An ultrasonic detection and positioning system based on beat frequency coded multi-wavelength fiber laser and its demodulation method.

Abstract

This invention discloses an ultrasonic detection and positioning system using beat-frequency encoded multi-wavelength fiber lasers, comprising an optical pumping device, an optical acquisition device, a signal demodulation device, and a fiber laser ultrasonic sensing array. The fiber laser ultrasonic sensing array includes at least three dual-polarized fiber lasers not distributed on the same straight line, used to generate at least three single-frequency dual-polarized lasers under the excitation of the pump light, with each dual-polarized fiber laser having a different beat frequency and output wavelength. This ultrasonic detection and positioning system is based on frequency modulation and demodulation of the beat frequency signals of each laser polarization using ultrasonic signals. It has high sensitivity and does not require additional time-division or wavelength-division multiplexing modules, greatly reducing the complexity of the system. This invention also discloses a demodulation method for the above-mentioned ultrasonic detection and positioning system.

Description

Technical Field

[0001] This invention relates to ultrasonic detection and positioning technology, and more particularly to an ultrasonic detection and positioning system and its demodulation method. Background Technology

[0002] In aerospace, nuclear power, and oil and gas exploration, ensuring the safe and stable operation of equipment structures is crucial. Ultrasonic detection technology plays a vital role in structural health monitoring by detecting ultrasonic signals within a structure to reflect its internal characteristics. Traditional ultrasonic detection technology relies on piezoelectric ultrasonic sensors, which suffer from drawbacks such as large size, susceptibility to electromagnetic interference, poor high-temperature performance, and difficulty in multiplexing and networking. Fiber optic ultrasonic sensors, on the other hand, offer unique advantages such as small size, resistance to electromagnetic interference, a wide operating temperature range, and the ability to be multiplexed, demonstrating enormous potential in engineering applications.

[0003] Currently, the fiber optic ultrasonic sensor commonly used in engineering applications is the fiber Bragg grating (FBG). This type of sensor detects ultrasound based on changes in resonant wavelength. By using a wavelength division multiplexing (WDM) FBG array and arranging it in a specific spatial configuration, combined with phase demodulation technology, the location of the ultrasonic point of action can be determined, thus achieving ultrasonic source localization. However, the low Q value of FBG limits its detection sensitivity.

[0004] Fiber lasers have advantages such as narrow linewidth, high signal-to-noise ratio, and high sensitivity, which can effectively solve the problem of low sensitivity in FBG ultrasonic detection.

[0005] Harbin Institute of Technology proposed a multi-wavelength fiber laser ultrasonic detection system in Chinese patent publication number CN112857554A. The multi-wavelength fiber laser in this system adopts a ring cavity structure. The resonant cavity of the ring cavity fiber laser is relatively long, which makes it prone to mode switching or even multi-longitudinal mode operation, affecting the stability of the detection system.

[0006] Jinan University proposed an ultrasonic imaging system and its imaging method in Chinese patent publication number CN114190974A. The linear cavity fiber laser has a short cavity length and usually supports stable operation of a single longitudinal mode, which can realize the fabrication of high-performance multi-wavelength fiber lasers. However, this type of ultrasonic detection system uses wavelength division multiplexing linear cavity fiber laser array, which requires the use of time division multiplexing and wavelength division multiplexing modules corresponding to each laser wavelength, increasing the complexity of the system.

[0007] In summary, existing multi-channel fiber ultrasonic detection systems based on FBG, ring cavity fiber lasers, or linear cavity fiber lasers suffer from technical shortcomings such as low sensitivity, poor stability, and complex and bulky structures. Summary of the Invention

[0008] To address the shortcomings of the prior art, this invention provides an ultrasonic detection and positioning system that modulates and demodulates the frequency of each laser polarization beat frequency signal based on ultrasonic signals. This system is highly sensitive and requires no additional time-division or wavelength-division multiplexing modules, greatly reducing the complexity of the system.

[0009] The present invention also provides a demodulation method for the above-mentioned ultrasonic detection and positioning system.

[0010] The technical problem to be solved by the present invention is achieved through the following technical solution:

[0011] An ultrasonic detection and positioning system using beat-frequency coded multi-wavelength fiber laser includes an optical pumping device, an optical acquisition device, a signal demodulation device, and a fiber laser ultrasonic sensing array.

[0012] The output end of the optical pumping device is connected to the input end of the fiber laser ultrasonic sensing array, and is used to emit pump light into the fiber laser ultrasonic sensing array to excite the fiber laser ultrasonic sensing array to output multi-wavelength dual-polarization laser.

[0013] The input end of the optical acquisition device is connected to the output end of the fiber laser ultrasonic sensor array, and is used to acquire the multi-wavelength dual-polarization laser output by the fiber laser ultrasonic sensor array.

[0014] The input end of the signal demodulation device is connected to the output end of the optical acquisition device, and is used to demodulate the multi-wavelength dual-polarization laser acquired by the optical acquisition device in order to calculate the location information of the ultrasonic signal source;

[0015] The fiber laser ultrasonic sensing array includes at least three dual-polarized fiber lasers that are not distributed on the same straight line, which are used to generate at least three single-frequency dual-polarized lasers under the excitation of the pump light, and each dual-polarized fiber laser has a different beat frequency and output wavelength.

[0016] Furthermore, the dual-polarization fiber lasers are connected end to end in sequence. The first dual-polarization fiber laser serves as both the input and output of the fiber laser ultrasonic sensing array, and is simultaneously connected to the output of the optical pumping device and the input of the optical acquisition device.

[0017] Furthermore, the ultrasonic detection and positioning system also includes a wavelength division multiplexer, which has a first port, a second port, and a third port. The output of the optical pumping device is connected to the first port of the wavelength division multiplexer, and the input of the optical acquisition device is connected to the third port of the wavelength division multiplexer. One end of the first dual-polarization fiber laser, which serves as both the input and output of the fiber laser ultrasonic sensing array, is connected to the second port of the wavelength division multiplexer.

[0018] Furthermore, each dual-polarization fiber laser includes an active fiber and a laser resonator, with the laser resonator fabricated within the active fiber; the laser resonators of each dual-polarization fiber laser have different output wavelengths but the same birefringence, or both different output wavelengths and birefringence.

[0019] Furthermore, the beat frequency f of the dual-polarization fiber laser satisfies the following formula:

[0020]

[0021] Where c is the speed of light, B is the birefringence of the laser resonator, n is the average refractive index of the active optical fiber, and λ is the output wavelength of the laser resonator.

[0022] Furthermore, the laser resonant cavity uses femtosecond fiber gratings as reflective cavity mirrors on both sides.

[0023] Furthermore, the femtosecond fiber grating is a fiber Bragg grating written with a femtosecond laser, making the laser resonant cavity a phase-shifted fiber grating or an FBG-FP cavity. During writing, the birefringence of the fiber Bragg grating is adjusted by the femtosecond laser, thereby adjusting the beat frequency of the dual-polarization fiber laser.

[0024] Furthermore, a femtosecond laser is used to write the fiber Bragg grating in the active optical fiber, and the focused spot of the femtosecond laser satisfies the following formula:

[0025]

[0026] Where, ω x and ω y W represents the lengths of the focused spot of the femtosecond laser along its major and minor axes, respectively. x and W y λ0 represents the length of the laser spot of the femtosecond laser in the long axis and short axis directions, respectively, f is the focal length of the femtosecond laser, and λ0 is the wavelength of the femtosecond laser.

[0027] Furthermore, the center wavelength λ of the fiber Bragg grating satisfies the following formula:

[0028]

[0029] Wherein, neff is the effective refractive index of the fiber Bragg grating, Λ is the grating period of the fiber Bragg grating, and m is the order of the fiber Bragg grating.

[0030] Furthermore, the fiber Bragg grating includes multiple elliptical modulation planes distributed along the fiber axis, each elliptical modulation plane being perpendicular to the fiber axis and having a major axis direction and a minor axis direction.

[0031] Furthermore, the birefringence B of the fiber Bragg grating satisfies the following formula:

[0032]

[0033] Where, n x and n y λ represents the effective refractive index of the fiber Bragg grating along its major and minor axes, respectively. x and λ y Λ represents the center wavelength of the fiber Bragg grating along its long axis and short axis, respectively, and Λ represents the grating period of the fiber Bragg grating.

[0034] A demodulation method for ultrasonic detection and localization includes the following steps:

[0035] Step 100: Obtain the time-domain signal of the multi-wavelength dual-polarization laser output by the fiber laser ultrasonic sensing array;

[0036] Step 200: Demodulate the time-domain signal of the multi-wavelength dual-polarized laser to obtain the frequency shift-time variation diagram of each dual-polarized fiber laser;

[0037] Step 300: Based on the frequency shift-time variation diagrams of each dual-polarized fiber laser, obtain the frequency shift-time difference between at least three dual-polarized fiber lasers, and calculate the position information of the ultrasonic source.

[0038] Furthermore, in step 200, the demodulation step for the time-domain signal of the multi-wavelength dual-polarization laser is as follows:

[0039] Step 210: Use a local microwave source to generate at least three microwave signals, one microwave signal corresponding to one single-frequency dual-polarization laser, and the corresponding microwave signal and the single-frequency dual-polarization laser have the same frequency and a phase difference of π / 2.

[0040] Step 220: Mix the time-domain signal of the multi-wavelength dual-polarization laser with each microwave signal to achieve down-conversion and obtain at least three mixed signals;

[0041] Step 230: Perform low-pass filtering on each mixing signal to obtain the same-direction branch signal I(t) and the quadrature branch signal Q(t);

[0042] Step 240: Perform arctangent operations on the in-phase branch signal I(t) and quadrature branch signal Q(t) of each mixing signal to obtain the phase information of the ultrasonic signal.

[0043] Step 250: Phase information of each mixing signal After performing differentiation processing, the frequency shift-time variation diagram of the ultrasonic signal acting on each dual-polarization fiber laser is obtained by demodulation.

[0044] Furthermore, in step 300, a Cartesian coordinate system is constructed to obtain the coordinate values ​​(xi, yi) of each dual-polarization fiber laser 50 in the Cartesian coordinate system. Assuming the coordinate value of the ultrasonic source is (x0, y0), the distance d between each dual-polarization fiber laser 50 and the ultrasonic source is then determined. i as follows:

[0045]

[0046] Selecting any one of the dual-polarization fiber lasers 50 as the reference laser (x', y'), the hyperbolic equation between the other dual-polarization fiber lasers 50 and the reference laser is constructed as follows:

[0047]

[0048] Among them, v u Let d be the propagation speed of the ultrasonic signal. t(i) The frequency shift time difference between each dual-polarization fiber laser 50 and the reference laser;

[0049] Choose at least two hyperbolic equations to form the following system of equations.

[0050]

[0051] Finally, by solving for the intersection of at least two hyperbolic equations, the location information (x0, y0) of the ultrasonic source can be obtained.

[0052] The present invention has the following advantages: The ultrasonic detection and positioning system of the present invention uses at least three dual-polarized fiber lasers not distributed on the same straight line to form the fiber laser ultrasonic sensing array. Since each dual-polarized fiber laser has a different output wavelength and beat frequency, it can generate different single-frequency dual-polarized lasers when excited by the optical pumping device. Each single-frequency dual-polarized laser has a different beat frequency and they are mixed to form the multi-wavelength dual-polarized laser, which is output to the optical acquisition device for acquisition. The signal demodulation device demodulates the signal and calculates the position information of the ultrasonic signal source. It has the advantages of high sensitivity and no need for additional time-division and wavelength-division multiplexing modules, which greatly reduces the complexity of the system. Furthermore, since each single-frequency dual-polarized laser has a different output wavelength, there is no crosstalk between the single-frequency dual-polarized lasers in the multi-wavelength dual-polarized laser. Attached Figure Description

[0053] Figure 1 This is a schematic diagram of the ultrasonic detection and positioning system provided by the present invention.

[0054] Figure 2 This is a schematic diagram of the structure of the fiber laser ultrasonic sensor array in the ultrasonic detection and positioning system provided by the present invention.

[0055] Figure 3 This is a schematic diagram of the axial and radial planes of the fiber Bragg grating in the ultrasonic detection and positioning system provided by the present invention.

[0056] Figure 4 A flowchart illustrating the demodulation method of the ultrasonic detection and positioning system provided by this invention.

[0057] Figure 5 This is a step-by-step flowchart of step 200 in the demodulation method of the ultrasonic detection and positioning system provided by the present invention. Detailed Implementation

[0058] The present invention will now be described in detail with reference to the accompanying drawings and embodiments, examples of which are shown in the drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0059] In the description of this invention, it should be understood that the terms "length", "width", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They 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. Therefore, they should not be construed as limitations on this invention.

[0060] Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first," "second," or "third" may explicitly or implicitly include one or more of that feature. In the description of this invention, "multiple" means two or more, unless otherwise explicitly specified.

[0061] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," "fixing," and "setting," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; 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 communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0062] Example 1

[0063] like Figure 1 The aforementioned ultrasonic detection and positioning system using beat-frequency coded multi-wavelength fiber laser includes an optical pumping device 1, an optical acquisition device 3, a signal demodulation device 4, and a fiber laser ultrasonic sensing array 5.

[0064] The output end of the optical pumping device 1 is connected to the input end of the fiber laser ultrasonic sensing array 5, and is used to emit pump light into the fiber laser ultrasonic sensing array 5 to excite the fiber laser ultrasonic sensing array 5 to output multi-wavelength dual-polarization laser.

[0065] The input end of the optical acquisition device 3 is connected to the output end of the fiber laser ultrasonic sensor array 5, and is used to acquire the multi-wavelength dual-polarization laser output by the fiber laser ultrasonic sensor array 5.

[0066] The input end of the signal demodulation device 4 is connected to the output end of the optical acquisition device 3, and is used to demodulate the multi-wavelength dual-polarization laser acquired by the optical acquisition device 3 in order to calculate the location information of the ultrasonic signal source.

[0067] The fiber laser ultrasonic sensing array 5 includes at least three dual-polarized fiber lasers 50 that are not distributed on the same straight line, which are used to generate at least three single-frequency dual-polarized lasers under the excitation of the pump light, and each dual-polarized fiber laser 50 has a different beat frequency.

[0068] The ultrasonic detection and positioning system of the present invention uses at least three dual-polarized fiber lasers 50 not distributed on the same straight line to form the fiber laser ultrasonic sensing array 5. Each dual-polarized fiber laser 50 can generate its own corresponding single-frequency dual-polarized laser under the excitation of the optical pumping device 1. Each single-frequency dual-polarized laser has a different beat frequency and is mixed to form the multi-wavelength dual-polarized laser, which is output to the optical acquisition device 3 for acquisition and demodulated by the signal demodulation device 4 to calculate the position information of the ultrasonic signal source. It has the advantages of high sensitivity and no need for additional time division and wavelength division multiplexing modules, which greatly reduces the complexity of the system. Furthermore, since each single-frequency dual-polarized laser has a different output wavelength, there is no crosstalk between the single-frequency dual-polarized lasers in the multi-wavelength dual-polarized laser.

[0069] During demodulation, since the beat frequency and output wavelength of each single-frequency dual-polarized laser are different, the single-frequency dual-polarized laser can be identified among the multi-wavelength dual-polarized lasers based on the differences in beat frequency and output wavelength.

[0070] Each dual-polarization fiber laser 50 has a polarization plane perpendicular to the light transmission direction, and each dual-polarization fiber laser 50 has a first polarization direction x and a second polarization direction y that are orthogonal to each other on the polarization plane. Due to the non-uniformity of the gain medium in the first polarization direction x and the second polarization direction y, there are two polarization modes oscillating simultaneously in each dual-polarization fiber laser 50, and the frequency difference between the two polarization modes forms different beat frequencies.

[0071] When the ultrasonic signal acts on each of the dual-polarization fiber lasers 50, the force can be decomposed into two components in the first polarization direction x and the second polarization direction y, causing each dual-polarization fiber laser 50 to undergo different degrees of deformation in the first polarization direction x and the second polarization direction y, thereby changing the non-uniformity of the gain medium and ultimately causing the beat frequency of each single-frequency dual-polarization laser to drift.

[0072] Since the various dual-polarization fiber lasers 50 are not on the same straight line, the ultrasonic signal arrives at each dual-polarization fiber laser 50 at different times, resulting in a time difference in the beat frequency drift of each dual-polarization fiber laser 50. Based on the frequency shift time difference, the position information of the ultrasonic signal source can be calculated.

[0073] Each dual-polarized fiber laser 50 is connected end to end in sequence. In this embodiment, each dual-polarized fiber laser 50 is a reflection-emitting laser. The first dual-polarized fiber laser 50 serves as both the input and output of the fiber laser ultrasonic sensing array 5, and is connected to both the output of the optical pumping device 1 and the input of the optical acquisition device 3.

[0074] The ultrasonic detection and positioning system further includes a wavelength division multiplexer 2, which has a first port 21, a second port 22, and a third port 23. The output of the optical pumping device 1 is connected to the first port 21 of the wavelength division multiplexer 2, and the input of the optical acquisition device 3 is connected to the third port 23 of the wavelength division multiplexer 2. The first dual-polarization fiber laser 50 is connected to the second port of the wavelength division multiplexer 2.

[0075] The pump light emitted by the optical pumping device 1 is incident on the first dual-polarized fiber laser 50 through the first port 21 and the second port 22 of the wavelength division multiplexer 2, and then sequentially incident on the other dual-polarized fiber lasers 50, thereby exciting each dual-polarized fiber laser 50 to generate single-frequency dual-polarized lasers with different beat frequencies. When the single-frequency dual-polarized lasers are emitted, they are mixed to form the multi-wavelength dual-polarized laser. The multi-wavelength dual-polarized laser is then incident on the optical acquisition device 3 through the second port 22 and the third port 23 of the wavelength division multiplexer 2.

[0076] In some examples, each dual-polarized fiber laser 50 is a transmission-emitting laser. In the sequentially connected dual-polarized fiber lasers 50, the first dual-polarized fiber laser 50 serves only as the input end of the fiber laser ultrasonic sensing array 5 and is connected to the output end of the optical pumping device 1, while the last dual-polarized fiber laser 50 serves as the output end of the fiber laser ultrasonic sensing array 5 and is connected to the input end of the optical acquisition device 3.

[0077] like Figure 2 As shown, each dual-polarization fiber laser 50 includes an active fiber 51 and a laser resonator 52, wherein the laser resonator 52 is fabricated in the active fiber 51; the laser resonators 51 of each dual-polarization fiber laser 50 have different output wavelengths but the same birefringence, or both output wavelengths and birefringence are different.

[0078] The ultrasonic detection and positioning system of the present invention uses the active optical fiber 51 as the gain medium of the dual-polarized fiber laser 50. When the pump light passes through the active optical fiber 51 of each dual-polarized fiber laser 50, the rare earth elements doped in the active optical fiber 51 absorb the energy of the pump light and jump to the upper energy level of the laser, thereby generating population inversion. When the inverted particles jump to the lower energy level of the laser, they radiate laser light of a specific wavelength band outward by stimulated emission or spontaneous emission. The laser light oscillates and is amplified in the laser resonant cavity 52 of each dual-polarized fiber laser 50 before being output.

[0079] The beat frequency f of the dual-polarization fiber laser 50 satisfies the following formula:

[0080]

[0081] Where c is the speed of light, B is the birefringence of the laser resonant cavity 52, n is the average refractive index of the active optical fiber 51, and λ is the output wavelength of the laser resonant cavity 52.

[0082] As can be seen from the above formula, changes in birefringence and output wavelength will cause different beat frequencies of each laser resonator 52, thus resulting in different beat frequencies for each single-frequency dual-polarized laser. Therefore, when fabricating the fiber laser ultrasonic sensing array 5, different beat frequencies for each single-frequency dual-polarized laser can be achieved by designing at least three laser resonators 52 with different output wavelengths and the same birefringence, or by designing at least three laser resonators 52 with different output wavelengths and birefringence.

[0083] When an ultrasonic signal is applied to each of the dual-polarized lasers 50, the active optical fiber 51 will undergo slight deformation along both the fiber axis z and the fiber radial direction xy, causing the effective refractive index of each laser resonator 52 to change to different degrees in the first polarization direction x and the second polarization direction y, i.e., the birefringence changes, thereby causing the beat frequency of each single-frequency dual-polarized laser to drift.

[0084] The active optical fibers 51 of each dual-polarization fiber laser 50 are connected end to end in sequence through transmission optical fibers 53. The first end of the active optical fiber 51 of the first dual-polarization fiber laser 50 serves as both the input and output end of the fiber laser ultrasonic sensing array, and is connected to both the output end of the optical pumping device and the input end of the optical acquisition device. The tail end is connected to the first end of the active optical fiber 51 of the next dual-polarization fiber laser 50 through the transmission optical fiber 53.

[0085] The transmission optical fiber 53 may be, but is not limited to, a single-mode optical fiber.

[0086] The active optical fiber 51 may be, but is not limited to, a single-mode fiber, few-mode fiber, multimode fiber, or large-core-diameter large-mode-field fiber doped with rare earth elements, such as thulium-doped fiber, neodymium-doped fiber, holmium-doped fiber, erbium-doped fiber, ytterbium-doped fiber, or thulium-holmium co-doped fiber.

[0087] Preferably, the laser resonant cavity 52 uses femtosecond fiber gratings as reflective cavity mirrors 521 on both sides.

[0088] The femtosecond fiber grating is formed by writing it in the active fiber 51 using a femtosecond laser. Compared with the traditional ultraviolet phase mask method, the femtosecond laser can write fiber gratings of arbitrary periods in the active fiber 51, and the modulation plane morphology of the fiber grating can be flexibly controlled during writing, thereby enabling the fabrication of a laser resonator 52 with arbitrary output wavelength, and the birefringence of the laser resonator 52 can be flexibly controlled.

[0089] The output wavelength of each laser resonator 52 depends on the center wavelength of the femtosecond fiber grating used, and the two are equal. Furthermore, the output wavelength of the laser resonator 52 varies with the center wavelength of the femtosecond fiber grating. The birefringence of each laser resonator 52 mainly depends on the birefringence of the femtosecond fiber grating used, but the two are not equal. The birefringence inherent in the active fiber 51 itself will have a certain influence on the birefringence of the laser resonator 52.

[0090] Preferably, the femtosecond fiber grating is a fiber Bragg grating written using a femtosecond laser, such that the laser resonant cavity 52 is a phase-shifted fiber grating or an FBG-FP cavity, that is, the dual-polarization fiber laser 50 is a distributed feedback laser (DFB) or a distributed Bragg reflector laser (DBR). During writing, the birefringence of the fiber Bragg grating is adjusted by the femtosecond laser, thereby adjusting the beat frequency of the dual-polarization fiber laser.

[0091] Both the phase-shifting fiber grating and the FBG-FP cavity have two fiber Bragg gratings. The difference is that the two fiber Bragg gratings of the phase-shifting fiber grating are seamlessly cascaded, and there is a phase-shift transition point at the cascade point. The laser in the laser resonator 52 can oscillate at this phase-shift transition point. In contrast, there is a short section of optical fiber between the two fiber Bragg gratings of the FBG-FP cavity, and the laser in the laser resonator 52 can oscillate in this short section of optical fiber.

[0092] like Figure 3 As shown, the fiber Bragg grating includes a plurality of elliptical modulation planes 520 arranged sequentially along the fiber axis z. Each elliptical modulation plane 520 is perpendicular to the fiber axis z and has a major axis direction x and a minor axis direction y.

[0093] The major axis direction x of the elliptical modulation plane 520 corresponds to the first polarization direction x of the dual-polarization fiber laser 50, and the minor axis direction y of the elliptical modulation plane 520 corresponds to the second polarization direction y of the dual-polarization fiber laser 50.

[0094] The center wavelength λ of the fiber Bragg grating satisfies the following formula:

[0095]

[0096] Wherein, neff is the effective refractive index of the fiber Bragg grating, Λ is the grating period of the fiber Bragg grating, and m is the order of the fiber Bragg grating.

[0097] As can be seen from the above formula, when preparing the fiber laser ultrasonic sensing array 5, fiber Bragg gratings with different grating periods can be designed as reflective cavity mirrors 521 for each laser resonator 52, so that the output wavelengths of each laser resonator 52 are different.

[0098] Due to the non-uniformity of the elliptical modulation plane 520, the fiber Bragg grating has different effective refractive indices in the major axis direction x and the minor axis direction y, respectively, to form birefringence, thereby modulating the laser to different degrees in the first polarization direction x and the second polarization direction y, so that the output laser forms bipolarity.

[0099] Because the fiber Bragg grating has different effective refractive indices along its major axis (x) and minor axis (y), its center wavelengths along these axes are actually different, according to the formula for the center wavelength mentioned above. The birefringence B of the fiber Bragg grating satisfies the following formula:

[0100]

[0101] Where, n x and n y λ represents the effective refractive index of the fiber Bragg grating along its major axis (x) and minor axis (y), respectively. x and λ y Λ represents the center wavelength of the fiber Bragg grating along its major axis x and minor axis y, respectively, and Λ represents the grating period of the fiber Bragg grating.

[0102] When the fiber Bragg grating is written using the femtosecond laser, a displacement platform is used to move the active fiber 51 along the fiber axis z. The grating period Λ of the fiber Bragg grating is determined by the moving speed v of the displacement platform and the repetition frequency f of the femtosecond laser, i.e., Λ = v / f. Therefore, laser resonators 52 with different output wavelengths can be prepared by setting different moving speeds v of the displacement platform and different repetition frequencies f of the femtosecond laser.

[0103] The lengths of the focused spot of the femtosecond laser along its major axis (x) and minor axis (y) satisfy the following formulas:

[0104]

[0105] Where, ω x and ω yW represents the length of the focused spot of the femtosecond laser along its major axis (x) and minor axis (y), respectively. x and W y λ0 represents the length of the laser spot of the femtosecond laser in the long axis and short axis directions, respectively, f is the focal length of the femtosecond laser, and λ0 is the wavelength of the femtosecond laser.

[0106] Furthermore, the femtosecond laser can be combined with a beam shaper to shape the focused spot of the femtosecond laser, thereby adjusting the length of the focused spot in the major axis direction x and the minor axis direction y to form an elliptical modulation plane with arbitrary ellipticity. This allows an elliptical modulation plane 520 with arbitrary ellipticity to be written in the active optical fiber 51, realizing different birefringences of the fiber Bragg grating.

[0107] like Figure 1 As shown, the light acquisition device 3 includes an optical isolator 31, an optical amplifier 32, a photodetector 33, and a signal acquisition device 34 connected in sequence.

[0108] The optical isolator 31 is used to isolate the reverse signal of the multi-wavelength dual-polarized laser. It only allows the multi-wavelength dual-polarized laser to enter the optical acquisition device 3 from the fiber laser ultrasonic sensor array 5, but does not allow the reverse signal of the multi-wavelength dual-polarized laser to enter the fiber laser ultrasonic sensor array 5 from the optical acquisition device 3.

[0109] The optical amplifier 32 is used to amplify the multi-wavelength dual-polarized laser to improve the signal-to-noise ratio of the multi-wavelength dual-polarized laser;

[0110] The photodetector 33 is used to receive the multi-wavelength dual-polarization laser and convert it into photoelectric signals;

[0111] The signal acquisition device 34 is used to acquire the electrical signal converted by the photodetector 33 and output the acquired electrical signal to the signal demodulation device 4 for demodulation processing.

[0112] Example 2

[0113] like Figure 4 As shown, a demodulation method for ultrasonic detection and positioning is used in the ultrasonic detection and positioning system described in Embodiment 1; the demodulation method includes the following steps:

[0114] Step 100: Obtain the time-domain signal of the multi-wavelength dual-polarization laser output by the fiber laser ultrasonic sensor array 5.

[0115] Step 200: Demodulate the time-domain signal of the multi-wavelength dual-polarized laser to obtain the frequency shift-time variation diagram of each dual-polarized fiber laser 50.

[0116] In step 200, such as Figure 5 As shown, the signal demodulation device 4 uses the IQ demodulation method to demodulate the time-domain signal of the multi-wavelength dual-polarization laser. The steps are as follows:

[0117] Step 210: Use a local microwave source to generate at least three microwave signals, one microwave signal corresponding to one single-frequency dual-polarization laser, and the corresponding microwave signal and the single-frequency dual-polarization laser have the same frequency and a phase difference of π / 2.

[0118] Step 220: Mix the time-domain signal of the multi-wavelength dual-polarization laser with each microwave signal to achieve down-conversion and obtain at least three mixed signals;

[0119] Step 230: Perform low-pass filtering on each mixing signal to obtain the same-direction branch signal I(t) and the quadrature branch signal Q(t);

[0120] Step 240: Perform arctangent operations on the in-phase branch signal I(t) and quadrature branch signal Q(t) of each mixing signal to obtain the phase information of the ultrasonic signal.

[0121] Step 250: Phase information of each mixing signal After performing differentiation processing, the frequency shift-time variation diagram of the ultrasonic signal acting on each dual-polarization fiber laser is obtained by demodulation.

[0122] Step 300: Based on the frequency shift-time variation diagram of each dual-polarized fiber laser 50, obtain the frequency shift time difference between at least three dual-polarized fiber lasers 50, and calculate the position information of the ultrasonic source.

[0123] In step 300, the time when each dual-polarized fiber laser 50 experiences beat frequency drift can be directly obtained from the frequency shift-time variation graph. Then, by subtracting the time when each dual-polarized fiber laser 50 experiences beat frequency drift, the frequency shift time difference between each dual-polarized fiber laser 50 can be obtained.

[0124] Then, a Cartesian coordinate system is constructed to obtain the coordinates (xi, yi) of the i-th dual-polarization fiber laser 50 in the Cartesian coordinate system. Assuming the coordinates of the ultrasonic source are (x0, y0), the distance d between the i-th dual-polarization fiber laser 50 and the ultrasonic source is... i as follows:

[0125]

[0126] Choosing any dual-polarization fiber laser 50 as the reference laser (x', y'), the hyperbolic equation between the i-th dual-polarization fiber laser 50 and the reference laser is constructed as follows:

[0127]

[0128] Among them, v u Let d be the propagation speed of the ultrasonic signal. t(i) The frequency shift time difference between the i-th dual-polarization fiber laser 50 and the reference laser;

[0129] Select at least two hyperbolic equations of dual-polarization fiber lasers 50 to form the following system of equations.

[0130]

[0131] Finally, by solving for the intersection of at least two hyperbolic equations, the location information (x0, y0) of the ultrasonic source can be obtained.

[0132] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the embodiments of the present invention and not to limit them. Although the embodiments of the present invention have been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the embodiments of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A beat-frequency coded multi-wavelength fiber laser ultrasonic detection and positioning system, characterized in that, It includes an optical pumping device, an optical acquisition device, a signal demodulation device, and a fiber laser ultrasonic sensing array. The output end of the optical pumping device is connected to the input end of the fiber laser ultrasonic sensing array, and is used to emit pump light into the fiber laser ultrasonic sensing array to excite the fiber laser ultrasonic sensing array to output multi-wavelength dual-polarization laser. The input end of the optical acquisition device is connected to the output end of the fiber laser ultrasonic sensor array, and is used to acquire the multi-wavelength dual-polarization laser output by the fiber laser ultrasonic sensor array. The input end of the signal demodulation device is connected to the output end of the optical acquisition device, and is used to demodulate the multi-wavelength dual-polarization laser acquired by the optical acquisition device in order to calculate the location information of the ultrasonic signal source; The fiber laser ultrasonic sensing array includes at least three dual-polarized fiber lasers that are not distributed on the same straight line, which are used to generate at least three single-frequency dual-polarized lasers under the excitation of the pump light, and each dual-polarized fiber laser has a different beat frequency and output wavelength. Each dual-polarization fiber laser is connected end to end in sequence. The first dual-polarization fiber laser serves as both the input and output of the fiber laser ultrasonic sensing array, and is simultaneously connected to the output of the optical pumping device and the input of the optical acquisition device. The ultrasonic detection and positioning system further includes a wavelength division multiplexer, which has a first port, a second port, and a third port. The output of the optical pumping device is connected to the first port of the wavelength division multiplexer, and the input of the optical acquisition device is connected to the third port of the wavelength division multiplexer. One end of the first dual-polarization fiber laser, which serves as both the input and output of the fiber laser ultrasonic sensing array, is connected to the second port of the wavelength division multiplexer.

2. The ultrasonic detection and positioning system according to claim 1, characterized in that, Each dual-polarization fiber laser includes an active fiber and a laser resonator, with the laser resonator fabricated within the active fiber. The laser resonators of each dual-polarization fiber laser may have different output wavelengths but the same birefringence, or both different output wavelengths and birefringence.

3. The ultrasonic detection and positioning system according to claim 2, characterized in that, The beat frequency of the dual-polarization fiber laser Satisfy the following formula: in, At the speed of light, The birefringence of the laser resonant cavity is given. The average refractive index of the active optical fiber is... The output wavelength of the laser resonant cavity is denoted as .

4. The ultrasonic detection and positioning system according to claim 2, characterized in that, The laser resonant cavity uses femtosecond fiber gratings as reflective cavity mirrors on both sides.

5. The ultrasonic detection and positioning system according to claim 4, characterized in that, The femtosecond fiber grating is a fiber Bragg grating written with a femtosecond laser, making the laser resonant cavity a phase-shifted fiber grating or an FBG-FP cavity. During writing, the birefringence of the fiber Bragg grating is adjusted by the femtosecond laser, thereby adjusting the beat frequency of the dual-polarization fiber laser.

6. The ultrasonic detection and positioning system according to claim 5, characterized in that, The fiber Bragg grating is written in the active optical fiber using a femtosecond laser, and the focused spot of the femtosecond laser satisfies the following formula: ， in, and These are the lengths of the focused spot of the femtosecond laser along its major and minor axes, respectively. and These represent the lengths of the femtosecond laser spot along its major and minor axes, respectively, before it is focused. The focal length of the femtosecond laser is the focusing focal length. The wavelength of the femtosecond laser is denoted as λ.

7. The ultrasonic detection and positioning system according to claim 5, characterized in that, The center wavelength of the fiber Bragg grating Satisfy the following formula: in, The effective refractive index of the fiber Bragg grating is... denoted as the grating period of the fiber Bragg grating, and m as the order of the fiber Bragg grating.

8. The ultrasonic detection and positioning system according to claim 5, characterized in that, The fiber Bragg grating includes multiple elliptical modulation planes distributed along the fiber axis. Each elliptical modulation plane is perpendicular to the fiber axis and has a major axis direction and a minor axis direction.

9. The ultrasonic detection and positioning system according to claim 8, characterized in that, The birefringence of the fiber Bragg grating Satisfy the following formula: in, and These are the effective refractive indices of the fiber Bragg grating along its major and minor axes, respectively. and These are the center wavelengths of the fiber Bragg grating along its major and minor axes, respectively. The grating period of the fiber Bragg grating is given.

10. A demodulation method for ultrasonic detection and positioning, characterized in that, In the ultrasonic detection and positioning system according to claim 1, the demodulation method includes the following steps: Step 100: Obtain the time-domain signal of the multi-wavelength dual-polarization laser output by the fiber laser ultrasonic sensing array; Step 200: Demodulate the time-domain signal of the multi-wavelength dual-polarized laser to obtain the frequency shift-time variation diagram of each dual-polarized fiber laser; Step 300: Based on the frequency shift-time variation diagrams of each dual-polarized fiber laser, obtain the frequency shift-time difference between at least three dual-polarized fiber lasers, and calculate the position information of the ultrasonic source.

11. The demodulation method according to claim 10, characterized in that, In step 200, the demodulation steps for the time-domain signal of the multi-wavelength dual-polarization laser are as follows: Step 210: Use a local microwave source to generate at least three microwave signals, one microwave signal corresponding to one single-frequency dual-polarization laser, and the corresponding microwave signal and the single-frequency dual-polarization laser have the same frequency and a phase difference of π / 2. Step 220: Mix the time-domain signal of the multi-wavelength dual-polarization laser with each microwave signal to achieve down-conversion and obtain at least three mixed signals; Step 230: Perform low-pass filtering on each mixing signal to obtain the same-direction branch signal I(t) and the quadrature branch signal Q(t); Step 240: Perform arctangent operation on the same-direction branch signal I(t) and the orthogonal branch signal Q(t) of each mixing signal to obtain the phase information j(t)=arctan[Q(t) / I(t)] of the ultrasonic signal; Step 250: Differentiate the phase information j(t) of each mixing signal and demodulate to obtain the frequency shift-time variation diagram of the ultrasonic signal acting on each dual polarization fiber laser.

12. The demodulation method according to claim 10, characterized in that, In step 300, a Cartesian coordinate system is constructed, and the coordinates (xi, yi) of the i-th dual-polarization fiber laser in the Cartesian coordinate system are obtained. Assuming the coordinates of the ultrasonic source are (x0, y0), the distance between the i-th dual-polarization fiber laser and the ultrasonic source is calculated. as follows: Choosing any dual-polarization fiber laser as the reference laser (x', y'), the hyperbolic equation between the i-th dual-polarization fiber laser and the reference laser is constructed as follows: in, The speed at which the ultrasonic signal propagates. The frequency shift time difference between the i-th dual-polarization fiber laser and the reference laser; Select the hyperbolic equations of at least two dual-polarization fiber lasers to form the following system of equations. ，i≥2 Finally, by solving the intersection of at least two hyperbolic equations, the location information (x0, y0) of the ultrasound source can be obtained.

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