitive hydrogel segment I is accurately controlled by the temperature of the temperature-sensitive hydrogel segment I, and further other parameters such as the acoustic refractive index and the like are controlled by the formula (1) and the formula (2); in the process, the processing temperature is different according to different functions of the acoustic lens;

after different temperature-sensitive hydrogel segments I realize the specific sound velocity characteristic, assembling the temperature-sensitive hydrogel segments I and the rubber film II together according to the inner diameter of the ring from large to small, and adhering the adjacent temperature-sensitive hydrogel segments I and the rubber film II together to obtain a curved surface acoustic lens;

thirdly, building and testing an experimental platform

3.1) directly placing the curved surface acoustic lens assembled in the second step into a water tank (14), wherein only the periphery of the bottom plate (1) in the curved surface acoustic lens is directly contacted with water, and a curved surface formed by the gradient curved surface shell (2-10) is positioned above the water surface and isolated from the water area;

3.2) establishing a test coordinate system, taking the vertical projection coordinate of the highest point of the curved surface acoustic lens in the z-axis direction on the bottom plate (1) as an original point, taking the longer side of the bottom plate (1) as the x direction, taking the right side of the curved surface acoustic lens as the positive direction, and taking the shorter side of the bottom plate (1) as the y direction;

3.3) placing an ultrasonic transducer (11) in a water tank (14) for generating a sound wave source on the left side of the curved acoustic lens, wherein the center position coordinate of the sound wave source is (-0.15, 0, 0) and the unit is meter; the method comprises the following steps that a linear slide rail (16) is erected above a water tank (14), is parallel to the y direction and is located between a curved surface acoustic lens and the edge of the water tank (14), and sliders A (17) and B (18) are installed above the linear slide rail (16), wherein the slider A (17) is fixed and is aligned with the edge of the curved surface acoustic lens in the-y direction, and the slider B (18) is installed on one side of the slider A (17) in the + y direction;

3.4) in order to obtain the instantaneous sound pressure distribution state, hydrophones A (12) and B (13) are adopted for simultaneous measurement; cables of hydrophones A (12) and B (13) are respectively fixed on the sliding blocks A (17) and B (18) and are simultaneously connected with an upper computer (15) for signal acquisition and analysis, and measuring heads of the hydrophones A (12) and B (13) are flush with the curved acoustic lens baseplate (1) in the z direction; in the test process, the sliding block A (17) and the hydrophone A (12) are kept still, signals collected by the hydrophone A (12) are used as reference signals, the sliding block B (18) drives the hydrophone B (13) to move along the linear slide rail (16), the length of a moving track L1 is the same as the width of the curved acoustic lens, and 1 transmission sound pressure signal collection is carried out every 2 mm; and performing smooth denoising processing on the obtained sound pressure signals, referring to the sound pressure signals collected by the hydrophone A (12), calculating the signal phase difference between the hydrophone A (12) and the hydrophone B (13), and drawing an instantaneous sound pressure intensity distribution curve.

4. The method for implementing a gradient-index-based three-dimensional multifunctional curved acoustic lens according to claim 3, wherein the curved acoustic lens can implement different acoustic functions, including stealth, focusing and wavefront conversion; wherein the gradient refractive index distribution N (rho) of different acoustic functions of the curved acoustic lens is given in Table 1, wherein N_bThe acoustic refractive index of the temperature-sensitive hydrogel fragment I relative to water under the state of lowest water content is expressed, and rho is the radius of a circular ring of the curved shell (2-10);

TABLE 1 gradient refractive index profiles of different acoustic functions

5. The method for realizing a gradient-index-based three-dimensional curved acoustic lens as claimed in claim 3, wherein the water depth in the water tank (14) in the third step is 5 mm.

Technical Field

The invention belongs to the field of acoustic measurement and control, and relates to a three-dimensional multifunctional acoustic lens based on gradient refractive index regulation and a realization method thereof.

Background

The acoustic Gradient Refractive Index (GRIN) lens is a special acoustic superstructure, can change the propagation path of an acoustic beam due to the fact that the spatial distribution state of the refractive index of the GRIN lens in a medium is changed constantly, has application potential of ultrasonic imaging below a diffraction limit and the like, and provides infinite possibility for the fields of medical instruments, secret communication, acoustic detection and the like. The 'local resonance type acoustic metamaterial focusing lens' in the 2018 patent combines a new structural form by utilizing the basic principle of photonic crystal local resonance, and can achieve good and medium-low broadband sound wave focusing effects in various media such as air or water. The 'broadband focusing lens based on the acoustic resonance super surface' in 2019 patent regulates and controls the phase of a transmitted wave according to the combination mode of different resonance structural units and the structural size of the resonance structural units, realizes focusing of the lens at different positions, extracts and processes the sound pressure intensity on a focusing plane, and analyzes the broadband focusing characteristic of the lens. However, although the acoustic lens has novel and singular characteristics, most of the existing acoustic lenses are all two-dimensional lenses, the acoustic performance depends on a complex geometric array structure, the manufacturing difficulty is high, and the performance is fixed after the structure is determined, so that the practical application of the device is limited.

The invention provides a three-dimensional curved surface acoustic lens which is purely dependent on gradient change of material refractive index, a layered simple structure is adopted, different refractive index segments form a curved surface shell structure, based on the mapping relation between the phase state of a temperature-sensitive hydrogel material and the refractive index of a curved surface shell, the gradient refractive index distribution state is changed by regulating and controlling the temperature of the material, the control of a sound wave propagation path is realized, and a multifunctional waveform conversion acoustic device capable of being switched adjustably is formed.

Disclosure of Invention

The invention aims to solve the problem that once the structure of the existing acoustic lens is determined, the function is determined (namely, the acoustic phenomenon of a single function is only determined), and provides a new technology for realizing the curved-surface gradient-index acoustic lens by using a tunable temperature-sensitive hydrogel material. When the temperature of different temperature-sensitive hydrogels of the curved surface acoustic lens is in a gradient rule, the acoustic lens with different functions can be realized and can be repeatedly realized; in addition, different from the mode that the traditional acoustic lens depends on the gradual change of a geometric array structure, the curved surface acoustic lens adjusts the refractive index of the material in a circuit mode of providing bias voltage, and has the characteristics of simple debugging, convenient operation and low realization cost.

In order to achieve the purpose, the invention adopts the technical scheme that:

the gradient-refractive-index curved-surface acoustic lens comprises 10 temperature-sensitive hydrogel segments I and 9 rubber films II, wherein the 10 temperature-sensitive hydrogel segments I with different acoustic refractive indexes are arranged according to a gradient change rule, one rubber film II is added between every two layers of temperature-sensitive hydrogel segments I, and low-temperature-resistant glue is adhered together to form the curved-surface acoustic lens. Wherein, the rubber film II is used for blocking the heat transfer between every two temperature-sensitive hydrogel segments I.

The temperature-sensitive hydrogel segment I is formed by injecting agarose solution with 65% of water content into a 3D printing mold for solidification, the refractive index of the temperature-sensitive hydrogel segment I is related to temperature, and the refractive index of the temperature-sensitive hydrogel segment I can be controlled through physical cooling.

A method for realizing a three-dimensional multifunctional curved surface acoustic lens based on a gradient refractive index comprises the following steps:

firstly, designing a multifunctional curved surface acoustic lens

The curved surface acoustic lens is mainly formed by arranging 10 temperature-sensitive hydrogel segments I and 9 rubber films II according to a rule, and the acoustic refractive indexes of the 10 temperature-sensitive hydrogel segments I are in gradient change along the central axial direction of the lens. The temperature-sensitive hydrogel segment I comprises a square bottom plate 1 and 9 annular gradient curved surface shells 2-10, wherein a circular hole is arranged in the middle of the bottom plate 1, and the radius rho of the hole is₀(ii) a The curved surface shells 2-10 are respectively gradient circles with different sizesThe radius of the ring thin layer is gradually reduced along the axial direction of the lens, the height of each layer is the same, and the wall thickness of 9 circular rings is the same; the thickness of the bottom plate 1 is the same as that of the curved surface shell 2-10; and a layer of rubber film II for heat insulation is added between the adjacent temperature-sensitive hydrogel segments I, and the thickness of the rubber film II is far smaller than that of the circular ring.

The curved acoustic lens can realize different acoustic functions, including stealth, focusing and wavefront conversion. Wherein the gradient refractive index distribution N (rho) of different acoustic functions of the curved acoustic lens is given in Table 1, wherein N_bThe acoustic refractive index of the temperature-sensitive hydrogel fragment I relative to water in the state of lowest water content is expressed, and rho is the radius of a circular ring of the curved shell 2-10.

TABLE 1 gradient refractive index profiles of different acoustic functions

Second, lens material selection and preparation

2.1) a temperature-sensitive hydrogel material formed by polymerizing agarose in water is adopted to prepare the curved surface acoustic lens, the sound velocity of the material has strong dependence on temperature, and the relationship between the acoustic refractive index and the sound velocity is as follows:

wherein, c₀The acoustic velocity of water is obtained, v is the acoustic velocity of the temperature-sensitive hydrogel material, and n is the acoustic refractive index of the temperature-sensitive hydrogel relative to water.

2.2) when the temperature is reduced, the free water in the temperature-sensitive hydrogel is frozen to cause the sound velocity to be increased, so that the sound velocity of the material is changed, and the acoustic refractive index is changed, and vice versa. Firstly, pre-cooling temperature-sensitive hydrogel with the water content of 65 percent to-30 ℃ by using liquid nitrogen; then, sticking a temperature sensor on the side wall of the temperature-sensitive hydrogel material; and finally, stopping liquid nitrogen injection, measuring the sound velocity of the temperature-sensitive hydrogel material in the temperature rise process in real time at the room temperature of 20 ℃, and fitting the ultrasonic sound velocity curve by adopting a least square method to obtain accurate material parameters, wherein the fitting formula is as follows:

v＝0.6937t²-12.782t+2054.1 (2)

wherein t is temperature, and v is the sound velocity of the temperature-sensitive hydrogel material.

And 2.3) calculating the corresponding relation between the temperature of the surface of the temperature-sensitive hydrogel fragment I and the acoustic refractive index of the temperature-sensitive hydrogel fragment I according to the formulas (1) and (2).

2.4) assembling a curved acoustic lens: and 3D printing is utilized to manufacture a layered die with a Rinehart curved surface, and the layered die is used for fixing the shape of the temperature-sensitive hydrogel. Putting the agarose medium into distilled water, heating the agarose medium to 50 ℃ on a heating table, and uniformly stirring to obtain a homogeneous hydrogel solution with the water content of 65%. And injecting the hydrogel solution into a 3D printing mold to wait for cooling in a room temperature environment. And (3) stripping the hydrogel solution from the mold after the hydrogel solution is in a gel state to obtain different temperature-sensitive hydrogel segments I.

The circuit is used for providing bias voltage to accurately control the temperature and the acoustic refractive index of different temperature-sensitive hydrogel segments I of the curved acoustic lens: firstly, temperature-sensitive hydrogel segments I are cooled, a plurality of conductive electrode plates are pasted on the inner surface of each temperature-sensitive hydrogel segment I, and the temperature-sensitive hydrogel segments I are placed in an environment with the constant temperature of-35 ℃. And secondly, conducting the conductive electrode plate, heating the temperature-sensitive hydrogel segment I when current passes through the conductive electrode plate, and generating Joule heat to cause the temperature-sensitive hydrogel material to change the state of combined water in the structure of the temperature-sensitive hydrogel material and change the sound velocity of the temperature-sensitive hydrogel segment I material. And finally, according to the corresponding relation between the temperature of the temperature-sensitive hydrogel segment I determined in the step 2.3) and the acoustic refractive index thereof, accurately controlling the sound velocity of each temperature-sensitive hydrogel segment I according to the temperature of the temperature-sensitive hydrogel segment I, and further controlling other parameters such as the acoustic refractive index and the like according to the formula (1) and the formula (2). In this process, the processing temperature differs depending on the function of the acoustic lens.

After different temperature-sensitive hydrogel segments I realize the specific sound velocity characteristic, the temperature-sensitive hydrogel segments I and the rubber film II are assembled together according to the inner diameter of the ring from large to small, and the adjacent temperature-sensitive hydrogel segments I and the rubber film II are adhered together by low-temperature-resistant glue to obtain the curved-surface acoustic lens.

Thirdly, building and testing an experimental platform

3.1) directly placing the curved surface acoustic lens assembled in the second step into a water tank 14, wherein the water depth in the water tank 14 is 5mm, in this case, only the periphery of a bottom plate 1 in the acoustic lens is directly contacted with water, and a curved surface formed by the gradient curved surface shells 2-10 is positioned above the water surface, namely the upper surface and the lower surface of the curved surface shells 2-10 of the curved surface acoustic lens are isolated from the water area.

3.2) establishing a test coordinate system, taking the vertical projection coordinate of the highest point of the curved surface acoustic lens in the z-axis direction on the base plate 1 as an original point, taking the longer side of the base plate 1 as the x direction, taking the right side of the curved surface acoustic lens as the positive direction (the right side of the curved surface acoustic lens is a sound pressure value test area specified in the experiment), and taking the shorter side of the base plate 1 as the y direction.

3.3) the ultrasonic transducer 11 is placed in the water tank 14 with a central position coordinate of (-0.15, 0, 0) (in meters) for generating the acoustic wave source on the left side of the curved acoustic lens. The linear slide rail 16 is erected above the water tank 14, is parallel to the y direction and is positioned between the curved acoustic lens and the edge of the water tank 14, and sliders A17 and B18 are installed above the linear slide rail 16, wherein the slider A17 is fixed and is aligned with the edge of the curved acoustic lens in the-y direction, and the slider B18 is installed on one side of the slider A17 in the + y direction.

3.4) in order to obtain the instantaneous sound pressure distribution state, the hydrophones A12 and B13 are adopted for simultaneous measurement. Cables of the hydrophones A12 and B13 are respectively fixed on the sliders A17 and B18 and are simultaneously connected with the upper computer 15 for signal acquisition and analysis, and measuring heads of the hydrophones A12 and B13 are flush with the curved acoustic lens baseplate 1 in the z direction. In the test process, the slider A17 and the hydrophone A12 are kept still, signals collected by the hydrophone A12 serve as reference signals, the slider B18 drives the hydrophone B13 to move along the linear slide rail 16, the length of the moving track L1 is the same as the width of the curved acoustic lens, and 1 transmission sound pressure signal collection is carried out every 2 mm. And performing smooth denoising treatment on the obtained sound pressure signals, referring to the sound pressure signals collected by the hydrophone A12, calculating the signal phase difference between the hydrophone A12 and the hydrophone B13, and drawing an instantaneous sound pressure intensity distribution curve.

The invention has the beneficial effects that:

the invention designs the curved surface acoustic lens which depends on the gradient change of the refractive index of the material, and adopts a layered simple structure to simplify the geometric structure of the three-dimensional acoustic lens; a temperature control method of the acoustic refractive index of the temperature-sensitive hydrogel material is provided, the mapping relation between the temperature and the acoustic refractive index is established, and the acoustic refractive index is accurately controllable; the refractive index distribution model of the acoustic lens with different functions is established, the refractive index distribution model comprises the functions of stealth, focusing and wavefront conversion, and temperature-sensitive hydrogel materials under different temperature control form an adjustable switching multifunctional waveform conversion acoustic device. Through the establishment of an experimental platform and the sound pressure test, a corresponding sound pressure curve is obtained, the effect of the curved surface acoustic lens is verified, and the method is high in reliability.

Drawings

Fig. 1(a) is a structural view of a gradient index-based curved acoustic lens, and fig. 1(b) is a sectional view of the curved acoustic lens, in which: a is the layer height distance of the temperature-sensitive hydrogel segment I, b is the layer height distance of the rubber film II, and c is the thickness of the bottom plate.

Fig. 2 is an experimental scene diagram of a three-dimensional multifunctional acoustic lens based on a gradient refractive index provided by the invention, wherein: l1-movement trajectory.

Fig. 3 is an acoustic canopy test analysis.

Fig. 4 is a diagram of a luneberg lens test analysis.

Fig. 5 is a focused acoustic lens sound pressure test analysis diagram.

In the figure: the temperature-sensitive hydrogel comprises a temperature-sensitive hydrogel segment I, a rubber film II, a bottom plate 1, a 2-10 curved-surface shell, an 11 ultrasonic transducer, a12 hydrophone A, a 13 hydrophone B, a 14 water tank, a 15 upper computer, a 16 linear slide rail, a17 slide block A and a 18 slide block B.

Detailed Description

The embodiments of the present invention will be described in detail with reference to the accompanying drawings and technical solutions.

The gradient-refractive-index curved-surface acoustic lens comprises 10 temperature-sensitive hydrogel segments I and 9 rubber films II, wherein the 10 temperature-sensitive hydrogel segments I with different acoustic refractive indexes are arranged according to a gradient change rule, one rubber film II is added between every two layers of temperature-sensitive hydrogel segments I, and low-temperature-resistant glue is adhered together to form the curved-surface acoustic lens.

The following examples are used to illustrate the gradient index based three-dimensional multifunctional acoustic lens construction and effect testing process.

Firstly, designing a multifunctional curved surface acoustic lens:

the curved surface acoustic lens is mainly formed by arranging temperature-sensitive hydrogel segments I and rubber films II according to the radius of a circular ring, the radius of the circular ring of different temperature-sensitive hydrogel segments I is gradually reduced along the axial direction of the lens, and the acoustic refractive index of the curved surface acoustic lens is in gradient change along the central axial direction of the lens. The temperature-sensitive hydrogel segment I comprises a bottom plate 1 and a gradient curved surface shell 2-10. The bottom plate 1 is a cuboid, the length is 400mm, the width is 300mm, the wall thickness is 4mm, a circular hole is arranged in the middle of the bottom plate 1, and the radius rho of the hole is₀100 mm. The curved surface shells 2-10 are respectively gradient ring thin layers with different sizes, the layer height a is 6.8mm, the wall thickness of the ring is 4mm, and the gradient size distribution of the ring is shown in table 2. A layer of rubber film II is added between the adjacent temperature-sensitive hydrogel segments I for heat insulation, and the thickness b is 0.5 mm.

TABLE 2 gradient size of circular ring of curved surface shell 2-10 in temperature sensitive hydrogel fragment I

Temperature sensitive hydrogel fragment I	2	3	4	5	6	7	8	9	10
										Upper ring external diameter (mm)	99.5	97.7	94.5	89.5	82.5	73	60	39	-
Outer diameter of lower ring (mm)	100	99.4	97.6	94.4	89.3	82.3	72.8	59.7	38.7

Secondly, selecting materials and preparing the lens:

2.1) manufacturing a layered die with a Rinehart curved surface by using a 3D printing process, and fixing the shape of the temperature-sensitive hydrogel. Putting the agarose medium into distilled water, heating the agarose medium to 50 ℃ on a heating table, and uniformly stirring to obtain a homogeneous hydrogel solution with the water content of 65%. And injecting the hydrogel solution into a 3D printing mold to wait for cooling in a room temperature environment. And stripping the hydrogel solution from the mold after the hydrogel solution is in a gel state to obtain the bottom plate 1 and the gradient curved surface shell 2-10 of the temperature-sensitive hydrogel segment I.

2.2) to realize the stealth function of the curved surface acoustic lens, firstly, the gradient refractive index distribution is calculated according to the table 1 and the table 2, and the formula (1) is combined, wherein the underwater sound velocity c₀At 1500m/s, calculating the sound velocity distribution of the bottom plate 1 and the curved surface shell 2-10 in the temperature-sensitive hydrogel segment I, as shown in Table 3:

TABLE 3 Acoustic velocity distribution of temperature sensitive hydrogel segment I for realizing stealth function

2.3) substituting the sound velocity distribution of the bottom plate 1 and the curved surface shell 2-10 in the temperature-sensitive hydrogel segment I into the formula (2), and calculating the control temperature of the bottom plate 1 and the curved surface shell 2-10 in the temperature-sensitive hydrogel segment I as shown in figure 4. Firstly, putting a temperature-sensitive hydrogel fragment I in an environment with the temperature of-35 ℃ until the temperature is balanced; secondly, sticking a plurality of conductive electrode plates on the inner surface of each temperature-sensitive hydrogel segment I; and finally, conducting the conductive electrode plate, heating the temperature-sensitive hydrogel segment I when current passes through the conductive electrode plate, and generating joule heat to cause the temperature-sensitive hydrogel material to change the state of combined water in the structure of the temperature-sensitive hydrogel material and change the sound velocity of the temperature-sensitive hydrogel segment I material.

TABLE 4 temperature distribution of temperature-sensitive hydrogel fragments I for achieving stealth function

Temperature sensitive hydrogel fragment I	1	2	3	4	5	6	7	8	9	10
											Temperature/. degree.C	0	-3	-7.5	-13.5	-15	-17.5	-19	-21	-23	-26

And 2.4) after different temperature-sensitive hydrogel segments I realize the specific sound velocity characteristic, assembling the temperature-sensitive hydrogel segments I and the rubber film II together according to the inner diameter of the ring from large to small, and adhering the adjacent temperature-sensitive hydrogel segments I and the rubber film II together by using low-temperature-resistant glue to obtain the curved-surface acoustic lens.

Thirdly, building and testing an experimental platform

3.1) directly placing the curved surface acoustic lens assembled in the second step into a water tank 14, wherein the water depth in the water tank 14 is 5mm, in this case, only the periphery of a bottom plate 1 in the acoustic lens is directly contacted with water, and a curved surface formed by the gradient curved surface shells 2-10 is positioned above the water surface, namely the upper surface and the lower surface of the curved surface shells 2-10 of the curved surface acoustic lens are isolated from the water area. A cylindrical steel block with the radius of 50mm and the height of 10mm is adopted to simulate an obstacle and is placed in the center of the interior of the curved surface acoustic lens.

3.2) establishing a test coordinate system, taking the vertical projection coordinate of the highest point of the curved surface acoustic lens on the bottom plate 1 in the z-axis direction as an original point, taking the longer side of the bottom plate 1 as the x direction, taking the right side of the curved surface acoustic lens as the positive direction, and taking the shorter side of the bottom plate 1 as the y direction.

3.3) the ultrasonic transducer 11 is placed in the water tank 14 with a central position coordinate of (-0.15, 0, 0) (in meters) for generating the acoustic wave source on the left side of the curved acoustic lens. The linear slide rail 16 is mounted above the water tank 14, is positioned between the x-direction boundary of the curved acoustic lens and the edge of the water tank 14 and is parallel to the y direction, and sliders A17 and B18 are mounted above the linear slide rail, wherein the slider A17 is fixed and is aligned with the edge of the curved acoustic lens in the-y direction, and the slider B18 is mounted on the + y direction side of the slider A17.

3.4) obtaining the instantaneous sound pressure distribution state, and simultaneously measuring by using hydrophones A12 and B13. Cables of the hydrophones A12 and B13 are respectively fixed on the sliders A17 and B18 and are simultaneously connected with the upper computer 15 for signal acquisition and analysis, and measuring heads of the hydrophones A12 and B13 are flush with the curved acoustic lens baseplate 1 along the z direction. In the test process, the slide block A17 and the hydrophone A12 are kept still, the slide block B18 drives the hydrophone B13 to move along the linear slide rail 16, the length of the moving track L1 is the same as the width of the curved acoustic lens, and 1 transmission sound pressure signal acquisition is carried out every 2 mm. And performing smooth denoising treatment on the obtained sound pressure signals, calculating the signal phase difference between the hydrophone A12 and the hydrophone B13 according to the reference sound pressure signals collected by the hydrophone A12, and drawing an instantaneous sound pressure intensity distribution curve. When the curved surface acoustic lens is not provided, the sound pressure of the track L1 is random in the range of-0.1 to 0.1m of the y axis, the sound pressure is lower, and the phase distribution is irregular; when the curved surface acoustic lens exists, the sound pressure distribution of the track L1 shows regular change in the y-axis-0.1 m interval, the phase distribution shows a smooth curve from-pi to pi, the phase distribution accords with the phase characteristic of cylindrical waves, and the stealth function of the curved surface acoustic lens is proved.

Fourthly, other functional demonstration analysis of the multifunctional curved surface acoustic lens

In order to test the wavefront conversion function of the curved acoustic lens, the lens preparation process of the second step is repeated, the refractive index formula calculated above is substituted into the formulas (1) and (2), and the sound velocity of the temperature-sensitive hydrogel segment I is uniformly controlled at 0 ℃. And repeating the experimental test process of the third step. The test curve is shown in fig. 4, wherein when the curved surface acoustic lens is not provided, the sound pressure of the track L1 shows regular change in the y-axis range from-0.1 m to 0.1m, and the phase distribution shows a smooth curve from-pi to pi, which conforms to the phase characteristic of cylindrical waves; when the curved surface acoustic lens exists, the sound pressure distribution of the track L1 is relatively even in the range of-0.1 m of the y axis, the phase distribution is consistent, the phase characteristics of plane waves are met, and the wave front conversion function of the curved surface acoustic lens is proved.

In order to verify the focusing function of the curved acoustic lens, the lens preparation process of the second step is repeated, and the temperature requirements of the curved acoustic lens for realizing the focusing function on different temperature-sensitive hydrogel segments I are as follows:

TABLE 5 temperature distribution of temperature-sensitive hydrogel fragments I that fulfill the focusing function

Temperature sensitive hydrogel fragment I	1	2	3	4	5	6	7	8	9	10
											Temperature/. degree.C	-26	-25.7	-25	-23	-22.5	-21.3	-18	-13.8	-8.3	0

And repeating the experimental test process of the third step to verify the realization effect of the focusing function. The test curve is shown in fig. 5, when the curved surface acoustic lens is not provided, the sound pressure of the track L1 shows regular change in the y-axis-0.1 m interval, and accords with the phase characteristic of cylindrical waves; when the curved acoustic lens exists, the sound pressure distribution of the track L1 is obviously concentrated in the range of-0.05 m of the y axis and is higher than the sound pressure amplitude of cylindrical waves on the L1, and the focusing function of the curved acoustic lens is proved.

In summary, the invention provides a gradient-refractive-index-based three-dimensional multifunctional acoustic lens, which can change the refractive index and the sound velocity of temperature-sensitive hydrogel segments I in different layers in a circuit temperature control manner, and provides a device and an implementation method for implementing a tunable three-dimensional multifunctional acoustic lens.

The above-mentioned embodiments only express the embodiments of the present invention, but not should be understood as the limitation of the scope of the invention patent, it should be noted that, for those skilled in the art, many variations and modifications can be made without departing from the concept of the present invention, and these all fall into the protection scope of the present invention.