A laser synchronous detection system for surface / subsurface defects of transparent materials and its operation method

Abstract

The application discloses a kind of laser synchronous detection system and its operating method for transparent material surface / subsurface defect.The linear polarized laser emitted by laser is focused on the surface of transparent material by polarizing beam splitter, integrating sphere and lower focusing lens.Precision displacement platform drives transparent material to move, and realizes laser automatic scanning.Surface polarized light sensor acquires surface scattering signal.Subsurface defect scattering light is reflected by mirror at the bottom of transparent material, and is reflected by polarizing beam splitter.After focusing by upper focusing lens, it is collected by subsurface polarized light sensor through pinhole.Computer carries out filtering processing and real-time imaging to the collected signal, and realizes the simultaneous detection to transparent material surface and subsurface defect.The application has simple structure, can realize real-time imaging to transparent material surface and subsurface defect synchronously, and improves detection efficiency.A mirror is additionally arranged at the bottom of transparent material, reduces the influence of sample stage surface on detection, and solves the problem that material high light transmittance influences subsurface detection.

Description

TECHNICAL FIELD

[0001] The application provides a laser synchronization system for surface and subsurface defects of transparent materials and a method for operating the same, and belongs to the field of non-destructive testing of transparent materials. BACKGROUND

[0002] Transparent materials are increasingly important in the fields of tablet computers, smart phones, solar panels, medicine and optics. Transparent materials are mainly divided into two categories: transparent inorganic materials and transparent organic materials. Transparent inorganic materials include glass, ceramics and quartz, etc., which have high hardness, high temperature resistance and corrosion resistance, etc.; transparent organic materials mainly include various transparent plastics such as polymethyl methacrylate (PMMA), polycarbonate (PC) and polystyrene (PS), etc., which have the advantages of light weight, easy processing and impact resistance. With the rapid development of optical and semiconductor technologies at home and abroad, optical high-precision glass and other transparent devices with smooth surfaces have been widely used in science and technology, military, semiconductors and daily life, and the quality requirements for these devices are also getting higher and higher. In the manufacturing and processing of optical elements, surface defects and subsurface defects are inevitably produced. These defects can reduce the quality of the coating, transmission performance and damage threshold of the optical element, thereby seriously affecting the imaging quality of the optical system and the stability of the system. In addition, during the production of transparent materials, various defects may be produced due to factors such as temperature and mold. These defects usually include scratches, cracks, bubbles, inclusions and dirt, etc. Their presence can reduce the light transmittance and visibility of transparent materials, and thus lead to a decline in product quality. Therefore, the demand for defect detection of transparent materials is becoming more and more urgent.

[0003] The defect detection methods of transparent materials are divided into two categories: destructive detection and non-destructive detection. Destructive detection methods expose the subsurface defects of the material by destructive means, and then use microscopic techniques to observe to obtain subsurface information. These methods include cross-section microscopic observation, angle polishing, magnetic rheological polishing observation and constant chemical etching rate method, etc. However, destructive detection methods cannot be integrated into online detection systems, which reduces the overall production efficiency. In addition, destructive detection methods are greatly influenced by experimental conditions and operator experience. Even if the same destructive detection method is used, different operators may get different results. In addition, there is no uniform standard for destructive detection methods at home and abroad at present.

[0004] Non-destructive testing methods use physical properties such as light and electricity to detect defects on the surface and subsurface of samples without damaging the samples. Due to the high light transmittance of transparent materials, internal defects such as bubbles and micro-cracks are difficult to distinguish visually. Traditional optical microscopes are difficult to detect these internal defects because of their low optical contrast, which is easily obscured by transmitted light. Moreover, the high light transmittance of transparent materials during detection can affect the results. In addition, due to the existence of the diffraction limit, the resolution of optical microscopes is limited, making it difficult to detect sub-wavelength scale defects. At the same time, the interference of ambient light can affect the detection accuracy. The high reflectivity of transparent materials makes it easy for ambient light to enter the detection system, increasing background noise and reducing the signal-to-noise ratio of defect detection.

[0005] CN116660154A relates to a transparent glass cover 2D slight scratch defect detection system and method. The method arranges a line array camera and a line array light source at a predetermined angle on the workbench to realize imaging and detection of weak scratches. This patent effectively improves the detection accuracy and efficiency of weak scratches, but cannot detect subsurface defects of transparent glass. Patent CN106501279A discloses a low subsurface damage detection method for hard and brittle optical materials, which realizes the characterization of material surface roughness by using linearly polarized laser. Patent CN115356265A discloses a device for detecting silicon wafer subsurface damage signals using two detectors. The use of two detectors achieves the splitting of median cracks and side cracks signals, achieving more accurate subsurface damage detection. The above methods can only detect the surface or subsurface respectively, and cannot realize simultaneous detection. Since the surface roughness of hard and brittle materials affects the subsurface damage signal, these methods are difficult to accurately detect the subsurface damage of hard and brittle materials with rough surface. SUMMARY

[0006] The purpose of the invention is to solve the above-mentioned problems existing in the detection method of transparent materials. The invention proposes a laser synchronization system for detecting surface / subsurface defects of transparent materials and its operation method. A mirror is added at the bottom of the transparent sample to reduce the influence of the sample stage surface on the detection. The surface and subsurface defects of the transparent material are detected simultaneously, improving the detection accuracy and efficiency.

[0007] Technical solution: In order to achieve the above-mentioned purpose of the invention, the invention adopts the following technical solution:

[0008] In one embodiment of the invention, a laser detection system for surface / subsurface defects of transparent materials is proposed, which includes a laser, a polarization beam splitter, an integrating sphere, a focusing lens, a mirror, a pinhole, a sensor, a precision displacement platform, and a computer.

[0009] The laser emitted by the laser is a linearly polarized infrared light beam, which is located at the top of the system.

[0010] The polarization beamsplitter is located below the laser and has a polarization selection function. The linearly polarized light emitted by the laser directly passes through the polarization beamsplitter.

[0011] The integrating sphere is located below the polarization beamsplitter and has a surface scattering light collection function. After the infrared laser passes through the polarization beamsplitter, it passes through the interior of the integrating sphere in a straight line along the optical path.

[0012] The focusing lens includes an upper focusing lens and a lower focusing lens. The lower focusing lens is located below the integrating sphere and focuses the laser beam onto the surface of the material, increasing the power density and energy concentration of the laser. The upper focusing lens is located at the left end of the polarization beamsplitter. The upper and lower focusing lenses are of the same model and form a conjugate during detection.

[0013] The mirror is located below the material and above the stage, reducing the influence of the stage surface on the detection results while reflecting the light scattered by the subsurface defects of the transparent material.

[0014] The pinhole is located at the focal point of the upper focusing lens, i.e., the left end of the lens. By blocking scattered light outside the focal point, it reduces background noise and interference signals, thereby improving the signal-to-noise ratio of the image.

[0015] The sensor includes a surface polarized light sensor and a subsurface polarized light sensor. The subsurface polarized light sensor is used to collect signals scattered by subsurface defects, and the surface polarized light sensor is used to collect signals scattered by surface defects.

[0016] The precision displacement platform is composed of a servo motor, a ball screw, a stage, and a controller. The precision displacement platform moves the material relative to the focal point, keeping the beam position unchanged, and realizes scanning detection of the laser.

[0017] The computer realizes the design of the detection scanning path through programming and transmits instructions to the controller to drive the precision displacement platform to move the material, realizing automatic scanning. At the same time, the computer is used to receive the signals collected by the sensor, filter and process the surface and subsurface scattered light signals, and image them, and determine whether there are defects in the image.

[0018] The operation method of the above-mentioned laser detection system for transparent material surface / subsurface defects is as follows:

[0019] (1) The laser emits a linearly polarized infrared light beam, which passes through the polarization beamsplitter, the integrating sphere, and the lower focusing lens and is focused on the surface of the transparent material.

[0020] (2) a part of linearly polarized light is reflected and scattered on the surface, and after being collected by the integrating sphere, uniform light is formed after multiple reflections inside the integrating sphere, so that the surface polarized light sensor accurately collects the signal of the surface polarized light.

[0021] (3) another part of linearly polarized light penetrates the surface and enters the subsurface of the transparent material, and after multiple scattering by the subsurface defects, the polarization state changes to become partially polarized light. The light reflected by the mirror returns along the original light path, and the light with the same polarization state as the emitted laser passes through the polarizing beam splitter, and the remaining polarized light is reflected by the polarizing beam splitter. After focusing by the focusing lens, the reflected light passes through the pinhole and is collected by the subsurface polarized light sensor.

[0022] (4) During the detection process, the precision displacement platform moves according to the pre-set scanning path of the computer to realize automatic scanning of the laser on the material.

[0023] (5) After the two sensors collect the polarized light detection signals, the voltage signals are converted into digital signals by the data acquisition card. The digital signals are input into the computer, filtered and processed to form an image, and finally the defects on the surface / subsurface of the transparent material are visualized and the defect positions are accurately detected.

[0024] Advantages: Compared with the existing detection method, the present application has the following advantages:

[0025] 1. When detecting transparent materials, due to the high light transmittance of the materials, the light tends to be forward scattered in the cracks, and the light is scattered in the same direction or close to the same direction as the incident light. The present application adds a mirror above the object table and below the transparent material to be detected, so that the light is reflected twice, and the scattered polarized light is collected by the sensor. Unlike the detection of opaque materials, which is based on backscattering of light in the cracks, the scattered polarized light is collected by the sensor. By adding a mirror, the influence of the object table and the detection environment is reduced, and the problem of the influence of the high light transmittance of the materials on the subsurface detection is solved, while the accuracy of the defect detection of the transparent materials is improved.

[0026] 2. The linearly polarized laser emitted by the laser is scattered multiple times at the subsurface defects, and the polarization state changes. By detecting the polarization state of the scattered light, the subsurface defects of the transparent material can be effectively detected.

[0027] 3. Since the present application can simultaneously collect the polarized light of the surface and subsurface of the transparent material, different sensors simultaneously collect the polarized light signals of different surfaces, and the computer performs real-time signal processing and imaging, and the defects on the surface / subsurface are obtained at the same time, thereby improving the detection efficiency.

[0028] 4、The device structure in the application is relatively simple, easy to install, and suitable for real-time detection in industrial automation field. Its design is simple, and operation is convenient. The device can be rapidly deployed without affecting the existing production process, and effectively improves the detection efficiency and production quality.

[0029] 5、The transparent material involved in the application includes but is not limited to transparent glass, quartz glass, and semiconductor materials such as monocrystalline silicon. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 is a schematic diagram of the laser detection system adopted in the application;

[0031] Figure 2 is a laser detection imaging diagram of the subsurface defect of the transparent glass in Example 1;

[0032] Figure 3 is a laser detection imaging diagram of the surface defect of the transparent glass in Example 1;

[0033] Figure 4 is a sample quartz glass surface detection result diagram of Example 2;

[0034] Figure 5 is a sample quartz glass subsurface detection result diagram of Example 2.

[0035] In the figure, 1-laser, 2-linearly polarized laser, 3-polarization beamsplitter, 4-integral sphere, 5-lower focusing lens, 6-partially polarized light, 7-transparent material, 8-reflected light, 9-mirror, 10-precision displacement platform, 11-subsurface polarized light, 12-upper focusing lens, 13-needle hole, 14-subsurface polarized light sensor, 15-surface polarized light, 16-surface polarized light sensor, and 17-computer. DETAILED DESCRIPTION

[0036] The application will be further described below in combination with the drawings.

[0037] The specific detection object involved in the application is transparent glass. The following examples will be described around this material.

[0038] As shown in Figure 1 , a laser detection system for surface / subsurface defects of transparent material, the system includes a laser 1, a polarization beamsplitter 3, an integral sphere 4, a focusing lens, a mirror 9, a needle hole 13, a sensor, a precision displacement platform 10, and a computer 17.

[0039] The linearly polarized laser 2 emitted by the laser 1 is a linearly polarized infrared light beam, which is located at the top end of the system.

[0040] The polarizing beam splitter 3 is located below the laser 1 and has a polarization selection function. The linearly polarized laser 2 emitted by the laser 1 directly passes through the polarizing beam splitter 3.

[0041] The integrating sphere 4 is located below the polarizing beam splitter 3 and is used to collect the surface scattered light of the material. After the linearly polarized laser 2 passes through the polarizing beam splitter 3, it linearly passes through the inside of the integrating sphere 4 along the light path.

[0042] The focusing lens includes a lower focusing lens 5 and an upper focusing lens 11 (the lower focusing lens 5 and the upper focusing lens 9 are of the same type). The lower focusing lens 5 is located below the integrating sphere 4 and focuses the linearly polarized laser 2 to the surface of the transparent material 7, thereby improving the power density and energy concentration of the laser. The upper focusing lens 12 is located at the left end of the polarizing beam splitter 3.

[0043] The mirror 9 is located below the transparent material 7 and above the stage 10, which reduces the influence of the surface of the stage 10 on the detection results and reflects the light scattered by the subsurface defects of the transparent material 7.

[0044] The pinhole 13 is located at the left end of the upper focusing lens 12 and is at the focal point of the lens. By blocking the scattered light outside the focal point, the pinhole 13 reduces background noise and interference signals, thereby improving the signal-to-noise ratio of the imaging.

[0045] The sensor includes a subsurface polarized light sensor 14 and a surface polarized light sensor 16. The subsurface polarized light sensor 14 is used to collect the signal of the subsurface defect scattered light 11, and the surface polarized light sensor 16 is used to collect the signal of the surface defect scattered light 15.

[0046] The precision displacement platform 10 is composed of a servo motor, a ball screw, a stage, and a controller. The precision displacement platform 10 drives the transparent material 7 to move relative to the focal point, and the position of the light beam remains unchanged, thereby realizing the scanning detection of the laser.

[0047] The computer 17 realizes the design of the detection scanning path through programming and transmits instructions to the controller, so that the precision displacement platform 10 drives the material 7 to realize automatic scanning. At the same time, the computer 17 is used to receive the signals collected by the surface / subsurface polarized light sensor, to image the surface and subsurface scattered light signals after filtering processing, and to judge whether there are defects in the image.

[0048] The specific operation steps of the above-mentioned laser detection system for transparent material surface / subsurface defects are as follows:

[0049] The laser 1 emits linearly polarized laser 2, which is focused on the surface of the transparent material 7 through the polarizing beam splitter 3, the integrating sphere 4 and the lower focusing lens 5. A part of the linearly polarized light is reflected and scattered on the surface, and after being collected by the integrating sphere 4, it forms uniform light after multiple reflections inside the integrating sphere 4, so that the surface polarized light sensor 16 accurately collects the signal of the surface polarized light 15. Another part of the linearly polarized light penetrates the surface and enters the subsurface of the transparent material, and after multiple scattering through the subsurface defects, the polarization state changes and becomes partially polarized light. The reflected light 8 reflected by the mirror 9 returns along the original light path, and the light with the same polarization state as the emitted light passes through the polarizing beam splitter 3, and the rest of the polarized light is reflected by the polarizing beam splitter 3. The reflected subsurface polarized light 11 is focused by the upper focusing lens 12 and passes through the pinhole 13 to be collected by the subsurface polarized light sensor 14. During the detection process, the precision displacement platform 10 moves according to the pre-set scanning path of the computer 17, realizing the automatic scanning of the laser on the material. After the two sensors collect the polarized light detection signals, the data acquisition card converts the voltage signals into digital signals. The digital signals are input into the computer 17, and after filtering processing, imaging is performed, and finally the surface / subsurface defects of the transparent material 7 are visualized and detected.

[0050] Example 1

[0051] In the embodiment, the detected material is a square glass sheet with a side length of 50 mm and a thickness of 1 mm. By using a pressure head, a pressure mark is pressed on the surface of the glass to obtain a glass sample with surface and subsurface defects. The surface and subsurface cracks of the pressure mark glass are scanned and detected by using the laser detection system built. The detection scheme is as follows Figure 1 . The beam of the detection laser is fixed, and after the computer presets the scanning trajectory, the precision displacement platform drives the glass sample to be detected to move. The surface polarized light sensor collects the light scattering signals of the glass surface in real time, and the subsurface polarized light sensor collects the light scattering signals of the glass subsurface in real time. After the light power signals collected by the two sensors are converted into voltage signals, the continuous analog signals are converted into discrete digital signals by the data acquisition card, which are read by the computer in real time, and the sampling frequency is set to 1000 Hz. The computer performs low-pass filtering processing on the read subsurface polarized light signals, and the cutoff frequency is set to 10 Hz, and real-time imaging is performed, as shown in Figure 2 . The laser detection imaging diagram of the subsurface defects of the transparent glass is shown in Figure 2 . It can be seen that the part of the glass with defects has a strong polarized light signal, and the part without defects has a weak polarized light signal. By comparing the signal strength, the shape and position of the defects can be judged. The surface polarized light sensor collects the surface scattering light signals collected by the integrating sphere in real time, and the computer processes the signals collected by the sensor to perform real-time imaging, as shown in Figure 3 . The defects on the surface can be clearly seen from Figure 3 . FromFigure 2 and Figure 3 It can be seen that due to the addition of the mirror on the stage, the influence of the stage surface on the detection is reduced, and the imaging result of the defect is clear.

[0052] Example 2

[0053] In the embodiment, a cup-shaped diamond wheel with metal bond is selected for the plane grinding of the quartz glass, and the grain size of the wheel is 170#. During the grinding process, the wheel speed is 8000r / min, the grinding depth is 0.005mm, and the feed speed is 200mm / min. A spot is polished on the ground surface of the quartz glass by using the magnetorheological polishing technology. The middle part of the spot passes through the subsurface crack layer to fully expose the subsurface crack.

[0054] In this embodiment, the laser detection device completes the detection in the dark environment provided by the optical shading cloth to prevent the influence of stray light on the detection result.

[0055] During the detection, the quartz glass is placed above the mirror, the computer sends instructions to control the movement of the Z-axis guide rail, and the laser is focused on the surface of the quartz glass. The laser power is adjusted to make the laser output 10mW of linearly polarized light. The running track of the laser focus point is set on the computer, and the data detected by the surface polarization light sensor and the subsurface polarization light sensor are collected by the acquisition card while the guide rail is moving. Then, the collected data is processed and analyzed in the computer, and the detection result is as shown in Figure 4 and 5 As shown in Figure 4 , it is the surface detection result of the sample, and the spot morphology in the figure is the spot obtained by using the magnetorheological polishing technology. The light-colored part in the figure is the magnetorheological polishing area, and the dark-colored part is the surface ground by the grinding wheel, which has a certain roughness. As shown in Figure 5 , it is the subsurface detection result of the sample, and the dark-colored part in the figure is the undamaged area, and the rest is the damaged area, including the cracks generated during grinding. Through the design of the double sensors, the accuracy and efficiency of the detection are improved, and a more reliable technical means is provided for the evaluation of the defects and damages of the transparent material.

[0056] The lateral resolution of the laser detection system is:

[0057]

[0058] The axial resolution of the laser detection system is:

[0059]

[0060] As can be seen from the resolution formula of the laser detection system, the numerical aperture NA of the focusing lens is inversely proportional to the resolution, so a focusing lens with a large numerical aperture is selected as far as possible. In the laser detection device, the numerical aperture and the focal length of the focusing lens are 0.55 and 4.51 mm respectively, so the theoretical lateral resolution of the laser detection device is 850 nm, and the axial resolution is 1420 nm. Because the laser detection system needs to form a conjugate between the lower focusing lens and the upper focusing lens during detection, the same model of the two lenses is selected.

[0061] The present application is not limited to the embodiments, and any equivalent concept or change within the technical scope disclosed in the present application is included in the protection scope of the present application.

Claims

1. A laser synchronization detection system for transparent material surface and subsurface defects, characterized in that, The system comprises a laser, a polarization beam splitter, an integrating sphere, a focusing lens, a mirror, a pinhole, a sensor, a precision displacement platform, and a computer; The laser emits linearly polarized infrared light, which is located at the top of the system; The polarization beam splitter is located below the laser and has a polarization selection function; the linearly polarized light emitted by the laser directly passes through the polarization beam splitter; The integrating sphere is located below the polarization beam splitter and is used to collect the surface scattering light of the transparent material; after the infrared laser passes through the polarization beam splitter, it linearly passes through the inside of the integrating sphere along the light path; The focusing lens comprises an upper focusing lens and a lower focusing lens; the lower focusing lens is located below the integrating sphere and focuses the laser beam on the surface of the transparent material, thereby increasing the power density and energy concentration of the laser; The upper focusing lens is located at the left end of the polarization beam splitter; The mirror is located below the transparent material and above the stage, which reduces the influence of the surface of the stage on the detection results and allows the light scattered by the subsurface defects of the transparent material to be reflected; The pinhole is located at the left end of the upper focusing lens and is located at the focal point of the focusing lens; by blocking the scattered light outside the focal point, the background noise and interference signals are reduced, thereby improving the signal-to-noise ratio of the imaging; The sensor is connected to the computer and is used to collect the signals of the surface / subsurface defect scattering light; the computer is programmed to design the detection scanning path and transmits instructions to the controller, so that the precision displacement platform drives the material to realize automatic scanning; at the same time, the computer is used to receive the signals collected by the sensor, filter and process the surface / subsurface scattering light signals, and image the signals; the computer is also used to determine whether there are defects in the image; the sensor comprises a surface polarized light sensor and a subsurface polarized light sensor; the surface polarized light sensor (16) is arranged on one side of the integrating sphere and is used to collect the signals of the subsurface defect scattering light; the subsurface polarized light sensor (14) is arranged on one side of the upper focusing lens and behind the pinhole and is used to collect the signals of the surface defect scattering light.

2. A laser synchronization detection system for transparent material surface and subsurface defects according to claim 1, characterized in that, The transparent material comprises transparent inorganic materials and transparent organic materials; the upper focusing lens and the lower focusing lens are of the same type.

3. The system for laser synchronous detection of surface and subsurface defects in transparent materials according to claim 1, wherein, The laser, the polarization beam splitter, the integrating sphere, the focusing lens, the pinhole, and the sensor are installed in an electric displacement platform that can move up and down; the optical axes of the optical elements are located in the same plane.

4. The system for laser synchronous detection of surface and subsurface defects in transparent materials according to claim 1, wherein, The integrating sphere comprises three light transmission holes, i.e., an upper hole, a lower hole, and a left hole, which serve as the entrance and exit of the laser beam.

5. The system for laser synchronous detection of surface and subsurface defects in transparent materials according to claim 1, wherein, The precision displacement platform comprises a servo motor, a ball screw, a stage, and a controller; the precision displacement platform drives the transparent material to move horizontally relative to the focal point, and the position of the light beam remains unchanged, thereby realizing the scanning detection of the laser.

6. The method of claim 1, wherein the method further comprises: The method comprises the following steps: (1) The laser emits linearly polarized infrared light, which passes through the polarization beam splitter, the integrating sphere, and the lower focusing lens and is focused on the surface of the transparent material; (2) A part of the linearly polarized light is reflected and scattered on the transparent surface, is collected by the integrating sphere, and is uniformly formed after multiple reflections in the integrating sphere, so that the surface polarized light sensor accurately collects the signals of the surface polarized light; (3) Another part of the linearly polarized light penetrates the surface and enters the subsurface of the transparent material. After multiple scattering by the subsurface defects, the polarization state changes and becomes partially polarized light. The reflected light passes through the mirror and returns along the original light path. The light with the same polarization state as the emitted laser passes through the polarizing beam splitter, and the rest of the polarized light is reflected by the polarizing beam splitter. After focusing by the focusing lens, the reflected light passes through the pinhole and is collected by the subsurface polarized light sensor. (4) After the subsurface polarized light sensor and the surface polarized light sensor collect the polarized light detection signals, the data acquisition card converts the voltage signals into digital signals. The digital signals are input into the computer, filtered, and then imaged to realize the visualization and detection of the defects on the surface and subsurface of the transparent material simultaneously. During the detection process, the precision displacement platform moves according to the pre-set scanning path of the computer to realize the automatic scanning of the laser on the material.

7. A method of operating a laser synchronization detection system for surface and subsurface defects in transparent materials according to claim 6, characterized in that, The upper and lower focusing lenses form a conjugate during the detection process. The lateral resolution of the laser detection system is: （1） The axial resolution of the laser detection system is: （2） Therefore, the numerical aperture NA of the focusing lens is inversely proportional to the resolution, and a focusing lens with a large numerical aperture needs to be selected.

Images