A flexible fiber optic Mach-Zehnder interferometric ultrasonic sensor

Abstract

The present invention relates to a flexible fiber optic Mach-Zehnder interferometric ultrasonic sensor, which includes a first single-mode fiber, a sensing fiber, and a second single-mode fiber. The first single-mode fiber, the sensing fiber, and the second single-mode fiber are all cylindrical, wherein: the first single-mode fiber, the sensing fiber, and the second single-mode fiber are connected in sequence; and both between the first single-mode fiber and the sensing fiber and between the sensing fiber and the second single-mode fiber are connected by a coupling material. The present invention can achieve multi-directional ultrasonic detection by utilizing the superior elasticity of the coupling material and the compactness of the structure; the ultrasonic stress change of the coupling material makes the sensor highly sensitive to external ultrasonic signals; the low thermal conductivity of the coupling material can eliminate the temperature cross-sensitivity effect of the sensor. The packaging structure is a through-hole ring, and both the first single-mode fiber and the second single-mode fiber are fixed at the through-hole connection, and the application scenario range is large.

Description

Technical Field

[0001] The present invention belongs to the technical field of fiber optic sensing, and relates to a flexible fiber optic Mach-Zehnder interferometric ultrasonic sensor. Background Art

[0002] Ultrasonic non-destructive testing technology is widely used in engineering safety monitoring, geological exploration, bridge detection and other fields due to its advantages of strong penetration ability, high sensitivity, accurate positioning of internal reflectors, safe operation, and portable equipment. At present, piezoelectric ceramic transducers (PZT) are the main ultrasonic detection devices, occupying the market position and being widely used in ultrasonic medical diagnosis, structural non-destructive testing, underwater sonar and other fields. However, the internal piezoelectric ceramic material is easily damaged, the acoustic impedance matching needs to be improved, it is vulnerable to electromagnetic interference, and the detection system is bulky. As a new type of sensing technology developed in recent years, fiber optic ultrasonic sensing technology has a wider response frequency band, higher detection sensitivity, smaller size, lighter weight, stronger anti-electromagnetic interference ability, longer signal transmission distance, and better fidelity compared with traditional PZT sensors.

[0003] Fiber optic ultrasonic sensing technology mainly includes three types: fiber Bragg grating ultrasonic sensing technology, intensity-type ultrasonic sensing technology, and interferometric ultrasonic sensing technology. Among them, interferometric fiber optic ultrasonic sensors mainly obtain ultrasonic information by the change of the phase of the light propagating in the fiber caused by ultrasonic waves. Compared with the other two technologies, interferometric fiber optic sensing technology not only has great advantages in detection sensitivity, but also has the advantages of flexible and diverse structures, a wide range of detection parameters, and rich demodulation methods. Existing fiber optic ultrasonic sensors are restricted in practical applications due to structural design reasons and cannot detect multi-directional ultrasonic fields. Especially for high-sensitivity fiber optic ultrasonic sensors of the interferometric type, due to the limitation of the interference structure, they can only detect ultrasonic signals in a specific direction. At the same time, temperature changes in the measurement environment will also cause the working point of the sensor to drift, resulting in a series of problems such as reduced sensitivity, signal distortion, and decreased linearity of the fiber optic ultrasonic sensor.

[0004] Therefore, it is necessary to study a fiber optic ultrasonic sensor with high sensitivity, capable of detecting multi-directional ultrasonic signals, and strong anti-temperature interference ability. Summary of the Invention

[0005] In order to solve the above problems existing in the prior art, the present invention provides a flexible fiber optic Mach-Zehnder interferometric ultrasonic sensor. The technical problems to be solved by the present invention are realized through the following technical solutions:

[0006] An embodiment of the present invention provides a flexible fiber optic Mach-Zehnder interferometric ultrasonic sensor. The interferometric ultrasonic sensor includes a first single-mode fiber, a sensing fiber, and a second single-mode fiber. The first single-mode fiber, the sensing fiber, and the second single-mode fiber are all cylindrical, where:

[0007] The first single-mode fiber, the sensing fiber, and the second single-mode fiber are connected in sequence;

[0008] The first single-mode fiber, the sensing fiber, and the second single-mode fiber all include a cladding and a central core. The central cores of the first single-mode fiber, the sensing fiber, and the second single-mode fiber are correspondingly arranged on the central axis of the cladding, and the central cores arranged in the first single-mode fiber, the sensing fiber, and the second single-mode fiber are connected in sequence;

[0009] And the first single-mode fiber and the sensing fiber, and the sensing fiber and the second single-mode fiber are both connected by a coupling material.

[0010] In an embodiment of the present invention, the coupling material includes polycarbonate or polyethylene naphthalate.

[0011] In an embodiment of the present invention, the sensing fiber includes a first dual-core fiber. In the axial direction within the first dual-core fiber, a first side core is provided. The first side core and the central core of the first dual-core fiber have a first preset distance, and the first preset distance is greater than zero.

[0012] In an embodiment of the present invention, the refractive indices of the central core and the first side core of the first dual-core fiber are the same, and the refractive indices of the central core and the first side core of the first dual-core fiber are greater than the refractive index of the cladding of the first dual-core fiber; the diameter of the central core of the first dual-core fiber is smaller than that of the first side core.

[0013] In an embodiment of the present invention, the sensing fiber includes a first triple-core fiber. In the axial direction within the first triple-core fiber, a second side core and a third side core are provided. The second side core and the third side core are asymmetrically distributed on both sides of the central core of the first triple-core fiber.

[0014] In one embodiment of the present invention, the refractive indices of the central core of the first three-core optical fiber, the second side core, and the third side core are the same, and the refractive indices of the central core of the first three-core optical fiber, the second side core, and the third side core are greater than the refractive index of the cladding of the first three-core optical fiber. The diameters of the second side core and the third side core are the same and greater than the diameter of the central core of the first three-core optical fiber. The centers of the three end faces at the same end of the second side core, the third side core, and the central core of the first three-core optical fiber form an isosceles triangle, and the apex angle of the isosceles triangle is 120°.

[0015] In one embodiment of the present invention, the sensing optical fiber includes a second two-core optical fiber and a second three-core optical fiber. The first single-mode optical fiber, the second two-core optical fiber, the second three-core optical fiber, and the second single-mode optical fiber are connected in sequence, and the second two-core optical fiber and the second three-core optical fiber are connected by polycarbonate or polyethylene naphthalate;

[0016] In the axial direction of the second two-core optical fiber, a fourth side core is provided. The fourth side core and the central core of the second two-core optical fiber have a first preset distance, and the first preset distance is greater than zero;

[0017] In the axial direction of the second three-core optical fiber, a fifth side core and a sixth side core are provided. The fifth side core and the sixth side core are asymmetrically distributed on both sides of the central core of the second three-core optical fiber.

[0018] In one embodiment of the present invention, the refractive indices of the central core of the second two-core optical fiber and the fourth side core are the same, and the refractive indices of the central core of the second two-core optical fiber and the fourth side core are greater than the refractive index of the cladding of the second two-core optical fiber; the diameter of the central core of the second two-core optical fiber is smaller than that of the fourth side core;

[0019] The refractive indices of the central core of the second three-core optical fiber, the fifth side core, and the sixth side core are the same, and the refractive indices of the central core of the second three-core optical fiber, the fifth side core, and the sixth side core are less than the refractive index of the cladding of the second three-core optical fiber. The diameters of the fifth side core and the sixth side core are the same and greater than the diameter of the central core of the second three-core optical fiber. The centers of the three end faces at the same end of the fifth side core, the sixth side core, and the central core of the second three-core optical fiber form an isosceles triangle, and the apex angle of the isosceles triangle is 120°.

[0020] One embodiment of the present invention further provides a sensor packaging structure, including the interferometric ultrasonic sensor described in any one of the above embodiments and a ring with a through hole. The interferometric ultrasonic sensor passes through the through hole of the ring.

[0021] In one embodiment of the present invention, the material of the ring includes polyurethane elastomer.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0023] The flexible interferometric ultrasonic sensor structure designed by the present invention can receive ultrasonic signals from different directions. The overall structure is simple, easy to fabricate, and has a low cost. Moreover, due to the high elasticity, flexibility, and high light transmittance of the coupling material, the sensor can detect a wide range of ultrasonic frequencies and has high sensitivity.

[0024] The coupling material adopted by the present invention has low thermal conductivity, and the ultrasonic stress change of PC / PEN is several orders of magnitude larger than the change caused by thermal expansion. At the same time, since most of the energy of the core mode and the cladding mode is located in the silica microfibers, which have similar thermo-optic coefficients, the sensor has strong resistance to temperature interference.

[0025] The interferometric ultrasonic sensor provided by the present invention is composed of all optical fibers, and the fiber ultrasonic sensor is a passive device. Therefore, it overcomes the drawback that traditional ultrasonic sensors cannot work under strong electromagnetic interference. Compared with traditional ultrasonic sensors, it also has the advantages of small size and light weight. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a schematic structural diagram of a flexible fiber optic Mach-Zehnder interferometric ultrasonic sensor provided by an embodiment of the present invention;

[0027] Figure 2 It is a schematic structural diagram of another flexible fiber optic Mach-Zehnder interferometric ultrasonic sensor provided by an embodiment of the present invention;

[0028] Figure 3 It is a schematic structural diagram of yet another flexible fiber optic Mach-Zehnder interferometric ultrasonic sensor provided by an embodiment of the present invention;

[0029] Figure 4 It is a schematic diagram of the sensor under the action of ultrasonic waves in different directions provided by an embodiment of the present invention;

[0030] Figures 5a - 5c They are respectively schematic diagrams of the results of the total sound pressure field, stress, sound pressure level, and displacement of the ultrasonic field simulation of the sensor provided by an embodiment of the present invention;

[0031] Figure 6 It is a schematic structural diagram of the sensor package provided by an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0032] The following further describes the present invention in detail with specific embodiments, but the embodiments of the present invention are not limited thereto.

[0033] Example 1

[0034] At present, due to the limitation of the interference structure, the existing fiber optic interferometric ultrasonic sensors cannot detect multi-directional ultrasonic fields and are restricted in practical applications. Moreover, the existing fiber optic interferometric ultrasonic sensors are greatly affected by temperature.

[0035] Based on this, please refer to Figures 1 to 3 , Figure 1 which is a schematic structural diagram of a flexible fiber optic Mach-Zehnder interferometric ultrasonic sensor provided by an embodiment of the present invention, Figure 2 which is another schematic structural diagram of a flexible fiber optic Mach-Zehnder interferometric ultrasonic sensor provided by an embodiment of the present invention, Figure 3 which is still another schematic structural diagram of a flexible fiber optic Mach-Zehnder interferometric ultrasonic sensor provided by an embodiment of the present invention. An embodiment of the present invention provides a flexible fiber optic Mach-Zehnder interferometric ultrasonic sensor. The interferometric ultrasonic sensor includes a first single-mode fiber 1 (the full English name of the single-mode fiber is Single Mode Fiber, abbreviated as SMF), a sensing fiber, and a second single-mode fiber 2. The first single-mode fiber 1, the sensing fiber, and the second single-mode fiber 2 are all cylindrical. Among them:

[0036] The first single-mode fiber 1, the sensing fiber, and the second single-mode fiber 2 are connected in sequence;

[0037] The first single-mode fiber 1, the sensing fiber, and the second single-mode fiber 2 all include a cladding 3 and a core 4. The cores 4 of the first single-mode fiber 1, the sensing fiber, and the second single-mode fiber 2 are correspondingly arranged on the central axis of the cladding 3, and the cores arranged in the first single-mode fiber 1, the sensing fiber, and the second single-mode fiber are connected in sequence;

[0038] Moreover, the first single-mode fiber 1 and the sensing fiber, and the sensing fiber and the second single-mode fiber 2 are all connected through a coupling material.

[0039] Furthermore, the materials of the cladding and the core of the first single-mode fiber 1, the sensing fiber, and the second single-mode fiber 2 are all silica microfibers.

[0040] Furthermore, the coupling material includes polycarbonate (PC) or polyethylene naphthalate two formic acid glycol ester (PEN). That is to say, the first single-mode fiber 1 and the sensing fiber, and the sensing fiber and the second single-mode fiber are connected through polycarbonate or polyethylene naphthalate two formic acid glycol ester.

[0041] The fiber optic Mach-Zehnder interferometric ultrasonic sensor proposed by the present invention includes two single-mode optical fibers (i.e., the first single-mode optical fiber and the second single-mode optical fiber) and a sensing optical fiber, and soft splicing is carried out between the two single-mode optical fibers and the sensing optical fiber using PC or PEN (coupling material). When light is injected from the first single-mode optical fiber 1 into the sensing optical fiber through the coupling material, due to the high light transmittance and certain width of the coupling material, scattering occurs when the light reaches the coupling material. Due to the different core diameters and positions of the single-mode optical fiber and the sensing optical fiber, a part of the light in the core enters the cladding of the sensing optical fiber to excite the cladding mode of the optical fiber, and the other part of the light continues to propagate along the core of the sensing optical fiber. After the core fundamental mode and the cladding mode propagate a certain distance in the sensing optical fiber, they are transmitted through the coupling material again and finally reach the second single-mode optical fiber for output. Since the effective refractive indices of the core fundamental mode and the cladding mode are different, this results in an optical path difference when they propagate in the optical fiber, thereby generating interference.

[0042] In addition, for a better understanding of the present invention, the present invention also provides a preparation method for the sensor, specifically: during preparation, first, the end faces of the first single-mode optical fiber 1, the sensing optical fiber, and the second single-mode optical fiber 2 are cut flat and cleaned with alcohol. Then, the first section of the first single-mode optical fiber 1 and the sensing optical fiber are aligned under a microscope so that their cut surfaces are in contact with each other. A drop of PC or PEN solution is added at the connection point between the first single-mode optical fiber 1 and the sensing optical fiber, and the PC or PEN solution is allowed to fully penetrate into the contact surface between the first single-mode optical fiber 1 and the sensing optical fiber, and the excess solution is removed. Soft splicing is carried out by heating and curing. After curing, PC or PEN has a high ultimate tensile strength, making the structure have better elasticity and flexibility, while still maintaining sufficient strength for optical fiber connection. The other end of the sensing optical fiber is soft-spliced with the second single-mode optical fiber 2 in the same way, and finally an interferometric ultrasonic sensor is formed.

[0043] The interference intensity between the core fundamental mode and the cladding mode of the interferometric ultrasonic sensor can be defined as follows:

[0044]

[0045] Where, I core is the light intensity of the core fundamental mode, I clad is the light intensity of the cladding mode, and ΔΦ is the phase difference between the core fundamental mode and the cladding mode.

[0046] The phase difference between the core fundamental mode and the cladding mode can be expressed as:

[0047]

[0048] Where, L is the length of the sensing optical fiber, λ is the optical wavelength in the optical fiber, and Δn eff is the effective refractive index difference between the core fundamental mode and the cladding mode.

[0049] When the phase difference satisfies ΔΦ = (2k + 1)π, (k = 0, ±1, ±2, L), the interference intensity is minimized, and the interference valley wavelength at this time is:

[0050]

[0051] When measuring the ultrasonic signal, the SMF parts at both ends are fixed. Due to the high elasticity and flexibility of the coupling material at the connection, the sensing optical fiber will displace when receiving ultrasonic signals in any one direction, resulting in a change in length L. At the same time, due to the elasto-optic effect, the effective refractive index of the transmission mode will change. Therefore, the effective refractive index difference between the core fundamental mode and the cladding mode changes with the ultrasonic signal, and according to formula (2), the phase difference ΔΦ will change. Therefore, ultrasonic detection can be achieved by detecting the change in the output interference spectrum.

[0052] In summary, the present invention uses PC or PEN material as the coupler of the sensor to form an interferometric ultrasonic sensor. Since the light transmittance of the coupling material PC or PEN used is high, when light enters the sensing optical fiber from the first single-mode optical fiber through PC or PEN, multiple cladding modes are excited due to core mismatch. These cladding modes and the core fundamental mode are coupled into PC or PEN after transmission in the sensing optical fiber, and then output via the second single-mode optical fiber to form a fiber Mach-Zehnder interferometer. The SMFs at both ends of the sensor are fixed, and the ultrasonic signals received by the coupling material and the free-end sensing optical fiber will cause strain, thereby modulating the intensity or wavelength change of the sensor signal. The present invention utilizes the superior elasticity of PC or PEN and the compactness of the structure to achieve multi-directional ultrasonic detection without the need for additional external vibration receivers, making the sensor highly sensitive to external ultrasonic signals or mechanical vibrations. Moreover, the low thermal conductivity of PC or PEN can eliminate the temperature cross-sensitivity effect of the sensor.

[0053] Embodiment 2

[0054] Please refer to Figure 1 , based on the above embodiments, the present invention provides a specific flexible fiber Mach-Zehnder interferometric ultrasonic sensor. The sensing optical fiber of the interferometric ultrasonic sensor is the first dual-core fiber 5 (the full English name of the dual-core fiber is dual-core fiber, abbreviated as DCF). That is, the interferometric ultrasonic sensor is composed of a first single-mode optical fiber 1, a first dual-core fiber 5, and a second single-mode optical fiber 2 connected in sequence. The first single-mode optical fiber 1 is connected to the first dual-core fiber 5 through PC or PEN, and the first dual-core fiber 5 and the second single-mode optical fiber 2 are connected through PC or PEN.

[0055] Furthermore, the refractive index of the central cores of the first single-mode optical fiber 1 and the second single-mode optical fiber 2 is greater than that of the claddings of the first single-mode optical fiber 1 and the second single-mode optical fiber 2. Only in this way can the optical signal form total reflection at the junction of the core and the cladding and be transmitted.

[0056] Optionally, the refractive index of the central cores of the first single-mode optical fiber 1 and the second single-mode optical fiber 2 is 1.4682, and the diameter of the central cores is 9 μm.

[0057] Optionally, the refractive index of the claddings of the first single-mode optical fiber 1 and the second single-mode optical fiber 2 is 1.4628, and the diameter of the claddings is 125 μm.

[0058] Furthermore, a first side core 6 is arranged in the axial direction within the first dual-core optical fiber 5. The first side core 6 and the central core 4 of the first dual-core optical fiber 5 have a first preset distance, and the first preset distance is greater than zero.

[0059] Optionally, the first preset distance is 36.5 μm.

[0060] Furthermore, the refractive indices of the central core 4 and the first side core 6 of the first dual-core optical fiber 5 are the same, and the refractive indices of the central core 4 and the first side core 6 of the first dual-core optical fiber 5 are greater than that of the cladding 3 of the first dual-core optical fiber 5. Only in this way can the optical signal form total reflection at the junction of the core and the cladding and be transmitted; the diameter of the central core 4 of the first dual-core optical fiber 5 is smaller than that of the first side core 6.

[0061] Optionally, the diameters of the central core 4 and the first side core 6 of the first dual-core optical fiber 5 are 5.5 μm and 8.5 μm respectively.

[0062] Optionally, the refractive indices of the central core 4 and the first side core 6 of the first dual-core optical fiber 5 are 1.457.

[0063] Optionally, the diameter of the cladding of the first dual-core optical fiber 5 is 125 μm, and the refractive index is 1.444.

[0064] The present invention uses a PC or PEN material as a coupler of the sensor to form an interferometric ultrasonic sensor. Since the coupling materials PC or PEN used have a high light transmittance, when light enters the first dual-core fiber from the first single-mode fiber through PC or PEN, multiple cladding modes are excited due to core mismatch. These cladding modes and the core fundamental mode are transmitted in the sensing fiber and then coupled into PC or PEN, and then output via the second single-mode fiber, forming a fiber Mach-Zehnder interferometer. The single-mode fibers at both ends of the sensor are fixed, and the ultrasonic signals received by the coupling material and the free-end sensing fiber will cause strain, thereby modulating the intensity or wavelength change of the sensor signal. The present invention utilizes the excellent elasticity of PC or PEN and the compactness of the structure to achieve multi-directional ultrasonic detection without the need for an additional external vibration receiver, making the sensor highly sensitive to external ultrasonic signals or mechanical vibrations. Moreover, the low thermal conductivity of PC or PEN can eliminate the temperature cross-sensitivity effect of the sensor.

[0065] Embodiment III

[0066] Please refer to Figure 2 , on the basis of the above embodiments, the present invention provides a specific flexible fiber Mach-Zehnder interferometric ultrasonic sensor. The sensing fiber of the interferometric ultrasonic sensor is the first three-core fiber 7 (the full English name of the three-core fiber is three-core fiber, abbreviated as 3CF). That is, the interferometric ultrasonic sensor is composed of a first single-mode fiber 1, a first three-core fiber 7, and a second single-mode fiber 2 connected in sequence. The first single-mode fiber 1 is connected to the first three-core fiber 7 through PC or PEN, and the first three-core fiber 7 and the second single-mode fiber 2 are connected through PC or PEN.

[0067] Furthermore, the refractive index of the central core of the first single-mode fiber 1 and the second single-mode fiber 2 is greater than the refractive index of the cladding of the first single-mode fiber 1 and the second single-mode fiber 2, so that the optical signal can form total reflection at the junction of the core and the cladding for transmission.

[0068] Optionally, the refractive index of the central core of the first single-mode fiber 1 and the second single-mode fiber 2 is 1.4682, and the diameter of the central core is 9 μm.

[0069] Optionally, the refractive index of the cladding of the first single-mode fiber 1 and the second single-mode fiber 2 is 1.4628, and the diameter of the cladding is 125 μm.

[0070] Furthermore, a second side core 8 and a third side core 9 are arranged in the axial direction inside the first three-core fiber 7, and the second side core 8 and the third side core 9 are asymmetrically distributed on both sides of the central core 4 of the first three-core fiber 7.

[0071] The refractive indices of the central core 4, the second side core 8, and the third side core 9 of the first triple-core optical fiber 7 are the same, and the refractive indices of the central core 4, the second side core 8, and the third side core 9 of the first triple-core optical fiber 7 are greater than the refractive index of the cladding 3 of the first triple-core optical fiber 7. Only in this way can the optical signal form total reflection at the core-cladding junction and be transmitted. The diameters of the second side core 8 and the third side core 9 are the same and greater than the diameter of the central core 4 of the first triple-core optical fiber 7. The central points of the three end faces at the same end of the second side core 8, the third side core 9, and the central core 4 of the first triple-core optical fiber 7 form an isosceles triangle, and the apex angle of the isosceles triangle is 120°.

[0072] Optionally, the diameter of the cladding 3 of the first triple-core optical fiber 7 is 125 μm, the diameter of the central core of the first triple-core optical fiber 7 is 5.5 μm, and the diameters of the second side core 8 and the third side core 9 are 8.1 μm.

[0073] Optionally, the distance between the central core of the first triple-core optical fiber 7 and the second side core 8, and the distance between the central core of the first triple-core optical fiber 7 and the third side core 9 are 31.5 μm, and the distance between the second side core 8 and the third side core 9 is 54.5 μm.

[0074] Optionally, the refractive index of the cladding 3 of the first triple-core optical fiber 7 is 1.447, and the refractive indices of the central core, the second side core 8, and the third side core 9 of the first triple-core optical fiber 7 are 1.457.

[0075] Under the action of ultrasonic waves, there are mainly two reasons for the change in the intensity or wavelength of the sensor signal. One is the deformation of the coupling material caused by the ultrasonic signal. When an ultrasonic signal is emitted from a certain direction and acts on the sensor, due to the high elasticity and flexibility of the coupling material, slight deformation will occur. Here, taking Figure 2 the sensing structure as an example, the sensing principle of the sensor is introduced, and other sensors can refer to this principle. As Figure 4 shown, when the ultrasonic wave acts on the sensor from the A or F direction, the coupling material at ① shrinks, and the coupling material at ② stretches; when the ultrasonic wave acts on the sensor from the C or D direction, the coupling material at ① stretches, and the coupling material at ② shrinks; when the ultrasonic wave acts on the sensor from B, the upper side of the coupling material at ① and ② shrinks, and the lower side stretches; when the ultrasonic wave acts on the sensor from E, the upper side of the coupling material at ① and ② stretches, and the lower side shrinks. The second is that the ultrasonic signal can directly act on the sensing optical fiber through the coupling agent water, and the sensing optical fiber generates slight deformation. At the same time, due to the elasto-optic effect, the effective refractive index of the transmission mode will change. Therefore, the effective refractive index difference between the core fundamental mode and the cladding mode changes with the ultrasonic signal.

[0076] To illustrate the effect of the interferometric ultrasonic sensor of the present invention, an acoustic simulation of the sensor is carried out using finite element software to simulate the performance of the ultrasonic sensor. Here, taking the sensing optical fiber as the first three-core optical fiber as an example, the corresponding simulation results are shown in Figures 5(a), 5(b) and 5(c) respectively. Figure 5(a) is the XZ cross-sectional view of the spherical total sound pressure field, from which it can be clearly seen from which direction the sound wave is incident into the domain. Figure 5(b) is the stress diagram of the ultrasonic signal corresponding to the sensor structure. Since the two ends of the single mode are fixed, a certain stress is applied to the middle sensing optical fiber part, about 1 Pa, corresponding to the pressure amplitude of the incident ultrasonic signal. Figure 5(c) is the sound pressure level of the spherical sound field and the displacement diagram corresponding to the sensor. The results show that the displacement is larger at the coupling materials on both sides, and the maximum displacement is about 8×10 -8 mm. There is also a concentrated sound pressure level in the coupling material parts on both sides, and the maximum sound pressure level can reach 120 dB, which indicates that PC / PEN is more likely to detect ultrasonic signals compared with optical fiber materials. When the sensor structure receives ultrasonic signals, the deformation of PC / PEN will affect the change of L in formula (2) and the change of the effective refractive index of the transmission mode, thereby detecting the change of the output spectrum.

[0077] In summary, the present invention uses PC or PEN materials as the coupler of the sensor to form an interferometric ultrasonic sensor. Since the coupling materials PC or PEN used have a high light transmittance, when light passes from the first single-mode optical fiber through PC or PEN into the first three-core optical fiber 7, multiple cladding modes are excited due to core mismatch. These cladding modes and the core fundamental mode are coupled into PC or PEN after transmission in the sensing optical fiber, and then output via the second single-mode optical fiber to form a fiber Mach-Zehnder interferometer. The single-mode optical fibers at both ends of the sensor are fixed, and the ultrasonic signals received by the coupling material and the free-end sensing optical fiber will cause strain, thereby modulating the intensity or wavelength change of the sensor signal. The present invention utilizes the excellent elasticity of PC or PEN and the compactness of the structure to achieve multi-directional ultrasonic detection without the need for additional external vibration receivers, making the sensor highly sensitive to external ultrasonic signals or mechanical vibrations. Moreover, the low thermal conductivity of PC or PEN can eliminate the temperature cross-sensitivity effect of the sensor.

[0078] Example 4

[0079] Please refer to Figure 3, on the basis of the above embodiments, the present invention provides a specific flexible fiber optic Mach-Zehnder interferometric ultrasonic sensor. The sensing fiber of the interferometric ultrasonic sensor is the second dual-core fiber 10 and the second triple-core fiber 11. That is, the interferometric ultrasonic sensor is composed of a first single-mode fiber 1, a second dual-core fiber 10, a second triple-core fiber 11, and a second single-mode fiber 2 connected in sequence. The first single-mode fiber 1 and the second dual-core fiber 10, the second dual-core fiber 10 and the second triple-core fiber 11, and the second triple-core fiber 11 and the second single-mode fiber 2 are all connected by PC or PEN.

[0080] Further, the refractive index of the central core of the first single-mode fiber 1 and the second single-mode fiber 2 is greater than the refractive index of the cladding of the first single-mode fiber 1 and the second single-mode fiber 2. Only in this way can the optical signal form total reflection at the junction of the core and the cladding and be transmitted.

[0081] Optionally, the refractive index of the central core of the first single-mode fiber 1 and the second single-mode fiber 2 is 1.4682, and the diameter of the central core is 9μm.

[0082] Optionally, the refractive index of the cladding of the first single-mode fiber 1 and the second single-mode fiber 2 is 1.4628, and the diameter of the cladding is 125μm.

[0083] Further, a fourth side core 12 is arranged in the axial direction in the second dual-core fiber 10. The fourth side core 12 and the central core 4 of the second dual-core fiber 10 have a first preset distance, and the first preset distance is greater than zero.

[0084] Optionally, the first preset distance is 36.5μm.

[0085] Further, the refractive indices of the central core 4 of the second dual-core fiber 10 and the fourth side core 12 are the same, and the refractive indices of the central core 4 of the second dual-core fiber 10 and the fourth side core 12 are greater than the refractive index of the cladding 3 of the second dual-core fiber 10; the diameter of the central core 4 of the second dual-core fiber 10 is smaller than that of the fourth side core 12.

[0086] Optionally, the diameters of the central core 4 of the second dual-core fiber 10 and the fourth side core 12 are 5.5μm and 8.5μm respectively.

[0087] Optionally, the refractive indices of the central core 4 of the second dual-core fiber 10 and the fourth side core 12 are 1.457.

[0088] Optionally, the cladding diameter of the second dual-core fiber 10 is 125μm, and the refractive index is 1.444.

[0089] Furthermore, a fifth side core 13 and a sixth side core 14 are arranged in the axial direction within the second three-core optical fiber 11, and the fifth side core 13 and the sixth side core 14 are asymmetrically distributed on both sides of the central core 4 of the second three-core optical fiber 11.

[0090] The refractive indices of the central core 4, the fifth side core 13, and the sixth side core 14 of the second three-core optical fiber 11 are the same, and the refractive indices of the central core 4, the fifth side core 13, and the sixth side core 14 of the second three-core optical fiber 11 are less than the refractive index of the cladding 3 of the second three-core optical fiber 11. The diameters of the fifth side core 13 and the sixth side core 14 are the same and larger than the diameter of the central core 4 of the second three-core optical fiber 11. The central points of the three end faces at the same end of the fifth side core 13, the sixth side core 14, and the central core 4 of the second three-core optical fiber 11 form an isosceles triangle, and the apex angle of the isosceles triangle is 120°.

[0091] Optionally, the diameter of the cladding 3 of the second three-core optical fiber 11 is 125 μm, the diameter of the central core of the second three-core optical fiber 11 is 5.5 μm, and the diameters of the fifth side core 13 and the sixth side core 14 are 8.1 μm.

[0092] Optionally, the distance between the central core of the second three-core optical fiber 11 and the fifth side core 13, and the distance between the central core of the second three-core optical fiber 11 and the sixth side core 14 are 31.5 μm, and the distance between the fifth side core 13 and the sixth side core 14 is 54.5 μm.

[0093] Optionally, the refractive index of the cladding 3 of the second three-core optical fiber 11 is 1.447, and the refractive indices of the central core, the fifth side core 13, and the sixth side core 14 of the second three-core optical fiber 11 are 1.457.

[0094] The present invention uses PC or PEN material as the coupler of the sensor to form an interferometric ultrasonic sensor. Due to the high light transmittance of the coupling material PC or PEN used, when light enters the second two-core optical fiber 10 and the second three-core optical fiber 11 from the first single-mode optical fiber through PC or PEN in sequence, multiple cladding modes will be excited due to core mismatch. These cladding modes and the core fundamental mode are coupled into PC or PEN after transmission in the sensing optical fiber, and then output via the second single-mode optical fiber to form a fiber Mach-Zehnder interferometer. The single-mode optical fibers at both ends of the sensor are fixed, and the ultrasonic signals received by the coupling material and the free-end sensing optical fiber will cause strain, thereby modulating the intensity or wavelength change of the sensor signal. The present invention utilizes the excellent elasticity of PC or PEN and the compactness of the structure to achieve multi-directional ultrasonic detection without the need for additional external vibration receivers, making the sensor highly sensitive to external ultrasonic signals or mechanical vibrations. Moreover, the low thermal conductivity of PC or PEN can eliminate the temperature cross-sensitivity effect of the sensor.

[0095] Embodiment 5

[0096] On the basis of the above embodiments, the present invention further provides a sensor packaging structure, which includes the interferometric ultrasonic sensor described in any one of the above embodiments and a ring with a through hole, and the interferometric ultrasonic sensor passes through the through hole of the ring. For example, please refer to Figure 6 , and its interferometric ultrasonic sensor is Figure 3 the structure shown. Among them, the ① end is the incident light wave, which is connected to a tunable wavelength laser. The ② end is the outgoing light wave, which is connected to a photodiode, as well as a current-voltage conversion high-speed amplification circuit, an oscilloscope, etc. The ③ structure is a ring packaging structure.

[0097] The present invention is mainly made by soft splicing other sensing optical fibers between two single-mode optical fibers with a coupling material (polycarbonate (PC), polyethylene naphthalate glycol ester (PEN)) with high light transmittance, high heat resistance, and good flexibility. A certain width of PC / PEN material is used as the coupler of the interference system, and finally an interference structure of SMF-(PC / PEN)-(DCF / 3CF / DCF+3CF)-(PC / PEN)-SMF is formed. The packaging module is designed as a ring structure. The thickness of the ring structure is 5μm. The diameter of the ring structure is equal to the sum of the widths of the coupling materials on both sides and the length of the sensing optical fiber. The diameters of the two holes of the ring are 126μm respectively, slightly larger than the cladding diameter of the single mode, which is 125μm. The ring structure can not only fix the single-mode part of the sensor, but also drive the sensor to rotate 360° together to receive ultrasonic signals from different directions. The overall structure is simple, easy to manufacture, and has a low cost. Moreover, due to the high elasticity, flexibility, and high light transmittance of the coupling material, the sensor can detect a wide range of ultrasonic frequencies and has high sensitivity.

[0098] The material used for the ring structure in the packaging module is polyurethane elastomer. The polyurethane elastomer has good sound transmission performance, which can make the ultrasonic wave pass through the ring structure without reflection and loss when it is incident. At the same time, the polyurethane elastomer has the characteristics of good matching of characteristic acoustic impedance with water and low acoustic attenuation constant, which fully meets the basic requirements of the design of underwater acoustic sound transmission materials, enabling the sensor to operate underwater and increasing the application scenarios of the sensor. The single-mode optical fiber and the ring hole are fixed with epoxy resin, which can enable the sensor to absorb the residual ultrasonic field and has a large application scenario range.

[0099] The interferometric ultrasonic sensor provided by the present invention is composed of all optical fibers, and its diameter is composed of 125μm silica microfibers (the materials of the cladding and the core of the interferometric ultrasonic sensor are both silica microfibers), and the diameter of the ring structure can be adjusted according to the actual ultrasonic detection environment. Therefore, it not only has a high spatial resolution but also meets the measurement requirements of narrow spaces.

[0100] In the description of the present invention, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, features defined with "first" and "second" may explicitly or implicitly include one or more of such features. In the description of the present invention, "a plurality of" means two or more unless otherwise specifically defined.

[0101] In the description of this specification, the description with reference to terms such as "an embodiment", "some embodiments", "examples", "specific examples", or "some examples" means that the specific features, structures, materials, or specific data points described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or specific data points described may be combined in a suitable manner in any one or more embodiments or examples. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification.

[0102] The above content is a further detailed description of the present invention in conjunction with specific preferred embodiments, and it cannot be determined that the specific implementation of the present invention is limited only to these descriptions. For those of ordinary skill in the technical field to which the present invention pertains, without departing from the concept of the present invention, several simple deductions or substitutions can be made, and all should be regarded as belonging to the protection scope of the present invention.

Claims

1. A flexible fiber optic Mach-Zehnder interferometric ultrasonic sensor, characterized in that, The interferometric ultrasonic sensor includes a first single-mode optical fiber (1), a sensing optical fiber, and a second single-mode optical fiber (2). The first single-mode optical fiber (1), the sensing optical fiber, and the second single-mode optical fiber (2) are all cylindrical. Among them: The first single-mode optical fiber (1), the sensing optical fiber, and the second single-mode optical fiber (2) are connected in sequence; The first single-mode optical fiber (1), the sensing optical fiber, and the second single-mode optical fiber (2) all include a cladding (3) and a central core (4). The central cores (4) of the first single-mode optical fiber (1), the sensing optical fiber, and the second single-mode optical fiber (2) are correspondingly arranged on the central axis of the cladding (3), and the central cores arranged in the first single-mode optical fiber (1), the sensing optical fiber, and the second single-mode optical fiber (2) are connected in sequence; And the first single-mode optical fiber (1) and the sensing optical fiber, the sensing optical fiber and the second single-mode optical fiber (2) are all connected through a coupling material. The coupling material includes polycarbonate or polyethylene naphthalate; Among them, the sensing optical fiber includes a first dual-core optical fiber (5). In the axial direction of the first dual-core optical fiber (5), a first side core (6) is arranged. The first side core (6) and the central core (4) of the first dual-core optical fiber (5) have a first preset distance, and the first preset distance is greater than zero; Alternatively, the sensing optical fiber includes a first triple-core optical fiber (7). In the axial direction of the first triple-core optical fiber (7), a second side core (8) and a third side core (9) are arranged. The second side core (8) and the third side core (9) are asymmetrically distributed on both sides of the central core (4) of the first triple-core optical fiber (7); Alternatively, the sensing optical fiber includes a second dual-core optical fiber (10) and a second triple-core optical fiber (11). The first single-mode optical fiber (1), the second dual-core optical fiber (10), the second triple-core optical fiber (11), and the second single-mode optical fiber (2) are connected in sequence, and the second dual-core optical fiber (10) and the second triple-core optical fiber (11) are connected through polycarbonate or polyethylene naphthalate; In the axial direction of the second dual-core optical fiber (10), a fourth side core (12) is arranged. The fourth side core (12) and the central core (4) of the second dual-core optical fiber (10) have a first preset distance, and the first preset distance is greater than zero; In the axial direction of the second triple-core optical fiber (11), a fifth side core (13) and a sixth side core (14) are arranged. The fifth side core (13) and the sixth side core (14) are asymmetrically distributed on both sides of the central core (4) of the second triple-core optical fiber (11).

2. The flexible fiber optic Mach-Zehnder interferometric ultrasonic sensor according to claim 1, wherein When the sensing optical fiber includes a first dual-core optical fiber (5), the refractive indices of the central core (4) and the first lateral core (6) of the first dual-core optical fiber (5) are the same, and the refractive indices of the central core (4) and the first lateral core (6) of the first dual-core optical fiber (5) are greater than the refractive index of the cladding (3) of the first dual-core optical fiber (5); the diameter of the central core (4) of the first dual-core optical fiber (5) is smaller than that of the first lateral core (6).

3. The flexible fiber optic Mach-Zehnder interferometric ultrasonic sensor according to claim 1, wherein When the sensing optical fiber includes a first triple-core optical fiber (7), the refractive indices of the central core (4), the second lateral core (8), and the third lateral core (9) of the first triple-core optical fiber (7) are the same, and the refractive indices of the central core (4), the second lateral core (8), and the third lateral core (9) of the first triple-core optical fiber (7) are greater than the refractive index of the cladding (3) of the first triple-core optical fiber (7), the diameters of the second lateral core (8) and the third lateral core (9) are the same and greater than the diameter of the central core (4) of the first triple-core optical fiber (7), the centers of the three end faces at the same end of the second lateral core (8), the third lateral core (9), and the central core (4) of the first triple-core optical fiber (7) form an isosceles triangle, and the apex angle of the isosceles triangle is 120°.

4. The flexible fiber optic Mach-Zehnder interferometric ultrasonic sensor according to claim 1, wherein When the sensing optical fiber includes a second dual-core optical fiber (10) and a second triple-core optical fiber (11), the refractive indices of the central core (4) and the fourth lateral core (12) of the second dual-core optical fiber (10) are the same, and the refractive indices of the central core (4) and the fourth lateral core (12) of the second dual-core optical fiber (10) are greater than the refractive index of the cladding (3) of the second dual-core optical fiber (10); the diameter of the central core (4) of the second dual-core optical fiber (10) is smaller than that of the fourth lateral core (12); The refractive indices of the central core (4), the fifth lateral core (13), and the sixth lateral core (14) of the second triple-core optical fiber (11) are the same, and the refractive indices of the central core (4), the fifth lateral core (13), and the sixth lateral core (14) of the second triple-core optical fiber (11) are smaller than the refractive index of the cladding (3) of the second triple-core optical fiber (11), the diameters of the fifth lateral core (13) and the sixth lateral core (14) are the same and greater than the diameter of the central core (4) of the second triple-core optical fiber (11), the centers of the three end faces at the same end of the fifth lateral core (13), the sixth lateral core (14), and the central core (4) of the second triple-core optical fiber (11) form an isosceles triangle, and the apex angle of the isosceles triangle is 120°.

5. A sensor packaging structure, characterized in that, An interferometric ultrasonic sensor according to any one of claims 1 to 4 and a ring having a through hole, wherein the interferometric ultrasonic sensor passes through the through hole of the ring.

6. The sensor package according to claim 5, characterized in that, The material of the ring includes a polyurethane elastomer.

Images