Compact orbital angular momentum classifier based on laser direct writing

Abstract

The utility model discloses a compact orbital angular momentum classifier based on laser direct writing, which relates to the technical field of compact orbital angular momentum classification, and comprises a substrate and a plurality of micro-nano structure units prepared on the substrate through laser direct writing, and the plurality of micro-nano structure units are distributed in an annular array. Forming an orbital angular momentum classifier unit; and a plurality of orbital angular momentum classifier units are stacked along the direction of an optical axis to form the orbital angular momentum classifier. According to the orbital angular momentum classifier, the micro-nano structure unit is prepared on the substrate through the laser direct writing technology, miniaturization and integration of the orbital angular momentum classifier are achieved, integration and application of a system are facilitated, parameters of the micro-nano structure unit and the array vortex grating are reasonably designed, and efficient classification of orbital angular momentum light beams with different topological charges can be achieved. In addition, the laser direct writing technology and the femtosecond laser direct writing technology have the advantages of being low in cost and high in efficiency.

Description

Technical Field

[0001] The utility model relates to the technical field of compact orbital angular momentum classification equipment, in particular to a compact orbital angular momentum classifier based on laser direct writing. Background Art

[0002] Orbital angular momentum is increasingly used in the fields of communications, quantum information processing, and optical imaging. However, existing orbital angular momentum classifiers have the problems of large size, complex structure, and high cost, which limit their practical applications. Therefore, the utility model aims to provide a compact orbital angular momentum classifier based on laser direct writing to solve these problems. Utility Model Content

[0003] In order to achieve the above-mentioned purpose, the utility model specifically adopts the following technical solutions:

[0004] A compact orbital angular momentum classifier based on laser direct writing comprises a substrate and a plurality of micro-nano structure units prepared on the substrate by laser direct writing, wherein the plurality of micro-nano structure units are distributed in a ring array to form an orbital angular momentum classifier unit; a plurality of the orbital angular momentum classifier units are stacked along an optical axis direction to form the orbital angular momentum classifier.

[0005] Furthermore, the micro-nano structure unit is a metal nanostructure with a specific geometric shape and size, which produces a specific scattering response to the incident orbital angular momentum light beam. The material of the metal nanostructure is gold, and the substrate material is quartz.

[0006] Furthermore, in the orbital angular momentum classifier, the radius of the annular array, the number of micro-nano structure units, and the number of stacked orbital angular momentum classifier units can be adjusted according to actual needs.

[0007] Furthermore, the mathematical expression of the transfer function of the array vortex grating is: .in , , Represent the weight coefficient, order, and spatial position of the mth orbital angular momentum mode respectively. To achieve pure phase modulation, this complex transfer function is approximated as a pure phase function: ,in , n represents the total number of orbital angular momentum modes, Re represents the real part, yes The determining factor of . Expanding the above equation by Fourier coefficients yields: , the Fourier coefficients can be expressed as; , the conversion efficiency is defined as ,here represents the total energy of all OAM modes. It can be understood as the total energy of the OAM mode at the target position; the uniformity value is used to characterize the intensity uniformity between each beam in the grating diffraction field and is defined as ,here and Respectively The maximum and minimum values ​​in .

[0008] Furthermore, the design process of the array vortex grating includes: first, initialization setting, iteration counter n = 1, calculate the conversion efficiency of the grating at the m-th order; second, calculate the uniformity value, if the number of iterations n < N, it will be updated according to and, wherein represents the update rate constant, and the purpose of optimizing the pure phase grating is achieved by calculating the modulus length calculation and continuously updating, and at the same time increasing n to n + 1, and continuing the loop calculation until the n < N condition is not met, jump out of the loop, and end the iteration; finally, obtain the final phase transfer function, design the phase distribution of the phase element to, and obtain the array vortex grating that can realize decoding of different OAM modes at an ideal position.

[0009] Furthermore, the parameters of the array vortex grating are: resolution size is 256×256, pixel size is 500nm, wavelength , n = 8 corresponds to l = -16, -12, -8, -4, 4, 8, 12, 16 respectively.

[0010] Furthermore, the method includes the following steps: converting the phase information corresponding to each pixel of a phase-type diffractive optical element of suitable size and resolution obtained by program calculation into height information; using MATLAB software to design the height information according to the processing trajectory route to be realized, and generating a TXT file with spatial point information, height information and shutter switch information settings that can be recognized by the processing system end software, wherein 0 represents the shutter is on, and the laser propagates normally along the optical path; 1 represents the shutter is off, and the laser is blocked; according to the principle of light transmission in the medium, the mathematical expression of the relationship between the height information of the optical element and the phase information at the corresponding position is, wherein (x, y) represents the position coordinates on the plane, represents the phase value, n is the refractive index of the processing material, and represents the refractive index of the surrounding medium; in the experiment, IP-Dip photoresist is used to prepare the phase plate, and the material refractive index n = 1.55, the surrounding medium is air, so the wavelength is designed. Under this set of specific parameters, the maximum height corresponding to the maximum phase value of the processed phase plate is 2 is about 1μm, the resolution size of the designed phase element is 256×256, the scanning step length of the laser on the substrate plane is set to 0.5μm, and the overall size of the phase plate is 128μm×128μm×1μm; the femtosecond laser direct writing platform based on the principle of two-photon polymerization adopts an immersion, point-by-point scanning, and three-dimensional displacement platform working alone. The processing is carried out at a laser processing speed and power of about 100μm / s and 10mw. The phase plate is prepared according to the process flow, and the experimental processing takes about 150min

[0011] Further, the design process for decomposing different orders of OAM to different positions of the array vortex grating is as follows: First, the initialization settings , iteration counter n=1. The above formula can be used to calculate the conversion efficiency of the grating at the m-order .,The second step is to calculate the uniformity value If the number of iterations n<N, Will be updated as follows: in, represents the update rate constant, which is calculated by the modulus calculate So as to continuously update Optimized pure phase grating At the same time, n is increased to n+1, and the loop calculation continues until the condition n<N is not met, then the loop is jumped out and the iteration ends. Finally, the final phase transfer function can be obtained. Then the phase distribution of the phase element is designed as , we can obtain array vortex light at the ideal position, which can decode different OAM modes.

[0012] The beneficial effects of the utility model are as follows:

[0013] 1. Small size and compact structure: Micro-nano structural units are prepared on the substrate through laser direct writing technology, which realizes the miniaturization and integration of the orbital angular momentum classifier, which is conducive to the integration and application of the system.

[0014] 2. High classification efficiency: Reasonable design of the parameters of micro-nano structural units and array vortex gratings can achieve efficient classification of orbital angular momentum beams with different topological charges, thereby improving the accuracy and reliability of classification.

[0015] 3. Low cost: Laser direct writing technology and femtosecond laser direct writing technology have the advantages of low cost and high efficiency, which can reduce the manufacturing cost of orbital angular momentum classifiers and facilitate their promotion in practical applications. High flexibility: The structure and parameters of the orbital angular momentum classifier can be flexibly adjusted according to different application requirements to achieve the classification of different orbital angular momentum beams. Good performance: The designed array vortex grating has good uniformity and conversion efficiency, and can achieve efficient classification and regulation of orbital angular momentum beams. In summary, the compact orbital angular momentum classifier based on laser direct writing of the utility model has important application value and broad market prospects, and provides a new solution for the development and application of orbital angular momentum technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is the design flow chart of the array vortex grating of the utility model;

[0017] Figure 2 It is a classification effect diagram of the array vortex grating of the utility model for orbital angular momentum light beams l=-16, -12, -8, -4, 4, 8, 12, 16 respectively;

[0018] Figure 3 It is a classification principle diagram of orbital angular momentum based on array vortex grating of the utility model;

[0019] Figure 4 It is a classification effect diagram of the utility model array vortex grating for two mixed orbital angular momentum light beams l=8 and l=-12, l=-4 and l=16, l=4 and l=-16, l=-8 and l=12 respectively; DETAILED DESCRIPTION

[0020] In order to make the purpose, technical solution and advantages of the embodiments of the present utility model clearer, the technical solution in the embodiments of the present utility model will be clearly and completely described below in conjunction with the drawings in the embodiments of the present utility model.

[0021] Embodiment 1:

[0022] A compact orbital angular momentum classifier based on laser direct writing comprises a substrate and a plurality of micro-nano structure units prepared on the substrate by laser direct writing, wherein the plurality of micro-nano structure units are distributed in a ring array to form an orbital angular momentum classifier unit; a plurality of the orbital angular momentum classifier units are stacked along an optical axis to form the orbital angular momentum classifier, wherein the micro-nano structure unit is a metal nanostructure with a specific geometric shape and size, which generates a specific scattering response to an incident orbital angular momentum light beam, wherein the material of the metal nanostructure is gold, and the material of the substrate is quartz.

[0023] like Figure 1-4 As shown, in some embodiments, the transfer function mathematical expression of the array vortex grating is: .in , , Represent the weight coefficient, order, and spatial position of the mth orbital angular momentum mode respectively. To achieve pure phase modulation, this complex transfer function is approximated as a pure phase function: ,in , n represents the total number of orbital angular momentum modes, Re represents the real part, yes The determining factor of . Expanding the above equation by Fourier coefficients yields: , the Fourier coefficients can be expressed as; , the conversion efficiency is defined as ,here represents the total energy of all OAM modes. It can be understood as the total energy of the OAM mode at the target position; the uniformity value is used to characterize the intensity uniformity between each beam in the grating diffraction field and is defined as ,here and Respectively The maximum and minimum values ​​in .

[0024] like Figure 1 As shown, in some embodiments, the design process of the array vortex grating includes: first, initialization settings, iteration counter n = 1, calculate the conversion efficiency of the grating at the m-order; second, calculate the uniformity value, if the number of iterations n < N, it will be updated according to and , wherein represents the update rate constant, and the purpose of optimizing the pure phase grating is achieved by calculating the modulus length calculation, and n is increased to n + 1 at the same time, and the loop calculation is continued until the n < N condition is not met, and the loop is jumped out and the iteration is ended; finally, the final phase transfer function is obtained, and the phase distribution of the phase element is designed to obtain an array vortex grating that can realize decoding of different OAM modes at an ideal position.

[0025] like Figure 2 As shown, in some embodiments, the parameters of the array vortex grating are: resolution size is 256×256, pixel size is 500nm, wavelength is , n = 8 corresponds to l = -16, -12, -8, -4, 4, 8, 12, 16 respectively.

[0026] like Figure 1 As shown, in some embodiments, the following steps are included: converting the phase information corresponding to each pixel of the phase-type diffractive optical element of suitable size and resolution obtained by the program into height information; using MATLAB software to design the height information according to the processing trajectory route to be implemented, and generating a TXT file with spatial point information, height information and shutter switch information settings that can be recognized by the processing system end software, wherein 0 represents shutter on, and the laser propagates normally along the optical path; 1 represents shutter off, and the laser is blocked; according to the principle of light transmission in the medium, the mathematical expression of the relationship between the height information of the optical element and the phase information at the corresponding position is, wherein (x, y) represents the position coordinates on the plane, represents the phase value, n is the refractive index of the processing material, and represents the refractive index of the surrounding medium; in the experiment, IP-Dip photoresist is used to prepare the phase plate, and the material refractive index n = 1.55, the surrounding medium is air, so the wavelength is designed. Under this set of specific parameters, the maximum height corresponding to the maximum phase value of the processed phase plate is 2 is about 1μm, the resolution of the designed phase element is 256×256, the scanning step of the laser in the substrate plane is set to 0.5μm, and the overall size of the phase plate is 128μm×128μm×1μm. The femtosecond laser direct writing platform based on the principle of two-photon polymerization adopts an immersion, point-by-point scanning, and three-dimensional displacement platform working alone. The processing is carried out at a laser processing speed and power of about 100μm / s and 10mw. The phase plate is prepared according to the process flow, and the experimental processing takes about 150min.

[0027] like Figure 1 As shown in Figure 1, the design process of decomposing array vortex gratings of different orders of OAM to different positions in space. First, the initialization settings , iteration counter n=1. The above formula can be used to calculate the conversion efficiency of the grating at the m-order .,The second step is to calculate the uniformity value If the number of iterations n<N, Will be updated as follows: in, represents the update rate constant, which is calculated by the modulus calculate So as to continuously update Optimized pure phase grating At the same time, n is increased to n+1, and the loop calculation continues until the condition n<N is not met, then the loop is jumped out and the iteration ends. Finally, the final phase transfer function can be obtained. Then the phase distribution of the phase element is designed as , we can obtain an array vortex grating at an ideal position that can decode different OAM modes.

[0028] Working step 1: Prepare the base

[0029] First, prepare the substrate and select a suitable substrate material, such as quartz, glass or silicon, cut it into the required size, and clean it to ensure that the substrate surface is clean and flat. Then design the micro-nano structural unit. According to the requirements of orbital angular momentum classification, design a metal nanostructure unit with a specific geometric shape and size. The material can be selected from one of gold, silver and copper. Determine the parameters of the micro-nano structural unit through computer simulation and theoretical analysis.

[0030] Working step 2: Preparation of micro-nano structural units

[0031] Design laser direct writing to prepare micro-nano structural units, use the laser direct writing system to prepare metal nanostructure units according to the designed pattern on the substrate. Adjust the parameters of the laser, such as power, scanning speed, etc., to accurately control the size and shape of the micro-nano structural unit, and then prepare the orbital angular momentum classifier unit. Then, distribute the prepared micro-nano structural units on the substrate in a circular array to form an orbital angular momentum classifier unit, ensuring the uniformity and accuracy of the distribution of the micro-nano structural units. Finally, stack multiple orbital angular momentum classifier units along the optical axis to construct a complete orbital angular momentum classifier. Pay attention to the alignment and connection between the units to ensure the performance and stability of the classifier.

[0032] Working step 3: Designing array vortex grating

[0033] According to the design principle and process of the above-mentioned array vortex grating, the parameters and structure of the array vortex grating are determined through mathematical calculation and simulation analysis to achieve efficient classification of orbital angular momentum.

[0034] Working step 4: Making the phase plate

[0035] Then the phase plate can be prepared. According to the method described above, the phase information of the phase-type diffractive optical element is converted into height information. Using MATLAB software, the height information is designed into a TXT file containing spatial point information, height information and shutter switch information according to the processing trajectory route. According to the principle of light transmission in the medium, the relationship between the height information and the phase information of the optical element is determined. IP-Dip photoresist is selected as the material for preparing the phase plate. The corresponding material refractive index, the refractive index of the surrounding medium, the design wavelength and other parameters are set. According to these parameters, the height corresponding to the phase plate when the phase value is 2π at the maximum, as well as the resolution of the phase element, the laser scanning step length and the overall size of the phase plate are calculated. Then, a femtosecond laser direct writing platform based on the principle of two-photon polymerization is used. The processing method of immersion, point-by-point scanning, and three-dimensional displacement platform working alone is used to prepare the phase plate according to the process flow under the conditions of laser processing speed of about 100μm / s and power of about 10mw. The entire experimental processing process takes about 150min.

[0036] Work step 5: performance testing and evaluation

[0037] The prepared orbital angular momentum classifier is tested for performance. By injecting orbital angular momentum beams with different topological charges into the classifier, detecting its output results, analyzing the performance indicators of the classifier such as classification accuracy, efficiency, and uniformity, and evaluating and optimizing the performance of the classifier based on the test results to meet the needs of practical applications.

[0038] Work step 6: Characterization results and analysis

[0039] Structural characterization: Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to test the structure of the processed phase plate samples. The AFM equipment model is Bruker Dimension Icon from Germany, and the tapping measurement mode is used to extract the morphological features in an air environment. The SEM equipment is ZEISS Sigma 300 from Germany, and the samples need to be gold-plated before testing.

[0040] Result analysis: According to the optical characterization results of SEM, the surface profile of the phase plate sample is consistent with the designed phase map results. AFM was further used to characterize the specific area of ​​the phase plate sample. The results showed that the height results of the characterization were roughly consistent with the theoretical height trend, but there was a certain height gap, which may be due to the measurement error of AFM. Despite this gap, the overall analysis shows that the processing results of the designed phase plate sample are good. In order to ensure the integrity and fault tolerance of the processing phase, the phase plate is added on a 0.8μm thick substrate, and the maximum height tested by AFM is about 1.8μm.

[0041] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A compact orbital angular momentum classifier based on laser direct writing, characterized in that: It comprises a substrate and a plurality of micro-nano structure units prepared on the substrate by laser direct writing, wherein the plurality of micro-nano structure units are distributed in a ring array to form an orbital angular momentum classifier unit; and a plurality of the orbital angular momentum classifier units are stacked along the optical axis direction to form the orbital angular momentum classifier.

2. A compact orbital angular momentum classifier based on laser direct writing according to claim 1, characterized in that: The micro-nano structure unit is a metal nanostructure with a specific geometric shape and size, which produces a specific scattering response to an incident orbital angular momentum light beam.

3. A compact orbital angular momentum classifier based on laser direct writing according to claim 2, characterized in that: The material of the metal nanostructure is gold.

4. The compact orbital angular momentum classifier based on laser direct writing according to claim 1, characterized in that: The substrate material is quartz.

5. The compact orbital angular momentum classifier based on laser direct writing according to claim 1, characterized in that: In the orbital angular momentum classifier, the radius of the annular array, the number of micro-nano structure units, and the number of stacked orbital angular momentum classifier units can be adjusted according to actual needs.

6. The compact orbital angular momentum classifier based on laser direct writing according to claim 1, characterized in that: The parameters of the array vortex grating are: resolution size is 256×256, pixel size is 500nm, wavelength is , n = 8 corresponds to l = -16, -12, -8, -4, 4, 8, 12, 16 respectively.

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