Heterodyne interferometer for two-dimensional angular displacement measurement and measurement method

Abstract

The invention relates to the technical field of ultra-precision measurement, in particular to a heterodyne interferometer for two-dimensional angular displacement measurement and a measurement method.The heterodyne interferometer comprises a double-frequency laser light source used for providing a cross-polarized double-frequency laser beam; the device is characterized in that a depolarization splitting prism is arranged on an output light path of the double-frequency laser light source, a double-frequency laser beam is split into a first double-frequency beam and a second double-frequency beam after entering the depolarization splitting prism, and the two double-frequency beams respectively enter a plane interference measurement group and a vertical plane interference measurement group; according to the invention, the interference light path of each dimension is two axes, so that the situation of measurement errors caused by parallelism reduction along with the increase of the number of axes in a multi-axis measurement mode is avoided. Meanwhile, the two beams of light in each dimension are measuring light, so that the resolution is improved, the common-mode noise can be suppressed, and the anti-interference capability of the system is enhanced.

Description

Technical Field

[0001] The invention relates to the technical field of ultra-precision measurement, and in particular to a heterodyne interferometer and a measurement method for two-dimensional angular displacement measurement. Background Art

[0002] Laser has many advantages such as strong directionality, small beam angle, concentrated energy, high brightness, good monochromaticity and good coherence. Therefore, compared with microwave measurement equipment, ranging equipment with laser as light source has obvious advantages in measurement directionality and accuracy, and is widely used in non-contact, high-precision measurement fields.

[0003] Since the invention of the first laser interferometer in 1967, laser interferometer has been widely used in high-precision instrument calibration and verification, precision motion testing, and high-end equipment integration due to its wide range, high precision, traceability, and non-contact characteristics. Its working principle is coherent detection, which is divided into heterodyne detection and homodyne detection in terms of detection method. The essence of the heterodyne laser interferometer system is to use phase difference to demodulate displacement measurement information. The obtained photoelectric signal has a high signal-to-noise ratio, strong anti-interference ability, and is easy to achieve high-resolution measurement. It has attracted great attention and in-depth research in the field of measurement research and industrial application, and has made great progress. It is now widely used in the field of geometric measurement and is enduring.

[0004] At present, the mature products in the world mainly include the XL series single-frequency laser interferometers of Renishaw of the UK and the XD series of API of the United States. The heterodyne laser interferometer products mainly include the 5530 series of Keysight (formerly Agilent) of the United States and the ZMI series of ZYGO of the United States.

[0005] Domestically, many research institutions and universities have also gradually carried out research on laser interferometers, which have been developed for 20 years. The mature products mainly include S1500 of Harbin Ultra-Precision Engineering Technology Center Co., Ltd. (referred to as Harbin Ultra-Precision) and MJS6A of Chengdu Tool Research Institute (referred to as Chengdu Tool Institute). Heterodyne laser interferometers mainly include HUE series of Harbin Ultra-Precision Engineering Technology Center Co., Ltd. and MJS5 series of Chengdu Tool Institute. In addition, universities including Tsinghua University and Harbin Institute of Technology have also conducted targeted research and improvement on the core components and key indicators of zero-difference and heterodyne interferometers, and have achieved breakthroughs in many aspects.

[0006] After the rapid development of homodyne and heterodyne interferometers, in order to meet the measurement needs in more fields, it is necessary to detect the object to be measured in multiple directions at the same time, so a multi-axis measurement interferometer was designed and developed. At present, multi-axis measurement is generally achieved by using traditional prism group step-by-step splitting, which is extremely difficult to design and process. The parallelism error in the splitting process will accumulate with the increase in the number of incident beams. Keysight, ZYGO and other companies have achieved high-precision parallel splitting through step-by-step splitting of prism groups, and developed multi-axis interferometers on this basis.

[0007] The parallelism between the optical axes of Keysight's three-axis interferometer system is as high as 25urad, but the parallelism of the optical axes of the five-axis interferometer system has been reduced to 100mrad. The reduction in parallelism greatly increases the linear error of the measurement. In addition, when the measurement distance is large, the turbulence in the air will also cause different degrees of disturbance to the measurement beam of each axis, affecting the measurement stability of displacement and angle. Summary of the invention

[0008] The purpose of the present invention is to address the shortcomings of the existing heterodyne laser interferometry measurement method and propose a heterodyne interferometer and measurement method for two-dimensional angular displacement measurement. The device has the advantages of easy processing of components; more convenient optical path adjustment; two optical axes in each dimension are both measuring optical paths, which can improve the anti-interference ability of the system; the optical path is highly symmetrically designed to avoid temperature drift, etc., and finally realizes high-precision two-dimensional angular displacement measurement.

[0009] In order to achieve the above object, the present invention adopts the following technical solutions:

[0010] A heterodyne interferometer for two-dimensional angular displacement measurement, comprising a dual-frequency laser light source for providing a dual-frequency laser light beam with orthogonal polarization, characterized in that: a depolarizing beam splitter prism is arranged on the output optical path of the dual-frequency laser light source, the dual-frequency laser light beam is incident on the depolarizing beam splitter prism and is divided into a first dual-frequency light beam and a second dual-frequency light beam, and the two dual-frequency light beams enter a plane interferometry group and a vertical interferometry group respectively;

[0011] The plane interference measurement group includes a first half-wave plate, a first right-angle prism and a first polarization beam splitter prism, the first half-wave plate is arranged on one side of the depolarization beam splitter prism, the first dual-frequency light beam passes through the first half-wave plate and the first right-angle prism in sequence and then enters the first polarization beam splitter prism, and forms a first beam of measurement light and a second beam of measurement light respectively after being transmitted and reflected by the first polarization beam splitter prism, a first target retroreflector is arranged on the outgoing light path of the first beam of measurement light, a second right-angle prism and a second target retroreflector are arranged on the outgoing light path of the second beam of measurement light, a first polarizer is attached to the rear end surface of the first polarization beam splitter prism, a first photodetector is arranged on the outgoing light path of the first polarization beam splitter prism, a second half-wave plate is arranged on one side of the first polarization beam splitter prism, and a laser length measuring instrument is arranged after the second half-wave plate;

[0012] The vertical interferometer measurement group includes a third half-wave plate and a second polarization beam splitter prism, the third half-wave plate is arranged on one side of the depolarization beam splitter prism, the second dual-frequency light beam enters the second polarization beam splitter prism after passing through the third half-wave plate, and forms a third beam of measurement light and a fourth beam of measurement light respectively after being transmitted and reflected by the second polarization beam splitter prism, a third right-angle prism and a third target retroreflector are arranged in sequence on the outgoing light path of the fourth beam of measurement light, a fourth right-angle prism, a fifth right-angle prism, and a fourth target retroreflector are arranged in sequence on the outgoing light path of the third beam of measurement light, a second polarizer is attached to the rear end surface of the second polarization beam splitter prism, and a second photodetector is arranged on the outgoing light path of the second polarizer;

[0013] The first target retroreflective mirror, the second target retroreflective mirror, the third target retroreflective mirror and the fourth target retroreflective mirror are all arranged on a rotating platform, and horizontal rotating shafts are arranged on both sides of the rotating platform, and the horizontal rotating shafts are rotatably connected to the middle rotating platform, and vertical rotating shafts are arranged on the upper and lower sides of the middle rotating platform, and the vertical rotating shafts are rotatably connected to the fixed platform, and the vertical rotating shafts provide vertical rotation power for the middle rotating platform through an external servo motor, and the horizontal rotating shafts provide horizontal rotation power for the rotating platform through an external servo motor, and the first target retroreflective mirror and the second target retroreflective mirror are distributed near the vertical rotating shafts. The third target retroreflective mirror and the fourth target retroreflective mirror are distributed at positions close to the horizontal rotation axis, and the angle adjustment of the first target retroreflective mirror, the second target retroreflective mirror, the third target retroreflective mirror and the fourth target retroreflective mirror in three-dimensional space is achieved through the cooperation of the vertical rotation axis and the horizontal rotation axis; the interference light path in each dimension is two-axis, which avoids the situation in which the parallelism decreases as the number of axes increases in the multi-axis measurement method and causes measurement errors; at the same time, the two beams of light in each dimension are both measurement lights, which can suppress common mode noise while improving the resolution, thereby enhancing the anti-interference ability of the system.

[0014] Furthermore, the polarization axis directions of the first polarizer and the second polarizer are both 45° to the horizontal direction.

[0015] Furthermore, the first half wave plate, the second half wave plate and the third half wave plate are all installed in a rotationally connected manner.

[0016] A two-dimensional angular displacement measurement method comprises the following steps:

[0017] S1, after the dual-frequency laser beam is incident on the depolarization beam splitter prism, the transmitted beam forms the second dual-frequency beam, and the reflected beam forms the first dual-frequency beam;

[0018] S2, the first dual-frequency light beam passes through the first half-wave plate and the first right-angle prism in sequence and then enters the first polarization beam splitter, and its transmitted light beam forms the first measurement light beam, and its reflected light beam forms the second measurement light beam; the second dual-frequency light beam passes through the third half-wave plate and enters the second polarization beam splitter, and its transmitted light beam forms the third measurement light beam, and its reflected light beam forms the fourth measurement light beam;

[0019] S3, the first beam of measuring light contacts the first target retroreflector with Doppler frequency shift and is transmitted through the first polarization beam splitter for output; the second beam of measuring light contacts the second target retroreflector after being reflected by the second right-angle prism, and then contacts the first polarization beam splitter with Doppler frequency shift and is reflected through the second right-angle prism for output; the third beam of measuring light contacts the third target retroreflector after being reflected by the fourth right-angle prism and the fifth right-angle prism, and then contacts the second polarization beam splitter with Doppler frequency shift and is reflected through the fifth right-angle prism and the fourth right-angle prism for output; the fourth beam of measuring light contacts the fourth target retroreflector after being reflected by the third right-angle prism, and then contacts the second polarization beam splitter with Doppler frequency shift and is reflected through the third right-angle prism for output;

[0020] S4, the paths of the first measuring light beam and the second measuring light beam coincide after passing through the first polarization beam splitter, and then pass through the first polarizer to perform beat frequency interference to form a first measuring signal Im1; the paths of the third measuring light beam and the fourth measuring light beam coincide after passing through the second polarization beam splitter, and then pass through the second polarizer to perform beat frequency interference to form a second measuring signal Im2.

[0021] S5, the first photodetector receives a first measurement signal Im1, and the second photodetector receives a second measurement signal Im2. After signal processing of the first measurement signal Im1 and the second measurement signal Im2, the angular displacement of the measured object on two axes can be obtained.

[0022] Furthermore, the dual-frequency laser beam is generated by a dual acousto-optic modulation method.

[0023] Furthermore, the interference light path in each dimension consists of two axes, which avoids the situation in which the parallelism decreases and causes measurement errors as the number of axes increases in multi-axis measurement. At the same time, the two beams of light in each dimension are measurement lights, which can suppress common mode noise while improving the resolution, thereby enhancing the system's anti-interference ability.

[0024] Furthermore, the angular displacement of the measured object on two axes can be used to calibrate its initial position and final position in three-dimensional space in a spherical coordinate system, thereby achieving high-precision measurement of two-dimensional angular displacement.

[0025] Compared with the prior art, the present invention has the following advantages:

[0026] 1. The present invention only includes two-dimensional interference optical paths. On the one hand, the volume is smaller than the current multi-axis interferometer. On the other hand, the component processing is more convenient and the optical path adjustment is simpler.

[0027] 2. In the present invention, the interference optical path in each dimension is two-axis, which avoids the situation in which the parallelism decreases and causes measurement errors as the number of axes increases in the multi-axis measurement method. At the same time, the two beams of light in each dimension are both measurement lights, which can suppress common mode noise while improving the resolution and enhance the anti-interference ability of the system.

[0028] 3. The vertical interferometer measurement group in the present invention is highly symmetrically designed to avoid the idle path error caused by the different initial optical path lengths of the two arms. For the plane interferometer measurement group, a laser length measuring instrument is introduced to measure the optical path lengths of the two arms, and the initial optical path difference is calculated, which can be compensated in data processing to eliminate the idle path error. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a structural schematic diagram of the present invention;

[0030] Figure 2 is the optical path diagram of the plane interferometry measurement group in the present invention;

[0031] Figure 3 The optical path diagram of the neutral plane interferometry measurement group of the present invention;

[0032] Figure 4 It is a schematic diagram of the installation structure of the rotating table in the present invention;

[0033] Figure 5 This is a measurement result diagram of the zero point of the horizontal dimension of the present invention;

[0034] Figure 6 This is a measurement result diagram of the vertical dimension zero point of the present invention;

[0035] Figure 7 This is a diagram showing the verification measurement results of the horizontal dimension angular displacement of the present invention;

[0036] Figure 8 This is a diagram showing the verification measurement results of the vertical dimension angular displacement of the present invention;

[0037] Fig. 9 It is a two-dimensional angular displacement geometric space representation diagram of the present invention;

[0038] In the figure: dual-frequency laser light source 1, depolarizing beam splitter prism 2, plane interferometry group 3, vertical interferometry group 4, rotating table 5, vertical rotation axis 6, horizontal rotation axis 7, intermediate rotating table 8, fixed table 9, first half-wave plate 301, first right-angle prism 302, first polarizing beam splitter prism 303, first target retroreflector 304, second right-angle prism 305, second target retroreflector 306, first polarizer 307, first photodetector 308, second half-wave plate 309, laser length measuring instrument 310, third half-wave plate 401, second polarizing beam splitter prism 402, third right-angle prism 403, third target retroreflector 404, fourth right-angle prism 405, fifth right-angle prism 406, fourth target retroreflector 407, second polarizer 408, second photodetector 409. DETAILED DESCRIPTION

[0039] In order to further illustrate the technical solution of the present invention, the present invention is further described below through embodiments.

[0040] A heterodyne interferometer for two-dimensional angular displacement measurement, comprising a dual-frequency laser light source 1 for providing a dual-frequency laser light beam with orthogonal polarization, characterized in that: a depolarizing beam splitter prism 2 is arranged on the output optical path of the dual-frequency laser light source 1, and the dual-frequency laser light beam is divided into a first dual-frequency light beam and a second dual-frequency light beam after being incident on the depolarizing beam splitter prism 2, and the two dual-frequency light beams enter a plane interference measurement group 3 and a vertical interference measurement group 4 respectively;

[0041] The plane interference measurement group 3 includes a first half wave plate 301, a first right angle prism 302 and a first polarization beam splitter prism 303. The first half wave plate 301 is arranged on one side of the depolarization beam splitter prism 2. The first dual-frequency light beam passes through the first half wave plate 301 and the first right angle prism 302 in sequence and then enters the first polarization beam splitter prism 303. After being transmitted and reflected by the first polarization beam splitter prism 303, the first measurement light beam and the second measurement light beam are formed respectively. A first target retroreflector 304 is arranged on the top, a second right-angle prism 305 and a second target retroreflector 306 are arranged on the outgoing light path of the second measuring light beam, a first polarizing plate 307 is attached to the rear end surface of the first polarizing beam splitter prism 303, a first photodetector 308 is arranged on the outgoing light path of the first polarizing plate 307, a second half-wave plate 309 is arranged on one side of the first polarizing beam splitter prism 303, and a laser length measuring instrument 310 is arranged behind the second half-wave plate 309;

[0042] The vertical interferometer measurement group 4 includes a third half-wave plate 401 and a second polarization beam splitter prism 402. The third half-wave plate 401 is arranged on one side of the depolarization beam splitter prism 2. The second dual-frequency light beam enters the second polarization beam splitter prism 402 after passing through the third half-wave plate 401, and forms a third beam of measurement light and a fourth beam of measurement light respectively after being transmitted and reflected by the second polarization beam splitter prism 402. A third right-angle prism 403 and a third target retroreflector 404 are sequentially arranged on the outgoing light path of the fourth beam of measurement light. A fourth right-angle prism 405, a fifth right-angle prism 406, and a fourth target retroreflector 407 are sequentially arranged on the outgoing light path of the third beam of measurement light. A second polarizer 408 is attached to the rear end surface of the second polarization beam splitter prism 402, and a second photodetector 409 is arranged on the outgoing light path of the second polarizer 408.

[0043] The polarization axis directions of the first polarizer 307 and the second polarizer 408 are both 45 degrees to the horizontal direction. The first half wave plate 301, the second half wave plate 309 and the third half wave plate 401 are all installed in a rotationally connected manner.

[0044] The first target retroreflective mirror 305, the second target retroreflective mirror 306, the third target retroreflective mirror 404 and the fourth target retroreflective mirror 407 are all arranged on the rotating table 5, and horizontal rotation shafts 7 are arranged on both sides of the rotating table 5, and the horizontal rotation shaft 7 is rotatably connected to the intermediate turntable. Vertical rotation shafts 6 are arranged on the upper and lower sides of the intermediate turntable, and the vertical rotation shaft 6 is rotatably connected to the fixed table. The vertical rotation shaft 6 provides vertical rotation power for the intermediate turntable through an external servo motor, and the horizontal rotation shaft 7 provides horizontal rotation power for the rotating table 5 through an external servo motor. The first target retroreflective mirror 305 and the second target retroreflective mirror 306 are distributed at positions close to the vertical rotation shaft 6, and the third target retroreflective mirror 404 and the fourth target retroreflective mirror 407 are distributed at positions close to the horizontal rotation shaft 7. Through the cooperation of the vertical rotation shaft 6 and the horizontal rotation shaft 7, the angle adjustment of the first target retroreflective mirror 305, the second target retroreflective mirror 306, the third target retroreflective mirror 404 and the fourth target retroreflective mirror 407 in three-dimensional space is realized. The interference optical path in each dimension consists of two axes, which avoids the situation in which the parallelism decreases and causes measurement errors as the number of axes increases in multi-axis measurement. At the same time, the two beams of light in each dimension are measurement lights, which can suppress common mode noise while improving the resolution, thereby enhancing the system's anti-interference ability.

[0045] A two-dimensional angular displacement measurement method comprises the following steps:

[0046] S1, after the dual-frequency laser beam is incident on the depolarization beam splitter prism 2, the transmitted beam forms a second dual-frequency beam, and the reflected beam forms a first dual-frequency beam, and the dual-frequency laser beam is generated by a dual acousto-optic modulation method;

[0047] S2, the first dual-frequency light beam passes through the first half-wave plate 301 and the first right-angle prism 302 in sequence and then enters the first polarization beam splitter 303, and its transmitted light beam forms the first measurement light beam, and its reflected light beam forms the second measurement light beam; the second dual-frequency light beam passes through the third half-wave plate 401 and then enters the second polarization beam splitter 402, and its transmitted light beam forms the third measurement light beam, and its reflected light beam forms the fourth measurement light beam;

[0048] S3, the first beam of measuring light contacts the first target retroreflector 304 and carries the Doppler frequency shift to be transmitted through the first polarization beam splitter 303 and output; the second beam of measuring light contacts the second target retroreflector 306 after being reflected by the second right-angle prism 305, and then reflects through the second right-angle prism 305 and reflects through the first polarization beam splitter 303 and output; the third beam of measuring light contacts the third target retroreflector 407 after being reflected by the fourth right-angle prism 405 and the fifth right-angle prism 406, and then reflects through the fifth right-angle prism 406 and the fourth right-angle prism 405 and transmits through the second polarization beam splitter 402 and output; the fourth beam of measuring light contacts the fourth target retroreflector 404 after being reflected by the third right-angle prism 403, and then reflects through the second polarization beam splitter 402 and output with the Doppler frequency shift;

[0049] S4, the paths of the first measuring light beam and the second measuring light beam coincide after passing through the first polarization beam splitter 303, and then pass through the first polarizer 307 for beat frequency interference to form a first measuring signal Im1; the paths of the third measuring light beam and the fourth measuring light beam coincide after passing through the second polarization beam splitter 402, and then pass through the second polarizer 408 for beat frequency interference to form a second measuring signal Im2.

[0050] S5, the first photodetector 308 receives the first measurement signal Im1, and the second photodetector 409 receives the second measurement signal Im2. After signal processing, the angular displacement of the object to be measured on two axes can be obtained by processing the first measurement signal Im1 and the second measurement signal Im2.

[0051] The interference optical path in each dimension is two-axis, which avoids the situation in which the parallelism decreases as the number of axes increases in the multi-axis measurement method, causing measurement errors; at the same time, the two beams of light in each dimension are measurement lights, which can suppress common mode noise while improving the resolution, and enhance the anti-interference ability of the system. The angular displacement of the measured object on the two axes can be calibrated in the spherical coordinate system to its initial position and final position in the three-dimensional space, realizing high-precision measurement of two-dimensional angular displacement.

[0052] like Figure 5 , Figure 6 As shown, the standard deviation of the original angular displacement information obtained by the present invention is about 3mas, which can represent the stability of the entire system;

[0053] Figure 7 This is a verification measurement result diagram of the horizontal dimension angular displacement of the present invention; the turntable angular displacement range is: 0-3.5°, and the red line part is the standard deviation of the static moment obtained by selecting part of the data, in seconds; Figure 8This is the verification measurement result of the vertical angular displacement of the present invention; the angular displacement range of the turntable is: 0-3.5°, and the standard deviation of each static moment is one order of magnitude different from the horizontal measurement result, because the vertical system stability is poor. Figure 7 , Figure 8 It can be seen that the present invention can realize the measurement of two-dimensional angular displacement.

[0054] like Fig. 9 As shown, the initial and final states of the object to be measured when the angular displacement is generated are measured by the present invention. The normal line of the object to be measured is used to represent its direction. The angular displacement state of the object to be measured can be described in the spherical coordinate system. If the initial position of the object to be measured is located in the XOZ plane, its normal line can be represented as (1, π, π). The object to be measured generates an angular displacement of φ around the Z axis and an angular displacement of θ around the X axis. The final position can be represented as (1, π+θ, π+φ) in the spherical coordinate system. Through this representation method, the initial position and final position of the object to be measured in three-dimensional space can be calibrated to achieve accurate description of two-dimensional angular displacement.

[0055] Fig. 9 The two-dimensional angular displacement geometric space representation diagram of the present invention; the above shows and describes the main features and advantages of the present invention. For those skilled in the art, it is obvious that the specific implementation of the present invention is not limited to the details of the above exemplary embodiments, and without departing from the spirit or basic characteristics of the present invention, the creative ideas and design ideas of the present invention can be realized in other specific forms, which should be equivalent to the protection scope disclosed in the technical solution of the present invention. Therefore, no matter from which point of view, the embodiments should be regarded as exemplary and non-restrictive. The scope of the present invention is defined by the attached claims rather than the above description, and it is intended to include all changes that fall within the meaning and scope of the equivalent elements of the claims in the present invention.

[0056] In addition, it should be understood that although the present specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This description of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

Claims

1. A heterodyne interferometer for two-dimensional angular displacement measurement, comprising a dual-frequency laser light source (1) for providing a dual-frequency laser beam with orthogonal polarizations, characterized in that: A depolarizing beam splitter prism (2) is arranged on the output optical path of the dual-frequency laser light source (1); the dual-frequency laser beam is incident on the depolarizing beam splitter prism (2) and is split into a first dual-frequency beam and a second dual-frequency beam; the two dual-frequency beams enter a plane interferometry group (3) and a plane interferometry group (4) respectively; The plane interference measurement group (3) comprises a first half-wave plate (301), a first right-angle prism (302) and a first polarization beam splitter prism (303); the first half-wave plate (301) is arranged on one side of the depolarization beam splitter prism (2); the first dual-frequency light beam passes through the first half-wave plate (301) and the first right-angle prism (302) in sequence and then enters the first polarization beam splitter prism (303); after being transmitted and reflected by the first polarization beam splitter prism (303), a first beam of measurement light and a second beam of measurement light are respectively formed; on the outgoing optical path of the first beam of measurement light A first target retroreflector (304) is provided, a second right-angle prism (305) and a second target retroreflector (306) are provided on the outgoing light path of the second beam of measuring light, a first polarizing plate (307) is attached to the rear end surface of the first polarizing beam splitter prism (303), a first photodetector (308) is provided on the outgoing light path of the first polarizing plate (307), a second half-wave plate (309) is provided on one side of the first polarizing beam splitter prism (303), and a laser length measuring instrument (310) is provided behind the second half-wave plate (309); The vertical interferometer measurement group (4) comprises a third half-wave plate (401) and a second polarization beam splitter prism (402), wherein the third half-wave plate (401) is arranged on one side of the depolarization beam splitter prism (2), and the second dual-frequency light beam enters the second polarization beam splitter prism (402) after passing through the third half-wave plate (401), and forms a third beam of measurement light and a fourth beam of measurement light respectively after being transmitted and reflected by the second polarization beam splitter prism (402), and a third right-angle prism (403) and a third target retroreflector (404) are arranged in sequence on the outgoing light path of the fourth beam of measurement light, and a fourth right-angle prism (405), a fifth right-angle prism (406), and a fourth target retroreflector (407) are arranged in sequence on the outgoing light path of the third beam of measurement light, a second polarizer (408) is attached to the rear end surface of the second polarization beam splitter prism (402), and a second photodetector (409) is arranged on the outgoing light path of the second polarizer (408); The first target retroreflective mirror (305), the second target retroreflective mirror (306), the third target retroreflective mirror (404) and the fourth target retroreflective mirror (407) are all arranged on a rotating platform (5). Horizontal rotating shafts (7) are arranged on both sides of the rotating platform (5). The horizontal rotating shafts (7) are rotatably connected to the intermediate rotating platform (8). Vertical rotating shafts (6) are arranged on both upper and lower sides of the intermediate rotating platform (8). The vertical rotating shafts (6) are rotatably connected to the fixed platform (9). The vertical rotating shafts (6) provide vertical rotation power for the intermediate rotating platform (8) through an external servo motor. The horizontal rotating shafts (7) are arranged on the upper and lower sides of the intermediate rotating platform (8). ) provides horizontal rotation power for the rotating table (5) through an external servo motor, the first target retroreflective mirror (305) and the second target retroreflective mirror (306) are distributed at positions close to the vertical rotation axis (6), and the third target retroreflective mirror (404) and the fourth target retroreflective mirror (407) are distributed at positions close to the horizontal rotation axis (7). Through the cooperation of the vertical rotation axis (6) and the horizontal rotation axis (7), the angle adjustment of the first target retroreflective mirror (305), the second target retroreflective mirror (306), the third target retroreflective mirror (404) and the fourth target retroreflective mirror (407) in three-dimensional space is achieved.

2. A heterodyne interferometer for two-dimensional angular displacement measurement according to claim 1, characterized in that: The polarization axis directions of the first polarizer (307) and the second polarizer (408) are both 45 degrees to the horizontal direction.

3. A heterodyne interferometer for two-dimensional angular displacement measurement according to claim 1, characterized in that: The first half wave plate (301), the second half wave plate (309) and the third half wave plate (401) are all installed in a rotationally connected manner.

4. A two-dimensional angular displacement measurement method using the heterodyne interferometer of claim 1, characterized in that: The following steps are involved: S1, after the dual-frequency laser beam is incident on the depolarization beam splitter prism (2), the transmitted beam forms a second dual-frequency beam, and the reflected beam forms a first dual-frequency beam; S2, the first dual-frequency light beam passes through the first half-wave plate (301) and the first right-angle prism (302) in sequence and then enters the first polarization beam splitter (303), wherein the transmitted light beam forms the first beam of measurement light, and the reflected light beam forms the second beam of measurement light; the second dual-frequency light beam passes through the third half-wave plate (401) and then enters the second polarization beam splitter (402), wherein the transmitted light beam forms the third beam of measurement light, and the reflected light beam forms the fourth beam of measurement light; S3, the first beam of measuring light contacts the first target retroreflector (304) with a Doppler frequency shift and is transmitted and outputted from the first polarization beam splitter (303); the second beam of measuring light is reflected by the second right-angle prism (305) and contacts the second target retroreflector (306); thereafter, it is reflected by the second right-angle prism (305) with a Doppler frequency shift and is reflected and outputted from the first polarization beam splitter (303); the third beam of measuring light contacts the third target retroreflector (407) with a Doppler frequency shift and is reflected by the fifth right-angle prism (406) and the fourth right-angle prism (405) with a Doppler frequency shift and is transmitted and outputted from the second polarization beam splitter (402); the fourth beam of measuring light contacts the fourth target retroreflector (404) with a Doppler frequency shift and is reflected by the third right-angle prism (403) and is reflected and outputted from the second polarization beam splitter (402); S4, the paths of the first beam of measurement light and the second beam of measurement light coincide after passing through the first polarization beam splitter (303), and then pass through the first polarizer (307) to perform beat frequency interference to form a first measurement signal Im1; the paths of the third beam of measurement light and the fourth beam of measurement light coincide after passing through the second polarization beam splitter (402), and then pass through the second polarizer (408) to perform beat frequency interference to form a second measurement signal Im2. S5, the first photodetector (308) receives the first measurement signal Im1, and the second photodetector (409) receives the second measurement signal Im2. After signal processing of the first measurement signal Im1 and the second measurement signal Im2, the angular displacement of the measured object on two axes can be obtained.

5. A two-dimensional angular displacement measurement method according to claim 4, characterized in that: The dual-frequency laser beam is generated by a dual-acoustic-optical modulation method.

6. A two-dimensional angular displacement measurement method according to claim 4, characterized in that: The interference optical path in each dimension consists of two axes, which avoids the situation in which the parallelism decreases and causes measurement errors as the number of axes increases in multi-axis measurement. At the same time, the two beams of light in each dimension are measurement lights, which can suppress common mode noise while improving the resolution, thereby enhancing the system's anti-interference ability.

7. A two-dimensional angular displacement measurement method according to claim 4, characterized in that: The angular displacement of the measured object on two axes can be used to calibrate its initial position and final position in three-dimensional space in a spherical coordinate system, thereby achieving high-precision measurement of two-dimensional angular displacement.

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