Low frequency vibration measuring device and measuring method

Abstract

The present application relates to the technical field of vibration measurement, and particularly relates to a low-frequency vibration measurement device and a measurement method, comprising a multi-wavelength light source module, a vortex light beam reference arm module, a Gaussian light beam detection arm module and an interference pattern collection and measurement module. The vortex light beam reference arm module is used for preparing a multi-wavelength vortex light beam as a reference light beam. The present application proposes an interference vibration measurement configuration in which the Gaussian light beam is used as a detection light beam and the vortex light beam is used as the reference light beam, and adopts a method of performing spatial filtering on the returned detection light beam and then performing interference measurement, so that the interference pattern distortion caused by static aberration can be effectively filtered out.

Description

Technical Field

[0001] This invention relates to the field of vibration measurement technology, specifically to a low-frequency vibration measurement device and method. Background Technology

[0002] Mechanical vibration is a common phenomenon in industrial production and daily life, and its measurement plays a crucial role in monitoring the health status, operational efficiency, and fault diagnosis of electromechanical equipment. This is especially true for large structures such as large hydroelectric generators and reservoir dams, where low-frequency vibration characteristics are extremely important for health status monitoring. However, in measuring low-frequency micro-vibrations, the low velocity of the target results in a small Doppler frequency shift, making commonly used laser Doppler vibration measurement techniques less suitable. Laser interferometry, on the other hand, can measure the target's high-precision position information in real time, independent of the Doppler effect, and is therefore suitable for online monitoring of low-frequency vibrations in large power machinery such as hydroelectric generators. Optical vibration measurement techniques, due to their ability to provide comprehensive vibration information without interference and their high resolution and accuracy, offer fast and robust solutions in various applications. The key technologies for these solutions are various forms of optical metrology, such as interferometry, shearing, speckle interferometry, and holographic interferometry, each offering unique advantages in vibration analysis and materials testing. Optical interferometers, with their short wavelengths and extremely high phase sensitivity, have become an important foundation for vibration measurement. With the rapid development of optics and computer technology, optical interferometric vibration measurement technology continues to evolve and progress.

[0003] In recent years, significant progress has been made in the field of structured light, represented by vortex beams. 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 definite orbital angular momentum (OAM) and has been applied in fields including holographic displays, optical storage, and optical communication. In the field of optical sensing, vortex beams also possess 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 phase difference between the two arms causes a change in the optical power at the interferometer's output port. After calibration, the relative phase shift can be calculated based on the measured optical power. However, the output optical power at the port is not only related to the relative phase shift but is also directly affected by the total power of the laser source and the beam splitter's splitting ratio. In contrast, in a vortex beam interferometer, the output is an interference pattern with a specific angular intensity distribution. Changes in the relative phase shift are directly reflected in the rotation angle of the interference pattern, while changes in the light source power and splitting ratio do not affect the rotation angle, but only the overall brightness and contrast of the interference pattern. This achieves decoupling of the relative phase shift as the measured quantity, exhibiting significant robustness. Based on this, vortex beam interferometers have already seen initial applications in fields such as rotation measurement, refractive index measurement, sample concentration measurement, and optical surface shape measurement.

[0004] In the field of vibration measurement, the application of vortex beam interferometry is still in its early stages of research. Currently, 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, demonstrating significant advantages in accuracy and robustness, as well as a simple and compact structure. However, literature review and analysis of practical application scenarios reveal that the main mechanism of these measurement methods involves using a typical vortex beam (Laguerre-Gaussian mode beam) as the probe light incident on the object under test, and measuring by collecting the petal-shaped pattern formed by the interference between the reflected light and the reference light. For the reference beam, most studies use a Laguerre-Gaussian beam conjugate to the probe light, resulting in good mode matching, but lower measurement accuracy and dynamic range. Furthermore, when beam alignment is off, the interference pattern is severely distorted, making data extraction difficult. Using a Gaussian beam as the reference beam can improve measurement accuracy and dynamic range, and reduce the requirements for beam alignment. However, the significant difference in intensity distribution between the Laguerre-Gaussian beam and the Gaussian beam leads to poor mode matching, hindering the improvement of the signal-to-noise ratio of the interference pattern. Furthermore, in practical optical systems, vortex beams are extremely sensitive to aberrations such as coma and astigmatism, resulting in significant distortion and even singularity splitting of their intensity distribution. In most practical vibration measurement applications, the propagation path of the probe beam often contains external factors such as rough reflecting surfaces, airflow disturbances, and particulate dust, leading to unpredictable aberrations. Therefore, existing vibration measurement schemes using vortex beams as probe beams may struggle to obtain stable interferometric data in practical applications. Another significant limitation of vortex beam interferometry in vibration measurement stems from the inherent 2π phase periodicity of optical interferometers. Consequently, the amplitude of vibration measured by this scheme is limited to less than half the wavelength of the probe light, i.e., several hundred nanometers, which is insufficient to meet the requirements of many industrial applications (on the order of 10 micrometers). Summary of the Invention

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

[0006] This invention discloses a low-frequency vibration measurement device, including a multi-wavelength light source module, a vortex beam reference arm module, a Gaussian beam detection arm module, and an interferogram acquisition and measurement module.

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

[0008] Furthermore, the multi-wavelength light source module includes at least two laser light sources, each of which is equipped with a half-wave plate and is combined using a first dichroic mirror.

[0009] By setting at least two laser sources, it is possible to provide input light with different center wavelengths and good coherence and collimation. The polarization direction of the linearly polarized light is adjusted by the corresponding half-wave plate, and then the beam is combined by the first dichroic mirror and output to the vortex beam reference arm module.

[0010] Furthermore, there are two laser sources, with a difference in their center wavelengths ranging from 10 nm to 50 nm.

[0011] A smaller difference in center wavelength can not only increase the equivalent wavelength of the dual wavelengths, 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 quarter-wave plate, and a 0-degree reflector.

[0013] In use, the incident beam is split into two beams by a polarization beam splitter. 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 a first quarter wave plate. It then returns along the original path through a 0-degree mirror and passes through the first quarter wave plate and spiral phase plate again, transforming into a horizontally polarized vortex beam with a topological charge of 2l. Finally, it is transmitted through the polarization beam splitter into the interferometric 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 configuration that passes through the spiral phase plate twice.

[0015] Its order is l, and it adopts a configuration that passes through a spiral phase plate twice to obtain a vortex reference beam with a topological charge of 2l.

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

[0017] Furthermore, the rear 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 probe beam is focused by the first probe lens and passes through the center of the pinhole to obtain an ideal Gaussian beam. It is then collimated by the second probe lens and converted to circularly polarized light by the second quarter-wave plate. Finally, it is 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 near the back focal plane of the lens. The probe light reflected from the sample then travels back through the above optical path, becoming vertically polarized, and is reflected by the polarization beam splitter into the interferogram acquisition and measurement module.

[0019] Furthermore, the interferometric pattern acquisition and measurement module includes a first measuring lens, a polarizer, a second measuring lens, a second dichroic mirror, and a camera, the number of which corresponds to the number of laser light sources.

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

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

[0022] The first and second measuring lenses form a 4F imaging optical path structure. Based on the ABCD matrix theory of beam propagation, the light field distribution on the camera's photosensitive surface of the reference vortex beam is identical to that on the front surface of the spiral phase plate, exhibiting a Gaussian intensity distribution and a spiral phase distribution. The probe Gaussian beam, after passing through this optical path, remains a collimated Gaussian beam on the camera's photosensitive surface. Multiple beams, after passing through the polarizer, exhibit consistent polarization states, resulting in interference and an interference pattern. By selecting the angle between the polarizer's principal axis and the horizontal plane, the power ratio between the reference and probe beams participating in the interference can be adjusted. Since the intensity distributions of the reference and probe beams are essentially identical, the mode matching is good, facilitating full utilization of optical power and improving the signal-to-noise ratio of the interference pattern measurement.

[0023] A low-frequency vibration measurement method based on multi-wavelength vortex beam interferometry is proposed, which uses a Gaussian beam as the probe beam and a vortex beam as the reference beam.

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

[0025] Furthermore, the aforementioned low-frequency vibration measurement device is used.

[0026] Compared with the prior art, the beneficial effects of the present invention are:

[0027] 1. This invention patent proposes an interferometric vibration measurement configuration that uses a Gaussian beam as the probe beam and a vortex beam as the reference beam. It employs a method of spatially filtering the returned probe beam before performing interferometric measurements, effectively filtering out interference pattern distortion caused by static aberrations.

[0028] 2. A 4f-type imaging optical path is used to generate a vortex reference beam with a Gaussian intensity distribution, replacing the commonly used Laguerre-Gaussian vortex beam, thereby achieving optimized matching between the probe beam and the reference beam mode;

[0029] 3. A multi-wavelength vortex beam interferometer configuration is proposed to increase the equivalent wavelength of the interferometer, thereby effectively improving the range of vibration amplitudes that the system can measure. Attached Figure Description

[0030] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

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

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

[0033] Figure 3 This is a graph showing the relationship between the rotation angle of the interference pattern measured by the camera and the displacement of the object under test in Embodiment 1 of the present invention.

[0034] Figure 4 In Embodiment 1 of the present invention, the vibration frequency and amplitude are obtained by fitting the data of relative displacement over time.

[0035] In the above figures, the meanings of the various markings are as follows: 100-Multi-wavelength light source module, 101-Laser light source, 102-1 / 2 waveplate, 103-First dichroic mirror, 200-Vortex beam reference arm module, 201-Polarization beam splitter, 202-Spiral phase plate, 203-First 1 / 4 waveplate, 204-0 degree reflector, 300-Gaussian beam detection arm module, 301-Detection first lens, 302-Pinhole, 303-Detection second lens, 304-Second 1 / 4 waveplate, 305-Focusing lens, 306-Sample, 400-Interference pattern acquisition and measurement module, 401-Measurement first lens, 402-Polarizer, 403-Measurement second lens, 404-Second dichroic mirror, 405-First camera, 406-Second camera. Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0037] Example 1

[0038] The low-frequency vibration measurement device used in this embodiment is one such device. Figures 1-2 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 interferogram acquisition and measurement module 400.

[0039] In this embodiment, the multi-wavelength light source module 100 is provided with two laser light sources 101 to provide laser beams of two wavelengths, with center wavelengths of 10nm to 50nm. The smaller difference in center wavelengths can not only increase the equivalent wavelength of the two wavelengths, thereby improving the amplitude measurement range, but also reduce the negative impact of chromatic aberration of optical devices on the measurement results. Each laser light source is provided with a half-wave plate 102, and the beams are combined using a first dichroic mirror 103.

[0040] By setting two laser light sources 101, input light with different center wavelengths and good coherence and collimation can be provided. The polarization direction of the linearly polarized light is adjusted by the corresponding half-wave plate 102. Then, the beam is combined by 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 quarter wave plate 203, and a 0-degree reflector 204. The first quarter wave plate 203 is an achromatic wave plate covering the spectral range of a dual-wavelength light source. The spiral phase plate 202 can be an achromatic vortex phase plate or a vortex half-wave plate (q-plate) with an order of l. By using a configuration that passes through the spiral phase plate 202 twice, a vortex reference beam with a topological charge of 2l is obtained.

[0042] In use, the incident beam is split into two beams by the polarization beam splitter 201. 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 quarter wave plate 203. It then returns along the original path by the 0-degree reflector 204 and passes through the first quarter wave plate 203 and the spiral phase plate 202 again, transforming into a horizontally polarized vortex beam with a topological charge of 2l. Finally, it is transmitted through the polarization beam splitter 201 into the interferometric pattern acquisition and measurement module 400.

[0043] The Gaussian beam detection arm module 300 consists of a first detection lens 301, a pinhole 302, a second detection lens 303, a second quarter-wave plate 304, and a focusing lens 305. The sample 306 is located at the rear focal plane of the focusing lens. The rear 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. The horizontally polarized detection beam is focused by the first detection lens 301 and passes through the center of the pinhole 302 to obtain an ideal Gaussian beam. It is then collimated by the second detection lens 303 and converted into circularly polarized light by the second quarter-wave plate 304. After that, it is focused by the focusing lens 305 and 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 back through the above optical path again, becomes vertically polarized, and is reflected by the polarization beam splitter 201 into the interferogram acquisition and measurement module 400.

[0044] The interference pattern acquisition and measurement module 400 consists of a first measuring lens 401, a polarizer 402, a second measuring lens 403, a second dichroic mirror 404, a first camera 405, and a second camera 406. The first measuring lens 401 and the second measuring lens 403 form a 4F imaging optical path structure. The front focal plane of the first measuring lens 401 is located on the front surface of the spiral phase plate 202 in the vortex beam reference arm module 200. The rear focal plane of the first measuring lens 401 coincides with the front focal plane of the second measuring lens 403, and the rear focal plane of the second measuring lens 403 coincides with the photosensitive surface of the first camera 405. 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, their polarization states are consistent, resulting in interference and the generation of an interference pattern. By selecting the angle between the principal axis of the polarizer 402 and the horizontal plane, the power ratio between the reference light and the probe light participating in the interference can be adjusted.

[0045] like Figure 1 As shown, according to the ABCD matrix theory of beam propagation, for the reference vortex beam, the light field distribution on the photosensitive surfaces 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 a Gaussian intensity distribution and a spiral phase distribution, E GV (r,θ)=A1exp(-r 2 / w 2 )exp(ilθ); and the Gaussian beam being detected remains a collimated Gaussian beam on the photosensitive surfaces of the first camera 405 and the second camera 406 after passing through the aforementioned optical path. Since the intensity distributions of the reference beam and the probe beam are basically the same, the mode matching is good, and the intensity distribution of the interference pattern is as follows: It is beneficial to make full use of optical power and improve the signal-to-noise ratio of interferometric pattern measurement.

[0046] The interference between the probe Gaussian beam and the reference vortex beam produces a petal-shaped interference pattern. The vibration of the sample 306 causes a change in the optical path difference between the probe Gaussian beam and the reference vortex beam, which in turn alters the relative phase of the two beams during interference, resulting in a corresponding change in the spatial pointing angle of the petal-shaped interference pattern. It can be seen that, for the same displacement change, the relative phase of beams of different wavelengths is inversely proportional to the wavelength. Therefore, the rotation angle of the interference patterns of beams of different wavelengths is also different, such as... Figure 3 As shown.

[0047] Furthermore, 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 vibrational optical path difference reaches Δz = λ / 2, the rotated interference pattern will coincide with the interference pattern when Δz = 0, thus failing to correctly obtain the information on the change in optical path difference. Therefore, the vibrational amplitude of single-wavelength interferometry is limited to the half-wavelength range. For a dual-wavelength interferometry system with wavelengths λ1 and λ2, its equivalent optical wavelength is λ0 = λ1λ2 / |λ1 - λ2|. Taking a common semiconductor laser tube with wavelengths λ1 = 785 nm and λ2 = 808 nm as an example, its equivalent optical wavelength is λ0 = λ1λ2 / |λ1 - λ2| = 27.57 μm. Within the equivalent half-wavelength range (λ0 / 2 = 13.78 μm), the combination of the rotation angles of the two wavelength interference patterns... The variation will not repeat, thus allowing for a unique determination of the change in optical path difference.

[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 under test over time can be obtained. By fitting the data of the displacement over time, the vibration frequency and vibration amplitude information of the sample can be obtained, thus providing a low-frequency vibration measurement system based on multi-wavelength vortex beam interference.

[0049] The above are the embodiments listed in this example. However, this example is not limited to the optional embodiments described above. Those skilled in the art can arbitrarily combine the above methods to obtain other various embodiments. Anyone can derive other various forms of embodiments based on the inspiration of this example. The above specific embodiments should not be construed as limiting the scope of protection of this example. The scope of protection of this example should be determined by the claims, and the specification can be used to interpret the claims.

Claims

1. A low-frequency vibration measuring device, characterized in that: 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 interferogram 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; The multi-wavelength light source module (100) includes at least two laser light sources (101), each laser light source (101) is provided with a 1 / 2 wave plate (102), and the beam is combined using a first dichroic mirror (103); The vortex beam reference arm module (200) includes a polarization beam splitter (201), a spiral phase plate (202), a first quarter wave plate (203), and a 0-degree mirror (204); The Gaussian beam detection arm module (300) includes a first detection lens (301), a pinhole (302), a second detection lens (303), a second quarter-wave plate (304), and a focusing lens (305).

2. The low-frequency vibration measuring device according to claim 1, characterized in that: There are two laser sources (101), and the difference in their center wavelengths is in the range of 10nm to 50nm.

3. The low-frequency vibration measuring device according to claim 1, characterized in that: The spiral phase plate (202) is an achromatic vortex phase plate or a vortex half-wave plate, and adopts a configuration that passes through the spiral phase plate (202) twice.

4. The low-frequency vibration measuring device according to claim 1, characterized in that: The rear 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 at the point where the rear focal plane of the first detection lens (301) coincides with the front focal plane of the second detection lens (303).

5. A low-frequency vibration measuring device according to claim 1, characterized in that: The interferogram acquisition and measurement module (400) includes a first measuring lens (401), a polarizer (402), a second measuring lens (403), a second dichroic mirror (404), and a camera, the number of which corresponds to the number of laser light sources.

6. A low-frequency vibration measuring device according to claim 5, characterized in that: The front focal plane of the first measuring 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 measuring lens (401) coincides with the front focal plane of the second measuring lens (403). The back focal plane of the second measuring lens (403) coincides with the photosensitive surface of a camera. All cameras are at the same distance from the second dichroic mirror (404).

7. A low-frequency vibration measurement method based on multi-wavelength vortex beam interference, characterized in that: Using a low-frequency vibration measuring device according to any one of claims 1-6, a Gaussian beam is used as the probe beam, and a vortex beam is used as the reference beam.

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