System and method for calibrating air refractive index based on self-tracing grating interferometer

Abstract

The invention discloses a system and a method for calibrating an air refractive index based on a self-tracing grating interferometer, and belongs to the technical field of air refractive index measurement. According to the system and the method for calibrating the air refractive index based on the self-traceability grating interferometer, the grating is used as a displacement measurement reference, interference of air factors on measurement data is avoided, the system and the method are more stable than a laser interferometer using laser wavelength as a reference in the same measurement environment, the comparability of measured data is higher, and the measurement accuracy is higher. And the measurement repetition stability is higher. The traditional calibration method depending on a large number of environmental sensors (such as temperature, humidity and air pressure sensors) is avoided, the calibration process is simplified, the calibration efficiency is improved, the calibration process and the measurement process are carried out in parallel, and independent calibration operation does not need to be executed without interrupting measurement. In a dynamically changing measurement environment, calibration parameters can be adjusted in real time to respond to fluctuation of the environment, and accuracy and reliability of measurement data are ensured.

Description

Technical Field

[0001] The present invention relates to the technical field of air refractive index measurement, and in particular to a system and method for calibrating air refractive index based on a self-traceable grating interferometer. Background Art

[0002] The status of precision and ultra-precision measurement technology in the field of modern processing and manufacturing cannot be ignored. The improvement of its range and accuracy directly affects the size and accuracy of the products obtained in the manufacturing process. Laser interferometer is widely used in the field of displacement measurement due to its non-contact measurement and large-range travel measurement characteristics. However, the measurement accuracy of laser interferometer is based on the accuracy of wavelength, and its measurement results are based on wavelength. In long-distance displacement measurement, environmental factors, especially changes in temperature, pressure and humidity, have a significant impact on the refractive index of air, thereby causing disturbances to the wavelength, which directly affects the accuracy of displacement measurement results. Therefore, accurate measurement of the refractive index of air is crucial to ensure the accuracy of displacement measurement. By accurately measuring and compensating for changes in the refractive index of air, the measurement range and application scenarios of laser interferometers under various environmental conditions are expanded, and its applicability in diversified measurement scenarios is enhanced.

[0003] The main existing method for measuring the refractive index of air is the empirical formula method, which relies on the measurement of environmental parameters (such as temperature, pressure, humidity, carbon dioxide concentration, etc.) by environmental sensors, and calculates the refractive index of air through empirical formulas such as the Edlén equation. However, the main drawback of this measurement method is that it is highly dependent on the measurement accuracy of environmental parameters, and there is a problem that local measurements cannot be equivalent to the average value of the entire optical path. At the same time, the determination of environmental parameters is based on the measurement of a large number of environmental sensors, which is costly and inefficient.

[0004] Grating interferometers are resistant to environmental interference and have strong robustness, and they have the basic conditions for providing on-site calibration for laser interferometers. Since grating interferometers rely on traditional gratings for measurement, traditional gratings are resistant to environmental interference and have high stability, and they have the basic conditions for on-site measurement of air refractive index, but their grating period uncertainty contains the component of air refractive index and is not traceable. Summary of the invention

[0005] The purpose of the present invention is to provide a system and method for calibrating the air refractive index based on a self-traceable grating interferometer, which not only reduces the dependence on local environmental sensors, but also enables calibration under complex conditions, thus achieving the traceability and reliability of the air refractive index measurement results. It provides an innovative and efficient solution for on-site air refractive index measurement, which is more suitable for on-site and online measurement in enterprises, and promotes the industrial application of nano-metrology.

[0006] To achieve the above-mentioned purpose, the present invention provides a system for calibrating the refractive index of air based on a self-traceable grating interferometer, comprising a self-traceable grating interferometer, wherein the light source on the self-traceable grating interferometer is refracted onto the zero expansion glass on the nano-displacement stage 1, and then a measurement light path that affects the standard wavelength of the laser interferometer is formed through a reflector and connected to the nano-displacement stage 2.

[0007] A method for calibrating air refractive index based on a self-traceable grating interferometer comprises the following steps:

[0008] S1, after the periodic direction of the self-traceable grating and the measurement axis direction of the reflector are aligned according to the Abbe principle, they are fixed on the nano-displacement stage 2, and the host computer controller issues a measurement command to drive the nano-displacement stage 1 carrying the self-traceable grating and the reflector to perform precise displacement;

[0009] S2. Sample the self-traceable grating interferometer signal and the laser interferometer signal according to the Nyquist sampling theorem, and extract the interference phase;

[0010] S3. Based on the displacement measurement result of the self-traceable grating interferometer, the equivalent wavelength of the laser interferometer on the measurement path is calculated;

[0011] S4. The ratio of the standard wavelength of the laser interferometer light source to the equivalent wavelength is used as the average air refractive index on the laser interferometer measurement path.

[0012] Preferably, in step S1, the zero Abbe principle is specifically achieved by mechanically designing a nano-displacement stage adapter so that the period direction of the self-traceable grating and the measuring axis of the reflector are collinear.

[0013] Preferably, in step S1, the self-traceable grating is a grating structure prepared by atomic lithography technology with extremely high period accuracy and uniformity. During this process, the metal material is heated and sublimated to a gas state in a vacuum environment and ejected in an effusion manner, wherein the ejected atomic beam locks the laser wavelength at the transition frequency of two energy levels of the metal atoms through a frequency-stabilized laser, and the ejected periodically distributed atoms are deposited on the zero-expansion glass, and the formed grating structure enables its period to be directly traced back to the transition frequency of two energy levels of the metal atoms.

[0014] Preferably, in step S2, the wavelength used for measurement by the laser interferometer is not compensated for the influence of the air refractive index. During use, the measurement result of the laser interferometer directly reflects the influence of the air refractive index on the laser wavelength without any form of calibration or adjustment.

[0015] Preferably, in step S2, the host computer samples the signal at a specific frequency according to the Nyquist sampling theorem, and converts the interference signal into a displacement signal through a signal processing algorithm, and the relationship between the phase and displacement of the self-traceable grating interferometer and the laser interferometer is as follows:

[0016]

[0017] Where d is the grating pitch, m is the subdivision factor, n is the refractive index of air, λ is the laser wavelength in air, and λ0 is the laser wavelength in vacuum.

[0018] Preferably, in step S3, the displacement measurement reference is the period of the self-traceable grating, and the frequency difference of the two beams of light generated by the Doppler effect of the grating movement is used to generate an interference measurement signal after beam combination, and the signal is converted into a displacement based on the grating period, thereby realizing an interferometer for micro-nanoscale measurement. The self-traceable grating interferometer uses the grating spacing as the measurement reference, and its result is not affected by changes in the air environment, while the laser interferometer uses the laser wavelength in the air as the reference, and the wavelength is affected by changes in environmental conditions.

[0019] Preferably, in step S4, the laser standard wavelength is a standard wavelength of the laser interferometer in a vacuum environment. In an air environment, the laser wavelength of the laser interferometer will be smaller than the standard wavelength due to the refractive index of the air. The laser interferometer is used to measure and calculate the influence of the refractive index of the air in a non-vacuum environment.

[0020] Preferably, in step S4, the average air refractive index is calculated by comparing the standard wavelength of the laser interferometer light source with the equivalent wavelength on the laser interferometer measurement path measured based on the self-traceable grating to obtain the average air refractive index of the measurement path.

[0021] Therefore, the present invention adopts a system and method for calibrating the refractive index of air based on a self-traceable grating interferometer of the above structure, which has the following beneficial effects:

[0022] (1) The present invention uses a grating as a displacement measurement reference, avoiding the interference of air factors on the measurement data. Under the same measurement environment, it is more stable than a laser interferometer that uses laser wavelength as a reference, the measured data is more comparable, and the measurement repeatability is greater.

[0023] (2) The present invention reduces the reliance on local environmental sensors: it avoids the traditional calibration method that relies on a large number of environmental sensors (such as temperature, humidity, and air pressure sensors), simplifies the calibration process, and improves calibration efficiency.

[0024] (3) The present invention has real-time calibration capability: the calibration process is carried out in parallel with the measurement process, and there is no need to interrupt the measurement to perform independent calibration operations. In a dynamically changing measurement environment, the calibration parameters can be adjusted instantly to respond to environmental fluctuations, ensuring the accuracy and reliability of the measurement data.

[0025] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a schematic diagram of the structure of a system for calibrating the refractive index of air based on a self-traceable grating interferometer according to the present invention;

[0027] Figure 2 A schematic diagram of a self-traceable grating preparation process of a system and method for calibrating the refractive index of air based on a self-traceable grating interferometer according to the present invention;

[0028] Reference numerals

[0029] 1. Self-traceable grating interferometer, 2. Nano-displacement stage, 3. Nano-displacement stage, 4. Zero expansion glass, 5. Reflector, 6. Measurement optical path that affects the standard wavelength of the laser interferometer. DETAILED DESCRIPTION

[0030] The technical solution of the present invention is further described below through the accompanying drawings and embodiments.

[0031] 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.

[0032] Example

[0033] like Figure 1 As shown, the present invention provides a system for calibrating the refractive index of air based on a self-traceable grating interferometer, including a self-traceable grating interferometer, wherein the light source on the self-traceable grating interferometer is refracted onto the zero expansion glass on the nano-displacement stage 1, and then a measuring light path that affects the standard wavelength of the laser interferometer is formed through a reflector and connected to the nano-displacement stage 2.

[0034] A method for calibrating air refractive index based on a self-traceable grating interferometer comprises the following steps:

[0035] S1, after the periodic direction of the self-traceable grating and the measurement axis direction of the reflector are aligned according to the Abbe principle, they are fixed on the nano-displacement stage 2, and the host computer controller issues a measurement command to drive the nano-displacement stage 1 carrying the self-traceable grating and the reflector to perform precise displacement;

[0036] S2. Sample the self-traceable grating interferometer signal and the laser interferometer signal according to the Nyquist sampling theorem, and extract the interference phase;

[0037] S3. Based on the displacement measurement result of the self-traceable grating interferometer, the equivalent wavelength of the laser interferometer on the measurement path is calculated;

[0038] S4. The ratio of the standard wavelength of the laser interferometer light source to the equivalent wavelength is used as the average air refractive index on the laser interferometer measurement path.

[0039] In step S1, the zero Abbe principle is specifically to make the period direction of the self-traceable grating and the measuring axis of the reflector collinear by mechanically designing the nano-translation stage adapter.

[0040] In step S1, the self-traceable grating is a grating structure prepared by atomic lithography technology with extremely high period accuracy and uniformity. During this process, the metal material is heated and sublimated to a gas state in a vacuum environment and ejected in an effusion manner, wherein the ejected atomic beam locks the laser wavelength to the transition frequency of the two energy levels of the metal atoms through a frequency-stabilized laser, and the ejected periodically distributed atoms are deposited on the zero-expansion glass, and the formed grating structure enables its period to be directly traced back to the transition frequency of the two energy levels of the metal atoms.

[0041] In step S2, the wavelength used for measurement by the laser interferometer is not compensated for the influence of the air refractive index. During use, the measurement result of the laser interferometer directly reflects the influence of the air refractive index on the laser wavelength without any form of calibration or adjustment.

[0042] In step S2, the host computer samples the signal at a specific frequency according to the Nyquist sampling theorem, and converts the interference signal into a displacement signal through a signal processing algorithm. The relationship between the phase and displacement of the self-traceable grating interferometer and the laser interferometer is as follows:

[0043]

[0044] Where d is the grating pitch, m is the subdivision factor, n is the refractive index of air, λ is the laser wavelength in air, and λ0 is the laser wavelength in vacuum.

[0045] In step S3, the displacement measurement reference is the period of the self-traceable grating. The frequency difference of the two beams of light generated by the Doppler effect of the grating movement is used to generate an interference measurement signal after beam combination. The signal is converted into a displacement based on the grating period, thereby realizing an interferometer for micro-nanoscale measurement. The self-traceable grating interferometer uses the grating spacing as the measurement reference, and its result is not affected by changes in the air environment. The laser interferometer uses the laser wavelength in the air as the reference, and the wavelength is affected by changes in environmental conditions.

[0046] In step S4, the laser standard wavelength is a standard wavelength of the laser interferometer in a vacuum environment. In an air environment, the laser wavelength of the laser interferometer will be affected by the refractive index of the air, and the actual wavelength of the laser will be smaller than the standard wavelength. The laser interferometer is used to measure and calculate the influence of the refractive index of the air in a non-vacuum environment.

[0047] In step S4, the average air refractive index is calculated by comparing the standard wavelength of the laser interferometer light source with the equivalent wavelength on the laser interferometer measurement path measured based on the self-traceable grating to measure the average air refractive index of the measurement path.

[0048] Among them, the self-traceable grating interferometer refers to an interferometer that uses a self-traceable grating as a displacement scale, utilizes the frequency difference of two beams of light generated by the Doppler effect of grating movement, generates an interference measurement signal after beam combining, and converts the signal into a displacement based on the grating period, thereby realizing micro-nano scale measurement. Its signal processing method undergoes signal filtering, differential amplification, orthogonalization and regularization processing, and finally uses the inverse tangent method to calculate the displacement value.

[0049] A self-traceable grating is a grating whose characteristic parameters (such as period) can be directly traced back to some natural reference. The chromium atomic lithography grating that can be traced back to the transition frequency of chromium atoms is a typical example of a self-traceable grating. Figure 2 This is the schematic diagram of chromium atom lithography technology. Specifically, the chromium transition frequency (7S3→7P4) is selected as the natural reference, and the laser wavelength is strictly locked at the wavelength (425.55nm) corresponding to the transition frequency, so that the collimated chromium atomic beam is orthogonally deposited on the substrate through the standing wave field formed by the mutual reflection of the above wavelengths. The chromium atoms will be gathered to the antinode or node position corresponding to the standing wave field, forming a periodic parallel grating structure on the substrate. The grating period error can theoretically be less than 0.001nm, and the period error of the actual product does not exceed 0.1nm. The line density of the chromium atom lithography grating can reach 4700 lines / mm, and its surface shape is sinusoidal, and it can be repeatedly prepared. It has the characteristics of high consistency, high uniformity, and strong resistance to environmental changes under the condition of maintaining high line density, and the self-traceable grating has been approved as a national first-class standard material.

[0050] Phase unwrapping is a technique used to extract continuous phase information from the interference pattern of an interferometer. It mainly includes:

[0051] (1) Wrapping phase extraction

[0052] Wrap Phase It is calculated from the intensity image I of the interference pattern, usually using the following formula:

[0053]

[0054] Among them I sin ,I cos are the sine and cosine components of the interference pattern, respectively.

[0055] (2) Signal error correction

[0056] The collected voltage signal is passed through a digital low-pass filter to remove high-frequency noise signals, and then the Heydemann method is used to calculate and correct the nonlinear effect of the signal combined with the interference principle to obtain a standard orthogonal interference signal;

[0057] (3) Phase unwrapping

[0058] The purpose of phase expansion is to Transition to the continuous phase of practice It is expressed by the following formula:

[0059]

[0060] in is the decimal part, and j is an integer used to adjust the phase of each point to ensure phase continuity.

[0061] (4) Displacement calculation

[0062] After the optical phase is obtained, the Doppler effect signal of the grating interferometer makes the phase directly linearly related to the displacement. The displacement information can be directly obtained through this transformation. The specific transformation relationship is described as:

[0063]

[0064] Similarly, the displacement information formula of the laser interferometer is:

[0065]

[0066] Based on the phase ratio diagrams of self-traceable grating interferometer and laser interferometer under different environments, the correction factor and air refractive index were calculated.

[0067] Therefore, the present invention adopts the above-mentioned system and method for calibrating the refractive index of air based on a self-traceable grating interferometer, and adopts a grating as a displacement measurement reference, thereby avoiding the interference of air factors on the measurement data. Under the same measurement environment, it is more stable than a laser interferometer using the laser wavelength as a reference, and the measured data is more comparable and the measurement repetition stability is greater. The calibration process is simplified, the calibration efficiency is improved, and the calibration process is carried out in parallel with the measurement process, without interrupting the measurement to perform an independent calibration operation. In a dynamically changing measurement environment, the calibration parameters can be adjusted in real time to respond to environmental fluctuations, ensuring the accuracy and reliability of the measurement data.

[0068] 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 system for calibrating the refractive index of air based on a self-traceable grating interferometer, characterized in that: It comprises a self-traceable grating interferometer, the light source on the self-traceable grating interferometer is refracted onto the zero expansion glass on the first nano displacement stage, and then a measuring optical path affecting the standard wavelength of the laser interferometer is formed through a reflector and connected to the second nano displacement stage.

2. The method for calibrating the air refractive index based on a self-traceable grating interferometer according to claim 1, characterized in that: The following steps are involved: S1, after the periodic direction of the self-traceable grating and the measurement axis direction of the reflector are aligned according to the Abbe principle, they are fixed on the nano-displacement stage 2, and the host computer controller issues a measurement command to drive the nano-displacement stage 1 carrying the self-traceable grating and the reflector to perform precise displacement; S2. Sample the self-traceable grating interferometer signal and the laser interferometer signal according to the Nyquist sampling theorem, and extract the interference phase; S3. Based on the displacement measurement result of the self-traceable grating interferometer, the equivalent wavelength of the laser interferometer on the measurement path is calculated; S4. The ratio of the standard wavelength of the laser interferometer light source to the equivalent wavelength is used as the average air refractive index on the laser interferometer measurement path.

3. The method for calibrating the air refractive index based on a self-traceable grating interferometer according to claim 2, characterized in that: In step S1, the zero Abbe principle is specifically to make the period direction of the self-traceable grating and the measuring axis of the reflector collinear by mechanically designing the nano-translation stage adapter.

4. The method for calibrating the air refractive index based on a self-traceable grating interferometer according to claim 2, characterized in that: In step S1, the self-traceable grating is a grating structure prepared by atomic lithography technology with extremely high period accuracy and uniformity. During this process, the metal material is heated and sublimated to a gas state in a vacuum environment and ejected in an effusion manner, wherein the ejected atomic beam locks the laser wavelength to the transition frequency of the two energy levels of the metal atoms through a frequency-stabilized laser, and the ejected periodically distributed atoms are deposited on the zero-expansion glass, and the formed grating structure enables its period to be directly traced back to the transition frequency of the two energy levels of the metal atoms.

5. The method for calibrating the air refractive index based on a self-traceable grating interferometer according to claim 2, characterized in that: In step S2, the wavelength used for measurement by the laser interferometer is not compensated for the influence of the air refractive index. During use, the measurement result of the laser interferometer directly reflects the influence of the air refractive index on the laser wavelength without any form of calibration or adjustment.

6. The method for calibrating the air refractive index based on a self-traceable grating interferometer according to claim 2, characterized in that: In step S2, the host computer samples the signal at a specific frequency according to the Nyquist sampling theorem, and converts the interference signal into a displacement signal through a signal processing algorithm. The relationship between the phase and displacement of the self-traceable grating interferometer and the laser interferometer is as follows: Where d is the grating pitch, m is the subdivision factor, n is the refractive index of air, λ is the laser wavelength in air, and λ0 is the laser wavelength in vacuum.

7. The method for calibrating the air refractive index based on a self-traceable grating interferometer according to claim 2, characterized in that: In step S3, the displacement measurement reference is the period of the self-traceable grating. The frequency difference of the two beams of light generated by the Doppler effect of the grating movement is used to generate an interference measurement signal after beam combining. The signal is converted into a displacement based on the grating period, thereby realizing an interferometer for micro-nano scale measurement.

8. The method for calibrating the air refractive index based on a self-traceable grating interferometer according to claim 2, characterized in that: In step S4, the laser standard wavelength is a standard wavelength of the laser interferometer in a vacuum environment. In an air environment, the laser wavelength of the laser interferometer will be affected by the refractive index of the air, and the actual wavelength of the laser will be smaller than the standard wavelength. The laser interferometer is used to measure and calculate the influence of the refractive index of the air in a non-vacuum environment.

9. The method for calibrating the air refractive index based on a self-traceable grating interferometer according to claim 2, characterized in that: In step S4, the average air refractive index is calculated by comparing the standard wavelength of the laser interferometer light source with the equivalent wavelength on the laser interferometer measurement path measured based on the self-traceable grating to measure the average air refractive index of the measurement path.

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