System and method for detecting off-axis paraboloid error through laser interference

Abstract

The invention discloses a system and method for detecting an off-axis paraboloid error through laser interference, and the method comprises the steps: dividing an emergent light path of a laser into two beams, enabling one beam to pass through a frequency sweeping detection device, and enabling a signal to be received by a computer; the other beam sequentially passes through the laser collimation beam expander, the focusing lens, the off-axis parabolic mirror to be measured, the main plane mirror, the Mach-Zehnder shear interference device and the imaging lens to the CCD camera, and then a signal is received by the computer; by setting the frequency stabilization or frequency sweep of the emergent laser of the tunable external cavity semiconductor laser, the system is switched to be in a shear amount calibration mode or a surface shape detection mode; the shearing directions of the two output laser beams are adjusted to be orthogonal, synthesis of the differential wave surface and the shearing amount in the two orthogonal directions is obtained, measurement of distribution of the whole reflection wave surface on the off-axis parabolic mirror to be measured is achieved, and the paraboloid surface shape error is obtained. According to the invention, the defects in the aspects of phase extraction and disturbance rejection are overcome, and high-precision surface shape measurement can be carried out on the measured off-axis paraboloid.

Description

Technical Field

[0001] The present invention belongs to the field of precision laser interferometric measurement, and particularly relates to a system and method for detecting the error of an off-axis paraboloid by laser interference. Background Art

[0002] With the development of modern optical technology, the demand for the precision and reliability of optical systems is constantly increasing. At the same time, traditional spherical optical elements are difficult to meet the design requirements of miniaturization and light weight of optical systems. Aspherical optical systems have unique advantages such as compact system structure and portability. As an off-axis quadratic aspherical optical element, the parabolic mirror has also been more widely used. However, compared with traditional spherical optical elements, the high-precision manufacturing of aspherical mirrors faces more difficult challenges. The accuracy of their surface shape error often affects the detection performance of the entire optical system. Existing detection technologies are mainly divided into contact measurement and non-contact measurement methods. Contact measurement is easy to scratch the measured part and has low measurement efficiency; non-contact measurement methods usually indirectly reflect the surface shape error through their wavefront aberration. Existing technical means of non-contact interferometric measurement mainly include: 1) Interferometric measurement is difficult to have both anti-disturbance ability and high measurement sensitivity. A measurement system using a non-common optical path design is easily affected by air flow disturbance, thus affecting the measurement accuracy; 2) Using a standard reference flat mirror and a standard reference spherical mirror as reference objects, the measurement accuracy is limited by the absolute accuracy of the reference surface. Using a high-precision reference surface shape increases the cost of the entire detection system; 3) Using a mechanical phase-shifting system for phase-shifting interferometric measurement, its phase-shifting accuracy is limited by the accuracy of the mechanical micro-displacement system, and it is difficult to avoid generating non-linear phase-shifting errors, thus affecting the phase extraction accuracy and limiting the accuracy of the entire detection system. Summary of the Invention

[0003] To solve the above-mentioned defects in the prior art, the purpose of the present invention is to provide a system and method for detecting the error of an off-axis paraboloid by laser interference, so as to solve the deficiencies in anti-disturbance and phase extraction in the above-mentioned prior art. The present invention can perform high-precision surface shape measurement on the measured off-axis paraboloid.

[0004] The present invention is realized through the following technical solutions.

[0005] In one aspect of the present invention, a system for detecting the error of an off-axis paraboloid by laser interference is provided. The system includes a main beam splitter connected to a tunable external cavity semiconductor laser. The emitted laser beam of the tunable external cavity semiconductor laser is split into two beams by the main beam splitter. One beam passes through a swept-frequency detection device, and the laser beam signal is connected to a computer; the other beam passes through a laser collimation and beam expander to the measured off-axis paraboloid mirror. The laser beam reflected by the measured off-axis paraboloid mirror passes through a main plane mirror to a Mach-Zehnder shear interferometer to a CCD camera, and the laser beam signal is connected to a computer;

[0006] Output frequency-stabilized laser and frequency-swept laser respectively through a tunable external cavity semiconductor laser, and switch the system to the shear amount calibration mode or the surface shape detection mode; by adjusting the parallel plate in the Mach-Zehnder shear interference device, make the shear directions of the two output laser beams orthogonal, obtain the synthesis of the differential wavefront and the shear amount in two orthogonal directions, realize the measurement of the entire reflected wavefront distribution on the off-axis parabolic mirror to be measured, and obtain the parabolic surface shape error.

[0007] Preferably, the laser collimating and beam expanding device is arranged perpendicular to the output laser beam, the off-axis parabolic mirror to be measured and the main plane mirror are arranged at 45° relative to the incident laser beam, and the directions are opposite.

[0008] Preferably, a focusing lens is provided on the laser collimating and beam expanding device; an imaging lens is provided in front of the CCD camera.

[0009] Preferably, the Mach-Zehnder shear interference device is arranged parallel to the incident laser beam, and the imaging lens and the CCD camera are arranged parallel to the incident laser beam.

[0010] Preferably, the Mach-Zehnder shear interference device includes a first beam splitter, a first plane mirror, a second plane mirror, a parallel plate and a second beam splitter; the first beam splitter and the second beam splitter are arranged in sequence on the extended optical path of the plane mirror laser beam;

[0011] The first plane mirror and the second plane mirror respectively form the four corners of a rectangle with the first beam splitter and the second beam splitter; a parallel plate is provided between the first beam splitter and the second beam splitter, and the parallel plate can rotate.

[0012] Preferably, the first beam splitter and the second beam splitter, the first plane mirror and the second plane mirror are all arranged at 45° relative to the incident laser beam, and the arrangement directions of the first beam splitter and the second beam splitter, the first plane mirror and the second plane mirror are opposite.

[0013] Preferably, the frequency-swept detection device includes a Fabry-Perot cavity and a photodetector.

[0014] Preferably, the frequency-swept detection device is placed in a sealed cavity, and a temperature sensor, a humidity sensor and a pressure sensor are built in the sealed cavity.

[0015] Another aspect of the present invention provides a method for measuring the off-axis parabolic error by laser interference detection of the system, including:

[0016] Step 1, place the frequency-swept detection device in a sealed cavity, build a temperature sensor, a humidity sensor and a pressure sensor in the sealed cavity; fix the off-axis parabolic mirror to be measured in the system optical path;

[0017] Step 2: Set the tunable external cavity semiconductor laser to output frequency-stabilized laser, and the system is in the shear amount calibration mode;

[0018] Step 3: One beam of the frequency-stabilized laser beam after being split by the main beam splitter passes through the laser collimating and beam expanding device and is emitted to the off-axis parabolic mirror to be measured. The frequency-stabilized laser beam is reflected by the main plane mirror and then enters the Mach-Zehnder shear interferometer;

[0019] Step 4: Adjust the parallel plate in the Mach-Zehnder shear interferometer to rotate in the pitch direction so that the shear direction is the vertical direction; After adjusting the frequency-stabilized laser beam after shearing by the Mach-Zehnder shear interferometer, collect the static interference image through the CCD camera, perform spot edge detection and fit the center coordinate value of the image to obtain the shear amount in the first direction;

[0020] Step 5: Set the output mode of the tunable external cavity semiconductor laser to frequency-swept laser, control the tunable external cavity semiconductor laser to output tunable laser with linearly varying frequency, and switch the system to the surface shape detection mode;

[0021] Step 6: One beam of the frequency-swept laser beam enters the frequency-swept detection device after passing through the main beam splitter, and the photodetector receives the signal; Another beam of the frequency-swept laser beam enters the optical path to be measured, and after passing through the laser collimating and beam expanding device, the focusing lens, the off-axis parabolic mirror to be measured, the main plane mirror, the Mach-Zehnder shear interferometer and the imaging lens in sequence, collect a continuous dynamic interference image through the CCD camera (13); Obtain the photoelectric signal and the interference image, calculate the wave aberration of a single measurement point on the differential wavefront; Fit the wave aberration of a single measurement point to obtain the differential wavefront in the first direction;

[0022] Step 7: Set the output mode of the tunable external cavity semiconductor laser 1 to frequency-stabilized laser, and switch the system to the shear amount calibration mode;

[0023] Step 8: Reset the parallel plate and rotate it in the rotation direction so that the shear direction is the horizontal direction; Collect the static interference image through the CCD camera, perform spot edge detection and fit the center coordinate value of the image to obtain the shear amount in the second direction;

[0024] Step 9: Repeat the adjustment of the tunable external cavity semiconductor laser to output frequency-swept laser, switch the system to the surface shape detection mode, and sequentially pass through the optical path to be measured steps to obtain the photoelectric signal and the interference image;

[0025] Fit the differential wavefront in the first direction through the obtained photoelectric signal and interference image, calculate the wave aberration of a single measurement point on the differential wavefront; Fit the wave aberration of a single measurement point to obtain the differential wavefront in the second direction;

[0026] Through the photoelectric detection of the differential wavefront in the first direction and the differential wavefront in the second direction, perform Zernike fitting to calculate the Zernike coefficients of the wave aberration, and obtain the surface shape error.

[0027] Due to the above technical solutions adopted by the present invention, it has the following beneficial effects:

[0028] 1. The present invention proposes a system and method for detecting the error of an off-axis paraboloid by laser interference. Through the common-path design of the main optical path section combined with Mach-Zehnder shear interference, the system has high sensitivity and certain anti-disturbance ability at the same time.

[0029] 2. The system completes self-coherent interference through a shear interference device without the need for a high-precision reference surface. Therefore, the external errors introduced by the reference surface are avoided, and the detection cost is reduced on the premise of ensuring the detection accuracy of the aspherical wavefront.

[0030] 3. The system generates phase changes through dynamic scanning by a tunable external cavity semiconductor laser, overcomes the limitation of mechanical phase shifting required in interference measurement, improves the measurement efficiency, and can avoid the errors introduced by the mechanical phase shifting mechanism at the same time. The measurement accuracy no longer depends on a high-precision micro-displacement system, and can complete the rapid and high-precision detection of the parabolic mirror surface error. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The drawings described herein are used to provide a further understanding of the present invention, form a part of this application, and do not constitute an improper limitation to the present invention. In the drawings:

[0032] Figure 1 is the system schematic diagram of laser interference for detecting the error of an off-axis paraboloid;

[0033] Figure 2 is the overall manufacturing method flowchart of the present invention;

[0034] FIG. 3(a) and (b) are respectively the off-axis paraboloid interference diagrams in direction one and direction two obtained by using the present invention in the embodiment;

[0035] FIG. 4(a) and (b) are respectively the differential wavefront distribution diagrams in direction one and direction two of the off-axis paraboloid obtained by using the present invention in the embodiment;

[0036] FIG. 5(a) and (b) are respectively the surface error distribution diagrams of the off-axis paraboloid before treatment and obtained by using the present invention in the embodiment.

[0037] Among them: 1 - tunable external cavity semiconductor laser, 2 - main beam splitter, 3 - laser collimation and beam expander, 4 - focusing lens, 5 - off-axis paraboloid mirror to be measured, 6 - main plane mirror, 7 - first beam splitter, 8 - first plane mirror, 9 - second plane mirror, 10 - parallel plate, 11 - second beam splitter, 12 - imaging lens, 13 - CCD camera, 14 - Fabry-Perot cavity, 15 - photodetector, 16 - computer. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0038] The present invention will be described in detail below in conjunction with the accompanying drawings and specific embodiments. The schematic embodiments and descriptions of the present invention are used to explain the present invention, but do not limit the present invention.

[0039] Please refer to Figure 1 As shown, a system for detecting the error of an off-axis paraboloid by laser interference provided by an embodiment of the present invention includes a tunable external cavity semiconductor laser 1, a main beam splitter 2, a laser collimating and beam expanding device 3, a focusing lens 4, a to-be-tested off-axis paraboloid mirror 5, a main plane mirror 6, an imaging lens 12, a CCD camera 13, a Mach-Zehnder shear interference device, a swept-frequency detection device, and a computer 16.

[0040] The swept-frequency detection device includes a Fabry-Perot cavity 14 and a photodetector 15. The swept-frequency detection device is fixed with a cage frame and placed in a sealed cavity with a stable environment. A temperature sensor, a humidity sensor, and a pressure sensor are built in the sealed cavity.

[0041] The tunable external cavity semiconductor laser 1 is connected to the main beam splitter 2. The outgoing laser light path of the tunable external cavity semiconductor laser 1 is divided into two beams by the main beam splitter 2. After one laser beam passes through the swept-frequency detection device, the laser beam signal is connected to the computer 16; the other laser beam passes through the laser collimating and beam expanding device 3 to the to-be-tested off-axis paraboloid mirror 5. The laser beam reflected by the to-be-tested off-axis paraboloid mirror 5 passes through the main plane mirror 6 to the Mach-Zehnder shear interference device to the CCD camera 13, and the laser beam signal is connected to the computer 16. An imaging lens 12 is provided in front of the CCD camera 13. A focusing lens 4 is provided on the laser collimating and beam expanding device 3.

[0042] Among them, the laser collimating and beam expanding device 3 is arranged perpendicular to the outgoing laser beam. The to-be-tested off-axis paraboloid mirror 5 and the main plane mirror 6 are arranged at 45° with respect to the incident laser beam and in opposite directions. The Mach-Zehnder shear interference device is arranged parallel to the incident laser beam. The imaging lens 12 and the CCD camera 13 are arranged parallel to the incident laser beam.

[0043] The Mach-Zehnder shear interference device includes a first beam splitter 7, a first plane mirror 8, a second plane mirror 9, a parallel plate 10, and a second beam splitter 11; the first beam splitter 7 and the second beam splitter 11 are arranged in parallel in sequence on the extended light path of the laser beam of the plane mirror 6; the first plane mirror 8 and the second plane mirror 9 are arranged in parallel respectively, forming the four corners of a rectangle with the first beam splitter 7 and the second beam splitter 11; a parallel plate 10 is provided between the first beam splitter 7 and the second beam splitter 11, and the parallel plate 10 can be rotated respectively in the pitch and yaw directions. The first beam splitter 7 and the second beam splitter 11, the first plane mirror 8 and the second plane mirror 9 are all arranged at 45° with respect to the incident laser beam, and the arrangement directions of the first beam splitter 7 and the second beam splitter 11, the first plane mirror 8 and the second plane mirror 9 are opposite.

[0044] In the present invention, a tunable external cavity semiconductor laser 1 is used as a light source to output a tuned laser with a linearly changing laser frequency. The frequency-tuned laser is divided into two beams by a main beam splitter 2. One beam of laser enters a frequency sweep detection device composed of a Fabry-Perot cavity 14 and a photodetector 15 and transmits the signal to a computer 16; the other beam of laser passes through a laser collimating beam expander 3, a focusing lens 4, an off-axis parabolic mirror to be measured 5, and a main plane reflector 6 in sequence and then enters a Mach-Zehnder shear phase shifting device, and then obtains an interference image through an imaging lens 12 and a CCD camera 13 and finally transmits it to a computer 16. By adjusting the parallel plate 10 to rotate in the pitch and rotation directions respectively, interference images in two orthogonal directions can be obtained respectively.

[0045] A series of interference images and frequency sweep signals that continuously change in two orthogonal directions during frequency sweeping are extracted by computer 16, and the shearing amount in the two orthogonal directions, the differential wavefront in the directions, and the frequency sweeping range of the corresponding time interval are calculated. The surface error of the off-axis parabola to be measured is further calculated by the shear interference wavefront fitting method.

[0046] See also Figure 2 The embodiment of the present invention provides a method for measuring an off-axis parabola error by laser interferometry, comprising the following steps:

[0047] Step 1, placing the frequency sweep detection device in a sealed cavity with a stable environment, and installing a temperature sensor, a humidity sensor and a pressure sensor in the sealed cavity, and fixing the off-axis parabolic mirror 5 to be tested in the optical path of the system;

[0048] Step 2, setting the output mode of the tunable external cavity semiconductor laser 1 to a frequency-stabilized laser, the system to a shear calibration mode, and the tunable external cavity semiconductor laser 1 to a New focus TLB-6813-P laser with a central wavelength of 780 nm;

[0049] Step 3, a frequency-stabilized laser beam split by the main beam splitter 2 is emitted to the off-axis parabolic mirror 5 to be measured through the laser collimator 3, and the frequency-stabilized laser beam is reflected to the Mach-Zehnder shearing interferometer device through the main plane reflector 6;

[0050] Step 4, adjusting the parallel plate 10 in the Mach-Zehnder shearing interferometer to rotate along the pitch direction so that the shearing direction is the vertical direction; the stabilized frequency laser beam after shearing is adjusted by adjusting the Mach-Zehnder shearing interferometer to collect a static interference image through the CCD camera 13, and the spot edge detection is performed and the image center coordinate value is fitted to obtain the direction one shearing amount p;

[0051] Step 5, setting the output mode of the tunable external cavity semiconductor laser 1 to a swept frequency laser, controlling the tunable external cavity semiconductor laser 1 to output a laser frequency that changes linearly, and switching the system to a surface shape detection mode;

[0052] Step 6: A beam of laser passes through the main beam splitter 2 and then enters the swept-frequency detection device, and the signal generated by it is received by the photodetector 15; another beam of laser enters the path to be measured, and successively passes through the laser collimating and beam expanding device 3, the focusing lens 4, the off-axis parabolic mirror 5 to be measured, the main plane mirror 6, the Mach-Zehnder shear interferometer device and the imaging lens 12, and then a continuous dynamic interference image is collected by the CCD camera 13 to obtain an optoelectronic signal and an interference pattern; calculate the wave aberration of a single measurement point on the differential wavefront; fit the wave aberration of the single measurement point to obtain the differential wavefront in direction one;

[0053] Step 7: Set the output mode of the tunable external cavity semiconductor laser 1 to a frequency-stabilized laser, and switch the system to the shear amount calibration mode;

[0054] Step 8: The parallel plate 10 is reset and rotated in the rotation direction so that the shear direction is the horizontal direction; a static interference image is collected by the CCD camera 13, the edge of the light spot is detected and the center coordinate value of the image is fitted to obtain the optoelectronic signal and the shear amount q in direction two of the interference image;

[0055] Step 9: Switch the system to the surface shape detection mode, and repeat Steps 5 to 6 to obtain the optoelectronic signal in direction two and the interference image;

[0056] Step 10: Fit the differential wavefront in direction one through the optoelectronic signal and the interference image obtained in Step 6; calculate the wave aberration U of a single measurement point on the differential wavefront m,n , as shown in the following formula:

[0057]

[0058] where c is the speed of light, is the phase change of a section of interference signal corresponding to the point (m, n), and n p is the number of wave peaks detected in the laser swept-frequency output interval, and FSR is the free spectral range of the Fabry-Perot cavity.

[0059] Fit the differential wavefront U in direction one obtained from a single measurement point, as shown in the following formula:

[0060]

[0061] Similarly, fit the differential wavefront V in direction two through the optoelectronic signal and the interference image obtained in Step 9, as shown in the following formula:

[0062]

[0063] Through the optoelectronic detection of the differential wavefronts U and V, and combining the shear amounts obtained in Steps 4 and 8, perform Zernike fitting to calculate the Zernike coefficients to obtain the surface shape error W, as shown in the following formula:

[0064]

[0065] Among them, C is the Zernike coefficient vector, which can be calculated from the differential wavefront U and V Zernike coefficient vectors, and Z k is the Zernike matrix of order k.

[0066] The present invention will be further described below by means of a specific embodiment.

[0067] 1) Place the swept-frequency detection device in a sealed cavity with a stable environment, and install a temperature, humidity and pressure sensor in the sealed cavity. Fix an off-axis parabolic mirror to be measured with a diameter of 25.4 mm, a reflection focal length of 50.8 mm, and an off-axis angle of 15° in the system optical path through a mirror mount.

[0068] 2) Set the output mode of the tunable external cavity semiconductor laser 1 to a frequency-stabilized laser with a frequency of 380 THz.

[0069] 3) A beam of frequency-stabilized laser beam after being split by the main beam splitter 2 is emitted through the laser collimating and beam expanding device 3 to the off-axis parabolic mirror 5 to be measured. The frequency-stabilized laser beam is reflected by the main plane mirror 6 to the Mach-Zehnder shear interference device;

[0070] 4) Adjust the parallel plate 10 in the Mach-Zehnder shear interference device to rotate in the pitch direction so that the shear direction is the vertical direction; after adjusting the Mach-Zehnder shear interference device to shear, the frequency-stabilized laser beam is collected by the CCD camera 13 to obtain a static interference image, the edge of the light spot is detected and the center coordinate value of the image is fitted, and the shear amount p in the first direction is obtained as 918.5 μm.

[0071] 5) Adjust the output mode of the tunable external cavity semiconductor laser 1 to the swept-frequency mode, with a maximum optical frequency tuning range of 180 GHz, an optical frequency tuning rate of 0.5 Hz, a center power of the laser of 7 mW, and a frame rate of the CCD camera 13 of 500 FPS.

[0072] 6) Referring to Fig. 3(a), collect a continuous dynamic interference image through the CCD camera 13, and the acquisition start and end times should correspond to the swept-frequency time.

[0073] 7) Set the output mode of the tunable external cavity semiconductor laser 1 to a frequency-stabilized laser with a frequency of 380 THz.

[0074] 8) Reset the parallel plate 10 and rotate it in the rotation direction, and the interference image is sheared by 91 pixels in the horizontal direction; collect a static interference image through the CCD camera 13, detect the edge of the light spot and fit the center coordinate value of the image, and obtain the shear amount q in the second direction as 500.5 μm.

[0075] 9) Adjust the output mode of the tunable external cavity semiconductor laser 1 to the frequency-sweeping mode. The maximum optical frequency tuning range is 180 GHz, the optical frequency tuning rate is 0.5 Hz, the central power of the laser is 7 mW, and the acquisition frame rate of the CCD camera 13 is 500 FPS.

[0076] Refer to Fig. 3(b). Collect a continuous dynamic interference image through the CCD camera 13. The start and end times of the acquisition should correspond to the frequency-sweeping time.

[0077] Calculate the parabolic surface shape error W from the interference image.

[0078] Refer to Fig. 4(a) and (b). The obtained optoelectronic signal and the differential wavefronts in the first and second fitting directions of the interference image are given.

[0079] Refer to Table 1. The calculated Zernike coefficient vector C of the surface shape error is given as shown in Table 1.

[0080] Table 1 Zernike coefficient vector C of the surface shape error

[0081]

[0082] Refer to Fig. 5(a). The distribution of the surface shape error W and the surface shape error value obtained without processing are given. The result has a large error and is significantly affected by environmental disturbances. Refer to Fig. 5(b). Substitute the obtained Zernike coefficient vector C in Table 1 into the surface shape error W, and the distribution of the surface shape error W and the surface shape error value obtained by this method are obtained. It can be seen from this that the anti-disturbance ability of the system of the present invention is significantly improved while maintaining high sensitivity.

[0083] The present invention is not limited to the above embodiments. Based on the technical solutions disclosed in the present invention, those skilled in the art can make some substitutions and deformations of some technical features without creative labor according to the disclosed technical content, and these substitutions and deformations are all within the protection scope of the present invention.

Claims

1. A system for detecting the errors of an off-axis paraboloid by laser interference, characterized in that, The system includes a main beam splitter (2) connected to a tunable external cavity semiconductor laser (1). The output laser beam of the tunable external cavity semiconductor laser (1) is split into two beams by the main beam splitter (2). One beam passes through a frequency-sweeping detection device, and then the laser beam signal is connected to a computer (16). The other beam passes through a laser collimation and beam expander (3) to an off-axis parabolic mirror (5) to be measured. The laser beam reflected by the off-axis parabolic mirror (5) to be measured passes through a main plane mirror (6) to a Mach-Zehnder shear interferometer device to a CCD camera (13), and the laser beam signal is connected to the computer (16). The tunable external cavity semiconductor laser (1) outputs stable-frequency laser and frequency-sweeping laser respectively, and the system is switched to a shear amount calibration mode or a surface shape detection mode. By adjusting the parallel plate in the Mach-Zehnder shear interferometer device, the shear directions of the two output laser beams are made orthogonal, and the synthesis of the differential wavefront and the shear amount in two orthogonal directions is obtained, so as to realize the measurement of the entire reflected wavefront distribution on the off-axis parabolic mirror (5) to be measured and obtain the parabolic surface shape error.

2. The system for detecting the errors of an off-axis paraboloid by laser interferometry according to claim 1, wherein, The laser collimation and beam expander (3) is arranged perpendicular to the output laser beam. The off-axis parabolic mirror (5) to be measured and the main plane mirror (6) are set at 45° relative to the incident laser beam and in opposite directions.

3. The system for detecting the error of an off-axis paraboloid by laser interference according to claim 1, wherein A focusing lens (4) is provided on the laser collimation and beam expander (3); an imaging lens (12) is provided in front of the CCD camera (13).

4. The system for detecting the error of an off-axis paraboloid by laser interferometry according to claim 1, wherein The Mach-Zehnder shear interferometer device is arranged parallel to the incident laser beam, and the imaging lens (12) and the CCD camera (13) are arranged parallel to the incident laser beam.

5. The system for detecting the error of an off-axis paraboloid by laser interference according to claim 1, characterized in that, The Mach-Zehnder shear interferometer device includes a first beam splitter (7), a first plane mirror (8), a second plane mirror (9), a parallel plate (10) and a second beam splitter (11). The first beam splitter (7) and the second beam splitter (11) are arranged in parallel in sequence on the extended optical path of the laser beam of the plane mirror (6). The first plane mirror (8) and the second plane mirror (9) respectively form the four corners of a rectangle with the first beam splitter (7) and the second beam splitter (11). A parallel plate (10) is provided between the first beam splitter (7) and the second beam splitter (11), and the parallel plate (10) can rotate.

6. The system for detecting the error of an off-axis paraboloid by laser interferometry according to claim 5, characterized in that, The first beam splitter (7) and the second beam splitter (11), the first plane mirror (8) and the second plane mirror (9) are all set at 45° relative to the incident laser beam, and the arrangement directions of the first beam splitter (7) and the second beam splitter (11), the first plane mirror (8) and the second plane mirror (9) are opposite.

7. The system for detecting the error of an off-axis paraboloid by laser interference according to claim 1, wherein, The frequency-sweeping detection device includes a Fabry-Perot cavity (14) and a photodetector (15).

8. The system for detecting the error of an off-axis paraboloid by laser interference according to claim 1, wherein The frequency-sweeping detection device is placed in a sealed cavity, and a temperature sensor, a humidity sensor and a pressure sensor are built in the sealed cavity.

9. A method for measuring the error of an off-axis paraboloid by laser interference detection of the system according to any one of claims 1-8, characterized in that, Including: Place the frequency-sweeping detection device in a sealed cavity, and build a temperature sensor, a humidity sensor and a pressure sensor in the sealed cavity; fix the off-axis parabolic mirror (5) to be measured in the optical path of the system; Set the tunable external cavity semiconductor laser (1) to output stable-frequency laser, and the system is in the shear amount calibration mode; A beam of frequency-stabilized laser beam after being split by the main beam splitter (2) is emitted from the laser collimating and beam expanding device (3) and incident on the off-axis parabolic mirror to be measured (5). The frequency-stabilized laser beam is reflected by the main plane mirror (6) and then incident on the Mach-Zehnder shear interferometer device; Adjust the parallel plate (10) in the Mach-Zehnder shear interferometer device to rotate in the pitch direction so that the shear direction is the vertical direction; After adjusting the Mach-Zehnder shear interferometer device to shear the frequency-stabilized laser beam, collect the static interference image through the CCD camera (13), perform spot edge detection and fit the center coordinate value of the image to obtain the shear amount in direction one; Set the output mode of the tunable external cavity semiconductor laser (1) to frequency-swept laser, control the tunable external cavity semiconductor laser (1) to output a tunable laser with a linearly changing frequency, and switch the system to the surface shape detection mode; A beam of frequency-swept laser beam enters the frequency-swept detection device after passing through the main beam splitter (2), and the photodetector (15) receives the signal; Another beam of frequency-swept laser beam enters the optical path to be measured, and successively passes through the laser collimating and beam expanding device (3), the focusing lens (4), the off-axis parabolic mirror to be measured (5), the main plane mirror (6), the Mach-Zehnder shear interferometer device and the imaging lens (12), and then a continuous dynamic interference image is collected through the CCD camera (13); Obtain the photoelectric signal and the interference pattern; Calculate the wave aberration of a single measurement point on the differential wavefront; Fit the wave aberration of a single measurement point to obtain the differential wavefront in direction one; Set the output mode of the tunable external cavity semiconductor laser (1) to frequency-stabilized laser, and switch the system to the shear amount calibration mode; The parallel plate (10) is reset and rotated in the rotational direction so that the shear direction is the horizontal direction; Collect the static interference image through the CCD camera (13), perform spot edge detection and fit the center coordinate value of the image to obtain the shear amount in direction two; Repeat the adjustment of the tunable external cavity semiconductor laser to output frequency-swept laser, switch the system to the surface shape detection mode, and successively pass through the steps of the optical path to be measured to obtain the photoelectric signal and the interference image; Fit the differential wavefront in direction one through the obtained photoelectric signal and the interference image, calculate the wave aberration of a single measurement point on the differential wavefront; Fit the wave aberration of a single measurement point to obtain the differential wavefront in direction two; Through the photoelectric detection of the differential wavefront in direction one and the differential wavefront in direction two, perform Zernike fitting to calculate the Zernike coefficients of the wave aberration, and obtain the surface shape error.

10. The method for measuring the off-axis paraboloid error by laser interference detection according to claim 9, wherein, Calculate the wave aberration U of a single measurement point on the differential wavefront m,n , as follows: where c is the speed of light, is the phase change of an interference signal corresponding to the point (m, n), and n p is the number of detected wave peaks in the laser sweeping output range, and FSR is the free spectral range of the Fabry-Perot cavity.

Images