Double-frequency heterodyne three-axis grating ruler

Abstract

The invention relates to the technical field of precision optical measurement, and discloses a double-frequency heterodyne three-axis grating ruler. The device mainly comprises a double-frequency orthogonal polarization light source, a plane mirror, a combined grating, a two-dimensional reflection grating, a one-dimensional grating, a polaroid, a quarter-wave plate, a half-wave plate, a heterodyne photoelectric conversion unit and a signal acquisition and processing device. The double-frequency light source generates S and P polarized light which are different in frequency and vertical in polarization, the S and P polarized light enters the combined grating partition through the plane mirror to be diffracted, and X, Y and Z three-axis displacement signals are synchronously acquired in cooperation with a Littrow angle auto-collimation light path of the two-dimensional reflection grating. Y-axis displacement is obtained by heterodyne interference signals formed by beam combination of light returned by the two-dimensional reflection grating in the position space of the one-dimensional grating, and X-axis displacement and Z-axis displacement are obtained by joint calculation of interference signals diffracted and separated by the combined grating. The system is symmetrical in light path, compact in structure, quasi-common in light path, capable of effectively restraining environment temperature excursion and mechanical vibration, and suitable for multi-degree-of-freedom high-precision positioning measurement.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of precision optical measuring instruments, and particularly relates to a dual-frequency heterodyne three-axis grating ruler for multi-degree-of-freedom displacement synchronous measurement. TECHNICAL BACKGROUND In industrial production and scientific research, the demand for measuring the position and attitude of objects in three-dimensional space is increasing. For example, in a three-coordinate measuring machine, it is necessary to accurately measure the coordinate values of a workpiece in the X, Y and Z directions to realize accurate detection of the size, shape and position of the workpiece. Traditional single-axis or dual-axis grating rulers cannot meet the demand of such three-axis measurement, so it is necessary to develop a grating ruler capable of simultaneously measuring three degrees of freedom, i.e. a three-axis grating ruler. At the same time, in order to improve the measurement speed and accuracy, dual-frequency heterodyne technology is introduced into the three-axis grating ruler, which can more accurately measure the displacement of the grating and improve the measurement resolution and anti-interference ability by using two beams of light of different frequencies to generate a heterodyne interference signal.

[0002] In the patent US4776701, Heidenhain Company of Germany proposed a reflective Moire fringe displacement measurement scheme, which realized the design of 120° fixed phase difference, UV cured polymer manufacturing grating by combining the regulation of the sawtooth mark / gap ratio of the indicator grating and the 0.5 μm preferred height, solved the problem of phase separation relying on gap in early technology and improved the single-axis measurement accuracy, but this patent only supports single-axis measurement, and the design based on single-frequency light interference has weak anti-environmental interference ability.

[0003] In the patent EP1762828B1, Heidenhain proposed an optical position measurement scheme based on two-dimensional cross grating and multiple sets of non-collinear scanning systems, which obtained multi-degree-of-freedom position signals of the relative motion component in the horizontal and vertical directions through two or three interference field scanning light paths and phase interpolation processing, and was applied in precise positioning of semiconductor manufacturing equipment, solving the problem that traditional single-axis grating measurement cannot simultaneously detect multi-direction displacement. However, this scheme relies on stable single-frequency coherent light source and light path symmetry to ensure measurement accuracy, and the structure is complex and multiple sets of scanning systems need to work together for multi-degree-of-freedom measurement, which limits the integration and stability of the system.

[0004] The patent CN114111587B of Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, proposes a three-axis high-optical subdivision grating ruler scheme based on a dual-frequency orthogonal polarization light source, a two-dimensional high-density diffraction grating and multiple near-Littrow collimation diffraction The scheme realizes single-read head X / Y / Z three-axis measurement by combining Doppler frequency multiplication of a two-dimensional subdivision prism assembly and heterodyne detection, and realizes single-read head X / Y / Z three-axis measurement by combining Doppler frequency multiplication of a two-dimensional subdivision prism assembly and heterodyne detection. Displacement and angle measurement with optical subdivision of 8x or higher in three degrees of freedom features a compact structure and zero optical path difference to reduce dead path errors caused by environmental parameters. However, the optical path of this patent relies on multiple round-trip diffractions and the relative positioning of precision components, making assembly and adjustment complex and the optical path stability sensitive to mechanical vibration and temperature drift. At the same time, the improvement of subdivision magnification depends on a specific truncated quadrangular prism combination structure, which increases manufacturing and assembly costs.

[0005] ZYGO Inc.'s patent EP3207339B1 proposes an interferometric encoder that generates a heterodyne signal by utilizing the frequency difference between a measurement beam and a reference beam. This encoder achieves high-precision target position detection through optical beam splitting, double diffraction, and a quasi-reflection structure. Furthermore, its optical path design reduces the space occupied by the encoder scale and its sensitivity to grating tilt, improving stability in single-axis or two-dimensional scales. However, this approach primarily targets one-dimensional or two-dimensional reflective gratings, and its optical path structure relies on multiple sets of optical elements arranged along a fixed plane. It lacks optical path coupling and synchronization design for three-axis encoders, making it difficult to achieve three-axis dual-frequency heterodyne measurement within the same system.

[0006] Patent CN108286943B from the Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, proposes a displacement measurement optical system for a lithography system stage. It utilizes difference-frequency lasers of red and blue wavelengths to construct dual difference-frequency interferometer arms. A geometric compensation arm, combined with temperature and humidity sensors, compensates for air refractive index disturbances and thermal expansion errors, improving the interferometric displacement measurement accuracy of a long-stroke two-dimensional stage. However, this solution targets a two-dimensional single-point interferometric measurement architecture and cannot achieve multi-axis continuous encoding and synchronous output of the grating ruler.

[0007] Canon Corporation of Japan proposed a movable diffraction grating encoder scheme in patent US5038032. This scheme detects changes in interference light intensity by repeatedly introducing a semiconductor laser beam into a movable diffraction grating with a non-coplanar incident direction, combined with a reflector and a cat's-eye optical system. This effectively suppresses zero-order reflected light from entering the detector, thereby improving the signal-to-noise ratio and sub-micron accuracy of single-axis displacement measurement. However, this scheme still uses the single-frequency optical interference principle, limiting the number of measurement axes to single-axis or two-dimensional, making it difficult to achieve multi-axis synchronous measurement.

[0008] In summary, existing triaxial or multi-degree-of-freedom grating measurement systems generally suffer from problems such as complex optical path structures, a large number of components, difficult assembly and adjustment, sensitivity to environmental disturbances, and low system integration. Therefore, there is an urgent need to develop a compact, optically symmetrical, interference-resistant, and easily implemented triaxial synchronous displacement measurement scheme to meet the industrial and scientific research needs for high-precision, multi-degree-of-freedom positioning measurements. Summary of the Invention

[0009] To overcome the problems of complex optical path structure, high installation and debugging difficulty, susceptibility to temperature drift and mechanical vibration, and insufficient system integration in existing triaxial grating measurement systems, this invention proposes a dual-frequency heterodyne triaxial grating ruler for synchronous measurement of multi-degree-of-freedom displacement, achieving high-precision synchronous measurement of triaxial displacement with compact structure, symmetrical optical path, and quasi-common optical path.

[0010] To achieve the above objectives, the technical solution of the present invention is as follows: The present invention employs a compact three-axis synchronous measurement optical path structure, including a dual-frequency orthogonal polarization light source, a plane mirror, a combined grating, a two-dimensional reflection grating, a one-dimensional grating, several polarizers, a quarter-wave plate, a half-wave plate, a photodetector, and a signal processing system.

[0011] A dual-frequency orthogonal polarization source simultaneously outputs two linearly polarized beams with independent frequencies and mutually perpendicular polarization directions: one beam is S-polarized light, and the other is P-polarized light, with frequencies of [missing information]. and .

[0012] The key structure of this invention lies in the partitioned layout of the combined grating and the coupling mode of multiple diffraction and reflection. The combined grating consists of three one-dimensional gratings: the grating lines in the middle diffraction region are along the Y-axis, and the grating lines in the two side diffraction regions are along the X-axis. Dual-frequency orthogonally polarized light, after its propagation direction is adjusted by a plane mirror, is incident on the middle diffraction region, generating 0th-order and ±1st-order diffracted light in the first diffraction. The 0th-order diffracted light, after passing through a quarter-wave plate, is partially split and returned by a partial mirror, then incident on the combined grating again for secondary diffraction; the other part passes through a polarizer and a quarter-wave plate into a two-dimensional reflection grating, generating ±1st-order diffracted light. The generated ±1st-order light enters the left and right diffraction regions of the combined grating respectively, and after secondary diffraction, spatially combines at the one-dimensional grating position, forming a heterodyne interference signal for Y-axis displacement measurement. Simultaneously, the ±1st order light generated by the initial diffraction of the combined grating is modulated with a specific polarization and then incident on the two-dimensional reflection grating at a Littrow angle, returning after collimation. After a second diffraction by the combined grating, it forms an interference signal with the second diffraction of the 0th order diffracted light. This signal is collected by a polarizer and a corresponding photodetector, respectively, enabling the joint calculation of X-axis and Z-axis displacement information. This optical path design ensures the synchronous acquisition of the three-axis signals, maintains optical path symmetry and quasi-common optical path, and effectively reduces the effects of temperature drift and mechanical vibration.

[0013] A dual-frequency heterodyne triaxial grating ruler, characterized in that it includes: A dual-frequency orthogonal polarization light source is used to generate two beams of linearly polarized light with different frequencies and perpendicular polarization directions. A plane mirror is disposed in the output light path of the dual-frequency orthogonal polarization light source; A combined grating is disposed on the reflected light path of the plane mirror and consists of three adjacent one-dimensional grating regions. The grating lines of the middle diffraction region are along the Y-axis, while the grating lines of the left and right diffraction regions are both along the X-axis, forming a partitioned diffraction structure. The middle diffraction region simultaneously generates 0th-order diffraction light and ±1st-order diffraction light for the incident dual-frequency orthogonal polarized light source. A two-dimensional reflection grating is disposed on the diffraction path of the combined grating, and its grating plane is perpendicular to the Z-axis of the measurement coordinate system; Part of the reflector is disposed on the 0th order diffraction path in the middle diffraction region of the combined grating; The polarization control component is configured to: guide a portion of the beam diffracted through the middle diffraction region of the combined grating to the two-dimensional reflection grating, and cause the returning diffracted beam to undergo secondary diffraction in the diffraction regions on both sides of the combined grating to spatially combine, thereby generating a first heterodyne interference signal containing displacement information in the first direction; simultaneously, guide another portion of the beam diffracted through the middle diffraction region of the combined grating to the two-dimensional reflection grating in a self-collimated manner, and return along the original path in a self-collimated manner to form quasi-common path interference, thereby generating second and third heterodyne interference signals containing displacement information in the second and third directions; A one-dimensional grating is disposed on the secondary diffraction optical path in the left and right diffraction regions of the combined grating; The photoelectric detection system includes a first photodetector, a second photodetector, and a third photodetector, which respectively receive first, second, and third heterodyne interference signals formed through different optical paths; The signal processing system, connected to the photoelectric detection system, is used to synchronously calculate the displacement of the X, Y, and Z axes.

[0014] Furthermore, the polarization control component includes a second quarter-wave plate disposed on the transmission optical path of the partial reflector; a second polarizer and a third polarizer disposed on the ±1st order diffraction optical paths of the intermediate diffraction region; and a half-wave plate disposed on the +1st order diffraction optical path of the two-dimensional reflection grating.

[0015] The 0th-order diffracted light generated in the intermediate diffraction region is split by the first quarter-wave plate and a portion of the reflecting mirrors. A portion of the transmitted light passes through the second quarter-wave plate and is incident on the two-dimensional reflection grating. The -1st-order diffracted light generated by the two-dimensional reflection grating enters the left diffraction region of the combined grating for secondary diffraction. The +1st-order diffracted light generated by the two-dimensional reflection grating is phase-modulated by the half-wave plate and enters the right diffraction region of the combined grating for secondary diffraction. The other portion of the reflected light returns to the intermediate diffraction region through the first quarter-wave plate for secondary diffraction, thus realizing optical path multiplexing. The ±1st order diffracted light generated in the intermediate diffraction region is modulated by the second and third polarizers, respectively, and then incident on the two-dimensional reflection grating at a Littrow angle. It then returns to the intermediate diffraction region of the combined grating in a self-collimated manner for secondary diffraction, forming a quasi-common-path interference structure with the re-diffracted light of the 0th order diffracted light generated in the intermediate diffraction region.

[0016] Furthermore, the polarization control component also includes a first polarizer, a fourth polarizer, and a fifth polarizer respectively disposed on the three probe optical paths; The ±1st order diffracted light generated by the two-dimensional reflection grating undergoes secondary diffraction in the left and right diffraction regions of the combined grating, and is then combined by the one-dimensional grating before entering the first photodetector through the first polarizer to form the first heterodyne interference signal. The +1st order diffracted light from the quasi-common-path interference enters the second photodetector via the fourth polarizer, forming the second heterodyne interference signal. The -1st order diffracted light from the quasi-common-path interference enters the third photodetector via the fifth polarizer, forming the third heterodyne interference signal.

[0017] Furthermore, the two output beams of the dual-frequency orthogonal polarization light source have independent frequencies and mutually perpendicular polarization directions, wherein the S-polarized light and the P-polarized light correspond to... and ,and .

[0018] Furthermore, the two-dimensional reflective grating is either reflective or transmissive. When it is transmissive, a reflector is also provided on its transmissive light path to achieve retroreflection.

[0019] Furthermore, the signal processing system calculates the X-axis and Z-axis displacements using the following formulas: In the formula, and These are the displacement information received by the second and third photodetectors, respectively.

[0020] Furthermore, after the 0th-order diffracted light generated in the intermediate diffraction region is split by the first quarter-wave plate and a portion of the reflecting mirrors, a portion of the transmitted light is incident on the two-dimensional reflection grating through the second quarter-wave plate, with its incident angle deviating from normal incidence. The preferred angle is 5° to 8° to avoid reflection interference.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention utilizes dual-frequency heterodyne technology to superimpose two beams of light with different frequencies and perpendicular polarization directions to form a heterodyne signal. This not only significantly improves the displacement measurement resolution but also enhances the suppression of incoherent interference. By coupling combined gratings with two-dimensional and one-dimensional gratings, the system achieves a compact, quasi-common optical path, and symmetrical multi-degree-of-freedom measurement scheme. This invention avoids the complexity of parallel measurements with multiple optical paths, reduces the number of components and the difficulty of installation and adjustment, and improves overall stability and industrial feasibility. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the optical path of the dual-frequency heterodyne triaxial grating ruler of the present invention.

[0023] In the figure: 1-Dual-frequency orthogonal polarization source, 2-Plane mirror, 3-Combined grating (31-Left diffraction region, 32-Middle diffraction region, 33-Right diffraction region), 4-First quarter-wave plate, 5-Partial mirror, 6-Second quarter-wave plate, 7-Two-dimensional reflection grating, 8-Second polarizer, 9-Third polarizer, 10-Half-wave plate, 11-One-dimensional grating, 12-First polarizer, 13-First photodetector, 14-Fourth polarizer, 15-Fifth polarizer, 16-Second photodetector, 17-Third photodetector, 18-Signal acquisition and processor. Detailed Implementation

[0024] To make the objectives, technical solutions, and beneficial effects of this invention clearer, the following description is provided in conjunction with the appendix. Figure 1 The specific embodiments of the present invention will be described in detail below. These embodiments are only used to explain the principles and structure of the present invention and do not constitute a limitation on the scope of protection of the present invention. Any structural improvements or substitutions made by those skilled in the art without departing from the spirit of the present invention should be considered within the scope of protection of the present invention.

[0025] refer to Figure 1 The dual-frequency heterodyne triaxial grating ruler of this embodiment utilizes a dual-frequency orthogonal polarization light source 1 and a coupled diffraction optical path. Through polarization encoding and optical path multiplexing techniques, it synchronously generates three physically separate and informationally decoupled heterodyne interference signals, corresponding to X, Y, and Z axis displacements, respectively. The specific structure is as follows: The system comprises a dual-frequency orthogonal polarization light source 1, a plane mirror 2, a combined grating 3 (including a central diffraction region 32, a left diffraction region 31, and a right diffraction region 33), a first quarter-wave plate 4, a partial mirror 5, a second quarter-wave plate 6, a two-dimensional reflection grating 7, a second polarizer 8, a third polarizer 9, a half-wave plate 10, a one-dimensional grating 11, a first polarizer 12, a first photodetector 13, a fourth polarizer 14, a fifth polarizer 15, a second photodetector 16, a third photodetector 17, and a signal acquisition and processor 18. The combined grating 3 consists of three adjacent one-dimensional gratings, where the grating lines of the central grating region 32 are along the Y-axis, and the grating lines of the left diffraction region 31 and the right diffraction region 33 are along the X-axis. The two grating line directions of the two-dimensional reflection grating 7 correspond to the X-axis and Y-axis of the three axes, respectively, and the plane of the two-dimensional reflection grating is perpendicular to the Z-axis of the rectangular coordinate system, with the central axis of symmetry set as the Z-axis.

[0026] In the operation of this embodiment, the dual-frequency orthogonal polarization light source 1 emits light with a frequency of... and Two linearly polarized beams, one S-polarized and the other P-polarized, are incident on a combined grating 3 after their transmission directions are changed by a plane mirror 2. The beams diffract on the grating along the Y-axis, forming 0th-order and ±1st-order diffracted beams. The 0th-order diffracted beam is split by a first quarter-wave plate 4 and a partial mirror 5. The reflected beam then passes back through the first quarter-wave plate 4, where the P and S beams interchange and return to the combined grating's central diffracting region 32 for further diffraction. The -1st-order diffracted beam passes through a fourth polarizer 14 and enters a second photodetector 16, while the +1st-order diffracted beam passes through a fifth polarizer 15 and enters a third photodetector 17. The remaining transmitted beam passes through a second quarter-wave plate 6, where the P and S beams interchange and are incident orthogonally on a two-dimensional reflection grating 7, where diffraction occurs. The -1st order diffracted light generated by the two-dimensional reflection grating 7 enters the left region 31 of the combined grating 3 for further diffraction, while the +1st order diffracted light first passes through the half-wave plate 10 to introduce a specific phase change, preparing the phase for subsequent interference, and then enters the right region 33 of the combined grating 3 for further diffraction. The ±1st order diffracted light returned by the two-dimensional reflection grating 7 is spatially combined at the position of the one-dimensional grating 11, and then enters the first photodetector 13 through the first polarizer 12 to form a heterodyne interference signal. The displacement in the Y-axis direction is calculated by the signal acquisition and processor 18.

[0027] Simultaneously, the ±1st order light generated by the initial diffraction of the combined grating 3 is modulated by the second polarizer 8 (transmission axis parallel to the Y-axis) and the third polarizer 9 (transmission axis parallel to the X-axis), respectively, and then incident on the two-dimensional reflection grating 7 at a Littrow angle. It then returns along the same path to the diffraction region 32 in the middle of the combined grating 3 for further diffraction. This design allows the return light and the path of the second diffracted light from the 0th order diffracted light to form a quasi-common optical path, achieving near-zero optical path difference and effectively reducing the influence of temperature drift and mechanical vibration on the measurement. The -1st order diffracted light from the three beams formed in this optical path enters the second photodetector 16 via the fourth polarizer 14, and the +1st order diffracted light enters the third photodetector 17 via the fifth polarizer 15. The interference signals received by the second and third photodetectors 16 are input to the signal acquisition and processor 18. After joint analysis and calculation, displacement information in the X and Z axes is obtained, thereby achieving synchronous three-axis measurement.

[0028] The displacement information of the X-axis and Z-axis is calculated as follows: and The displacement information received by the second photodetector 16 and the third photodetector 17 are respectively: Through the above-mentioned optical path and optical component configuration, this embodiment achieves high-precision synchronous measurement of three degrees of freedom (X, Y, Z) displacement while ensuring a compact system structure and symmetrical optical path. It also utilizes dual-frequency heterodyne technology to improve the system's resolution and anti-interference capability, making it particularly suitable for scenarios requiring high-precision positioning of multiple degrees of freedom, such as coordinate measuring machines, precision machine tools, and semiconductor manufacturing equipment.

[0029] Specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention. Where there is no conflict, the above embodiments and features described therein can be combined with each other.

Claims

1. A dual-frequency heterodyne triaxial grating ruler, characterized in that, include: A dual-frequency orthogonal polarization light source is used to generate two beams of linearly polarized light with different frequencies and perpendicular polarization directions. A plane mirror is disposed in the output light path of the dual-frequency orthogonal polarization light source; A combined grating is disposed on the reflected light path of the plane mirror and consists of three adjacent one-dimensional grating regions. The grating lines of the middle diffraction region are along the Y-axis, while the grating lines of the left and right diffraction regions are both along the X-axis, forming a partitioned diffraction structure. The middle diffraction region simultaneously generates 0th-order diffraction light and ±1st-order diffraction light for the incident dual-frequency orthogonal polarized light source. A two-dimensional reflection grating is disposed on the diffraction path of the combined grating, and its grating plane is perpendicular to the Z-axis of the measurement coordinate system; Part of the reflector is disposed on the 0th order diffraction path in the middle diffraction region of the combined grating; The polarization control component is configured to: guide a portion of the beam diffracted through the middle diffraction region of the combined grating to the two-dimensional reflection grating, and cause the returning diffracted beam to undergo secondary diffraction in the diffraction regions on both sides of the combined grating to spatially combine, thereby generating a first heterodyne interference signal containing displacement information in the first direction; simultaneously, guide another portion of the beam diffracted through the middle diffraction region of the combined grating to the two-dimensional reflection grating in a self-collimated manner, and return along the original path in a self-collimated manner to form quasi-common path interference, thereby generating second and third heterodyne interference signals containing displacement information in the second and third directions; A one-dimensional grating is disposed on the secondary diffraction optical path in the left and right diffraction regions of the combined grating; The photoelectric detection system includes a first photodetector (13), a second photodetector (16) and a third photodetector (17), which respectively receive first, second and third heterodyne interference signals formed through different optical paths; The signal processing system, connected to the photoelectric detection system, is used to synchronously calculate the displacement of the X, Y, and Z axes.

2. The dual-frequency heterodyne triaxial grating ruler according to claim 1, characterized in that, The polarization control component includes a second quarter-wave plate (6) disposed on the transmission optical path of the partial reflector; a second polarizer (8) and a third polarizer (9) disposed on the ±1st order diffraction optical path of the intermediate diffraction region (32); and a half-wave plate (10) disposed on the +1st order diffraction optical path of the two-dimensional reflection grating (7). The 0th order diffracted light generated in the intermediate diffraction region (32) is split by the first quarter-wave plate (4) and a partial reflector (5). A portion of the transmitted light is incident on the two-dimensional reflection grating (7) through the second quarter-wave plate (6). The -1st order diffracted light generated by the two-dimensional reflection grating (7) enters the left diffraction region (31) of the combined grating for secondary diffraction. The +1st order diffracted light generated by the two-dimensional reflection grating (7) is phase-modulated by the half-wave plate (10) and enters the right diffraction region (33) of the combined grating for secondary diffraction. Another portion of the reflected light returns to the intermediate diffraction region (32) through the first quarter-wave plate (4) for secondary diffraction, thus realizing optical path multiplexing. The ±1st order diffracted light generated by the intermediate diffraction region (32) is modulated by the second polarizer (8) and the third polarizer (9) respectively, and then incident on the two-dimensional reflection grating (7) at the Littrow angle. It returns to the intermediate diffraction region (32) of the combined grating in the original path in a self-collimated manner for secondary diffraction, and forms a quasi-common path interference structure with the second diffracted light of the 0th order diffracted light returned by the intermediate diffraction region (32).

3. The dual-frequency heterodyne triaxial grating ruler according to claim 2, characterized in that, The polarization control component also includes a first polarizer (12), a fourth polarizer (14) and a fifth polarizer (15) respectively disposed on the three probe optical paths. The ±1st order diffracted light generated by the two-dimensional reflection grating (7) undergoes secondary diffraction in the left diffraction region (31) and the right diffraction region (33) of the combined grating, and is then combined by the one-dimensional grating (11) and enters the first photodetector (13) through the first polarizer (12) to form the first heterodyne interference signal. The +1st order diffracted light from the quasi-common path interference enters the second photodetector (16) via the fourth polarizer (14) to form the second heterodyne interference signal; The -1st order diffracted light from the quasi-common path interference enters the third photodetector (17) via the fifth polarizer (15), forming the third heterodyne interference signal.

4. The dual-frequency heterodyne triaxial grating ruler according to claim 1, characterized in that, The two output beams of the dual-frequency orthogonal polarization light source have independent frequencies and perpendicular polarization directions, wherein the S-polarized light and the P-polarized light correspond to respectively and ,and .

5. The dual-frequency heterodyne triaxial grating ruler according to claim 1, characterized in that, The two-dimensional reflective grating can be of the transmissive type, and a reflector can be set in its transmissive light path to achieve retroreflection.

6. The dual-frequency heterodyne triaxial grating ruler according to any one of claims 1-5, characterized in that, The signal processing system calculates the X-axis and Z-axis displacements using the following formulas: In the formula, and These are the displacement information received by the second photodetector (16) and the third photodetector (17), respectively.

7. The dual-frequency heterodyne triaxial grating ruler according to claim 2, characterized in that, The zero-order diffraction light generated by the intermediate diffraction region (32) is split by the first quarter-wave plate (4) and a partial reflector (5). A portion of the transmitted light is incident on the two-dimensional reflection grating (7) through the second quarter-wave plate (6). Its incident angle deviates from normal incidence, with a preferred angle of 5° to 8° to avoid reflection interference.

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