Terahertz beam phase lens manufacturing method and system based on phase recovery algorithm

Abstract

The application provides a terahertz beam phase lens manufacturing method and system based on a phase recovery algorithm, relates to the technical field of terahertz wave imaging, and comprises: a lens design method based on a phase recovery algorithm, which makes incident light form a desired distribution and corrects a non-uniform phase distribution on an output plane, so that the flat-top distribution of the light beam can remain unchanged over a relatively long distance. A suitable flat-top beam field intensity distribution is selected, the actual light intensity distribution of a terahertz emission source is tested and analyzed based on experiments, the initial value of the incident Gaussian beam of an iterative algorithm is fitted through an algorithm, iterative calculation is performed by using a phase recovery algorithm, and the homogenization root mean square and flat-top energy proportion of the reshaped flat-top beam are required to meet the requirements of terahertz wave beam expansion, homogenization and detector imaging, the terahertz beam radiated by the terahertz radiation source is shaped into a flat-top beam, and the homogenized light spot diameter is greater than 50mm, and the homogenization degree is greater than 70%.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of terahertz imaging, in particular to a terahertz beam phase lens manufacturing method and system based on a phase recovery algorithm. BACKGROUND

[0002] Terahertz (THz) is a general term for electromagnetic radiation in a specific frequency band of the electromagnetic spectrum, with a frequency range of 0.1 THz to 10 THz (1 THz equals 10^12 Hz) and a single photon energy of 0.41 meV to 41 meV, corresponding to a wavelength range of 0.03 mm to 3 mm. Because its frequency is between microwave and infrared radiation, terahertz waves are a special area of transition from macroscopic classical theory to microscopic quantum theory, combining the characteristics of both electronics and photonics. Prior to the mid-1980s, the terahertz band, with a frequency of 0.1 THz to 10 THz, was not thoroughly studied due to the lack of high-power terahertz radiation sources and high-sensitivity detectors. In contrast, infrared radiation and microwaves have been widely used in many fields such as communication, detection, spectroscopy, and imaging. However, with the rapid development of electronic photonics since the 1990s, reliable laser emission sources can now be easily obtained to generate terahertz pulses, which has led to the rapid development of terahertz technology.

[0003] Most existing terahertz imaging systems use Gaussian beams as illumination sources. Due to the non-uniform intensity, rapid decay, defocusing effect, and difficulty in control of Gaussian beams, their practical application in some application scenarios is limited. Their application is limited in illumination imaging of large-size targets. To address the above problems, a flat-top beam is a potential solution. The present application proposes a general framework for designing a terahertz flat-top beam generation system based on a phase recovery algorithm. Using scalar diffraction theory as the calculation theory and the phase recovery algorithm as the core design algorithm, a circular terahertz flat-top beam system is designed, and the time-domain finite difference method is used to numerically simulate the system to verify the correctness of the designed system.

[0004] Currently, terahertz flat-top beams have wide application potential in fields such as imaging, communication, sensing, and medical diagnosis, and are a field of interest in recent years in the study of the terahertz band. Research on terahertz flat-top beams also involves the design and synthesis of new materials, as well as the development and optimization of terahertz devices, to achieve the feasibility and stability of terahertz flat-top beam technology in practical applications. Overall, the study of terahertz flat-top beams is in a stage of continuous deepening and expansion, and in the future it is expected to bring new breakthroughs and opportunities for the development and application of terahertz technology. SUMMARY

[0005] The purpose of the present application is to provide a terahertz beam phase lens manufacturing method and system based on phase recovery algorithm. First, the terahertz flat-top beam generation system is constructed based on the phase recovery algorithm. Through the research and analysis of the light intensity distribution of the terahertz emission source, the research and scheme design of the terahertz beam homogenization irradiation technology based on the light intensity distribution are carried out. The homogenization root mean square and flat-top energy ratio of the shaped flat-top beam meet the requirements of terahertz beam expansion, homogenization and detector imaging, and the research, test and system optimization design of the terahertz beam shaping system are carried out according to the scheme.

[0006] Secondly, the present application studies the high-efficiency phase recovery algorithm suitable for the terahertz wave band, and realizes the phase recovery of the terahertz wave by spatially discrete sampling the terahertz complex amplitude and using fast Fourier transform and other tools.

[0007] Thirdly, the present application can study and design the applicable diffractive element, establish a high-precision computer three-dimensional model of the phase plate according to the design process of the diffractive element, and realize high-precision and high-efficiency phase plate processing by using 3D printing technology, so as to realize shaping of the terahertz source and obtain a flat-top beam. Meanwhile, the present application proposes to use two phase lenses to realize the terahertz flat-top beam: the first phase lens makes the incident light form the expected distribution, and the second phase lens is used to correct the non-uniform phase distribution on the output plane, so that the flat-top distribution of the light beam can remain unchanged within a certain distance.

[0008] Specifically, in one aspect, the present application provides a terahertz beam phase lens manufacturing method based on a phase recovery algorithm, which comprises the following steps:

[0009] S1, determining the flat-top beam field intensity distribution;

[0010] S2, testing the actual light intensity distribution of the beam emitted by the terahertz source, and fitting the incident Gaussian beam initial value of the iterative algorithm based on the Gaussian curve algorithm:

[0011] S3, using the incident Gaussian beam initial value to perform iteration on the phase distribution based on the phase recovery algorithm to obtain the thickness distribution of the first phase lens and the second phase lens, and the specific steps are as follows:

[0012] S31, using the phase recovery algorithm to perform iterative calculation, and letting the electric field estimation values of the diffraction plane and the image plane at the kth iteration be and respectively.

[0013] wherein, and are the amplitude estimation values of the diffraction plane and the image plane respectively, and are the phase distribution estimation values of the diffraction plane and the image plane respectively, and i is the imaginary part;

[0014] S32, calculating the electric field value of the diffraction plane at the k+1th iteration according to the following formula

[0015]

[0016] wherein a1 is an initial value of amplitude, I0 is the incident plane light intensity distribution, j is the imaginary part;

[0017] S33, constructing the plane p1' and the plane p2, so that the light intensity in the range of r1 on the p1' plane is equal to the light intensity in the range of r2 on the p2 plane, r1 and r2 satisfy the following formula:

[0018]

[0019] wherein ω0 is the waist radius of the Gaussian beam, R FL is the radius at half the maximum light intensity, r1 is the distance of the Gaussian beam from the p1' plane, and r2 is the distance of the Gaussian beam from the p2 plane;

[0020] S34, determining that the phase increment distributions corresponding to the first phase lens and the second phase lens are respectively:

[0021]

[0022] wherein λ0 is the wavelength in vacuum, n is the refractive index, and d is the thickness of the phase lens, according to the laser theory, the electric field intensity distribution of the Gaussian beam satisfies:

[0023]

[0024] wherein R is the radial coordinate, a AMP is the field strength at the waist, ω0 is the waist radius of the Gaussian beam, z0 is the waist position, z is the distance from the waist of the Gaussian beam in the wave propagation direction, and ω is the beam radius corresponding to z;

[0025] The initial value of the incident Gaussian beam obtained by fitting is brought into the formula to obtain the estimated value of the phase distribution on the p1 plane The estimated value of the phase distribution on the p1' plane is Take as the initial value of the phase distribution and use the I-O algorithm for iterative calculation, since Therefore, the phase change amount distributions of the first phase lens and the second phase lens are:

[0026]

[0027] wherein, is the phase change amount distribution of the first phase lens, is the phase change amount distribution of the second phase lens;

[0028] ​The lens thickness distribution corresponding to the first phase lens and the second phase lens is:

[0029]

[0030] Wherein, h1 is the thickness distribution of the first phase lens, h2 is the thickness distribution of the second phase lens;

[0031] S4, according to the phase difference, the three-dimensional structure model of the diffraction lens is established, the finite difference method is used for simulation analysis and verification, and the light field distribution of the flat top beam is obtained, wherein the spot diameter is the diameter at half of the maximum light intensity, and the light intensity distribution uniformity U RMS ,

[0032] S5, selecting appropriate materials, using 3D printing device to prepare the first phase lens and the second phase lens respectively and using the detection system to test the intensity distribution of the outgoing beam.

[0033] Preferably, the flat top beam field intensity distribution determined in step S1 is a homogenized Lorentz function distribution, and the formula of the normalized homogenized Lorentz function distribution in polar coordinates is as follows:

[0034]

[0035] Wherein, r is the radius in polar coordinates, R FL is the radius at half of the maximum light intensity, q is the shape parameter, the larger the value of q is, the steeper the edge of the flat top distribution is, and the closer to the rectangle.

[0036] Preferably, step S2 specifically includes the following substeps:

[0037] S21, the intensity distribution of the source outgoing beam propagation direction section is measured, the strongest value point, i.e. the center point, is found, and the intensity distribution of the xz plane and the yz plane is scanned with the center point position as the reference;

[0038] S22, the intensity distribution data obtained by scanning is imported into the simulation software and fitted by using the Gaussian curve to obtain the initial value of the incident Gaussian beam.

[0039] Preferably, the flat top beam field intensity distribution determined in step S1 is a homogenized Lorentz function distribution, and the formula of the normalized homogenized Lorentz function distribution in polar coordinates is as follows:

[0040]

[0041] Wherein, r is the radius in polar coordinates, R FL is the radius at half of the maximum light intensity, q is the shape parameter, the larger the value of q is, the steeper the edge of the flat top distribution is, and the closer to the rectangle.

[0042] Preferably, the solution of the equation in step S33 is:

[0043]

[0044] Preferably, the Gaussian curve equation expression in step S22 is as follows:

[0045]

[0046] wherein r is the radius in polar coordinates, I AMP is a constant, z is the distance from the waist of the Gaussian beam in the wave propagation direction, ω0 is the waist radius of the Gaussian beam, ω is the beam radius corresponding to z, z0 is the waist position; the waist radius ω0 and the waist position z0 can be obtained by fitting.

[0047] On the other hand, the application provides a phase lens manufacturing system of a terahertz beam phase lens manufacturing method based on a phase recovery algorithm, comprising a flat-top beam field intensity distribution determination unit, an incident Gaussian beam initial value calculation unit, a phase lens thickness distribution determination unit, a flat-top beam light field distribution determination unit, and a phase lens preparation unit.

[0048] The flat-top beam field intensity distribution determination unit is used to determine the flat-top beam field intensity distribution.

[0049] The incident Gaussian beam initial value calculation unit is used to test the actual light intensity distribution of the beam emitted by the terahertz source, and obtain the incident Gaussian beam initial value of the iterative algorithm based on Gaussian curve algorithm fitting:

[0050] The phase lens thickness distribution determination unit is used to iteratively calculate the incident Gaussian beam initial value based on the phase recovery algorithm, and obtain the thickness distribution of the first phase lens and the second phase lens.

[0051] The flat-top beam light field distribution determination unit is used to establish a three-dimensional structure model of the diffractive lens according to the phase difference, perform simulation analysis and verification by using the finite difference method, and obtain the light field distribution of the flat-top beam.

[0052] The phase lens preparation unit is used to select a suitable material, prepare the first phase lens and the second phase lens by using a 3D printing device, and test the intensity distribution of the outgoing beam by using a detection system.

[0053] Compared with the prior art, the application has the following advantages:

[0054] (1) The application is a terahertz beam phase lens manufacturing method based on a phase recovery algorithm, which adopts scalar diffraction theory to decompose the complex vector wave transmission calculation problem into several scalar wave solving problems, adopts the Input-Output (I-O) algorithm in the phase recovery algorithm, obtains the phase change of the front and rear surfaces of the lens through iterative calculation, and adopts the finite difference time domain method for simulation verification, so that the parameters of the phase lens can be accurately calculated, and the accuracy of the terahertz beam phase lens is ensured.

[0055] (2) The terahertz beam phase lens manufacturing method based on the phase recovery algorithm is based on scalar diffraction theory and phase recovery algorithm, compared with the common ray tracing algorithm in the existing visible light band, the method starts from diffraction optics and is applicable to the low-frequency terahertz band where diffraction interference is more significant, and has great reference value for terahertz homogenization imaging research and terahertz special beam shaping research.

[0056] (3) The method of the application selects a suitable flat-top beam intensity distribution, analyzes the actual light intensity distribution of the terahertz emission source based on experimental testing, fits the initial value of the incident Gaussian beam of the iterative algorithm by algorithm, performs iterative calculation by using the phase recovery algorithm, and shapes the terahertz beam radiated by the terahertz radiation source into a flat-top beam according to the homogenization root mean square and flat-top energy proportion of the shaped flat-top beam to meet the needs of terahertz beam expansion and homogenization and detector imaging, so that the homogenization spot diameter is greater than 50 mm, and the homogenization degree is more than 70%. BRIEF DESCRIPTION OF DRAWINGS

[0057] Figure 1 The flowchart is designed based on the phase recovery algorithm theory of the application;

[0058] Figure 2 The flowchart is the Input-Output algorithm of the phase recovery algorithm of the application;

[0059] Figure 3 The schematic diagram of the terahertz flat-top beam shaping system of the application;

[0060] Figure 4 The lens group homogenization simulation effect display diagram designed by the application;

[0061] Figure 5a And Figure 5b The beam schematic diagram and intensity distribution diagram of the terahertz source of the application before shaping;

[0062] Figure 6a And Figure 6b The beam schematic diagram and intensity distribution diagram of the application after shaping by the lens group;

[0063] Figure 7 This is a schematic block diagram of the manufacturing system of the present invention. Detailed Implementation

[0064] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0065] On the one hand, this invention provides a method for manufacturing a terahertz beam phase lens based on a phase retrieval algorithm, such as... Figure 1 As shown, it includes the following steps:

[0066] S1. Determine the field strength distribution of the flat-top beam; In a specific embodiment, step S1 determines the field strength distribution of the flat-top beam as a homogenized Lorentz function distribution. The formula for the normalized homogenized Lorentz function distribution in polar coordinates is as follows:

[0067]

[0068] Where r is the radius in polar coordinates, R FL The radius is at half the maximum light intensity, and q is a shape parameter. The larger the value of q, the steeper the edge of the flat-top distribution, and the closer it is to a rectangle.

[0069] S2. Test the actual intensity distribution of the beam emitted by the terahertz source, and obtain the initial value of the incident Gaussian beam for the iterative algorithm based on Gaussian curve fitting; wherein, step S2 specifically includes the following sub-steps:

[0070] S21. Measure the intensity distribution of the cross section in the direction of the emitted beam, find the point of strongest intensity, i.e. the center point, and scan the intensity distribution of the xz plane and yz plane respectively with the center point as a reference.

[0071] S22. Import the intensity distribution data obtained from the scan into the simulation software and fit it using a Gaussian curve to obtain the initial value of the incident Gaussian beam.

[0072] The equation of the Gaussian curve is:

[0073]

[0074] Where r is the radius in polar coordinates, I AMP Let z be a constant, z be the distance from the waist of the Gaussian beam in the direction of wave propagation, ω0 be the waist radius of the Gaussian beam, ω be the beam radius at the corresponding z position, and z0 be the waist position. By fitting, the waist radius ω0 and the waist position z0 can be obtained.

[0075] S3, iteratively calculate the phase distribution based on the phase retrieval algorithm using the initial value of the incident Gaussian beam, and obtain the thickness distribution of the first phase lens and the second phase lens, the specific steps are as follows:

[0076] S31, iteratively calculate using the phase retrieval algorithm, and let the electric field estimation values of the diffraction plane and the image plane at the kth iteration be

[0077] wherein, and are the amplitude estimation values of the diffraction plane and the image plane respectively, and are the phase distribution estimation values of the diffraction plane and the image plane respectively, and i is the imaginary part.

[0078] S32, calculate the electric field value of the diffraction plane at the k+1th iteration according to the following formula

[0079]

[0080] wherein, a1 is the amplitude initial value, I0 is the incident plane light intensity distribution, and j is the imaginary part.

[0081] S33, construct the plane p1' and the plane p2, so that the light intensity in the range r1 on the p1' plane is equal to the light intensity in the range r2 on the p2 plane, r1 and r2 satisfy the following formula:

[0082]

[0083] wherein, ω0 is the waist radius of the Gaussian beam, R FL is the radius at half of the maximum light intensity, r1 is the distance of the Gaussian beam from the p1' plane, and r2 is the distance of the Gaussian beam from the p2 plane;

[0084] The solution of the equation in step S33 is:

[0085]

[0086] S34, determine that the phase increment distributions corresponding to the first phase lens and the second phase lens are respectively:

[0087]

[0088] wherein, λ0 is the wavelength in vacuum, n is the refractive index, and d is the thickness of the phase lens, according to the laser theory, the electric field intensity distribution of the Gaussian beam satisfies:

[0089]

[0090] wherein, R is the radial coordinate, a AMPis the waist radius of the Gaussian beam, z0 is the waist position, z is the distance from the waist of the Gaussian beam in the wave propagation direction, and ω is the beam radius corresponding to z.

[0091] The phase distribution estimation value on the p1 plane is obtained by bringing the initial value of the incident Gaussian beam obtained by fitting into the formula The phase distribution estimation value on the p1 plane is obtained by bringing the initial value of the incident Gaussian beam obtained by fitting into the formula The phase distribution estimation value on the p1 plane is obtained by bringing the initial value of the incident Gaussian beam obtained by fitting into the formula The I-O algorithm is used for iterative calculation as the phase distribution initial value, and since It is known that Therefore, the phase change distribution of the first phase lens and the second phase lens is:

[0092]

[0093] wherein, is the phase change distribution of the first phase lens, is the phase change distribution of the second phase lens.

[0094] The lens thickness distribution corresponding to the first phase lens and the second phase lens is

[0095]

[0096] wherein, h1 is the thickness distribution of the first phase lens, and h2 is the thickness distribution of the second phase lens.

[0097] S4, a three-dimensional structure model of the diffractive lens is established according to the phase difference, simulation analysis verification is carried out by using the finite difference method, and the light field distribution of the flat-top beam is obtained, wherein the spot diameter is taken as the diameter at the half of the maximum light intensity, and the light intensity distribution uniformity U RMS ,

[0098] S5, a suitable material is selected, the first phase lens and the second phase lens are prepared by using a 3D printing device, and the intensity distribution of the outgoing beam is tested by using a detection system.

[0099] On the other hand, the application provides a phase lens manufacturing system of a terahertz beam phase lens manufacturing method based on a phase recovery algorithm, as Figure 7 shown, which comprises a flat-top beam field intensity distribution determination unit 1, an incident Gaussian beam initial value calculation unit 2, a phase lens thickness distribution determination unit 3, a flat-top beam light field distribution determination unit 4, and a phase lens preparation unit 5.

[0100] The flat-top beam field intensity distribution determination unit 1 is used for determining the flat-top beam field intensity distribution.

[0101] The incident Gaussian beam initial value calculation unit 2 is used for testing the actual light intensity distribution of the beam emitted by the terahertz source, and the incident Gaussian beam initial value of the iterative algorithm is obtained based on the Gaussian curve algorithm fitting.

[0102] The phase lens thickness distribution determination unit 3 is used for iterating the incident Gaussian beam initial value based on the phase recovery algorithm, and the thickness distribution of the first phase lens and the second phase lens is obtained.

[0103] The flat-top beam light field distribution determination unit 4 is used for establishing a three-dimensional structure model of the diffraction lens according to the phase difference, performing simulation analysis verification by using the finite difference method, and obtaining the light field distribution of the flat-top beam.

[0104] The phase lens preparation unit 5 is used for selecting a suitable material, preparing the first phase lens and the second phase lens by using a 3D printing device, and testing the intensity distribution of the outgoing beam by using a detection system.

[0105] The embodiment provides a terahertz beam phase lens design method based on a phase recovery algorithm, Figure 1 is a flowchart designed based on a phase recovery algorithm theory. The method specifically comprises the following steps:

[0106] S1, selecting a suitable flat-top beam field intensity distribution. In the embodiment, an improved super-Lorentz function distribution, that is, a homogenized Lorentz function distribution, is used as an ideal flat-top beam light intensity distribution. The advantage of using this distribution function is that the first step of design needs to use a simple geometric optics principle to obtain a traditional aspheric phase distribution as an initial value, and in this process, a ray mapping function needs to be used. For the above homogenized Lorentz function distribution, the ray mapping function has an analytical expression, so that the calculation process becomes very simple.

[0107] The formula of the normalized homogenized Lorentz function distribution in the polar coordinate is as follows:

[0108]

[0109] In the formula, r is the radius in the polar coordinate, R FL is the radius at half of the maximum light intensity (FWHM), q is a shape parameter, and the greater the q value, the steeper the flat-top distribution edge, and the closer to the rectangle.

[0110] S2, obtaining the actual light intensity distribution of the terahertz wave source according to experimental measurement, and fitting the incident Gaussian beam initial value of the iterative algorithm by using an algorithm. In the experiment, the intensity distribution of the source outgoing beam in the propagation direction section is first measured, the strongest value point, that is, the center point is found, and the intensity distribution of the xz plane and the yz plane is scanned with the center point position as a reference. The outgoing beam of the terahertz source is in a Gaussian distribution in the radial direction. The data obtained by scanning is fitted by using a Gaussian curve, and the Gaussian curve expression is as follows:

[0111]

[0112] where r is the radial coordinate, I AMP is a constant, z is the distance from the waist of the Gaussian beam in the direction of wave propagation, ω0 is the waist radius of the Gaussian beam, ω is the beam radius at z, and z0 is the waist position. The waist radius ω0 and the waist position z0 can be obtained by fitting.

[0113] where, Figure 2 is the Input-Output algorithm flowchart, Figure 3 is a schematic diagram of a terahertz flat-top beam shaping system.

[0114] S3, iterative calculation, the system directly converts the non-collimated Gaussian light radiated by the terahertz source into collimated flat-top light. The key step in the design of the phase lens is to obtain the phase distributions of the incident surfaces (p1, p2) and the exit surfaces (p1', p2') of the two phase lenses. Based on the thin lens approximation, we theoretically consider the phase lens as a pure phase type device, ignoring the influence of its thickness on the amplitude; the incident light rays exit at approximately the same coordinates, i.e., the p1 plane and the p1' plane have the same spatial position coordinates, and the p2 and p2' planes also have the same spatial position coordinates.

[0115] Based on the above analysis, iterative calculation can be performed, and the estimated values of the electric field of the diffraction plane and the image plane at the kth iteration are where and are the estimated values of the amplitude and phase distribution of the diffraction plane and the image plane, respectively.

[0116] The calculation is performed according to the following formula:

[0117]

[0118] Assuming that the energy is conserved during the propagation of the light field, the energy distribution on the p1' and p2 planes has a mapping relationship. For a pure phase device, the absorption and reflection losses are ignored, and the r1 and r2 that satisfy the condition that the light intensity in the r1 range on the p1' plane is equal to the light intensity in the r2 range on the p2 plane are:

[0119]

[0120] The solution to the equation is:

[0121]

[0122] The phase increment distributions corresponding to the first phase lens and the second phase lens are:

[0123]

[0124] where λ0 is the wavelength in vacuum.

[0125] According to the laser theory, the electric field intensity distribution of the Gaussian beam satisfies:

[0126]

[0127] where R is the radial coordinate, a AMP is the field intensity at the beam waist, ω0 is the beam waist radius of the Gaussian beam, z0 is the beam waist position, z is the distance from the beam waist of the Gaussian beam in the wave propagation direction, and ω is the beam radius corresponding to z. According to the formula, the field intensity and phase distribution on any propagation section can be calculated. The estimated value of the phase distribution on the p1 plane can be obtained by bringing the fitted parameters of the terahertz Gaussian beam into the formula Therefore, the estimated value of the phase distribution on the p1' plane is Take I-O algorithm with as the initial value for iterative calculation, because Therefore, the phase change distribution of the first phase lens and the second phase lens is:

[0128]

[0129] The lens thickness distribution corresponding to the first phase lens and the second phase lens is:

[0130]

[0131] Figure 4 is the display picture of the homogenization simulation effect of the designed lens group. The finite difference method is used for simulation, and the beam diameter and the relative root mean square are calculated by the simulation data to quantitatively screen the flat-top beam quality generated by the simulation. Preliminary screening can obtain a homogenization lens group with a radius not less than 50 mm and a relative intensity distribution uniformity not less than 70% within a certain propagation distance.

[0132] Figure 5a and Figure 5b are the beam schematic diagram and the intensity distribution diagram of the terahertz source without shaping, i.e. the beam intensity schematic diagram emitted by the terahertz source detected by the detector scanning, and the intensity distribution is a standard Gaussian distribution.

[0133] Figure 6a and Figure 6b are the beam schematic diagram and the intensity distribution diagram after shaping by the lens group. The shaped beam intensity distribution is shown in the figure. The calculated beam diameter Φ = 101.5 mm, and U RMS = 72.00% has good shaping effect.

[0134] ​The method of the present application firstly selects a suitable flat-top beam field intensity distribution, based on experimental test of the actual light intensity distribution of the terahertz emission source and analysis, the initial value of the incident Gaussian beam of the iterative algorithm is fitted by an algorithm, the iterative calculation is carried out by using the phase recovery algorithm, and the homogenization root mean square and the flat-top energy proportion of the shaped flat-top beam meet the requirements of terahertz wave beam expansion and homogenization and detector imaging, the terahertz beam radiated by the terahertz radiation source is shaped into a flat-top beam, and the homogenization spot diameter is greater than 50mm, and the homogenization degree reaches more than 70%.

[0135] The above embodiments only describe the preferred embodiments of the present application, and do not limit the scope of the present application, and various modifications and improvements of the technical solutions of the present application made by those skilled in the art without departing from the design spirit of the present application shall fall within the protection scope determined by the claims of the present application.

Claims

1. A method for manufacturing a terahertz beam phase lens based on a phase retrieval algorithm, characterized in that: It includes the following steps: S1. Determine the field strength distribution of the flat-top beam; S2. Test the actual intensity distribution of the beam emitted by the terahertz source, and obtain the initial value of the incident Gaussian beam for the iterative algorithm based on Gaussian curve fitting: S3. Using the initial value of the incident Gaussian beam, the phase distribution is iterated based on the phase retrieval algorithm to obtain the thickness distribution of the first and second phase lenses. The specific steps are as follows: S31. Iterative calculations are performed using the phase retrieval algorithm, letting the... The electric field estimates for the diffraction plane and the image plane at the next iteration are respectively , ; in, , These are the amplitude estimates for the diffraction plane and the image plane, respectively. , These are the estimated phase distribution values ​​for the diffraction plane and the image plane, respectively. It is the imaginary part; S32. Calculate the following formula: Electric field value of the diffraction plane in the next iteration : ； in, The initial value of the amplitude, I0 represents the light intensity distribution on the incident plane. It is the imaginary part; S33, Constructing a Plane and plane ,make On a plane Light intensity within the range equals On a plane Light intensity within range, , Satisfy the following formula: ； in, The waist radius of the Gaussian beam. The radius is the point where the light intensity is at half its maximum value. Gaussian beam distance Distance between planes, Gaussian beam distance Distance between planes; S34. Determine the phase increment distributions corresponding to the first phase lens and the second phase lens as follows: ； in, Let λ be the wavelength in vacuum, n be the refractive index, and d be the thickness of the phase lens. According to laser theory, the electric field intensity distribution of a Gaussian beam satisfies: ； Where R is the radial coordinate. For the waist area to be strong, The waist radius of the Gaussian beam. For the waist area, Let be the distance from the waist of the Gaussian beam along the direction of wave propagation. For the corresponding Beam radius at location; Substituting the initial value of the incident Gaussian beam obtained from the fitting into the formula yields... Phase distribution estimate on the surface , The estimated phase distribution on the surface is ,Will As the initial value for the phase distribution, iterative calculations are performed using the IO algorithm. It is known that Therefore, the phase change distribution of the first phase lens and the second phase lens is as follows: ； in, The distribution of phase change for the first phase lens. The distribution of phase change for the second phase lens; The lens thickness distributions corresponding to the first-phase lens and the second-phase lens are as follows: ； in, The thickness distribution of the first phase lens. The thickness distribution of the second phase lens; S4. Establish a three-dimensional structural model of the diffraction lens based on the phase difference, and perform simulation analysis and verification using the finite difference method to obtain the light field distribution of the flat-top beam. The beam spot diameter is taken as the diameter at half the maximum light intensity, and the light intensity distribution uniformity U... RMS , ; S5. Select appropriate materials, use a 3D printing device to prepare the first phase lens and the second phase lens respectively, and use a detection system to test the intensity distribution of the outgoing beam.

2. The method for manufacturing a terahertz beam phase lens based on a phase retrieval algorithm according to claim 1, characterized in that: In step S1, the field strength distribution of the flat-top beam is determined to be a homogenized Lorentz function distribution. The formula for the normalized homogenized Lorentz function distribution in polar coordinates is as follows: ； in, The radius in polar coordinates. The radius is the point where the light intensity is at half its maximum value. For shape parameters, The larger the value, the steeper the edge of the flat-top distribution, and the closer it is to a rectangle.

3. The method for manufacturing a terahertz beam phase lens based on a phase retrieval algorithm according to claim 1, characterized in that: Step S2 specifically includes the following sub-steps: S21. Measure the intensity distribution of the cross section in the direction of the emitted beam, find the point of strongest intensity, i.e. the center point, and scan the intensity distribution of the xz plane and yz plane respectively with the center point as a reference. S22. Import the intensity distribution data obtained from the scan into the simulation software and fit it using a Gaussian curve to obtain the initial value of the incident Gaussian beam.

4. The method for manufacturing a terahertz beam phase lens based on a phase retrieval algorithm according to claim 1, characterized in that: The solution to the equation in step S33 is: 。 5. The method for manufacturing a terahertz beam phase lens based on a phase retrieval algorithm according to claim 3, characterized in that: The equation for the Gaussian curve in step S22 is as follows: ； in, The radius in polar coordinates. It is a constant. Let be the distance from the waist of the Gaussian beam along the direction of wave propagation. The waist radius of the Gaussian beam. For the corresponding Beam radius at location, The waist position is defined; the waist radius can be obtained through fitting. With waist position .

6. A phase lens manufacturing system for the terahertz beam phase lens manufacturing method based on the phase retrieval algorithm as described in claim 1, characterized in that: It includes a flat-top beam field intensity distribution determination unit, an incident Gaussian beam initial value calculation unit, a phase lens thickness distribution determination unit, a flat-top beam optical field distribution determination unit, and a phase lens fabrication unit; The flat-top beam field strength distribution determination unit is used to determine the flat-top beam field strength distribution. The incident Gaussian beam initial value calculation unit is used to test the actual light intensity distribution of the beam emitted by the terahertz source, and obtains the incident Gaussian beam initial value based on Gaussian curve fitting algorithm: The phase lens thickness distribution determination unit is used to iterate the initial value of the incident Gaussian beam based on the phase recovery algorithm to obtain the thickness distribution of the first phase lens and the second phase lens. The flat-top beam optical field distribution determination unit is used to establish a three-dimensional structural model of the diffraction lens based on the phase difference, and to perform simulation analysis and verification using the finite difference method to obtain the optical field distribution of the flat-top beam. The phase lens fabrication unit is used to select suitable materials, use a 3D printing device to fabricate a first phase lens and a second phase lens respectively, and use a detection system to test the intensity distribution of the outgoing beam.

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