An underwater acoustic field detection method and system based on terahertz vortex beam interference and topological charge regulation

Abstract

The present application relates to a kind of underwater acoustic field detection method and system based on terahertz vortex beam interference and topological charge regulation, belong to precision measurement technical field.The terahertz radiation source emits terahertz beam, after collimating by convex lens, it is incident to terahertz vortex beam generator, and incident beam is modulated as vortex beam of specific topological charge;Beam splitter divides vortex beam into reference wave and test wave;Test wave is reflected back by reflector after being, and carries the displacement information of water surface;Test wave carrying water surface displacement information and reference wave interfere at beam splitter;Interference signal is captured by terahertz probe, and according to interference signal, the amplitude, frequency information of water surface displacement is obtained, so as to obtain underwater acoustic field information.The present application is based on the design technology of vortex beam generation and topological charge modulation device of super surface, further improves the integration and detection precision of terahertz water surface displacement detection system, so as to obtain underwater acoustic field information.

Description

Technical Field

[0001] This invention relates to an underwater acoustic field detection method and system based on terahertz vortex beam interferometry and topological charge control, belonging to the field of precision measurement technology. Background Technology

[0002] In recent years, human exploration of the ocean has increased significantly. The detection of underwater acoustic sources can be applied to seabed mapping and depth sounding, underwater resource exploration, marine biodiversity monitoring, underwater target location and communication, and diver life support. The detection of underwater acoustic targets such as submarines and torpedoes has always been a key issue in maritime defense. The sound waves radiated by underwater acoustic sources such as submarines and torpedoes carry relevant information about underwater targets in the surface-excited perturbations. Detecting these perturbations and assessing their characteristics has become a new approach to underwater acoustic source detection. Furthermore, the detection of surface perturbations generated by sound wave excitation has shown potential applications in areas such as submarine communication, seabed mapping, wave spectrum research, and fluid medium characteristic parameter detection.

[0003] Underwater sound field detection technologies mainly include sonar technology and laser-acoustic joint detection technology. Compared with traditional sonar technology, laser-acoustic joint detection technology obtains characteristic information of the underwater sound field by detecting the surface disturbances caused by underwater sound sources. It has advantages such as higher detection accuracy, higher sensitivity, smaller system size, real-time operation, and non-contact operation. Regarding the detection of surface waves, based on different methods of acquiring and demodulating the detection signals, domestic and foreign scholars mainly employ four types of optical detection methods: laser diffraction, optical flux measurement, Doppler vibration measurement, and laser interferometry.

[0004] Terahertz detection technology has developed rapidly since the 19th century, with extensive research conducted in non-destructive testing, secure communications, anti-stealth, and battlefield reconnaissance. Early terahertz detection technologies were primarily based on the physical properties of terahertz waves in different materials and thickness measurements, targeting stationary or simply moving targets. Subsequent testing has expanded to terahertz radar, counter-terrorism security checks, and biomedicine. Terahertz-based vibration detection can be traced back to the detection of human vital signs by Wright State University in 2009. Subsequently, my country's National University of Defense Technology, University of Electronic Science and Technology of China, and Air Force Medical University have also conducted related research, but studies on perturbations to distant targets are limited. The only remaining method for measuring vibration displacement in the terahertz band using interferometry has an accuracy in the millimeter to centimeter range, but the testing frequency is only in the tens of hertz, requiring further development.

[0005] The wavefront of a vortex beam exhibits a helical structure and its intensity is distributed in a ring. This unique structure offers significant advantages and broad application potential in communication, precision measurement, and information security. The key to traditional displacement measurement technology lies in the high-precision measurement of target displacement. Vortex beam interferometry differs significantly from traditional interferometry, and its characteristics can be utilized to achieve higher detection accuracy and resolution, demonstrating great research and application potential. In 2008, Harke et al. combined vortex beams with microscopy, improving the imaging resolution to 25 nm. In 2020, North China University proposed a displacement measurement technique using vortex beams and spherical wave interferometry, achieving a high-precision displacement measurement with an error of 1.25 nm at an incident wave of 632.8 nm. In 2022, Shandong University proposed a dual-frequency vortex beam interferometry algorithm, improving measurement accuracy to the PM level.

[0006] The key to vortex beam measurement lies in the generation and modulation of the vortex beam. Numerous scientists both domestically and internationally have conducted research on vortex beam generation and modulation. Currently, the main methods for vortex beam generation can be divided into intracavity generation and extracavity generation methods. Intracavity generation has the advantage that the spatial phase of the generated vortex beam is unaffected by the frequency bandwidth, but its disadvantage is that it requires strict axisymmetry of the resonant cavity and is not suitable for generating high-order vortex beams. Extracavity generation methods can be further divided into spiral phase plate methods, computational holography, spatial light modulator methods, q-plate methods, and metasurface methods. The spiral phase plate method has high conversion efficiency, but it is limited by the processing precision and cannot be adjusted; the order of the vortex beam generated by the computational holography method is controllable, but it has strict requirements on the equipment; the order of the vortex beam generated by the spatial light modulator method is controllable, but the beam quality is affected by factors such as the resolution of the spatial light modulator and the beam diffraction efficiency; the q-plate method is applicable to a wide frequency band, but it is not suitable for the generation of high-order vortex beams; the metasurface method has small size, high conversion efficiency, and is adjustable, but it is also limited by the processing precision. Summary of the Invention

[0007] This invention comprehensively analyzes various existing displacement measurement methods, vortex beam generation methods, and sound signal processing methods, and proposes a terahertz ultrasensitive displacement measurement technology based on vortex beam interferometry, and a design technology based on metasurface vortex beam generation and topological charge modulation devices. The aim is to further improve the integration and detection accuracy of the terahertz water surface displacement detection system, thereby obtaining underwater sound field information.

[0008] The present invention adopts the following technical solution:

[0009] An underwater acoustic field detection method based on terahertz vortex beam interferometry and topological charge manipulation is used to detect the characteristic information of underwater acoustic fields, including the following steps:

[0010] (1) The terahertz radiation source emits a terahertz beam, which is collimated by a convex lens and then incident on the terahertz vortex beam generator. The incident beam is modulated into a vortex beam with a specific topological charge.

[0011] (2) The beam splitter divides the vortex beam into a reference wave and a test wave;

[0012] (3) The test wave is reflected back by the water surface after passing through the reflector, and carries the displacement information of the water surface.

[0013] (4) The test wave carrying the water surface displacement information interferes with the reference wave at the beam splitter;

[0014] (5) The interference signal is captured by the terahertz probe, and the amplitude and frequency information of the water surface displacement are obtained based on the interference signal.

[0015] Preferably, in step (1), the terahertz vortex beam generator adopts a transmission phase type metasurface and a double-layer moiré structure design, that is, two conjugate transmission phase type metasurfaces are placed facing each other to realize a joint transmission function and a joint phase. The joint transmission function and the joint phase are equal to the superposition of the transmission functions and phases of the two elements. Each transmission phase type metasurface includes a substrate and a metasurface radiation unit on the substrate. The metasurface radiation unit is cylindrical. Phase modulation is achieved by changing the optical path of the electromagnetic wave during transmission by using different cylinder diameters.

[0016] The modulation effect of topological charge can be obtained by rotating one of the transmission phase metasurfaces.

[0017] Preferably, the joint transfer function and joint phase of two conjugate phase elements are equal to the superposition of the transfer functions and phases of the two elements:

[0018]

[0019] Among them, T joint For joint transfer function, φ joint For joint phase, i is the imaginary unit;

[0020] Therefore, the phase distribution design of the two metasurfaces can be obtained:

[0021]

[0022] Where 'a' is a design parameter used to control the adjustment relationship between the rotation angle of the two surfaces and the topological load. Let be the angle from 0 to 2π in polar coordinates; when a metasurface is rotated by an angle θ, its phase becomes:

[0023]

[0024] The phase of the entire bilayer metasurface can be written as:

[0025]

[0026] Where the rotation angle θ is a known value, therefore the second term is... It is irrelevant and has no effect on the generation of vortex phase, so it can be ignored. Furthermore, the relationship between the phase, topological charge, and angle of the vortex beam is as follows:

[0027]

[0028] The topological charge l generated by the vortex beam can be derived as:

[0029] l=2aθ (6)

[0030] Using this vortex beam generator, vortex beams with different topological charges can be obtained by rotating one of the metasurfaces to generate an angle θ. Clockwise rotation generates positive topological charges, while counterclockwise rotation generates negative topological charges. Traditional vortex beam generators are often wavelength-dependent and cannot operate in broadband environments. The dual-layer metasurface generator in this invention only needs to calibrate the correspondence between the rotation angle and the change in topological charge at different wavelengths to achieve the generation and modulation of high-order topological charge vortex beams in broadband environments.

[0031] Preferably, the terahertz vortex beam generator is packaged and integrated using a rotary mounting base. One side of the rotary mounting base is equipped with a transmission phase type metasurface, and the other side is equipped with a second transmission phase type metasurface. The two transmission phase type metasurfaces are arranged facing each other. By symmetrically placing the two metasurfaces, the initial joint phase of the two metasurfaces is 0. The center of the rotary mounting base is provided with a central hole to allow the beam to pass through. The rotary mounting base serves to clamp and rotate one of the transmission phase type metasurfaces.

[0032] One side of the rotating mounting base is fixed, while the other side is equipped with a rotating sleeve. One of the transmission phase type metasurfaces is installed inside the rotating sleeve. The rotation of the rotating sleeve drives one of the transmission phase type metasurfaces to perform precise rotational positioning. The other metasurface is fixed to the non-rotating part of the rotating mounting base through a cage structure, thereby realizing the precise relative rotation of the two metasurfaces in the vortex beam generator. By changing the relative rotation angle θ, the topological charge can be controlled.

[0033] Preferably, the rotating sleeve is connected to a driver, which ensures the precise rotation of the rotating sleeve. The rotary mounting base of the present invention, combined with the driver, encapsulates and integrates a precise rotation positioning device, providing high accuracy and high repeatability.

[0034] It is worth noting that the rotary mounting base only needs to be able to rotate, and the driver only needs to be able to perform precise rotation control; there are no specific requirements for the structure.

[0035] This invention employs a transmission-phase metasurface and a double-layer moiré structure to design a vortex beam generator, modulating a terahertz beam emitted from a terahertz source while simultaneously achieving device miniaturization. The transmission-phase metasurface achieves phase modulation by altering the optical path length of the electromagnetic wave during propagation. Specifically, changing the geometric parameters of the metasurface's radiating units, such as the cylinder diameter or the length and width of the square prism, changes the duty cycle of the structure, thereby altering its equivalent refractive index and achieving different phase modulations at high gain.

[0036] By arranging different metasurface units into an array according to the phase modulation of the desired vortex beam, the designed metasurface vortex beam generator can be realized.

[0037] Preferably, the water surface displacement information carried in step (3) can be quantitatively analyzed through modeling. The assumption that the water surface perturbation caused by the underwater sound field generates surface transverse waves is based on the following premise: the water-air interface is not an ideal pressure release surface. Thus, the pressure change generated by the underwater sound field will cause perturbation on the water surface. Under the action of the underwater sound field, a "thin layer" of submicron perturbation will form on the water surface, that is, surface transverse microwaves, or surface microwaves for short, will be generated. The perturbation frequency is equal to the frequency of the underwater sound field. Therefore, as long as the perturbation information of the surface microwaves is extracted, the frequency information of the underwater sound field can be restored. When there is an underwater sound field, surface waves will be formed on the water surface:

[0038]

[0039] Among them, A x The amplitude of the surface wave excited by the underwater sound field; ωx and ω and t are the angular frequency and initial phase of the water surface wave excited by the underwater sound field, respectively (the initial phase can be set to 0 without affecting the generality), and t is the propagation time of the water surface wave.

[0040] In real water environments, due to the influence of external environmental disturbances, low-frequency noise with an amplitude in the micrometer range always exists on the water surface:

[0041]

[0042] Where A0 is the amplitude of the noisy water surface wave; ω0 and These are the angular frequency and initial phase of the noisy water surface wave, respectively (the initial phase can be set to 0 without affecting the generality).

[0043] Therefore, the surface disturbance of water is:

[0044] S(t)=S0(t)+S x (t)=A0sinω0t+A x sinω x t (9)

[0045] Underwater acoustic field information is extracted by measuring surface perturbations.

[0046] Preferably, a dual terahertz probe and a displacement device are used to detect the interference signal. Assuming that the vortex beam generated by the vortex beam generator is split, the reference wave can be expressed by the following formula:

[0047] E1(r, Θ)=Aexp(ilΘ)exp(ikz1) (10)

[0048] Where A is the amplitude, l is the topological charge number, and Θ is the azimuth angle. λ is the wave number, λ is the wavelength, and z1 is the optical path length of the reference arm;

[0049] The test beam is conjugate with the reference beam, which can be expressed by the following formula:

[0050] E2(r, Θ)=Aexp(-ilΘ)exp(ikz2) (11)

[0051] Where z2 is the optical path length of the test arm;

[0052] In the absence of an underwater sound field, the intensity of the interference between the two vortex wave beams is:

[0053] I0 = |E1 + E2| 2 =2A 2 +2A 2 cos[2lΘ+k(z1-z2)-2kS0(t)] (12)

[0054] Under the influence of surface waves in noise, the interference pattern will rotate. Let the rotation angle be ΔΘ0, then the following equality relationship exists:

[0055] 2lΔΘ0=2kS0(t) (13)

[0056] Noise surface waves can be represented as:

[0057]

[0058] When there is a slight disturbance on the water surface, the intensity of the interference becomes:

[0059] I = |E1 + E2| 2 =2A 2 +2A 2 coS[2lΘ+k(z1-z2)-2kS(t)] (15)

[0060] After an underwater sound field excites a perturbation on the water surface, the resulting interference pattern will rotate. Let the rotation angle be ΔΘ, then:

[0061] 2lΔΘ=2kS(t) (16)

[0062] Minor disturbances to the water surface can be represented as:

[0063]

[0064] Combining equations (9), (14), and (17), we can conclude that:

[0065]

[0066] Where l and λ are known quantities, the detection of surface perturbations is transformed into the precise extraction of the rotation angle of the interference pattern.

[0067] Preferably, in step (5), after the terahertz probe captures the interference signal, it is necessary to determine whether the "petal"-shaped interference fringes exceed the detection range. Considering the characteristics of vortex beam interference, that is, the longer the working wavelength of the measured radiation, the greater the topological charge of the vortex beam, and the smaller the fringe angle rotation at the same distance, the displacement amplitude response range can be adjusted by changing the working wavelength or modulating the topological charge of the vortex beam.

[0068] A system for implementing the above-mentioned underwater acoustic field detection method based on terahertz vortex beam interferometry and topological charge control includes a terahertz radiation source, a convex lens, a terahertz vortex beam generator, beam splitter A, beam splitter B, beam splitter C, a reflector A, a reflector B, the water surface to be measured, a terahertz probe, and a visible band laser.

[0069] The terahertz spherical wave emitted by the terahertz radiation source passes sequentially through a convex lens, a terahertz vortex beam generator, beam splitter A, beam splitter C, reflector A, the water surface to be measured, reflector A, beam splitter C, reflector B, beam splitter B, and the terahertz probe as a test arm.

[0070] The terahertz spherical wave emitted by the terahertz radiation source passes sequentially through a convex lens, a terahertz vortex beam generator, beam splitter A, beam splitter B, and a terahertz probe as a reference arm.

[0071] The visible band laser is a visible light fiber laser. The laser emitted by the visible band laser passes sequentially through beam splitter B, mirror B, beam splitter C, mirror A, the water surface to be measured, mirror A, beam splitter C, mirror B, and beam splitter B as an auxiliary aiming optical path.

[0072] The terahertz spherical beam emitted by the terahertz radiation source is first transformed into a planar beam by a convex lens, then modulated into a terahertz vortex beam with a specific topological charge by a terahertz vortex beam generator, and then split into a mutually conjugate transmitted terahertz vortex beam and a reflected terahertz vortex beam by a beam splitter A.

[0073] The terahertz vortex beam transmitted through beam splitter A is used as the test beam. After passing through beam splitter C and reflector A, it is incident on the water surface to be measured, carrying the displacement information of the water surface. After reflection, it is reflected by reflector A, beam splitter C, reflector B and beam splitter B before entering the terahertz probe.

[0074] The terahertz vortex beam reflected by beam splitter A serves as a reference beam. After passing through beam splitter B, it enters the probe. The reference beam and the test beam are combined to form "petal"-shaped interference fringes. The amplitude and frequency information of the water surface displacement are captured by the terahertz probe.

[0075] Preferably, the system is integrated in a modular manner, consisting of a radiation module, an auxiliary aiming module, and a detection module;

[0076] The radiation module is integrated with a terahertz radiation source, a convex lens, and a terahertz vortex beam generator. This module emits a terahertz vortex beam as the radiation source of the detection system.

[0077] The aiming assistance module consists of a visible light fiber laser, whose emitted light is visible light and is coaxial with the terahertz wave emitted by the radiation module. It is used to assist aiming and adjust the position of the detection module.

[0078] Considering the unknown initial fringe angle and the distortion of interference fringes caused by complex environmental factors to the test arm beam, the detection module consists of two terahertz probes and a displacement device. The two terahertz probes are mounted on the displacement device, and their positions can be adjusted via the device. The approximate position can be determined using a visible fiber laser in the auxiliary aiming module. While the terahertz beam is invisible to the naked eye, the visible laser beam is. The visible laser and the terahertz beam used for measurement are coaxial, allowing the approximate location of the terahertz beam center to be determined from the position of the visible laser beam. The interference intensity fringes are arranged around the beam's optical axis. Moving the detector around the optical axis near the interference pattern, the intensity value reaches its maximum when the center of the detector's detection area coincides with the center of a "petal-shaped" fringe. After moving around the optical axis, the detector's intensity value decreases and reaches its minimum level when it reaches the next "petal-shaped" fringe, indicating that there are no "petal-shaped" fringes in the detector's detection area. Therefore, the optimal position for the detector is when the intensity decreases to near its minimum level, indicating that the detector is at the edge of a "petal-shaped" fringe. Finally, based on the preliminary detection results, the topological charge of the vortex beam generated by the metasurface vortex beam generator was adjusted to determine the most suitable displacement amplitude sensitive range.

[0079] After adjusting the visible light fiber laser in the auxiliary aiming module to a rough position, the terahertz probe is then adjusted to the optimal position using the "petal-shaped" interference fringe displacement device.

[0080] Preferably, considering the aberrations caused by factors such as surface noise, component defects, and misalignment, as well as possible fringe distortion, twisting, and uneven distribution, the rate of change of the "petal" fringe angle at different locations may be inconsistent. That is, under the same displacement, the angle change may vary significantly. Therefore, this invention uses symmetrically distributed dual-intensity probes to detect and record the angle change of interference fringes. Two terahertz probes are placed at the edges of the "petals" of the fringes and are centrally symmetrically distributed. When the angle change of the "petal" fringe measured by one probe is too small due to aberration distortion to obtain intensity change information, the second probe can obtain sufficient change information. By detecting the increase or decrease in intensity, the interference fringe angle change information is obtained, thereby obtaining the frequency and amplitude information of the displacement.

[0081] However, when the "petal" angle changes too much, the fringes may completely fall outside the detector's monitoring range, making it impossible to measure information and distinguish the direction of the fringe angle change. Therefore, this invention provides an amplitude response range for displacement measurement. Considering the characteristics of vortex beam interferometry—that is, the longer the operating wavelength of the measured radiation, the greater the topological charge of the vortex beam, and the smaller the fringe angle rotation at the same distance—changing the operating wavelength or modulating the topological charge of the vortex beam can adjust the displacement amplitude response range. This invention employs topological charge modulation, adjusting the minimum and maximum values ​​of the displacement amplitude response range by increasing or decreasing the number of topological charges: when the underwater sound field is small, the amplitude of surface disturbances is small, and a low-topological-charge vortex beam is used for measurement; when the underwater sound field is large, the amplitude of surface disturbances may reach 5 μm or even larger, so as the signal strength increases, the number of topological charges is changed in real time, and a larger upper limit of the response amplitude is obtained through a vortex beam with a higher-order topological charge, thereby achieving a large amplitude range detection of underwater acoustic signals.

[0082] For any details not covered in this invention, please refer to the prior art.

[0083] The beneficial effects of this invention are as follows:

[0084] 1. Compared with the prior art, the terahertz vortex beam interferometry method used in this invention has the advantages of higher detection accuracy and higher sensitivity in measuring water surface perturbations, while its system size is smaller and its integration is higher.

[0085] 2. The vortex beam topology charge of the metasurface broadband adjustable vortex beam generator used in this invention is adjustable, with a larger displacement amplitude response range and a wider operating frequency band.

[0086] 3. The dual terahertz intensity probe detection scheme used in this invention can avoid interference fringe distortion caused by aberrations in the test arm beam due to environmental disturbances and other factors, as well as the influence of unknown initial "petal" angles caused by uncertain optical path differences between the test arm and the reference arm. Compared with the area array terahertz camera detection method, it has the advantages of high response speed, high detection frequency and low cost.

[0087] 4. Water molecules have a high absorption rate for the terahertz band, so the propagation distance of terahertz waves underwater is extremely short. This invention utilizes this characteristic to avoid interference from various underwater factors and has higher anti-interference capability. Attached Figure Description

[0088] Figure 1 This is a schematic diagram of the intensity distribution of the vortex beam;

[0089] Figure 2 This is a schematic diagram of the phase distribution of the vortex beam;

[0090] Figure 3 This is a flowchart of the underwater acoustic field detection method based on terahertz vortex beam interferometry and topological charge control of the present invention.

[0091] Figure 4 A schematic diagram showing the changes in interference fringes of a vortex beam under different displacements;

[0092] Figure 5(a) is a schematic diagram of detecting the change in the angle of interference fringes using a single intensity probe;

[0093] Figure 5(b) is a schematic diagram of detecting the change in the angle of interference fringes using a multi-intensity probe;

[0094] Figure 6 A schematic diagram of a centrally symmetrically distributed dual-intensity probe measuring aberrations in fringes;

[0095] Figure 7 A schematic diagram showing the variation of displacement response range under different topological load numbers;

[0096] Figure 8 The diagram shows the structure of a terahertz vortex beam generator, where (a) is a phase diagram of the transmission phase type metasurface with an overall spiral stepped shape; and (b) is a diagram of two terahertz beams with different topological charges obtained from the transmission phase type metasurface.

[0097] Figure 9 A schematic diagram of the rotating mounting base structure for a terahertz vortex beam generator;

[0098] Figure 10 A schematic diagram of the system structure for realizing an underwater acoustic field detection method based on terahertz vortex beam interferometry and topological charge modulation;

[0099] Among them, 1-terahertz radiation source, 2-convex lens, 3-terahertz vortex beam generator, 4-beam splitter A, 5-beam splitter B, 6-beam splitter C, 7-reflector A, 8-water surface to be measured, 9-reflector B, 10-terahertz probe, 11-visible fiber laser. Detailed implementation method:

[0100] To make the technical problems, technical solutions and advantages of the present invention clearer, a detailed description will be given below in conjunction with the accompanying drawings and specific embodiments. However, this description is not limited thereto. All aspects not described in detail in the present invention are based on conventional techniques in the field.

[0101] Example 1:

[0102] An underwater acoustic field detection method based on terahertz vortex beam interferometry and topological charge manipulation, such as Figures 1-7 As shown, it includes the following steps:

[0103] (1) The terahertz radiation source emits a terahertz beam, which is collimated by a convex lens and then incident on the terahertz vortex beam generator. The incident beam is modulated into a vortex beam with a specific topological charge.

[0104] (2) The beam splitter divides the vortex beam into a reference wave and a test wave;

[0105] (3) The test wave is reflected back by the water surface after passing through the reflector, and carries the displacement information of the water surface.

[0106] (4) The test wave carrying the water surface displacement information interferes with the reference wave at the beam splitter;

[0107] (5) The interference signal is captured by the terahertz probe, and the amplitude and frequency information of the water surface displacement are obtained based on the interference signal.

[0108] Example 2:

[0109] An underwater acoustic field detection method based on terahertz vortex beam interferometry and topological charge modulation, as described in Example 1, differs in that, in step (1), the terahertz vortex beam generator adopts a transmission phase type metasurface and a double-layer moiré structure design, that is, two conjugate transmission phase type metasurfaces are placed facing each other to realize a joint transmission function and a joint phase, which are equal to the superposition of the transmission functions and phases of the two elements; wherein, each transmission phase type metasurface includes a substrate and a metasurface radiation unit on the substrate, and the metasurface radiation unit is cylindrical, such as Figure 8 (b) Phase modulation is achieved by using different cylinder diameters to change the optical path of electromagnetic waves during transmission.

[0110] The modulation effect of topological charge can be obtained by rotating one of the transmission phase metasurfaces.

[0111] The joint transfer function and joint phase of two conjugate phase elements are equal to the superposition of the transfer functions and phases of the two elements:

[0112]

[0113] Among them, T joint For joint transfer function, φ joint For joint phase, i is the imaginary unit;

[0114] Therefore, the phase distribution design of the two metasurfaces can be obtained:

[0115]

[0116] Where 'a' is a design parameter used to control the adjustment relationship between the rotation angle of the two surfaces and the topological load. Let be the angle from 0 to 2π in polar coordinates; when a metasurface is rotated by an angle θ, its phase becomes:

[0117]

[0118] The phase of the entire bilayer metasurface can be written as:

[0119]

[0120] Where the rotation angle θ is a known value, therefore the second term is... It is irrelevant and has no effect on the generation of vortex phase, so it can be ignored. Furthermore, the relationship between the phase, topological charge, and angle of the vortex beam is as follows:

[0121]

[0122] The topological charge l generated by the vortex beam can be derived as:

[0123] l=2aθ (6)

[0124] Using this vortex beam generator, vortex beams with different topological charges can be obtained by rotating one of the metasurfaces to generate an angle θ. Clockwise rotation generates positive topological charges, while counterclockwise rotation generates negative topological charges. Traditional vortex beam generators are often wavelength-dependent and cannot operate in broadband environments. The dual-layer metasurface generator in this invention only needs to calibrate the correspondence between the rotation angle and the change in topological charge at different wavelengths to achieve the generation and modulation of high-order topological charge vortex beams in broadband environments.

[0125] Example 3:

[0126] An underwater acoustic field detection method based on terahertz vortex beam interferometry and topological charge modulation, as described in Example 2, differs in that the terahertz vortex beam generator is packaged and integrated using a rotary mounting base, as shown in Example 2. Figure 8 , 9 As shown, a transmission phase type metasurface is mounted on one side of the rotary mount, and a second transmission phase type metasurface is mounted on the other side. The two transmission phase type metasurfaces are arranged facing each other. By placing the two metasurfaces symmetrically, the initial joint phase of the two metasurfaces is 0. A central hole is provided in the center of the rotary mount to allow the beam to pass through. The rotary mount serves to clamp and rotate one of the transmission phase type metasurfaces.

[0127] One side of the rotating mounting base is fixed, while the other side is equipped with a rotating sleeve. One of the transmission phase type metasurfaces is installed inside the rotating sleeve. The rotation of the rotating sleeve drives one of the transmission phase type metasurfaces to perform precise rotational positioning. The other metasurface is fixed to the non-rotating part of the rotating mounting base through a cage structure, thereby realizing the precise relative rotation of the two metasurfaces in the vortex beam generator. By changing the relative rotation angle θ, the topological charge can be controlled.

[0128] Preferably, the rotating sleeve is connected to a driver, which ensures the precise rotation of the rotating sleeve. The rotary mounting base of the present invention, combined with the driver, encapsulates and integrates a precise rotation positioning device, providing high accuracy and high repeatability.

[0129] It is worth noting that the rotary mounting base only needs to be able to rotate, and the driver only needs to be able to perform precise rotation control; there are no specific requirements for the structure.

[0130] This invention employs a transmission-phase metasurface and a double-layer moiré structure to design a vortex beam generator, modulating a terahertz beam emitted from a terahertz source while simultaneously achieving device miniaturization. The transmission-phase metasurface achieves phase modulation by altering the optical path length of the electromagnetic wave during propagation. Specifically, changing the geometric parameters of the metasurface's radiating units, such as the cylinder diameter or the length and width of the square prism, changes the duty cycle of the structure, thereby altering its equivalent refractive index and achieving different phase modulations at high gain.

[0131] By arranging different metasurface units into an array according to the phase modulation of the desired vortex beam, the designed metasurface vortex beam generator can be realized.

[0132] Example 4:

[0133] An underwater acoustic field detection method based on terahertz vortex beam interferometry and topological charge modulation, as described in Example 1, differs in that the water surface displacement information carried in step (3) can be quantitatively analyzed through modeling. The assumption that the water surface perturbation caused by the underwater acoustic field generates surface transverse waves is based on the following premise: the water-air interface is not an ideal pressure release surface. Thus, the pressure change generated by the underwater acoustic field will cause perturbation on the water surface. Under the action of the underwater acoustic field, a "thin layer" of submicron perturbation will form on the water surface, that is, surface transverse microwaves, or surface microwaves for short, will be generated. The perturbation frequency is equal to the frequency of the underwater acoustic field. Therefore, as long as the perturbation information of the surface microwaves is extracted, the frequency information of the underwater acoustic field can be reconstructed. When an underwater acoustic field exists, surface waves will be formed on the water surface:

[0134]

[0135] Among them, A x ω represents the amplitude of the surface wave excited by the underwater sound field. x and ω and t are the angular frequency and initial phase of the water surface wave excited by the underwater sound field, respectively (the initial phase can be set to 0 without affecting the generality), and t is the propagation time of the water surface wave.

[0136] In real water environments, due to the influence of external environmental disturbances, low-frequency noise with an amplitude in the micrometer range always exists on the water surface:

[0137]

[0138] Where A0 is the amplitude of the noisy water surface wave; ω0 and These are the angular frequency and initial phase of the noisy water surface wave, respectively (the initial phase can be set to 0 without affecting the generality).

[0139] Therefore, the surface disturbance of water is:

[0140] S(t)=S0(t)+S x (t)=A0sinω0t+A x sinω x t (9)

[0141] Underwater acoustic field information is extracted by measuring surface perturbations.

[0142] Example 5:

[0143] An underwater acoustic field detection method based on terahertz vortex beam interferometry and topological charge modulation, as described in Example 4, differs in that it uses dual terahertz probes and a displacement device to detect the interference signal. Assuming that the vortex beam generated by the vortex beam generator is split, the reference wave can be expressed by the following formula:

[0144] E1(r, Θ)=Aexp(ilΘ)exp(ikz1) (10)

[0145] Where A is the amplitude, l is the topological charge number, and Θ is the azimuth angle. λ is the wave number, λ is the wavelength, and z1 is the optical path length of the reference arm;

[0146] The test beam is conjugate with the reference beam, which can be expressed by the following formula:

[0147] E2(r, Θ)=Aexp(-ilΘ)exp(ikz2) (11)

[0148] Where z2 is the optical path length of the test arm;

[0149] In the absence of an underwater sound field, the intensity of the interference between the two vortex wave beams is:

[0150] I0 = |E1 + E2| 2 =2A 2 +2A 2 cos[2lΘ+k(z1-z2)-2kS0(t)] (12)

[0151] Under the influence of surface waves in noise, the interference pattern will rotate. Let the rotation angle be ΔΘ0, then the following equality relationship exists:

[0152] 2lΔΘ0=2kS0(t) (13)

[0153] Noise surface waves can be represented as:

[0154]

[0155] When there is a slight disturbance on the water surface, the intensity of the interference becomes:

[0156] I = |E1 + E2| 2 =2A 2 +2A 2 cos[2lΘ+k(z1-z2)-2kS(t)] (15)

[0157] After an underwater sound field excites a perturbation on the water surface, the resulting interference pattern will rotate. Let the rotation angle be ΔΘ, then:

[0158] 2lΔΘ=2kS(t) (16)

[0159] Minor disturbances to the water surface can be represented as:

[0160]

[0161] Combining equations (9), (14), and (17), we obtain:

[0162]

[0163] Where l and λ are known quantities, the detection of surface perturbations is transformed into the precise extraction of the rotation angle of the interference pattern.

[0164] Example 6:

[0165] An underwater acoustic field detection method based on terahertz vortex beam interferometry and topological charge modulation is described in Example 5. The difference is that in step (5), after the terahertz probe captures the interference signal, it is necessary to determine whether the "petal"-shaped interference fringes exceed the detection range. Considering the characteristics of vortex beam interferometry, that is, the longer the working wavelength of the measured radiation, the greater the topological charge of the vortex beam, and the smaller the fringe angle rotation at the same distance, the displacement amplitude response range can be adjusted by changing the working wavelength or modulating the topological charge of the vortex beam.

[0166] Example 7:

[0167] A system for implementing an underwater acoustic field detection method based on terahertz vortex beam interferometry and topological charge manipulation, such as... Figure 10 As shown, it includes a terahertz radiation source 1, a convex lens 2, a terahertz vortex beam generator 3, a beam splitter A4, a beam splitter B5, a beam splitter C6, a reflector A7, a reflector B9, the water surface to be measured 8, a terahertz probe 10, and a visible band laser.

[0168] The terahertz spherical wave emitted by the terahertz radiation source 1 passes sequentially through the convex lens 2, the terahertz vortex beam generator 3, the beam splitter A4, the beam splitter C6, the reflector A7, the water surface to be measured 8, the reflector A7, the beam splitter C6, the reflector B9, the beam splitter B5, and the terahertz probe 10 as the test arm.

[0169] The terahertz spherical wave emitted by the terahertz radiation source 1 passes sequentially through the convex lens 2, the terahertz vortex beam generator 3, the beam splitter A4, the beam splitter B5, and the terahertz probe 10 as a reference arm.

[0170] The visible-band laser is a visible-light fiber laser 11. The laser emitted by the visible-band laser passes sequentially through beam splitter B5, reflector B9, beam splitter C6, reflector A7, the water surface to be measured 8, reflector A7, beam splitter C6, reflector B9, and beam splitter B5 as an auxiliary aiming optical path.

[0171] The terahertz spherical beam emitted by the terahertz radiation source is first transformed into a planar beam by a convex lens, then modulated into a terahertz vortex beam with a specific topological charge by a terahertz vortex beam generator, and then split into a mutually conjugate transmitted terahertz vortex beam and a reflected terahertz vortex beam by a beam splitter A.

[0172] The terahertz vortex beam transmitted through beam splitter A is used as the test beam. After passing through beam splitter C and reflector A, it is incident on the water surface to be measured, carrying the displacement information of the water surface. After reflection, it is reflected by reflector A, beam splitter C, reflector B and beam splitter B before entering the terahertz probe.

[0173] The terahertz vortex beam reflected by beam splitter A serves as a reference beam. After passing through beam splitter B, it enters the probe. The reference beam and the test beam are combined to form "petal"-shaped interference fringes. The amplitude and frequency information of the water surface displacement are captured by the terahertz probe.

[0174] Example 8:

[0175] An implementation system for an underwater acoustic field detection method based on terahertz vortex beam interferometry and topological charge modulation is described in Example 7. The difference is that the system is integrated in a modular manner and is divided into a radiation module, an auxiliary aiming module, and a detection module.

[0176] The radiation module is integrated with a terahertz radiation source 1, a convex lens 2, and a terahertz vortex beam generator 3. This module emits a terahertz vortex beam as the radiation source of the detection system.

[0177] The auxiliary aiming module consists of a visible light fiber laser 11, whose emitted light is visible light and is coaxial with the terahertz wave emitted by the radiation module, and is used to assist aiming and adjust the position of the detection module.

[0178] Considering the unknown initial fringe angle and the distortion of interference fringes caused by complex environmental factors to the test arm beam, the detection module consists of two terahertz probes and a displacement device. The two terahertz probes are mounted on the displacement device, and their positions can be adjusted via the device. The approximate position can be determined using a visible fiber laser in the auxiliary aiming module. While the terahertz beam is invisible to the naked eye, the visible laser beam is. The visible laser and the terahertz beam used for measurement are coaxial, allowing the approximate location of the terahertz beam center to be determined from the position of the visible laser beam. The interference intensity fringes are arranged around the beam's optical axis. Moving the detector around the optical axis near the interference pattern, the intensity value reaches its maximum when the center of the detector's detection area coincides with the center of a "petal-shaped" fringe. After moving around the optical axis, the detector's intensity value decreases and reaches its minimum level when it reaches the next "petal-shaped" fringe, indicating that there are no "petal-shaped" fringes in the detector's detection area. Therefore, the optimal position for the detector is when the intensity decreases to near its minimum level, indicating that the detector is at the edge of a "petal-shaped" fringe. Finally, based on the preliminary detection results, the topological charge of the vortex beam generated by the metasurface vortex beam generator was adjusted to determine the most suitable displacement amplitude sensitive range.

[0179] After adjusting the visible light fiber laser in the auxiliary aiming module to a rough position, the terahertz probe is then adjusted to the optimal position using the "petal-shaped" interference fringe displacement device.

[0180] Preferably, considering the aberrations caused by factors such as surface noise, component defects, and misalignment, as well as possible fringe distortion, twisting, and uneven distribution, the rate of change of the "petal" fringe angle at different locations may be inconsistent. That is, under the same displacement, the angle change may vary significantly. Therefore, this invention uses symmetrically distributed dual-intensity probes to detect and record the angle change of interference fringes. Two terahertz probes are placed at the edges of the "petals" of the fringes and are centrally symmetrically distributed. When the angle change of the "petal" fringe measured by one probe is too small due to aberration distortion to obtain intensity change information, the second probe can obtain sufficient change information. By detecting the increase or decrease in intensity, the interference fringe angle change information is obtained, thereby obtaining the frequency and amplitude information of the displacement.

[0181] However, when the "petal" angle changes too much, the fringes may completely fall outside the detector's monitoring range, making it impossible to measure information and distinguish the direction of the fringe angle change. Therefore, this invention provides an amplitude response range for displacement measurement. Considering the characteristics of vortex beam interferometry—that is, the longer the operating wavelength of the measured radiation, the greater the topological charge of the vortex beam, and the smaller the fringe angle rotation at the same distance—changing the operating wavelength or modulating the topological charge of the vortex beam can adjust the displacement amplitude response range. This invention employs topological charge modulation, adjusting the minimum and maximum values ​​of the displacement amplitude response range by increasing or decreasing the number of topological charges: when the underwater sound field is small, the amplitude of surface disturbances is small, and a low-topological-charge vortex beam is used for measurement; when the underwater sound field is large, the amplitude of surface disturbances may reach 5 μm or even larger, so as the signal strength increases, the number of topological charges is changed in real time, and a larger upper limit of the response amplitude is obtained through a vortex beam with a higher-order topological charge, thereby achieving a large amplitude range detection of underwater acoustic signals.

[0182] Figure 4 The interference fringes of the conjugate vortex beam rotate with the change of displacement, where d0 is the initial displacement and Δd is the change of displacement. As Δd increases, the rotation angle of the fringes also increases.

[0183] As shown in Figure 5, multiple probes can capture more rotation information and will not cause inaccurate rotation angle measurements due to insufficient detection points.

[0184] Figure 6This is a schematic diagram of the measurement using two centrally symmetrically distributed probes employed in this invention. By modifying the linear relationship between the fringe rotation information and the water surface vibration amplitude through topological charge modulation, the rotating interference fringes are always within the detection range of the detector. Thus, the rotation information can be measured using two probes, and the impact of interference fringe distortion on measurement accuracy is greatly reduced.

[0185] Figure 7 This diagram illustrates the displacement response range for different topological loads. d1 and d2 represent the minimum and maximum response displacement values ​​when the topological load is 1, respectively. Displacements smaller than d1 cannot be detected because the rotation angle is too small, and displacements larger than d2 also cannot be effectively detected because the rotation angle is too large.

[0186] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for underwater acoustic field detection based on terahertz vortex beam interferometry and topological charge manipulation, characterized in that, Includes the following steps: (1) The terahertz radiation source emits a terahertz beam, which is collimated by a convex lens and then incident on the terahertz vortex beam generator. The incident beam is modulated into a vortex beam with a specific topological charge. (2) The beam splitter divides the vortex beam into a reference wave and a test wave; (3) The test wave is reflected back by the water surface after passing through the reflector, and carries the displacement information of the water surface; (4) The test wave carrying the water surface displacement information interferes with the reference wave at the beam splitter; (5) The interference signal is captured by the terahertz probe, and the amplitude and frequency information of the water surface displacement are obtained based on the interference signal; In step (1), the terahertz vortex beam generator adopts a transmission phase type metasurface and a double-layer moiré structure design, that is, two conjugate transmission phase type metasurfaces are placed facing each other to realize the joint transmission function and joint phase. The joint transmission function and joint phase are equal to the superposition of the transmission functions and phases of the two elements. Each transmission phase type metasurface includes a substrate and a metasurface radiation unit on the substrate. The metasurface radiation unit is cylindrical. Phase modulation is achieved by changing the optical path of the electromagnetic wave during transmission by using different cylinder diameters. The modulation effect of topological charge can be obtained by rotating one of the phase-transmitting metasurfaces; The joint transfer function and joint phase of two conjugate phase elements are equal to the superposition of the transfer functions and phases of the two elements: （1） in, For joint transfer functions, For joint phase, i is the imaginary unit; Therefore, the phase distribution design of the two metasurfaces can be obtained: （2） in Here, φ is a design parameter used to control the adjustment relationship between the rotation angle of two surfaces and the topological load, where φ is a polar coordinate from 0 to 1. The angle; when a metasurface is rotated by an angle θ, its phase becomes: （3） The phase of the entire bilayer metasurface is written as: （4） Where the rotation angle θ is a known value, the second term is independent of φ and has no effect on the generation of the vortex phase, so it can be ignored. Furthermore, the relationship between the phase, topological charge, and angle of the vortex beam is as follows: （5） The topological charge l generated by the vortex beam is obtained as follows: （6） Using this vortex beam generator, vortex beams with different topological charges can be obtained by rotating one of the metasurfaces to generate an angle θ.

2. The underwater acoustic field detection method based on terahertz vortex beam interferometry and topological charge control according to claim 1, characterized in that, The terahertz vortex beam generator is packaged and integrated using a rotary mounting base. One side of the rotary mounting base is equipped with a transmission phase type metasurface, and the other side is equipped with a second transmission phase type metasurface. The two transmission phase type metasurfaces are arranged facing each other. By placing the two metasurfaces symmetrically, the initial joint phase of the two metasurfaces is 0. The center of the rotary mounting base is provided with a central hole to allow the beam to pass through. One side of the rotating mounting base is fixed, while the other side is equipped with a rotating sleeve. One of the transmission phase type metasurfaces is installed inside the rotating sleeve. The rotation of the rotating sleeve drives the transmission phase type metasurface to perform precise rotational positioning. The rotating sleeve is connected to a driver, which ensures the precise rotation of the rotating sleeve.

3. The underwater acoustic field detection method based on terahertz vortex beam interferometry and topological charge control according to claim 1, characterized in that, When an underwater sound field exists, it will create surface waves on the water surface: （7） Among them, A x ω represents the amplitude of the surface wave excited by the underwater sound field. x and φ x ω and t are the angular frequency and initial phase of the surface wave excited by the underwater sound field, respectively, and t is the propagation time of the surface wave. In real water environments, due to the influence of external environmental disturbances, low-frequency noise with an amplitude in the micrometer range always exists on the water surface: （8） Where A0 is the amplitude of the noisy water surface wave; ω0 and φ0 are the angular frequency and initial phase of the noisy water surface wave, respectively; Therefore, the surface disturbance of water is: （9） Underwater acoustic field information is extracted by measuring surface perturbations.

4. The underwater acoustic field detection method based on terahertz vortex beam interferometry and topological charge modulation according to claim 3, characterized in that, The interference signal is detected using a dual terahertz probe and a displacement device. Assuming that the vortex beam generated by the vortex beam generator is split, the reference wave is represented by the following formula: （10） Where A is the amplitude and l is the topological charge number. It is the azimuth angle. λ is the wave number, λ is the wavelength, and z1 is the optical path length of the reference arm; The test beam is conjugate with the reference beam, expressed by the following formula: （11） Where z2 is the optical path length of the test arm; In the absence of an underwater sound field, the intensity of the interference between the two vortex wave beams is: （12） Under the influence of surface waves in the noise, the interference pattern will rotate; let the angle of rotation be Δ. If the result is 0, then the following equality relationship exists: （13） Noise surface waves are represented as: （14） When there is a slight disturbance on the water surface, the intensity of the interference becomes: （15） After the underwater sound field excites a slight disturbance on the water surface, the resulting interference pattern will rotate. Let the angle of rotation be Δ. Then we have: （16） Minor disturbances to the water surface are represented as: （17） Combining equations (9), (14), and (17), we obtain: （18） Where l and λ are known quantities, the detection of surface perturbations is transformed into the precise extraction of the rotation angle of the interference pattern.

5. The underwater acoustic field detection method based on terahertz vortex beam interferometry and topological charge modulation according to claim 4, characterized in that, In step (5), after the terahertz probe captures the interference signal, it is necessary to determine whether the "petal"-shaped interference fringes are outside the detection range. Considering the characteristics of vortex beam interference, that is, the longer the working wavelength of the measured radiation, the greater the topological charge of the vortex beam, and the smaller the fringe angle rotation at the same distance, the displacement amplitude response range can be adjusted by changing the working wavelength or modulating the topological charge of the vortex beam.

6. A system for implementing the underwater acoustic field detection method based on terahertz vortex beam interferometry and topological charge manipulation as described in claim 1, characterized in that, It includes a terahertz radiation source, a convex lens, a terahertz vortex beam generator, beam splitter A, beam splitter B, beam splitter C, a reflector A, a reflector B, the water surface to be measured, a terahertz probe, and a visible band laser. The terahertz spherical wave emitted by the terahertz radiation source passes sequentially through a convex lens, a terahertz vortex beam generator, beam splitter A, beam splitter C, reflector A, the water surface to be measured, reflector A, beam splitter C, reflector B, beam splitter B, and the terahertz probe as a test arm. The terahertz spherical wave emitted by the terahertz radiation source passes sequentially through a convex lens, a terahertz vortex beam generator, beam splitter A, beam splitter B, and a terahertz probe as a reference arm. The visible band laser is a visible light fiber laser. The laser emitted by the visible band laser passes sequentially through beam splitter B, mirror B, beam splitter C, mirror A, the water surface to be measured, mirror A, beam splitter C, mirror B, and beam splitter B as an auxiliary aiming optical path. The terahertz spherical beam emitted by the terahertz radiation source is first transformed into a planar beam by a convex lens, then modulated into a terahertz vortex beam with a specific topological charge by a terahertz vortex beam generator, and then split into a mutually conjugate transmitted terahertz vortex beam and a reflected terahertz vortex beam by a beam splitter A. The terahertz vortex beam transmitted through beam splitter A is used as the test beam. After passing through beam splitter C and reflector A, it is incident on the water surface to be measured, carrying the displacement information of the water surface. After reflection, it is reflected by reflector A, beam splitter C, reflector B and beam splitter B before entering the terahertz probe. The terahertz vortex beam reflected by beam splitter A serves as a reference beam. After passing through beam splitter B, it enters the probe. The reference beam and the test beam are combined to form "petal"-shaped interference fringes. The amplitude and frequency information of the water surface displacement are captured by the terahertz probe.

7. The implementation system of the underwater acoustic field detection method based on terahertz vortex beam interferometry and topological charge control according to claim 6, characterized in that, It is integrated in a modular manner, and is divided into a radiation module, an auxiliary aiming module, and a detection module; The radiation module is integrated with a terahertz radiation source, a convex lens, and a terahertz vortex beam generator. This module emits a terahertz vortex beam as the radiation source of the detection system. The aiming assistance module consists of a visible light fiber laser, whose emitted light is visible light and is coaxial with the terahertz wave emitted by the radiation module. It is used to assist aiming and adjust the position of the detection module. The detection module consists of two terahertz probes and a displacement device. The two terahertz probes are mounted on the displacement device and their positions can be adjusted by the displacement device. After being adjusted to a rough position by the visible light fiber laser in the auxiliary aiming module, the terahertz probes are then adjusted to the optimal position by the displacement device moving the "petal-shaped" interference fringes.

8. The implementation system of the underwater acoustic field detection method based on terahertz vortex beam interferometry and topological charge control according to claim 7, characterized in that, Two terahertz probes are placed at the edges of the striped "petals" and are centrally symmetrically distributed.

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