A method for designing optical paths of near-field equivalent far-field laser spots

Abstract

A method for designing an optical path for a near-field equivalent to a far-field laser spot relates to the field of laser optical path design and specifically comprises the following steps: S1: determining the distance of an equivalent far-field laser as needed; S2: determining various parameters of a laser source; S3: determining the maximum experimental working distance in the near field, selecting two short-focus lenses and two long-focus lenses of the same focal length, the size of the lenses of which should allow the laser beam to pass completely; S4: placing attenuation plates with different attenuation magnifications, and achieving any desired attenuation magnification by combining attenuation plates with different attenuation magnifications; S5: designing an equivalent far-field optical path, constructing an optical path, and conducting a near-field equivalent far-field laser spot experiment; the present invention can flexibly adjust the distance between lenses to simulate laser spots of different far-field distances, and control the laser energy before reaching a photoelectric imaging system by adding attenuation plates in the optical path. The method is low-cost and easy to implement, and provides a new approach to achieving consistency of near-field and far-field laser spots.

Description

Technical Field

[0001] The present invention relates to the field of laser light path design, and in particular to a light path design method for a near-field equivalent far-field laser spot. Background Art

[0002] It is well known that when far-field lasers irradiate optoelectronic imaging equipment, in addition to the main spot focused by the optical system, the image plane often contains complex energy distributions such as circular fringes, speckles, ghost images, and regular dot patterns. This is because diffraction, scattering, and interference effects simultaneously occur when lasers irradiate optoelectronic imaging equipment. However, the near-field laser beams produced by laboratory lasers differ from far-field lasers in both spot size and beam divergence. This inevitably leads to differences in near-field experimental phenomena and far-field laser irradiation phenomena. Furthermore, experiments with far-field lasers irradiating optoelectronic imaging equipment are expensive and limited by space and personnel, making it difficult to obtain a large number of experimental data samples at different far-field distances.

[0003] Previous studies simulating the interference loss effects of far-field laser beams on optoelectronic devices under near-field conditions have simply passed the near-field laser through a beam expansion system before irradiating the optoelectronic imaging device. A typical beam expansion system consists of two mirrors or two single lenses. This system simultaneously expands the beam and compresses the laser beam's divergence angle, making the laser beam nearly parallel. However, in experiments, there is still a significant discrepancy between the laser beam spot simulated by beam expansion alone and the actual far-field laser spot. First, the typical beam expansion system cannot flexibly equate the laser spot size at different far-field distances. Second, even when the same laser spot size is achieved, the laser divergence angle varies due to different laser wavefront curvature radii, resulting in different interference effects. The ability to simulate the phenomenon of far-field laser irradiation on photoelectric detectors as realistically as possible demonstrates that the equivalence of near-field and far-field laser beams is a prerequisite for near-field experiments to accurately evaluate the interference effects of far-field laser beams on optoelectronic imaging equipment. Summary of the Invention

[0004] In order to overcome the deficiencies in the background technology, the present invention discloses a method for designing an optical path of a near-field equivalent far-field laser spot.

[0005] In order to achieve the above-mentioned purpose, the present invention adopts the following technical solutions:

[0006] A method for designing an optical path of a near-field equivalent far-field laser spot comprises the following steps:

[0007] S1: Determine the distance of the equivalent far-field laser as needed;

[0008] S2: Determine various parameters of the laser source, including the laser source beam waist, the distance from the laser source beam waist to the light outlet, and the laser wavelength;

[0009] S3: Determine the maximum experimental working distance in the near field, select two short-focus lenses with the same focal length and two long-focus lenses with the same focal length. The lens size should allow the laser beam to pass completely through.

[0010] S4: Place attenuation sheets with different attenuation ratios. By combining attenuation sheets with different attenuation ratios, any desired attenuation ratio can be achieved to adjust the energy entering the detector and simulate laser interference with photoelectric imaging equipment of different intensities and different atmospheric attenuation levels;

[0011] S5: Design an equivalent far-field optical path, build an optical path, and conduct a near-field equivalent far-field laser spot experiment; the steps of the equivalent far-field optical path design are as follows:

[0012] The propagation law of the Gaussian beam is calculated based on its q parameter.

[0013] 1 / q(z)=1 / R(z)-i*lambda / (pi*w(z)^2) (1)

[0014] Where z is the propagation distance, R is the curvature radius of the laser beam wavefront, w is the effective cross-sectional radius of the beam, i.e., the beam size, lambda is the laser wavelength, and i is the imaginary unit;

[0015] For a beam traveling a distance L in the far field, its q parameter becomes:

[0016] q=q0+L,q0=i*pi*w0^2 / lambda=iQ (2)

[0017] w0 is the beam waist of the laser, and Q is the imaginary part of q0. Under near-field conditions, the q parameter of the laser beam before reaching the optoelectronic imaging device after a series of beam transformations is:

[0018] q1=(A*q0+B) / (C*q0+D) (3)

[0019] A, B, C, and D are parameters to be determined. To achieve the same wavefront curvature radius and effective beam cross-section as in the far field, the q parameter in the near and far fields must be equal to q = q1:

[0020] (A*q0+B) / (C*q0+D)=q0+L (4)

[0021] Since q0 is a complex number, we can separate the real and imaginary parts to obtain two equations:

[0022] LD=B+CQ^2 (5)

[0023] A=D+LC (6)

[0024] If (5) is satisfied, the same wavefront curvature radius can be obtained in the near and far fields. If (6) is satisfied, the laser spot sizes in the near and far fields are also the same. Satisfying the above two constraints can ensure that the laser spots in the near and far fields before entering the detector are basically the same, which better meets the equivalence requirements.

[0025] In order to be universal for lasers with different beam waist sizes, C should be set to 0 in equations (5) and (6), thereby increasing the number of required equations to 3:

[0026] LD=B,A=D,C=0 (7)

[0027] When the number of lenses is 4, a symmetrical lens group is found with a better distance between lenses and lens focal length; under the symmetrical structure, the following order is given:

[0028] l1=l5,l2=l4 (8)

[0029] F1=F4,F2=F3 (9)

[0030] Wherein, l1 is the distance from the laser beam waist to the front surface of the first lens, l2 is the distance between the first lens and the second lens, l4 is the distance between the third lens and the fourth lens, l5 is the distance from the fourth lens to the optoelectronic imaging device, and F1, F2, F3, and F4 are the focal lengths of the lenses, respectively;

[0031] Substitute all equations (7), (8), (9) into the paraxial light transformation matrix to obtain the value of C. The calculation process of the paraxial light transformation matrix M is as follows:

[0032] M=M1*M2*M3*M4*M5*M4*M3*M2*M1 (10)

[0033] M1 to M5 and M are all 2*2 matrices. The multiplications in formula (10) are all matrix multiplications, where M1 = [1l1; 0 1], M2 = [1 0; -1 / F1 1], M3 = [1l2; 0 1], M4 = [1 0; -1 / F21], M5 = [1l3; 0 1], and M = [AB; CD]. Calculating M and setting the C term to zero yields the following equation:

[0034] (F1+F2-l2)(F1*l3+2*F2*l2+F2*l3-l2*l3-2*F1*F2) / (F1^2*F2^2)=0 (11)

[0035] In fact, l3 is the distance between the second lens and the third lens. According to the requirement of (11), l2=F1+F2 is required. Substitute this equation into the original paraxial light transformation matrix and simplify it to obtain:

[0036] A = D = 1 (12)

[0037] B = -(2F1^2*F2 - l3*F1^2 + 2F1*F2^2 - 2l1*F2^2) / (F2^2) (13)

[0038] Further, according to (7), it should be made that B = LD = L, thus obtaining equation (14)

[0039] l3 = (F2^2*L + 2*F1^2*F2 + 2*F1*F2^2 - 2*l1*F2^2) / (F1^2) (14)

[0040] Since l3 << L, so F2 << F1. It can be seen from the above formula that as the required equivalent far - field distance increases, l3 increases linearly. By controlling the change of l3, the laser spot at different far - field distances can be flexibly simulated; In order to control the total system length l, it is calculated that:

[0041] l = l1 + l2 + l3 + l4 + l5 = 2*F1 + (F2 / F1)^2*L + 2*l1 + 4F2 + 2F2^2 / F1 - 2(F2 / F1)^2*l1 ≈ 2*F1 + (F2 / F1)^2*L + 2*l1(15)

[0042] It can be seen from equation (15) that the distance l1 from the laser beam waist to the front surface of the first lens can be made as small as possible. Just place the lens in front of the laser light output port. When F1 ≈ (F2^2*L)^(1 / 3), the total system length is the shortest.

[0043] In the optical path design method of the near - field equivalent far - field laser spot, the distance from the laser source beam waist to the light output port in step S2 should be less than the distance from the laser beam waist to the first lens.

[0044] In the optical path design method of the near - field equivalent far - field laser spot, in step S3, according to the actual situation of the laboratory, determine the maximum near - field experimental working distance. According to the ratio of the maximum experimental working distance to the distance of the far - field laser, determine the ratio of the lens focal lengths, and it should be ensured that the focal length of the short - focus lens is much smaller than the focal length of the long - focus lens.

[0045] In the optical path design method of the near - field equivalent far - field laser spot, in step S4, the attenuation sheet is flexibly placed at a suitable position between the laser light output port and the third lens, as long as it does not affect the optical path design.

[0046] Due to the adoption of the above technical solutions, the present invention has the following beneficial effects:

[0047] The optical path design method for near-field equivalent far-field laser spot described in the present invention calculates the laser paraxial propagation process based on the q parameter of a Gaussian beam, uses a set of four lenses with a symmetrical structure to achieve an effect in which the near-field laser wavefront curvature radius and the effective beam cross-sectional radius are completely consistent with those of the far-field laser, can flexibly adjust the distance between lenses to simulate laser spots at different far-field distances, and controls the laser energy before reaching the optoelectronic imaging system by adding an attenuation plate in the optical path. The present invention is low-cost and easy to implement, providing a new approach to achieving consistency in near-field and far-field laser spots. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 Schematic diagram of the optical path of the equivalent far-field laser spot in an embodiment of the present invention.

[0049] Figure 2 This is the design parameter table of the equivalent far-field optical path system (unit: m).

[0050] Figure 3 This is the laser spot obtained by zemax simulation of the near-field equivalent far-field optical path system.

[0051] Figure 4 This is a three-dimensional schematic diagram of the near-field equivalent far-field optical path simulated by Zemax. DETAILED DESCRIPTION

[0052] The present invention is explained in detail by the following examples, the purpose of which is to protect all technical improvements within the scope of the present invention.

[0053] The optical path design method of the near-field equivalent far-field laser spot specifically includes the following steps:

[0054] S1: Determine the distance of the equivalent far-field laser as needed;

[0055] S2: Determine various parameters of the laser source, including the laser source beam waist, the distance from the laser source beam waist to the light outlet, and the laser wavelength;

[0056] S3: Determine the maximum experimental working distance in the near field, select two short-focus lenses with the same focal length and two long-focus lenses with the same focal length. The size of the lenses should allow the laser beam to pass completely. According to the actual situation of the laboratory, determine the maximum experimental working distance in the near field. According to the ratio of the maximum experimental working distance and the distance of the far-field laser, determine the ratio of the lens focal lengths. It should be ensured that the focal length of the short-focus lens is much smaller than that of the long-focus lens. The distance from the laser source beam waist to the light outlet should be smaller than the distance from the laser beam waist to the first lens.

[0057] S4: Place attenuation sheets with different attenuation ratios. By combining attenuation sheets with different attenuation ratios, any desired attenuation ratio can be achieved to adjust the energy entering the detector and simulate laser interference with optoelectronic imaging equipment of different intensities and different atmospheric attenuation levels. The attenuation sheet should be flexibly placed at a suitable position between the laser outlet and the third lens without affecting the optical path design.

[0058] S5: Design an equivalent far-field optical path, build an optical path, and conduct a near-field equivalent far-field laser spot experiment; the steps of the equivalent far-field optical path design are as follows:

[0059] The propagation law of the Gaussian beam is calculated based on its q parameter.

[0060] 1 / q(z)=1 / R(z)-i*lambda / (pi*w(z)^2) (1)

[0061] Where z is the propagation distance, R is the curvature radius of the laser beam wavefront, w is the effective cross-sectional radius of the beam, i.e., the beam size, lambda is the laser wavelength, and i is the imaginary unit;

[0062] For a beam traveling a distance L in the far field, its q parameter becomes:

[0063] q=q0+L,q0=i*pi*w0^2 / lambda=iQ (2)

[0064] w0 is the beam waist of the laser, and Q is the imaginary part of q0. Under near-field conditions, the q parameter of the laser beam before reaching the optoelectronic imaging device after a series of beam transformations is:

[0065] q1=(A*q0+B) / (C*q0+D) (3)

[0066] A, B, C, and D are parameters to be determined. To achieve the same wavefront curvature radius and effective beam cross-section as in the far field, the q parameter in the near and far fields must be equal to q = q1:

[0067] (A*q0+B) / (C*q0+D)=q0+L (4)

[0068] Since q0 is a complex number, we can separate the real and imaginary parts to obtain two equations:

[0069] LD=B+CQ^2 (5)

[0070] A=D+LC (6)

[0071] If (5) is satisfied, the same wavefront curvature radius can be obtained in the near and far fields. If (6) is satisfied, the laser spot sizes in the near and far fields are also the same. Satisfying the above two constraints can ensure that the laser spots in the near and far fields before entering the detector are basically the same, which better meets the equivalence requirements.

[0072] In order to be universal for lasers with different beam waist sizes, C should be set to 0 in equations (5) and (6), thereby increasing the number of required equations to 3:

[0073] LD=B,A=D,C=0 (7)

[0074] When the number of lenses is 4, a symmetrical lens group is found with a better distance between lenses and lens focal length; under the symmetrical structure, the following order is given:

[0075] l1=l5,l2=l4 (8)

[0076] F1=F4,F2=F3 (9)

[0077] Wherein, l1 is the distance from the laser beam waist to the front surface of the first lens, l2 is the distance between the first lens and the second lens, l4 is the distance between the third lens and the fourth lens, l5 is the distance from the fourth lens to the optoelectronic imaging device, and F1, F2, F3, and F4 are the focal lengths of the lenses, respectively;

[0078] Substitute all equations (7), (8), (9) into the paraxial light transformation matrix to obtain the value of C. The calculation process of the paraxial light transformation matrix M is as follows:

[0079] M=M1*M2*M3*M4*M5*M4*M3*M2*M1 (10)

[0080] M1 to M5 and M are all 2*2 matrices. The multiplications in formula (10) are all matrix multiplications, where M1 = [1l1; 0 1], M2 = [1 0; -1 / F1 1], M3 = [1l2; 0 1], M4 = [1 0; -1 / F21], M5 = [1l3; 0 1], and M = [AB; CD]. Calculating M and setting the C term to zero yields the following equation:

[0081] (F1+F2-l2)(F1*l3+2*F2*l2+F2*l3-l2*l3-2*F1*F2) / (F1^2*F2^2)=0 (11)

[0082] In fact, l3 is the distance between the second lens and the third lens. According to the requirement of (11), l2=F1+F2 is required. Substitute this equation into the original paraxial light transformation matrix and simplify it to obtain:

[0083] A=D=1 (12)

[0084] B=-(2F1^2*F2-l3*F1^2+2F1*F2^2-2l1*F2^2) / (F2^2) (13)

[0085] Furthermore, according to (7), it should be made that B = LD = L, thus obtaining equation (14).

[0086] l3 = (F2^2 * L + 2 * F1^2 * F2 + 2 * F1 * F2^2 - 2 * l1 * F2^2 / (F1^2)) (14)

[0087] Since l3 << L, then F2 << F1. As can be seen from the above formula, as the required equivalent far - field distance increases, l3 increases linearly. By controlling the change of l3, the laser spot at different far - field distances can be flexibly simulated. To control the total system length l, it is calculated that:

[0088] l = l1 + l2 + l3 + l4 + l5 = 2 * F1 + (F2 / F1)^2 * L + 2 * l1 + 4F2 + 2F2^2 / F1 - 2(F2 / F1)^(2) * l1 ≈ 2 * F1 + (F2 / F1)^2 * L + 2 * l1 (15)

[0089] As can be seen from equation (15), the distance l1 from the laser beam waist to the front surface of the first lens can be made as small as possible. Just place the lens in front of the laser light output port. When F1 ≈ (F2^2 * L)^(1 / 3), the total system length is the shortest.

[0090] Embodiment

[0091] An optical path design method for a near - field equivalent far - field laser spot, the method comprising the following steps:

[0092] S1: Determine the distance of the equivalent far - field laser according to requirements; as Figure 2 shown, the values of L1 are 200m, 1km, 3km, 5km, 10km;

[0093] S2: Set the laser source beam waist to be 10mm, the distance from the laser source beam waist to the light output port to be 50mm, and the laser wavelength to be 1.064um;

[0094] S3: Set the maximum near - field experimental working distance to be 10m, the focal lengths of the long - focal - length lenses to be 0.6m, 1m, 1.5m, 2m, and the focal length of the short - focal - length lens to be 35mm; according to Figure 1 shown, the size requirements of lenses F1, F2, and F3 allow the laser to pass through completely. Set the diameter of the first lens F1 to be 80mm, the diameters of the second lens F2 and the third lens F3 to be 25mm, and the diameter of the fourth lens F4 to be greater than the entrance pupil diameter of the optical system of the optoelectronic imaging device. In this embodiment, the diameter of the lens F4 is 120mm;

[0095] S4: As Figure 1As shown, the beam diameter between lens F2 and lens F3 is small, so the attenuation plate can be placed between lens F2 and lens F3. Figure 1 The attenuation ratio and number of the medium attenuation sheets can be flexibly combined according to actual conditions. In this embodiment, two 5dB attenuation sheets are used, which means that the laser light intensity is attenuated to 0.1 times the original intensity.

[0096] S5: Design the equivalent far-field optical path, build the optical path, and conduct the near-field equivalent far-field laser spot experiment;

[0097] Furthermore, the optical path design in step S5 is as follows:

[0098] In order to more conveniently describe the behavior of laser beams passing through free space and optical systems, the q parameter of Gaussian beams is used to calculate their propagation law:

[0099] 1 / q(z)=1 / R(z)-i*lambda / (pi*w(z)^2) (1)

[0100] Where z is the propagation distance, R is the curvature radius of the laser beam wavefront, w is the effective cross-sectional radius of the beam, i.e., the spot size, lambda is the laser wavelength, and i is the imaginary unit.

[0101] For a beam traveling a distance L in the far field, its q parameter becomes:

[0102] q=q0+L,q0=i*pi*w0^2 / lambda=iQ (2)

[0103] w0 is the beam waist of the laser, Q is the imaginary part of q0. Under near-field conditions, the q parameter of the laser beam before reaching the optoelectronic imaging device after a series of beam transformations is:

[0104] q_prime=(A*q0+B) / (C*q0+D) (3)

[0105] A, B, C, and D are parameters to be determined. To achieve the same wavefront curvature radius and effective beam cross-section as in the far field, the q parameters in the near and far fields need to be equal:

[0106] (A*q0+B) / (C*q0+D)=q0+L (4)

[0107] Since q0 is a complex number, we can separate the real and imaginary parts to obtain two equations:

[0108] LD=B+CQ^2 (5)

[0109] A=D+LC (6)

[0110] If (5) is satisfied, the same wavefront curvature radius can be obtained in the near and far fields. If (6) is satisfied, the laser spot sizes in the near and far fields are also the same. Meeting the above two constraints can ensure that the laser spots in the near and far fields before entering the detector are basically the same, which better meets the equivalence requirements.

[0111] Furthermore, in order to be universal for lasers with different beam waist sizes, C=0 in equations (5) and (6), thus increasing the number of required equations to three:

[0112] LD=B,A=D,C=0 (7)

[0113] When the number of lenses is 4, a symmetrical lens group is found with a better distance between lenses and lens focal length. The symmetrical structure is ordered as follows:

[0114] l1=l5,l2=l4 (8)

[0115] F1=F4,F2=F3 (9)

[0116] Wherein, l1 is the distance from the laser beam waist to the front surface of lens F1, l2 is the distance between lens F1 and lens F2, l4 is the distance between lens F3 and lens F4, l5 is the distance from lens F4 to the optoelectronic imaging device, and F1, F2, F3, and F4 are the focal lengths of the lenses, respectively;

[0117] Furthermore, all equations (7), (8), and (9) are substituted into the paraxial light transformation matrix to obtain the value of C. The calculation process of the paraxial light transformation matrix M is as follows:

[0118] M=M1*M2*M3*M4*M5*M4*M3*M2*M1 (10)

[0119] M1 to M5 and M are all 2*2 matrices. The multiplications in formula (10) are all matrix multiplications, where M1 = [1l1; 0 1], M2 = [1 0; -1 / F1 1], M3 = [1l2; 0 1], M4 = [1 0; -1 / F21], M5 = [1l3; 0 1], and M = [AB; CD]. Calculating M and setting the C term to zero yields the following equation:

[0120] (F1+F2-l2)(F1*l3+2*F2*l2+F2*l3-l2*l3-2*F1*F2) / (F1^2*F2^2)=0 (11)

[0121] In fact, l3 is the distance between lens F2 and lens F3. According to the requirements of formula (11), l2 = F1 + F2. Substitute this equation into the original paraxial ray transformation matrix and simplify it to obtain:

[0122] A = D = 1 (12)

[0123] B = -(2F1^2*F2 - l3*F1^2 + 2F1*F2^2 - 2l1*F2^2) / (F2^2) (13)

[0124] Further, according to (7), it should be made that B = LD = L, thus obtaining equation (14)

[0125] l3 = (F2^2*L + 2*F1^2*F2 + 2*F1*F2^2 - 2*l1*F2^2) / (F1^2) (14)

[0126] Because l3 << L, so F2 << F1; as can be seen from the above formula, as the required equivalent far - field distance increases, l3 increases linearly. By controlling the change of l3, the laser spot at different far - field distances can be flexibly simulated. In order to control the total system length l, it is calculated that:

[0127] l = l1 + l2 + l3 + l4 + l5 = 2*F1+(F2 / F1)^2*L + 2*l1 + 4F2 + 2F2^2 / F1 - 2(F2 / F1)^2*l1 ≈ 2*F,+(F2 / F1)^2*L + 2*l1(15)

[0128] As can be seen from equation (15), the distance l1 from the laser beam waist to the front surface of the first lens can be made as small as possible,

[0129] as long as the lens is placed in front of the laser light output port;

[0130] In this embodiment, l1 = 100mm is set. According to l5 = l1, the value of l5 can be determined. Since the short - focus lens F2 is selected as �5mm, according to the actual equivalent far - field distance and referring to when F1 ≈ (F2^2*L)^(1 / 3), the total system length is the shortest; the lens can be selected according to cost. In this embodiment, a single lens in a common lens library is selected to determine the value of F1. According to l2 = F1 + F2, the value of l2 can be determined, and similarly, according to l4 = l2, the value of l4 can be determined. Thus, all system parameters are determined. The results of all system parameters calculated in this embodiment are as Figure 2 shown.

[0131] From Figure 2 it can be seen that for an ideal Gaussian - mode laser, all lenses are selected as ideal lenses, that is, thin lenses. The wave - front curvature radius R(l) and the effective cross - section radius w(l) of the light beam at the entrance pupil of the optical system where the near - field equivalent optical path finally arrives are equal to the wave - front curvature radius R(L) and the effective cross - section radius w(L) of the laser spot at the far - field respectively; zemax software is used to simulate the optical path of the laser spot at the near - field equivalent 3km. All single lenses are set as paraxial surfaces, and the focal lengths and intervals are set. The system parameters are shown in Figure 2, physical optics analysis simulation to obtain the laser spot size ( Figure 3 ) is consistent with the theoretical calculation results. Figure 4 This is a three-dimensional schematic diagram of the near-field equivalent far-field optical path simulated by Zemax.

[0132] The parts not described in detail in this invention are prior art.

[0133] The embodiments selected herein for the purpose of disclosing the invention are presently considered suitable, but it should be understood that the invention is intended to include all variations and modifications of the embodiments that fall within the scope of the concept and invention.

Claims

1. A method for designing an optical path of a near-field equivalent far-field laser spot, characterized by: The specific steps include: S1: Determine the distance of the equivalent far-field laser as needed; S2: Determine various parameters of the laser source, including the laser source beam waist, the distance from the laser source beam waist to the light outlet, and the laser wavelength; S3: Determine the maximum experimental working distance in the near field, select two short-focus lenses with the same focal length and two long-focus lenses with the same focal length. The lens size should allow the laser beam to pass completely through. S4: Place attenuation sheets with different attenuation ratios. By combining attenuation sheets with different attenuation ratios, any desired attenuation ratio can be achieved to adjust the energy entering the detector and simulate laser interference with photoelectric imaging equipment of different intensities and different atmospheric attenuation levels; S5: Design an equivalent far-field optical path, build an optical path, and conduct a near-field equivalent far-field laser spot experiment; the steps of the equivalent far-field optical path design are as follows: The propagation law of the Gaussian beam is calculated based on its q parameter. 1 / q(z)=1 / R(z)-i*lambda / (pi*w(z)^2) (1) Where z is the propagation distance, R is the curvature radius of the laser beam wavefront, w is the effective cross-sectional radius of the beam, i.e., the beam size, lambda is the laser wavelength, and i is the imaginary unit; For a beam traveling a distance L in the far field, its q parameter becomes: q=q0+L,q0=i*pi*w0^2 / lambda=iQ (2) w0 is the beam waist of the laser, and Q is the imaginary part of q0. Under near-field conditions, the q parameter of the laser beam before reaching the optoelectronic imaging device after a series of beam transformations is: q1=(A*q0+B) / (C*q0+D) (3) A, B, C, and D are parameters to be determined. To achieve the same wavefront curvature radius and effective beam cross-section as in the far field, the q parameter in the near and far fields must be equal to q = q1: (A*q0+B) / (C*q0+D)=q0+L (4) Since q0 is a complex number, we can separate the real and imaginary parts to obtain two equations: LD=B+CQ^2 (5) A=D+LC (6) If (5) is satisfied, the same wavefront curvature radius can be obtained in the near and far fields. If (6) is satisfied, the laser spot sizes in the near and far fields are also the same. Satisfying the two constraints (5) and (6) can ensure that the laser spots in the near and far fields are basically the same before entering the detector, which better meets the equivalence requirements. In order to be universal for lasers with different beam waist sizes, C should be set to 0 in equations (5) and (6), thereby increasing the number of required equations to 3: LD=B,A=D,C=0 (7) When the number of lenses is 4, a symmetrical lens group is found with a better distance between lenses and lens focal length; under the symmetrical structure, the following order is given: l1＝l5,l2＝l4 (8) F1=F4,F2=F3 (9) Wherein, l1 is the distance from the laser beam waist to the front surface of the first lens, l2 is the distance between the first lens and the second lens, l4 is the distance between the third lens and the fourth lens, l5 is the distance from the fourth lens to the optoelectronic imaging device, and F1, F2, F3, and F4 are the focal lengths of the lenses, respectively; Substitute all equations (7), (8), (9) into the paraxial light transformation matrix to obtain the value of C. The calculation process of the paraxial light transformation matrix M is as follows: M=M1*M2*M3*M4*M5*M4*M3*M2*M1 (10) M1 to M5 and M are all 2*2 matrices. The multiplications in formula (10) are all matrix multiplications, where M1 = [1l1; 01], M2 = [1 0; -1 / F1 1], M3 = [1l2; 0 1], M4 = [1 0; -1 / F21], M5 = [1l3; 0 1], and M = [AB; CD]. Calculating M and setting the C term to zero yields the following equation: (F1+F2-l2)(F1*l3+2*F2*l2+F2*l3-l2*l3-2*F1*F2) / (F1^2*F2^2)=0 (11) In fact, l3 is the distance between the second lens and the third lens. According to the requirement of (11), l2=F1+F2 is required. Substitute this equation into the original paraxial light transformation matrix and simplify it to obtain: A=D=1 (12) B=-(2F1^2*F2-l3*F1^2+2F1*F2^2-2l1*F2^2) / (F2^2) (13) According to (7), we should set B = LD = L, thus obtaining equation (14) l3=(F2^2*L+2*F1^2*F2+2*F1*F2^2-2*l1*F2^2 / (F1^2) (14) Because l3 is much smaller than L, F2 is much smaller than F1. As can be seen from the above formula, as the required equivalent far-field distance increases, l3 increases linearly. By controlling the change of l3, laser spots with different far-field distances can be flexibly simulated. In order to control the total length of the system l, it is calculated that: l=l1+l2+l3+l4+l5=2*F1+(F2 / F1)^2*L+2*l1+4F2+2F2^2 / F1-2(F2 / F1)^2*l1≈2*F1+(F2 / F1)^2*L+2*l1(15) From formula (15), we can see that the distance l1 from the laser beam waist to the front surface of the first lens can be as small as possible, as long as the lens is placed in front of the laser light outlet. When F1 ≈ (F2^2*L)^(1 / 3), the total length of the system is the shortest.

2. The optical path design method for near-field equivalent far-field laser spot according to claim 1 is characterized by: In step S2, the distance from the laser source beam waist to the light outlet should be smaller than the distance from the laser source beam waist to the first lens.

3. The optical path design method for near-field equivalent far-field laser spot according to claim 1 is characterized by: In step S3, the maximum experimental working distance of the near field is determined according to the actual laboratory conditions, and the ratio of the focal lengths of the lenses is determined according to the ratio of the maximum experimental working distance to the distance of the far-field laser. The focal length of the short-focus lens should be much smaller than the focal length of the long-focus lens.

4. The optical path design method for near-field equivalent far-field laser spot according to claim 1 is characterized by: The attenuation plate described in step S4 is flexibly placed at a suitable position between the laser light outlet and the third lens, as long as it does not affect the optical path design.

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