Absolute inspection method for meter-level large-aperture horizontal plane interferometer

Abstract

The present invention discloses an absolute inspection method for a meter-level large-aperture horizontal plane interferometer, which belongs to the field of optical detection, integrates two detection means, and realizes absolute inspection of a meter-level horizontal plane interferometer through the coordinated work of a support mechanism and a lightweight plane reflector. The support mechanism adopts a pair of triangular blocks to realize a V-shaped support mode, and its working surface is tangent to the lightweight plane reflector and perpendicular to the ribs of the lightweight plane reflector. At the same time, the lightweight grid form of the plane reflector with 60° rotational symmetry is utilized to ensure that the reflector is in the best detection state during each rotation measurement. The Rich-Common method is used to compensate for the inaccurate defocused surface shape solved in the rotation and translation absolute inspection method, so as to realize absolute inspection of a meter-level large-aperture horizontal plane interferometer.

Description

Technical Field

[0001] The invention belongs to the field of optical detection, and in particular relates to an absolute inspection method for a meter-level large-caliber (caliber above 1m) horizontal plane interferometer. Background Art

[0002] The demand for large-aperture optical planes in high-power lasers, space telescopes and other fields is increasing, and the manufacturing difficulty is gradually increasing. The traditional optical plane inspection method is mainly based on the principle of relative inspection, that is, comparing the reflection plane with the reference plane, but this method has error accumulation. In order to further improve the detection accuracy, an absolute inspection method is proposed. It separates the reference surface and the test surface through multiple measurements and complex mathematical operations to obtain the absolute surface shape of the test surface. This method not only improves the detection accuracy, but also reduces the processing accuracy requirements of the reference plane in the interference system.

[0003] Among the absolute inspection methods, the liquid plane method, the three-crystal mutual inspection method, and the two-crystal mutual inspection method are common methods. Specifically, they include the classic three-face mutual inspection method, the Zernike polynomial fitting method proposed by Fritz, the odd-even function method, the two-crystal three-face mutual inspection method, the oblique incidence method, the rotation and translation method, etc. For large-aperture horizontal interferometers, the liquid plane method obviously cannot meet the requirements; the three-crystal mutual inspection method, its measurement steps involve large-scale flipping and rotation operations of the optical plane, the detection process is complicated and cumbersome, and due to the weight problem of the large-aperture plane, the rotation operation will affect the stress distribution of the large-aperture plane, resulting in changes in the surface shape, which will affect the measurement results. In addition, the processing and manufacturing of large-aperture planes is difficult, and it is difficult to have two transmission flat crystals; the rotation and translation method in the two-crystal mutual inspection method only requires two planes to achieve absolute inspection, and has high measurement accuracy. It is a more practical absolute inspection method, but it needs to solve several problems, including how to align the flat crystal with the ideal position after rotation, and the surface shape change of the flat crystal itself after rotation.

[0004] In the process of rotation and translation absolute inspection, the translation of the optical plane will inevitably introduce tilt error, which will lead to inaccurate solution of the defocus term in the test result. At present, the methods for eliminating tilt error can be divided into two categories: for small-diameter planes, the liquid level reference method can be used to eliminate the tilt error introduced by translation by taking advantage of the property that the liquid always remains horizontal; for larger-diameter planes, an angle measurement device (autocollimator) or a small interferometer can be used to monitor the tilt angle change of the reflector and compensate for it during calculation. For planes with an aperture of more than meter, if the absolute detection accuracy of the large-diameter plane interferometer is to reach 0.1λ (PV), the error introduced by the translation process cannot exceed 0.05λ (PV), which is equivalent to a tilt angle error of no more than 0.013 arc seconds. The detection accuracy of the autocollimator (such as Taylor Hobson's new ultra-high precision autocollimator) is generally 0.2 arc seconds, which obviously cannot meet the measurement requirements. The detection accuracy of a small-aperture interferometer is generally 0.05λ, which is equivalent to a tilt angle change of 0.065 arc seconds, which is greater than the detection value and cannot meet the measurement requirements. Therefore, for the absolute inspection of meter-level aperture planes, the method of measuring the tilt angle change is no longer applicable. CN112525099A discloses a test device for a large-aperture plane reflector, in which the bearing mechanism of the mirror body not only needs to fix the plane reflector, but also needs to realize the 8-times equal-angle rotation function of the plane reflector. Therefore, the overall structure of the mechanism is complex and the processing difficulty is relatively large. In addition, during the translation detection process, it is difficult to ensure that the tilt angle of the reflector changes slightly within the range of ±0.013". Therefore, while minimizing the large-aperture gravity deformation and support deformation as much as possible, a support method with a simple structure is sought to realize the function of rotation and translation, and at the same time avoid the ultra-high control requirements for the tilt error during the translation process, so that the absolute inspection of the large-aperture horizontal interferometer can be realized in a low-cost and highly operable manner.

[0005] The Richey-Common method is a commonly used detection method for large-aperture plane mirrors. This method requires a spherical mirror with high surface shape accuracy, and the detection method is intuitive and easy to implement. In addition, the processing cost of high-precision spherical mirrors is low, which is very suitable for detecting large-aperture planes. However, during the detection, due to the long interference cavity, the airflow disturbance has a significant impact. Even if hundreds of average measurements are taken, there are still problems such as air temperature gradient that affect the detection results. In order to quickly obtain the measurement results, a dynamic interferometer is used for polarization phase shift measurement. Polarization phase shift uses "space" to exchange "time". Compared with traditional phase shift, the wavefront resolution obtained by the dynamic interferometer is lower, and the measurement accuracy is slightly lower than that of the traditional phase shift interferometer. In addition, the optical path of this method will be tilted at a certain angle, so the full-aperture beam cannot be used. The measurement result is approximately an ellipse, which will cause the spatial resolution in a certain direction to decrease, thereby reducing the measurement accuracy.

[0006] Although the Richey-Common method has the above-mentioned problems, defocus, as a low-frequency surface shape, can be accurately obtained from the measurement results through fitting, effectively avoiding the problem of reduced accuracy caused by low spatial resolution of the measurement. Therefore, incorporating the Richey-Common method into the measurement results of the defocus amount can effectively avoid the impact of the tilt error during the translation process and avoid the use of ultra-high-precision translation mechanisms or measurement equipment. Therefore, this patent proposes to combine the detection results of the Richey-Common method and the rotation-translation method to achieve absolute inspection of meter-level large-aperture horizontal plane interferometers. Summary of the invention

[0007] The purpose of the present invention is to provide an absolute inspection method for a meter-level large-aperture horizontal plane interferometer, so as to solve the absolute inspection problem of the meter-level large-aperture horizontal plane interferometer under the conditions of high measurement accuracy and low cost.

[0008] The technical solution to achieve the purpose of the present invention is: an absolute inspection method for a meter-level large-aperture horizontal plane interferometer, the steps are as follows:

[0009] Step 1: Design a circular lightweight plane reflector.

[0010] Step 2, a meter-level large-aperture Fizeau phase-shifting interferometer, a transmission flat crystal and a lightweight plane reflector are arranged in sequence along the first optical axis, wherein the lightweight plane reflector is arranged on a supporting mechanism, and the supporting mechanism is fixed on a slider of a translation guide rail, and the surface shape information of the lightweight plane reflector after discarding the defocused surface shape is obtained by using a rotational translation absolute inspection method.

[0011] Step 2-1, rotating the lightweight plane reflector 6 times at equal intervals around the center normal direction, recording the measurement results at each rotation angle, and subtracting the measurement result at an angle of 0° from the average value of the 6 measurement results to obtain the rotationally asymmetric surface shape information of the lightweight plane reflector;

[0012] Step 2-2, rotate the lightweight plane reflector to 0°, and then translate the lightweight plane reflector along the x direction. The translation amount is 10% to 20% of the diameter of the lightweight plane reflector, and the measurement results before and after the translation are subtracted to obtain the rotationally symmetric surface information of the lightweight plane reflector.

[0013] Step 2-3: combining the rotationally asymmetric surface shape and the rotationally symmetric surface shape information of the lightweight plane reflector to obtain the surface shape information of the lightweight plane reflector after discarding the defocused surface shape.

[0014] Step 3: Use the Richey-Common method to obtain the defocused surface information of the lightweight plane reflector:

[0015] Step 3-1: Build the detection optical path and measure the system error of the detection optical path:

[0016] The detection optical path comprises a dynamic interferometer, an interferometer spherical lens and a standard spherical reflector which are sequentially arranged along a second optical axis.

[0017] Step 3-2, add a lightweight plane mirror between the interferometer spherical lens and the standard spherical mirror of the detection optical path, rotate the lightweight plane mirror to change the Richey angle θ between the second optical axis and the lightweight plane mirror, and measure the wave aberration of the pupil surface at two different Richey angles.

[0018] Step 3-3, respectively subtract the systematic error of the detection optical path from the wave aberration of the pupil surface at two different Richey angles, and obtain two corresponding wave aberration results with the systematic error eliminated. Then, the two wave aberration results are processed by the coordinate and amplitude conversion method to obtain the surface shape results of two lightweight plane mirrors with adjustment errors. Finally, the two surface shape results with adjustment errors are subjected to error separation to obtain the defocused surface shape information of the lightweight plane mirror.

[0019] Step 4, using the surface shape information of the lightweight plane reflector after discarding the defocused surface shape obtained in step 2 and the defocused surface shape information of the lightweight plane reflector obtained in step 3, the absolute surface shape of the lightweight plane reflector is obtained, and finally the absolute inspection of the meter-level large-aperture horizontal plane interferometer is realized.

[0020] Compared with the prior art, the present invention has the following significant advantages:

[0021] (1) In order to solve the problem of inaccurate solution of the defocused item surface shape by the two-plane rotation and translation method, the Richey-Common method is used to compensate for the surface shape information. Although the Richey-Common method has the problem of insufficient spatial resolution, since the defocused item belongs to low-frequency surface shape information, it can be extracted by fitting the detection results. Therefore, it can be considered that the accuracy of the extracted defocused item surface shape is not affected by the spatial resolution. Through this method, the absolute inspection accuracy of the surface shape can reach PV value 1 / 100 wavelength.

[0022] (2) Due to the difficulty and high cost of machining high-precision large-diameter planes, compared with the three-plane mutual inspection method, the combination of the two-plane rotation and translation method and the Richey-Common method requires fewer planes and has lower inspection costs.

[0023] (3) The plane reflector body is light in weight and has little shape change under the action of gravity. In addition, the mirror body adopts a lightweight structure with 60° rotational symmetry. During detection, a V-shaped support method is used. Its contact surface is perpendicular to the ribs of the lightweight structure, and the force is reasonable, which effectively reduces the surface shape change caused by the plane's own gravity and external support, and the error does not exceed 2nm. The V-shaped support method is implemented by a pair of triangular blocks, which does not require the design and processing of additional mechanical devices for the lightweight reflector, reducing the detection cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Schematic diagram of the V-block support mechanism and lightweight design of the plane reflector.

[0025] Figure 2 Schematic diagram of lightweight plane mirror.

[0026] Figure 3 Schematic diagram of the optical path for rotation and translation detection.

[0027] Figure 4 Schematic diagram of rotation and translation detection steps.

[0028] Figure 5 Schematic diagram of the Richey-Common cavity optical path.

[0029] Figure 6 Schematic diagram of the Richey-Common detection optical path.

[0030] Figure 7 The present invention is a flow chart of the absolute inspection method for a meter-level large-aperture horizontal plane interferometer.

[0031] In the figure: 1-lightweight plane reflector; 2-support mechanism; 3-triangular grid form; 4-contact surface; 5-circular grid form; 6-transmission flat crystal; 7-dynamic interferometer; 8-interferometer spherical lens; 9-standard spherical reflector; 10-Fizeau type phase shifting interferometer. DETAILED DESCRIPTION

[0032] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below in conjunction with the accompanying drawings.

[0033] Combination Figure 1 to Figure 7 , an absolute inspection method for meter-level large-aperture horizontal plane interferometer, the steps are as follows:

[0034] Step 1: Design a circular lightweight plane reflector 1:

[0035] The material of the lightweight plane reflector 1 is silicon carbide or microcrystal. Its back needs to be processed according to the lightweight grid, the silicon carbide material adopts the triangular grid form 3, and the microcrystal material adopts the circular grid form 5; the lightweight solution of the lightweight plane reflector 1 made of silicon carbide material, the mirror thickness is controlled at 4-6mm, the mirror body thickness is controlled at 115-125mm, the rib thickness is controlled at 4-5mm, and the triangular grid side length is controlled at 75-90mm, can obtain a lightweight plane reflector 1 with relatively small weight and gravity deformation.

[0036] Step 2: A meter-level large-aperture Fizeau phase-shifting interferometer 10 (aperture of 1m or more), a transmission flat crystal 6 (meter-level), and a lightweight plane reflector 1 are sequentially arranged on the common first optical axis, wherein the lightweight plane reflector 1 is arranged on a support mechanism 2, and the support mechanism 2 is fixed on a slider of a translation guide rail. The surface shape information of the lightweight plane reflector 1 can be obtained by using the rotation and translation absolute inspection method. However, due to the tilt error generated by translating the lightweight plane reflector 1 during the detection process, this will make the defocus item surface shape solution inaccurate. Therefore, the defocus item in the surface shape information of the lightweight plane reflector 1 needs to be discarded, as follows:

[0037] Step 2-1, perform 6 equally spaced rotations around the center normal direction of the lightweight plane reflector 1, record the measurement results at each rotation angle, and subtract the measurement result at an angle of 0° from the average value of the 6 measurement results to obtain the rotationally asymmetric surface information of the lightweight plane reflector 1.

[0038] Step 2-2, rotate the lightweight plane reflector 1 to an angle of 0°, and then translate the lightweight plane reflector 1 along the x direction (i.e., perpendicular to the paper surface), and the translation amount is 10% to 20% of the diameter of the lightweight plane reflector 1. The measurement results before and after the translation are subtracted to obtain the rotationally symmetric surface shape information of the lightweight plane reflector 1.

[0039] Step 2-3: combining the rotationally asymmetric surface shape and the rotationally symmetric surface shape information of the lightweight plane reflector 1 to obtain the surface shape information of the lightweight plane reflector 1 after discarding the defocused surface shape.

[0040] Among them, the support mechanism 2 uses a pair of triangular blocks to realize the V-shaped support mode. The working surfaces of the two triangular blocks are tangent to the two contact surfaces 4 of the lightweight plane reflector. The normal angle of the working surfaces of the two triangular blocks is 60°, and they are respectively perpendicular to the ribs of the lightweight plane reflector 1. The lightweight grid distribution of the lightweight plane reflector 1 is consistent before and after 6 equal-interval 60° rotations, and each rotation can make the ribs of the lightweight plane reflector 1 perpendicular to the working surface of the support mechanism 2, so that the lightweight plane reflector 1 is located at its optimal detection position every time it rotates. The translation guide rail is laid along the x direction.

[0041] Step 3: Using the Richey-Common method, obtain the defocused surface information of the lightweight plane reflector 1, as follows:

[0042] Step 3-1: Build the detection optical path and measure the system error of the detection optical path:

[0043] The detection optical path comprises a dynamic interferometer 7, an interferometer spherical lens 8 and a standard spherical reflector 9 which are sequentially arranged along the second optical axis.

[0044] Step 3-2, add the lightweight plane mirror 1 between the interferometer spherical lens 8 and the standard spherical mirror 9 of the detection optical path, rotate the lightweight plane mirror 1, change the Richey angle θ between the second optical axis and the lightweight plane mirror 1 (that is, the angle between the second optical axis and the center normal of the lightweight plane mirror 1), and measure the wave aberration of the pupil surface at two different Richey angles.

[0045] Step 3-3, respectively subtract the systematic error of the detection optical path from the wave aberration of the pupil plane at two different Richey angles, and obtain two corresponding wave aberration results with the systematic error eliminated. Then, the two wave aberration results are processed by the coordinate and amplitude conversion method to obtain the surface shape detection results of two lightweight plane reflectors 1 with adjustment errors. Finally, the two surface shape results with adjustment errors are separated to obtain the defocused surface shape information of the lightweight plane reflector 1.

[0046] Among them, as the Richey angle increases, the shape of the wave aberration will be severely compressed. When the Richey angle is too large, it will have a certain impact on the accuracy of the measured wave aberration, but when the Richey angle is too small, the detection light in the optical path will be blocked. In order to obtain more accurate detection results and ensure that the light in the optical path does not overlap, the selection range of the Richey angle is between 20° and 50°.

[0047] Step 4, using the surface shape information of the lightweight plane reflector 1 after discarding the defocused surface shape obtained in step 2 and the defocused surface shape information of the lightweight plane reflector 1 obtained in step 3, the absolute surface shape of the lightweight plane reflector 1 is obtained, and finally the absolute inspection of the meter-level large-aperture horizontal plane interferometer is realized.

Claims

1. An absolute inspection method for meter-level large-aperture horizontal plane interferometer, characterized in that: Here are the steps: Step 1, designing a circular lightweight plane reflector (1); Step 2: A meter-class large-aperture Fizeau phase-shifting interferometer (10), a transmission flat crystal (6), and a lightweight plane reflector (1) are sequentially arranged along a common first optical axis, wherein the lightweight plane reflector (1) is arranged on a support mechanism (2), and the support mechanism (2) is fixed on a slider of a translation guide rail, and a rotation-translation absolute inspection method is used to obtain surface shape information of the lightweight plane reflector (1) after discarding the defocused surface shape; Step 2-1, rotating the lightweight plane reflector (1) six times at equal intervals around the center normal direction, recording the measurement result at each rotation angle, and subtracting the measurement result at an angle of 0° from the average value of the six measurement results to obtain the rotationally asymmetric surface shape information of the lightweight plane reflector (1); Step 2-2, rotating the lightweight plane reflector (1) to an angle of 0°, and then translating the lightweight plane reflector (1) along the x direction, the translation amount being 10% to 20% of the diameter of the lightweight plane reflector (1), and subtracting the measurement results before and after the translation to obtain the rotationally symmetric surface shape information of the lightweight plane reflector (1); Step 2-3, combining the rotationally asymmetric surface shape and the rotationally symmetric surface shape information of the lightweight plane reflector (1), to obtain the surface shape information of the lightweight plane reflector (1) after discarding the defocused surface shape; Step 3: Using the Richey-Common method, obtain the defocused surface information of the lightweight plane reflector (1): Step 3-1: Build the detection optical path and measure the system error of the detection optical path: The detection optical path comprises a dynamic interferometer (7), an interferometer spherical lens (8) and a standard spherical reflector (9) which are arranged in sequence along a common second optical axis; Step 3-2, adding the lightweight plane reflector (1) between the interferometer spherical lens (8) and the standard spherical reflector (9) of the detection optical path, rotating the lightweight plane reflector (1) to change the Richey angle θ between the second optical axis and the lightweight plane reflector (1), and measuring the wave aberration of the pupil surface at two different Richey angles; Step 3-3, respectively subtracting the systematic error of the detection optical path from the wave aberration of the pupil plane at two different Richey angles, and obtaining two wave aberration results with the systematic error eliminated, and then processing the two wave aberration results using a coordinate and amplitude conversion method to obtain two surface shape results of the lightweight plane reflector (1) with adjustment errors, and finally performing error separation on the two surface shape results with adjustment errors, thereby obtaining the defocused surface shape information of the lightweight plane reflector (1); Step 4, using the surface shape information of the lightweight plane reflector (1) obtained in step 2 after discarding the defocused surface shape and the defocused surface shape information of the lightweight plane reflector (1) obtained in step 3, the absolute surface shape of the lightweight plane reflector (1) is obtained, and finally the absolute inspection of the meter-level large-aperture horizontal plane interferometer is realized; The lightweight plane reflector (1) is made of silicon carbide or microcrystals, and its back is processed according to a lightweight grid. The grid form (3) of silicon carbide is triangular, and the grid form (5) of microcrystals is circular. The lightweight grid distribution of the lightweight plane reflector (1) is consistent before and after six equal-interval 60° rotations, and each rotation can make the ribs of the lightweight plane reflector (1) perpendicular to the working surface of the support mechanism (2), so that the lightweight plane reflector (1) is located at its optimal detection position each time it rotates.

2. The absolute inspection method for a meter-level large-aperture horizontal plane interferometer according to claim 1 is characterized in that: The processing requirements for the lightweight plane reflector (1) made of silicon carbide material are as follows: the mirror surface thickness is controlled within 4 to 6 mm, the mirror body thickness is controlled within 115 to 125 mm, the rib thickness is controlled within 4 to 5 mm, and the triangular grid side length is controlled within 75 to 90 mm.

3. The absolute inspection method for a meter-level large-aperture horizontal plane interferometer according to claim 1 is characterized in that: The meter-class large-aperture Fizeau-type phase-shifting interferometer (10) has an aperture of more than 1 meter.

4. The absolute inspection method for a meter-level large-aperture horizontal plane interferometer according to claim 1 is characterized in that: The support mechanism (2) uses a pair of triangular blocks to realize a V-shaped support mode, and its working surface is tangent to the lightweight plane reflector (1). The normal angles of the working surfaces of the two triangular blocks are 60 degrees, and they are respectively perpendicular to the ribs of the lightweight plane reflector (1).

5. The absolute inspection method for a meter-level large-aperture horizontal plane interferometer according to claim 1 is characterized in that: The translation guide rail is laid along the x direction.

6. The absolute inspection method for a meter-level large-aperture horizontal plane interferometer according to claim 1, characterized in that: In step 3, the selection range of the Richey angle is between 20° and 50°.

Images