Optical coherence tomography spatial light path system in far-field laser processing

Abstract

The invention discloses an optical coherence tomography spatial light path system in far-field laser processing, which comprises a light source system formed by a light source and a collimating lens, an annular spectroscope, a first focusing lens, a second focusing lens and an achromatic lens group, and the anti-reflection area is arranged around the light splitting area and is plated with an anti-reflection film, and the second surface is plated with an anti-reflection film, so that the reflection loss of the non-light splitting area is reduced. On the basis, a new spatial light path is designed, and through the design of internal light splitting and external anti-reflection, the diameter of incident light can be reduced, the intensity of scattered light and the imaging depth of field can be remarkably improved, and the clear tomography capability in the whole process can be maintained. The innovation of the invention is reflected in the collaborative layout of the annular light splitting and anti-reflection functions, and the creative spatial light path design abandons the traditional optical fiber coupler, and the optical signal can be directly collected by the image sensor. The core difficulties that scattered light of a far-field laser processing scene is weak and the imaging depth of field is limited are solved, and a foundation is laid for high-quality in-situ detection and intelligent control of far-field laser processing.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical coherence tomography (OCT), and more specifically, relates to a spatial optical path system for optical coherence tomography in far-field laser processing. Background Art

[0002] Laser processing is widely used in the industrial field due to its high processing precision and programmability, as well as the advantages of thermodynamic precision control and digital integration. Far-field laser processing refers to the precision processing performed by laser under far-field conditions. Generally, a high-frequency vibration system is used to guide the laser beam for high-frequency flying processing. It has been widely used in the fields of new energy battery welding, laser 3D printing flat wire motor welding, etc. One of the bottleneck technologies of laser processing technology is defect detection. To achieve precise control of defects, the key lies in breaking through the limitations of traditional detection methods and obtaining dynamic evolution data of the processing field (such as the molten pool / keyhole) in real time during the manufacturing process. At present, the development of real-time in-situ imaging technology has become an important link in connecting process mechanism research, defect formation prediction and intelligent process control. Online monitoring of the molten pool status and real-time defect detection are the core requirements for ensuring processing quality.

[0003] In the field of laser processing monitoring, real-time diagnostic technology is mainly divided into two technical routes: indirect characterization methods based on visible light / thermal radiation and direct detection technology based on fiber-coupled optical coherence tomography (fiber-optic OCT). The former infers the state of the melt pool / keyhole through surface radiation field reconstruction, but due to the complex physical field coupling mechanism, it is difficult to accurately obtain the dynamic characteristics of the three-dimensional processing field. In contrast, fiber-optic OCT technology achieves tomographic imaging of the keyhole depth direction through the coaxial coupling of a low-coherence broadband light source and a high-power laser beam. Its axial resolution can reach the micron level, breaking through the depth detection limitations of traditional methods.

[0004] Fiber-optic OCT-based detection devices typically use a main beam splitter to split a beam of coherent light into two sub-beams of coherent light, which are transmitted to a reference arm and a measuring arm respectively. The reference arm provides reference light with a known optical path, which interferes with the detection light reflected by the measuring arm. The measuring arm is responsible for detecting the dynamic optical path of the internal structure of the keyhole during the machining process.

[0005] In the fiber-optic OCT optical path system for monitoring the laser processing process, the reference arm and the measuring arm show significant differences in optical path characteristics. As a reference interference path, the reference arm has an optical path structure that is independent of the processing area. It usually adopts a rigid spatial optical path design, showing excellent phase stability and environmental interference resistance. In contrast, the measuring arm needs to be directly exposed to the dynamic environment of the processing area. It must not only achieve precise focusing of the light beam, but also capture the scattered light field modulated by the molten pool / keyhole in real time. Its optical path stability is affected by multiple factors such as thermal radiation and mechanical vibration. This asymmetric stability architecture determines the core constraint of the system's imaging quality: the intensity and phase fidelity of the scattered light signal of the measuring arm directly determine the resolution and dynamic response characteristics of the tomographic image. In the near-field mirror laser processing scenario, the working distance of the measuring arm is limited, and the fiber-optic OCT detection device is sufficient to meet the imaging requirements. However, in far-field laser processing, the following problems exist:

[0006] 1. The scattered light signal of the measuring arm is weak in far-field laser processing

[0007] In far-field laser processing applications (typical working distance > 500mm), the optical path design of the measuring arm faces essential physical constraints. The detection beam needs to travel a long distance in free space to reach the processing surface, and its transmission optical path increases. According to the scattered light transmission attenuation law, the scattered light returning to the measuring arm attenuates with distance; at the same time, the inherent architecture of the fiber-optic OCT detection device further aggravates the signal attenuation: the scattered light needs to be spatially coupled, optically transmitted, and distributed twice by a splitter, and the composite loss of the entire process cannot be ignored. This cascaded signal attenuation is manifested as the inability to capture enough scattered light from the measuring arm, which ultimately makes it difficult for traditional fiber-optic OCT detection devices to obtain effective tomographic data in far-field processing monitoring.

[0008] 2. The far-field fiber-optic OCT detection device has insufficient depth of field and is difficult to adapt to complex processing surfaces.

[0009] In far-field laser machining monitoring scenarios, traditional fiber-optic optical coherence tomography (OCT) detection devices face performance contradictions. According to the diffraction limit theory, the depth of field of the detection beam is strictly inversely proportional to the beam diameter. However, to compensate for the exponential decay of scattered light intensity during far-field transmission, the device must increase the incident spot diameter to maintain the signal-to-noise ratio of the return signal. This design trade-off creates a dilemma: when optimizing the spot size to achieve sufficient scattered light intensity, the confocal parameter of the beam drops sharply, making it difficult for the system to maintain clear tomographic imaging capabilities across complex curved surfaces with millimeter-scale height fluctuations. This physical constraint is essentially the result of the coupling of the Rayleigh criterion and the scattering cross-section modulation effect. While increasing the spot diameter increases the integrated scattering cross-section area, the resulting depth-of-field compression effect severely weakens the system's longitudinal resolution of three-dimensional topography. This performance contradiction is particularly prominent when the machined surface exhibits multi-scale features such as steps and pores, becoming a major technical bottleneck restricting the monitoring accuracy of far-field fiber-optic OCT detection devices. Summary of the Invention

[0010] The purpose of the present invention is to overcome the shortcomings of the existing technology and provide a spatial optical path system for optical coherence tomography in far-field laser processing to improve the power of the scattered light signal of the measuring arm to maintain clear tomographic imaging capabilities throughout the entire process.

[0011] To achieve the above-mentioned object of the invention, the optical coherence tomography spatial optical path system in far-field laser processing of the present invention is characterized by comprising:

[0012] The light source system consists of a light source and a collimator. The light generated by the light source is converted into a collimated parallel beam through the collimator.

[0013] An annular beam splitter comprises an optical glass substrate having a first surface and a second surface. The first surface comprises a beam splitting region and an anti-reflection region. The beam splitting region is provided in the central region of the optical glass substrate and is coated with a beam splitting film for partially reflecting and transmitting incident light. The anti-reflection region is provided around the beam splitting region and is coated with an anti-reflection film. The second surface is coated with an anti-reflection film for reducing reflection loss in a non-beam splitting region to form an optical window region with high transmittance.

[0014] a first focusing lens, a second focusing lens, an achromatic lens group, and an image sensor;

[0015] The collimated parallel light beam emitted by the light source system is incident at 45 degrees to the spectroscopic area of ​​the first surface of the annular beam splitter. The incident light beam, i.e., the collimated parallel light beam, is reflected by the spectroscopic film to form the measuring light. The measuring light is then projected onto the measured processing surface through the first focusing lens. The measuring light is reflected by the processing surface to generate diffusely reflected scattered light, which returns along the original light path. The returned diffusely reflected scattered light passes through the first focusing lens and reaches the annular beam splitter. The outer area of ​​the spectroscopic area, i.e., the anti-reflection area, passes through it with high transmittance. After passing through the annular beam splitter, the diffusely reflected scattered light is focused by the achromatic lens group in sequence, and the image sensor finally completes the optical signal acquisition.

[0016] The incident light beam, that is, the collimated parallel light beam, is projected through the beam splitter film to form a reference light, which is then projected onto the mirror through the second focusing lens. The reference light is reflected by the mirror to produce reflected light, which returns in the opposite direction along the original light path. The reflected light reaches the beam splitting area of ​​the annular beam splitter, and after being reflected by the beam splitter film, it is focused by the achromatic lens group in sequence, and finally the image sensor completes the light signal acquisition.

[0017] The object of the invention of the present invention is achieved like this:

[0018] In response to the problems of limited incident light diameter, low scattered light collection efficiency, and insufficient imaging depth of field in traditional far-field fiber-optic OCT detection devices, the present invention proposes a spatial optical path system for optical coherence tomography in far-field laser processing, including a light source system consisting of a light source and a collimating mirror, an annular beam splitter, a first focusing lens, a second focusing lens, and an achromatic lens group. The central area of ​​the annular beam splitter is the beam splitting area, which is coated with a beam splitting film to achieve efficient beam splitting and is used to partially reflect and transmit the incident light. The anti-reflection area is arranged around the beam splitting area and is coated with an anti-reflection film. The second surface is coated with an anti-reflection film to reduce the reflection loss in the non-beam splitting area. On this basis, a new spatial optical path is designed. Through the design of internal beam splitting and external anti-reflection, the incident light diameter can be reduced, the scattered light intensity and imaging depth of field can be significantly improved, and clear tomographic imaging capabilities can be maintained throughout the process. The innovation of the present invention is reflected in the coordinated layout of the annular beam splitting and anti-reflection functions, which solves the core difficulties of weak scattered light and limited imaging depth of field in far-field laser processing scenarios, and lays the foundation for high-quality in-situ detection and intelligent control of far-field laser processing. In addition, when the annular beam splitter is applied to the OCT spatial optical path system, the annular beam splitter divides the incident light into a reference optical path and a measurement optical path. After the scattered light from the measuring arm returns, it does not need to pass through the optical fiber coupler and directly interferes with the image. The generated interference light signal can be collected by the image sensor, realizing tomographic imaging without optical fiber coupling.

[0019] At the same time, the present invention also has the following beneficial effects:

[0020] The annular spectroscopic structure realizes the dual functions of a single optical path: the central area reflects the measurement light, and the outer area transmits the scattered light; the anti-reflection film can effectively improve the scattered light collection efficiency according to the optimized layout of the optical path; the rectangular substrate design facilitates system integration, and other shapes of substrates can also be designed for easy installation; the film parameters can be customized to meet the detection needs of different wavelength ranges (400-1700nm). Through the coordinated design of the spot distribution in the spatial optical path and the film structure, the device innovatively solves the problem of spatial multiplexing of the measurement light path and the scattered light path, significantly improving the light utilization efficiency of the optical detection system. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a schematic diagram of the existing optical path design principle based on traditional fiber-based near-field OCT;

[0022] Figure 2 This is a schematic diagram of the scattering loop measurement principle of the existing fiber-optic OCT measurement arm;

[0023] Figure 3 This is a schematic structural diagram of a specific embodiment of the optical coherence tomography spatial optical path system in far-field laser processing of the present invention;

[0024] Figure 4 yes Figure 3 A schematic structural diagram of a specific embodiment of the annular beam splitter is shown;

[0025] Figure 5 yes Figure 4 The diagram shows the principle diagram of the scattering loop measurement of the measurement arm of the optical coherence tomography spatial optical path system in far-field laser processing;

[0026] Figure 6 It is a concrete physical picture of a common beam splitter and the annular beam splitter of the present invention;

[0027] Figure 7 This is a comparison data chart of the scattered light intensity obtained by the image sensor OV9280 in the focused and unfocused states. DETAILED DESCRIPTION

[0028] The following describes the specific embodiments of the present invention in conjunction with the accompanying drawings so that those skilled in the art can better understand the present invention. It should be noted that in the following description, when detailed descriptions of known functions and designs may dilute the main content of the present invention, such descriptions will be omitted here.

[0029] Figure 1 This is a schematic diagram of the existing design principle of the traditional fiber-based near-field OCT optical path.

[0030] like Figure 1As shown in the figure, the conventional fiber-based near-field OCT optical path design utilizes an interferometric architecture based on fiber integration. The specific process is as follows: low-coherence light emitted by a low-coherence light source (SLD) is transmitted via a single-mode fiber to a 50:50 fiber-coupled beamsplitter. After this, the light is split into two independent optical paths: a reference arm and a measurement arm. The reference arm light is collimated by a collimator and incident on a reflector. The measurement arm light is guided by a lens onto the sample surface and backscattered. After returning along their original paths, the two beams interfere at the beamsplitter. The resulting time-domain interferometric signal is collected by a high-resolution spectrometer, and a tomographic image is reconstructed using a fast Fourier transform (FFT) algorithm.

[0031] Figure 2 This is a schematic diagram of the measurement principle of the scattering loop of the existing fiber-optic OCT measurement arm.

[0032] like Figure 2 As shown in the figure, the measurement process of the measurement arm scattering circuit is as follows: low-coherence light emitted by the light source is transmitted to a 50:50 fiber-coupled splitter via a single-mode optical fiber. By cutting off the reference arm optical path, the measurement arm light is projected onto a standard scattering plate through a collimator. After the backscattered light returns along the original path, a high-sensitivity light intensity meter is used to directly collect the scattered light power. By comparing the difference with the measured light intensity of the spatial light, the scattering enhancement effect of the annular beam splitter is quantitatively verified.

[0033] Figure 3 It is a structural schematic diagram of a specific implementation of the optical coherence tomography spatial optical path system in far-field laser processing of the present invention.

[0034] In this embodiment, if Figure 3 As shown, the optical coherence tomography spatial optical path system in far-field laser processing of the present invention includes a light source system 1 consisting of a light source 101 and a collimating lens 102, an annular beam splitter 2, a first focusing lens 3, a second focusing lens 4, an achromatic lens group 5 and an image sensor 6.

[0035] The light source system 1 is composed of a light source 101 and a collimating lens 102 . The light generated by the light source 101 is converted into a collimated parallel light beam by the collimating lens 102 .

[0036] Figure 4 yes Figure 3 The structure diagram of a specific embodiment of the annular beam splitter is shown.

[0037] In this embodiment, if Figure 4As shown, the annular beam splitter 2 includes an optical glass substrate having a first surface and a second surface. The first surface includes a beam splitting area 201 and an anti-reflection area 202. The beam splitting area 201 is arranged in the central area of ​​the optical glass substrate. The beam splitting area 201 is coated with a beam splitting film S1 for partially reflecting and transmitting the incident light. The anti-reflection area 202 is arranged around the beam splitting area 201 and is coated with an anti-reflection film S2. The second surface is coated with an anti-reflection film S2 for reducing the reflection loss in the non-beam splitting area to form an optical window area with high transmittance.

[0038] In this embodiment, if Figure 4 As shown, the annular beam splitter 2 of the present invention is made of an H-K9L optical glass substrate. The rectangular substrate has a size of 12 mm × 17 mm × 1 mm. A beam splitting region 201 is provided in the central region of the rectangular substrate and is coated with an elliptical beam splitting film with a short axis of 3.9 mm and a long axis of 5.8 mm. In order to fully realize the beam splitting function for the collimated incident light with a diameter of 3 mm, an anti-reflection film is coated in the other region not coated with the beam splitting film, namely the anti-reflection region 202. The specific coating parameters are a transmittance of >99.2% and a wavelength range of 850±30 nm.

[0039] On the first surface and the second surface of the optical glass substrate, a light splitting area and a transmittance-enhancing area are formed by a multi-step mask coating process.

[0040] The collimated parallel light beam emitted by the light source system 1 is incident on the splitting area 201 of the first surface of the annular beam splitter 2 at 45°. The incident light beam, i.e., the collimated parallel light beam, is reflected by the beam splitter film S1 to form measurement light, which is projected onto the measured processing surface (object plane) through the first focusing lens 3. The measurement light is reflected by the processing surface to generate diffusely reflected scattered light, which returns in the opposite direction along the original light path. The returned diffusely reflected scattered light passes through the first focusing lens 3 to reach the annular beam splitter 2, and the outer area of ​​the splitting area 201, i.e., the anti-reflection area 202, passes through it with high transmittance. After passing through the annular beam splitter 2, the diffusely reflected scattered light is focused by the achromatic lens group 5 in sequence, and finally the image sensor 6 completes the light signal acquisition.

[0041] The incident light beam, i.e., the collimated parallel light beam, is projected through the beam splitter film S1 to form a reference light, which is then projected onto the mirror surface through the second focusing lens 4. The reference light is reflected by the mirror surface to generate reflected light, which returns in the opposite direction along the original light path. The reflected light reaches the beam splitting area 201 of the annular beam splitter 2, and after being reflected by the beam splitter film S1, it is focused by the achromatic lens group 5 in sequence, and finally the image sensor 6 completes the light signal acquisition.

[0042] In this embodiment, the light emitted by the light source 101 is shaped into a Φ3mm collimated parallel beam by the collimator 102, and is projected onto the splitting area 201 on the first surface of the annular beam splitter 2 at an incident angle of 45°. The working distance of the measurement optical path is set to 1000mm (from the annular beam splitter 2 to the measured processing surface, i.e., the object plane). The measurement light, i.e., the scattered light generated after the detection beam reaches the measured processing surface, returns along the original path, passes through the annular beam splitter 2 again, and is focused by the achromatic lens group 5 in sequence. Finally, the optical signal is collected by the OV9280 CMOS image sensor 6.

[0043] In this embodiment, the light source is a power-adjustable light source (812nm-870nm) with a maximum power of 25mW. The detection light receiver used in this embodiment is a CMOS image sensor OV9280, and the sample under test is a standard scattering surface. The OV9280 CMOS image sensor 6 has a 1280×800 pixel array output capability. This design maintains 50% spectral efficiency in the central measurement optical path while improving total light energy utilization through anti-reflection treatment in the annular region. Combined with an achromatic lens, it effectively captures the scattered light spot shape.

[0044] Figure 5 yes Figure 4 The figure shows the principle diagram of the scattering circuit measurement of the measurement arm of the optical coherence tomography spatial optical path system in far-field laser processing. A main beam with a wavelength of approximately 850 nm is emitted from light source 101, and a standard scattering surface is placed on the object surface. When the main beam is split and irradiated by the standard scattering surface, scattered light is emitted in various directions. After passing through a beam splitter, the light signal is collected by the OV9280 CMOS sensor. To compare the scattered light intensity under different light intensities, the light source intensity was adjusted in the experiment. The scattered light signal intensity was recorded for each experimental group in both the focused and defocused states of the object surface. The total light intensity value collected by the OV9280 under different light intensities was calculated and compared.

[0045] Figure 6 These are specific physical pictures of a common beam splitter (left) and the annular beam splitter in the present invention (right).

[0046] In this embodiment, in order to verify the enhancement effect of the annular beam splitter 2 on scattered light in the present invention, three groups of comparative experiments were set up. Figure 5 The optical path structure shown in the figure uses a 50R / 50T ordinary beam splitter. The optical path structure of experimental group 2 is the same as that of experimental group 1, and the annular beam splitter 2 of the present invention is used. The experimental group 3 uses Figure 2 As shown in the optical path diagram, the reference arm is blocked and the scattered light intensity received by the measuring arm is measured with a light intensity meter for comparison.

[0047] Light source intensity Experimental Group 1 Experimental Group 2 Experimental Group 3 25mw 83.1uw 96.5uw 0.821uw 15mw 50.6uw 61.3uw 0.633uw 10mw 34.5uw 38.8uw 0.315uw

[0048] Table 1

[0049] Table 1 shows the scattered light power of the three experimental groups. As shown in the experimental data in Table 1, a light intensity meter was used to quantitatively analyze the light power returned from the scattering surface. The results show that the measured scattered light power of the traditional fiber-optic near-field OCT optical path under far-field object plane conditions is only 0.821uW, which cannot meet the requirements of high dynamic range imaging. Compared with the optical coherence tomography spatial optical path system in the far-field laser processing of the present invention, the scattered light power is increased to 96.5uW under the same working conditions, breaking through the weak far-field signal, confirming the imaging failure problem caused by insufficient utilization of scattered light in the traditional solution.

[0050] like Figure 7 As shown, the scattered light intensity is quantitatively analyzed by OV9280. As shown in the experimental data in Table 1, under the same light source intensity conditions, the scattered light collection intensity of the two annular beam splitters in the experimental group is increased by 15%±3% in the focused state and 25%±3% in the defocused state compared to the ordinary beam splitter in the experimental group. This enhancement effect remains stable within the wide dynamic light intensity range of 5-25mW, confirming the strong adaptability of the geometric optical advantages of the annular beam splitter to complex working conditions. The annular beam splitter structure of the present invention effectively reduces the optical path loss and improves the collection efficiency of scattered light by spatially separating the light splitting area and the transmittance-enhancing area. Experimental data proves that this design can significantly enhance the scattered light signal and is suitable for application scenarios that are sensitive to weak light signals, such as online monitoring of laser processing, OCT imaging, and biomedical testing.

[0051] Although the above describes the illustrative specific embodiments of the present invention to facilitate understanding of the present invention by those skilled in the art, it should be clear that the present invention is not limited to the scope of the specific embodiments. For those skilled in the art, as long as various changes are within the spirit and scope of the present invention as defined and determined by the appended claims, these changes are obvious, and all inventions and creations using the concepts of the present invention are protected.

Claims

1. A spatial optical path system for optical coherence tomography in far-field laser processing, characterized in that: include: The light source system consists of a light source and a collimator. The light generated by the light source is converted into a collimated parallel beam through the collimator. An annular beam splitter comprises an optical glass substrate having a first surface and a second surface. The first surface comprises a beam splitting region and an anti-reflection region. The beam splitting region is provided in the central region of the optical glass substrate and is coated with a beam splitting film for partially reflecting and transmitting incident light. The anti-reflection region is provided around the beam splitting region and is coated with an anti-reflection film. The second surface is coated with an anti-reflection film for reducing reflection loss in a non-beam splitting region to form an optical window region with high transmittance. a first focusing lens, a second focusing lens, an achromatic lens group, and an image sensor; The collimated parallel light beam emitted by the light source system is incident at 45 degrees to the spectroscopic area of ​​the first surface of the annular beam splitter. The incident light beam, i.e., the collimated parallel light beam, is reflected by the spectroscopic film to form the measuring light. The measuring light is then projected onto the measured processing surface through the first focusing lens. The measuring light is reflected by the processing surface to generate diffusely reflected scattered light, which returns along the original light path. The returned diffusely reflected scattered light passes through the first focusing lens and reaches the annular beam splitter. The outer area of ​​the spectroscopic area, i.e., the anti-reflection area, passes through it with high transmittance. After passing through the annular beam splitter, the diffusely reflected scattered light is focused by the achromatic lens group in sequence, and the image sensor finally completes the optical signal acquisition. The incident light beam, that is, the collimated parallel light beam, is projected through the beam splitter film to form a reference light, which is then projected onto the mirror through the second focusing lens. The reference light is reflected by the mirror to produce reflected light, which returns in the opposite direction along the original light path. The reflected light reaches the beam splitting area of ​​the annular beam splitter, and after being reflected by the beam splitter film, it is focused by the achromatic lens group in sequence, and finally the image sensor completes the light signal acquisition.

2. The optical coherence tomography spatial optical path system for far-field laser processing according to claim 1, characterized in that: The optical glass substrate of the annular beam splitter is rectangular with dimensions of 12mm×17mm×1mm. The dimensions of the beam splitting area are 3.9mm on the short axis and 5.8mm on the long axis. The parameters of the antireflection film coated on the antireflection area and the second surface are transmittance>99.2% and the wavelength range is 850±30nm.

3. The optical coherence tomography spatial optical path system for far-field laser processing according to claim 1, characterized in that: On the first surface and the second surface of the optical glass substrate, a light splitting area and a transmittance-enhancing area are formed by a multi-step mask coating process.

4. The optical coherence tomography spatial optical path system for far-field laser processing according to claim 1, characterized in that: The annular beam splitter divides the incident light into a reference light path and a measurement light path. The scattered light from the measurement arm returns directly to interference imaging without passing through a fiber coupler. The generated interference light signal can be collected by an image sensor, realizing tomographic imaging without fiber coupling.

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