Coaxial monitoring system for ultrafast laser micromachining

Abstract

This invention belongs to the field of laser micromachining and relates to a coaxial monitoring system for ultrafast laser micromachining. The system includes an ultrafast laser, a beam-splitting plate, a laser focusing lens, an intermediate lens group, a filter, a fixed-magnification telecentric lens, and a high dynamic range camera. The ultrafast laser emits ultrafast laser light. The beam-splitting plate and the laser focusing lens are arranged sequentially from front to back along the optical path of the ultrafast laser. The laser focusing lens focuses the ultrafast laser light onto the workpiece, generating visible monitoring light on the workpiece. The visible monitoring light passes through the laser focusing lens and is reflected by the beam-splitting plate to form reflected visible monitoring light. The intermediate lens group, filter, fixed-magnification telecentric lens, and high dynamic range camera are arranged sequentially from front to back along the optical path of the reflected visible monitoring light. This invention provides a coaxial monitoring system for ultrafast laser micromachining that improves imaging clarity and resolution.

Description

Technical Field

[0001] This invention belongs to the field of laser micromachining and relates to an optical monitoring system, particularly a coaxial monitoring system for ultrafast laser micromachining. Background Technology

[0002] Ultrafast laser drilling technology is finding increasingly widespread applications in the machining of ultra-precision components for aerospace applications. Currently, the methods for controlling the quality of film condensation holes in blades during laser drilling are limited and generally ineffective. Therefore, achieving online coaxial monitoring and penetration detection of film condensation holes in blades is of paramount importance.

[0003] Currently, penetration monitoring technologies based on ultrafast laser drilling and vision sensors include coaxial monitoring (such as...). Figure 1 as well as Figure 2 ) and paraxial monitoring (e.g. Figure 3 Two schemes are shown above. Taking coaxial monitoring as an example, a laser processing coaxial monitoring system mainly consists of a beam splitter, an imaging lens, a sensor (camera), a data acquisition card, and a computer. A typical structural diagram of a laser processing monitoring system is shown above. Figure 1 as well as Figure 2 As shown in the reflection diagram, the beam splitter is a specially coated lens that allows laser light of a specific wavelength to pass through while reflecting some wavelengths. The main function of the beam splitter is to separate the laser light from the monitoring light signal. The working principle of the monitoring system is as follows: During laser processing, the laser light is focused onto the workpiece surface with almost no loss through the beam splitter and laser focusing lens for processing. Simultaneously, the light signal from the workpiece surface enters the sensor through the laser focusing lens, beam splitter, and focusing lens (in front of the sensor). The sensor converts the light signal into an electrical signal, which is then sampled, quantized, and encoded by the data acquisition card before being converted into a digital signal and input into the computer for further processing.

[0004] However, both coaxial and paraxial monitoring have the following main shortcomings:

[0005] 1) Currently developed laser processing monitoring systems are mostly applicable to laser processing technologies such as laser welding and laser cutting. However, there is little research on monitoring systems for ultrafast laser hole-making technology (film cooling holes in blades) both domestically and internationally. 2) Existing coaxial laser processing monitoring systems (such as laser welding and laser cutting monitoring systems) have low magnification and low resolution, making it difficult to meet the high-precision monitoring requirements of laser micromachining. 3) For vision sensor-based monitoring systems to achieve visual monitoring, an optical system capable of visual imaging is necessary. 4) Domestic and international literature only provides schematic diagrams of the optical path principle of the monitoring system. In practical applications, various problems are often encountered, especially in ultrafast laser hole-making technology. Coaxial imaging encounters various complex problems, such as: a) ghosting problem (because both surfaces of the beam-splitting plate can reflect the light field of the processing area, the camera image will show ghosting of varying intensities, seriously affecting the judgment of penetration. Ghosting issues are discussed in...). Figure 4 (b) Image blurring problem (After the laser is focused by the focusing lens, an ultra-high power density electric field is formed near the focal point. This electric field can ionize materials to form plasma. This plasma and its surrounding area have the ability to remove materials, i.e., processing capabilities. The scale of this area can even be ≥1mm, which means that the processing position in this area is not fixed, that is, the position that needs to be imaged is not fixed, and the image is often blurry. For details, see [link to documentation]). Figure 5 c) Low imaging resolution (generally, fixed-focus lenses can only achieve a relatively low magnification (β≦1), which cannot provide very detailed imaging of the processed area); and d) Inability to image deep holes (in ultrafast laser hole-making (blade film aperture) technology, the diameter of the film aperture is very small (≤0.5mm), and the maximum depth-to-diameter ratio is ≥17:1. Light from the processed area inside the deep hole has difficulty entering the camera, and the camera's sensitivity cannot detect this light intensity, thus preventing imaging). Especially in the process of ultrafast laser fabrication of blade film apertures, if the laser is not turned off in time after hole-making, it will cause damage to the wall, greatly increasing production costs. Therefore, it is necessary to optimize the coaxial monitoring scheme so that production personnel can optimize process parameters, effectively control the hole-making quality of the film aperture, and prevent damage to the wall. Summary of the Invention

[0006] In order to solve the above-mentioned technical problems in the background art, the present invention provides a coaxial monitoring system for ultrafast laser micromachining that improves imaging clarity and imaging resolution.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A coaxial monitoring system for ultrafast laser micromachining is characterized in that: the coaxial monitoring system for ultrafast laser micromachining includes an ultrafast laser, a beam-splitting plate, a laser focusing lens, an intermediate lens group, a filter, a fixed-magnification telecentric lens, and a high dynamic range camera; the ultrafast laser emits ultrafast laser light; the beam-splitting plate and the laser focusing lens are arranged sequentially from front to back along the optical path of the ultrafast laser; the laser focusing lens focuses the ultrafast laser light onto the workpiece; visible monitoring light is generated on the workpiece; the visible monitoring light passes through the laser focusing lens and is reflected by the beam-splitting plate to form reflected visible monitoring light; the intermediate lens group, the filter, the fixed-magnification telecentric lens, and the high dynamic range camera are arranged sequentially from front to back along the optical path of the reflected visible monitoring light.

[0009] The aforementioned intermediate lens group and the laser focusing lens constitute an optical 4F system.

[0010] The transmittance of the aforementioned intermediate lens group in the visible light band of 620nm-680nm is not less than 95%.

[0011] The filter described above is a bandpass filter.

[0012] The passband range of the above-mentioned filter is 620nm-680nm.

[0013] The aforementioned coaxial monitoring system for ultrafast laser micromachining also includes a coated mirror disposed between the intermediate lens group and the filter; the reflected visible monitoring light passes through the intermediate lens group and is reflected by the coated mirror to the filter.

[0014] The above-mentioned coated mirror has a reflectivity of no less than 95% for visible light in the 620nm-680nm wavelength range.

[0015] The aforementioned ultrafast laser is an LC femtosecond laser with a center wavelength of 1030 nm and a pulse width of 290 fs.

[0016] The upper and lower surfaces of the aforementioned beam-splitting plate have a transmittance of not less than 99.5% for laser light in the 1030nm band; the lower surface of the beam-splitting plate has a reflectance of not less than 95% for visible light in the 620nm-680nm band.

[0017] The aforementioned fixed-magnification telecentric lens has an imaging magnification of 3x, a depth of field ≥ 1mm, and a working distance of 65mm; the high dynamic range camera has a dynamic range of 140dB.

[0018] The advantages of this invention are:

[0019] This invention provides a coaxial monitoring system for ultrafast laser micromachining, comprising an ultrafast laser, a beam-splitting plate, a laser focusing lens, an intermediate lens group, a filter, a fixed-magnification telecentric lens, and a high dynamic range camera. The ultrafast laser emits ultrafast laser light. The beam-splitting plate and the laser focusing lens are arranged sequentially from front to back along the optical path of the ultrafast laser. The laser focusing lens focuses the ultrafast laser light onto the workpiece, generating visible monitoring light on the workpiece. The visible monitoring light passes through the laser focusing lens and is reflected by the beam-splitting plate to form reflected visible monitoring light. The intermediate lens group, filter, fixed-magnification telecentric lens, and high dynamic range camera are arranged sequentially from front to back along the optical path of the reflected visible monitoring light. The 4f system design in the coaxial monitoring system for ultrafast laser micromachining provided by this invention ensures that even if the processing position is not fixed during laser processing (position change ≥ 1 mm), the image of the processing area is locked within a certain range after passing through the 4f system. The fixed-magnification telecentric lens also has a depth of field ≥ 1 mm, effectively solving the problem of image blurring. Simultaneously, it greatly improves the imaging resolution, and the optical magnification of the entire system can reach β ≥ 3. Furthermore, this invention also employs a high dynamic range camera, capable of sensing a wide range of light intensities and operating normally in bright environments. Even in deep holes, it can clearly capture images of the processing area. The coaxial monitoring system for ultrafast laser micromachining provided by this invention avoids double reflection of the image by the beam-splitting plate, eliminating ghosting. Tests show that the ghosting phenomenon has disappeared. It can specifically solve a series of problems such as ghosting, image blurring, low image resolution, and inability to image deep holes. In other words, the coaxial monitoring system for ultrafast laser micromachining provided by this invention can improve image clarity and resolution, and reduce image blurring caused by excessive changes in object distance. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the optical path principle of a coaxial monitoring optical system (reflective type) in the prior art;

[0021] Figure 2 This is a schematic diagram of the optical path principle of a coaxial monitoring optical system (transmission type) in the prior art;

[0022] Figure 3 This is a schematic diagram of the optical path principle of a parietal monitoring optical system in the prior art;

[0023] Figure 4 This is a schematic diagram illustrating the principle of ghosting produced by optical systems in existing technologies;

[0024] Figure 5 This is a schematic diagram illustrating the principle of imaging blur produced by optical systems in existing technologies;

[0025] Figure 6 This is a schematic diagram of the optical path principle of the coaxial monitoring system for ultrafast laser micromachining provided by the present invention;

[0026] Figure 7 This is a comparison of ghosting between existing technologies and the coaxial monitoring system for ultrafast laser micromachining used in this invention;

[0027] Figure 8 These are comparison images of images taken by a regular camera and the high dynamic range camera used in this invention.

[0028] Figure 9 The image is a camera image (at the start of processing) of a blade film-forming hole in a coaxial monitoring system for ultrafast laser micromachining provided by this invention.

[0029] Figure 10 The image is a camera image (during processing) of a blade film pore during processing, based on the coaxial monitoring system for ultrafast laser micromachining provided by this invention.

[0030] Figure 11 The image is a camera image (after processing) of a blade film pore during the processing of the coaxial monitoring system for ultrafast laser micromachining provided by this invention.

[0031] in:

[0032] 1-Processed part; 2-Laser focusing lens; 3-Beam splitter plate; 4-Intermediate lens group; 5-Coated reflector; 6-Filter; 7-Fixed telecentric lens; 8-High dynamic range camera; 9-Ultrafast laser. Detailed Implementation

[0033] See Figure 6 This invention provides a coaxial monitoring system for ultrafast laser micromachining, solving problems such as ghosting, blurred imaging, low imaging accuracy, and inability to image deep holes. The monitoring system includes: an ultrafast laser, a beam-splitting plate 3, a laser focusing lens 2, an intermediate lens group 4, a filter 6, a fixed-magnification telecentric lens 7, and a high dynamic range camera 8; the ultrafast laser emits an ultrafast laser 9; the beam-splitting plate 3 and the laser focusing lens 2 are arranged sequentially from front to back along the optical path of the ultrafast laser 9; the laser focusing lens 2 focuses the ultrafast laser 9 onto the workpiece 1; visible monitoring light is generated on the workpiece 1; the visible monitoring light passes through the laser focusing lens 2 and is reflected by the beam-splitting plate 3 to form reflected visible monitoring light; the intermediate lens group 4, the filter 6, the fixed-magnification telecentric lens 7, and the high dynamic range camera 8 are arranged sequentially from front to back along the optical path of the reflected visible monitoring light.

[0034] The intermediate lens group 4 used in this invention has a visible light transmittance of no less than 95% in the 620nm-680nm wavelength range, and its optical design optimizes issues such as astigmatism, distortion, and chromatic aberration. Specifically, the laser focusing lens 2 focuses the incident ultrafast laser 9 onto the surface of the workpiece 1, forming a very small laser spot for laser processing. Furthermore, the laser focusing lens 2 also serves as the objective lens of the imaging system, forming a 4f system together with the intermediate lens 4 to assist in imaging the laser processing area. The visible monitoring light from the processing area inside the air film aperture is imaged at a specific location after passing through the laser focusing lens 2 and the intermediate lens group 4. Using this 4f system, the image at processing area a can be locked at point b, such as... Figure 6 As shown. The 4f system design in the coaxial monitoring system for ultrafast laser micromachining provided by this invention ensures that even if the processing position is not fixed during laser processing (position change ≥ 1mm), the image of the processing area is locked within a certain range after passing through the 4f system. The fixed-magnification telecentric lens 7 is selected, and its depth of field is also ≥ 1mm, which can effectively solve the problem of image blurring. At the same time, it also greatly improves the imaging resolution. The optical imaging magnification of the entire system can reach β ≥ 3.

[0035] Filter 6 is a bandpass filter with a bandpass range of 620nm-680nm.

[0036] The coaxial monitoring system for ultrafast laser micromachining also includes a coated mirror 5 positioned between the intermediate lens group 4 and the filter 6; the reflected visible monitoring light passes through the intermediate lens group 4 and is then reflected by the coated mirror 5 to the filter 6. The coated mirror 5 has a reflectivity of not less than 95% for visible light in the 620nm-680nm wavelength band.

[0037] The ultrafast laser is an LC femtosecond laser with a center wavelength of 1030nm and a pulse width of 290fs. This ultrafast laser can generate ultrafast lasers that meet the processing requirements, and it can operate stably with adjustable power.

[0038] The beam-splitter 3 is a specially coated lens that allows laser light of a specific wavelength (1030nm) to pass through, while reflecting light of a portion of the wavelength range (620nm-680nm). The main function of the beam-splitter 3 is to separate the ultrafast laser 9 from the visible monitoring light. The upper and lower surfaces of the beam-splitter 3 have a transmittance of no less than 99.5% for laser light of the 1030nm wavelength range; the lower surface of the beam-splitter 3 has a reflectance of no less than 95% for visible light of the 620nm-680nm wavelength range. The beam-splitter 3 has a high damage threshold to the laser.

[0039] The fixed-magnification telecentric lens 7 has an imaging magnification of 3x, a depth of field of ≥1mm, and a working distance of 65mm. The fixed-magnification telecentric lens 7 can perform secondary imaging of the processing area a.

[0040] The high dynamic range camera 8 has a dynamic range of 140dB. Its working principle involves acquiring images separately under overexposure, underexposure, and normal exposure conditions, and then fusing the three images. Its characteristic is that it can form a clearer image of the target in high-brightness environments. It is already widely used in the welding field. Introducing a high dynamic range camera into laser micromachining can help to better observe and judge the processing status. The coaxial monitoring system for ultrafast laser micromachining provided by this invention uses the high dynamic range camera 8, which can sense a very wide range of light intensity, operate normally in high-brightness environments, and can even capture images of the processing area in deep holes. See details... Figure 8 .

[0041] The coaxial monitoring system for ultrafast laser micromachining provided by this invention mainly comprises two parts: a laser focusing optical path (consisting of an ultrafast laser, a beam splitter 3, and a laser focusing mirror 2) and a visible light imaging optical path (consisting of an intermediate lens group 4, a coated reflector 5, a filter 6, a fixed-magnification telecentric lens 7, and a high dynamic range camera 8, etc.). During normal operation, the blade is placed on the XYZAC five-axis worktable and moves in three-dimensional space with the platform. The ultrafast laser 9 is focused onto the blade by the laser focusing mirror 2 for laser micromachining. Simultaneously, the processing status of the blade's film pores can be observed through the visible light imaging system. The working principles of these two parts are discussed in detail below.

[0042] Its working principle is as follows:

[0043] (1) The laser generates an ultrafast laser 9 of 1030nm, which is incident on the beam splitting plate 3 in the vertical direction. After being transmitted through the beam splitting plate 3, it is transmitted downward in the vertical direction and incident on the blade surface through the laser focusing lens 2 to perform laser processing on the blade.

[0044] (2) In order to observe the laser processing status on the blade, a camera is used for observation. The visible light generated in the processing area a is imaged at a specific position b by the 4f imaging system consisting of the laser focusing lens 2 and the intermediate lens group 4. The 620nm-680nm part of the visible light band is reflected by the coated reflector 5 to the filter 6 and then incident into the fixed magnification telecentric lens 7, and finally imaged on the target surface of the high dynamic range camera 8. The data is collected by the data acquisition card and sent to the computer and displayed on the monitor.

[0045] The coaxial monitoring system for ultrafast laser micromachining provided by this invention avoids double reflection of the image by the beam-splitting plate, thus eliminating ghosting. Testing shows that the ghosting phenomenon has disappeared. Figure 7 As shown.

[0046] The coaxial monitoring system for ultrafast laser micromachining provided by this invention can achieve more accurate monitoring of the ultrafast laser machining process of air film pores in blades, and can exhibit obvious penetration imaging characteristics, as detailed in the following figures. Figure 9 , Figure 10 as well as Figure 11 The camera image at the beginning of laser processing is relatively simple, showing that the laser is performing a rotary cutting scan. The middle part of the rotary cutting area is a black feature. After the laser penetrates the sidewall of the blade, due to the filling material inside the blade, the middle part of the processing area shows a relatively stable white bright spot. This can be used as a basis for penetration interpretation.

Claims

1. A coaxial monitoring system for ultrafast laser micromachining, characterized in that: The coaxial monitoring system for ultrafast laser micromachining includes an ultrafast laser, a beam splitter (3), a laser focusing lens (2), an intermediate lens group (4), a filter (6), a fixed-magnification telecentric lens (7), and a high dynamic range camera (8). The ultrafast laser emits an ultrafast laser (9). The beam splitter (3) and the laser focusing lens (2) are arranged sequentially from front to back on the optical path of the ultrafast laser (9). The laser focusing lens (2) focuses the ultrafast laser (9) onto the workpiece (1). Visible monitoring light is generated on the workpiece (1). The visible monitoring light passes through the laser focusing lens (2) and is reflected by the beam splitter (3) to form reflected visible monitoring light. The intermediate lens group (4), the filter (6), the fixed-magnification telecentric lens (7), and the high dynamic range camera (8) are arranged sequentially from front to back on the optical path of the reflected visible monitoring light. The intermediate lens group (4) and the laser focusing lens (2) constitute an optical 4F system; The coaxial monitoring system for ultrafast laser micromachining also includes a coated mirror (5) disposed between the intermediate lens group (4) and the filter (6); the reflected visible monitoring light passes through the intermediate lens group (4) and is reflected by the coated mirror (5) to the filter (6). The intermediate lens group (4) performs the first image of the processing area and locks it between the intermediate lens group (4) and the coated mirror (5); the fixed magnification telecentric lens (7) performs the second image of the processing area and finally presents it on the high dynamic range camera (8).

2. The coaxial monitoring system for ultrafast laser micromachining according to claim 1, characterized in that: The transmittance of the intermediate lens group (4) in the visible light band of 620nm-680nm is not less than 95%.

3. The coaxial monitoring system for ultrafast laser micromachining according to claim 2, characterized in that: The filter (6) is a bandpass filter.

4. The coaxial monitoring system for ultrafast laser micromachining according to claim 3, characterized in that: The passband range of the filter (6) is 620nm-680nm.

5. The coaxial monitoring system for ultrafast laser micromachining according to any one of claims 1-4, characterized in that: The coated mirror (5) has a reflectivity of not less than 95% for visible light in the 620nm-680nm band.

6. The coaxial monitoring system for ultrafast laser micromachining according to claim 5, characterized in that: The ultrafast laser is an LC femtosecond laser with a center wavelength of 1030 nm and a pulse width of 290 fs.

7. The coaxial monitoring system for ultrafast laser micromachining according to claim 6, characterized in that: The upper and lower surfaces of the beam splitter plate (3) have a transmittance of not less than 99.5% for laser light with a wavelength of 1030nm; the lower surface of the beam splitter plate (3) has a reflectance of not less than 95% for visible light with a wavelength of 620nm-680nm.

8. The coaxial monitoring system for ultrafast laser micromachining according to claim 7, characterized in that: The fixed-magnification telecentric lens (7) has an imaging magnification of 3x, a depth of field of ≥1mm, and a working distance of 65mm; the high dynamic range camera (8) has a dynamic range of 140dB.

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