A self-witnessed coherent imaging system for artifact removal and noise filtering

Abstract

The present invention discloses a self-witnessed coherent imaging system for artifact removal and noise filtering. The system comprises an interconnected processing light source, a broadband light source, a fiber optic beam splitter, a dichroic mirror, an optical lens, a spectrometer, a reference arm, and a sample arm. The optical beam generated by the broadband light source is split into two parts by the fiber optic beam splitter, one of which enters the reference arm; the other part is combined with the processing light by the dichroic mirror, and then focused onto the sample arm spot by the optical lens. The imaging light returning from the sample arm and the reference arm is recombined at the fiber optic beam splitter and enters the spectrometer. When the system provided by the present invention is applied to laser welding of 316L stainless steel, the keyhole tracking rate is improved by 67%, autocorrelation artifacts are completely eliminated, the signal-to-noise ratio is doubled, and there is no loss in imaging rate or spatial resolution. No balanced detection or dedicated light source is required.

Description

Technical Field

[0001] The present invention belongs to the field of morphological change measurement in laser processing, and in particular relates to a self-witnessing coherent imaging system for removing artifacts and filtering noise. Background Art

[0002] In high-power laser processing, such as welding and additive manufacturing, in-situ coherent imaging can measure morphological changes in real time at high speeds (over 300kHz line rates). However, this is affected by the interferometric imaging process, which is subject to speckle and image artifacts, and has high requirements for balance and lighting, which also leads to low work efficiency. High-power lasers are rapidly becoming a standard tool for automated advanced manufacturing applications, mainly laser welding, laser cutting, and laser additive manufacturing. Among these applications, the use of laser welding in electromobility (e-mobility) battery packs presents an unprecedented challenge to the manufacturing industry. The weld depth must be precisely controlled to ensure both joint strength and the weld does not perforate the lithium-ion battery. In addition, these applications often involve metals such as copper, which have properties that can lead to unstable welds, requiring unprecedented levels of control to ensure weld quality and depth. However, existing technologies cannot solve this problem. Summary of the Invention

[0003] In view of the above-mentioned deficiencies in the prior art, the object of the present invention is to provide a self-witnessing coherent imaging system comprising a second imaging channel for removing artifacts and noise filtering, thereby improving the control level to ensure welding quality and depth.

[0004] The technical solution provided by the present invention is:

[0005] A self-witness coherent imaging system for artifact removal and noise filtering includes a processing light source, a broadband light source, a fiber beam splitter, a dichroic mirror, an optical lens, a spectrometer, a reference arm, and a sample arm connected to each other;

[0006] The light beam generated by the broadband light source is divided into two parts by the fiber optic beam splitter, one of which enters the reference arm; the other part is combined with the processing beam by the dichroic mirror, and then the light beam is focused onto the sample arm light spot by the optical lens; the imaging light returned from the sample arm and the reference arm is recombined on the fiber optic beam splitter and enters the spectrometer.

[0007] The beam from the broadband light source passes through a 50 / 50 fiber optic beam splitter and enters the reference arm from one path; the other path is combined with the processing beam and passes through a dichroic mirror, which is the reference path; the beam is focused onto a light spot by a lens; the imaging light returning from the sample arm and reference arm is recombined on the fiber optic beam splitter and enters the spectrometer.

[0008] The system includes a second imaging channel that uses the same light source and spectrometer as the original ICI system and is automatically synchronized to the main channel.

[0009] The self-witness coherent imaging system for removing artifacts and noise filtering uses a mirror-doped fiber laser as the light source of the processing beam. The laser is coupled to a transmission fiber, and the fiber is connected to the processing head through a focal length collimator.

[0010] The described self-witnessed coherent imaging system for artifact removal and noise filtering uses two different reference arm configurations, a single-pass reference arm to generate a baseline of typical ICI data, and a double-pass reference arm using a 50 / 50 fiber beam splitter to redirect half of the light to a second gold mirror.

[0011] The self-witnessed coherent imaging system for artifact removal and noise filtering is combined with spectral measurements using a transmission diffraction grating and a high-speed CMOS line scan camera for sampling operations.

[0012] The self-witnessed coherent imaging system for artifact removal and noise filtering can improve the keyhole tracking rate in stainless steel laser welding.

[0013] The self-witnessed coherent imaging system for artifact removal and noise filtering employs a novel interferogram:

[0014]

[0015] In the interference pattern: A r1 and A r2 are the electric field amplitudes of the first reference arm channel and the second reference arm channel, respectively.

[0016] In the interference diagram, the fourth term is caused by the interference between the reference arm channels, the fifth and sixth terms are caused by the interference between the sample arm and the main reference arm and the second reference arm paths, respectively, and the last term is caused by the interference inside the sample arm. Since the frequency dependence of the first, second, and third terms is consistent with the light source spectrum envelope, that is, A after Fourier transform, z = 0, so they can be eliminated during processing. Using item 4, the main reference arm and the witness reference arm (A r ), and average background subtraction is then used to eliminate this term.

[0017] The present invention has the following beneficial effects:

[0018] 1. The present invention divides the light beam generated by the broadband light source into two parts through the fiber optic beam splitter, one of which enters the reference arm; the other part is combined with the processing beam through the dichroic mirror, and the light beam is focused onto the sample arm light spot through the optical lens; the imaging light returned from the sample arm and the reference arm is recombined on the fiber optic beam splitter and then enters the spectrometer, improving the control level to ensure welding quality and depth.

[0019] 2. When the system provided by the present invention is applied to laser welding of 316L stainless steel, the keyhole tracking rate is improved by 67%, and autocorrelation artifacts are completely eliminated. The signal-to-noise ratio is tripled, and there is no loss in imaging rate and spatial resolution. No balanced detection or dedicated light source is required. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 is a schematic diagram of a self-witnessing coherent imaging system for artifact removal and noise filtering according to an embodiment of the present invention;

[0021] Figure 2 Schematic diagram of the principle of a two-way reference arm configured in an embodiment of the present invention. DETAILED DESCRIPTION

[0022] The embodiments of the present invention are described in detail below. The following embodiments are implemented based on the technical solutions of the present invention, and provide detailed implementation methods and specific operating procedures. However, the protection scope of the present invention is not limited to the following embodiments.

[0023] Example:

[0024] An embodiment of the present invention provides a self-witnessed coherent imaging system for removing artifacts and noise filtering, which includes a processing light source, a broadband light source, a fiber optic beam splitter, a dichroic mirror, an optical lens, a spectrometer, a reference arm, and a sample arm that are interconnected. The light beam generated by the broadband light source is divided into two parts by the fiber optic beam splitter, one part of which enters the reference arm; the other part is combined with the processing light beam by the dichroic mirror, and then the light beam is focused onto the sample arm light spot by the optical lens; the imaging light returned from the sample arm and the reference arm is recombined on the fiber optic beam splitter and enters the spectrometer.

[0025] Figure 1 Schematic diagram of the process of the self-witness inline confocal imaging system provided by the present invention. Figure 1 As shown, the system includes the following:

[0026] The beam from the broadband light source passes through a 50 / 50 fiber optic beam splitter and enters the reference arm from one path; the other path is combined with the processing beam and passes through a dichroic mirror, which is the reference path; the beam is focused onto a light spot by a lens; the imaging light returning from the sample arm and reference arm is recombined on the fiber optic beam splitter and enters the spectrometer.

[0027] like Figure 1 As shown:

[0028] The beam from a fiber-coupled broadband light source (SUPERLUM BLM-S-840G-1-30, central wavelength: 840 nm, bandwidth: 32 nm, power: 18 mW) was passed through a 50 / 50 fiber beam splitter (Thorlabs FC850-40-50-APC 2×2 Broadband SM Coupler). Light enters the reference arm from one path, while the other path is combined with the processing beam (1070 nm) using a dichroic mirror; the path shared with the processing beam is called the sample arm; the light is focused to a 49 μm spot by a 150 mm lens; the imaging light returning from the sample and reference arms is recombined on a fiber optic beam splitter; a transmission diffraction grating (Wasatch Photonics HD-1800) and a high-speed CMOS line scan camera (Basler SprintspL4096-140 km) are used in combination with spectral measurements; 896 pixels are sampled at 200 kHz with an integration time of 2 μs and an axial resolution of 25 μm (limited by the light source bandwidth, the spectral characteristics of the optical components, and the imperfect dispersion matching between the sample and reference arms); strong backscattering from the high-power laser beam as well as strong blackbody radiation are filtered out by optics (by the dichroic grating and the diffraction grating) and by the coherent interferometry process; two different reference arm configurations are used, such as Figure 2 Shown: A single-pass reference arm is used to generate a baseline for typical ICI data. A double-pass reference arm uses a 50 / 50 free-space beamsplitter to redirect half of the light onto a second gold mirror. Experimentally, two peaks can be easily resolved at 90 μm. Choosing an adaptation that is an integer multiple of the depth bin size (10 μm) helps exploit correlations between pixel values ​​for image processing and subsequently reject uncorrelated noise and artifacts.

[0029] The system used a mirror-doped fiber laser (IPG Photonics YLS1000-IC) with a maximum power of 1 kW and a wavelength of 1070 nm as the processing light source. This was coupled into a 100 μm delivery fiber. This fiber was connected to the processing head (LaserMech Accufiber PLYDH0209) via a 50 mm focal length collimator. The beam was focused onto the sample using a 150 mm focal length lens, producing a spot size of 240 μm1 / e². Spot welds were performed on 3161 stainless steel samples prepared from NIST Standard Reference Material (SRM) 1155a at an average power of 360 W / 10 ms.

[0030] This system uses the following innovative interferograms:

[0031]

[0032] A r1 and A r2 are the electric field amplitudes of the primary reference arm channel and the second reference arm channel, respectively. The fourth term is caused by the interference between the reference arm channels, the fifth and sixth terms are caused by the interference between the sample arm and the primary reference arm and the second reference arm path, respectively, and the last term is caused by the interference inside the sample arm. Since the frequency dependence of the first, second, and third terms is consistent with the light source spectrum envelope (i.e., A after Fourier transform), z = 0), so they can be eliminated during processing. Using item 4, we can determine that the main reference arm and the witness reference arm (A r ), then use average background subtraction to eliminate this term. Since we know that A r , we can take advantage of this relationship.

[0033]

[0034] The average background (corresponding to terms 1, 2, and 4 in Eq. (3)) was collected by averaging 1000 acquisitions with the sample arm blocked. All subsequently measured interferograms were background subtracted, interpolated to constant k-space, and quickly extracted from k-space.

[0035]

[0036] Fast Fourier transform to z space. There are three remaining terms, not including the convolution required by the finite bandwidth of the light source.

[0037] If fA r1 ≈A r2 , when an interface is the result of light interference between the sample arm and the reference arm, the resulting function will have a distance A rIf a peak is due to noise or due to interference only in the sample arm, the witness peak will not appear.

[0038] To better control weld quality, in-situ monitoring of the welding process is needed. Off-site techniques such as X-ray micro-computed tomography (CT) can identify potentially catastrophic defects in parts and then address them. However, these techniques are costly and can produce side effects that can be repaired too late. Recent advances in high-brightness X-ray imaging have produced excellent high-speed tomographic images. Previously hidden dynamic principles are being revealed in laser welding and additive manufacturing, but only for highly constrained part geometries and within a few highly specialized research facilities (e.g., synchrotrons). In-line coherence imaging (ICI) is a process monitoring technique. It is an instrument that can measure sample height during laser processing in processes such as laser ablation, drilling, welding, and 3D additive manufacturing. This unit, integrated into a laser, can accurately position the sample in dynamic depth at a single point in time, or can raster scan the entire two-dimensional morphology. It will be used to measure process quality in materials such as metals, with speeds up to 312 kHz for silicon and bone, enabling closed-loop manual and dynamic control, providing a level of control to ensure weld quality and depth.

[0039] The present invention provides a self-witnessed coherent imaging system for artifact removal and noise filtering. The broadband light source's beam passes through a 50 / 50 fiber optic beam splitter and enters the reference arm along one path. The other path, combined with the processing beam, passes through a dichroic mirror and serves as the reference path. The beam is focused onto a spot by a lens. The imaging light returning from the sample and reference arms is recombined at the fiber optic beam splitter and enters a spectrometer. When applied to laser welding of 316L stainless steel, this system improved keyhole tracking efficiency by 67%, completely eliminated autocorrelation artifacts, and tripled the signal-to-noise ratio. There was no loss in imaging rate or spatial resolution, and no need for balanced detection or a dedicated light source.

[0040] The above describes in detail the preferred embodiments of the present invention. It should be understood that numerous modifications and variations based on the concepts of the present invention can be made by those skilled in the art without inventive effort. Therefore, any technical solution that can be derived by those skilled in the art based on the concepts of the present invention through logical analysis, reasoning, or limited experimentation based on the existing technology should be within the scope of protection defined by the claims.

Claims

1. A self-witnessed coherent imaging system for removing artifacts and noise filtering, characterized in that It includes interconnected A processing light source, a broadband light source, a fiber optic beam splitter, a dichroic mirror, an optical lens, a spectrometer, a reference arm, and a sample arm are connected; the light beam generated by the broadband light source is divided into two parts by the fiber optic beam splitter, one part of which enters the reference arm; the other part is combined with the processing light beam emitted by the processing light source through the dichroic mirror, and then the light beam is focused to the sample arm light spot through the optical lens; The imaging light returned from the sample arm and the reference arm is recombined on the fiber beam splitter and then enters the spectrometer; Two different reference arm configurations were used, including a single-pass reference arm and a double-pass reference arm. A baseline of typical ICI data was generated using the single-pass reference arm. The double-pass reference arm used a 50:50 fiber beam splitter to redirect half of the light onto a second gold mirror. The system uses the following interferogram: In the interference pattern: and are the electric field amplitudes of the one-way reference arm and the two-way reference arm, respectively; In the interference pattern: 2A r1 A r2 cos(Δr) is caused by the interference between the reference arm channels, and are caused by the interference between the sample arm and the single-pass reference arm and the double-pass reference arm paths, respectively. It is caused by interference inside the sample arm; The system also includes a second imaging channel that uses the same light source and spectrometer as the ICI system and is automatically synchronized to the main channel; The reference arm: The system includes a double-pass reference arm, which uses a fiber optic beam splitter to guide half of the light to the first gold mirror, forming an ICI system; and the other half to the second gold mirror, forming the second imaging channel.

2. The self-witnessed coherent imaging system for removing artifacts and noise filtering according to claim 1, characterized in that The processing light source is a mirror-doped laser with a maximum power of 1 kW and a wavelength of 1070 nm.

3. The self-witnessed coherent imaging system for removing artifacts and noise filtering according to claim 2, characterized in that The mirror-doped laser is coupled with a transmission optical fiber to output a processing light source, and the transmission optical fiber is connected to a processing head through a focal length collimator.

4. The self-witnessed coherent imaging system for removing artifacts and noise filtering according to claim 1, characterized in that The system also includes a transmission diffraction grating and a high-speed CMOS line scan camera, which are combined with spectral measurement to complete the sampling operation.

5. The self-witnessed coherent imaging system for removing artifacts and noise filtering according to claim 4, characterized in that The transmission diffraction grating is of the model Wasatch Photonics HD-1800; the high-speed CMOS line scan camera is of the model Basler Sprint spL4096-140KM.

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