A reflective point diffraction interferometer and a surface shape measurement method

Abstract

The present application relates to the field of interference measurement technology, and discloses a reflective point diffraction interferometer and a surface measurement method, wherein a pinhole with an elliptical cross-section is provided on a diffraction plate, wherein: a first polarized light is incident on a first principal optical axis of the pinhole, a second polarized light is incident on a second principal optical axis of the pinhole, and the polarization directions of the first polarized light and the second polarized light are perpendicular to each other; the projections of the pinhole in the directions of the first principal optical axis and the second principal optical axis are both circular; the first polarized light is diffracted through the pinhole to form a first spherical wave, which is used as a reference light for interference measurement; the second polarized light is diffracted through the pinhole to form a second spherical wave and is irradiated to a mirror to be measured, and after being reflected by the mirror to be measured, it is focused and reflected close to the pinhole, and is used as a test light for interference measurement, and the test light and the reference light are polarization-phase shifted to generate a transient phase-shifted interference pattern, thereby solving the technical problem that the related art cannot meet the requirements of high-precision surface detection of optical elements with large numerical apertures.

Description

Technical Field

[0001] The present application relates to the field of interference measurement technology, and in particular to a reflective point diffraction interferometer and a surface measurement method. Background Art

[0002] In the related technology, the most common high-precision optical surface detection method is the reflective pinhole point diffraction interferometer, such as Figure 1 As shown, it uses a tiny pinhole as a point diffraction to generate an outgoing spherical wave, a part of the outgoing spherical wave is used as reference light, and the other part is used as test light. The test light irradiates the mirror to be tested, and a metal reflective film is plated at the pinhole of the diffraction plate. The focused light reflected by the mirror to be tested returns to the pinhole and then reflects and interferes with the reference light of the other part of the spherical wave. By analyzing the interference pattern, high-precision detection of the mirror type to be tested can be achieved. Since the outgoing spherical wave needs to be divided into two parts, the range of measurable mirror numerical aperture is greatly limited. Summary of the invention

[0003] The present application provides a reflective point diffraction interferometer and a surface measurement method, which solve the technical problem in the related art that high-precision surface detection of large numerical aperture optical elements cannot be achieved, and achieves the technical effect of increasing the measurement numerical aperture range.

[0004] In order to achieve the above objectives, the main technical solutions adopted in this application include:

[0005] In a first aspect, an embodiment of the present application provides a reflective point diffraction interferometer, comprising a diffraction plate, on which a pinhole with an elliptical cross-section is provided, wherein: a first polarized light is incident on a first principal optical axis of the pinhole, a second polarized light is incident on a second principal optical axis of the pinhole, and the polarization directions of the first polarized light and the second polarized light are perpendicular to each other; the projections of the pinhole in the directions of the first principal optical axis and the second principal optical axis are both circular; the first polarized light is diffracted through the pinhole to form a first spherical wave, which serves as a reference light for interference measurement; the second polarized light is diffracted through the pinhole to form a second spherical wave and is irradiated toward a mirror to be measured, and after being reflected by the mirror to be measured, it is focused and reflected close to the pinhole, and serves as a test light for interference measurement, and the test light and the reference light are polarization-shifted to generate a transient phase-shift interference pattern, and the transient phase-shift interference pattern is used to extract the surface information of the mirror to be measured.

[0006] The embodiment of the present application provides a reflective point diffraction interferometer, comprising a diffraction plate, on which a pinhole with an elliptical cross-section is provided, wherein: a first polarized light is incident on a first principal optical axis of the pinhole, a second polarized light is incident on a second principal optical axis of the pinhole, and the polarization directions of the first polarized light and the second polarized light are perpendicular to each other; the projections of the pinhole in the directions of the first principal optical axis and the second principal optical axis are both circular; the first polarized light is diffracted through the pinhole to form a first spherical wave, which is used as a reference light for interference measurement; the second polarized light is diffracted through the pinhole to form a second spherical wave and is irradiated toward a mirror to be measured, and after being reflected by the mirror to be measured, it is focused and reflected close to the pinhole, and is used as a test light for interference measurement, and the test light and the reference light are polarization-phase shifted to generate a transient phase-shifted interference pattern, and the transient phase-shifted interference pattern is used to extract the surface information of the mirror to be measured, thereby solving the technical problem that the high-precision surface detection of optical elements with large numerical apertures cannot be satisfied in the related art, and achieving the technical effect of improving the measurement numerical aperture range.

[0007] Optionally, the interferometer further includes: a polarization phase shifting system, used to separate the linearly polarized light into the first polarized light and the second polarized light, and to achieve phase shifting by changing the polarization states of the reference light and the test light; and an imaging system, used to receive the reference light and the test light to obtain a transient phase shift interference pattern.

[0008] Optionally, the polarization phase shifting system specifically includes a half-wave plate, a polarization beam splitter, a quarter-wave plate, and a polarization array, wherein: the linear polarized light is divided into the first polarized light and the second polarized light with polarization directions perpendicular to each other after passing through the half-wave plate and the polarization beam splitter; wherein the half-wave plate is used to adjust the light intensity ratio of the first polarized light and the second polarized light; the reference light and the test light are converted into two circularly polarized lights with opposite rotation directions after passing through the quarter-wave plate, and are then phase-shifted by the polarization array.

[0009] Optionally, the structure of the imaging system adopts an image-side telecentric optical path, specifically including a third objective lens, a tube lens, an imaging lens and an image sensor, wherein the reference light and the test light generate a transient phase-shifting interference pattern in the image sensor after passing through the third objective lens, the tube lens and the imaging lens.

[0010] Optionally, the interferometer further includes: an alignment system for guiding the alignment of the test light to improve the measurement accuracy of the mirror to be tested, the alignment system specifically includes a first objective lens, a second objective lens, a third objective lens, a wide field of view objective lens, a tube lens, a dichroic prism and an industrial camera, wherein: the test light and the reference light are divided into reflected light waves and transmitted light waves after passing through the tube lens and the dichroic prism, wherein the reflected light waves are focused in the industrial camera to form a test light spot and a reference light spot, and the transmitted light waves are used to obtain a transient phase-shifting interference pattern; the wide field of view objective lens is used to obtain a transient phase-shifting interference pattern according to the relative positions of the test light spot and the reference light spot. , coarsely adjust the relative position of the mirror to be tested and the diffraction plate, the third objective lens is used to finely adjust the relative position of the mirror to be tested and the diffraction plate according to the relative positions of the test light spot and the reference light spot, so that the second spherical wave is focused close to the pinhole after being reflected by the mirror to be tested; the first objective lens and the second objective lens are used to adjust the relative positions of the first objective lens, the second objective lens and the diffraction plate according to the energy distribution of the test light spot and the reference light spot, so that the area with the strongest energy in the diffuse spot formed after the first polarized light and the second polarized light pass through the first objective lens and the second objective lens respectively is incident on the pinhole.

[0011] Optionally, the interferometer also includes a laser and a pre-processing system, wherein the pre-processing system is used to pre-process the laser light emitted by the laser, and the pre-processing system includes a beam expander, a fourth objective lens, a rotating frosted glass, a mask plate with small holes, a fifth objective lens and a polarizer, wherein: the laser light emitted by the laser is expanded and collimated by the beam expander, then focused on the small holes of the mask plate after passing through the fourth objective lens and the rotating frosted glass, becomes parallel light after passing through the fifth objective lens, and is emitted as linearly polarized light after passing through the polarizer.

[0012] Optionally, the diffraction plate is a glass substrate on which a chrome film is plated, and the pinholes are etched on the chrome film. The material of the glass substrate is C7980, and the thickness is set to 3 mm.

[0013] Optionally, the interferometer further includes a Hindle sphere, which is disposed between the diffraction plate and the mirror to be measured and is used to measure a convex spherical mirror to be measured.

[0014] In a second aspect, an embodiment of the present application provides a surface measurement method, which is implemented based on the above-mentioned reflective point diffraction interferometer, and the method includes: a diffraction plate with a pinhole is arranged on the interferometer, and the cross-section of the pinhole is arranged to be elliptical; linearly polarized light is divided into a first polarized light and a second polarized light whose polarization directions are perpendicular to each other; the first polarized light is incident on the first principal optical axis of the pinhole, and the second polarized light is incident on the second principal optical axis of the pinhole; the projections of the pinhole in the directions of the first principal optical axis and the second principal optical axis are both circular; the first polarized light is diffracted through the pinhole to form a first spherical wave, which serves as a reference light for interference measurement, the second polarized light is diffracted through the pinhole to form a second spherical wave and illuminates the mirror to be measured, is reflected by the mirror to be measured, is focused and reflected close to the pinhole, and serves as a test light for interference measurement, and the test light and the reference light are polarization-shifted to generate a transient phase-shift interference pattern; the transient phase-shift interference pattern is analyzed, and the surface information of the mirror to be measured is extracted according to the analysis result.

[0015] Optionally, the method also includes: dividing the test light and the reference light into reflected light waves and transmitted light waves after passing through a tube lens and a dichroic prism, wherein the reflected light waves are focused in an industrial camera to form a test light spot and a reference light spot, and the transmitted light waves are used to obtain an instantaneous phase-shift interference pattern; according to the relative positions of the test light spot and the reference light spot, adjusting the relative positions of the mirror to be tested and the diffraction plate so that the second spherical wave is focused close to the pinhole after being reflected by the mirror to be tested; according to the energy distribution of the test light spot and the reference light spot, adjusting the relative positions of the first objective lens, the second objective lens and the diffraction plate so that the area with the strongest energy in the diffuse spot formed after the first polarized light and the second polarized light pass through the first objective lens and the second objective lens respectively is incident on the pinhole.

[0016] In a third aspect, an embodiment of the present application provides a computer device, comprising: a memory and a processor, wherein the memory and the processor are communicatively connected to each other, computer instructions are stored in the memory, and the processor executes the above-mentioned face measurement method by executing the computer instructions.

[0017] In a fourth aspect, an embodiment of the present application provides a computer-readable storage medium, on which computer instructions are stored, and the computer instructions are used to enable a computer to execute the above-mentioned surface measurement method.

[0018] In a fifth aspect, an embodiment of the present application provides a computer program product, including computer instructions, wherein the computer instructions are used to enable a computer to execute the above-mentioned surface measurement method. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the specific implementation methods of the present application or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0020] Figure 1 A schematic diagram of a pinhole point diffraction interferometer in an application scenario;

[0021] Figure 2 A schematic diagram of a reflective point diffraction interferometer provided in an embodiment of the present application;

[0022] Figure 3 A schematic diagram of the positional relationship between an elliptical pinhole and its projection circle provided in an embodiment of the present application;

[0023] Figure 4 A schematic diagram of the optical paths of the first polarized light and the second polarized light incident on the pinhole substrate provided in an embodiment of the present application;

[0024] Figure 5 A schematic diagram of a simulation model of spherical wave aberration of elliptical pinhole diffraction provided in an embodiment of the present application;

[0025] Figure 6 A schematic diagram of scanning results of wave aberration of an elliptical pinhole with varying sizes provided in an embodiment of the present application;

[0026] Figure 7 An imaging optical path evaluation function curve diagram provided in an embodiment of the present application;

[0027] Figure 8 A schematic diagram of an imaging optical path distortion curve provided in an embodiment of the present application;

[0028] Fig. 9 A schematic diagram of a Hindle ball measuring a convex surface provided in an embodiment of the present application;

[0029] Fig.10 A schematic diagram of an effective measurement area of ​​a mirror to be measured provided in an embodiment of the present application;

[0030] Fig.11 The intensity diagram of the light spot entering the target surface of the industrial camera when the position of the mirror to be measured relative to the diffraction plate deviates by 1.82 mm in the direction perpendicular to the optical axis of the large-field objective lens during the coarse adjustment provided in the embodiment of the present application;

[0031] Fig.12 The position of the mirror to be measured relative to the diffraction plate during the coarse adjustment provided in the embodiment of the present application deviates by 1.82 in the direction perpendicular to the optical axis of the large field of view objective lens. The intensity diagram of the light spot entering the target surface of the industrial camera at that time;

[0032] Fig.13 During the fine adjustment provided in the embodiment of the present application, the position of the measured mirror relative to the diffraction plate deviates by 73° in the direction perpendicular to the optical axis of the large field of view objective lens. The intensity diagram of the light spot entering the target surface of the industrial camera at that time;

[0033] Fig.14 The position of the mirror to be measured relative to the diffraction plate during fine adjustment provided in the embodiment of the present application deviates by 0.7 in the direction perpendicular to the optical axis of the large field of view objective lens. The intensity diagram of the light spot entering the target surface of the industrial camera at that time;

[0034] Fig.15 A schematic diagram of the structure of a point diffraction transient interferometer provided in an embodiment of the present application;

[0035] Fig.16 A flow chart of a surface shape measurement method provided in an embodiment of the present application;

[0036] Fig.17 A schematic diagram of the structure of a computer device provided in an embodiment of the present application.

[0037] Figure numerals: 1-laser; 2-beam expander; 3-fourth objective lens; 4-rotating frosted glass; 5-mask; 6-fifth objective lens; 7-polarizer; 8-half-wave plate; 9-polarization beam splitter; 10-second objective lens; 11-first reflector; 12-second reflector; 13-first objective lens; 14-diffraction plate; 15-mirror to be tested; 16-third objective lens; 17-wide field of view objective lens; 18-quarter-wave plate; 19-tube lens; 20-beam splitter; 21-industrial camera; 22-imaging lens; 23-polarization array camera. DETAILED DESCRIPTION

[0038] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present application.

[0039] Interference detection method is a non-contact measurement method. Its principle is that one beam of light illuminates a standard reference plane as reference light, and another beam of light illuminates the mirror to be tested and returns with mirror information as test light. When the two beams of light interfere, due to the different light spots and different position phases, an optical path difference is generated, thereby generating curved interference fringes, which can be used to determine the surface shape of the mirror to be tested. The traditional spherical interference detection method uses a reference sphere with high surface accuracy to obtain the required reference light, and then compares it with the test light containing the mirror information of the mirror to be tested, thereby obtaining the surface data of the mirror to be tested. However, the surface accuracy of the reference sphere directly limits the detection accuracy that can be achieved by traditional interference, and the point diffraction interferometer uses the point diffraction principle to obtain an ideal spherical wave, which solves this problem well. The point diffraction spherical interference detection method is divided into the optical fiber point diffraction detection method and the pinhole point diffraction interference detection method. Generally, the core diameter of a single-mode optical fiber is 3-5um. It is difficult to manufacture a core diameter smaller than 2-3um, that is, the core diameter of the optical fiber cannot be made as small as a pinhole. This will cause the available numerical aperture (NA) of the optical fiber diffraction spherical wave to be generally less than 0.2, which cannot meet the high-precision surface detection of large numerical aperture optical components.

[0040] Typical pinhole diffraction interferometers are Figure 1 As shown in the figure, it uses a tiny pinhole to generate point diffraction to generate an outgoing spherical wave, a part of which is used as reference light, and the other part is used as test light. The test light illuminates the mirror to be tested and is reflected by the diffraction plate (i.e. Figure 1 The pinhole of the pinhole plate in the measuring instrument is plated with a metal reflective film. The light focused by the mirror to be measured after reflection returns to the place close to the pinhole and then overlaps with the reference light of another part of the spherical wave after reflection. The piezoelectric ceramic is used to push the mirror to be measured for mechanical phase shift measurement, thereby realizing the spherical surface detection. Since the outgoing spherical wave is the only spherical wave generated by the pinhole diffraction, it needs to be evenly divided into the test light and the reference light, that is, it needs to be "divided into two" (one part as the reference light and the other part as the test light). Therefore, the range of the measurable numerical aperture (NA) of the mirror is greatly limited. At the same time, the time phase shift method, including the piezoelectric phase shift and wavelength phase shift measurement method, does not obtain multiple interference patterns at the same time, and is easily disturbed by environmental factors such as air disturbance and vibration. In addition, in the related art, the imaging system including the imaging optical path is outside the interferometer, and is not integrated into the interferometer to achieve integration. The relative positions between the imaging system, the mirror to be measured and the interferometer are easily disturbed, and need to be repeatedly adjusted, which is not convenient for use.

[0041] The embodiment of the present application provides a reflective point diffraction interferometer, including a diffraction plate 14, on which a pinhole with an elliptical cross section is arranged, and a first polarized light and a second polarized light with mutually perpendicular polarization directions pass through the pinhole to generate a first spherical wave and a second spherical wave that are independent of each other, respectively, wherein the first spherical wave is used as a reference light for interference measurement, and the second spherical wave is irradiated to the mirror surface of the mirror to be measured 15, reflected by the mirror to be measured 15, focused close to the pinhole and reflected again, and then used as a test light for interference measurement, and the test light and the reference light are polarization-phase shifted to generate a transient phase-shifted interference pattern, and the transient phase-shifted interference pattern is used to extract the surface information of the mirror to be measured. Among them, the pinhole diffracts two independent spherical waves to generate test light and reference light respectively, instead of "dividing" a spherical wave into two, and compared with the typical pinhole point diffraction interferometer, high-precision measurement of larger caliber and larger numerical aperture can be achieved.

[0042] Please refer to Figure 2 , Figure 2 Schematic diagram of a reflective point diffraction interferometer provided in an embodiment of the present application. The embodiment of the present application provides a reflective point diffraction interferometer, including a diffraction plate 14, on which a pinhole with an elliptical cross-section is provided, wherein: a first polarized light is incident on the first principal optical axis of the pinhole, a second polarized light is incident on the second principal optical axis of the pinhole, and the polarization directions of the first polarized light and the second polarized light are perpendicular to each other; the projections of the pinhole in the directions of the first principal optical axis and the second principal optical axis are both circular; the first polarized light is diffracted through the pinhole to form a first spherical wave, which serves as a reference light for interference measurement; the second polarized light is diffracted through the pinhole to form a second spherical wave and shines on the mirror to be measured 15, and after being reflected by the mirror to be measured 15, it is focused and reflected close to the pinhole, and serves as a test light for interference measurement, and the test light and the reference light are polarization-phase shifted to generate a transient phase-shifted interferogram, and the transient phase-shifted interferogram is used to extract the surface information of the mirror to be measured.

[0043] In order to ensure that the diffraction waves generated by the first polarized light and the second polarized light passing through the diffraction plate 14 are spherical waves, the cross section of the pinhole needs to be set to an elliptical shape. Figure 3 , Figure 3Schematic diagram of the positional relationship between the elliptical pinhole and its projection circle provided in the embodiment of the present application. The projection of the pinhole on the first principal optical axis of the first polarized light incident on the pinhole is circular, and the first polarized light can generate a first spherical wave through the pinhole. The projection of the pinhole on the second principal optical axis of the second polarized light incident on the pinhole is circular, and the second polarized light can generate a second spherical wave through the pinhole, so the first spherical wave and the second spherical wave are independent of each other in space. The polarization directions of the first polarized light and the second polarized light are perpendicular to each other, so the first spherical wave and the second spherical wave are perpendicular to each other in the polarization direction. The elliptical shape of the pinhole is related to the normal of the first principal optical axis, the second principal optical axis and the plane where the diffraction plate 14 is located. The second spherical wave shines on the mirror to be tested 15. If the mirror to be tested 15 is a concave spherical surface, the reflected light passing through the mirror to be tested 15 returns along the original path, is focused on the diffraction plate 14 and reflected again as the test light. Here, focusing close to the pinhole means that the distance error between the focus point of the light reflected by the measured mirror 15 and the pinhole needs to be controlled in micrometer level to meet the measurement accuracy requirement.

[0044] Specifically, the first polarized light is p-light, and the second polarized light is s-light. The p-light is parallel to the xoz plane and propagates along the z-axis; the s-light is parallel to the yoz plane and propagates along the z-axis. The design process of setting the cross section of the pinhole to an ellipse specifically includes the following steps:

[0045] Step (1) Select the material and thickness of the diffraction plate 14 substrate. The diffraction plate 14 is a chrome film plated on a glass substrate with a pinhole etched on the chrome film. When the principal optical axis of the incident light is not perpendicular to the glass substrate, the thicker the glass substrate, the greater the aberration introduced, resulting in a larger spot incident on the pinhole surface, reducing the diffracted light energy passing through the pinhole; at the same time, the test light is reflected back to the chrome film from the test mirror 15, and the peak-to-valley value (PV) of the chrome film surface is required to be less than / 10, therefore, the glass substrate needs to have a certain thickness and hardness. In the embodiment of the present application, the material of the glass substrate of the diffraction plate 14 is selected to be C7980, and the thickness of the glass substrate is 3 mm.

[0046] Step (2) Design the shape of the pinhole. In order for the same pinhole to diffract two independent spherical waves at the same time, the projection of the pinhole on the principal optical axis of the incident first polarized light and the second polarized light is required to be circular. Therefore, the pinhole needs to be designed as an ellipse. The shape of the ellipse is related to the angle between the principal optical axis of the first polarized light and the second polarized light incident on the pinhole and the normal of the glass substrate. Please refer to Figure 4 , Figure 4 Schematic diagram of the optical path of the first polarized light and the second polarized light incident on the pinhole substrate provided in the embodiment of the present application. Figure 4 As shown, , according to the refraction theorem, , where n=1.457 is the refractive index of C7980 at 632.8nm, we get .because Figure 4 The two normals shown as dotted lines are parallel to each other, so According to symmetry, the angle between the reference principal optical axis and the normal line of the glass substrate is also 29.03° within the glass substrate.

[0047] like Figure 3 As shown, assuming that the plane where the pinhole is located is the xoy plane, the expression of the principal optical axis of the second polarized light is:

[0048]

[0049] The projection of the pinhole in the direction of the principal optical axis of the second polarized light is a circle with a radius of R. Therefore, the distance from all points on the pinhole to the principal optical axis of the second polarized light is R. According to the distance formula from a point to a straight line in three-dimensional space:

[0050]

[0051] in, Any point in space To a point on line l The point (x, y, 0) is the coordinate of any point on the pinhole, and the vector from the point (0, t, 1.80t) on the line l is:

[0052]

[0053] The cross product of v and l is:

[0054]

[0055] The module length is:

[0056]

[0057] The distance d from a point to a straight line satisfies the following relationship:

[0058]

[0059] The pinhole shape expression is:

[0060]

[0061] The major axis of the ellipse and the principal optical axes of the first polarized light and the second polarized light are in the same plane.

[0062] Step (3) Calculate the maximum diameter of the projection circle of the elliptical pinhole that meets the required diffraction angle based on the NA of the interference system design. Take the system design requirement of NA=0.5 as an example, that is, the NA of the diffracted spherical wave generated by the pinhole is ≥0.5. Convert the numerical aperture to the diffraction angle, and the diffraction half angle for:

[0063]

[0064] From the relationship between the diffraction half angle θ and the pinhole short axis D2:

[0065]

[0066] in, =0.6328μm is the wavelength of laser 1.

[0067] Step (4) determines the size of the elliptical pinhole when the diffracted spherical wave is closer to the ideal spherical wave. Figure 5 , Figure 5 Schematic diagram of the simulation model of spherical wave aberration of elliptical pinhole diffraction provided in the embodiment of the present application. Figure 5 As shown, first, a three-dimensional simulation model is established through the finite-difference time-domain method to calculate the near-field distribution of the pinhole. Then, the far-field distribution is obtained based on the diffraction propagation. The phase distribution of the far-field diffraction spherical wave is fitted to the ideal diffraction spherical wave. The peak-to-valley value (PV) and root mean square value (RMS) of the wavefront difference before and after fitting are calculated to analyze the quality of the diffraction spherical wave.

[0068] The 3D model of the finite difference time domain method includes an excitation source, an elliptical pinhole, and boundary conditions. The NA of the excitation source should be consistent with the NA of the first objective lens 13 and the second objective lens 10. Ignoring the aberration of the objective lens, a spherical wave with consistent NA and converging at the center of the pinhole is selected as the excitation source. When the influence of the glass substrate aberration is not considered, the major axis of the ellipse is the minor axis. The boundary condition is set as a perfectly matched layer (PML) to prevent the outgoing light field from returning to the simulation model and interfering with the results.

[0069] Scan the size (minor axis) of the elliptical pinhole to find the optimal solution for the elliptical pinhole. Figure 6 , Figure 6 The schematic diagram of the scanning results of the wavefront aberration of the elliptical pinhole with the size change provided in the embodiment of the present application. The short axis is scanned from 0.3μm to 1.4μm, with a step length of 0.1μm. The wavefront error of the wavefront aberration changes with the short axis on a plane with a length and width of 600μm at a distance of 500μm from the pinhole as shown in the figure. Figure 6As shown, the variation trends of PV and RMS are roughly consistent, and the wave aberration is the smallest when the minor axis length is 0.6 μm. Through the above steps, the shape and size of the elliptical pinhole are designed, so that the first spherical wave and the second spherical wave are closer to the ideal spherical wave, thereby achieving high-precision measurement.

[0070] The reflective point diffraction interferometer provided in the embodiment of the present application includes a diffraction plate 14, on which a pinhole with an elliptical cross-section is provided, wherein: a first polarized light is incident on a first principal optical axis of the pinhole, a second polarized light is incident on a second principal optical axis of the pinhole, and the polarization directions of the first polarized light and the second polarized light are perpendicular to each other; the projections of the pinhole in the directions of the first principal optical axis and the second principal optical axis are both circular; the first polarized light is diffracted through the pinhole to form a first spherical wave, which is used as a reference light for interference measurement; the second polarized light is diffracted through the pinhole to form a second spherical wave and is irradiated toward a mirror to be measured 15, and after being reflected by the mirror to be measured 15, it is focused and reflected close to the pinhole, and is used as a test light for interference measurement. The test light and the reference light are polarization-shifted to generate a transient phase-shifted interference pattern, and the transient phase-shifted interference pattern is used to extract the surface information of the mirror to be measured, which solves the technical problem in the related art that high-precision surface detection of optical elements with large numerical apertures cannot be satisfied, and achieves the technical effect of improving the measurement numerical aperture range.

[0071] In some embodiments, the reflective point diffraction interferometer further includes: a polarization phase shifting system for splitting linearly polarized light into a first polarized light and a second polarized light, and achieving phase shifting by changing the polarization states of the reference light and the test light; and an imaging system for receiving the reference light and the test light to obtain a transient phase shift interference pattern.

[0072] Among them, the polarization phase shifting system divides the linear polarized light into a first polarized light and a second polarized light whose polarization directions are perpendicular to each other. The first polarized light and the second polarized light are focused on the pinhole, and are diffracted through the pinhole on the diffraction plate 14 to generate a first spherical wave and a second spherical wave that are independent of each other, wherein the first spherical wave is used as a reference light, and the second spherical wave is directed to the mirror to be measured 15, and after being reflected by the mirror to be measured 15 and reflected again by the diffraction plate 14, it is used as a test light. The polarization phase shifting system is also used to change the polarization state of the reference light and the test light to achieve phase shifting. The imaging system is used to receive the reference light and the test light to generate a transient phase shifting interferogram. The transient phase shifting interferogram refers to obtaining multiple interference patterns with a fixed phase difference at the same time.

[0073] In some embodiments, the image sensor and the polarization array are integrated into a polarization array camera. The polarization array includes a plurality of polarizers with different polarization angles.

[0074] In some embodiments, the polarization phase shifting system specifically includes a half-wave plate 8, a polarization beam splitter prism 9, a quarter-wave plate 18, and a polarization array, wherein: the linear polarized light is divided into a first polarized light and a second polarized light whose polarization directions are perpendicular to each other after passing through the half-wave plate 8 and the polarization beam splitter prism 9; wherein the half-wave plate 8 is used to adjust the light intensity ratio of the first polarized light and the second polarized light; the reference light and the test light are converted into two circularly polarized lights with opposite rotation directions after passing through the quarter-wave plate 18, and are then phase shifted by the polarization array.

[0075] Among them, only when the light intensities of the two coherent light beams that interfere are equal, it is conducive to high-precision interference measurement. The first polarized light passes through the pinhole on the diffraction plate 14 to generate a first spherical wave as a reference light, and the second polarized light passes through the pinhole on the diffraction plate 14 to generate a second spherical wave. After the second spherical wave is reflected by the mirror to be measured 15 and reflected close to the pinhole, the energy of the test light is reduced, which may cause a large difference in the energy of the reference light and the test light, resulting in poor contrast of the interference fringes. The half-wave plate 8 can be used to change the polarization state without losing the total energy of the polarized light as much as possible, and then the polarization beam splitter prism 9 is used to balance the light intensity ratio between the measurement light and the reference light, thereby improving the contrast of the interference fringes, which is conducive to achieving high-precision interference measurement. The polarization beam splitter prism 9 is used to separate linearly polarized light into a first polarized light and a second polarized light whose polarization directions are perpendicular to each other.

[0076] The polarization phase shifting system uses a combination of a half-wave plate 8, a polarization beam splitter prism 9, a quarter-wave plate 18, and a polarization array camera 23 to separate linearly polarized light into a first polarized light and a second polarized light. Phase shifting is achieved by changing the polarization states of the reference light and the test light. A four-step phase shift result is obtained in a single shot, thereby achieving transient measurement and avoiding the influence of environmental factors such as air disturbance and vibration.

[0077] Specifically, the working process of the polarization phase shifting system is as follows:

[0078] Step (1) rotates the half-wave plate 8 so that the energy of the reference light and the test light incident on the polarization array of the polarization array camera 23 is equal to improve the contrast of the interference fringes. In the following steps, the changes in the intensity of the first polarized light and the second polarized light in the optical path are not considered.

[0079] Step (2) The linearly polarized light is separated into a first polarized light (p light) and a second polarized light (s light) after passing through the polarization beam splitter prism 9. The Jones vector of the first polarized light is , the Jones vector of the second polarized light is ,in, is the phase difference between the first polarized light and the second polarized light.

[0080] Step (3) The first polarized light and the second polarized light are respectively passed through the pinhole on the diffraction plate 14 to form a reference light and a test light. The reference light and the test light are incident on a quarter wave plate 18 whose angle between the fast axis and the X axis is 45 degrees. The Jones matrix of the quarter wave plate 18 is for:

[0081] ;

[0082] The reference light becomes right-handed circularly polarized light after passing through the quarter-wave plate 18, which can be expressed by the Jones matrix as follows:

[0083] ;

[0084] The test light becomes left-handed circularly polarized light after passing through the quarter-wave plate 18, which can be expressed by the Jones matrix as follows:

[0085] ;

[0086] Step (4) The reference light and the test light are finally incident on the polarization array on the target surface of the polarization array camera 23, which includes four adjacent polarizers to form a phase shift unit. The polarization angles of the four polarizers are 0°, 45°, 90°, and 135° respectively. The reference light and the test light are polarized in a direction with an angle of After the polarizer is installed, the outgoing light has a polarization direction with an angle of Linearly polarized light, ignoring the phase , its Jones vector can be expressed as:

[0087] ,

[0088] in, is the Jones matrix of the polarizer, and the obtained interference light intensity can be expressed as:

[0089] ;

[0090] Therefore, the phase shifts of 0, , , Four transient phase-shifting interference patterns.

[0091] In some embodiments, the structure of the imaging system adopts an image-side telecentric optical path, specifically including the third objective lens 16, the tube lens 19, the imaging lens 22, and the image sensor. After the reference light and the test light pass through the third objective lens 16, the tube lens 19, and the imaging lens 22, a transient phase-shifted interference pattern is generated in the image sensor.

[0092] The imaging system uses the image-side telecentric optical path as the imaging optical path, where the tube lens 19 and the imaging lens 22 form a 4f system, which plays the role of beam expansion and coherent imaging, and realizes the acquisition of medium and low frequency surface information and small interference fringe distortion. The reference light and the test light after polarization phase shifting can generate a transient phase shift interference pattern in the image sensor. The imaging optical path is integrated into the interferometer to achieve integration and more convenient use. The design process of the imaging optical path in the imaging system is as follows:

[0093] Step (1) Analyze the F number and magnification of the imaging system. The object field of the third objective lens 16 is set to 0.5 mm, the focal length is 4 mm, and the numerical aperture of the output light is set to:

[0094]

[0095] The exit pupil diameter is:

[0096]

[0097] The shortest side of the target surface of the polarization array camera 23 is about 7 mm, so the imaging system is a 4f system with an F number of 0.0625 and a magnification of 1.75.

[0098] Step (2) The image-side telecentric optical path is used as the design target of the imaging optical path. The analysis of each component is as follows: the focal length of the tube lens 19 is 200 mm, which can be replaced by a paraxial surface during design; according to the magnification, the focal length of the imaging lens 22 is 350 mm, and the distance from the tube lens 19 is 550 mm.

[0099] Step (3) Optimize the structural parameters of the imaging lens 22. The imaging lens 22 is a cemented lens, which includes four lenses. Through the ray tracing method, the vertical axis aberration of the image space in the X and Y directions is set to 0 as the evaluation function, and the curvature radius of the lens is set as a variable.

[0100] Step (4) Analyze the fine resolution frequency of the imaging optical path instrument transfer function (ITF). In addition to ITF, the coherence transfer function (CTF) and modulation transfer function (MTF) also describe the performance of the imaging system. ITF represents the response of amplitude at different spatial frequencies, CTF represents the response of complex amplitude at different spatial frequencies, and MTF represents the response of light intensity at different spatial frequencies. Ideally, the cutoff frequency of the coherence transfer function CTF is consistent with the cutoff frequency of the instrument transfer function ITF, and the cutoff frequency of CTF can be calculated by MTF. For an exit pupil with a circular hole of diameter D, the coherent imaging transfer function is:

[0101]

[0102] In the above formula, is the spatial frequency, is the cut-off frequency. The transfer function of the incoherent imaging system is related to the spatial frequency The relationship is:

[0103]

[0104] make ,but Therefore, the spatial frequency when the MTF of incoherent imaging is 0.391 is the cutoff frequency of coherent imaging, that is, the cutoff frequency of ITF.

[0105] Please refer to Figure 7 , Figure 7 This is a graph of the imaging optical path evaluation function provided in the embodiment of the present application. Figure 7 As shown, MTF (Modulation Transfer Function) at 55mm -1 The minimum modulus of each field of view is 0.391, so the cutoff frequency of CTF (coherence transfer function) and ITF (instrument transfer function) is 55mm -1 The expected ITF curve is generally convex. A linear decreasing straight line is used to replace the convex curve. The convex instrument transfer function curve is higher than the linear instrument transfer function. When the ITF is higher than 0.7, it is considered to achieve good resolution. Therefore, at a certain frequency A linear instrument transfer function can achieve good resolution, and a convex instrument transfer function can also achieve good resolution. In the embodiment of the present application, =16mm -1 , realizing the measurement of medium and low frequency surface information.

[0106] Step (5) Analyze the distortion of the imaging light path. In order to ensure that the distortion does not affect the pixel offset, the distortion of the imaging light path in the interference system provided in the embodiment of the present application usually needs to be controlled at the sub-pixel level. Please refer to Figure 8 , Figure 8 This is a schematic diagram of the imaging optical path distortion curve provided by the embodiment of the present application, wherein the horizontal axis represents the field of view position and the vertical axis represents the distortion. Figure 8 As shown, the pixel size of the image sensor in the polarization array camera 23 is 3.45 μm, and the maximum distortion is 0.1631 μm, which is at the sub-pixel level. The half width of the target surface of the polarization array camera 23 is 3.5 mm, which is converted to a distortion rate of 0.0045%.

[0107] In some embodiments, if the measured mirror 15 is a convex spherical surface, a Hindle ball needs to be arranged between the diffraction plate 14 and the measured mirror 15 so that the reflected light is focused close to the pinhole. The Hindle ball can transform the convex surface measurement problem into a concave surface measurement problem, thereby realizing the measurement of large-aperture convex spherical surfaces. Please refer to Fig. 9 , Fig. 9The schematic diagram of the Hindle ball measuring convex surface provided in the embodiment of the present application. Fig. 9 The specific design process of Hindle ball is as follows:

[0108] Step (1) Determine the optical path structure of the Hindle sphere measuring convex surface. Fig. 9 As shown, point S is the pinhole on the diffraction plate 14. The light emitted from point S is reflected by any point M1 on the measured mirror 15 and then incident on a point M2 on the Hindle sphere. The light beam returns along the original path and returns to point S again.

[0109] Step (2) Select the distance between the Hindle sphere and the pinhole S. The light is incident on the test mirror 15 through the Hindle sphere, so a hole needs to be opened at the center of the Hindle sphere, which causes part of the light beam incident on the test mirror 15 to be unable to return to point S. is the center of the circle, Draw a circle with radius , where The area within the circle cannot be detected because the boundary point of the area cannot be detected. When the Hindle ball is closer to the test mirror 15, the required Hindle ball size is smaller, but the larger the central opening is, the larger the undetectable area is. In order to ensure that the undetectable central area is smaller, the Hindle ball is made as close to the pinhole of the interferometer as possible, and considering the actual thickness of the Hindle ball, in the embodiment of the present application, the distance between the reflective surface of the Hindle ball and the pinhole S is 20 mm.

[0110] Step (3) calculates the shortest distance between the mirror to be measured 15 and the pinhole S. Taking the measurement of a convex spherical mirror with a diameter of 200 mm and a radius of curvature of 600 mm as an example, the numerical aperture of the interferometric system is set to 0.5. In order to measure the entire surface of the mirror to be measured 15, the distance between the pinhole S and the vertex O of the mirror to be measured 15 must meet the following conditions:

[0111]

[0112] Step (4) When the above distance conditions are met, the surface shape of the Hindle sphere is optimized by ray tracing, so that the incident direction of the light incident on the Hindle sphere is perpendicular to the surface, that is, it can eventually return to point S. In the embodiment of the present application, the distance between the pinhole S and the vertex O of the test mirror 15 is set to 200 mm, and the design result of the Hindle sphere is 409.8 mm in diameter, 348.52 mm in radius of curvature, 0.61 in cone coefficient, and the second-order term =2.32×10 -4 , fourth-order term =3.42×10 -9 , sixth-order term =1.08×10 -15 , eighth-order term =4.95×10 -21 Please refer to Fig.10 , Fig.10 A schematic diagram of the effective measurement area of ​​the test mirror 15 provided in the embodiment of the present application, that is, a light trace diagram reflected back to the test mirror 15 by the Hindle sphere, wherein the area with a central radius of 4.32 mm is not detected.

[0113] In some embodiments, the interferometer further includes an alignment system for guiding the alignment of the test light to improve the measurement accuracy of the test mirror 15. The alignment system specifically includes a first objective lens 13, a second objective lens 10, a third objective lens 16, a wide field of view objective lens 17, a tube lens 19, a beam splitter 20 and an industrial camera 21, wherein: the test light and the reference light are divided into a reflected light wave and a transmitted light wave after passing through the tube lens 19 and the beam splitter 20, wherein the reflected light wave is focused in the industrial camera 21 to form a test light spot and a reference light spot, and the transmitted light wave is used to obtain a transient phase-shifted interference pattern; the wide field of view objective lens 17 is used to coarsely adjust the phase of the test mirror 15 and the diffraction plate 14 according to the relative positions of the test light spot and the reference light spot. As for the position, the third objective lens 16 is used to finely adjust the relative position of the test mirror 15 and the diffraction plate 14 according to the relative positions of the test light spot and the reference light spot, so that the second spherical wave is focused close to the pinhole after being reflected by the test mirror 15; the first objective lens 13 and the second objective lens 10 are used to adjust the relative positions of the first objective lens 13, the second objective lens 10 and the diffraction plate 14 according to the energy distribution of the test light spot and the reference light spot, so that the area with the strongest energy in the diffuse spot formed after the test light and the reference light pass through the first objective lens 13 and the second objective lens 10 respectively is incident on the pinhole.

[0114] Among them, the third objective lens 16 and the wide field objective lens 17 have the same focal distance, and no other components need to be adjusted when the two are switched. The third objective lens 16 is used for both detection and fine-tuning steps when adjusting the posture of the test mirror 15 so that the test light spot and the reference light spot coincide. The test light and the reference light are divided into reflected light waves and transmitted light waves after passing through the tube lens 19 and the beam splitter prism 20, wherein the test light and the reference light in the reflected light wave are focused by the tube lens 19 in the industrial camera 21 to form a test light spot and a reference light spot. When the test light deviates from the pinhole, it is in an off-axis state. By observing the position where the test light spot deviates from the reference light spot on the target surface of the industrial camera 21, the posture of the reflector can be adjusted accordingly so that the test light is reflected close to the pinhole. According to the energy level of the test light spot and the reference light spot on the target surface of the industrial camera 21, the postures of the first objective lens 13 and the second objective lens 10 can be adjusted accordingly so that the area with the strongest energy in the diffuse spot is incident on the pinhole and diffracted. By adjusting the posture of the test lens 15, the test light spot and the reference light spot in the industrial camera 21 are made to enter the range of the third objective lens 16 or the wide field objective lens 17. When the test light spot and the reference light spot coincide, it indicates that the off-axis error of the test light is small and the test light is reflected close to the pinhole.

[0115] In order to avoid collision between the first objective lens 13, the second objective lens 10 and the diffraction plate 14, the first objective lens 13 and the second objective lens 10 need to be long working distance objective lenses. At the same time, since the glass substrate of the diffraction plate 14 will introduce aberrations and cause the light spot incident on the pinhole to become larger, the first objective lens 13 and the second objective lens 10 are required to be telephoto objective lenses at the same time to reduce the size of the light spot incident on the pinhole. For example, the parameters of the first objective lens 13 and the second objective lens 10 are selected as a magnification of 5X, a working distance of 33.61 mm, and a focal length of 40 mm.

[0116] Likewise, the third objective lens 16 needs to have a long working distance to avoid collision with the second objective lens 10 and the diffraction plate 14. For example, the selected parameters of the third objective lens 16 are NA 0.5 and a working distance of 18.9 mm.

[0117] Among them, the design process of the alignment optical path in the alignment system is as follows:

[0118] Step (1) Design the basic structure of the alignment optical path. The large field of view objective lens 17 needs to be a low-power objective lens for coarse adjustment and to provide a large object field of view. The parameters of the large field of view objective lens 17 are selected as 2X magnification, 100mm focal length, and 12.5mm object field of view. The third objective lens 16 needs to be a high-power objective lens for fine adjustment. The parameters of the third objective lens 16 are selected as 50X magnification, 4mm focal length, and 0.5mm object field of view. In order to facilitate the switching of the third objective lens 16 and the large field of view objective lens 17, the two have the same parfocal distance.

[0119] Step (2) Positioning the target surface of the industrial camera 21. In order to facilitate observation of the focal spot position and protect the target surface of the industrial camera 21, the spot diameter on the target surface of the industrial camera 21 is set to be approximately 80 pixels. Through ray tracing, it is found that the distance between the target surface of the industrial camera 21 and the tube lens 19 is 192 mm.

[0120] Step (3) During the coarse adjustment, the objective lens 17 with a large field of view is switched to work, according to the geometric relationship:

[0121]

[0122] In the above formula, is the focal length of the wide-field objective lens 17, It is the distance from the tube lens 19 to the target surface of the industrial camera 21.

[0123] Please refer to Fig.11 , Fig.11 This is a target surface intensity diagram of the light spot entering the industrial camera 21 when the position of the test mirror 15 relative to the diffraction plate 14 deviates by 1.82 mm in the direction perpendicular to the optical axis of the large field of view objective lens 17 during the coarse adjustment provided in the embodiment of the present application. Please refer to Fig.12 , Fig.12The position of the test mirror 15 relative to the diffraction plate 14 during the coarse adjustment provided in the embodiment of the present application deviates by 1.82 in the direction perpendicular to the optical axis of the large field of view objective lens 17. The intensity diagram of the light spot entering the target surface of the industrial camera 21. Deviation 18.2 At this time, the light spot is 10 pixels away from the center of the target surface of the industrial camera 21.

[0124] Step (4) switches to the third objective lens 16 for fine adjustment, according to the geometric relationship:

[0125]

[0126] In the above formula, is the focal length of the third objective lens 16. Please refer to Fig.13 , Fig.13 During the fine adjustment provided in the embodiment of the present application, the position of the test mirror 15 relative to the diffraction plate 14 deviates by 73% in the direction perpendicular to the optical axis of the large field of view objective lens 17. The intensity diagram of the light spot entering the target surface of the industrial camera 21. Please refer to Fig.14 , Fig.14 During the fine adjustment provided in the embodiment of the present application, the position of the test mirror 15 relative to the diffraction plate 14 deviates by 0.7 in the direction perpendicular to the optical axis of the large field of view objective lens 17. The intensity diagram of the light spot entering the target surface of the industrial camera 21 at the time of At this time, the light spot is 10 pixels away from the center of the target surface of the industrial camera 21.

[0127] In some embodiments, the interferometer also includes a laser 1 and a pre-processing system, wherein the pre-processing system is used to pre-process the laser emitted by the laser 1 to improve the polarization degree of the laser, and the pre-processing system includes a beam expander 2, a fourth objective lens 3, a rotating frosted glass 4, a mask plate 5 with a small hole, a fifth objective lens 6 and a polarizer 7, wherein: the laser emitted by the laser 1 is expanded and collimated by the beam expander 2, and then focused on the small hole of the mask plate 5 after passing through the fourth objective lens 3 and the rotating frosted glass 4, and becomes parallel light after passing through the fifth objective lens 6, and is emitted as linearly polarized light after passing through the polarizer 7.

[0128] Among them, the beam expander 2 is used to expand and collimate the laser emitted by the laser 1. In order to reduce the size of the Airy disk of the fourth objective lens 3 to improve the utilization rate of the laser energy, it is necessary to increase the diameter of the beam incident on the fourth objective lens 3 through the beam expander 2, and the beam diameter should not be greater than the pupil diameter of the fourth objective lens 3. In order to design more redundant space for the mechanical structure and facilitate the installation and adjustment of the system, the fourth objective lens 3 and the fifth objective lens 6 need to be low-magnification and long-working-distance microscope lenses. The fourth objective lens 3 and the fifth objective lens 6 form a 4f system, and a rotating frosted glass 4 and a mask plate 5 with a small hole are placed between the fourth objective lens 3 and the fifth objective lens 6. The rotating frosted glass 4 is arranged near the focal plane of the fourth objective lens 3 to reduce the temporal coherence of the laser to suppress the coherent noise. The small hole on the mask plate 5 is arranged on the focal plane of the fourth objective lens 3 to filter out high-frequency noise and aberrations of the frosted glass. The diameter of the small hole needs to be slightly smaller than the diffuse spot diameter of the fourth objective lens 3. The parallel light emitted by the fifth objective lens 6 is incident on the polarizer 7. The function of the polarizer 7 is to increase the polarization degree of the laser. Although the light beam emitted by the laser 1 is polarized light, the polarization degree is low and does not meet the system requirements. The linearly polarized light emitted by the polarizer 7 is incident on the half-wave plate 8 in the light splitting system.

[0129] In some embodiments, the wavelength of the laser 1 is set to 632.8 nm, the beam diameter is 0.8 mm, and the divergence angle is 1.4 mrad. The pupil diameter of the fourth objective lens 3 is set to 9 nm, and the beam expander 2 is a commercial 10X beam expander 2. After beam expansion, the beam diameter is 8 mm and the divergence angle is 0.14 mrad. The magnification of the third objective lens 16 and the fourth objective lens 3 are both 10X, and the working distance is 15.86 nm. The 632.8 nm extinction ratio is selected. The polarizer 7 is used to improve the polarization state of the laser. The diameter of the diffuse spot is calculated as follows:

[0130]

[0131] In the above formula, λ is the wavelength of the laser 1, NA3 is the numerical aperture of the objective lens 3, D′ is the pupil diameter of the objective lens 3, and D is the diameter of the light beam incident on the objective lens 3. Therefore, the diameter of the small hole on the mask 5 is selected to be 3 μm.

[0132] Please refer to Fig.15 , Fig.15 The schematic diagram of the structure of the point diffraction transient interferometer provided in the embodiment of the present application includes a laser 1, a beam expander 2, a fourth objective lens 3, a rotating frosted glass 4, a mask plate 5 with a small hole, a fifth objective lens 6, a polarizer 7, a half-wave plate 8, a polarization beam splitter prism 9, a first objective lens 13, a first reflector 11, a second reflector 12, a first objective lens 13, a diffraction plate 14, a mirror to be measured 15, a third objective lens 16, a large field of view objective lens 17, a quarter wave plate 18, a tube lens 19, a beam splitter prism 20, an industrial camera 21, an imaging lens 22, and a polarization array camera 23.

[0133] Among them, the laser emitted by the laser 1 is expanded and collimated by the beam expander 2. The expanded and collimated laser is incident on the 4f system composed of the fourth objective lens 3 and the fifth objective lens 6. The parallel light emitted by the fifth objective lens 6 is incident on the polarizer 7. The linearly polarized light emitted by the polarizer 7 is incident on the half-wave plate 8, and the polarization direction of the polarizer 7 is rotated to adjust the contrast of the interference light. Then it passes through the polarization beam splitter prism 9 to actively balance the energy ratio of the interference light and improve the contrast of the interference fringes. The light emitted by the half-wave plate 8 is incident on the polarization beam splitter prism 9 and is divided into the first polarized light and the second polarized light. Among them, the first polarized light is p light, which is incident on the first objective lens 13 after being refracted by the first reflector 11 and the second reflector 12, and is focused on the pinhole of the diffraction plate 14. The second polarized light is s light, which is also focused on the pinhole after passing through the second objective lens 10. The first polarized light is diffracted by the pinhole into the first spherical wave, which is collimated by the third objective lens 16. The second polarized light is diffracted by the pinhole to become a second spherical wave, and then incident on the test mirror 15 , reflected back to the chrome film on the diffraction plate 14 , and then incident on the third objective lens 16 .

[0134] Among them, the factors affecting the numerical aperture (NA) measurement range of the point diffraction transient interferometer provided in the embodiment of the present application include: the numerical apertures of the first spherical wave and the second spherical wave generated by the first polarized light and the second polarized light through the pinhole diffraction, and the numerical aperture of the third objective lens 16, wherein the NA of the third objective lens 16 determines the NA of the reference light and the test light that can be received by the polarization array camera 23. Specifically, the NA required for the mirror 15 to be tested needs to be smaller than the NA of the second spherical wave to ensure that the mirror surface of the mirror 15 to be tested can be effectively measured. The NA required for the mirror 15 to be tested needs to be smaller than the NA of the third objective lens 16. The NA of the third objective lens 16 needs to be greater than or equal to the NA of the interference system.

[0135] After the reference light and the test light pass through the quarter wave plate 18, they become right-handed and left-handed circularly polarized light, respectively, and are incident on the beam splitter prism 20 for splitting after passing through the tube lens 19. A part of the light split by the beam splitter prism 20 is incident on the industrial camera 21. The other part of the light split by the beam splitter prism 20 is incident on the imaging lens 22 for imaging of coherent information. The right-handed polarized light and the left-handed polarized light incident on the polarization array camera 23 pass through four groups of polarization arrays with polarization directions differing by 45 degrees in the polarization array camera 23. Four transient phase-shifting interference patterns can be obtained simultaneously in the image sensor in the polarization array camera 23, and the phase shift amounts are 0, π / 2, π, and 3π / 2, respectively. Among them, the four phase-shifting interference patterns will be used to solve the surface information of the mirror 15 to be tested. The embodiment of the present application adopts the above-mentioned polarization phase shifting method to realize transient measurement, avoiding the influence of environmental factors such as air disturbance and vibration.

[0136] The embodiment of the present application also provides an embodiment of a method for measuring the surface shape of the mirror 15 to be measured. It should be noted that the steps shown in the flowchart of the accompanying drawings can be executed in a computer system such as a set of computer executable instructions, and although a logical order is shown in the flowchart, in some cases, the steps shown or described can be executed in an order different from that shown here.

[0137] Please refer to Fig.16 , Fig.16 The flowchart of the surface measurement method of the mirror 15 to be measured provided in the embodiment of the present application is as follows: Fig.16 As shown, the surface shape measurement method of the mirror to be measured 15 includes the following steps:

[0138] Step S1: a diffraction plate 14 with a pinhole is arranged on the interferometer, and the cross section of the pinhole is arranged to be elliptical.

[0139] Step S3: dividing the linearly polarized light into a first polarized light and a second polarized light whose polarization directions are perpendicular to each other; the first polarized light is incident on the first principal optical axis of the pinhole, and the second polarized light is incident on the second principal optical axis of the pinhole; the projections of the pinhole in the directions of the first principal optical axis and the second principal optical axis are both circular.

[0140] Step S5: The first polarized light is diffracted through the pinhole to form a first spherical wave, which is used as the reference light for interference measurement. The second polarized light is diffracted through the pinhole to form a second spherical wave and is irradiated toward the mirror to be measured 15. After being reflected by the mirror to be measured 15, it is focused and reflected close to the pinhole and used as the test light for interference measurement. The test light and the reference light are polarization-shifted to generate a transient phase-shift interference pattern.

[0141] Step S7: Analyze the transient phase-shift interferogram, and extract the surface information of the mirror to be tested 15 according to the analysis result.

[0142] The phase information is obtained by analyzing the obtained multiple transient phase-shifting interferograms. Extracting the surface information of the mirror to be tested 15 includes calculating the peak-to-valley value (PV) and the root mean square value (RMS) of the surface error of the mirror to be tested according to the phase information, wherein PV or RMS are two important parameters for characterizing the surface error of the optical surface of the mirror to be tested 15.

[0143] In some embodiments, the above-mentioned surface measurement method of the mirror to be tested 15 further includes: dividing the test light and the reference light into reflected light waves and transmitted light waves after passing through the tube lens 19 and the beam splitter prism 20, wherein the reflected light waves are focused in the industrial camera 21 to form a test light spot and a reference light spot, and the transmitted light waves are used to obtain a transient phase-shifting interference pattern; according to the relative positions of the test light spot and the reference light spot, adjusting the relative positions of the mirror to be tested 15 and the diffraction plate 14 so that the test light spot and the reference light spot coincide with each other, so that the second spherical wave is focused close to the pinhole after being reflected by the mirror to be tested 15. According to the energy distribution of the test light spot and the reference light spot, adjusting the relative positions of the first objective lens 13, the second objective lens 10 and the diffraction plate 14 so that the area with the strongest energy in the diffuse spot formed after the first polarized light and the second polarized light respectively pass through the first objective lens and the second objective lens is incident on the pinhole.

[0144] Specifically, the relative position of the test mirror 15 and the diffraction plate 14 is adjusted so that the test light spot and the reference light spot coincide with each other, including: firstly coarsely adjusting the relative position between the test light spot and the reference light spot, and then finely adjusting the relative position between the test light spot and the reference light spot to achieve the coincidence of the test light spot and the reference light spot, so as to improve the measurement accuracy.

[0145] The further functional description of each of the above modules and units is the same as that of the above corresponding embodiments and will not be repeated here.

[0146] The reflective point diffraction interferometer in this embodiment is presented in the form of a functional unit, where the unit refers to an ASIC (Application Specific Integrated Circuit) circuit, a processor and memory that executes one or more software or fixed programs, and / or other devices that can provide the above functions.

[0147] See also Fig.17 , Fig.17 A schematic diagram of the structure of a computer device provided in an embodiment of the present application is shown in FIG. Fig.17As shown, the computer device includes: one or more processors, memory, and interfaces for connecting various components, including high-speed interfaces and low-speed interfaces. Various components utilize different buses to communicate with each other, and can be installed on a common mainboard or installed in other ways as required. The processor can process the instructions executed in the computer device, including instructions stored in the memory or on the memory to display the graphical information of the GUI on an external input / output device (such as, a display device coupled to the interface). In some optional embodiments, if necessary, multiple processors and / or multiple buses can be used together with multiple memories and multiple memories. Similarly, multiple computer devices can be connected, and each device provides part of the necessary operations (for example, as a server array, a group of blade servers, or a multi-processor system). Fig.17 A processor is taken as an example.

[0148] The processor may be a central processing unit, a network processor or a combination thereof. The processor may further include a hardware chip. The hardware chip may be a dedicated integrated circuit, a programmable logic device or a combination thereof. The programmable logic device may be a complex programmable logic device, a field programmable gate array, a general purpose array logic or any combination thereof.

[0149] The memory stores instructions executable by at least one processor, so that the at least one processor executes the method shown in the above embodiment.

[0150] The memory may include a program storage area and a data storage area, wherein the program storage area may store an operating system, an application required for at least one function; the data storage area may store data created according to the use of the computer device, etc. In addition, the memory may include a high-speed random access memory, and may also include a non-transient memory, such as at least one disk storage device, a flash memory device, or other non-transient solid-state storage device. In some optional embodiments, the memory may optionally include a memory remotely arranged relative to the processor, and these remote memories may be connected to the computer device via a network. Examples of the above-mentioned network include, but are not limited to, the Internet, an intranet, a local area network, a mobile communication network, and combinations thereof.

[0151] The memory may include volatile memory, such as random access memory; the memory may also include non-volatile memory, such as flash memory, hard disk or solid state drive; the memory may also include a combination of the above types of memory. The computer device also includes a communication interface for the computer device to communicate with other devices or communication networks.

[0152] The embodiment of the present application also provides a computer-readable storage medium. The above method according to the embodiment of the present application can be implemented in hardware, firmware, or can be implemented as a computer code that can be recorded in a storage medium, or can be implemented as a computer code that is originally stored in a remote storage medium or a non-temporary machine-readable storage medium and will be stored in a local storage medium through a network download, so that the method described herein can be stored in such software processing on a storage medium using a general-purpose computer, a dedicated processor, or programmable or dedicated hardware. Among them, the storage medium can be a magnetic disk, an optical disk, a read-only storage memory, a random access memory, a flash memory, a hard disk or a solid-state hard disk, etc.; further, the storage medium can also include a combination of the above types of memories. It can be understood that a computer, a processor, a microprocessor controller, or programmable hardware includes a storage component that can store or receive software or computer code. When the software or computer code is accessed and executed by a computer, a processor, or hardware, the method shown in the above embodiment is implemented.

[0153] The embodiment of the present application provides a computer program product, which includes computer instructions, which are stored in a computer-readable storage medium. A processor of a computer device reads the computer instructions from the computer-readable storage medium, and the processor executes the computer instructions, so that the computer device executes the method of any embodiment of the present application.

[0154] Although the embodiments of the present application are described in conjunction with the accompanying drawings, those skilled in the art may make various modifications and variations without departing from the spirit and scope of the present application, and such modifications and variations are all within the scope defined by the appended claims.

[0155] The systems or units described in the above embodiments may be implemented by computer chips or entities, or by products with certain functions. A typical implementation device is a computer. Specifically, the computer may be, for example, a personal computer, a laptop computer, a cellular phone, a camera phone, a smart phone, a personal digital assistant, a media player, a navigation device, an email device, a game console, a tablet computer, a wearable device, or a combination of any of these devices.

[0156] For the convenience of description, the above device is described in various units according to their functions. Of course, when implementing the present application, the functions of each unit can be implemented in the same or multiple software and / or hardware.

[0157] Those skilled in the art will appreciate that the embodiments of the present application may be provided as methods, systems, or computer program products. Therefore, the present application may adopt the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware. Moreover, the present application may adopt the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program codes.

[0158] The present application is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0159] These computer program instructions may also be stored in a computer-readable memory capable of directing a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer-readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 A process or multiple processes and / or boxes Figure 1 A function specified in one or more boxes.

[0160] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process in the computer or other programmable device. Figure 1 A process or multiple processes and / or boxes Figure 1 The steps for the functions specified in one or more boxes.

[0161] It should also be noted that the terms "include", "comprises" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, commodity or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, commodity or device. In the absence of more restrictions, the elements defined by the sentence "comprises a ..." do not exclude the existence of other identical elements in the process, method, commodity or device including the elements.

[0162] Each embodiment in this specification is described in a progressive manner, and the same or similar parts between the embodiments can be referred to each other, and each embodiment focuses on the differences from other embodiments. In particular, for the system embodiment, since it is basically similar to the method embodiment, the description is relatively simple, and the relevant parts can be referred to the partial description of the method embodiment.

[0163] The above are only embodiments of the present application and are not intended to limit the present application. For those skilled in the art, the present application may have various changes and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application should be included within the scope of the claims of the present application.

[0164] Although the embodiments of the present application have been described in conjunction with the accompanying drawings, those skilled in the art may make various modifications and variations without departing from the spirit and scope of the present application, and such modifications and variations are all within the scope defined by the appended claims.

Claims

1. A reflective point diffraction interferometer, characterized in that: It comprises a diffraction plate, on which a pinhole with an elliptical cross section is provided, wherein: The first polarized light is incident on the first principal optical axis of the pinhole, and the second polarized light is incident on the second principal optical axis of the pinhole, and the polarization directions of the first polarized light and the second polarized light are perpendicular to each other; The projections of the pinhole in the directions of the first principal optical axis and the second principal optical axis are both circular; The first polarized light is diffracted through the pinhole to form a first spherical wave, which is used as a reference light for interference measurement; the second polarized light is diffracted through the pinhole to form a second spherical wave and is directed to the mirror to be measured, and is reflected by the mirror to be measured, and after being focused and reflected close to the pinhole, it is used as a test light for interference measurement. The test light and the reference light are polarization-phase shifted to generate a transient phase-shifted interferogram, and the transient phase-shifted interferogram is used to extract the surface information of the mirror to be measured; The interferometer further includes: an alignment system for guiding the test light alignment to improve the measurement accuracy of the mirror to be tested, and the alignment system specifically includes a first objective lens, a second objective lens, a third objective lens, a large field of view objective lens, a tube lens, a beam splitter prism and an industrial camera, wherein: The test light and the reference light are divided into reflected light waves and transmitted light waves after passing through the tube lens and the beam splitter prism, wherein the reflected light waves are focused in the industrial camera to form test light spots and reference light spots, and the transmitted light waves are used to obtain transient phase-shift interference patterns; The large-field-of-view objective lens is used to roughly adjust the relative position of the mirror to be tested and the diffraction plate according to the relative positions of the test light spot and the reference light spot, and the third objective lens is used to finely adjust the relative position of the mirror to be tested and the diffraction plate according to the relative positions of the test light spot and the reference light spot, so that the second spherical wave is focused close to the pinhole after being reflected by the mirror to be tested; The first objective lens and the second objective lens are used to adjust the relative positions of the first objective lens, the second objective lens and the diffraction plate according to the energy distribution of the test light spot and the reference light spot, so that the area with the strongest energy in the diffuse spot formed after the first polarized light and the second polarized light pass through the first objective lens and the second objective lens respectively is incident on the pinhole.

2. The interferometer according to claim 1, characterized in that The interferometer further comprises: A polarization phase shifting system, used for splitting the linearly polarized light into the first polarized light and the second polarized light, and realizing phase shifting by changing the polarization states of the reference light and the test light; The imaging system is used to receive the reference light and the test light to obtain a transient phase-shift interference pattern.

3. The interferometer according to claim 2, characterized in that The polarization phase shifting system specifically includes a half-wave plate, a polarization beam splitter prism, a quarter-wave plate, and a polarization array, wherein: The linearly polarized light is divided into the first polarized light and the second polarized light whose polarization directions are perpendicular to each other after passing through the half-wave plate and the polarization splitter prism; wherein the half-wave plate is used to adjust the light intensity ratio of the first polarized light and the second polarized light; The reference light and the test light are transformed into two circularly polarized lights with opposite rotation directions by a quarter wave plate and then phase-shifted by a polarization array.

4. The interferometer according to claim 2, characterized in that The structure of the imaging system adopts an image-side telecentric optical path, and specifically includes a third objective lens, a tube lens, an imaging lens and an image sensor, wherein the reference light and the test light generate a transient phase-shifting interference pattern in the image sensor after passing through the third objective lens, the tube lens and the imaging lens.

5. The interferometer according to claim 2, characterized in that The interferometer further includes a laser and a pre-processing system, wherein the pre-processing system is used to pre-process the laser emitted by the laser, and the pre-processing system includes a beam expander, a fourth objective lens, a rotating frosted glass, a mask plate with a small hole, a fifth objective lens and a polarizer, wherein: The laser emitted by the laser is expanded and collimated by a beam expander, then focused on the small hole of the mask plate through the fourth objective lens and rotating frosted glass, becomes parallel light after passing through the fifth objective lens, and is emitted as linearly polarized light after passing through a polarizer.

6. The interferometer according to any one of claims 1 to 5, characterized in that: The diffraction plate is a glass substrate on which a chrome film is plated, and the pinholes are etched on the chrome film. The material of the glass substrate is C7980, and the thickness is set to 3 mm.

7. The interferometer according to any one of claims 1 to 5, characterized in that: The interferometer further comprises a Hindle sphere, which is arranged between the diffraction plate and the mirror to be measured and is used for measuring the convex spherical mirror to be measured.

8. A surface measurement method, the surface measurement method is implemented based on the reflective point diffraction interferometer according to any one of claims 1 to 7, characterized in that: The method comprises: A diffraction plate with a pinhole is arranged on the interferometer, wherein the cross section of the pinhole is arranged to be elliptical; The linearly polarized light is divided into a first polarized light and a second polarized light whose polarization directions are perpendicular to each other; the first polarized light is incident on the first principal optical axis of the pinhole, and the second polarized light is incident on the second principal optical axis of the pinhole; the projections of the pinhole in the directions of the first principal optical axis and the second principal optical axis are both circular; The first polarized light is diffracted through the pinhole to form a first spherical wave, which is used as the reference light for interference measurement; the second polarized light is diffracted through the pinhole to form a second spherical wave and is directed to the mirror to be measured, and is reflected by the mirror to be measured. After being focused and reflected close to the pinhole, it is used as the test light for interference measurement. The test light and the reference light are polarization-phase shifted to generate a transient phase-shifted interference pattern; Analyzing the transient phase-shift interferogram, and extracting the surface information of the mirror to be measured according to the analysis result; The test light and the reference light are separated into reflected light waves and transmitted light waves after passing through a tube lens and a beam splitter prism, wherein the reflected light waves are focused in an industrial camera to form a test light spot and a reference light spot, and the transmitted light waves are used to obtain a transient phase-shift interference pattern; According to the relative positions of the test light spot and the reference light spot, adjusting the relative positions of the test mirror and the diffraction plate, so that the second spherical wave is focused close to the pinhole after being reflected by the test mirror; According to the energy distribution of the test light spot and the reference light spot, the relative positions of the first objective lens, the second objective lens and the diffraction plate are adjusted so that the area with the strongest energy in the diffuse spot formed after the first polarized light and the second polarized light pass through the first objective lens and the second objective lens respectively is incident on the pinhole.

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