CN117704960A

CN117704960A - Quasi-common-path heterodyne grating interferometer and dead zone optical path assessment and calibration method thereof

Info

Publication number: CN117704960A
Application number: CN202311740224.1A
Authority: CN
Inventors: 王国超; 杨俊�; 李星辉; 颜树华
Original assignee: National University of Defense Technology
Current assignee: National University of Defense Technology
Priority date: 2023-12-18
Filing date: 2023-12-18
Publication date: 2024-03-15

Abstract

The invention provides a quasi-common-path heterodyne grating interferometer and a dead zone optical path assessment and calibration method thereof, which comprise a double-frequency laser generating unit, a first laser transmission optical path, a second laser transmission optical path, a first photoelectric detector and a second photoelectric detector: the dual-frequency laser generating unit is used for outputting a frequency f ₀₊ f ₁ And a first laser of frequency f ₀₊ f ₂ Is a second laser of (2); the first laser and the second laser are respectively projected to the same position of the reflection grating through a first laser transmission light path and a second laser transmission light path; on the basis of independent heterodyne laser input, a symmetrical oblique incidence structure is adopted to realize quasi-common path design of a first laser transmission optical path and a second laser transmission optical path, the advantages of quasi-common path and space separation heterodyne structure are fused, and the evaluation is carried out on the sub-nanometer levelThe effect of dead zone optical path on displacement measurement is estimated. The invention further reduces the periodic nonlinear error to be below sub-nanometer while maintaining high resolution, repeatability and long-term stability.

Description

Quasi-common-path heterodyne grating interferometer and dead zone optical path assessment and calibration method thereof

Technical Field

The invention mainly relates to the technical field of grating interferometers, in particular to a quasi-common-path heterodyne grating interferometer and dead zone optical path assessment and calibration method thereof.

Background

As science and technology have reached unprecedented levels in the last decades, the need for precision engineering has increased. Ultra-precise positioning is used as a basic branch of precise engineering, and becomes a foundation stone for various high-tech applications such as precise machinery, semiconductor, scanning probe microscope manufacturing and the like. However, current ultra-precise positioning presents significant challenges. A prominent example is a lithography machine used in semiconductor manufacturing. As Integrated Circuits (ICs) continue to break through boundaries, feature sizes shrink below 10 nanometers, and sub-nanometer precision is urgently needed for wafer scan positioning. Thus, development and enhancement of ultra-precise positioning technology are urgently required.

Currently, there are three main types of ultra-precise positioning techniques, namely capacitive sensors, laser interferometers and grating interferometers:

capacitive sensors can provide a high degree of accuracy over a limited range. However, as the measurement range increases, their applicability may decrease, especially in the context of a lithographic apparatus, because the displacement of the wafer scanner may extend to hundreds of millimeters.

Laser interferometry-based laser interferometers and grating interferometers show great potential in displacement and angle measurement. The application of laser interferometers is very widespread and the measurement results can be traced back to the wavelength of the laser source. However, due to the long optical path, its performance is greatly limited by the environment. In contrast, grating interferometers using a zoom pitch as a measurement reference are distinguished by their compactness, reduced noise level, multiple degree of freedom measurement function, absolute positioning capability, and elasticity against environmental disturbances. Pure differential interferometry is also the earliest and most popular technique, providing high sensitivity with minimal laser source preconditions and simple structure. But pure differential grating interferometers rely on tracking variations in interference intensity, which makes pure differential grating interferometers particularly susceptible to stray light interference and environmental interference.

Disclosure of Invention

Aiming at the technical problems existing in the prior art, the invention provides a quasi-common-path heterodyne grating interferometer and a dead zone optical path evaluation and calibration method thereof.

In order to achieve the above purpose, the technical scheme adopted by the invention is as follows:

in one aspect, the invention provides a quasi-common-path heterodyne grating interferometer, which comprises a dual-frequency laser generating unit, a first laser transmission light path, a second laser transmission light path, a first photoelectric detector and a second photoelectric detector:

The dual-frequency laser generating unit is used for outputting a frequency f ₀₊ f ₁ And a first laser of frequency f ₀₊ f ₂ Wherein the frequency is f ₀ The frequency f is obtained after beam splitting and frequency modulation of the laser of (2) ₀₊ f ₁ And a first laser of frequency f ₀₊ f ₂ Is a second laser of f ₁ 、f ₂ Frequency shifts of the corresponding modulations, respectively;

the first laser is divided into a first measuring beam and a first reference beam through a beam splitter of a first laser transmission light path, the second laser is divided into a second measuring beam and a second reference beam through a beam splitter of a second laser transmission light path, the first measuring beam and the second measuring beam are respectively projected onto the same position of the reflection grating through corresponding measuring light transmission light paths, and the first photoelectric detector is used for receiving reflected light from the reflection grating, namely measuring light beams; the first reference beam and the second reference beam are transmitted to the second photoelectric detector through corresponding reference light transmission light paths, and the second photoelectric detector receives the reference beam; the total optical path length of the measuring light transmission optical path corresponding to the first measuring light beam and the reference light transmission optical path corresponding to the first reference light beam is the quasi-common optical path length of the measuring light transmission optical path corresponding to the second measuring light beam and the reference light transmission optical path corresponding to the second reference light beam.

The invention can adopt the space optical structure quasi-common optical path heterodyne grating interferometer and the optical fiber structure quasi-common optical path heterodyne grating interferometer. The space light structure mainly refers to the transmission light paths of the measuring light beam and the reference light beam are space light path structures, and the optical fiber structure mainly refers to the transmission light path of the reference light beam and a part of the transmission light path of the measuring light beam are based on the optical fiber structure.

Regarding the quasi-common path heterodyne grating interferometer for spatial light structures, it is possible to have the following further design:

further, a first non-polarized beam splitter is arranged on the first laser transmission light path, the first non-polarized beam splitter divides first laser into a first measuring beam and a first reference beam, the first measuring beam is used for being projected onto a reflection grating, the first reference beam is transmitted to a third non-polarized beam splitter, a second non-polarized beam splitter is arranged on the second laser transmission light path, the second non-polarized beam splitter divides second laser into a second measuring beam and a second reference beam, the second measuring beam is used for being projected onto the reflection grating, the second reference beam is transmitted to the third non-polarized beam splitter, the third non-polarized beam splitter outputs a combined beam of the first reference beam and the second reference beam which are incident on the third non-polarized beam splitter, the optical path of the first measuring beam between the first non-polarized beam splitter and the reflection grating is quasi-common optical path of the second measuring beam between the second non-polarized beam splitter and the reflection grating, and the optical path of the first reference beam between the first non-polarized beam splitter and the third non-polarized beam splitter and the quasi-common optical path of the second reference beam between the second reference beam splitter and the second non-polarized beam splitter are quasi-common optical paths between the second non-polarized beam splitter and the third non-polarized beam.

Further, the first laser transmission optical path is further provided with a first half-wave plate and a first polarizer in sequence, and the first laser is transmitted to the first non-polarization beam splitter after passing through the first half-wave plate and the first polarizer in sequence; the second laser transmission light path is further provided with a second half-wave plate and a second polarizer in sequence, and the second laser is transmitted to the second non-polarization beam splitter after passing through the second half-wave plate and the second polarizer in sequence.

Further, an optical delay device for adjusting the optical path length is additionally arranged between the first non-polarized beam splitter and the reflection grating or/and between the second non-polarized beam splitter and the reflection grating or/and between the first reference beam and the third non-polarized beam splitter or/and between the second reference beam and the third non-polarized beam splitter.

Further, the optical delay device comprises a shell, an incident window and an emergent window are oppositely arranged on the shell, a series of lenses are arranged in the shell, the lenses comprise a first lens, a second lens, a third lens and a fourth lens, a light beam is incident to the first lens in the shell through the incident window, the first lens reflects the light beam to the second lens, the second lens reflects the light beam to the third lens, the third lens reflects the light beam to the fourth lens, the light beam reflected by the fourth lens exits through the emergent window, the first lens and the second lens are arranged on a moving platform, and the first lens and the second lens can move under the driving of the moving platform so as to change the optical path of the light beam in the shell.

Further, the first lens and the fourth lens are both high-reflection mirrors, the second lens and the third lens are plane mirrors, meanwhile, a diaphragm is arranged on a transmission light path of the first lens, light beams are incident to the first lens in the shell through the incident window, the light beams reflected by the first lens are reflected by the second lens and the third lens and then reach the fourth lens, the light beams transmitted by the first lens are transmitted to the fourth lens through the diaphragm, and the light path correction is carried out by utilizing the transmitted light transmitted by the first lens, so that the incident light beams and the emergent light beams of the optical delay device are coaxial.

Regarding the fiber structure, the quasi-common path heterodyne grating interferometer can have the following further design:

further, the first laser beam is split into two beams by the 1# optical fiber beam splitter, wherein one beam is used as a first measuring beam, and the other beam is used as a first reference beam; the second laser is split into two beams through a 2# optical fiber beam splitter, wherein one beam is used as a second measuring beam, and the other beam is used as a second reference beam;

the first reference beam and the second reference beam are respectively transmitted to the optical fiber combiner through the corresponding reference optical fibers, and are output to the second photoelectric detector through the optical fiber combiner.

Further, the first measuring light beam is output after being collimated by the first optical fiber collimator, a first half wave plate and a first polarizer are sequentially arranged on a transmission light path for transmitting the first measuring light beam, and the first measuring light is projected onto the reflection grating after passing through the first half wave plate and the first polarizer; the second measuring beam is output after being collimated by the second optical fiber collimator, a second half-wave plate and a second polarizer are sequentially arranged on a transmission light path for transmitting the second measuring beam, and the second measuring beam is projected onto the same position of the reflection grating after passing through the second half-wave plate and the second polarizer.

Further, optical fiber retarders are arranged on the reference light optical fiber transmission optical paths corresponding to the first reference light and the second reference light, the first reference light and the second reference light are respectively input to the optical fiber beam combiner after the optical paths of the first optical fiber retarders and the second optical fiber retarders on the corresponding reference light optical fiber transmission optical paths are adjusted, and the first reference light and the second reference light are output to the second photoelectric detector through the optical fiber beam combiner.

The quasi-common path heterodyne grating interferometer, whether in a fiber structure or a spatial optical structure, can have the following further design:

further, the quasi-common path heterodyne grating interferometer further comprises a phase meter, wherein the phase meter is electrically connected with the first photoelectric detector and the second photoelectric detector and is used for calculating the phase difference between output signals of the two photoelectric detectors.

Further, the quasi-common-path heterodyne grating interferometer further comprises a grating displacement resolving unit, the phase meter outputs a phase difference between output signals of the two photodetectors to the grating displacement resolving unit, and the grating displacement resolving unit resolves the grating displacement according to the phase difference between the output signals of the two photodetectors.

Further, in the quasi-common-path heterodyne grating interferometer, the dual-frequency laser generating unit comprises a laser source, a beam splitter, a first light modulator, a second light modulator, a first collimator and a second collimator; the output frequency of the laser source is f ₀ The laser beam is divided into at least two laser beams with the same polarization and electric field amplitude through a beam splitter, the two laser beams with the same polarization and electric field amplitude are respectively input into a first optical modulator and a second optical modulator, and the first optical modulator and the second optical modulator respectively carry out the input laser beamFrequency modulation, f ₁ 、f ₂ The frequency shift of the first light modulator and the second light modulator is respectively modulated, and the frequency of the laser output by the first light modulator is f ₀₊ f ₁ The frequency of the laser output by the second light modulator is f ₀₊ f ₂ The first light modulator outputs laser after being collimated by the first collimator to serve as first laser, and the second light modulator outputs laser after being collimated by the second collimator to serve as second laser.

Further, in the quasi-common-path heterodyne grating interferometer, the dual-frequency laser generating unit further comprises a feedback control module, laser output by the laser source is divided into three beams through the beam splitter, one beam is input into the feedback control module, and the feedback control module controls the laser source based on modulation transmission spectrum, so that the laser wavelength output by the laser source is stable.

On the other hand, the invention provides a dead zone optical path assessment method of a quasi-common optical path heterodyne grating interferometer, which comprises the following steps:

And carrying out frequency scanning experiments on the quasi-common-path heterodyne grating interferometer to obtain dead zone optical path of the quasi-common-path heterodyne grating interferometer by calculation, wherein the calculation method comprises the following steps:

wherein DeltaPhi _d The amount of change in phase difference due to dead zone optical path c represents the speed of light, Δf represents the laser frequency fluctuation due to wavelength, and n is the refractive index of air.

On the other hand, the invention provides a dead zone optical path calibration method of a quasi-common optical path heterodyne grating interferometer, which comprises the steps of adjusting the optical path of a first measuring beam on a corresponding measuring light transmission optical path or/and the optical path of a second measuring beam on the corresponding measuring light transmission optical path; or, adjusting the optical path of the first reference beam on the corresponding reference light transmission optical path or/and the optical path of the second reference beam on the corresponding reference light transmission optical path;

and minimizing the dead zone optical path of the quasi-common-path heterodyne grating interferometer or enabling the dead zone optical path to be lower than a set threshold value until the dead zone optical path is lower than the set threshold value, so as to obtain the quasi-common-path heterodyne grating interferometer meeting the set measurement precision after the dead zone optical path is calibrated. According to the invention, the displacement measurement error of the quasi-common-path heterodyne grating interferometer can be reduced by reducing the dead zone optical path, and the displacement measurement precision of the quasi-common-path heterodyne grating interferometer is improved.

The heterodyne grating interferometer adopts the double-frequency laser generating unit to perform heterodyne detection, and has stronger robustness and higher signal-to-noise ratio. One of the main challenges facing heterodyne grating interferometers is the separation and polarization adjustment of lasers of different frequencies, which can lead to periodic nonlinear errors. In order to achieve sub-nanometer precision, periodic nonlinear errors will become a critical issue to be overcome. In order to solve the problem, the invention provides a dual-frequency laser which stabilizes the wavelength of a laser source, outputs dual-frequency laser with wavelength stability meeting the requirement, namely two lasers with the same polarization and electric field amplitude but different frequencies, and spatially separates the dual-frequency laser beams, so that the two lasers are respectively transmitted through respective light paths, and the periodic nonlinear error can be effectively reduced. Thus, although the frequency and polarization are mixed rarely, the two laser beams are transmitted separately through the respective optical paths, and then the dead zone optical path is unavoidable. Quasi-common path heterodyne grating interferometers are susceptible to environmental interference, and interference phases deviate from the laser wavelength unstably, so that the anti-interference capability of the interference phase deviation is weakened, and phase errors are accumulated during long-term measurement. Therefore, the invention provides the method for evaluating the dead zone optical path of the alignment common-path heterodyne grating interferometer through the frequency scanning interferometry test, and the influence of the dead zone optical path on displacement measurement is evaluated on the sub-nanometer level measurement precision. The influence of laser frequency fluctuation and environmental interference on the measurement precision of the alignment common-path heterodyne grating interferometer can be further reduced through the optimization design of the laser frequency fluctuation and the dead zone optical path, and the design ensures that the proposed quasi-common-path heterodyne grating interferometer can reduce the periodic nonlinear error to be below sub-nanometer while maintaining high resolution, repeatability and long-term stability.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings that are required in the embodiments or the description of the prior art will be briefly described, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and other drawings may be obtained according to the structures shown in these drawings without inventive effort for a person skilled in the art.

FIG. 1 is a schematic diagram of a quasi-common-path heterodyne grating interferometer according to an embodiment of the present invention;

FIG. 2 is a schematic diagram of a quasi-common-path heterodyne grating interferometer according to an embodiment of the present invention;

FIG. 3 is a schematic diagram of a quasi-common-path heterodyne grating interferometer according to an embodiment of the present invention;

FIG. 4 is a schematic diagram of a quasi-common-path heterodyne grating interferometer according to an embodiment of the present invention;

FIG. 5 is a schematic diagram of an optical delay device according to an embodiment of the present invention;

FIG. 6 is a schematic diagram of a fiber-structured quasi-common-path heterodyne grating interferometer according to an embodiment of the present invention;

FIG. 7 is a schematic diagram of a fiber-structured quasi-common-path heterodyne grating interferometer according to an embodiment of the present invention;

Legend description:

100. a laser; 1. a laser source; 2. a power amplifier; 3. a frequency multiplier; 4. a beam splitter; 5. a first light modulator; 6. a second light modulator; 7. a signal generating unit; 8. a first collimator, 9, a second collimator; 10. a first half wave plate; 11. a first polarizer; 12. a first non-polarizing beam splitter; 13. a reflection grating; 14. a second half-wave plate; 15. a second polarizer; 16. a second non-polarizing beam splitter; 17. a third non-polarizing beam splitter; 18. a second photodetector; 19. a first photodetector; 20. a phase meter; 21. a computer; 22. a feedback control module; 23. a first optical delay device; 24. a second optical delay device; 25. a 1# fiber optic splitter; 26. a 1# fiber optic splitter; 27. a first optical fiber retarder; 28. a second fiber optic retarder; 29. an optical fiber combiner;

2301. an entrance window; 2302. a first lens; 2303. a second lens; 2304. a third lens; 2305. a fourth lens; 2306. a diaphragm; 2307. an exit window.

Detailed Description

The following description of the embodiments of the present invention will be made clearly and fully with reference to the accompanying drawings, in which it is evident that the embodiments described are only some, but not all embodiments of the invention. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.

An embodiment provides a quasi-common-path heterodyne grating interferometer, which comprises a dual-frequency laser generating unit, a first laser transmission light path, a second laser transmission light path, a first photoelectric detector and a second photoelectric detector:

the dual-frequency laser generating unit is used for outputting a frequency f ₁ And a first laser of frequency f ₂ Is a second laser of (2);

In one embodiment, a quasi-common-path heterodyne grating interferometer is provided, which adopts a symmetrical oblique incidence structure to realize quasi-common-path design based on independent heterodyne laser input, and combines the advantages of quasi-common-path and spatially separated heterodyne structures. Referring to fig. 1, an embodiment provides a spatially structured quasi-common-path heterodyne grating interferometer including a dual frequency laser generating unit, a first laser transmission optical path, a second laser transmission optical path, a first photodetector 19, a second photodetector 18, and so on.

The dual-frequency laser generating unit is used for outputting a frequency f ₀₊ f ₁ And a first laser of frequency f ₀₊ f ₂ Wherein the frequency is f ₀ The frequency f is obtained after beam splitting and frequency modulation of the laser of (2) ₀₊ f ₁ And a first laser of frequency f ₀₊ f ₂ Is a second laser of f ₁ 、f ₂ Respectively the frequency shifts of the corresponding modulations.

The dual-frequency laser generating unit comprises a laser 100, wherein the output frequency of the laser 100 is f ₀ At least two laser beams with the same polarization and electric field amplitude are split by the beam splitter 4, the two laser beams with the same polarization and electric field amplitude are respectively input into the first optical modulator 5 and the second optical modulator 6, the first optical modulator 5 and the second optical modulator 6 respectively carry out frequency modulation on the input laser beams under the control of the signal generating unit 7, and f ₁ 、f ₂ The frequency shift of the first optical modulator 5 and the second optical modulator 6 are respectively modulated, and the frequency of the laser output by the first optical modulator 5 is f ₀₊ f ₁ The second optical modulator 6 outputs laser light with a frequency f ₀₊ f ₂ The first light modulator 5 outputs laser light after being collimated by the first collimator 8, and outputs the laser light as first laser light, and the second light modulator 6 outputs laser light after being collimated by the second collimator 9, and outputs the laser light as second laser light. Thus, two laser beams with different frequencies, which are spatially separated and have stable wavelengths, can be obtained. The specific structural form of the laser 100 is not limited.

The first measuring beam and the first reference beam are used for being projected onto the reflection grating, the first reference beam is transmitted to the third non-polarized beam splitter, the second non-polarized beam splitter is arranged on a transmission light path of the second laser, the second non-polarized beam splitter divides the second laser into a second measuring beam and a second reference beam, the second measuring beam is used for being projected onto the reflection grating, the second reference beam is transmitted to the third non-polarized beam splitter, the third non-polarized beam splitter outputs a combined beam of the first reference beam and the second reference beam which are incident on the third non-polarized beam splitter, an optical path between the first non-polarized beam splitter and the reflection grating and an optical path between the second non-polarized beam splitter and the reflection grating are quasi-common optical paths, and an optical path between the first reference beam and the second reference beam between the first non-polarized beam splitter and the third non-polarized beam splitter are quasi-common optical paths.

The first laser and the second laser are transmitted through a first laser transmission light path and a second laser transmission light path respectively, wherein a first non-polarizing beam splitter 12 is arranged on the first laser transmission light path, the first non-polarizing beam splitter 12 divides the first laser into two parts, namely a first measuring beam and a first reference beam. The first measuring beam is used for being projected onto the reflection grating 13, the first reference beam is transmitted to the third non-polarizing beam splitter 17, the second non-polarizing beam splitter 16 is arranged on the transmission path of the second laser, and the second non-polarizing beam splitter 16 divides the second laser into two parts, namely the second measuring beam and the second reference beam. Wherein the second measuring beam is for projection onto the reflection grating 13, the second reference beam is transmitted to a third non-polarizing beam splitter 17, and the third non-polarizing beam splitter 17 combines the first reference beam and the second reference beam incident on the third non-polarizing beam splitter 17 for output. The position of the device can be adjusted so that the optical path between the first measuring beam and the first non-polarizing beam splitter 12 and the reflection grating 13 and the optical path between the second measuring beam and the second non-polarizing beam splitter 16 and the reflection grating 13 are quasi-common optical paths, and the optical path between the first reference beam and the second reference beam and the optical path between the first non-polarizing beam splitter 12 and the third non-polarizing beam splitter 17 and the optical path between the second reference beam and the second non-polarizing beam splitter 16 and the third non-polarizing beam splitter 17 are quasi-common optical paths.

The first photodetector 19 is configured to receive the reflected light from the reflection grating 13, i.e., the measuring beam;

the second photodetector 18 is configured to receive the laser beam output by the third non-polarizing beam splitter 17, i.e., the reference beam.

Referring to fig. 2, an embodiment provides a quasi-common-path heterodyne grating interferometer including a dual-frequency laser generating unit, a first laser transmission optical path, a second laser transmission optical path, a first photodetector, a second photodetector, and so on. The dual-frequency laser generating unit is used for outputting a frequency f ₀₊ f ₁ And a first laser of frequency f ₀₊ f ₂ Wherein the frequency is f ₀ The frequency f is obtained after beam splitting and frequency modulation of the laser of (2) ₀₊ f ₁ And a first laser of frequency f ₀₊ f ₂ Is a second laser of f ₁ 、f ₂ Respectively the frequency shifts of the corresponding modulations. The dual-frequency laser generating unit outputs two laser beams having the same polarization and electric field amplitude but different frequencies, and referring to fig. 1, the dual-frequency laser generating unit emits two laser beams of different frequencies through the first collimator 8 and the second collimator 9, respectively.

Then, the symmetrical oblique incidence space light path structure design of the quasi-common light path enables the two laser beams emitted by the first collimator 8 and the second collimator 9 to be spatially separated and spread in a symmetrical mode. Specifically, the first laser and the second laser are transmitted to the same position on the reflection grating 13 in a quasi-common path at a certain inclination angle through symmetrical oblique incidence space optical paths.

Referring to fig. 1, a first half-wave plate 10, a first polarizer 11, and a first non-polarizing beam splitter 12 are sequentially disposed on a first laser transmission path that transmits a first laser, the first non-polarizing beam splitter 12 dividing the first laser into two parts, a first measuring beam and a first reference beam. Wherein the first measuring beam is intended to be projected onto the reflection grating 13 and the first reference beam is transmitted to a third non-polarizing beam splitter 17. A second half-wave plate 14, a second polarizer 15, and a second non-polarizing beam splitter 16 are sequentially disposed on a second laser transmission optical path for transmitting the second laser, and the second non-polarizing beam splitter 16 divides the second laser into two parts, a second measuring beam and a second reference beam. Wherein the second measuring beam is intended to be projected onto the reflection grating 13 and the second reference beam is transmitted to a third non-polarizing beam splitter 17. The third non-polarizing beam splitter 17 combines the first reference beam and the second reference beam incident on the third non-polarizing beam splitter 17 to output. The position of the device can be adjusted so that the optical path between the first measuring beam and the first non-polarizing beam splitter 12 and the reflection grating 13 and the optical path between the second measuring beam and the second non-polarizing beam splitter 16 and the reflection grating 13 are quasi-common optical paths, and the optical path between the first reference beam and the third non-polarizing beam splitter 12 and 17 and the optical path between the second reference beam and the second non-polarizing beam splitter 12 and the third non-polarizing beam splitter 17 are quasi-common optical paths.

The phase meter 20 is electrically connected to the first photodetector 19 and the second photodetector 18, and is configured to calculate a phase difference between output signals of the two photodetectors, and output a grating displacement to the computer 21.

It can be understood that the optical path between the first non-polarizing beam splitter 12 and the reflection grating 13 and the optical path between the second non-polarizing beam splitter 16 and the reflection grating 13 of the first measuring beam are common optical paths, and the optical path between the first non-polarizing beam splitter 12 and the third non-polarizing beam splitter 17 of the first reference beam and the optical path between the second non-polarizing beam splitter 16 and the third non-polarizing beam splitter 17 of the second reference beam are common optical paths, which is the most ideal case, and the strict common optical paths can reduce the measurement error caused by environmental interference and improve the measurement accuracy. However, in practical application, strict common-path under ideal conditions cannot be achieved, so that a concept of quasi-common-path is provided, namely, a common-path with a certain error exists, namely, a dead zone optical path exists in the quasi-common-path heterodyne grating interferometer. In particular, to achieve high precision measurements at the sub-nanometer level, it is desirable to minimize dead zone light in a quasi-common path heterodyne grating interferometer The dead zone optical path is the optical path L between the first non-polarizing beam splitter 12 and the reflection grating 13 ₁ Optical path L between second non-polarizing beam splitter 16 and reflection grating 13 ₂ The difference and the optical path L of the first reference beam between the first 12 to third 17 non-polarizing beam splitters ₄ An optical path L with the second reference beam between the second non-polarizing beam splitter 16 to the third non-polarizing beam splitter 17 ₃ The difference determines the dead zone optical path L _d ＝L ₁ -L ₂ +L ₃ -L ₄ 。

Referring to fig. 2, an embodiment proposes that the two laser beams emitted by the first collimator 8 and the second collimator 9 come from the same laser source. The dual-frequency laser generating unit comprises a laser source 1, a power amplifier 2, a frequency multiplier 3 beam splitter 4, a first optical modulator 5, a second optical modulator 6, a signal generating unit 7, a first collimator 8 and a second collimator 9. Specifically, the laser source 1 outputs a frequency f ₀ The laser of (2) sequentially enters a power amplifier 2 and a frequency multiplier 3 to respectively amplify and multiply the power. Then the laser beams are divided into at least two beams with the same polarization and electric field amplitude through a beam splitter 4, the two beams of laser beams with the same polarization and electric field amplitude are respectively input into a first optical modulator 5 and a second optical modulator 6, the first optical modulator 5 and the second optical modulator 6 respectively carry out frequency modulation on the input laser beams under the control of a signal generating unit 7, and f ₁ 、f ₂ The frequency shift of the first optical modulator 5 and the second optical modulator 6 are respectively modulated, and the frequency of the laser output by the first optical modulator 5 is f ₀₊ f ₁ The second optical modulator 6 outputs laser light with a frequency f ₀₊ f ₂ . The output laser of the first light modulator 5 is collimated by the first collimator 8 and then output as first laser, and the output laser of the second light modulator 6 is collimated by the second collimator 9 and then output as second laser. Thus, two laser beams with different frequencies, which are spatially separated and have stable wavelengths, can be obtained.

Referring to fig. 3, an embodiment proposes a dual-frequency laser generating unit comprising a laser source 1, a power amplifier 2, a frequency multiplier 3, a beam splitter 4, a first optical modulator 5,A second light modulator 6, a signal generating unit 7, a feedback control module 22, a first collimator 8 and a second collimator 9. Specifically, the output frequency of the laser source 1 is f ₀ The laser of (2) sequentially enters a power amplifier 2 and a frequency multiplier 3 to respectively amplify and multiply the power. Then, the laser beam is split into three laser beams with the same polarization and electric field amplitude through the beam splitter 4, one laser beam is input into the feedback control module 22, and the feedback control module 22 is in control connection with the laser source 1 and is used for carrying out feedback control on the laser source, so that the laser source outputs laser with stable wavelength, and the wavelength stabilizing method is based on Modulation Transfer Spectrum (MTS). The other two beams of laser beams with the same polarization and electric field amplitude are respectively input into a first optical modulator and a second optical modulator, the first optical modulator 5 and the second optical modulator 6 respectively carry out frequency modulation on the input laser beams under the control of a signal generating unit 7, and f ₁ 、f ₂ The frequency shift of the first optical modulator 5 and the second optical modulator 6 are respectively modulated, and the frequency of the laser output by the first optical modulator 5 is f ₀₊ f ₁ The second optical modulator 6 outputs laser light with a frequency f ₀₊ f ₂ . The output laser of the first light modulator 5 is collimated by the first collimator 8 and then output as first laser, and the output laser of the second light modulator 6 is collimated by the second collimator 9 and then output as second laser. Thus, two laser beams with different frequencies, which are spatially separated and have stable wavelengths, can be obtained. The method can obtain two laser beams with different frequencies, which are spatially separated and have stable wavelengths.

In the embodiments shown in fig. 1 or fig. 2 or 3, two spatially separated laser beams from a dual-frequency laser generating unit are projected onto a reflection grating 13 and then combined by a highly symmetrical quasi-common path spatial light path. At a frequency f of emergence of the first collimator 8 ₀₊ f ₁ For example, the polarized laser beam is p-polarized after passing through the first half-wave plate 10 and the first polarizer 11, and the output power is changed. Further, the output laser beam is split by the first non-polarizing beam splitter 12 to become reflected light and transmitted light, wherein the reflected light is received by the second photodetector 18 after passing through the third non-polarizing beam splitter 17, and the transmitted light is projected from the first-order diffraction angle Onto the reflection grating 13, the first order diffracted light propagates to the first photodetector 19 due to the grating diffraction phenomenon. After a symmetrical structure, the laser beam from the second collimator 9 is combined with the laser beam from the first collimator 8 at the first photodetector 19, the second photodetector 18. The first photodetector 19 receives a laser beam composed of a combination of reflection gratings, which is called a measuring beam. The laser beam combined with the third non-polarizing beam splitter 17 and the second photodetector 18 is referred to as a reference beam.

Due to the doppler effect, the received laser frequency is affected by the grating displacement, which can be observed in the form of a phase change. According to the grating equation, the phase change Φ caused by the grating displacement can be expressed as:

where s is the grating displacement and g is the grating period.

In order to accurately obtain the phase change phi caused by the grating displacement, a dual-frequency laser generating unit is used. Two laser beams with different frequencies, same polarization and electric field amplitude are overlapped to generate a light beat. This method can detect a slight frequency shift caused by the grating displacement. The average energy of the two laser beams combined is received by the photodetector during the response time τ, and the current signal on the photodetector can be noted as:

Wherein E is the wave function of the combined laser beams, k is the photovoltaic conversion efficiency, and t is the times.

Output signal I of the first photodetector _PD1 And the output signal I of the second photodetector _PD2 Can be expressed as:

in U ₀ The electric field amplitude, n is the refractive index of air, lambda is the vacuum wavelength of laser, and t is the time.

Phase difference phi existing between output signals of the first photodetector and the second photodetector ₀ The value of which can be expressed as:

wherein L is _d For dead zone optical path of the system, phi _d Is dead zone optical path L _d The initial phase difference caused can be regarded as zero.

The grating displacement s can then be expressed as:

in this case, the grating displacement variation Δs and the phase difference variation Δφ ₀ The relationship between these can be noted as:

theoretically, phi _d Not a constant, it is affected by fluctuations in the vacuum wavelength of the laser and the refractive index of air. In reality, however, phi _d Is generally considered to be a constant, Δφ _d Is considered zero. Thus Δφ _d The presence of (a) causes a grating displacement error deltas _error 。Δφ _d The value of (2) can be expressed as:

where Δn represents the fluctuation of the air reflection index, c represents the speed of light, and Δf represents the fluctuation of the laser frequency due to the wavelength.

From delta phi _d Induced grating displacement error Δs _error The value can be expressed as:

Δφ _d the value of (2) contributes to the system noise level, emphasizing that its value is minimized. Dead zone optical path L when the system is subject to environmental interference or laser wavelength fluctuation _d Is an important influencing factor and can be used for evaluating environmental disturbance. On the other hand, frequency fluctuations are also limited. In order to realize sub-nanometer precision displacement measurement, it is important to minimize dead zone optical path, stabilize laser wavelength and reduce environmental interference.

In one embodiment, a dead zone optical path evaluation method of a quasi-common-path heterodyne grating interferometer is provided, including:

frequency scanning experiments are carried out by aligning the common-path heterodyne grating interferometer, and dead zone optical path of the quasi-common-path heterodyne grating interferometer is obtained through calculation, and the calculation method is as follows:

In one embodiment, a dead zone optical path calibration method of a quasi-common optical path heterodyne grating interferometer is provided, wherein the optical path of a first measuring beam on a corresponding measuring light transmission optical path or/and the optical path of a second measuring beam on the corresponding measuring light transmission optical path are adjusted; or, adjusting the optical path of the first reference beam on the corresponding reference light transmission optical path or/and the optical path of the second reference beam on the corresponding reference light transmission optical path;

And minimizing the dead zone optical path of the quasi-common-path heterodyne grating interferometer or enabling the dead zone optical path to be lower than a set threshold value until the dead zone optical path is lower than the set threshold value, so as to obtain the quasi-common-path heterodyne grating interferometer meeting the set measurement precision after the dead zone optical path is calibrated. The displacement measurement error of the quasi-common-path heterodyne grating interferometer can be reduced by minimizing the dead zone optical path, and the displacement measurement precision of the quasi-common-path heterodyne grating interferometer is improved.

In one embodiment, a dead zone optical path calibration method of a quasi-common optical path heterodyne grating interferometer is provided, including the following steps:

wherein DeltaPhi _d The phase difference change amount caused by dead zone optical path is represented by c, the light velocity is represented by deltaf, the laser frequency fluctuation caused by wavelength is represented by deltaf, and n is the air refractive index;

in view of the high precision measurement on the sub-nanometer level, it is desirable to minimize the dead zone optical path in the quasi-common path heterodyne grating interferometer, where the dead zone optical path is the optical path L between the first non-polarizing beam splitter 12 and the reflective grating 13 by the first measuring beam ₁ Optical path L between second non-polarizing beam splitter 16 and reflection grating 13 ₂ The difference and the optical path L of the first reference beam between the first 12 to third 17 non-polarizing beam splitters ₄ An optical path L with the second reference beam between the second non-polarizing beam splitter 16 to the third non-polarizing beam splitter 17 ₃ The difference determines the dead zone optical path L _d ＝L ₁ -L ₂ +L ₃ -L ₄ 。

By adjusting the optical path L of the first measuring beam between the first non-polarizing beam splitter 12 and the reflection grating 13 ₁ Or/and the optical path L of the second measuring beam between the second non-polarizing beam splitter 16 and the reflection grating 13 ₂ Or adjusting the optical path L of the first reference beam between the first 12 to third 17 non-polarizing beam splitters ₄ Or/and the optical path L of the second reference beam between the second non-polarizing beam splitter 16 and the third non-polarizing beam splitter 17 ₃ And until the dead zone optical path is lower than a set threshold, the set threshold is determined according to the set measurement precision, and the quasi-common optical path heterodyne grating interferometer meeting the set measurement precision after dead zone optical path calibration is obtained.

In order to facilitate the adjustment of the dead zone optical path, an optical delay device for adjusting the optical path can be added between the first non-polarizing beam splitter 12 and the reflection grating 13 or/and between the second non-polarizing beam splitter 16 and the reflection grating 13 in the quasi-common path heterodyne grating interferometer provided by any embodiment of the structure shown in fig. 1 or 2 or 3. Or an optical delay device for adjusting the optical path is additionally arranged between the first non-polarizing beam splitter 12 and the third non-polarizing beam splitter 17 or/and between the second non-polarizing beam splitter 16 and the third non-polarizing beam splitter 17. Therefore, the dead zone optical path of the quasi-common-path heterodyne grating interferometer can be conveniently adjusted, and the influence of the dead zone optical path on the quasi-common-path heterodyne grating interferometer is avoided as much as possible.

Referring to fig. 4, in one embodiment, a quasi-common-path heterodyne grating interferometer is provided, in which a first optical delay device 23 and a second optical delay device 24 for adjusting optical paths are respectively added between the first non-polarizing beam splitter 12 and the third non-polarizing beam splitter 17 and between the second non-polarizing beam splitter 16 and the third non-polarizing beam splitter 17. The first optical delay device 23 and the second optical delay device 24 can adjust the optical path length, thereby adjusting the dead zone optical path length of the quasi-common-path heterodyne grating interferometer, and avoiding the influence of the dead zone optical path length on the quasi-common-path heterodyne grating interferometer as much as possible by reducing the dead zone optical path length as much as possible.

It will be appreciated that the specific configuration of the optical retardation device is not limited and may consist of a series of mirrors or the like, the main purpose of which is to adjust the optical path, and that some of the mirrors may be designed as displaceable mirrors, so that the adjustment of the optical path can be achieved. The person skilled in the art can make flexible designs based on common general knowledge and conventional technical means in the art.

Referring to fig. 5, an optical delay device according to an embodiment includes a housing, an incident window 2301 and an exit window 2307 are disposed on the housing, a series of lenses including a first lens 2302, a second lens 2303, a third lens 2304 and a fourth lens 2305 are disposed in the housing, a light beam is incident on the first lens 2302 in the housing through the incident window 2301, the first lens 2302 reflects the light beam to the second lens 2303, the second lens 2303 reflects the light beam to the third lens 2304, the third lens 2304 reflects the light beam to the fourth lens 2305, and the light beam reflected by the fourth lens 2305 exits through the exit window 2307. The first lens 2302 and the second lens 2303 are disposed on the moving platform 2308, and the first lens 2302 and the second lens 2303 can be driven by the moving platform 2308 to move, so as to change the optical path of the light beam in the housing. The structural form of the mobile platform is not limited, in the field of mechanical design, there are many designs for realizing precise and controllable movement, such as designing an electric control guide rail, and the like, and the invention is not limited to the specific structural form and the specific driving form of the mobile platform, and certainly not limited to the electric driving, and also can be used for precisely and manually adjusting the mobile platform.

Further, in fig. 5, the first lens 2302 and the fourth lens 2305 are all high-reflection lenses, the second lens 2303 and the third lens 2304 are all plane mirrors, meanwhile, a diaphragm 2306 is disposed on a transmission path of the first lens 2302, a light beam enters the first lens 2302 in the housing through an incident window 2301, most of the light beam is reflected at the first lens 2302, the light beam reflected by the first lens 2302 is reflected by the second lens 2303 and the third lens 2304 twice and then reaches the fourth lens 2305, and the light beam transmitted by the first lens 2302 is transmitted by the diaphragm 2306 to the fourth lens 2305, so that the light path of the light beam transmitted by the first lens 2302 can be corrected, and the incident light beam and the emergent light beam of the optical delay device are coaxial. The two plane mirrors of the second mirror 2303 and the third mirror 2304 are fixed to a guide rail, and move up and down by the guide rail, thereby adjusting the optical path length.

The quasi-common-path heterodyne grating interferometer provided by any of the embodiments of the structures shown in fig. 1, 2, 3 and 4 is realized based on a spatial light path, and the invention can be realized not only based on the spatial light path but also based on an optical fiber structure, and further can adjust the dead zone optical path through an optical fiber delay line. The quasi-common-path heterodyne grating interferometer with the optical fiber structure comprises a double-frequency laser generating unit, a first laser transmission light path, a second laser transmission light path, a first photoelectric detector 19, a second photoelectric detector 18 and the like. The structure and implementation manner of the dual-frequency laser generating unit provided in any of the foregoing embodiments may be adopted, and are not described herein.

The dual-frequency laser generating unit in the embodiments shown in fig. 3 and 4 is used in the dual-frequency laser generating unit of the quasi-common-path heterodyne grating interferometer with the optical fiber structure, and is not described herein again. Wherein the first optical modulator 5 and the second optical modulator 6 respectively perform frequency modulation on the input laser under the control of the signal generating unit 7, f ₁ 、f ₂ The frequency shift of the first optical modulator 5 and the second optical modulator 6 are respectively modulated, and the frequency of the laser output by the first optical modulator 5 is f ₀₊ f ₁ The second optical modulator 6 outputs laser light with a frequency f ₀₊ f ₂ . The output frequency of the first optical modulator 5 is f ₀₊ f ₁ Is split into two beams by the 1# optical fiber beam splitter 25, wherein one beam is used as a first measuring beam, and the other beam is used as a first reference beam; the frequency output by the second optical modulator 6 is f ₀₊ f ₂ Is split into two beams, one of which is the second measuring beam and the other is the second reference beam, via a 2# fiber beam splitter 26. The first measuring beam is output after being collimated by the first collimator 8, the second measuring beam output laser is output after being collimated by the second collimator 9, and the first collimator 8 and the second collimator 9 are optical fiber collimators.

Referring to fig. 6, a quasi-common-path heterodyne grating interferometer with an optical fiber structure according to an embodiment, the dual-frequency laser generating unit in the embodiment shown in fig. 3 and 4 is adopted as the dual-frequency laser generating unit, and is not described herein.

Wherein the first optical modulator 5 and the second optical modulator 6 respectively perform frequency modulation on the input laser under the control of the signal generating unit 7, f ₁ 、f ₂ The frequency shift of the first optical modulator 5 and the second optical modulator 6 are respectively modulated, and the frequency of the laser output by the first optical modulator 5 is f ₀₊ f ₁ The second optical modulator 6 outputs laser light with a frequency f ₀₊ f ₂ . The output frequency of the first optical modulator 5 is f ₀₊ f ₁ Is split into two beams by the 1# optical fiber beam splitter 25, wherein one beam is used as a first measuring beam, and the other beam is used as a first reference beam; the frequency output by the second optical modulator 6 is f ₀₊ f ₂ Is split into two beams by the 2# fiber beam splitter 26, one of which is the second measuring beam and the other is the second reference beam. The first measuring beam is output after being collimated by the first collimator 8, the second measuring beam is output after being collimated by the second collimator 9, and the first collimator 8 and the second collimator 9 are optical fiber collimators. The first measuring beam transmission optical path for transmitting the first measuring beam is sequentially provided with a first half-wave plate 10 and a first polarizer 11, the first measuring beam is projected onto the reflection grating 13 after passing through the first half-wave plate 10 and the first polarizer 11, the second measuring beam transmission optical path for transmitting the second measuring beam is sequentially provided with a second half-wave plate 14 and a second polarizer 15, and the second laser is projected onto the reflection grating 13 after passing through the second half-wave plate 14 and the second polarizer 15. The first measuring beam and the second measuring beam are transmitted to the same position on the reflection grating 13 in a quasi-common path by a certain inclination angle through a symmetrical oblique incidence space optical path. The first photodetector 19 is configured to receive reflected light, i.e., a measuring beam, from the reflection grating 13.

The first reference beam and the second reference beam are respectively input to the optical fiber combiner 29 after passing through the corresponding optical fiber transmission optical paths, and output to the second photoelectric detector 18 through the optical fiber combiner 29. The second photodetector 18 is configured to receive a reference beam.

In view of the high precision measurement on the sub-nanometer level, it is desirable to minimize the dead zone optical path in the quasi-common path heterodyne grating interferometer, where the dead zone optical path is the optical path L between the first collimator 8 and the reflection grating 13 by the first measuring beam ₁ An optical path L between the second collimator 9 and the reflection grating 13 with the second measuring beam ₂ The difference and the optical path length L of the first reference beam between the 1# optical fiber beam splitter 25 and the optical fiber combiner 29 ₄ From the second reference beam at the 2# fiber beam splitter 26 to lightOptical path L between fiber combiners 29 ₃ The difference determines the dead zone optical path L _d ＝L ₁ -L ₂ +L ₃ -L ₄ . In practical application, by reasonably selecting the optical fibers for transmitting the two reference beams and the lengths of the optical fibers, and adjusting the optical path length between the first collimator 8 and the reflection grating 13 of the first measuring beam and the optical path length between the first collimator 9 and the reflection grating 13 of the second measuring beam to be equal and symmetrical, the dead zone optical path length can be reduced to a certain extent. But the dead zone optical path cannot be completely eliminated in a strict sense.

By adjusting the optical path L of the first measuring beam between the first collimator 9 and the reflection grating 13 ₁ Or/and the optical path L of the second measuring beam between the second collimator 9 and the reflection grating 13 ₂ Or by adjusting the optical path L of the first reference beam between the 1# fiber splitter 25 to the fiber combiner 29 ₄ Or/and the optical path L of the second reference beam between the 2# fiber beam splitter 26 and the fiber combiner 29 ₃ And until the dead zone optical path is lower than a set threshold, the set threshold is determined according to the set measurement precision, and the quasi-common optical path heterodyne grating interferometer meeting the set measurement precision after dead zone optical path calibration is obtained.

Fig. 7 shows a quasi-common-path heterodyne grating interferometer with an optical fiber structure according to an embodiment, and the dual-frequency laser generating unit in the embodiments shown in fig. 3 and 4 is used as the dual-frequency laser generating unit, which is not described herein. Wherein the first optical modulator 5 and the second optical modulator 6 respectively perform frequency modulation on the input laser under the control of the signal generating unit 7, f ₁ 、f ₂ The frequency shift of the first optical modulator 5 and the second optical modulator 6 are respectively modulated, and the frequency of the laser output by the first optical modulator 5 is f ₀₊ f ₁ The second optical modulator 6 outputs laser light with a frequency f ₀₊ f ₂ The output frequency of the first optical modulator 5 is f ₀₊ f ₁ Is split into two beams by the 1# optical fiber beam splitter 25, wherein one beam is used as a first measuring beam, and the other beam is used as a first reference beam; the frequency output by the second optical modulator 6 is f ₀₊ f ₂ Is split into two beams by a 2# fiber beam splitter 26, whichOne of the beams is used as a second measuring beam, and the other beam is used as a second reference beam. The first measuring beam is output after being collimated by the first collimator 8, the second measuring beam is output after being collimated by the second collimator 9, and the first collimator 8 and the second collimator 9 are optical fiber collimators.

The first half-wave plate 10 and the first polarizer 11 are sequentially arranged on a transmission light path for transmitting the first measuring light beam, the first measuring light beam is projected onto the reflection grating 13 after passing through the first half-wave plate 10 and the first polarizer 11, the second half-wave plate 14 and the second polarizer 15 are sequentially arranged on a transmission light path for transmitting the second measuring light beam, and the second measuring light beam is projected onto the reflection grating 13 after passing through the second half-wave plate 14 and the second polarizer 15. The first measuring beam and the second measuring beam are transmitted to the same position on the reflection grating 13 in a quasi-common path by a certain inclination angle through a symmetrical oblique incidence space optical path. The first photodetector 19 is configured to receive reflected light, i.e., a measuring beam, from the reflection grating 13.

In order to facilitate the adjustment of the optical path length, in fig. 6, the first reference beam and the second reference beam are respectively input to the optical fiber combiner 29 after the optical path lengths are adjusted by the first optical fiber delay device 27 and the second optical fiber delay device 28 on the corresponding optical fiber transmission optical paths, and are output to the second photodetector 18 through the optical fiber combiner 29. The second photodetector 18 is configured to receive a reference beam.

In the quasi-common-path heterodyne grating interferometer with the optical fiber structure, in order to replace an original space optical path by using an optical fiber device, a beam splitter is used for separating a light beam serving as a reference from a light beam used for measurement, and two laser beams used for measurement respectively exit through a first collimator 8 and a second collimator 9, are diffracted on a reflection grating 13 and are transmitted to a first photoelectric detector 19. The two laser beams as references are received by the second photodetector 18 after being combined after the optical path lengths of the two laser beams are adjusted by the two optical fiber retarders along the optical fibers, respectively. The structure adjusts the dead zone optical path through the optical fiber retarder, and in addition, the space optical path of the measuring beam is effectively shortened, which is more beneficial to optical path adjustment.

The dead zone optical path evaluation method of the quasi-common-path heterodyne grating interferometer with the optical fiber structure is the same as that of the quasi-common-path heterodyne grating interferometer with the spatial optical path structure, namely, the dead zone optical path evaluation method comprises the following steps:

The dead zone optical path calibration method of the quasi-common-path heterodyne grating interferometer with the optical fiber structure is similar to the dead zone optical path calibration method of the quasi-common-path heterodyne grating interferometer with the spatial optical path structure, and comprises the following steps:

In the quasi-common-path heterodyne grating interferometer with the optical fiber structure shown in FIG. 6, the optical path L between the first collimator and the reflection grating is adjusted by adjusting the optical path L of the first measuring beam ₁ Or/and the optical path L of the second measuring beam between the first collimator and the reflection grating ₂ Or by adjusting the optical path L of the first reference beam between the No. 1 optical fiber splitter and the optical fiber combiner ₄ Or/and the optical path L of the second reference beam between the No. 2 optical fiber beam splitter and the optical fiber combiner ₃ Until the dead zone optical path is lower than a set threshold, the set threshold is determined according to the set measurement precision, and the dead zone optical path is calibrated and then meets the set measurementA precise quasi-common path heterodyne grating interferometer.

In the quasi-common-path heterodyne grating interferometer with the optical fiber structure shown in fig. 7, the first reference beam and the second reference beam are respectively input to the optical fiber combiner 29 after the optical paths of the first reference beam and the second reference beam are adjusted by the first optical fiber delay 27 and the second optical fiber delay 28 on the corresponding optical fiber transmission optical paths, so that the optical path L of the first reference beam between the 1# optical fiber beam splitter 25 and the optical fiber combiner 29 can be respectively adjusted by the first optical fiber delay and the second optical fiber delay ₄ Or/and the optical path L of the second reference light between the 2# optical fiber beam splitter 26 and the optical fiber combiner 29 ₃ And further adjusting the dead zone optical path until the dead zone optical path is lower than a set threshold, wherein the set threshold is determined according to the set measurement precision, and the quasi-common-path heterodyne grating interferometer meeting the set measurement precision after dead zone optical path calibration is obtained.

The invention can measure the linear displacement with sub-nanometer precision. Because the optical path structure of the laser source with stable wavelength and the quasi-common optical path is adopted, the dead zone optical path is quantitatively evaluated, and the dead zone optical path calibration of the common optical path heterodyne grating interferometer is further aligned, so that the quasi-common optical path heterodyne grating interferometer meeting the set measurement precision is obtained. The quasi-common-path heterodyne grating interferometer provided by the invention can reduce periodic nonlinear errors, can realize sub-nanometer precision displacement measurement, and is used for ultra-precise positioning.

The invention is not a matter of the known technology.

The technical features of the above embodiments may be arbitrarily combined, and all possible combinations of the technical features in the above embodiments are not described for brevity of description, however, as long as there is no contradiction between the combinations of the technical features, they should be considered as the scope of the description.

The above examples merely represent a few embodiments of the present application, which are described in more detail and are not to be construed as limiting the scope of the invention. It should be noted that it would be apparent to those skilled in the art that various modifications and improvements could be made without departing from the spirit of the present application, which would be within the scope of the present application. Accordingly, the scope of protection of the present application is to be determined by the claims appended hereto.

The above description is only of the preferred embodiments of the present invention and is not intended to limit the present invention, but various modifications and variations can be made to the present invention by those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. The quasi-common-path heterodyne grating interferometer is characterized by comprising a double-frequency laser generating unit, a first laser transmission light path, a second laser transmission light path, a first photoelectric detector and a second photoelectric detector:

2. The quasi-common optical path heterodyne grating interferometer according to claim 1, wherein a first non-polarizing beam splitter is disposed on a first laser transmission path, the first non-polarizing beam splitter splits a first laser beam into a first measurement beam and a first reference beam, wherein the first measurement beam is used for being projected onto a reflection grating, the first reference beam is transmitted to a third non-polarizing beam splitter, a second non-polarizing beam splitter is disposed on a second laser transmission path, the second non-polarizing beam splitter splits the second laser beam into a second measurement beam and a second reference beam, wherein the second measurement beam is used for being projected onto the reflection grating, the second reference beam is transmitted to a third non-polarizing beam splitter, the third non-polarizing beam splitter outputs the first reference beam and the second reference beam incident on the third non-polarizing beam splitter, an optical path between the first measurement beam and the second measurement beam between the first non-polarizing beam splitter and the reflection grating is quasi-common, and an optical path between the first reference beam and the second reference beam between the first non-polarizing beam splitter and the third non-polarizing beam splitter is quasi-common to the third non-polarizing beam splitter.

3. The quasi-common-path heterodyne grating interferometer according to claim 2, wherein the first laser transmission optical path is further provided with a first half-wave plate and a first polarizer in sequence, and the first laser is transmitted to the first non-polarizing beam splitter after passing through the first half-wave plate and the first polarizer in sequence; the second laser transmission light path is further provided with a second half-wave plate and a second polarizer in sequence, and the second laser is transmitted to the second non-polarization beam splitter after passing through the second half-wave plate and the second polarizer in sequence.

4. A quasi-common path heterodyne grating interferometer according to claim 3, wherein an optical delay device for adjusting an optical path length is added between the first non-polarizing beam splitter and the reflection grating or/and between the second non-polarizing beam splitter and the reflection grating or/and between the first reference beam and the first non-polarizing beam splitter and/or between the second reference beam and the third non-polarizing beam splitter.

5. The quasi-common path heterodyne grating interferometer according to claim 4, wherein the optical delay device comprises a housing, an incident window and an emergent window are relatively arranged on the housing, a series of lenses including a first lens, a second lens, a third lens and a fourth lens are arranged in the housing, the light beam is incident to the first lens in the housing through the incident window, the first lens reflects the light beam to the second lens, the second lens reflects the light beam to the third lens, the third lens reflects the light beam to the fourth lens, the light beam reflected by the fourth lens exits through the emergent window, the first lens and the second lens are arranged on the moving platform, and the first lens and the second lens can move under the driving of the moving platform, so that the optical path of the light beam in the housing is changed.

6. The quasi-common-path heterodyne grating interferometer according to claim 5, wherein the first lens and the fourth lens are both high-reflection mirrors, the second lens and the third lens are both plane mirrors, a diaphragm is disposed on a transmission path of the first lens, a light beam is incident to the first lens in the housing through the incident window, the light beam reflected by the first lens is reflected by the second lens and the third lens and then reaches the fourth lens, the light beam transmitted by the first lens is corrected in an optical path by using the transmitted light through the first lens through the diaphragm to the fourth lens, and the incident light beam and the outgoing light beam of the optical delay device are coaxial.

7. The quasi-common path heterodyne grating interferometer according to claim 1, wherein the first laser beam is split into two beams by a 1# fiber beam splitter, one of the two beams is a first measurement beam, and the other beam is a first reference beam; the second laser is split into two beams through a 2# optical fiber beam splitter, wherein one beam is used as a second measuring beam, and the other beam is used as a second reference beam;

8. The quasi-common-path heterodyne grating interferometer according to claim 7, wherein the first measuring beam is output after being collimated by the first fiber collimator, a first half wave plate and a first polarizer are sequentially arranged on a transmission light path for transmitting the first measuring beam, and the first measuring beam is projected onto the reflection grating after passing through the first half wave plate and the first polarizer; the second measuring beam is output after being collimated by the second optical fiber collimator, a second half-wave plate and a second polarizer are sequentially arranged on a transmission light path for transmitting the second measuring beam, and the second measuring beam is projected onto the same position of the reflection grating after passing through the second half-wave plate and the second polarizer.

9. The quasi-common-path heterodyne grating interferometer according to claim 7, wherein the reference light fiber transmission paths corresponding to the first reference light and the second reference light are respectively provided with a fiber retarder, the first reference light and the second reference light are respectively input to the fiber combiner after the optical path lengths of the first fiber retarder and the second fiber retarder on the corresponding reference light fiber transmission paths are adjusted, and are output to the second photodetector through the fiber combiner.

10. The quasi-common path heterodyne grating interferometer according to any one of claims 1 to 9, further comprising a phase meter electrically connected to the first photodetector and the second photodetector for calculating a phase difference between output signals of the two photodetectors.

11. The quasi-common path heterodyne grating interferometer according to claim 10, further comprising a grating shift resolving unit, wherein the phase meter outputs a phase difference between output signals of the two photodetectors to the grating shift resolving unit, and wherein the grating shift resolving unit resolves the grating shift from the phase difference between the output signals of the two photodetectors.

12. The quasi-common path heterodyne grating interferometer according to claim 1 or claim 2 or claim 3 or claim 4 or claim 5 or claim 6 or claim 7 or claim 8 or claim 9 or claim 11, wherein the dual frequency laser generation unit comprises a laser source, a beam splitter, a first light modulator, a second light modulator, a first collimator, and a second collimator; the output frequency of the laser source is f ₀ The laser beam is divided into at least two laser beams with the same polarization and electric field amplitude through a beam splitter, the two laser beams with the same polarization and electric field amplitude are respectively input into a first optical modulator and a second optical modulator, the first optical modulator and the second optical modulator respectively carry out frequency modulation on the input laser beams, and f ₁ 、f ₂ The frequency shift of the first light modulator and the second light modulator is respectively modulated, and the frequency of the laser output by the first light modulator is f ₀₊ f ₁ The frequency of the laser output by the second light modulator is f ₀₊ f ₂ The first light modulator outputs laser after being collimated by the first collimator to serve as first laser, and the second light modulator outputs laser after being collimated by the second collimator to serve as second laser.

13. The quasi-common path heterodyne grating interferometer according to claim 12, wherein the dual frequency laser generating unit further comprises a feedback control module, the laser output from the laser source is split into three beams by the beam splitter, one of the three beams is input to the feedback control module, and the feedback control module controls the laser source based on the modulation transmission spectrum, so that the laser output from the laser source is stable in wavelength.

14. The dead zone optical path evaluation method of a quasi-common path heterodyne grating interferometer as set forth in claim 1, comprising:

15. The dead zone optical path calibration method of the quasi-common path heterodyne grating interferometer according to claim 1, wherein the optical path length of the first measuring beam on the corresponding measuring light transmission optical path or/and the optical path length of the second measuring beam on the corresponding measuring light transmission optical path are adjusted; or, adjusting the optical path of the first reference beam on the corresponding reference light transmission optical path or/and the optical path of the second reference beam on the corresponding reference light transmission optical path;

And minimizing the dead zone optical path of the quasi-common-path heterodyne grating interferometer or enabling the dead zone optical path to be lower than a set threshold value until the dead zone optical path is lower than the set threshold value, so as to obtain the quasi-common-path heterodyne grating interferometer meeting the set measurement precision after the dead zone optical path is calibrated.