Zero-optical-path-difference double-frequency heterodyne grating ruler and displacement measurement method

Abstract

A zero-optical-path-difference double-frequency heterodyne grating ruler comprises a light source, a signal detecting and processing system, a polarization splitting prism, a first quarter-wave plate, a rectangular prism, a second quarter-wave plate, a third quarter-wave plate, a total reflection mirror, a long-strip prism, a reference grating and a 45-degree-placed polaroid. The two measuring light paths of the system always keep zero optical path difference, so that the measuring precision can be ensured, and the environmental stability of the system is also remarkably improved. And a high-precision and high-robustness measurement scheme is provided for precision equipment such as a photoetching machine and a precision motion platform.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of precision optical measurement, and particularly relates to a zero optical path difference double-frequency heterodyne grating ruler and a displacement measurement method, which is suitable for photolithography machines, precision motion platforms, nanometer positioning systems and other high-precision displacement measurement scenes. BACKGROUND

[0002] In recent years, with the continuous development of precision manufacturing technology, higher requirements have been put forward for high-precision displacement and angle measurement systems, specifically in terms of high-frequency dynamic response characteristics, higher system integration and stronger environmental interference resistance. The precision of precision measurement instruments has become one of the key factors restricting the further improvement of process level in semiconductor manufacturing, precision machining and other industries.

[0003] At present, the widely used laser interferometer has a measurement reference of beam wavelength and the reference light path and the measurement light path cannot maintain strict zero optical path difference, so it is easily disturbed by environmental factors such as air turbulence and temperature gradient, resulting in a decrease in measurement accuracy and long-term stability. In contrast, the grating interferometer has a measurement reference of grating period and therefore has better environmental interference resistance, and has become a widely used core measurement module in high-precision motion platforms, precision optical machine systems and nanometer measurement systems.

[0004] However, the existing grating ruler system based on a double-frequency heterodyne grating interferometer still has room for further optimization in terms of optical path structure, especially in terms of the design of a zero optical path difference optical path for realizing large-range displacement measurement, which to some extent limits the further improvement of the ultimate measurement precision and environmental robustness.

[0005] Chinese patent CN 110360931 A proposes a symmetric zero optical path difference heterodyne interferometric grating displacement measurement system, which uses the high extinction ratio and natural beam splitting characteristics of a Wollaston prism to realize beam splitting and convergence of the measurement signal. However, the designed system has low tolerance during assembly and adjustment, and if errors are introduced during assembly and adjustment, the measurement data will not be accurate.

[0006] Chinese patent CN 112097649 A proposes a heterodyne grating displacement measurement optical system, which uses polarization beam splitting and a rotating light path to realize 4 times optical subdivision in a single diffraction. However, the system has high requirements for device assembly and adjustment accuracy, and has low tolerance.

[0007] Chinese patent CN 117704960 A proposes a quasi-common optical path heterodyne grating interferometer, which manages dead zone optical paths through spatial separation of light beams and active evaluation / calibration to improve precision. However, the system is complex and has high cost, and the system is in a discrete architecture and is easily affected by environmental interference.

[0008] Chinese patent CN 119124001 A proposes a heterodyne precision displacement measurement system based on a self-traceable grating, which improves measurement accuracy by introducing a self-traceable grating into heterodyne interferometry. However, the system design is relatively complex and its discrete architecture makes it still sensitive to environmental factors.

[0009] The American company ZYGO proposed a heterodyne grating interferometer in patent US20120194824A1. It uses birefringent optical elements to introduce the input beam into a small angle to suppress signal errors caused by stray light. However, the designed system has a non-zero optical path difference between the measurement light and the reference light, which is easily affected by environmental interference, resulting in inaccurate measurement data.

[0010] In summary, existing grating interferometers still face several bottlenecks in achieving high precision, high stability, high integration, and high environmental robustness, especially the need for zero optical path difference design and high tolerance. Summary of the Invention

[0011] This invention addresses the problem of inaccurate measurement signals caused by the non-zero optical path difference between the two measurement beams in traditional dual-frequency heterodyne grating interferometers by providing a zero-optical-path-difference dual-frequency heterodyne grating ruler. By introducing a long strip prism into the optical path and combining it with the synergistic effect of a waveplate and prism, a strictly zero optical path difference between the two measurement beams is achieved. Simultaneously, through a self-collimating optical path design, the system's assembly tolerance, environmental stability, and overall performance are significantly improved, providing a reliable high-precision displacement measurement solution for high-end precision equipment.

[0012] The technical solution of this invention is as follows: A zero-optical-path-difference dual-frequency heterodyne grating ruler includes: a light source and signal detection and processing system, a polarizing beam splitter, a first quarter-wave plate, a right-angle prism, a second quarter-wave plate, a third quarter-wave plate, a total reflection mirror, a strip prism and a reference grating, and a polarizer placed at 45°. Its characteristic lies in that the light source and signal detection and processing system are used to generate a frequency of , It receives and processes the returned interference signal from dual-frequency linearly polarized light with mutually orthogonal polarization directions. The polarizing beam splitter is used to split the incident dual-frequency beam according to its polarization state and to recombine the two returning measurement beams. The elongated prism is configured to have at least a first reflecting slope and a second reflecting slope; and to receive two spatially separated measurement beams emitted from the second quarter-wave plate, and to ensure that the two measurement beams, after being reflected once by the first reflecting slope and the second reflecting slope respectively, exit at different, preset Littrow angles and are incident on the reference grating; simultaneously, it is ensured that the total optical path length of the two measurement beams from their starting point after being split in the polarizing beam splitter to their ending point after being reflected by the elongated prism, diffracted by the reference grating, and returning along the original path to converge again in the polarizing beam splitter is strictly equal, thereby achieving zero optical path difference between the two measurement beams; The polarizing beam splitter, the first quarter-wave plate, the right-angle prism, the third quarter-wave plate, and the total reflection mirror constitute a polarization state cyclic modulation and optical path reversal module. This module is used to separate the dual-frequency orthogonally polarized light emitted by the light source and signal detection and processing system into two paths according to their polarization states. The two paths are then guided to pass sequentially through the elongated prism and the reference grating, and then return along the original path with opposite polarization state conversion sequences. Finally, they are combined in the polarizing beam splitter, and after being combined by a polarizer placed at 45°, a common optical path interference signal is generated and returned to the light source and signal detection and processing system.

[0013] Furthermore, the light source and signal detection and processing system emits a frequency of... , Furthermore, the frequency difference of P and S light beams with mutually orthogonal polarization directions is... The light is split into two beams by a polarizing beam splitter, and the light reflected by the polarizing beam splitter ( The S-frequency light (at a certain frequency) passes through the first quarter-wave plate and then enters the right-angle prism. After being reflected by the right-angle prism, it passes through the first quarter-wave plate again, becoming the P-frequency light. After being incident on the polarizing beam-splitting prism, it is transmitted and exits, passing through the second quarter-wave plate and then entering the lower inclined surface of the long strip prism. The light then strikes the surface of the lower inclined surface of the long strip prism with a certain frequency. After angular reflection The light is incident on the reference grating at a (Littrow) angle. After diffraction by the reference grating, it carries the Doppler frequency shift information of the reference grating and is reflected back along the original optical path. It then passes through the second quarter-wave plate and becomes S-light. After reflection by the polarizing beam splitter, it passes through the third quarter-wave plate and is incident on the total reflection mirror. After reflection by the total reflection mirror, it returns along the original optical path and passes through the third quarter-wave plate again, becoming P-light. Light transmitted through a polarizing beam splitter ( The P-frequency light passes through the third quarter-wave plate and is incident on the total reflection mirror. After reflection, it returns along the original path, passes through the third quarter-wave plate again, and becomes S-frequency light, which is then incident on the polarizing beam splitter. After reflection, it passes through the second quarter-wave plate and is incident on the inclined surface of the long prism. After angular reflection, with The light beam, incident at a (Littrow) angle, is diffracted by the reference grating and returns along its original path carrying the Doppler frequency shift information of the reference grating. It then passes through the second quarter-wave plate and becomes a P-beam, which is transmitted through the polarizing beam splitter. After passing through the first quarter-wave plate, it is reflected by a right-angle prism and then passes through the first quarter-wave plate again, becoming an S-beam. Finally, the two beams carrying the Doppler frequency shift information of the reference grating converge at the inclined surface of the polarizing beam splitter and exit along a common path. After being combined by a polarizer placed at 45°, they generate an interference signal. This signal is then detected, processed, and calculated by the light source and signal detection and processing system to obtain the displacement of the reference grating.

[0014] Furthermore, the elongated prism is an integrated prism made from a single piece of optical glass through molding and polishing. Its cross-sectional shape is a polygon containing the first reflective slope and the second reflective slope. Both the first reflective slope and the second reflective slope are coated with an optical thin film to improve reflectivity.

[0015] Furthermore, the elongated prism satisfies the following condition: Let the grating period of the reference grating be... The diffraction order is positive and negative 1st order, and the frequency of the dual-frequency light emitted by the light source and signal detection and processing system is... The corresponding wavelength is ,frequency The corresponding wavelength is ; The angle at which the light beam is incident on the first reflecting slope of the elongated prism is The angle at which the light is incident on the second reflecting slope is The angle between the first reflecting slope of the elongated prism and the horizontal reference plane. The angle between the first reflecting slope and the second reflecting slope of the elongated prism The light beam reflected by the first reflecting slope is at a first Littoral angle. Incident on the reference grating, and The light beam reflected by the second reflecting slope moves at a second Littoral angle. Incident on the reference grating, and .

[0016] Furthermore, the polarizing beam splitter, the first quarter-wave plate, and the right-angle prism are arranged sequentially along the first optical axis; the polarizing beam splitter, the third quarter-wave plate, and the total reflection mirror are arranged sequentially along the second optical axis; the polarizing beam splitter, the second quarter-wave plate, and the strip prism are arranged sequentially along the third optical axis; the first optical axis, the second optical axis, and the third optical axis are perpendicular to each other and intersect inside the polarizing beam splitter.

[0017] Furthermore, the light source and signal detection and processing system integrates a dual-frequency laser, a photodetector, and a signal processing circuit, wherein the output frequency of the dual-frequency laser is... and Linearly polarized light with mutually orthogonal polarization directions is used to receive the common-path interference signal, and the signal processing circuit is used to perform heterodyne demodulation on the interference signal to obtain the displacement information of the reference grating.

[0018] Furthermore, the reference grating uses a material with a low coefficient of thermal expansion as a substrate, and its grating surface is positioned facing the elongated prism.

[0019] Secondly, the present invention also provides a method for displacement measurement using a zero optical path difference dual-frequency heterodyne grating ruler, characterized by comprising the following steps: The light source and signal detection and processing system generate dual-frequency orthogonally polarized light; The dual-frequency orthogonally polarized light is separated according to polarization state and guided to form a first measurement optical path and a second measurement optical path by using a polarization beam splitter and a polarization state cyclic modulation and optical path reversal module. The two measurement optical paths are spatially separated but their polarization states periodically orthogonally change with time. The first and second measurement optical paths are guided to the same measurement area of ​​the reference grating by a long strip prism and incident at different Littoral angles. The long strip prism ensures that the first and second measurement optical paths maintain equal physical optical path lengths before reaching the reference grating and during their return. After being diffracted by the reference grating, the first and second measurement optical paths, carrying Doppler frequency shift information caused by relative motion, return along the original optical paths; After the first and second measurement optical paths return and are recombined in the polarization beam splitter module, they form a common optical path interference beam through a polarizer placed at 45°. The light source and signal detection and processing system receives the interference beam and calculates the displacement of the reference grating using a heterodyne interference signal processing method.

[0020] Furthermore, on the X / Y motion stage, two sets of the grating rulers of this invention are used. In the X and Y directions, respectively, long strip prisms are used to replace the long strip reference mirror in the traditional laser interferometer. With the help of reference gratings fixed on the motion stage frame and a customized interferometer composed of waveplates and polarizing beam splitters, two-dimensional large-range displacement measurement can be realized.

[0021] Compared with the prior art, the beneficial effects of the present invention are: 1. Zero optical path difference is achieved using a specially designed elongated prism. By combining a specially designed elongated prism with waveplates and other prisms, the two measurement optical paths of the designed dual-frequency heterodyne optical system maintain zero optical path difference. Furthermore, the reference grating period made of zero-expansion glass is used as the measurement benchmark, fundamentally eliminating the influence of air fluctuations. Therefore, it effectively eliminates the dead-path measurement errors commonly found in traditional laser interferometers caused by environmental factors such as air fluctuations. Compared to other designs, it can achieve more accurate high-precision displacement measurement of large-stroke motion stages.

[0022] 2. Through the self-collimating optical path design, the system's assembly and adjustment tolerance, environmental stability, and overall performance are significantly improved, providing a reliable high-precision displacement measurement solution for high-end precision equipment.

[0023] 3. By using two sets of the grating rulers of this invention on the X / Y motion stage, long strip prisms are used to replace the long strip reference mirror in the traditional laser interferometer in the X and Y directions, respectively. With the help of reference gratings fixed on the motion stage frame and a customized interferometer composed of waveplates and polarizing beam splitters, two-dimensional large-range displacement measurement with zero optical path difference can be realized. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the structure of a zero-optical-path-difference dual-frequency heterodyne grating ruler according to the present invention. Detailed Implementation 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.

[0025] like Figure 1 As shown, an embodiment of the present invention provides a zero optical path difference dual-frequency heterodyne grating ruler, including a light source and signal detection and processing system 1, a polarizing beam splitter 2, a first quarter-wave plate 3, a right-angle prism 4, a second quarter-wave plate 5, a third quarter-wave plate 6, a total reflection mirror 7, a strip prism 8, a reference grating 9, and a polarizer 10 placed at 45°.

[0026] The elongated prism 8 is used in conjunction with the aforementioned optical elements such as waveplates and beam splitters to polarize and split the incident beam and maintain a zero optical path difference between the two measurement beams.

[0027] With the polarizing beam splitter 2 as the origin reference, a first quarter-wave plate 3 and a right-angle prism 4 are arranged parallel to it on the left; a second quarter-wave plate and a strip prism 8 are arranged parallel to it on the right; a third quarter-wave plate 6 and a total reflection mirror 7 are arranged parallel to it above; and a light source and signal detection and processing system 1 are arranged parallel to it below. A reference grating 9 is arranged above the strip prism 8 to form a dual-frequency heterodyne grating ruler structure with zero optical path difference.

[0028] The elongated prism 8 can be drawn according to the specifications.Figure 1 The structure is set as a quadrilateral with two right-angled sides and two inclined planes, but it is not limited to this structure. It can be a triangle, pentagon, or other structure containing two inclined planes, as long as the elongated prism 8 structure has at least two inclined planes. This ensures that the two beams exiting the second quarter-wave plate 5 are reflected by the upper and lower inclined planes and incident on the reference grating 9 at a Littrow angle, returning along the original optical path with zero optical path difference after carrying the Doppler frequency shift information of the reference grating 9. Both the upper and lower inclined planes of the elongated prism 8 are coated with anti-reflection coatings to increase reflectivity.

[0029] like Figure 1 The structure shown is a quadrilateral with two right-angled sides and two inclined planes. The angle at which the beam enters the lower inclined plane of the long strip prism 8 is... The angle at which the beam enters the inclined surface of the long strip prism 8 The angle between the lower inclined plane of the long prism 8 and the horizontal plane. The angle between the upper and lower inclined planes of the long prism 8 The Littrow angle of the incident reference grating 9 after reflection from the lower inclined surface of the long strip prism 8. The Littrow angle of the incident reference grating 9 after reflection from the inclined surface of the elongated prism 8. The following conditions must be met (assuming the grating period of reference grating 9 is...). The diffraction order is positive and negative 1st order, and the beam frequency is... The corresponding wavelength is Beam frequency The corresponding wavelength is ): The workflow of this embodiment is as follows: (Combined with...) Figure 1 As shown, the light source and signal detection and processing system 1 emits a frequency of , And a beam of P and S light with mutually orthogonal polarization directions (frequency difference: The light is split into two beams by the polarizing beam splitter 2, of which the light reflected by the polarizing beam splitter 2 ( The S-frequency light (at a certain frequency) passes through the first quarter-wave plate 3 and then enters the right-angle prism 4. After being reflected by the right-angle prism 4, it passes through the first quarter-wave plate 3 again and becomes P-frequency light. After being incident on the polarizing beam splitter prism 2, it is transmitted and exits, and then enters the lower inclined surface of the long strip prism 8 through the second quarter-wave plate 5. On the surface of the lower inclined surface of the long strip prism 8, it is reflected by the first quarter-wave plate 3 and then by the second quarter-wave plate 5. After angular reflection The light is incident on the reference grating 9 at a (Littrow) angle. After diffraction by the reference grating 9, it carries the Doppler frequency shift information of the reference grating 9 and is reflected back along the original optical path. It then passes through the second quarter-wave plate 5 and becomes S-beam. After reflection by the polarizing beam splitter prism 2, it passes through the third quarter-wave plate 6 and is incident on the total reflection mirror 7. After reflection by the total reflection mirror, it returns along the original optical path and passes through the third quarter-wave plate 6 again, becoming P-beam.

[0030] The light transmitted through polarizing beam splitter 2 ( The P-frequency light (passing through the third quarter-wave plate 6) is incident on the total reflection mirror 7, and after reflection by the total reflection mirror 7, it returns along the original optical path. It then passes through the third quarter-wave plate 6 again, becoming S-frequency light, and is incident on the polarizing beam splitter 2. After reflection by the polarizing beam splitter 2, it passes through the second quarter-wave plate 5 and is incident on the inclined surface of the long prism 8. After angular reflection, with The light beam is incident on the reference grating 9 at a (Littrow) angle. After diffraction by the reference grating 9, it carries the Doppler frequency shift information of the reference grating 9 and returns along the original optical path. After passing through the second quarter-wave plate 5, it becomes P-beam and is incident on the polarizing beam splitter 2. After being transmitted through the polarizing beam splitter 2, it is emitted and passes through the first quarter-wave plate 3 before being incident on the right-angle prism 4. After being reflected by the right-angle prism 4, it passes through the first quarter-wave plate 3 again and becomes S-beam. Finally, the two beams carrying the Doppler frequency shift information of the reference grating 9 converge at the inclined surface of the polarizing beam splitter 2 and then exit along the same optical path. After being combined by the polarizer (10) placed at 45°, a coherent signal is generated. Then, the displacement of the reference grating 9 is obtained by the signal detection, processing and calculation of the light source and the signal detection and processing system 1.

[0031] On the X / Y motion stage, two sets of grating rulers of the present invention are used. In the X and Y directions, long strip prisms 8 replace the long strip reference mirror in the traditional laser interferometer. With the help of reference gratings 9 fixed on the motion stage frame and a customized interferometer composed of waveplates and polarizing beam splitters, two-dimensional large-range displacement measurement can be realized.

[0032] 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 zero-optical-path-difference dual-frequency heterodyne grating ruler, comprising: Light source and signal detection and processing system (1), polarizing beam splitter (2), first quarter-wave plate (3), right angle prism (4), second quarter-wave plate (5), third quarter-wave plate (6), total reflection mirror (7), strip prism (8) and reference grating (9), polarizer placed at 45° (10). The feature is that the light source and signal detection and processing system (1) is used to generate a frequency of , It receives and processes the returned interference signal from dual-frequency linearly polarized light with mutually orthogonal polarization directions. The polarizing beam splitter (2) is used to split the incident dual-frequency beam according to its polarization state and recombin the two returning measurement beams. The elongated prism (8) is configured to have at least a first reflecting slope and a second reflecting slope; and to receive two spatially separated measurement beams emitted from the second quarter-wave plate (5), and to make the two measurement beams, after being reflected once by the first reflecting slope and the second reflecting slope respectively, exit at different, preset Littrow angles and be incident on the reference grating (9); at the same time, to ensure that the total optical path experienced by the two measurement beams from the starting point after being split in the polarizing beam splitter (2) to the ending point after being reflected by the elongated prism (8), diffracted by the reference grating (9) and returning along the original path to reconverge in the polarizing beam splitter (2), is strictly equal, thereby achieving zero optical path difference between the two measurement beams; The polarization beam splitter (2), the first quarter-wave plate (3), the right-angle prism (4), the third quarter-wave plate (6), and the total reflection mirror (7) constitute a polarization state cyclic modulation and optical path reversal module, which is used to separate the dual-frequency orthogonally polarized light emitted by the light source and signal detection and processing system (1) into two paths according to the polarization state, and guide the two paths of light to pass through the long strip prism (8) and the reference grating (9) in sequence, and then return along the original path with opposite polarization state conversion sequences. Finally, they are combined in the polarization beam splitter (2), and after being combined by the polarizer (10) placed at 45°, a common optical path interference signal is generated and returned to the light source and signal detection and processing system (1).

2. The zero optical path difference dual-frequency heterodyne grating ruler according to claim 1, characterized in that, The light source and signal detection and processing system (1) emits a frequency of , Furthermore, the frequency difference of P and S light beams with mutually orthogonal polarization directions is... The light is split into two beams by a polarizing beam splitter (2), and the light reflected by the polarizing beam splitter (2) is... The S-frequency light passes through the first quarter-wave plate (3) and then enters the right-angle prism (4). After being reflected by the right-angle prism (4), it passes through the first quarter-wave plate (3) again and becomes P-frequency light. After entering the polarizing beam splitter (2), it is transmitted and exited, and after passing through the second quarter-wave plate (5), it enters the lower inclined surface of the long strip prism (8). On the surface of the lower inclined surface of the long strip prism (8) at a frequency of S-frequency, it enters the lower inclined surface of the long strip prism (8) at a frequency of S-frequency. After angular reflection The light is incident on the reference grating (9) at a (Littrow) angle. After being diffracted by the reference grating (9), it carries the Doppler frequency shift information of the reference grating (9) and is reflected back along the original optical path. It then passes through the second quarter-wave plate (5) and becomes S light. After being reflected by the polarizing beam splitter (2), it passes through the third quarter-wave plate (6) and is incident on the total reflection mirror (7). After being reflected by the total reflection mirror, it returns along the original optical path and passes through the third quarter-wave plate (6) again, becoming P light. The light transmitted through the polarizing beam splitter (2) The P-frequency light passes through the third quarter-wave plate (6) and is incident on the total reflection mirror (7). After being reflected by the total reflection mirror (7), it returns along the original optical path and passes through the third quarter-wave plate (6) again to become S-frequency light, which is then incident on the polarizing beam splitter (2). After being reflected by the polarizing beam splitter (2), it passes through the second quarter-wave plate (5) and is incident on the inclined surface of the long strip prism (8). After angular reflection, with The light beam (Littrow) is incident on the reference grating (9) at an angle. After diffraction by the reference grating (9), it carries the Doppler frequency shift information of the reference grating (9) and returns along the original optical path. After passing through the second quarter-wave plate (5), it becomes P-beam and is incident on the polarizing beam splitter (2). After being transmitted through the polarizing beam splitter (2), it passes through the first quarter-wave plate (3) and is incident on the right-angle prism (4). After being reflected by the right-angle prism (4), it passes through the first quarter-wave plate (3) again and becomes S-beam. Finally, the two beams carrying the Doppler frequency shift information of the reference grating (9) converge at the inclined surface of the polarizing beam splitter (2) and then exit along the same optical path. After being combined by the polarizer (10) placed at 45°, they generate an interference signal. Then, the light source and the signal detection and processing system (1) perform signal detection, processing and calculation to obtain the displacement of the reference grating (9).

3. The zero optical path difference dual-frequency heterodyne grating ruler according to claim 1 or 2, characterized in that, The elongated prism (8) is an integrated prism made from a single piece of optical glass through molding and polishing. Its cross-sectional shape is a polygon containing the first reflective slope and the second reflective slope. Both the first reflective slope and the second reflective slope are coated with an optical thin film to improve reflectivity.

4. The zero optical path difference dual-frequency heterodyne grating ruler according to claim 1, characterized in that, The elongated prism (8) satisfies the following conditions: Let the grating period of the reference grating (9) be... The diffraction order is positive and negative 1st order, and the frequency of the dual-frequency light emitted by the light source and signal detection and processing system (1) is... The corresponding wavelength is ,frequency The corresponding wavelength is ; The angle at which the light beam is incident on the first reflecting surface of the elongated prism (8) is The angle at which the light is incident on the second reflecting slope is The angle between the first reflecting slope of the elongated prism (8) and the horizontal reference plane. The angle between the first reflecting slope and the second reflecting slope of the elongated prism (8) The light beam reflected by the first reflecting slope is at a first Littoral angle. Incident on the reference grating (9), and The light beam reflected by the second reflecting slope moves at a second Littoral angle. Incident on the reference grating (9), and .

5. The zero optical path difference dual-frequency heterodyne grating ruler according to claim 1, characterized in that, The polarizing beam splitter (2), the first quarter-wave plate (3), and the right-angle prism (4) are arranged sequentially along the first optical axis; the polarizing beam splitter (2), the third quarter-wave plate (6), and the total reflection mirror (7) are arranged sequentially along the second optical axis; the polarizing beam splitter (2), the second quarter-wave plate (5), and the strip prism (8) are arranged sequentially along the third optical axis; the first optical axis, the second optical axis, and the third optical axis are perpendicular to each other and intersect inside the polarizing beam splitter (2).

6. The zero optical path difference dual-frequency heterodyne grating ruler according to claim 1, characterized in that, The light source and signal detection and processing system (1) integrates a dual-frequency laser, a photodetector, and a signal processing circuit. The output frequency of the dual-frequency laser is... and Linearly polarized light with mutually orthogonal polarization directions, the photodetector is used to receive the common optical path interference signal, and the signal processing circuit is used to perform heterodyne demodulation on the interference signal to obtain the displacement information of the reference grating (9).

7. The zero optical path difference dual-frequency heterodyne grating ruler according to claim 1, characterized in that, The reference grating (9) uses a material with a low coefficient of thermal expansion as a substrate, and its grating surface is set facing the elongated prism (8).

8. A method for displacement measurement using a zero-optical-path-difference dual-frequency heterodyne grating ruler as described in any one of claims 1-7, characterized in that, Includes the following steps: The light source and signal detection and processing system (1) generates dual-frequency orthogonally polarized light; The dual-frequency orthogonal polarized light is separated according to polarization state and guided to form a first measurement optical path and a second measurement optical path by using a polarization beam splitter (2) and a polarization state cyclic modulation and optical path reversal module. The two measurement optical paths are spatially separated but the polarization state changes periodically orthogonally with time. The first measurement optical path and the second measurement optical path are respectively guided to the same measurement area of ​​the reference grating (9) by a long strip prism (8) and incident at different Littoral angles, wherein the long strip prism (8) ensures that the first measurement optical path and the second measurement optical path always maintain equal physical optical path before reaching the reference grating (9) and during the return process; After being diffracted by the reference grating (9), the first and second measurement optical paths carry Doppler frequency shift information caused by relative motion and return along the original optical paths; After the first and second measurement optical paths return to the polarization state cyclic modulation and optical path reversal module, they are recombined in the polarization beam splitter (2) and form a common optical path interference beam through the polarizer (10) placed at 45°. The light source and signal detection and processing system (1) receives the interference beam and calculates the displacement of the reference grating (9) by heterodyne interference signal processing method.

9. The displacement measurement method according to claim 1, characterized in that, On the X / Y motion stage, two sets of the grating rulers of the present invention are used. In the X and Y directions, long strip prisms (8) are used to replace the long strip reference mirror in the traditional laser interferometer. With the help of reference gratings (9) fixed on the motion stage frame and a customized interferometer composed of wave plates and polarizing beam splitters, two-dimensional large-range displacement measurement can be realized.

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