A method and system for measuring the production accuracy of high-density CGH

Abstract

In the field of optical detection technology, the present invention provides a method and system for measuring the manufacturing accuracy of high-line density CGH, including: obtaining the structural parameters of an ideal CGH and the structural parameters of the manufactured CGH; establishing an ideal structural model and a measured structural model of the CGH; discretizing the coordinate axes of the above-mentioned ideal structural model and the measured structure, decomposing them into unit grids, and obtaining the near-field physical field distribution of the diffractive optical element; calculating the far-field wavefront distribution after long-distance propagation based on the Huygens principle, fitting and calculating the RMS number of the far-field wavefront error caused by the manufacturing error of the manufactured CGH, so as to calculate its manufacturing accuracy. The method provided by the present application can more accurately analyze the influence of manufacturing errors on the wavefront accuracy of high-line density CGH and more accurately measure the manufacturing accuracy of CGHs with different parameters, which is conducive to promoting the development of high-steepness and large-deviation aspheric detection technology and processing technology in actual engineering applications.

Description

Technical Field

[0001] The present invention relates to the field of optical measurement technology, and in particular to a method and system for measuring the manufacturing accuracy of a high-line-density CGH. Background Art

[0002] Aspheric surfaces are special surfaces with many unique properties. Using aspheric surfaces in optical systems can greatly increase the degree of freedom of optical design optimization, effectively simplify the optical system, and improve the imaging quality of the system. It is precisely because of these unique advantages that aspheric surfaces are increasingly widely used in various fields, such as high-quality cameras, high-precision optical measuring instruments, large astronomical telescope systems, etc. With the development of optical precision processing technology, the application of special types of aspheric surfaces is becoming more and more intense, such as optical aspheric surfaces with large deviation and high steepness used in infrared reconnaissance and early warning cameras. Generally speaking, the deviation of aspheric surfaces is about tens to hundreds of microns, and from the current development trend, there is still a trend of continued increase. However, sub-nanometer high-precision detection of such high-steepness, large-deviation aspheric surfaces has always been a major problem in the field of optical detection.

[0003] At present, the computer-generated hologram method is generally used to achieve high-precision detection of aspheric surfaces. That is, the standard spherical wave emitted by the interferometer is converted into an ideal aspheric wavefront that matches the aspheric surface using a computer-generated hologram (CGH), thereby achieving zero-position detection. The optical path diagram is shown in Figure 1 As shown. Computational holography is a feasible detection method for high-steepness and large-deviation aspheric surfaces. A problem that needs to be overcome in this application is that as the aspheric surface steepness increases, the spatial frequency of the CGH etched relief structure required for detection will also increase. In the actual processing process, when the line density is higher than 700lp / mm, it is already very difficult to process. As the line density increases, the structural period will also decrease, and the smaller the structure, the more sensitive it is to the introduced error.

[0004] Therefore, the impact of manufacturing errors on high-line-density CGHs cannot be ignored. For high-precision detection technology, the existence of manufacturing errors will directly affect the detection accuracy of the wavefront, so the accurate measurement of the manufacturing accuracy of high-line-density CGHs is of great significance and value for high-precision detection. The current method for measuring the manufacturing accuracy of CGHs is to use the same manufacturing process to make a CGH for detecting random balls, and then the manufacturing accuracy of all components made by this manufacturing process is assumed to be this accuracy value. The problem with this measurement method is that it ignores the uniqueness of CGHs made for different design requirements and design parameters in actual applications. For example, when the aspheric parameters are different, the line density and line width of the CGH are different, and the impact of manufacturing errors will also be different. Therefore, this measurement method can only roughly estimate the manufacturing accuracy and cannot solve the problem of CGH customization. In addition to the above problems, there is also such a problem in the analysis and measurement of the manufacturing errors of high-line-density CGHs. Usually, the error analysis of CGH is based on the linear grating model of scalar diffraction theory. As the line density of CGH increases, the period of the structure may reach a level close to or even smaller than the wavelength of the incident light. Since the scalar diffraction theory ignores the interaction between structures and the polarization response of the structure to the incident light, when the period of the structure is less than about 4λ, there will be a large deviation between the scalar diffraction theory and the calculated value of the strict vector diffraction theory. Therefore, the application of scalar diffraction theory in high-line density CGH has certain limitations.

[0005] Therefore, it is necessary to propose a new technical solution to solve the above-mentioned technical problems. Summary of the invention

[0006] In view of this, the present invention proposes a method and system for measuring the manufacturing accuracy of high-density CGH, which solves the influence of manufacturing errors introduced by the limited manufacturing process technology level on the high-precision detection wavefront, and also solves the limitation of scalar diffraction theory for high-density CGH with high steepness and large deviation, thereby improving the detection accuracy. The method is based on the strict vector diffraction theory and can accurately analyze the wavefront deviation introduced by the manufacturing error, and can also accurately measure and calculate the manufacturing accuracy of different CGHs according to different practical application situations.

[0007] A first aspect of an embodiment of the present invention provides a method for measuring the manufacturing accuracy of a high-density CGH, the method comprising:

[0008] Obtaining the structural parameters of the ideal CGH element and the structural parameters of the manufactured CGH;

[0009] The ideal structure model and the measured structure model of the equivalent grating are established according to the structure parameters of the ideal CGH and the structure parameters of the manufactured CGH respectively;

[0010] Discretize the coordinate axes of the above ideal structure model and the measured structure, divide them into unit grids, establish and solve the Maxwell equations, so as to obtain the near-field physical field distribution of the ideal structure model and the measured structure model of the equivalent grating;

[0011] Specifically, an ideal structure model and a measured structure model are constructed according to the ideal structure parameters and the measured CGH production parameters respectively, the coordinate axes of the model are discretized and decomposed into a unit grid structure, and the propagation process of the electric field and magnetic field vectors of the incident light wave in the structure satisfies the Maxwell equations. The solution of the equations is solved according to the different physical processes of the ideal structure model and the measured structure model and the boundary conditions they meet. Solving the Maxwell equations is actually to find the distribution of the electric field in space after the electric field vector interacts with the structure, so as to obtain the near-field physical field of the diffraction grating structure model.

[0012] Based on the Huygens principle, the far-field wavefront distribution of the ideal structure model and the measured structure model after long-distance propagation is calculated as follows:

[0013]

[0014] The wavefront deviation between the far-field distribution of the measured model and the far-field distribution of the ideal model is fitted and the RMS value is calculated to obtain the production accuracy of the actual CGH.

[0015] Optionally, the step of obtaining the structural parameters of an ideal CGH and the structural parameters of a manufactured CGH includes:

[0016] Obtain the ideal high-density area of ​​the CGH and the minimum structural size of the CGH;

[0017] After the CGH is produced, the structural parameters of the actually produced CGH are obtained.

[0018] Optionally, the structural parameters of the CGH actually manufactured include period, line width, duty cycle, etching groove depth and structural sidewall angle.

[0019] Optionally, the calculating, based on the Huygens principle, the far-field wavefront distribution of the near-field wavefronts of the ideal structure model and the measured structure model after long-distance propagation includes:

[0020] The near-field physical field distribution is decomposed into:

[0021]

[0022] Among them, a x ,a y are the structural periods in the x and y directions, is the electric field vector distribution in the xy plane at the near field distance z0, k x , k y are the direction vectors in the x and y directions respectively, m, n are integers, and i is an imaginary number;

[0023] Assuming that each sub-wave propagates freely to the distant space as a wave source, the far-field wavefront distribution of the near-field wavefront of the equivalent grating model after long-distance propagation is:

[0024]

[0025] Optionally, the coordinate axis discretization of the ideal structure model and the measured structure is performed to divide the ideal structure model and the measured structure into unit grids, and Maxwell equations are established and solved, including:

[0026] Discretize the coordinate axes of the three-dimensional space where the ideal structural model and the measured structural model are located into unit grids;

[0027] After discretization, the vector equation can be transformed into six scalar equations:

[0028]

[0029]

[0030] The partial differential equation in the above formula is differentially discretized by the differential approximation method. After discretizing it in the space domain and time domain, the boundary conditions and excitation conditions are combined to obtain the solution of the above discretized Maxwell equations (that is, the propagation process of the electric field and the magnetic field in the established model structure satisfies the Maxwell equations, which are decomposed into a unit grid structure after coordinate discretization, and the satisfied vector equations can be decomposed into these six scalar equations).

[0031] Optionally, the near-field physical field distribution includes amplitude and phase information of the incident light wave after being modulated by the diffractive optical element.

[0032] Optionally, when establishing the measured structure model, different positions of the manufactured CGH are measured multiple times, and the measurement data that meets the preset conditions are selected as input for establishing the measured structure model.

[0033] A second aspect of an embodiment of the present invention provides a system for measuring the manufacturing accuracy of a high-density CGH, the system comprising:

[0034] A parameter acquisition module, used to acquire the structural parameters of an ideal CGH element and the structural parameters of the manufactured CGH;

[0035] A model building module, used to build an ideal structural model and a measured structural model of CGH according to the structural parameters of the ideal CGH and the structural parameters of the manufactured CGH respectively;

[0036] A solution module is used to discretize the coordinate axes of the above-mentioned ideal structure model and the measured structure, divide them into unit grids, establish and solve the Maxwell equations, so as to obtain the near-field physical field distribution of the ideal structure model and the measured structure model of the equivalent grating;

[0037] It is also used to calculate the far-field wavefront distribution of the near-field wavefront of the ideal structure model and the measured structure model after long-distance propagation based on the Huygens principle:

[0038]

[0039] The wavefront deviation between the far-field distribution of the measured model and the far-field distribution of the ideal model is fitted and the RMS value is calculated to obtain the production accuracy of the actual CGH.

[0040] Optionally, the parameter acquisition module is specifically used to:

[0041] Obtain the ideal high-density area of ​​the CGH and the minimum structural size of the CGH;

[0042] After the CGH is produced, the structural parameters of the actually produced CGH are obtained.

[0043] Optionally, the structural parameters of the CGH actually manufactured include period, line width, duty cycle, etching groove depth and structural sidewall angle.

[0044] The embodiments of the present invention have the following beneficial effects compared with the prior art: the measurement method provided by the present application solves the influence of the manufacturing error introduced by the limited manufacturing process technology level on the high-precision detection wavefront, and also solves the limitation of the scalar diffraction theory for high-steepness, large-deviation high-line-density CGH, thereby improving the detection accuracy. The method is based on the strict vector diffraction theory and can accurately analyze the wavefront deviation introduced by the manufacturing error, and can also accurately measure and calculate the manufacturing accuracy of different CGHs according to different practical application situations. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] In order to more clearly illustrate the technical method of the embodiment of the present invention, the drawings required in the embodiment or the prior art description are briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0046] Figure 1 The optical path diagram of the computer-generated holography detection method provided by the present invention;

[0047] Figure 2A grating structure model with ideal design parameters and a grating structure model with manufacturing errors provided by an embodiment of the present invention;

[0048] Figure 3 A flowchart of the method for measuring the production accuracy of a high-density CGH provided in an embodiment of the present invention;

[0049] Figure 4 A schematic diagram of the structure of a high-line-density CGH production accuracy measurement system provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0050] In the following description, specific details such as specific structures and techniques are provided for the purpose of illustration rather than limitation, so as to provide a thorough understanding of the embodiments provided by the present invention. However, it should be clear to those skilled in the art that the present invention may be implemented in other embodiments without these specific details. In other cases, detailed descriptions of well-known systems, devices, circuits, and methods are omitted to prevent unnecessary details from obstructing the description of the present invention.

[0051] Embodiment 1:

[0052] See also Figure 2-3 The implementation process of the method for measuring the production accuracy of high-density CGH provided in the embodiment of the present application is as follows:

[0053] Determine the high-line density area and minimum structural size of CGH according to design requirements; after making CGH, measure its structural parameters.

[0054] Specifically, the actual structural parameters of the manufactured CGH are measured using a confocal microscope, including period, line width, duty cycle, etching groove depth, and structural sidewall angle. Due to the limitation of processing technology, the relief structure on the surface of the etched component is generally a trapezoidal structure, so it is better to choose the center position when measuring the line width.

[0055] Establishment of theoretical and measured models: Establish an ideal structural model based on the ideal structural parameters, and select representative structural parameters from different positions of multiple measurements as input for establishing the measured model. The ideal model and the manufacturing error model are as follows: Figure 2 As shown in (a) and (b).

[0056] Solve Maxwell's equations based on the finite difference time domain method, including:

[0057] The three-dimensional space where the research target is located is discretized into coordinate axes and divided into unit grids. After discretization, the vector equation can be transformed into six scalar equations:

[0058]

[0059]

[0060] The difference approximation method is then introduced to perform differential discretization on the partial differential equations in (1) and (2). After discretizing them in the space domain and time domain, the solution of the above discretized Maxwell equations can be obtained by combining the boundary conditions and excitation conditions.

[0061] Obtaining the physical field distribution in the near-field area and extracting the amplitude and phase information, including: obtaining the near-field physical field distribution of the diffractive optical element based on the solution of the Maxwell equations, the physical field distribution includes the amplitude and phase information of the incident light wave after being modulated by the diffractive optical element.

[0062] The far-field distribution is calculated based on the Huygens principle, including: According to the Huygens principle, the near-field wavefront can be regarded as the superposition of countless wave sources, so the near-field physics can be decomposed into:

[0063]

[0064] Among them, a x ,a y are the structural periods in the x-direction and y-direction respectively. Then, let each sub-wave propagate freely in the long-distance space as a wave source. The superposition of each sub-wave after propagation in space is the far-field wavefront distribution after the near-field wavefront propagates over a long distance, as shown in (4).

[0065]

[0066] The calculation and measurement of CGH manufacturing accuracy can obtain the far-field wavefront deviation caused by the manufacturing error generated by actual processing according to the above process, and the RMS value of the deviation is mathematically calculated to obtain the manufacturing accuracy of the actual CGH measured. Using this method to accurately calculate and measure the manufacturing accuracy of the actual CGH is of great value in improving the detection accuracy.

[0067] The present invention adopts the finite time domain difference method in the strict vector diffraction theory. At present, the finite time domain difference method has been widely used in the analysis of electromagnetic field propagation and diffraction problems. Its core idea is to discretize the three-dimensional space where the research target is located along the coordinate axis, divide it into small hexahedral unit grids, and use medium parameters ε, μ, σ, etc. to describe the electrical parameters of each unit grid. Then the calculation time is discretized to make it a series of time steps with a certain order. The algebraic equations about the electromagnetic field components can be approximated by the differential formulas of the three space coordinate axes and the time coordinate axis. According to the specific problem, the boundary conditions and excitation conditions are appropriately combined, and iterative operations are performed to solve the Maxwell algebraic equations to obtain the near-field distribution of the time-domain electromagnetic field of the research target, that is, the process and results of the interaction between the electromagnetic wave and the diffraction optical element are calculated as time goes by. The amplitude and phase information can be extracted from the obtained near-field electromagnetic field distribution, and then based on the Huygens principle, any point on the wavefront can be regarded as a new secondary wave source. The secondary wave source propagates a certain distance to a long distance in free space. The envelope formed by the superposition of the secondary waves emitted by these points is the new wavefront of the original wavefront after propagating a certain time and distance, and finally the electromagnetic field distribution of the target in the far field is obtained. Then, the wavefront deviation of the far field caused by the actual manufacturing error can be obtained by combining the finite time-domain difference method with the Huygens principle. If there is an unmarked manufacturing error in the CGH, the error will be reflected in the actual detection result during detection, which will affect the accuracy of the detection. The present invention can more accurately analyze and calculate the wavefront error caused by the manufacturing error, and then more accurately measure the manufacturing accuracy of the CGH, which is convenient for judging the degree of influence of the CGH manufacturing error on the actual detection result. In addition, the method of the present invention also effectively solves the problem of the application limitations of the scalar diffraction theory in high line density conditions, so that the calculated measurement results have higher accuracy. The present invention is of great significance for promoting the development of high-precision detection technology for high-steepness and large-deviation aspheric surfaces in practical engineering applications.

[0068] Embodiment 2:

[0069] Figure 4 The schematic diagram of the structure of the measurement system for high line density CGH production accuracy provided by the present application is shown, and the measurement system includes:

[0070] A parameter acquisition module 41 is used to acquire the structural parameters of an ideal CGH element and the structural parameters of a manufactured CGH;

[0071] A model building module 42, for building an ideal structural model of CGH and a measured structural model of CGH according to the structural parameters of the ideal CGH and the structural parameters of the manufactured CGH respectively;

[0072] A solution module 43 is used to discretize the coordinate axes of the above-mentioned ideal structure model and the measured structure, divide them into unit grids, establish and solve the Maxwell equations to obtain the near-field physical field distribution of the diffractive optical element;

[0073] It is also used to calculate the far-field wavefront distribution of the near-field wavefront of the CGH made based on the Huygens principle after long-distance propagation:

[0074]

[0075] The RMS number is calculated by fitting the wavefront error between the far-field wavefront distribution of the near-field wavefront of the manufactured CGH after long-distance propagation and the ideal wavefront to obtain the manufacturing accuracy of the manufactured CGH.

[0076] It should be understood that when used in this specification and the appended claims, the term "comprising" indicates the presence of described features, integers, steps, operations, elements and / or components, but does not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or groups thereof.

[0077] It should also be understood that the terms used in this specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. As used in the specification of the present invention and the appended claims, unless the context clearly indicates otherwise, the singular forms "a", "an" and "the" are intended to include plural forms.

[0078] It should be further understood that the term "and / or" used in the present description and the appended claims refers to any and all possible combinations of one or more of the associated listed items, and includes these combinations.

[0079] The above embodiments are only used to illustrate the present invention, not to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, a person skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for measuring the production accuracy of high-density CGH, characterized in that: The measuring method comprises: Obtaining the structural parameters of the ideal CGH element and the structural parameters of the manufactured CGH; The ideal structure model and the measured structure model of the equivalent grating are established according to the structure parameters of the ideal CGH and the structure parameters of the manufactured CGH respectively; Discretize the coordinate axes of the above ideal structure model and the measured structure, divide them into unit grids, establish and solve the Maxwell equations, so as to obtain the near-field physical field distribution of the ideal structure model and the measured structure model of the equivalent grating; Based on the Huygens principle, the far-field wavefront distribution of the ideal structure model and the measured structure model after long-distance propagation is calculated as follows: The wavefront deviation between the far-field distribution of the measured model and the far-field distribution of the ideal model is fitted and the RMS value is calculated to obtain the production accuracy of the actual CGH.

2. The measuring method according to claim 1, characterized in that: The step of obtaining the structural parameters of an ideal CGH and the structural parameters of the manufactured CGH includes: Obtain the ideal high-density area of ​​the CGH and the minimum structural size of the CGH; After the CGH is produced, the structural parameters of the actually produced CGH are obtained.

3. The measuring method according to claim 2, characterized in that: The structural parameters of the CGH actually manufactured include period, line width, duty cycle, etching groove depth and structural side wall angle.

4. The measuring method according to claim 1, characterized in that: The method of calculating the far-field wavefront distribution of the near-field wavefronts of the ideal structure model and the measured structure model after long-distance propagation based on the Huygens principle includes: The near-field physical field distribution is decomposed into: Among them, a x ,a y are the structural periods in the x and y directions respectively; Assuming that each sub-wave propagates freely to the distant space as a wave source, the far-field wavefront distribution of the near-field wavefront of the equivalent grating model after long-distance propagation is:

5. The measuring method according to claim 1, characterized in that: The above-mentioned ideal structure model and measured structure are discretized into coordinate axes, divided into unit grids, and Maxwell equations are established and solved, including: Discretize the coordinate axes of the three-dimensional space where the ideal structural model and the measured structural model are located into unit grids; After discretization, the vector equation can be transformed into six scalar equations: The partial differential equation in the above formula is discretized by differential approximation method. After discretizing it in space domain and time domain, the solution of the above discretized Maxwell equations is obtained by combining boundary conditions and excitation conditions.

6. The measuring method according to claim 5, characterized in that: The near-field physical field distribution includes the amplitude and phase information of the incident light wave after being modulated by the diffractive optical element.

7. The measuring method according to any one of claims 1 to 6, characterized in that: When establishing the measured structure model, different positions of the manufactured CGH are measured multiple times, and the measurement data that meet the preset conditions are selected as the input for establishing the measured structure model.

8. A system for measuring the production accuracy of high-density CGH, characterized in that: The measuring system comprises: A parameter acquisition module, used to acquire the structural parameters of an ideal CGH element and the structural parameters of the manufactured CGH; A model building module, used to build an ideal structural model and a measured structural model of CGH according to the structural parameters of the ideal CGH and the structural parameters of the manufactured CGH respectively; A solution module is used to discretize the coordinate axes of the above-mentioned ideal structure model and the measured structure, divide them into unit grids, establish and solve the Maxwell equations, so as to obtain the near-field physical field distribution of the ideal structure model and the measured structure model of the equivalent grating; It is also used to calculate the far-field wavefront distribution of the near-field wavefront of the ideal structure model and the measured structure model after long-distance propagation based on the Huygens principle: The wavefront deviation between the far-field distribution of the measured model and the far-field distribution of the ideal model is fitted and the RMS value is calculated to obtain the production accuracy of the actual CGH.

9. The measuring system according to claim 8, characterized in that The parameter acquisition module is specifically used for: Obtain the ideal high-density area of ​​the CGH and the minimum structural size of the CGH; After the CGH is produced, the structural parameters of the actually produced CGH are obtained.

10. The measuring system according to claim 9, characterized in that The structural parameters of the CGH actually manufactured include period, line width, duty cycle, etching groove depth and structural side wall angle.

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