Low-coherence optical freeform surface topography interferometric scanning measurement device and method

Abstract

The application discloses a low-coherence optical free-form surface topography interference scanning measurement device and method, and belongs to the field of optical free-form surface measurement. The device comprises a low-coherence illumination system for generating a pair of orthogonally polarized light with adjustable optical path difference, an interference generating system for generating a reference light beam and a test light beam, a scanning system for controlling a three-dimensional topography scanning interference measurement of a measured object, and an imaging system for interference fringe imaging reception. The application does not need an aberration compensator, is not affected by an interference return error, has the advantages of high precision, high flexibility, low cost and the like, and can be effectively used for high-precision measurement of optical free-form surface three-dimensional topography.

Description

Technical Field

[0001] This invention belongs to the field of optical interferometric measurement technology, and in particular to a low-coherence optical freeform surface topography interferometric scanning measurement device and method. Background Technology

[0002] Optical freeform surfaces, due to their greater design freedom, can significantly improve the optical performance of systems while enabling compact and lightweight system designs. Therefore, freeform surface components have gradually become key optical devices in major national engineering projects such as defense, astronomical observation, and aerospace. However, there is currently a lack of high-precision, flexible, and low-cost topography measurement solutions for optical freeform surfaces, severely restricting their in-depth application in high-precision fields. Therefore, there is an urgent need to achieve high-precision interferometric measurement of optical freeform surfaces in optical manufacturing.

[0003] The most commonly used high-precision interferometry method for optical freeform surfaces is computational holography (CGH). This method involves designing a computational hologram for the freeform surface under test and adjusting the spherical test wavefront to match the freeform surface wavefront of the test surface to achieve null-position interferometry. While this approach is simple in principle, it has low measurement flexibility; each CGH can only be used for a specific surface shape, and the cost is high. Non-null-position interferometry typically utilizes a non-null-position compensator to compensate for some low-order aberrations on the freeform surface under test, combined with a sub-aperture stitching method to achieve full-aperture measurement. This type of technology requires complex system hysteresis error calibration methods and places high demands on the manufacturing and control of the non-null-position compensator, resulting in a complex system. Summary of the Invention

[0004] The purpose of this invention is to address the problems existing in the current technology by providing a low-coherence optical freeform surface topography interferometric scanning measurement device and method that does not require an aberration compensator, is unaffected by interference backlash error, and has advantages such as high precision, high flexibility, and low cost.

[0005] The technical solution for achieving the objective of this invention is as follows: On one hand, a low-coherence optical freeform surface topography interferometric scanning measurement device is provided, the device comprising a low-coherence illumination system, an interferometric generation system, a scanning system, and an imaging system.

[0006] The low-coherence illumination system is used to generate a pair of orthogonally polarized lights, and the optical path difference between the two orthogonally polarized lights is adjustable;

[0007] The interference generation system is used to generate a reference beam and a test beam;

[0008] The scanning system is used to control the test piece to perform three-dimensional topographic scanning interferometry.

[0009] The imaging system is used for imaging and receiving interference fringes.

[0010] Furthermore, the device also includes a housing, within which the low-coherence illumination system, the interference generation system, and the imaging system are mounted.

[0011] Furthermore, the low-coherence illumination system includes a low-coherence laser and a polarization beam splitter arranged coaxially in sequence, a first reflector arranged along the direction of transmitted light from the polarization beam splitter, and a second reflector arranged along the direction of reflected light from the polarization beam splitter.

[0012] The interference generation system includes a non-polarizing beam splitter arranged along the direction of transmitted light from the polarizing beam splitter, and a converging mirror group and a hemispherical lens arranged sequentially along the direction of reflected light from the non-polarizing beam splitter.

[0013] The scanning system includes a horizontal displacement mechanism and a vertical displacement mechanism, which are used together to control the test piece to perform three-dimensional topographic scanning interferometry.

[0014] The imaging system includes an imaging mirror group and a polarizing camera arranged sequentially along the transmission light direction of the non-polarizing beam splitter. The imaging mirror group images the interference fringes onto the detection target surface of the polarizing camera, and the polarizing camera forms four spatial phase-shifted interferograms with a phase shift of π / 4.

[0015] Furthermore, the position of the second reflector is adjustable, and it can move along its axis, i.e., the direction of the reflected light from the polarizing beam splitter.

[0016] Furthermore, the hemispherical lens includes a partially reflecting surface, the light beam reflected by which the light beam serves as a reference beam for interference.

[0017] Furthermore, the hemispherical lens further converges the emitted beam from the converging lens group and forms the focal point of the test beam near the center of the hemispherical lens.

[0018] Furthermore, the nominal normal distance between the focal point of the test beam and the test point of the device under test is equal to the distance difference between the first reflector, the second reflector, and the polarizing beam splitter.

[0019] Furthermore, the test piece is placed on a horizontal displacement mechanism located outside the housing, which drives the test piece to move horizontally along the x and y directions. The vertical displacement mechanism is mounted on the housing and drives the housing to move vertically along the z direction.

[0020] On the other hand, a low-coherence optical freeform surface topography interferometric scanning measurement method is provided, characterized in that the method includes the following steps:

[0021] Step 1: The low-coherence laser beam polarized light is split into a pair of orthogonally polarized light: s-wave and p-wave after passing through a polarization beam splitter.

[0022] Step 2: After being reflected by the non-polarized beam splitter, the S-wave and P-wave are incident on the hemispherical lens through the converging lens group. Part of the S-wave and P-wave are reflected by part of the reflecting surface of the hemispherical lens to form an interference reference beam, and the other part is transmitted through the hemispherical lens to form a test beam, which converges at the focal point of the test beam. The test beam is emitted from the focal point of the test beam and reaches the initial test point of the device under test.

[0023] Step 3: Adjust the axial position of the second reflector so that the optical path difference between the second reflector and the first reflector and the polarization beam splitter is equal to the optical path difference from the focal point of the test beam to the initial test point;

[0024] Step 4: The reference beam reflected by the partially reflecting surface and the test beam reflected by the device under test are incident on the polarization camera through the imaging lens group. The S-wave reflected by the partially reflecting surface and the P-wave reflected by the device under test are optically path matched and interference is formed. The interference fringes are bullseye rings. The remaining reflected beams are used as background light intensity and do not participate in the interference.

[0025] Step 5: Based on the interferogram obtained in Step 4, calculate the phase distribution of the initial test points of the device under test;

[0026] Step 6: Use the scanning system to complete the interferometric scanning measurement of all test points of the test piece. The height of non-initial test points is calculated from the phase change of the point relative to the initial test point, the offset of the bullseye ring center, and the motion parameters of the displacement stage.

[0027] Step 7: Perform polynomial fitting on the test point cloud data to obtain the three-dimensional surface distribution of the test part.

[0028] Furthermore, step 6 calculates the height of the non-initial test points, specifically including:

[0029] The phase change of the bullseye ring relative to the initial test point is δh, and the center offsets of the bullseye ring in the x and y directions are x, respectively. d and y d The spatial positions of the X, Y, and Z axis translation stages are respectively X p Y p and Z p ;

[0030] Step 6-1, calculate the azimuth angle α and zenith angle θ between the test point and the focal point of the test beam:

[0031]

[0032] In the formula, γ is the transformation coefficient of the object space, which is obtained by system calibration;

[0033] Step 6-2, calculate the spatial location of the test point:

[0034]

[0035] Compared with the prior art, the significant advantages of this invention are:

[0036] 1) The optical path for freeform surface interferometric scanning measurement is constructed by using low coherence interference and spatial point scanning, which solves the problems of compensator limitations and backlash error correction in traditional freeform surface measurement, and has high accuracy and flexibility.

[0037] 2) A large NA test wavefront is formed using a hemispherical lens, which has the ability to measure freeform surfaces with large slopes.

[0038] 3) The bullseye ring fringes at the test points are obtained using a low-coherence polarization interference method, which is not affected by environmental vibration.

[0039] 4) The testing process only requires three-dimensional translational motion, the system control is simple, the cost is low, and the practicality is high.

[0040] The present invention will now be described in further detail with reference to the accompanying drawings. Attached Figure Description

[0041] Figure 1 This is a schematic diagram of the optical path structure of the low-coherence optical freeform surface topography interferometric scanning measurement device of the present invention.

[0042] Figure 2 This is a schematic diagram of the measurement of the initial test point in one embodiment.

[0043] Figure 3 This is a schematic diagram showing the spatial relationship between the focal point of the test beam and the device under test in one embodiment.

[0044] Figure 4 This is a schematic diagram showing the center offset of the bullseye ring interference fringes on the detector target surface in one embodiment. Detailed Implementation

[0045] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0046] It should be noted that if the embodiments of the present invention involve directional indication, the directional indication is only used to explain the relative positional relationship and movement of the components in a certain specific posture. If the specific posture changes, the directional indication will also change accordingly.

[0047] Furthermore, if the embodiments of this invention involve descriptions such as "first" or "second," these descriptions are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined with "first" or "second" may explicitly or implicitly include at least one of those features. Additionally, the technical solutions of the various embodiments can be combined with each other, but this must be based on the ability of those skilled in the art to implement them. If the combination of technical solutions is contradictory or impossible to implement, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection claimed by this invention.

[0048] In one embodiment, combined Figure 1 A low-coherence optical freeform surface topography interferometric scanning measurement device is provided. The device includes a housing 16, a scanning system, and a low-coherence illumination system 5, an interference generation system 10, and an imaging system 15 disposed within the housing 16.

[0049] The low-coherence illumination system 5 is used to generate a pair of orthogonally polarized lights, and the optical path difference between the two orthogonally polarized lights is adjustable.

[0050] The interference generation system 10 is used to generate a reference beam and a test beam;

[0051] The scanning system is used to control the test piece 11 to perform three-dimensional topographic scanning interferometry.

[0052] The imaging system 15 is used for imaging and receiving interference fringes.

[0053] Furthermore, in one embodiment, the low-coherence illumination system 5 includes a low-coherence laser 1 and a polarization beam splitter 2 arranged coaxially in sequence, a first reflector 3 arranged along the direction of transmitted light from the polarization beam splitter 2, and a second reflector 4 arranged along the direction of reflected light from the polarization beam splitter 2.

[0054] The interference generation system 10 includes a non-polarized beam splitter 6 arranged along the transmitted light direction of the polarized beam splitter 2, a converging mirror group 7 and a hemispherical lens 8 arranged sequentially along the reflected light direction of the non-polarized beam splitter 6.

[0055] The scanning system includes a horizontal displacement mechanism 12a and a vertical displacement mechanism 12b. The test piece 11 is placed on the horizontal displacement mechanism 12a. The horizontal displacement mechanism 12a is located outside the housing 16 and is used to drive the test piece 11 to move horizontally along the x and y directions. The vertical displacement mechanism 12b is installed on the housing 16 and is used to drive the housing 16 to move vertically along the z direction.

[0056] The imaging system 15 includes an imaging mirror group 13 and a polarization camera 14 arranged sequentially along the transmission light direction of the non-polarization beam splitter 6. The imaging mirror group 13 images interference fringes onto the detection target surface of the polarization camera 14, and the polarization camera 14 forms four spatial phase-shifted interferograms with a phase shift of π / 4.

[0057] Here, the position of the second reflector 4 is adjustable and can be moved along its axis, i.e., the direction of the reflected light from the polarizing beam splitter 2.

[0058] Here, the hemispherical lens 8 includes a partially reflecting surface 8a, and the light beam reflected by the partially reflecting surface 8a serves as the reference beam for interference.

[0059] Here, the hemispherical lens 8 further converges the emitted beam from the converging lens group 7 and forms the test beam focal point 9 at a position close to the center of the hemispherical lens 8.

[0060] Furthermore, in one embodiment, combined with Figure 2 The nominal normal distance between the focal point 9 of the test beam and the test point 11a of the device under test 11 is equal to the distance difference between the first reflecting mirror 3, the second reflecting mirror 4, and the polarizing beam splitter 2. This allows for the selection of the reference beam and the test beam, forming interference fringes.

[0061] It should be noted that, Figure 2 The test piece 11 is a plane, which is not necessary and is only used to illustrate the spatial positional relationship of the initial test point 11a. It can replace the optical freeform surface.

[0062] Furthermore, in one embodiment, combined with Figure 3 The spatial coordinates of the test beam focus 9 at test point 11a of the test piece 11 are given, and the normal distance between it and test point 11a is H+δh, where δh is the phase change of the bullseye ring fringe of the non-initial test point relative to the initial test point. In this embodiment, the spatial positional relationship between test point 11a and test beam focus 9 is as follows:

[0063] X S = (H+δh)sinθcosα+X P ,

[0064] Y S = (H+δh)sinθsinα+Y P ,

[0065] Z S = (H+δh)cosθ+Z P ,

[0066] In the formula, α and θ are the azimuth and zenith angles of the spatial positional relationship between test point 11a and test beam focus 9, respectively, and θ is the spatial coordinate of test point 11a.

[0067] Furthermore, in one embodiment, combined with Figure 4 The non-initial test point bullseye ring stripe center C on the target surface of the polarization camera 14 n Compared to the initial test point, the center of the bullseye ring stripe, C0, has shifted, with a shift amount of x. d and y d The offset of the bullseye ring stripe center can be calculated. Figure 3 The azimuth and zenith angles of the spatial relationship between test point 11a and test beam focus 9:

[0068]

[0069] In the formula, γ is the transformation coefficient of the object space, obtained by system calibration. In this embodiment, the center of the bullseye ring fringe is obtained by phase fitting.

[0070] In one embodiment, a low-coherence optical freeform surface interferometry scanning measurement method is provided, the method comprising:

[0071] Step 1: The low-coherence laser 1 emits polarized light, which is then split into two orthogonally polarized beams, s-wave and p-wave, by polarization beam splitter 2.

[0072] Step 2: After being reflected by the non-polarizing beam splitter 6, the S-wave and P-wave are incident on the hemispherical lens 8 through the converging mirror group 7. Part of the S-wave and P-wave are reflected by part of the reflecting surface 8a of the hemispherical lens 8 to form an interference reference beam, and the other part is transmitted through the hemispherical lens 8 to become a test beam, which converges at the focal point 9 of the test beam. After the test beam is emitted from the focal point 9, it reaches the initial test point of the device under test 11.

[0073] Step 3: Adjust the axial position of the second reflector 4 so that the optical path difference between the second reflector 4 and the first reflector 3 and the polarization beam splitter 2 is equal to the optical path difference from the beam focus 9 to the initial test point;

[0074] Step 4: The reference beam reflected by the partially reflecting surface 8a and the test beam reflected by the device under test 11 are incident on the polarization camera 14 through the imaging lens group 13. The S-wave reflected by the partially reflecting surface 8a and the P-wave reflected by the device under test 11 are optically matched and thus form interference. The interference fringes are bullseye rings. The remaining reflected beams serve as background light intensity and do not participate in the interference.

[0075] Step 5: Based on the interferogram obtained in Step 4, calculate the phase distribution of the initial test points of the test piece 11;

[0076] Step 6: Use the scanning system to complete the interferometric scanning measurement of all test points of the test piece. The height of non-initial test points is calculated from the phase change of the point relative to the initial test point, the offset of the bullseye ring center and the motion parameters of the displacement stage. Polynomial fitting will be performed on the test point cloud data to reconstruct the surface shape of the test piece.

[0077] Preferably, Zernike polynomial fitting is used to reconstruct the surface shape to be measured.

[0078] Furthermore, in one embodiment, step 6, calculating the height of the non-initial test point, specifically includes:

[0079] The phase change of the bullseye ring relative to the initial test point is δh, and the center offsets of the bullseye ring in the x and y directions are x, respectively. d and y d The spatial positions of the X, Y, and Z axis translation stages (the three-axis translation stages formed by the scanning system) are respectively X p Y p and Z p ;

[0080] Step 6-1, calculate the azimuth angle α and zenith angle θ between the test point and the focal point 9 of the test beam:

[0081]

[0082] In the formula, γ is the transformation coefficient of the object space, which is obtained by system calibration;

[0083] Step 6-2, calculate the spatial coordinates of the test point:

[0084] X S = (H+δh)sinθcosα+X P

[0085] Y S = (H+δh)sinθsinα+Y P

[0086] Z S = (H+δh)cosθ+Z P

[0087] Where H is the nominal normal distance between the test point and the focal point 9 of the test beam.

[0088] This invention enables high-precision and high-flexibility three-dimensional topographic interferometric scanning measurement of optical freeform surfaces.

[0089] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention without departing from its spirit and scope should be included within the protection scope of the present invention.

Claims

1. A low-coherence optical freeform surface topography interferometric scanning measurement device, characterized in that, The device includes a low-coherence illumination system, an interferometry generation system, a scanning system, and an imaging system. The low-coherence illumination system is used to generate a pair of orthogonally polarized lights, and the optical path difference between the two orthogonally polarized lights is adjustable; The interference generation system is used to generate a reference beam and a test beam; The scanning system is used to control the test piece to perform three-dimensional topographic scanning interferometry. The imaging system is used for imaging and receiving interference fringes; The low-coherence illumination system includes a low-coherence laser and a polarization beam splitter arranged coaxially in sequence, a first reflector arranged along the direction of transmitted light from the polarization beam splitter, and a second reflector arranged along the direction of reflected light from the polarization beam splitter. The interference generation system includes a non-polarizing beam splitter arranged along the direction of transmitted light from the polarizing beam splitter, and a converging mirror group and a hemispherical lens arranged sequentially along the direction of reflected light from the non-polarizing beam splitter. The scanning system includes a horizontal displacement mechanism and a vertical displacement mechanism, which are used together to control the test piece to perform three-dimensional topographic scanning interferometry. The imaging system includes an imaging mirror group and a polarization camera arranged sequentially along the transmission light direction of the non-polarization beam splitter. The imaging mirror group images interference fringes onto the detection target surface of the polarization camera, and the polarization camera forms four spatial phase-shifted interferograms with a phase shift of π / 4. The hemispherical lens includes a partially reflecting surface, the light beam reflected by which the light beam is used as a reference beam for interference; The hemispherical lens further converges the outgoing beam from the converging lens group and forms the focal point of the test beam near the center of the hemispherical lens. The nominal normal distance between the focal point of the test beam and the test point of the device under test is equal to the distance difference between the first reflector, the second reflector, and the polarizing beam splitter.

2. The low-coherence optical freeform surface topography interferometric scanning measurement device according to claim 1, characterized in that, The device also includes a housing, within which the low-coherence illumination system, the interference generation system, and the imaging system are mounted.

3. The low-coherence optical freeform surface topography interferometric scanning measurement device according to claim 1, characterized in that, The position of the second reflector is adjustable and can be moved along its axis, i.e., along the direction of the reflected light from the polarizing beam splitter.

4. The low-coherence optical freeform surface topography interferometric scanning measurement device according to claim 1, characterized in that, The test piece is placed on a horizontal displacement mechanism located outside the housing and is used to drive the test piece to move horizontally along the x and y directions. The vertical displacement mechanism is mounted on the housing and is used to drive the housing to move vertically along the z direction.

5. A low-coherence optical freeform surface topography interferometric scanning measurement method based on the apparatus described in any one of claims 1 to 4, characterized in that, The method includes the following steps: Step 1: The low-coherence laser beam polarized light is split into a pair of orthogonally polarized light: s-wave and p-wave after passing through a polarization beam splitter. Step 2: After being reflected by the non-polarized beam splitter, the S-wave and P-wave are incident on the hemispherical lens through the converging lens group. Part of the S-wave and P-wave are reflected by part of the reflecting surface of the hemispherical lens to form an interference reference beam, and the other part is transmitted through the hemispherical lens to form a test beam, which converges at the focal point of the test beam. The test beam is emitted from the focal point of the test beam and reaches the initial test point of the device under test. Step 3: Adjust the axial position of the second reflector so that the optical path difference between the second reflector and the first reflector and the polarization beam splitter is equal to the optical path difference from the focal point of the test beam to the initial test point; Step 4: The reference beam reflected by the partially reflecting surface and the test beam reflected by the device under test are incident on the polarization camera through the imaging lens group. The S-wave reflected by the partially reflecting surface and the P-wave reflected by the device under test are optically path matched and interference is formed. The interference fringes are bullseye rings. The remaining reflected beams are used as background light intensity and do not participate in the interference. Step 5: Based on the interferogram obtained in Step 4, calculate the phase distribution of the initial test points of the device under test; Step 6: Use the scanning system to complete the interferometric scanning measurement of all test points of the test piece. The height of non-initial test points is calculated from the phase change of the point relative to the initial test point, the offset of the bullseye ring center, and the motion parameters of the displacement stage. Step 7: Perform polynomial fitting on the test point cloud data to obtain the three-dimensional surface distribution of the test part; Step 6 involves calculating the height of non-initial test points, specifically including: The phase change of the bullseye ring relative to the initial test point is δh, and the center offsets of the bullseye ring in the x and y directions are x, respectively. d and y d The spatial positions of the X, Y, and Z axis translation stages are respectively X p Y p and Z p ; Step 6-1, calculate the azimuth angle α and zenith angle θ between the test point and the focal point of the test beam: In the formula, γ is the transformation coefficient of the object space, which is obtained by system calibration; Step 6-2, calculate the spatial coordinates of the test point: Where H is the nominal normal distance between the test point and the focal point of the test beam.

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