Heterodyne precision displacement measurement system based on self-tracing grating

Abstract

The invention discloses a heterodyne precision displacement measurement system based on a self-tracing grating, and belongs to the technical field of precision displacement measurement. Comprising a single-frequency laser module, a frequency shift module, a heterodyne displacement measurement module, a self-tracing grating and a signal processing module. The single-frequency laser module generates single-frequency laser, and the single-frequency laser passes through the frequency shift module to obtain two linearly polarized lights with different frequencies and mutually orthogonal polarization states to form a heterodyne light source. Heterodyne laser enters the self-tracing grating through the heterodyne displacement measurement module, diffracted light returns to the heterodyne displacement measurement module along an original light path, a measurement signal and a reference signal with displacement information are obtained, and finally the phase difference of the two signals is calculated through the signal processing module and displacement conversion is carried out. The ultra-precise displacement measurement system has high stability and resolution, displacement values are independently traced, and the ultra-precise displacement measurement system has the environmental interference resistance of a heterodyne grating interferometer and has the advantages of high measurement resolution, independent measurement results and reliable traceability.

Description

Technical Field

[0001] This invention relates to the field of precision displacement measurement technology, and in particular to a heterodyne precision displacement measurement system based on a self-traceable grating. Background Technology

[0002] Since the beginning of the 21st century, the fields of integrated circuits and semiconductor manufacturing have developed rapidly. The level of advanced precision manufacturing represents a country's equipment manufacturing and modern industrial production level. Among them, ultra-precision measurement technology is the foundation of modern advanced manufacturing. The measurement accuracy and range determine the manufacturing precision and size.

[0003] As research into the field of nanometry deepens, various precision displacement measurement methods have emerged. Among them, laser interferometers and grating interferometers, due to their high precision and fast measurement speed, are widely used in various precision manufacturing industries such as ultra-precision displacement measurement systems for lithography machines and ultra-precision CNC machine tools. Laser interferometers use wavelength as the measurement reference. In non-vacuum environments, the measurement reference wavelength changes with fluctuations in environmental factors such as temperature and pressure gradients, which affects the measurement accuracy of laser interferometers. Grating interferometers, on the other hand, use the grating pitch as the measurement reference, easily achieving single-point multi-degree-of-freedom measurement, and have a shorter optical path, offering advantages such as high displacement measurement accuracy and resistance to environmental interference. Under the same environmental control conditions, grating interferometers exhibit higher measurement stability than laser interferometers. In addition, in the detection scheme of the interferometer, the zero-difference measurement system is a DC system. When the measurement environment is unstable, there are problems such as DC offset and amplitude transformation. The heterodyne measurement system performs measurements in a higher frequency range, so it can effectively avoid low-frequency noise sources. Moreover, the displacement is only represented by the phase difference of the interference signal and is not easily affected by changes in light intensity. In comparison, the heterodyne measurement system has stronger anti-interference ability.

[0004] Despite the increasing importance of grating interferometers, two significant issues remain: the lack of direct or independent traceability of the gratings and the scribe-line errors within them. On one hand, because different samples have different grating pitches, the pitch and its deviations require calibration using metrological methods. Calibration instruments demand harsh operating environments, making calibration difficult under field or general laboratory conditions. On the other hand, from a grating manufacturing perspective, the accuracy of the grating pitch directly impacts the interferometer's measurement accuracy. Gratings typically manufactured using mechanical scribing, scanning interference lithography, or electron beam lithography are susceptible to pitch errors, wavelength drift, and nanometer positioning errors, all of which reduce the accuracy of the grating pitch. Simultaneously, the demand for high-density gratings is continuously increasing to improve the measurement resolution of grating interferometers.

[0005] To address the aforementioned issues, a novel approach is offered for a deposition-based self-tracing grating with a grating pitch strictly traced back to a natural constant. It is fabricated using atomic lithography, employing Cr atoms as the grating material, and specifically utilizing Cr atoms... 7 S3→ 7 P4 0 The transition, as the resonant wavelength, has its grating pitch traced back to the Cr atom. 7 S3→ 7 P4 0 The transition wavelength (425.55nm) is half that of the standard grating. Theoretically, the grating pitch error can reach the order of 0.001nm, and the actual sample pitch error does not exceed 0.1nm, giving the pitch value used as the measurement reference for the grating interferometer self-traceability. Furthermore, the grating's line density can reach 4700 lines / mm, meeting the requirements of high-resolution grating interferometers, and exhibiting high consistency, high uniformity, and strong resistance to environmental changes while maintaining high line density. Previously, a self-traceable grating interferometric precision displacement measurement system was proposed. This system combines a self-traceable grating interferometer with a zero-difference measurement method, achieving not only the same measurement speed and resolution as existing grating interferometers, but also overcoming the problems of non-traceability of displacement measurement results and low line density in existing grating interferometers, thus improving the reliability of precision displacement measurement. Further, we combine a self-traceable grating with a heterodyne measurement method to propose an ultra-precision displacement measurement system with good stability, high resolution, and directly traceable measurement results. Summary of the Invention

[0006] The purpose of this invention is to provide a heterodyne precision displacement measurement system based on a self-traceable grating, which combines the environmental interference resistance of a heterodyne grating interferometer with the advantages of high measurement resolution, independent measurement results, and reliable traceability.

[0007] To achieve the above objectives, this invention provides a heterodyne precision displacement measurement system based on a self-traceable grating, comprising a single-frequency laser module, a frequency shifting module, a heterodyne displacement measurement module, a self-traceable grating, and a signal processing module connected in sequence. The single-frequency laser module emits a single-frequency laser beam, which, after processing by the frequency shifting module, generates linearly polarized light with a stable frequency difference and perpendicular polarization states, forming a heterodyne source. This light then enters the heterodyne displacement measurement module, propagates to the self-traceable grating, and undergoes diffraction. The light returns to the photodetector in the heterodyne measurement module, where photoelectric conversion yields a reference signal and a measurement signal. These two signals are then connected to the signal processing module for phase calculation to obtain the displacement and direction of motion. The single-frequency laser module and the frequency shifting module are connected in free space.

[0008] Preferably, the frequency shifting module includes a beam splitter 1, which is connected to a beam reducer 1 and a beam reducer 2 respectively. The beam reducer 1 is connected to the polarizing beam splitter 1 via an acousto-optic modulator 1, a reflector 1, a reflector 3, a half-wave plate 1, and a polarizer 1. The beam reducer 2 is connected to the polarizing beam splitter 1 via an acousto-optic modulator 2, a reflector 2, a reflector 4, a half-wave plate 2, and a polarizer 2.

[0009] Preferably, the single-frequency laser is incident on the beam splitter and split into two beams of the same frequency f. The two beams are then passed through the beam reducer and the beam reducer to reduce the beam diameter, so as to match the aperture requirements of the acousto-optic modulator and the acousto-optic modulator.

[0010] The center frequencies of the first and second acoustic-optical modulators are set to f1 and f2, respectively. The two beams of light emitted after emission produce frequency shifts of f1 and f2, respectively, with the frequencies being:

[0011] f+f1

[0012] f+f2

[0013] Two beams of light with different frequencies are then adjusted by reflectors one, two, three, and four, and each beam is perpendicularly incident on half-wave plate one, half-wave plate two, polarizer one, and polarizer two. The two beams that emerge are horizontally polarized and vertically polarized, respectively. They are then incident on polarizing beam splitter one and combined to form a dual-frequency beam with mutually orthogonal polarization directions. The exiting beam consists of two perpendicularly polarized beams with a frequency difference of:

[0014] f1-f2.

[0015] Preferably, the heterodyne displacement measurement module includes a second beam splitter prism and a second polarizing beam splitter prism. The second beam splitter prism is connected to a fourth polarizer and the second polarizing beam splitter prism. The fourth polarizer prism is connected to a first photodetector. The second polarizing beam splitter prism is connected to a third polarizer, a first quarter-wave plate, and a second quarter-wave plate in three other directions. The third polarizer prism is connected to the second photodetector. The first quarter-wave plate is connected to a fifth reflector. The second quarter-wave plate is connected to a sixth reflector. Both the fifth and sixth reflectors are connected to the self-traceable grating.

[0016] Preferably, the two polarized beams of the dual-frequency light source are incident on a beam splitter prism and split into two equal-order dual-frequency beams. One beam passes through polarizer four and is received by photodetector one, which converts it into a reference signal via photoelectric conversion. The other beam is incident on polarizing beam splitter two and split into one vertically linearly polarized beam and one horizontally linearly polarized beam. Each beam is separated by quarter-wave plate one and quarter-wave plate two, transforming from linearly polarized to circularly polarized beams. These beams then pass through mirror five and mirror six and are incident at a Littrow angle on a self-tracing grating on a nanometer-displacement stage. The negative first-order diffracted beams return along the original optical path to the beat frequency interference of polarizer three and are then received by photodetector two to form a measurement signal.

[0017] The beam splitter prism two proportionally splits the incident dual-frequency laser into two paths. One of the split beams is directly incident on the polarizer at four beat frequencies to form an interference signal. This signal is then converted by photodetector one as a reference signal. A1 and D1 are the amplitude and DC bias of the reference signal, respectively. Therefore, the reference signal I... r The expression is:

[0018] I r =A1cos[2π(f1-f2)t]+D1;

[0019] The other dual-frequency laser beam is split into p-beams and s-beams by a polarizing beam splitter. The s-beam is converted from linearly polarized to circularly polarized by the first quarter-wave plate and incident on the self-tracing grating at a Littrow angle by the sixth reflector. The first-order diffracted beam returns to the first quarter-wave plate, where it is converted from circularly polarized to p-beams and transmitted through the second polarizing beam splitter. Due to the Doppler frequency shift effect of the grating, where Δf is the Doppler frequency shift, the frequency expression of the returned p-beam is:

[0020] f+f1+Δf

[0021] The other p-beam, following a symmetrical propagation path, after diffraction by a quarter-wave plate and a self-tracing grating, returns with the following frequency expression for the s-beam:

[0022] f+f2-Δf

[0023] The two diffracted beams return and undergo beat frequency interference after passing through polarizer three. This interference is received by photodetector two, forming a measurement signal. A2 and D2 represent the amplitude and DC bias of the measurement signal, respectively. The expression for the measurement signal is:

[0024] I m =A2cos[2π(f1-f2±2Δf)t]+D2

[0025] The measured signal and the reference signal are acquired by the signal processing module, and the phase difference generated by the Doppler frequency shift is calculated and converted to obtain the motion displacement information.

[0026] Preferably, the self-traceable grating is prepared by heating and sublimating a metal material to a gaseous state in a vacuum environment using atomic lithography, and then extracting a metal atomic beam in a cascading manner. The raw material for its preparation is Cr atoms.

[0027] Preferably, the single-frequency laser module is a positive and negative first-order diffraction beam capable of obtaining a self-traceable grating, and the laser wavelength needs to satisfy the condition λ < 2g = 425.6 nm.

[0028] Preferably, the heterodyne displacement measurement module adopts a Littrow optical path structure to adapt to the diffraction characteristics of the self-traceable grating, achieving optical subdivision while having a large motion tolerance.

[0029] Therefore, the heterodyne precision displacement measurement system based on a self-traceable grating, which adopts the above structure, has the following beneficial effects:

[0030] This invention uses an atomic photolithography self-traceable grating as the displacement measurement reference grating, selects Cr atoms as the grating fabrication material, and simultaneously selects Cr atoms 7 S3→ 7 P4 0 The transition, as the resonant wavelength, has its grating pitch traced back to the Cr atom. 7 S3→ 7 P4 0 The transition wavelength (425.55nm) is half of the standard wavelength. Theoretically, the grating pitch error can reach the order of 0.001nm, and the actual sample pitch error does not exceed 0.1nm, giving the pitch value used as the measurement reference for the grating interferometer a self-traceable characteristic. Furthermore, the grating's line density can reach 4700 lines / mm, meeting the requirements of high-resolution grating interferometers, and exhibiting high consistency, high uniformity, and strong resistance to environmental changes while maintaining high line density.

[0031] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the structure of a heterodyne precision displacement measurement system based on a self-traceable grating according to the present invention;

[0033] Figure 2 This is a schematic diagram of the frequency shifting module of a heterodyne precision displacement measurement system based on a self-traceable grating according to the present invention.

[0034] Figure 3 This is a schematic diagram of the heterodyne displacement measurement module of a heterodyne precision displacement measurement system based on a self-traceable grating according to the present invention.

[0035] Figure 4This is a schematic diagram illustrating the overall principle of a heterodyne precision displacement measurement system based on a self-traceable grating according to the present invention.

[0036] Figure 5 This is a schematic diagram of the measurement signal acquisition of a heterodyne precision displacement measurement system based on a self-traceable grating according to the present invention;

[0037] Figure 6 This is a schematic diagram of reference signal acquisition for a heterodyne precision displacement measurement system based on a self-traceable grating according to the present invention.

[0038] Figure 7 This is a schematic diagram of displacement calculation for a heterodyne precision displacement measurement system based on a self-traceable grating according to the present invention.

[0039] Figure Labels

[0040] 1. Single-frequency laser module; 2. Frequency shifting module; 3. Heterodyne displacement measurement module; 4. Self-traceable grating; 5. Signal processing module; 201. Beam splitter 1; 202. Beam reducer 1; 203. Beam reducer 2; 204. Acousto-optic modulator 1; 205. Acousto-optic modulator 2; 206. Mirror 1; 207. Mirror 2; 208. Mirror 3; 209. Mirror 4; 210. Half-wave plate 1; 211. Polarizer 1; 212. Half-wave plate 2; 213. Polarizer II. 214. Polarizing beam splitter 1; 301. Beam splitter 2; 302. Polarizing beam splitter 2; 303. Quarter wave plate 1; 304. Quarter wave plate 2; 305. Mirror 5; 306. Mirror 6; 307. Polarizer 3; 308. Polarizer 4; 309. Photodetector 1; 310. Photodetector 2; 311. FPGA phase calculation board; 312. Dual acousto-optic adjustment driver; 313. Plane mirror; 314. Nanoscale displacement stage. Detailed Implementation

[0041] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0042] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning understood by one of ordinary skill in the art to which this invention pertains. The terms "first," "second," and similar terms used in this invention do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Terms such as "comprising" or "including" mean that the element or object preceding the word encompasses the elements or objects listed following the word and their equivalents, without excluding other elements or objects. Terms such as "connected" or "linked" are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. Terms such as "upper," "lower," "left," and "right" are used only to indicate relative positional relationships; when the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0043] Example

[0044] like Figure 1-7 As shown, this invention provides a heterodyne precision displacement measurement system based on a self-traceable grating, comprising a single-frequency laser module 1, a frequency shifting module 2, a heterodyne displacement measurement module 3, a self-traceable grating 4, and a signal processing module 5 connected in sequence. After emitting a single-frequency laser beam from the single-frequency laser module 1, it is processed by the frequency shifting module 2 to generate linearly polarized light with a stable frequency difference and perpendicular polarization states, forming a heterodyne source. This light then enters the heterodyne displacement measurement module 3, propagates to the self-traceable grating 4, and undergoes diffraction. The light returns to the photodetector in the heterodyne measurement module, where it is converted into a reference signal and a measurement signal. These two signals are then connected to the signal processing module 5 for phase calculation to obtain the displacement and direction of motion. The single-frequency laser module and the frequency shifting module are connected in free space.

[0045] The frequency shifting module 2 includes a beam splitter 201, which is connected to a beam reducer 202 and a beam reducer 203. The beam reducer 202 is connected to a polarizing beam splitter 214 via an acousto-optic modulator 204, a reflector 206, a reflector 208, a half-wave plate 210, and a polarizer 211. The beam reducer 203 is connected to a polarizing beam splitter 214 via an acousto-optic modulator 205, a reflector 207, a reflector 209, a half-wave plate 212, and a polarizer 213.

[0046] A single-frequency laser is incident on a beam splitter and splits into two beams of the same frequency f. The two beams are then passed through a beam reducer 1 202 and a beam reducer 203 to reduce the beam diameter in order to match the aperture requirements of the acousto-optic modulator 1 204 and acousto-optic modulator 205.

[0047] The center frequencies of the acoustic-optical modulator 1 (204) and the acoustic-optical modulator 2 (205) are set to f1 and f2, respectively. The two beams of light emitted after emission produce frequency shifts of f1 and f2, respectively, with the following frequencies:

[0048] f+f1

[0049] f+f2

[0050] Two beams of light with different frequencies are then adjusted by reflectors 1-206, 2-207, 3-208, and 4-209, and are perpendicularly incident on half-wave plates 1-210, 2-212, 1-211, and 2-213, respectively. The resulting two beams are horizontally polarized and vertically polarized, respectively, and are then incident on polarizing beam splitter 1-214 to be combined into dual-frequency light with mutually orthogonal polarization directions. The resulting light consists of two perpendicularly polarized linearly polarized beams with a frequency difference of:

[0051] f1-f2.

[0052] The heterodyne displacement measurement module 3 includes a beam splitter prism 2 301 and a polarizing beam splitter prism 2 302. The beam splitter prism 2 301 is connected to the polarizer 4 308 and the polarizing beam splitter prism 2 302 respectively. The polarizer 4 308 is connected to the photodetector 1 309. The polarizing beam splitter prism 2 302 is connected to the polarizer 3 307, the quarter-wave plate 1 303 and the quarter-wave plate 2 304 in the other three directions respectively. The polarizer 3 307 is connected to the photodetector 2 310. The quarter-wave plate 1 303 is connected to the reflector 5 305. The quarter-wave plate 2 304 is connected to the reflector 6 306. The reflectors 5 305 and 6 306 are both connected to the self-traceable grating 4.

[0053] Two polarized beams from a dual-frequency light source are incident on a beam splitter prism and split into two equal-order dual-frequency beams. One beam passes through polarizer 4 (308) and is received by photodetector 1 (309), which converts the light into a reference signal. The other beam is incident on polarizing beam splitter 2 (302) and split into two beams: one vertically linearly polarized and the other horizontally linearly polarized. Each beam is then separated by quarter-wave plate 1 (303) and quarter-wave plate 2 (304), transforming the linearly polarized beams into circularly polarized beams. These beams then pass through mirror 5 (305) and mirror 6 (306) and are incident at a Littrow angle on a self-tracing grating 4 on a nanometer-displacement stage. The negative first-order diffracted beams return along the original optical path to polarizer 3 (307) for beat frequency interference, and are then received by photodetector 2 (310) to form a measurement signal.

[0054] Beam splitter prism 2 (301) splits the incident dual-frequency laser into two proportional paths. One path, after splitting, is directly incident on polarizer 4 (308) to form an interference signal through beat frequency. This signal is then converted by photodetector 1 (309) to serve as a reference signal. A1 and D1 represent the amplitude and DC bias of the reference signal, respectively. Therefore, the reference signal I... r The expression is:

[0055] I r =A1cos[2π(f1-f2)t]+D1;

[0056] Another dual-frequency laser beam is split into p-beams and s-beams by a polarizing beam splitter. The s-beam is converted from linearly polarized to circularly polarized by a quarter-wave plate 303, and then incident on the self-tracing grating 4 at a Littrow angle by a mirror 306. The first-order diffracted beam returns to the quarter-wave plate 303, where it is converted from circularly polarized to p-beams and transmitted through a polarizing beam splitter 301. Due to the Doppler frequency shift effect of the grating, where Δf is the Doppler frequency shift, the frequency expression of the returned p-beam is:

[0057] f+f1+Δf

[0058] The other p-beam, following a symmetrical propagation path, after diffraction by the quarter-wave plate 304 and the self-tracing grating 4, returns with the s-beam frequency expression as follows:

[0059] f+f2-Δf

[0060] The two diffracted beams, after passing through polarizer 3213, undergo beat frequency interference and are received by photodetector 2310 to form a measurement signal. A2 and D2 represent the amplitude and DC bias of the measurement signal, respectively. The expression for the measurement signal is:

[0061] I m =A2cos[2π(f1-f2±2Δf)t]+D2

[0062] After the measurement signal and the reference signal are acquired by the signal processing module, the phase difference caused by the Doppler frequency shift is calculated and converted to obtain the motion displacement information.

[0063] The self-traceable grating 4 is prepared by heating and sublimating a metal material to a gaseous state in a vacuum environment using atomic lithography, and then extracting a metal atomic beam in a cascading manner. The raw material for its preparation is Cr atoms.

[0064] The single-frequency laser module 1 is capable of obtaining positive and negative first-order diffraction beams from the self-traceable grating 4, and the laser wavelength needs to satisfy the condition λ < 2g = 425.6nm.

[0065] To adapt to the diffraction characteristics of the self-traceable grating, the heterodyne displacement measurement module 3 adopts a Littrow optical path structure, achieving optical subdivision while maintaining a large motion tolerance.

[0066] Working principle: The 405nm wavelength single-frequency laser 1 has a laser wavelength λ < 2g = 425.6nm. This invention preferably uses a 405nm wavelength semiconductor laser. This type of laser not only has rich optical properties but is also easier to operate and debug in the visible light range.

[0067] The first beam reducer 202 and the second beam reducer 203 are each composed of a pair of plano-convex lenses with a focal length of 20mm and a focal length of 40mm and a wavelength of 405nm.

[0068] The adjustable frequency acoustic-optic modulator 1 204 and adjustable frequency acoustic-optic modulator 205 are acoustic-optic modulators with a frequency band containing a wavelength of 405nm and center frequencies of 130MHz and 135MHz respectively. They adjust the output of the dual acoustic-optic modulation driver 312 to sine signals of 130MHz and 135MHz respectively, and are connected through an SMA radio frequency line.

[0069] The self-traceable grating 4 is prepared by heating and sublimating a metal material to a gaseous state in a vacuum environment using atomic lithography and then extracting a metal atom beam in a cascading manner. Its grating pitch is 212.8 nm and can be traced back to the transition frequency of chromium atoms.

[0070] The nanostage 314 has a range of 100 micrometers, X / Y motion axes, closed-loop resolution of 0.1 nm, and linearity of 0.03%.

[0071] The FPGA phase calculation board 311 uses a Xilinx-XKU60 processor and a dual-channel AD9646 acquisition module, and is connected to photodetector 309 and photodetector 310 via an SMA RF cable.

[0072] A 405nm wavelength single-frequency laser 1 emits a single-frequency laser beam, which is then split into two beams of the same frequency, denoted as f1 and f2, by a beam splitter prism 201. The two beams are then reduced to a diameter of 2mm by beam reducers 202 and 203, respectively. A dual acousto-optic modulation driver 312 outputs driving frequencies f1 and f2 to tunable acousto-optic modulators 204 and 205. The frequencies of the two emitted beams are as follows:

[0073] f+f1

[0074] f+f2

[0075] Adjust mirrors 206, 207, 208, and 209 so that the two frequencies of light are incident perpendicularly onto half-wave plates 210 and 212, polarizer 211 and polarizer 213, respectively. Adjust the incident axes of the two pairs of wave plates and polarizer groups to be perpendicular, so that the two beams are reflected and transmitted through polarizing beam splitter 301, respectively. This results in the two beams exiting as linearly polarized light with a frequency difference of f1-f2, perpendicular to each other, serving as heterodyne sources.

[0076] The emitted dual-frequency light passes through plane mirror 313 and is incident on beam splitter prism 301, where it is proportionally split into two beams. One beam is reflected and directly interfered with by polarizer 307, then received by photodetector 309. After photoelectric conversion, it serves as reference signal I. r The reference signal is transmitted to port a of the FPGA phase calculation board 311.

[0077] Another dual-frequency beam is incident on polarizing beam splitter prism 2 302, splitting into reflected s-beams and transmitted p-beams. These beams pass through quarter-wave plate 1 303 and quarter-wave plate 2 304 respectively, changing from linearly polarized to circularly polarized light. Reflectors 5 305 and 6 306 are adjusted so that the two beams are incident at a Littrow angle of θ = 72.10° onto a self-tracing grating placed on a nano-displacement stage 314. This drives the nano-displacement stage 314 to move, causing the incident light to interfere with the Doppler effect. The first-order diffracted beams return to quarter-wave plate 1 303 and quarter-wave plate 2 304 via their original paths. The circularly polarized light is converted to linearly polarized light, the originally transmitted p-beams become reflected s-beams, and the originally reflected s-beams become transmitted p-beams. The returning two beams carry Doppler frequencies, causing beat interference at polarizer 3 307. Finally, the signals are received by photodetector 2 310, converted into a measurement signal I after photoelectric conversion. m The measurement signal is transmitted to port b of the FPGA phase calculation board 311.

[0078] The two signals are sampled at 100MHz and converted to digital signals by a 14-bit AD9643 dual-channel acquisition module. The acquisition results of the measured signal and the reference signal are as follows: Figure 5 As shown in Figure 6. The FPGA then performs displacement calculations on the two signals, and the corresponding displacement measurement results are shown below. Figure 7 As shown.

[0079] Therefore, this invention employs a heterodyne precision displacement measurement system based on a self-traceable grating, as described above. A single-frequency laser module generates a single-frequency laser, which, after passing through a frequency-shifting module, yields two linearly polarized lights with different frequencies and mutually orthogonal polarization states, forming a heterodyne source. The heterodyne laser, after passing through the heterodyne displacement measurement module, is incident on the self-traceable grating. The diffracted light returns to the heterodyne displacement measurement module along the original optical path, resulting in a measurement signal and a reference signal containing displacement information. Finally, the phase difference between the two signals is calculated by the signal processing module, and displacement conversion is performed. This achieves an ultra-precision displacement measurement system with high stability and resolution, and independent traceability of displacement values. It also possesses the environmental interference resistance of a heterodyne grating interferometer, offering advantages such as high measurement resolution and independent and reliable traceability of measurement results.

[0080] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A heterodyne precision displacement measurement system based on a self-traceable grating, characterized in that: The invention comprises a single-frequency laser module, a frequency shift module, a heterodyne displacement measurement module, a self-traceable grating and a signal processing module which are connected in sequence. The single-frequency laser module generates laser light, which passes through the frequency shift module to obtain a heterodyne light source. The heterodyne light source is incident on the heterodyne displacement measurement module, and returns to the heterodyne displacement measurement module after being diffracted by the self-traceable grating to obtain a displacement resolution electrical signal. The signal processing module resolves the phase to obtain the motion displacement of the self-traceable grating.

2. The heterodyne precision displacement measurement system based on self-traceable grating according to claim 1, characterized in that: The frequency shift module includes a beam splitter prism 1, which is connected to a beam reducer 1 and a beam reducer 2 respectively. The beam reducer 1 is connected to a polarization beam splitter prism 1 through an acousto-optic adjuster 1, a reflector 1, a reflector 3, a half-wave plate 1 and a polarizer 1. The beam reducer 2 is connected to a polarization beam splitter prism 1 through an acousto-optic adjuster 2, a reflector 2, a reflector 4, a half-wave plate 2 and a polarizer 2.

3. The heterodyne precision displacement measurement system based on self-traceable grating according to claim 2, characterized in that: The single-frequency laser is incident on the first beam splitter prism and is equally divided into two beams of the same frequency light with the frequency f. The two beams of light are respectively reduced in diameter by the first beam reducer and the second beam reducer to match the aperture requirements of the first acousto-optic adjuster and the second acousto-optic adjuster. The center frequencies of the A / O adjuster 1 and A / O adjuster 2 are set to f1 and f2 respectively. The two light beams after emission produce frequency shifts of f1 and f2 respectively, and their frequencies are: f+f1 f+f2 The two beams of light with different frequencies are then adjusted by reflector 1, reflector 2, reflector 3, and reflector 4, and are vertically incident on half-wave plate 1, half-wave plate 2, polarizer 1, and polarizer 2. The two beams of light after exiting are horizontally polarized light and vertically polarized light, respectively. Then, they are respectively incident on polarization beam splitter 1 to combine into dual-frequency light with mutually orthogonal polarization directions. The light after exiting is two beams of vertical linear polarized light, and the frequency difference is: f1-f2.

4. The heterodyne precision displacement measurement system based on self-traceable grating according to claim 1, characterized in that: The heterodyne displacement measurement module includes a second beam splitter prism and a second polarization beam splitter prism. The second beam splitter prism is connected to a fourth polarizer and the second polarization beam splitter prism respectively. The fourth polarizer is connected to a first photodetector. The other three directions of the second polarization beam splitter prism are respectively connected to a third polarizer, a quarter wave plate and a quarter wave plate. The third polarizer is connected to a second photodetector. The quarter wave plate is connected to a fifth reflector. The quarter wave plate is connected to a sixth reflector. The fifth reflector and the sixth reflector are both connected to the self-traceable grating. The first photodetector and the second photodetector are both connected to an FPGA phase solver board. The first acousto-optic adjuster and the second acousto-optic adjuster are both connected to a dual acousto-optic adjustment driver.

5. The heterodyne precision displacement measurement system based on self-traceable grating according to claim 4, characterized in that: The two polarized beams of the dual-frequency light source are incident on the second beam splitter prism and divided into two dual-frequency beams in equal proportion. One beam passes through the fourth polarizer and is received by the first photodetector, and is used as a reference signal after photoelectric conversion. The other dual-frequency beam is incident on the second polarization beam splitter prism and is divided into one vertical linear polarized light and one horizontal linear polarized light. Each of them is converted from linear polarized light to circular polarized light by the first and second quarter wave plates, and then passes through the fifth and sixth reflectors and is incident on the self-traceable grating on the nano-displacement stage at the Littrow angle. Each negative first-order diffracted light returns along the original optical path to the polarizer for three-beat frequency interference, and is then received by the second photodetector to form a measurement signal.

6. The heterodyne precision displacement measurement system based on self-traceable grating according to claim 1, characterized in that: The self-traceable grating is prepared by heating and sublimating a metal material to a gas state in a vacuum environment through atomic lithography technology, and then drawing out a metal atom beam in an effusion manner, and the preparation raw material is Cr atoms.

7. The heterodyne precision displacement measurement system based on self-traceable grating according to claim 1, characterized in that: The single-frequency laser module is capable of obtaining positive and negative first-order diffraction beams from a traceable grating, and the laser wavelength needs to satisfy the condition of λ<2g=425.6nm.

8. The heterodyne precision displacement measurement system based on self-traceable grating according to claim 1, characterized in that: The heterodyne displacement measurement module is adapted to the diffraction characteristics of the self-traceable grating and adopts a Littrow optical path structure to achieve optical binary subdivision while having a large motion tolerance.

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