A two-axis homodyne grating interferometry displacement measurement method and system based on a wollaston prism and a four-quadrant detector

Abstract

A two-axis homodyne grating interference displacement measurement method and system based on a Wollaston prism and a four-quadrant detector belong to the field of grating interference displacement measurement and are suitable for two-axis homodyne grating interference displacement measurement for synchronous measurement of horizontal and vertical displacements. The method is used for solving the problem that a Wollaston prism (WP) and a four-quadrant detector (QPD) are not matched in the prior art in the two-axis homodyne grating interference displacement measurement occasion. In the method, a certain device in an optical path or a polarization direction of a light beam is rotated, so that a direction of a connecting line of two light beams is perpendicular to a normal direction of a dividing surface of the Wollaston prism. At this time, the two light beams pass through the Wollaston prism to form four light spots in a square distribution, so that the problem of optical path mismatch between the two-axis parallel light and the rotating Wollaston prism is solved, and the four pixels of the four-quadrant detector are effectively utilized.

Description

Technical Field

[0001] The present invention belongs to the field of grating interferometry, in particular to the field of two-axis homodyne grating interferometry displacement measurement for synchronous measurement of horizontal and vertical displacements. Background Art

[0002] Grating interferometry is a commonly used precision displacement measurement scheme. Compared with traditional dual-beam interferometry schemes such as the Michelson interferometry scheme, the biggest feature of grating interferometry is that it uses the grating pitch as a reference and can measure the displacement within the grating plane. The grating pitch reference is insensitive to the refractive index of air and is suitable for more demanding measurement environments. On this basis, the laser wavelength reference can measure the displacement perpendicular to the grating plane, and grating interferometry is often used for two-axis displacement measurement. With the rapid development of advanced manufacturing, two-axis grating displacement measurement technology that simultaneously measures the grating plane within and perpendicular to the grating plane has been applied in high-end manufacturing equipment such as lithography machines. In-depth research has also been conducted at home and abroad on two-axis grating displacement measurement methods.

[0003] In principle, grating interferometers can be divided into homodyne grating interferometers and heterodyne grating interferometers. Homodyne grating interferometer is the earliest, most basic and most widely used grating displacement measurement system, which is implemented using a single-frequency light source. The principle structure is simple, but it cannot distinguish the direction of movement, and a four-channel detection structure needs to be added, which will result in a large optical path volume and limit integration in high-end equipment systems. The heterodyne grating interferometer light source uses a dual-frequency laser. Although only two signals, reference and measurement, are needed to distinguish the direction of movement and no four-channel detection structure is required, the dual-frequency light source is more complex and expensive than a single-frequency light source. Therefore, in actual applications, homodyne grating interferometry is more widely used, and the miniaturization and integration of the two-axis homodyne grating interferometer optical path is an important topic in this field.

[0004] At present, the domestic patent layout in the field of two-axis homodyne grating interferometry for synchronous measurement of horizontal and vertical displacement is not mature:

[0005] For example, Tsinghua University and Beijing Huazhuo Jingke Technology Co., Ltd., as applicants, have published two patent documents: CN103759654A and CN103759655A, which disclose two two-axis displacement measurement systems based on homodyne grating interferometry, which can achieve optical four-division. However, both use a structure of three dichroic prisms and three wave plates to form a four-channel detection structure, which has many optical path components, is large in size, and is difficult to assemble and adjust.

[0006] Related public research includes: In their research paper "Two-degree-of-freedom displacement measurement system based on doublediffraction gratings," Lu Zhengang and others from Harbin Institute of Technology proposed a two-axis displacement measurement method based on the principle of homodyne laser grating interferometry. This method uses zero-order diffracted light to form a laser interference optical path to measure the vertical displacement of the grating. The displacement of the grating plane is measured by forming a grating interference optical path using positive and negative first-order diffracted light. In this scheme, the laser interference optical path and the grating interference optical path share a grating, but the measurement optical paths are independent of each other, making the system complex and large in size. It also uses a four-channel detection structure with three dichroic prisms and wave plates. In their research paper, "Position and out-of-straightness measurement of a precision linear air-bearing stage by using a two-degree-of-freedom linear encoder," Wei Gao and his team from Tohoku University in Japan proposed a vertically incident differential measurement homodyne grating interferometry scheme. This scheme can achieve two-axis measurement within the same grating interferometry optical path, but still uses three beam splitters and wave plates to form a four-channel detection structure. Furthermore, the vertical displacement of the grating in this scheme will cause an optical axis offset, resulting in a smaller vertical range.

[0007] The four-channel detection structure formed by three beam-splitting prisms and wave plates is the main reason for the complex optical structure and large size of two-axis homodyne grating interferometry, making the miniaturization of the optical path difficult and the difficulty of assembly and adjustment. In recent years, the field of interferometry has proposed a new combined structure based on a Wollaston prism and a four-quadrant detector, which can realize interference and beam splitting in one, with fewer components and a small size, which is conducive to the miniaturization of the optical path. For example, in his master's thesis "Research on Key Technologies of Single-Frequency Laser Interferometry Vibration Measurement," Guo Xuanbiao of Harbin Institute of Technology proposed a laser interferometry optical path based on a Wollaston prism and a four-quadrant detector, which can realize single-axis displacement measurement. However, because the Wollaston prism needs to be rotated 45° for installation, this solution is currently only applicable to single-axis measurement and cannot achieve multi-axis measurement. In addition, this solution only uses two pixels of a four-quadrant detector device, which is wasteful.

[0008] Therefore, the present invention proposes a two-axis homodyne grating interferometry displacement measurement method and system based on a Wollaston prism and a four-quadrant detector, which solves the problem that the combined structure of the Wollaston prism and the four-quadrant detector is not compatible with two-axis displacement measurement. Summary of the Invention

[0009] The invention is used to solve the problem in the prior art of incompatibility between a Wollaston prism (WP) and a four-quadrant detector (QPD) in two-axis homodyne grating interferometric displacement measurement.

[0010] To achieve the above object, the present invention provides the following technical solutions:

[0011] The present invention provides a two-axis homodyne grating interferometric displacement measurement method based on a Wollaston prism and a four-quadrant detector, the measurement method comprising the following steps:

[0012] Step S1: Splitting the light beam emitted by the light source into biaxial parallel light;

[0013] Step S2: The biaxial parallel light passes through the interferometer assembly and is divided into reference light and measurement light. The reference light is reflected back to the interferometer assembly. The measurement light passes through the refractive element, is incident on the measurement grating, is diffracted, and returns to the interferometer assembly along the same path, where it overlaps with the reference light. The two overlapping light beams enter the four-channel interference light receiving unit, are first split into two parts by a non-polarizing beam splitter prism and wave plate (QWP) assembly, and then pass through the first WP and the second WP, ​​respectively. By adjusting the posture of the optical elements in the optical path or the polarization direction of the incident light, the two light beams each produce a set of four square-distributed light spots on the QPD.

[0014] Step S3: Perform photoelectric detection and signal processing on the four light spot signals to realize the calculation of displacement information.

[0015] Furthermore, in another preferred embodiment, the above-mentioned method for adjusting the posture of an optical element can be designed as follows:

[0016] The non-polarization beam splitter NPBS1, the first WP, the second WP, ​​the first QPD, and the second QPD are all rotated 45° around the optical axis.

[0017] Furthermore, the present invention provides a two-axis homodyne grating interferometry displacement measurement system with a rotating beam splitter prism, which is used to realize two-axis homodyne grating interferometry displacement measurement through the optical element posture adjustment method described in the preferred embodiment above, and the displacement measurement system includes a single-frequency laser light source, a non-polarizing beam splitter prism NPBS1, an interferometer group, a measuring grating and a four-channel interference light receiving unit;

[0018] The interferometer group includes a polarization beam splitter prism PBS, a first QWP, a second QWP, a reflector, and a refractive element;

[0019] The four-channel interference light receiving unit includes a non-polarization beam splitter prism NPBS2, a third QWP, a first WP, a second WP, ​​a first QPD, and a second QPD;

[0020] A first QWP and a second QWP are respectively provided on both sides of the polarization beam splitter prism PBS;

[0021] A third QWP is provided on one surface of the non-polarizing beam splitter prism NPBS2;

[0022] A single-frequency laser light source emits a single-frequency laser. The single-frequency laser is split into two-axis parallel light after passing through a non-polarizing beam splitter prism NPBS1. The two-axis parallel light enters a polarizing beam splitter prism PBS and is split into four beams. Two of the s-polarized light beams are reflected back to the polarizing beam splitter prism PBS by a reflector, becoming p-polarized light and serving as reference light. The other two p-polarized light beams pass through a refractive element and enter a measurement grating at the Littrow angle. They return to the polarizing beam splitter prism PBS along the original path and serve as measurement light, where they overlap with the reference light. The two overlapping light beams are first split into two parts by a non-polarizing beam splitter prism NPBS2 and a third QWP assembly. The two parts then pass through a first WP and a second WP, ​​respectively, each generating a set of four square-distributed light spots on the first QPD and the second QPD.

[0023] Furthermore, the above-mentioned single-frequency laser is split into two-axis parallel light after passing through the non-polarization beam splitter prism NPBS1, and the angle between the optical axis connecting the two parallel light and the x-axis is 45°.

[0024] Furthermore, in another preferred embodiment, the above-mentioned method for adjusting the polarization direction of the incident light can be designed as follows:

[0025] Rotate the polarization direction of incident light by 45°.

[0026] Furthermore, the present invention provides a two-axis homodyne grating interferometry displacement measurement system with a rotating polarization direction, the displacement measurement system is used to realize two-axis homodyne grating interferometry displacement measurement by the incident light polarization direction adjustment method described in the preferred embodiment above, the displacement measurement system includes a single-frequency laser light source, a non-polarizing beam splitter prism NPBS1, an interferometer group, a measuring grating and a four-channel interference light receiving unit;

[0027] The interferometer group includes a polarization beam splitter prism PBS, a first QWP, a second QWP, a reflector, and a refractive element;

[0028] The four-channel interference light receiving unit includes a non-polarization beam splitter prism NPBS2, a third QWP, a first WP, a second WP, ​​a first QPD, and a second QPD;

[0029] A first QWP and a second QWP are respectively provided on both sides of the polarization beam splitter prism PBS;

[0030] A third QWP is provided on one surface of the non-polarizing beam splitter prism NPBS2;

[0031] A single-frequency laser light source emits a single-frequency laser. The single-frequency laser is split into two-axis parallel light after passing through a non-polarizing beam splitter prism NPBS1. The two-axis parallel light enters a polarizing beam splitter prism PBS and is split into four beams. Two of the s-polarized light beams are reflected back to the polarizing beam splitter prism PBS by a reflector, becoming p-polarized light and serving as reference light. The other two p-polarized light beams pass through a refractive element and enter a measurement grating at the Littrow angle. They return to the polarizing beam splitter prism PBS along the original path and serve as measurement light, where they overlap with the reference light. The two overlapping light beams are first split into two parts by a non-polarizing beam splitter prism NPBS2 and a third QWP assembly. The two parts then pass through a first WP and a second WP, ​​respectively, each generating a set of four square-distributed light spots on the first QPD and the second QPD.

[0032] Furthermore, the angle between the s-polarized light and the p-polarized light and the optical axis of the Wollaston prism is 45°.

[0033] Furthermore, in another preferred embodiment, the above optical element posture adjustment method can also be designed as follows:

[0034] The polarizing beam splitter prism PBS, the non-polarizing beam splitter prism NPBS2, the first WP, the second WP, ​​the first QPD, and the second QPD are all rotated 45° around the optical axis.

[0035] Furthermore, the present invention provides a two-axis homodyne grating interferometric displacement measurement system of a rotating interference prism, the system being used to implement two-axis homodyne grating interferometric displacement measurement through the optical element posture adjustment method described in the preferred embodiment above, the displacement measurement system comprising a single-frequency laser light source, a non-polarizing beam splitter prism NPBS1, an interferometer group, a measuring grating, and a four-channel interference light receiving unit;

[0036] The interferometer group includes a polarization beam splitter prism PBS, a first QWP, a second QWP, a reflector, and a refractive element;

[0037] The four-channel interference light receiving unit includes a non-polarization beam splitter prism NPBS2, a third QWP, a first WP, a second WP, ​​a first QPD, and a second QPD;

[0038] A first QWP and a second QWP are respectively provided on both sides of the polarization beam splitter prism PBS;

[0039] A third QWP is provided on one surface of the non-polarizing beam splitter prism NPBS2;

[0040] A single-frequency laser light source emits a single-frequency laser. The single-frequency laser is split into two-axis parallel light after passing through a non-polarizing beam splitter prism NPBS1. The two-axis parallel light enters a polarizing beam splitter prism PBS and is split into four beams. Two of the s-polarized light beams are reflected back to the polarizing beam splitter prism PBS by a reflector, becoming p-polarized light and serving as reference light. The other two p-polarized light beams pass through a refractive element and enter a measurement grating at the Littrow angle. They return to the polarizing beam splitter prism PBS along the original path and serve as measurement light, where they overlap with the reference light. The two overlapping light beams are first split into two parts by a non-polarizing beam splitter prism NPBS2 and a third QWP assembly. The two parts then pass through a first WP and a second WP, ​​respectively, each generating a set of four square-distributed light spots on the first QPD and the second QPD.

[0041] Furthermore, the angle between the optical axis of the light beam split into two beams by the non-polarization beam splitting prism NPBS2 and the third QWP assembly and the optical axis of the Wollaston prism is 45°.

[0042] The beneficial effects of the present invention are:

[0043] Most two-axis homodyne interferometric displacement measurement optical paths use three beam splitters and wave plates to form a four-channel detection structure, but the structure is complex, the volume is large, and the adjustment is difficult, which is not conducive to the miniaturization of the optical path. The combined module of Wollaston prism (WP) and quadrant detector (QPD) has only been used for single-degree-of-freedom interferometric displacement measurement. If used for two-axis homodyne interferometric displacement measurement, there will be a problem that the combined structure of Wollaston prism and quadrant detector is not compatible with the two-axis displacement measurement. The present invention combines two-axis homodyne grating interferometric displacement measurement with the combined module of Wollaston prism and quadrant detector. For the first time, by adjusting the posture of the optical elements in the optical path or rotating the polarization direction of the light beam, it not only solves the optical path mismatch problem between the dual-axis parallel light and the rotating Wollaston prism, but also realizes the effective utilization of all pixels of the quadrant detector (QPD).

[0044] Furthermore, the present invention organically combines a combination module of a Wollaston prism (WP) and a four-quadrant detector (QPD) to achieve miniaturization of the optical path.

[0045] The present invention is suitable for realizing horizontal and vertical two-axis displacement measurement of a line grating by utilizing the principle of single-frequency grating interference through a four-channel detection structure based on a Wollaston prism (WP) and a four-quadrant detector (QPD). BRIEF DESCRIPTION OF THE DRAWINGS

[0046] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following is a brief introduction to the drawings required for use in the specific embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0047] Figure 1 This is a diagram of the optical path structure of the two-axis homodyne grating interferometry displacement measurement system of the rotating beam splitter prism of the present invention;

[0048] Figure 2 yes Figure 1 A three-dimensional diagram of the optical path structure of the two-axis homodyne grating interferometric displacement measurement system with a rotating beam splitter prism;

[0049] Figure 3 Schematic diagram of the angle between the s-polarized light and the p-polarized light and the x-axis being 45°;

[0050] Figure 4 This is a diagram of the optical path structure of the two-axis homodyne grating interferometry displacement measurement system for rotating polarization direction according to the present invention;

[0051] Figure 5 yes Figure 4 A three-dimensional diagram of the optical path structure of the two-axis homodyne grating interferometric displacement measurement system with a rotating polarization direction. DETAILED DESCRIPTION

[0052] The specific embodiments of the present invention are further described in detail below with reference to the accompanying drawings. The following embodiments will help those skilled in the art further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that those skilled in the art may make various changes and improvements without departing from the scope of the present invention, and these are all within the scope of protection of the present invention.

[0053] Implementation 1: This implementation addresses the problem of incompatibility between the Wollaston prism (WP) and the four-quadrant detector (QPD) in the prior art of two-axis homodyne grating interferometry displacement measurement. Therefore, a two-axis homodyne grating interferometry displacement measurement method based on a Wollaston prism and a four-quadrant detector is proposed. The method comprises the following steps:

[0054] Step S1: Splitting the light beam emitted by the light source into biaxial parallel light;

[0055] Step S2: The biaxial parallel light passes through the interferometer assembly and is divided into reference light and measurement light. The reference light is reflected back to the interferometer assembly. The measurement light passes through the refractive element, is incident on the measurement grating, is diffracted, and returns to the interferometer assembly along the same path, where it overlaps with the reference light. The two overlapping light beams enter the four-channel interference light receiving unit, are first split into two parts by a non-polarizing beam splitter prism and wave plate (QWP) assembly, and then pass through the first WP and the second WP, ​​respectively. By adjusting the posture of the optical elements in the optical path or the polarization direction of the incident light, the two light beams each produce a set of four square-distributed light spots on the QPD.

[0056] Step S3: Perform photoelectric detection and signal processing on the four light spot signals to realize the calculation of displacement information.

[0057] The two-axis homodyne grating interferometric displacement measurement method proposed in this embodiment uses a combination structure of a Wollaston prism (WP) and a four-quadrant detector (QPD) as a four-channel interference light receiving unit of a two-axis homodyne grating interferometer; and by adjusting the posture of certain optical elements in the optical path or the polarization direction of the incident light, the direction of the optical axis connecting the two light beams entering the four-channel interference light receiving unit is made perpendicular to the direction of the Wollaston prism interface normal, so that the two light beams pass through the Wollaston prism and each form a group of four square-distributed light spots on the four-quadrant detector, thereby solving the optical path mismatch problem between the two-axis parallel light and the rotating Wollaston prism, that is, the problem that the combination structure of the Wollaston prism and the four-quadrant detector is not compatible with two-axis displacement measurement.

[0058] Implementation method 2, see Figure 1 and Figure 2 This embodiment describes a two-axis homodyne grating interferometry displacement measurement system using a rotating beam splitter prism. The displacement measurement system implements the two-axis homodyne grating interferometry displacement measurement method based on a Wollaston prism and a four-quadrant detector described in the first embodiment.

[0059] The displacement measurement system is as follows Figure 1 and Figure 2 As shown, it includes a single-frequency laser light source, a non-polarization beam splitter prism NPBS1, an interferometer group, a measuring grating and a four-channel interference light receiving unit;

[0060] The interferometer group includes a polarization beam splitter prism PBS, a first QWP, a second QWP, a reflector, and a refractive element;

[0061] The four-channel interference light receiving unit includes a non-polarization beam splitter prism NPBS2, a third QWP, a first WP, a second WP, ​​a first QPD, and a second QPD;

[0062] A first QWP and a second QWP are respectively provided on both sides of the polarization beam splitter prism PBS;

[0063] A third QWP is provided on one surface of the non-polarization beam splitting prism NPBS2.

[0064] In order to realize two-axis homodyne grating interferometric displacement measurement, this embodiment uses a combination structure of a Wollaston prism (WP) and a four-quadrant detector (QPD) as a four-channel interference light receiving unit of a two-axis homodyne grating interferometer. To achieve this goal, it is necessary to rotate the Wollaston prism (WP) 45° so that both light beams have components on the optical axis of the Wollaston prism (WP) and interfere with each other to realize two-axis homodyne interferometric displacement measurement. At the same time, the corresponding four-quadrant detector (QPD) also needs to be rotated 45°. At this time, due to the Wollaston prism (WP), the two light beams have components on the optical axis of the Wollaston prism (WP). ) rotation will cause an optical path mismatch problem. In this embodiment, the non-polarization beam splitter prism NPBS1 is rotated 45° around the optical axis, and the Wollaston prism (WP) and the four-quadrant detector (QPD) are rotated in the same manner. This makes the direction of the optical axis connection line of the two light beams entering the four-channel interference light receiving unit perpendicular to the normal direction of the Wollaston prism interface. As a result, the two light beams pass through the Wollaston prism and form a group of four square-distributed light spots on the four-quadrant detector, thereby solving the optical path mismatch problem between the dual-axis parallel light and the rotating Wollaston prism.

[0065] The specific optical path structure is:

[0066] A single-frequency laser source emits a single-frequency laser. After passing through the non-polarizing beam splitter prism NPBS1, the single-frequency laser is split into two parallel beams. Since the non-polarizing beam splitter prism NPBS1 is rotated 45° about its optical axis, the angle between the optical axes of the two parallel beams and the x-axis is 45°. This means that the direction of the optical axes of the two parallel beams is perpendicular to the normal direction of the Wollaston prism interface. After entering the polarizing beam splitter prism PBS, the two parallel beams are split into four beams. Two s-polarized beams are reflected back to the polarizing beam splitter prism PBS by a reflector, becoming p-polarized and serving as reference beams. The other two p-polarized beams pass through a refractive element and enter the measurement grating at the Littrow angle. They then return to the polarizing beam splitter prism PBS along the same path as the measurement beams, where they overlap with the reference beams. The two overlapping beams are split into two parts by the non-polarizing beam splitter prism NPBS2 and the third QWP assembly. These two parts then interfere with each other through the first and second WPs, respectively, generating a set of four square-shaped light spots on the first and second QPDs.

[0067] Implementation method three, see Figure 3 and Figure 4 This embodiment describes a two-axis homodyne grating interferometry displacement measurement system with a rotating polarization direction. The displacement measurement system implements the two-axis homodyne grating interferometry displacement measurement method based on a Wollaston prism and a four-quadrant detector described in the first embodiment above.

[0068] like Figure 3 and Figure 4 As shown, the optical elements required for the displacement measurement system described in this embodiment are the same as those of the displacement measurement system described in the above embodiment 2, and also include a single-frequency laser light source, a non-polarizing beam splitter prism NPBS1, an interferometer group, a measuring grating, and a four-channel interference light receiving unit;

[0069] The interferometer group includes a polarization beam splitter prism PBS, a first QWP, a second QWP, a reflector, and a refractive element;

[0070] The four-channel interference light receiving unit includes a non-polarization beam splitter prism NPBS2, a third QWP, a first WP, a second WP, ​​a first QPD, and a second QPD;

[0071] A first QWP and a second QWP are respectively provided on both sides of the polarization beam splitter prism PBS;

[0072] A third QWP is provided on one surface of the non-polarization beam splitting prism NPBS2.

[0073] The difference from the above-mentioned embodiment 2 is that this embodiment does not rotate the Wollaston prism (WP) and the four-quadrant detector (QPD) by 45°, but rotates the polarization direction of the incident light by 45°, so that the angle between the s-polarized light and the p-polarized light and the optical axis of the Wollaston prism is also 45°, as shown in FIG. Figure 4 As shown, there's no need to rotate the Wollaston prism 45°. S- and p-polarized light also have components in the directions of o- and e-light, allowing for interference. Therefore, the optical path mismatch problem that exists with biaxial parallel light and a rotated Wollaston prism is eliminated. In practical applications, the optical axis directions of the polarization beamsplitter (PBS) and wave plates (QPW1, QPW2, and QWP3) in the optical path need to change as the polarization direction of the beam changes.

[0074] Embodiment 4: This embodiment provides a two-axis homodyne grating interferometry displacement measurement system using a rotating interference prism. The displacement measurement system is used to implement the two-axis homodyne grating interferometry displacement measurement method based on a Wollaston prism and a four-quadrant detector described in the above embodiment 1.

[0075] The optical components required for the displacement measurement system described in this embodiment are the same as those of the displacement measurement system described in the second embodiment, and also include a single-frequency laser light source, a non-polarizing beam splitter prism NPBS1, an interferometer lens assembly, a measurement grating, and a four-channel interference light receiving unit;

[0076] The interferometer group includes a polarization beam splitter prism PBS, a first QWP, a second QWP, a reflector, and a refractive element;

[0077] The four-channel interference light receiving unit includes a non-polarization beam splitter prism NPBS2, a third QWP, a first WP, a second WP, ​​a first QPD, and a second QPD;

[0078] A first QWP and a second QWP are respectively provided on both sides of the polarization beam splitter prism PBS;

[0079] A third QWP is provided on one surface of the non-polarization beam splitting prism NPBS2.

[0080] The difference from the above-mentioned embodiment 2 is that, in this embodiment, the Wollaston prism (WP) and the four-quadrant detector (QPD) are also rotated 45°, but the non-polarizing beam splitter prism NPBS1 is not rotated. Instead, the polarizing beam splitter prism PBS, the non-polarizing beam splitter prism NPBS2, the Wollaston prism (WP) and the four-quadrant detector (QPD) are rotated 45° around the optical axis. This can also make the optical axis connection direction of the two light beams entering the four-channel interference light receiving unit perpendicular to the normal direction of the Wollaston prism interface, so that the two light beams pass through the Wollaston prism and each form a group of four square-distributed light spots on the four-quadrant detector, solving the optical path mismatch problem between the dual-axis parallel light and the rotating Wollaston prism.

[0081] The specific optical path structure is:

[0082] A single-frequency laser light source emits a single-frequency laser, which is then split into two-axis parallel light after passing through the non-polarizing beam splitter prism NPBS1. The two-axis parallel light enters the polarizing beam splitter prism PBS and is split into four beams. Two s-polarized light beams are reflected back to the polarizing beam splitter prism PBS by a reflector, and the s-polarized light is converted into p-polarized light, which serves as reference light. The other two p-polarized light beams pass through a refractive element and enter the measuring grating at the Littrow angle and return to the polarizing beam splitter prism PBS along the original path, serving as measuring light and overlapping with the reference light. The two overlapping light beams are split into two parts by the non-polarizing beam splitter prism NPBS2 and the third QWP assembly, and are respectively injected into the first WP and the second WP. Since the Wollaston prism (WP) and the quadrant detector (QPD) are both rotated 45 degrees around the optical axis, the two light beams incident on the Wollaston prism (WP) can interfere with each other; at the same time, since the polarization beam splitter prism PBS and the non-polarization beam splitter prism NPBS2 are also rotated 45 degrees around the optical axis, the direction of the optical axis connection line of the two light beams incident on the Wollaston prism (WP) is perpendicular to the normal direction of the Wollaston prism interface, so that the two light beams pass through the first WP and the second WP, ​​respectively, and generate a group of four square-distributed light spots on the first QPD and the second QPD, thereby solving the optical path mismatch problem between the biaxial parallel light and the rotating Wollaston prism.

[0083] In summary, in order to use a combination module of a Wollaston prism (WP) and a four-quadrant detector (QPD) as a two-axis homodyne interferometer displacement measurement, the present invention adjusts the posture of certain components in the optical path or rotates the polarization direction of the light beam so that the direction of the Wollaston prism interface normal to the direction of the line connecting the optical axes of the two light beams is perpendicular. At this time, the two light beams pass through the Wollaston prism to generate a group of four light spots distributed in a square on the four-quadrant detector, thereby solving the optical path mismatch problem existing between the two-axis parallel light and the rotating Wollaston prism.

[0084] In the above description, specific details such as specific system structures and techniques are provided for illustration rather than limitation to facilitate a thorough understanding of the embodiments of the present application. However, it will be apparent to those skilled in the art that the present application may be implemented in other embodiments without these specific details. In other cases, detailed descriptions of well-known optical paths, devices, circuits, and methods are omitted to prevent unnecessary details from obscuring the description of the present application.

[0085] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be understood to indicate or imply relative importance or implicitly specify the quantity of the technical features indicated. Thus, features defined as "first" or "second" may explicitly or implicitly include at least one of these features.

[0086] The foregoing description is merely an embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that the present invention is susceptible to various modifications and variations. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention shall be included within the scope of the claims.

Claims

1. A two-axis homodyne grating interferometry displacement measurement method based on a Wollaston prism and a four-quadrant detector, characterized in that: The method is: S1: Split the light beam emitted by the light source into biaxial parallel light; S2: The biaxial parallel light passes through the interferometer group and is divided into reference light and measurement light. The reference light is reflected back to the interferometer group. The measurement light passes through the refractive element, is incident on the measurement grating, is diffracted, and returns to the interferometer group along the same path, overlapping with the reference light. The two overlapping light beams are incident on the four-channel interference light receiving unit, first passing through the non-polarizing beam splitter prism NPBS2 and the third wave plate QWP assembly to be divided into two parts. The two parts then pass through the first WP and the second WP respectively. By adjusting the posture of the optical elements in the optical path or the polarization direction of the incident light, the two parts of light each produce a set of four square-distributed light spots on the QPD. S3: Perform photoelectric detection and signal processing on the four light spot signals to realize the calculation of displacement information.

2. The two-axis homodyne grating interferometry displacement measurement method based on Wollaston prism and four-quadrant detector according to claim 1, wherein The method for adjusting the optical element posture is: The non-polarization beam splitter NPBS1, the first WP, the second WP, ​​the first QPD, and the second QPD are all rotated 45° around the optical axis.

3. A two-axis homodyne grating interferometry displacement measurement system with a rotating beam splitter prism, characterized in that: The displacement measurement system is used to realize two-axis homodyne grating interferometric displacement measurement by the optical element posture adjustment method according to claim 2, and the displacement measurement system includes a single-frequency laser light source, a non-polarizing beam splitter prism NPBS1, an interferometer group, a measuring grating and a four-channel interference light receiving unit; The interferometer group includes a polarization beam splitter prism PBS, a first wave plate QWP, a second wave plate QWP, a reflector, and a refractive element; The four-channel interference light receiving unit includes a non-polarization beam splitter prism NPBS2, a third wave plate QWP, a first WP, a second WP, ​​a first QPD, and a second QPD; The first wave plate QWP and the second wave plate QWP are respectively provided on both sides of the polarization beam splitter prism PBS; A third wave plate QWP is provided on one surface of the non-polarizing beam splitter prism NPBS2; A single-frequency laser light source emits a single-frequency laser, which is then split into two-axis parallel light after passing through a non-polarizing beam splitter prism NPBS1. The two-axis parallel light enters a polarizing beam splitter prism PBS and is split into four beams. Two of the s-polarized light beams are reflected back to the polarizing beam splitter prism PBS by a reflector, where they are converted into p-polarized light and serve as reference light. The other two p-polarized light beams pass through a refractive element and enter a measuring grating at the Littrow angle, returning to the polarizing beam splitter prism PBS along the original path as measuring light, and coincide with the reference light. The two coincident light beams are first split into two parts by passing through a non-polarizing beam splitter prism NPBS2 and a third wave plate QWP assembly, and then pass through a first WP and a second WP, ​​respectively, each generating a set of four square-distributed light spots on the first QPD and the second QPD.

4. The two-axis homodyne grating interferometry displacement measurement system with a rotating beam splitter prism according to claim 3, characterized in that: After passing through the non-polarizing beam splitter NPBS1, the single-frequency laser is split into two-axis parallel light, and the angle between the optical axis and the x-axis is 45°.

5. A two-axis homodyne grating interferometry displacement measurement method based on a Wollaston prism and a four-quadrant detector according to claim 1, characterized in that: The method for adjusting the polarization direction of incident light is: Rotate the polarization direction of incident light by 45°.

6. A two-axis homodyne grating interferometric displacement measurement system with a rotating polarization direction, characterized in that: The displacement measurement system is used to realize two-axis homodyne grating interferometric displacement measurement by the incident light polarization direction adjustment method according to claim 5, and the displacement measurement system includes a single-frequency laser light source, a non-polarizing beam splitter prism NPBS1, an interferometer group, a measuring grating and a four-channel interference light receiving unit; The interferometer group includes a polarization beam splitter prism PBS, a first wave plate QWP, a second wave plate QWP, a reflector, and a refractive element; The four-channel interference light receiving unit includes a non-polarization beam splitter prism NPBS2, a third wave plate QWP, a first WP, a second WP, ​​a first QPD, and a second QPD; The first wave plate QWP and the second wave plate QWP are respectively provided on both sides of the polarization beam splitter prism PBS; A third wave plate QWP is provided on one surface of the non-polarizing beam splitter prism NPBS2; A single-frequency laser light source emits a single-frequency laser, which is then split into two-axis parallel light after passing through a non-polarizing beam splitter prism NPBS1. The two-axis parallel light enters a polarizing beam splitter prism PBS and is split into four beams. Two of the s-polarized light beams are reflected back to the polarizing beam splitter prism PBS by a reflector, where they are converted into p-polarized light and serve as reference light. The other two p-polarized light beams pass through a refractive element and enter a measuring grating at the Littrow angle, returning to the polarizing beam splitter prism PBS along the original path as measuring light, and coincide with the reference light. The two coincident light beams are first split into two parts by passing through a non-polarizing beam splitter prism NPBS2 and a third wave plate QWP assembly, and then pass through a first WP and a second WP, ​​respectively, each generating a set of four square-distributed light spots on the first QPD and the second QPD.

7. The two-axis homodyne grating interferometry displacement measurement system with polarization rotation according to claim 6, characterized in that: The angle between the s-polarized light and the p-polarized light and the optical axis of the Wollaston prism is 45°.

8. The two-axis homodyne grating interferometry displacement measurement method based on a Wollaston prism and a four-quadrant detector according to claim 1, characterized in that: The optical element posture adjustment method can also be: The polarizing beam splitter prism PBS, the non-polarizing beam splitter prism NPBS2, the first WP, the second WP, ​​the first QPD, and the second QPD are all rotated 45° around the optical axis.

9. A two-axis homodyne grating interferometric displacement measurement system with a rotating interference prism, characterized in that: The system is used to realize two-axis homodyne grating interferometric displacement measurement by the optical element posture adjustment method according to claim 8, and the displacement measurement system includes a single-frequency laser light source, a non-polarizing beam splitter prism NPBS1, an interferometer group, a measuring grating and a four-channel interference light receiving unit; The interferometer group includes a polarization beam splitter prism PBS, a first wave plate QWP, a second wave plate QWP, a reflector, and a refractive element; The four-channel interference light receiving unit includes a non-polarization beam splitter prism NPBS2, a third wave plate QWP, a first WP, a second WP, ​​a first QPD, and a second QPD; The first wave plate QWP and the second wave plate QWP are respectively provided on both sides of the polarization beam splitter prism PBS; A third wave plate QWP is provided on one surface of the non-polarizing beam splitter prism NPBS2; A single-frequency laser light source emits a single-frequency laser, which is then split into two-axis parallel light after passing through a non-polarizing beam splitter prism NPBS1. The two-axis parallel light enters a polarizing beam splitter prism PBS and is split into four beams. Two of the s-polarized light beams are reflected back to the polarizing beam splitter prism PBS by a reflector, where they are converted into p-polarized light and serve as reference light. The other two p-polarized light beams pass through a refractive element and enter a measuring grating at the Littrow angle, returning to the polarizing beam splitter prism PBS along the original path as measuring light, and coincide with the reference light. The two coincident light beams are first split into two parts by passing through a non-polarizing beam splitter prism NPBS2 and a third wave plate QWP assembly, and then pass through a first WP and a second WP, ​​respectively, each generating a set of four square-distributed light spots on the first QPD and the second QPD.

10. The two-axis homodyne grating interferometric displacement measurement system of the rotating interference prism according to claim 9, characterized in that: The angle between the optical axis connecting the two beams separated by the non-polarizing beam splitter NPBS2 and the third wave plate QWP assembly and the optical axis of the Wollaston prism is 45°.

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