Method for calculating laser propagation distance in water cloud system based on plasma-optical field coupling model

Abstract

The application discloses a method for calculating laser propagation distance in a water cloud system based on a plasma-optical field coupling model, comprising the following steps: calculating the number density of liquid droplets of different sizes in different water cloud systems; according to the difference in the sizes of the liquid droplets, calculating the distribution of the electron density of the plasma generated by the laser in a single liquid droplet; according to the change in the electron density distribution, calculating the feedback of the liquid droplet to the optical field and the change in the optical property of the liquid droplet; and through the calculation of the change in the optical property, calculating the nonlinear absorption of the liquid droplet to different laser intensities. Through the consideration of the scattering and absorption of the liquid droplet to the optical field, the propagation distance of the laser in different atmospheric water cloud systems can be obtained.

Description

Technical Field

[0001] The present invention belongs to the field of laser plasma technology, and in particular relates to a method for calculating the propagation distance of laser in a water cloud system based on a plasma-light field coupling model. Background Art

[0002] For nearly two decades, laser propagation in water-cloud systems has garnered widespread attention. On the one hand, water clouds can shield Earth from direct sunlight, contributing to the evolution and development of the ecological environment. On the other hand, their presence can interfere with the stability and accuracy of atmospheric remote sensing, satellite positioning, radar detection, and remote spectroscopy. Recent advances in free-space optical communication and laser-induced water condensation in air have also spurred further research into the interaction between lasers and water clouds.

[0003] Traditional research usually focuses on two aspects: (1) the propagation of laser optical filaments in the atmosphere and water clouds. (2) the propagation of lasers with lower energy thresholds in water clouds. The latter can be divided into two categories: the influence of water clouds on the propagation of lasers in the visible band to the near-infrared band and on the infrared band. However, although the nature of the effects of visible band or near-infrared band lasers on the propagation of lasers in water clouds is similar to that of the infrared band, which is a microscopic manifestation of the interaction between lasers and droplets, theoretical research is still relatively lacking. Experimentally, since the laser energy cannot be accurately controlled below the breakdown threshold of the medium, filaments are generated, which also limits the measurement of the propagation distance of lasers at low energy thresholds in water clouds. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for calculating the propagation distance of laser in a water-cloud system based on a plasma-light field coupling model, so as to obtain the propagation distance of laser in different water-cloud systems.

[0005] The technical solution for achieving the purpose of the present invention is as follows: In a first aspect, the present invention provides a method for calculating the propagation distance of a laser in a water-cloud system based on a plasma-light field coupling model, comprising the following steps:

[0006] Calculate the droplet number density of different water cloud systems at different sizes;

[0007] Calculate the electron density distribution of the plasma generated in the droplet;

[0008] Calculate the droplet's feedback to the light field and the changes in its optical properties;

[0009] Calculate the nonlinear absorption rate of the droplet under different laser intensities;

[0010] Calculate the propagation distance of the laser in different atmospheric water cloud systems.

[0011] In a second aspect, the present application provides a computer device, comprising a memory, a processor and a computer program stored in the memory and executable on the processor, wherein the processor implements the steps of the method of the first aspect when executing the program.

[0012] In a third aspect, the present application provides a computer readable storage medium, which stores a computer program, wherein the program is executable on a processor to implement the steps of the method of the first aspect.

[0013] In a fourth aspect, the present application provides a computer program product, comprising a computer program, wherein the computer program is executable on a processor to implement the steps of the method of the first aspect.

[0014] Compared with the prior art, the present application has the following beneficial effects: (1) the present application reasonably considers the relationship between the droplet distribution and the droplet size in different water cloud systems, reasonably separates each different water cloud system, and calculates the laser propagation distance in at least three water cloud systems; (2) the present application considers the growth of electron density in the process of laser propagation and the feedback effect of droplet clusters on the laser light field, and reasonably considers the absorption effect of the water cloud system on the laser; (3) the present application obtains the nonlinear absorption of each water cloud system on the laser by calculating the droplet absorption coefficient under different parameter conditions, and calculates the attenuation of the plasma on the laser; (4) the present application calculates the laser propagation distance in at least three water cloud systems, and since it is applicable to the water cloud system, the attenuation of four or even more water cloud systems on the laser and the calculation of the laser propagation distance can be derived. BRIEF DESCRIPTION OF DRAWINGS

[0015] In order to more clearly illustrate the technical solutions of the present application, the following will briefly introduce the drawings needed in the embodiments. Of course, for those skilled in the art, other drawings can also be obtained without creative labor on the basis of these drawings.

[0016] Figure 1 The method block diagram for calculating the laser propagation distance in the water cloud system based on the plasma-light field coupling model in the embodiment of the present application.

[0017] Figure 2 The free electron density distribution in the droplet after the interaction of the laser and the droplet in the embodiment of the present application.

[0018] Figure 3 The feedback effect of the droplet and the plasma on the light field in the embodiment of the present application.

[0019] Figure 4 Changes in the absorption coefficient of the plasma to laser light according to an embodiment of the present invention.

[0020] Figure 5 The propagation distance of the laser in different water-cloud systems according to the embodiment of the present invention. DETAILED DESCRIPTION

[0021] like Figure 1 As shown, the present invention proposes a method for calculating the propagation distance of laser in a water-cloud system based on a plasma-light field coupling model, the method comprising the following steps:

[0022] Q1: Calculate the droplet number density of different water cloud systems with different sizes;

[0023] Q2: Calculate the electron density distribution of the plasma generated in the droplet;

[0024] Q3: Calculate the droplet’s feedback to the light field and the changes in its optical properties;

[0025] Q4: Calculate the nonlinear absorption rate of the droplet at different laser intensities;

[0026] Q5: Calculate the propagation distance of laser in different atmospheric water cloud systems.

[0027] The following is a detailed explanation of each step:

[0028] The first step is to calculate the number density of droplets of different sizes in different water cloud systems, relying on the equation:

[0029]

[0030] Where N represents the total number density, a represents the normalization constant in different water cloud systems, r0 represents the radius used for droplet modeling, r0≤10μm, and r0 is considered as a constant value that changes with the water cloud system during the calculation process, with values ​​of 5μm, 6μm, 8μm, and 10μm. α and γ describe the slope of the droplet size distribution, and in this embodiment, they are fixed values ​​of 4 and 2.34, respectively. Generally, the distribution of droplets of different sizes is also different. Regardless of the water cloud system, its absorption of laser light is relatively small at the beginning, until plasma is generated and the electron density approaches or reaches the threshold value (~10 21 cm -3 ) above, the droplet's absorption of subsequent laser light begins to increase, while its transmission decreases, ultimately completely shielding the laser. Furthermore, to obtain relatively accurate results, subsequent calculations typically select the droplet size with the highest distribution density in each system.

[0031] In the second step, the main calculation formula for calculating the electron density distribution of the plasma generated in the droplet is:

[0032]

[0033] Among them, the first two terms on the right represent the proliferation of the number of electrons caused by multiphoton ionization and avalanche ionization, and the last two terms represent the loss of electrons due to recombination and diffusion effects during the proliferation process. K is the K-order multiphoton absorption cross section, ,ω,I,represent the reduced Planck constant, laser frequency, and laser intensity, respectively; D,ρ t , g, V P , is the electric displacement vector, the number density of neutral molecules, the recombination coefficient, and the effective ionization potential of water under undisturbed conditions. In addition, σ A is the ionization cross section of avalanche ionization, expressed as:

[0034]

[0035] In the above formula, τ c is the electron collision time parameter, which defaults to 1 fs. c, m, ε0, and n0 represent the speed of light in vacuum, the mass of the electron, the dielectric constant of vacuum, and the refractive index of water in undisturbed conditions. τ and e are the pulse width and the charge carried by a single electron, respectively.

[0036] The third step is to calculate the droplet's feedback to the light field and the changes in its optical properties;

[0037] The basic method is:

[0038]

[0039] In the above formula, ε rL , μ0, J represents the linear vacuum dielectric constant, ε0, vacuum magnetic permeability, free current density; E is the electric field amplitude, P NL is the nonlinear polarization, and the expression is:

[0040] P NL =2n0n2ε0IE+icn0E(β K I K-1 +σ c ρ)ω -1

[0041] Where n2 is the nonlinear optical Kerr coefficient; ρ is the free electron number density during the evolution process.

[0042] Changes in the optical properties of droplets:

[0043]

[0044] Among them, σc is a plural expression:

[0045]

[0046] By combining the above equations and comsol numerical simulation, we can obtain the electron density distribution generated during the interaction between the laser and the droplet, as well as the feedback effect of the plasma on the light field. For a given laser intensity I, the change in the absorption coefficient of the droplet under different parameters can be obtained.

[0047] The fourth step is to calculate the nonlinear absorption rate of the droplet under different laser intensities;

[0048] Changes in plasma absorption coefficient under different parameters:

[0049]

[0050] N in the above formula r Represents the effect of plasma on the refractive index of the medium, N i is the plasma extinction coefficient. In the near-infrared band, the reaction of water to laser is mainly scattering, and the absorption coefficient is almost ignored. However, under high-power laser intensity, the rapid increase in electron density further reduces the scattering coefficient, and the extinction coefficient (absorption + scattering) becomes dominant.

[0051] The absorption coefficient is calculated as:

[0052]

[0053] The absorption coefficient of plasma for laser energy is obtained:

[0054]

[0055] By combining the above equations, we can obtain the change in the absorption coefficient of the droplet for different laser intensities under the condition of a given wavelength, as well as the relationship between the change in energy absorption of the droplet and time when a given laser intensity is incident.

[0056] The fifth step is to calculate the propagation distance of the laser in different atmospheric water cloud systems;

[0057] The relevant calculation formula is:

[0058]

[0059] In the above formula, l is the beam length, r is the initial radius of the laser, R and N are the droplet radius and the droplet number density when the radius is R, α is the absorption coefficient corresponding to different laser intensities, and τ p is the laser pulse width.

[0060] By combining the equations in the above steps (1, 2, 3, 4, 5), the propagation distance of laser in different water cloud systems can be obtained, and the decay relationship of the propagation distance with time t under the condition of a given pulse width τ p is obtained.

[0061] In order to make the person in the technical field better understand the technical solutions in the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below in combination with the drawings in the embodiments of the present application.

[0062] Referring to Table 1, the droplet distribution of three common water cloud systems is given in the embodiment, which are marine stratocumulus, continental cumulus, marine cumulus and common water mist. In this embodiment, we do not give the traditional range value, because it is not conducive to subsequent calculation, so we give the average radius R and the number density N in each water cloud system.

[0063] Table 1 Number density of different droplet sizes in different water cloud systems

[0064]

[0065] Referring to Figure 2 , the distribution of electron density in the process of interaction between laser with a wavelength of 1064 nm and an intensity of 5.1 TW / cm 2 and droplet is given. Due to the lens-like effect of the droplet, the laser is usually focused on the right side of the droplet, and the intensity is even as high as 100 times the intensity of the laser source. At this time, the light field distribution is as shown in Figure 3 , the light field inside the droplet is relatively uniform except the focusing area, because the diffraction and dispersion do not change significantly. In the focusing area, due to the change of its optical properties, the nonlinear absorption of the droplet and the focusing area to the laser gradually increases, and the energy and field amplitude of the laser propagating through the droplet begin to decay. The change of the absorption coefficient of the plasma generated in the whole droplet focusing area to the laser is shown in Figure 4 . In the initial stage of laser action, the electron density increases, at this time the absorption begins to increase, until the electron density reaches the critical threshold, the plasma in the focusing area begins to shield the laser, the absorption coefficient begins to stabilize, and finally decays.

[0066] Referring to Figure 5 , the present application finally combines the above five steps, ignores the dispersion of the droplet in the process of laser propagation, considers the influence of nonlinear absorption on the laser, combines the equation in Q5, and obtains the propagation distance of laser in different water cloud systems. Among them, Figure 5 It is also shown in Q5 that for different pulse widths, the laser propagation distance is also different, and generally for long pulse width laser, the peak power is lower, and the propagation distance in the water cloud system is longer.

[0067] As far as the water mist results are concerned, they are close to the experimental values ​​recently published in "Nature Photonics", with an error of less than 5%.

[0068] The embodiments described above are only specific implementation methods of the present invention, which are used to illustrate the technical solutions of the present invention, rather than to limit them. The scope of protection of the present invention is not limited thereto. Although the present invention has been described in detail with reference to the aforementioned embodiments, it should be understood by those skilled in the art that any person skilled in the art can still modify or easily conceive of changes to the technical solutions described in the aforementioned embodiments within the technical scope disclosed by the present invention, or perform equivalent replacements on some of the technical features thereof. However, these modifications, changes or replacements do not deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should all be included in the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be subject to the scope of protection of the claims.

Claims

1. A method for calculating the propagation distance of laser in a water-cloud system based on a plasma-light field coupling model, characterized in that: The method comprises the following steps: Calculate the droplet number density of different water cloud systems with different sizes: Where N represents the total number density, a represents the normalization constant in different water cloud systems, r0 represents the radius used for droplet modeling, and α and γ describe the slope of the droplet size distribution; Calculate the electron density distribution of the plasma generated in the droplet. The specific calculation formula is: Among them, the first two terms on the right represent the proliferation of the number of electrons caused by multiphoton ionization and avalanche ionization, and the last two terms represent the loss of electrons due to recombination and diffusion effects during the proliferation process; β K is the K-order multiphoton absorption cross section, ω, I represent the reduced Planck constant, laser frequency, and laser intensity, respectively. D, ρ t , g, V P are the electric displacement vector, the number density of neutral molecules, the recombination coefficient, and the effective ionization potential of water under undisturbed conditions. In addition, σ A is the ionization cross section of avalanche ionization, expressed as: In the above formula, τ c is the electron collision time parameter, c, m, ε0, and n0 represent the speed of light in vacuum, the mass of electron, the dielectric constant of vacuum, and the refractive index of water in undisturbed conditions, respectively; τ and e are the pulse width and the charge carried by a single electron, respectively; Calculate the droplet's feedback to the light field and the change in its optical properties. The specific method is: In the above formula, ε rL , μ0, J represent the linear vacuum dielectric constant, vacuum magnetic permeability, and free current density; E is the electric field amplitude; P NL is the nonlinear polarization, and the expression is: P NL =2n0n2ε0IE+icn0E(β K I K-1 +s c p)w -1 Where n2 is the nonlinear optical Kerr coefficient, and ρ is the free electron number density during the evolution process; Changes in the optical properties of droplets: Among them, σ c is a plural expression: By combining the above equations and numerical simulations using COMSOL, we can obtain the electron density distribution generated during the interaction between the laser and the droplet, as well as the feedback effect of the plasma on the light field. For a given laser intensity I, we can obtain the variation in the droplet's absorption coefficient under different parameters. Calculate the nonlinear absorption coefficient of the droplet under different laser intensities. The calculation process is: Changes in plasma absorption coefficient under different parameters: In the above formula, N r Represents the effect of plasma on the refractive index of the medium, N i is the extinction coefficient of the plasma; The absorption coefficient is calculated as: The absorption coefficient of plasma for laser energy is obtained: By combining the above equations, we can obtain the change of the droplet's absorption coefficient for different laser intensities under the condition of a given wavelength, as well as the relationship between the droplet's energy absorption and time when a given laser intensity is incident. Calculate the propagation distance of laser in different atmospheric water cloud systems using the following formula: In the above formula, l is the beam length, r is the initial radius of the laser, R and N are the droplet radius and the droplet number density when the radius is R, respectively, α is the absorption coefficient corresponding to different laser intensities, τ p is the laser pulse width.

2. A computer device comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein: When the processor executes the program, the steps of the method according to claim 1 are implemented.

3. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the program is executed by a processor, the steps of the method according to claim 1 are implemented.

4. A computer program product comprising a computer program, characterized in that When the computer program is executed by a processor, the steps of the method according to claim 1 are implemented.

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