Knife edge calibration method in photoacoustic measurement process

Abstract

The invention relates to the technical field of ultrasonic detection, and particularly provides a knife edge calibration method in a photoacoustic measurement process, which comprises the following steps of: moving a knife edge to deviate from signal light, adjusting a light spot distance between pump light and probe light on a measured surface, moving the probe light to a full width at half maximum position of the pump light, determining a knife edge calibration scheme according to system noise, and calibrating the knife edge according to the system noise. If the system noise is smaller than a preset first noise threshold value, the knife edge is moved to enable the signal-to-noise ratio of the photoacoustic signals obtained through PSD mode detection to reach the maximum value, if the system noise is larger than the preset first noise threshold value, the knife edge is moved to enable the amplitude of the photoacoustic signals obtained through PSD mode detection to reach the maximum value, and the position of the knife edge is adjusted again in the set interval to obtain the photoacoustic signals. The signal-to-noise ratio of the photoacoustic signal reaches the maximum value, and the knife edge calibration is completed. According to the invention, the precision and normalization of knife edge calibration are improved, and the signal-to-noise ratio of a measurement signal is obviously improved.

Description

Technical Field

[0001] This invention relates to the field of ultrasonic testing technology, and specifically provides a knife edge calibration method in the photoacoustic measurement process. Background Technology

[0002] The principle of femtosecond laser ultrasonic testing of wafer surface metal film thickness is mainly based on using a femtosecond laser beam to excite ultrasound on the wafer surface; this laser beam is called the pump beam. Then, another beam is used to probe the ultrasonic signal on the wafer surface; this beam is called the probe beam. Ultrasonic detection can be achieved by directly measuring reflectivity, known as the REF mode; or by detecting the light deflection signal caused by ultrasonic displacement, known as the PSD mode. The probe beam with deflection signal has higher sensitivity than the former and can detect weaker ultrasonic signals. The difference between the two methods is that one detects the photoelastic effect: the change in reflectivity caused by ultrasound; the other detects the light deflection effect: proportional to the ultrasonic displacement signal. The PSD mode generally uses a knife-edge method to measure the change in light intensity caused by the deflection of the probe beam due to ultrasound.

[0003] However, in the optical path of PSD mode, the position of the knife prism is very sensitive to the signal-to-noise ratio of the signal. Therefore, the positional accuracy of the knife prism is required to be high. The existing optical path calibration schemes for the knife prism position are all manual calibrations without precise calibration standards. The position is usually determined by observing the detected ultrasonic signal waveform. Manual calibration has low accuracy and the calibration process is extremely complicated. Summary of the Invention

[0004] To address the aforementioned problems, this invention provides a knife-edge calibration method in the photoacoustic measurement process. Based on system noise, two specific calibration routes are given, which significantly improves the accuracy and efficiency of the measurement and is of great significance for enhancing the application value of femtosecond laser ultrasonic detection technology.

[0005] To achieve the above objectives, the technical solution created by this invention is implemented as follows: The knife-edge calibration method in the photoacoustic measurement process provided by this invention includes: S1: Construct the femtosecond laser ultrasonic detection optical path in PSD mode. The detection optical path includes a pump light and a probe light. After the probe light is incident on the sample to be tested, it is reflected to form a signal light. Move the blade to deviate from the signal light. Adjust the spot spacing of the pump light and probe light on the surface to be tested and detect the intensity of the signal light until the signal light intensity is half of the maximum signal light intensity. The maximum signal light intensity is the signal light obtained when the spots of the pump light and probe light on the surface to be tested coincide. S2: Adjust the blade to the initial beam splitting position in any PSD mode so that the signal light is guided to the detector through the blade and the system noise of the detection optical path is obtained. If the system noise is less than the preset first noise threshold, the knife edge position is adjusted so that the signal light is guided to the detector through the knife edge, and the photoacoustic signal obtained by the detector satisfies the following condition: the signal-to-noise ratio of the photoacoustic signal reaches the maximum value, and the knife edge calibration is completed. If the system noise is greater than the first noise threshold, adjust the knife edge position so that the signal light is guided to the detector through the knife edge and the amplitude of the photoacoustic signal obtained by the detector reaches the maximum value; set a fine adjustment range that includes the maximum amplitude, and adjust the knife edge position within the fine adjustment range so that the signal-to-noise ratio of the photoacoustic signal reaches the maximum value, thus completing the knife edge calibration.

[0006] Preferably, the power of the pump light remains constant during the calibration process.

[0007] Preferably, if the system noise is greater than the first noise threshold, the amplitude of the photoacoustic signal at the initial beam splitting position is calculated, the blade is adjusted to move the same distance in both the forward and reverse directions, and the forward and reverse amplitudes are calculated accordingly. The direction of blade movement is determined by comparing the forward and reverse amplitudes.

[0008] Preferably, if the positive amplitude is greater than the negative amplitude, the blade edge is adjusted to... The step size moves in the positive direction until the amplitude of the detected photoacoustic signal reaches its maximum value; If the reverse amplitude is greater than the forward amplitude, adjust the blade edge accordingly. The step size is reversed until the amplitude of the detected photoacoustic signal reaches its maximum value.

[0009] Preferably, the fine-tuning range is Where 0 represents the value of 0. The position of the knife edge when the maximum amplitude of the photoacoustic signal is obtained by adjusting the step size of the knife edge.

[0010] Preferably, adjusting the knife edge position within the fine-tuning range to maximize the signal-to-noise ratio of the photoacoustic signal includes: In the fine-tuning range Inside, with To adjust the blade position by step size, the final calibrated position is selected when the signal-to-noise ratio of the detected photoacoustic signal reaches its maximum value; where n is a positive integer greater than 1.

[0011] Preferably, if the system noise is less than a preset first noise threshold, the photoacoustic signal-to-noise ratio at the initial beam splitting position is calculated, the blade is adjusted to move the same distance in both the forward and reverse directions, and the forward and reverse signal-to-noise ratios are calculated accordingly. The blade movement direction is determined by comparing the forward and reverse signal-to-noise ratios.

[0012] Preferably, if the forward signal-to-noise ratio is greater than the reverse signal-to-noise ratio, the blade is adjusted to move forward until the signal-to-noise ratio of the detected photoacoustic signal reaches its maximum value; If the reverse signal-to-noise ratio is greater than the forward signal-to-noise ratio, adjust the blade to move in the reverse direction until the signal-to-noise ratio of the detected photoacoustic signal reaches its maximum value.

[0013] Preferably, the noise of the standard sample is measured by detecting the optical path as the system noise; and a second noise threshold is set. If the system noise is greater than the second noise threshold, the knife edge calibration is stopped and the detection optical path is checked.

[0014] Preferably, the first noise threshold is 10% of the photoacoustic signal amplitude; the second noise threshold is 50% of the acoustic signal amplitude.

[0015] Compared with the prior art, the present invention can achieve the following beneficial effects: This invention adjusts the beam spacing between the pump light and the probe light on the measured surface, moving the probe light to the half-width at half-maximum (WHM) of the pump light. This positions the probe light at the point of maximum thermal expansion. Two specific knife-edge calibration routes are designed based on system noise. If the system noise is less than a preset noise threshold, the knife-edge position is adjusted to maximize the signal-to-noise ratio (SNR). If the system noise exceeds the threshold, the knife-edge position is first adjusted to maximize the signal amplitude. Then, within the fine-tuning range, the maximum SNR is used as the final calibration standard to obtain the optimal knife-edge position. The most suitable method can be selected based on the actual situation, increasing the flexibility of the solution and significantly improving the accuracy and standardization of knife-edge calibration. By optimizing the knife-edge position, the SNR of the measurement signal is significantly improved, thereby enhancing the reliability of the measurement results.

[0016] Furthermore, this invention can be combined with a drive device and a control system. The drive device adjusts the blade position, and the control system automatically completes the blade calibration according to the blade calibration route provided by this invention. This achieves automatic calibration of the blade position, improves the efficiency and accuracy of calibration, and simplifies the operation steps and reduces the difficulty of operation through an automated and standardized calibration process. This is of great significance for improving the application value of femtosecond laser ultrasonic testing technology. Attached Figure Description

[0017] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments and descriptions of the invention are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 This is a flowchart of knife edge calibration provided according to an embodiment of the present invention; Figure 2 This is a schematic diagram of the PSD mode femtosecond laser ultrasonic detection optical path provided in an embodiment of the present invention; Figure 3 This is a schematic diagram of the optical deflection theory provided in an embodiment of the present invention; Figure 4 This is a schematic diagram of the structure of the first differential detection module provided in the embodiment of the present invention; Figure 5 This is a schematic diagram of the structure of the second differential detection module provided in an embodiment of the present invention; Figure 6 This is a schematic diagram illustrating the changes in thermal expansion and displacement slope according to an embodiment of the present invention; Figure 7 This is a schematic diagram of the acoustic-optical signal when the system noise is less than a first noise threshold, according to an embodiment of the present invention. Figure 8 This is a schematic diagram of the acoustic-optical signal when the system noise is greater than a first noise threshold, according to an embodiment of the present invention. Figure 9 This is a comparison chart of signal-to-noise ratio and amplitude curves of signals at different knife-edge positions according to an embodiment of the present invention; Figure 10 This is a schematic diagram of the acoustic-optical signal when the system noise is greater than the second noise threshold according to an embodiment of the present invention.

[0018] The reference numerals in the figures include: First reflector 101, second reflector 102, blade 103, electric moving platform 104, balance detector 105, first photodetector 201, second photodetector 202. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only for explaining the invention and do not constitute a limitation thereof. Similar elements in different embodiments are referred to by associated similar element reference numerals. In the following embodiments, many details are described to facilitate a better understanding of the invention. However, those skilled in the art will readily recognize that some features may be omitted in different situations, or may be replaced by other elements, materials, or methods. In some cases, some operations related to the invention are not shown or described in the specification. This is to avoid obscuring the core parts of the invention with excessive description. For those skilled in the art, detailed description of these related operations is not necessary; they can fully understand the related operations based on the description in the specification and general technical knowledge in the art.

[0020] It should be noted that, unless otherwise specified, the embodiments and features described in this invention can be combined to form various implementations. Furthermore, the order of the steps or actions in the method description can be changed or adjusted in a manner readily apparent to those skilled in the art. Therefore, the various orders in the specification and drawings are merely for the clear description of a particular embodiment and do not imply a mandatory order, unless otherwise stated that a particular order must be followed.

[0021] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," and "counterclockwise," etc., indicating orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation on this invention. Furthermore, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, features defined with "first," "second," etc., may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0022] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art will understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0023] The invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0024] In one embodiment of the present invention, a knife-edge calibration method is provided during photoacoustic measurement to solve the problem of difficulty in calibrating the knife-edge position during differential measurement. Please refer to [link to relevant documentation]. Figure 1 The specific calibration process is as follows: S1: First, build as follows Figure 2The PSD mode femtosecond laser ultrasonic testing optical path shown is an existing design. Its main working principle is as follows: A femtosecond laser beam is emitted from a femtosecond laser, and after splitting and modulating the beam, a pump beam and a probe beam are obtained. The pump beam is transmitted through the optical path and then illuminates the surface of the sample under test. Similarly, the probe beam is transmitted through the optical path and then illuminates the surface under test. The probe beam is reflected by the surface under test to form a signal beam, which is transmitted through the optical path to the differential detection module. The differential detection module reads the surface information carried by the signal beam.

[0025] Please see Figure 2 When the pump light shines on the surface under test, the sample absorbs the energy of the pump light and excites ultrasound. As the ultrasound propagates within the sample, it is reflected at the interface between different propagation media. With the propagation of the ultrasound, the surface under test undergoes thermal expansion, resulting in bulges of varying heights. This causes a change in the angle of incidence of the probe light after it shines on the surface, leading to an angular deflection of the signal light reflected from the surface. Figure 3 The angle θ changes. When the angle of the signal light deflects, the relative position of the blade used to split the signal light in the differential detection module also changes.

[0026] Specifically, two differential detection modules are shown in this embodiment of the invention. For the first differential detection module, please refer to [link to relevant documentation]. Figure 4The first differential detection module includes a first reflector 101, a second reflector 102, a knife edge 103, an electric moving platform 104, and a balance detector 105, also known as a differential detector. During differential measurement, the incident light path of the signal light is perpendicularly incident on the apex of the knife edge 103. The knife edge 103 splits the detection light into two beams, which then illuminate two sides of the apex of the knife edge 103. These two sides reflect the two beams of light onto the first reflector 101 and the second reflector 102, respectively. In this differential detection module, both sides of the knife edge 103 participate in the light path propagation. The first and second reflectors 101 and 102 then reflect both beams of light onto the balance detector 105. The balance detector 105 performs differential calculations on the light intensities of these two beams to obtain a high signal-to-noise ratio light deflection signal. Since the detected light intensity change is proportional to the deflection angle, and the deflection angle is proportional to the ultrasonic displacement... Therefore, the signal detected by the balance detector 105 is an ultrasonic displacement signal, i.e., a photoacoustic signal. Furthermore, to facilitate the position adjustment of the blade 103, the blade 103 is fixed to the electric moving platform 104, and the electric moving platform 104 is equipped with a corresponding host computer controller. The host computer controller drives the electric moving platform 104 to move the blade 103 relative to both sides of the signal light axis, adjusting the size of the two beams of light to achieve the desired effect. Figure 4 Taking the placement of the blade 103 as an example, the blade moves in a straight line, moving up and down.

[0027] For the second type of differential detection module, please refer to [link / reference]. Figure 5The second differential detection module structure includes a knife edge 103, an electric moving platform 104, a first photodetector 201, and a second photodetector 202. This design has a simpler optical path structure but requires two detectors. Specifically, during differential measurement, the incident light path is perpendicular to one side of the knife edge 103, with part of the signal light illuminating the side of the knife edge 103, while the remaining portion does not illuminate the knife edge 103 but continues to propagate along the original optical path. The signal light illuminating the side of the knife edge 103 is reflected by the knife edge 103 towards the first photodetector 201. The signal light continuing to propagate along the original optical path illuminates the second photodetector 202, which is also split into two beams by the knife edge 103. The light intensities of these two beams are detected by the first photodetector 201 and the second photodetector 202. Differential calculations are performed on the light intensities to obtain a high signal-to-noise ratio light deflection signal, thereby obtaining a photoacoustic signal. Similarly, to facilitate the position adjustment of the blade 103, the blade 103 is fixed on the electric moving platform 104, and the electric moving platform 104 is equipped with a corresponding host computer controller. The host computer controller drives the electric moving platform 104 to move the blade 103 in a direction perpendicular to the optical axis of the signal light, thereby adjusting the size of the two beams of light split off. Figure 5 Taking the placement of the blade 103 as an example, the blade moves in a straight line, moving up and down.

[0028] Before calibrating at the knife edge 103 position, in order to avoid the change in deflection angle caused by thermal expansion affecting the accuracy of the calibration results, it is necessary to fix the power of the pump light during the calibration process, so that the power of the pump light remains constant, and to prevent the height of the thermal expansion bulge on the measured surface from changing due to different pump light power, which in turn causes the deflection angle of the signal light to change.

[0029] After the pump light power is fixed, the knife edge 103 in the femtosecond laser ultrasonic detection optical path needs to be moved to deviate from the signal light, so that the transmission path of the signal light is not affected by the knife edge 103. After the knife edge 103 is moved, it can no longer split the signal light. This position is described as a non-splitting position, allowing the probe light to be directly incident on the detector (PD) after being reflected from the side. For both the first and second differential detection modules, the knife edge 103 needs to be driven downward to prevent the signal light from being split by the knife edge 103. At this time, the spot spacing between the pump light and the probe light on the measured surface is adjusted by adjusting the components in the femtosecond laser ultrasonic detection optical path, i.e., adjusting the position of the probe light incident on the measured surface. During the adjustment process, the signal light is collected by the detector to obtain the light reflectivity signal. This process is actually similar to REF mode. The adjustment of the spot spacing is stopped when the intensity of the detected signal light is equal to half of the maximum signal light intensity, i.e., the intensity of the signal light when the spots of the pump light and the probe light on the measured surface coincide. At this point, the probe beam is considered to have moved to the half-width at half-maximum (WHM) position of the pump beam, which is where the slope of thermal expansion is maximum. See also... Figure 6 In this embodiment of the invention, the beam spacing at the half-width at half-maximum (WHM) of the Gaussian curve is theoretically calculated and compared with the beam spacing obtained from actual testing when the signal light intensity drops to half of its maximum value. The results show that the two are very close. Figure 6 The peaks of the two curves are quite close. Therefore, it can be considered that during the adjustment of the spot spacing, when the intensity of the detected signal light is half of the maximum signal light intensity when the spots of the pump light and the probe light overlap on the surface being measured, the probe light moves to the half-width at half-maximum position of the pump light, thereby improving the sensitivity and accuracy of the subsequent knife-edge 103 calibration, accurately characterizing the thermal expansion properties of the material, optimizing the experimental results, and enhancing the contrast of the signal.

[0030] S2: After the beam spacing is adjusted, the electric moving platform 104 moves the blade 103 to the beam splitting position, which is the initial beam splitting position. The initial beam splitting position refers to the position where the blade 103 splits the signal light into two parts. The blade 103 can guide the signal light to the first photodetector 201 and the second photodetector 202 respectively. At this time, the electric moving platform 104 continues to drive the blade 103 to move, and the photoacoustic signal is acquired through the detector. The host computer controller, which controls the movement of the electric moving platform 104, automatically adjusts the position of the blade 103 according to the following calibration strategy stored in its internal memory: First, the noise level of the photoacoustic signal from the standard sample (i.e., the standard wafer) is measured at the initial beam splitting position to determine the current system noise level. This system noise is the system noise in PSD mode. In PSD mode, the signal detected by the photodetector in the femtosecond laser ultrasonic detection optical path is a curve with time and signal amplitude as the horizontal and vertical axes, respectively. Due to the presence of system noise, the signal amplitude deviates from the theoretical value at this point. The fluctuation value of the signal amplitude deviation at this point is the system noise. The maximum value of the photoacoustic signal amplitude is multiplied by a specific ratio to obtain the first noise threshold.

[0031] Further, the subsequent knife-edge calibration route is determined based on the system noise. Specifically, a first noise threshold (i.e., threshold 1) is set. In this embodiment of the invention, the first noise threshold is selected as 10% of the maximum amplitude of the photoacoustic signal. The system noise is compared with the first noise threshold to determine whether the calibration standard is the signal-to-noise ratio of the photoacoustic signal or the amplitude of the photoacoustic signal. like Figure 7 As shown, if the system noise is less than a preset first noise threshold, the signal-to-noise ratio (SNR) fluctuation is small, and the optimal position of the cutting edge can be determined based on the SNR. The photoacoustic signal SNR at the current initial position is calculated, and the cutting edge 103 is moved the same distance in both the forward and reverse directions by the electric moving platform 104. The signal-to-noise ratio (SNR) during the forward and reverse movements is calculated separately. The SNR during the forward movement is recorded as the forward SNR, and the SNR during the reverse movement is recorded as the reverse SNR. By observing the changes in the forward and reverse SNR during the forward and reverse movements, the adjustment direction of the blade 103 is determined. Specifically, if the forward SNR is greater than the reverse SNR, it is determined that the blade 103 should move in the forward direction, and the blade 103 is driven from the initial beam-splitting position by the electric moving platform 104. The blade 103 moves forward by a step size. During this forward movement, the signal-to-noise ratio (SNR) gradually increases. The blade 103 continues to move forward until the SNR of the detected photoacoustic signal reaches its maximum value. This position is then determined to be the optimal position for the blade 103, and the blade calibration is complete. The criterion for determining that the photoacoustic signal SNR has reached its maximum value is that the SNR gradually decreases with continued movement, indicating that the SNR curve has reached its peak. If the reverse SNR is greater than the forward SNR, the blade 103 should move in the reverse direction. The blade 103 is driven to move in the reverse direction from its initial beam-splitting position by the motorized moving platform 104. During this reverse movement, the SNR gradually increases. The blade 103 continues to move in the reverse direction until the SNR of the detected photoacoustic signal reaches its maximum value. This position is then determined to be the optimal position for the blade 103, and the blade calibration is complete.

[0032] like Figure 8As shown, if the system noise exceeds a preset first noise threshold, the signal-to-noise ratio (SNR) fluctuates significantly, making it impossible to accurately determine the optimal tool edge position based solely on the SNR. In this situation, moving the tool edge 103 will be severely affected by the system noise; therefore, the position with the highest SNR is not the optimal tool edge 103 position. Consequently, calibrating the tool edge 103 based on the SNR will lead to inaccurate calibration. See [link to documentation] for details. Figure 9 The position where the photoacoustic signal amplitude is maximum is close to the actual optimal position, while the position where the signal-to-noise ratio is maximum differs from the actual optimal position by a certain distance. If the maximum amplitude is used as the initial calibration standard for preliminary calibration, and then the maximum signal-to-noise ratio is used as the final calibration standard based on the preliminary calibration, the optimal knife-edge position can be obtained. The specific calibration process is as follows: calculate the photoacoustic signal amplitude at the current initial position, and drive the knife-edge 103 to move the same distance in both the forward and reverse directions using the electric moving platform 104. The amplitude values ​​during the forward and reverse movements are calculated separately, with the amplitude during the forward movement recorded as the forward amplitude and the amplitude during the reverse movement recorded as the reverse amplitude. By observing the changes in the forward and reverse amplitude values ​​during the forward and reverse movements, the adjustment direction of the blade 103 is determined. Specifically, if the forward amplitude is greater than the reverse amplitude, it is determined that the blade 103 should move in the forward direction, and the blade 103 is driven from the initial beam-splitting position by the electric moving platform 104. For example, the blade 103 moves in a positive direction by a step size. During the forward movement, the amplitude gradually increases, and the knife edge 103 continues to move forward until the amplitude of the detected photoacoustic signal reaches its maximum value. The criterion for determining that the photoacoustic signal amplitude has reached its maximum value is that the amplitude gradually decreases with continued movement, indicating that the amplitude curve has reached its peak. If the reverse amplitude is greater than the forward amplitude, it is determined that the knife edge 103 should move in the reverse direction. The electric moving platform 104 drives the knife edge 103 to move in the reverse direction from its initial beam-splitting position. During the reverse movement, the amplitude gradually increases, and the knife edge 103 continues to move in the reverse direction until the amplitude of the detected photoacoustic signal reaches its maximum value, completing the initial calibration of the knife edge 103.

[0033] A further fine-tuning interval is defined, including the position of the maximum amplitude. Specifically, a certain distance is taken to the left and right of the blade edge position at the maximum amplitude position to form the fine-tuning interval. The blade edge position is adjusted within this fine-tuning interval. The fine-tuning interval is designed as follows: Where 0 represents the value of 0. The position of knife edge 103 is obtained when the photoacoustic signal amplitude is at its maximum value by adjusting the step size of knife edge 103. Within the fine-tuning range... Inside, with To adjust the position of the blade 103 by step size, the final calibrated position is selected when the signal-to-noise ratio of the detected photoacoustic signal reaches its maximum value; where n is a positive integer greater than 1, and in this embodiment of the invention, n is 10. The process of adjusting the blade 103 within the fine-tuning range is the same as the calibration process when the system noise is less than the preset first noise threshold, and will not be described in detail here.

[0034] Furthermore, a second noise threshold was set, which is 50% of the maximum amplitude of the photoacoustic signal. For example... Figure 10 As shown, if the system noise is greater than the second noise threshold, the signal amplitude will be greatly affected by the noise. Whether the amplitude is at its maximum or the signal-to-noise ratio is at its maximum, the optimal knife edge position cannot be obtained. This may be due to a problem with the system hardware, requiring the device to alarm, stop knife edge calibration, and check the hardware of the detection optical path.

[0035] In summary, the above description is merely a preferred embodiment of this specification and is not intended to limit the scope of protection of this specification. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this specification should be included within the scope of protection of this specification.

[0036] The systems, apparatuses, modules, or units described in one or more of the above embodiments may be implemented by a computer chip or entity, or by a product having a certain function. A typical implementation device is a computer. Specifically, a computer may be, for example, a personal computer, a laptop computer, a cellular phone, a camera phone, a smartphone, a personal digital assistant, a media player, a navigation device, an email device, a game console, a tablet computer, a wearable device, or any combination of these devices.

[0037] It should also be noted that the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0038] The various embodiments in this specification are described in a progressive manner. Similar or identical parts between embodiments can be referred to mutually. Each embodiment focuses on describing the differences from other embodiments. In particular, the system embodiments are basically similar to the method embodiments, so the description is relatively simple; relevant parts can be referred to the descriptions in the method embodiments.

[0039] The foregoing has described specific embodiments of this specification. Other embodiments are within the scope of the appended claims. In some cases, the actions or steps recited in the claims may be performed in a different order than that shown in the embodiments and may still achieve the desired result. Furthermore, the processes depicted in the drawings do not necessarily require the specific or sequential order shown to achieve the desired result. In some embodiments, multitasking and parallel processing are possible or may be advantageous.

Claims

1. A knife-edge calibration method in a photoacoustic measurement process, characterized in that, include: S1: Construct a femtosecond laser ultrasonic detection optical path in PSD mode. The detection optical path includes a pump light and a probe light. After the probe light is incident on the sample to be tested, it is reflected to form a signal light. Move the blade to deviate from the signal light. Adjust the spot spacing of the pump light and the probe light on the surface to be tested and detect the intensity of the signal light until the signal light intensity is half of the maximum signal light intensity. The maximum signal light intensity is the signal light obtained when the spots of the pump light and the probe light on the surface to be tested coincide. S2: Adjust the blade to the initial beam splitting position in any PSD mode so that the signal light is guided to the detector through the blade and the system noise of the detection optical path is obtained. If the system noise is less than the preset first noise threshold, the knife edge position is adjusted so that the photoacoustic signal obtained by the detector meets the following condition: the signal-to-noise ratio of the photoacoustic signal reaches the maximum value, and the knife edge calibration is completed. If the system noise is greater than the first noise threshold, the knife edge position is adjusted so that the signal light is guided to the detector through the knife edge and the amplitude of the photoacoustic signal obtained by the detector reaches the maximum value; a fine-tuning range containing the maximum amplitude is set, and the knife edge position is adjusted within the fine-tuning range so that the signal-to-noise ratio of the photoacoustic signal reaches the maximum value, thus completing the knife edge calibration.

2. The knife-edge calibration method in the photoacoustic measurement process as described in claim 1, characterized in that, The power of the pump light remains constant during calibration.

3. The knife-edge calibration method in the photoacoustic measurement process as described in claim 1, characterized in that, If the system noise is greater than the first noise threshold, calculate the photoacoustic signal amplitude at the initial beam splitting position, adjust the blade to move the same distance in both the forward and reverse directions, and calculate the forward and reverse amplitudes accordingly. The blade movement direction is determined by comparing the forward and reverse amplitudes.

4. The knife-edge calibration method in the photoacoustic measurement process as described in claim 3, characterized in that, If the positive amplitude is greater than the negative amplitude, then adjust the blade edge to... The step size moves in the positive direction until the amplitude of the detected photoacoustic signal reaches its maximum value; If the reverse amplitude is greater than the forward amplitude, then adjust the blade to... The step size is reversed until the amplitude of the detected photoacoustic signal reaches its maximum value.

5. The knife-edge calibration method in the photoacoustic measurement process as described in claim 4, characterized in that, The fine-tuning interval is Where 0 represents the value of 0. The position of the knife edge when the maximum amplitude of the photoacoustic signal is obtained by adjusting the step size of the knife edge.

6. The knife-edge calibration method in the photoacoustic measurement process as described in claim 5, characterized in that, Adjusting the blade position within the fine-tuning range to maximize the signal-to-noise ratio of the photoacoustic signal includes: In the fine-tuning range Inside, with To adjust the blade position by step size, the blade position at which the signal-to-noise ratio of the detected photoacoustic signal reaches its maximum value is selected as the final calibrated position; where n is a positive integer greater than 1.

7. The knife-edge calibration method in the photoacoustic measurement process as described in claim 5, characterized in that, If the system noise is less than a preset first noise threshold, calculate the photoacoustic signal-to-noise ratio at the initial beam splitting position, adjust the blade to move the same distance in both the forward and reverse directions, and calculate the forward and reverse signal-to-noise ratios accordingly. The blade movement direction is determined by comparing the forward and reverse signal-to-noise ratios.

8. The knife-edge calibration method in the photoacoustic measurement process as described in claim 1, characterized in that, If the forward signal-to-noise ratio is greater than the reverse signal-to-noise ratio, the blade is adjusted to move forward until the detected photoacoustic signal-to-noise ratio reaches its maximum value. If the reverse signal-to-noise ratio is greater than the forward signal-to-noise ratio, the blade is adjusted to move in the reverse direction until the detected photoacoustic signal-to-noise ratio reaches its maximum value.

9. The knife-edge calibration method in the photoacoustic measurement process as described in claim 1, characterized in that, The noise of the standard sample is measured through the detection optical path and used as the system noise. A second noise threshold is set. If the system noise exceeds the second noise threshold, the blade calibration is stopped and the detection optical path is checked.

10. The knife-edge calibration method in the photoacoustic measurement process as described in claim 9, characterized in that, The first noise threshold is 10% of the photoacoustic signal amplitude; the second noise threshold is 50% of the acoustic signal amplitude.

Images