A heterodyne interferometry apparatus and method based on wavefront manipulation of a target mirror

Abstract

A heterodyne interferometer device and method based on wavefront regulation target mirror belong to laser interferometer measurement technical field, the present application solves the problem of insufficient system interference keeping ability under the condition of large angle deflection of target mirror. The device comprises a heterodyne interference mirror group and a wavefront regulation target mirror, the heterodyne interference mirror group separates a bundle of input light beams into a first measurement light beam and a first reference light beam, and separates another bundle of input light beams into a second measurement light beam and a second reference light beam, the wavefront regulation target mirror is installed on the measured object, and the second measurement light beam is reflected to form a return measurement light beam and returns to the heterodyne interference mirror group, when the measured object occurs angle deflection such as pitching or yawing in the movement process, the wavefront regulation target mirror deflects synchronously with the measured object, the return measurement light beam is parallel to the second measurement light beam, the change of the return measurement light beam caused by large angle deflection of the wavefront regulation target mirror is more manifested as the position translation of the optical axis, thereby weakening the angle mismatch and wavefront mismatch.

Description

Technical Field

[0001] This invention belongs to the field of laser interferometry technology, and particularly relates to a heterodyne interferometry device and method based on wavefront-controlled target mirror. Background Technology

[0002] Heterodyne laser interferometry (HIL) technology is widely used in ultra-precision displacement measurement of ultra-precision CNC machine tools, lithography machines, coordinate measuring machines, and other high-end equipment due to its advantages such as non-contact operation, high precision, high dynamic range, and direct traceability of measurement results. With the continuous development of high-end equipment manufacturing and precision measurement applications, existing single-axis heterodyne laser interferometry systems not only require higher measurement accuracy and resolution, but also place higher demands on system miniaturization, integration, thermal stability, and ease of assembly and adjustment.

[0003] Current improvements primarily focus on the heterodyne interferometer's mirror assembly itself, optimizing aspects such as beam-splitting and beam-guiding structure integration, optical frequency aliasing suppression, reduction of periodic nonlinearity errors, and ease of assembly and adjustment. However, for single-axis heterodyne interferometry, existing structures can generally maintain good interference when the target mirror undergoes only a small angle change; but when the target mirror deflects at a large angle, the return path of the measurement light changes significantly. This leads to problems such as increased angle between the returned measurement light and the reference light, decreased spatial overlap, and exacerbated wavefront mismatch, resulting in decreased interference contrast. In severe cases, it may even prevent the formation of a stable, demodulated interference signal. Existing single-axis heterodyne laser interferometers have not yet proposed an effective solution directly addressing the interference preservation problem under large-angle target mirror deflection conditions.

[0004] Looking at the common structures of existing single-axis heterodyne laser interferometers, the target end typically uses a common plane mirror as the target mirror. Its main function is to reflect the measurement light, but it lacks the ability to actively control the propagation state and wavefront distribution of the returned measurement light. In other words, while existing technologies can improve the overall system performance by optimizing the heterodyne interferometer mirror assembly, there is still a lack of direct and effective solutions for improving interference preservation capabilities at large angles in a single-axis structure through the optical structure of the target end. In particular, how to reduce the impact of changes in the propagation state of the returned measurement light on the interference state when the target mirror undergoes a large-angle deflection remains a technical problem that has not yet been fully solved in existing technologies.

[0005] Existing technologies also include schemes that use cat's-eye retroreflective structures to improve the tolerance angle at the target end. These structures typically utilize the two-way propagation relationship between the lens and the reflecting surface, allowing incident light to return approximately along the incident direction even when it is deflected at a certain angle at the target end, thus reducing the impact of target end tilt on the propagation direction of the returned light. Previous studies have introduced cat's-eye retroreflectors into interferometry systems and analyzed their anti-mirror tilt characteristics. For example, Peña-Arellano FE et al. published "Mirror tilt immunity interferometry with a cat's-eye retroreflector" in *Applied Optics*, which points out that the cat's-eye structure can improve the interferometer's adaptability to mirror tilt at specific target mirror positions.

[0006] However, for a single-axis heterodyne interferometric displacement measurement system, stable return light direction does not equate to stable formation of the measurement interference signal. In a practical heterodyne interferometer, the return measurement light needs to undergo optical path stages such as polarization splitting, polarization state conversion, splitting and combining, and finite aperture reception before being guided to the predetermined measurement output channel and achieving sufficient spatial overlap and wavefront matching with the reference light at the predetermined combining position. Traditional cat-eye retroreflective structures, when the lens and reflecting surface satisfy a near-focal plane relationship, can suppress, to some extent, the change in return light direction caused by the target end angle deflection, making this change more apparent as a change in the lateral position, optical axis position, or receiving coupling state of the return beam; however, these beam state changes still need to meet the receiving, combining, and phase demodulation conditions of the heterodyne interferometric system. If the return light propagation state does not match the measurement light's output path, polarization conversion relationship, combining position, and receiving coupling conditions, it may lead to difficulties in effectively entering the measurement interference channel, a decrease in the overlap between the measurement light and the reference light, a reduction in interference contrast, or unstable phase demodulation. In their article "Effects of incident beam deviation from the center of a cat's eye retro-reflector on the measurement accuracy of a laser tracing system" published in Optics and Lasers in Engineering[J], Chen H. F. et al. pointed out that incident light deviating from the center of a cat's eye affects the measurement accuracy of a laser tracing system. Therefore, existing cat's eye retroreflection structures mainly address the problem of target end return direction stability, but cannot directly solve the system matching problem between return measurement light introduction, reference light combination, receiver coupling, and phase demodulation in a single-axis heterodyne interferometry system, nor can they directly meet the requirements.

[0007] Therefore, it is necessary to propose a new single-axis large-angle ultra-precision heterodyne interferometry measurement device to solve the problem of insufficient system interference preservation capability under large-angle deflection of the target mirror. Summary of the Invention

[0008] To address the problems existing in the prior art, this invention provides a single-axis large-angle heterodyne interferometry device and method based on a wavefront-controlled target mirror. In a single-axis heterodyne interferometry system, by setting a wavefront-controlled target mirror at the target end, the effect of the large-angle deflection of the wavefront-controlled target mirror on the propagation state of the returning measurement light is altered. This makes the change in the direction of the returning measurement light caused by the deflection of the wavefront-controlled target mirror more of a translation of the optical axis position, thereby reducing the decrease in interference contrast between the returning measurement light and the reference light due to changes in the included angle and wavefront mismatch. This improves the interference preservation capability and displacement measurement stability of the single-axis heterodyne interferometry displacement measurement system under the condition of large-angle deflection of the wavefront-controlled target mirror.

[0009] The key technical focus of this invention lies in the control of the propagation state of the return measurement light in a single measurement channel within a uniaxial heterodyne interferometric displacement measurement system. The measurement result is the uniaxial displacement information of the wavefront-controlled target mirror along the measurement direction. This invention does not obtain multiple measured displacements from multiple spatially distributed measurement positions, nor does it calculate angular degrees of freedom based on the geometric relationship between multiple measured displacements. The technical solution adopted by this invention is as follows:

[0010] A heterodyne interferometry device based on wavefront-controlled target mirror includes:

[0011] Heterodyne light source, used to emit two input beams with different frequencies;

[0012] Heterodyne interferometer array is used to separate an input beam into a first measurement beam and a first reference beam, and another input beam into a second measurement beam and a second reference beam.

[0013] The wavefront-controlled target mirror is used to be installed on the object under test. It reflects the second measurement beam to form a return measurement beam and returns the return measurement beam to the heterodyne interferometer group. When the object under test moves and the pitch or yaw angle deflects, the wavefront-controlled target mirror deflects synchronously with the object under test, and the return measurement beam remains parallel to the second measurement beam.

[0014] The heterodyne interferometer array couples the returned measurement beam with the first reference beam to form a measurement interference light signal, and couples the first measurement beam with the second reference beam to form a reference interference light signal;

[0015] The photoelectric conversion unit is used to receive the measurement interference optical signal and the reference interference optical signal, and convert them into corresponding measurement interference electrical signal and reference interference electrical signal;

[0016] The electronic signal processing unit is used to demodulate and process the measured interference electrical signal and the reference interference electrical signal to obtain the uniaxial displacement information of the wavefront-controlled target mirror along the measurement direction.

[0017] Furthermore, the heterodyne light source includes a single-frequency laser, an optical isolator, and a Wollaston prism arranged sequentially. An optical isolator is provided between the emitting end of the single-frequency laser and the incident end of the Wollaston prism. The two refractive ends of the Wollaston prism are respectively oriented towards the input ends of two acousto-optic frequency shifters. The output ends of the two acousto-optic frequency shifters are respectively connected to one end of two first fiber couplers via optical fibers.

[0018] Furthermore, the heterodyne interferometer assembly includes a base and a first fiber collimator, a second fiber collimator, a first polarizing beam splitter, a second polarizing beam splitter, a second fiber coupler, and a third fiber coupler fixed on the base. The reflecting ends of the first and second polarizing beam splitters are positioned opposite each other. A half-wave plate is provided between the first and second polarizing beam splitters. The incident ends of the first and second polarizing beam splitters are aligned in the same direction. The output end of the first fiber collimator faces the incident end of the first polarizing beam splitter, and the output end of the second fiber collimator faces the incident end of the second polarizing beam splitter. The output end of the second fiber coupler... The input end of the third fiber coupler faces the reflecting end of the second polarizing beam splitter, and the input end of the third fiber coupler faces the reflecting end of the first polarizing beam splitter. The transmission end of the first polarizing beam splitter is equipped with a first reflecting mirror, and a first quarter-wave plate is provided between the first reflecting mirror and the first polarizing beam splitter. The transmission end of the second polarizing beam splitter is equipped with a second quarter-wave plate, and a wavefront-controlled target mirror is provided on the path of the transmission end of the second polarizing beam splitter. The output ends of the second fiber coupler and the third fiber coupler are respectively connected to the electronic signal processing unit through corresponding photoelectric conversion units. The input ends of the first fiber collimator and the second fiber collimator are respectively connected to the other ends of the two first fiber couplers through optical fibers.

[0019] Furthermore, the first polarizing beam splitter has a first polarizer at its incident end, a third quarter-wave plate on the side of the first polarizing beam splitter facing the second fiber coupler, a third polarizer between the third quarter-wave plate and the first polarizing beam splitter, a second polarizer at its incident end, a fourth quarter-wave plate on the side of the second polarizing beam splitter facing the third fiber coupler, and a fourth polarizer between the fourth quarter-wave plate and the second polarizing beam splitter.

[0020] Furthermore, the wavefront-controlled target mirror includes a lens assembly and a second reflecting mirror, with the lens assembly positioned closer to the second polarizing beam splitter relative to the second reflecting mirror.

[0021] This invention also provides a heterodyne interferometry method based on a wavefront-controlled target mirror, implemented using the aforementioned heterodyne interferometry device based on a wavefront-controlled target mirror, comprising the following steps:

[0022] Step 1: Based on the predetermined axial displacement measurement range of the object under test, the allowable deflection range of the wavefront control target mirror, the diameter of the second measurement beam, the effective receiving range of the heterodyne interferometer group, and the focal length of the lens assembly, determine the axial distance between the lens assembly and the second reflector, and determine the installation position of the wavefront control target mirror relative to the heterodyne interferometer group, so that when the wavefront control target mirror moves within the predetermined deflection range, the returning measurement beam remains within the effective receiving range of the heterodyne interferometer group;

[0023] The axial distance d between the lens assembly and the second reflector is determined by the following formula:

[0024] (1);

[0025] In the formula, α is the allowable deflection angle of the wavefront-controlled target mirror. max The maximum value of α is determined based on the maximum permissible deflection angle of the object being measured during the measurement process;

[0026] β is the allowable residual exit tilt angle of the heterodyne interferometer group. max The maximum value of β is determined based on the tilt angle of the maximum residual return measurement beam that the heterodyne interferometer group is allowed to receive;

[0027] Therefore, the range of the axial distance d between the lens assembly and the second reflecting mirror can be obtained as follows:

[0028] (2);

[0029] Therefore, when it is desirable to prioritize suppressing the exit tilt angle of the returning measurement beam, d = f is taken; when compensation is required based on the beam center position, receiving aperture, or installation space, d = f + δ is taken. d δ d δ is the distance d is offset from the focal length f by the axial spacing d. d Satisfy the following formula:

[0030] (3);

[0031] First, determine the maximum allowable deflection angle α of the wavefront-controlled target mirror. max The maximum residual tilt angle β of the returned measurement beam that the heterodyne interferometer group can receive. max The focal length f of the lens assembly is then used to determine the selectable range of the axial spacing d using equations (1)-(3), wherein β max Determined based on the effective receiving angle of the heterodyne interference mirror group, the minimum permissible interference contrast, or experimental calibration results;

[0032] Assume the wavefront-controlled target mirror is at its maximum deflection angle α. max At that time, the maximum lateral position change of the returned measurement beam within the receiving plane of the heterodyne interferometer group is ΔX. max The effective receiving radius of the heterodyne interferometer group is A. rec The radius of the returning measurement beam within the receiving plane is w. rec If the assembly and thermal drift allowance is m, then the installation position of the wavefront-controlled target mirror relative to the heterodyne interferometer group is determined as follows:

[0033] (4);

[0034] ΔX max Calculate using the following formula:

[0035] (5);

[0036] In the formula, z rec z is the distance from the lens assembly to the receiving plane or equivalent combining plane of the heterodyne interferometer group. rec Corresponds to the working distance parameter of the wavefront-controlled target mirror relative to the heterodyne interferometer group;

[0037] Step 2: Install the wavefront control target mirror onto the object under test, so that the wavefront control target mirror moves synchronously with the object under test along the measurement direction. When the object under test undergoes pitch or yaw angle deflection during the movement, the wavefront control target mirror deflects synchronously with the object under test.

[0038] Step 3: The heterodyne light source emits two input beams with different frequencies and a fixed frequency difference. One of the input beams is collimated and polarized by the first fiber collimator and the first polarization beam splitter in sequence to form a first measurement beam and a first reference beam. The first reference beam is emitted from the reflecting end of the first polarization beam splitter, first undergoes polarization state transformation by a half-wave plate, and then is emitted to the third fiber coupler by the second polarization beam splitter. The first measurement beam is emitted from the transmitting end of the first polarization beam splitter, first undergoes polarization state transformation by a first quarter-wave plate, then is emitted to the first reflecting mirror and reflected back to the first polarization beam splitter, and finally is emitted to the second fiber coupler after being reflected by the cemented surface of the first polarization beam splitter.

[0039] The other input beam is collimated, polarization-state converted, and polarization-splitting sequentially by the second fiber collimator and the second polarization beam splitter to form a second measurement beam and a second reference beam. The second reference beam exits through the reflecting end of the second polarization beam splitter, undergoes polarization state transformation by a half-wave plate, and then passes through the first polarization beam splitter to the second fiber coupler. The second measurement beam exits through the transmitting end of the second polarization beam splitter, undergoes polarization state transformation by a second quarter-wave plate, and then passes through the lens assembly to the second reflecting mirror. The second measurement beam is reflected by the second reflecting mirror to form a return measurement beam. The return measurement beam passes through the lens assembly to the cemented surface of the second polarization beam splitter and is reflected to the third fiber coupler. The frequencies of the first reference beam and the second reference beam remain unchanged.

[0040] Step 4: The first measurement beam directed towards the second fiber coupler at least partially overlaps with the second reference beam to form a reference interference optical signal. The first reference beam directed towards the third fiber coupler at least partially overlaps with the returning measurement beam to form a measurement interference optical signal. The reference interference optical signal and the measurement interference optical signal are respectively input into the photoelectric conversion unit, which converts them into corresponding measurement interference electrical signals and reference interference electrical signals.

[0041] Step 5: The electronic signal processing unit performs phase demodulation on the reference interference electrical signal and the measurement interference electrical signal to obtain the phase change information of the measurement interference optical signal relative to the reference interference optical signal, and calculates the single-axis displacement information of the wavefront-controlled target mirror along the measurement direction based on the phase change information.

[0042] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0043] 1. This invention sets a wavefront-controlled target mirror on the object under test, so that the change in the returned measurement beam caused by the large-angle deflection of the wavefront-controlled target mirror is more manifested as a translation of the optical axis position, rather than a significant increase in the angle between the returned measurement beam and the first reference beam. This reduces angle mismatch and wavefront mismatch, reduces the decrease in interference contrast, and improves the large-angle interference retention capability of the heterodyne interferometric displacement measurement system.

[0044] 2. While maintaining the compact structure of the single-axis heterodyne interferometry measurement system, this invention combines an integrated heterodyne interferometer mirror group with a wavefront-controlled target mirror. This retains the advantages of the heterodyne interferometer mirror group, such as high integration, good thermal stability, and high measurement accuracy and resolution, while enhancing the system's adaptability to large-angle deflection of the wavefront-controlled target mirror. It is more suitable for single-axis ultra-precision heterodyne interferometric displacement measurement under conditions where the wavefront-controlled target mirror has a large-angle deflection.

[0045] 3. The present invention also provides a method for determining the parameters between the distance between the lens assembly and the second reflector, the installation position of the wavefront-controlled target mirror, the effective light transmission size of the lens assembly, the effective reflection size of the second reflector, and the interference reception conditions. This makes the wavefront-controlled target mirror no longer just a conceptual target end structure, but a measurement structure that can be actually configured according to the allowable deflection angle of the target end of the object under test, the effective reception range of the heterodyne interferometer group, and the interference preservation requirements. Attached Figure Description

[0046] Figure 1 This is a schematic diagram of the structure of the device of the present invention;

[0047] Figure 2 This is a schematic diagram of the heterodyne light source.

[0048] Figure 3 This is a schematic diagram of the heterodyne interference mirror assembly.

[0049] Figure 4 A schematic diagram showing the placement of polarizers and waveplates on the second polarizing beam splitter and the first polarizing beam splitter;

[0050] Figure 5 This is a schematic diagram of the wavefront-controlled target mirror.

[0051] Figure 6 This is a schematic diagram illustrating the working principle of wavefront-controlled target mirrors.

[0052] In the diagram, 1. Heterodyne light source, 11. Single-frequency laser, 12. Optical isolator, 13. Wollaston prism, 14. Acousto-optic frequency shifter, 15. First fiber coupler, 2. Heterodyne interferometer group, 21. First fiber collimator, 22. Second fiber collimator, 23. First polarizer, 24. Second polarizer, 25. First polarizing beam splitter, 26. Second polarizing beam splitter, 27. First quarter-wave plate, 28. Second quarter-wave plate, 29. Third polarizer, 210. Fourth polarizer, 211. Third quarter-wave plate, 212. Fourth quarter-wave plate, 213. Second fiber coupler, 214. Third fiber coupler, 215. Half-wave plate, 216. First reflector, 3. Wavefront-controlled target mirror, 31. Lens assembly, 32. Second reflector, 4. Photoelectric conversion unit, 5. Electronic signal processing unit, 61. 62. Second measuring beam, 63. Return measuring beam, 64. First reference beam, 65. Observation plane. Detailed Implementation

[0053] To make the objectives, technical solutions, and advantages of this invention clearer, the invention is described below with reference to specific embodiments shown in the accompanying drawings. However, it should be understood that these descriptions are merely exemplary and not intended to limit the scope of the invention. Furthermore, descriptions of well-known structures and technologies are omitted in the following description to avoid unnecessarily obscuring the concept of the invention.

[0054] The connections mentioned in this invention are divided into fixed connections and detachable connections. Fixed connections, also known as non-detachable connections, include but are not limited to conventional fixed connection methods such as folded connections, riveted connections, adhesive connections, and welded connections. Detachable connections include but are not limited to conventional disassembly methods such as bolted connections, snap-fit ​​connections, pin connections, and hinged connections. When a specific connection method is not explicitly defined, it is assumed that at least one existing connection method can be found to achieve this function, and those skilled in the art can choose according to their needs. For example, a welded connection can be chosen for fixed connections, and a bolted connection can be chosen for detachable connections.

[0055] The present invention will be further described in detail below with reference to the accompanying drawings. The following embodiments are explanations of the present invention, but the present invention is not limited to the following embodiments.

[0056] Example 1: As Figures 1-6 As shown, a heterodyne interferometry measurement device based on wavefront-controlled target mirror includes:

[0057] Heterodyne source 1 is used to emit two input beams with different frequencies;

[0058] Heterodyne interferometer group 2 is used to separate one input beam into a first measurement beam and a first reference beam 63, and to separate another input beam into a second measurement beam 61 and a second reference beam;

[0059] The wavefront-controlled target mirror 3 is used to be installed on the object under test. It reflects the second measurement beam 61 to form a return measurement beam 62 and makes the return measurement beam 62 return to the heterodyne interferometer group 2. When the object under test undergoes pitch or yaw during its movement, the wavefront-controlled target mirror 3 deflects synchronously with the object under test, so that the return measurement beam 62 is parallel to the second measurement beam 61.

[0060] The heterodyne interferometer group 2 refracts the returned measurement beam 62 through the cemented surface. The refracted returned measurement beam 62 propagates in the same direction as the first reference beam 63. The heterodyne interferometer group 2 couples the returned measurement beam 62 with the first reference beam 63 to form a measurement interference light signal. The first measurement beam and the second reference beam are coupled through the heterodyne interferometer group 2 to form a reference interference light signal.

[0061] The photoelectric conversion unit 4 is used to receive the measurement interference optical signal and the reference interference optical signal, and convert them into corresponding measurement interference electrical signal and reference interference electrical signal;

[0062] The electronic signal processing unit 5 is used to demodulate and process the measured interference electrical signal and the reference interference electrical signal to obtain the single-axis displacement information of the wavefront-controlled target mirror 3 along the measurement direction.

[0063] The heterodyne light source 1 includes a single-frequency laser 11, an optical isolator 12, and a Wollaston prism 13 arranged in sequence. An optical isolator 12 is provided between the emitting end of the single-frequency laser 11 and the incident end of the Wollaston prism 13. The two refractive ends of the Wollaston prism 13 are respectively facing the input ends of two acousto-optic frequency shifters 14. The output ends of the two acousto-optic frequency shifters 14 are respectively connected to one end of two first fiber optic couplers 15 through optical fibers.

[0064] The single-frequency light emitted by the single-frequency laser 11 is isolated by the optical isolator 12 and then enters the Wollaston prism 13, where it is split into two single-frequency beams of the same frequency. The two single-frequency beams are then frequency-shifted by the acousto-optic frequency shifter 14 to form two input beams of different frequencies, which are then transmitted to the heterodyne interferometer group 2 through the first fiber coupler 15 and the optical fiber.

[0065] The heterodyne interferometer assembly 2 includes a base and a first fiber collimator 21, a second fiber collimator 22, a first polarizing beam splitter 25, a second polarizing beam splitter 26, a second fiber coupler 213, and a third fiber coupler 214 fixed on the base. The first polarizing beam splitter 25 and the second polarizing beam splitter 26 are both cubic prisms formed by cementing two right-angled triangular prisms together. The reflecting ends of the first polarizing beam splitter 25 and the second polarizing beam splitter 26 are positioned opposite each other. A half-wave plate 215 is provided between the first polarizing beam splitter 25 and the second polarizing beam splitter 26. The incident ends of the first polarizing beam splitter 25 and the second polarizing beam splitter 26 are aligned in the same direction. The output end of the first fiber collimator 21 faces the incident end of the first polarizing beam splitter 25, and the output end of the second fiber collimator 22 faces the incident end of the second polarizing beam splitter 26. At the incident end of 6, the input end of the second fiber coupler 213 faces the reflection end of the second polarizing beam splitter 26, and the input end of the third fiber coupler 214 faces the reflection end of the first polarizing beam splitter 25. The transmission end of the first polarizing beam splitter 25 is provided with a first reflecting mirror 216. A first quarter-wave plate 27 is provided between the first reflecting mirror 216 and the first polarizing beam splitter 25. The transmission end of the second polarizing beam splitter 26 is provided with a second quarter-wave plate 28. A wavefront-controlled target mirror 3 is provided on the path of the transmission end of the second polarizing beam splitter 26. The output ends of the second fiber coupler 213 and the third fiber coupler 214 are respectively connected to the electronic signal processing unit 5 through the corresponding photoelectric conversion unit 4. The input ends of the first fiber collimator 21 and the second fiber collimator 22 are respectively connected to the other ends of the two first fiber couplers 15 through optical fibers.

[0066] The first polarizing beam splitter 25 has a first polarizer 23 at its incident end, a third quarter-wave plate 211 on the side of the first polarizing beam splitter 25 facing the second fiber coupler 213, a third polarizer 29 between the third quarter-wave plate 211 and the first polarizing beam splitter 25, a second polarizer 24 at its incident end, a fourth quarter-wave plate 212 on the side of the second polarizing beam splitter 26 facing the third fiber coupler 214, and a fourth polarizer 210 between the fourth quarter-wave plate 212 and the second polarizing beam splitter 26.

[0067] The wavefront control target mirror 3 includes a lens assembly 31 and a second reflecting mirror 32. The lens assembly 31 is closer to the second polarizing beam splitter 26 than the second reflecting mirror 32. The wavefront control target mirror 3 is used to control the return measurement beam 62 of the measured object in a single-axis measurement channel. It is set at the measurement position corresponding to a single measured object, and its control object is the propagation state of the return measurement beam 62 in a single measurement channel. Since the second measurement beam 61 and the return measurement beam 62 each undergo propagation once between the lens assembly 31 and the second reflecting mirror 32, the lens assembly 31 and the second reflecting mirror 32 can change the way the angle deflection of the wavefront control target mirror 3 affects the propagation state of the return measurement beam 62. This makes the change in the direction of the return measurement beam 62 caused by the angle deflection of the wavefront control target mirror more of a translation of the optical axis position. The exit tilt angle of the return measurement beam 62 can be expressed as:

[0068] ;

[0069] In the formula, β is the outgoing tilt angle of the return measurement beam 62 after passing through the lens assembly 31, α is the deflection angle of the wavefront control target mirror, d is the axial distance between the lens assembly 31 and the second reflecting mirror 32, and f is the focal length of the lens assembly 31.

[0070] When the returning measurement beam 62 reaches the plane of the lens assembly 31, the lateral offset relative to the second measurement beam 61 can be expressed as:

[0071] ;

[0072] In the formula, Δx1 is the lateral offset relative to the original optical axis when the returning measurement beam 62 reaches the lens assembly plane for the second time;

[0073] Under the small-angle approximation condition, we have:

[0074] ;

[0075] If we take any plane with a distance z behind the lens assembly 31 as the observation plane 64, then the lateral position of the returning measurement beam 62 relative to the second measurement beam 61 within the observation plane 64 can be expressed as:

[0076] ;

[0077] In the formula, x(z) is the lateral position of the return measurement beam 62 in the observation plane 64 at a distance z from the lens assembly;

[0078] When d approaches f, β decreases significantly; when d = f, β = 0, and the above equation can be further written as:

[0079] ;

[0080] That is, under the condition d=f, the change in the returned measurement beam 62 caused by the angular deflection of the wavefront-controlled target mirror 3 is mainly manifested as a lateral translation of the optical axis, and the amount of translation is:

[0081] ;

[0082] In the formula, Δx is the optical axis translation between the return measurement beam 62 and the second measurement beam 61;

[0083] Therefore, when the wavefront-controlled target mirror 3 deflects at a large angle, the change between the returning measurement beam 62 and the first reference beam 63 is no longer mainly manifested as a significant increase in the included angle, but rather as a shift in the optical axis position that has a smaller impact on interference. This reduces the angle mismatch and wavefront mismatch between the returning measurement beam 62 and the first reference beam under large-angle deflection conditions.

[0084] Example 2: Figures 1-6 As shown, a heterodyne interferometry method based on a wavefront-controlled target mirror is implemented using a heterodyne interferometry device based on a wavefront-controlled target mirror as described in Example 1, and includes the following steps:

[0085] Step 1: Based on the predetermined axial displacement measurement range of the object under test, the allowable deflection range of the wavefront-controlled target mirror 3, the diameter of the second measurement beam 61, the effective receiving range of the heterodyne interferometer group 2, and the focal length of the lens assembly 31, determine the axial distance between the lens assembly 31 and the second reflecting mirror 32, and determine the installation position of the wavefront-controlled target mirror 3 relative to the heterodyne interferometer group 2, so that when the wavefront-controlled target mirror 3 moves within the predetermined deflection range, the returning measurement beam 62 remains within the effective receiving range of the heterodyne interferometer group 2;

[0086] The axial distance d between the lens assembly 31 and the second reflecting mirror 32 is determined by the following formula:

[0087] (1);

[0088] In the formula, α is the allowable deflection angle of the wavefront-controlled target mirror 3. max The maximum value of α is determined based on the maximum permissible deflection angle of the object being measured during the measurement process;

[0089] β is the allowable residual exit tilt angle of heterodyne interferometer group 2. max The maximum value of β is determined based on the tilt angle of the maximum residual return measurement beam 62 that the heterodyne interferometer group 2 is allowed to receive;

[0090] Therefore, the range of the axial distance d between the lens assembly 31 and the second reflecting mirror 32 can be obtained as follows:

[0091] (2);

[0092] Therefore, when it is desirable to prioritize suppressing the exit tilt angle of the returning measurement beam 62, d = f is taken; when compensation is required based on the beam center position, receiving aperture, or installation space, d = f + δ is taken. d δ d δ is the distance d is offset from the focal length f by the axial spacing d. d Satisfy the following formula:

[0093] (3);

[0094] The above relationship gives the specific method for determining the axial spacing d, that is, first determine the maximum allowable deflection angle α of the wavefront-controlled target mirror 3. max The maximum residual tilt angle β of the returned measurement beam 62 that the heterodyne interferometer group 2 can receive. max The focal length f of the lens assembly 31 is then used to determine the selectable range of the axial spacing d using equations (1)-(3), wherein β max Determined based on the effective receiving angle, minimum permissible interference contrast, or experimental calibration results of heterodyne interferometer group 2;

[0095] Assume that the wavefront-controlled target mirror 3 is at its maximum deflection angle α. max At that time, the maximum lateral position change of the returned measurement beam 62 within the receiving plane of the heterodyne interferometer group 2 is ΔX. max The effective receiving radius of heterodyne interferometer group 2 is A. rec The radius of the returning measurement beam 62 within the receiving plane is w. rec Given that the assembly and thermal drift allowance is m, the installation position of the wavefront-controlled target mirror 3 relative to the heterodyne interferometer group 2 is determined as follows:

[0096] (4);

[0097] ΔX max Calculate using the following formula:

[0098] (5);

[0099] In the formula, z rec z is the distance from lens assembly 31 to the receiving plane or equivalent combining plane of heterodyne interference mirror group 2. rec Corresponds to the working distance parameter of wavefront-controlled target mirror 3 relative to heterodyne interferometer group 2;

[0100] Therefore, given the known effective receiving radius A of the heterodyne interferometer group 2 rec The focal length f, axial spacing d, and maximum permissible deflection angle α of lens assembly 31 max and the return measurement beam with a radius of 62w recIn this case, the allowable installation distance range of the wavefront-controlled target mirror 3 relative to the heterodyne interferometer group 2 can be determined by the above inequality;

[0101] The effective reflection area of ​​the second reflector 32 should cover the area on the surface of the second reflector 32 where the second measuring beam 61 acts under the maximum deflection state, and allowance should be reserved for assembly error and thermal drift; the effective light transmission area of ​​the lens assembly 31 should cover the passage area of ​​the incident second measuring beam 61 and the returning measuring beam 62 under the maximum deflection state, and the size of the second measuring beam 61 can be expressed by the actual beam radius, the equivalent beam radius, or the effective beam size obtained according to the system calibration;

[0102] Furthermore, to ensure the demodulation capability of the measured interference signal, the above parameters can be checked based on the minimum permissible interference contrast. If an interference signal meeting the phase demodulation requirements cannot be formed at the maximum permissible deflection angle at the target end of the measured object, then the interference contrast can be reduced. Corrections can be made by increasing the effective receiving radius of the heterodyne interferometer group 2, increasing the light transmission aperture of the lens assembly 31, increasing the effective reflection area of ​​the second reflecting mirror 32, or adjusting the installation distance of the wavefront control target mirror 3.

[0103] Step 2: Install the wavefront control target mirror 3 on the object under test, so that the wavefront control target mirror 3 moves synchronously with the object under test along the measurement direction. When the object under test undergoes pitch or yaw angle deflection during the movement, the wavefront control target mirror 3 deflects synchronously with the object under test.

[0104] Step 3: Heterodyne light source 1 emits two input beams with different frequencies and a fixed frequency difference. One of the input beams is collimated and polarized by the first fiber collimator 21 and the first polarization beam splitter 25 in sequence to form a first measurement beam and a first reference beam 63. The first reference beam 63 is emitted from the reflecting end of the first polarization beam splitter 25, first undergoes polarization state transformation by the half-wave plate 215, and then is emitted to the third fiber coupler 214 by the second polarization beam splitter 26. The first measurement beam is emitted from the transmitting end of the first polarization beam splitter 25, first undergoes polarization state transformation by the first quarter-wave plate 27, then is emitted to the first reflecting mirror 216 and reflected back to the first polarization beam splitter 25, and finally is emitted to the second fiber coupler 213 after being reflected by the cemented surface of the first polarization beam splitter 25.

[0105] Another input beam is collimated, polarization-state converted, and polarization-splitting by the second fiber collimator 22 and the second polarization beam splitter 26 in sequence to form a second measurement beam 61 and a second reference beam. The second reference beam is emitted from the reflecting end of the second polarization beam splitter 26, undergoes polarization state transformation by the half-wave plate 215, and then passes through the first polarization beam splitter 25 to the second fiber coupler 213. The second measurement beam 61 is emitted from the transmitting end of the second polarization beam splitter 26, undergoes polarization state transformation by the second quarter-wave plate 28, and then passes through the lens assembly 31 to the second reflecting mirror 32. The second measurement beam 61 is reflected by the second reflecting mirror 32 to form a return measurement beam 62. The return measurement beam 62 passes through the lens assembly 31 to the cemented surface of the second polarization beam splitter 26 and is reflected to the third fiber coupler 214. The frequencies of the first reference beam 63 and the second reference beam remain unchanged.

[0106] Step 4: The first measurement beam directed towards the second fiber coupler 213 at least partially overlaps with the second reference beam to form a reference interference optical signal. The first reference beam 63 directed towards the third fiber coupler 214 at least partially overlaps with the returning measurement beam 62 to form a measurement interference optical signal. The reference interference optical signal and the measurement interference optical signal are respectively input into the photoelectric conversion unit 4, and are converted by the photoelectric conversion unit 4 into the corresponding measurement interference electrical signal and reference interference electrical signal.

[0107] Step 5: The electronic signal processing unit 5 performs phase demodulation on the reference interference electrical signal and the measurement interference electrical signal to obtain the phase change information of the measurement interference optical signal relative to the reference interference optical signal, and calculates the single-axis displacement information of the wavefront-controlled target mirror 3 along the measurement direction based on the phase change information. The single-axis displacement information is the axial displacement measurement result in a single measurement channel, excluding the angular degree of freedom information calculated based on multiple spatially distributed measurement displacements.

[0108] This invention, by setting a wavefront-modulated target mirror 3 on the object under test, makes the change in the returned measurement beam 62 caused by the large-angle deflection of the wavefront-modulated target mirror 3 more manifest as a translation of the optical axis position, rather than a significant increase in the angle between the returned measurement beam 62 and the first reference light. This reduces angle mismatch and wavefront mismatch, decreases the decrease in interference contrast, and improves the large-angle interference retention capability of the heterodyne interferometric displacement measurement system.

[0109] While maintaining the compact structure of the single-axis heterodyne interferometric measurement system, this invention combines the integrated heterodyne interferometer mirror group 2 with the wavefront-controlled target mirror 3. This retains the advantages of the heterodyne interferometer mirror group, such as high integration, good thermal stability, and high measurement accuracy and resolution, while enhancing the system's adaptability to large-angle deflection of the wavefront-controlled target mirror. It is more suitable for single-axis ultra-precision heterodyne interferometric displacement measurement under conditions where the wavefront-controlled target mirror has a large-angle deflection.

[0110] The present invention also provides a method for determining the parameters between the axial distance d between the lens assembly 31 and the second reflector 32, the installation position of the wavefront-controlled target mirror 3, the effective light transmission size of the lens assembly 31, the effective reflection size of the second reflector 32, and the interference reception conditions. This makes the wavefront-controlled target mirror 3 no longer just a conceptual target end structure, but a measurement structure that can be actually configured according to the allowable deflection angle of the target end of the measured object, the effective reception range of the heterodyne interferometer group 2, and the interference preservation requirements.

[0111] The above embodiments are merely illustrative examples of the present invention and do not limit its scope of protection. Those skilled in the art can make partial changes to them, as long as they do not exceed the spirit and essence of the present invention, they are all within the scope of protection of the present invention.

Claims

1. A heterodyne interferometry method based on a wavefront-controlled target mirror, implemented using a heterodyne interferometry device based on a wavefront-controlled target mirror, the heterodyne interferometry device comprising: Heterodyne source (1) is used to emit two input beams with different frequencies; Heterodyne interferometer group (2) is used to separate an input beam into a first measurement beam and a first reference beam (63), and to separate another input beam into a second measurement beam (61) and a second reference beam; The wavefront-controlled target mirror (3) is used to be installed on the object under test. It reflects the second measurement beam (61) to form a return measurement beam (62) and makes the return measurement beam (62) return to the heterodyne interferometer group (2). When the object under test moves and the pitch or yaw angle deflects, the wavefront-controlled target mirror (3) deflects synchronously with the object under test. The return measurement beam (62) remains parallel to the second measurement beam (61). The heterodyne interferometer group (2) couples the returned measurement beam (62) with the first reference beam (63) to form a measurement interference light signal, and couples the first measurement beam with the second reference beam to form a reference interference light signal; The photoelectric conversion unit (4) is used to receive the measurement interference optical signal and the reference interference optical signal, and convert them into the corresponding measurement interference electrical signal and the reference interference electrical signal; The electronic signal processing unit (5) is used to demodulate and process the measurement interference electrical signal and the reference interference electrical signal to obtain the single-axis displacement information of the wavefront-controlled target mirror (3) along the measurement direction; The heterodyne interferometer assembly (2) includes a base and a first fiber collimator (21), a second fiber collimator (22), a first polarizing beam splitter (25), a second polarizing beam splitter (26), a second fiber coupler (213), and a third fiber coupler (214) fixed on the base. The reflecting ends of the first polarizing beam splitter (25) and the second polarizing beam splitter (26) are arranged opposite to each other. A half-wave plate (215) is provided between the first polarizing beam splitter (25) and the second polarizing beam splitter (26). The incident ends of the first polarizing beam splitter (25) and the second polarizing beam splitter (26) are arranged in the same direction. The output end of the first fiber collimator (21) faces the first polarizing beam splitter (26). The first polarization beam splitter (25) has an incident end, the output end of the second fiber collimator (22) faces the incident end of the second polarization beam splitter (26), the input end of the second fiber coupler (213) faces the reflection end of the second polarization beam splitter (26), the input end of the third fiber coupler (214) faces the reflection end of the first polarization beam splitter (25), the first polarization beam splitter (25) has a first reflecting mirror (216) at its transmission end, a first quarter-wave plate (27) is provided between the first reflecting mirror (216) and the first polarization beam splitter (25), the second polarization beam splitter (26) has a second quarter-wave plate (28) at its transmission end, and a wavefront-controlled target mirror (3) is provided on the path of the transmission end of the second polarization beam splitter (26). The wavefront-controlled target mirror (3) includes a lens assembly (31) and a second reflector (32), with the lens assembly (31) being closer to the second polarizing beam splitter (26) than the second reflector (32). The heterodyne interferometry method is characterized by comprising the following steps: Step 1: Based on the predetermined axial displacement measurement range of the object under test, the allowable deflection range of the wavefront control target mirror (3), the diameter of the second measurement beam (61), the effective receiving range of the heterodyne interferometer group (2), and the focal length of the lens assembly (31), determine the axial distance between the lens assembly (31) and the second reflector (32), and determine the installation position of the wavefront control target mirror (3) relative to the heterodyne interferometer group (2), so that when the wavefront control target mirror (3) moves within the predetermined deflection range, the returning measurement beam (62) remains within the effective receiving range of the heterodyne interferometer group (2); The axial distance d between the lens assembly (31) and the second reflector (32) is determined by the following formula: （1）； In the formula, α is the allowable deflection angle of the wavefront-controlled target mirror (3), α max The maximum value of α is determined based on the maximum permissible deflection angle of the object being measured during the measurement process; β is the allowable residual exit tilt angle of the heterodyne interferometer group (2), β max The maximum value of β is determined based on the tilt angle of the maximum residual return measurement beam (62) that the heterodyne interferometer group (2) is allowed to receive; Therefore, the range of the axial distance d between the lens assembly (31) and the second reflecting mirror (32) can be obtained as follows: （2）； Therefore, when it is desirable to prioritize suppressing the exit tilt angle of the returning measurement beam (62), d = f is taken; when compensation is required based on the beam center position, receiving aperture, or installation space, d = f + δ is taken. d δ d δ is the distance d is offset from the focal length f by the axial spacing d. d Satisfy the following formula: （3）； First, determine the maximum allowable deflection angle α of the wavefront-controlled target mirror (3). max The maximum residual tilt angle β of the returned measurement beam (62) that the heterodyne interferometer group (2) can receive. max The focal length f of the lens assembly (31) is then used to determine the selectable range of the axial spacing d using equations (1)-(3), wherein β max Determined based on the effective receiving angle, minimum permissible interference contrast, or experimental calibration results of the heterodyne interferometer group (2); Suppose that the wavefront-controlled target mirror (3) is at the maximum deflection angle α max At that time, the maximum lateral position change of the returned measurement beam (62) in the receiving plane of the heterodyne interferometer group (2) is ΔX. max The effective receiving radius of the heterodyne interferometer group (2) is A. rec The radius of the returning measurement beam (62) in the receiving plane is w. rec If the assembly and thermal drift allowance is m, then the installation position of the wavefront-controlled target mirror (3) relative to the heterodyne interferometer group (2) is determined as follows: （4）； ΔX max Calculate using the following formula: （5）； In the formula, z rec z is the distance from the lens assembly (31) to the receiving plane or equivalent combining plane of the heterodyne interference mirror group (2). rec The working distance parameter of the wavefront-controlled target mirror (3) relative to the heterodyne interferometer group (2) corresponds to the working distance parameter of the wavefront-controlled target mirror (3). Step 2: Install the wavefront control target mirror (3) on the object under test, so that the wavefront control target mirror (3) moves synchronously with the object under test along the measurement direction. When the object under test moves and the pitch or yaw angle deflects, the wavefront control target mirror (3) deflects synchronously with the object under test. Step 3: The heterodyne light source (1) emits two input beams with different frequencies and a fixed frequency difference. One of the input beams is collimated and polarized by the first fiber collimator (21) and the first polarization beam splitter (25) in sequence to form a first measurement beam and a first reference beam (63). The first reference beam (63) is emitted from the reflecting end of the first polarization beam splitter (25), first passes through a half-wave plate (215) to obtain a polarization state transformation, and then passes through the second polarization beam splitter (26) to be emitted towards the third fiber coupler (214). The first measurement beam is emitted from the transmitting end of the first polarization beam splitter (25), first passes through the first quarter-wave plate (27) to obtain a polarization state transformation, then passes through the first reflector (216) and is reflected back to the first polarization beam splitter (25), and finally passes through the cemented surface of the first polarization beam splitter (25) and is emitted towards the second fiber coupler (213). The other input beam is collimated, polarization-state converted, and polarization-splitting by the second fiber collimator (22) and the second polarization beam splitter (26) in sequence to form a second measurement beam (61) and a second reference beam. The second reference beam is emitted from the reflecting end of the second polarization beam splitter (26), first undergoes polarization state conversion by a half-wave plate (215), and then is directed to the second fiber coupler (213) by the first polarization beam splitter (25). The second measurement beam (61) passes through the second polarization beam splitter (26). The beam is emitted from the transmission end, first undergoes polarization transformation through the second quarter-wave plate (28), and then passes through the lens assembly (31) to the second reflector (32). The second measurement beam (61) is reflected by the second reflector (32) to form a return measurement beam (62). The return measurement beam (62) passes through the lens assembly (31) to the cemented surface of the second polarizing beam splitter (26) and is reflected towards the third fiber coupler (214). The frequencies of the first reference beam (63) and the second reference beam remain unchanged. Step 4: The first measurement beam directed towards the second fiber coupler (213) at least partially overlaps with the second reference beam to form a reference interference optical signal. The first reference beam (63) directed towards the third fiber coupler (214) at least partially overlaps with the returning measurement beam (62) to form a measurement interference optical signal. The reference interference optical signal and the measurement interference optical signal are respectively input into the photoelectric conversion unit (4) and converted by the photoelectric conversion unit (4) into the corresponding measurement interference electrical signal and reference interference electrical signal. Step 5: The electronic signal processing unit (5) performs phase demodulation on the reference interference electrical signal and the measurement interference electrical signal to obtain the phase change information of the measurement interference optical signal relative to the reference interference optical signal, and calculates the single-axis displacement information of the wavefront-controlled target mirror (3) along the measurement direction based on the phase change information.

2. The heterodyne interferometry method based on wavefront-controlled target mirror according to claim 1, characterized in that: The heterodyne light source (1) includes a single-frequency laser (11), an optical isolator (12) and a Wollaston prism (13) arranged in sequence. An optical isolator (12) is provided between the emitting end of the single-frequency laser (11) and the incident end of the Wollaston prism (13). The two refractive ends of the Wollaston prism (13) are respectively facing the input ends of two acousto-optic frequency shifters (14). The output ends of the two acousto-optic frequency shifters (14) are respectively connected to one end of two first fiber couplers (15) through optical fibers.

3. The heterodyne interferometry method based on wavefront-controlled target mirror according to claim 2, characterized in that: The output ends of the second fiber optic coupler (213) and the third fiber optic coupler (214) are connected to the electronic signal processing unit (5) through the corresponding photoelectric conversion unit (4), and the input ends of the first fiber optic collimator (21) and the second fiber optic collimator (22) are connected to the other ends of the two first fiber optic couplers (15) through optical fibers.

4. The heterodyne interferometry measurement method based on wavefront-controlled target mirror according to claim 3, characterized in that: The first polarizing beam splitter (25) has a first polarizer (23) at its incident end, a third quarter-wave plate (211) on the side of the first polarizing beam splitter (25) facing the second fiber coupler (213), a third polarizer (29) between the third quarter-wave plate (211) and the first polarizing beam splitter (25), a second polarizer (24) at its incident end, a fourth quarter-wave plate (212) on the side of the second polarizing beam splitter (26) facing the third fiber coupler (214), and a fourth polarizer (210) between the fourth quarter-wave plate (212) and the second polarizing beam splitter (26).

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