A method for measuring the numerical aperture of an optical system based on grating shearing interferometry

Abstract

A method for measuring the numerical aperture of an optical system based on grating shearing interferometer, the grating shearing interferometer system used in this method includes: an optical and illumination system, an optical system to be measured, an object plane diffraction grating plate, an image plane diffraction grating plate, a two-dimensional photoelectric sensor and a computing and processing unit. The object plane diffraction grating plate and the image plane diffraction grating plate are respectively placed on the object plane and the image plane of the optical system to be measured. During the measurement, the differential wavefront at two or more axial positions is measured by axial scanning and the tilt term coefficient is obtained by fitting. The difference in the tilt term coefficient of the differential wavefront at adjacent positions and the axial distance of adjacent positions are used to directly calculate the numerical aperture of the system to be measured. This method does not require the production of additional gratings. On the basis of wave aberration measurement, it only needs to add a step of shear phase measurement to obtain the measurement of the numerical aperture of the optical system to be measured. It has the advantages of simple measurement process and convenient operation.

Description

Technical Field

[0001] The present invention relates to the field of optical measurement technology, and in particular to a method for measuring the numerical aperture (NA) of an optical system based on grating shearing interferometer, which is suitable for measuring the numerical aperture of a lithography machine projection objective or other optical imaging systems based on a grating shearing interferometer. Background Art

[0002] Ronchi grating shearing interferometer is a shearing interferometer that uses an extended light source and a grating on the object plane to modulate the coherence of the light source. It has the advantages of common optical path, large dynamic range, no need for a separate ideal reference wavefront, high precision, simple structure, etc. For example, prior art 1 (Lu Yunjun, Tang Feng, Wang Xiangchao, Grating Shearing Interferometer Optical Imaging System Wave Aberration Detection Method, Chinese Invention Patent, Patent No.: 109900201B) proposed a differential wavefront extraction algorithm for Ronchi shearing interferometer, which can process the phase-shifted interference pattern to obtain the differential wavefront, and on this basis, use the wavefront reconstruction to obtain the wave aberration of the measured system.

[0003] As an important parameter of the optical system, the numerical aperture (NA) determines the imaging resolution of the optical system. When using a dual-grating Ronchi shearing interferometer to measure the wavefront aberration of the optical system to be measured, the NA also needs to be accurately calibrated in advance. In addition, for the development of a high-NA shearing interferometer system, it is also necessary to calibrate the NA to establish a non-uniformly distributed shear model to achieve high-precision measurement of wavefront aberrations.

[0004] Traditional NA measurement methods require calibration of focal length and exit pupil diameter. Although there are many methods for measuring focal length, exit pupil diameter cannot be directly measured in many cases. A method for measuring f-number based on Ronchi shearing interferometer proposed in prior art 2 (Sukmock Lee, Direct determination off-number by using Ronchi test, Optics Express, 17 (7), 5107-5111) can establish the relationship between the defocus term and f in the reconstructed wavefront through wave aberrations at two different positions. The numerical aperture of the object lens to be measured can be obtained by conversion from the f-number. At that time, the method needed to measure the shear phase in the x-direction and y-direction at the same time, and then reconstruct the wavefront to obtain the reconstructed wavefront. The test process and data processing were relatively complicated, and the x-direction and y-direction gratings needed to be switched multiple times during the measurement process, which easily introduced errors in the wavefront reconstruction process, reducing the accuracy of NA calibration.

[0005] Currently, there is no numerical aperture detection method with simple measurement process and high precision based on grating shearing interferometer. Summary of the invention

[0006] The purpose of the present invention is to overcome the shortcomings of the above-mentioned prior art and propose a method for measuring the numerical aperture of an optical system with a simpler process and higher measurement accuracy. By measuring the differential wavefront in the x or y direction at two different axial positions, the NA of the optical system to be measured is directly calculated through the difference in the coefficients of the tilt term Z2 (or Z3) of the differential wavefront. This method does not require wavefront reconstruction and only requires measuring the differential wavefront in the x or y direction at two different positions, thereby simplifying the measurement process and data processing, improving the measurement efficiency and measurement accuracy, and the method is easily integrated into a Ronchi shearing interferometer system.

[0007] In order to achieve the above object, the technical solution of the present invention is as follows:

[0008] A method for measuring the numerical aperture of an optical system based on grating shearing interference, wherein the grating shearing interferometer used in the method comprises: a light source and an illumination system, an object plane diffraction grating plate, a first three-dimensional displacement stage, an image plane diffraction grating plate, a second three-dimensional displacement stage, a two-dimensional photoelectric sensor and a computing processing unit, wherein the light source and the illumination system output spatially incoherent light, the object plane diffraction grating plate is fixed on the first three-dimensional displacement stage, the image plane diffraction grating plate is fixed on the second three-dimensional displacement stage, the image plane diffraction grating plate is fixed on the second three-dimensional displacement stage, and the object plane diffraction grating plate comprises two groups of one-dimensional gratings with perpendicular grating directions. The image plane diffraction grating plate includes a group of chessboard gratings or two groups of one-dimensional gratings with perpendicular grating lines. The output end of the two-dimensional photoelectric sensor is connected to the calculation processing unit, and an xyz coordinate system is established. The z-axis direction is along the direction of the system optical axis, the x-axis is along the grating line direction of the second grating on the object plane diffraction grating plate, and the y-axis is along the grating line direction of the first grating on the object plane diffraction grating plate. The motion axes of the first three-dimensional displacement stage and the second three-dimensional displacement stage are set to be the x-axis, the y-axis and the z-axis respectively; the angle between the diagonal direction of the chessboard grating and the x-axis (or the y-axis) is 45 degrees; the method is characterized in that the steps are as follows:

[0009] (1) placing the optical system to be measured in the grating shearing interferometer, so that the light source and the illumination system are located on the object side of the optical system to be measured, and the image plane diffraction grating plate is located on the image side of the optical system to be measured, adjusting the first three-dimensional translation stage so that the object plane diffraction grating plate is located on the object plane of the optical system to be measured, and adjusting the second three-dimensional translation stage so that the image plane diffraction grating plate is located on the image plane of the optical system to be measured;

[0010] (2) Adjust the first three-dimensional translation stage so that the second grating or the first grating on the object plane diffraction grating plate enters the field of view of the optical system to be measured, and adjust the second three-dimensional translation stage so that the chessboard grating or the one-dimensional grating in the corresponding direction on the image plane diffraction grating plate enters the field of view and is conjugate with the position of the second grating on the object plane diffraction grating plate. Record the position of the image plane grating at this time, which is recorded as P1;

[0011] (3) Taking P1 as the starting point, define N positions (N>=2) scanned by the second 3D translation stage along the z direction as the axial scanning position, denoted as P i , where i = 1, 2, 3 ... N;

[0012] (4) If the second grating 102 enters the field of view of the optical system 3 to be measured, the prior art 1 is used to obtain a series of shearing interference patterns in the x-axis direction by phase shifting the grating on the object plane or the image plane, and the differential wavefront in the x-axis direction is measured. right Perform Zernike fitting and extract the Z2 coefficient c 2,i , where i = 1, 2, 3, ... N;

[0013] If the first grating 101 enters the field of view of the optical system 3 to be measured, the prior art 1 is used to obtain a series of shearing interference patterns in the y-axis direction by phase shifting the grating on the object plane or the image plane, and the differential wavefront in the y-axis direction is measured. right Perform Zernike fitting and extract the Z3 coefficient c 3,i , where i = 1, 2, 3, ... N;

[0014] (5) If the current z-position P of the second three-dimensional translation stage i P N , then proceed to step 6), otherwise the second three-dimensional translation stage moves to the next position P i+1 , so that P i =P i+1 , return to step 4);

[0015] (6) If the second grating 102 enters the field of view of the optical system 3 to be measured, the Z2 coefficient c 2,i The Z2 coefficient c of the previous position 2,i-1 Subtract and get the difference Δc of N-1 Z2 coefficients 2,i , position P i With the previous position P i-1 Subtract and get the corresponding axial distance ΔZ i , where i = 2, 3 ... N; the numerical aperture NA of N-1 groups of optical systems to be tested is calculated according to formula (1):

[0016]

[0017] If the first grating 101 enters the field of view of the optical system 3 to be measured, the Z3 coefficient c 3,i The Z3 coefficient c of the previous position 3,i-1 Subtract and get the difference Δc of N-1 Z3 coefficients 3,i , position Pi With the previous position P i-1 Subtract and get the corresponding axial distance ΔZ i , where i = 2, 3 ... N. At this time, the numerical aperture NA of the N-1 groups of optical systems to be tested (3) is calculated according to formula (2):

[0018]

[0019] (7) The final numerical aperture of the optical system (3) to be tested is the average of these N-1 groups of NA.

[0020] In the method for measuring the numerical aperture of an optical system based on grating shearing interference, the ratio of the period of the one-dimensional grating on the object plane diffraction grating plate to the period of the chessboard grating or one-dimensional grating on the image plane diffraction grating plate is equal to the magnification of the optical system to be measured.

[0021] In the method for measuring the numerical aperture of an optical system based on grating shearing interference, the grating duty ratios on the object plane diffraction grating plate and the image plane diffraction grating plate are both 50%.

[0022] The technical effect of the present invention is that, based on a grating shearing interferometer, the differential wavefront at two or more axial positions is measured by an axial scanning method, and the tilt term of the differential wavefront (Z2 of the differential wavefront in the x direction or Z3 of the differential wavefront in the y direction) coefficient is fitted, and the NA of the optical system to be measured is directly calculated by using the distance between two adjacent positions and the difference in the tilt term coefficient of the differential wavefront. The method does not require the production of an additional grating, and the structure of the shearing interferometer does not need to be changed. It only needs to add a shear phase measurement, and has the characteristics of simple measurement process and simple output processing. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 Schematic diagram of a device for measuring the numerical aperture of an optical system based on grating shearing interferometry;

[0024] Figure 2 is a schematic diagram of the object plane diffraction grating plate;

[0025] Figure 3 It is a schematic diagram of the image plane diffraction grating chessboard grating;

[0026] Figure 4 Schematic diagram of grating defocus optical path difference;

[0027] Figure 5 Schematic diagram of the Z2 coefficient of the differential wavefront in the x-direction, the Z3 coefficient of the differential wavefront in the y-direction and the z-direction position of the grating;

[0028] Among them, 1. object plane diffraction grating plate; 2. first three-dimensional translation stage; 3. optical system to be measured; 4. image plane diffraction grating plate; 5. second three-dimensional translation stage; 6. two-dimensional photoelectric sensor; 7. computing and processing unit; 8. light source and lighting system. DETAILED DESCRIPTION

[0029] In order to better understand the purpose, technical solutions and advantages of the present invention, the present invention is further described below in conjunction with the accompanying drawings and embodiments, but the protection scope of the present invention should not be limited thereto.

[0030] The present invention discloses a method for measuring the numerical aperture of an optical system based on grating shearing interferometer. The grating shearing interferometer used in the method is as follows: Figure 1 As shown, the system comprises: a light source and illumination system 8, an object plane diffraction grating plate 1, a first three-dimensional displacement stage 2, an image plane diffraction grating plate 4, a second three-dimensional displacement stage 5, a two-dimensional photoelectric sensor 6 and a calculation processing unit 7, wherein the light source and illumination system 8 outputs spatially incoherent light, the object plane diffraction grating plate 1 is fixed on the first three-dimensional displacement stage 2, the image plane diffraction grating plate 4 is fixed on the second three-dimensional displacement stage 5, and the output end of the two-dimensional photoelectric sensor 6 is connected to the calculation processing unit 7;

[0031] Establish an xyz coordinate system, where the z-axis direction is along the optical axis direction of the shearing interferometer, the x-axis is along the grating line direction of the second grating 102 on the object plane diffraction grating plate 1, and the y-axis is along the grating line direction of the first grating 101 on the object plane diffraction grating plate 1, and the motion axes of the first three-dimensional translation stage 2 and the second three-dimensional translation stage 5 are the x-axis, y-axis and z-axis respectively;

[0032] The first three-dimensional translation stage 2 is used to move the first grating 101 and the second grating 102 in the object plane diffraction grating plate 1 to the center of the object plane field of view of the optical system 3 to be measured;

[0033] The second three-dimensional translation stage 5 is used to move the chessboard grating in the image plane diffraction grating plate 4 to the center of the image plane field of view of the optical system 3 to be measured, and to perform specific periodic movements on the image plane diffraction grating plate 4 in the x-axis direction and the y-axis direction;

[0034] The two-dimensional photoelectric sensor 6 may be a charge coupled device CCD or a CMOS image sensor, and the detection surface receives the shearing interference fringes generated by the chessboard grating diffraction;

[0035] The computing and processing unit 7 is used to collect and store interference patterns, and process and analyze the interference patterns;

[0036] Figure 2is a schematic diagram of the object plane diffraction grating plate 1, comprising two one-dimensional diffraction gratings, namely a first grating 101 with grating lines along the y-axis direction and a second grating 102 with grating lines along the x-axis direction, the period of the one-dimensional diffraction grating is P1, and the duty cycle is 50%;

[0037] The first grating 101 and the second grating 102 are phase gratings or amplitude gratings;

[0038] Figure 3 Schematic diagram of the image plane diffraction grating plate 4 chessboard grating, which is a chessboard grating with a period of P2 and a duty cycle of 50%; the chessboard grating is composed of square grids, and the diagonal direction of the square is along the x-axis direction or the y-axis direction;

[0039] The period P1 of the one-dimensional grating and the period P2 of the two-dimensional grating satisfy:

[0040] P1=M·P2 (1)

[0041] Wherein, M is the magnification of the optical system 3 to be measured.

[0042] Figure 4 The figure shows the optical path difference at any point M on the CCD when the image plane grating is out of focus. The distance between the image plane grating 4 and the focal plane is ΔZ. Taking the shearing in the x direction as an example, the optical path difference introduced by the +1st order light and the -1st order light due to the defocusing of the image plane grating is:

[0043]

[0044] Where P is the image plane grating period, NA is the numerical aperture of the optical system, and λ is the wavelength. It can be seen that the main component of the optical path difference is the tilt term (the Z2 term in the Zernike polynomial). Similarly, if the shear is in the y direction, the main component of the optical path difference is the tilt term (the Z3 term).

[0045] Figure 5 Shown is a curve showing the relationship between the shear phase tilt term coefficient (the Z2 term coefficient of the shear phase in the x direction or the Z3 term coefficient of the shear phase in the y direction) and the image plane grating defocus distance.

[0046] Embodiment 1:

[0047] The method for measuring the numerical aperture of an optical system based on grating shearing interferometry is characterized in that the method comprises the following steps:

[0048] (1) placing the optical system 3 to be measured in the grating shearing interferometer, so that the light source and the illumination system 8 are located on the object side of the optical system 3 to be measured, and the image plane diffraction grating plate 4 is located on the image side of the optical system 3 to be measured, adjusting the first three-dimensional translation stage 2 so that the object plane diffraction grating plate 1 is located on the object plane of the optical system 3 to be measured, and adjusting the second three-dimensional translation stage 5 so that the image plane diffraction grating plate 4 is located on the image plane of the optical system 3 to be measured;

[0049] (2) Adjust the first three-dimensional translation stage 2 so that the second grating 102 on the object plane diffraction grating plate 1 enters the field of view of the optical system 3 to be measured, and adjust the second three-dimensional translation stage 5 so that the chessboard grating or the one-dimensional grating in the corresponding direction on the image plane diffraction grating plate 4 enters the field of view and is conjugate with the position of the second grating 102 on the object plane diffraction grating plate 1. Record the position of the image plane grating at this time, which is recorded as P1;

[0050] (3) Taking P1 as the starting point, define N positions (N>=2) scanned along the z direction by the second three-dimensional translation stage 5 as axial scanning positions, denoted as P i , where i = 1, 2, 3 ... N;

[0051] (4) Using the prior art 1, a series of shearing interference patterns in the x-axis direction are obtained by phase shifting the object plane or image plane grating, and the differential wavefront in the x-axis direction is measured. right Perform Zernike fitting and extract the Z2 coefficient c 2,i , where i = 1, 2, 3, ... N;

[0052] (5) If the current z-position P of the second three-dimensional translation stage 5 is i P N , then proceed to step 6), otherwise the second three-dimensional displacement stage 5 moves to the next position P i+1 , so that P i =P i+1 , return to step 4);

[0053] (6) The Z2 coefficient c 2,i The Z2 coefficient c of the previous position 2,i-1 Subtract and get the difference Δc of N-1 Z2 coefficients 2,i , position P i With the previous position P i-1 Subtract and get the corresponding axial distance ΔZ i , where i = 2, 3, ... N;

[0054] (7) According to formula (1), the numerical aperture NA of the N-1 groups of optical systems 3 to be tested is calculated as follows:

[0055]

[0056] The final numerical aperture of the optical system 3 to be tested is the average value of these N-1 groups of NA.

[0057] Embodiment 2:

[0058] The method for measuring the numerical aperture of the optical system to be measured using the Ronchi grating shearing interferometer comprises the following steps:

[0059] (1) placing the optical system 3 to be measured in the grating shearing interferometer, so that the light source and the illumination system 8 are located on the object side of the optical system 3 to be measured, and the image plane diffraction grating plate 4 is located on the image side of the optical system 3 to be measured, adjusting the first three-dimensional translation stage 2 so that the object plane diffraction grating plate 1 is located on the object plane of the optical system 3 to be measured, and adjusting the second three-dimensional translation stage 5 so that the image plane diffraction grating plate 4 is located on the image plane of the optical system 3 to be measured;

[0060] (2) Adjust the first three-dimensional translation stage 2 so that the first grating 101 on the object plane diffraction grating plate 1 enters the field of view of the optical system 3 to be measured, and adjust the second three-dimensional translation stage 5 so that the chessboard grating or the one-dimensional grating in the corresponding direction on the image plane diffraction grating plate 4 enters the field of view and is conjugate with the position of the first grating 101 on the object plane diffraction grating plate 1. Record the position of the image plane grating at this time, which is recorded as P1;

[0061] (3) Taking P1 as the starting point, define N positions (N>=2) scanned along the z direction by the second three-dimensional translation stage 5 as axial scanning positions, denoted as P i , where i = 1, 2, 3 ... N;

[0062] (4) Using the prior art 1, a series of shearing interference patterns in the y-axis direction are obtained by phase shifting the object plane or image plane grating, and the differential wavefront in the y-axis direction is measured. right Perform Zernike fitting and extract the Z3 coefficient c 3,i , where i = 1, 2, 3, ... N;

[0063] (5) If the current z-position P of the second three-dimensional translation stage 5 is i P N , then proceed to step 6), otherwise the second three-dimensional displacement stage 5 moves to the next position P i+1 , so that P i =P i+1 , return to step 4);

[0064] (6) The Z3 coefficient c 3,i The Z3 coefficient c of the previous position 3,i-1 Subtract and get the difference Δc of N-1 Z3 coefficients 3,i , position P i With the previous position P i-1Subtract and get the corresponding axial distance ΔZ i , where i = 2, 3, ... N;

[0065] (7) According to formula (1), the numerical aperture NA of the N-1 groups of optical systems 3 to be tested is calculated as follows:

[0066]

[0067] The final numerical aperture of the optical system 3 to be tested is the numerical aperture of these N-1 groups of NA i The mean of .

[0068] The present invention proposes a method for measuring the numerical aperture of an optical system based on grating shearing interferometry, which measures the differential wavefront at two or more axial positions and fits the tilt term of the differential wavefront, and combines the difference of the tilt term coefficient and the distance between adjacent positions to calculate the numerical aperture of the objective lens to be measured. The method does not require the production of an additional grating, and only needs to add a step of shearing phase measurement on the basis of wave aberration measurement to obtain the measurement of the numerical aperture of the optical system to be measured, and has the advantages of simple measurement process and convenient operation.

Claims

1. A method for measuring the numerical aperture of an optical system based on grating shearing interferometry, wherein the grating shearing interferometer used in the method comprises: a light source and illumination system (8), an object plane diffraction grating plate (1), a first three-dimensional displacement stage (2), an image plane diffraction grating plate (4), a second three-dimensional displacement stage (5), a two-dimensional photoelectric sensor (6) and a computing processing unit (7), wherein the light source and illumination system (8) outputs spatially incoherent light, the object plane diffraction grating plate (1) is fixed on the first three-dimensional displacement stage (2), the image plane diffraction grating plate (4) is fixed on the second three-dimensional displacement stage (5), and the object plane diffraction grating plate (1) comprises two sets of light A one-dimensional grating with grating lines perpendicular to each other, the image plane diffraction grating plate (4) comprises a group of chessboard gratings or two groups of one-dimensional gratings with grating lines perpendicular to each other, the output end of the two-dimensional photoelectric sensor (6) is connected to a calculation processing unit (7), an xyz coordinate system is established, the z-axis direction is along the optical axis direction of the grating shearing interferometer, the x-axis is along the grating line direction of the second grating (102) on the object plane diffraction grating plate (1), and the y-axis is along the grating line direction of the first grating (101) on the object plane diffraction grating plate (1), and the movement axes of the first three-dimensional displacement stage (2) and the second three-dimensional displacement stage (5) are the x-axis, the y-axis and the z-axis respectively, characterized in that: The steps of this method are as follows: Step 1) placing the optical system to be measured (3) in the grating shearing interferometer, so that the light source and the illumination system (8) are located on the object side of the optical system to be measured (3), and the image plane diffraction grating plate (4) is located on the image side of the optical system to be measured (3), adjusting the first three-dimensional displacement stage (2) so that the object plane diffraction grating plate (1) is located on the object plane of the optical system to be measured (3), and adjusting the second three-dimensional displacement stage (5) so that the image plane diffraction grating plate (4) is located on the image plane of the optical system to be measured (3); Step 2) adjusting the first three-dimensional displacement stage (2) so that the second grating (102) or the first grating (101) on the object plane diffraction grating plate (1) enters the field of view of the optical system (3) to be measured, and adjusting the second three-dimensional displacement stage (5) so that the chessboard grating or the one-dimensional grating in the corresponding direction on the image plane diffraction grating plate (4) enters the field of view and is conjugate with the position of the second grating (102) on the object plane diffraction grating plate (1), and recording the position of the image plane grating at this time, which is recorded as P1; Step 3) Taking P1 as the starting point, define N positions of the second three-dimensional translation stage (5) along the z direction scanning, N>=2, as the axial scanning position, denoted as P i , where i = 1, 2, 3 ... N; Step 4) If the second grating (102) enters the field of view of the optical system (3) to be measured, a series of shearing interference patterns in the x-axis direction are obtained by phase shifting the object plane or image plane grating, and the differential wavefront in the x-axis direction is measured. right Perform Zernike fitting and extract the Z2 coefficient c 2,i , where i = 1, 2, 3, ... N; If the first grating (101) enters the field of view of the optical system (3) to be measured, a series of shearing interference patterns in the y-axis direction are obtained by phase shifting the grating on the object plane or the image plane, and the differential wavefront in the y-axis direction is measured. right Perform Zernike fitting and extract the Z3 coefficient c 3,i , where i = 1, 2, 3, ... N; Step 5) If the current z-position P of the second three-dimensional translation stage (5) is i P N , then proceed to step 6), otherwise the second three-dimensional displacement stage (5) moves to the next position P i+1 , so that P i =P i+1 , return to step 4); Step 6) If the second grating (102) enters the field of view of the optical system (3) to be measured, the Z2 coefficient c 2,i The Z2 coefficient c of the previous position 2,i-1 Subtract and get the difference Δc of N-1 Z2 coefficients 2,i , position P i With the previous position P i-1 Subtract and get the corresponding axial distance ΔZ i , where i=2, 3...N; at this time, the numerical aperture NA of the N-1 groups of optical systems to be measured (3) is calculated according to formula (1): Wherein, P is the image plane grating period, NA is the numerical aperture of the optical system, and λ is the wavelength. If the first grating (101) enters the field of view of the optical system (3) to be measured, the Z3 coefficient c 3,i The Z3 coefficient c of the previous position 3,i-1 Subtract and get the difference Δc of N-1 Z3 coefficients 3,i , position P i With the previous position P i-1 Subtract and get the corresponding axial distance ΔZ i , where i = 2, 3 ... N. At this time, the numerical aperture NA of the N-1 groups of optical systems to be tested (3) is calculated according to formula (2): Step 7) The final numerical aperture of the optical system (3) to be tested is the average of the N-1 groups of NA.

2. The method for measuring the numerical aperture of an optical system based on grating shearing interferometry according to claim 1, characterized in that: The ratio of the period of the one-dimensional grating on the object plane diffraction grating plate (1) to the period of the chessboard grating or the one-dimensional grating on the image plane diffraction grating plate (4) is equal to the magnification of the optical system (3) to be measured.

3. The method for measuring the numerical aperture of an optical system based on grating shearing interferometry according to claim 1, characterized in that: The duty ratio of the object plane diffraction grating and the image plane diffraction grating is 50%.

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