A multi-point scanning photoacoustic imaging system for underwater fouling detection

Abstract

The present invention belongs to the field of optical contamination detection, specifically a multi-point scanning photoacoustic imaging system for underwater contamination detection. The system comprises a laser, a function generator, a photoacoustic sealing plate, an ultrasonic transducer, a data acquisition unit, and a computing unit. Laser light emitted by the laser is transmitted through an optical fiber into the function generator, where it is amplitude-modulated and then enters an optical switch. The optical switch then enters the photoacoustic sealing plate, where a photoacoustic signal is generated. The signal then passes through an ultrasonic transducer and a lock-in amplifier and enters the data acquisition unit, which then sends the collected data to the computing unit for processing and display. This method has the advantage of using modulated light, with the laser light emitted by each fiber collimator having a different modulation frequency, thus generating different photoacoustic signal frequencies. This allows for simultaneous excitation and imaging of multiple points, and even single-shot imaging. Furthermore, more information can be obtained based on the response of each point to the modulated laser light at each frequency.

Description

Technical Field

[0001] The present invention belongs to the field of optical contamination detection, and in particular relates to a multi-point scanning photoacoustic imaging underwater contamination detection system. Background Art

[0002] Due to the limitations of underwater lighting conditions and the strong scattering, reflection, and refraction of light in water, it is difficult to effectively implement in situ monitoring of surface biofouling using underwater instruments and equipment based on traditional optical detection. Summary of the Invention

[0003] Based on the above problems, this application provides an underwater fouling system that does not require on-site sampling, can perform simultaneous excitation and imaging of multiple points, and can even achieve one-time imaging. Its technical solution is:

[0004] A multi-point scanning photoacoustic imaging system for underwater fouling detection includes a laser, a function generator, a photoacoustic sealing plate, an ultrasonic transducer, a lock-in amplifier, a data acquisition unit, and a computing unit. Laser light emitted by the laser enters the function generator through an optical fiber, is amplitude-modulated, and then enters an optical switch. From the optical switch, the laser enters the photoacoustic sealing plate, which generates a photoacoustic signal. After passing through an ultrasonic transducer and a lock-in amplifier, the signal enters the data acquisition unit. The data acquisition unit sends the collected data to the computing unit for data processing and display.

[0005] Preferably, the photoacoustic sealing plate is made of a transparent waterproof material and is equipped with multiple optical fibers. One end of all the optical fibers is bundled into a connector to form a bundling head, which can be inserted into the optical switch connector. The other ends are scattered on the same surface of the waterproof transparent material. The surface is integrated with a micro-fiber collimator. The surface is divided into multiple virtual grids. Each micro-fiber collimator is located in the center of the virtual grid. The micro-fiber collimator does not directly contact water and is connected to the optical fiber.

[0006] Preferably, the laser output is adjusted to a high-frequency periodic pulsed sine or cosine wave using a function generator, and the laser emits laser light of a specific wavelength, which is determined by the wavelength band where the light absorption coefficient of the fouling material is the strongest.

[0007] Preferably, a plurality of optical switches are provided, and the plurality of switches are opened or closed simultaneously as required to control the order in which the laser enters the optical fiber in the bundling head and ensure that the laser traverses each micro-fiber collimator.

[0008] Preferably, there is at least one ultrasonic transducer, and each ultrasonic transducer can simultaneously receive photoacoustic signals generated from multiple points.

[0009] Preferably, the specific data processing steps are:

[0010] S1. Obtaining the amplitude information of the photoacoustic signal using a lock-in amplifier or an amplifier plus a lock-in algorithm;

[0011] S2. Invert the amplitude of the original photoacoustic signal obtained based on the distance between the micro-fiber collimator and the ultrasonic transducer and the ultrasonic attenuation coefficient of seawater at the temperature at which the data was collected;

[0012] S3. According to the strength of each photoacoustic signal, the virtual grid where each micro-fiber collimator is located is assigned, and the brightness value of each grid pixel in the photoacoustic sealing plate is normalized;

[0013] S4. Take the inverse of the normalized data for each grid and add 1 to obtain an image whose expression habits are consistent with normal sensory perception;

[0014] S5. The data calculation unit adds the images obtained in step S4 by each ultrasonic transducer in the ultrasonic transducer array, and finally composites the images obtained by all ultrasonic transducers to obtain the final detection result.

[0015] Preferably, in step S3, the virtual grid assignment step is:

[0016] The maximum amplitude of the original photoacoustic signal obtained by a single scan is taken as 1, and the amplitudes of other micro-fiber collimators are converted into numbers from 0 to 1 according to the corresponding proportions. That is, the normalized data n is obtained by using the following formula: i :

[0017]

[0018] Among them, A0i max is the maximum value of the original amplitude A0i,

[0019] The calculation formula of the original amplitude A0i is:

[0020]

[0021] Wherein, α is the ultrasonic attenuation coefficient of seawater, Li is the distance between the ultrasonic transducer and the micro-fiber collimator, and Ai is the amplitude of the i-th point.

[0022] Preferably, the brighter the image obtained in step S3, the more grid attachments there are.

[0023] Preferably, the brightest point in the image is rounded up or rounded up, and the resulting number is the number of ultrasonic transducers participating in the data collection, so as to detect whether the ultrasonic transducer is not working.

[0024] Beneficial effects

[0025] Because this method uses modulated light, the lasers emitted by each fiber collimator have different modulation frequencies, resulting in different photoacoustic signal frequencies. This allows for simultaneous excitation and imaging of multiple points, even enabling one-shot imaging. Furthermore, more information can be obtained based on how each point responds to the modulated laser light at each frequency. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is the system structure diagram of this application.

[0027] Figure 2 This is the structural diagram of the photoacoustic sealing plate.

[0028] In the figure, 1-laser, 2-optical fiber, 22-optical fiber bundle, 3-function generator, 4-optical switch, 41-optical switch connector, 5-photoacoustic sealing plate, 51-body, 52-micro-fiber collimator, 53-virtual grid, 6-ultrasonic transducer, 7-lock-in amplifier, 8-data acquisition unit, 9-computing unit. DETAILED DESCRIPTION

[0029] The following detailed description is illustrative and is intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which this application belongs. It should be noted that the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the exemplary embodiments according to this application.

[0030] Photoacoustic imaging is a new nondestructive imaging technology based on the photoacoustic effect. Frequency-domain photoacoustic imaging uses an amplitude-modulated continuous laser as a light source. By amplitude-modulating a carrier wave of a specific wavelength or using laser pulses, the laser is irradiated onto the target object. The object absorbs heat and elastically expands and contracts, inducing ultrasonic waves. An ultrasonic transducer is then used to receive the acoustic signal and image the target. Materials that absorb heat and produce thermoelastic expansion are often called chromophores. Because the photoacoustic effect does not exist for transparent media, the more opaque the material or the more light-absorbing the attached material, the stronger the photoacoustic signal generated. In other words, the photoacoustic signal intensity should be correlated with the thickness of the attached layer and the attached material within a certain range, allowing verification of these two parameters.

[0031] Figure 1As shown, a multi-point scanning photoacoustic imaging system for detecting underwater contamination includes a laser 1, a function generator 3, a photoacoustic sealing plate 5, an ultrasonic transducer 6, a phase-locked amplifier 7, a data acquisition unit 8, and a computing unit 9. Laser light emitted by the laser 1 is transmitted through an optical fiber 2 into the function generator 3, where the laser light is amplitude-modulated and then enters an optical switch 4. From the optical switch 4, the laser light enters the photoacoustic sealing plate 5, where a photoacoustic signal is generated. The photoacoustic signal passes through the super acoustic transducer 6 and the phase-locked amplifier 7 and then enters the data acquisition unit 8. The data acquisition unit 8 sends the collected data to the computing unit 9 for data processing and display.

[0032] Figure 2 As shown, the photoacoustic sealing plate 5 is made of a transparent waterproof material (plexiglass) and includes a body 51. A plurality of optical fibers 2 are arranged inside the body 51 to form an optical fiber bundle 22. One end of all optical fibers 2 is bundled into a connector to form a bundle head, which can be inserted into the optical switch connector 41. The other ends are dispersed on the same surface of the waterproof transparent material. The surface integrates micro-fiber collimators 52. The surface is divided into multiple virtual grids 53. Each micro-fiber collimator 52 is located in the center of the virtual grid 53 (the virtual grid facilitates finite element expression when assigning values). The micro-fiber collimator 52 does not directly contact water and is connected to the optical fiber 2.

[0033] The function generator 3 (a multi-channel function generator is used, which can generate multiple waveforms at the same time, corresponding to multiple photoacoustic signals) is used to adjust the output of the laser 1 to a sine or cosine wave with a high-frequency periodic pulse. The laser 1 emits a laser of a specific wavelength, and the wavelength of the laser is determined by the band with the strongest light absorption coefficient of the contaminant.

[0034] The optical switches 4 are provided in plurality and are opened or closed simultaneously as required to control the order in which the laser enters the optical fiber in the bundling head and ensure that the laser traverses each micro-fiber collimator 52 .

[0035] There are multiple ultrasonic transducers 6 forming an ultrasonic transducer array. The ultrasonic transducer array is also designed according to the shape of the photoacoustic sealing enclosure. Each ultrasonic transducer 6 corresponds to a micro-fiber collimator 52 .

[0036] The working principle is:

[0037] Laser light emitted by laser 1 is transmitted through optical fiber 2 into function generator 3 (either single-channel or multi-channel). The laser light is amplitude-modulated before entering optical switch 4 (amplitude modulation allows different fiber collimators to input laser light with different modulation frequencies). The other end of the optical switch is connected to the beamforming head of the photoacoustic enclosure 5. Optical switch 4 controls the order in which the laser light enters the beamforming head (multiple beams can be opened or closed simultaneously as needed). Laser light is then transmitted from the inside out through micro-fiber collimators 52 onto the outer surface of the enclosure, stimulating a photoacoustic signal. The photoacoustic signal is collected by an ultrasonic transducer array, passed through an amplifier (or lock-in amplifier 7), and then into a data acquisition unit 8. The data is then input into a calculation unit 9 for processing and display.

[0038] The specific data processing is:

[0039] S1. Obtaining the amplitude information of the photoacoustic signal using a lock-in amplifier or an amplifier plus a lock-in algorithm;

[0040] S2. The original photoacoustic signal amplitude is inverted based on the distance between the micro-fiber collimator and the ultrasonic transducer and the ultrasonic attenuation coefficient of seawater at the temperature at which the data was collected. The calculation formula for the original amplitude A0i is:

[0041]

[0042] Wherein, α is the ultrasonic attenuation coefficient of seawater, Li is the distance between the ultrasonic transducer and the micro-fiber collimator, and Ai is the amplitude of the i-th point.

[0043] S3. According to the strength of each photoacoustic signal, the virtual grid where each micro-fiber collimator is located is assigned, and the brightness value of each grid pixel in the photoacoustic sealing plate is normalized;

[0044] The idea of ​​assignment is to take the maximum amplitude of the original photoacoustic signal obtained in a single scan as 1, and convert the amplitudes of other points into numbers between 0 and 1 according to the corresponding proportions, that is, to obtain the normalized data n using the following formula for normalization: i :

[0045]

[0046] Among them, A0i max is the maximum value of the original amplitude A0i.

[0047] For example, for a sealed cover with 12 micro-fiber collimators, there are 12 photoacoustic signals generated after one traversal. After measurement, the amplitudes of the 12 photoacoustic signals are in microvolts and the values ​​are

[0048] 8.929673 0.32434 5.458739 7.320641 0.507903 6.44582 1.796288 2.328697 9.212476 6.142287 7.254594 7.105292

[0049] The maximum amplitude is 9.212476μV, so after normalization, the amplitude matrix becomes

[0050] 0.969302 0.035207 0.592538 0.794644 0.055132 0.699684 0.194984 0.252776 1 0.666736 0.787475 0.771268

[0051] This is the value of each virtual grid in the brightness map. Using MATLAB, the matrix is ​​assigned to the corresponding virtual grid to obtain the brightness map. The larger the grid value, the brighter the image.

[0052] S4. Since the image obtained in step S3 shows that brighter grids have more attachments, which is inconsistent with normal expression habits, the normalized data of each grid is negated and then added to 1 to obtain an image with expression habits consistent with normal perception;

[0053] S5. The data calculation unit adds the images obtained in step S4 for each ultrasonic transducer, ultimately combining the images obtained by all ultrasonic transducers to obtain the final detection result. Normalization is not required in this step, as theoretically, the points with strong photoacoustic signals are the same in each image. The brightest point is rounded up (or rounded to the nearest integer), and the resulting number should be the number of participating ultrasonic transducers. This method can detect whether an ultrasonic transducer is not working (if the ultrasonic transducer is working, the data acquisition card will obtain empty data).

[0054] The above description is merely a preferred embodiment of the present application and is not intended to limit the present application. Various modifications and variations are possible for those skilled in the art. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present application shall be included within the scope of protection of the present application.

Claims

1. A multi-point scanning photoacoustic imaging system for underwater fouling detection, characterized in that: The system includes a laser, a function generator, a photoacoustic sealing plate, an ultrasonic transducer, a lock-in amplifier, a data acquisition unit, and a computing unit. The laser light emitted by the laser enters the function generator through an optical fiber, is amplitude-modulated, and then enters the optical switch. The optical switch then enters the photoacoustic sealing plate, which generates a photoacoustic signal. The signal passes through the ultrasonic transducer and the lock-in amplifier and then enters the data acquisition unit. The data acquisition unit sends the collected data to the computing unit for data processing and display. The photoacoustic sealing plate is made of a transparent waterproof material and is equipped with multiple optical fibers. One end of all optical fibers is bundled into a connector to form a bundle head, which can be inserted into the optical switch connector. The other ends are dispersed on the same surface of the waterproof transparent material. The surface is integrated with micro-fiber collimators. The surface is divided into multiple virtual grids. Each micro-fiber collimator is located in the center of the virtual grid. The micro-fiber collimators do not directly contact water and are connected to optical fibers. Amplitude modulation can input lasers of different modulation frequencies to different micro-fiber collimators. The data processing process is: S1. Obtaining the amplitude information of the photoacoustic signal using a lock-in amplifier or an amplifier plus a lock-in algorithm; S2. Invert the amplitude of the original photoacoustic signal obtained based on the distance between the micro-fiber collimator and the ultrasonic transducer and the ultrasonic attenuation coefficient of seawater at the temperature at which the data was collected; S3. According to the strength of each photoacoustic signal, the virtual grid where each micro-fiber collimator is located is assigned, and the brightness value of each grid pixel in the photoacoustic sealing plate is normalized; Assign values ​​to the virtual grid where each micro-fiber collimator is located. The steps are as follows: The maximum amplitude of the original photoacoustic signal obtained by a single scan is taken as 1, and the amplitudes of other micro-fiber collimators are converted into numbers between 0 and 1 according to the corresponding proportions. That is, the normalized data is obtained by using the following formula: : ； in, is the maximum value of the original amplitude A0i, The calculation formula of the original amplitude A0i is: ； Wherein, α is the ultrasonic attenuation coefficient of seawater, Li is the distance between the ultrasonic transducer and the micro-fiber collimator, is the amplitude of the i-th point; S4. Take the inverse of the normalized data for each grid and add 1 to obtain an image whose expression habits are consistent with normal sensory perception; S5. The data calculation unit adds the images obtained in step S4 by each ultrasonic transducer in the ultrasonic transducer array, and finally composites the images obtained by all ultrasonic transducers to obtain the final detection result.

2. The multi-point scanning photoacoustic imaging underwater fouling detection system according to claim 1, characterized in that: The laser output is adjusted to a high-frequency periodic pulsed sine or cosine wave using a function generator. The laser emits laser light of a specific wavelength, which is determined by the wavelength band with the strongest light absorption coefficient of the contaminant.

3. The multi-point scanning photoacoustic imaging underwater fouling detection system according to claim 2, characterized in that: The optical switches are provided in plurality and are opened or closed simultaneously as required to control the order in which the laser enters the optical fiber in the bundling head and ensure that the laser traverses each micro-fiber collimator.

4. The multi-point scanning photoacoustic imaging underwater fouling detection system according to claim 2, characterized in that: There is at least one ultrasonic transducer, and each ultrasonic transducer can simultaneously receive photoacoustic signals generated from multiple points.

5. The multi-point scanning photoacoustic imaging underwater fouling detection system according to claim 1, characterized in that: The brighter the image obtained in step S3, the more mesh attachments there are.

6. The multi-point scanning photoacoustic imaging underwater fouling detection system according to claim 1, characterized in that: The brightest point in the image is rounded up or rounded up, and the resulting number is the number of ultrasonic transducers involved in this data collection, which is used to detect whether the ultrasonic transducer is malfunctioning.

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