Optical fiber vibration sensor, preparation method, detection system and application method

Abstract

The present invention proposes an optical fiber vibration sensor, preparation method, detection system and application method, including a capillary, a single-mode optical fiber, a few-mode optical fiber and a film layer, wherein the capillary has an air cavity; the single-mode optical fiber and the few-mode optical fiber each have one end plugged into the capillary, the end faces of the single-mode optical fiber and the few-mode optical fiber are opposite, and the single-mode optical fiber and the few-mode optical fiber do not contact the capillary; the film layer is arranged on one end of the few-mode optical fiber away from the single-mode optical fiber. The present invention uses single-mode and few-mode optical fibers to prepare sensors, and the types of transmission modes are relatively small, the number of modes is controllable, and the interference components are simple, and it is easy to obtain interference spectra with high contrast and regular shapes, which is conducive to demodulating the spectrum and adjusting the free spectral range of the spectrum; at the same time, because the optical fiber does not contact the capillary, the optical fiber has good vibration performance and the advantage of high sensitivity; by controlling the thickness of the film layer, it is also convenient to adjust the detection frequency range of the sensor.

Description

Technical Field

[0001] The present invention relates to the field of sensor technology, and in particular to an optical fiber vibration sensor, a preparation method, a detection system and an application method. Background Art

[0002] Fiber optic sensing technology is an advanced sensing technology with many advantages. First of all, fiber optic sensing technology has the characteristics of high precision and high sensitivity, and can detect and measure small changes. Secondly, fiber optic sensing technology has strong anti-interference ability, can resist the influence of electromagnetic interference and external environmental conditions, and has high stability. In addition, fiber optic sensing technology can achieve long-distance signal transmission and is suitable for scenarios that require long-distance monitoring and measurement. The fiber optic sensor itself is very thin and light, and can be flexibly applied in various complex environments. In addition, fiber optic sensing technology can realize information perception within the entire sensor length and realize global monitoring and measurement. Fiber optic sensing technology can also work under harsh environmental conditions, can adapt to harsh conditions such as high temperature, low temperature, high pressure, corrosion, etc., and has a wide range of applications.

[0003] The invention patent with the existing authorization announcement number CN107941390B discloses an optical fiber Fabry-Perot sensor and a manufacturing method thereof. The optical fiber Fabry-Perot sensor comprises: a hollow tube body, which has a first tube body, a cavity portion and a second tube body arranged in sequence along the axial direction; a first optical fiber, which is arranged in the first tube body along the axial direction, and has a first light-guiding end face arranged in the cavity portion; and a second optical fiber, which is arranged in the second tube body along the axial direction, and has a second light-guiding end face arranged in the cavity portion, the first light-guiding end face and the second light-guiding end face are arranged opposite to each other at a preset distance, and the inner diameter of the cavity portion is larger than the inner diameter of any one of the first tube body and the second tube body.

[0004] As in the above technical solution, the sensor uses single-mode, multi-mode optical fiber, polarization-maintaining optical fiber or photonic crystal fiber, and when integrated, the optical fiber is fixed in the hollow tube. Sensors with this structure will have problems such as too many or too few modes, uncontrollable number of modes, and poor transmission performance. At the same time, since the optical fiber is fixed to the hollow tube, this will affect the sensitivity of the optical fiber vibration, resulting in reduced detection accuracy; therefore, a new optical fiber vibration sensor structure is urgently needed. Summary of the invention

[0005] In view of this, the present invention proposes an optical fiber vibration sensor, a preparation method, a detection system and an application method with a small and controllable number of modes and high detection sensitivity, so as to solve the problems of poor controllability of the number of modes and low detection accuracy in existing optical fiber vibration sensors.

[0006] The technical solution of the present invention is achieved in this way:

[0007] In one aspect, the present invention provides an optical fiber vibration sensor, comprising a capillary, a single-mode optical fiber, a few-mode optical fiber and a membrane layer, wherein:

[0008] Capillaries have air cavities;

[0009] One end of the single-mode optical fiber and the few-mode optical fiber is respectively inserted into the capillary tube, the end faces of the single-mode optical fiber and the few-mode optical fiber are opposite to each other, and the single-mode optical fiber and the few-mode optical fiber do not contact the capillary tube;

[0010] The film layer is arranged on one end of the few-mode optical fiber away from the single-mode optical fiber, and is used to increase the light intensity and improve the vibration sensitivity.

[0011] On the basis of the above technical solution, preferably, the length of the air cavity is 20-24 μm, and the inner diameter of the capillary is 300-320 μm.

[0012] On the basis of the above technical solution, preferably, the thickness of the film layer is 80 to 120 nm.

[0013] On the basis of the above technical solution, preferably, it further comprises a carrier plate, and the capillary, the single-mode optical fiber and the few-mode optical fiber are all arranged on the carrier plate.

[0014] On the basis of the above technical solution, preferably, the film layer is a gold film and the carrier sheet is a glass sheet.

[0015] In another aspect, the present invention provides a method for preparing an optical fiber vibration sensor, comprising the following steps:

[0016] S1, stripping a section of the coating layer of the single-mode optical fiber and the few-mode optical fiber, and cutting the end faces of the single-mode optical fiber and the few-mode optical fiber to keep the end faces flat;

[0017] S2. Use a FSM-60S fusion splicer to place the single-mode fiber and the few-mode fiber on the fusion splicer and observe whether the end faces are flat. If not, re-cut the fiber end faces until the end face inclination angle is less than 1 degree.

[0018] S3, fixing the capillary onto the carrier sheet;

[0019] S4, fixing the single-mode optical fiber and the few-mode optical fiber at two ends of the micro-displacement platform, and placing a carrier together with the capillary under the ends of the single-mode optical fiber and the few-mode optical fiber;

[0020] S5, connecting the broadband light source to the port 1 of the optical fiber circulator, the single-mode optical fiber to the port 2 of the optical fiber circulator, and the spectrometer to the port 3 of the optical fiber circulator, adjusting the distance between the end face of the single-mode optical fiber and the end face of the few-mode optical fiber through the micro-displacement platform, and simultaneously observing the free spectrum range of the interference spectrum on the spectrometer to adjust the cavity length of the air cavity;

[0021] S6. After adjusting the cavity length of the air cavity, use ultraviolet curing glue to cure the single-mode optical fiber and the few-mode optical fiber on the carrier sheet to complete the preparation of the optical fiber vibration sensor.

[0022] On the basis of the above technical solution, preferably, step S1 further includes coating a film layer, and the specific operations are as follows:

[0023] Select the appropriate coating material, according to the detection frequency range, and preset the coating thickness;

[0024] A coating is applied to one end of the few-mode optical fiber away from the single-mode optical fiber until the coating reaches a certain thickness and the quality of the coating meets the detection requirements to form a film layer.

[0025] On the other hand, the present invention provides a detection system, including the above-mentioned optical fiber vibration sensor.

[0026] On the basis of the above technical solution, preferably, it also includes a narrow line width light source, an optical fiber circulator, a photodetector and an oscilloscope, wherein:

[0027] A narrow linewidth light source is connected to port one of the optical fiber circulator;

[0028] The single-mode optical fiber of the optical fiber vibration sensor is connected to the second port of the optical fiber circulator;

[0029] The photodetector is connected to port three of the optical fiber circulator;

[0030] An oscilloscope is connected to the photodetector.

[0031] In another aspect, the present invention provides an application method of the above detection system, comprising the following steps:

[0032] P1. The carrier sheet in the optical fiber vibration sensor is placed on the side of the pipeline, and the single-mode optical fiber and the few-mode optical fiber are parallel to the pipeline;

[0033] P2. Use a spectrometer to find the light wavelength corresponding to the maximum slope on the cascade spectrum of the vibration sensor and record the value of the light wavelength;

[0034] P3, adjusting the wavelength of the narrow linewidth light source to the wavelength of the light recorded in step P2;

[0035] P4. Give the pipeline a certain amount of external excitation. When the pipeline is vibrated, it will cause the optical fiber vibration sensor to vibrate, causing the single-mode optical fiber and the few-mode optical fiber to bend and deform, thereby changing the cavity length of the air cavity. The change in cavity length will change the phase difference of the interference mode of the optical fiber vibration sensor, which will then manifest as a change in the output light intensity, thereby realizing vibration sensing. At the same time, the bending deformation of the few-mode optical fiber causes the cavity length of the MI cavity to change, and the air cavity and the MI cavity form a cascade cursor, thereby improving the sensing sensitivity.

[0036] The optical fiber vibration sensor, preparation method, detection system and application method of the present invention have the following beneficial effects compared with the prior art:

[0037] (1) By using single-mode and few-mode optical fibers to prepare sensors, the types of transmission modes are relatively small, the number of modes is controllable, the interference components are simple, and it is easy to obtain interference spectra with high contrast and regular shape, which is conducive to demodulating the spectrum and adjusting the free spectral range of the spectrum; at the same time, since the optical fiber does not contact the capillary, the optical fiber has good vibration performance and has the advantage of high sensitivity; and by setting a film layer at the end of the few-mode optical fiber, it can reflect light, thereby increasing the contrast of mode interference and ensuring detection accuracy; at the same time, the film layer plays a role in increasing mass to enhance the corresponding sensitivity of the sensor to vibration, and by controlling the thickness of the film layer, it is also convenient to adjust the detection frequency range of the sensor;

[0038] (2) In the preparation method of the sensor, the coating thickness is preset according to the detection frequency range, and then the film layer is prepared. The spacing between the single-mode optical fiber and the few-mode optical fiber can also be adjusted. In this way, the sensor can adjust the detection frequency range by different means, so that it has an accurate detection frequency range after assembly and good performance.

[0039] (3) In this sensor, a film layer is set to reflect light. When the light is transmitted back to the air cavity interface, a few-mode MI cavity is formed. Both the few-mode MI cavity and the air cavity respond to vibration, thereby forming a cascade cursor, which can significantly improve the sensitivity of the sensor. At the same time, the film layer increases the intensity of the reflected light, thereby increasing the contrast of the mode interference and ensuring accurate detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0041] Figure 1 is a structural diagram of the optical fiber vibration sensor of the present invention;

[0042] Figure 2 is a structural diagram of the detection system of the present invention;

[0043] Figure 3 This is the frequency response diagram of the optical fiber vibration sensor when the external excitation is 1kHz;

[0044] Figure 4 is a frequency response diagram of the vibration sensor when the external excitation is 2kHz;

[0045] Figure 5 is a frequency response diagram of the vibration sensor when the external excitation is 3kHz;

[0046] Figure 6 is a frequency response diagram of the vibration sensor when the external excitation is 4kHz;

[0047] Figure 7 is a frequency response diagram of the vibration sensor when the external excitation is 5kHz;

[0048] Figure 8 is a frequency response diagram of the vibration sensor when the external excitation is 6kHz;

[0049] Fig. 9 is a frequency response diagram of the vibration sensor when the external excitation is 7kHz;

[0050] Fig.10 is a frequency response diagram of the vibration sensor when the external excitation is 8kHz;

[0051] Fig.11 is a frequency response diagram of the vibration sensor when the external excitation is 9kHz;

[0052] Fig.12 is a frequency response diagram of the vibration sensor when the external excitation is 10kHz;

[0053] Fig.13 is a frequency response diagram of the vibration sensor when the external excitation is 11kHz;

[0054] Fig.14 is a frequency response diagram of the vibration sensor when the external excitation is 12kHz;

[0055] Fig.15 is a frequency response diagram of the vibration sensor when the external excitation is 13kHz;

[0056] Fig.16 is a frequency response diagram of the vibration sensor when the external excitation is 14kHz;

[0057] Fig.17 is a frequency response diagram of the vibration sensor when the external excitation is 15kHz;

[0058] In the figure: 1. capillary; 101. air cavity; 2. single-mode optical fiber; 3. few-mode optical fiber; 4. film layer; 5. carrier plate; 6. narrow linewidth light source; 7. optical fiber circulator; 8. photodetector; 9. oscilloscope. DETAILED DESCRIPTION

[0059] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0060] like Figure 1 As shown, the optical fiber vibration sensor of the present invention includes a capillary 1, an air cavity 101, a single-mode optical fiber 2, a few-mode optical fiber 3, a film layer 4 and a carrier sheet 5.

[0061] like Figure 1 As shown, the capillary 1 has an air cavity 101; one end of the single-mode optical fiber 2 and the few-mode optical fiber 3 are respectively inserted into the capillary 1, and the end faces of the single-mode optical fiber 2 and the few-mode optical fiber 3 are opposite, and the single-mode optical fiber 2 and the few-mode optical fiber 3 do not contact the capillary 1; the membrane layer 4 is arranged on the end of the few-mode optical fiber 3 away from the single-mode optical fiber 2, and the membrane layer 4 is used to increase the light intensity and improve the vibration sensitivity;

[0062] As in the above structure, the single-mode optical fiber 2 and the few-mode optical fiber 3 are inserted into the capillary 1, and the end faces of the single-mode optical fiber 2 and the few-mode optical fiber 3 are opposite to form an air Fabry–Pérot microcavity, that is, an air cavity 101, and the capillary 1 plays a protective role to prevent the external stress from affecting the air cavity 101; specifically, the capillary 1 is a quartz capillary;

[0063] Since the single-mode optical fiber 2 and the few-mode optical fiber 3 do not contact the capillary 1, the capillary 1 will not interfere with the vibration of the single-mode optical fiber 2 and the few-mode optical fiber 3, thereby ensuring the corresponding sensitivity and further ensuring the detection accuracy;

[0064] The film layer 4 is used to reflect light. Specifically, the film layer 4 is made of gold film to ensure the light reflectivity.

[0065] When the sensor detects vibration, light is input from the single-mode optical fiber 2, transmitted through the single-mode optical fiber 2 and the air cavity 101 in sequence, and then enters the few-mode optical fiber 3. Finally, when it reaches the rear end face of the few-mode optical fiber 3, it is reflected by the film layer 4, transmitted in the reverse direction, and finally enters the single-mode optical fiber 2 again.

[0066] During the test, if vibration is detected, since the optical fibers are not fused, the end faces of the single-mode optical fiber 2 and the few-mode optical fiber 3 are slightly misaligned, and when the light enters the few-mode optical fiber 3 through the air cavity 101, the LP in the few-mode optical fiber 3 is simultaneously excited. 01 LP 11Mode. The excited mode is transmitted in the few-mode fiber 3, then reflected by the film layer 4 on the rear end face, and when it is transmitted back to the interface of the air cavity 101-few-mode fiber 3, it will be combined and interfered to form a few-mode MI cavity, forming an MI interferometer. The strength of the mode excitation is closely related to the vibration intensity, which affects the intensity of the interference spectrum. Vibration sensing can be achieved based on the change of this intensity;

[0067] At the same time, since both the single-mode optical fiber 2 and the few-mode optical fiber 3 are deformed, the deformation causes the cavity length of the air cavity 101 to change, and the bending deformation of the few-mode optical fiber 3 also causes the MI cavity length to change; the change in cavity length will change the phase difference of the interference mode of the optical fiber vibration sensor, which will then be manifested as a change in the output light intensity;

[0068] Both the air cavity 101 and the MI cavity respond to vibrations, thereby forming a cascade vernier that can significantly increase sensor sensitivity.

[0069] In the vibration sensor, when the single-mode optical fiber 2 and the few-mode optical fiber 3 are provided, the spacing between the single-mode optical fiber 2 and the few-mode optical fiber 3 is controlled, thereby controlling the cavity length of the air cavity 101, so as to adjust the cavity length of the air cavity 101 to adjust the free spectrum range of the interference spectrum of the air cavity 101 and determine the detection frequency range;

[0070] The film layer 4 is used as a reflection enhancement film to enhance the intensity of reflected light, thereby increasing the contrast of mode interference;

[0071] Specifically, the membrane layer 4 can also play a role in increasing mass and improving the response sensitivity of the optical fiber to vibration. By adjusting the thickness of the membrane layer 4 and then adjusting the mass of the membrane layer 4 to change the vibration performance of the optical fiber, the adjustment of the detection frequency range can be achieved, making the sensor suitable for different detection needs.

[0072] In the vibration sensor, the length of the air cavity 101 is 20-24 μm, the inner diameter of the capillary 1 is 300-320 μm, and the thickness of the film layer 4 is 80-120 nm.

[0073] like Figure 1 As shown, the capillary 1, the single-mode optical fiber 2 and the few-mode optical fiber 3 are all arranged on a carrier sheet 5;

[0074] As in the above structure, by setting the carrier sheet 5, the capillary 1 can be fixed, which facilitates the subsequent assembly of the single-mode optical fiber 2 and the few-mode optical fiber 3, and also provides support for the single-mode optical fiber 2 and the few-mode optical fiber 3 to prevent the single-mode optical fiber 2 and the few-mode optical fiber 3 from contacting the capillary 1, thereby ensuring the accuracy of the detection;

[0075] When in use, the optical fiber vibration sensor can be attached to the pipeline through the carrier sheet 5 , so the carrier sheet 5 facilitates the installation and use of the optical fiber vibration sensor.

[0076] Specifically, the carrier sheet 5 is a glass sheet.

[0077] The method for preparing the optical fiber vibration sensor of the present invention comprises the following steps:

[0078] S1, stripping a section of the coating layer of the single-mode optical fiber 2 and the few-mode optical fiber 3, and cutting the end faces of the single-mode optical fiber 2 and the few-mode optical fiber 3 to keep the end faces flat;

[0079] S2. Use a fusion splicer model FSM-60S to place the single-mode optical fiber 2 and the few-mode optical fiber 3 on the fusion splicer and observe whether the end faces are flat. If not, re-cut the optical fiber end faces until the end face inclination angle is less than 1 degree.

[0080] S3, fixing the capillary 1 to the carrier sheet 5;

[0081] S4, fixing the single-mode optical fiber 2 and the few-mode optical fiber 3 at both ends of the micro-displacement platform, and placing the carrier plate 5 together with the capillary 1 under the ends of the single-mode optical fiber 2 and the few-mode optical fiber 3;

[0082] S5, connecting the broadband light source to the port 1 of the optical fiber circulator 7, connecting the single-mode optical fiber 2 to the port 2 of the optical fiber circulator 7, and connecting the spectrometer to the port 3 of the optical fiber circulator 7, adjusting the distance between the end face of the single-mode optical fiber 2 and the end face of the few-mode optical fiber 3 by means of the micro-displacement platform, and simultaneously observing the free spectrum range of the interference spectrum on the spectrometer to adjust the cavity length of the air cavity 101;

[0083] S6. After adjusting the cavity length of the air cavity 101, use ultraviolet curing glue to cure the single-mode optical fiber 2 and the few-mode optical fiber 3 on the carrier sheet 5, and complete the preparation of the optical fiber vibration sensor.

[0084] As in the above method, when preparing the optical fiber vibration sensor, the capillary 1 is first fixed to the carrier 5, and then the single-mode optical fiber 2 and the few-mode optical fiber 3 are assembled by a fusion splicer;

[0085] In this process, the distance between the end faces of the single-mode optical fiber 2 and the few-mode optical fiber 3 is controlled to adjust the cavity length of the air cavity 101, thereby adjusting the free spectrum range of the interference spectrum and determining the detection frequency range.

[0086] Step S1 also includes coating a film layer 4, and the specific operations are: selecting a suitable coating material, according to the detection frequency range, and presetting the coating thickness; coating the end of the few-mode optical fiber 3 away from the single-mode optical fiber 2, and the coating reaches a certain thickness, and the quality of the coating meets the detection requirements, so as to form a film layer 4;

[0087] In this method, when the film layer 4 is provided, the coating is performed according to the detection requirements, thereby ensuring the sensitivity and natural frequency of the optical fiber vibration sensor, ensuring the accuracy of the detection, and making the sensor have a larger application range.

[0088] In the present method, further, the length of the few-mode optical fiber 3 is adjusted to control the optical path difference for different mode transmissions, thereby adjusting the free spectral range of the interference spectrum.

[0089] like Figure 2 As shown, the detection system of the present invention includes the above-mentioned optical fiber vibration sensor, and also includes a narrow line width light source 6, an optical fiber circulator 7, a photodetector 8 and an oscilloscope 9, wherein the narrow line width light source 6 is connected to the port 1 of the optical fiber circulator 7; the single-mode optical fiber 2 of the optical fiber vibration sensor is connected to the port 2 of the optical fiber circulator 7; the photodetector 8 is connected to the port 3 of the optical fiber circulator 7; and the oscilloscope 9 is connected to the photodetector 8;

[0090] The application method of the detection system of the present invention comprises the following steps:

[0091] P1. The carrier sheet 5 in the optical fiber vibration sensor is placed on the side of the pipeline, and the single-mode optical fiber 2 and the few-mode optical fiber 3 are parallel to the pipeline;

[0092] P2. Use a spectrometer to find the light wavelength corresponding to the maximum slope on the cascade spectrum of the vibration sensor and record the value of the light wavelength;

[0093] P3, adjusting the wavelength of the narrow line width light source 6 to the light wavelength recorded in step P2; the light wavelength recorded in step P2 is the working point of the narrow line width light source 6;

[0094] P4. Give the pipeline a certain external excitation. When the pipeline is vibrated, it will cause the optical fiber vibration sensor to vibrate, causing the single-mode optical fiber 2 and the few-mode optical fiber 3 to bend and deform, thereby changing the cavity length of the air cavity 101. The change in cavity length will change the phase difference of the interference mode of the optical fiber vibration sensor, which will then manifest as a change in the output light intensity, thereby realizing vibration sensing. At the same time, the bending deformation of the few-mode optical fiber 3 causes the cavity length of the MI cavity to change, and the air cavity 101 and the MI cavity form a cascade cursor to improve the sensing sensitivity.

[0095] In specific applications, when the object to be detected vibrates, the intensity of the interference light will change, which in turn changes the electrical signal converted by the photodetector 8. The oscilloscope 9 displays and records the electrical signal, and the frequency and amplitude of the vibration signal can be analyzed by performing Fourier transform FFT on the output electrical signal. Vibration detection is achieved by observing the frequency and amplitude of the vibration signal.

[0096] The few-mode optical fiber 3 transmission mode adopted by the present invention has fewer types and controllable number of modes. The interference component is simple, and it is easy to obtain a high-contrast, regular-shaped interference spectrum, which is conducive to demodulating the spectrum and adjusting the free spectral range of the spectrum. The connection method of the single-mode optical fiber 2 and the few-mode optical fiber 3 is a non-fusion method. When there is vibration, the single-mode optical fiber 2 and the few-mode optical fiber 3 produce a mode of dislocation excitation of the core of the few-mode optical fiber 3. The non-fusion optical fiber adopts a mechanical connection method, does not require professional technical equipment, is simple to make, and has strong repeatability. In addition, compared with the traditional fusion method, the non-fusion method can replace and reconnect the optical fiber at any time without the need for fusion, which improves the flexibility of the connection and the convenience of maintenance.

[0097] like Figures 3 to 17 As shown, it is a frequency response diagram of a specific embodiment of the optical fiber vibration sensor. Experiments show that the vibration sensor based on non-fused macro-distance heterogeneous optical fibers can detect the corresponding frequency peak under external excitation of 1kHz-15Hz;

[0098] Specifically, in this embodiment, the relevant parameters of the optical fiber vibration sensor are: the cavity length of the air cavity 101 is 22 μm, the inner diameter of the capillary 1 is 300 μm, and the thickness of the film layer 4 is 100 nm.

[0099] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. An optical fiber vibration sensor, characterized in that: It comprises a capillary (1), a single-mode optical fiber (2), a few-mode optical fiber (3) and a film layer (4), wherein: The capillary tube (1) has an air cavity (101); One end of each of the single-mode optical fiber (2) and the few-mode optical fiber (3) is inserted into the capillary (1), the end faces of the single-mode optical fiber (2) and the few-mode optical fiber (3) are opposite to each other, and the single-mode optical fiber (2) and the few-mode optical fiber (3) do not contact the capillary (1); The film layer (4) is arranged on an end of the few-mode optical fiber (3) away from the single-mode optical fiber (2), and the film layer (4) is used to increase light intensity and improve vibration sensitivity.

2. The optical fiber vibration sensor according to claim 1, characterized in that: The cavity length of the air cavity (101) is 20 to 24 μm, and the inner diameter of the capillary tube (1) is 300 to 320 μm.

3. The optical fiber vibration sensor according to claim 2, characterized in that: The thickness of the film layer (4) is 80-120 nm.

4. The optical fiber vibration sensor according to claim 3, characterized in that: It also comprises a carrier plate (5), on which the capillary (1), the single-mode optical fiber (2) and the few-mode optical fiber (3) are all arranged.

5. The optical fiber vibration sensor according to claim 4, characterized in that: The film layer (4) is a gold film, and the carrier sheet (5) is a glass sheet.

6. A method for preparing the optical fiber vibration sensor according to claim 5, characterized in that: The following steps are involved: S1, stripping a section of the coating layer of the single-mode optical fiber (2) and the few-mode optical fiber (3), and cutting the end faces of the single-mode optical fiber (2) and the few-mode optical fiber (3) so that the end faces remain flat; S2, using a fusion splicer of model FSM-60S, placing the single-mode optical fiber (2) and the few-mode optical fiber (3) on the fusion splicer, and observing whether the end faces are flat; if not, re-cutting the optical fiber end faces until the end face inclination angle is less than 1 degree; S3, fixing the capillary (1) onto the carrier sheet (5); S4, fixing the single-mode optical fiber (2) and the few-mode optical fiber (3) at two ends of the micro-displacement platform, and placing the carrier plate (5) together with the capillary (1) below the ends of the single-mode optical fiber (2) and the few-mode optical fiber (3); S5, connecting a broadband light source to port 1 of the optical fiber circulator (7), connecting the single-mode optical fiber (2) to port 2 of the optical fiber circulator (7), and connecting a spectrometer to port 3 of the optical fiber circulator (7), adjusting the distance between the end face of the single-mode optical fiber (2) and the end face of the few-mode optical fiber (3) by means of a micro-displacement platform, and simultaneously observing the free spectrum range of the interference spectrum on the spectrometer, so as to adjust the cavity length of the air cavity (101); S6. After adjusting the cavity length of the air cavity (101), the single-mode optical fiber (2) and the few-mode optical fiber (3) are cured on the carrier sheet (5) using ultraviolet curing glue, thereby completing the preparation of the optical fiber vibration sensor.

7. The preparation method according to claim 6, characterized in that: The step S1 further includes plating the film layer (4), and the specific operations are as follows: Select the appropriate coating material, according to the detection frequency range, and preset the coating thickness; A coating is applied to one end of the few-mode optical fiber (3) away from the single-mode optical fiber (2), and the coating reaches a certain thickness and the quality of the coating meets the detection requirements, so as to form the film layer (4).

8. A detection system, characterized in that: Comprising the optical fiber vibration sensor as claimed in claim 5.

9. The detection system according to claim 8, characterized in that: It also includes a narrow line width light source (6), an optical fiber circulator (7), a photodetector (8) and an oscilloscope (9), wherein: The narrow linewidth light source (6) is connected to port one of the optical fiber circulator (7); The single-mode optical fiber (2) of the optical fiber vibration sensor is connected to port 2 of the optical fiber circulator (7); The photoelectric detector (8) is connected to port three of the optical fiber circulator (7); The oscilloscope (9) is connected to the photoelectric detector (8).

10. An application method of the detection system according to claim 9, characterized in that: The following steps are involved: P1. The carrier sheet (5) in the optical fiber vibration sensor is placed on the side of a pipeline, and the single-mode optical fiber (2) and the few-mode optical fiber (3) are parallel to the pipeline; P2. Using a spectrometer, find the light wavelength corresponding to the maximum slope on the cascade spectrum of the vibration sensor, and record the value of the light wavelength; P3, adjusting the wavelength of the narrow linewidth light source (6) to the light wavelength recorded in step P2; P4. A certain external excitation is given to the pipeline. When the pipeline is vibrated, the optical fiber vibration sensor will vibrate, causing the single-mode optical fiber (2) and the few-mode optical fiber (3) to bend and deform, thereby changing the cavity length of the air cavity (101). The change in cavity length will change the phase difference of the interference mode of the optical fiber vibration sensor, which will then be manifested as a change in output light intensity, thereby achieving vibration sensing; At the same time, the bending deformation of the few-mode optical fiber (3) causes the cavity length of the MI cavity to change, and the air cavity (101) and the MI cavity form a cascade cursor, thereby improving the sensing sensitivity.

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