Low-frequency vibration measuring device and measuring method

Abstract

The invention relates to the technical field of vibration measurement, in particular to a low-frequency vibration measurement device and method, and the device comprises a multi-wavelength light source module, a vortex beam reference arm module, a Gaussian beam detection arm module and an interference pattern collection and measurement module. The vortex beam reference arm module is used for preparing a multi-wavelength vortex beam as a reference beam. According to the invention, an interference vibration measurement configuration in which a Gaussian beam is used as a detection beam and a vortex beam is used as a reference beam is provided, a method of carrying out interference measurement after carrying out spatial filtering on returned detection light is adopted, and interference pattern distortion caused by static aberration is effectively filtered out.

Description

Technical Field

[0001] The present invention relates to the field of vibration measurement technology, and in particular to a low-frequency vibration measurement device and a measurement method. Background Art

[0002] Mechanical vibration is a common phenomenon in industrial production and daily life. The measurement of vibration plays a key role in monitoring the health status, operating efficiency and fault diagnosis of electromechanical equipment. In particular, for large structures such as large hydroelectric generators and reservoir dams, their low-frequency vibration characteristics are extremely important in health status monitoring. In the measurement of low-frequency micro-vibration, due to the low movement speed of the target to be measured, the Doppler frequency shift is small, and the commonly used laser Doppler vibration measurement technology is less applicable. Laser interferometry can measure the high-precision position information of the target in real time, and does not rely on the Doppler effect. Therefore, it is suitable for online monitoring of low-frequency vibration of large power machinery such as hydroelectric generators. Optical-based vibration measurement technology provides fast and robust solutions in various applications because it can provide comprehensive vibration information without interference and has the characteristics of high resolution and accuracy. The key technologies of these solutions are various forms of optical metrology, such as interferometry, shearing, speckle interferometry and holographic interferometry, each of which provides unique advantages in the field of vibration analysis and material testing. Among them, optical interferometer has become an important basis for vibration measurement due to its shorter wavelength and extremely high phase sensitivity. With the rapid development of optics and computer technology, optical interferometry vibration measurement technology is still in continuous development and progress.

[0003] In recent years, the field of structured light represented by vortex beams has made significant progress. The phase of a vortex beam has an angular distribution in the form of exp(ilθ), where θ is the azimuth angle and l is the topological charge (TC). It carries a certain orbital angular momentum (OAM) and has been applied in fields including holographic display, optical storage, and optical communication. In the field of optical sensing, vortex beams also have unique advantages. By replacing the fundamental mode Gaussian vortex beam in a conventional laser interferometer with a vortex beam, a vortex beam interferometer can be constructed. In a conventional laser interferometer, the change in the phase difference between the two arms will cause a change in the optical power at the output port of the interferometer. After calibration, the relative phase shift can be calculated based on the measured optical power. However, the output optical power of the port is not only related to the relative phase shift, but is also directly affected by the total power of the laser light source and the splitting ratio of the beam splitter. In contrast, in a vortex beam interferometer, the interferometer outputs an interference pattern with a specific angular light intensity distribution. The change in relative phase shift is directly reflected in the rotation angle of the interference pattern, while the change in light source power and splitting ratio does not affect the rotation angle, but only affects the overall brightness and contrast of the interference pattern. This achieves the decoupling of the relative phase shift to be measured, and has significant robustness. Based on this, vortex beam interferometers have been initially applied in fields such as rotation measurement, refractive index measurement, sample concentration measurement, and optical surface measurement.

[0004] In the field of vibration measurement, the application of vortex beam interferometry technology is still in the preliminary research stage. At present, only a few research papers have conducted theoretical analysis and preliminary experimental verification of the basic principles of vortex beam interferometry vibration measurement in a laboratory environment, indicating that the scheme has significant advantages in accuracy and robustness, and the structure is simple and compact. However, through literature research and analysis of actual application scenarios, it can be found that the main mechanism of these measurement methods is to use a typical vortex beam (Laguerre-Gaussian mode beam) as a probe light to be incident on the object to be measured, and to measure by collecting the petal-shaped pattern formed by the interference between the reflected light and the reference light. For the reference beam, most of the research work uses a Laguerre-Gaussian beam conjugate with the probe light. The two have good mode matching, but the measurement accuracy and dynamic range are low, and when the beam alignment deviates, the interference pattern is severely deformed, making it difficult to effectively extract data. Using a Gaussian beam as a reference beam can improve the measurement accuracy and dynamic range, and reduce the requirements for beam alignment, but the light intensity distribution between the Laguerre-Gaussian beam and the Gaussian beam is significantly different, and the mode matching effect is poor, which hinders the improvement of the signal-to-noise ratio of the interference pattern. In addition, in actual optical systems, vortex beams are extremely sensitive to aberrations such as coma and astigmatism of optical systems, and their light intensity distribution will be significantly distorted or even split at singular points. In most practical application scenarios of vibration measurement, there are often external factors such as rough reflective surfaces, airflow disturbances, and particulate dust in the propagation path of the detection beam, which leads to unpredictable aberrations. Therefore, for existing vibration measurement schemes that use vortex beams as detection beams, it may be difficult to obtain stable interference pattern data in practical applications. Another important limitation of vortex beam interferometer vibration measurement comes from the inherent 2π phase periodicity of the optical interferometer. Therefore, the amplitude of the vibration measured by this scheme is limited to less than half the wavelength of the detection light, that is, hundreds of nanometers, which is difficult to meet the needs of many industrial application scenarios (on the order of 10 microns). Summary of the invention

[0005] The purpose of the present invention is to provide a low-frequency vibration measurement device and measurement method to solve the technical problem in the prior art that the amplitude of the measured vibration is limited to less than half the wavelength of the detection light, that is, hundreds of nanometers, which is difficult to meet the needs of many industrial application scenarios (on the order of 10 microns).

[0006] The invention discloses a low-frequency vibration measuring device, which comprises a multi-wavelength light source module, a vortex beam reference arm module, a Gaussian beam detection arm module and an interference pattern acquisition and measurement module.

[0007] The vortex beam reference arm module is used to prepare a multi-wavelength vortex beam as a reference beam.

[0008] Furthermore, the multi-wavelength light source module includes at least two laser light sources, each of which is provided with a 1 / 2 wave plate, and the beams are combined by a first dichroic mirror.

[0009] By setting up at least two laser light sources, it is possible to provide input lights with different central wavelengths and good coherence and collimation. The polarization directions of the linearly polarized lights are adjusted using corresponding 1 / 2 wave plates, and then the beams are combined using the first dichroic mirror and output to the vortex beam reference arm module.

[0010] Furthermore, there are two laser light sources, and the difference between their central wavelengths is in the range of 10nm to 50nm.

[0011] A smaller difference between the central wavelengths can not only increase the dual-wavelength equivalent wavelength, thereby improving the amplitude measurement range, but also reduce the negative impact of chromatic aberration of optical devices on the measurement results.

[0012] Furthermore, the vortex beam reference arm module includes a polarization beam splitter, a spiral phase plate, a first 1 / 4 wave plate and a 0-degree reflector.

[0013] When in use, the incident light beam is divided into two beams by a polarization beam splitter, wherein the reflected vertically polarized reference beam is transformed into a circularly polarized vortex beam with a topological charge of l after passing through a spiral phase plate of order l and the first 1 / 4 wave plate, and then returns to the original path through a 0-degree reflector and passes through the first 1 / 4 wave plate and the spiral phase plate again, and is transformed into a horizontally polarized vortex beam with a topological charge of 2l, and then is transmitted through the polarization beam splitter into the interference pattern acquisition and measurement module.

[0014] Furthermore, the spiral phase plate is an achromatic vortex phase plate or a vortex half-wave plate (q-plate), and adopts a double-pass spiral phase plate configuration.

[0015] Its order is l, and a vortex reference beam with a topological charge of 2l is obtained by passing through the spiral phase plate configuration twice.

[0016] Furthermore, the Gaussian beam detection arm module includes a first detection lens, a pinhole, a second detection lens, a second 1 / 4 wave plate and a focusing lens.

[0017] Further, the back focal plane of the first detection lens coincides with the front focal plane of the second detection lens, and the pinhole is placed there.

[0018] The horizontally polarized detection beam is focused by the first detection lens and passes through the center of the pinhole to obtain an ideal Gaussian beam. It is then collimated by the second detection lens and converted into circularly polarized light by the second 1 / 4 wave plate. It is then focused by the focusing lens and incident on the sample to be measured. The position of the focusing lens is adjusted so that the sample is located near the back focal plane of the lens. The detection light reflected from the sample passes through the above optical path again in the opposite direction, is converted into vertical polarization, and is reflected by the polarization beam splitter into the interference pattern acquisition and measurement module.

[0019] Furthermore, the interference pattern acquisition and measurement module includes a first lens, a polarizer, a second lens, a second dichroic mirror and a camera, and the number of the cameras corresponds to the laser light source.

[0020] Further, the front focal plane of the first lens is located on the front surface of the spiral phase plate in the vortex beam reference arm module, and the back focal plane of the first lens is coincident with the front focal plane of the second lens.

[0021] Furthermore, the back focal plane of the second lens is measured to coincide with the photosensitive surface of a camera, and the distances between all cameras and the second dichroic mirror are the same.

[0022] Among them, the first measuring lens and the second measuring lens form a 4F imaging optical path structure. According to the ABCD matrix theoretical analysis of beam propagation, for the reference vortex beam, its light field distribution on the camera photosensitive surface has the same distribution as the light field on the front surface of the spiral phase plate, that is, a vortex light field with Gaussian light intensity distribution and spiral phase distribution; and the detection Gaussian beam passes through the above optical path and is still a collimated Gaussian beam on the camera photosensitive surface. After multiple beams of light are transmitted through the polarizer, the polarization states are consistent, and interference occurs to produce interference patterns. By selecting the angle between the main axis of the polarizer and the horizontal plane, the power ratio between the reference light and the detection light participating in the interference can be adjusted. Since the light intensity distribution of the reference beam and the detection beam is basically consistent, the mode matching is good, which is conducive to the full utilization of the optical power and improves the signal-to-noise ratio of the interference pattern measurement.

[0023] A low-frequency vibration measurement method based on multi-wavelength vortex beam interference uses a Gaussian beam as a detection beam and a vortex beam as a reference beam.

[0024] Furthermore, the reference beam is a vortex light field with Gaussian intensity distribution and spiral phase distribution.

[0025] Furthermore, the above-mentioned low-frequency vibration measuring device is used.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] 1. The present invention proposes an interferometric vibration measurement configuration using a Gaussian beam as a detection beam and a vortex beam as a reference beam. The method of spatially filtering the returned detection light before interferometric measurement is adopted to effectively filter out the interference pattern distortion caused by static aberrations.

[0028] 2. A 4f-type imaging optical path is used to generate a vortex reference light with a Gaussian light intensity distribution, replacing the commonly used Laguerre-Gaussian vortex light, to achieve optimal matching between the detection light and the reference light mode;

[0029] 3. A multi-wavelength vortex beam interferometer configuration is proposed to increase the equivalent wavelength of the interferometer, thereby effectively improving the vibration amplitude range that the system can measure. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below. It should be understood that the following drawings only represent some embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without creative work.

[0031] Figure 1 It is a schematic structural diagram of the low-frequency vibration measuring device of the present invention.

[0032] Figure 2 It is a schematic diagram of the optical structure of the vortex reference light field with Gaussian light intensity distribution of the present invention.

[0033] Figure 3 This is a diagram showing the relationship between the rotation angle of the interference pattern measured by the camera in Example 1 of the present invention and the displacement of the object to be measured.

[0034] Figure 4 The vibration frequency and amplitude are obtained by fitting the data of the relative displacement varying with time in Example 1 of the present invention.

[0035] In the above drawings, the meanings of the various marks are as follows: 100-multi-wavelength light source module, 101-laser light source, 102-1 / 2 wave plate, 103-first dichroic mirror, 200-vortex beam reference arm module, 201-polarization beam splitter, 202-spiral phase plate, 203-first 1 / 4 wave plate, 204-0 degree reflector, 300-Gaussian beam detection arm module, 301-detection of the first lens, 302-pinhole, 303-detection of the second lens, 304-second 1 / 4 wave plate, 305-focusing lens, 306-sample, 400-interference pattern acquisition and measurement module, 401-measurement of the first lens, 402-polarizer, 403-measurement of the second lens, 404-second dichroic mirror, 405-first camera, 406-second camera. DETAILED DESCRIPTION

[0036] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention are clearly and completely described below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments.

[0037] Example 1

[0038] A low-frequency vibration measuring device of the above device used in this embodiment is as follows: Figure 1-Figure 2 As shown, it includes a multi-wavelength light source module 100, a vortex beam reference arm module 200, a Gaussian beam detection arm module 300 and an interference pattern acquisition and measurement module 400.

[0039] In the present embodiment, the multi-wavelength light source module 100 is provided with two laser light sources 101 to provide laser light beams of two wavelengths, whose center wavelengths are respectively, and the difference between the center wavelengths is in the range of 10nm to 50nm; the smaller difference between the center wavelengths can not only increase the dual-wavelength equivalent wavelength, thereby improving the amplitude measurement range, but also reduce the negative impact of the chromatic aberration of the optical device on the measurement result; each laser light source is provided with a 1 / 2 wave plate 102, and the beams are combined by using the first dichroic mirror 103.

[0040] By setting up two laser light sources 101, it is possible to provide input lights with different central wavelengths and good coherence and collimation. The polarization directions of the linearly polarized lights are adjusted using the corresponding 1 / 2 wave plates 102, and then the beams are combined using the first dichroic mirror 103 and output to the vortex beam reference arm module 200.

[0041] The vortex beam reference arm module 200 includes a polarization beam splitter 201, a spiral phase plate 202, a first 1 / 4 wave plate 203 and a 0-degree reflector 204. The first 1 / 4 wave plate 203 is an achromatic wave plate covering the spectral range of the dual-wavelength light source. The spiral phase plate 202 can adopt an achromatic vortex phase plate or a vortex half-wave plate (q-plate) with an order of l. By passing through the spiral phase plate 202 twice, a vortex reference beam with a topological charge of 2l is obtained.

[0042] When in use, the incident light beam is divided into two beams by the polarization beam splitter 201, wherein the reflected vertically polarized reference beam is transformed into a circularly polarized vortex beam with a topological charge of l after passing through the spiral phase plate 202 of order l and the first 1 / 4 wave plate 203, and then returns to the original path by the 0-degree reflector 204 and passes through the first 1 / 4 wave plate 203 and the spiral phase plate 202 again, and is transformed into a horizontally polarized vortex beam with a topological charge of 2l, and then is transmitted through the polarization beam splitter 201 into the interference pattern acquisition and measurement module 400.

[0043] The Gaussian beam detection arm module 300 is composed of a detection first lens 301, a pinhole 302, a detection second lens 303, a second 1 / 4 wave plate 304 and a focusing lens 305, and the sample 306 is located at the rear focal plane of the focusing lens. The rear focal plane of the detection first lens 301 coincides with the front focal plane of the detection second lens 303, and the detection pinhole 302 is placed there. The horizontally polarized detection beam is focused by the detection first lens 301 and passes through the center of the pinhole 302 to obtain an ideal Gaussian beam, and then collimated by the detection second lens 303, and converted into circularly polarized light by the second 1 / 4 wave plate 304, and then focused by the focusing lens 305 to be incident on the sample 306 to be measured. The position of the focusing lens 305 is adjusted so that the sample 306 is located near the rear focal plane of the lens. The detection light reflected from the sample passes through the above-mentioned optical path again in the opposite direction, is converted into vertical polarization, and is reflected by the polarization beam splitter 201 into the interference pattern acquisition and measurement module 400;

[0044] The interference pattern acquisition and measurement module 400 is composed of a first lens 401, a polarizer 402, a second lens 403, a second dichroic mirror 404, a first camera 405, and a second camera 406. Among them, the first lens 401 and the second lens 403 form a 4F imaging optical path structure, the front focal plane of the first lens 401 is located on the front surface of the spiral phase plate 202 in the vortex beam reference arm module 200, the back focal plane of the first lens 401 coincides with the front focal plane of the second lens 403, the back focal plane of the second lens 403 coincides with the photosensitive surface of the first camera 405, and the distances between the first camera 405, the second camera 406 and the second dichroic mirror 404 are the same. After the two beams of light are transmitted through the polarizer 402, the polarization states are consistent, and interference occurs to produce interference patterns. By selecting the angle between the main axis of the polarizer 402 and the horizontal plane, the power ratio between the reference light and the detection light participating in the interference can be adjusted.

[0045] like Figure 1 As shown, according to the ABCD matrix theoretical analysis of beam propagation, for the reference vortex beam, its light field distribution on the photosensitive surface of the first camera 405 and the second camera 406 has the same distribution as the light field on the front surface of the spiral phase plate 202, that is, a vortex light field with Gaussian light intensity distribution and spiral phase distribution, E GV (r,θ)=A1exp(-r 2 / w 2 )exp(ilθ); and the detection Gaussian beam passes through the above optical path and is still a collimated Gaussian beam on the photosensitive surface of the first camera 405 and the second camera 406, Since the light intensity distribution of the reference beam and the detection beam is basically the same, the mode matching is good, and the light intensity distribution of the interference pattern is It is beneficial to fully utilize the optical power and improve the signal-to-noise ratio of interference pattern measurement.

[0046] The interference between the detection Gaussian beam and the reference vortex beam will produce a petal-shaped interference pattern. The vibration of the sample 306 to be tested will cause the optical path difference of the detection Gaussian beam relative to the reference vortex beam to change, thereby causing the relative phase of the two beams to change when they interfere, and then causing the spatial pointing angle of the petal-shaped interference pattern to change accordingly. It can be seen that under the same displacement change, the relative phase of light beams of different wavelengths is inversely proportional to the wavelength, so the rotation angles of the interference patterns of light beams of different wavelengths are also different, such as Figure 3 shown.

[0047] In addition, since the measurement is achieved by extracting the spatial orientation angle of the petal-shaped interference pattern, it can be seen from the above formula that when the change in the vibration optical path difference reaches Δz=λ / 2, the rotated interference pattern will overlap with the interference pattern of Δz=0, making it impossible to correctly obtain the change information of the optical path difference. Therefore, the vibration amplitude of the single-wavelength interference measurement is limited to the half-wavelength range. For a dual-wavelength interference system with wavelengths λ1 and λ2, the equivalent light wavelength is λ0=λ1λ2 / |λ1-λ2|. Taking the common semiconductor laser tube wavelengths λ1=785nm and λ2=808nm as an example, the equivalent light wavelength is λ0=λ1λ2 / |λ1-λ2|=27.57μm. Within the equivalent half-wavelength range (λ0 / 2=13.78μm), the combination of the rotation angles of the two-wavelength interference patterns It will not recur, so the change value of the optical path difference can be uniquely determined.

[0048] like Figure 4 As shown, by measuring the change of the spatial pointing angle of the interference pattern over time, the change of the displacement of the sample 306 to be tested over time can be obtained. By fitting the data of the displacement change over time, the vibration frequency and vibration amplitude information of the sample can be obtained, thereby providing a low-frequency vibration measurement system based on multi-wavelength vortex beam interference.

[0049] The above are the implementation methods listed in this embodiment, but this embodiment is not limited to the above optional implementation methods. Those skilled in the art can arbitrarily combine the above methods to obtain other various implementation methods. Anyone can derive other various forms of implementation methods under the inspiration of this embodiment. The above specific implementation methods should not be understood as limiting the scope of protection of this embodiment. The scope of protection of this embodiment shall be based on the definition in the claims, and the description can be used to interpret the claims.

Claims

1. A low-frequency vibration measuring device, characterized in that: It comprises a multi-wavelength light source module (100), a vortex beam reference arm module (200), a Gaussian beam detection arm module (300) and an interference pattern acquisition and measurement module (400); The vortex beam reference arm module (200) is used to prepare a multi-wavelength vortex beam as a reference beam.

2. A low-frequency vibration measuring device according to claim 1, characterized in that: The multi-wavelength light source module (100) comprises at least two laser light sources (101), each laser light source (101) is provided with a 1 / 2 wave plate (102), and beams are combined using a first dichroic mirror (103).

3. A low-frequency vibration measuring device according to claim 2, characterized in that: There are two laser light sources (101), and the difference between their central wavelengths is in the range of 10 nm to 50 nm.

4. A low-frequency vibration measuring device according to claim 1, characterized in that: The vortex beam reference arm module (200) comprises a polarization beam splitter (201), a spiral phase plate (202), a first quarter wave plate (203) and a 0-degree reflector (204).

5. A low-frequency vibration measuring device according to claim 4, characterized in that: The spiral phase plate (202) is an achromatic vortex phase plate or a vortex half-wave plate, and adopts a double-pass spiral phase plate (202) configuration.

6. A low-frequency vibration measuring device according to claim 1, characterized in that: The Gaussian beam detection arm module (300) comprises a first detection lens (301), a pinhole (302), a second detection lens (303), a second 1 / 4 wave plate (304) and a focusing lens (305).

7. A low-frequency vibration measuring device according to claim 6, characterized in that: The back focal plane of the first detection lens (301) coincides with the front focal plane of the second detection lens (303), and the pinhole (302) is placed there.

8. A low-frequency vibration measuring device according to claim 1, characterized in that: The interference pattern acquisition and measurement module (400) comprises a first lens (401) for measuring, a polarizing plate (402), a second lens (403) for measuring, a second dichroic mirror (404), and cameras, wherein the number of cameras corresponds to the number of laser light sources.

9. A low-frequency vibration measuring device according to claim 8, characterized in that: The front focal plane of the first lens (401) is located on the front surface of the spiral phase plate (202) in the vortex beam reference arm module (200), the back focal plane of the first lens (401) is coincident with the front focal plane of the second lens (403), the back focal plane of the second lens (403) is coincident with the photosensitive surface of a camera, and the distances between all cameras and the second dichroic mirror (404) are the same.

10. A low-frequency vibration measurement method based on multi-wavelength vortex beam interferometry, characterized in that: The Gaussian beam was used as the probe beam, and the vortex beam was used as the reference beam.

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