A heterodyne precision displacement measurement system based on self-traceable grating

Abstract

The present invention discloses a heterodyne precision displacement measurement system based on a self-traceable grating, and belongs to the technical field of precision displacement measurement. It includes a single-frequency laser module, a frequency shift module, a heterodyne displacement measurement module, a self-traceable grating, and a signal processing module. The single-frequency laser module generates a single-frequency laser, and after passing through the frequency shift module, two linearly polarized lights with different frequencies and mutually orthogonal polarization states are obtained to form a heterodyne light source. The heterodyne laser is incident on the self-traceable grating through the heterodyne displacement measurement module, and the diffracted light returns to the heterodyne displacement measurement module along the original optical path to obtain a measurement signal and a reference signal with displacement information. Finally, the phase difference of the two signals is solved through the signal processing module and the displacement conversion is performed. An ultra-precision displacement measurement system with high stability and resolution and independent traceability of displacement values is realized, which has the anti-environmental interference capability of a heterodyne grating interferometer, and has the advantages of high measurement resolution, independent measurement results, and reliable traceability.

Description

Technical Field

[0001] The present invention relates to the technical field of precision displacement measurement, and in particular to a heterodyne precision displacement measurement system based on a self-traceable grating. Background Art

[0002] Since the beginning of the 21st century, fields such as integrated circuits and semiconductor manufacturing have developed rapidly. The level of high-end 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. Measurement accuracy and range determine the accuracy and size of manufacturing.

[0003] As people's exploration of the field of nano-measurement continues to deepen, various precision displacement measurement methods have been derived. Among them, laser interferometers and grating interferometers are widely used in various precision manufacturing industries such as ultra-precision displacement measurement systems of lithography machines and ultra-precision CNC machine tools due to their high precision and fast measurement speed. Laser interferometers use wavelength as the measurement reference. In a non-vacuum environment, the measurement reference wavelength changes with the fluctuation of environmental factors such as temperature and pressure gradient, which will affect the measurement accuracy of the laser interferometer. The grating interferometer uses the grating pitch as the measurement reference, which is easy to achieve single-point multi-degree-of-freedom measurement, and has a short optical path. It has the advantages of high displacement measurement accuracy and resistance to environmental interference. Under the same environmental control indicators, grating interferometers have higher measurement stability than laser interferometers. In addition, in the interferometer detection scheme, the zero-difference measurement system is a DC system. When the measurement environment is unstable, there are problems such as DC offset and amplitude conversion. The heterodyne measurement system performs measurements within a higher frequency range, so it can effectively avoid low-frequency noise sources. 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 capabilities.

[0004] Despite the growing importance of grating interferometers, the lack of direct or independent traceability of gratings and the line errors in the gratings are two important issues. On the one hand, due to the different grating pitches of different samples, the grating pitch and its deviation need to be calibrated by metrological methods. Calibration-type measuring instruments require a strict working environment and are difficult to calibrate on site or under general laboratory conditions. On the other hand, from the perspective of grating manufacturing, the accuracy of the grating pitch directly affects the measurement accuracy of the interferometer. Generally, for gratings manufactured by mechanical ruling, scanning interferometer lithography or electron beam lithography, line errors, wavelength drift and nanopositioning errors will reduce the accuracy of the grating pitch. At the same time, in order to improve the measurement resolution of grating interferometers, the demand for high-density gratings is also increasing.

[0005] In response to the above problems, a deposition-type self-traceable grating with a grating pitch strictly traced back to the constant in nature provides a new idea. It is prepared by atomic lithography, using Cr atoms as the grating preparation material, and selecting Cr atoms 7 S 3 → 7 P 4 0 The transition is used as the resonant wavelength, and its grating pitch is traced back to the Cr atom 7 S 3 → 7 P 4 0 Half of the transition wavelength (425.55nm). Theoretically, the grating pitch error can reach the order of 0.001nm, and the actual sample pitch error does not exceed 0.1nm, which makes the pitch value used as the measurement benchmark of the grating interferometer have self-traceability. In addition, the line density of the self-traceable grating can reach 4700 lines / mm, which meets the requirements of high-resolution grating interferometers. It has the characteristics of high consistency, high uniformity, and strong resistance to environmental changes under the condition of maintaining high line density. Prior to this, a self-traceable grating interferometer precision displacement measurement system has been proposed. This system combines the self-traceable grating interferometer with the zero-difference measurement method, and achieves not only the same measurement speed and resolution as the existing grating interferometer, but also overcomes the problem that the displacement measurement results in the existing grating interferometer cannot be traced and the line density is low, thereby improving the credibility of the precision displacement measurement. Further, we combine the self-traceable grating with the heterodyne measurement method to propose an ultra-precision displacement measurement system with good stability and high resolution, and the measurement results can be directly traced. Summary of the invention

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

[0007] To achieve the above-mentioned purpose, the present invention provides a heterodyne precision displacement measurement system based on a self-traceable grating, comprising a single-frequency laser module, a frequency shift module, a heterodyne displacement measurement module, a self-traceable grating and a signal processing module connected in sequence, wherein the single-frequency laser module emits a single-frequency laser, and after being processed by the frequency shift module, generates two linearly polarized lights with stable frequency difference and perpendicular polarization states, forming a heterodyne light source, which is then incident on the heterodyne displacement measurement module and propagates to the self-traceable grating to generate diffraction, and returns to the photoelectric detector in the heterodyne measurement module, and after photoelectric conversion, a reference signal and a measurement signal are obtained, and the two signals are connected to the signal processing module for phase resolution to obtain the motion displacement and motion direction. The single-frequency laser module and the frequency shift module are connected in free space.

[0008] Preferably, the frequency shifting module includes a beam splitter prism 1, which is respectively connected to a beam reducer 1 and a beam reducer 2, and 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, and the beam splitter 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.

[0009] Preferably, a single-frequency laser is incident on a first beam splitter prism and is equally divided into two beams of light of the same frequency, both of which have a frequency of f. The two beams of light are respectively reduced in diameter by a first beam reducer and a second beam reducer to match the aperture requirements of the first acousto-optic adjuster and the second acousto-optic adjuster.

[0010] Set the center frequencies of the A / O adjuster 1 and A / O adjuster 2 to be f 1 and f 2 The two beams of light after emission generate f 1 and f 2 The frequency shift is:

[0011] f+f 1

[0012] f+f 2

[0013] 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:

[0014] f 1 -f 2 .

[0015] Preferably, the heterodyne displacement measurement module includes a second beam splitter prism and a second polarization beam splitter prism, the second beam splitter prism is respectively connected to a fourth polarizer and the second polarization beam splitter prism, 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, and the fifth reflector and the sixth reflector are both connected to the self-traceable grating.

[0016] Preferably, two polarized beams of the dual-frequency light source are incident on a beam splitter prism and divided into two dual-frequency lights in equal proportion, one of which passes through a polarizer plate 4 and is received by a photodetector 1, and is used as a reference signal after photoelectric conversion. The other dual-frequency light is incident on a polarization beam splitter prism 2 and is divided into a vertical linear polarized light and a horizontal linear polarized light, each of which is converted from a linearly polarized light to a circularly polarized light by a quarter wave plate 1 and a quarter wave plate 2, and then passes through a reflector 5 and a reflector 6, and is incident on the self-traceable grating on the nano-displacement stage at a Littrow angle, and each negative first-order diffracted light returns along the original optical path to the polarizer for three-way frequency interference, and is then received by a photodetector 2 to form a measurement signal;

[0017] The beam splitter prism 2 divides the incident dual-frequency laser into two paths in equal proportion. After the beam splitting, one path is directly incident on the polarizer 4 to form an interference signal, which is converted into a reference signal by the photoelectric detector 1. 1 , D 1 is the amplitude and DC bias of the reference signal, then the reference signal I r The expression is:

[0018] I r =A 1 cos[2π(f 1 -f 2 )t]+D 1 ;

[0019] The other dual-frequency laser is split into p light and s light by a polarization beam splitter prism. The s light is transformed from linearly polarized light to circularly polarized light by the first quarter wave plate 1, and is incident on the self-traceable grating at a Littrow angle by the reflector 6. The first-order diffracted light after diffraction returns to the quarter wave plate 1 along the original optical path, and is transformed from circularly polarized light to p light and transmits through the polarization beam splitter prism 2. Due to the Doppler frequency shift effect of the grating, Δf is the Doppler frequency shift, and the frequency expression of the returned p light is:

[0020] f+f 1 +Δf

[0021] The other path of p light follows the symmetrical propagation path, and after diffraction by the 1 / 4 wave plate 2 and the self-traceable grating, the frequency expression of the returned s light is:

[0022] f+f 2 -Δf

[0023] The two beams of diffracted light return after passing through polarizer 3 and then beat frequency interference, and are received by photodetector 2 to form a measurement signal, A 2 , D 2 To measure the amplitude and DC offset of the signal, the expression of the measured signal is:

[0024] I m =A 2 cos[2π(f1 -f 2 ±2Δf)t]+D 2

[0025] The measurement signal and the reference signal are collected by the signal processing module to calculate the phase difference caused by the Doppler frequency shift and convert it into motion displacement information.

[0026] Preferably, 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.

[0027] Preferably, 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.

[0028] Preferably, 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 larger motion tolerance.

[0029] Therefore, the heterodyne precision displacement measurement system based on the self-traceable grating adopting the above structure in the present invention has the following beneficial effects:

[0030] The present invention adopts atomic photolithography self-traceable grating as displacement measurement reference grating, selects Cr atoms as grating preparation material, and selects Cr atoms as grating preparation material. 7 S 3 → 7 P 4 0 The transition is used as the resonant wavelength, and its grating pitch is traced back to the Cr atom 7 S 3 → 7 P 4 0 Half of the transition wavelength (425.55nm). Theoretically, the grating pitch error can reach 0.001nm, and the actual sample pitch error does not exceed 0.1nm, making the pitch value used as the measurement benchmark of the grating interferometer self-traceable. In addition, the line density of the self-traceable grating can reach 4700 lines / mm, meeting the requirements of high-resolution grating interferometers, and has the characteristics of high consistency, high uniformity, and strong resistance to environmental changes while maintaining high line density.

[0031] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0033] Figure 2 Schematic diagram of the frequency shift module principle of a heterodyne precision displacement measurement system based on a self-tracing grating according to the present invention;

[0034] Figure 3 Schematic diagram of the heterodyne displacement measurement module principle of a heterodyne precision displacement measurement system based on a self-tracing grating according to the present invention;

[0035] Figure 4 Schematic diagram of the overall example principle of a heterodyne precision displacement measurement system based on a self-tracing grating according to the present invention;

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

[0037] Figure 6 Schematic diagram of the reference signal acquisition of a heterodyne precision displacement measurement system based on a self-tracing grating according to the present invention;

[0038] Figure 7 Schematic diagram of the displacement calculation of a heterodyne precision displacement measurement system based on a self-tracing grating according to the present invention;

[0039] Reference numerals

[0040] 1. Single-frequency laser module, 2. Frequency shift module, 3. Heterodyne displacement measurement module, 4. Self-tracing grating, 5. Signal processing module, 201. Beam splitter prism 1, 202. Beam expander 1, 203. Beam expander 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 2, 214. Polarizing beam splitter prism 1, 301. Beam splitter prism 2, 302. Polarizing beam splitter prism 2, 303. Quarter-wave plate 1, 304. Quarter-wave plate 2, 305. Mirror 5, 306. Mirror 6, 307. Polarizer 3, 308. Polarizer 4, 309. Photoelectric detector 1, 310. Photoelectric detector 2, 311. FPGA phase calculation board, 312. Dual acousto-optic modulator driver, 313. Plane mirror, 314. Nanometer displacement stage. Specific embodiments

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

[0042] Unless otherwise defined, the technical terms or scientific terms used in the present invention should be understood by people with ordinary skills in the field to which the present invention belongs. The words "first", "second" and similar words used in the present invention do not indicate any order, quantity or importance, but are only used to distinguish different components. "Include" or "comprise" and similar words mean that the elements or objects appearing before the word include the elements or objects listed after the word and their equivalents, without excluding other elements or objects. "Connect" or "connected" and similar words are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. "Up", "down", "left", "right" and the like are only used 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, the present invention provides a heterodyne precision displacement measurement system based on self-traceable grating, comprising a single-frequency laser module 1, a frequency shift module 2, a heterodyne displacement measurement module 3, a self-traceable grating 4 and a signal processing module 5 connected in sequence. After the single-frequency laser module 1 emits a single-frequency laser, it is processed by the frequency shift module 2 to generate two linear polarized lights with stable frequency difference and perpendicular polarization states, forming a heterodyne light source, which is then incident on the heterodyne displacement measurement module 3 and propagates to the self-traceable grating 4 to generate diffraction, and returns to the photoelectric detector in the heterodyne measurement module. After photoelectric conversion, a reference signal and a measurement signal are obtained. The two signals are connected to the signal processing module 5 for phase resolution to obtain the motion displacement and motion direction. The single-frequency laser module and the frequency shift module are connected in free space.

[0045] The frequency shift module 2 includes a beam splitter prism 201, which is connected to a beam reducer 202 and a beam reducer 203 respectively. The beam reducer 202 is connected to a polarization beam splitter prism 214 via an acousto-optic adjuster 204, a reflector 206, a reflector 3 208, a half-wave plate 210 and a polarizer 211. The beam splitter 203 is connected to a polarization beam splitter prism 214 via an acousto-optic adjuster 205, a reflector 207, a reflector 4 209, a half-wave plate 212 and a polarizer 213.

[0046] 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 passed through the first beam reducer 202 and the second beam reducer 203 to reduce the beam diameter to match the aperture requirements of the first acousto-optic adjuster 204 and the second acousto-optic adjuster 205.

[0047] The center frequencies of the acousto-optic adjuster 1 204 and the acousto-optic adjuster 2 205 are set to be f 1 and f 2The two beams of light after emission generate f 1 and f 2 The frequency shift is:

[0048] f+f 1

[0049] f+f 2

[0050] The two beams of light with different frequencies are then adjusted by the reflector 1 206, the reflector 2 207, the reflector 3 208, and the reflector 4 209, and are vertically incident on the half-wave plate 1 210, the half-wave plate 212, the polarizer 1 211, and the polarizer 2 213. The two beams of light after being emitted are horizontally polarized light and vertically polarized light, respectively, and then are respectively incident on the polarization beam splitter 1 214 to be combined into dual-frequency light with mutually orthogonal polarization directions. The emitted light is two beams of vertical linear polarized light, and the frequency difference is:

[0051] f 1 -f 2 .

[0052] The heterodyne displacement measurement module 3 includes a second beam splitter prism 301 and a second polarization beam splitter prism 302. The second beam splitter prism 301 is connected to a fourth polarizer 308 and a second polarization beam splitter prism 302 respectively. The fourth polarizer 308 is connected to a first photodetector 309. The other three directions of the second polarization beam splitter prism 302 are connected to a third polarizer 307, a quarter wave plate 303 and a quarter wave plate 304 respectively. The third polarizer 307 is connected to a second photodetector 310. The quarter wave plate 303 is connected to a fifth reflector 305. The quarter wave plate 304 is connected to a sixth reflector 306. Both the fifth reflector 305 and the sixth reflector 306 are connected to a self-traceable grating 4.

[0053] The two polarized beams of the dual-frequency light source are incident on the beam splitter prism and are divided into two dual-frequency lights in equal proportion. One of the beams passes through the fourth polarizer 308 and is received by the first photodetector 309, and is used as a reference signal after photoelectric conversion. The other dual-frequency light is incident on the second polarization beam splitter prism 302 and is divided into one vertical linear polarized light and one horizontal linear polarized light. Each of the two is converted from linear polarized light to circular polarized light by the first 1 / 4 wave plate 303 and the second 1 / 4 wave plate 304, and then passes through the fifth reflector 305 and the sixth reflector 306, and is incident on the self-traceable grating 4 on the nano-displacement stage at the Littrow angle. Each negative first-order diffracted light returns to the third polarizer 307 along the original optical path to beat frequency interference, and is then received by the second photodetector 310 to form a measurement signal;

[0054] The second beam splitter 301 divides the incident dual-frequency laser into two paths in equal proportion. After the beam splitting, one path is directly incident on the fourth polarizer 308 to form an interference signal, which is converted into a reference signal by the photoelectric detector 309. 1 , D1 is the amplitude and DC bias of the reference signal, then the reference signal I r The expression is:

[0055] I r =A 1 cos[2π(f 1 -f 2 )t]+D 1 ;

[0056] Another dual-frequency laser beam is split into p-light and s-light by a polarization beam splitter prism. The s-light is transformed from linearly polarized light to circularly polarized light by a quarter wave plate 303 and incident on the self-traceable grating 4 at a Littrow angle by a reflector 6 306. The first-order diffracted light after diffraction returns to the quarter wave plate 303 along its original optical path and transforms from circularly polarized light to p-light and transmits out of the polarization beam splitter prism 2 301. Due to the Doppler frequency shift effect of the grating, Δf is the Doppler frequency shift, and the frequency expression of the returned p-light is:

[0057] f+f 1 +Δf

[0058] The other p light follows the symmetrical propagation path, and after diffracting through the 1 / 4 wave plate 2 304 and the self-traceable grating 4, the frequency expression of the returned s light is:

[0059] f+f 2 -Δf

[0060] The two beams of diffracted light return through the polarizer 3 213 and then interfere with each other. They are received by the photodetector 2 310 to form a measurement signal, A 2 , D 2 To measure the amplitude and DC offset of the signal, the expression of the measured signal is:

[0061] I m =A 2 cos[2π(f 1 -f 2 ±2Δf)t]+D 2

[0062] The measurement signal and the reference signal are collected by the signal processing module to calculate the phase difference caused by the Doppler frequency shift and convert it into motion displacement information.

[0063] The self-traceable grating 4 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. The raw material for the preparation is Cr atoms.

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

[0065] The heterodyne displacement measurement module 3 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.

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

[0067] The beam reducer 1 202 and the beam reducer 203 are each composed of a pair of plano-convex lenses with a focal length of 20 mm and a focal length of 40 mm and a wavelength of 405 nm.

[0068] The adjustable frequency acousto-optic adjuster 1 204 and the adjustable frequency acousto-optic adjuster 205 are acousto-optic adjusters with a frequency band including a wavelength of 405nm and a center frequency of 130Mhz and 135Mhz respectively. The outputs of the dual acousto-optic modulation driver 312 are adjusted to sinusoidal signals of 130Mhz and 135Mhz respectively, and are connected via an SMA radio frequency cable.

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

[0070] The nano-displacement stage 314 has a measuring range of 100 microns, an X / Y motion axis, a closed-loop resolution of 0.1 nm, and a linearity of 0.03%.

[0071] The FPGA phase solver board 311 uses a Xilinx-XKU60 processor and a dual-channel AD9646 acquisition module, and is connected to a photodetector 1 309 and a photodetector 2 310 via an SMA radio frequency line.

[0072] 405nm wavelength single-frequency laser 1, after emitting single-frequency laser, the beam splitter 201 divides the single-frequency light into two beams of the same frequency with equal intensity, expressed as f 1 、f 2 The two beams of light are respectively reduced to 2 mm in diameter by the beam reducer 1 202 and the beam reducer 203. The dual acousto-optic modulation driver 312 outputs a driving frequency f 1 、f 2 To frequency-adjustable acousto-optic adjuster 1 204 and frequency-adjustable acousto-optic adjuster 2 205. The frequencies of the two optical paths after emission are:

[0073] f+f 1

[0074] f+f2

[0075] Then adjust the reflector 1 206, reflector 2 207, plane reflector 3 208, and plane reflector 4 209 so that the two frequency lights are respectively incident perpendicularly to the half-wave plate 1 210, half-wave plate 212, polarizer 1 211, and polarizer 2 213. Adjust the two wave plates and the incident axis of the polarizer group perpendicularly so that the two paths of light are respectively reflected and transmitted by the polarization beam splitter prism 1 301, so that the two paths of light have a frequency difference of f after being emitted. 1 -f 2 , linearly polarized lights that are perpendicular to each other are used as heterodyne light sources.

[0076] The dual-frequency light after being emitted passes through the plane reflector 313, enters the beam splitter prism 1 301 and is divided into two beams of light in equal proportion. After being reflected directly from the polarizer 3 307, it is received by the photodetector 1 309 and converted into a reference signal I after photoelectric conversion. r The reference signal is transmitted to port a of the FPGA phase solver card 311.

[0077] Another dual-frequency light is incident on the second polarization beam splitter prism 302, and is divided into reflected s light and transmitted p light, which are respectively passed through the 1 / 4 wave plate 1 303 and the 1 / 4 wave plate 2 304, and are changed from linear polarization to circular polarization. The reflector 5 305 and the reflector 6 306 are adjusted so that the two light beams are incident on the self-traceable grating placed on the nano-displacement stage 314 at a Littrow angle of θ=72.10°, and the nano-displacement stage 314 is driven to move, so that the incident light and the interference produce a Doppler effect, and the original optical path of each first-order diffracted light returns to the 1 / 4 wave plate 1 303 and the 1 / 4 wave plate 2 304. The circularly polarized light is converted into linearly polarized light, the originally transmitted p light is converted into reflected s light, and the originally reflected s light is converted into transmitted p light. The frequencies of the two returned light beams carry the Doppler frequency, and beat frequency interference occurs at the polarizer 3 307. Finally, they are received by the photodetector 2 310, and are used as the measurement signal I after photoelectric conversion. m , the measurement signal is transmitted to port b of FPGA phase solver card 311.

[0078] The two signals are converted into digital signals through the 100Mhz sampling rate and 14-bit AD9643 dual-channel acquisition module. The acquisition effect of the measurement signal and the reference signal is as follows: Figure 5 , as shown in 6. Then the FPGA performs displacement calculation on the two signals, and the corresponding displacement measurement effect is as follows: Figure 7 shown.

[0079] Therefore, the present invention adopts the above-mentioned heterodyne precision displacement measurement system based on self-traceable grating. The single-frequency laser module generates a single-frequency laser, and passes through the frequency shift module to obtain two linearly polarized lights with different frequencies and mutually orthogonal polarization states, forming a heterodyne light source. The heterodyne laser is incident on the self-traceable grating through the heterodyne displacement measurement module, and the diffracted light returns to the heterodyne displacement measurement module along the original optical path to obtain a measurement signal and a reference signal with displacement information. Finally, the phase difference of the two signals is solved by the signal processing module and the displacement conversion is performed. An ultra-precision displacement measurement system with high stability and resolution and independent traceability of displacement values ​​is realized, which has the anti-environmental interference ability of the heterodyne grating interferometer and has the advantages of high measurement resolution, independent and reliable traceability of measurement results.

[0080] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution 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 2, 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: Two beams of polarized light from the dual-frequency light source are incident on the second beam splitter prism and divided into two dual-frequency lights in equal proportion. One of the beams 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 light 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 the two beams is converted from linearly polarized light to circularly polarized light by the first and second quarter wave plates, and then passes through the fifth and sixth reflectors to be 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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