Beat frequency coding type multi-wavelength fiber laser ultrasonic detection positioning system and demodulation method thereof

Abstract

The invention discloses a beat frequency coding type multi-wavelength optical fiber laser ultrasonic detection positioning system, which comprises an optical pumping device, an optical acquisition device, a signal demodulation device and an optical fiber laser ultrasonic sensing array, and is characterized in that the optical fiber laser ultrasonic sensing array comprises at least three dual-polarization optical fiber lasers which are not distributed on the same straight line; and the dual-polarization fiber lasers are used for generating at least three paths of single-frequency dual-polarization laser under the excitation of the pump light, and each dual-polarization fiber laser has different beat frequencies and output wavelengths. The ultrasonic detection positioning system modulates and demodulates the frequency of each laser polarization beat frequency signal based on an ultrasonic signal, is high in sensitivity, does not need extra time division and wavelength division multiplexing modules, and greatly reduces the complexity of the system. The invention further discloses a demodulation method of the ultrasonic detection positioning system.

Description

Technical Field

[0001] The invention relates to ultrasonic detection and positioning technology, and in particular to an ultrasonic detection and positioning system and a demodulation method thereof. Background Art

[0002] In the fields of aerospace, nuclear power, oil and gas exploration, etc., it is crucial to ensure the safe and stable operation of equipment structures. Ultrasonic detection technology can reflect the internal characteristics of the structure by detecting ultrasonic signals in the structure, and plays an important role in structural health monitoring. Traditional ultrasonic detection technology is based on ultrasonic sensors made of piezoelectric materials. Such sensors have shortcomings such as large size, susceptibility to electromagnetic interference, poor high temperature performance, and difficulty in multiplexing and networking. Fiber optic ultrasonic sensors have unique advantages such as small size, resistance to electromagnetic interference, wide temperature range, and multiplexing, showing great potential in engineering applications.

[0003] The fiber optic ultrasonic sensor commonly used in engineering applications is the fiber Bragg grating (FBG), which senses ultrasound based on the change of the resonant wavelength. By using wavelength division multiplexing FBG arrays and arranging them in a specific space, combined with phase demodulation technology, the position of the ultrasound action point can be determined and the ultrasound source can be located. However, the low Q value of FBG limits its detection sensitivity.

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

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

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

[0007] In general, the existing multi-channel fiber-optic ultrasonic detection systems based on FBG, ring-cavity fiber lasers or linear-cavity fiber lasers have technical deficiencies such as low sensitivity, poor stability, and complex and bulky structure. Summary of the invention

[0008] In order to solve the above-mentioned deficiencies of the prior art, the present invention provides an ultrasonic detection and positioning system, which is based on the frequency modulation and demodulation of each laser polarization beat signal by ultrasonic signals, has high sensitivity and does not require additional time division and wavelength division multiplexing modules, greatly reducing the complexity of the system.

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

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

[0011] A beat frequency coded multi-wavelength fiber laser ultrasonic detection and positioning system comprises an optical pumping device, an optical collection device, a signal demodulation device and a fiber laser ultrasonic sensor array.

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

[0013] The input end of the light collection device is connected to the output end of the fiber laser ultrasonic sensor array, and is used to collect 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 light collection device, and is used to demodulate the multi-wavelength dual-polarization laser collected by the light collection device to calculate the position information of the ultrasonic signal source;

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

[0016] Furthermore, each dual-polarization fiber laser is connected end to end in sequence, wherein the first dual-polarization fiber laser serves as both the input and output of the fiber laser ultrasonic sensor array and is simultaneously connected to the output of the optical pumping device and the input of the light collection 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 end of the optical pumping device is connected to the first port of the wavelength division multiplexer, and the input end of the light collection device is connected to the third port of the wavelength division multiplexer; one end of the first dual-polarization fiber laser serves as both the input and output ends of the fiber laser ultrasonic sensor array and is connected to the second port of the wavelength division multiplexer.

[0018] Furthermore, each dual-polarization fiber laser comprises an active fiber and a laser resonant cavity, wherein the laser resonant cavity is fabricated in the active fiber; the laser resonant cavities of each dual-polarization fiber laser have different output wavelengths and the same birefringence, or have different output wavelengths and birefringence.

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

[0020]

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

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

[0023] Furthermore, the femtosecond fiber grating is a fiber Bragg grating written by a femtosecond laser, so that the laser resonant cavity is a phase-shifted fiber grating or a 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 focusing spot of the femtosecond laser satisfies the following formula:

[0025]

[0026] Among them, ω x and ω y are the lengths of the focused spot of the femtosecond laser in the long axis direction and the short axis direction, W x and W y are the lengths of the laser spot of the femtosecond laser before being focused in the major axis direction and the minor axis direction, f is the focusing focal length of the femtosecond laser, and λ0 is the wavelength of the femtosecond laser.

[0027] Furthermore, the central 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 a plurality of elliptical modulation planes distributed along the axial direction of the optical fiber, each elliptical modulation plane is perpendicular to the axial direction of the optical fiber and has 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] Among them, n x and n y are the effective refractive index of the fiber Bragg grating in the long axis direction and the short axis direction, λ x and λ y are the central wavelengths of the fiber Bragg grating in the major axis direction and the minor axis direction respectively, and Λ is the grating period of the fiber Bragg grating.

[0034] A demodulation method for ultrasonic detection and positioning comprises the following steps:

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

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

[0037] Step 300: Obtain the frequency shift time difference between at least three dual-polarization fiber lasers according to the frequency shift-time variation diagram of each dual-polarization fiber laser, and calculate the position information of the ultrasound source.

[0038] Furthermore, in step 200, the steps of demodulating the time domain signal of the multi-wavelength dual-polarization laser are as follows:

[0039] Step 210: using a local microwave source to generate at least three microwave signals, one microwave signal corresponds to one single-frequency dual-polarization laser, 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: Mixing the time domain signal of the multi-wavelength dual-polarization laser with each microwave signal to obtain at least three mixed signals after down-conversion;

[0041] Step 230: performing low-pass filtering on each mixed signal to obtain an in-phase branch signal I(t) and an orthogonal branch signal Q(t);

[0042] Step 240: Perform arc tangent operations on the in-phase branch signal I(t) and the orthogonal 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 Differentiation processing is performed respectively and demodulation is performed to obtain the frequency shift-time variation diagram of the ultrasonic signal acting on each dual-polarization fiber laser.

[0044] Furthermore, in step 300, a plane rectangular coordinate system is constructed to obtain the coordinate values ​​(xi, yi) of each dual-polarization fiber laser 50 in the plane rectangular coordinate system. Assuming that 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 i as follows:

[0045]

[0046] Select any dual-polarization fiber laser 50 as the reference laser (x', y'), and construct the hyperbolic equation between the other dual-polarization fiber lasers 50 and the reference laser as follows:

[0047]

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

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

[0050]

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

[0052] The present invention has the following beneficial effects: the ultrasonic detection and positioning system of the present invention adopts at least three dual-polarization fiber lasers distributed on different lines to form the fiber laser ultrasonic sensor array. Since the output wavelengths and beat frequencies of the dual-polarization fiber lasers are different, different single-frequency dual-polarization lasers can be generated respectively when excited by the optical pumping device. Each single-frequency dual-polarization laser has a different beat frequency and is mixed to form the multi-wavelength dual-polarization laser, which is output to the optical collection device for collection, and is demodulated by the signal demodulation device to calculate the position information of the ultrasonic signal source. The system 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. Since each single-frequency dual-polarization laser has a different output wavelength, each single-frequency dual-polarization laser in the multi-wavelength dual-polarization laser will not crosstalk with each other. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 This is a schematic diagram of the structure 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 structural diagram of the axial plane and radial plane of the fiber Bragg grating in the ultrasonic detection and positioning system provided by the present invention.

[0056] Figure 4 A block diagram of the steps of the demodulation method of the ultrasonic detection and positioning system provided by the present invention.

[0057] Figure 5 This is a block diagram of the steps of step 200 in the demodulation method of the ultrasonic detection and positioning system provided by the present invention. DETAILED DESCRIPTION

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

[0059] In the description of the present invention, it should be understood that the terms "length", "width", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside" and "outside" etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present invention.

[0060] In addition, the terms "first", "second", and "third" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first", "second", and "third" may explicitly or implicitly include one or more of the features. In the description of the present invention, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0061] In the present invention, unless otherwise clearly specified and limited, the terms "install", "connect", "connect", "fix", "set" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or an indirect connection through an intermediate medium, or it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0062] Embodiment 1

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

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

[0065] The input end of the light collection device 3 is connected to the output end of the fiber laser ultrasonic sensor array 5, and is used to collect 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 light collection device 3, and is used to demodulate the multi-wavelength dual-polarization laser collected by the light collection device 3 to calculate the position information of the ultrasonic signal source;

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

[0068] The ultrasonic detection and positioning system of the present invention uses at least three dual-polarization fiber lasers 50 distributed on different lines to form the fiber laser ultrasonic sensor array 5. Each dual-polarization fiber laser 50 can generate its own corresponding single-frequency dual-polarization laser under the excitation of the optical pumping device 1. Each single-frequency dual-polarization laser has a different beat frequency and is mixed to form the multi-wavelength dual-polarization laser, which is output to the optical collection device 3 for collection 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. Since each single-frequency dual-polarization laser has a different output wavelength, each single-frequency dual-polarization laser in the multi-wavelength dual-polarization laser will not crosstalk with each other.

[0069] During demodulation, since the beat frequencies and output wavelengths of the single-frequency dual-polarization lasers are different, the single-frequency dual-polarization lasers can be identified in the multi-wavelength dual-polarization lasers according to the differences in beat frequencies and output wavelengths.

[0070] The polarization plane of each dual-polarization fiber laser 50 is 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 a different beat frequency.

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

[0072] Furthermore, since the 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. The position information of the ultrasonic signal source can be calculated based on the frequency shift time difference.

[0073] The dual-polarization fiber lasers 50 are connected end to end in sequence. In the present embodiment, each dual-polarization fiber laser 50 is a reflection-emitting laser, wherein the first dual-polarization fiber laser 50 serves as both the input and output ends of the fiber laser ultrasonic sensor array 5 and is simultaneously connected to the output end of the optical pumping device 1 and the input end of the optical collection device 3.

[0074] The ultrasonic detection and positioning system also includes a wavelength division multiplexer 2, which has a first port 21, a second port 22 and a third port 23. The output end of the optical pumping device 1 is connected to the first port 21 of the wavelength division multiplexer 2, and the input end of the light collection 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-polarization fiber laser 50 through the first port 21 and the second port 22 of the wavelength division multiplexer 2, and is incident on the other dual-polarization fiber lasers 50 in sequence through the first dual-polarization fiber laser 50, thereby stimulating each dual-polarization fiber laser 50 to generate a single-frequency dual-polarization laser with a different beat frequency. When each single-frequency dual-polarization laser is emitted, it is mixed to form the multi-wavelength dual-polarization laser, and the multi-wavelength dual-polarization laser is then incident on the light collection device 3 through the second port 22 and the third port 23 of the wavelength division multiplexer 2.

[0076] In some examples, each dual-polarization fiber laser 50 is a transmission-output laser. Among the dual-polarization fiber lasers 50 connected end to end in sequence, the first dual-polarization fiber laser 50 serves only as the input end of the fiber laser ultrasonic sensor array 5 and is connected to the output end of the optical pumping device 1, while the last dual-polarization fiber laser 50 serves as the output end of the fiber laser ultrasonic sensor array 5 and is connected to the input end of the light collection device 3.

[0077] like Figure 2 As shown, each dual-polarization fiber laser 50 includes an active fiber 51 and a laser resonant cavity 52, and the laser resonant cavity 52 is made in the active fiber 51; the laser resonant cavity 51 of each dual-polarization fiber laser 50 has different output wavelengths and the same birefringence, or both the output wavelength and the 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-polarization fiber laser 50. When the pump light passes through the active optical fiber 51 of each dual-polarization fiber laser 50, the rare earth element doped in the active optical fiber 51 absorbs the energy of the pump light and then transitions to the upper energy level of the laser, thereby generating a particle number inversion. When the inverted particles transition to the lower energy level of the laser, they radiate laser light of a specific wavelength band outwardly in the form of stimulated radiation or spontaneous radiation. The laser light oscillates in the laser resonant cavity 52 of each dual-polarization fiber laser 50 and is amplified and then output.

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

[0080]

[0081] Wherein, 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] It can be seen from the above formula that changes in birefringence and output wavelength will cause the beat frequencies of each laser resonant cavity 52 to be different, so that each single-frequency dual-polarization laser has a different beat frequency. Therefore, when manufacturing the fiber laser ultrasonic sensor array 5, the beat frequencies of each single-frequency dual-polarization laser can be different by designing at least three laser resonant cavities 52 with different output wavelengths and the same birefringence, or by designing at least three laser resonant cavities 52 with different output wavelengths and birefringence.

[0083] When the ultrasonic signal acts on each dual-polarization laser 50, the active optical fiber 51 will produce a slight deformation along the optical fiber axial direction z and the optical fiber radial direction xy, causing the effective refractive index of each laser resonant cavity 52 in the first polarization direction x and the second polarization direction y to change to varying degrees, that is, the birefringence changes, thereby causing the beat frequency of each single-frequency dual-polarization laser to drift.

[0084] The active optical fibers 51 of the various dual-polarization fiber lasers 50 are connected end to end in sequence through the transmission optical fiber 53, wherein the head end of the active optical fiber 51 of the first dual-polarization fiber laser 50 serves as both the input and output ends of the fiber laser ultrasonic sensor array, and is simultaneously connected to the output end of the optical pumping device and the input end of the optical collection device, and the tail end is connected to the head 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, single-mode optical fiber, few-mode optical fiber, multi-mode optical fiber or large-core large-mode-field optical fiber doped with rare earth elements such as thulium-doped optical fiber, neodymium-doped optical fiber, holmium-doped optical fiber, erbium-doped optical fiber, ytterbium-doped optical fiber or thulium-holmium co-doped optical fiber.

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

[0088] The femtosecond fiber grating is formed by writing in the active optical fiber 51 using a femtosecond laser. Compared with the conventional ultraviolet phase mask method, the femtosecond laser can be used to write a fiber grating of any period in the active optical fiber 51, and the modulation plane morphology of the fiber grating can be flexibly adjusted during writing, so that a laser resonant cavity 52 with any output wavelength can be prepared, and the birefringence of the laser resonant cavity 52 can be flexibly adjusted.

[0089] The output wavelength of each laser resonant cavity 52 depends on the central wavelength of the adopted femtosecond fiber grating, and the two are equal, and the output wavelength of the laser resonant cavity 52 changes with the central wavelength of the femtosecond fiber grating. The birefringence of each laser resonant cavity 52 mainly depends on the birefringence of the adopted femtosecond fiber grating, but the two are not equal. The birefringence of the active fiber 51 itself will have a certain influence on the birefringence of the laser resonant cavity 52.

[0090] Preferably, the femtosecond fiber grating is a fiber Bragg grating written by a femtosecond laser, so that the laser resonant cavity 52 is a phase-shifted fiber grating or a FBG-FP cavity, that is, the dual-polarization fiber laser 50 is a distributed feedback laser (DFB) or a distributed Bragg reflection 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] The phase-shifted fiber Bragg grating or FBG-FP cavity has two fiber Bragg gratings. The difference is that the two fiber Bragg gratings of the phase-shifted fiber Bragg grating are seamlessly cascaded, and there is a phase shift transition point at the cascade. The laser in the laser resonant cavity 52 can oscillate in the phase shift transition point, and there is a short section of optical fiber between the two fiber Bragg gratings of the FBG-FP cavity. The laser in the laser resonant cavity 52 can oscillate in the short section of optical fiber.

[0092] like Figure 3 As shown, the fiber Bragg grating includes a plurality of elliptical modulation planes 520 sequentially distributed along the fiber axial direction z. Each elliptical modulation plane 520 is perpendicular to the fiber axial direction 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 central 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] It can be seen from the above formula that when preparing the fiber laser ultrasonic sensor array 5, the output wavelengths of each laser resonant cavity 52 can be different by designing a fiber Bragg grating with a different grating period as a reflective cavity mirror 521 for each laser resonant cavity 52.

[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 having different degrees of modulation effects on the laser in the first polarization direction x and the second polarization direction y, so that the output laser forms dual polarization.

[0099] Since the fiber Bragg grating has different effective refractive indices in the major axis direction x and the minor axis direction y, according to the above central wavelength formula, its central wavelengths in the major axis direction x and the minor axis direction y are actually different. The birefringence B of the fiber Bragg grating satisfies the following formula:

[0100]

[0101] Among them, n x and n y are the effective refractive index of the fiber Bragg grating in the major axis direction x and the minor axis direction y, respectively, x and λ y are the center wavelengths of the fiber Bragg grating in the major axis direction x and the minor axis direction y respectively, and Λ is the grating period of the fiber Bragg grating.

[0102] When the femtosecond laser is used to write the fiber Bragg grating, a displacement platform is used to drive the active optical fiber 51 to move along the optical fiber axis z, and the grating period Λ of the fiber Bragg grating is determined by the moving speed v of the displacement platform and the repetition rate f of the femtosecond laser, that is, Λ=v / f. Therefore, by setting different moving speeds v of the displacement platform and different repetition rates f of the femtosecond laser, a laser resonant cavity 52 with different output wavelengths can be prepared.

[0103] The lengths of the focused spot of the femtosecond laser in the long axis direction x and the short axis direction y satisfy the following formula:

[0104]

[0105] Among them, ω x and ω yare the lengths of the focused spot of the femtosecond laser in the long axis direction x and the short axis direction y, respectively, x and W y are the lengths of the laser spot of the femtosecond laser before being focused in the major axis direction and the minor axis direction, f is the focusing focal length of the femtosecond laser, and λ0 is the wavelength of the femtosecond laser.

[0106] In addition, the femtosecond laser can also be combined with a beam shaper, and the beam shaper is used 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, thereby writing an elliptical modulation plane 520 with arbitrary ellipticity in the active optical fiber 51 to achieve different birefringence degrees of the fiber Bragg grating.

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

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

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

[0110] The photoelectric detector 33 is used to receive the multi-wavelength dual-polarization laser and convert the photoelectric signal;

[0111] The signal collector 34 is used to collect the electrical signal converted by the photodetector 33 and output the collected electrical signal to the signal demodulation device 4 for demodulation processing.

[0112] Embodiment 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 Example 1; the demodulation method comprises the following steps:

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

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

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

[0117] Step 210: using a local microwave source to generate at least three microwave signals, one microwave signal corresponds to one single-frequency dual-polarization laser, 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: Mixing the time domain signal of the multi-wavelength dual-polarization laser with each microwave signal to obtain at least three mixed signals after down-conversion;

[0119] Step 230: performing low-pass filtering on each mixed signal to obtain an in-phase branch signal I(t) and an orthogonal branch signal Q(t);

[0120] Step 240: Perform arc tangent operations on the in-phase branch signal I(t) and the orthogonal 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 Differentiation processing is performed respectively and demodulation is performed to obtain the frequency shift-time variation diagram of the ultrasonic signal acting on each dual-polarization fiber laser.

[0122] Step 300: According to the frequency shift-time variation diagram of each dual-polarization fiber laser 50, the frequency shift time difference between at least three dual-polarization fiber lasers 50 is obtained, and the position information of the ultrasound source is calculated.

[0123] In step 300, the time when the beat frequency drift occurs in each dual polarization fiber laser 50 can be directly obtained from the frequency shift-time variation diagram, and then the time when the beat frequency drift occurs in each dual polarization fiber laser 50 is subtracted to obtain the frequency shift time difference between each dual polarization fiber laser 50.

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

[0125]

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

[0127]

[0128] Among them, v u is the propagation speed of ultrasonic signal, d t(i) is 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 the dual-polarization fiber laser 50 to form the following set of equations:

[0130]

[0131] Finally, by solving the intersection of the at least two hyperbolic equations, the position information (x0, y0) of the ultrasound 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 rather than to limit them. Although the embodiments of the present invention have been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the embodiments of the present invention can still be modified or replaced by equivalents, and these modifications or equivalent replacements 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 collection device, a signal demodulation device and a fiber laser ultrasonic sensor array. The output end of the optical pumping device is connected to the input end of the fiber laser ultrasonic sensor array, and is used to emit pumping light into the fiber laser ultrasonic sensor array to excite the fiber laser ultrasonic sensor array to output multi-wavelength dual-polarization laser; The input end of the light collection device is connected to the output end of the fiber laser ultrasonic sensor array, and is used to collect 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 light collection device, and is used to demodulate the multi-wavelength dual-polarization laser collected by the light collection device to calculate the position information of the ultrasonic signal source; The fiber laser ultrasonic sensor array includes at least three dual-polarization fiber lasers that are not distributed on the same straight line, and is used to generate at least three single-frequency dual-polarization lasers under the excitation of the pump light, and each dual-polarization fiber laser has a different beat frequency and output wavelength.

2. The ultrasonic detection and positioning system according to claim 1, characterized in that: The dual-polarization fiber lasers are connected end to end in sequence, wherein the first dual-polarization fiber laser serves as both the input and output ends of the fiber laser ultrasonic sensor array and is simultaneously connected to the output end of the optical pumping device and the input end of the optical collection device.

3. The ultrasonic detection and positioning system according to claim 2, characterized in that: 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 end of the optical pumping device is connected to the first port of the wavelength division multiplexer, and the input end of the light collection 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 ends of the fiber laser ultrasonic sensor array, is connected to the second port of the wavelength division multiplexer.

4. The ultrasonic detection and positioning system according to claim 1, characterized in that: Each dual-polarization fiber laser comprises an active fiber and a laser resonant cavity, wherein the laser resonant cavity is made in the active fiber; the laser resonant cavities of each dual-polarization fiber laser have different output wavelengths and the same birefringence, or different output wavelengths and different birefringence.

5. The ultrasonic detection and positioning system according to claim 4, characterized in that: The beat frequency f of the dual-polarization fiber laser satisfies the following formula: Wherein, c is the speed of light, B is the birefringence of the laser resonant cavity, n is the average refractive index of the active optical fiber, and λ is the output wavelength of the laser resonant cavity.

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

7. The ultrasonic detection and positioning system according to claim 6, characterized in that: The femtosecond fiber grating is a fiber Bragg grating written by a femtosecond laser, so that the laser resonant cavity is a phase-shifted fiber grating or a 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.

8. The ultrasonic detection and positioning system according to claim 7, 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: Among them, ω x and ω y are the lengths of the focused spot of the femtosecond laser in the long axis direction and the short axis direction, W x and W y are the lengths of the laser spot of the femtosecond laser before being focused in the major axis direction and the minor axis direction, respectively, f is the focusing focal length of the femtosecond laser, and λ0 is the wavelength of the femtosecond laser.

9. The ultrasonic detection and positioning system according to claim 7, characterized in that: The central wavelength λ of the fiber Bragg grating satisfies the following formula: 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.

10. The ultrasonic detection and positioning system according to claim 7, characterized in that: The fiber Bragg grating comprises a plurality of elliptical modulation planes distributed along the axial direction of the optical fiber. Each elliptical modulation plane is perpendicular to the axial direction of the optical fiber and has a long axis direction and a short axis direction.

11. The ultrasonic detection and positioning system according to claim 10, characterized in that: The birefringence B of the fiber Bragg grating satisfies the following formula: Among them, n x and n y are the effective refractive index of the fiber Bragg grating in the long axis direction and the short axis direction, λ x and λ y are the central wavelengths of the fiber Bragg grating in the major axis direction and the minor axis direction respectively, and Λ is the grating period of the fiber Bragg grating.

12. A demodulation method for ultrasonic detection and positioning, characterized in that: The steps include: Step 100: Acquire the time domain signal of the multi-wavelength dual-polarization laser output by the fiber laser ultrasonic sensor array; Step 200: Demodulate the time domain signal of the multi-wavelength dual-polarization laser to obtain a frequency shift-time variation diagram of each dual-polarization fiber laser; Step 300: Obtain the frequency shift time difference between at least three dual-polarization fiber lasers according to the frequency shift-time variation diagram of each dual-polarization fiber laser, and calculate the position information of the ultrasound source.

13. The demodulation method according to claim 12, characterized in that: In step 200, the steps of demodulating the time domain signal of the multi-wavelength dual-polarization laser are as follows: Step 210: using a local microwave source to generate at least three microwave signals, one microwave signal corresponds to one single-frequency dual-polarization laser, the corresponding microwave signal and the single-frequency dual-polarization laser have the same frequency and a phase difference of π / 2; Step 220: Mixing the time domain signal of the multi-wavelength dual-polarization laser with each microwave signal to obtain at least three mixed signals after down-conversion; Step 230: performing low-pass filtering on each mixed signal to obtain an in-phase branch signal I(t) and an orthogonal branch signal Q(t); Step 240: Perform arc tangent operations on the in-phase branch signal I(t) and the orthogonal branch signal Q(t) of each mixing signal to obtain the phase information of the ultrasonic signal. Step 250: Phase information of each mixing signal Differentiation processing is performed respectively and demodulation is performed to obtain the frequency shift-time variation diagram of the ultrasonic signal acting on each dual-polarization fiber laser.

14. The demodulation method according to claim 12, characterized in that: In step 300, a plane rectangular coordinate system is constructed to obtain the coordinate value (xi, yi) of the i-th dual-polarization fiber laser in the plane rectangular coordinate system. Assuming that the coordinate value of the ultrasonic source is (x0, y0), the distance d between the i-th dual-polarization fiber laser and the ultrasonic source is i as follows: Select any dual-polarization fiber laser as the reference laser (x', y'), and construct the hyperbolic equation between the i-th dual-polarization fiber laser and the reference laser as follows: Among them, v u is the propagation speed of ultrasonic signal, d t(i) is the frequency shift time difference between the i-th dual-polarization fiber laser and the reference laser; Select at least two hyperbolic equations of dual-polarization fiber lasers to form the following set of equations: Finally, by solving the intersection of the at least two hyperbolic equations, the position information (x0, y0) of the ultrasound source can be obtained.

Images