Compact double-frequency heterodyne triaxial grating ruler and precision motion system thereof

Abstract

The invention discloses a compact double-frequency heterodyne triaxial grating ruler, and belongs to the technical field of precision optical measurement. The grating ruler adopts an integrated compact structure formed by stacking a double-frequency cross-polarization laser light source, a prism and detector module, a beam combining prism module and a two-dimensional reflecting grating from top to bottom. The method is characterized in that measurement light is made to deviate from the center of a beam combining prism in a beam combining prism module for normal incidence, the geometrical configuration of the beam combining prism is combined, and multi-stage diffraction light beams generated after the measurement light is irradiated to a two-dimensional reflection grating are spatially separated in the prism; and two beam combining gratings with orthogonal scribed line directions are utilized to carry out common-path beam combination on + / -1 level diffraction beams representing X and Y axial displacements to form X, Y and Z three-axis measurement beams which are independent from each other on a spatial light path. And finally, photoelectric conversion and signal processing are carried out through a prism and a detector module, and the three-degree-of-freedom displacement is calculated synchronously. The device effectively inhibits inter-axis crosstalk, has the advantages of being compact in structure, easy and convenient to install and adjust and high in environment interference resistance, and is suitable for the ultra-precision measurement field of photoetching machines, precision motion platforms and the like.

Description

Technical Field

[0001] This invention belongs to the field of precision optical measuring instruments, and specifically relates to a dual-frequency heterodyne grating ruler and its precision motion system for three-degree-of-freedom displacement measurement. Background Technology

[0002] Optical encoders (a type of optical encoder) are widely used in precision displacement and angle measurement and have become core sensing units in precision motion platforms, high-precision optomechanical systems, and nanoscale metrology equipment. In recent years, with the continuous development of precision manufacturing technology, there has been a growing demand for simultaneous measurement of multi-degree-of-freedom displacements, improving the dynamic real-time response performance of measurement systems, and reducing system complexity. Technological breakthroughs in this field will further enhance the performance of industries such as semiconductor manufacturing and precision machining. Therefore, there is an urgent need to develop displacement measurement optical encoders towards multi-axis and smaller sizes.

[0003] ASML's patent NL2015826A proposes an optical encoding system for lithography equipment. This system improves measurement accuracy by using a single-polarization fiber to transmit the light beam, combined with a beam splitter and scale diffraction. However, it can only perform one-dimensional measurements.

[0004] Japanese Patent JP2010091515A discloses a heterodyne grating displacement measurement device that uses a rotating diffraction grating to generate two orthogonally polarized diffracted beams with different frequencies. However, using polarization-maintaining optical fibers makes installation and alignment very difficult, and can lead to unstable light intensity signals during movement.

[0005] Chinese patent CN 106247947 A proposes a heterodyne two / three-dimensional grating displacement measurement system, which, through different combinations of grating periods and optical path design, can meet the needs of both large-range coarse measurement and small-range fine measurement. However, the designed system has a complex structure and contains a large number of optical components, resulting in high cost and difficult assembly and adjustment.

[0006] Chinese patent CN 103604376 A proposes an anti-optical aliasing dual-frequency laser grating interferometric three-dimensional measurement system. It utilizes a single-mode polarization-maintaining fiber to spatially separate dual-frequency lasers to eliminate polarization aliasing caused by incomplete polarization splitting, thereby reducing periodic nonlinear errors. However, the system is essentially a Michelson interferometer, making it susceptible to environmental influences.

[0007] Chinese patent CN 119085478 A proposes a heterodyne six-DOF grating interferometer, which reduces errors by eliminating polarization aliasing and achieves system miniaturization. However, the system is essentially a Michelson interferometer, and its discrete optical path makes it susceptible to environmental influences.

[0008] Chinese patent CN 112097647 A proposes a heterodyne grating displacement measurement device, which reduces environmental sensitivity by simplifying the complex structure of traditional multiple diffraction and reduces signal errors caused by temperature drift by utilizing symmetrical equal optical path lengths. However, the system relies on multiple sets of polarization elements, which are complex to assemble and adjust, have low fault tolerance, and can only perform one-dimensional measurements.

[0009] In summary, existing grating ruler systems generally suffer from the following problems in multi-axis measurement: complex system structure, large size, and difficult assembly and adjustment; sensitive to the environment and poor anti-interference ability; crosstalk between multi-axis signals, affecting measurement accuracy; low integration, making it difficult to meet the requirements of modern precision equipment for multi-axis measurement, compact, and highly robust measurement systems. Summary of the Invention

[0010] This invention addresses the problems of discrete optical paths, large size, and orthogonality deviation in existing multi-axis grating rulers by proposing a compact dual-frequency heterodyne triaxial grating ruler, aiming to achieve three-degree-of-freedom displacement measurement with single measurement point, high precision, high stability, and high integration.

[0011] The technical solution of this invention is as follows: A compact dual-frequency heterodyne triaxial grating ruler includes: a dual-frequency orthogonal polarization laser source, a prism and detector module, a beam combining prism module, and a two-dimensional reflection grating. Signal acquisition and processing.

[0012] The beam combining prism module and the prism and detector module can be integrated into one module, stacked along the beam propagation direction to form an integrated structure; it is connected to the optical fiber of the dual-frequency orthogonal polarization laser source, and then electrically connected to the signal acquisition and processor.

[0013] The prism and detector module includes: a non-polarizing beam splitter prism, a quarter-wave plate, a reflector, a first polarizing beam splitter prism, a first photodetector, a second photodetector; a second polarizing beam splitter prism, a third photodetector, a fourth photodetector; a third polarizing beam splitter prism, a fifth photodetector, and a sixth photodetector.

[0014] The beam combining prism module includes: a beam combining prism, a first half-wave plate, a second half-wave plate, and a beam combining grating. beam combiner grating The first compensation plate and the second compensation plate have the same optical path length as the first half-wave plate and the second half-wave plate.

[0015] The beam combining grating in the beam combining prism module With beam combining grating The grating lines are orthogonal to each other and are attached to the upper surface of the beam combiner prism. Their positions are referenced to the incident beam. One beam combiner grating is fixed to the left or right of the center of the incident beam, and the other is fixed in front of or behind the center of the incident beam.

[0016] The two-dimensional reflection grating With beam combining grating beam combiner grating The two-dimensional grating pitch can be determined according to the specific situation, and setting it to a grating with equal grating pitch in both dimensions is the optimal choice.

[0017] The beam combining prisms in the beam combining prism module can be configured according to the drawn... Figure 1 The structure is set to a cuboid, but is not limited to this. It can be an isosceles trapezoid, rhombus, etc., as long as the beam-combining prism is axisymmetric. This ensures that the ±1st order diffracted light along the X-axis and ±1st order diffracted light along the Y-axis are separated within the beam-combining prism and can be incident on the beam-combining grating respectively. beam combiner grating Output is then possible. The four sides of the beam combining prism are coated with total internal reflection films or utilize only the condition of total internal reflection of the beam.

[0018] The optical path principle of the compact dual-frequency heterodyne triaxial grating ruler is as follows: The dual-frequency orthogonally polarized laser source emits a frequency of... , And a beam of P and S light with mutually orthogonal polarization directions (frequency difference: The beam is then split into two beams by a non-polarizing beam splitter. The light reflected by the non-polarizing beam splitter is incident on a beam combiner prism; the light transmitted through the non-polarizing beam splitter enters a quarter-wave plate, is reflected by a mirror, and returns to the non-polarizing beam splitter along its original path, serving as the Z-axis reference beam. The incident beam is positioned off-center from the beam combiner prism's center and incident directly onto a two-dimensional reflection grating. Diffraction occurs, resulting in five beams: a left beam, a right beam, a front beam, a rear beam, and a central 0th-order beam. The left and right beams are symmetrical about the central 0th-order beam axis in the plane formed by the left and right beams, and the front and rear beams are symmetrical about the central 0th-order beam axis in the plane formed by the front and rear beams.

[0019] The central 0th order beam among the five beams contains the frequency. , It is a beam with two components: P-ray and S-ray. The central 0th-order beam carries a two-dimensional reflection grating. The Z-axis Doppler frequency shift signal returns along the original optical path, forming a two-dimensional reflection grating. The measurement beam for Z-axis displacement, after passing through a beam combiner prism, is incident on a non-polarizing beam splitter prism. Passing through the non-polarizing beam splitter prism, it converges with the Z-axis reference beam. The converged beam then enters a third polarizing beam splitter prism, where the beam reflected from the third polarizing beam splitter is of frequency... , The S-beam enters the fifth photodetector; the beam transmitted from the third polarizing beam splitter has a frequency of , The P-beam enters the sixth photodetector. Then, all photodetectors perform photoelectric conversion on the received signal, converting the optical beat frequency signal generated by the displacement into an electrical beat frequency signal. Finally, after signal acquisition, processing, and calculation by the processor, a two-dimensional reflection grating can be obtained. The Z-axis displacement.

[0020] Two-dimensional reflection grating The left and right beams produced by diffraction form a two-dimensional reflection grating. The Y-axis measurement light beam is refracted after passing through the first half-wave plate into the beam combiner prism, and then reflected after entering the left side of the beam combiner prism. The right beam is refracted after passing through the first compensator plate into the beam combiner prism, and then reflected after entering the right side of the beam combiner prism. The left and right beams, after being reflected by the beam combiner prism, converge at the beam combiner grating. The beams are then combined and emitted. The left and right beams carry positive and negative Y-axis Doppler frequency shift signals, respectively, including frequencies... , It consists of two components: P-ray and S-ray. The left and right beams pass through a beam combiner grating. After exiting the common optical path, the beam enters the first polarizing beam splitter, where the beam reflected from the first polarizing beam splitter has a frequency of... , The S-beam enters the first photodetector; the beam transmitted from the first polarizing beam splitter has a frequency of , The P-beam enters the second photodetector. The photodetector then performs photoelectric conversion on the received signal, converting the optical beat frequency signal into an electrical beat frequency signal. Finally, after signal acquisition, processing, and calculation by a processor, a two-dimensional reflection grating can be obtained. The Y-axis displacement.

[0021] Similar to two-dimensional reflection gratings The front and back beams generated by diffraction form a two-dimensional reflection grating. The X-axis measurement light beam is refracted after passing through the second half-wave plate into the beam combiner prism, and then reflected after hitting the front side of the beam combiner prism. The rear beam is refracted after passing through the second compensator plate into the beam combiner prism, and then reflected after hitting the rear side of the beam combiner prism. The front and rear beams, after being reflected by the beam combiner prism, converge at the beam combiner grating. The beams are then combined and emitted. The front and rear beams carry positive and negative Doppler frequency shift signals along the X-axis, respectively, including frequencies... , It consists of two components: P-ray and S-ray. The front and rear beams pass through a beam combiner grating. The beam exiting through the common optical path enters the second polarizing beam splitter, where the beam reflected from the second polarizing beam splitter has a frequency of... , The S-beam enters the third photodetector; the beam transmitted from the second polarizing beam splitter has a frequency of , The P-beam enters the fourth photodetector. Then, all photodetectors perform photoelectric conversion on the received signal, converting the optical beat frequency signal generated by the displacement into an electrical beat frequency signal. Finally, after signal acquisition, processing, and calculation by the processor, a two-dimensional reflection grating can be obtained. The X-axis displacement.

[0022] Compared with the prior art, the beneficial effects of the present invention are: 1. By adopting a beam combining prism module and a beam separation path design, the incident light is set to be incident normally off-center from the beam combining prism, so that the X, Y, and Z axis measurement lights are separated by the beam combining prism. This effectively solves the problem of mixing and separation of the backlight of single-point triaxial measurement lights, and realizes single-point triaxial measurement under normal incidence conditions.

[0023] 2. Based on this design, the X / Y axis measurement optical path in a single-point triaxial grating ruler can be made to have zero optical path difference, achieving high environmental robustness, and is very compact, making it easy to install and adjust.

[0024] 3. This design adopts dual-frequency heterodyne interferometry technology, which inherits the anti-interference advantages of AC interferometers. Attached Figure Description

[0025] Figure 1 Schematic diagram of a compact dual-frequency heterodyne triaxial grating ruler Figure 2 Two-dimensional reflection grating Schematic diagram of the optical path (ZX plane) for Y-axis measurement In the picture: 1- Dual-frequency orthogonally polarized laser source; 101-Prism and detector module; 2-Unpolarized beam splitter prism, 3-Quarter-wave plate, 4-Reflector, 16-First polarization beam splitter prism, 17-First photodetector, 18-Second photodetector, 19-Second polarization beam splitter prism, 20-Third photodetector, 21-Fourth photodetector, 22-Third polarization beam splitter prism, 23-Fifth photodetector, 24-Sixth photodetector; 102 - Beam combiner prism module; 5 - Beam combiner prism, 7 - 1 First half-wave plate, 8 - 1 Second half-wave plate, 14 - First beam combiner grating 15-Second beam combiner grating 7-2 First compensation piece, 8-2 Second compensation piece.

[0026] 6-Two-dimensional reflection grating

[0027] 25 - Signal Acquisition and Processing; Detailed Implementation

[0028] The present invention will be further described in detail below with reference to embodiments, but the implementation of the present invention is not limited thereto.

[0029] like Figure 1 As shown, the present invention provides a compact dual-frequency heterodyne triaxial grating ruler, comprising: a dual-frequency orthogonal polarization laser source 1, a prism and detector module 101, a beam combining prism module 102, and a two-dimensional reflection grating. 6 and signal acquisition and processor 25.

[0030] The prism and detector module 101 detects and processes the X-axis, Y-axis, and Z-axis measurement beams separated by the beam combining prism module 102. The beam combining prism module 102 is positioned parallel to and below the prism and detector module 101, forming a compact dual-frequency heterodyne triaxial grating ruler reading head structure. The prism and detector module 101 and the beam combining prism module 102 are arranged in a compact, vertically stacked layout: the prism and detector module 101 is located on top, and it is fixed to the beam combining prism module 102 below by optical bonding or precision mechanical positioning. The two-dimensional reflective grating 6 is parallel to and opposite the lower surface of the beam combining prism module 102, maintaining a stable measurement gap, and the two move relative to each other.

[0031] The prism and detector module 101, with the non-polarizing beam splitter 2 as the origin reference, also includes: a quarter-wave plate 3, a reflector 4, a first polarizing beam splitter 16, a first photodetector 17, a second photodetector 18; a second polarizing beam splitter 19, a third photodetector 20, a fourth photodetector 21; a third polarizing beam splitter 22, a fifth photodetector 23, and a sixth photodetector 24.

[0032] The unpolarized beam splitter prism 2 serves as the core beam splitting element, splitting the dual-frequency laser beam incident from the dual-frequency orthogonally polarized laser source 1 into two paths. The reflected light serves as the measurement light and is transmitted downwards to the beam combining prism module 102. The transmitted light serves as the reference light and enters the fixed reference arm composed of a quarter-wave plate 3 and a reflector 4.

[0033] On the lower left side of the unpolarized beam splitter prism 2, within the ZY plane, are arranged a first polarizing beam splitter prism 16, a first photodetector 17, and a second photodetector 18; on the lower right side of the unpolarized beam splitter prism 2, within the ZX plane, are arranged a second polarizing beam splitter prism 19, a third photodetector 20, and a fourth photodetector 21; above the unpolarized beam splitter prism 2 are a third polarizing beam splitter prism 22, a fifth photodetector 23, and a sixth photodetector 24. A dual-frequency orthogonally polarized laser source 1 is arranged parallel to and connected to the prism and detector module 101 via an optical fiber. The prism and detector module 101 is electrically connected to the signal acquisition and processor 25 above it.

[0034] Z-axis reference optical path: The transmitted light from the unpolarized beam splitter prism 2 passes through the quarter-wave plate 3, is perpendicularly reflected by the mirror 4, and then passes through the quarter-wave plate 3 again. The polarization directions of both the P-ray and S-ray are rotated by 90° (i.e., the P-ray becomes the S-ray, and the S-ray becomes the P-ray). This beam is totally internally reflected back to the unpolarized beam splitter prism 2 along the original path, is reflected again, and thus merges with the subsequently returning Z-axis measurement light. It is then split by the third polarized beam splitter prism 22 and detected and processed by the fifth and sixth photodetectors 23 and 24, respectively.

[0035] Three-axis signal detection unit: The three beams after merging are guided to three different detection units.

[0036] The Z-axis detection unit consists of a third polarizing beam splitter 22 and fifth and sixth photodetectors 23 and 24. It receives interference signals from the 0th-order diffracted light and is used to calculate the Z-axis displacement.

[0037] The Y-axis detection unit consists of a first polarizing beam splitter 16 and first and second photodetectors 17 and 18. It receives the Y-axis interference signal emitted from the first beam combiner grating 14.

[0038] The X-axis detection unit consists of a second polarizing beam splitter 19 and third and fourth photodetectors 20 and 21. It receives the X-axis interference signal emitted from the second beam combiner grating 15.

[0039] The beam combining prism module 102, with the beam combining prism 5 as the origin reference, also includes a first half-wave plate 7-1, a second half-wave plate 8-2, and a beam combining grating. 14 and beam combining grating 15. First compensating plate 7-2, second compensating plate 8-2. The four sides of the beam combining prism 5 are coated with total reflection film or only utilize the condition of total reflection of the beam.

[0040] In this embodiment, the beam-combining prism 5 is a cuboid glass prism. The measurement light is not incident perpendicularly from the center of its upper surface, but rather is incident normally at a specific distance d away from the center point. This "eccentric incidence" design is the geometrical optical prerequisite for the subsequent spatial separation of all ±1 order diffracted light within the prism.

[0041] Below the beam combiner prism 5, parallel to it, are arranged a first half-wave plate 7-1 and a second half-wave plate 8-1. A first compensation plate 7-2 and a second compensation plate 8-2 are also present. The first half-wave plate 7-1 and the second half-wave plate 8-1 are mounted on the lower surface of the beam combiner prism 5 with a specific orientation (e.g., the element surface is perpendicular to the beam propagation direction and the fast axis forms a 22.5° angle with the P-beam or S-beam). Their function is to rotate the polarization direction of a specific diffracted beam, ensuring effective interference at the polarization beam splitter. The first compensation plate 7-2 and the second compensation plate 8-2 are used to compensate for the increased optical path difference caused by the waveplates, making the system optical path difference zero, thereby achieving insensitivity to environmental fluctuations.

[0042] First beam combiner 14 and the second beam combiner grating 15 are transmission diffraction gratings with mutually orthogonal etched lines, respectively attached to the left or right side and the back or front side of the upper surface of the beam combiner prism 5 (with the incident beam as a reference). Their function is to combine two spatially separated ±1st order diffraction beams carrying opposite Doppler frequency shifts into a common optical path, forming a pure heterodyne interference signal.

[0043] Dual-frequency orthogonally polarized laser source 1 emits a frequency of , And a beam of P and S light with mutually orthogonal polarization directions (frequency difference: The beam is then split into two beams by the unpolarized beam splitter 2. The measurement beam reflected by the unpolarized beam splitter 2 is incident on the beam combiner 5. The beam transmitted through the unpolarized beam splitter 2 enters the quarter-wave plate 3, is reflected by the mirror 4, and returns to the unpolarized beam splitter 2 along the original optical path. The incident beam on the beam combiner 5 is set to be incident off-center from the center of the beam combiner 5.

[0044] like Figure 2 As shown, the normally incident light beam, passing through the beam combining prism 5, is also normally incident on the two-dimensional reflection grating. The beam is 6, and diffraction occurs, resulting in five diffracted beams: left beam 9, right beam 13, front beam 10, rear beam 12, and central 0th order beam 11. The left beam 9 and right beam 13 are symmetrical about the central 0th order beam 11 in the plane formed by them, and the front beam 10 and rear beam 12 are symmetrical about the central 0th order beam 11 in the plane formed by them.

[0045] The central 0th order beam 11 of the five diffraction beams contains frequencies. , It is a beam with two components: P-beam and S-beam. The central 0th-order beam 11 carries a two-dimensional reflection grating. The 6Z-axis Doppler frequency shift signal returns along the original optical path to form a two-dimensional reflection grating. The beam measuring the Z-axis displacement is incident on the unpolarized beam splitter 2 through the beam combiner prism 5; the light transmitted through the unpolarized beam splitter 2 enters the quarter-wave plate 3, is reflected by the mirror 4, and returns to the unpolarized beam splitter 2 along the original optical path to form a two-dimensional reflection grating. A reference beam for 6Z-axis displacement. The reference beam and the measurement beam exit through the unpolarized beam splitter 2 and enter the third polarized beam splitter 22 via the same optical path. The beam reflected from the third polarized beam splitter 22 is of frequency... , The S-beam enters the fifth photodetector 23; the beam transmitted from the third polarizing beam splitter 22 has a frequency of , The P-beam enters the sixth photodetector 24. The photodetector then performs photoelectric conversion on the received signal, converting the optical beat frequency signal into an electrical beat frequency signal. Finally, after processing and calculation by the signal acquisition and processor 25, a two-dimensional reflection grating can be obtained. The Z-axis displacement of 6.

[0046] Two-dimensional reflection grating 6. The left beam 9 and right beam 13 produced by diffraction are as follows: Figure 2 As shown, a two-dimensional reflection grating is formed in the ZY plane. The 6X-axis measurement beams are as follows: the left beam 9 passes through the first half-wave plate 7-1 and enters the beam combiner prism, where it is refracted. It then reflects off the left side of the beam combiner prism 5. The right beam 13 passes through the first compensator 7-2 and enters the beam combiner prism, where it is refracted. It then reflects off the right side of the beam combiner prism 5. The left beam 9 and right beam 13, after reflection by the beam combiner prism 5, converge at the beam combiner grating. 14 beams exit after firing. The left beam 9 and right beam 13 carry positive and negative Doppler frequency shift signals along the Y-axis, respectively, including frequencies... , The beam has two components: P-ray and S-ray. The beam then passes through a beam combiner grating. 14 beams exiting the common optical path enter the first polarizing beam splitter 16, wherein the beam reflected from the first polarizing beam splitter 16 is of frequency... , The S-beam enters the second photodetector 17; the beam transmitted from the first polarizing beam splitter 16 has a frequency of , The P-beam enters the first photodetector 18. The photodetector then performs photoelectric conversion on the received signal, converting the optical beat frequency signal into an electrical beat frequency signal. Finally, after processing and calculation by the signal acquisition and processor 25, a two-dimensional reflection grating can be obtained. The Y-axis displacement of 6.

[0047] Similar to two-dimensional reflection gratings The front beam 10 and rear beam 12 generated by diffraction form a two-dimensional reflection grating in the ZX plane. The 6Y-axis measurement beams are refracted as follows: the front beam 10 passes through the second half-wave plate 8-1 and enters the beam combiner prism 5, where it is reflected after entering the front side of the beam combiner prism 5. The rear beam 12 passes through the second compensator 8-2 and enters the beam combiner prism 5, where it is refracted after entering the front side of the beam combiner prism 5, where it is reflected after entering the rear side of the beam combiner prism 5. After reflection by the beam combiner prism 5, the front beam 10 and the rear beam 12 converge at the beam combiner grating. The beam exits after beam 15. The front beam 10 and the rear beam 12 carry positive and negative Doppler frequency shift signals along the X-axis, respectively, including frequencies... , The beam has two components: P-ray and S-ray. Within the ZX plane, the beam passes through a beam combiner grating. The beam emitted from the common optical path 15 enters the second polarizing beam splitter 19, wherein the beam reflected from the second polarizing beam splitter 19 is of frequency... , The S-beam enters the third photodetector 20; the beam transmitted from the second polarizing beam splitter 19 has a frequency of , The P-beam enters the fourth photodetector 21. The photodetector then performs photoelectric conversion on the received signal, converting the optical beat frequency signal into an electrical beat frequency signal. Finally, after processing and calculation by the signal acquisition and processor 25, a two-dimensional reflection grating can be obtained. The X-axis displacement of 6.

[0048] 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 compact dual-frequency heterodyne triaxial grating ruler, characterized in that, include: A dual-frequency orthogonal polarization laser source (1) is used to generate a dual-frequency orthogonal polarization beam with frequencies f1 and f2 and orthogonal polarization states and is connected to the prism and detector module (101) by optical fiber; An optical reading head, comprising a prism and detector module (101) and a beam combining prism module (102) arranged sequentially along the optical path. A two-dimensional reflective grating (6) is positioned opposite to the optical reading head and has a measurement gap and relative motion; The signal processing unit (25) is electrically connected to the prism and detector module (101); The prism and detector module (101) and the beam combining prism module (102) are stacked along the beam propagation direction to form an integrated structure. The beam combining prism module (102) includes a beam combining prism (5), which is configured to: receive incident measurement light from the prism and detector module (101) and guide it to the two-dimensional reflection grating (6); at the same time, separate and combine the multi-level diffracted beams located at different spatial positions diffracted back by the two-dimensional reflection grating (6) to form three independent measurement beam channels on the spatial optical path corresponding to the X, Y, and Z axis displacements respectively, and return the three measurement beam channels to the prism and detector module (101).

2. The compact dual-frequency heterodyne triaxial grating ruler according to claim 1, characterized in that, The prism and detector module (101) includes: a non-polarizing beam splitter (2), a quarter-wave plate (3), a mirror (4), a first polarizing beam splitter (16), a first photodetector (17), a second photodetector (18); a second polarizing beam splitter (19), a third photodetector (20), a fourth photodetector (21); a third polarizing beam splitter (22), a fifth photodetector (23), and a sixth photodetector (24); The first polarizing beam splitter (16), the second polarizing beam splitter (19) and the third polarizing beam splitter (22) are respectively disposed on the optical paths of the three measurement beam channels returned by the beam combining prism module (102) to separate the measurement beam of each channel according to its orthogonal polarization state and guide it to the corresponding photodetector pair to generate heterodyne interference electrical signals.

3. The compact dual-frequency heterodyne triaxial grating ruler according to claim 2, characterized in that, The beam combiner prism (5) is configured such that the measurement light from the unpolarized beam splitter (2) is incident off its optical axis of symmetry, thereby causing the optical paths of at least two pairs of ±1st order diffracted beams generated by the diffraction of the two-dimensional reflection grating (6) to be spatially separated from each other inside the beam combiner prism (5).

4. The compact dual-frequency heterodyne triaxial grating ruler according to claim 3, characterized in that, The beam combining prism module (102) also includes a first beam combining grating. (14) and the second beam combiner grating (15), the first beam combiner grating (14) and the second beam combiner grating (15) is fixed on the upper surface of the beam combining prism (5) in such a way that the directions of its grating lines are orthogonal to each other. Its position is referenced to the incident beam. One beam combining grating is fixed to the left or right of the center of the incident beam, and the other is fixed in front of or behind the center of the incident beam. The beam combiner prism (5) converges a pair of ±1st order diffracted beams in the first direction generated by the diffraction of the two-dimensional reflection grating (6) to the first beam combiner grating after internal reflection. (14) to form the Y-axis measurement beam channel; and to converge the other pair of ±1st order diffracted beams in the second direction generated by the diffraction of the two-dimensional reflection grating (6) to the second beam combiner grating after internal reflection. (15) to form the X-axis measurement beam channel; the second direction plane is orthogonal to the first direction plane. According to claim 3 or 4, the compact dual-frequency heterodyne triaxial grating ruler is characterized in that the beam combining prism module (102) further includes a first half-wave plate (7-1) and a second half-wave plate (8-1), a first compensation plate (7-2) and a second compensation plate (8-2); the first half-wave plate (7-1), the second half-wave plate (8-1), the first compensation plate (7-2) and the second compensation plate (8-2) are mounted on the lower surface of the beam combining prism 5 in a specific orientation, and the surface of the element is perpendicular to the beam transmission direction. The first compensation plate (7-2) and the second compensation plate (8-2) have the same optical path length as the first half-wave plate (7-1) and the second half-wave plate (8-1).

5. The compact dual-frequency heterodyne triaxial grating ruler according to claim 4, characterized in that, The Z-axis measurement beam channel is formed by the 0th order diffracted beam returned by the two-dimensional reflection grating (6). The 0th order diffracted beam returns to the prism and detector module (101) along the original incident light path and interferes with the reference beam returned by the reference mirror (4).

6. The compact dual-frequency heterodyne triaxial grating ruler according to claim 4, characterized in that, The beam combiner prism (5) is a single optical prism with axisymmetry in a plane perpendicular to the incident optical axis.

7. The compact dual-frequency heterodyne triaxial grating ruler according to claim 7, characterized in that, The beam-combining prism (5) is a cuboid, isosceles trapezoid, or rhomboid prism, and its four sides are coated with a total reflection film or utilize only the conditions of total reflection.

8. The compact dual-frequency heterodyne triaxial grating ruler according to claim 4, characterized in that, The two-dimensional reflection grating (6) is a two-dimensional orthogonal grating, and the grating periods of its two dimensions are the same as those of the first beam combining grating. (14) and the second beam combiner (15) The grating period is matched.

9. A precision motion system, characterized in that, The system integrates a compact dual-frequency heterodyne triaxial grating ruler as described in any one of claims 1 to 9, wherein the two-dimensional reflective grating (6) is mounted on the moving part of the system and the optical reading head is mounted on the stationary base of the system, forming a closed-loop feedback control system for three-degree-of-freedom displacement measurement.

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