A cantilevered microfiber grating ultrasonic sensor

Abstract

This invention discloses a cantilevered microfiber grating ultrasonic sensor, belonging to the field of sensing. This invention is a microfiber grating ultrasonic sensor with a cantilever structure and variable diameter. The sensor consists of a small-diameter cantilever section, a transition section with a gradually increasing diameter, and a standard-diameter fiber optic pigtail. The fiber optic pigtail is fixed to the test object, forming a cantilever structure between the fiber waist and the transition section. The sensor can resonate at a specific frequency; changing parameters such as the length and diameter of the fiber waist and transition section alters the fiber's resonant frequency. Furthermore, the sensor amplifies signal amplitude; the amplification factor can be controlled by changing parameters such as the rate of change of the transition section's diameter.

Description

Technical Field

[0001] This invention belongs to the field of sensing, specifically referring to a cantilevered microfiber grating ultrasonic sensor. Background Technology

[0002] Fiber Bragg gratings (FBGs) offer advantages such as light weight, corrosion resistance, electromagnetic interference immunity, and multiplexing capability. However, they still suffer from low ultrasonic sensitivity, making it difficult to detect ultrasonic guided wave signals with small amplitudes or long distances. When FBGs form a cantilever structure, they resonate with ultrasonic waves of specific frequencies, but the signal amplitude is small, requiring further amplification. Micro-FBGs are not only small in size but also possess ultrasonic amplification capabilities through remote attachment. Summary of the Invention

[0003] This invention addresses the problems existing in the prior art by proposing a cantilevered microfiber grating ultrasonic sensor. This sensor combines the advantages of cantilevered fiber gratings and microfiber gratings, significantly increasing the ultrasonic amplitude at the resonant frequency, and allowing both the resonant frequency and the signal amplification factor to be designed and modified.

[0004] This invention's cantilever microfiber grating not only resonates at a specific frequency, but also exhibits significantly improved ultrasonic amplification compared to cantilever fiber gratings and microfiber gratings. This sensor allows for the design of a corresponding resonant frequency for specific needs, and the sensor amplification can be adjusted according to signal strength, offering high design flexibility and sensitivity, enabling multifunctional applications of fiber gratings.

[0005] This invention is implemented as follows:

[0006] A cantilevered microfiber grating ultrasonic sensor, characterized in that the sensor comprises an optical fiber waist (1), an optical fiber transition section (2), and an optical fiber pigtail (3); the diameter of the optical fiber waist (1) is greater than 10 μm and less than 125 μm; the grating region of the fiber grating is located in the optical fiber waist (1), and the optical fiber waist (1) has no coating layer; the diameter of the optical fiber transition section (2) gradually increases, the small diameter section of the optical fiber transition section (2) is connected to the optical fiber waist (1), and the large diameter section of the optical fiber transition section (2) is connected to the optical fiber pigtail (3), and the optical fiber transition section (2) has no coating layer; the diameter of the optical fiber pigtail (3) is 125 μm, and the optical fiber pigtail (3) can retain a coating layer to give the sensor better toughness, and the diameter is greater than 125 μm after retaining the coating layer;

[0007] The fiber pigtail (3) is fixed to the test piece using adhesive (4), ensuring that the fiber waist (1) and fiber transition section (2) are suspended to form a cantilever structure. Ultrasonic waves propagate from the test piece to the adhesive, then to the cantilever end and through the grating region, and are then reflected at the free end of the fiber waist (1). The incident wave and the reflected wave have the same frequency but opposite directions, forming a standing wave that resonates at a specific frequency.

[0008] Furthermore, this cantilevered microfiber grating ultrasonic sensor needs to be connected to a demodulation system to convert the dynamic strain caused by ultrasound into a high-frequency electrical signal output and record the real-time waveform. When the sensor performs an active ultrasonic experiment, the ultrasonic generator (12) is fixed at one end of the test piece, the signal generator (7) is connected to input the ultrasonic signal and amplified 10 times by the power amplifier (6), the ordinary fiber grating (13) and the microfiber grating (14) are fixed at the other end of the test piece, the demodulation system is connected and the 3dB point of the reflection spectrum is locked, the demodulation system inputs a single wavelength laser from the laser (11), which is transmitted to the grating by the circulator (10), reflected at the fiber grating and converted into an electrical signal by the photodetector (9) and output on the oscilloscope (8). When the ultrasound causes the Bragg wavelength shift, the fiber grating can measure the ultrasonic signal in real time and has high sensitivity.

[0009] Furthermore, its resonant frequency becomes detuned near the resonant frequency of a conventional cantilever fiber grating; the resonant frequency of a conventional cantilever fiber grating is:

[0010]

[0011] Where f is the frequency, n is the frequency order, l is the cantilever length, and v is the ultrasonic wave velocity; when the sensor is a micro fiber grating, the frequencies of f will not increase exponentially.

[0012] Furthermore, the length l of the fiber waist (1) w and fiber transition section (2) length l t The ratio l w / l t The first-order resonant frequency is affected by l. w / l t Increase and decrease; when l w / l t When l is less than 7.5, the two have an exponential relationship; when l w / l t When the value is greater than 7.5, the two have a linear function relationship.

[0013] Furthermore, the diameter of the fiber waist (1) can change the resonant frequency; the first-order resonant frequency decreases as the waist diameter increases, and the two are related by a linear function; the signal amplitude increases as the diameter of the fiber waist (1) decreases, and the two are related by a linear function.

[0014] Furthermore, the optical fiber is made of materials with low ultrasonic attenuation coefficients, including quartz glass, single-crystal alumina, and sulfide glass; the grating type includes uniform gratings and ultrasonically sensitive grating structures such as phase-shifted gratings.

[0015] The advantages of this invention compared to the prior art are as follows:

[0016] The cantilever structure of the fiber optic grating in this invention enables the sensor to resonate at a specific frequency, thereby improving the signal-to-noise ratio and amplifying the amplitude.

[0017] The resonant frequency is affected by parameters such as the length of the transition section and waist, and the waist diameter. By changing the ratio of the length of the fiber waist to the length of the transition section, and the waist diameter, ultrasonic sensors that resonate at different frequencies can be made.

[0018] The sensor has a simple structure; the finished sensor can be obtained by making a transition section and waist of a specific length and diameter using ordinary optical fiber. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the cantilever microfiber grating structure of a cantilever microfiber grating ultrasonic sensor according to the present invention.

[0020] Figure 2 A physical image of a cantilevered microfiber grating fabricated according to an embodiment of the cantilevered microfiber grating ultrasonic sensor of the present invention;

[0021] Figure 3 This is an enlarged view of the transition section of the finished cantilever microfiber grating ultrasonic sensor according to the present invention.

[0022] Figure 4 This is a schematic diagram of an ultrasonic experiment of an embodiment of a cantilevered microfiber grating ultrasonic sensor of the present invention;

[0023] Figure 5 The following are time-domain response diagrams of a cantilever microfiber grating and a conventional cantilever fiber grating in an embodiment of the cantilever microfiber grating ultrasonic sensor of the present invention: (a) is a microfiber grating, and (b) is a conventional fiber grating.

[0024] Figure 6 The images show the frequency domain response diagrams of a cantilever microfiber grating and a conventional cantilever fiber grating in an embodiment of the cantilever microfiber grating ultrasonic sensor of the present invention. (a) is a microfiber grating, and (b) is a conventional fiber grating.

[0025] Among them, 1-fiber waist, 2-fiber transition section, 3-fiber pigtail, 4-adhesive, 5-aluminum plate, 6-power amplifier, 7-signal generator, 8-oscilloscope, 9-photodetector, 10-circulator, 11-laser, 12-ultrasonic generator, 13-ordinary fiber Bragg grating, 14-micro fiber Bragg grating. Detailed Implementation

[0026] To make the objectives, technical solutions, and effects of this invention clearer and more explicit, the following examples provide a more detailed description of the invention. It should be noted that the specific embodiments described herein are merely illustrative and not intended to limit the scope of the invention.

[0027] like Figure 1 As shown, the sensor of the present invention includes an optical fiber waist 1, an optical fiber transition section 2, and an optical fiber pigtail 3. The diameter of the optical fiber waist 1 is greater than 10 μm and less than 125 μm. The grating region of the fiber grating is located in the portion of the optical fiber waist 1, and the optical fiber waist 1 has no coating. The diameter of the optical fiber transition section 2 gradually increases. The small diameter portion of the optical fiber transition section 2 connects to the optical fiber waist 1, and the large diameter portion of the optical fiber transition section 2 connects to the optical fiber pigtail 3. The optical fiber transition section 2 has no coating. The diameter of the optical fiber pigtail 3 is 125 μm. The optical fiber pigtail 3 can retain a coating to give the sensor better toughness. After retaining the coating, the diameter is greater than 125 μm. The optical fiber pigtail 3 is fixed to the test piece only with adhesive 4, ensuring that the optical fiber waist 1 and the optical fiber transition section 2 are suspended to form a cantilever structure. Ultrasonic waves propagate from the test piece to the adhesive, then to the cantilever end and through the grating region, and are then reflected at the free end of the optical fiber waist 1. The incident wave and the reflected wave have the same frequency but opposite directions, forming a standing wave that resonates at a specific frequency.

[0028] The ultrasonic experiment steps for a cantilevered microfiber grating ultrasonic sensor of the present invention are as follows:

[0029] Step 1: Determine the first-order resonant frequency of the fiber Bragg grating to be 150kHz, and the signal to be amplified by at least 2.5 times;

[0030] Step 2: The fiber transition section 2 is determined to be 2.25mm, the fiber waist is 18.25mm, and the ratio of fiber waist 1 to the transition section is 3.67.

[0031] Step 3: The diameter of the fiber waist is determined to be 34.5 μm. The actual parameters of the finished fiber grating are almost identical to the design parameters. Figure 2 and Figure 3 As shown;

[0032] Step 4: Use adhesive 4 to fix the fiber Bragg grating pigtail only to the test piece, i.e., the aluminum plate 5 in this embodiment, to form a cantilever structure, such as... Figure 1 As shown;

[0033] Step 5: Perform active ultrasound testing, such as... Figure 4As shown, an ultrasonic generator 12 is fixed to one end of an aluminum plate 5, and an ultrasonic signal is input to a signal generator 7 and amplified 10 times by a power amplifier 6. A common fiber grating 13 and a micro fiber grating 14 are fixed to the other end of the aluminum plate, and a demodulation system is connected to lock the 3dB point of the reflection spectrum. The demodulation system receives a single-wavelength laser from a laser 11, which is transmitted to the grating by a circulator 10. After reflection at the fiber grating, the laser signal is converted into an electrical signal by a photodetector 9 and output on an oscilloscope 8. When the ultrasonic wave causes a Bragg wavelength shift, the fiber grating can measure the ultrasonic signal in real time and has high sensitivity.

[0034] Step 6, Experimental Results Figure 5 After performing a Fast Fourier Transform on the signal, we obtain Figure 6 (a) is a microfiber grating with an amplitude of 95 Hz at its first resonant frequency, and (b) is a conventional fiber grating with an amplitude of 35 Hz at its first resonant frequency. Compared to a conventional fiber grating 13 with the same cantilever length, the signal amplitude is amplified by 2.6 times, meeting the design requirements. Because the structure of the cantilever microfiber grating differs from that of the conventional cantilever fiber grating, their resonant frequencies also differ.

[0035] The above are merely preferred embodiments of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principles of the present invention should be considered within the scope of protection of the present invention.

Claims

1. A cantilevered microfiber grating ultrasonic sensor, characterized in that, The sensor includes an optical fiber waist (1), an optical fiber transition section (2), and an optical fiber pigtail (3); The fiber waist (1) has a diameter greater than 10 μm and less than 125 μm; the grating region of the fiber grating is located in the fiber waist (1), and the fiber waist (1) has no coating layer; the fiber transition section (2) has a diameter that increases from small to large, the fiber transition section (2) is connected to the fiber waist (1) at the small diameter, and the fiber transition section (2) is connected to the fiber pigtail (3) at the large diameter, and the fiber transition section (2) has no coating layer; the fiber pigtail (3) has a diameter of 125 μm, and the fiber pigtail (3) can retain a coating layer to make the sensor more resilient, and the diameter is greater than 125 μm after retaining the coating layer; The fiber pigtail (3) is fixed to the test piece only with adhesive (4), ensuring that the fiber waist (1) and fiber transition section (2) are suspended to form a cantilever structure; the ultrasonic wave propagates from the test piece to the adhesive, then to the cantilever end and through the grid region, and is then reflected at the free end of the fiber waist (1); the incident wave and the reflected wave have the same frequency and opposite direction, forming a standing wave that causes the fiber to resonate; The fiber waist (1) has a length l w and fiber transition section (2) length l t The ratio l w / l t The first-order resonant frequency is affected by l. w / l t Increase and decrease; when l w / l t When the value is less than 7.5, the relationship between the two is exponential. When l w / l t When the value is greater than 7.5, the two have a linear function relationship.

2. The cantilevered microfiber grating ultrasonic sensor according to claim 1, characterized in that, The cantilevered micro fiber grating ultrasonic sensor needs to be connected to a demodulation system to convert the dynamic strain caused by ultrasound into a high-frequency electrical signal output and record the real-time waveform. When the ultrasound causes the Bragg wavelength shift, the fiber grating can measure the ultrasonic signal in real time and has high sensitivity.

3. The cantilevered microfiber grating ultrasonic sensor according to claim 1, characterized in that, Its resonant frequency is detuned near the resonant frequency of a conventional cantilever fiber grating; the resonant frequency of a conventional cantilever fiber grating is: Where f is the frequency, n is the frequency order, l is the cantilever length, and v is the ultrasonic wave velocity; when the sensor is a micro fiber grating, the frequencies of f will not increase exponentially.

4. The cantilevered microfiber grating ultrasonic sensor according to claim 1, characterized in that, The diameter of the fiber waist (1) can change the resonant frequency; the first-order resonant frequency decreases as the waist diameter increases, and the two are related by a linear function; the signal amplitude increases as the diameter of the fiber waist (1) decreases, and the two are related by a linear function.

5. The cantilevered microfiber grating ultrasonic sensor according to claim 1, characterized in that, The low ultrasonic attenuation coefficient materials of the optical fiber include quartz glass, single-crystal alumina, and sulfide glass; the types of ultrasonically sensitive gratings include uniform gratings and phase-shifting gratings.

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