Mode locking system and mode locking method suitable for microcavity ultrasonic detection

Abstract

The invention discloses a mode locking system and a mode locking method suitable for microcavity ultrasonic detection. The system comprises a control module, a laser output module and a detection module, the laser output module comprises a single-frequency laser, a temperature adjusting unit, a temperature detection unit and a heat transfer unit, and the detection module is used for detecting a transmission spectral line of the microcavity ultrasonic probe and transmitting the transmission spectral line to the control module; the control module is used for controlling the temperature in the cavity of the single-frequency laser through the linear interpolation data, realizing linear regulation and scanning of the wavelength entering the cavity, and screening and locking the microcavity mode of the microcavity ultrasonic probe; the device is also used for detecting a locking site of a microcavity mode by adopting a temperature detection and light intensity voltage feedback design, and automatically adjusting the temperature in a mode drift process to adjust the wavelength so as to correct the mode drift of the microcavity. The functions of mode scanning, active mode selection and mode locking of the microcavity ultrasonic probe can be autonomously completed, and a stable, reliable, low-cost and high-sensitivity new scheme is provided for ultrasonic detection.

Description

Technical Field

[0001] Embodiments of the present invention relate to the technical field of ultrasonic detection, and in particular, to a mode-locking system and a mode-locking method applicable to microcavity ultrasonic detection. Background Art

[0002] Research on ultrasound has spread across various fields such as scientific research, medicine, industry, and the military. Among them, ultrasonic detection, as a non-contact, high-resolution, and highly sensitive non-destructive detection method, is of great significance for many application scenarios such as underwater target detection, semiconductor package testing, and non-destructive flaw detection of metal materials. Efficient, sensitive, and accurate ultrasonic detection technologies have now been widely applied in many interdisciplinary fields. It has been found that whispering-gallery mode (WGM) microcavities have good acoustic detection capabilities. Microcavity ultrasonic probes, as a type of ultra-high-sensitivity microprobe device, provide new possibilities for ultrasonic detection.

[0003] For a microcavity ultrasonic probe, the whispering-gallery mode (hereinafter referred to as the mode) is a specific standing wave structure formed by light in the cavity. The mode of the microcavity ultrasonic probe is closely related to the wavelength of the incident laser. By changing the wavelength of the laser entering the microcavity ultrasonic probe or adjusting the physical parameters of the cavity (such as temperature or external pressure), different modes can be selectively excited, and the Q values (quality factors) of different modes are different. In a laser resonator cavity, the Q value represents the ratio of the energy stored in the cavity to the energy lost per unit cycle. In ultrasonic detection, to obtain better detection sensitivity, it is beneficial to select a mode with a higher Q value.

[0004] For the same mode, when the microcavity ultrasonic probe works at different positions of the mode, its acoustic detection sensitivity will vary greatly. Therefore, the measurement sensitivity of the microcavity ultrasonic probe is closely related to its own working point. During complex and moving measurement processes, the microcavity ultrasonic probe is affected by external factors such as temperature and pressure, and it is difficult to work at the same mode position for a long time and cannot maintain stability. Currently, there is no small and complete integration system that can integrate the complex optical path system of the microcavity ultrasonic probe and maintain the mode locking of the microcavity ultrasonic probe.

[0005] The prior art has at least the following problems and defects:

[0006] (1) Microcavity ultrasonic probes usually have multiple modes, and the Q values of different modes are different, which leads to different response sensitivities of different modes to the same ultrasonic signal. The use of microcavity ultrasonic probes must perform mode scanning, scan the modes within a certain wavelength range, and perform screening. Generally speaking, the narrower the mode width, the higher the sensitivity to ultrasonic signals; however, the narrower the mode width, the less likely the microcavity ultrasonic probe is to be stable within this mode, which is not conducive to the detection of complex and variable ultrasonic signals outside the laboratory. Moreover, the mode scanning of microcavity ultrasonic probes relies on a tunable laser to change the laser wavelength entering the cavity, which increases the usage cost and actual volume of the entire system. Currently, there is no general algorithm for mode scanning of microcavity ultrasonic probes that can scan and screen suitable modes without using a tunable laser for practical applications.

[0007] (2) The mode of the microcavity ultrasonic probe is related to both the laser wavelength entering the cavity and the physical parameters of the microcavity itself. Existing microcavity ultrasonic probes usually select microcavities with high Q values to provide high-sensitivity ultrasonic detection, which makes it extremely sensitive to perturbations during the measurement process. When the physical parameters of the microcavity itself, such as temperature, pressure, etc., change due to the influence of the external environment, even if the laser wavelength entering the cavity does not change, the mode of the microcavity will change. Therefore, mode drift often occurs during the long-term use of microcavity ultrasonic probes, and users often need to manually control the mode stability, which increases the usage difficulty and complexity of microcavity ultrasonic probes.

[0008] Therefore, there is an urgent need for a sensitive and efficient mode locking algorithm and a highly integrated and highly stable measurement system supporting it to provide portable, durable, highly robust, and highly sensitive ultrasonic detection. Summary of the Invention

[0009] The present invention provides a mode locking system and a mode locking method suitable for microcavity ultrasonic detection, which can independently complete the functions of mode scanning, active mode selection, and mode locking of microcavity ultrasonic probes, and provides a new stable, reliable, low-cost, and high-sensitivity solution for ultrasonic detection.

[0010] This system is applicable to microcavity ultrasonic probes made of various materials, and replaces expensive tunable lasers with low-cost single-frequency lasers.

[0011] In the first aspect, the present invention provides a mode locking system suitable for microcavity ultrasonic detection, including a control module, a laser output module, and a detection module; the control module is respectively connected to the laser output module and the detection module; the laser output module includes:

[0012] A single-frequency laser for generating single-frequency laser; a microcavity ultrasonic probe is arranged at the output end of the single-frequency laser; a temperature detection unit for detecting the intracavity temperature of the single-frequency laser;

[0013] A temperature regulation unit, configured to compare the error value between the temperature inside the cavity and the given temperature output by the control module and output a temperature regulation instruction; a heat transfer unit, configured to receive the temperature regulation instruction and adjust the temperature inside the cavity of the single-frequency laser; wherein, the central wavelength of the single-frequency laser has a linear relationship with the temperature inside the cavity;

[0014] The detection module is configured to detect the transmission spectrum line of the microcavity ultrasonic probe and transmit it to the control module;

[0015] The control module is configured to control the temperature inside the cavity of the single-frequency laser through linear interpolation data, realize linear regulation and scanning of the wavelength entering the cavity, screen and lock the microcavity mode of the microcavity ultrasonic probe; and is also configured to adopt a temperature detection and light intensity voltage feedback design to detect the lock position point of the microcavity mode, automatically regulate the temperature during the mode drift process for wavelength adjustment, and realize the correction of the microcavity mode drift.

[0016] Optionally, the detection module includes:

[0017] A photodetector, configured to detect the transmission spectrum line of the microcavity ultrasonic probe and the mode drift amount and convert them into electrical signals; a high-pass filter, configured to receive the electrical signals and filter out low-frequency noise and baseline drift to obtain high-frequency signals; an acquisition card, configured to acquire and record the high-frequency signals and transmit them to the control module.

[0018] Optionally, it further includes a polarization controller, which is arranged at the output end of the single-frequency laser and is connected to the microcavity ultrasonic probe; and is configured to adjust and control the polarization characteristics of the single-frequency laser entering.

[0019] Optionally, the single-frequency laser and the microcavity ultrasonic probe are connected through an optical fiber, and the optical fiber includes a polarization-maintaining optical fiber.

[0020] In a second aspect, an embodiment of the present invention further provides a mode locking system applicable to microcavity ultrasonic detection, which performs microcavity ultrasonic detection mode locking by using the mode locking system applicable to microcavity ultrasonic detection provided by the first invention. The mode locking method is characterized in that it includes:

[0021] Controlling the temperature regulation unit to linearly regulate the central wavelength of the single-frequency laser output by the single-frequency laser according to the linear relationship between the temperature inside the cavity and the central wavelength, and perform mode scanning on the microcavity ultrasonic probe;

[0022] Obtaining the transmission spectrum lines corresponding to each scanning wavelength, calculating the Q values corresponding to each transmission spectrum line, screening the high-Q value modes of the single-frequency laser, and using the high-Q value modes as the optimal modes of the microcavity ultrasonic probe;

[0023] The control temperature regulation unit regulates the single-frequency laser to operate in the high-Q mode, and locks the microcavity ultrasonic probe at the midpoint of the hypotenuse of the optimal mode;

[0024] Obtain the transmission spectrum line in the optimal mode, obtain the mode drift amount of the microcavity ultrasonic probe based on the transmission spectrum line, and based on the linear relationship between the intracavity temperature and the central wavelength, adjust the central wavelength of the single-frequency laser to compensate for the mode drift when the microcavity ultrasonic probe works.

[0025] Optionally, when performing a mode scan on the microcavity ultrasonic probe, the scan methods include a wide temperature scan and a narrow temperature scan;

[0026] First, control the temperature regulation unit to perform a wide temperature scan to obtain the wide-scan temperature of the microcavity ultrasonic probe in the optimal mode; based on the wide-scan temperature as a reference, control the temperature regulation unit to perform a narrow temperature scan to obtain the narrow-scan temperature of the microcavity ultrasonic probe in the optimal mode; use the high-Q mode corresponding to the narrow-scan temperature as the optimal mode of the microcavity ultrasonic probe; wherein, the wide-scan temperature and the narrow-scan temperature are determined by the Q value of the transmission spectrum line.

[0027] Optionally, when performing a mode scan on the microcavity ultrasonic probe, the control module controls the temperature regulation unit to perform a wide temperature scan and a narrow temperature scan respectively along the temperature increasing direction and the temperature decreasing direction, and screen the Q values corresponding to the obtained transmission spectrum lines.

[0028] Optionally, when screening the high-Q mode of the single-frequency laser, according to the mode thermal broadening phenomenon, when there are multiple high-Q modes and corresponding multiple spectral lines, select the dual side of the single side with the largest mode slope k in the spectral curve as the optimal mode of the microcavity ultrasonic probe; wherein, the dual side is determined by the symmetry of the transmission spectrum line.

[0029] Optionally, when screening the high-Q mode of the single-frequency laser, the control module presets the rated depth and rated width of the spectral line, and screens the spectral curves with a depth greater than the rated depth and a width less than the rated width.

[0030] Optionally, obtaining the mode drift amount of the microcavity ultrasonic probe based on the transmission spectrum line and based on the linear relationship between the intracavity temperature and the central wavelength, adjusting the central wavelength of the single-frequency laser, adjusting the central wavelength of the single-frequency laser, specifically including:

[0031] Obtain the mode slope k, the locking voltage U and the actual voltage U' of the transmission spectrum line in the optimal mode, and calculate the mode drift amount ΔU and its corresponding temperature adjustment amount d according to the first formula;

[0032] Wherein, the first formula satisfies: ΔU = U′ - U, d = (U′ - U) / k;

[0033] Judge that ΔU > 0 and k > 0; or, ΔU < 0 and k < 0, and control the temperature adjustment unit to increase the intracavity temperature of the single-frequency laser according to the temperature adjustment amount to adjust the central wavelength of the single-frequency laser;

[0034] Judge that ΔU < 0 and k > 0; or, ΔU > 0 and k < 0, and control the temperature adjustment unit to decrease the intracavity temperature of the single-frequency laser according to the temperature adjustment amount to adjust the central wavelength of the single-frequency laser.

[0035] In summary, the mode-locking system applicable to microcavity ultrasonic detection provided by the present invention, the control module controls the temperature adjustment unit to adjust the intracavity temperature of the single-frequency laser through linear interpolation data, so as to realize the linear regulation and uniform scanning of the incident wavelength, thereby realizing the detailed scanning and accurate reproduction of the microcavity mode, and can automatically screen out the most suitable mode for ultrasonic detection through the program, further calculate the ultrasonic signal based on the detected effective data, receive the mode drift of the microcavity ultrasonic probe, judge the compensation method of the mode, calculate the temperature change amount required to compensate the mode drift, and transmit it to the temperature adjustment unit for mode drift correction, realizing the mode scanning of the microcavity ultrasonic probe, and can actively screen out the best mode for easy use, without using expensive tunable lasers, reducing the system cost. Brief Description of the Drawings

[0036] Figure 1 is a schematic diagram of a mode-locking system applicable to microcavity ultrasonic detection provided by the present invention;

[0037] Figure 2 is a test diagram of the ultrasonic signal strength of the microcavity ultrasonic probe provided by the present invention at different working points;

[0038] Figure 3 is a linear relationship diagram of the intracavity temperature - central wavelength of the single-frequency laser provided by the present invention;

[0039] Figure 4 is a mode result diagram of the microcavity ultrasonic probe scanned by the temperature change of the single-frequency laser;

[0040] Figure 5 is a test diagram of the intracavity temperature control effect of the single-frequency laser provided by the present invention;

[0041] Figure 6 is a schematic diagram of a mode-locking method applicable to microcavity ultrasonic detection provided by the present invention;

[0042] Figure 7 is a test diagram of the temperature scanning result of the mode-locking system provided by the present invention.

[0043] Figure 8 is a schematic diagram of the mode thermal broadening of the microcavity ultrasonic probe of the mode-locking system;

[0044] Figure 9 It is a flowchart of a mode locking method suitable for microcavity ultrasonic detection provided by the present invention. Specific embodiments

[0045] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It can be understood that the specific embodiments described herein are only used to explain the present invention, rather than limiting the present invention. In addition, it should be noted that for the convenience of description, only the parts related to the present invention are shown in the drawings, rather than all the structures.

[0046] In view of one or more of the above problems existing in the prior art, the present invention proposes a mode locking system suitable for microcavity ultrasonic detection. Figure 1 It is a schematic diagram of a mode locking system suitable for microcavity ultrasonic detection provided by the present invention. Refer to Figure 1 , the mode locking system suitable for microcavity ultrasonic detection provided by the embodiments of the present invention includes a control module 10, a laser output module 20 and a detection module 30; the control module 10 is respectively connected to the laser output module 20 and the detection module 30. The control module 10 includes at least one processor, and the processor can be a microprocessor, such as a central processing unit (CPU), or other general-purpose processors, digital signal processors (DSPs), application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), or other programmable logic devices, transistor logic devices, hardware components, or any combination thereof. The control module 10 is built-in with an independently written intelligent program, which can receive the drift signal of the mode, automatically judge the compensation method of the mode, calculate the temperature change required to compensate for the mode drift, and transmit it to the temperature adjustment unit 23 to actively correct the drift. Specifically, the control module 10 controls the intracavity temperature of the single-frequency laser through linear interpolation data, realizes linear regulation and scanning of the incident wavelength, screens and locks the microcavity mode of the microcavity ultrasonic probe; also adopts temperature detection and light intensity voltage feedback design to detect the lock position of the microcavity mode, and automatically adjusts the temperature to perform wavelength adjustment during the mode drift process, so as to correct the microcavity mode drift and lock the mode.

[0047] Specifically, the laser output module 20 includes:

[0048] The single-frequency laser 21 is used to generate single-frequency laser. The present invention adopts a low-cost single-frequency laser 21 as the light source of the microcavity ultrasonic probe. The single-frequency laser 21 can be temperature-tuned through a hardware module, such as a single-frequency laser 21 with a laser center wavelength of 1064 nm or 1550 nm.

[0049] The microcavity ultrasonic probe 40 is arranged at the output end of the single-frequency laser 21. Among them, the output mode of the microcavity ultrasonic probe 40 changes with the change of the center wavelength of the single-frequency laser output by the single-frequency laser 21. Exemplarily, it is specifically manifested as the change of the shape and peak position of the transmission spectrum line.

[0050] When the ultrasonic signal is transmitted to the vicinity of the microcavity ultrasonic probe 40, the internal coupling state of the microcavity ultrasonic probe 40 changes, resulting in a change in the output mode, which can reflect the intensity and frequency of the ultrasonic signal. In the ultrasonic detection with the microcavity ultrasonic probe as the main body, for the same signal (that is, the intensity and frequency of the external ultrasonic signal input to the microcavity ultrasonic probe 40 are the same), when the microcavity ultrasonic probe works at different positions of the mode, the measured ultrasonic response amplitude is not the same, as Figure 2 shown. It can be seen that when the microcavity ultrasonic probe stably works at the midpoint of the hypotenuse of the mode curve (that is, Figure 2 (point C in (c)), corresponding to Figure 2 (g), the ultrasonic response to the same signal is the strongest.

[0051] Among them, Figure 2 (a) to Figure 2 (d) are the transmission spectrum lines of the microcavity ultrasonic probe locked at points A, B, C, and D of the microcavity mode respectively, Figure 2 (e) corresponds to the ultrasonic signal intensity at point A, Figure 2 (f) corresponds to the ultrasonic signal intensity at point B, Figure 2 (g) corresponds to the ultrasonic signal intensity at point C, Figure 2 (h) corresponds to the ultrasonic signal intensity at point D.

[0052] The temperature detection unit 22 is used to detect the intracavity temperature of the single-frequency laser 21 and feed it back to the temperature adjustment unit 23. The temperature detection unit 22 detects the intracavity temperature of the single-frequency laser 21 in real time. The temperature adjustment unit 23 is used to compare the error value between the actual intracavity temperature in the resonator of the single-frequency laser 21 and the given temperature output by the control module 10, and output a temperature adjustment instruction. Exemplarily, the error value is calculated by ΔT = T 实际 -T 给定 . Among them, the judgment basis given by the temperature adjustment instruction is:

[0053] When the intracavity temperature of the resonator > the given temperature of the control module 10, the temperature adjustment unit 23 will control the heat transfer unit 24 to lower the intracavity temperature of the resonator of the single-frequency laser 21.

[0054] When the temperature inside the resonant cavity is < the given temperature of the control module 10, the temperature regulation unit 23 will increase the temperature inside the resonant cavity of the single-frequency laser 21 by adjusting the heat transfer unit 24.

[0055] Among them, the temperature adjustment instruction is transmitted to the heat transfer unit 24 through the temperature regulation unit 23, directly regulating the magnitude and direction of the current of the heat transfer unit 24 to control the temperature of the laser.

[0056] Among them, the heat transfer unit 24 can be composed of a sensitive thermal conductive sticker and a thermoelectric cooler (TEC). When an electric current passes through, the heat transfer unit 24 can cool or heat the single-frequency laser 21. Specifically, the heat transfer unit 24 can be closely attached to the lower part of the resonant cavity of the single-frequency laser 21. The heat transfer unit 24 receives the temperature adjustment instruction and adjusts the temperature inside the resonant cavity of the single-frequency laser 21 through the thermoelectric cooler (TEC). The TEC realizes heating or cooling according to the direction of the current.

[0057] The central wavelength of the single-frequency laser output by the single-frequency laser 21 provided in the embodiment of the present invention has a linear relationship with the temperature inside the cavity. Specifically, Figure 3 is a linear relationship diagram of the temperature inside the cavity - central wavelength of the single-frequency laser provided by the present invention, where Figure 3 the abscissa is the scanned temperature, and the ordinate is the central wavelength of the single-frequency laser output by the single-frequency laser 21. Referring to Figure 3 , when the temperature inside the resonant cavity of the single-frequency laser 21 changes, the cavity length of the resonant cavity changes slightly, and the cavity length of the internal resonant cavity expands or contracts accordingly, thereby changing the output wavelength of the laser to achieve the effect of temperature tuning. This system finely tunes the heat transfer unit 24 through the linear interpolation result of the laser wavelength - temperature to achieve uniform scanning and linear regulation of the central wavelength of the single-frequency laser output by the single-frequency laser 21.

[0058] Specifically, the central wavelength of the single-frequency laser output by the single-frequency laser 21 increases with the increase of temperature. This is because the increase in temperature causes the cavity length of the resonant cavity to expand, thereby changing the resonance condition and causing the output wavelength to redshift. The embodiment of the present invention performs linear fitting on this change through linear interpolation, can confirm the linear relationship between the central wavelength of the single-frequency laser output by the single-frequency laser 21 and the temperature inside the cavity of the laser, and generates a set of corresponding data tables of wavelength - temperature for fine temperature tuning. In the present invention, referring to Figure 3 , the temperature inside the cavity - wavelength of the single-frequency laser 21 has a linear relationship, and the goodness of fit R 2 = 0.99. Thus, it further proves that the change of the output wavelength of the single-frequency laser 21 with temperature is uniform and linear.

[0059] In terms of the temperature control accuracy, the system uses a PID (Proportional-Integral-Derivative) control algorithm to precisely control the temperature of the single-frequency laser 21. By adjusting the proportional, integral, and derivative parameters, the temperature tuning accuracy is achieved to be above 0.01 °C / s, and the wavelength adjustment interval is less than 10 pm. By tuning the temperature of the single-frequency laser 21, the central wavelength of the single-frequency laser output by the single-frequency laser 21 can be changed, thereby exciting different modes of the microcavity ultrasonic probe 40 to achieve mode scanning.

[0060] Furthermore, referring to Figure 1 , the detection module 30 is used to detect the transmission spectrum line of the microcavity ultrasonic probe 40 and transmit it to the control module 10. Specifically, Figure 4 is a mode result diagram of the microcavity ultrasonic probe obtained by scanning with the change of the single-frequency laser temperature. Among them, Figure 4 the abscissa in Figure 4 is the scanning time, and the ordinate is the transmission spectrum line voltage. Referring to Figure 4 , the temperature scanning range is 12 °C to 33 °C, and the entire scanning time is 0 to 20 s. The mode of the microcavity ultrasonic probe 40 can be scanned in detail by changing the temperature of the single-frequency laser 21. The mode-locking program autonomously selects the best mode by comparing the Q value and slope of the transmission spectrum line. After the mode scanning is completed, the system stores the mode scanning signal and transfers it to the mode selection program for selection.

[0061] Based on the above embodiments, continue to refer to Figure 1, the detection module 30 includes a photodetector 31, a high-pass filter 32, and an acquisition card 33. The photodetector 31 is used to detect the transmission spectrum line and the mode drift amount of the microcavity ultrasonic probe 40 and convert them into electrical signals. Specifically, in the embodiment of the present invention, the photodetector 31 is composed of a photodiode. When designing and manufacturing the photodiode, a PN junction with a larger area is used to receive the incident light. Among them, under the reverse voltage working condition, when there is no light, the reverse current of the photodiode is extremely weak, which is called the dark current; when there is light, the reverse current rapidly increases to dozens of microamperes, which is called the photocurrent. The greater the intensity of the light, the greater the reverse current. The change of light causes the change of the current of the photodiode, so as to realize the conversion of the optical signal into an electrical signal. In this system, the output of the photodetector 31 is divided into two paths. The unfiltered part is used to monitor the drift of the mode baseline of the microcavity ultrasonic probe 40, and the part passing through the high-pass filter 32 is used to extract the effective ultrasonic signal to enhance the signal-to-noise ratio of the signal. The high-pass filter 32 is used to receive the electrical signal, filter out the low-frequency noise and baseline drift, and obtain the high-frequency signal (i.e., the ultrasonic signal). In this system, the selected frequency band of the high-pass filter 32 covers the ultrasonic frequency band, so that the ultrasonic signal can be effectively enhanced and the static mode baseline can be suppressed. The acquisition card 33 is used to collect and record the high-frequency signal and transmit it to the control module 10. Specifically, the acquisition card 33 can collect the effective data in the high-pass filter 32, and the control module 10 calculates the effective data to obtain the ultrasonic signal.

[0062] Figure 5 It is a test diagram of the intracavity temperature regulation effect of the single-frequency laser provided by the present invention. Among them, Figure 5 the abscissa is the scanning time, and the ordinate is the scanning temperature. Refer to Figure 5 , when the heat transfer unit 24 receives the temperature regulation instruction, the heat transfer unit 24 controls the temperature of the single-frequency laser 21 to rise uniformly, and maintains a uniform rise or fall during each temperature fine-tuning. When it is necessary to maintain at a specific temperature, the temperature fluctuation of the heat transfer unit 24 is less than 1 mK.

[0063] During the mode locking process, the mode drift of the microcavity ultrasonic probe 40 is detected by the photodetector 31, and the mode drift is converted into an electrical signal. The acquisition card 33 collects the electrical signal, and the control module 10 automatically identifies whether there is a mode drift phenomenon through the electrical signal feedback in real time by the acquisition card 33, and issues an instruction to the temperature regulation unit 23 to change the output wavelength by dynamically tuning the intracavity temperature of the resonator of the single-frequency laser 21, so as to reversely adjust the mode of the microcavity ultrasonic probe 40 and compensate for the mode drift.

[0064] Through the program design of temperature regulation and light intensity voltage feedback, the present invention realizes the mode scanning of the microcavity through the single-frequency laser 21, automatically searches for the optimal mode curve, and after finding the optimal mode, can automatically lock the microcavity ultrasonic probe 40 at the midpoint of the hypotenuse of the optimal mode. Based on the software and hardware design provided by this solution, highly sensitive measurement of ultrasonic signals can be achieved.

[0065] Among them, the mode locking system provided by the embodiments of the present invention is applicable to microcavity ultrasonic probes of various materials. It replaces expensive tunable lasers with low-cost single-frequency lasers, and linear tuning of temperature and wavelength can be achieved through hardware module control. This system can independently complete the functions of mode scanning, active mode selection, and mode locking of the microcavity ultrasonic probe, providing a new solution for ultrasonic detection that is stable, reliable, low-cost, and highly sensitive.

[0066] It should be noted that the system provided by the embodiments of the present invention is adapted to microcavity ultrasonic probes 40 of various materials, such as including but not limited to materials such as polymers, polymer-silica double shells, etc. The microcavity ultrasonic probe 40 in the system can be independently replaced with microcavity ultrasonic probes 40 of other materials.

[0067] On the basis of the above embodiments, continue to refer to Figure 1 , the mode locking system further includes a polarization controller 50, which is arranged at the output end of the single-frequency laser 21 and is connected to the microcavity ultrasonic probe 40; it is used to adjust and control the polarization characteristics of the single-frequency laser entering the microcavity ultrasonic probe 40. Specifically, the polarization controller 50 adjusts the single-frequency laser output by the single-frequency laser 21 to linear polarization, improving the detection accuracy of laser scanning.

[0068] On the basis of the above embodiments, continue to refer to Figure 1 , the single-frequency laser 21 and the microcavity ultrasonic probe 40 are connected by an optical fiber, and the optical fiber includes a polarization-maintaining optical fiber. Specifically, the optical fiber in the system can be changed to a polarization-maintaining optical fiber so that the polarization of the optical path remains unchanged, replacing the polarization controller 50. The optical fiber connector can be a common (commercial) standard interface, such as FC / APC, ST / UPC, etc. Among them, the optical fiber matches the laser wavelength of the single-frequency laser, such as it can transmit lasers with wavelengths of 1064nm and 1550nm.

[0069] Based on the same inventive concept, the embodiments of the present invention provide a mode locking system applicable to microcavity ultrasonic detection, and use the mode locking system applicable to microcavity ultrasonic detection provided by the above embodiments for mode locking of microcavity ultrasonic detection. The following details the method for mode locking of microcavity ultrasonic detection by applying the mode locking system applicable to microcavity ultrasonic detection provided by the embodiments of the present invention. Figure 6 It is a schematic diagram of a mode locking method applicable to microcavity ultrasonic detection provided by the present invention. Refer to Figures 1 - 6, the mode locking method includes:

[0070] S101. The control module controls the temperature adjustment unit to linearly adjust the central wavelength output by the single-frequency laser according to the linear relationship between the intracavity temperature and the central wavelength, and performs mode scanning on the microcavity ultrasonic probe.

[0071] Specifically, when the control module 10 starts to run, each module is powered on in sequence. The control module 10 queries whether each module is working properly and initializes each module.

[0072] Exemplarily, the control module 10 controls the temperature adjustment unit 23 to adjust the temperature scanning direction. For example, by default, it is in the reverse direction (i.e., the temperature decreasing direction), and a large-range temperature wide scan is performed. According to the linear relationship between the intracavity temperature and the central wavelength, the scanning temperature is linearly adjusted so that the central wavelength of the single-frequency laser output by the single-frequency laser 21 (hereinafter referred to as the scanning wavelength) performs a uniform scan within a wide range. The temperature scanning range is defaulted to 20°C, the scanning speed is 1°C / s, and the wavelength tuning range is about 2 nm. The temperature wide scan range or the sampling time of the scan can also be modified according to actual needs. Among them, the wavelength-temperature data table provided by linear interpolation and the temperature control accuracy of the temperature adjustment unit 23 jointly ensure the accuracy and uniformity of the temperature scan.

[0073] S102. The control module obtains the transmission spectra corresponding to each scanning wavelength, calculates the Q values corresponding to each transmission spectrum, screens the high-Q value modes of the single-frequency laser, and uses the high-Q value mode as the optimal mode of the microcavity ultrasonic probe.

[0074] Specifically, during the scanning process, the photodetector 31 detects the transmission spectrum of the microcavity ultrasonic probe 40 and converts it into an electrical signal. The high-pass filter 32 receives the electrical signal and filters out low-frequency noise and baseline drift to obtain a high-frequency signal (ultrasonic signal). The acquisition card 33 acquires and records the high-frequency signal and transmits it to the control module 10 to establish a database of each scanning temperature, scanning wavelength, and transmission spectrum.

[0075] Among them, the sensitivity of the microcavity ultrasonic probe 40 is related to the Q value (quality factor). A high Q value means that the energy loss of the cavity is lower, and the photons stay in the cavity for a longer time, thereby enhancing the interaction with the environment or the sample. Therefore, the higher the Q value, the higher the sensitivity of the microcavity ultrasonic probe 40. In the embodiment of the present invention, when selecting the mode of the microcavity ultrasonic probe 40, the mode with a high Q value is used as the optimal mode.

[0076] Specifically, the Q value of the microcavity ultrasonic probe 40 is calculated by the following formula:

[0077] Q = λ / Δλ.

[0078] Wherein, λ is the resonant wavelength of the mode, i.e., the scanning wavelength, and Δλ is the full width at half maximum of the mode. It can be understood that when the overall change in the resonant wavelength is not significant, the deeper the mode depth and the narrower the width, the smaller the full width at half maximum, and the higher the corresponding Q value. Therefore, in the embodiments of the present invention, through the mode selection program, high-Q value modes with large depth and narrow width are preferentially screened. Optionally, when screening high-Q value modes of single-frequency lasers, the control module 10 pre-sets the rated depth and rated width of the spectral line, and screens the spectral curves with a depth greater than the rated depth and a width greater than the rated width.

[0079] It should be noted that the Q value of the mode is not the higher the better. High-Q value modes may have the problem of violent changes and it is difficult to maintain stability in a complex external environment. In the actual application of ultrasonic detection in the present invention, it is only necessary to set the Q value of the selected mode to be higher than the fifth power to meet the measurement requirements. Based on this, a mode selection program is set in the control module 10 of the present invention to set the lower limit of the width of the mode. Only modes with a width higher than the set value can be locked through the program, and modes with a width lower than the set value are not conducive to long-term stability and are usually not adopted. Among them, the setting amounts of the rated depth and rated width of the transmission spectral line can be reasonably set according to actual needs, or the best dynamic values can be obtained according to the mode scanning results.

[0080] S103. The control module controls the temperature adjustment unit to adjust the single-frequency laser to operate in a high-Q value mode, and locks the microcavity ultrasonic probe at the midpoint of the hypotenuse of the best mode.

[0081] Specifically, after the best mode is confirmed without error, the control module 10 sets a central point of the best mode, which is usually located at the midpoint of the hypotenuse of one side of the mode (point C in Figure 2 (c)), and calculates the cavity temperature of the single-frequency laser 21 corresponding to the central point according to the linear interpolation result of wavelength-temperature. Refer to Figure 1 . Subsequently, the control module 10 issues a mode locking instruction, controls the temperature adjustment unit 23 to start the tuning program to issue a temperature adjustment instruction for temperature scanning, and controls the heat transfer unit 24 to adjust the temperature of the single-frequency laser 21 to this temperature, so that the working point of the microcavity ultrasonic probe 40 enters the best mode. After the working point of the microcavity ultrasonic probe 40 enters the best mode, the system records the transmission spectral line of the microcavity ultrasonic probe 40 at this time, and records the current voltage value, which is set as the locked voltage U. At the same time, the mode slope k of the transmission spectral line in this locked mode is recorded. The two constants, the locked voltage U and the mode slope k, will be important references for subsequent locking.

[0082] S104. The control module obtains the transmission spectral line in the best mode, obtains the mode drift amount of the microcavity ultrasonic probe based on the transmission spectral line, and adjusts the central wavelength of the single-frequency laser based on the linear relationship between the cavity temperature and the central wavelength to compensate for the mode drift during the operation of the microcavity ultrasonic probe.

[0083] Specifically, after the microcavity ultrasonic probe 40 is locked in the optimal mode, the acquisition card 33 starts to record the actual voltage U' of the current transmission spectrum line of the microcavity ultrasonic probe 40 at fixed intervals, and compares the actual voltage U' with the locked voltage U. The maximum allowable error between the two is set to 0.8% of the mode depth. Among them, the transmission spectrum line of the microcavity ultrasonic probe 40 can reflect the mode drift of the microcavity ultrasonic probe 40 in real time. When the mode is stable, the transmission spectrum line of the microcavity ultrasonic probe 40 should remain stable accordingly. When the voltage error value is higher than 0.8% of the mode depth, the system corrects the mode drift and tunes the temperature of the single-frequency laser 21 to make the microcavity ultrasonic probe 40 stable in the optimal mode.

[0084] Optionally, in step S104, based on the transmission spectrum line to obtain the mode drift amount of the microcavity ultrasonic probe and based on the linear relationship between the intracavity temperature and the central wavelength, adjust the central wavelength of the single-frequency laser, specifically including:

[0085] Step S11: The control module obtains the mode slope k, the locked voltage U, and the actual voltage U' of the transmission spectrum line in the optimal mode, and calculates the mode drift amount ΔU and its corresponding temperature adjustment amount d according to the first formula.

[0086] Among them, the first formula satisfies: ΔU = U′ - U, d = (U′ - U) / k.

[0087] Step S12: The control module determines that ΔU > 0 and k > 0; or, ΔU < 0 and k < 0, and controls the temperature adjustment unit to increase the intracavity temperature of the single-frequency laser according to the temperature adjustment amount to adjust the central wavelength of the single-frequency laser.

[0088] Specifically, when d > 0, it means that the actual voltage U' > the locking voltage U, and the mode slope k > 0; or the actual voltage U' < the locking voltage U, and k < 0. In both cases, it is necessary to control the laser wavelength to actively follow the mode drift amount so that the operating point of the microcavity ultrasonic probe 40 returns to the locked position. At this time, the system adjusts the intracavity temperature of the single-frequency laser 21 resonator in the positive direction, with a temperature step of 0.002 °C, and it can also be adjusted according to actual needs to ensure the speed of temperature tuning. Since the relationship between wavelength and temperature is linear, as the output wavelength of the single-frequency laser 21 increases, the operating point of the microcavity ultrasonic probe 40 moves upward, and the mode drift is corrected. After the first correction, the system compares the actual voltage U' of the transmission spectrum of the microcavity ultrasonic probe 40 with the locking voltage U again to calculate the difference. If the difference ΔU is still greater than the maximum tolerable error, the temperature adjustment operation is cycled. The temperature detection unit 22 detects the intracavity temperature of the single-frequency laser 21 resonator in real time and transmits it to the temperature adjustment unit 23, and the acquisition card 33 records the actual voltage U' of the current transmission spectrum of the microcavity ultrasonic probe 40 at fixed intervals for tuning feedback until the difference ΔU between the actual voltage U' and the locking voltage U is less than the maximum tolerable error.

[0089] Step S13: The control module determines that ΔU < 0 and k > 0; or, ΔU > 0 and k < 0, and controls the temperature adjustment unit to lower the intracavity temperature of the single-frequency laser 21 according to the temperature adjustment amount to adjust the central wavelength of the single-frequency laser.

[0090] Specifically, when d < 0, it means that the actual voltage U' < the locking voltage U, and the mode slope k > 0; or the actual voltage U' > the locking voltage U, and k < 0. At this time, the system adjusts the intracavity temperature of the single-frequency laser 21 resonator in the reverse direction, with a temperature step of 0.002 °C. The intracavity temperature of the single-frequency laser 21 decreases, the output wavelength of the single-frequency laser 21 decreases, the operating point of the microcavity ultrasonic probe 40 moves downward, and the mode drift is corrected. After that, the system compares the actual voltage U' of the transmission spectrum of the microcavity ultrasonic probe 40 with the locking voltage U again to calculate the difference. If the difference ΔU is still greater than the maximum tolerable error, the temperature adjustment operation is cycled. The temperature detection unit 22 detects the intracavity temperature of the single-frequency laser 21 resonator in real time and transmits it to the temperature adjustment unit 23, and the acquisition card 33 records the actual voltage U' of the current transmission spectrum of the microcavity ultrasonic probe 40 at fixed intervals for tuning feedback until the difference ΔU between the actual voltage U' and the locking voltage U is less than the maximum tolerable error. The mode locking program runs in a loop until it is manually turned off.

[0091] Based on the above embodiments, in step S101, when performing a mode scan on the microcavity ultrasonic probe, the scanning methods include temperature wide scan and temperature narrow scan. The control module first controls the temperature adjustment unit to perform a temperature wide scan to obtain the wide scan temperature of the microcavity ultrasonic probe in the optimal mode; based on the wide scan temperature, the control module controls the temperature adjustment unit to perform a temperature narrow scan to obtain the narrow scan temperature of the microcavity ultrasonic probe in the optimal mode; the high-Q value mode corresponding to the narrow scan temperature is used as the optimal mode of the microcavity ultrasonic probe. Among them, the wide scan temperature and the narrow scan temperature are determined by the Q value of the transmission spectrum line.

[0092] Figure 7 is a test chart of the temperature scan result of the mode locking system provided by the present invention, where Figure 7 (a) shows the mode result chart of the wide scan of the microcavity ultrasonic probe, Figure 7 (b) shows the mode result chart of the narrow scan of the microcavity ultrasonic probe, Figure 7 (c) shows the mode locking chart of the wide scan of the microcavity ultrasonic probe. Refer to Figures 1 - 7 shown, specifically:

[0093] First, the control module 10 controls the temperature adjustment unit 23 to perform a temperature wide scan in sequence.

[0094] Step S21: The control module 10 controls the temperature adjustment unit 23 to output a first temperature adjustment instruction according to the first temperature range, and the heat transfer unit 24 linearly adjusts the cavity temperature of the single-frequency laser 21 based on the first temperature adjustment instruction to output the single-frequency laser corresponding to each first scan temperature.

[0095] Exemplarily, the first temperature range can be 12°C to 33°C, the scanning speed is 1°C / s, and the wavelength tuning range is about 2 nm. The control module 10 controls the temperature adjustment unit 23 to adjust the temperature scanning direction, which is defaulted to reverse (cooling), to perform a large-range temperature wide scan, so that the output wavelength of the single-frequency laser 21 performs a uniform scan within a wide range. The range is defaulted to 20°C, the scanning speed is 1°C / s, and the wavelength tuning range is about 2 nm. The temperature wide scan range or the sampling time of the scan can also be modified according to actual needs. The wavelength-temperature data table provided by linear interpolation and the temperature control accuracy of the temperature adjustment unit 23 jointly ensure the accuracy and uniformity of the temperature scan, as Figure 7 (a) shown.

[0096] It should be noted that when the system adjusts the temperature of the single-frequency laser 21 to near the optimal mode, the adjustment direction needs to be the same as the scanning direction corresponding to the optimal mode. For example, if the optimal mode is obtained during the temperature reverse scan, the system will first quickly raise the temperature and then slowly lower the temperature to near the temperature corresponding to the optimal mode to ensure that the microcavity ultrasonic probe 40 enters the mode during the temperature reverse scan.

[0097] Step S22: The control module 10 acquires the first transmission spectra corresponding to each first scanning wavelength, calculates the first Q values corresponding to the first transmission spectra, screens the first high-Q value mode of the single-frequency laser, and obtains the first high-Q value scanning temperature corresponding to the first high-Q value mode.

[0098] Further, the control module 10 controls the temperature adjustment unit 23 to perform a narrow temperature scan, including:

[0099] Step S23: The control module 10 controls the temperature adjustment unit 23 to output a second temperature adjustment command according to a second temperature range. The heat transfer unit 24 linearly adjusts the intracavity temperature of the single-frequency laser 21 based on the second temperature adjustment command, and outputs the single-frequency laser corresponding to each second scanning temperature.

[0100] Among them, the first high-Q value scanning temperature is within the second temperature range, the second temperature range is within the first temperature range, and the number of second scanning temperatures is greater than the number of first scanning temperatures.

[0101] Exemplarily, the second temperature range is 20°C to 25°C, the scanning speed is 0.1°C / s, and the wavelength tuning range is about 0.2 nm. The control module 10 narrows the tuning range, controls the temperature adjustment unit 23 to adjust the temperature scanning direction, which is defaulted to reverse (cooling), and performs a narrow temperature scan in a small range, so that the output wavelength of the single-frequency laser 21 performs a uniform scan within a narrow range, with the range defaulted to 5°C and the scanning speed being 0.1°C / s, and the wavelength tuning range is about 0.2 nm. The scanning range of the narrow temperature scan or the sampling time of the scan can also be modified according to actual requirements. The wavelength-temperature data table provided by linear interpolation and the temperature control accuracy of the temperature adjustment unit 23 jointly ensure the accuracy and uniformity of the narrow temperature scan, as shown in Figure 7 (b).

[0102] Step S24: Acquire the second transmission spectra corresponding to each second scanning wavelength, calculate the second Q values corresponding to the second transmission spectra, screen the second high-Q value mode of the single-frequency laser, and use the second high-Q value mode as the best mode of the microcavity ultrasonic probe 40.

[0103] It should be noted that the temperature scan is performed twice to further confirm the specific position of the best mode by increasing the sampling data, and at the same time, as much as possible to avoid the mode drift that may occur during the two-way scan and mode screening of the system. After the narrow temperature scan, the system compares the data of the two scans, accurately locates the position of the best mode, and calculates the depth and width of the best mode, and finally locks the microcavity ultrasonic probe 40 in the best mode, as shown in Figure 7 (c).

[0104] Based on the above embodiments, during the wide temperature scan, the thermal broadening phenomenon of the mode needs to be considered. Among them, thermal broadening is a thermal nonlinear effect in the microcavity. The positive accumulation of the thermal nonlinear effect is jointly achieved by factors such as high Q value, small mode volume, miniature cavity size, and temperature-sensitive polymer cavity material, which leads to the bistable effect of the mode of the microcavity ultrasonic probe 40. Specifically, when the laser scans forward (heating up) and backward (cooling down), the high-Q mode will exhibit broadening or compression phenomena.

[0105] Figure 8 It is a schematic diagram of the thermal broadening of the mode of the microcavity ultrasonic probe of the mode locking system. Refer to Figure 8 , since thermal broadening is essentially the forward and backward scans of the same single mode, thermal broadening does not change the Q value of the mode, but will change the line shapes of the forward and backward scans of the mode. Specifically, it is manifested as the narrowing of one side of the mode and the broadening of the other side. Among them, Figure 8 (a) is the mode in the normal state, Figure 8 (b) is the same mode that generates thermal broadening. It can be seen that there is a single-sided slope of the mode (hereinafter simply referred to as the mode single side) in the broadened mode. Without changing the Q value of the mode, the mode slope k of the mode single side is reduced, so it is more convenient for the system to lock and can be one of the preferred modes. In addition, since the direction of the thermal broadening of the mode is related to the material used in the microcavity and is not completely fixed, optionally, when performing a mode scan on the microcavity ultrasonic probe 40, the control module 10 controls the temperature adjustment unit 23 to perform temperature narrow scans along the temperature increase direction and the temperature decrease direction respectively, and screen out all Q values corresponding to the transmission spectral lines. It can also be understood that the mode locking system provided by the embodiment of the present invention performs two wavelength scans in the forward heating and reverse cooling directions during the temperature scan to find the best mode line shape.

[0106] In the above step S102, the mode Q value is calculated. After the wide temperature scan is completed, the system records the scan data, stores the voltage values of the transmission spectral lines of the microcavity ultrasonic probe 40 at each temperature, and calculates the Q values of all modes within the scan range to screen the best mode. During the data processing process, the calculation of the mode Q value is simplified to the calculation of the depth U, width Δλ, and mode slope k of the mode single side.

[0107] Among them, the depth of the mode single side can be directly obtained from the voltage value of the transmission spectral line. By traversing the maximum and minimum values of the data through the peak search function, the minimum value obtained is the valley point of each mode, and all modes can be screened out therefrom.

[0108] The mode width is defined as the half-width Δλ of the mode side. Based on the linear relationship between wavelength and temperature, it can be represented by the temperature of the single-frequency laser 21. The mode with a temperature range of 0.05°C corresponding to the half-width means scanning the entire range of the mode half-width. The temperature of the single-frequency laser 21 needs to be adjusted by 0.05°C. The mode slope k of the mode side is obtained by the mode depth / full width of the mode side, that is, k = U / 2Δλ. Since thermal broadening will cause the shapes of the oblique edges on both sides of the mode to be different, when screening the best mode, the two sides of the same mode are regarded as different parts, and the screening result of the best mode will be accurate to the mode side.

[0109] For a single side of the mode, the deeper the maximum depth U of the mode and the higher the mode slope k of the single side of the mode, the higher the Q value of the mode. Optionally, when screening a high Q value mode of a single-frequency laser, the control module 10 pre-sets the rated depth and rated width of the spectral line, and screens the spectral curve with a depth greater than the rated depth and a width less than the rated width. That is to say, when the system is screening, the rated depth and rated width will be pre-set, and only the single side of the mode with a depth greater than the set rated depth and a width less than the set rated width will be selected. At the same time, because the narrow single side of the mode changes too drastically, it is not conducive to mode locking, so the system will set the minimum width of the single side of the mode according to the adjustable accuracy of the temperature control module, and only the mode with a width higher than the set value can be locked through the program.

[0110] After the Q value calculation is completed, the system gives a set of mode edges that meet the above conditions, and finds the mode edge with the largest mode slope k. Considering that the phenomenon of thermal broadening is the narrowing of one side of the mode and the broadening of the opposite side, and the mode without thermal broadening has symmetry, the mode edge finally selected by the present invention is the dual edge of the mode edge with the largest mode slope k as the best mode. Optionally, when screening high Q value modes of single-frequency lasers, when there are multiple high Q value modes and corresponding to multiple spectral lines, the dual edge of the mode edge with the largest mode slope in multiple spectral curves is selected as the best mode of the microcavity ultrasound probe 40.

[0111] It should be noted that the control module 10 adjusts the temperature of the single-frequency laser 21 to near the optimal mode, and the direction of adjustment must be the same as the scanning direction corresponding to the optimal mode. For example, the optimal mode is obtained when the temperature is reversed. The system will first quickly raise the temperature, then slowly lower the temperature, and adjust it to near the temperature corresponding to the optimal mode to ensure that the microcavity ultrasonic probe 40 enters the mode during the temperature reverse scan. After performing a wide temperature scan, narrow the tuning range and perform a narrow temperature scan. The second temperature scan is to further confirm the specific position of the optimal mode by increasing the sampling data, while avoiding as much as possible the mode drift that may occur during the time when the system performs bidirectional scanning and mode screening. After the narrow temperature scan, the system compares the data of the two scans, accurately locates the position of the optimal mode, and verifies the depth and width of the optimal mode.

[0112] Figure 9 This is a flowchart of a mode-locking method applicable to microcavity ultrasonic detection provided by the present invention. Refer to Figures 1 - 9 As shown below, the following lists the process regulation of the mode-locking method applicable to microcavity ultrasonic detection provided by the embodiments of the present invention:

[0113] Step S31: The system starts to run. The control module 10 starts to run, and each module is powered on in sequence. The control module 10 queries whether each module is working properly and initializes each module.

[0114] Step S32: Determine that each module is working properly.

[0115] Step S33: Adjust the temperature scanning direction, which is defaulted to reverse (cooling). The control module 10 controls the temperature adjustment unit 23 to adjust the temperature scanning direction, which is defaulted to reverse (i.e., the temperature decreasing direction).

[0116] Step S34: The temperature adjustment unit performs a wide temperature scan.

[0117] The temperature adjustment unit 23 starts the scanning program to issue a scanning instruction for a wide temperature scan, and controls the heat transfer unit 24 to uniformly scan the temperature of the single-frequency laser 21 within a wide range. The temperature scanning range is defaulted to 20 °C, the scanning speed is 1 °C / s, and the wavelength tuning range is about 2 nm. Among them, the single-frequency laser 21 is abbreviated as DFB (Distributed Feedback).

[0118] Step S35: Wide signal processing. The control module acquires the high-frequency signal transmitted by the acquisition card 33 and establishes a database of parameters such as each scanned temperature, scanned wavelength, and transmission spectrum line.

[0119] Step S36: The temperature scan has been performed twice. If it is Y, execute Step S37; if it is N, execute Step S38.

[0120] Step S37: Calculate the mode Q value to screen the best mode. The control module 10 calculates the Q value of the mode according to the formula Q = λ / Δλ of the Q value of the microcavity ultrasonic probe 40. Screen the mode with a high Q value as the best mode. When counting the transmission spectrum line, preferentially screen the high Q value mode with a large depth and a narrow width.

[0121] When screening the high Q value mode of the single-frequency laser, the control module 10 pre-sets the rated depth and rated width of the spectral line, and screens the spectral curve with a depth greater than the rated depth and a width greater than the rated width.

[0122] When there are multiple high-Q modes corresponding to multiple spectral lines, select the dual side of the side with the largest mode slope k in the spectral curve as the optimal mode of the microcavity ultrasonic probe. Among them, the dual side is determined by the symmetry of the transmission spectrum line.

[0123] Step S38: Adjust the temperature scanning direction to the positive direction (temperature increase). The control module 10 controls the temperature adjustment unit 23 to adjust the temperature scanning direction to the positive direction (i.e., the temperature increase direction). Then, sequentially execute steps S34 - S36.

[0124] Step S39: Adjust the DFB temperature to near the optimal mode. The control module 10 controls the temperature adjustment unit 23 to adjust the intracavity temperature of the single-frequency laser 21 to near the scanning temperature corresponding to the optimal mode.

[0125] Step S310: Narrow the temperature range for temperature narrow scanning. The temperature adjustment unit 23 starts the scanning program to issue a scanning instruction for temperature narrow scanning, and controls the heat transfer unit 24 to uniformly scan the temperature of the single-frequency laser 21 within a narrow range. The default temperature scanning range is 5°C, the scanning speed is 0.1°C / s, and the wavelength tuning range is approximately 0.2 nm. By increasing the sampling data for secondary temperature scanning, the specific position of the optimal mode can be further confirmed, and at the same time, the mode drift that may occur during the two-way scanning and mode screening of the system can be avoided as much as possible. After the temperature narrow scanning, the system compares the data of the two scans, including O value calculation, accurately locates the position of the optimal mode, and calculates the depth and width of the optimal mode. Finally, the microcavity ultrasonic probe 40 is locked on the optimal mode.

[0126] Step S311: Lock the optimal mode and transfer the working point to this mode. The control module 10 controls to lock the microcavity ultrasonic probe 40 on the optimal mode to achieve mode locking and enter the mode drift compensation program.

[0127] Step S312: Record the voltage at this time as the locked voltage.

[0128] Step S313: Mode voltage detection. The control module 10 obtains the voltage value of the transmission spectrum line of the microcavity ultrasonic probe 40 in real time.

[0129] Step S314: Judge whether |actual voltage - locked voltage| > error value. If yes, execute step S315; if no, execute step S313.

[0130] Step S315: Calculate the temperature adjustment amount d = (actual voltage - locked voltage) / mode slope. Calculate the mode drift amount ΔU and its corresponding temperature adjustment amount d according to the first formula. Among them, the first formula satisfies: ΔU = U′ - U, d = (U′ - U) / k. If d > 0, sequentially execute steps S317 - S319; if d ≤ 0, sequentially execute steps S320 - S321.

[0131] Step S317: The DFB temperature is increased.

[0132] Step S318: The DFB output wavelength increases.

[0133] Step S319: The operating point moves upward.

[0134] Step S320: The DFB temperature is decreased.

[0135] Step S321: The DFB output wavelength decreases.

[0136] Step S322: The operating point moves downward.

[0137] In summary, for the mode-locking method applicable to microcavity ultrasonic detection provided by the embodiments of the present invention, the control module controls the intracavity temperature of the single-frequency laser through linearly interpolated data to control the temperature regulation unit, so as to achieve linear regulation and uniform scanning of the incident wavelength, thereby realizing detailed scanning and accurate reproduction of the microcavity mode. Moreover, the most suitable mode for ultrasonic detection can be automatically selected through a program without using expensive tunable lasers, reducing the system cost. Further, through the mode-locking method, mode scanning and automatic locking of microcavity ultrasonic probes of various materials are achieved. The mode drift of the microcavity ultrasonic probe is received, the mode compensation method is judged, the temperature change amount required to compensate the mode drift is calculated, and it is transmitted to the temperature regulation unit for mode drift correction, so that the microcavity ultrasonic probe can autonomously and stably stay at the optimal operating point, and the temperature stability is at the 1 mK level. Based on the stability of the operating point, the ultrasonic signal can be inverted from the effective data detected by the microcavity ultrasonic probe. The high-stability mode-locking program design enables the microcavity ultrasonic probe to work in the optimal mode for a long time, has good anti-interference performance, can maintain high consistency in complex environments outside the laboratory, and reduces the mode-locking difficulty of the microcavity ultrasonic probe.

[0138] Note that the above is only the preferred embodiment of the present invention and the applied technical principle. Those skilled in the art will understand that the present invention is not limited to the specific embodiments described here. Various obvious changes, re-adjustments, mutual combinations, and substitutions can be made by those skilled in the art without departing from the protection scope of the present invention. Therefore, although the present invention has been described in detail through the above embodiments, the present invention is not limited to the above embodiments. Without departing from the inventive concept, it can include more other equivalent embodiments, and the scope of the present invention is determined by the scope of the appended claims.

Claims

1. A mode locking system suitable for microcavity ultrasonic detection, characterized in that: It includes a control module, a laser output module and a detection module; the control module is connected to the laser output module and the detection module respectively; the laser output module includes: A single-frequency laser is used to generate a single-frequency laser; a microcavity ultrasonic probe is arranged at the output end of the single-frequency laser; a temperature detection unit is used to detect the cavity temperature of the single-frequency laser; A temperature adjustment unit, used to compare the error value between the cavity temperature and the given temperature output by the control module and output a temperature adjustment instruction; a heat transfer unit, used to receive the temperature adjustment instruction and adjust the cavity temperature of the single-frequency laser; wherein the central wavelength of the single-frequency laser is linearly related to the cavity temperature; The detection module is used to detect the transmission spectrum of the microcavity ultrasound probe and transmit it to the control module; The control module is used to control the intra-cavity temperature of the single-frequency laser through linear interpolation data, realize linear regulation and scanning of the wavelength entering the cavity, and screen and lock the microcavity mode of the microcavity ultrasound probe; it is also used to adopt temperature detection and light intensity voltage feedback design to detect the locking position of the microcavity mode, automatically adjust the temperature to adjust the wavelength during the mode drift, and realize correction of the microcavity mode drift.

2. The mode locking system according to claim 1, characterized in that: The detection module comprises: A photoelectric detector is used to detect the transmission spectrum and mode drift of the microcavity ultrasound probe and convert them into electrical signals; a high-pass filter is used to receive the electrical signals and filter out low-frequency noise and baseline drift to obtain high-frequency signals; an acquisition card is used to acquire and record the high-frequency signals and transmit them to the control module.

3. The mode locking system according to claim 1, characterized in that: It also includes a polarization controller, which is arranged at the output end of the single-frequency laser and connected to the microcavity ultrasound probe; and is used to adjust and control the polarization characteristics of the single-frequency laser entering.

4. The mode locking system according to claim 1, characterized in that: The single-frequency laser and the microcavity ultrasound probe are connected via an optical fiber, and the optical fiber comprises a polarization-maintaining optical fiber.

5. A mode locking system suitable for microcavity ultrasonic detection, wherein the mode locking system suitable for microcavity ultrasonic detection according to any one of claims 1 to 4 is used to perform mode locking for microcavity ultrasonic detection, characterized in that: The mode locking method comprises: The temperature regulating unit is controlled to linearly regulate the central wavelength of the single-frequency laser output by the single-frequency laser according to the linear relationship between the temperature in the cavity and the central wavelength, so as to perform mode scanning on the microcavity ultrasonic probe; Obtain the transmission spectrum corresponding to each scanning wavelength, calculate the Q value corresponding to each transmission spectrum, select the high Q value mode of the single-frequency laser, and use the high Q value mode as the optimal mode of the microcavity ultrasound probe; Controlling the temperature adjustment unit to adjust the single-frequency laser to work in a high-Q value mode, and locking the microcavity ultrasound probe at the midpoint of the hypotenuse of the optimal mode; The transmission spectrum under the optimal mode is obtained, and the mode drift of the microcavity ultrasound probe is obtained based on the transmission spectrum and based on the linear relationship between the temperature in the cavity and the central wavelength, the central wavelength of the single-frequency laser is adjusted to compensate for the mode drift of the microcavity ultrasound probe when it is working.

6. The mode locking system according to claim 5, characterized in that: When the microcavity ultrasound probe is subjected to pattern scanning, the scanning modes include wide temperature scanning and narrow temperature scanning; First, the temperature regulating unit is controlled to perform a wide temperature scan to obtain the wide-scan temperature of the microcavity ultrasound probe in the optimal mode; based on the wide-scan temperature, the temperature regulating unit is controlled to perform a narrow temperature scan to obtain the narrow-scan temperature of the microcavity ultrasound probe in the optimal mode; the high Q value mode corresponding to the narrow-scan temperature is used as the optimal mode of the microcavity ultrasound probe; The wide scan temperature and the narrow scan temperature are determined by the Q value of the transmission spectrum.

7. The mode locking system according to claim 6, characterized in that: When the microcavity ultrasonic probe is subjected to pattern scanning, the control module controls the temperature regulating unit to perform wide temperature scanning and narrow temperature scanning respectively in the temperature increasing direction and the temperature decreasing direction, and screens the Q value corresponding to the transmission spectrum obtained by the scanning.

8. The mode locking system according to claim 5, characterized in that: When screening high-Q value modes of single-frequency lasers, based on the phenomenon of mode thermal broadening, when there are multiple high-Q value modes and corresponding to multiple spectral lines, the dual edge of the single side of the mode with the largest mode slope k in the spectral curve is selected as the optimal mode of the microcavity ultrasound probe; wherein the dual edge is determined by the symmetry of the transmission spectrum.

9. The mode locking system according to claim 6, characterized in that: When screening the high Q value mode of the single-frequency laser, the control module pre-sets the rated depth and rated width of the spectral line, and screens the spectral curve with a depth greater than the rated depth and a width less than the rated width.

10. The mode locking system according to claim 6, characterized in that: The mode drift of the microcavity ultrasonic probe is obtained based on the transmission spectrum and the central wavelength is adjusted based on the linear relationship between the temperature in the cavity and the central wavelength, specifically including: Obtain the mode slope k, the locking voltage U and the actual voltage U' of the transmission spectrum in the optimal mode, and calculate the mode drift amount ΔU and its corresponding temperature adjustment amount d by the first formula; Wherein, the first formula satisfies: ΔU=U′-U, d=(U′-U) / k; Determine that ΔU>0 and k>0; or, ΔU<0 and k<0, control the temperature adjustment unit to increase the cavity temperature of the single-frequency laser according to the temperature adjustment amount to adjust the central wavelength of the single-frequency laser; It is determined that ΔU<0 and k>0; or, ΔU>0 and k<0, the temperature adjustment unit is controlled to lower the cavity temperature of the single-frequency laser according to the temperature adjustment amount to adjust the central wavelength of the single-frequency laser.

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