A system and method for calibrating the refractive index of air based on a self-tracing grating interferometer

Abstract

The present invention discloses a system and method for calibrating the air refractive index based on a self-tracing grating interferometer, belonging to the technical field of air refractive index measurement. By adopting the above-mentioned system and method for calibrating the air refractive index based on a self-tracing grating interferometer, a grating is used as the displacement measurement reference, avoiding the interference of air factors on the measurement data. It is more stable than a laser interferometer using a laser wavelength as the reference in the same measurement environment, the measured data has higher comparability, and the measurement repeatability and stability are greater. It avoids the traditional calibration method that relies on a large number of environmental sensors (such as temperature, humidity, and pressure sensors), simplifies the calibration process, improves the calibration efficiency, 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, it can instantaneously adjust the calibration parameters to respond to environmental fluctuations, ensuring the accuracy and reliability of the measurement data.

Description

Technical Field

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

[0002] Precision and ultra-precision measurement technologies play an important role in the modern processing and manufacturing fields. The improvement of their measurement range and accuracy directly affects the size and accuracy of the products obtained during the manufacturing process. Laser interferometers are widely used in the field of displacement measurement due to their non-contact measurement and large-range measurement characteristics. However, the measurement accuracy of a laser interferometer is based on the accuracy of the wavelength, and its measurement results are based on the wavelength as a reference. In long-distance displacement measurement, environmental factors, especially the changes in temperature, pressure, and humidity, have a significant impact on the air refractive index, thereby causing perturbations to the wavelength. This perturbation directly affects the accuracy of the displacement measurement results. Therefore, accurately measuring the air refractive index is crucial for ensuring the accuracy of displacement measurement. By accurately determining and compensating for the changes in the air refractive index, the measurement range and application scenarios of the laser interferometer are extended under various environmental conditions, and its applicability in diverse measurement scenarios is enhanced.

[0003] The main existing method for measuring the air refractive index is the empirical formula method. This method relies on environmental sensors to measure environmental parameters (such as temperature, pressure, humidity, carbon dioxide concentration, etc.) and calculates the air refractive index through empirical formulas such as the Edlén equation. However, the main defect of this measurement method is its high dependence 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 measurements of a large number of environmental sensors, with high costs and low efficiency.

[0004] The grating interferometer has the characteristics of anti-environmental interference and strong robustness, providing the basic conditions for on-site calibration of the laser interferometer. Since the grating interferometer relies on traditional gratings for measurement, traditional gratings have strong anti-environmental interference and high stability, providing the basic conditions for on-site measurement of the air refractive index. However, the grating period uncertainty of traditional gratings contains components of the air refractive index and does not have traceability. 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 can be calibrated under complex conditions, 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, is more suitable for on-site measurement and online measurement in enterprises, and promotes the industrial application of nanometrology.

[0006] To achieve the above object, the present invention provides a system for calibrating the air refractive index based on a self-tracing grating interferometer, including a self-tracing grating interferometer. The light source on the self-tracing grating interferometer is refracted onto the zero-expansion glass on the first nano-displacement stage, and then forms a measurement optical path that affects the standard wavelength of the laser interferometer through a mirror and is connected to the second nano-displacement stage.

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

[0008] S1. After the self-tracing grating period direction and the mirror measurement axis direction are aligned according to the Abbe principle, they are fixed on the second nano-displacement stage. The upper computer controller issues a measurement instruction to drive the first nano-displacement stage carrying the self-tracing grating and the mirror to perform precise displacement.

[0009] S2. Sample the self-tracing 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-tracing grating interferometer, calculate the equivalent wavelength of the laser interferometer on the measurement path.

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

[0012] Preferably, in step S1, the zero Abbe principle specifically means that through mechanical design of the nano-displacement stage adapter, the self-tracing grating period direction and the mirror measurement axis are collinear.

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

[0014] Preferably, in step S2, the wavelength of the laser interferometer used in the measurement has not been 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. The relationship between the phase and displacement of the self-tracing grating interferometer and the laser interferometer is as follows:

[0016]

[0017] where d represents the grating pitch, m represents the subdivision multiple, n represents the air refractive index, λ represents the laser wavelength in air, and λ 0 is the laser wavelength in vacuum.

[0018] Preferably, in step S3, the displacement measurement reference is the period of the self-tracing grating. Using the frequency difference between two beams of light generated by the Doppler effect of the grating movement, an interference measurement signal is generated after beam combination, and the signal is converted into a displacement quantity based on the grating period quantity, thereby realizing an interferometer for micro-nano scale measurement. The self-tracing grating interferometer uses the grating pitch 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 air as the reference, and the wavelength is affected by changes in environmental conditions.

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

[0020] Preferably, in step S4, the method for calculating the average air refractive index is to take the ratio of the standard wavelength of the laser interferometer light source to the equivalent wavelength on the measurement path of the laser interferometer measured based on the self-tracing grating, and measure the average air refractive index of the measurement path.

[0021] Therefore, the system and method for calibrating the air refractive index based on the self-tracing grating interferometer with the above structure of the present invention have the following beneficial effects:

[0022] (1) The present invention uses a grating as the displacement measurement reference, avoiding the interference of air factors on the measurement data, being more stable than a laser interferometer using the laser wavelength as the reference in the same measurement environment, having higher comparability of the measured data, and greater measurement repeatability and stability.

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

[0024] (3) The present invention has real-time calibration capabilities: the calibration process runs in parallel with the measurement process, eliminating the need to interrupt the measurement for an independent calibration operation. In a dynamically changing measurement environment, calibration parameters can be adjusted instantaneously in response to environmental fluctuations to ensure the accuracy and reliability of measurement data.

[0025] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Description of the Drawings

[0026] Figure 1 It is a schematic structural diagram of a system for calibrating the air refractive index based on a self-tracing grating interferometer of the present invention;

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

[0028] Reference Signs

[0029] 1. Self-tracing grating interferometer, 2. Nanometer displacement stage, 3. Nanometer displacement stage, 4. Zerodur glass, 5. Reflecting mirror, 6. Measurement optical path affecting the standard wavelength of the laser interferometer. Detailed Embodiments

[0030] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0031] Unless otherwise defined, the technical terms or scientific terms used in the present invention shall have the ordinary meanings understood by those of ordinary skill in the field to which the present invention pertains. The "first", "second" and similar terms used in the present invention do not denote any order, quantity or importance, but are only used to distinguish different components. The terms such as "including" or "comprising" mean that the elements or items appearing before this word cover the elements or items listed after this word and their equivalents, without excluding other elements or items. The terms such as "connected" or "coupled" are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. The terms such as "upper", "lower", "left" and "right" are only used to indicate relative positional relationships, and when the absolute position of the object being described changes, the relative positional relationship may also change accordingly.

[0032] Embodiment

[0033] As Figure 1 shown, the present invention provides a system for calibrating the air refractive index based on a self-tracing grating interferometer, including a self-tracing grating interferometer. The light source on the self-tracing grating interferometer is refracted onto the Zerodur glass on the first nanometer displacement stage, and then forms a measurement optical path affecting the standard wavelength of the laser interferometer through the reflecting mirror and is connected to the second nanometer displacement stage.

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

[0035] S1. After the self-traceable grating period direction and the mirror measurement axis direction are aligned according to the Abbe principle, they are fixed on the second nano-displacement stage. The host computer controller issues a measurement instruction to drive the first nano-displacement stage carrying the self-traceable grating and the mirror 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, calculate the equivalent wavelength of the laser interferometer on the measurement path.

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

[0039] In step S1, the zero Abbe principle specifically means that through mechanical design of the nano-displacement stage adapter, the self-traceable grating period direction and the mirror measurement axis are collinear.

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

[0041] In step S2, the wavelength of the laser interferometer used in the measurement has not been 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 represents the grating pitch, m represents the subdivision multiple, n represents the air refractive index, λ represents the laser wavelength in air, and λ 0is the laser wavelength in vacuum.

[0045] In step S3, the displacement measurement reference is the period of the self-traceable grating. By using the frequency difference between two beams of light generated by the Doppler effect of the grating movement, an interference measurement signal is generated through beam combination. Based on the grating period quantity, the signal is converted into a displacement quantity, thus realizing an interferometer for micro-nano scale measurement. The self-traceable grating interferometer uses the grating pitch as the measurement reference, and its result is not affected by the change of the air environment, while the laser interferometer uses the laser wavelength in the air as the reference, and the wavelength is affected by the change of environmental conditions.

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

[0047] In step S4, the method for calculating the average air refractive index is to take the ratio of the standard wavelength of the laser interferometer light source to the equivalent wavelength on the measurement path of the laser interferometer measured with the self-traceable grating as the reference, and measure the average air refractive index of the measurement path.

[0048] Among them, the self-traceable grating interferometer refers to an interferometer that uses the self-traceable grating as the displacement scale, uses the frequency difference between two beams of light generated by the Doppler effect of the grating movement, generates an interference measurement signal through beam combination, and converts the signal into a displacement quantity based on the grating period quantity, thus realizing micro-nano scale measurement. Its signal processing method includes signal filtering, differential amplification, orthogonalization processing and regularization processing, and finally calculates the displacement value by the arctangent method.

[0049] A self-traceable grating refers to a grating whose characteristic parameters (such as period) can be directly traced back to some natural references. The chromium atom lithography grating that can be traced back to the chromium atom transition frequency is a typical representative of the self-traceable grating. Figure 2 is the schematic diagram of the chromium atom lithography technology. Specifically, select the chromium transition frequency (7S3→7P4) as the natural reference, strictly lock the laser wavelength at the wavelength (425.55nm) corresponding to this transition frequency, and deposit the collimated chromium atom beam orthogonally through the standing wave field formed by the mutual counter-propagation of the above wavelengths onto the substrate. The chromium atoms will be concentrated at the antinode or node positions 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, the period error of the actual product does not exceed 0.1nm, and the line density of the chromium atom lithography grating can reach 4700 lines / mm. Its surface profile is sinusoidal, and it can be prepared repeatedly. It has the characteristics of high consistency, high uniformity, strong anti-environmental change ability under the condition of maintaining a high line density, and the self-traceable grating has been approved as a national first-class standard substance.

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

[0051] (1) Wrapped phase extraction

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

[0053]

[0054] where 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 non-linear effect of the signal in combination with the interference principle to obtain a standard orthogonal interference signal;

[0057] (3) Phase unwrapping

[0058] The purpose of unwrapping the phase is to convert it to a practical continuous phase which is expressed by the following formula:

[0059]

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

[0061] (4) Displacement calculation

[0062] After obtaining the optical phase, due to the Doppler effect signal of the grating interferometer, the phase is directly linearly related to the displacement. Through this transformation, the displacement information can be directly obtained. 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 the self-tracing grating interferometer and the laser interferometer in different environments, the correction factor and the air refractive index were calculated.

[0067] Therefore, the present invention adopts the above-mentioned system and method for calibrating the air refractive index based on a self-tracing grating interferometer. By using a grating as the displacement measurement reference, it avoids the interference of air factors on the measurement data. It is more stable than a laser interferometer using a laser wavelength as the reference in the same measurement environment, and the measured data has higher comparability and greater measurement repeatability and stability. It simplifies the calibration process, improves the calibration efficiency, 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, it can instantaneously adjust the calibration parameters to respond to environmental fluctuations and ensure 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 solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that they can still modify or equivalently replace the technical solutions of the present invention, and these modifications or equivalent replacements cannot make the modified technical solutions deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for calibrating the refractive index of air based on a self-traceable grating interferometer, characterized in that: It includes a self-traceable grating interferometer, the light source on the self-traceable grating interferometer is refracted onto the zero expansion glass on the nano displacement stage 1, the nano displacement stage 1 is connected to the reflector, and then the reflector forms a measuring optical path that affects the standard wavelength of the laser interferometer and is connected to the nano displacement stage 2; The method for calibrating the refractive index of air comprises the following steps: S1. After the period 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 1. 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, the self-traceable grating interferometer and the laser interferometer sample the displacement signal on the nano-displacement stage 1 according to the Nyquist sampling theorem and extract the interference phase; 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: in, represents the grating spacing, Represents the subdivision multiple, is the refractive index of air, represents the laser wavelength in air, and is the wavelength of the laser in vacuum; represents the phase change of the self-traceable grating interferometer, represents the phase change of the laser interferometer, It represents the displacement change of nano-stage 1; 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; 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. The signal is converted into a displacement based on the grating period, thereby realizing an interferometer for micro-nanoscale measurement; S4, taking the ratio of the standard wavelength of the laser interferometer light source to the equivalent wavelength as the average air refractive index on the laser interferometer measurement path; 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.

2. The method for calibrating the air refractive index based on a self-traceable grating interferometer according to claim 1, 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.

3. The method for calibrating the air refractive index based on a self-traceable grating interferometer according to claim 1, 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.

4. The method for calibrating the air refractive index based on a self-traceable grating interferometer according to claim 1, 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.

5. The method for calibrating the air refractive index based on a self-traceable grating interferometer according to claim 1, 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.

Images