Two-degree-of-freedom displacement simultaneous measurement system and method based on orbital angular momentum beam interferometer

Abstract

The invention discloses a two-degree-of-freedom displacement simultaneous measurement system and method based on an orbital angular momentum beam interferometer. The system comprises a laser; the light field regulation and control module is used for converting the linearly polarized light into a light beam with orbital angular momentum; an optical isolator; a first beam splitter; a second beam splitter; the third beam splitter is used for combining the first reference light beam and the first measurement light beam to generate conjugate interference; a fourth beam splitter; a fifth beam splitter; the sixth beam splitter is used for combining the second reference light beam and the second measurement light beam to generate conjugate interference; a seventh beam splitter; a first reflector; a second reflector; a third reflector; a first Weive prism; a second Weive prism; a reflection grating; and the image acquisition and processing module is used for acquiring a petal-shaped interference pattern formed by conjugate interference. The system is compact in structure and high in demodulation sensitivity, and can realize non-contact and high-resolution synchronous two-degree-of-freedom measurement.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of laser precision measurement, and relates to a multi-degree-of-freedom displacement measurement technology of a reflection grating, in particular to a two-degree-of-freedom displacement simultaneous measurement system and method based on an orbital angular momentum beam interferometer. BACKGROUND

[0002] With the rapid development of precision engineering, micro-nano manufacturing and advanced measurement technology, the application demand of multi-degree-of-freedom displacement measurement in high-end equipment manufacturing, microstructure vibration monitoring and micro-nano operation system is increasing. Such applications not only require the measurement system to have nanometer-level or even higher resolution, but also require it to have good environmental stability, compact structure and easy-to-integrate system architecture. Traditional laser interferometers use laser wavelength as the measurement reference, which can theoretically achieve extremely high precision, but the measurement results are easily affected by environmental disturbances such as temperature fluctuations, air pressure changes and vibration noise, resulting in poor long-term stability of the system, which limits its application in complex industrial environments.

[0003] In order to solve the above problems, grating interferometers are widely used, which use grating pitch instead of wavelength as a measurement reference, significantly reducing the influence of environmental interference on measurement results, thereby improving the stability of the system. In recent years, orbital angular momentum (OAM) beams have shown great potential in high-sensitivity displacement demodulation due to their unique spiral phase structure and spin-symmetric petal-shaped interference patterns. By monitoring the rotation angle change of the interference pattern, sub-wavelength non-contact displacement measurement can be achieved. However, existing OAM interferometers mostly focus on single-axis displacement detection, and there are still many challenges for two-degree-of-freedom or even multi-degree-of-freedom displacement measurement, such as complex optical path structure, strong coupling in interference pattern demodulation process, and difficulty in real-time data processing algorithm. Therefore, there is an urgent need for an OAM grating interferometer system that has high stability, simple structure and high precision decoupling capability to meet the engineering needs of multi-degree-of-freedom displacement measurement in complex application environments. SUMMARY

[0004] The purpose of the application is to overcome the deficiencies in the prior art and provide a two-degree-of-freedom displacement simultaneous measurement system and method based on an orbital angular momentum beam interferometer. The system has a compact structure, good optical path symmetry and high demodulation sensitivity, and can realize non-contact, high-resolution synchronous two-degree-of-freedom measurement by combining the measurement method, which is suitable for precision assembly, nanometer positioning and microstructure vibration analysis fields.

[0005] Technical scheme: In order to achieve the above purpose, the application provides a two-degree-of-freedom displacement simultaneous measurement system based on an orbital angular momentum beam interferometer, which comprises:

[0006] a laser for generating linearly polarized laser light;

[0007] The light field modulation module is used to convert linearly polarized light into a beam with orbital angular momentum;

[0008] Optical isolators are used to maintain unidirectional beam transmission and prevent optical feedback;

[0009] The first beam splitter is used to split a beam with orbital angular momentum into a second reference beam and a principal axis beam.

[0010] The second beam splitter is used to split a beam with orbital angular momentum into a first reference beam and a principal axis beam.

[0011] The third beam splitter is used to combine the first reference beam and the first measurement beam to produce conjugate interference.

[0012] The fourth beam splitter is used to split the principal beam with orbital angular momentum into a first measurement beam and a second measurement beam.

[0013] The fifth beam splitter is used to reflect the main axis beam to the second mirror and transmit the first measurement beam to the third beam splitter.

[0014] The sixth beam splitter is used to combine the second reference beam and the second measurement beam to produce conjugate interference;

[0015] The seventh beam splitter is used to reflect the main axis beam to the third mirror and transmit the second measurement beam to the sixth beam splitter.

[0016] A first reflecting mirror is used to change the direction of the first measuring beam.

[0017] The second mirror is used for the transmission of the first measurement beam, including the +1st order Littrow diffraction angle incident and return.

[0018] The third reflecting mirror is used for the transmission of the second measurement beam, including the -1st order Littrow diffraction angle incident and return.

[0019] The first prism is used to change the topological charge of the first reference beam from positive to negative;

[0020] The second prism is used to change the topological charge of the second reference beam from positive to negative;

[0021] A reflective grating is used to diffract incident light and return the beam incident at the Littrow diffraction angle along its original path.

[0022] The first image acquisition and processing module is used to acquire the petal-shaped interference pattern formed by the conjugate interference of the +1st order Littrow diffraction angle;

[0023] The second image acquisition processing module is used for acquiring a petal-shaped interference pattern formed by the Littrow diffraction angle conjugate interference.

[0024] Further, the light field regulation module comprises an optical attenuator, an optical expander, a quarter wave plate and a vortex wave plate.

[0025] The optical attenuator is used for adjusting the laser light intensity to match the detection threshold of the image acquisition processing module.

[0026] The optical expander is used for adjusting the laser spot size to match the imaging unit size of the image acquisition processing module.

[0027] The quarter wave plate is used for converting linearly polarized light into circularly polarized light.

[0028] The vortex wave plate is used for converting the circularly polarized light into a light beam with orbital angular momentum.

[0029] The application further provides a two-degree-of-freedom displacement simultaneous measurement method based on an orbital angular momentum light beam interferometer, comprising the following steps:

[0030] S1: linearly polarized laser output by a laser passes through a light field regulation module to obtain a light beam with orbital angular momentum;

[0031] S2: the light beam with orbital angular momentum is kept in one-way transmission by an optical isolator;

[0032] S3: the light beam with orbital angular momentum is divided into a second reference light beam and a main axis light beam by a first beam splitter, the second reference light beam is converted into an OAM light beam with a topological charge number of -l after passing through a second Wollaston prism, and reaches a sixth beam splitter;

[0033] After the light beam with orbital angular momentum passes through the first beam splitter, the light beam is divided into a first reference light beam and a main axis light beam by a second beam splitter, the first reference light beam is converted into an OAM light beam with a topological charge number of -l after passing through a first Wollaston prism, and reaches a third beam splitter;

[0034] S4: the main axis light beam is divided into a first measurement light beam and a second measurement light beam by a fourth beam splitter;

[0035] The first measurement light beam reaches a fifth beam splitter through a first mirror, and then passes through the fifth beam splitter reflection, a second mirror and then reaches a reflection grating, and then passes through the reflection grating reflection, a second mirror and then reaches the third beam splitter through the fifth beam splitter transmission;

[0036] The second measurement light beam passes through a seventh beam splitter and a third mirror in sequence and then reaches the reflection grating, and then passes through the reflection grating reflection, the third mirror and then reaches the sixth beam splitter through the seventh beam splitter transmission;

[0037] S5: the first measurement beam and the first reference beam are subjected to conjugate interference at the third beam splitter to form a petal-shaped interference pattern, and a conjugate interference image of the +1 order measurement beam is obtained by the first CCD detector;

[0038] the second measurement beam and the second reference beam are subjected to conjugate interference at the sixth beam splitter to form a petal-shaped interference pattern, and a conjugate interference image of the -1 order measurement beam is obtained by the second CCD detector;

[0039] S6: after the reflective grating is moved, steps S3-S5 are repeated to obtain measurement conjugate interference images, the measurement conjugate interference images are compared with corresponding reference conjugate interference images, a rotation angle of the conjugate interference images is demodulated, and two degrees of freedom of in-plane and out-of-plane displacement of the reflective grating are simultaneously measured.

[0040] Further, the light field E l (r, θ) of the beam with the orbital angular momentum in the step S1 is

[0041] E l (r, θ) = R l (r)exp(ilθ)

[0042] wherein, R l (r) represents a radial distribution of the light field, r represents a radial coordinate, i.e. a distance from a center of the beam to a point, l represents a topological charge number of the beam with the orbital angular momentum, θ represents an azimuth angle of rotation around an optical axis, and i represents an imaginary unit, which is used to express a phase term in the light field.

[0043] Further, the light field E re (r, θ) of the first reference beam and the second reference beam in the step S3 is

[0044] E re (r, θ) = R -l (r)exp(-ilθ).

[0045] Further, the light field E me (r, θ) of the first measurement beam and the second measurement beam in the step S4 is

[0046]

[0047] wherein, and respectively represent phase differences of the measurement beams caused by in-plane and out-of-plane displacement of the reflective grating.

[0048] Further, the first measurement beam and the second measurement beam in the step S4 are incident to the reflective grating at ±1 order Littrow diffraction angles, and then return along the original path, and an incident angle α is

[0049] a = arcsin (l / 2d)

[0050] wherein, l is the wavelength of linearly polarized laser, d is the grating pitch of the reflection grating;

[0051] Phase changes of the first and second measurement beams after the displacement change of the grating and respectively

[0052]

[0053] wherein, and respectively represent the total phase difference caused by the in-plane displacement and out-of-plane displacement of the reflection grating surface to the phase change of the measurement beam corresponding to the +1 order Littrow diffraction angle and the -1 order Littrow diffraction angle; and respectively correspond to the phase changes caused by the Doppler frequency shift of the two measurement beams by the in-plane displacement, and respectively correspond to the phase changes caused by the Doppler frequency shift and the optical path change of the two measurement beams by the out-of-plane displacement; Δx is the in-plane displacement of the reflection grating, Δz is the out-of-plane displacement of the reflection grating, l is the wavelength of linearly polarized laser, d is the grating pitch of the reflection grating, and a is the incident angle.

[0054] Further, the light intensity expressions of the two pairs of conjugate beams interference in the step S5 are respectively

[0055]

[0056] wherein, I1(r, θ) represents the light intensity expression of the conjugate interference of the measurement beam and the reference beam of the +1 order Littrow diffraction angle, E me1 (r, θ) and E re1 (r, θ) are respectively the light intensity expressions of the measurement beam and the reference beam of the +1 order Littrow diffraction angle; I2(r, θ) represents the light intensity expression of the conjugate interference of the measurement beam and the reference beam of the -1 order Littrow diffraction angle, E me2 (r, θ) and E re2 (r, θ) are respectively the light intensity expressions of the measurement beam and the reference beam of the -1 order Littrow diffraction angle.

[0057] Further, the measurement of the two degrees of freedom displacement of the reflection grating in-plane and out-of-plane in the step S6 is expressed as follows:

[0058]

[0059] Where Δx is the in-plane displacement of the reflection grating, Δz is the out-of-plane displacement of the reflection grating, Δθ is the rotation angle of the conjugate interference image, Δθ1 is the rotation angle of the first CCD image, Δθ2 is the rotation angle of the second CCD image, λ is the wavelength of the linearly polarized laser, d is the grating pitch of the reflection grating, and α is the incident angle.

[0060] Beneficial effects: Compared with existing technologies, this invention achieves simultaneous extraction of two degrees of freedom displacement information (in-plane and out-of-plane displacement) of the reflected grating through a bidirectional Littrow diffraction angle path and a conjugate interference structure design, improving system measurement efficiency and space utilization. The ±1-order Littrow diffraction angle path and dual CCD independent acquisition channels eliminate the cross-coupling error between rotation angle and displacement based on optical path symmetry, enhancing measurement independence, strong decoupling, and low error. The bidirectional Littrow design realizes a compact, highly symmetrical interference path, facilitating integration into high-end equipment manufacturing platforms. The method of this invention is entirely based on optical measurement of interferometric images, enabling non-contact, high-resolution simultaneous measurement of two degrees of freedom displacement, and is applicable to fields such as precision assembly, nano-positioning, and microstructure vibration analysis. Attached Figure Description

[0061] Figure 1 This is a schematic diagram of the structural connections of the system of the present invention;

[0062] Figure 2 When l is 1, and the in-plane and out-of-plane displacements are simultaneously 50nm and 100nm, respectively, the petal-shaped interference pattern at the first image acquisition and processing module CCD1 and the second image acquisition and processing module CCD2 is shown in the diagram.

[0063] Figure 3 When l is 2, and the in-plane and out-of-plane displacements are 50nm and 100nm respectively, the diagram shows the petal-shaped interference pattern at the first image acquisition and processing module CCD1 and the second image acquisition and processing module CCD2. Detailed Implementation

[0064] The present invention will be further illustrated below with reference to the accompanying drawings and specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. After reading this invention, any modifications of the invention in various equivalent forms by those skilled in the art will fall within the scope defined by the appended claims.

[0065] Example 1:

[0066] like Figure 1 As shown, this embodiment provides a two-degree-of-freedom displacement simultaneous measurement system based on an orbital angular momentum beam interferometer, including:

[0067] Laser 1 is used to generate linearly polarized laser light;

[0068] A light field modulation module for converting linearly polarized light into a light beam with orbital angular momentum, comprising an optical attenuator 2, an optical expander 3, a quarter wave plate 4 and a vortex wave plate 5, wherein,

[0069] The optical attenuator 2 is used to adjust the intensity of the laser light to match the detection threshold of the image acquisition and processing module;

[0070] The optical expander 3 is used to adjust the size of the laser spot to match the size of the imaging unit of the image acquisition and processing module;

[0071] The quarter wave plate 4 is used to convert linearly polarized light into circularly polarized light;

[0072] The vortex wave plate 5 is used to convert circularly polarized light into a light beam with orbital angular momentum;

[0073] The optical isolator 6 is used to maintain the unidirectional output of the laser;

[0074] The first beam splitter 7 is used to divide the light beam with orbital angular momentum into a second reference light beam and a main axis light beam;

[0075] The second beam splitter 8 is used to divide the light beam with orbital angular momentum into a first reference light beam and a main axis light beam;

[0076] The first Dove prism 9 is used to change the topological charge number of the first reference light beam from positive to negative;

[0077] The third beam splitter 10 is used to combine the first reference light beam with the first measurement light beam to form a conjugate interference;

[0078] The first image acquisition and processing module 11 is used to acquire the petal-shaped interference pattern formed by the +1 order Littrow diffraction angle conjugate interference;

[0079] The fourth beam splitter 12 is used to divide the main axis light beam with orbital angular momentum into a first measurement light beam and a second measurement light beam;

[0080] The first mirror 13 is used to change the direction of the first measurement light beam;

[0081] The fifth beam splitter 14 is used to reflect the first measurement light beam to the second mirror 15 and transmit the first measurement light beam to the third beam splitter 10;

[0082] The second mirror 15 is used for the transmission of the first measurement light beam, including the incidence of the +1 order Littrow diffraction angle and the return along the original path;

[0083] The reflective grating 16 is used to diffract the ±1 order Littrow diffraction angle incident light and return along the original path;

[0084] The second Wollaston prism 17 is used for changing the topological charge number of the second reference light beam from positive to negative;

[0085] The sixth beam splitter 18 is used for combining the second reference light beam with the second measurement light beam to generate conjugate interference;

[0086] The second image acquisition and processing module 19 is used for acquiring the petal-shaped interference pattern formed by the -1 order Littrow diffraction angle conjugate interference;

[0087] The seventh beam splitter 20 is used for reflecting the second measurement light beam to the third mirror 21 and transmitting the second measurement light beam to the sixth beam splitter 18;

[0088] The third mirror 21 is used for the transmission of the second measurement light beam, including -1 order Littrow diffraction angle incidence and returning the original route.

[0089] In the embodiment, the laser 1 is a helium-neon laser, and the first image acquisition and processing module 11 and the second image acquisition and processing module 19 are CCD image sensors. The polarization ratio of the linearly polarized laser output by the laser 1 is greater than 500:1, and the wavelength λ of the linearly polarized laser is less than the grating pitch d of the reflection grating.

[0090] Embodiment 2:

[0091] In the embodiment, the system of embodiment 1 is used to realize the synchronous measurement of the two degrees of freedom displacement of the reflection grating 16 in the plane and out of the plane, and a two-degree-of-freedom displacement simultaneous measurement method based on the orbital angular momentum beam interferometer is provided, including the following steps:

[0092] S1: The linearly polarized laser output by the laser 1 is attenuated by the optical attenuator 2, adjusted to a size matched with the CCD image sensor by the optical beam expander 3, changed to circularly polarized light after the quarter-wave plate 4, and then converted into an optical beam with orbital angular momentum (OAM light beam) by the vortex wave plate 5, which is called OAM(+l) light beam;

[0093] The light field E of the OAM(+l) light beam l (r,θ) is

[0094] E l (r,θ)=R l (r)exp(ilθ)

[0095] Wherein, R l (r) represents the radial distribution of the light field, r represents the radial coordinate, i.e. the distance from the center of the light beam to a point, l represents the topological charge number of the light beam with orbital angular momentum, θ represents the azimuth angle of rotation around the optical axis, and i represents the imaginary unit, which is used to express the phase term in the light field.

[0096] S2: The OAM(+l) beam maintains unidirectional transmission through optical isolator 6;

[0097] S3: The OAM(+l) beam is split into a second reference beam and a main axis beam by the first beam splitter 7. The second reference beam is converted into an OAM beam with a topological charge of -l after passing through the second prism 17 and reaches the sixth beam splitter 18.

[0098] After the OAM(+l) beam passes through the first beam splitter 7, it is split into a first reference beam and a main axis beam by the second beam splitter 8. The first reference beam is converted into an OAM beam with a topological charge of -l after passing through the first prism 9 and reaches the third beam splitter 10.

[0099] The light fields E of the first and second reference beams re (r,θ) is

[0100] E re (r,θ)=R -l (r)exp(-ilθ).

[0101] S4: The main axis beam is split into a first measurement beam and a second measurement beam by the fourth beam splitter 12;

[0102] The first measuring beam passes through the first reflector 13 to the fifth beam splitter 14, and after being reflected by the fifth beam splitter 14 and the second reflector 15, it reaches the reflection grating 16. After being reflected by the reflection grating 16, it passes through the second reflector 15 and the fifth beam splitter 14 in sequence before reaching the third beam splitter 10.

[0103] The second measuring beam passes through the seventh beam splitter 20 and the third reflector 21 in sequence before reaching the reflector grating 16. After being reflected by the reflector grating 16, it passes through the third reflector 21 and the seventh beam splitter 20 in sequence before reaching the sixth beam splitter 18.

[0104] The light field E of the first and second measuring beams me (r,θ) is

[0105]

[0106] in, and These represent the phase difference of the measurement beam caused by in-plane and out-of-plane displacements of the reflective grating, respectively.

[0107] The first and second measuring beams are incident on the reflecting grating 16 at ±1 order Littrow diffraction angles and then return along the original path, with an incident angle α of...

[0108] α = arcsin(λ / 2d)

[0109] Where λ is the wavelength of the linearly polarized laser, and d is the grating pitch of the reflection grating 16;

[0110] Phase changes of the first and second measuring beams after the grating undergoes a displacement change and They are respectively

[0111]

[0112] in, and These represent the total phase difference caused by the in-plane and out-of-plane displacements of the reflection grating corresponding to the +1st and -1st order Littrow diffraction angles, respectively, resulting in the phase change of the measurement beam. and These correspond to the phase changes caused by the Doppler frequency shift resulting from the in-plane displacement of the two measurement beams. and These correspond to the Doppler frequency shift and optical path change caused by the out-of-plane displacement of the two measurement beams, respectively; Δx represents the in-plane displacement of the reflection grating 16, and Δz represents the out-of-plane displacement of the reflection grating 16.

[0113] S5: The first measuring beam and the first reference beam undergo conjugate interference at the third beam splitter 10, forming a petal-shaped interference pattern. The conjugate interference image of the +1 order measuring beam is detected and acquired by the first CCD image sensor.

[0114] The second measuring beam and the second reference beam undergo conjugate interference at the sixth beam splitter 18, forming a petal-shaped interference pattern. The conjugate interference image of the -1st order measuring beam is detected and acquired by the second CCD image sensor.

[0115] The expressions for the light intensity of the interference of two pairs of conjugate beams are as follows:

[0116]

[0117] Where I1(r,θ) represents the intensity expression of the conjugate interference between the measurement beam and the reference beam at the +1st order Littrow diffraction angle, and E me1 (r,θ) and E re1 (r,θ) represent the intensity expressions for the +1st order Littrow diffraction angle measurement beam and the reference beam, respectively; I2(r,θ) represents the intensity expression for the conjugate interference of the -1st order Littrow diffraction angle measurement beam and the reference beam, E me2 (r,θ) and E re2 (r,θ) are the light intensity expressions for the measurement beam and the reference beam of the -1st order Littrow diffraction angle, respectively.

[0118] S6: After the reflective grating 16 moves, repeat steps S3 to S5 to obtain the measured conjugate interference image, compare it with the corresponding reference conjugate interference image, demodulate the rotation angle of the conjugate interference image, and realize the simultaneous measurement of the two degrees of freedom displacement of the reflective grating 16 in-plane and out-of-plane.

[0119] The measurement of the two-degree-of-freedom displacements, both in-plane and out-of-plane, of the reflecting grating is expressed as follows:

[0120]

[0121] Where Δx is the in-plane displacement of the reflection grating 16, Δz is the out-of-plane displacement of the reflection grating 16, Δθ is the rotation angle of the conjugate interference image, Δθ1 is the rotation angle of the first CCD image, Δθ2 is the rotation angle of the second CCD image, λ is the wavelength of the linearly polarized laser, d is the grating pitch of the reflection grating, and α is the incident angle.

[0122] Example 3:

[0123] This embodiment applies the method of Embodiment 2 to a specific example, and the specific data and analysis are as follows:

[0124] The laser wavelength λ used is 632.8 nm, and the grating pitch d is 833.333 nm. Theoretical calculations show that the ±1st order Littrow diffraction angle α is 22.32°. When the topological charge l is 1, a 1° rotation of the conjugate petals in the first image acquisition and processing module 11 corresponds to an in-plane displacement of 4.628 nm and an out-of-plane displacement of 1.901 nm; a 1° rotation of the conjugate petals in the second image acquisition and processing module 19 corresponds to an in-plane displacement of -4.628 nm and an out-of-plane displacement of 1.901 nm. In this embodiment, the CCD image sensor resolution is 2592*1944, and the single pixel size is 2.2 μm. Arctan(1 / 972) = 0.058°, so the theoretical resolution for in-plane displacement measurement of the experimental system is approximately 0.134 nm, and the theoretical resolution for out-of-plane displacement measurement is approximately 0.055 nm. To improve the performance of CCD image sensors, such as achieving an angular resolution of 0.01°, when l is 1, the measurement resolution for in-plane displacement is 23 pm, and the measurement resolution for out-of-plane displacement is 9.5 pm.

[0125] like Figure 2 As shown, when l is 1, the interference pattern is such that when the in-plane and out-of-plane displacements are both 50 nm, the conjugate petals in the first image acquisition and processing module 11 rotate by 37.118°, and the conjugate petals in the second image acquisition and processing module 19 rotate by 15.508°; when the in-plane and out-of-plane displacements are both 100 nm, the interference pattern is such that when the conjugate petals in the first image acquisition and processing module 11 rotate by 74.244°, and the conjugate petals in the second image acquisition and processing module 19 rotate by 31.016°.

[0126] like Figure 3 As shown, when l is 2, the interference pattern is such that when the in-plane and out-of-plane displacements are both 50 nm, the conjugate petals in the first image acquisition and processing module 11 rotate by 18.561°, and the conjugate petals in the second image acquisition and processing module 19 rotate by 7.749°; when the in-plane and out-of-plane displacements are both 100 nm, the interference pattern is such that when the conjugate petals in the first image acquisition and processing module 11 rotate by 37.142°, and the conjugate petals in the second image acquisition and processing module 19 rotate by 15.509°.

Claims

1. A system for simultaneously measuring two degrees of freedom displacement based on an orbital angular momentum beam interferometer, characterized in that, include: Laser, used to generate linearly polarized laser light; The light field modulation module is used to convert linearly polarized light into a beam with orbital angular momentum; Optical isolators are used to maintain unidirectional beam transmission and prevent optical feedback; The first beam splitter is used to split a beam with orbital angular momentum into a second reference beam and a principal axis beam. The second beam splitter is used to split a beam with orbital angular momentum into a first reference beam and a principal axis beam. The third beam splitter is used to combine the first reference beam and the first measurement beam to produce conjugate interference. The fourth beam splitter is used to split the principal beam with orbital angular momentum into a first measurement beam and a second measurement beam. The fifth beam splitter is used to reflect the main axis beam to the second mirror and transmit the first measurement beam to the third beam splitter. The sixth beam splitter is used to combine the second reference beam and the second measurement beam to produce conjugate interference; The seventh beam splitter is used to reflect the main axis beam to the third mirror and transmit the second measurement beam to the sixth beam splitter. A first reflecting mirror is used to change the direction of the first measuring beam. The second mirror is used for the transmission of the first measurement beam, including the +1st order Littrow diffraction angle incident and return. The third reflecting mirror is used for the transmission of the second measurement beam, including the -1st order Littrow diffraction angle incident and return. The first prism is used to change the topological charge of the first reference beam from positive to negative; The second prism is used to change the topological charge of the second reference beam from positive to negative; A reflective grating is used to diffract incident light and return the beam incident at the Littrow diffraction angle along its original path. The first image acquisition and processing module is used to acquire the petal-shaped interference pattern formed by the conjugate interference of the +1st order Littrow diffraction angle; The second image acquisition and processing module is used to acquire petal-shaped interference patterns formed by the conjugate interference of the -1st order Littrow diffraction angle.

2. The two-degree-of-freedom displacement simultaneous measurement system based on an orbital angular momentum beam interferometer according to claim 1, characterized in that, The optical field manipulation module includes an optical attenuator, an optical beam expander, a quarter-wave plate, and a vortex wave plate; wherein... An optical attenuator is used to adjust the laser intensity to match the detection threshold of the image acquisition and processing module. An optical beam expander is used to adjust the size of the laser spot to match the size of the imaging unit in the image acquisition and processing module. A quarter-wave plate is used to convert linearly polarized light into circularly polarized light. A vortex waveplate is used to convert circularly polarized light into a beam with orbital angular momentum.

3. The method for simultaneous measurement of two degrees of freedom displacement based on an orbital angular momentum beam interferometer according to claim 1, characterized in that, Includes the following steps: S1: The linearly polarized laser output from the laser passes through the optical field modulation module to obtain a beam with orbital angular momentum; S2: A beam with orbital angular momentum is maintained in unidirectional transmission through an optical isolator; S3: The beam with orbital angular momentum is split into a second reference beam and a principal axis beam by the first beam splitter. The second reference beam is converted into an OAM beam with a topological charge of -1 after passing through the second Vickers prism and then reaches the sixth beam splitter. After the beam with orbital angular momentum passes through the first beam splitter, the second beam splitter splits the beam into a first reference beam and a principal axis beam. The first reference beam is converted into an OAM beam with a topological charge of -1 after passing through the first prism and then reaches the third beam splitter. S4: The main axis beam is split into a first measurement beam and a second measurement beam by the fourth beam splitter; The first measuring beam passes through the first reflector to the fifth beam splitter, and after being reflected by the fifth beam splitter and the second reflector, it reaches the reflection grating. After being reflected by the reflection grating, it passes through the second reflector and the fifth beam splitter before reaching the third beam splitter. The second measuring beam passes through the seventh beam splitter and the third reflector in sequence to reach the reflection grating. After being reflected by the reflection grating, it passes through the third reflector and the seventh beam splitter in sequence to reach the sixth beam splitter. S5: The first measurement beam and the first reference beam undergo conjugate interference at the third beam splitter, forming a petal-shaped interference pattern. The conjugate interference image of the +1 order measurement beam is obtained by the first CCD detection. The second measuring beam and the second reference beam undergo conjugate interference at the sixth beam splitter, forming a petal-shaped interference pattern. The conjugate interference image of the -1st order measuring beam is obtained by the second CCD detection. S6: After the reflective grating moves, repeat steps S3 to S5 to obtain the measured conjugate interference image, compare it with the corresponding reference conjugate interference image, demodulate the rotation angle of the conjugate interference image, and realize the simultaneous measurement of the two degrees of freedom displacement of the reflective grating in and out of the plane.

4. The method for simultaneous measurement of two degrees of freedom displacement based on an orbital angular momentum beam interferometer according to claim 3, characterized in that, The light field E of the beam with orbital angular momentum in step S1 l (r,θ) is From l (r,θ)=R l (r)exp(ilθ) Among them, R l (r) represents the radial distribution of the light field, r represents the radial coordinate, i.e. the distance from the center of the beam to a certain point, l represents the number of topological charges of the beam with orbital angular momentum, θ represents the azimuth angle of rotation around the optical axis, and i represents the imaginary unit, used to express the phase term in the light field.

5. The method for simultaneous measurement of two degrees of freedom displacement based on an orbital angular momentum beam interferometer according to claim 4, characterized in that, In step S3, the light fields E of the first and second reference beams re (r,θ) is From re (r,θ)=R -l (r)exp(-ilθ).

6. The method for simultaneous measurement of two degrees of freedom displacement based on an orbital angular momentum beam interferometer according to claim 5, characterized in that, In step S4, the light fields E of the first and second measuring beams are... me (r,θ) is in, and These represent the phase difference of the measurement beam caused by in-plane and out-of-plane displacements of the reflected grating, respectively.

7. The method for simultaneous measurement of two degrees of freedom displacement based on an orbital angular momentum beam interferometer according to claim 6, characterized in that, In step S4, the first and second measuring beams are incident on the reflection grating at ±1 order Littrow diffraction angles and then return along the original path. The incident angle α is α = arcsin(λ / 2d). Where λ is the wavelength of the linearly polarized laser, and d is the grating pitch of the reflection grating; Phase changes of the first and second measuring beams after the grating undergoes a displacement change and They are respectively in, and These represent the total phase difference caused by the in-plane and out-of-plane displacements of the reflection grating corresponding to the +1st and -1st order Littrow diffraction angles, respectively, resulting in the phase change of the measurement beam. and These correspond to the phase changes caused by the Doppler frequency shift resulting from the in-plane displacement of the two measurement beams. and These correspond to the Doppler frequency shift and phase changes caused by the out-of-plane displacement of the two measurement beams, respectively; Δx is the in-plane displacement of the reflection grating, Δz is the out-of-plane displacement of the reflection grating, λ is the wavelength of the linearly polarized laser, d is the grating pitch of the reflection grating, and α is the incident angle.

8. The method for simultaneous measurement of two degrees of freedom displacement based on an orbital angular momentum beam interferometer according to claim 7, characterized in that, The intensity expressions for the interference of the two pairs of conjugate beams in step S5 are as follows: Where I1(r,θ) represents the intensity expression of the conjugate interference between the measurement beam and the reference beam at the +1st order Littrow diffraction angle, and E me1 (r,θ) and E re1 (r,θ) represent the intensity expressions for the +1st order Littrow diffraction angle measurement beam and the reference beam, respectively; I2(r,θ) represents the intensity expression for the conjugate interference of the -1st order Littrow diffraction angle measurement beam and the reference beam, E me2 (r,θ) and E re2 (r,θ) are the light intensity expressions for the measurement beam and the reference beam of the -1st order Littrow diffraction angle, respectively.

9. The method for simultaneous measurement of two degrees of freedom displacement based on an orbital angular momentum beam interferometer according to claim 8, characterized in that, The measurement of the two-degree-of-freedom displacements in and out of the reflective grating in step S6 is expressed as follows: Where Δx is the in-plane displacement of the reflection grating, Δz is the out-of-plane displacement of the reflection grating, Δθ is the rotation angle of the conjugate interference image, Δθ1 is the rotation angle of the first CCD image, Δθ2 is the rotation angle of the second CCD image, λ is the wavelength of the linearly polarized laser, d is the grating pitch of the reflection grating, and α is the incident angle.

Images