Underwater laser-induced breakdown spectroscopy scanning imaging device and method

Abstract

The invention belongs to the technical field of underwater in-situ measurement, and discloses an underwater laser-induced breakdown spectroscopy scanning imaging device and method. The method comprises the following steps: a laser emission and beam shaping unit is used for high-energy pulse laser generation and beam shaping; the high-speed two-dimensional laser galvanometer scanning system is used for two-dimensional dynamic scanning control of the laser beams; the pressure-resistant optical emission and signal collection window is used for pressure-resistant optical emission, aberration correction and signal collection; the plasma emission spectrum acquisition and analysis unit is used for plasma emission spectrum acquisition and transmission; the sample surface morphology real-time imaging unit is used for real-time imaging and diagnosis of the sample surface morphology, clear imaging of the surface of an underwater target sample is carried out, real-time observation of the sample surface morphology and morphology change is realized, and a surface state image at a specific moment is captured. According to the invention, high-speed and high-precision element distribution imaging of an underwater target is completed, and the environmental adaptability of the system and the compatibility of a carrying platform are remarkably improved.

Description

Technical Field

[0001] This invention belongs to the field of underwater in-situ measurement technology, and particularly relates to an underwater laser-induced breakdown spectral scanning imaging device and method. Background Technology

[0002] Laser-induced breakdown spectroscopy (LIBS) focuses a high-intensity pulsed laser beam onto the sample surface, generating transient plasma. The light emitted by particles within this plasma is then collected to analyze the material's composition. Compared to commonly used elemental detection methods such as inductively coupled plasma atomic emission spectrometry (ICP-AES), LIBS offers faster detection speeds, requires no sample pretreatment, allows for real-time in-situ detection, and can be applied to liquids, making it highly promising for deep-sea exploration.

[0003] In the field of deep-sea exploration technology, existing technologies have focused on systematic equipment development and sea trials centered around LIBS (Liquid In-Situ Biometry) technology. However, existing LIBS in-situ detection systems are generally limited to single-point detection modes and cannot achieve spatial scanning imaging capabilities. This results in inherent defects such as low spatial resolution, insufficient detection efficiency, and lack of imaging capabilities when acquiring information on the elemental distribution on the sample surface. To improve spatial resolution, traditional LIBS scanning imaging technology typically uses a mechanical displacement platform to drive the sample to translate point by point, or uses a displacement device to move the optical probe to achieve coordinate scanning.

[0004] However, existing technologies face technical challenges in underwater in-situ detection scenarios: sample displacement schemes require precise mechanical manipulation of the target, which has extremely low engineering feasibility in complex underwater environments; while probe displacement schemes avoid moving the sample, they require the entire optical system to be mounted on the motion mechanism, significantly increasing the structural complexity, weight, and power consumption of the in-situ detection system, and imposing stringent requirements on the load capacity and motion stability of the underwater transport platform, which severely restricts its application in lightweight and miniaturized mobile platforms. Summary of the Invention

[0005] To overcome the problems existing in related technologies, the present invention discloses an underwater laser-induced breakdown spectral scanning imaging device and method, specifically relating to an underwater laser-induced breakdown spectral scanning imaging device based on a laser galvanometer.

[0006] The technical solution is as follows: an underwater laser-induced breakdown spectral scanning imaging device, the device comprising: Laser emission and beam shaping unit, used for high-energy pulsed laser generation and beam shaping; A high-speed two-dimensional laser galvanometer scanning system is used for two-dimensional dynamic scanning control of laser beams. Pressure-resistant optical emission and signal collection window, used for pressure-resistant optical emission, aberration correction and signal collection; The plasma emission spectrum acquisition and analysis unit is used for plasma emission spectrum acquisition and transmission. The real-time imaging unit for sample surface morphology is used for real-time imaging and diagnosis of sample surface morphology. It can clearly image the surface of underwater target samples, realize real-time observation of the surface morphology and morphological changes of samples, and capture surface state images at specific moments.

[0007] Furthermore, the laser emission and beam shaping unit includes a laser for generating high-energy pulsed lasers. The pulsed lasers generated by the laser are sequentially expanded by a beam expander, and the expanded laser beams are reflected by a first reflector and then incident on a high-speed two-dimensional laser galvanometer scanning system.

[0008] The high-speed two-dimensional laser galvanometer scanning system includes a laser galvanometer, which includes an X-axis galvanometer motor, a Y-axis galvanometer motor, an X-axis galvanometer reflector rigidly connected to the X-axis galvanometer motor, and a Y-axis galvanometer reflector rigidly connected to the Y-axis galvanometer motor. The X-axis galvanometer motor and Y-axis galvanometer motor receive external control signals and drive the X-axis galvanometer reflector and Y-axis galvanometer reflector to deflect around their axes, dynamically changing the reflection angle of the incident laser beam, thereby achieving two-dimensional scanning of the underwater sample surface within a specified area.

[0009] The pressure-resistant optical emission and signal collection window includes an optical emission head with an integrated pressure-resistant window. The internal optical path of the optical emission head is used to receive the scanning laser beam from the laser galvanometer and focus it onto the sample surface located underwater to excite and generate plasma. The optical path contains a scanning focusing lens group to eliminate optical aberrations caused by the deflection of the galvanometer reflector, so that the scanning laser beam can be accurately focused on the same focal plane on the sample surface throughout the entire scanning area. At the same time, the optical path design of the optical emission head is also used to collect the emission spectrum signal generated by the laser-induced plasma.

[0010] The scanning focusing lens group consists of two lenses: one is the main focusing lens, which focuses the laser beam onto the sample surface to achieve ablation; the other is a thin lens with weak negative power from the same glass family as the main focusing lens, which uses a negative meniscus to suppress astigmatism and coma; the focal length and refractive index of the two main focusing lenses must satisfy Petzval and be 0.

[0011] The plasma emission spectrum acquisition and analysis unit includes a spectrum acquisition module; the spectrum acquisition module, through an optical fiber probe, couples the plasma emission spectrum signal collected by the pressure-resistant optical emission and signal collection window with a dichroic mirror, a beam splitter, and a second focusing lens; the coupled optical signal is transmitted to the spectrometer via a transmission optical fiber; the spectrometer disperses the received spectral signal and converts it into a corresponding electrical signal.

[0012] The real-time imaging unit for sample surface morphology includes a plasma image acquisition CCD camera; the plasma image acquisition CCD camera is equipped with an appropriate imaging lens and filter; the optical signal of the sample surface collected by the optical output head passes through a dichroic mirror, a beam splitter, a second reflecting mirror and a first focusing lens in sequence and is then received by the plasma image acquisition CCD camera for real-time observation of the morphological features, state changes and spatial distribution of the sample surface. The underwater laser-induced breakdown spectral scanning imaging device further includes: a housing and a substrate, wherein each unit is integrated on the substrate, and the substrate is fixedly connected to the rear cover plate; the housing is fixedly connected to the rear cover plate and the front cover plate.

[0013] Another object of the present invention is to provide an underwater laser-induced breakdown spectral scanning imaging method, comprising: S1, High-energy pulsed laser generation and beam shaping; S2, Two-dimensional dynamic scanning control of the laser beam; S3, pressure-resistant optical emission, aberration correction and signal collection; S4, Plasma emission spectrum acquisition and transmission; S5, Real-time imaging and diagnosis of sample surface morphology.

[0014] In step S2, the two-dimensional dynamic scanning control of the laser beam includes: The beam is deflected by rotating the Y-axis and X-axis mirrors of a high-speed two-dimensional laser galvanometer scanning system; let the mechanical rotation angles of the X-axis and Y-axis galvanometer motors be respectively... , The optical deflection angles are respectively ; ; ; ; ; In the formula, The X-axis coordinates of the laser spot on the scanning plane. The Y-axis coordinate of the laser spot on the scanning plane. The distance from the center of the Y-axis galvanometer mirror to the center of the scanning plane is [missing information]. The center-to-center distance between the Y-axis galvanometer reflector and the X-axis galvanometer reflector; When the Y-axis galvanometer reflector and the X-axis galvanometer reflector deflect... , At that time, the coordinates of the focal point of the laser beam on the sample surface are: ; Size of the light spot on the sample surface for: ; ; ; In the formula, The new beam waist radius after lens transformation. The beam quality factor. The incident laser wavelength, The focal length of the lens. Let be the waist radius of the incident Gaussian beam. The radius of the light spot is... Center value Waist radius at the location, The distance from the beam waist along the optical axis. Rayleigh range indicates that the beam size is expanded to the beam waist. Distance at times; According to the central limit theorem, the variance of the overall mean decreases with the number of particles: ; In the formula, The variance of the sample mean. For the variance of single-particle parameters, This represents the number of independent particles contained within the scanned area. This represents the average value of the particle parameters. This represents the typical area of ​​a single particle. This represents the total area scanned. When the relative error of the measured average is less than a certain threshold Then it satisfies: ; The results were: ; In step S3, the pressure-resistant optical emission, aberration correction, and signal collection include: the focal length and refractive index of the primary focusing lens in the scanning focusing lens group and the weak negative power thin lens must satisfy Petzval and 0, as expressed by: ; ; In the formula, The refractive index of the main focusing mirror The focal length of the main focusing lens. For the refractive index of a weak negative power focusing mirror, The focal length of a weak negative power focusing lens; In step S4, plasma emission spectrum acquisition and transmission includes: setting up an optical fiber probe at the conjugate focal position or the optimal signal collection position of the internal optical path; the optimized spectral acquisition module efficiently focuses the plasma emission spectrum signal onto the end face of the optical fiber probe, that is, the image point size formed by the lens is less than or equal to the core diameter of the transmission fiber, and the end face of the fiber is placed on the clear image point formed by the plasma source through the lens, so that all the collected light can enter the transmission fiber to maximize the total optical flux; the optical signal is then led to the spectrometer for detection through a low-loss transmission fiber.

[0015] Combining all the above technical solutions, the beneficial effects of this invention are as follows: First, this invention provides an underwater laser-induced breakdown spectroscopy (LIBS) scanning imaging device based on a laser galvanometer, aiming to solve the problems of high system complexity, poor motion stability, and heavy platform load caused by the reliance on mechanical displacement in traditional underwater in-situ scanning imaging technology. By achieving non-mechanical beam scanning through a laser galvanometer, high-speed, high-precision elemental distribution imaging of underwater targets can be completed without moving the sample or optical probe, significantly improving the system's environmental adaptability and platform compatibility.

[0016] Secondly, this invention utilizes a laser galvanometer to achieve high-speed underwater laser scanning, upgrading underwater LIBS from "single-point detection" to "area array scanning imaging," enabling the acquisition of high-resolution elemental distribution maps in a short time. This technology can significantly improve the efficiency of seabed resource exploration and is more easily integrated into digital twin platforms such as marine surveys and smart marine engineering. Currently, underwater LIBS is mostly point-based analysis with fixed optical paths, lacking a high-speed area scanning system based on galvanometers. This invention organically couples the advantages of underwater LIBS technology, galvanometer scanning technology, and elemental imaging methods within the same device, forming a technical system not addressed in existing literature and engineering.

[0017] Third, underwater LIBS faces severe plasma confinement and cavitation bubble effects, leading to spectral line broadening, enhanced continuous background, and significant signal fluctuations. Existing systems primarily rely on single-point quantitative analysis. Even with improved quantitative accuracy through repeated measurements and signal normalization, high-resolution two-dimensional elemental imaging of large-area targets remains extremely time-consuming and inaccurate. This invention utilizes a galvanometer for rapid scanning, transforming the slow, coarse scanning previously performed by a displacement stage or ROV robotic arm into high-speed, precise spot jumping. This allows for multi-point measurements before cavitation bubbles accumulate or diffuse extensively, reducing environmental fluctuation differences between adjacent points. Accurate registration of spectral data with spatial coordinates on complex underwater target surfaces has always been a challenge in engineering applications. Traditional methods rely on mechanical displacement stages or ROV position feedback, resulting in large accumulated errors and slow response times. This invention utilizes the precise angle control and calibration model of the galvanometer to map the direction angle of each laser emission to the target plane coordinates, naturally establishing a correspondence between the spectrum and pixels, greatly simplifying the registration process and improving spatial reconstruction accuracy. Attached Figure Description

[0018] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this disclosure and, together with the description, serve to explain the principles of this disclosure; Figure 1 This is a flowchart of the underwater laser-induced breakdown spectral scanning imaging method provided in an embodiment of the present invention; Figure 2 This is a schematic diagram of the galvanometer principle of the high-speed two-dimensional laser galvanometer scanning system provided in an embodiment of the present invention; Figure 3 This is a schematic diagram of the underwater laser-induced breakdown spectral scanning imaging device provided in an embodiment of the present invention; Figure 4 This is a schematic diagram of the underwater laser-induced breakdown spectral scanning imaging device provided by the present invention mounted on an ROV. Figure 5 This is the experimental optical path diagram provided by the present invention; Figure 6 This is the actual lapis lazuli sample provided by the present invention; Figure 7 This is a signal intensity distribution diagram of calcium element at a wavelength of 396.84 nm, as presented in this invention. Figure 8 This is a signal intensity distribution diagram of sodium element at a wavelength of 588.96 nm, as presented in this invention. Figure 9 This is a signal intensity distribution diagram of iron at 358.14 nm, as shown in the present invention. Figure 10 This is a signal intensity distribution diagram of sulfur at a wavelength of 564.06 nm, as presented in this invention. In the diagram: 1. Laser; 2. Beam expander; 3. First reflecting mirror; 4. X-axis galvanometer motor; 5. Y-axis galvanometer motor; 6. Y-axis galvanometer reflecting mirror; 7. X-axis galvanometer reflecting mirror; 8. Optical output head; 9. Scanning focusing lens group; 10. Fiber optic probe; 11. Dichroic mirror; 12. Beam splitter; 13. First focusing lens; 14. Transmission fiber; 15. Spectrometer; 16. Plasma image acquisition CCD camera; 17. Second reflecting mirror; 18. Second focusing lens; 19. Substrate; 20. Rear cover; 21. Housing; 22. Front cover. Detailed Implementation

[0019] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a thorough understanding of the present invention. However, the present invention can be practiced in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0020] The innovation of this invention lies in its proposed underwater laser-induced breakdown spectroscopy (LIBS) scanning imaging device and method. This method utilizes a laser galvanometer to achieve high-speed dual-axis scanning of the laser beam underwater, transforming the traditional point-based analysis mode into area array scanning imaging, significantly improving the efficiency and spatial resolution of underwater elemental imaging. To adapt to the complex underwater high-pressure environment, this device employs an underwater sealing structure and refractive compensation design to ensure stable operation of the laser galvanometer under extreme conditions, overcoming the challenges posed by the underwater environment to the accuracy and reliability of optical equipment. Furthermore, by combining galvanometer angle control and geometric calibration techniques, precise registration of spectral data with the spatial position of the target surface is achieved, solving the problem of spectral and spatial registration in underwater LIBS imaging. This device can simultaneously acquire elemental distribution maps and spectral information of underwater targets, providing an efficient and accurate data acquisition method for underwater resource exploration, marine engineering structure health monitoring, and environmental assessment. This technical solution fills a gap in underwater LIBS imaging technology and has significant academic value and broad engineering application prospects.

[0021] Example 1, as Figure 1 Underwater laser-induced breakdown spectral scanning imaging methods include: S1, High-energy pulsed laser generation and beam shaping; A pulsed Nd:YAG laser 1 (wavelength 1064nm) is used to generate a high-energy pulsed laser that meets the requirements for underwater excitation. The laser beam is first expanded by a beam expander 2 to improve the collimation of the laser beam and provide more favorable laser beam characteristics for subsequent optical systems (especially galvanometer scanning), while reducing the risk of damage to optical components.

[0022] S2, Two-dimensional dynamic scanning control of the laser beam; To achieve efficient, large-scale, and non-contact elemental distribution analysis underwater, the core innovation of this invention lies in the adoption of a high-speed two-dimensional laser galvanometer scanning system. The expanded laser beam is precisely incident on this high-speed two-dimensional laser galvanometer scanning system, i.e., the laser galvanometer. This system completely replaces the bulky, slow, and unreliable mechanical scanning probe or sample stage movement schemes used in traditional underwater LIBS devices. The precision X-axis galvanometer motor 4, Y-axis galvanometer motor 5, Y-axis galvanometer reflector 6, and X-axis galvanometer reflector 7 are strictly sealed in the dry, atmospheric pressure environment of the main cavity of the device, completely isolating them from external high-pressure water and moisture, ensuring the long-term stability and reliability of its precise deflection control. A schematic diagram of the galvanometer principle of the high-speed two-dimensional laser galvanometer scanning system is shown below. Figure 2 ; The high-speed two-dimensional laser galvanometer scanning system consists of an X-axis galvanometer motor 4, a Y-axis galvanometer motor 5, a Y-axis galvanometer reflector 6, and an X-axis galvanometer reflector 7. The beam deflection is achieved by rotating the Y-axis galvanometer reflector 6 and the X-axis galvanometer reflector 7. Assume the mechanical rotation angles of the X-axis galvanometer motor 4 and the Y-axis galvanometer motor 5 are respectively... , Then the optical deflection angle is In the engineering field, there is a standard algorithm for the geometry of the two Y-axis galvanometer reflectors 6 and 7, namely: ; ; ; ; In the formula, The X-axis coordinates of the laser spot on the scanning plane. The Y-axis coordinate of the laser spot on the scanning plane. The distance from the center of the Y-axis galvanometer mirror 6 to the center of the scanning plane is... The center-to-center distance between the Y-axis galvanometer reflector 6 and the X-axis galvanometer reflector 7; When the Y-axis galvanometer reflector 6 and the X-axis galvanometer reflector 7 deflect... , At that time, the coordinates of the focal point of the laser beam on the sample surface are: ; Outside the Rayleigh range, the cross-sectional size of a Gaussian beam is proportional to the distance from the focal point. An ideal thin lens (scanning focusing lens group 9) does not alter the coherence and transverse mode structure of the light, but only changes the wavefront curvature of the beam. Furthermore, due to diffraction limitations, the beam forms a Fraunhofer diffraction image. Therefore, the size of the light spot on the sample surface... for: ; ; ; In the formula, The new beam waist radius after lens transformation. The beam quality factor. The incident laser wavelength, The focal length of the lens. Let be the waist radius of the incident Gaussian beam. The radius of the light spot is... Center value Waist radius at the location, The distance from the beam waist along the optical axis. Rayleigh range indicates that the beam size is expanded to the beam waist. Distance at times; Submarine rocks / cold seep products are highly heterogeneous, and single-point LIBS is easily affected by the laser strike location, reflecting only local mineral composition and failing to represent the overall chemical characteristics. Scanning imaging LIBS achieves statistical averaging through multi-point sampling, significantly improving the representativeness and accuracy of the analysis and reducing systematic errors caused by signal instability. The scan area determines the representativeness and structural information integrity of the analytical results. A sufficiently large area is necessary to reflect spatial distribution characteristics such as mineral coexistence, rock texture, and hydrothermal alteration. Too small an area results in distortion, while too large an area reduces efficiency. Mineral grain size and sample heterogeneity are the main controlling factors. The scan area needs to balance spatial resolution and representativeness to cover a sufficient number of mineral grains, ensuring statistical significance and the reliability of geochemical interpretation.

[0023] To include hundreds to thousands of particles in a single scan, spanning multiple stripes / phase boundaries, the scan area must be on the order of cm². According to the central limit theorem, the variance of the overall mean decreases with the number of particles: ; In the formula, The variance of the sample mean. For the variance of single-particle parameters, This represents the number of independent particles contained within the scanned area. This represents the average value of the particle parameters. This represents the typical area of ​​a single particle. This represents the total area scanned. When the relative error of the desired average value is less than a certain threshold Then it satisfies: ; The results were: ; Due to the large fluctuations in the composition of mineral or rock grains ( For example, fine-grained basalt matrix is ​​10–50 µm, medium- and coarse-grained gabbro / anthracite has an average grain size of 0.2–1 mm, and hydrothermal sulfide / altered host rocks are banded / vein-like. We can then obtain: That is, the scanning area is at least the average scale of a single particle. When the value is multiplied by 1, the statistical results tend to stabilize.

[0024] If the average particle size ,but: ; Therefore, the scanning range is determined to be To avoid overscanning that could cause overlap / voids in ablation points, the step size was determined to be 1.2 times the size of the ablation pit (the diameter of an ablation pit on seabed rocks is approximately 200µm). At this point, the number of ablation points is 125×125=15600, which statistically covers enough particles / dilute phases, spatially spans more boundaries / bands, and averages out single-point instability with more "fresh pixels".

[0025] S3, pressure-resistant optical emission, aberration correction and signal collection; The key to this invention lies in its highly integrated pressure-resistant optical emission and signal collection module. This is a specially designed sealed cavity with a high-strength, high-transmittance pressure-resistant optical emission and signal collection window (such as sapphire or fused silica) installed at the front end to withstand the high pressure of deep water and isolate it from the water medium. Addressing the critical challenges of long underwater working distances and severe optical aberrations introduced by the pressure-resistant window, this invention features a custom-designed dedicated scanning focusing lens group 9. This scanning focusing lens group 9 undergoes a special optical design, suppressing wavefront aberrations by optimizing the aspherical coefficient of the lens, constraining the material combination to meet achromatic conditions to reduce axial chromatic aberration, and effectively compensating for the main aberrations (such as spherical aberration and chromatic aberration) introduced by the pressure-resistant optical window material at underwater working wavelengths. This ensures f-theta distortion control (scan angle distortion) across the entire designed scanning field of view. With focus displacement satisfy Even after the laser beam travels a long distance through the pressure-resistant window in the water, it can still form a focused spot on the sample surface with significantly optimized size that meets the requirements of high spatial resolution.

[0026] Specifically, the scanning focusing lens group 9 consists of two lenses. One is the main focusing lens, which focuses the laser beam onto the sample surface to achieve ablation; the other is a thin, low-power lens of the same glass family as the main focusing lens, employing a negative meniscus to suppress astigmatism and coma. The focal length and refractive index of the two lenses must satisfy a Petzval sum of 0, i.e.: ; ; In the formula, The refractive index of the main focusing mirror The focal length of the main focusing lens. For the refractive index of a weak negative power focusing mirror, The focal length of a weak negative power focusing lens; The working distance of the scanning focusing lens group 9 is designed to accommodate the range required for typical underwater detection.

[0027] The spectral acquisition module is optimized with a shared scanning focusing lens group 9 and a common aperture layout. The optical path is separated by a dichroic mirror 11 (long-pass, cutoff wavelength 980nm). Its optical path layout is further optimized based on the aberration coordination principle to ensure efficient capture of emission spectral signals generated by plasma on the sample surface, which pass in the opposite direction through the same pressure-resistant optical exit and signal collection window. Its collection field of view is precisely matched to the dynamic position of the scanning spot of the high-speed two-dimensional laser galvanometer scanning system (laser galvanometer). This integrated design minimizes the underwater optical path, reduces laser energy attenuation and signal scattering loss, while ensuring the overall stability, reliability, and compactness of the optical path. It is the core structural support for the device's adaptability to high-pressure underwater environments.

[0028] S4, Plasma emission spectrum acquisition and transmission; An optical fiber probe 10 is positioned at the conjugate focal point or optimal signal collection location of the internal optical path. An optimized spectral acquisition module efficiently focuses the plasma emission spectral signal onto the end face of the optical fiber probe 10, ensuring that the image point size formed by the lens is less than or equal to the fiber core diameter and that the fiber end face is placed on the clear image point (plane) formed by the lens from the plasma source. This ensures that all collected light enters the transmission fiber 14 (fiber core) to maximize the total optical flux. The optical signal is then transmitted to the spectrometer 15 for detection via the low-loss transmission fiber 14. For example, the spectral acquisition module couples the plasma emission spectral signal collected by the pressure-resistant optical output and signal collection window through the optical fiber probe 10, using a dichroic mirror 11, a beam splitter 12, and a second focusing lens 18. The coupled optical signal is then transmitted to the spectrometer 15 via the transmission fiber 14. The spectrometer 15 disperses the received spectral signal and converts it into a corresponding electrical signal for subsequent data processing and analysis.

[0029] The total efficiency that the spectrometer 15 can receive Collection efficiency by fiber end face Fiber endface coupling efficiency Lens transmittance Fiber optic transmission efficiency Determined jointly. Among them, lens transmittance... and fiber optic transmission efficiency It is determined by the properties of the lens and the optical fiber itself.

[0030] The actual collection efficiency of optical fiber is limited by the numerical aperture of the fiber. and lens numerical aperture The smaller value in the range. Lens numerical aperture. , The aperture of the lens. This is the focal length of the lens.

[0031] If the numerical aperture of the lens Numerical aperture smaller than that of optical fiber Then the maximum receiving angle of the optical fiber is: ; If the numerical aperture of the lens Numerical aperture larger than that of optical fiber Then the maximum receiving angle of the optical fiber is: ; Right now: ; When considering the environmental refractive index When the impact ( and (representing the refractive indices of the core and cladding, respectively): ; ; Assuming the light source is an isotropic Lambertian source (light intensity varies with the cosine of the angle), the solid angle received by the optical fiber in a half-space (hemispherical) is... for: ; Then the fiber end face collection efficiency for: ; ; Fiber coupling efficiency The fiber coupling efficiency depends on the overlap area between the optical spot and the fiber core. When the center of the optical spot is aligned with the center of the fiber core, the fiber coupling efficiency is... The calculation is as follows: if ( The diameter of the light spot. (where the core diameter is...) ; if ,but: ; When the spot position deviates from the optical fiber axis, there is an offset distance between the spot center and the fiber core center. Therefore, it is necessary to calculate the overlap area between the light spot and the fiber core to determine the fiber coupling efficiency. Assume the equivalent diameter of the plasma is... The focal length of the lens is The object distance is Image distance is ,but: ; ; The overlap area between the light spot and the fiber core can be calculated using the formulas for calculating the overlap area and diffraction effect. The actual area of ​​the imaging spot and fiber coupling efficiency The core radius here Spot radius Center offset distance ,but: ; ; ; ; ; In reality, light spots, whether laser beams or light spots imaged through lenses, rarely have a uniform intensity distribution. The most common distribution is a Gaussian distribution (i.e., the light spot is brightest at the center and decreases exponentially towards the edges). The formula for light intensity under a Gaussian distribution is: ; In the formula, The radius of the light spot (the light intensity decays to the center value). (location) If both the incident light field and the fiber mode field are perfectly Gaussian distributed and the wavefronts are matched, the fiber end-face coupling efficiency is... The formula can be simplified to: ; ; In the formula, For optimal coupling efficiency during alignment. Let be the waist radius of the incident Gaussian beam. The radius of the fiber mode field is denoted as .

[0032] The total power for fiber optic collection efficiency is: ; S5, Real-time imaging and diagnosis of sample surface morphology; The device integrates a plasma image acquisition CCD camera 16. By reasonably setting its optical axis direction, it can clearly image the surface of underwater target samples, thereby realizing real-time observation of the surface morphology and changes of the sample, and capturing surface state images at specific moments.

[0033] Example 2, as Figure 3 As shown, the underwater laser-induced breakdown spectral scanning imaging device includes: The laser emission and beam shaping unit includes a laser 1 for generating high-energy pulsed laser light. The pulsed laser light generated by the laser 1 is sequentially expanded by a beam expander 2 to form a collimated beam that meets the requirements of subsequent optical systems. The expanded laser beam is reflected by a first reflecting mirror 3 and then incident on a high-speed two-dimensional laser galvanometer scanning system.

[0034] A high-speed two-dimensional laser galvanometer scanning system includes a laser galvanometer. The laser galvanometer includes an X-axis galvanometer motor 4, a Y-axis galvanometer motor 5, an X-axis galvanometer reflector 7 rigidly connected to the X-axis galvanometer motor 4, and a Y-axis galvanometer reflector 6 rigidly connected to the Y-axis galvanometer motor 5.

[0035] The X-axis galvanometer motor 4 and Y-axis galvanometer motor 5 receive external control signals and drive the X-axis galvanometer reflector 7 and Y-axis galvanometer reflector 6 to deflect precisely around the axis, thereby dynamically changing the reflection angle of the incident laser beam and realizing two-dimensional scanning of the underwater sample surface within a specified area.

[0036] The pressure-resistant optical emission and signal collection window includes an optical emission head 8 with an integrated pressure-resistant window. The internal optical path of the optical emission head 8 is used to receive the scanning laser beam from the laser galvanometer and focus it onto the sample surface located underwater to excite plasma generation. This optical path includes a scanning focusing lens group 9 to eliminate optical aberrations caused by the deflection of the galvanometer's reflecting mirrors, ensuring that the scanning laser beam is precisely focused on the same focal plane on the sample surface throughout the entire scanning area. Simultaneously, the optical path design of the optical emission head 8 also collects the emission spectral signal generated by the laser-induced plasma.

[0037] The plasma emission spectrum acquisition and analysis unit includes a spectrum acquisition module. The spectrum acquisition module, via an optical fiber probe 10, couples the plasma emission spectrum signal collected by the pressure-resistant optical emission and signal collection window through a dichroic mirror 11, a beam splitter 12, a second reflecting mirror 17, and a second focusing lens 18 for reception. The coupled optical signal is then transmitted to a spectrometer 15 via a transmission optical fiber 14. The spectrometer 15 disperses the received spectral signal and converts it into a corresponding electrical signal for subsequent data processing and analysis.

[0038] The real-time imaging unit for sample surface morphology includes a plasma image acquisition CCD camera 16. The plasma image acquisition CCD camera 16 is equipped with an appropriate imaging lens and filter, and its optical axis is rationally set to enable clear imaging of the underwater target sample surface. The optical signals from the sample surface collected by the pressure-resistant optical emission and signal collection window are sequentially passed through a dichroic mirror 11, a beam splitter 12, and a first focusing lens 13 before being received by the plasma image acquisition CCD camera 16. This data is used to observe the morphological features, state changes, and spatial distribution of the sample surface in real time, providing intuitive auxiliary information for data acquisition and analysis.

[0039] All units are integrated on the substrate 19, which is fixedly connected to the rear cover plate 20. The outer casing 21 is fixedly connected to the rear cover plate 20 and the front cover plate 22.

[0040] The underwater laser-induced breakdown spectral scanning imaging device of this invention is mounted on an ROV for in-situ measurements, such as... Figure 4 A schematic diagram of an underwater laser-induced breakdown spectral scanning imaging device mounted on an ROV. Carefully inspect the underwater laser-induced breakdown spectral scanning imaging device and all components of the ROV for damage, confirming there are no physical damages, cable breaks, or other issues. Pay particular attention to the core components that enable the scanning process, such as the laser galvanometer, ensuring they are functioning correctly. Simultaneously, conduct a comprehensive inspection of the ROV's power system, communication system, and navigation system to guarantee the ROV's normal operational capability.

[0041] The ROV's robotic arm precisely positions and locks onto the underwater laser-induced breakdown spectral scanning imaging device at its end effector. A hydraulically driven multi-finger gripper system applies a preset torque to smoothly grasp the device. A compatible data cable is then used to connect the underwater laser-induced breakdown spectral scanning imaging device to the ROV's internal PC. During the connection process, strict adherence to the interface markings is crucial to ensure correct connection and prevent damage from incorrect insertion. After connection, all connections are checked again to ensure they are secure.

[0042] Once the underwater laser-induced breakdown spectral scanning imaging device is mounted on an ROV and physically connected to the PC inside the ROV, it can perform two-dimensional scanning operations on target objects in the marine environment according to the instructions from the PC. The specific process is as follows: The pulsed laser emitted by laser 1 is expanded by beam expander 2, transmitted through dichroic mirror 11, and then enters laser galvanometer. The reflected beam is focused on the sample by lens to produce ablation. As the galvanometer angle changes continuously, the focal point moves accordingly. The plasma emission generated by ablation is collected by lens, reflected by galvanometer, and split proportionally by beam splitter 12. The beams are collected by plasma image acquisition CCD camera 16 and spectrometer 15 through lenses respectively. The former is used to observe plasma images at different coordinate points, and the latter is used to detect plasma emission spectra. The control of each device in the system is achieved through pulse generator and related software. Taking the rising edge of the laser pump pulse as the time start, the acquisition channel of plasma image acquisition CCD camera 16 is first turned on, and then the gain of laser 1 is controlled to Q-switched to output laser pulse. At the same time, the timing controller triggers the shutter of plasma image acquisition CCD camera 16 to capture plasma image. After the laser pulse is output, spectrometer 15 starts working. After the acquisition is completed, the software controls the movement of laser galvanometer. After the galvanometer is in position, the pulse generator is restarted to enter the next cycle. After each process, the system pauses to buffer data until the scanning of the set points is completed.

[0043] To verify the feasibility of the proposed scheme, a LIBS scanning system based on a laser galvanometer was built in the laboratory. The experimental optical path is as follows: Figure 5 As shown, a pulsed laser with a wavelength of 1064 nm was used to ablate the sample.

[0044] The specific optical path design is as follows: The 1064nm laser emitted by laser 1 is expanded by beam expander 2, then reflected by long-pass dichroic mirror 11 (DM1, cutoff wavelength 1180nm), and then transmitted through another long-pass dichroic mirror 11 (DM2, cutoff wavelength 980nm) before finally entering the laser galvanometer. The Y-axis galvanometer reflector 6 and X-axis galvanometer reflector 7 of this laser galvanometer both employ metal film structures to ensure high reflectivity within the 500-2000nm wavelength range. The laser light reflected by the Y-axis and X-axis galvanometer reflectors 6 and 7 is focused onto the sample surface by scanning focusing lens group 9 (L3), thereby achieving sample ablation. The scanning angle range of the galvanometer is ±11°, and with a focusing lens of 100mm focal length, a scanning area of ​​50mm×50mm can be achieved. Since the dynamic change of the laser galvanometer angle causes continuous shift in the focal point, the lateral signal acquisition method has significant limitations; therefore, the system adopts a backward acquisition scheme to collect signals. The plasma emitted by ablation is collected backward by a field mirror, reflected by a laser galvanometer and another long-pass dichroic mirror DM2, and then split into beams at a ratio of 1:9 by a beam splitter 12 (BS). The beams are then focused by a first focusing lens 13 (L1) and a second focusing lens 18 (L2), and received by a plasma image acquisition CCD camera 16 and a spectrometer 15. The plasma image acquisition CCD camera 16 is used to observe plasma images at different coordinate points, while the spectrometer 15 is used to detect the emission spectrum of the plasma.

[0045] The deflection of the laser galvanometer and the timing control of the system are both implemented using dedicated software, which allows for the setting of parameters for the scanning range and scanning path. Lapis lazuli was used as the sample in the experiment, and its surface was polished with 5000-grit sandpaper before scanning. Lapis lazuli is a complex mineral aggregate, mainly composed of silicate minerals, and its main chemical elements include sodium and calcium. The actual lapis lazuli sample is shown below. Figure 6 As shown; Using this system, a serpentine 50×50 dot matrix can be scanned within a 15mm×15mm sample area. After data processing, an intensity distribution map at a specific wavelength can be obtained, which can intuitively characterize the spatial distribution characteristics of the corresponding elements on the sample surface. Figure 7 The image shows the signal intensity distribution of calcium at a wavelength of 396.84 nm. Figure 8 The image shows the signal intensity distribution of sodium at a wavelength of 588.96 nm. Figure 9 The image shows the signal intensity distribution of iron at 358.14 nm. Figure 10 The image shows the signal intensity distribution of sulfur at a wavelength of 564.06 nm. The results can intuitively reflect the distribution of each element in the sample scanning area: calcium (wavelength 396.84nm) is unevenly distributed, indicating the spatial distribution of calcite impurities; sodium (wavelength 588.96nm) is uniformly distributed; iron (358.14nm) is dotted, corresponding to the location of pyrite inclusions; and sulfur (wavelength 564.06nm) distribution reflects the characteristics of the mineral matrix.

[0046] As described above, this method solves the problems of system complexity, poor motion stability, and heavy load on the transport platform caused by the reliance on mechanical displacement in traditional underwater LIBS scanning imaging. By using a laser galvanometer to achieve non-contact beam scanning, high-speed, high-precision two-dimensional elemental distribution imaging of underwater targets can be completed without moving the sample or probe, significantly improving the system's environmental adaptability and platform compatibility.

[0047] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any modifications, equivalent substitutions and improvements made by those skilled in the art within the scope of the technology disclosed in the present invention and within the spirit and principles of the present invention should be covered within the scope of protection of the present invention.

Claims

1. An underwater laser-induced breakdown spectral scanning imaging device, characterized in that, The device includes: Laser emission and beam shaping unit, used for high-energy pulsed laser generation and beam shaping; A high-speed two-dimensional laser galvanometer scanning system is used for two-dimensional dynamic scanning control of laser beams. Pressure-resistant optical emission and signal collection window, used for pressure-resistant optical emission, aberration correction and signal collection; The plasma emission spectrum acquisition and analysis unit is used for plasma emission spectrum acquisition and transmission. The real-time imaging unit for sample surface morphology is used for real-time imaging and diagnosis of sample surface morphology. It can clearly image the surface of underwater target samples, realize real-time observation of the surface morphology and morphological changes of samples, and capture surface state images at specific moments.

2. The underwater laser-induced breakdown spectral scanning imaging device according to claim 1, characterized in that, The laser emission and beam shaping unit includes a laser (1) for generating high-energy pulsed lasers. The pulsed lasers generated by the laser (1) are sequentially expanded by a beam expander (2). The expanded laser beams are reflected by a first reflector (3) and then incident on a high-speed two-dimensional laser galvanometer scanning system.

3. The underwater laser-induced breakdown spectral scanning imaging device according to claim 1, characterized in that, The high-speed two-dimensional laser galvanometer scanning system includes a laser galvanometer, which includes an X-axis galvanometer motor (4), a Y-axis galvanometer motor (5), an X-axis galvanometer reflector (7) rigidly connected to the X-axis galvanometer motor (4), and a Y-axis galvanometer reflector (6) rigidly connected to the Y-axis galvanometer motor (5). The X-axis galvanometer motor (4) and Y-axis galvanometer motor (5) receive external control signals and drive the X-axis galvanometer reflector (7) and Y-axis galvanometer reflector (6) to deflect around the axis, dynamically changing the reflection angle of the incident laser beam, and realizing two-dimensional scanning of the underwater sample surface in a specified area.

4. The underwater laser-induced breakdown spectral scanning imaging device according to claim 1, characterized in that, The pressure-resistant optical emission and signal collection window includes: an optical emission head (8) with an integrated pressure-resistant window, wherein the internal optical path of the optical emission head (8) is used to receive the scanning laser beam from the laser galvanometer and focus it onto the sample surface located underwater to excite and generate plasma; the optical path contains a scanning focusing lens group (9) to eliminate optical aberrations caused by the deflection of the galvanometer reflector, so that the scanning laser beam can be accurately focused on the same focal plane of the sample surface throughout the entire scanning area; at the same time, the optical path design of the optical emission head (8) is also used to collect the emission spectrum signal generated by the laser-induced plasma.

5. The underwater laser-induced breakdown spectral scanning imaging device according to claim 4, characterized in that, The scanning focusing lens group (9) consists of two lenses: one is the main focusing lens, which focuses the laser beam onto the sample surface to achieve ablation; the other is a thin lens with weak negative power of the same glass family as the main focusing lens, which uses a negative meniscus to suppress astigmatism and coma; the focal length and refractive index of the two main focusing lenses must satisfy Petzval and be 0.

6. The underwater laser-induced breakdown spectral scanning imaging device according to claim 1, characterized in that, The plasma emission spectrum acquisition and analysis unit includes: a spectrum acquisition module; the spectrum acquisition module receives the plasma emission spectrum signal collected by the pressure-resistant optical emission and signal collection window through the fiber optic probe (10), and then couples it through the dichroic mirror (11), the beam splitter (12), and the second focusing lens (18); the coupled optical signal is transmitted to the spectrometer (15) through the transmission fiber (14); the spectrometer (15) disperses the received spectral signal and converts it into a corresponding electrical signal.

7. The underwater laser-induced breakdown spectral scanning imaging device according to claim 1, characterized in that, The real-time imaging unit for sample surface morphology includes: a plasma image acquisition CCD camera (16); the plasma image acquisition CCD camera (16) is equipped with an appropriate imaging lens and filter; the optical signal of the sample surface collected by the optical output head (8) passes through a dichroic mirror (11), a beam splitter (12), a second reflecting mirror (17) and a first focusing lens (13) in sequence and is then received by the plasma image acquisition CCD camera (16) for real-time observation of the morphological features, state changes and spatial distribution of the sample surface; The device also includes: a housing (21) and a substrate (19), all of which are integrated on the substrate (19), and the substrate (19) is fixedly connected to the rear cover (20); the housing (21) is fixedly connected to the rear cover (20) and the front cover (22).

8. A method for underwater laser-induced breakdown spectral scanning imaging, characterized in that, This method is applied to the underwater laser-induced breakdown spectral scanning imaging device according to any one of claims 1-7, and the method includes the following steps: S1, High-energy pulsed laser generation and beam shaping; S2, Two-dimensional dynamic scanning control of the laser beam; S3, pressure-resistant optical emission, aberration correction and signal collection; S4, Plasma emission spectrum acquisition and transmission; S5, Real-time imaging and diagnosis of sample surface morphology.

9. The underwater laser-induced breakdown spectral scanning imaging method according to claim 8, characterized in that, In step S2, the two-dimensional dynamic scanning control of the laser beam includes: The beam is deflected by rotating the Y-axis mirror reflector (6) and X-axis mirror reflector (7) of the high-speed two-dimensional laser galvanometer scanning system; the mechanical rotation angles of the X-axis galvanometer motor (4) and the Y-axis galvanometer motor (5) are respectively , The optical deflection angles are respectively ; ； ； ； ； In the formula, The X-axis coordinates of the laser spot on the scanning plane. The Y-axis coordinate of the laser spot on the scanning plane. The distance from the center of the Y-axis galvanometer reflector (6) to the center of the scanning plane is... The center distance between the Y-axis galvanometer reflector (6) and the X-axis galvanometer reflector (7); When the Y-axis galvanometer reflector (6) and the X-axis galvanometer reflector (7) deflect... , At that time, the coordinates of the focal point of the laser beam on the sample surface are: ； Size of the light spot on the sample surface for: ； ； ； In the formula, The new beam waist radius after lens transformation. The beam quality factor. The incident laser wavelength, The focal length of the lens. Let be the waist radius of the incident Gaussian beam. Where is the radius of the light spot. Center value Waist radius at the location, The distance from the beam waist along the optical axis. Rayleigh range indicates that the beam size is expanded to the beam waist. Distance at times; According to the central limit theorem, the variance of the overall mean decreases with the number of particles: ； In the formula, The variance of the sample mean. For the variance of single-particle parameters, This represents the number of independent particles contained within the scanned area. This represents the average value of the particle parameters. This represents the typical area of ​​a single particle. This represents the total area scanned. When the relative error of the measured average is less than a certain threshold Then it satisfies: ； The results were: 。 10. The underwater laser-induced breakdown spectral scanning imaging method according to claim 8, characterized in that, In step S3, the pressure-resistant optical emission, aberration correction, and signal collection include: the focal length and refractive index of the primary focusing lens in the scanning focusing lens group (9) and the weak negative power thin lens must satisfy Petzval and 0, as expressed by: ； ； In the formula, The refractive index of the main focusing mirror The focal length of the main focusing lens. For the refractive index of a weak negative power focusing mirror, The focal length of a weak negative power focusing lens; In step S4, plasma emission spectrum acquisition and transmission includes: setting up an optical fiber probe (10) at the conjugate focal position or the optimal signal collection position of the internal optical path; the optimized spectral acquisition module efficiently focuses the plasma emission spectrum signal onto the end face of the optical fiber probe (10), that is, the image point size formed by the lens is less than or equal to the core diameter of the transmission optical fiber (14) and the end face of the optical fiber is placed on the clear image point formed by the plasma light source through the lens, so that all the collected light can enter the transmission optical fiber (14) to maximize the total light flux; the optical signal is then led to the spectrometer (15) for detection through the low-loss transmission optical fiber (14).

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