A laser quantitative moire imaging sensitivity improvement method, system, medium and electronic equipment

Abstract

The application discloses a phase modulation laser quantitative schlieren imaging sensitivity improvement method and system, and the method comprises the following steps: constructing a phase modulation iterative design physical model of a Gaussian beam to a flat-top beam; based on the improved algorithm of the complex amplitude double constraint in the physical model, extracting the optimal spatial phase solution of the phase modulation plate which can efficiently convert the incident Gaussian beam into a flat-top uniform laser; based on the optimal spatial phase solution, processing the phase modulation plate and building a quantitative schlieren high-time-resolution diagnostic system with a uniform flat-top laser background; based on the quantitative schlieren high-time-resolution diagnostic system, generating a uniform reference background image and a perturbation schlieren image, and extracting the light deflection angle based on the uniform reference background image and the perturbation schlieren image; based on the light deflection angle, performing multi-physical field three-dimensional inversion reconstruction of the fluid density field and the transient temperature field.

Description

Technical Field

[0001] This invention relates to the field of laser imaging technology, and more specifically, to a method, system, medium, and electronic device for improving the sensitivity of laser quantitative schlieren imaging. Background Technology

[0002] Quantitative schlieren imaging, as a non-invasive measurement method, is widely used for high-frequency dynamic diagnosis of refractive index, density, and temperature fields in extreme environments such as long air gap plasma discharges, aerospace wind tunnels, and combustion flow fields. To accurately capture the dramatic transient evolution processes within the channel, such as during the initial stage of a leader discharge, the system must possess extremely high temporal and spatial resolution. In existing quantitative schlieren parameter inversion research and engineering practice, background light source technologies mainly include two approaches: using LED light sources and using laser light sources. The advantages of using LED light sources, including those driven by constant power or pulsed current sources, are their low spatial coherence, reduced speckle interference noise, and relatively simple system setup. However, LED light source technology faces significant bottlenecks in achieving high spatiotemporal resolution in practical applications. According to the optical principles of quantitative schlieren systems, the relative sensitivity of the system measurement is closely related to the cumulative effect of light flux, and the system sensitivity is positively correlated with the background light intensity. To capture the rapid surge of plasma channels within the hundreds of nanoseconds, camera exposure times must be drastically compressed to the sub-microsecond level. Simultaneously, to ensure sampling points for extremely small stream channels, spatial resolution must be sufficiently small, leading to an exponentially precipitous drop in system sensitivity. Secondly, LED light sources face severe physical limitations in spatial collimation and the maximum luminous flux of a single chip. Even with pulsed drive, the light intensity cannot compensate for the light attenuation caused by the extremely short exposure time, resulting in a significant reduction in the schlieren image signal-to-noise ratio. This ultimately leads to substantial temperature field inversion errors, making it impossible to support large field-of-view, high-precision micron-level optical diagnostics.

[0003] To overcome the shortcomings of insufficient light intensity and collimation of LED light sources, academia and industry have developed schlieren measurement technology based on high-power short-pulse lasers as diagnostic light sources. The greatest advantage of laser light sources lies in their extremely high peak brightness and spatial collimation, theoretically capable of supporting extreme transient detection requirements such as nanosecond-level exposure and micrometer-level resolution. However, currently, most commercial or engineered laser schlieren systems merely introduce unshaped or simply expanded fundamental-mode Gaussian (TEM00) laser beams. This direct illumination method not only completely ignores the uneven energy distribution characteristics of the Gaussian beam cross-section; more importantly, within the framework of quantitative schlieren theory, the extraction of the light deflection angle strictly depends on the relative grayscale change rate of the image. When a Gaussian laser is directly used as an undisturbed background light, the value in the outer region of the light spot rapidly approaches zero. This not only causes frequent mathematical divergence due to "division by zero" when the computer calculates the deflection angle, but also, because the actual acquired signal inevitably contains background shot noise and system readout noise, the extremely small denominator will nonlinearly amplify these noises by tens of thousands of times, causing traditional inversion to easily collapse. This inherent physical defect of the Gaussian light field leads to a severe deterioration of the signal-to-noise ratio at the edge of the field of view in traditional laser quantitative schlieren systems, severely narrowing the effective high-fidelity observation field and making it impossible to achieve full-field, high-precision multi-physics parameter reconstruction.

[0004] In summary, existing LED light source measurement methods are limited by the low signal-to-noise ratio and low sensitivity caused by the physical upper limits of luminous intensity and collimation, while existing laser light source detection methods are limited by the theoretical bottlenecks of edge calculation divergence and drastic noise amplification caused by Gaussian energy distribution. Summary of the Invention

[0005] This invention proposes a method and system for improving the sensitivity of laser quantitative schlieren imaging, in order to solve the problem of how to achieve multi-physics three-dimensional inversion and reconstruction of fluid density field and transient temperature field, thereby improving imaging sensitivity.

[0006] To address the aforementioned problems, according to one aspect of the present invention, a method for improving the sensitivity of laser quantitative schlieren imaging is provided, characterized in that the method comprises: A physical model for phase modulation iterative design of Gaussian beam to flat-top beam conversion is constructed. The physical model incorporates an improved algorithm based on complex amplitude dual constraints to solve the spatial phase distribution of the adapted phase modulation plate. Based on the improved algorithm, the optimal spatial phase solution of the phase modulation plate that efficiently converts the incident Gaussian beam into a flat-top uniform laser is extracted. Based on the optimal spatial phase demodulation phase modulation board, a quantitative schlieren high temporal resolution diagnostic system with a uniform flat-top laser background is built. Based on the quantitative schlieren high temporal resolution diagnostic system, a uniform reference background image and a perturbed schlieren image are generated, and the light deflection angle is extracted based on the uniform reference background image and the perturbed schlieren image; Based on the light deflection angle, a high-sensitivity multiphysics three-dimensional inversion reconstruction of the fluid density field and transient temperature field is performed to improve the overall detection sensitivity of laser quantitative schlieren imaging.

[0007] Preferably, the physical model for constructing the phase modulation iterative design of the Gaussian beam to flat-top beam conversion includes: The preset beam conversion target is clearly defined, which is to convert the fundamental mode Gaussian pulse laser into a flat-top uniform laser with high efficiency. The conversion target is the construction standard of complex amplitude dual constraint. An improved Gerchberg-Saxton algorithm was selected as the improved algorithm based on complex amplitude dual constraints for iterative optimization of the optimal spatial phase distribution of the phase modulation board. Construct the incident spatial boundary with dual complex amplitude constraints, define the complex amplitude distribution of the incident Gaussian beam, and set the ideal complex amplitude distribution of the incident beam in the incident plane. The ideal complex amplitude distribution Satisfying the fundamental Gaussian beam model: in, Let be the waist radius of the Gaussian beam; The central peak amplitude; assuming the initial wavefront is an ideal isophase surface, the initial phase... ; This represents the amplitude distribution of the incident Gaussian beam at the incident plane. The imaginary unit; These are the spatial coordinates along the horizontal direction within the incident plane, representing different positions in the horizontal direction within that plane; These are the spatial coordinates along the vertical direction within the incident plane, representing different positions in the vertical direction within that plane; Constructing a target-end spatial boundary with dual complex amplitude constraints, and defining the ideal complex amplitude distribution of the target flat-top beam on the target observation plane. The ideal complex amplitude distribution A high-order super-Gaussian function is used to smooth the edge transition region of the target spot: in, The macroscopic effective radius of the target flat-top beam; It is a super-Gaussian order, and To ensure the steepness of the edges; The spatial coordinates along the horizontal direction within the target observation plane represent different positions in the horizontal direction within that plane. The spatial coordinates along the vertical direction within the target observation plane represent different positions in the vertical direction within that plane. This represents the stable amplitude of the flat-top beam. The superimposed energy conservation physical constraint restricts the complex amplitude under a dual constraint. This energy conservation constraint strictly follows the energy conservation law of optical propagation, and its expression is:

[0008] Preferably, the step of extracting the optimal spatial phase solution of the phase modulation plate that efficiently converts the incident Gaussian beam into a flat-top uniform laser based on the improved algorithm specifically includes extracting the optimal spatial phase solution through a nonlinear iterative optimization process, wherein the iterative optimization process sequentially executes the following steps: Initial state and forward propagation: Let For the current iteration number, the additional delay phase distribution matrix introduced in space by the phase modulation board is: ;No. At the start of the next iteration, the amplitude distribution of the incident Gaussian beam at the incident plane is used as a basis. Current iteration phase By combining these elements, the complex amplitude of the modulation plate's projection surface was constructed. The expression is: Using the Fresnel-Kirchhoff diffraction integral formula, the light field is propagated from the modulation plate plane into free space in the forward direction to the target plane, and the complex amplitude distribution of the target plane is calculated. : in, For Fresnel-Kirchhoff diffraction integral calculation; For the first Amplitude distribution of the target plane during the next iteration; For the first The phase wavefront of the target plane at the next iteration; To determine the horizontal spatial coordinates within the target observation plane, The vertical spatial coordinates within the target observation plane; Core complex amplitude physical constraint replacement: Preserve the phase wavefront propagating in the positive direction to reach the target plane Keep it unchanged, and set the target plane amplitude. According to the preset relaxation factor The ideal super-Gaussian target amplitude defined in the phase modulation iterative design physical model Perform linear fusion update to obtain the complex amplitude after linear fusion update. : Backpropagation and phase extraction: updating the complex amplitude after linear fusion Perform reverse propagation, tracing back to the input plane where the phase modulation plate is located, to obtain a new complex amplitude distribution on the input plane. : in, For Fresnel-Kirchhoff inverse diffraction integral calculation, For the first Phase distribution of the input target plane after each iteration of backpropagation; Extracting the phase component from the new complex amplitude distribution As the first The phase input of the next iteration, and the Gaussian incident amplitude. Recombined to impose amplitude constraints on the input plane; Convergence Criteria: Each iteration follows the physical constraint of energy conservation; the root mean square error of light intensity is calculated on the target plane, and the conversion of the beam into the target is used as the convergence criterion; when the root mean square error of the output light intensity on the target plane meets the preset convergence threshold, the iteration terminates, and the current phase input is used as the optimal spatial phase solution. .

[0009] Preferably, the quantitative schlieren high temporal resolution diagnostic system based on the optimal spatial phase demodulation phase modulation plate and the uniform flat-top laser background includes the fabrication of the phase modulation plate and the construction of the system optical path, and the construction of the uniform flat-top laser reference background, specifically: Based on the optimal spatial phase solution, a matching phase modulation plate is prepared. The pulsed laser, the beam expanding and collimating lens group, the phase modulation plate, the first schlieren spherical mirror, the flow field to be measured, the second schlieren spherical mirror, the knife edge, the narrowband filter and the camera are arranged in sequence according to the beam propagation order to form an improved Z-type quantitative schlieren optical path. Based on an improved Z-type quantitative schlieren optical path, the wavefront phase of the incident Gaussian laser is reconstructed using a phase modulation plate, shaping the beam into a flat-topped parallel beam with uniform cross-sectional energy. Using the uniform flat-topped laser as the imaging reference background, flat-topped background images and schlieren images containing flow field disturbances are acquired separately to obtain the global constant field background light intensity. The system optical path was built and a uniform flat-top laser reference background was constructed.

[0010] Preferably, the step of generating a uniform reference background image and a perturbed schlieren image based on the quantitative schlieren high temporal resolution diagnostic system, and extracting the light deflection angle based on the uniform reference background image and the perturbed schlieren image, includes: Based on the improved Z-type quantitative schlieren optical path, a uniform reference background image is generated; based on the introduced flow field disturbance, a disturbed schlieren image is generated. The uniform reference background image and the perturbation schlieren image are processed as follows: the background light intensity is... The shape is converted into a global constant field to determine the local grayscale changes in the probed image. Deflection angle caused solely by the flow field And it is determined by the effective cutting amount of the mechanical blade; Based on the processed uniform reference background image and the disturbed schlieren image, assuming that the light in the flow field propagates along the positive z-axis, the edge of the mechanical cutting edge is parallel to the x-axis, and the deflection angle of the light in the y-direction is... Expressed by the integral of the refractive index gradient of the medium: in, The absolute reference refractive index of the undisturbed gas in the environment; integration path The optical thickness of the flow field disturbance region; Let be the refractive index gradient of the medium in the flow field along the y-direction; Within the linear response range of the schlieren system, the deflection angle The relative contrast of a schlieren image is proportional to the deflection angle. The formula for calculating the light deflection angle is as follows: in, The grayscale values ​​of the schlieren image acquired under perturbation. The grayscale value of the flat-top background image without disturbance; This represents the system sensitivity coefficient.

[0011] Preferably, the multi-physics three-dimensional inversion and reconstruction of the fluid density field and transient temperature field based on the light deflection angle includes: Obtain the deflection vector field of the entire field of view Orthogonal components , They are respectively directional light deflection angle and Angle of deflection of directional light rays; Based on the theory of optical tomography in fluid dynamics, a two-dimensional scalar potential function is introduced to characterize the optical path difference. By utilizing the spatial gradient characteristic of the optical path difference based on the deflection angle, a fundamental correlation equation is established; the two-dimensional spatial divergence of both sides of the fundamental correlation equation is calculated to construct a two-dimensional Poisson equation characterizing the distribution of the optical path difference: The two-dimensional Poisson equation is solved using any one of the finite difference method, multigrid method, or conjugate gradient algorithm, combined with the Neumann boundary conditions of the undisturbed flow field boundary, based on the two-dimensional scalar potential function. Determine the absolute change in the integral projected refractive index. ; Introducing the Gladstone-Dale relation: in, The Gladstone-Dale constant is given for a specific laser wavelength and gas mixture composition. Let n be the spatial absolute density distribution of the gas medium; n is the absolute refractive index of the disturbed gas; the Abel inverse transform back projection algorithm is used to transform the two-dimensional... Restored to the true refractive index of each point in three-dimensional space The true refractive index Substituting the Gladstone-Dale relation, the transient three-dimensional flow density distribution field is analyzed. ; Assuming that the discharge expansion process locally satisfies the ideal gas law, the three-dimensional flow field density distribution field is used as an example. Inversely derived transient evolution temperature field of neutral gas in local plasma : in, The molar mass of the component gases Let be the ideal gas constant. 7. This invention provides a laser quantitative schlieren imaging sensitivity enhancement system, the system comprising: The physical model building unit is used to build a physical model for phase modulation iterative design of Gaussian beam to flat-top beam conversion. The physical model has a built-in improved algorithm based on complex amplitude dual constraints to solve the spatial phase distribution of the adapted phase modulation plate. The optimal spatial phase solution extraction unit is used to extract the optimal spatial phase solution of the phase modulation plate that efficiently converts the incident Gaussian beam into a flat-top uniform laser based on the improved algorithm. The diagnostic system building unit is used to build a quantitative schlieren high temporal resolution diagnostic system with a uniform flat-top laser background by processing the phase modulation board based on the optimal spatial phase solution. The light deflection angle extraction unit is used to generate a uniform reference background image and a perturbed schlieren image based on the quantitative schlieren high temporal resolution diagnostic system, and to extract the light deflection angle based on the uniform reference background image and the perturbed schlieren image. The three-dimensional inversion and reconstruction unit is used to perform high-sensitivity multi-physics three-dimensional inversion and reconstruction of the fluid density field and transient temperature field based on the light deflection angle, thereby improving the overall detection sensitivity of laser quantitative schlieren imaging.

[0012] Preferably, the physical model building unit is used to build a physical model for the phase modulation iterative design of the Gaussian beam to flat-top beam conversion, including: The preset beam conversion target is clearly defined, which is to convert the fundamental mode Gaussian pulse laser into a flat-top uniform laser with high efficiency. The conversion target is the construction standard of complex amplitude dual constraint. An improved Gerchberg-Saxton algorithm was selected as the improved algorithm based on complex amplitude dual constraints for iterative optimization of the optimal spatial phase distribution of the phase modulation board. Construct the incident spatial boundary with dual complex amplitude constraints, define the complex amplitude distribution of the incident Gaussian beam, and set the ideal complex amplitude distribution of the incident beam in the incident plane. The ideal complex amplitude distribution Satisfying the fundamental Gaussian beam model: in, Let be the waist radius of the Gaussian beam; The central peak amplitude; assuming the initial wavefront is an ideal isophase surface, the initial phase... ; This represents the amplitude distribution of the incident Gaussian beam at the incident plane. The imaginary unit; These are the spatial coordinates along the horizontal direction within the incident plane, representing different positions in the horizontal direction within that plane; These are the spatial coordinates along the vertical direction within the incident plane, representing different positions in the vertical direction within that plane; Constructing a target-end spatial boundary with dual complex amplitude constraints, and defining the ideal complex amplitude distribution of the target flat-top beam on the target observation plane. The ideal complex amplitude distribution A high-order super-Gaussian function is used to smooth the edge transition region of the target spot: in, The macroscopic effective radius of the target flat-top beam; It is a super-Gaussian order, and To ensure the steepness of the edges; The spatial coordinates along the horizontal direction within the target observation plane represent different positions in the horizontal direction within that plane. The spatial coordinates along the vertical direction within the target observation plane represent different positions in the vertical direction within that plane. This represents the stable amplitude of the flat-top beam. The superimposed energy conservation physical constraint restricts the complex amplitude under a dual constraint. This energy conservation constraint strictly follows the energy conservation law of optical propagation, and its expression is:

[0013] Preferably, the optimal spatial phase solution extraction unit is used to extract the optimal spatial phase solution of the phase modulation plate that efficiently converts the incident Gaussian beam into a flat-top uniform laser based on the improved algorithm. Specifically, it includes extracting the optimal spatial phase solution through a nonlinear iterative optimization process, wherein the iterative optimization process sequentially executes the following steps: Initial state and forward propagation: Let For the current iteration number, the additional delay phase distribution matrix introduced in space by the phase modulation board is: ;No. At the start of the next iteration, the amplitude distribution of the incident Gaussian beam at the incident plane is used as a basis. Current iteration phase By combining these elements, the complex amplitude of the modulation plate's projection surface was constructed. The expression is: Using the Fresnel-Kirchhoff diffraction integral formula, the light field is propagated from the modulation plate plane into free space in the forward direction to the target plane, and the complex amplitude distribution of the target plane is calculated. : in, For Fresnel-Kirchhoff diffraction integral calculation; For the first Amplitude distribution of the target plane during the next iteration; For the first The phase wavefront of the target plane at the next iteration; To determine the horizontal spatial coordinates within the target observation plane, The vertical spatial coordinates within the target observation plane; Core complex amplitude physical constraint replacement: Preserve the phase wavefront propagating in the positive direction to reach the target plane Keep it unchanged, and set the target plane amplitude. According to the preset relaxation factor The ideal super-Gaussian target amplitude defined in the phase modulation iterative design physical model Perform linear fusion update to obtain the complex amplitude after linear fusion update. : Backpropagation and phase extraction: updating the complex amplitude after linear fusion Perform reverse propagation, tracing back to the input plane where the phase modulation plate is located, to obtain a new complex amplitude distribution on the input plane. : in, For Fresnel-Kirchhoff inverse diffraction integral calculation, For the first Phase distribution of the input target plane after each iteration of backpropagation; Extracting the phase component from the new complex amplitude distribution As the first The phase input of the next iteration, and the Gaussian incident amplitude. Recombined to impose amplitude constraints on the input plane; Convergence Criteria: Each iteration follows the physical constraint of energy conservation; the root mean square error of light intensity is calculated on the target plane, and the conversion of the beam into the target is used as the convergence criterion; when the root mean square error of the output light intensity on the target plane meets the preset convergence threshold, the iteration terminates, and the current phase input is used as the optimal spatial phase solution. .

[0014] Preferably, the diagnostic system construction unit is used to construct a quantitative schlieren high temporal resolution diagnostic system with a uniform flat-top laser background based on the optimal spatial phase solution, including the fabrication of a phase modulation plate and the construction of the system optical path, and the construction of a uniform flat-top laser reference background, specifically: Based on the optimal spatial phase solution, a matching phase modulation plate is prepared. The pulsed laser, the beam expanding and collimating lens group, the phase modulation plate, the first schlieren spherical mirror, the flow field to be measured, the second schlieren spherical mirror, the knife edge, the narrowband filter and the camera are arranged in sequence according to the beam propagation order to form an improved Z-type quantitative schlieren optical path. Based on an improved Z-type quantitative schlieren optical path, the wavefront phase of the incident Gaussian laser is reconstructed using a phase modulation plate, shaping the beam into a flat-topped parallel beam with uniform cross-sectional energy. Using the uniform flat-topped laser as the imaging reference background, flat-topped background images and schlieren images containing flow field disturbances are acquired separately to obtain the global constant field background light intensity. The system optical path was built and a uniform flat-top laser reference background was constructed.

[0015] Preferably, the light deflection angle extraction unit is used to extract the light deflection angle based on the flat-top parallel beam with uniform cross-sectional energy generated by the quantitative schlieren high temporal resolution diagnostic system, including: Based on the improved Z-type quantitative schlieren optical path, a uniform reference background image is generated; based on the introduced flow field disturbance, a disturbed schlieren image is generated. The uniform reference background image and the perturbation schlieren image are processed as follows: the background light intensity is... The shape is converted into a global constant field to determine the local grayscale changes in the probed image. Deflection angle caused solely by the flow field And it is determined by the effective cutting amount of the mechanical blade; Based on the processed uniform reference background image and the disturbed schlieren image, assuming that the light in the flow field propagates along the positive z-axis, the edge of the mechanical cutting edge is parallel to the x-axis, and the deflection angle of the light in the y-direction is... Expressed by the integral of the refractive index gradient of the medium: in, The absolute reference refractive index of the undisturbed gas in the environment; integration path The optical thickness of the flow field disturbance region; Let be the refractive index gradient of the medium in the flow field along the y-direction; Within the linear response range of the schlieren system, the deflection angle The relative contrast of a schlieren image is proportional to the deflection angle. The formula for calculating the light deflection angle is as follows: in, The grayscale values ​​of the schlieren image acquired under perturbation. The grayscale value of the flat-top background image without disturbance; This represents the system sensitivity coefficient.

[0016] Preferably, the three-dimensional inversion and reconstruction unit is used to perform multi-physics three-dimensional inversion and reconstruction of the fluid density field and transient temperature field based on the light deflection angle, including: Obtain the deflection vector field of the entire field of view Orthogonal components , They are respectively directional light deflection angle and Angle of deflection of directional light rays; Based on the theory of optical tomography in fluid dynamics, a two-dimensional scalar potential function is introduced to characterize the optical path difference. By utilizing the spatial gradient characteristic of the optical path difference based on the deflection angle, a fundamental correlation equation is established; the two-dimensional spatial divergence of both sides of the fundamental correlation equation is calculated to construct a two-dimensional Poisson equation characterizing the distribution of the optical path difference: The two-dimensional Poisson equation is solved using any one of the finite difference method, multigrid method, or conjugate gradient algorithm, combined with the Neumann boundary conditions of the undisturbed flow field boundary, based on the two-dimensional scalar potential function. Determine the absolute change in the integral projected refractive index. ; Introducing the Gladstone-Dale relation: in, The Gladstone-Dale constant is given for a specific laser wavelength and gas mixture composition. Let n be the spatial absolute density distribution of the gas medium; n is the absolute refractive index of the disturbed gas; the Abel inverse transform back projection algorithm is used to transform the two-dimensional... Restored to the true refractive index of each point in three-dimensional space The true refractive index Substituting the Gladstone-Dale relation, the transient three-dimensional flow density distribution field is analyzed. ; Assuming that the discharge expansion process locally satisfies the ideal gas law, the three-dimensional flow field density distribution field is used as an example. Inversely derived transient evolution temperature field of neutral gas in local plasma : in, The molar mass of the component gases Let be the ideal gas constant. The pressure is the gas pressure. Based on another aspect of the invention, the invention provides a computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the steps of a method for improving the sensitivity of phase laser quantitative schlieren imaging.

[0017] According to another aspect of the present invention, the present invention provides an electronic device, comprising: The aforementioned computer-readable storage medium; and One or more processors for executing a program in the computer-readable storage medium.

[0018] This invention provides a method and system for improving the sensitivity of laser quantitative schlieren imaging. The method includes: constructing a physical model for phase modulation iterative design of Gaussian beam to flat-top beam conversion, with the physical model incorporating an improved algorithm based on complex amplitude dual constraints to solve for the spatial phase distribution of a suitable phase modulation plate; extracting the optimal spatial phase solution of the phase modulation plate that efficiently converts the incident Gaussian beam into a flat-top uniform laser based on the improved algorithm; processing the phase modulation plate based on the optimal spatial phase solution to build a high temporal resolution diagnostic system for quantitative schlieren imaging with a uniform flat-top laser background; generating a uniform reference background map and a perturbed schlieren map based on the high temporal resolution diagnostic system for quantitative schlieren imaging, and extracting the light deflection angle based on the uniform reference background map and the perturbed schlieren map; and performing high-sensitivity multi-physics three-dimensional inversion reconstruction of the fluid density field and transient temperature field based on the light deflection angle to improve the overall detection sensitivity of laser quantitative schlieren imaging. This invention introduces an improved algorithm based on hybrid complex amplitude constraints to design a diffraction spatial phase mask, efficiently and smoothly reconstructing the fundamental mode Gaussian pulse laser into a flat-top beam with absolutely uniform cross-sectional energy. This invention uses a flat-top uniform light field as the detection background for quantitative schlieren, completely severing the division-to-zero singularity and noise nonlinear amplification chain caused by the background light intensity approaching zero at the edge of a large field of view from the physical source. It also raises the signal-to-noise ratio of the light deflection angle extraction to the theoretical limit within the entire physical field of view, thereby breaking through the sensitivity bottleneck under high spatiotemporal resolution conditions and achieving high frame rate, high precision, and full-field three-dimensional inversion reconstruction of extreme transient physical fields. Attached Figure Description

[0019] Exemplary embodiments of the present invention can be more fully understood by referring to the following figures: Figure 1 This is a flowchart of a laser quantitative schlieren imaging sensitivity enhancement method according to an embodiment of the present invention; Figure 2 This is a flowchart of a phase-modulated laser quantitative schlieren imaging sensitivity enhancement method according to an embodiment of the present invention; Figure 3 This is a global optimization data flow graph of the improved Gerchberg-Saxton algorithm based on hybrid complex amplitude constraints according to an embodiment of the present invention; Figure 4 This is a schematic diagram of a phase-modulated laser quantitative schlieren fluid density and temperature field diagnostic system according to an embodiment of the present invention; Figure 5 This is a structural diagram of a laser quantitative schlieren imaging sensitivity enhancement system according to an embodiment of the present invention. Detailed Implementation

[0020] Exemplary embodiments of the invention will now be described with reference to the accompanying drawings. However, the invention may be embodied in many different forms and is not limited to the embodiments described herein. These embodiments are provided to fully and completely disclose the invention and to fully convey its scope to those skilled in the art. The terminology used in the exemplary embodiments illustrated in the drawings is not intended to limit the invention. In the drawings, the same units / elements are referred to by the same reference numerals.

[0021] Unless otherwise stated, the terms used herein (including technical terms) have their common meaning as understood by one of ordinary skill in the art. Furthermore, it is understood that terms defined in commonly used dictionaries should be understood to have a meaning consistent with the context of their relevant field, and not to be interpreted as having an idealized or overly formal meaning.

[0022] Figure 1 This is a flowchart of a laser quantitative schlieren imaging sensitivity enhancement method according to an embodiment of the present invention; In view of the physical limits of brightness and collimation of traditional LED light sources in existing technologies, and the technical defects of high-power laser light sources that cause division-to-zero divergence and severe noise amplification at the edge of the field of view due to Gaussian energy distribution, this invention proposes a method to improve the sensitivity of phase-modulated laser quantitative schlieren imaging.

[0023] This invention abandons traditional LED light sources or unprocessed Gaussian laser light sources. Instead, it introduces an improved Gerchberg-Saxton algorithm based on hybrid complex amplitude constraints to design a diffraction spatial phase mask. This mask efficiently and smoothly reconstructs the fundamental mode Gaussian pulse laser into a flat-top beam with absolutely uniform cross-sectional energy. Using this flat-top uniform light field as the detection background for quantitative schlieren, the invention completely severs the division-to-zero singularity and noise nonlinear amplification chain caused by the background light intensity approaching zero at the edge of a large field of view from its physical source. This raises the signal-to-noise ratio of the light deflection angle extraction across the entire physical field of view to the theoretical limit, thereby breaking through the sensitivity bottleneck under high spatiotemporal resolution conditions and achieving high frame rate, high precision, and full-field three-dimensional inversion reconstruction of extreme transient physical fields.

[0024] This invention provides a method for improving the sensitivity of phase-modulated laser quantitative schlieren imaging, comprising a complete optical shaping algorithm, optical path construction, deflection angle extraction, and multi-physics parameter inversion process. It includes the following five core steps.

[0025] like Figure 1 As shown, the present invention provides a method for improving the sensitivity of laser quantitative schlieren imaging, the method comprising: Step S1: Construct a physical model for the phase modulation iterative design of Gaussian beam to flat-top beam conversion. The physical model incorporates an improved algorithm based on complex amplitude dual constraints to solve the spatial phase distribution of the adapted phase modulation plate. Preferably, a physical model for the phase modulation iterative design of the Gaussian beam to flat-top beam conversion is constructed, including: The predefined beam conversion target is to efficiently convert the fundamental mode Gaussian pulse laser into a flat-top uniform laser, and the conversion target is the construction standard of complex amplitude dual constraint; The improved Gerchberg-Saxton algorithm was selected as the improved algorithm based on complex amplitude dual constraints to iteratively optimize the solution of the optimal spatial phase distribution of the phase modulation board. Construct the incident spatial boundary with dual complex amplitude constraints, define the complex amplitude distribution of the incident Gaussian beam, and set the ideal complex amplitude distribution of the incident beam in the incident plane. Ideal complex amplitude distribution Satisfying the fundamental Gaussian beam model: in, Let be the waist radius of the Gaussian beam; The central peak amplitude; assuming the initial wavefront is an ideal isophase surface, the initial phase... ; This represents the amplitude distribution of the incident Gaussian beam at the incident plane. The imaginary unit; These are the spatial coordinates along the horizontal direction within the incident plane, representing different positions in the horizontal direction within that plane; These are the spatial coordinates along the vertical direction within the incident plane, representing different positions in the vertical direction within that plane; Constructing a target-end spatial boundary with dual complex amplitude constraints, and defining the ideal complex amplitude distribution of the target flat-top beam on the target observation plane. Ideal complex amplitude distribution A high-order super-Gaussian function is used to smooth the edge transition region of the target spot: in, The macroscopic effective radius of the target flat-top beam; It is a super-Gaussian order, and To ensure the steepness of the edges; The spatial coordinates along the horizontal direction within the target observation plane represent different positions in the horizontal direction within that plane. The spatial coordinates along the vertical direction within the target observation plane represent different positions in the vertical direction within that plane. This represents the stable amplitude of the flat-top beam. The superimposed physical constraint of energy conservation restricts the complex amplitude under a dual constraint. This energy conservation constraint strictly follows the energy conservation law of optical propagation, expressed as:

[0026] In step S1 of this invention, a physical model for phase modulation iterative design of a Gaussian beam to a flat-top beam is constructed.

[0027] First, this invention designs a spatial phase mask capable of efficiently converting a fundamental mode (TEM00) Gaussian pulse laser into a flat-top uniform laser. Since the phase and amplitude in the optical system are deeply coupled during diffraction propagation, this invention employs an improved Gerchberg-Saxton algorithm based on complex amplitude dual constraints to obtain the optimal spatial phase distribution.

[0028] Define the initial light source of the pulsed laser output in the incident plane (input plane, The ideal complex amplitude distribution is It satisfies the fundamental Gaussian beam model: In the formula, The waist radius of the Gaussian beam ( (where the light intensity is high) The central peak amplitude is assumed to be an ideal isophase surface, i.e., the initial phase. Secondly, the target observation plane (i.e., the front surface of the first schlieren spherical mirror, set as a plane) is set. The ideal flat-top beam amplitude distribution required is To avoid strong Gibbs oscillations and edge diffraction rings caused by the ideal rectangular step function during Fourier transform and spatial propagation, this invention employs a high-order super-Gaussian function to smooth the edge transition region of the target spot: In the formula, The macroscopic effective radius of the target flat-top beam; It is a super-Gaussian order. To ensure the steepness of the edges; This represents the stable amplitude of the flat-top beam. Its magnitude is strictly determined according to the law of conservation of energy in optical propagation: Step S2: Based on the improved algorithm, extract the optimal spatial phase solution of the phase modulation plate that efficiently converts the incident Gaussian beam into a flat-top uniform laser. Preferably, based on the improved algorithm, the optimal spatial phase solution of the phase modulation plate that efficiently converts the incident Gaussian beam into a flat-top uniform laser is extracted. Specifically, this includes extracting the optimal spatial phase solution through a nonlinear iterative optimization process, in which the iterative optimization process sequentially executes the following steps: Initial state and forward propagation: Let For the current iteration number, the additional delay phase distribution matrix introduced in space by the phase modulation board is: ;No. At the start of the next iteration, the amplitude distribution of the incident Gaussian beam at the incident plane is used as a basis. Current iteration phase By combining these elements, the complex amplitude of the modulation plate's projection surface was constructed. The expression is: Using the Fresnel-Kirchhoff diffraction integral formula, the light field is propagated from the modulation plate plane into free space in the forward direction to the target plane, and the complex amplitude distribution of the target plane is calculated. : in, For Fresnel-Kirchhoff diffraction integral calculation; For the first Amplitude distribution of the target plane during the next iteration; For the first The phase wavefront of the target plane at the next iteration; To determine the horizontal spatial coordinates within the target observation plane, The vertical spatial coordinates within the target observation plane; Core complex amplitude physical constraint replacement: Preserve the phase wavefront propagating in the positive direction to reach the target plane Keep it unchanged, and set the target plane amplitude. According to the preset relaxation factor The ideal super-Gaussian target amplitude defined in the phase modulation iterative design physical model Perform linear fusion update to obtain the complex amplitude after linear fusion update. : Backpropagation and phase extraction: updating the complex amplitude after linear fusion Perform reverse propagation, tracing back to the input plane where the phase modulation plate is located, to obtain a new complex amplitude distribution on the input plane. : in, For Fresnel-Kirchhoff inverse diffraction integral calculation, For the first Phase distribution of the input target plane after each iteration of backpropagation; Extracting the phase component from the new complex amplitude distribution As the first The phase input of the next iteration, and the Gaussian incident amplitude. Recombined to impose amplitude constraints on the input plane; Convergence Criteria: Each iteration follows the physical constraint of energy conservation; the root mean square error of light intensity is calculated on the target plane, and the conversion of the beam to the target is used as the convergence criterion; when the root mean square error of the output light intensity on the target plane meets the preset convergence threshold, the iteration terminates, and the current phase input is used as the optimal spatial phase solution. .

[0029] Step S2: Extract the optimal spatial phase mask using the improved Gerchberg-Saxton algorithm based on complex amplitude constraints.

[0030] In step S2 of this invention, a hybrid region amplitude degree of freedom constraint is used to calculate the amplitude propagating forward to the target plane. According to the preset relaxation factor With the ideal super-Gaussian target amplitude Perform linear fusion and update the complex amplitude as follows Then reverse propagation is performed.

[0031] This invention inputs the system physical parameters defined in step S1 into the nonlinear iterative optimization process. Let... For the current iteration number, the additional delay phase distribution matrix introduced in space by the phase modulation board is: The execution logic of the iterative algorithm is as follows: Initial State and Forward Propagation: The first At the start of the next iteration, the complex amplitude of the modulation plate's exit surface is formed by combining the Gaussian incident amplitude with the phase of the current iteration: Subsequently, using the Fresnel-Kirchhoff diffraction integral formula, the light field was propagated from the modulation plate plane into free space to the target plane, and the complex amplitude distribution of the target plane was calculated. : Core Complex Amplitude Physical Constraint Replacement: Traditional GS algorithms directly replace the calculated amplitude with the target amplitude, which is prone to getting trapped in local optima and leading to near-field coupling errors. This invention introduces adaptive hybrid region amplitude degree-of-freedom constraints, maintaining the phase wavefront of the propagation arrival. The calculated amplitude remains unchanged. According to the preset relaxation factor Compared with the ideal super-Gaussian target amplitude defined in step S1 Perform linear fusion update: Backpropagation and phase extraction: for the updated complex amplitude Performing reverse propagation, tracing back to the input plane of beam shaping, i.e., the phase modulation plate, yields a new complex amplitude distribution: At this point, the phase portion obtained from the backward propagation is extracted. , take it as the first The phase input for the next iteration. At the start of the next iteration (step s1 of the next iteration), this phase will be compared with the known Gaussian incident amplitude. The components are recombined to form new incident complex amplitudes, thereby forcibly imposing amplitude constraints on the input plane.

[0032] Convergence Criterion: Repeat the above sub-steps, calculating the root mean square error (RMS) of the light intensity and the flat-top uniformity on the target plane for each iteration. The iteration terminates when the RMS of the output light intensity on the target plane and the ideal super-Gaussian flat-top distribution satisfy the convergence threshold. The output is the globally optimal phase solution. This is the height mapping diagram of the surface relief microstructure required for the phase modulation board.

[0033] Step S3: Based on the optimal spatial phase demodulation, a phase modulation board is processed to build a quantitative schlieren high temporal resolution diagnostic system with a uniform flat-top laser background; Preferably, based on the optimal spatial phase solution, a quantitative schlieren high temporal resolution diagnostic system with a uniform flat-top laser background is constructed, including the fabrication of a phase modulation plate and the construction of the system optical path, and the construction of a uniform flat-top laser reference background, specifically: A phase modulation plate matching is prepared based on the optimal spatial phase solution. The pulsed laser, the beam expanding and collimating lens group, the phase modulation plate, the first schlieren spherical mirror, the flow field to be measured, the second schlieren spherical mirror, the knife edge, the narrowband filter and the camera are arranged in sequence according to the beam propagation order to form an improved Z-type quantitative schlieren optical path. Based on an improved Z-type quantitative schlieren optical path, the wavefront phase of the incident Gaussian laser is reconstructed using a phase modulation plate, shaping the beam into a flat-topped parallel beam with uniform cross-sectional energy. Using the uniform flat-topped laser as the imaging reference background, flat-topped background images and schlieren images containing flow field disturbances are acquired separately to obtain the global constant field background light intensity. The system optical path was built and a uniform flat-top laser reference background was constructed.

[0034] Based on the optimal spatial phase solution A phase modulation plate is fabricated on a high damage threshold substrate using any one of mask lithography, electron beam direct writing or deep reactive ion etching techniques, and then embedded into an optical platform. An improved Z-shaped quantitative schlieren optical path is arranged around the physical field to be monitored. The schlieren optical path consists of a high-power short-pulse laser, a beam expander collimating lens group, a phase modulation plate, a first schlieren spherical mirror, a fluid / plasma physics diagnostic area, a second schlieren spherical mirror, a high-precision mechanical blade, an ultra-narrow band interference filter, and an ultra-high-speed digital CCD / CMOS camera, arranged in sequence according to beam propagation. After being expanded and collimated, the high-power nanosecond pulsed laser is incident on the phase modulation plate. The phase modulation plate redistributes the wavefront phase of the incident laser. After propagating in free space and being reflected by the first schlieren spherical mirror, the incident laser is transformed into a flat-top parallel beam with absolutely uniform cross-sectional energy. After the flat-top parallel beam penetrates the diagnostic area, it carries the light deflection information of the diagnostic area. It is then converged to the mechanical blade edge by the second schlieren spherical mirror to achieve spatial high-pass filtering. After filtering out plasma autoluminescence by an ultra-narrow band interference filter, a high signal-to-noise ratio schlieren grayscale image is captured by an ultra-high-speed digital camera within a tens of nanosecond exposure window. At the same time, a undisturbed flat-top background image is acquired to obtain the global constant field background light intensity. .

[0035] In step S3 of this invention, a quantitative schlieren high temporal resolution diagnostic system with a uniform flat-top laser background is constructed.

[0036] The quantitative schlieren diagnostic system constructed in step S3 of this invention includes, in sequence according to the beam propagation order: a pulsed laser, a beam expanding and collimating lens group, a phase modulation plate, a first schlieren spherical mirror, the flow field to be measured, a second schlieren spherical mirror, a knife edge, a narrowband filter, and a camera.

[0037] This invention obtains Subsequently, using mask lithography, electron beam direct writing, or deep reactive ion etching techniques, corresponding multi-step diffractive optical elements, i.e. phase modulation plates, are fabricated on substrates with high damage thresholds such as fused silica, and then embedded into the optical platform.

[0038] An improved Z-shaped quantitative schlieren optical path is arranged around the high-voltage discharge generator (such as a rod-plate long air gap discharge chamber) or wind tunnel physics field to be monitored. The system includes, in order of beam propagation: a high-power short-pulse laser, a beam expander and collimating lens group, a custom-fabricated phase modulation plate, a first schlieren spherical mirror (collimating mirror), a fluid / plasma physics diagnostic area, a second schlieren spherical mirror (focusing mirror), a high-precision mechanical blade, an ultra-narrow band interference filter, and an ultra-high-speed digital CCD / CMOS camera.

[0039] After passing through a beam expander and phase modulator, the wavefront phase of a high-power nanosecond pulsed laser is redistributed. As the beam propagates in free space, the irreversible phase evolution pushes the light energy from the central Gaussian peak outwards. By the time it reaches the first schlieren spherical mirror, it has perfectly diffracted into a flat-topped parallel beam with absolutely uniform cross-sectional energy. This flat-topped background beam penetrates the discharge test area without damage. When the discharge evolves to the streamer or leader dark region, extreme Joule heating induces violent gas expansion, generating a local refractive index gradient. The light from the flat-top beam is deflected by a fraction of an arc in the medium. The distorted beam, carrying information about the deflection, is refocused at the knife-edge surface at the focal point by a second spherical mirror. The knife-edge ablates a portion of the undeflected zero-order light (achieving spatial high-pass frequency filtering). Subsequently, the beam passes through an extremely narrow-band interference filter (whose center wavelength is strictly matched to the laser wavelength, and whose bandwidth is...). The broadband self-luminescence of the discharge plasma was completely filtered out, and the high signal-to-noise ratio schlieren grayscale image was finally captured by an ultra-high-speed digital camera within an exposure window of tens of nanoseconds.

[0040] Step S4: Generate a uniform reference background image and a perturbed schlieren image based on the quantitative schlieren high temporal resolution diagnostic system, and extract the light deflection angle based on the uniform reference background image and the perturbed schlieren image; Preferably, a uniform reference background image and a perturbed schlieren image are generated based on the quantitative schlieren high temporal resolution diagnostic system, and the light deflection angle is extracted based on the uniform reference background image and the perturbed schlieren image, including: A uniform reference background map is generated based on an improved Z-type quantitative schlieren optical path; a perturbation schlieren map is generated based on the introduced flow field disturbance. Processing the uniform reference background image and the perturbed schlieren image: Increasing the background light intensity... The shape is converted into a global constant field to determine the local grayscale changes in the probed image. Deflection angle caused solely by the flow field And it is determined by the effective cutting amount of the mechanical blade; Based on the processed uniform reference background image and perturbation schlieren image, assume that the light rays in the flow field propagate along the positive z-axis, the edge of the mechanical cutting edge is parallel to the x-axis, and the deflection angle of the light rays in the y-direction is... Expressed by the integral of the refractive index gradient of the medium: in, The absolute reference refractive index of the undisturbed gas in the environment; integration path The optical thickness of the flow field disturbance region; Let be the refractive index gradient of the medium in the flow field along the y-direction; Within the linear response range of the schlieren system, the deflection angle The relative contrast of a schlieren image is proportional to the deflection angle. The formula for calculating the light deflection angle is as follows: in, The grayscale values ​​of the schlieren image acquired under perturbation. The grayscale value of the flat-top background image without disturbance; This represents the system sensitivity coefficient.

[0041] In step S4 of this invention, the light deflection angle is extracted with high fidelity based on the flat-top light field intensity analytical framework.

[0042] In step S4 of the present invention, the light deflection angle The extraction formula is Thanks to phase modulation, the grayscale of the background image... It is a global high-level constant field, thus avoiding the divergence and nonlinear amplification of noise caused by the denominator approaching zero in the formula.

[0043] Since this system has already controlled the background light intensity at the source... The image is shaped into a global constant field to detect local grayscale changes. Strictly constrained to be the deflection angle caused solely by the flow field It is determined by the effective cutting amount of the blade.

[0044] Assume the light rays in the flow field propagate along the positive z-axis, and the cutting edge is parallel to the x-axis (i.e., the cutting edge cuts the light rays that are deflected along the y-direction). According to geometrical optics and Fermat's principle, the angle of deflection of the light ray in the y-direction... It can be expressed by the integral of the refractive index gradient of the medium: In the formula, The absolute reference refractive index of the undisturbed gas in the environment; integration path The optical thickness of the flow field disturbance region.

[0045] In schlieren systems, the angle of deflection This causes a slight shift in the light source image on the blade plane. As the image is obscured to varying degrees by the blade, it creates grayscale differences on the camera sensor. Within the linear response range, the relative contrast of the schlieren image is proportional to the deflection angle. in, The grayscale values ​​of the schlieren image acquired under perturbation. When there is no disturbance ( The grayscale value of the flat-top background image acquired at (time). The system sensitivity coefficient is determined by the focal length of the second schlieren spherical mirror, the cutting depth of the blade, and the geometric dimensions of the light source image on the blade plane. It can be obtained through theoretical calculation or experimental calibration.

[0046] Thanks to the flat-top beam shaping mechanism introduced in step S2, the denominator matrix It maintains a high constant value throughout the entire physical field of view. This completely avoids the field-of-view edge artifacts found with conventional Gaussian lasers. The divergence in "dividing by zero" calculations caused by approaching zero, and the nonlinear amplification of environmental background shot noise and system readout noise by tens of thousands of times. Therefore, the deflection angle The extracted signal-to-noise ratio reached the theoretical physical limit across the entire large field of view.

[0047] Step S5: Based on the light deflection angle, perform high-sensitivity multiphysics three-dimensional inversion reconstruction of the fluid density field and transient temperature field to improve the overall detection sensitivity of laser quantitative schlieren imaging.

[0048] Preferably, based on the light deflection angle, a multi-physics three-dimensional inversion reconstruction of the fluid density field and transient temperature field is performed, including: Obtain the deflection vector field of the entire field of view Orthogonal components , They are respectively directional light deflection angle and Angle of deflection of directional light rays; Based on the theory of optical tomography in fluid dynamics, a two-dimensional scalar potential function is introduced to characterize the optical path difference. By utilizing the spatial gradient characteristic of the optical path difference based on the deflection angle, a fundamental correlation equation is established; the two-dimensional spatial divergence of both sides of the fundamental correlation equation is calculated to construct a two-dimensional Poisson equation characterizing the distribution of the optical path difference: Using any one of the finite difference method, multigrid method, or conjugate gradient algorithm, combined with the Neumann boundary conditions of the undisturbed flow field boundary, the two-dimensional Poisson equation is solved based on the two-dimensional scalar potential function. Determine the absolute change in the integral projected refractive index. ; Introducing the Gladstone-Dale relation: in, The Gladstone-Dale constant is given for a specific laser wavelength and gas mixture composition. Let n be the spatial absolute density distribution of the gas medium; n is the absolute refractive index of the disturbed gas; the Abel inverse transform back projection algorithm is used to transform the two-dimensional... Restored to the true refractive index of each point in three-dimensional space The true refractive index Substituting the Gladstone-Dale relation, the transient three-dimensional flow density distribution field is analyzed. ; Assuming that the discharge expansion process locally satisfies the ideal gas law, the three-dimensional flow field density distribution field is used as an example. Inversely derived transient evolution temperature field of neutral gas in local plasma : in, The molar mass of the component gases Let be the ideal gas constant. This refers to gas pressure.

[0049] Step S5: Multiphysics 3D Inversion and Reconstruction of Fluid Density Field and Transient Temperature Field In step S5 of this invention, the two-dimensional Poisson equation characterizing the optical path difference is solved numerically. The absolute change in integral refractive index is obtained, and then the three-dimensional transient temperature field is inverted by combining the inverse Abel transform and the Gladstone-Dale relation.

[0050] This invention obtains a high-fidelity deflection angle vector field across the entire field of view. (After obtaining the orthogonal components by rotating the blade or using a color multi-directional phase-amplitude filter), the fluid dynamics optical tomography theory is used to reverse map them back to the physical state space.

[0051] The propagation of light in a weakly gradually varying refractive index medium follows the paraxial approximation of the wave equation. A two-dimensional scalar potential function is introduced to characterize the change in optical thickness, i.e., the optical path difference. Since the deflection angle is actually the spatial gradient of the optical path difference (i.e., By simultaneously calculating the two-dimensional spatial divergence on both sides of the equation, we can construct the following two-dimensional Poisson equation characterizing the distribution of optical path difference: By combining the finite difference method, multigrid method, or conjugate gradient algorithm with Neumann boundary conditions of the undisturbed flow field boundary, the Poisson equation can be numerically solved to obtain the absolute change in integral projected refractive index stably and smoothly. .

[0052] This invention introduces the Gladstone-Dale relation: In the formula, The GD constant is given for a specific laser wavelength and gas mixture composition. This represents the spatial absolute density distribution of the gaseous medium. Using back-projection algorithms such as the Abel inverse transform, the two-dimensional... The inverse transformation yields a three-dimensional refractive index distribution, which is then substituted into the GD formula to analytically derive the transient three-dimensional flow field density distribution. .

[0053] This invention employs the inverse Abel transform for axisymmetric flow fields (such as long air gap discharge channels, cylindrical jets, and other typical diagnostic objects), and its output is a scalar field in a three-dimensional Cartesian coordinate system. Or, more simply, a cylindrical coordinate system scalar field ( Refractive index and azimuth angle under axisymmetry (Irrelevant)

[0054] The specific expression for the inverse Abel transform is: in, The maximum radial radius of the flow field is . The integral is a radial variable. The output of this expression is... It is any radial position in three-dimensional space Axial position The instantaneous refractive index value at a given point is a continuous three-dimensional scalar field.

[0055] The basis for substituting the instantaneous expression into the Gladstone-Dale relation: Gladstone-Dale Relationship It is a universal empirical formula for gas optics, applicable to gaseous media at any spatial location. It describes the one-to-one correspondence between refractive index and density at a single point, regardless of whether that point is in an integral projection state.

[0056] The inverse Abel transform is precisely used to transform the "two-dimensional projection quantity integrated along the optical axis" "Restored to the true refractive index of each point in three-dimensional space" "Restored" It is an instantaneous physical quantity at a spatial point, which perfectly meets the input requirements of the Gladstone-Dale relation. Therefore, it can be directly substituted into the solution to calculate the density at the corresponding point. .

[0057] The GD constant and molar mass in this invention refer to the weighted average of the multi-component gases. For example, in air, if the mixed gas components are nitrogen and oxygen, then the GD constant and molar mass are the weighted average of 80% nitrogen and 20% oxygen.

[0058] Finally, in the macroscopic gas dynamics evolution, assuming that the discharge expansion process locally satisfies the ideal gas law, the transient evolution temperature field of the local plasma neutral gas is deduced from the density field. : In the formula, This represents the molar mass of the component gas. is the ideal gas constant.

[0059] Figure 2 This is a flowchart of a method for improving the sensitivity of phase-modulated laser quantitative schlieren imaging provided by the present invention. It includes the following core steps: S1.1 Construct an iterative physical model for phase modulation from a Gaussian beam to a flat-top beam. Determine parameters such as the beam waist radius ω0 of the Gaussian beam output from the pulsed laser, and use a higher-order super-Gaussian function to define the ideal amplitude distribution Aaim of the flat-top beam required for the target observation plane, so as to smooth the transition region at the edge of the beam spot.

[0060] S1.2 Extracting the optimal phase mask using the improved Gerchberg-Saxton algorithm based on hybrid complex amplitude constraints. Referring to Figure 3, which shows the global optimization data flow diagram, Figure 3 fully illustrates the numerical calculation process and data transfer relationships for extracting the optimal spatial phase solution of the phase modulation board according to this invention. Figure 3 The following explains the various identifiers and their corresponding technical steps: When the algorithm starts, it first enters the initial input stage marked S2.1, where three core initial parameters are input as the physical boundary conditions of the algorithm, including the incident Gaussian amplitude. That is, the amplitude distribution and initial phase of the fundamental mode Gaussian beam on the incident plane of the phase modulation plate. Assuming the incident laser is an ideal plane wave with no wavefront distortion, an initial phase distribution of all zeros, and a target super-Gaussian amplitude... A higher-order super-Gaussian function is used to smooth the edges of the light spot and suppress Gibbs oscillations. After the initial parameter input is completed, the core nonlinear optimization iterative loop is entered. Each module is executed sequentially in the direction of the arrow to form a closed loop. The first module is the combined complex amplitude module marked S2.2, which sets the phase distribution of the current iteration. With a fixed incident Gaussian amplitude The complex amplitude of the modulation plate's exit surface is generated by combining the amplitudes, expressed as follows: Then it enters the forward Fresnel diffraction propagation module marked S2.3, and performs Fresnel-Kirchhoff diffraction integral calculations. The complex amplitude of the modulation plate plane Propagating forward through free space to the target observation plane, the complex amplitude distribution of the target plane is calculated. The expression is as follows; next is the core improvement of this invention, the S2.4 hybrid region amplitude constraint replacement module, where the relaxation factor is clearly marked in the figure. The module maintains the phase wavefront that propagates in the positive direction to reach the target plane as input. The calculated amplitude obtained by forward propagation remains unchanged. With the ideal super-Gaussian target amplitude Weighted linear fusion generates updated target plane complex amplitudes. The expression is This method, which only applies flexible constraints to the amplitude and preserves the phase information of the target plane, avoids the local optima and diffraction ring problems caused by rigid replacement in traditional algorithms. Then, it enters the inverse Fresnel diffraction propagation module (S2.5), where Fresnel-Kirchhoff inverse diffraction integration is performed. The updated target plane complex amplitude Tracing back to the input plane where the phase modulation plate is located, we obtain a new complex amplitude distribution on the input plane. Then, using the phase extraction module identified as S2.6, the amplitude information obtained from the reverse propagation is discarded, and only the phase component of the new complex amplitude distribution is extracted. This phase is used for subsequent iterations or output. After phase extraction, the system enters the convergence determination module (S2.7) to calculate the root mean square error (RMS) between the output light intensity of the target plane and the ideal super-Gaussian distribution, and determines whether the preset convergence threshold is met. If the convergence condition is not met, the system enters the phase update module (S2.9) to update the extracted phase. As the phase input for the next iteration, return to the complex amplitude combination module S2.2 to start a new iteration; if the convergence condition is met, proceed to the output stage marked S2.8, the iteration terminates and the current optimal spatial phase solution is output. This result is a height mapping map of the surface relief microstructure of the phase modulation board, which is used for the subsequent fabrication of physical components.

[0061] S1.3. Construct a quantitative schlieren high-temporal-resolution diagnostic system with a uniform flat-top laser background. (See reference...) Figure 4 The physical architecture topology diagram shows that the system sequentially arranges a pulsed laser, a beam expander and collimating lens group, a custom phase modulation plate, a first schlieren mirror, the flow field region to be measured, a second schlieren mirror, a knife edge, a narrowband filter, and a high-speed camera. After the Gaussian laser passes through the phase modulation plate, its wavefront phase is redistributed. When it reaches the first schlieren mirror, it diffracts and evolves into a flat-topped parallel beam with absolutely uniform cross-sectional energy, completely filtering out plasma self-emission before being captured by the camera.

[0062] S1.4 Extracting the high-fidelity deflection angle vector field. Background and schlieren images are acquired under undisturbed and flow-perturbed conditions, respectively. The deflection angle is calculated using a flat-top light field intensity analytical framework. Since the background light intensity in this embodiment has been shaped into a high-constant field that does not drastically decrease with spatial position, the division-by-zero divergence and noise exponential amplification phenomena that occur in peripheral pixels when using traditional Gaussian lasers are effectively suppressed, achieving high signal-to-noise ratio deflection angle analysis across the entire physical field of view.

[0063] S1.5. Three-dimensional inversion and reconstruction of fluid density and temperature fields. The two-dimensional Poisson equation is solved using a finite difference algorithm to stably and smoothly obtain the optical path difference and integral refractive index change. The three-dimensional absolute fluid density field is calculated by combining the inverse Abel transform and the Gladstone-Dale relation. Finally, based on the ideal gas law, the transient temperature distribution under multiple physical environments, such as the discharge channel, is reconstructed from the density field. This invention abandons traditional light sources with limited light emission and uses a self-developed algorithm to flatten the top of the Gaussian laser at the physical hardware level. This embodiment completely eliminates the problem of edge signal-to-noise ratio degradation in laser schlieren systems, ensuring high reliability of deflection angle extraction under sub-microsecond exposure limits, and providing an innovative technical route for high spatial and temporal resolution three-dimensional diagnosis of extreme transient physical fields.

[0064] In existing technologies, whether using LEDs to enhance transient light intensity or directly introducing a laser source, the system is either limited by low sensitivity due to insufficient light flux or by edge calculation divergence caused by the Gaussian light field. This invention proposes a method for improving the sensitivity of quantitative schlieren imaging based on phase-modulated laser light, with the following technical effects: 1. This invention overcomes the physical limitations of Gaussian light fields, eliminating edge noise distortion. It innovatively integrates a diffraction spatial phase mask at the optical path front end, efficiently reconstructing the fundamental Gaussian laser into a high-order super-Gaussian flat-top beam. This technical solution ensures that the undisturbed background image acquired by the system maintains a high-level, constant grayscale distribution throughout the entire effective field of view, fundamentally eliminating the risk of the denominator in the deflection angle formula approaching zero, suppressing the phenomenon of local amplification of environmental and circuit noise, and enabling the optical signal-to-noise ratio at any position in the entire field of view to reach its optimal limit.

[0065] 2. The adaptive complex amplitude constraint algorithm achieves high convergence accuracy and avoids secondary diffraction interference from the optical field. In the phase mask iterative design stage, this invention abandons the traditional rigid amplitude replacement and adopts a hybrid region amplitude degree-of-freedom constraint mechanism. The calculated amplitude from forward propagation is linearly fused and updated with the ideal flat-top amplitude. This algorithmic improvement effectively suppresses high-frequency diffraction rings and Gibbs oscillations, generating an extremely smooth and uniform optical background without sacrificing laser energy.

[0066] 3. The challenge of the trade-off between spatiotemporal resolution and sensitivity has been overcome, enabling high-precision reconstruction of transient physical fields at the micro-nano scale and sub-microsecond level. Thanks to the high-quality, divergence-free signal-to-noise ratio guaranteed by the flat-top high-power laser within the ultra-short sub-microsecond exposure time, the backend deflection angle extraction, Poisson equation numerical solution, and Abel inverse transform process can maintain extremely low mathematical propagation errors. This significantly improves the analytical accuracy and stability of the system for retrieving the three-dimensional density and temperature of extreme transient flow fields.

[0067] Figure 5 This is a structural diagram of a laser quantitative schlieren imaging sensitivity enhancement system according to an embodiment of the present invention.

[0068] like Figure 5 As shown, the present invention provides a laser quantitative schlieren imaging sensitivity enhancement system, the system comprising: The physical model building unit 501 is used to build a physical model for the phase modulation iterative design of the Gaussian beam to flat-top beam conversion. The physical model has a built-in improved algorithm based on complex amplitude dual constraints to solve the spatial phase distribution of the adapted phase modulation plate. Preferably, the physical model building unit 501 is used to build a physical model for the phase modulation iterative design of the Gaussian beam to flat-top beam conversion, including: The predefined beam conversion target is to efficiently convert the fundamental mode Gaussian pulse laser into a flat-top uniform laser, and the conversion target is the construction standard of complex amplitude dual constraint; The improved Gerchberg-Saxton algorithm was selected as the improved algorithm based on complex amplitude dual constraints to iteratively optimize the solution of the optimal spatial phase distribution of the phase modulation plate. Construct the incident spatial boundary with dual complex amplitude constraints, define the complex amplitude distribution of the incident Gaussian beam, and set the ideal complex amplitude distribution of the incident beam in the incident plane. Ideal complex amplitude distribution Satisfying the fundamental Gaussian beam model: in, Let be the waist radius of the Gaussian beam; The central peak amplitude; assuming the initial wavefront is an ideal isophase surface, the initial phase... ; This represents the amplitude distribution of the incident Gaussian beam at the incident plane. The imaginary unit; These are the spatial coordinates along the horizontal direction within the incident plane, representing different positions in the horizontal direction within that plane; These are the spatial coordinates along the vertical direction within the incident plane, representing different positions in the vertical direction within that plane; Constructing a target-end spatial boundary with dual complex amplitude constraints, and defining the ideal complex amplitude distribution of the target flat-top beam on the target observation plane. Ideal complex amplitude distribution A high-order super-Gaussian function is used to smooth the edge transition region of the target spot: in, The macroscopic effective radius of the target flat-top beam; It is a super-Gaussian order, and To ensure the steepness of the edges; The spatial coordinates along the horizontal direction within the target observation plane represent different positions in the horizontal direction within that plane. The spatial coordinates along the vertical direction within the target observation plane represent different positions in the vertical direction within that plane. This represents the stable amplitude of the flat-top beam. The superimposed physical constraint of energy conservation restricts the complex amplitude under a dual constraint. This energy conservation constraint strictly follows the energy conservation law of optical propagation, expressed as:

[0069] The optimal spatial phase solution extraction unit 502 is used to extract the optimal spatial phase solution of the phase modulation plate that efficiently converts the incident Gaussian beam into a flat-top uniform laser based on an improved algorithm. Preferably, the optimal spatial phase extraction unit 502 is used for based on The improved algorithm extracts the optimal spatial phase solution for a phase modulation plate that efficiently converts an incident Gaussian beam into a flat-top uniform laser. Specifically, it involves extracting the optimal spatial phase solution through a nonlinear iterative optimization process, which sequentially executes the following steps: Initial state and forward propagation: Let For the current iteration number, the additional delay phase distribution matrix introduced in space by the phase modulation board is: ;No. At the start of the next iteration, the amplitude distribution of the incident Gaussian beam at the incident plane is used as a basis. Current iteration phase By combining these elements, the complex amplitude of the modulation plate's projection surface was constructed. The expression is: Using the Fresnel-Kirchhoff diffraction integral formula, the light field is propagated from the modulation plate plane into free space in the forward direction to the target plane, and the complex amplitude distribution of the target plane is calculated. : in, For Fresnel-Kirchhoff diffraction integral calculation; For the first Amplitude distribution of the target plane during the next iteration; For the first The phase wavefront of the target plane at the next iteration; To determine the horizontal spatial coordinates within the target observation plane, The vertical spatial coordinates within the target observation plane; Core complex amplitude physical constraint replacement: Preserve the phase wavefront propagating in the positive direction to reach the target plane Keep it unchanged, and set the target plane amplitude. According to the preset relaxation factor The ideal super-Gaussian target amplitude defined in the phase modulation iterative design physical model Perform linear fusion update to obtain the complex amplitude after linear fusion update. : Backpropagation and phase extraction: updating the complex amplitude after linear fusion Perform reverse propagation, tracing back to the input plane where the phase modulation plate is located, to obtain a new complex amplitude distribution on the input plane. : in, For Fresnel-Kirchhoff inverse diffraction integral calculation, For the first Phase distribution of the input target plane after each iteration of backpropagation; Extracting the phase component from the new complex amplitude distribution As the first The phase input of the next iteration, and the Gaussian incident amplitude. Recombined to impose amplitude constraints on the input plane; Convergence Criteria: Each iteration follows the physical constraint of energy conservation; the root mean square error of light intensity is calculated on the target plane, and the conversion of the beam to the target is used as the convergence criterion; when the root mean square error of the output light intensity on the target plane meets the preset convergence threshold, the iteration terminates, and the current phase input is used as the optimal spatial phase solution. .

[0070] The diagnostic system building unit 503 is used to build a quantitative schlieren high temporal resolution diagnostic system with a uniform flat-top laser background based on the optimal spatial phase solution. Preferably, the diagnostic system construction unit 503 is used to construct a quantitative schlieren high temporal resolution diagnostic system based on the optimal spatial phase solution and a uniform flat-top laser background, including the fabrication of a phase modulation plate and the construction of the system optical path, and the construction of a uniform flat-top laser reference background, specifically: A phase modulation plate matching is prepared based on the optimal spatial phase solution. The pulsed laser, the beam expanding and collimating lens group, the phase modulation plate, the first schlieren spherical mirror, the flow field to be measured, the second schlieren spherical mirror, the knife edge, the narrowband filter and the camera are arranged in sequence according to the beam propagation order to form an improved Z-type quantitative schlieren optical path. Based on an improved Z-type quantitative schlieren optical path, the wavefront phase of the incident Gaussian laser is reconstructed using a phase modulation plate, shaping the beam into a flat-topped parallel beam with uniform cross-sectional energy. Using the uniform flat-topped laser as the imaging reference background, flat-topped background images and schlieren images containing flow field disturbances are acquired separately to obtain the global constant field background light intensity. The system optical path was built and a uniform flat-top laser reference background was constructed.

[0071] The light deflection angle extraction unit 504 is used to extract the light deflection angle based on the flat-top parallel beam with uniform cross-sectional energy generated by the quantitative schlieren high temporal resolution diagnostic system. Preferably, the light deflection angle extraction unit 504 is used to extract the light deflection angle based on the flat-top parallel beam with uniform cross-sectional energy generated by the quantitative schlieren high temporal resolution diagnostic system, including: A uniform reference background map is generated based on an improved Z-type quantitative schlieren optical path; a perturbation schlieren map is generated based on the introduced flow field disturbance. Processing the uniform reference background image and the perturbed schlieren image: Increasing the background light intensity... The shape is converted into a global constant field to determine the local grayscale changes in the probed image. Deflection angle caused solely by the flow field And it is determined by the effective cutting amount of the mechanical blade; Based on the processed uniform reference background image and perturbation schlieren image, assume that the light rays in the flow field propagate along the positive z-axis, the edge of the mechanical cutting edge is parallel to the x-axis, and the deflection angle of the light rays in the y-direction is... Expressed by the integral of the refractive index gradient of the medium: in, The absolute reference refractive index of the undisturbed gas in the environment; integration path The optical thickness of the flow field disturbance region; Let be the refractive index gradient of the medium in the flow field along the y-direction; Within the linear response range of the schlieren system, the deflection angle The relative contrast of a schlieren image is proportional to the deflection angle. The formula for calculating the light deflection angle is as follows: in, The grayscale values ​​of the schlieren image acquired under perturbation. The grayscale value of the flat-top background image without disturbance; This represents the system sensitivity coefficient.

[0072] The 3D inversion and reconstruction unit 505 is used to perform high-sensitivity multi-physics 3D inversion and reconstruction of fluid density field and transient temperature field based on light deflection angle, thereby improving the overall detection sensitivity of laser quantitative schlieren imaging.

[0073] Preferably, the three-dimensional inversion and reconstruction unit 505 is used for multi-physics three-dimensional inversion and reconstruction of the fluid density field and transient temperature field based on the light deflection angle, including: Obtain the deflection vector field of the entire field of view Orthogonal components , They are respectively directional light deflection angle and Angle of deflection of directional light rays; Based on the theory of optical tomography in fluid dynamics, a two-dimensional scalar potential function is introduced to characterize the optical path difference. By utilizing the spatial gradient characteristic of the optical path difference based on the deflection angle, a fundamental correlation equation is established; the two-dimensional spatial divergence of both sides of the fundamental correlation equation is calculated to construct a two-dimensional Poisson equation characterizing the distribution of the optical path difference: Using any one of the finite difference method, multigrid method, or conjugate gradient algorithm, combined with the Neumann boundary conditions of the undisturbed flow field boundary, the two-dimensional Poisson equation is solved based on the two-dimensional scalar potential function. Determine the absolute change in the integral projected refractive index. ; Introducing the Gladstone-Dale relation: in, The Gladstone-Dale constant is given for a specific laser wavelength and gas mixture composition. Let n be the spatial absolute density distribution of the gas medium; n is the absolute refractive index of the disturbed gas; the Abel inverse transform back projection algorithm is used to transform the two-dimensional... Restored to the true refractive index of each point in three-dimensional space The true refractive index Substituting the Gladstone-Dale relation, the transient three-dimensional flow density distribution field is analyzed. ; Assuming that the discharge expansion process locally satisfies the ideal gas law, the three-dimensional flow field density distribution field is used as an example. Inversely derived transient evolution temperature field of neutral gas in local plasma : in, The molar mass of the component gases Let be the ideal gas constant. The gas pressure is [not specified]. This invention provides a computer-readable storage medium storing a computer program that, when executed by a processor, implements steps such as those in a laser quantitative schlieren imaging sensitivity enhancement method.

[0074] This invention provides an electronic device, comprising: The aforementioned computer-readable storage medium; and One or more processors for executing a program in a computer-readable storage medium.

[0075] The present invention has been described with reference to a few embodiments. However, it will be apparent to those skilled in the art that other embodiments besides those disclosed above fall equivalently within the scope of the present invention.

[0076] Generally, all terms used in this invention are interpreted according to their ordinary meaning in the art, unless otherwise expressly defined herein. All references to “a / the / the [device, component, etc.]” ​​are openly interpreted as at least one instance of said device, component, etc., unless otherwise expressly stated. The steps of any method disclosed herein need not be performed in the exact order disclosed unless explicitly stated otherwise.

[0077] Those skilled in the art will understand that embodiments of the present invention can be provided as methods, systems, or computer program products. Therefore, the present invention can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the present invention can take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0078] This invention is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart illustrations and / or block diagrams. Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0079] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0080] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0081] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for improving the sensitivity of laser quantitative schlieren imaging, characterized in that, The method includes: A physical model for phase modulation iterative design of Gaussian beam to flat-top beam conversion is constructed. The physical model incorporates an improved algorithm based on complex amplitude dual constraints to solve the spatial phase distribution of the adapted phase modulation plate. Based on the improved algorithm, the optimal spatial phase solution of the phase modulation plate that efficiently converts the incident Gaussian beam into a flat-top uniform laser is extracted. Based on the optimal spatial phase demodulation phase modulation board, a quantitative schlieren high temporal resolution diagnostic system with a uniform flat-top laser background is built. Based on the quantitative schlieren high temporal resolution diagnostic system, a uniform reference background image and a perturbed schlieren image are generated, and the light deflection angle is extracted based on the uniform reference background image and the perturbed schlieren image; Based on the light deflection angle, a high-sensitivity multiphysics three-dimensional inversion reconstruction of the fluid density field and transient temperature field is performed to improve the overall detection sensitivity of laser quantitative schlieren imaging.

2. The method according to claim 1, characterized in that, The physical model for the phase modulation iterative design of the Gaussian beam to flat-top beam conversion includes: The preset beam conversion target is clearly defined, which is to convert the fundamental mode Gaussian pulse laser into a flat-top uniform laser with high efficiency. The conversion target is the construction standard of complex amplitude dual constraint. An improved Gerchberg-Saxton algorithm was selected as the improved algorithm based on complex amplitude dual constraints to iteratively optimize the solution of the optimal spatial phase distribution of the phase modulation plate. Construct the incident spatial boundary with dual complex amplitude constraints, define the complex amplitude distribution of the incident Gaussian beam, and set the ideal complex amplitude distribution of the incident beam in the incident plane. The ideal complex amplitude distribution Satisfying the fundamental Gaussian beam model: in, Let be the waist radius of the Gaussian beam; The central peak amplitude; assuming the initial wavefront is an ideal isophase surface, the initial phase... ; This represents the amplitude distribution of the incident Gaussian beam at the incident plane. The imaginary unit; These are the spatial coordinates along the horizontal direction within the incident plane, representing different positions in the horizontal direction within that plane; These are the spatial coordinates along the vertical direction within the incident plane, representing different positions in the vertical direction within that plane; Constructing a target-end spatial boundary with dual complex amplitude constraints, and defining the ideal complex amplitude distribution of the target flat-top beam on the target observation plane. The ideal complex amplitude distribution A high-order super-Gaussian function is used to smooth the edge transition region of the target spot: in, The macroscopic effective radius of the target flat-top beam; It is a super-Gaussian order, and To ensure the steepness of the edges; The spatial coordinates along the horizontal direction within the target observation plane represent different positions in the horizontal direction within that plane. The spatial coordinates along the vertical direction within the target observation plane represent different positions in the vertical direction within that plane. This represents the stable amplitude of the flat-top beam. The superimposed energy conservation physical constraint restricts the complex amplitude under a dual constraint. This energy conservation constraint strictly follows the energy conservation law of optical propagation, and its expression is: 。 3. The method according to claim 2, characterized in that, The step of extracting the optimal spatial phase solution for the phase modulation plate that efficiently converts the incident Gaussian beam into a flat-top uniform laser based on the improved algorithm specifically includes extracting the optimal spatial phase solution through a nonlinear iterative optimization process, wherein the iterative optimization process sequentially executes the following steps: Initial state and forward propagation: Let For the current iteration number, the additional delay phase distribution matrix introduced in space by the phase modulation board is: ;No. At the start of the next iteration, the amplitude distribution of the incident Gaussian beam at the incident plane is used as a basis. Current iteration phase By combining these elements, the complex amplitude of the modulation plate's projection surface was constructed. The expression is: Using the Fresnel-Kirchhoff diffraction integral formula, the light field is propagated from the modulation plate plane into free space in the forward direction to the target plane, and the complex amplitude distribution of the target plane is calculated. : in, For Fresnel-Kirchhoff diffraction integral calculation; For the first Amplitude distribution of the target plane during the next iteration; For the first The phase wavefront of the target plane at the next iteration; To determine the horizontal spatial coordinates within the target observation plane, The vertical spatial coordinates within the target observation plane; Core complex amplitude physical constraint replacement: Preserve the phase wavefront propagating in the positive direction to reach the target plane Keep it unchanged, and set the target plane amplitude. According to the preset relaxation factor The ideal super-Gaussian target amplitude defined in the phase modulation iterative design physical model Perform linear fusion update to obtain the complex amplitude after linear fusion update. : Backpropagation and phase extraction: updating the complex amplitude after linear fusion Perform reverse propagation, tracing back to the input plane where the phase modulation plate is located, to obtain a new complex amplitude distribution on the input plane. : in, For Fresnel-Kirchhoff inverse diffraction integral calculation, For the first Phase distribution of the input target plane after each iteration of backpropagation; Extracting the phase component from the new complex amplitude distribution As the first The phase input of the next iteration, and the Gaussian incident amplitude. Recombined to impose amplitude constraints on the input plane; Convergence Criteria: Each iteration follows the physical constraint of energy conservation; the root mean square error of light intensity is calculated on the target plane, and the conversion of the beam into the target is used as the convergence criterion; when the root mean square error of the output light intensity on the target plane meets the preset convergence threshold, the iteration terminates, and the current phase input is used as the optimal spatial phase solution. .

4. The method according to claim 1, characterized in that, The quantitative schlieren high temporal resolution diagnostic system based on the optimal spatial phase demodulation phase modulation plate and the uniform flat-top laser background includes the fabrication of the phase modulation plate and the construction of the system optical path, and the construction of the uniform flat-top laser reference background, specifically: Based on the optimal spatial phase solution, a matching phase modulation plate is prepared. The pulsed laser, the beam expanding and collimating lens group, the phase modulation plate, the first schlieren spherical mirror, the flow field to be measured, the second schlieren spherical mirror, the knife edge, the narrowband filter and the camera are arranged in sequence according to the beam propagation order to form an improved Z-type quantitative schlieren optical path. Based on an improved Z-type quantitative schlieren optical path, the wavefront phase of the incident Gaussian laser is reconstructed using a phase modulation plate, shaping the beam into a flat-topped parallel beam with uniform cross-sectional energy. Using the uniform flat-topped laser as the imaging reference background, flat-topped background images and schlieren images containing flow field disturbances are acquired separately to obtain the global constant field background light intensity. The system optical path was built and a uniform flat-top laser reference background was constructed.

5. The method according to claim 4, characterized in that, The process of generating a uniform reference background image and a perturbed schlieren image based on the quantitative schlieren high temporal resolution diagnostic system, and extracting the light deflection angle based on the uniform reference background image and the perturbed schlieren image, includes: Based on the improved Z-type quantitative schlieren optical path, a uniform reference background image is generated; based on the introduced flow field disturbance, a disturbed schlieren image is generated. The uniform reference background image and the perturbation schlieren image are processed as follows: the background light intensity is... The shape is converted into a global constant field to determine the local grayscale changes in the probed image. Deflection angle caused solely by the flow field And it is determined by the effective cutting amount of the mechanical blade; Based on the processed uniform reference background image and the disturbed schlieren image, assuming that the light in the flow field propagates along the positive z-axis, the edge of the mechanical cutting edge is parallel to the x-axis, and the deflection angle of the light in the y-direction is... Expressed by the integral of the refractive index gradient of the medium: in, The absolute reference refractive index of the undisturbed gas in the environment; integration path The optical thickness of the flow field disturbance region; Let be the refractive index gradient of the medium in the flow field along the y-direction; Within the linear response range of the schlieren system, the deflection angle The relative contrast of a schlieren image is proportional to the deflection angle. The formula for calculating the light deflection angle is as follows: in, The grayscale values ​​of the schlieren image acquired under perturbation. The grayscale value of the flat-top background image without disturbance; This represents the system sensitivity coefficient.

6. The method according to claim 1, characterized in that, The multi-physics three-dimensional inversion and reconstruction of the fluid density field and transient temperature field based on the light deflection angle includes: Obtain the deflection vector field of the entire field of view Orthogonal components , They are respectively directional light deflection angle and Angle of deflection of directional light rays; Based on the theory of optical tomography in fluid dynamics, a two-dimensional scalar potential function is introduced to characterize the optical path difference. By utilizing the spatial gradient characteristic of the optical path difference based on the deflection angle, a fundamental correlation equation is established; the two-dimensional spatial divergence of both sides of the fundamental correlation equation is calculated to construct a two-dimensional Poisson equation characterizing the distribution of the optical path difference: The two-dimensional Poisson equation is solved using any one of the finite difference method, multigrid method, or conjugate gradient algorithm, combined with the Neumann boundary conditions of the undisturbed flow field boundary, based on the two-dimensional scalar potential function. Determine the absolute change in the integral projected refractive index. ; Introducing the Gladstone-Dale relation: in, The Gladstone-Dale constant is given for a specific laser wavelength and gas mixture composition. Let n be the spatial absolute density distribution of the gas medium; n is the absolute refractive index of the disturbed gas; the Abel inverse transform back projection algorithm is used to transform the two-dimensional... Restored to the true refractive index of each point in three-dimensional space The true refractive index Substituting the Gladstone-Dale relation, the transient three-dimensional flow density distribution field is analyzed. ; Assuming that the discharge expansion process locally satisfies the ideal gas law, the three-dimensional flow field density distribution field is used as an example. Inversely derived transient evolution temperature field of neutral gas in local plasma : in, The molar mass of the component gases Let be the ideal gas constant. This refers to gas pressure.

7. A laser quantitative schlieren imaging sensitivity enhancement system, characterized in that, The system includes: The physical model building unit is used to build a physical model for phase modulation iterative design of Gaussian beam to flat-top beam conversion. The physical model has a built-in improved algorithm based on complex amplitude dual constraints to solve the spatial phase distribution of the adapted phase modulation plate. The optimal spatial phase solution extraction unit is used to extract the optimal spatial phase solution of the phase modulation plate that efficiently converts the incident Gaussian beam into a flat-top uniform laser based on the improved algorithm. The diagnostic system building unit is used to build a quantitative schlieren high temporal resolution diagnostic system with a uniform flat-top laser background by processing the phase modulation board based on the optimal spatial phase solution. The light deflection angle extraction unit is used to generate a uniform reference background image and a perturbed schlieren image based on the quantitative schlieren high temporal resolution diagnostic system, and to extract the light deflection angle based on the uniform reference background image and the perturbed schlieren image. The three-dimensional inversion and reconstruction unit is used to perform high-sensitivity multi-physics three-dimensional inversion and reconstruction of the fluid density field and transient temperature field based on the light deflection angle, thereby improving the overall detection sensitivity of laser quantitative schlieren imaging.

8. The system according to claim 7, characterized in that, The physical model building unit is used to build a physical model for the phase modulation iterative design of the Gaussian beam to flat-top beam conversion, including: The preset beam conversion target is clearly defined, which is to convert the fundamental mode Gaussian pulse laser into a flat-top uniform laser with high efficiency. The conversion target is the construction standard of complex amplitude dual constraint. An improved Gerchberg-Saxton algorithm was selected as the improved algorithm based on complex amplitude dual constraints to iteratively optimize the solution of the optimal spatial phase distribution of the phase modulation plate. Construct the incident spatial boundary with dual complex amplitude constraints, define the complex amplitude distribution of the incident Gaussian beam, and set the ideal complex amplitude distribution of the incident beam in the incident plane. The ideal complex amplitude distribution Satisfying the fundamental Gaussian beam model: in, Let be the waist radius of the Gaussian beam; The central peak amplitude; assuming the initial wavefront is an ideal isophase surface, the initial phase... ; This represents the amplitude distribution of the incident Gaussian beam at the incident plane. The imaginary unit; These are the spatial coordinates along the horizontal direction within the incident plane, representing different positions in the horizontal direction within that plane; These are the spatial coordinates along the vertical direction within the incident plane, representing different positions in the vertical direction within that plane; Constructing a target-end spatial boundary with dual complex amplitude constraints, and defining the ideal complex amplitude distribution of the target flat-top beam on the target observation plane. The ideal complex amplitude distribution A high-order super-Gaussian function is used to smooth the edge transition region of the target spot: in, The macroscopic effective radius of the target flat-top beam; It is a super-Gaussian order, and To ensure the steepness of the edges; The spatial coordinates along the horizontal direction within the target observation plane represent different positions in the horizontal direction within that plane. The spatial coordinates along the vertical direction within the target observation plane represent different positions in the vertical direction within that plane. This represents the stable amplitude of the flat-top beam. The superimposed energy conservation physical constraint restricts the complex amplitude under a dual constraint. This energy conservation constraint strictly follows the energy conservation law of optical propagation, and its expression is: 。 9. The system according to claim 8, characterized in that, The optimal spatial phase solution extraction unit is used to extract the optimal spatial phase solution of the phase modulation plate that efficiently converts the incident Gaussian beam into a flat-top uniform laser based on the improved algorithm. Specifically, it includes extracting the optimal spatial phase solution through a nonlinear iterative optimization process, wherein the iterative optimization process sequentially executes the following steps: Initial state and forward propagation: Let For the current iteration number, the additional delay phase distribution matrix introduced in space by the phase modulation board is: ;No. At the start of the next iteration, the amplitude distribution of the incident Gaussian beam at the incident plane is used as a basis. Current iteration phase By combining these elements, the complex amplitude of the modulation plate's projection surface was constructed. The expression is: Using the Fresnel-Kirchhoff diffraction integral formula, the light field is propagated from the modulation plate plane into free space in the forward direction to the target plane, and the complex amplitude distribution of the target plane is calculated. : in, For Fresnel-Kirchhoff diffraction integral calculation; For the first Amplitude distribution of the target plane during the next iteration; For the first The phase wavefront of the target plane at the next iteration; To determine the horizontal spatial coordinates within the target observation plane, The vertical spatial coordinates within the target observation plane; Core complex amplitude physical constraint replacement: Preserve the phase wavefront propagating in the positive direction to reach the target plane Keep it unchanged, and set the target plane amplitude. According to the preset relaxation factor The ideal super-Gaussian target amplitude defined in the phase modulation iterative design physical model Perform linear fusion update to obtain the complex amplitude after linear fusion update. : Backpropagation and phase extraction: updating the complex amplitude after linear fusion Perform reverse propagation, tracing back to the input plane where the phase modulation plate is located, to obtain a new complex amplitude distribution on the input plane. : in, For Fresnel-Kirchhoff inverse diffraction integral calculation, For the first Phase distribution of the input target plane after each iteration of backpropagation; Extracting the phase component from the new complex amplitude distribution As the first The phase input of the next iteration, and the Gaussian incident amplitude. Recombined to impose amplitude constraints on the input plane; Convergence Criteria: Each iteration follows the physical constraint of energy conservation; the root mean square error of light intensity is calculated on the target plane, and the conversion of the beam into the target is used as the convergence criterion; when the root mean square error of the output light intensity on the target plane meets the preset convergence threshold, the iteration terminates, and the current phase input is used as the optimal spatial phase solution. .

10. The system according to claim 7, characterized in that, The diagnostic system construction unit is used to construct a quantitative schlieren high temporal resolution diagnostic system with a uniform flat-top laser background based on the optimal spatial phase solution. This includes the fabrication of a phase modulation plate and the construction of the system optical path, as well as the construction of a uniform flat-top laser reference background. Specifically: Based on the optimal spatial phase solution, a matching phase modulation plate is prepared. The pulsed laser, the beam expanding and collimating lens group, the phase modulation plate, the first schlieren spherical mirror, the flow field to be measured, the second schlieren spherical mirror, the knife edge, the narrowband filter and the camera are arranged in sequence according to the beam propagation order to form an improved Z-type quantitative schlieren optical path. Based on an improved Z-type quantitative schlieren optical path, the wavefront phase of the incident Gaussian laser is reconstructed using a phase modulation plate, shaping the beam into a flat-topped parallel beam with uniform cross-sectional energy. Using the uniform flat-topped laser as the imaging reference background, flat-topped background images and schlieren images containing flow field disturbances are acquired separately to obtain the global constant field background light intensity. The system optical path was built and a uniform flat-top laser reference background was constructed.

11. The system according to claim 10, characterized in that, The light deflection angle extraction unit is used to extract the light deflection angle based on the flat-top parallel beam with uniform cross-sectional energy generated by the quantitative schlieren high temporal resolution diagnostic system, including: Based on the improved Z-type quantitative schlieren optical path, a uniform reference background image is generated; based on the introduced flow field disturbance, a disturbed schlieren image is generated. The uniform reference background image and the perturbation schlieren image are processed as follows: the background light intensity is... The shape is converted into a global constant field to determine the local grayscale changes in the probed image. Deflection angle caused solely by the flow field And it is determined by the effective cutting amount of the mechanical blade; Based on the processed uniform reference background image and the disturbed schlieren image, assuming that the light in the flow field propagates along the positive z-axis, the edge of the mechanical cutting edge is parallel to the x-axis, and the deflection angle of the light in the y-direction is... Expressed by the integral of the refractive index gradient of the medium: in, The absolute reference refractive index of the undisturbed gas in the environment; integration path The optical thickness of the flow field disturbance region; Let be the refractive index gradient of the medium in the flow field along the y-direction; Within the linear response range of the schlieren system, the deflection angle The relative contrast of a schlieren image is proportional to the deflection angle. The formula for calculating the light deflection angle is as follows: in, The grayscale values ​​of the schlieren image acquired under perturbation. The grayscale value of the flat-top background image without disturbance; This represents the system sensitivity coefficient.

12. The system according to claim 7, characterized in that, The three-dimensional inversion and reconstruction unit is used to perform multi-physics three-dimensional inversion and reconstruction of the fluid density field and transient temperature field based on the light deflection angle, including: Obtain the deflection vector field of the entire field of view Orthogonal components , They are respectively directional light deflection angle and Angle of deflection of directional light rays; Based on the theory of optical tomography in fluid dynamics, a two-dimensional scalar potential function is introduced to characterize the optical path difference. By utilizing the spatial gradient characteristic of the optical path difference based on the deflection angle, a fundamental correlation equation is established; the two-dimensional spatial divergence of both sides of the fundamental correlation equation is calculated to construct a two-dimensional Poisson equation characterizing the distribution of the optical path difference: The two-dimensional Poisson equation is solved using any one of the finite difference method, multigrid method, or conjugate gradient algorithm, combined with the Neumann boundary conditions of the undisturbed flow field boundary, based on the two-dimensional scalar potential function. Determine the absolute change in the integral projected refractive index. ; Introducing the Gladstone-Dale relation: in, The Gladstone-Dale constant is given for a specific laser wavelength and gas mixture composition. Let n be the spatial absolute density distribution of the gas medium; n is the absolute refractive index of the disturbed gas; the Abel inverse transform back projection algorithm is used to transform the two-dimensional... Restored to the true refractive index of each point in three-dimensional space The true refractive index Substituting the Gladstone-Dale relation, the transient three-dimensional flow density distribution field is analyzed. ; Assuming that the discharge expansion process locally satisfies the ideal gas law, the three-dimensional flow field density distribution field is used as an example. Inversely derived transient evolution temperature field of neutral gas in local plasma : in, The molar mass of the component gases Let be the ideal gas constant. This refers to gas pressure.

13. A computer-readable storage medium having a computer program stored thereon, characterized in that, When the program is executed by the processor, it implements the steps of the method as described in any one of claims 1-6.

14. An electronic device, characterized in that, include: The computer-readable storage medium as described in claim 13; as well as One or more processors for executing a program in the computer-readable storage medium.

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