F-P sensor based on change of refractive index of medium in cavity and preparation method of F-P sensor

Abstract

The invention discloses an F-P (Fabry-Perot) sensor based on change of refractive index of a medium in a cavity and a preparation method thereof, the F-P sensor comprises a rigid sleeve with an F-P inner cavity, and one end of the rigid sleeve is provided with a through hole communicated with the F-P inner cavity so as to allow a liquid medium to enter the F-P inner cavity; the cavity wall of the inner cavity is provided with a first end face and a second end face which are parallel and opposite to each other; the hole axis of the through hole is parallel to the first end face and the second end face; the reflecting diaphragm is fixedly arranged on the first end surface; one end of the optical fiber insertion core is perpendicular to the second end face and is inserted into the F-P inner cavity, and the other end of the optical fiber insertion core is located outside the rigid sleeve. According to the invention, the detection frequency band of the F-P sensor can be effectively broadened, and the F-P sensor can be flexibly installed near or even on the surface of a device which is easy to generate partial discharge in transformer equipment so as to realize the measurement of partial discharge signals in the transformer.

Description

Technical Field

[0001] The invention belongs to the field of ultrasonic sensing of partial discharge of electrical equipment, and in particular relates to a FP sensor based on the change of the refractive index of a medium in a cavity and a preparation method thereof, and an ultrasonic measurement system and method. Background Art

[0002] Power transformers are one of the core equipment for the operation of power systems. Ensuring the safe and reliable operation of power transformer equipment is the primary condition for ensuring the safe operation of power grids. During the long-term operation of power transformers, they will be interfered by strong electromagnetic environment and insulation aging. If the solid medium inside the power transformer is aged and damaged, the gas content of the oil component is too high, metal particles and winding faults, etc., will cause partial discharge faults in the power transformer. Smaller partial discharge faults can be restored within a few days by relying on the recoverability of the power transformer insulating oil itself. Once the partial discharge fault is too large and converted into arc discharge and causes the power transformer to break down, a large-scale power accident will occur. A large number of statistical data show that insulation failure is the main cause of power transformer failure. If the insulation fault of the power transformer can be handled in time, thereby eliminating the insulation fault problem, the reliable and stable operation of the power system can be effectively guaranteed.

[0003] At present, the partial discharge detection of power transformers is mainly divided into electrical detection method and non-electrical detection method. Electrical detection method mainly includes pulse current detection method and ultra-high frequency detection method. The pulse current detection method has low measurement frequency, narrow detection frequency band, weak anti-interference ability, and cannot be used for localization of partial discharge. The ultra-high frequency detection method has high sensitivity and accurate positioning, but it is easily interfered by the electromagnetic field of the operating equipment. Non-electrical detection methods mainly include ultrasonic method, gas chromatography method, and decomposition product method. Gas chromatography has strong anti-electromagnetic interference ability and high sensitivity, but it takes a long time to degas and it is difficult to reflect sudden faults. The decomposition product method is mainly difficult to detect early discharge faults and has low timeliness. The ultrasonic detection method is considered to be one of the most mature partial discharge detection methods at present. It has the characteristics of strong anti-electromagnetic interference ability, ultrasonic positioning, and wide detection frequency band. However, piezoelectric ceramic sensors (PZT) are currently mostly used to install on the outer wall of the transformer for detection, resulting in reduced sensitivity. Compared with traditional piezoelectric sensors, fiber optic ultrasonic sensors use optical fiber as a signal transmission medium and a signal receiving medium, and have many advantages such as small size, corrosion resistance, high temperature resistance, anti-electromagnetic interference, and easy reuse. They are particularly suitable for electrical equipment working in high voltage, high current, and strong magnetic field environments. Among them, fiber optic Fabry-Perot (FP) sensors are a type of ultrasonic sensor based on the Fabry-Perot Interference principle, with a higher upper frequency response limit and higher detection sensitivity.

[0004] The principle of the existing FP sensor sensing ultrasonic signals is mainly that when the FP sensor detects the change of medium density in the FP cavity, it is called direct coupling FP sensing, and local discharge detection is achieved by detecting the change of the cavity length. However, the high-sensitivity frequency response range of this FP sensor is narrow, resulting in a limited measurement range and reduced application flexibility. Summary of the invention

[0005] In order to solve the deficiencies in the prior art, the purpose of the present invention is to provide a Fabry-Perot sensor based on the change of the refractive index of the medium in the cavity and its preparation method and ultrasonic measurement system, so as to broaden the detection band of the existing Fabry-Perot sensor, and can be flexibly installed near or even on the surface of devices inside the transformer equipment that are prone to partial discharge, so as to realize the measurement of partial discharge signals inside the transformer.

[0006] The present invention adopts the following technical solution.

[0007] According to a first aspect of the present invention, a FP sensor based on a change in the refractive index of a medium in a cavity is provided. The FP sensor based on a change in the refractive index of a medium in a cavity comprises:

[0008] A rigid sleeve having an FP inner cavity; one end of the rigid sleeve has a through hole communicating with the FP inner cavity to allow a liquid medium to enter the FP inner cavity; a cavity wall of the inner cavity has a first end face and a second end face that are parallel and opposite to each other; a hole axis of the through hole is parallel to the first end face and the second end face;

[0009] A reflective film, fixedly arranged on the first end surface;

[0010] An optical fiber ferrule, one end of which is inserted into the FP inner cavity perpendicularly to the second end face through the opening on the second end face, and the other end of which is located outside the rigid sleeve.

[0011] Preferably, when the ultrasonic signal is transmitted into the FP cavity, the density of the liquid medium in the FP cavity is modulated by the sound pressure of the ultrasonic signal and changes.

[0012] Preferably, the rigid sleeve is a quartz sleeve.

[0013] Preferably, the cavity length of the FP inner cavity is 195-205 μm; the cavity length refers to the distance between the end of the optical fiber ferrule inserted into the FP inner cavity and the reflective film.

[0014] Preferably, the reflective film is made of Corning glass coated with a high-reflective dielectric film with a reflectivity greater than or equal to 99%.

[0015] Preferably, the refractive index of the liquid medium in the FP cavity is:

[0016]

[0017] Among them, n0 and ρ0 are the refractive index and density of the liquid medium in equilibrium state, respectively; c0 is the sound velocity of the liquid medium in equilibrium state; p is the sound pressure of the ultrasonic signal.

[0018] According to a second aspect of the present invention, a method for preparing a FP sensor based on a change in the refractive index of a medium in a cavity as described in the first aspect of the present invention is provided. The preparation method comprises the following steps:

[0019] The reflective film is fixedly packaged on the first end surface of the rigid sleeve by using ultraviolet curing glue;

[0020] Using a three-axis micro-displacement stage, insert the optical fiber ferrule into the FP inner cavity of the rigid sleeve perpendicular to the second end surface;

[0021] By observing the reflection spectrum of the FP sensor using a broadband light source and a spectrometer, the cavity length of the FP sensor is set to a predetermined value; wherein the cavity length is the distance between the end of the optical fiber ferrule inserted into the FP inner cavity and the reflective diaphragm;

[0022] After the cavity length of the FP sensor is adjusted to the predetermined value, the optical fiber ferrule is fixed to the rigid sleeve using ultraviolet curing glue.

[0023] According to a third aspect of the present invention, an ultrasonic measurement system is provided. The ultrasonic measurement system comprises the FP sensor based on the change of the refractive index of the medium in the cavity as described in the first aspect of the present invention. The ultrasonic measurement system also comprises: an optical fiber circulator, a photodetector, and a narrow-band laser;

[0024] The fiber circulator has a first port and a second port on a straight line and a third port perpendicular to the straight line; the narrowband laser and the fiber optic ferrule are respectively connected to the first port and the second port of the fiber circulator; the input end of the photodetector is connected to the third port of the fiber circulator.

[0025] According to a fourth aspect of the present invention, an ultrasonic measurement method is provided. The measurement method uses the ultrasonic measurement system as described in the third aspect of the present invention, and comprises the following steps:

[0026] Placing the FP sensor based on the change of the refractive index of the medium in the cavity in a liquid medium, so as to allow the liquid medium to enter the FP cavity through the through hole;

[0027] The narrowband laser is activated, so that after the narrowband laser generated by the narrowband laser enters the optical fiber ferrule through the optical fiber circulator, a part of the laser is reflected by the end face of the optical fiber ferrule, and another part of the laser is transmitted through the optical fiber ferrule into the FP inner cavity and then reflected by the reflective film and enters the optical fiber ferrule again, and interferes with the part of the laser reflected by the end face of the optical fiber ferrule at the optical fiber circulator to generate interference light;

[0028] The interference light is detected by photoelectric detection and converted into an electrical signal.

[0029] The beneficial effect of the present invention is that, compared with the prior art, a rigid sleeve, an optical fiber insert, and a reflective membrane are used, wherein the rigid sleeve has an FP inner cavity, and one end of the rigid sleeve has a through hole connected to the FP inner cavity to allow liquid medium to enter the FP inner cavity; wherein the inner cavity has a first end face and a second end face that are parallel and oppositely arranged, and the hole axis of the through hole is parallel to the first end face and the second end face; the reflective membrane is fixedly arranged on the first end face, and one end of the optical fiber insert is inserted into the FP inner cavity perpendicularly to the second end face, and the other end is located outside the rigid sleeve. In this way, when the partial discharge ultrasonic signal of the power transformer is transmitted to the FP cavity, the elastic strain of the medium in the FP inner cavity will cause periodic changes in time and space, thereby causing the density of the medium in the FP inner cavity to change due to the sound pressure modulation of the ultrasonic signal. Since the FP cavity length of the rigid sleeve used in the present invention does not change, the medium refractive index is only affected by the medium density, so the medium refractive index also changes accordingly, thereby changing the interference phase and interference intensity. Therefore, the response performance of the FP sensor of the present invention mainly depends on the properties of the medium in the cavity (density, refractive index, temperature and viscosity, etc.), so that the FP sensor of the present invention has the advantages of wide bandwidth (1-200kHz), wide dynamic range, simple structure and anti-electromagnetic interference, etc., and can effectively detect all types of partial discharge of power transformers, solving the problem of the narrow high-sensitivity response range (generally less than 30kHz) of the existing indirect coupling Fabry-Perot sensor.

[0030] In addition, compared with traditional external ultrasonic sensors, due to the interference of solid structures such as transformer windings, the sound waves are already very weak when they are transmitted to the transformer casing, and many weak local discharge ultrasonic signals cannot be effectively measured. The FP sensor based on the change of the refractive index of the medium in the cavity of the present invention can be built into the location inside the transformer where local discharge is prone to occur, thereby realizing the measurement of free sound field weak ultrasonic signals such as tip discharge, free particle discharge and surface discharge inside the transformer. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a comparison of the sensing principle of the FP sensor in the present invention and the FP sensor in the prior art;

[0032] Figure 2 It is the design drawing and physical picture of the FP sensor in the present invention;

[0033] Figure 3 is the interference spectrum of the FP sensor in the present invention;

[0034] Figure 4 is the frequency response of the FP sensor in the present invention. DETAILED DESCRIPTION

[0035] The present application is further described below in conjunction with the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and cannot be used to limit the protection scope of the present application.

[0036] As attached Figure 1 FIG. 4 is a comparison of the sensing principles of the FP sensor of the present invention and the existing FP sensor.

[0037] Fiber Fabry-Perot sensing is an ultrasonic signal detection method based on the Fabry-Perot interference principle. When narrowband light is incident on two parallel reflectors at a certain distance, the two beams of light are reflected back and coherently interfere at a certain point. The working principle of the fiber FP sensor is shown in the attached figure. Figure 1 As shown, the narrowband laser outputs narrowband laser to the single-mode fiber ferrule, and the laser is reflected back to the first beam of light at the end face of the ferrule (the reflectivity of the end face of the single-mode fiber ferrule is about 3.5%). The transmitted light enters the cavity of the FP sensor and finally enters the second reflector. Usually, the reflectivity of the second reflector is higher (generally, the reflectivity is selected to be above 95%). After reflection, the second laser beam enters the ferrule again and interferes with the first reflected light at the circulator. The interference light is output from the third port of the circulator to the photodetector, which converts the optical signal into an electrical signal and outputs the data to a data acquisition card or oscilloscope.

[0038] According to the optical characteristics of the fiber FP sensing structure, it can be analyzed through the multi-beam interference model. According to the multi-beam interference principle of physical optics, the light intensity Iout of the synthesized output of the multi-beam interference fringes is:

[0039]

[0040] Among them, R1 and R2 are the reflectivity of the two end faces of the optical fiber FP, is the phase difference between adjacent light beams, Iin is the input light intensity, n is the refractive index of the medium liquid in the FP cavity, L is the FP cavity length, and λ is the wavelength of the incident narrow-band laser.

[0041] When the FP sensor static operating point is Figure 1 When it is at the midpoint of the linear operating range in the circuit, its sensitivity is the highest, which is called the orthogonal operating point (Q point).

[0042] It can be seen that when the wavelength λ of the incident narrowband laser is fixed, the intensity of the output spectrum is only related to the FP cavity length L and the medium refractive index n. Therefore, when external signals (sound, pressure, vibration, dynamic strain, etc.) act on the FP sensor, it will cause the FP cavity length to change ΔL (indirect coupling type FP) or the refractive index of the cavity medium to change Δn (direct coupling type FP). As a result, the phase difference of the FP interference light changes, and then the interference light intensity changes. The external signal can be obtained by demodulating the light intensity or phase change.

[0043] In the prior art, the FP sensing structure is composed of an optical fiber end face, a sound-sensitive membrane, and an interference cavity wall, wherein no liquid medium enters the cavity between the optical fiber end face and the sound-sensitive membrane. The external signal changes the cavity length between the two reflection surfaces by causing the vibration of the sound-sensitive membrane, resulting in changes in the interference phase and interference intensity. Therefore, the response performance of the existing FP sensor mainly depends on the mechanical and optical properties of the sound-sensitive membrane. The existing FP sensor has a narrow high-sensitivity area (generally less than 30kHz). The first-order natural resonant frequency and static pressure sensitivity of the sound-sensitive membrane are shown in equations (1) and (2), respectively:

[0044]

[0045] Among them, ρ, h, E, and v are the density, thickness, Young's modulus, and Poisson's ratio of the sound-sensitive diaphragm, respectively; r0 is the effective vibration radius of the diaphragm; r is the distance from the center of the diaphragm; ρ f is the fluid mass density (the density of insulating oil is taken as 895kg / m 3 ).

[0046] The FP sensor of the present invention is also composed of an optical fiber end face, a reflective diaphragm, and an interference cavity wall. The cavity between the optical fiber end face and the reflective diaphragm is connected to the outside world and can allow liquid media to enter. It senses ultrasonic signals in a different way. When the ultrasonic signal is transmitted to the FP cavity, it will cause the elastic strain of the medium in the FP cavity to change periodically in time and space, thereby causing the density of the medium in the FP cavity of the present invention to change due to the sound pressure modulation of the ultrasonic signal. Since the refractive index of the medium is only affected by the density of the medium, the refractive index of the medium also changes accordingly, thereby changing the interference phase and interference intensity. Therefore, the response performance of the FP sensor of the present invention mainly depends on the properties of the medium in the cavity (density, refractive index, temperature and viscosity, etc.). The FP sensor of the present invention has the advantages of wide bandwidth and wide dynamic range.

[0047] According to the Clausius-Mosotti relationship, the refractive index n of the liquid medium changes with the density ρ, and the refractive index change rate of the liquid medium is obtained as follows:

[0048]

[0049] Among them, n0 and ρ0 are the refractive index and density of the liquid medium in equilibrium state, respectively; c0 is the sound velocity of the liquid medium in equilibrium state; p is the sound pressure of the ultrasonic signal.

[0050] The sensitivity S of the FP sensor of the present invention is proportional to the free spectral range FSR and the finesse F:

[0051]

[0052] Wherein, R1 and R2 are the reflectivity of the two reflection surfaces respectively; n is the refractive index of the medium; and L is the FP cavity length.

[0053] Formula (3) shows that the FP sensing signal of the present invention is only proportional to the sound pressure of the ultrasonic signal and has nothing to do with the frequency. The response performance of the FP sensor of the present invention mainly depends on the properties of the medium in the cavity (density, refractive index, temperature and viscosity, etc.), and has the advantages of wide bandwidth and wide dynamic range.

[0054] Example 1

[0055] This embodiment relates to a FP sensor based on the change of the refractive index of the medium in the cavity.

[0056] Figure 2 The design drawings and actual pictures of the FP sensor based on the change of the refractive index of the medium in the cavity are shown.

[0057] like Figure 2 The FP sensor based on the change of the refractive index of the medium in the cavity includes: a rigid sleeve, a reflective membrane and an optical fiber ferrule.

[0058] The rigid sleeve has a rigid sleeve of an FP inner cavity, and one end of the rigid sleeve has a through hole connected to the FP inner cavity to allow a liquid medium to enter the FP inner cavity. The cavity wall of the inner cavity has a first end face and a second end face that are parallel and opposite to each other. The reflective diaphragm is fixedly arranged on the first end face. One end of the optical fiber ferrule is inserted into the FP inner cavity perpendicularly to the second end face through an opening on the second end face, and the other end of the optical fiber ferrule is located outside the rigid sleeve.

[0059] By setting a rigid sleeve, the sleeve can maintain its original shape and structure when subjected to external force, and hardly deforms. At the same time, the through holes on the rigid sleeve allow the liquid medium to enter the inner cavity of the sleeve. Therefore, when the ultrasonic signal is transmitted to the FP inner cavity, the elastic strain of the medium in the FP inner cavity changes periodically in time and space, so that the density of the medium in the FP inner cavity is modulated by the sound pressure of the ultrasonic signal and changes.

[0060] Preferably, the other end of the rigid sleeve opposite to the through hole is closed.

[0061] Preferably, the rigid sleeve is a quartz sleeve. As an example, the size of the quartz sleeve is 15×11×11 mm, the thickness of the sleeve wall is 3 mm, the length of the optical fiber ferrule is 2 m, the diameter of the ferrule is 2.5 mm, and the diameter of the through hole is 5 mm.

[0062] Preferably, the cavity length of the FP cavity is 195-205 μm, and the cavity length refers to the distance between the end of the optical fiber ferrule inserted into the FP cavity and the reflective diaphragm. This cavity length range helps to ensure that the FP sensor operates in a linear working area, thereby maintaining a high sensitivity. More preferably, the cavity length of the FP cavity is 200 μm.

[0063] Preferably, the reflective film is made of Corning glass coated with a high reflective dielectric film with a reflectivity greater than or equal to 99%. More preferably, the density of the Corning glass is ρ = 2380 kg / m 3 , Young's modulus E = 73.6 GPa, Poisson's ratio v = 0.23.

[0064] Furthermore, the refractive index of the liquid medium in the FP cavity Δn is:

[0065]

[0066] Among them, n0 and ρ0 are the refractive index and density of the liquid medium in equilibrium state, respectively; c0 is the sound velocity of the liquid medium in equilibrium state; p is the sound pressure of the ultrasonic signal.

[0067] Example 2

[0068] This embodiment provides a method for preparing a FP sensor based on the change of the refractive index of the medium in the cavity as in Embodiment 1.

[0069] Specifically, the preparation method of the FP sensor includes the following steps:

[0070] Step A1: fix and package the reflective film on the first end surface of the rigid sleeve using ultraviolet curing glue.

[0071] Step A2: Use a three-axis micro-displacement stage to insert the optical fiber ferrule into the FP inner cavity of the rigid sleeve perpendicular to the second end surface.

[0072] Step A3: Observe the reflection spectrum of the FP sensor using a broadband light source and a spectrometer to set the cavity length of the FP sensor to a predetermined value, wherein the cavity length is the distance between the end of the optical fiber ferrule inserted into the FP cavity and the reflective diaphragm.

[0073] The predetermined value is preferably 195-205 μm, and more preferably 200 μm. The purpose of reserving a longer cavity length is to allow the cavity to better sense the change in the refractive index of the medium and improve the sensor's ability to receive ultrasonic signals.

[0074] Step A4: After adjusting the cavity length of the FP sensor to the set position, fix the optical fiber ferrule on the rigid sleeve with ultraviolet curing glue.

[0075] Example 3

[0076] This embodiment provides an ultrasonic measurement system, which includes the FP sensor based on the change of the refractive index of the medium in the cavity as in Embodiment 1, and also includes an optical fiber circulator, a photodetector, and a narrow-band laser.

[0077] The optical fiber circulator has a first port and a second port on a straight line and a third port perpendicular to the straight line. The narrowband laser and the optical fiber ferrule are connected to the first port and the second port of the optical fiber circulator respectively. The input end of the photodetector is connected to the third port of the optical fiber circulator.

[0078] Preferably, the ultrasonic measurement system further comprises an oscilloscope or a data acquisition card connected to the output end of the photoelectric detection to display or acquire the signal output by the photoelectric detector.

[0079] Example 4

[0080] This embodiment provides an ultrasonic measurement method. The ultrasonic measurement method uses the ultrasonic measurement system of embodiment 3, and includes the following steps:

[0081] Step B1, placing the FP sensor based on the change of the refractive index of the medium in the cavity in a liquid medium, so as to allow the liquid medium to enter the FP cavity through the through hole;

[0082] Step B2, activating the narrow-band laser, so that after the narrow-band laser generated by the narrow-band laser enters the optical fiber ferrule through the optical fiber circulator, a part of the laser is reflected by the end face of the optical fiber ferrule, and another part of the laser is transmitted through the optical fiber ferrule into the FP inner cavity and then reflected by the reflective film and enters the optical fiber ferrule again, and interferes with the part of the laser reflected by the end face of the optical fiber ferrule at the optical fiber circulator to generate interference light;

[0083] Step B3: Detect the interference light through photoelectric detection and convert the interference light into an electrical signal.

[0084] The drawings and the actual object of the FP sensor of the present invention designed according to Example 1 are as follows: Figure 2(c)-(d) are shown. The actual cavity length of the direct-coupled FP sensor is calculated to be 202.567 μm using the peak tracking method. The interference spectrum is shown in the attached figure. Figure 3 shown.

[0085] Attached Figure 4 The frequency response of the FP sensor of the present invention is given, and it can be found that the frequency response of the FP sensor designed by the present invention is relatively flat, and has a wide frequency band of 1-200 kHz.

[0086] The beneficial effect of the present invention is that, compared with the prior art, a rigid sleeve, an optical fiber insert, and a reflective membrane are used, wherein the rigid sleeve has an FP inner cavity, and one end of the rigid sleeve has a through hole connected to the FP inner cavity to allow liquid medium to enter the FP inner cavity; wherein the inner cavity has a first end face and a second end face that are parallel and oppositely arranged, and the hole axis of the through hole is parallel to the first end face and the second end face; the reflective membrane is fixedly arranged on the first end face, and one end of the optical fiber insert is inserted into the FP inner cavity perpendicularly to the second end face, and the other end is located outside the rigid sleeve. In this way, when the partial discharge ultrasonic signal of the power transformer is transmitted to the FP cavity, the elastic strain of the medium in the FP inner cavity will cause periodic changes in time and space, thereby causing the density of the medium in the FP inner cavity to change due to the sound pressure modulation of the ultrasonic signal. Since the present invention adopts a rigid sleeve, the FP cavity length does not change, and the refractive index of the medium therein is only affected by the density of the medium. Therefore, the response performance of the FP sensor of the present invention mainly depends on the properties of the medium in the cavity (density, refractive index, temperature and viscosity, etc.), so that the FP sensor of the present invention has the advantages of wide frequency band (1-200kHz), wide dynamic range, simple structure and anti-electromagnetic interference, etc., and can effectively detect all types of partial discharge of power transformers, solving the problem of the narrow high-sensitivity response range (generally less than 30kHz) of the existing indirect coupling Fabry-Perot sensor.

[0087] In addition, compared with traditional external ultrasonic sensors, due to the interference of solid structures such as transformer windings, the sound waves are already very weak when they are transmitted to the transformer casing, and many weak local discharge ultrasonic signals cannot be effectively measured. The FP sensor based on the change of the refractive index of the medium in the cavity of the present invention can be built into the location inside the transformer where local discharge is prone to occur, thereby realizing the measurement of free sound field weak ultrasonic signals such as tip discharge, free particle discharge and surface discharge inside the transformer.

[0088] The applicant of the present invention has made a detailed explanation and description of the implementation examples of the present invention in conjunction with the drawings in the specification. However, those skilled in the art should understand that the above implementation examples are only preferred implementation schemes of the present invention, and the detailed description is only to help readers better understand the spirit of the present invention, but not to limit the scope of protection of the present invention. On the contrary, any improvements or modifications based on the inventive spirit of the present invention should fall within the scope of protection of the present invention.

Claims

1. A FP sensor based on the change of the refractive index of the medium in the cavity, characterized in that: include: A rigid sleeve having an FP inner cavity; one end of the rigid sleeve has a through hole communicating with the FP inner cavity to allow a liquid medium to enter the FP inner cavity; a cavity wall of the inner cavity has a first end face and a second end face that are parallel and opposite to each other; a hole axis of the through hole is parallel to the first end face and the second end face; A reflective film, fixedly arranged on the first end surface; An optical fiber ferrule, one end of which is inserted into the FP inner cavity perpendicularly to the second end face through the opening on the second end face, and the other end of which is located outside the rigid sleeve.

2. The FP sensor based on the change of the refractive index of the medium in the cavity according to claim 1, characterized in that: When the ultrasonic signal is transmitted into the FP cavity, the density of the liquid medium in the FP cavity is modulated by the sound pressure of the ultrasonic signal and changes.

3. The FP sensor based on the change of the refractive index of the medium in the cavity according to claim 1, characterized in that: The other end of the rigid sleeve opposite to the through hole is closed.

4. The FP sensor based on the change of the refractive index of the medium in the cavity according to claim 1, characterized in that: The rigid sleeve is a quartz sleeve.

5. The FP sensor based on the change of the refractive index of the medium in the cavity according to claim 1, characterized in that: The cavity length of the FP inner cavity is 195-205 μm; the cavity length refers to the distance between the end of the optical fiber ferrule inserted into the FP inner cavity and the reflective film.

6. The FP sensor based on the change of the refractive index of the medium in the cavity according to claim 1, characterized in that: The reflective film is made of Corning glass coated with a high-reflective dielectric film with a reflectivity greater than or equal to 99%.

7. The FP sensor based on the change of the refractive index of the medium in the cavity according to any one of claims 1 to 6, characterized in that: The refractive index of the liquid medium in the FP cavity is: Among them, n0 and ρ0 are the refractive index and density of the liquid medium in equilibrium state, respectively; c0 is the sound velocity of the liquid medium in equilibrium state; p is the sound pressure of the ultrasonic signal.

8. A method for preparing a FP sensor based on the change of the refractive index of the medium in the cavity as claimed in any one of claims 1 to 7, characterized in that: The steps include: The reflective film is fixedly packaged on the first end surface of the rigid sleeve by using ultraviolet curing glue; Using a three-axis micro-displacement stage, insert the optical fiber ferrule into the FP inner cavity of the rigid sleeve perpendicular to the second end surface; By observing the reflection spectrum of the FP sensor using a broadband light source and a spectrometer, the cavity length of the FP sensor is set to a predetermined value; wherein the cavity length is the distance between the end of the optical fiber ferrule inserted into the FP inner cavity and the reflective diaphragm; After the cavity length of the FP sensor is adjusted to the predetermined value, the optical fiber ferrule is fixed to the rigid sleeve using ultraviolet curing glue.

9. An ultrasonic measurement system, characterized in that: The FP sensor based on the change of the refractive index of the medium in the cavity as claimed in any one of claims 1 to 7 further comprises: an optical fiber circulator, a photodetector, and a narrow-band laser; The fiber circulator has a first port and a second port on a straight line and a third port perpendicular to the straight line; the narrowband laser and the fiber optic ferrule are respectively connected to the first port and the second port of the fiber circulator; the input end of the photodetector is connected to the third port of the fiber circulator.

10. An ultrasonic measurement method, using the ultrasonic measurement system according to claim 9, characterized in that: The steps include: Placing the FP sensor based on the change of the refractive index of the medium in the cavity in a liquid medium, so as to allow the liquid medium to enter the FP cavity through the through hole; The narrowband laser is activated, so that after the narrowband laser generated by the narrowband laser enters the optical fiber ferrule through the optical fiber circulator, a part of the laser is reflected by the end face of the optical fiber ferrule, and another part of the laser is transmitted through the optical fiber ferrule into the FP inner cavity and then reflected by the reflective film and enters the optical fiber ferrule again, and interferes with the part of the laser reflected by the end face of the optical fiber ferrule at the optical fiber circulator to generate interference light; The interference light is detected by photoelectric detection and converted into an electrical signal.

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