Mirror surface shape measurement system and method based on ultrafast high-precision laser scanning

Abstract

The invention discloses a mirror surface shape measurement system and method based on ultrafast high-precision laser scanning, and aims to solve the problems that the surface shape detection precision is low, the surface shape detection size is small, the absolute positioning precision is insufficient, and the system and method are liable to be vibrated in the environment in the conventional surface shape detection technology. The system specifically comprises a pulse laser, a beam expanding and splitting module, an interference module, a four-step phase shift module, a detection module, a receiving module and an upper computer. The pulse laser emits pulse laser; the beam expanding and light splitting module divides the pulse laser into 0-level diffraction light and + / -1-level diffraction light which are parallel to each other; the interference module receives the 0-level diffraction light and the + / -1-level diffraction light to form multi-level diffraction interference light; the four-step phase shift module performs four-step phase shift light splitting on the multi-stage diffraction interference light; the detection module receives the phase-shifted light beam and performs photoelectric conversion to form a 0-level phase-shifted signal, a + 1-level phase-shifted signal and a-1-level phase-shifted signal; the receiving module receives a 0-level phase shift signal, a + 1-level phase shift signal and a-1-level phase shift signal; and the upper computer is used for restoring the surface shape information of the to-be-measured mirror.

Description

Technical Field

[0001] The invention relates to a mirror surface shape measurement system and method, and in particular to a mirror surface shape measurement system and method based on ultrafast high-precision laser scanning. Background Art

[0002] The surface measurement using pulsed laser is mainly based on the large-aperture measuring beam composed of Michelson structure, while the laser scanning interferometer uses the laser beam to scan the measuring substrate, continuously records the surface information of the measured surface, and uses the shape and position of the laser beam interference fringes to feedback the height information of the measured surface to restore the measured surface information. Compared with the large-aperture surface measurement of Zygo interferometer and 4D interferometer, it avoids the processing of ultra-precision standard mirrors, and can further improve the surface aperture of the scanning mirror as the scanning range and accuracy increase. In view of the technical problem that the surface size continues to increase in actual measurement, but the measurement accuracy continues to decrease, the use of ultrashort pulse laser and dual-cavity mirror laser probe for surface scanning is one of the key technologies to solve this problem.

[0003] In the research of surface scanning interferometers, Michelson, Milau, Linnik and Fizeau are more common. On the market, the more prominent equipment is the British Taylor Luphoscan 850HD interferometer, which has a measuring aperture of up to 850mm, a measuring height range of up to 210mm, a surface error accuracy of up to λ / 20, an RMS better than 5nm, and a maximum slope change of the surface of up to ±8°. After real-time compensation using active anti-vibration systems, temperature sensors and air pressure sensors, high repeatability measurements of 40nm can be achieved; in addition, there are the 650mm*650mm sub-nanometer scanning interferometer of Zygo Company in the United States and the high-precision laser interferometer of Zeiss in Germany with an RMS better than 0.02nm. In scientific research, the more prominent ones are the INF800-LP-WM laser interferometers of Beijing Institute of Technology and Taike Company, with an effective aperture of 800mm, a PV better than λ / 12, and a system RMS repeatability measurement accuracy better than 0.63nm. At the same time, TMC air flotation is used for real-time vibration compensation. In addition, there are also the spliced ​​interferometer of the National University of Defense Technology, the processing capability below 300mm of the Chengdu Institute of Optoelectronics, and the 24-inch laser interferometer of the Institute of Optoelectronics Technology with a 600mm aperture. At present, the EUV band lithography machine has entered the manufacturing accuracy of 3nm, or even PM level, so it is necessary to use multi-cross technology to solve a series of problems such as low surface monitoring accuracy, high difficulty in surface processing, high difficulty in surface calibration, and the influence of environmental vibration, temperature, and air pressure interference. Summary of the invention

[0004] The purpose of the present invention is to provide a mirror surface measurement system and method based on ultrafast high-precision laser scanning to solve the technical problems existing in the existing surface detection technology, such as low surface detection accuracy, small surface detection size, insufficient absolute positioning accuracy, and susceptibility to interference from vibration, air pressure and temperature in the environment.

[0005] In the present invention: firstly, an ultrashort pulse laser of 500ps is used for interference measurement; secondly, multiple reflections of a double-cavity mirror are used to improve measurement accuracy; thirdly, phase coupling of ±1st-order diffraction light of a one-dimensional transmission grating is used to reduce the influence of environmental errors; fourthly, the 0th-order diffraction light of a one-dimensional transmission grating is used for absolute positioning and real-time precise positioning measurement, so as to achieve the purpose of ultra-precision surface measurement.

[0006] In order to achieve the above object, the present invention adopts the following technical solution:

[0007] A mirror surface shape measurement system based on ultrafast high-precision laser scanning is used for measuring the surface shape of the mirror to be measured. The system is special in that it includes a pulse laser, a beam expansion and splitting module, an interference module, a four-step phase shift module, a detection module, a receiving module and a host computer.

[0008] The pulse laser emits pulse laser;

[0009] The beam expansion and splitting module splits the pulsed laser into mutually parallel 0th order diffraction light and ±1st order diffraction light;

[0010] The interference module receives the 0th order diffraction light and the ±1st order diffraction light, and makes the three enter the measurement light path and the reference light path of the interference module respectively for interference, thereby forming multi-order diffraction interference light; the mirror to be measured is arranged on the measurement light path of the 0th order diffraction light and the ±1st order diffraction light;

[0011] The four-step phase shift module is used to perform four-step phase shift splitting on the multi-order diffraction interference light to form a phase-shifted light beam; the phase-shifted light beam includes 0th order diffraction interference light and ±1st order diffraction interference light with a phase of 0°, 0th order diffraction interference light and ±1st order diffraction interference light with a phase of 90°, 0th order diffraction interference light and ±1st order diffraction interference light with a phase of 180°, and 0th order diffraction interference light and ±1st order diffraction interference light with a phase of 270°.

[0012] The detection module is used to receive the phase-shifted light beam and perform photoelectric conversion to form a 0-level phase-shifted signal, a +1-level phase-shifted signal and a -1-level phase-shifted signal;

[0013] The receiving module receives the 0-level phase shift signal, the +1-level phase shift signal and the -1-level phase shift signal respectively, and uploads them to the host computer;

[0014] The host computer is used to perform phase unwrapping calculation on the 0-level phase shift signal, the +1-level phase shift signal and the -1-level phase shift signal, display linear measurement data, and then use the linear measurement data to restore the surface information of the mirror to be measured.

[0015] Furthermore, the pulse laser is an ultrashort pulse laser, which is used to emit 532nm pulse laser with a repetition frequency of 1-1000Hz, a pulse width of 500ps, and a single pulse energy of 1mJ.

[0016] Furthermore, the beam expansion and splitting module includes a first converging lens, a beam expansion device, a one-dimensional transmission grating, a second converging lens and an aperture, all of which are located in the optical path of the pulsed laser and are sequentially arranged along the emission direction thereof;

[0017] The first converging lens is used to make the pulsed laser incident on the beam expansion device in parallel;

[0018] The beam expansion device is used to expand the beam of parallel incident pulsed laser;

[0019] The one-dimensional transmission grating is used to form 0th order diffraction light and ±1st order diffraction light;

[0020] The second converging lens is used to make the 0th order diffracted light and the ±1st order diffracted light respectively incident on the aperture in parallel;

[0021] The aperture is used to isolate the overlapping part of the light beams of the 0th order diffraction light and the ±1st order diffraction light, and at the same time change the energy size and the aperture size of the incident light spot.

[0022] Further, the interference module comprises a first polarization beam splitter prism having four sides, wherein the first side is opposite to the third side, and the second side is opposite to the fourth side;

[0023] The first side corresponds to the aperture and is used to receive the 0th order diffraction light and the ±1st order diffraction light;

[0024] The second side corresponds to the four-step phase shift module, and a half-wave plate is provided on the optical path between the second side and the four-step phase shift module for emitting multi-order diffraction interference light;

[0025] The third side is provided with a second quarter wave plate, a second partially transmitting cavity mirror and a high reflective mirror in sequence along a direction away from the third side, forming a reference optical path;

[0026] The fourth side is provided with a first quarter wave plate, a first partially transmissive cavity mirror, and a first focusing lens in sequence along a direction away from the fourth side; a side of the first focusing lens away from the first partially transmissive cavity mirror is used to set a mirror to be measured; the first quarter wave plate, the first partially transmissive cavity mirror, the first focusing lens, and the mirror to be measured constitute a measuring optical path;

[0027] The first partially transmissive cavity mirror and the second partially transmissive cavity mirror are both double cavity mirrors, and one side of the two close to the first polarization beam splitter prism is coated with a reflective film, and the other side is coated with an anti-reflection film, the reflectivity of the reflective film is ≥90%, and the transmittance of the anti-reflection film is ≥90%;

[0028] The beam splitting surface of the first polarization beam splitter prism is at an equal distance from the rear surfaces of the first partially transmissive cavity mirror and the second partially transmissive cavity mirror; the distance from the rear surface of the first partially transmissive cavity mirror to the reflective surface of the mirror to be measured is equal to the distance from the rear surface of the second partially transmissive cavity mirror to the reflective surface of the high reflective mirror; the rear surface is the surface away from the first polarization beam splitter prism.

[0029] Further, the four-step phase shift module includes a beam splitter prism, a second polarization beam splitter prism, and a third polarization beam splitter prism;

[0030] The beam splitter prism is used to split the energy of the multi-order diffraction interference light emitted by the half-wave plate to form reflected light and transmitted light; the beam splitter prism has a splitting ratio of 5:5;

[0031] The second polarization beam splitter prism is located on the optical path of the transmitted light, and a third quarter wave plate is provided on the optical path between the second polarization beam splitter prism and the beam splitter prism, for polarization-splitting the transmitted light to form 0th order diffraction interference light and ±1st order diffraction interference light with a phase of 90°, and 0th order diffraction interference light and ±1st order diffraction interference light with a phase of 270°;

[0032] The third polarization beam splitter prism is located on the optical path of the reflected light, and a fourth quarter-wave plate is arranged on the optical path between the third polarization beam splitter prism and the beam splitter prism, for polarization-splitting the reflected light to form 0th-order diffraction interference light and ±1st-order diffraction interference light with a phase of 0°, and 0th-order diffraction interference light and ±1st-order diffraction interference light with a phase of 180°.

[0033] Furthermore, the polarization angles of the first quarter wave plate, the second quarter wave plate, the third quarter wave plate, and the fourth quarter wave plate are all 45°.

[0034] Further, the detection module includes first to twelfth photodetectors;

[0035] The first photodetector is used to receive the 0th order diffraction interference light with a phase of 0° and perform photoelectric conversion to form a first 0th order phase shift signal;

[0036] The second photodetector is used to receive the 0th order diffraction interference light with a phase of 180° and perform photoelectric conversion to form a second 0th order phase shift signal;

[0037] The third photodetector is used to receive the 0th order diffraction interference light with a phase of 90° and perform photoelectric conversion to form a third 0th order phase shift signal;

[0038] The fourth photodetector is used to receive the 0th order diffraction interference light with a phase of 270° and perform photoelectric conversion to form a fourth 0th order phase shift signal;

[0039] The fifth photodetector is used to receive the +1-order diffraction interference light with a phase of 0° and perform photoelectric conversion to form a first +1-order phase shift signal;

[0040] The sixth photodetector is used to receive the +1st order diffraction interference light with a phase of 180° and perform photoelectric conversion to form a second +1st order phase shift signal;

[0041] The seventh photodetector is used to receive the +1-order diffraction interference light with a phase of 90° and perform photoelectric conversion to form a third +1-order phase shift signal;

[0042] The eighth photodetector is used to receive the +1-order diffraction interference light with a phase of 270° and perform photoelectric conversion to form a fourth +1-order phase shift signal;

[0043] The ninth photodetector is used to receive the -1 order diffraction interference light with a phase of 0° and perform photoelectric conversion to form a first -1 order phase shift signal;

[0044] The tenth photodetector is used to receive the -1st order diffraction interference light with a phase of 180° and perform photoelectric conversion to form a second -1st order phase shift signal;

[0045] The eleventh photodetector is used to receive the -1 order diffraction interference light with a phase of 90° and perform photoelectric conversion to form a third -1 order phase shift signal;

[0046] The twelfth photodetector is used to receive the -1 order diffraction interference light with a phase of 270° and perform photoelectric conversion to form a fourth -1 order phase shift signal;

[0047] The first to fourth 0-level phase-shifted signals are the 0-level phase-shifted signals; the first to fourth +1-level phase-shifted signals are the +1-level phase-shifted signals; and the first to fourth -1-level phase-shifted signals are -1-level phase-shifted signals.

[0048] Further, the receiving module includes a first oscilloscope, a second oscilloscope and a third oscilloscope;

[0049] The first oscilloscope is used to receive the 0-level phase shift signal and upload it to the host computer;

[0050] The second oscilloscope is used to receive the +1 level phase shift signal and upload it to the host computer;

[0051] The third oscilloscope is used to receive the -1 level phase shift signal and upload it to the host computer.

[0052] Furthermore, the interference module also includes a second focusing lens;

[0053] The second focusing lens is arranged on the optical path between the second partially transmitting cavity mirror and the high reflecting mirror.

[0054] A mirror surface shape measurement method based on ultrafast high-precision laser scanning, using the above-mentioned mirror surface shape measurement system based on ultrafast high-precision laser scanning, is special in that it includes the following steps:

[0055] Step 1, without installing the mirror to be tested and the high reflective mirror, the pulse laser emits pulse laser;

[0056] Step 2: Adjust the pulse laser so that the pulse laser emitted by it is horizontally incident on the beam expansion and spectrometer module, which divides the pulse laser into parallel 0th order diffraction light and ±1st order diffraction light, and adjust the interference module so that the 0th order diffraction light and the 0th order reflected light of the ±1st order diffraction light can be completely incident on the center of the detection photosensitive surface of the detection module after passing through the interference module and the four-step phase shift module, thereby completing the preliminary surface positioning;

[0057] Step 3, install a high reflector, and adjust the spatial position of the high reflector so that the multi-order light of the 0th order diffraction light and the multi-order light of the ±1st order diffraction light formed on the reference light path of the interference module can be completely incident on the center of the detection photosensitive surface of the detection module, thereby completing the precise surface positioning of the reference light path;

[0058] Step 4, install the mirror to be measured, and adjust the spatial position of the mirror to be measured so that the multi-order light of the 0th order diffraction light and the multi-order light of the ±1st order diffraction light formed on the measurement optical path of the interference module are parallel to and overlap with the light formed on the reference optical path, and can be completely incident on the center of the detection photosensitive surface of the detection module, thereby completing the precise surface positioning of the measurement optical path;

[0059] Step 5, repeatedly cross linearly move the mirror to be tested along the horizontal line and the vertical line, and after each movement, the detection module obtains the 0-level phase shift signal, +1-level phase shift signal and -1-level phase shift signal of the mirror to be tested, and uploads them to the host computer through the receiving module, wherein the 0-level phase shift signal is used as the control group, and the ±1-level phase shift signal is calibrated in real time for the surface interferometry information;

[0060] Step 6: The host computer performs phase unwrapping calculations on all acquired 0-level phase shift signals, +1-level phase shift signals, and -1-level phase shift signals, displays multiple sets of linear measurement data, and then uses the multiple sets of linear measurement data to restore the surface information of the mirror to be measured.

[0061] Beneficial effects of the present invention:

[0062] 1. The mirror surface measurement system and method based on ultrafast high-precision laser scanning provided by the present invention improve the surface detection accuracy, can adapt to larger surface detection sizes, and avoid the influence of phase noise and environmental errors.

[0063] 2. The present invention utilizes an ultrashort pulse laser with a pulse width of 500 ps, ​​so that the resolution can reach the ps level in the time domain; secondly, the present invention uses a dual-cavity mirror structure and consistent optical path characteristics to achieve multiple reflection measurements, and each reflection can carry surface information once, greatly improving the surface measurement accuracy; then, the present invention uses a one-dimensional transmission grating, through the phase decoupling of the ±1st order diffraction light, to reduce the influence of air pressure, vibration and temperature during the propagation of the light beam, further improving the surface measurement accuracy.

[0064] 3. The present invention utilizes multiple reflections of a dual-cavity mirror and intends to use pulse information from 100 reflections to perform 100 surface shape information superpositions. At the same time, by utilizing the cross-linear movement of the measuring mirror, the measurement size can be expanded as the measuring mirror moves. By performing multiple continuous scanning measurements, the surface shape information of the entire measuring mirror can be restored, thereby improving the detection size of the measuring surface shape.

[0065] 4. In the present invention, firstly, the 0th order light of the 0th order diffraction light and the 0th order light of the ±1st order diffraction light of the partially transmitted cavity mirror of the dual cavity mirror in the laser probe are used to perform preliminary precision surface positioning; secondly, the 0th order diffraction light in the laser probe is used to perform precise surface positioning; thirdly, the 0th order diffraction light is used to perform real-time surface interference information calibration under the state of real-time measurement of multi-order light of the ±1st order diffraction light, so as to achieve the purpose of ultra-precision interferometer surface detection and positioning measurement.

[0066] 5. First, by using ultrashort pulse laser, each pulse width is 500ps, and the pulse width of 100 round trips is about 300ps. Each pulse recognition can form an equal comb sequence measurement signal, and equal order recognition can avoid the influence of phase noise; second, by utilizing the mutual multiple reflections and interference of ±1-order diffraction light, after phase decoupling, equal phase recognition information of 2Δφ, 4Δφ...2nΔφ is formed within a pulse; third, by using four-step phase shift, different diffraction interference information is formed through different channels, so as to realize the difference in the effective recognition time of the pulse signal and the effective recognition time of the stray light, and effectively avoid the influence of the stray light on the detection results. BRIEF DESCRIPTION OF THE DRAWINGS

[0067] Figure 1 It is a structural schematic diagram of Embodiment 1 of a mirror surface shape measurement system and method based on ultrafast high-precision laser scanning of the present invention;

[0068] Figure 2 is a schematic diagram of light beam diffraction of a one-dimensional transmission grating in Embodiment 1 of the present invention;

[0069] Figure 3 1 is a schematic diagram of the transmission of the light beam of the second part of the reference light path through the multi-level reflected light of the cavity mirror in the first embodiment of the present invention;

[0070] Figure 4 1 is a schematic diagram of the transmission of the light beam of the multi-level reflected light of the first part of the measuring light path through the cavity mirror in the first embodiment of the present invention;

[0071] Figure 5 It is a structural schematic diagram of Embodiment 2 of the mirror surface shape measurement system and method based on ultrafast high-precision laser scanning of the present invention.

[0072] Figure Number:

[0073] 1-pulse laser, 201-first converging lens, 202-beam expander, 203-one-dimensional transmission grating, 204-second converging lens, 205-aperture, 3-interference module, 4-four-step phase shift module, 5-detection module, 6-receiving module, 301-first polarization beam splitter prism, 404-second polarization beam splitter prism, 405-third polarization beam splitter prism, 305-first quarter wave plate, 302-second quarter wave plate, 402-third quarter wave plate, 403-fourth quarter wave plate, 306-first partial transmission cavity mirror, 303-second partial transmission cavity mirror, 307-first focusing lens, 309-second Focusing lens, 304-high reflective mirror, 8-mirror to be tested, 308-half wave plate, 401-beam splitter prism, 501-first photodetector, 502-second photodetector, 503-third photodetector, 504-fourth photodetector, 505-fifth photodetector, 506-sixth photodetector, 507-seventh photodetector, 508-eighth photodetector, 509-ninth photodetector, 510-tenth photodetector, 511-eleventh photodetector, 512-twelfth photodetector, 601-first oscilloscope, 602-second oscilloscope, 603-third oscilloscope, 7-host computer. DETAILED DESCRIPTION

[0074] The technical solution of the present invention will be clearly and completely described below in conjunction with the accompanying drawings and embodiments. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0075] Embodiment 1:

[0076] The embodiment of the present invention provides a mirror surface measurement system based on ultrafast high-precision laser scanning, which is used for measuring the surface shape of a mirror 8 to be measured. Figure 1 As shown, it includes a pulse laser 1, a beam expansion and splitting module 2, an interference module 3, a four-step phase shift module 4, a detection module 5, a receiving module 6 and a host computer 7;

[0077] The pulse laser 1 is an ultrashort pulse laser, which is used to emit a 532nm pulse laser with a repetition frequency of 1-1000Hz, a pulse width of 500ps, and a single pulse energy of 1mJ.

[0078] The beam expansion and spectroscopic module 2 includes a first converging lens 201, a beam expansion device 202, a one-dimensional transmission grating 203, a second converging lens 204 and an aperture 205, which are all located in the optical path of the pulse laser and are arranged in sequence along its emission direction; the first converging lens 201 is used to make the pulse laser incident on the beam expansion device 202 in parallel; the beam expansion device 202 is used to expand the parallel incident pulse laser; the one-dimensional transmission grating 203 is used to form 0th order diffraction light and ±1st order diffraction light; the second converging lens 204 is used to make the 0th order diffraction light and ±1st order diffraction light respectively incident on the aperture 205 in parallel; the aperture 205 is used to isolate the overlapping part of the beams of the 0th order diffraction light and the ±1st order diffraction light, and at the same time change its energy size and the aperture size of the incident light spot.

[0079] The interference module 3 receives the 0th order diffraction light and the ±1st order diffraction light, and makes the three enter the measurement optical path and the reference optical path of the interference module 3 respectively for interference, so as to form multi-order diffraction interference light; the mirror to be measured 8 is arranged on the measurement optical path of the 0th order diffraction light and the ±1st order diffraction light; the interference module 3 includes a first polarization beam splitter prism 301 with four sides, wherein the first side is opposite to the third side, and the second side is opposite to the fourth side; the first side corresponds to the aperture 205; the second side corresponds to the four-step phase shift module 4, and a half wave plate 308 is arranged on the optical path between the second side and the four-step phase shift module 4; the third side is provided with a second quarter wave plate 302, a second partially transmitting cavity mirror 303 and a high reflector 304 in sequence along the direction away from the third side; the fourth side is provided with a first quarter wave plate 305, a first partially transmitting cavity mirror 306 and a high reflector 307 in sequence along the direction away from the fourth side A partially transmitting cavity mirror 306 and a first focusing lens 307; the side of the first focusing lens 307 away from the first partially transmitting cavity mirror 306 is used to set the mirror to be measured 8; the first partially transmitting cavity mirror 306 and the second partially transmitting cavity mirror 303 are both double cavity mirrors, and the side of the two close to the first polarization beam splitter prism 301 is coated with a reflective film, and the other side is coated with an anti-reflection film, the reflectivity of the reflective film is ≥90%, and the transmittance of the anti-reflection film is ≥90%; the splitting surface of the first polarization beam splitter prism 301 is at the same distance from the rear surface of the first partially transmitting cavity mirror 306 and the second partially transmitting cavity mirror 303; the distance from the rear surface of the first partially transmitting cavity mirror 306 to the reflective surface of the mirror to be measured 8 is equal to the distance from the rear surface of the second partially transmitting cavity mirror 303 to the reflective surface of the high reflector 304; the rear surface is the surface away from the first polarization beam splitter prism 301.

[0080] The first polarization beam splitter prism 301 performs orthogonal polarization beam splitting, wherein the P light is transmitted and the S light is reflected;

[0081] The first quarter wave plate 305 and the second quarter wave plate 302 are used to change the polarization state, and the P light and the S light become circularly polarized light, and the polarization angles are both set to 45°;

[0082] The first focusing lens 307 is used to converge the light beam;

[0083] The high reflective mirror 304 is coated with a high reflective film, and the reflectivity is ≥ 98%;

[0084] The half wave plate 308 is used to change the energy of the P light and the S light;

[0085] The four-step phase shift module 4 is used to perform four-step phase shift splitting on the multi-order diffraction interference light to form a phase-shifted light beam; the phase-shifted light beam includes 0th order diffraction interference light and ±1st order diffraction interference light with a phase of 0°, 0th order diffraction interference light and ±1st order diffraction interference light with a phase of 90°, 0th order diffraction interference light and ±1st order diffraction interference light with a phase of 180°, and 0th order diffraction interference light and ±1st order diffraction interference light with a phase of 270°; the four-step phase shift module 4 includes a beam splitter prism 401, a second polarization beam splitter prism 404, and a third polarization beam splitter prism 405; the beam splitter prism 401 is used to perform energy splitting on the multi-order diffraction interference light emitted by the half wave plate 308 to form reflected light and transmitted light; the beam splitter prism 401 has a splitting ratio of 5:5; the second polarization beam splitter prism The mirror 404 is located on the optical path where the transmitted light is located, and a third quarter wave plate 402 is arranged on the optical path between the second polarization beam splitter prism 404 and the beam splitter prism 401, which is used to perform polarization beam splitting on the transmitted light to form 0th order diffraction interference light and ±1st order diffraction interference light with a phase of 90°, and 0th order diffraction interference light and ±1st order diffraction interference light with a phase of 270°; the third polarization beam splitter prism 405 is located on the optical path where the reflected light is located, and a fourth quarter wave plate 403 is arranged on the optical path between the third polarization beam splitter prism 405 and the beam splitter prism 401, which is used to perform polarization beam splitting on the reflected light to form 0th order diffraction interference light and ±1st order diffraction interference light with a phase of 0°, and 0th order diffraction interference light and ±1st order diffraction interference light with a phase of 180°.

[0086] The second polarization beam splitter prism 404 and the third polarization beam splitter prism 405 perform orthogonal polarization beam splitting, wherein the P light is transmitted and the S light is reflected;

[0087] The third quarter wave plate 402 and the fourth quarter wave plate 403 are used to change the polarization state, and the P light and the S light become circularly polarized light, and the polarization angles are both set to 45°;

[0088] The detection module 5 is used to receive the phase-shifted light beam and perform photoelectric conversion to form a 0-order phase-shifted signal, a +1-order phase-shifted signal and a -1-order phase-shifted signal; the detection module 5 includes first to twelfth photodetectors; the first photodetector 501 is used to receive the 0-order diffraction interference light with a phase of 0°, and perform photoelectric conversion to form a first 0-order phase-shifted signal; the second photodetector 502 is used to receive the 0-order diffraction interference light with a phase of 180°, and perform photoelectric conversion to form a second 0-order phase-shifted signal; the third photodetector 503 is used to receive the 0-order diffraction interference light with a phase of 90°, and perform photoelectric conversion to form a second 0-order phase-shifted signal. The fourth photodetector 504 is used to receive the 0th order diffraction interference light with a phase of 270°, and perform photoelectric conversion to form a third 0th order phase shift signal; the fourth photodetector 504 is used to receive the 0th order diffraction interference light with a phase of 270°, and perform photoelectric conversion to form a fourth 0th order phase shift signal; the fifth photodetector 505 is used to receive the +1st order diffraction interference light with a phase of 0°, and perform photoelectric conversion to form a first +1st order phase shift signal; the sixth photodetector 506 is used to receive the +1st order diffraction interference light with a phase of 180°, and perform photoelectric conversion to form a second +1st order phase shift signal; the seventh photodetector 506 is used to receive the +1st order diffraction interference light with a phase of 180°, and perform photoelectric conversion to form a second +1st order phase shift signal; The electrical detector 507 is used to receive the +1st order diffraction interference light with a phase of 90°, and perform photoelectric conversion to form a third +1st order phase shift signal; the eighth photodetector 508 is used to receive the +1st order diffraction interference light with a phase of 270°, and perform photoelectric conversion to form a fourth +1st order phase shift signal; the ninth photodetector 509 is used to receive the -1st order diffraction interference light with a phase of 0°, and perform photoelectric conversion to form a first -1st order phase shift signal; the tenth photodetector 510 is used to receive the -1st order diffraction interference light with a phase of 180°, and perform photoelectric conversion to form a first -1st order phase shift signal. Photoelectric conversion is performed to form a second -1 order phase shift signal; the eleventh photodetector 511 is used to receive the -1 order diffraction interference light with a phase of 90°, and perform photoelectric conversion to form a third -1 order phase shift signal; the twelfth photodetector 512 is used to receive the -1 order diffraction interference light with a phase of 270°, and perform photoelectric conversion to form a fourth -1 order phase shift signal; the first to fourth 0th order phase shift signals are 0th order phase shift signals; the first to fourth +1st order phase shift signals are +1st order phase shift signals; the first to fourth -1st order phase shift signals are -1st order phase shift signals.

[0089] The receiving module 6 receives the 0-order phase shift signal, the +1-order phase shift signal and the -1-order phase shift signal respectively, and uploads them to the host computer 7; the receiving module 6 includes a first oscilloscope 601, a second oscilloscope 602 and a third oscilloscope 603; the first oscilloscope 601 is used to receive the 0-order phase shift signal, that is, the interference measurement signal of the 0-order diffracted light, and uploads it to the host computer 7, and the four-step phase shift information comes from: the first photodetector 501 to the fourth photodetector 504 respectively; the second oscilloscope 602 is used to receive the +1-order phase shift signal, that is, the interference measurement signal of the +1-order diffracted light, and the four-step phase shift information comes from: the fifth photodetector 505 to the eighth photodetector 508, and uploads it to the host computer 7; the third oscilloscope 603 is used to receive the -1-order phase shift signal, that is, the interference measurement signal of the -1-order diffracted light, and the four-step phase shift information comes from: the ninth photodetector 509 to the twelfth photodetector 512, and uploads it to the host computer 7.

[0090] The host computer 7 is used to perform phase unwrapping calculation on the interference measurement signal (i.e., 0-level phase shift signal, +1-level phase shift signal and -1-level phase shift signal), display linear measurement data, and then use multiple linear measurement data to restore the surface information of the mirror to be measured 8.

[0091] Combination Figure 1-Figure 4 As shown, the pulse laser emitted by the pulse laser 1 is presented as a horizontal beam through the first converging lens 201, the incident spot size is expanded by the beam expander 202, and the incident light is incident on the one-dimensional transmission grating 203 for splitting, and is divided into 0-order diffraction light and ±1-order diffraction light. The three beams are transformed into parallel beams through the second converging lens 204, screened by the aperture 205, split at the first polarization splitting prism 301, and enter the reference optical path and the measurement optical path respectively;

[0092] The reference optical path includes a second quarter wave plate 302, a second partially transmitting cavity mirror 303 and a high reflective mirror 304;

[0093] The transmitted light P light entering the reference light path is transmitted through the second quarter wave plate 302, and the polarization state is changed. The P light becomes circularly polarized light, and is split in the second part transmission cavity mirror 303 to form reflected light and transmitted light; the reflected light is transmitted through the second quarter wave plate 302, and the polarization state is changed. The circularly polarized light becomes S light, which is reflected by the first polarization splitting prism 301 and transmitted by the half wave plate 308 to form 0-level light incident on the four-step phase shift module 4; the transmitted light is reflected by the high reflector 304, returns along the original path, and is split in the second part The transmission cavity mirror 303 splits the light to form the transmission light and the reflected light again; the transmission light is transmitted through the second quarter wave plate 302, the polarization state is changed, the circularly polarized light is changed into S light, and it is reflected through the first polarization beam splitter prism 301 and transmitted through the half wave plate 308 to form the first-level light incident on the four-step phase shift module 4; the reflected light is reflected multiple times through the second partial transmission cavity mirror 303 and the high reflector 304, and a reflection signal is obtained each time, and the second-level light, the third-level light, the fourth-level light ... ... n-level light are obtained in sequence, and n is greater than or equal to 6;

[0094] The measuring optical path includes a first quarter wave plate 305, a first partially transmitting cavity mirror 306, a first focusing lens 307 and a mirror to be measured 8;

[0095] The reflected light S light entering the measuring optical path is transmitted through the first quarter wave plate 305, and the polarization state is changed. The S light becomes circularly polarized light, and is split at the first partial transmission cavity mirror 306. The reflected light forms the 0th order light, and the transmitted light forms the 1st order light.

[0096] The 0-order light is transmitted through the first quarter-wave plate 305, the polarization state is changed, the circularly polarized light is changed into P light, and is transmitted through the first polarization beam splitter prism 301 and the half-wave plate 308, and is incident on the four-step phase shift module 4;

[0097] The first-order light is converged by the first focusing lens 601, reflected by the mirror to be measured 8, and emitted at a reflection angle opposite to the incident angle, parallel to the first focusing lens 307, and split by the first partial transmission cavity mirror 306, into transmitted light and reflected light; the transmitted light is transmitted by the first quarter-wave plate 401, the polarization state is changed, and the circularly polarized light is converted into P light, which is transmitted by the first polarization splitting prism 301 and the half-wave plate 308 to form the first-order light incident on the four-step phase shift module 4; the reflected light is reflected multiple times by the first partial transmission cavity mirror 306, the first focusing lens 307 and the mirror to be measured 8, and a reflection signal is obtained each time, and the second-order light, the third-order light, the fourth-order light, and the n-order light are obtained in sequence;

[0098] The multi-level lights of the reference optical path and the measurement optical path are combined and then split by the beam splitter prism 401. The first transmitted light changes its polarization state by the third quarter wave plate 402, and polarization splits by the second polarization beam splitter prism 404. The second transmitted light is incident on the fourth photodetector 504, the eighth photodetector 508 and the twelfth photodetector 512, and the second reflected light is incident on the third photodetector 503, the seventh photodetector 507 and the eleventh photodetector 511; the first reflected light changes its polarization state by the fourth quarter wave plate 403, and polarization splits by the third polarization beam splitter prism 405. The third transmitted light is incident on the first photodetector 501, the fifth photodetector 505 and the ninth photodetector 509, and the third reflected light is incident on the second photodetector 502, the sixth photodetector 506 and the tenth photodetector 510.

[0099] The optical paths of the three light beams passing through the splitting surface of the first polarization beam splitting prism 301 to the rear surface of the first partial transmission cavity mirror 306 and the rear surface of the second partial transmission cavity mirror 303 are equal;

[0100] The optical path from the rear surface of the first partially transmissive cavity mirror 301 on the measuring optical path to the front surface of the mirror to be measured 8 is equal to the optical path from the rear surface of the second partially transmissive cavity mirror 303 on the reference optical path to the front surface of the high reflective mirror 304;

[0101] The multi-level reference light on the reference light path are:

[0102] 0th order diffraction light: 0th order light, 1st order light, 2nd order light...nth order light;

[0103] +1st order diffraction light: 0th order light, 1st order light, 2nd order light...nth order light;

[0104] -1st order diffraction light: 0th order light, 1st order light, 2nd order light...nth order light;

[0105] The multi-level reference light on the measurement optical path are:

[0106] 0th order diffraction light: 0th order light, 1st order light, 2nd order light...nth order light;

[0107] Multi-order reflected light on the left: 0th order light of +1st order diffraction light, 1st order light of -1st order diffraction light, 2nd order light of +1st order diffraction light…(-1) n n-th order light of the first-order diffracted light;

[0108] Multi-order reflected light on the right: 0th order light of -1st order diffraction light, 1st order light of +1st order diffraction light, 2nd order light of -1st order diffraction light…(-1) n+1 n-th order light of the first-order diffracted light;

[0109] When the reflected light is reflected by the measuring mirror 8 once, it carries the mirror phase information Δφ once, and after n reflections, the mirror phase information carried is nΔφ;

[0110] When the structures of the reference optical path and the measurement optical path are inconsistent, the phase measurement information of the second photodetector 502 and the third photodetector 503 of the interference light beam of the multi-level reflected light are respectively: nΔφ and -nΔφ. Phase coupling operation is performed on the two beams of interference information to reduce the influence of environmental errors in the measurement system. The phase coupling information is: 2nΔφ.

[0111] The first photodetector 501, the second photodetector 502, the third photodetector 503 and the fourth photodetector 504 receive respectively: 0°, 90°, 180° and 270° phase shifts of the 0th order diffracted light: 0th order light, 1st order light, 2nd order light, 3rd order light, 4th order light…nth order light of the 0th order diffracted light on the reference optical path; 0th order light, 1st order light, 2nd order light, 3rd order light, 4th order light…nth order light of the 0th order diffracted light on the measuring optical path; mutual interference, photoelectric conversion to the first oscilloscope 601, and transmission to the host computer 7;

[0112] The fifth photodetector 505, the sixth photodetector 506, the seventh photodetector 507 and the eighth photodetector 508 receive respectively: 0°, 90°, 180° and 270° phase shifts of ±1st order diffraction light: 0th order light, 1st order light, 2nd order light, 3rd order light, 4th order light…nth order light of +1st order diffraction light on the reference light path; 0th order light of +1st order diffraction light, 1st order light of -1st order diffraction light, 2nd order light of +1st order diffraction light…(-1) on the measurement light path n The n-order light of the first-order diffracted light interferes with each other, is photoelectrically converted to the second oscilloscope 602, and is transmitted to the host computer 7;

[0113] The ninth photodetector 509, the tenth photodetector 510, the eleventh photodetector 511 and the twelfth photodetector 512 receive respectively: 0°, 90°, 180° and 270° phase shifts of ±1st order diffraction light: 0th order light, 1st order light, 2nd order light, 3rd order light, 4th order light ... ... nth order light of -1st order diffraction light on the reference light path; 0th order light of -1st order diffraction light, 1st order light of +1st order diffraction light, 2nd order light of -1st order diffraction light on the measurement light path ... ... (-1) n+1 The n-level light of the first-order diffracted light interferes with each other, is photoelectrically converted to the third oscilloscope 603, and is transmitted to the host computer 7;

[0114] The reference optical path is +1-order diffraction light, 0-order diffraction light and -1-order diffraction light from top to bottom. The three diffraction light beams are split by the second partial transmission cavity mirror 303, and the first reflected light returns along the original path to form 0-order light of +1-order diffraction light, 0-order diffraction light and -1-order diffraction light; the first transmitted light is vertically incident on the high reflection mirror 304, returns along the original path, and is split by the second partial transmission cavity mirror 303, and the second transmitted light forms 1-order light of +1-order diffraction light, 0-order diffraction light and -1-order diffraction light; the second reflected light is vertically incident on the high reflection mirror 304, returns along the original path, and is split by the second partial transmission cavity mirror 303, and the third transmitted light forms 2-order light of +1-order diffraction light, 0-order diffraction light and -1-order diffraction light; and so on, n-order light of +1-order diffraction light, 0-order diffraction light and -1-order diffraction light is formed;

[0115] The measuring optical path is initially incident +1-order diffraction light, 0-order diffraction light, and -1-order diffraction light from left to right; the three diffraction light beams are split by the first partially transmitted cavity mirror 306, and the first reflected light returns along the original path to form +1-order diffraction light, 0-order diffraction light, and 0-order light of -1-order diffraction light; the first transmitted light is converged by the first focusing lens 307, the 0-order diffraction light is vertically incident on the measuring mirror 8 for reflection, and returns along the original path, the ±1-order diffraction light is incident on the measured mirror 8 at opposite and equal angles for reflection, returns at equal and opposite angles, converges by the first focusing lens 307, and is vertically incident on the first partially transmitted cavity mirror 306 for splitting, and the second transmitted light is sequentially incident from left to right. The first reflected light is incident on the mirror 8 to be measured at opposite and equal angles, and returns at equal and opposite angles, and is converged by the first focusing lens 307, and is incident on the first partially transmitted cavity mirror 306 for splitting. The third transmitted light is formed from left to right: the second light of the +1-order diffraction light, the second light of the 0-order diffraction light, and the second light of the -1-order diffraction light; and so on, from left to right: (-1) n n-order light of the first-order diffracted light, n-order light of the 0th-order diffracted light, (-1) n+1 The nth order light of the first order diffracted light.

[0116] The method for measuring the surface shape using the above-mentioned mirror surface shape measurement system comprises the following steps:

[0117] Step 1, without installing the test mirror 8 and the high reflective mirror 304, the pulse laser 1 emits pulse laser;

[0118] Step 2, adjust the pulse laser 1 so that the pulse laser emitted by it is horizontally incident on the beam expansion and spectrometer module 2, the beam expansion and spectrometer module 2 divides the pulse laser into mutually parallel 0th order diffraction light and ±1st order diffraction light, adjust the interference module 3 so that the 0th order diffraction light and the 0th order reflected light of the ±1st order diffraction light can be completely incident on the center of the detection photosensitive surface of the detection module 5 after passing through the interference module 3 and the four-step phase shift module 4, and complete the preliminary surface positioning;

[0119] Step 3, install the high reflector 304, and adjust the spatial position of the high reflector 304 so that the spatial position states of the first partial transmission cavity mirror 306 and the second partial transmission cavity mirror 303 in the interference module 3 enable the multi-order light of the 0th order diffraction light and the multi-order light of the ±1st order diffraction light formed on the reference light path to be completely incident on the center of the detection photosensitive surface of the detection module 5, thereby completing the precise surface positioning of the reference light path;

[0120] Step 4, install the mirror to be measured 8, and adjust the spatial position of the mirror to be measured 8, so that the multi-order light of the 0th order diffraction light and the multi-order light of the ±1st order diffraction light formed on the measurement light path of the interference module 3 are parallel to and overlap with the light formed on the reference light path, and can be completely incident on the center of the detection photosensitive surface of the detection module 5, so as to complete the precise surface positioning of the measurement light path;

[0121] Step 5, repeatedly cross-linearly move the mirror 8 to be tested along the horizontal line and the vertical line, and after each movement, the detection module 5 obtains the 0-level phase shift signal, +1-level phase shift signal and -1-level phase shift signal of the mirror 8 to be tested, and uploads them to the host computer 7 through the receiving module 6, wherein the 0-level phase shift signal is used as a control group, and the ±1-level phase shift signal is calibrated in real time for the surface interferometry information;

[0122] The number of movements and the number of movement steps are positively correlated with the size of the mirror surface of the test mirror 8. The larger the mirror surface size, the more the number of movements if the number of movement steps is constant; if the number of movements is constant, the larger the number of movement steps; first move horizontally linearly to obtain horizontal linear curve 1, then take the end point of the horizontal linear movement as the starting point, perform vertical linear nano-step movement, and then move horizontally linearly to obtain horizontal linear curve 2, and then perform reciprocating cross movement to obtain multiple horizontal linear curve information of the entire surface shape according to the size of the test mirror 8. If the mirror surface size of the test mirror 8 is 100mm and the number of movement steps is 0.1mm, it needs to move 1000 times;

[0123] Step 6: The host computer 7 performs phase unwrapping calculation on all acquired 0-level phase shift signals, +1-level phase shift signals and -1-level phase shift signals, displays multiple sets of linear measurement data, and then uses the multiple sets of linear measurement data to restore the surface information of the mirror to be measured 8.

[0124] Embodiment 2:

[0125] The structure of the mirror surface shape measurement system based on ultrafast high-precision laser scanning of this embodiment is basically the same as that of the mirror surface shape measurement system based on ultrafast high-precision laser scanning of the first embodiment, except that:

[0126] like Figure 5 As shown, the interference module 3 further includes a second focusing lens 309 ; the second focusing lens 309 is arranged on the optical path between the second partially transmitting cavity mirror 303 and the high reflective mirror 304 .

[0127] When the reference optical path includes the second focusing lens 309, the direction of the reference optical path light beam changes, and the direction of the light beam is completely consistent with the measurement optical path. The outgoing pulse lasers from top to bottom are: 0th order light of +1st order diffraction light, 1st order light of -1st order diffraction light, 2nd order light of +1st order diffraction light, 3rd order light of -1st order diffraction light…(-1) n n-order light of the first-order diffraction light; 0-order light of the 0-order diffraction light, 1-order light of the 0-order diffraction light, 2-order light of the 0-order diffraction light, 3-order light of the 0-order diffraction light…n-order light of the 0-order diffraction light; 0-order light of the -1-order diffraction light, 1-order light of the +1-order diffraction light, 2-order light of the -1-order diffraction light, 3-order light of the +1-order diffraction light…(-1) n+1 The nth order light of the first order diffracted light.

[0128] The present invention proposes to use ultrashort pulse laser and dual-cavity mirror to perform multiple reflection oscillations to increase the accuracy of surface measurement, and at the same time use the 0th order diffraction light and ±1st order diffraction light of a one-dimensional transmission grating to construct phase decoupling and absolute positioning to achieve real-time surface positioning measurement and phase information sequence recognition, thereby realizing ultra-precision mirror surface scanning detection.

[0129] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A mirror surface measurement system based on ultrafast high-precision laser scanning, used for measuring the surface shape of a mirror (8) to be measured, characterized in that: It comprises a pulse laser (1), a beam expansion and splitting module (2), an interference module (3), a four-step phase shift module (4), a detection module (5), a receiving module (6) and a host computer (7); The pulse laser (1) emits pulse laser; The beam expansion and splitting module (2) splits the pulsed laser into mutually parallel 0th-order diffraction light and ±1st-order diffraction light; The interference module (3) receives the 0th order diffraction light and the ±1st order diffraction light, and makes the three enter the measurement light path and the reference light path of the interference module (3) respectively for interference, thereby forming multi-order diffraction interference light; the measured mirror (8) is arranged on the measurement light path of the 0th order diffraction light and the ±1st order diffraction light; The four-step phase shift module (4) is used to perform four-step phase shift splitting on the multi-order diffraction interference light to form a phase-shifted light beam; the phase-shifted light beam includes 0th-order diffraction interference light and ±1st-order diffraction interference light with a phase of 0°, 0th-order diffraction interference light and ±1st-order diffraction interference light with a phase of 90°, 0th-order diffraction interference light and ±1st-order diffraction interference light with a phase of 180°, and 0th-order diffraction interference light and ±1st-order diffraction interference light with a phase of 270°. The detection module (5) is used to receive the phase-shifted light beam and perform photoelectric conversion to form a 0-level phase-shifted signal, a +1-level phase-shifted signal and a -1-level phase-shifted signal; The receiving module (6) receives a 0-level phase shift signal, a +1-level phase shift signal and a -1-level phase shift signal respectively, and uploads them to a host computer (7); The host computer (7) is used to perform phase unwrapping calculation on the 0-level phase shift signal, the +1-level phase shift signal and the -1-level phase shift signal, display linear measurement data, and then use the linear measurement data to restore the surface information of the mirror to be measured (8).

2. The mirror surface measurement system based on ultrafast high-precision laser scanning according to claim 1, characterized in that: The pulse laser (1) is an ultrashort pulse laser, which is used to emit 532nm pulse laser with a repetition frequency of 1-1000Hz, a pulse width of 500ps and a single pulse energy of 1mJ.

3. The mirror surface measurement system based on ultrafast high-precision laser scanning according to claim 2, characterized in that: The beam expansion and splitting module (2) comprises a first converging lens (201), a beam expansion device (202), a one-dimensional transmission grating (203), a second converging lens (204) and an aperture (205), all of which are located in the optical path of the pulsed laser and are arranged in sequence along the emission direction thereof; The first converging lens (201) is used to make the pulsed laser incident on the beam expanding device (202) in parallel; The beam expansion device (202) is used to expand the beam of parallel incident pulsed laser light; The one-dimensional transmission grating (203) is used to form 0th order diffraction light and ±1st order diffraction light; The second converging lens (204) is used to make the 0th order diffracted light and the ±1st order diffracted light respectively incident on the aperture (205) in parallel; The aperture (205) is used to isolate the overlapping part of the light beams of the 0th order diffraction light and the ±1st order diffraction light, and at the same time change the energy size and the aperture size of the incident light spot.

4. The mirror surface measurement system based on ultrafast high-precision laser scanning according to claim 1, 2 or 3, characterized in that: The interference module (3) comprises a first polarization beam splitting prism (301) having four sides, wherein the first side is opposite to the third side, and the second side is opposite to the fourth side; The first side corresponds to the aperture (205) and is used to receive the 0th order diffraction light and ±1st order diffraction light; The second side corresponds to the four-step phase shift module (4), and a half-wave plate (308) is provided on the optical path between the second side and the four-step phase shift module (4) for emitting multi-order diffraction interference light; The third side is provided with a second quarter wave plate (302), a second partially transmitting cavity mirror (303) and a high reflective mirror (304) in sequence along a direction away from the third side, forming a reference optical path; The fourth side is provided with a first quarter wave plate (305), a first partially transmissive cavity mirror (306), and a first focusing lens (307) in sequence along a direction away from the fourth side; a side of the first focusing lens (307) away from the first partially transmissive cavity mirror (306) is used to arrange a mirror to be measured (8); the first quarter wave plate (305), the first partially transmissive cavity mirror (306), the first focusing lens (307), and the mirror to be measured (8) constitute a measuring optical path; The first partially transmissive cavity mirror (306) and the second partially transmissive cavity mirror (303) are both double cavity mirrors, and the sides of the two close to the first polarization beam splitting prism (301) are coated with a reflective film, and the other sides are coated with an anti-reflection film, the reflectivity of the reflective film is ≥90%, and the transmittance of the anti-reflection film is ≥90%; The beam splitting surface of the first polarization beam splitting prism (301) is at an equal distance from the rear surfaces of the first partially transmissive cavity mirror (306) and the second partially transmissive cavity mirror (303); the distance from the rear surface of the first partially transmissive cavity mirror (306) to the reflective surface of the mirror to be measured (8) is equal to the distance from the rear surface of the second partially transmissive cavity mirror (303) to the reflective surface of the high reflective mirror (304); and the rear surface is a surface away from the first polarization beam splitting prism (301).

5. The mirror surface measurement system based on ultrafast high-precision laser scanning according to claim 4, characterized in that: The four-step phase shift module (4) comprises a beam splitter prism (401), a second polarization beam splitter prism (404), and a third polarization beam splitter prism (405); The beam splitter prism (401) is used to perform energy splitting on the multi-order diffraction interference light emitted by the half-wave plate (308) to form reflected light and transmitted light; the beam splitter prism (401) has a splitting ratio of 5:5; The second polarization beam splitter prism (404) is located on the optical path of the transmitted light, and a third quarter wave plate (402) is provided on the optical path between the second polarization beam splitter prism (404) and the beam splitter prism (401) for performing polarization beam splitting on the transmitted light to form 0th order diffraction interference light and ±1st order diffraction interference light with a phase of 90°, and 0th order diffraction interference light and ±1st order diffraction interference light with a phase of 270°; The third polarization beam splitter prism (405) is located on the optical path of the reflected light, and a fourth quarter wave plate (403) is provided on the optical path between the third polarization beam splitter prism (405) and the beam splitter prism (401) for performing polarization beam splitting on the reflected light to form 0th order diffraction interference light and ±1st order diffraction interference light with a phase of 0°, and 0th order diffraction interference light and ±1st order diffraction interference light with a phase of 180°.

6. The mirror surface measurement system based on ultrafast high-precision laser scanning according to claim 5, characterized in that: The polarization angles of the first quarter wave plate (305), the second quarter wave plate (302), the third quarter wave plate (402), and the fourth quarter wave plate (403) are all 45°.

7. The mirror surface measurement system based on ultrafast high-precision laser scanning according to claim 6, characterized in that: The detection module (5) comprises first to twelfth photodetectors; The first photodetector (501) is used to receive the 0th order diffraction interference light with a phase of 0° and perform photoelectric conversion to form a first 0th order phase shift signal; The second photodetector (502) is used to receive the 0th order diffraction interference light with a phase of 180° and perform photoelectric conversion to form a second 0th order phase shift signal; The third photodetector (503) is used to receive the 0th order diffraction interference light with a phase of 90° and perform photoelectric conversion to form a third 0th order phase shift signal; The fourth photodetector (504) is used to receive the 0th order diffraction interference light with a phase of 270° and perform photoelectric conversion to form a fourth 0th order phase shift signal; The fifth photodetector (505) is used to receive the +1-order diffraction interference light with a phase of 0° and perform photoelectric conversion to form a first +1-order phase shift signal; The sixth photodetector (506) is used to receive the +1st order diffraction interference light with a phase of 180° and perform photoelectric conversion to form a second +1st order phase shift signal; The seventh photodetector (507) is used to receive the +1st order diffraction interference light with a phase of 90° and perform photoelectric conversion to form a third +1st order phase shift signal; The eighth photodetector (508) is used to receive the +1-order diffraction interference light with a phase of 270° and perform photoelectric conversion to form a fourth +1-order phase shift signal; The ninth photodetector (509) is used to receive the -1 order diffraction interference light with a phase of 0° and perform photoelectric conversion to form a first -1 order phase shift signal; The tenth photodetector (510) is used to receive the -1st order diffraction interference light with a phase of 180° and perform photoelectric conversion to form a second -1st order phase shift signal; The eleventh photodetector (511) is used to receive the -1st order diffraction interference light with a phase of 90° and perform photoelectric conversion to form a third -1st order phase shift signal; The twelfth photodetector (512) is used to receive the -1st order diffraction interference light with a phase of 270° and perform photoelectric conversion to form a fourth -1st order phase shift signal; The first to fourth 0-level phase-shifted signals are the 0-level phase-shifted signals; the first to fourth +1-level phase-shifted signals are the +1-level phase-shifted signals; and the first to fourth -1-level phase-shifted signals are -1-level phase-shifted signals.

8. The mirror surface measurement system based on ultrafast high-precision laser scanning according to claim 7, characterized in that: The receiving module (6) comprises a first oscilloscope (601), a second oscilloscope (602) and a third oscilloscope (603); The first oscilloscope (601) is used to receive the 0-level phase shift signal and upload it to the host computer (7); The second oscilloscope (602) is used to receive the +1-level phase shift signal and upload it to the host computer (7); The third oscilloscope (603) is used to receive the -1 level phase shift signal and upload it to the host computer (7).

9. The mirror surface shape measurement system based on ultrafast high-precision laser scanning according to claim 7, characterized in that: The interference module (3) further comprises a second focusing lens (309); The second focusing lens (309) is arranged on the optical path between the second partially transmitting cavity mirror (303) and the high reflective mirror (304).

10. A mirror surface shape measurement method based on ultrafast high-precision laser scanning, using the mirror surface shape measurement system based on ultrafast high-precision laser scanning according to any one of claims 1 to 9, characterized in that: The following steps are involved: Step 1: The pulse laser (1) emits pulse laser without installing the mirror to be tested (8) and the high reflective mirror (304); Step 2, adjusting the pulse laser (1) so that the pulse laser emitted by it is horizontally incident on the beam expansion and splitting module (2), the beam expansion and splitting module (2) splits the pulse laser into mutually parallel 0th order diffraction light and ±1st order diffraction light, adjusting the interference module (3) so that the 0th order diffraction light and the 0th order reflected light of the ±1st order diffraction light can be completely incident on the center of the detection photosensitive surface of the detection module (5) after passing through the interference module (3) and the four-step phase shift module (4), thereby completing the preliminary surface positioning; Step 3, installing a high reflector (304), and adjusting the spatial position of the high reflector (304) so ​​that the multi-order light of the 0th order diffraction light and the multi-order light of the ±1st order diffraction light formed on the reference light path of the interference module (3) can be completely incident on the center of the detection photosensitive surface of the detection module (5), thereby completing the precise surface positioning of the reference light path; Step 4, installing the mirror to be measured (8), adjusting the spatial position of the mirror to be measured (8), so that the multi-order light of the 0th order diffraction light and the multi-order light of the ±1st order diffraction light formed on the measurement light path of the interference module (3) are parallel to and overlap with the light formed on the reference light path, and can be completely incident on the center of the detection photosensitive surface of the detection module (5), thereby completing the precise surface positioning of the measurement light path; Step 5, repeatedly cross-linearly move the mirror to be tested (8) along the horizontal line and the vertical line, and after each movement, the detection module (5) obtains the 0-level phase shift signal, the +1-level phase shift signal and the -1-level phase shift signal of the mirror to be tested (8), and uploads them to the host computer (7) through the receiving module (6), wherein the 0-level phase shift signal is used as a control group, and the ±1-level phase shift signal is calibrated in real time for the surface interference information; Step 6: The host computer (7) performs phase unwrapping calculation on all acquired 0-level phase shift signals, +1-level phase shift signals and -1-level phase shift signals, displays multiple sets of linear measurement data, and then uses the multiple sets of linear measurement data to restore the surface information of the measured mirror (8).

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