Optical material defect identification method for vibration deformation measurement by time average holographic method

Abstract

The invention discloses an optical material defect identification method for vibration deformation measurement through a time average holographic method, and relates to the technical field of digital holographic measurement, the optical material defect identification method specifically comprises the following steps: simplifying an optical material into a circular thin plate, and obtaining the influence of the surface defect of the circular thin plate on the amplitude; obtaining the influence relation of the exposure time change of the circular thin plate under sinusoidal vibration excitation on the reconstruction effect of the time-averaged hologram; constructing a digital holographic reflective optical path; respectively shooting time average holograms before and after vibration deformation of the to-be-detected sample; extracting interference information of the to-be-detected sample; inverse Fourier transform is carried out to obtain complex amplitude distribution, phase extraction is carried out, and the phase difference before and after vibration deformation is calculated; and based on the phase difference before and after the vibration deformation, obtaining the occurrence position of the surface defect of the to-be-detected sample to realize defect identification. According to the invention, the identification of different types of defects is realized by positioning the areas with obviously abnormal phase difference stripes.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of digital holographic measurement technology, and particularly to an optical material defect identification method for vibration deformation measurement by time-averaged holography. BACKGROUND

[0002] Optical materials may have various types of defects due to improper operation during use, which may affect the performance of optical materials such as light transmittance, reflectivity, and refractive index, and may also affect the long-term reliability of the materials. A large part of optical materials have rough surfaces, so when obtaining defect information by light reflection, the imaging process of the rough surface object needs to be considered, that is, after the illumination of a coherent light source, random speckle interference phenomena will be formed on the image plane. Some of the current traditional non-destructive testing methods require long-time scanning, some have high requirements for equipment and experimental environment, and most of the detection methods are limited to specific materials. When most optical materials are excited to vibrate, the surface defect positions will exhibit abnormal local vibration modes, so it is of great significance to develop a method for identifying defects of rough surface optical materials by combining digital holographic interferometry with surface vibration physical fields, which is important for the universality, high sensitivity, and durability of optical material defects.

[0003] Digital holography is a non-contact, non-destructive, and full-range real-time quantitative imaging technology that can capture and analyze the wavefront information of three-dimensional objects. The digital holographic imaging light path takes the imaging process of the measured object as the object light path, and introduces reference light for interference, uses a charge-coupled device (CCD) instead of a chemical film to record the interference pattern, and then obtains the three-dimensional image and phase information of the measured object by numerical reconstruction. In order to solve the problem of mutual interference of zero-order images and conjugate images in holographic images, generally, the first-order positive image is extracted in the frequency domain, and then inverse Fourier transform is performed to obtain the complex amplitude distribution. In order to realize the accurate extraction of the first-order positive spectrum, the best filtering window needs to be determined after determining the center of the first-order positive spectrum. Some people propose to use histogram analysis technology for adaptive filtering, but the parameter setting in the analysis process still needs manual intervention.

[0004] Digital holographic technology can detect objects in real time and dynamically, and can obtain the three-dimensional topographic information of the object through numerical reconstruction, so it can be used for the detection of vibrating deformation objects. At present, digital holographic technology has been widely used in the measurement of vibrating object surface vibration mode, real-time vibration change, amplitude distribution, etc. Powell et al. first applied digital holography to the measurement field and proposed a time-averaged holography (TAH) method for non-contact and non-destructive testing of deformations caused by stress, vibration and heating. With the development of information technology, in 2003, Picart et al. extended the TAH technology to use digital CMOS or CCD camera sensors instead of holographic film. This new method is called time-averaged digital holography (TADH). In 2024, Fu et al. proposed a method for characterizing nanometer / micro film resonators by analyzing the average interference fringes obtained by continuous light measurement. This technology effectively establishes a direct relationship between the vibration amplitude and the blur degree of the interference fringes. Therefore, the vibration measurement method based on digital holography obtains the interference fringes after the sample vibration by using the time-averaged method, and realizes the quantitative measurement and defect positioning of the vibrating deformation object through fringe analysis and numerical operation.

[0005] Since most of the optical materials that need defect identification are rough surfaces, the design of the reflected object light path has relatively high requirements. When specifically combining digital holographic technology with vibration measurement, on the one hand, phase extraction is needed when performing digital holographic interference measurement on vibrating objects, and the time phase shift technology of on-axis holography is not suitable for non-static detection, so a defect detection method suitable for various environments is needed. SUMMARY

[0006] The purpose of the present application is to provide a method for identifying defects in optical materials for vibration deformation measurement by time-averaged holography to solve the problems in the prior art.

[0007] To achieve the above purpose, the present application provides the following technical solution: a method for identifying defects in optical materials for vibration deformation measurement by time-averaged holography, which specifically includes the following steps: Step S100, simplify the optical material into a circular thin plate, and obtain the influence of the surface defects of the circular thin plate on the vibration amplitude; Step S200, obtain the influence relationship of the circular thin plate changing the exposure time on the reconstruction effect of the time-averaged hologram under sinusoidal vibration excitation; Step S300, construct a digital holographic reflection light path composed of an interference system and a vibration excitation source; Step S400, respectively shoot the time-averaged holograms of the sample to be measured before and after the vibration deformation caused by the vibration excitation; Step S500: Using the time-averaged hologram before and after vibration deformation caused by vibration excitation as a reference, extract the interference information of the sample to be tested; Step S600: Perform inverse Fourier transform on the extracted interference information to obtain the complex amplitude distribution of the interference information before and after vibration deformation, extract the phase from the complex amplitude distribution and calculate the phase difference before and after vibration deformation; Step S700: Based on the phase difference before and after the vibration deformation, the location of the surface defect on the sample under test is obtained by using the influence of the surface defect on the amplitude obtained in step S100, thereby achieving defect identification.

[0008] In step S100, the rough surface optical material is simplified to a circular thin plate, and the influence of surface defects of the circular thin plate on the amplitude is investigated, specifically: The surface vibration process of the circular thin plate after being excited by a sinusoidal wave was simulated using COMSOL software to explore the influence of surface defects on the amplitude and simplify the parameter setting process.

[0009] In step S200, the influence of changing the exposure time of the circular thin plate under sinusoidal vibration excitation on the reconstruction effect of the time-averaged hologram is obtained, specifically as follows: The recording process of time-averaged holograms was acquired by simulating the sinusoidal vibration of a circular thin plate with its edge fixed using software simulation. The recording process of the time-averaged hologram is analyzed to obtain the camera exposure time requirements of the digital holographic optical path when recording the time-averaged hologram. The camera exposure time needs to be much longer than the vibration period of the circular thin plate; The intensity of the time-averaged hologram acquired after the circular thin plate vibrates is directly proportional to the magnitude of the first-order zero-order Bessel function.

[0010] In step S300, a digital holographic reflective optical path consisting of an interference system and a vibration excitation source is constructed. The specific steps are as follows: Step S301: Design a digital holographic reflective optical path consisting of an interference system and a vibration excitation source; Step S302: Adjust the parameters using the laser source assembly and the beam expander / filter assembly; Step S303: Set the imaging lens and aperture to achieve focusing, limit the divergence angle and adjust the off-axis angle, and complete the construction of the digital holographic reflective optical path.

[0011] In step S400, time-averaged holograms of the sample under test are captured before and after vibrational deformation caused by vibrational excitation, specifically: Step S401: Using an audio signal generator and a matching speaker, adjust the sine wave signal and set the frequency to vibrate and excite the sample under test, so that the sample under test produces surface vibration deformation. Step S402: Set the camera exposure time and use the digital holographic reflective optical path constructed in step S300 to collect time-averaged holograms before and after vibration deformation.

[0012] Specifically: The interference information is the positive first-order spectral term obtained by Fourier transforming the time-averaged holograms before and after vibration deformation.

[0013] Based on the time-averaged holograms of the sample before and after vibrational deformation caused by vibrational excitation, interference information of the sample before and after vibrational deformation caused by vibrational excitation is extracted, specifically: Step S501: Perform Fourier transform on the time-averaged holograms before and after the vibration deformation. Step S502: Complete extraction of the positive first-order spectral term of the spectral domain obtained by Fourier transform is achieved by bandpass filtering; Step S503: Place the extracted positive first-order spectrum term at the center of the spectrum.

[0014] The extracted interference information is subjected to inverse Fourier transform to obtain the complex amplitude distribution of the interference information before and after vibration deformation. Phase extraction is then performed from the complex amplitude distribution, and the phase difference before and after vibration deformation is calculated. Specifically: Step S601: Perform inverse Fourier transform on the extracted positive first-order spectral term to obtain the complex amplitude distribution that includes only the interference information before and after vibration deformation; Step S602: Perform conjugate multiplication on the complex amplitude distribution of the interference information before and after vibration deformation to retain the complex modulus information; Step S603: Extract the phase from the complex amplitude distribution and calculate the phase difference before and after vibration deformation.

[0015] The parameters of the constructed digital holographic reflective optical path are adjusted according to the actual situation, specifically including the following steps: The phase difference before and after tilting the mirror is calculated. The phase difference before and after vibration is calculated by striking an optically excited material harmonic oscillator. Parameters are adjusted based on the distribution characteristics of phase difference fringes.

[0016] In step S700, based on the phase difference before and after the vibration deformation, and through the influence of the surface defects obtained in step S100 on the amplitude, the location of the surface defects on the sample under test is obtained, thereby achieving defect identification. Specifically: Step S701: Collect time-averaged holograms of the sample before and after vibration deformation. Step S702: Calculate the phase difference before and after vibration deformation; Step S703: Using the influence of surface defects on amplitude obtained in step S100, identify the abnormal positions of phase difference fringes before and after vibration deformation, and obtain the location of surface defects on the sample to be tested. Step S704: Achieve defect identification of the sample to be tested.

[0017] Compared with existing technologies, the beneficial effects of this invention are as follows: This invention is based on acoustically excited time-averaged digital holographic vibration measurement technology and focuses on the problem of defect identification in optical materials. First, through finite element numerical simulation analysis of the excited vibration process of an object, it is proven that defects affect the vibration deformation displacement at corresponding positions on the surface, thus leading to local amplitude anomalies. Subsequently, simulation investigation is conducted based on the experimental recording theory of time-averaged holography to determine that the camera exposure time should be much longer than the object's vibration period. Next, a reflective digital holographic measurement system is designed, and the optical path is adjusted through steps such as laser filtering and beam expansion, adjusting the intensity ratio of the reference light and the object light, setting an imaging lens and aperture in the object light optical path to achieve accurate focusing and limit the divergence angle, and optimizing the off-axis angle. Before the formal experiment, the sensitivity of the optical path to obtaining deformation information is verified by measuring the micro-tilt of the reflector and the vibration of the harmonic oscillator. Finally, the adjusted optical path is used to detect the vibration surface deformation field of samples such as optical sandwich composite materials and 3D printed glass under acoustic excitation, and the phase difference before and after vibration is obtained by combining the double exposure method, digital holographic reconstruction, and phase extraction method. By locating areas with obvious abnormalities in phase difference fringes, different types of defects can be identified, providing an important reference for the complete identification of surface depressions, internal debonding, and subsurface defects in optical materials. Attached Figure Description

[0018] Figure 1 This is a flowchart illustrating the optical material defect identification method based on time-averaged holographic vibration deformation measurement according to the present invention. Figure 2 This is a schematic diagram of the reflective off-axis digital holographic optical path system in the optical material defect identification method for vibration deformation measurement using the time-averaged holographic method of the present invention. Figure 3 This is a numerical simulation diagram illustrating the amplitude difference between points in the area surrounding the defect and points far from the defect in an embodiment of the optical material defect identification method for time-averaged holographic vibration deformation measurement according to the present invention. Figure 4 This is a schematic diagram illustrating the influence of subsurface defects on the surface point displacement field through numerical simulation in an embodiment of the optical material defect identification method for vibration deformation measurement using the time-averaged holographic method of the present invention. Figure 5This is a schematic diagram illustrating the influence of defects of different circular hole depths on the vibration displacement field through numerical simulation in an embodiment of the optical material defect identification method for vibration deformation measurement using the time-averaged holographic method of the present invention. Figure 6 In an embodiment of the optical material defect identification method for vibration deformation measurement using the time-averaged holographic method of the present invention, a numerical simulation study was conducted to compare holographic fringe patterns under different camera exposure times. Figure 7 This is a schematic diagram of the optical path adjustment process in an embodiment of the optical material defect identification method for vibration deformation measurement using the time-averaged holographic method of the present invention. Figure 8 This is a diagram illustrating the process of using Kronecker product interpolation to eliminate zero orders in holograms before and after vibration, as part of the optical material defect identification method for vibration deformation measurement using the time-averaged holographic method of the present invention. Figure 9 This is a diagram illustrating the process of extracting the positive first-order term through spectral filtering in an embodiment of the optical material defect identification method for vibration deformation measurement using the time-averaged holographic method of the present invention. Figure 10 This is a diagram showing the sensitivity results of the optical path verification by micro-tilting of the reflector in an embodiment of the optical material defect identification method for vibration deformation measurement using the time-averaged holographic method of the present invention. Figure 11 This is a diagram showing the sensitivity results of the optical path for verifying the vibration of a harmonic oscillator in an embodiment of the optical material defect identification method for measuring vibration deformation using the time-averaged holographic method of the present invention. Figure 12 This is a schematic diagram of defects in multilayer adhesive composite materials and an image showing the results of surface depression defect identification in an embodiment of the optical material defect identification method for vibration deformation measurement using the time-averaged holographic method of the present invention. Figure 13 This is a diagram showing the debonding defect identification result of a multilayer adhesive composite material in an embodiment of the optical material defect identification method for vibration deformation measurement using the time-averaged holographic method of the present invention. Figure 14 This image shows the subsurface hole defect identification results of a multilayer adhesive composite material in an embodiment of the optical material defect identification method for vibration deformation measurement using the time-averaged holographic method of the present invention. Figure 15 This is a diagram showing the identification result of the 3D printed glass processing trajectory in an embodiment of the optical material defect identification method for vibration deformation measurement using the time-averaged holographic method of the present invention. In the figure: 1. Laser source assembly; 21. Plano-convex lens; 22. Pinhole aperture; 23. Cemented doublet lens; 24. Concave lens; 25. Convex lens; 3. Stage and sample to be tested; 41 and 42. Beam splitter prism; 43, 44 and 45. Reflector; 51. Adjustable aperture; 52. Imaging lens; 61. Half-wave plate; 62. Polarizer; 7. CMOS image sensor; 8. Computer. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] Example: Figures 1-15 As shown, the present invention provides a technical solution. To achieve the above objectives, the present invention provides the following technical solution: a method for identifying optical material defects by measuring vibration deformation using time-averaged holographic methods, wherein the optical material defect identification method specifically includes the following steps: Step S100: Simplify the optical material into a circular thin plate and obtain the influence of the surface defects of the circular thin plate on the amplitude; Step S200: Obtain the relationship between the effect of changing the exposure time of the circular thin plate under sinusoidal vibration excitation on the reconstruction effect of the time-averaged hologram; Step S300: Construct a digital holographic reflective optical path consisting of an interference system and a vibration excitation source; Step S400: Take time-averaged holograms of the sample before and after vibration deformation caused by vibration excitation. Step S500: Using the time-averaged hologram before and after vibration deformation caused by vibration excitation as a reference, extract the interference information of the sample to be tested; Step S600: Perform inverse Fourier transform on the extracted interference information to obtain the complex amplitude distribution of the interference information before and after vibration deformation, extract the phase from the complex amplitude distribution and calculate the phase difference before and after vibration deformation; Step S700: Based on the phase difference before and after the vibration deformation, the location of the surface defect on the sample under test is obtained by using the influence of the surface defect on the amplitude obtained in step S100, thereby achieving defect identification.

[0021] In step S100, the rough surface optical material is simplified to a circular thin plate, and the influence of surface defects of the circular thin plate on the amplitude is investigated, specifically: The surface vibration process of the circular thin plate after being excited by a sinusoidal wave was simulated by finite element method using COMSOL software. The influence of surface defects on the amplitude was investigated and the parameter setting process was simplified. In the initial settings of COMSOL, select the three-dimensional structure, solid mechanics module, and transient solution. A simplified circular thin plate object is simulated to create a geometric model as a cylinder with a diameter of 10 mm and a thickness of 0.2 mm. At the same time, a geometric model with circular holes and cracks is set. Then, by setting the basic parameters of the physical field, mesh generation, and solution time step, the surface vibration process of the circular thin plate and the circular thin plate with different defects after being excited by a sinusoidal wave signal is simulated by finite element numerical simulation. First, select specific coordinate points to set up a one-dimensional plotting group. Export the point displacement data of samples without defects and samples with circular holes and cracks at the same time. It can be seen that the displacement fields of the selected points at different times are different due to the presence of defects. Figure 3 As shown, a-1 is a schematic diagram of the selection of specific points around the defect; b-1 is a schematic diagram of the numerical differences in the displacement field at different points after vibration excitation. It can be seen that the vibration amplitude is greater closer to the defect area, and the difference in vibration amplitude at the defect under sinusoidal excitation can be used as an important basis for defect identification. To investigate whether non-surface defects affect surface vibration deformation, a cylinder with a radius of 0.5 mm was added starting from the location corresponding to the geometric back side of the selected point. The cylinder did not penetrate the surface. The difference between the original geometry and the newly added cylinder was calculated, and a union was formed. This allowed the investigation of the influence of subsurface defects on the vibration field of the surface point. The amplitude variation data of this point over multiple periods were obtained with and without subsurface defects, and plotted as follows: Figure 4 The diagram shows the effect of subsurface defects on the displacement field of surface points. a-2 is a schematic diagram of the geometric setting of subsurface defects; b-2 is a schematic diagram of the difference curve of vibration displacement of the same surface point with and without subsurface defects over time. It can be seen that the presence of subsurface defects also affects the surface point displacement field. The surface point displacement amplitude is larger in the region where subsurface defects exist, which proves that subsurface defects can be effectively identified based on the region of abnormal amplitude. To investigate the influence of defect size on the surface vibration field, circular holes of different depths were incorporated into the modeled circular plate geometry: 1 / 100 of the plate thickness, 1 / 20 of the plate thickness, and complete perforation. The effect of the hole depth on the displacement field was explored. Specific points around the defect were selected, and the differences in the displacement field at these points over time were analyzed as follows: Figure 5 As shown; a-3 is a schematic diagram of the geometric setting of circular hole defects at different depths; b-3 is a schematic diagram of the difference curve of vibration displacement of the same surface point with time when the circular hole depth is different; It can be seen that as the depth of the circular hole increases, the displacement field value, i.e. the vibration amplitude, of the points around the defect increases. In particular, when the circular hole defect penetrates the plate surface, the amplitude increases sharply. This may be because the presence of the circular hole defect reduces the rigidity near the defect, leading to an increase in amplitude.

[0022] In step S200, the influence of changing the exposure time of the circular thin plate under sinusoidal vibration excitation on the reconstruction effect of the time-averaged hologram is obtained, specifically as follows: The recording process of time-averaged holograms was acquired by simulating the sinusoidal vibration of a circular thin plate with its edge fixed using software simulation. The recording process of the time-averaged hologram is analyzed to obtain the camera exposure time requirements of the digital holographic optical path when recording the time-averaged hologram. The camera exposure time needs to be much longer than the vibration period of the circular thin plate; The intensity of the time-averaged hologram acquired after the circular thin plate vibrates is directly proportional to the magnitude of the first-order zero-order Bessel function.

[0023] To verify the effect of changing the exposure time on the reconstruction effect of the time-averaged hologram under sinusoidal vibration excitation, the fundamental frequency vibration of a circular thin film fixed at its edge was simulated. The vibration displacement was determined by a first-order Bessel function J1(β). 11 r) describes the radial variation, cosφ describes the angular four-lobed symmetry, the laser wavelength is set to 632.8 nm (helium-neon laser wavelength), and the sensitivity vector γ represents the phase modulation caused by vibration, proportional to the derivative of displacement and laser wavelength. The complex amplitude envelope is calculated for each pixel of the hologram, with a series truncation of 20 terms to ensure that the contribution of higher-order Bessel terms can be ignored. Let the vibration period be T, then the holographic fringe simulation results for different exposure times α are as follows: Figure 6 As shown, a) Exposure time α = 0.08T; b) Exposure time α = 0.12T; c) Exposure time α = 0.25T; d) Exposure time α = 0.50T; e) Exposure time α = 0.67T; f) Exposure time α = 1T; g) Exposure time α = 2T; h) Exposure time α = 3T; i) Exposure time α = 4T; It can be seen that the contrast of the stripes gradually increases from short exposure to long exposure, and gradually presents clear rings, proving that the time averaging effect gradually becomes dominant. When holographically recording a periodically vibrating object, if the exposure time is much longer than the vibration period and is an integer multiple of the vibration period, the digital holographic optical path records a time-averaged hologram; the intensity of the time-averaged hologram acquired after the object vibrates is proportional to the magnitude of the first-order zero Bessel function J0.

[0024] In step S300, a digital holographic reflective optical path consisting of an interference system and a vibration excitation source is constructed. The specific steps are as follows: Step S301: Design a digital holographic reflective optical path consisting of an interference system and a vibration excitation source; Step S302: Adjust the parameters using the laser source assembly and the beam expander / filter assembly; Step S303: Set the imaging lens and aperture to achieve focusing, limit the divergence angle and adjust the off-axis angle, and complete the construction of the digital holographic reflective optical path.

[0025] A reflective off-axis digital holographic optical path vibration deformation measurement system based on the Mach-Zehnder interferometer, such as... Figure 2 As shown, it includes a laser source assembly 1; a beam expander and filter assembly 2; a sample platform 3; a beam splitter and reflector assembly 4; an imaging assembly 5; a light intensity tuning assembly 6; a CMOS image sensor 7; and a computer 8. The beam expander filter assembly 2 includes a plano-convex lens 21, a pinhole aperture 22, a cemented doublet lens 23, a concave lens 24, and a convex lens 25; Beam splitter and reflector assembly 4 includes beam splitters 41 and 42 and reflectors 44 and 45; Imaging assembly 5 includes an adjustable aperture 51 and an imaging lens 52; The light intensity modulation assembly 6 includes a half-wave plate 61 and a polarizer 62; Among them: laser source component 1 is a He-Ne laser, used as the light source for holographic imaging optical path; The beam expanding and filtering component 2 is used to perform beam expanding, filtering and collimation operations on the He-Ne laser beam. Since speckle interferometry imaging has high requirements for the quality of the reference light, the laser beam needs to be filtered and expanded and collimated before entering the reference optical path after beam splitting. In the reference optical path, a Keplerian beam expander is formed by a 30mm focal length plano-convex lens 21, a 5µm pinhole aperture 22, and a 200mm focal length cemented achromatic lens 23, which also serves to filter stray light. However, the pinhole aperture filtering causes a significant loss of light intensity. Therefore, an integrated Galilean beam expander collimator is used separately in the object optical path to obtain relatively bright laser illumination. The integrated beam expander collimator includes a concave lens 24 and a convex lens 25. Sample platform 3 is used to carry and hold the optical material sample to be tested, and can perform translation and focusing actions; In the beam splitter and reflector assembly 4, beam splitters 41 and 42 are used to split the beam to form a Mach-Zehnder system, and reflectors 43-45 are used to perform total internal reflection of the beam. The imaging component 5 includes an adjustable aperture 51 and an imaging lens 52. Based on the principle of imaging by reflection from rough surface objects, the adjustable aperture limits the reflection divergence angle and the imaging lens enables the object under test to be clearly imaged on the CMOS.

[0026] The intensity adjustment assembly 6 includes a half-wave plate 61 and a polarizer 62, used to adjust the intensity ratio of the reference light to the object light.

[0027] The CMOS image sensor 7 is used to record the interference information generated by the interferometric imaging system and form an interference image; the CMOS image sensor 6 has a 2048×2048 rectangular pixel array with high quantum efficiency and 16-bit depth. The CMOS image sensor 7 is connected to the computer 8, and its exposure time parameters are controlled by the computer 8 to transmit the acquired interference image to the computer 8 for processing. Computer 8 is used to store and analyze the interference image.

[0028] The working process of this optical path is as follows: First, the laser beam is split into a reference beam and an object beam by the beam splitter prism 41. The reference beam is reflected by the mirror 43 and then enters the beam expander 2. The Keplerian beam expander, consisting of a 30mm focal length plano-convex lens 21, a 5µm pinhole aperture 22, and a 200mm focal length cemented achromatic lens 23, expands and collimates the beam and filters out stray light. It is then adjusted by the intensity adjustment components 61 and 62 to be basically consistent with the intensity of the object beam, and then reflected by the mirror 45 before entering the beam splitter prism 42. The object beam is expanded and collimated by the integrated Galilean beam expander collimator consisting of a concave lens 24 and a convex lens 25 with minimal intensity loss. It is then reflected by the mirror 44 to illuminate the object under test. The reflected light from the object under test is limited by the divergence angle by the adjustable aperture 51, and then enters the beam splitter prism 42 after passing through the imaging lens 52. The object beam and the reference beam interfere at the re-splitter 42 and are recorded on the CMOS image sensor, and then saved by the computer 8 for subsequent image processing.

[0029] In actual optical path adjustment, the object's position is first adjusted according to the imaging rules of convex lenses, ensuring that the rough surface object is clearly focused and imaged on the CMOS after passing through the imaging lens. Simultaneously, an aperture stop is placed at a suitable position in front of the object to limit the divergence angle. After blocking the reference optical path, the aperture and position are adjusted until the spectrum of the object light image is a complete, bright circular spot without speckle effects, such as... Figure 7 The spectrum diagrams for a-4 and the pure object light are shown in the image. Excessive exposure time adjustment can lead to overly bright reference light, and an excessively high intensity ratio between the reference and object lights can also result in unclear imaging. Therefore, a polarizer and a half-glass plate need to be placed in the reference light path to attenuate the light intensity and adjust the intensity ratio. If the directions of the object and reference lights are not adjusted, the interference terms formed after coaxial interference will severely overlap with the zero-order terms in the spectral domain. To facilitate the extraction of interference terms and obtain interference information, the off-axis angle is adjusted by adjusting the pitch and horizontal position of the reference light path mirror before actual optical path acquisition, so that all terms in the spectral domain are completely separated.Figure 7 In the middle, b-4, the spectral separation spectrum of each term in the holographic interference after adjusting the off-axis angle is shown.

[0030] In step S400, time-averaged holograms of the sample under test are captured before and after vibrational deformation caused by vibrational excitation, specifically: Step S401: Using an audio signal generator and a matching speaker, adjust the sine wave signal and set the frequency to vibrate and excite the sample under test, so that the sample under test produces surface vibration deformation. Step S402: Set the camera exposure time and use the digital holographic reflective optical path constructed in step S300 to collect time-averaged holograms before and after vibration deformation.

[0031] The processing of time-averaged holograms generally employs a double-exposure method, capturing holograms of the object before and after vibration, and calculating the phase difference to obtain the three-dimensional deformation information of the vibrating object. Therefore, this invention uses a non-destructive acoustic excitation method to induce vibration deformation on the surface of the sample under test. The vibration excitation source uses an audio signal generator and a matching speaker from NTI Corporation. The frequency range of the audio signal generator is 10-20Hz, and the output can be adjusted to a sine wave signal. According to the requirements for exposure time proposed in step S200, the camera exposure time is set to acquire holograms of the sample before and after the excited vibration.

[0032] Specifically: The interference information is the positive first-order spectral term obtained by Fourier transforming the time-averaged holograms before and after vibration deformation.

[0033] Based on the time-averaged holograms of the sample before and after vibrational deformation caused by vibrational excitation, interference information of the sample before and after vibrational deformation caused by vibrational excitation is extracted, specifically: Step S501: Perform Fourier transform on the time-averaged holograms before and after the vibration deformation. Step S502: By using the method of spectrum center positioning and adaptive filtering window, the complete extraction of the positive first-order spectrum term of the spectrum domain obtained by Fourier transform is achieved. Step S503: Place the extracted positive first-order spectrum term at the center of the spectrum.

[0034] In actual speckle interference fringe patterns, adjusting the directions of the reference and object beams can separate the positive and negative first-order spectra from the zero-order spectrum in the spectral domain. However, due to the rough surface of the optical material being measured, spectral aliasing is inevitable during the actual interference image formation process. Therefore, to reduce the impact of spectral aliasing, before calculating the complex amplitude before and after vibration, a 3×3 Kronecker product interpolation of the holograms before and after vibration is performed to suppress the zero-order spectrum. The spectral result of removing the zero-order term using the Kronecker interpolation is shown below. Figure 8As shown; a-5 is the original spectrum; b-5 is the spectrum after zero-level elimination processing; After the zero-order term is suppressed, the positive first-order term needs to be extracted as completely as possible in the spectral domain using spectral filtering. The positive first-order spectrum is extracted from the holograms before and after vibration acquired by the reflective holographic optical path in step S300 in the spectral domain. The extraction process is as follows: Figure 9 As shown in Figure a-6, which is a schematic diagram of the spectral distribution of each term in the spectral domain, it can be seen that after adjusting the angle between the object beam and the reference beam multiple times, the positive and negative first-order terms and the zero-order term in the spectral domain have been completely separated. The red boxes in the figure represent the positive and negative first-order terms, and the blue boxes represent the zero-order terms. The zero-order terms are the low-frequency components in the spectral domain, while the positive first-order terms containing interference information are the high-frequency components. Therefore, to fully obtain the interference information, a bandpass filter can be used to completely extract the positive first-order spectral terms in the spectral domain. Figure b-6 shows the result of the bandpass filter extraction of the positive first-order terms. Subsequently, placing the extracted high-frequency components at the center of the spectral origin can eliminate the carrier wave. Figure c-6 is a schematic diagram of the centering process for the extracted positive first-order terms. The extracted interference information is subjected to inverse Fourier transform to obtain the complex amplitude distribution of the interference information before and after vibration deformation. Phase extraction is then performed from the complex amplitude distribution, and the phase difference before and after vibration deformation is calculated. Specifically: Step S601: Perform inverse Fourier transform on the extracted positive first-order spectral term to obtain the complex amplitude distribution that includes only the interference information before and after vibration deformation; Step S602: Perform conjugate multiplication on the complex amplitude distribution of the interference information before and after vibration deformation to retain the complex modulus information; Step S603: Extract the phase from the complex amplitude distribution and calculate the phase difference before and after vibration deformation.

[0035] After extracting the complete positive first-order spectrum from the holograms acquired before and after vibration, the complex amplitude before and after vibration can be obtained through inverse Fourier transform. Since the object-light imaging optical path is significantly affected by noise during hologram acquisition, and directly extracting the phase from the complex amplitude may lead to abrupt phase changes, conjugate multiplication of the complex amplitudes before and after vibration is performed to preserve the complex modulus information, while ensuring relatively high computational stability under low signal-to-noise ratio conditions. Therefore, the phase difference before and after vibration can be expressed as: ; Where * represents the conjugate symbol. and These are the complex amplitudes obtained by reconstructing the holograms before and after the vibration, respectively. The parameters of the constructed digital holographic reflective optical path are adjusted according to the actual situation, specifically including the following steps: The phase difference before and after tilting the mirror is calculated. The phase difference before and after vibration is calculated by striking an optically excited material harmonic oscillator. Parameters are adjusted based on the distribution characteristics of phase difference fringes.

[0036] In the initial stage of the experiment, the off-axis angle was adjusted to completely separate the various spectra in the spectral domain. The angle between the object beam and the reference beam was adjusted to completely separate the positive first-order spectrum from the zero-order spectrum. With the spectrum completely separated, a mirror was placed on the sample stage to observe the spectra. The spectrum at this point is shown in the image. Figure 10 In the diagram, a-7, a schematic diagram of the spectrum separation with the reflector horizontally placed is shown; it can be seen that there is basically no aliasing in the spectral region. In this state, the sample stage with the reflector is adjusted to a slight tilt. The complex amplitude of the holograms before and after the tilt is obtained through spectrum extraction and reconstruction, and the phase difference before and after the tilt is calculated as follows: Figure 10 In the diagram, b-7, the phase difference before and after the mirror is horizontal and slightly tilted is shown. It can be seen that the phase difference fringes are caused by tilting. Since the surface of the mirror is flat and the fringes have no obvious abnormal areas, it can be verified that the optical path can accurately identify the phase difference caused by the deformation of objects in the field of view. The hemispherical gyroscope resonator is a crucial component of the gyroscope, a core inertial navigation element. Resonators are typically made of quartz coated with a metal film, resulting in a non-flat surface. Furthermore, resonators possess frequency-stable vibration characteristics. Therefore, measuring the amplitude distribution of a local surface region of the resonator using the optical path described in this paper further demonstrates the feasibility of the optical path. First, the resonator is fixed with a support, and a hologram of the stationary state is acquired after aligning the illumination beam. Then, the resonator is gently tapped to induce vibration, and a hologram of the vibrating state is acquired. The amplitude distribution of the local surface region at the center of the resonator is calculated using the phase difference before and after vibration. Figure 11 As shown, a-8 is a schematic diagram of the amplitude distribution in a local area of ​​the harmonic oscillator; b-8 is a schematic diagram of the amplitude distribution in a local area of ​​the harmonic oscillator. It can be seen that the amplitude in the harmonic oscillator surface region gradually increases and the stripes are relatively uniform, thus proving that the optical path has high sensitivity for measuring vibration changes.

[0037] In step S700, based on the phase difference before and after the vibration deformation, and through the influence of the surface defects obtained in step S100 on the amplitude, the location of the surface defects on the sample under test is obtained, thereby achieving defect identification. Specifically: Step S701: Collect time-averaged holograms of the sample before and after vibration deformation. Step S702: Calculate the phase difference before and after vibration deformation; Step S703: Using the influence of surface defects on amplitude obtained in step S100, identify the abnormal positions of phase difference fringes before and after vibration deformation, and obtain the location of surface defects on the sample to be tested. Step S704: Achieve defect identification of the sample to be tested.

[0038] The structural integrity of optical sandwich composite materials depends on the tight bonding between the layers. Surface depressions, internal debonding, and subsurface voids all affect their mechanical properties. Therefore, this experimental optical path is used to identify common defects in simplified double-layer bonded wood panels. Schematic diagrams of various defects are shown below. Figure 12 In the diagram, a-9, a schematic diagram of common material defects in multilayer adhesives is shown. The sample to be tested is placed at the focusing position of the object-optical path. First, the interference pattern of the sample in a static state is acquired. Then, the frequency of the sine wave is changed by the acoustic excitation device to excite the sample to vibrate on the surface. During the excitation process, the frequency sweep mode is selected to find the frequency band with a large vibration amplitude. Then, the interference pattern of the sample vibration state is acquired in this frequency band. The phase difference before and after vibration can be obtained by the double exposure method. Phase difference calculation for samples with surface depressions, such as Figure 12 As shown in the phase difference diagram of surface depression defect (b-9), it can be seen that there is an obvious amplitude anomaly region at the location of the surface depression.

[0039] Internal debonding refers to the partial detachment of the bonded area in the middle of a multilayer material, resulting in separation or gaps between the layers. Internal debonding defects in optical adhesives affect light scattering and transmission properties, as well as the overall mechanical strength; therefore, identifying internal debonding defects is crucial. In this experiment, internal debonding was artificially created in different locations within the adhesive layer of the material, ensuring that the debonding locations were not visible on the surface. Samples with internal debonding at different locations were then subjected to acoustic excitation using a frequency band with a relatively large excitation amplitude as determined above. The phase difference before and after vibration was measured as follows: Figure 13 The diagrams shown are a-10, left-side debonding phase difference diagram, b-10, middle-side debonding phase difference diagram, and c-10, right-side debonding phase difference diagram.

[0040] It can be seen that the deformation anomaly area changes as the internal debonding location changes, and the internal debonding location can be identified using this method.

[0041] Subsurface defects can also occur during the use of materials. As shown in the numerical simulation results in step S100, subsurface defects also affect the point displacement field of the surface. In this experiment, holes that do not penetrate to the surface are created on the back side of the material. Holograms of the sample with subsurface defects before and after vibration are acquired under different frequency acoustic wave excitations, such as... Figure 14 The images shown are the results of subsurface porosity defect identification for multilayer adhesive composite materials. a-11 is the phase difference diagram at an acoustic excitation frequency of 200 Hz; b-11 is the phase difference diagram at an acoustic excitation frequency of 225 Hz; c-11 is the phase difference diagram at an acoustic excitation frequency of 250 Hz; and d-11 is the phase difference diagram at an acoustic excitation frequency of 275 Hz. It can be seen that the stripe density changes from sparse to dense and then back to sparse as the frequency increases. This is because the frequency range includes the resonant frequency of the sample under test, and the vibration amplitude will increase significantly at the material's resonant frequency. However, regardless of the change in stripe density, the abnormal position of the stripes corresponding to the subsurface hole region remains unchanged. Therefore, the location of the subsurface hole defect can be accurately located by observing the abnormal region of the amplitude.

[0042] Laser bed fusion (LPBF), an important branch of laser additive manufacturing, is primarily applied to metallic materials. In recent years, with the increasing demand for product personalization and structural complexity in the manufacturing industry, this technology, with its unique ability to customize complex structures, has gradually expanded into the glass manufacturing field. However, the unique physical and chemical properties of glass itself bring many challenges to LPBF technology in the processing of quartz glass. To improve the forming quality of quartz glass in the LPBF process, reducing the strip width has become an effective strategy. While this method optimizes the forming effect to some extent, it inevitably leaves processing marks at the starting position of the strip pattern. This is because during laser scanning, the starting and ending ends are respectively formed glass and unformed powder, with a significant difference in their thermal conductivity, resulting in uneven heat distribution. At the same time, the inherent rapid heating and cooling characteristics of LPBF technology further exacerbate the heat unevenness, ultimately leading to stress concentration in local areas and thus residual processing marks that are invisible to the naked eye. Considering the brittle nature of glass materials, traditional destructive testing methods such as drilling can easily cause irreversible damage to the sample, seriously affecting subsequent research and applications. Therefore, the method presented in this paper can induce minute vibrations on the glass surface through acoustic excitation, and the phase difference before and after the vibration can be calculated.

[0043] like Figure 15 As shown, a-12 is a schematic diagram of the microscopic structure of 3D printed glass; b-12 is a schematic diagram of processing mark identification. It can be seen that the amplitude of the processing mark area caused by stress concentration is significantly different from that of the surrounding area. Therefore, the processing marks of 3D printed glass can be displayed in the phase difference fringe pattern. By reproducing the marks, a certain reference can be provided for the adjustment of laser processing parameters.

[0044] In summary, this invention investigates the defect identification problem of optical materials based on acoustically excited time-averaged digital holographic vibration measurement technology. First, a finite element numerical simulation of the excited vibration process of an object was conducted, demonstrating that defects affect the vibrational deformation displacement at corresponding locations on the surface, leading to local amplitude anomalies. Then, the theoretical basis of time-averaged holography experimental recording was explored through simulation, determining that the camera exposure time should be set much longer than the object's vibration period. Subsequently, a reflective digital holographic measurement system was designed, and the optical path was adjusted through steps such as laser filtering and beam expansion, adjusting the intensity ratio of the reference light and object light, setting an imaging lens and aperture in the object light path to achieve accurate focusing and limit the divergence angle, and adjusting the off-axis angle. Before conducting experiments on the optical material, the sensitivity of the optical path to acquiring deformation information was verified by measuring the micro-tilt of the reflector and the vibration of the harmonic oscillator. Finally, the calibrated optical path was used to detect the vibration surface deformation field of samples such as optical sandwich composite materials and 3D printed glass after being excited by acoustic waves. The phase difference before and after vibration was obtained by combining the double exposure method with digital holographic reconstruction and phase extraction method. By locating the obvious abnormal area of ​​the phase difference stripe, the identification of different types of defects was realized. This provides a reference for the non-destructive and high-sensitivity identification of different types of defects in optical materials and has important practical significance.

[0045] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

Claims

1. A method for identifying optical material defects by measuring vibration deformation using time-averaged holography, characterized in that: The optical material defect identification method specifically includes the following steps: Step S100: Simplify the optical material into a circular thin plate and obtain the influence of the surface defects of the circular thin plate on the amplitude; Step S200: Obtain the relationship between the effect of changing the exposure time of the circular thin plate under sinusoidal vibration excitation on the reconstruction effect of the time-averaged hologram; Step S300: Construct a digital holographic reflective optical path consisting of an interference system and a vibration excitation source; Step S400: Take time-averaged holograms of the sample before and after vibration deformation caused by vibration excitation. Step S500: Using the time-averaged hologram before and after vibration deformation caused by vibration excitation as a reference, extract the interference information of the sample to be tested; Step S600: Perform inverse Fourier transform on the extracted interference information to obtain the complex amplitude distribution of the interference information before and after vibration deformation, extract the phase from the complex amplitude distribution and calculate the phase difference before and after vibration deformation; Step S700: Based on the phase difference before and after the vibration deformation, the location of the surface defect on the sample under test is obtained by using the influence of the surface defect on the amplitude obtained in step S100, thereby achieving defect identification.

2. The optical material defect identification method based on time-averaged holographic vibration deformation measurement according to claim 1, characterized in that: In step S100, the rough surface optical material is simplified to a circular thin plate, and the influence of surface defects of the circular thin plate on the amplitude is investigated, specifically: The surface vibration process of the circular thin plate after being excited by a sinusoidal wave was simulated using COMSOL software to explore the influence of surface defects on the amplitude and simplify the parameter setting process.

3. The optical material defect identification method based on time-averaged holographic vibration deformation measurement according to claim 2, characterized in that: In step S200, the influence of changing the exposure time of the circular thin plate under sinusoidal vibration excitation on the reconstruction effect of the time-averaged hologram is obtained, specifically as follows: Simulation of the sinusoidal vibration of a circular thin plate with its edge fixed, and the recording process of time-averaged holograms; The recording process of the time-averaged hologram is analyzed to obtain the camera exposure time requirements of the digital holographic optical path when recording the time-averaged hologram. The camera exposure time is greater than the vibration period of the circular thin plate; The intensity of the time-averaged hologram acquired after the circular thin plate vibrates is directly proportional to the magnitude of the first-order zero-order Bessel function.

4. The optical material defect identification method based on time-averaged holographic vibration deformation measurement according to claim 3, characterized in that: In step S300, a digital holographic reflective optical path consisting of an interference system and a vibration excitation source is constructed. The specific steps are as follows: Step S301: Design a digital holographic reflective optical path consisting of an interference system and a vibration excitation source; Step S302: Adjust the parameters using the laser source assembly and the beam expander / filter assembly; Step S303: Set the imaging lens and aperture to achieve focusing, limit the divergence angle and adjust the off-axis angle, and complete the construction of the digital holographic reflective optical path.

5. The optical material defect identification method based on time-averaged holographic vibration deformation measurement according to claim 4, characterized in that: In step S400, time-averaged holograms of the sample under test are captured before and after vibrational deformation caused by vibrational excitation, specifically: Step S401: Using an audio signal generator and a matching speaker, adjust the sine wave signal and set the frequency to vibrate and excite the sample under test, so that the sample under test produces surface vibration deformation. Step S402: Set the camera exposure time and use the digital holographic reflective optical path constructed in step S300 to collect time-averaged holograms before and after vibration deformation.

6. The optical material defect identification method based on time-averaged holographic vibration deformation measurement according to claim 1, characterized in that: Specifically: The interference information is the positive first-order spectral term obtained by Fourier transforming the time-averaged holograms before and after vibration deformation.

7. The optical material defect identification method based on time-averaged holographic vibration deformation measurement according to claim 6, characterized in that: Based on the time-averaged holograms of the sample before and after vibrational deformation caused by vibrational excitation, interference information of the sample before and after vibrational deformation caused by vibrational excitation is extracted, specifically: Step S501: Perform Fourier transform on the time-averaged holograms before and after the vibration deformation. Step S502: By using bandpass filtering, the positive first-order spectral term of the spectral domain obtained by Fourier transform is completely extracted; Step S503: Place the extracted positive first-order spectrum term at the center of the spectrum.

8. The optical material defect identification method based on time-averaged holographic vibration deformation measurement according to claim 7, characterized in that: The extracted interference information is subjected to inverse Fourier transform to obtain the complex amplitude distribution of the interference information before and after vibration deformation. Phase extraction is then performed from the complex amplitude distribution, and the phase difference before and after vibration deformation is calculated. Specifically: Step S601: Perform inverse Fourier transform on the extracted positive first-order spectral term to obtain the complex amplitude distribution that includes only the interference information before and after vibration deformation; Step S602: Perform conjugate multiplication on the complex amplitude distribution of the interference information before and after vibration deformation to retain the complex modulus information; Step S603: Extract the phase from the complex amplitude distribution and calculate the phase difference before and after vibration deformation.

9. The optical material defect identification method based on time-averaged holographic vibration deformation measurement according to claim 4, characterized in that: The parameters of the constructed digital holographic reflective optical path are adjusted according to the actual situation, specifically including the following steps: The phase difference before and after tilting the mirror is calculated. The phase difference before and after vibration is calculated by striking an optically excited material harmonic oscillator. Parameters are adjusted based on the distribution characteristics of phase difference fringes.

10. The optical material defect identification method based on time-averaged holographic vibration deformation measurement according to claim 9, characterized in that: In step S700, based on the phase difference before and after the vibration deformation, and through the influence of the surface defects obtained in step S100 on the amplitude, the location of the surface defects on the sample under test is obtained, thereby achieving defect identification. Specifically: Step S701: Collect time-averaged holograms of the sample before and after vibration deformation. Step S702: Calculate the phase difference before and after vibration deformation; Step S703: Using the influence of surface defects on amplitude obtained in step S100, identify the abnormal positions of phase difference fringes before and after vibration deformation, and obtain the location of surface defects on the sample to be tested. Step S704: Achieve defect identification of the sample to be tested.

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