Coaxial monitoring system for ultrafast laser micromachining

Abstract

The invention belongs to the field of laser micromachining, and relates to a coaxial monitoring system for ultrafast laser micromachining, which comprises an ultrafast laser, a light splitting flat plate, a laser focus lens, a middle lens group, a filter, a fixed-time telecentric lens and a high-dynamic camera, the ultrafast laser emits ultrafast laser; the light splitting flat plate and the laser focus lens are sequentially arranged on a light path where the ultrafast laser is located from front to back; the laser focus lens is used for converging the ultrafast laser on a machined part; visible monitoring light is generated on the machined part; the visible monitoring light penetrates through the laser focus lens and then is reflected by the light splitting flat plate to form reflected visible monitoring light; the middle lens group, the filter, the fixed-time telecentric lens and the high-dynamic camera are sequentially arranged on a light path where the reflected visible monitoring light is located from front to back. The coaxial monitoring system for ultrafast laser micromachining provided by the invention can be used for improving the imaging definition and the imaging resolution.

Description

Technical Field

[0001] The invention belongs to the field of laser micromachining and relates to an optical monitoring system, in particular to a coaxial monitoring system for ultrafast laser micromachining. Background Art

[0002] Ultrafast laser hole making technology is increasingly widely used in the field of ultra-precision parts processing in aviation and aerospace. At present, there are not many means to control the quality of blade film holes during laser hole making and the effect is average. How to achieve online coaxial monitoring and penetration detection of blade film holes is particularly important.

[0003] At present, the penetration monitoring technologies based on ultrafast laser hole making and visual sensors include coaxial monitoring (such as Figure 1 as well as Figure 2 ) and rangefinder monitoring (such as Figure 3 Taking coaxial monitoring as an example, the laser processing coaxial monitoring system is mainly composed of a spectroscopic plate, an imaging lens, a sensor (camera), a data acquisition card, and a computer. The structural diagram of a typical laser processing monitoring system is shown above. Figure 1 as well as Figure 2 As shown in the reflection type. The spectroscopic plate is a special lens that has been coated. It allows lasers of a specific wavelength band to be transmitted, while light of some wavelength bands is reflected. The main function of the spectroscope is to separate the laser from the monitoring light signal. The working principle of the monitoring system is as follows: during laser processing, the laser is focused on the surface of the workpiece almost without loss through the spectroscopic plate and the laser focusing mirror for processing the workpiece; at the same time, the light signal on the surface of the workpiece enters the sensor through the laser focusing mirror, the spectroscopic plate and the focusing lens (in front of the sensor), and the sensor converts the light signal into an electrical signal, which is converted into a digital signal after sampling, quantization and encoding by the data acquisition card and input into the computer for subsequent processing.

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

[0005] 1) Currently developed laser processing monitoring systems are mostly suitable for laser processing technologies such as laser welding and laser cutting, while there are few studies on related monitoring systems for ultrafast laser hole making technology (blade air film holes) at home and abroad; 2) The existing laser coaxial processing monitoring systems (such as laser welding and laser cutting monitoring systems) have low magnification and low resolution, which makes it difficult to meet the high-precision monitoring requirements of laser microprocessing during the process; 3) To achieve the purpose of visual monitoring, the monitoring system based on visual sensors must have an optical system that can complete visual imaging. 4) Domestic and foreign literature only provides a schematic diagram 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 will encounter various complex problems, such as: a) Ghosting problem (because both surfaces of the spectroscopic plate can reflect the light field of the processing area, the camera will present ghost images of different intensities after imaging, which seriously affects the judgment of penetration. The ghosting situation is shown in Figure 4 ); b) Image blur (after the laser is focused by the focusing lens, an ultra-high power density electric field is formed near the focus. This electric field can ionize the material to form plasma. This plasma and its surrounding area have the ability to remove materials, that is, processing ability. The scale of this area is even ≧1mm, which means that the processing position in this area is not fixed, that is, the position to be imaged is not fixed, and the imaging is often blurred. The specific reasons can be seen Figure 5 ); c) low imaging resolution (the imaging magnification that a general fixed-focus lens can achieve is often low (β≦1), and it is not possible to image the processing area in great detail); and d) the inside of a deep hole cannot be imaged (in ultrafast laser hole making (blade air film hole) technology, the diameter of the air film hole is very small ≦0.5mm, and the maximum depth-to-diameter ratio is ≧17:1. It is difficult for light from the processing area inside the deep hole to enter the camera, and the camera’s sensitivity cannot recognize this light intensity, so it cannot be imaged). Especially in the process of ultrafast laser making of blade air film holes, if the light cannot be turned off in time after the hole making is completed, it will cause damage to the wall, greatly increasing the production cost. Therefore, it is necessary to optimize the coaxial monitoring solution so that production personnel can optimize the process parameters, effectively control the hole making quality of the air film hole, and prevent damage to the wall. Summary of the invention

[0006] In order to solve the above technical problems existing in the background technology, the present invention provides a coaxial monitoring system for ultrafast laser micromachining with improved imaging clarity and imaging resolution.

[0007] In order to achieve the above object, the present invention adopts the following technical solution:

[0008] A coaxial monitoring system for ultrafast laser micromachining, characterized in that: the coaxial monitoring system for ultrafast laser micromachining comprises an ultrafast laser, a spectroscopic plate, a laser focusing mirror, an intermediate lens group, a filter, a fixed-magnification telecentric lens and a high-dynamic camera; the ultrafast laser emits ultrafast laser; the spectroscopic plate and the laser focusing mirror are arranged in sequence from front to back on the optical path where the ultrafast laser is located; the laser focusing mirror converges the ultrafast laser on a workpiece; generates visible monitoring light on the workpiece; the visible monitoring light passes through the laser focusing mirror and is reflected by the spectroscopic plate to form reflected visible monitoring light; the intermediate lens group, the filter, the fixed-magnification telecentric lens and the high-dynamic camera are arranged in sequence from front to back on the optical path where the reflected visible monitoring light is located.

[0009] The above-mentioned intermediate lens group and laser focusing lens constitute an optical 4F system.

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

[0011] The above-mentioned filters are bandpass filters.

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

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

[0014] The reflectivity of the above-mentioned coated reflector to visible light in the 620nm-680nm band is not less than 95%.

[0015] The ultrafast laser is an LC femtosecond laser, and the central wavelength of the ultrafast laser is 1030 nm and the pulse width is 290 fs.

[0016] The transmittance of the upper and lower surfaces of the above-mentioned splitter plate to lasers with a wavelength of 1030nm is not less than 99.5%; the reflectivity of the lower surface of the splitter plate to visible light with a wavelength of 620nm-680nm is not less than 95%.

[0017] The imaging magnification of the fixed-magnification telecentric lens is 3 times, the depth of field is ≧1mm, and the working distance is 65mm; the dynamic range of the high dynamic camera is 140dB.

[0018] The advantages of the present invention are:

[0019] The present invention provides a coaxial monitoring system for ultrafast laser micromachining, comprising an ultrafast laser, a spectroscopic plate, a laser focusing mirror, an intermediate lens group, a filter, a fixed-magnification telecentric lens and a high-dynamic camera; the ultrafast laser emits an ultrafast laser; the spectroscopic plate and the laser focusing mirror are sequentially arranged from front to back on the optical path where the ultrafast laser is located; the laser focusing mirror converges the ultrafast laser on the workpiece; a visible monitoring light is generated on the workpiece; the visible monitoring light passes through the laser focusing mirror and is reflected by the spectroscopic plate to form a reflected visible monitoring light; the intermediate lens group, the filter, the fixed-magnification telecentric lens and the high-dynamic camera are sequentially arranged from front to back on the optical path where the reflected visible monitoring light is located. The 4f system design in the coaxial monitoring system for ultrafast laser micromachining provided by the present invention, even if the processing position is not fixed during the laser processing process (position change ≧1mm), after passing through the 4f system, the image of the processing area is locked within a certain range, and a fixed-magnification telecentric lens is selected, and its depth of field is also ≧1mm, which can well solve the problem of imaging blur, and at the same time greatly improve the imaging resolution, and the optical imaging magnification of the entire system can reach β≧3. Moreover, the present invention also uses a high-dynamic camera, which can sense a wide range of light intensities and work normally in a high-brightness environment. Even in a deep hole, the image of the processing area can be clearly captured. The coaxial monitoring system for ultrafast laser micromachining provided by the present invention avoids the double reflection of the image by the spectroscopic plate and can eliminate ghosting. After testing, it can be seen that the ghosting phenomenon has disappeared, and a series of problems such as ghosting, blurred imaging, low imaging resolution, and inability to image deep holes can be specifically solved. That is, the coaxial monitoring system for ultrafast laser micromachining provided by the present invention can improve imaging clarity and imaging resolution, and can reduce imaging blur caused by excessive changes in object distance. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

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

[0023] Figure 4 It is a schematic diagram of the principle analysis of ghosting produced by an optical system in the prior art;

[0024] Figure 5 It is a schematic diagram of the principle analysis of imaging blur produced by an optical system in the prior art;

[0025] Figure 6 It 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 It is a comparison diagram of the ghosting between the coaxial monitoring system of the ultrafast laser micromachining used in the prior art and the present invention;

[0027] Figure 8 It is a comparison diagram of the imaging of an ordinary camera and the high dynamic camera used in the present invention;

[0028] Fig. 9 It is a camera image of the coaxial monitoring system for ultrafast laser micromachining provided by the present invention when machining the air film holes of the blade (at the beginning of machining);

[0029] Fig.10 It is a camera image (during the processing) of the coaxial monitoring system for ultrafast laser micromachining provided by the present invention when machining the air film holes of the blade;

[0030] Fig.11 It is a camera image of the coaxial monitoring system for ultrafast laser micromachining provided by the present invention when machining the air film holes of the blade (after the machining is completed);

[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 magnification telecentric lens; 8-high dynamic camera; 9-ultrafast laser. DETAILED DESCRIPTION

[0033] See also Figure 6 The present invention provides a coaxial monitoring system for ultrafast laser micromachining, which solves the problems of ghosting, blurred imaging, low imaging accuracy, and inability to image deep holes. The monitoring system includes: an ultrafast laser, a spectroscopic plate 3, a laser focusing mirror 2, an intermediate lens group 4, a filter 6, a fixed-magnification telecentric lens 7, and a high-dynamic camera 8; the ultrafast laser emits an ultrafast laser 9; the spectroscopic plate 3 and the laser focusing mirror 2 are sequentially arranged from front to back on the optical path where the ultrafast laser 9 is located; the laser focusing mirror 2 converges the ultrafast laser 9 on a workpiece 1; generates visible monitoring light on the workpiece 1; the visible monitoring light passes through the laser focusing mirror 2 and is reflected by the spectroscopic 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 camera 8 are sequentially arranged from front to back on the optical path where the reflected visible monitoring light is located.

[0034] The intermediate lens group 4 used in the present invention has a transmittance of visible light in the 620nm-680nm band of not less than 95%, and the intermediate lens group 4 can optimize problems such as astigmatism, distortion and chromatic aberration through optical design. Among them, the laser focusing mirror 2 converges the incident ultrafast laser 9 onto the surface of the workpiece 1 to form a laser spot with a very small diameter for laser processing; in addition, the laser focusing mirror 2 is also the objective lens of the imaging system, and together with the intermediate lens 4, it constitutes a 4f system to assist in imaging the laser processing area. The visible monitoring light of the processing area inside the air film hole is imaged at a specific position once after passing through the laser focusing mirror 2 and the intermediate lens group 4. The 4f system can be used to lock the image at processing area a at b, such as Figure 6 As shown. The 4f system design in the coaxial monitoring system for ultrafast laser micromachining provided by the present invention, even if the processing position is not fixed during the laser processing (position change ≧1mm), after passing through the 4f system, the image of the processing area is locked within a certain range, and a fixed-magnification telecentric lens 7 is selected, and its depth of field is also ≧1mm, which can well solve the problem of imaging blur, and at the same time greatly improve the imaging resolution, and the optical imaging magnification of the entire system can reach β≧3.

[0035] The filter 6 is a bandpass filter, and the bandpass range of the filter 6 is 620nm-680nm.

[0036] The coaxial monitoring system for ultrafast laser micromachining also includes a coated reflector 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 reflector 5 to the filter 6. The reflectivity of the coated reflector 5 to visible light in the 620nm-680nm band is not less than 95%.

[0037] The ultrafast laser is an LC femtosecond laser with a central wavelength of 1030 nm and a pulse width of 290 fs. The ultrafast laser can generate ultrafast laser light 9 that meets the processing requirements, can operate stably, and has adjustable power.

[0038] The spectroscopic plate 3 is a special lens with a coating, which allows the laser of a specific wavelength band (1030nm) to be transmitted, while the light of a partial wavelength band (620nm-680nm) is completely reflected. The main function of the spectroscopic plate 3 is to separate the ultrafast laser 9 from the visible monitoring light; the transmittance of the upper and lower surfaces of the spectroscopic plate 3 to the laser of the wavelength band of 1030nm is not less than 99.5%; the reflectivity of the lower surface of the spectroscopic plate 3 to the visible light of the wavelength band of 620nm-680nm is not less than 95%. The spectroscopic plate 3 has a high damage threshold to the laser.

[0039] The imaging magnification of the fixed-magnification telecentric lens 7 is 3 times, the depth of field is ≧1mm, and the working distance is 65mm. The fixed-magnification telecentric lens 7 can perform secondary imaging on the processing area a.

[0040] The dynamic range of the high dynamic camera 8 is 140dB. Its working principle is to collect images under overexposure, underexposure and normal exposure respectively, and then fuse the three images. Its characteristic is that it can form a clearer image of the target in a high-brightness environment. It has been widely used in the field of welding. Introducing high dynamic cameras into laser micromachining can help better observe and judge the processing status. The coaxial monitoring system for ultrafast laser micromachining provided by the present invention uses a high dynamic camera 8, which can sense a wide range of light intensity, work normally in a high-brightness environment, and can capture the image of the processing area even in a deep hole. For details, see Figure 8 .

[0041] The coaxial monitoring system for ultrafast laser micromachining provided by the present invention mainly includes two parts: a laser focusing optical path part (consisting of an ultrafast laser, a beam splitter plate 3 and a laser focusing mirror 2) and a visible light imaging optical path part (consisting of an intermediate lens group 4, a coated reflector 5, a filter 6, a fixed-magnification telecentric lens 7 and a high-dynamic camera 8 and other structures). When the equipment is working normally, the blade is placed on the XYZAC five-axis workbench and moves in three-dimensional space with the platform. The ultrafast laser 9 converges on the blade after passing through the laser focusing mirror 2 for laser micromachining. At the same time, the processing status of the blade air film hole 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:

[0043] (1) The laser generates an ultrafast laser 9 of 1030 nm, which is incident on the beam splitter plate 3 in the vertical direction, transmitted downward in the vertical direction after being transmitted through the beam splitter plate 3, and incident on the surface of the blade through the laser focusing mirror 2, thereby performing laser processing on the blade;

[0044] (2) In order to observe the laser processing conditions 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 through the 4f imaging system composed of the laser focusing mirror 2 and the intermediate lens group 4. The 620nm-680nm part of the visible light band is reflected from the light at b by the coated reflector 5 to the filter 6, and then incident on the fixed-magnification telecentric lens 7, and finally imaged on the target surface of the high-dynamic camera 8. It 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 the present invention avoids the double reflection of the image by the beam splitter plate and can eliminate ghosting. After testing, it can be seen that the ghosting phenomenon has disappeared. Figure 7 shown.

[0046] The coaxial monitoring system for ultrafast laser micromachining provided by the present invention can realize more accurate monitoring of the process of ultrafast laser machining of blade air film holes, and can present obvious penetration imaging features. Fig. 9 , Fig.10 as well as Fig.11 The camera image at the beginning of laser processing is relatively simple. It can be seen that the laser is performing peeling scanning. The middle part of the peeling area is a black feature. When the laser penetrates the side wall of the blade, due to the filling material inside the blade, the middle part of the processing area presents a relatively stable white bright spot, which can be used as a basis for penetration judgment.

Claims

1. A coaxial monitoring system for ultrafast laser micromachining, characterized in that: The coaxial monitoring system for ultrafast laser micromachining comprises an ultrafast laser, a spectroscopic plate (3), a laser focusing mirror (2), an intermediate lens group (4), a filter (6), a fixed-magnification telecentric lens (7) and a high-dynamic camera (8); the ultrafast laser emits an ultrafast laser (9); the spectroscopic plate (3) and the laser focusing mirror (2) are arranged in sequence from front to back on the optical path where the ultrafast laser (9) is located; the laser focusing mirror (2) focuses the ultrafast laser (9) on the workpiece (1); and generates visible monitoring light on the workpiece (1); the visible monitoring light passes through the laser focusing mirror (2) and is reflected by the spectroscopic 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 camera (8) are arranged in sequence from front to back on the optical path where the reflected visible monitoring light is located.

2. The coaxial monitoring system for ultrafast laser micromachining according to claim 1, characterized in that: The intermediate lens group (4) and the laser focusing lens (2) form an optical 4F system.

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

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

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

6. The coaxial monitoring system for ultrafast laser micromachining according to any one of claims 1 to 5, characterized in that: The coaxial monitoring system for ultrafast laser micromachining further comprises a coated reflector (5) arranged 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 reflector (5) to the filter (6).

7. The coaxial monitoring system for ultrafast laser micromachining according to claim 6, characterized in that: The reflectivity of the coated reflector (5) to visible light in the 620nm-680nm band is not less than 95%.

8. The coaxial monitoring system for ultrafast laser micromachining according to claim 7, characterized in that: The ultrafast laser is an LC femtosecond laser, and the central wavelength of the ultrafast laser is 1030nm and the pulse width is 290fs.

9. The coaxial monitoring system for ultrafast laser micromachining according to claim 8, characterized in that: The transmittance of the upper and lower surfaces of the splitter plate (3) to lasers with a wavelength of 1030 nm is not less than 99.5%; the reflectivity of the lower surface of the splitter plate (3) to visible light with a wavelength of 620 nm-680 nm is not less than 95%.

10. The coaxial monitoring system for ultrafast laser micromachining according to claim 9, characterized in that: The imaging magnification of the fixed-magnification telecentric lens (7) is 3 times, the depth of field is ≧1 mm, and the working distance is 65 mm; the dynamic range of the high-dynamic camera (8) is 140 dB.

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