A flexible optical fiber fabry-perot interferometric ultrasonic sensor

Abstract

A kind of flexible optical fiber Fabry-Perot interference type ultrasonic sensor, high sound transmission package shell is provided with optical fiber ultrasonic sensing structure, optical fiber ultrasonic sensing structure is single mode optical fiber one end and is provided with flexible probe, the material of the flexible probe is polydimethylsiloxane, single mode optical fiber other end extends outside package shell, the flexible probe is cylinder, axial through hole is arranged in the center of flexible probe, the axial through hole of flexible probe is provided with sound transmission body away from single mode optical fiber one end, the cavity between the sound transmission body and single mode optical fiber end face is Fabry-Perot air cavity.The optical fiber ultrasonic sensing structure of the present application adopts single mode optical fiber one end and is provided with flexible probe, with the characteristics of high transmittance, low thermal conductivity, can eliminate the temperature cross sensitivity influence of sensor, the sensor of the present application is passive device, and the anti-electromagnetic interference ability is strong, with the advantages of small size, light weight, meet the measurement requirements of narrow space.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of optical fiber sensing, and particularly relates to a flexible optical fiber Fabry-Perot interferometric ultrasonic sensor. BACKGROUND

[0002] Ultrasonic signals are widely used in ultrasonic flaw detection, imaging, welding, medical examination and industrial automation control due to their strong penetration, good directivity, long propagation distance, no harm to human body and equipment and other advantages. At present, piezoelectric ceramic transducers (PZT) are the main ultrasonic detection devices, which occupy an important position in the market and are widely used in ultrasonic cleaning, humidifiers, underwater sonar and other fields. However, the piezoelectric ceramic material inside is easy to be damaged, the acoustic impedance matching needs to be improved, it is easy to be affected by electromagnetic interference, and the detection system is bulky. As a new type of sensing technology developed in recent years, compared with the traditional PZT sensor, the optical fiber ultrasonic sensing technology has a wide response band, high detection sensitivity, small size, light weight, strong anti-electromagnetic interference ability, long signal transmission distance, good fidelity and other advantages.

[0003] Optical fiber ultrasonic sensing technology mainly includes optical fiber grating type ultrasonic sensing technology, intensity type ultrasonic sensing technology and interference type ultrasonic sensing technology. Among them, the interference type optical fiber ultrasonic sensor mainly obtains ultrasonic information through the phase change of the propagating light in the optical fiber caused by ultrasonic waves. Compared with the other two technologies, the interference type optical fiber ultrasonic sensor not only has great advantages in detection sensitivity, but also has the advantages of flexible and diverse structure, wide detection parameters and rich demodulation methods. The interference type optical fiber ultrasonic sensor mainly includes Mach-Zehnder interferometric ultrasonic sensor, Michelson interferometric ultrasonic sensor and Fabry-Perot interferometric ultrasonic sensor.

[0004] Due to the diameter limitation of the ordinary optical fiber used at present, most of the optical fiber ultrasonic sensors draw the sensing optical fiber into a small diameter thin cone, which means that the mechanical strength of the sensing head is low and needs to be accurately controlled in the manufacturing process. The ultrasonic sensor based on Fabry-Perot interference uses hollow optical fiber, capillary and Fabry-Perot interference ultrasonic sensor structure filled with acoustic sensitive material by using existing technology to occupy the main research field due to its high sensitivity and easy detection of ultrasonic signals. However, since the principle of demodulating ultrasonic signals mainly depends on the change of the cavity length of the filling material, the detection of ultrasonic signals depends on the filling material, and although the structure outside the cavity is affected by the ultrasonic signal, it is not efficiently utilized, thereby causing a series of problems such as reduction of sensitivity of the optical fiber ultrasonic sensor, signal distortion and reduction of linearity. Therefore, it is necessary to study an optical fiber ultrasonic sensor with high sensitivity, easy detection of ultrasonic signals and strong ultrasonic response. SUMMARY

[0005] The technical problem to be solved by the present application is to overcome the shortcomings of the existing optical fiber ultrasonic sensor, and provide a flexible optical fiber Fabry-Perot interference type ultrasonic sensor with high sensitivity, easy detection of ultrasonic signals and strong ultrasonic response.

[0006] The technical solution adopted to solve the above technical problems is: a flexible optical fiber Fabry-Perot interference type ultrasonic sensor, a high sound transmission packaging shell is provided with an optical fiber ultrasonic sensing structure, the optical fiber ultrasonic sensing structure is provided with a flexible probe at one end of a single mode optical fiber, the material of the flexible probe is polydimethylsiloxane, the other end of the single mode optical fiber extends out of the packaging shell, the flexible probe is a cylinder, an axial through hole is arranged in the center of the flexible probe, a sound transmission body is arranged in the axial through hole of the flexible probe away from the single mode optical fiber, and the cavity between the sound transmission body and the end face of the single mode optical fiber is a Fabry-Perot air cavity.

[0007] As a preferred technical solution, a conical hole is processed on the end of the packaging shell where the flexible probe is located.

[0008] As a preferred technical solution, the material of the sound transmission body is neoprene or butyl rubber.

[0009] As a preferred technical solution, the material of the packaging shell is glass steel.

[0010] As a preferred technical solution, the manufacturing method of the optical fiber ultrasonic sensing structure is: a single mode optical fiber with stripped coating is prepared, the end face of the single mode optical fiber is cut flat and cleaned with alcohol, then the end of the single mode optical fiber is inserted into a flexible probe preparation mold, a mixed solution of polydimethylsiloxane and curing agent is injected into the flexible probe preparation mold, and the polydimethylsiloxane probe integrated with the single mode optical fiber is formed by heating and curing, the flexible probe preparation mold is disassembled, the sound transmission body is filled under a microscope, the neoprene or butyl rubber solution is dropped on a glass slide, the flexible probe is moved constantly by using a precision displacement table, the neoprene or butyl rubber solution is fully soaked into the axial through hole of the polydimethylsiloxane, and the excess neoprene or butyl rubber solution is removed and solidified, and finally the optical fiber ultrasonic sensing structure with the single mode optical fiber-polydimethylsiloxane-neoprene or butyl rubber Fabry-Perot interference structure is formed.

[0011] As a preferred technical solution, the flexible probe preparation mold includes a tube body and a center column, the length of the center column is less than the length of the tube body, and the diameter of the center column is less than the inner diameter of the tube body.

[0012] As a preferred technical solution, the preparation method of the mixed solution of polydimethylsiloxane and curing agent is: the polydimethylsiloxane and the curing agent are mixed in a ratio of 10:1, and are uniformly stirred in a blender for 10 minutes, so that the polydimethylsiloxane and the curing agent are fully mixed and uniform, and the preparation is completed.

[0013] The beneficial effects of the present application are as follows:

[0014] The optical fiber ultrasonic sensing structure of the present application is provided with a flexible probe at one end of a single-mode optical fiber, the flexible probe is made of polydimethylsiloxane, has high light transmittance and low thermal conductivity, the ultrasonic stress change is several orders of magnitude larger than the change caused by thermal expansion, and the temperature cross-sensitivity of the sensor can be eliminated; when light is incident on the Fabry-Perot air cavity from the single-mode optical fiber, the light beam is reflected at the end face of the single-mode optical fiber and the end face of the sound-transparent body to cause a double-beam interference phenomenon, the interference output signal is related to the length of the Fabry-Perot air cavity, the polydimethylsiloxane flexible probe has excellent elastic properties, when external ultrasonic signals act on the flexible probe, the length of the Fabry-Perot air cavity changes, and thus the interference output signal changes, so that the length change of the Fabry-Perot cavity can be obtained from the output interference signal, and the external ultrasonic change information can be obtained. The present application is highly sensitive to external ultrasonic signals or mechanical vibrations, and has the advantages of high detection efficiency and sensitivity.

[0015] The sensor of the present application is a passive device, and thus it overcomes the disadvantage that the conventional ultrasonic sensor cannot work under strong electromagnetic interference, and has the advantages of small size and light weight compared with the conventional ultrasonic sensor, and meets the measurement requirements in a narrow space. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 is a structural schematic diagram of the present application.

[0017] Figure 2 is a structural schematic diagram of the flexible probe preparation mold of the present application.

[0018] Among them: the packaging shell 1, the single-mode optical fiber 2, the flexible probe 3, the Fabry-Perot air cavity 4, the sound-transparent body 5, the conical hole 6, the tube body 7, the center column 8; DETAILED DESCRIPTION

[0019] The present application will be further described in detail below in combination with the drawings and examples, but the present application is not limited to the following embodiments.

[0020] In Figure 1 , the flexible optical fiber Fabry-Perot interference type ultrasonic sensor of the present embodiment is connected by a packaging shell 1, a single-mode optical fiber 2, a flexible probe 3, and a sound-transparent body 5.

[0021] The material of the encapsulation shell 1 is glass fiber reinforced plastic, and the optical fiber ultrasonic sensing structure is installed in the encapsulation shell 1. The optical fiber ultrasonic sensing structure is provided with a flexible probe 3 at one end of a single-mode optical fiber 2. The core diameter of the single-mode optical fiber 2 is 9 μm, the cladding diameter is 125 μm, the refractive index of the core is 1.4682, the refractive index of the cladding is 1.4628, the material of the flexible probe 3 is polydimethylsiloxane, the other end of the single-mode optical fiber 2 extends out of the encapsulation shell 1, the flexible probe 3 is a cylinder with an outer diameter of 125 μm, an axial through hole is arranged in the center of the flexible probe 3, the diameter of the axial through hole is 50 μm, the refractive index of the flexible probe 3 is 1.3997, a sound-transparent body 5 is arranged in the axial through hole of the flexible probe 3 away from the single-mode optical fiber 2, the material of the sound-transparent body 5 is neoprene, and the material of the sound-transparent body 5 can also be butyl rubber, the cavity between the sound-transparent body 5 and the end face of the single-mode optical fiber 2 is a Fabry-Perot air cavity 4, and a conical hole 6 is processed at the end of the encapsulation shell 1 where the flexible probe 3 is located, which is used for focusing a large amount of acoustic signals to the flexible probe 3, greatly improving the ultrasonic detection efficiency and sensitivity.

[0022] The material of the encapsulation shell 1 is glass fiber reinforced plastic, and the optical fiber ultrasonic sensing structure is installed in the encapsulation shell 1. The optical fiber ultrasonic sensing structure is provided with a flexible probe 3 at one end of a single-mode optical fiber 2. The core diameter of the single-mode optical fiber 2 is 9 μm, the cladding diameter is 125 μm, the refractive index of the core is 1.4682, the refractive index of the cladding is 1.4628, the material of the flexible probe 3 is polydimethylsiloxane, the other end of the single-mode optical fiber 2 extends out of the encapsulation shell 1, the flexible probe 3 is a cylinder with an outer diameter of 125 μm, an axial through hole is arranged in the center of the flexible probe 3, the diameter of the axial through hole is 50 μm, the refractive index of the flexible probe 3 is 1.3997, a sound-transparent body 5 is arranged in the axial through hole of the flexible probe 3 away from the single-mode optical fiber 2, the material of the sound-transparent body 5 is neoprene, and the material of the sound-transparent body 5 can also be butyl rubber, the cavity between the sound-transparent body 5 and the end face of the single-mode optical fiber 2 is a Fabry-Perot air cavity 4, and a conical hole 6 is processed at the end of the encapsulation shell 1 where the flexible probe 3 is located, which is used for focusing a large amount of acoustic signals to the flexible probe 3, greatly improving the ultrasonic detection efficiency and sensitivity.

[0023] The preparation method of the optical fiber ultrasonic sensing structure of the embodiment is as follows: a single-mode optical fiber 2 with a stripped coating is prepared, the end face of the single-mode optical fiber 2 is cut flat and cleaned with alcohol, then the end of the single-mode optical fiber 2 is inserted into a flexible probe 3 preparation mold, a mixed solution of polydimethylsiloxane and a curing agent is injected into the flexible probe 3 preparation mold, and the polydimethylsiloxane probe integrated with the single-mode optical fiber 2 is formed by heating and curing. The flexible probe preparation mold is disassembled, the sound-transparent body 5 is filled under a microscope, the neoprene or butyl rubber solution is dropped on a glass slide, the precise displacement table is used to move the flexible probe 3 constantly, the neoprene or butyl rubber solution is fully soaked into the axial through hole of the polydimethylsiloxane, and the excess neoprene or butyl rubber solution is removed and solidified, so that the optical fiber ultrasonic sensing structure with the Fabry-Perot interference structure of the single-mode optical fiber 2-polydimethylsiloxane-neoprene or butyl rubber is finally formed.

[0024] The preparation method of the mixed solution of polydimethylsiloxane and a curing agent of the embodiment is as follows: the polydimethylsiloxane and the curing agent are mixed at a ratio of 10:1, and are uniformly stirred in a blender for 10 minutes, so that the polydimethylsiloxane and the curing agent are fully mixed and uniform, and the preparation is completed.

[0025] The flexible probe 3 preparation mold of the embodiment includes a tube body 7 and a center column 8. The length of the center column 8 is 0.5 times the length of the tube body 7, and the diameter of the center column is 50 μm. The inner diameter of the tube body 7 is 125 μm.

[0026] The working principle of the present application is as follows:

[0027] When light is incident from the single-mode optical fiber 2 to the Fabry-Perot air cavity 4, part of the light is reflected at the end face of the single-mode optical fiber 2 to form first reflected light, and the remaining light is transmitted to the Fabry-Perot air cavity 4 and reflected at the end face of the sound-transmitting body 5 to form second reflected light. The two reflected light beams will be recombined at the single-mode optical fiber 2 and output to the spectrometer connected to the single-mode optical fiber 2. Due to the different phase delays caused by the different optical path differences between the first reflected light and the second reflected light, the two reflected lights produce interference, and interference fringes are produced in the reflected light spectrum. According to the working principle of the Fabry-Perot interferometer, the intensity I of the interference light is:

[0028]

[0029] In the formula, I1 is the intensity of the first reflected light, I2 is the intensity of the second reflected light, and ΔΦ is the phase difference between the intensity of the first reflected light and the intensity of the second reflected light.

[0030] The phase difference ΔΦ between the intensity of the first reflected light and the intensity of the second reflected light is

[0031]

[0032] In the formula, n is the refractive index of the core of the single-mode optical fiber 2, ΔL is the optical path difference, and λ is the wavelength of the light in the single-mode optical fiber 2. When the phase difference satisfies ΔΦ=(2k+1)π, (k=nΔL / λ), the corresponding interference extreme wavelength is:

[0033]

[0034] In the formula, k is an integer.

[0035] Therefore, the free spectral range FSR between the two interference light beams can be:

[0036]

[0037] When the sensor is subjected to an ultrasonic wave, the refractive index n and the cavity length L of the Fabry-Perot air cavity 4 will change, thereby modulating the phase difference of the two reflected lights and causing the interference spectrum to change. At this time, the measurement of the ultrasonic signal can be realized by detecting the interference spectrum. When the external ultrasonic signal acts on the flexible probe 3, due to the superior elastic properties of dimethylsiloxane and chlorobutyl rubber or butyl rubber, the ultrasonic response area is increased at the flexible probe 3, causing the flexible probe 3 and the sound-transmitting body 5 to change, thereby causing the cavity between the sound-transmitting body 5 and the end face of the single-mode optical fiber 2 to change, and the interference output signal to change. Therefore, the length change amount of the Fabry-Perot air cavity 4 can be derived from the output interference signal, and the external ultrasonic change can be measured.

Claims

1. A flexible fiber optic Fabry-Perot interferometer ultrasonic sensor, characterized in that: A fiber optic ultrasonic sensing structure is housed within a high-transmission encapsulation shell. The fiber optic ultrasonic sensing structure consists of a flexible probe at one end of a single-mode fiber, the flexible probe being made of polydimethylsiloxane. The other end of the single-mode fiber extends outside the encapsulation shell. The flexible probe is cylindrical, with an axial through-hole at its center. A sound-transmitting element is located at the end of the axial through-hole away from the single-mode fiber. The cavity between the sound-transmitting element and the end face of the single-mode fiber is a Fabry-Perot air cavity. A conical hole is machined at the end of the encapsulation shell where the flexible probe is located. The sound-transmitting element is made of neoprene rubber or butyl rubber.

2. The flexible fiber optic Fabry-Perot interferometer ultrasonic sensor according to claim 1, characterized in that: The packaging shell is made of fiberglass.

3. The flexible fiber optic Fabry-Perot interferometric ultrasonic sensor according to claim 1, characterized in that, The fabrication method of the fiber optic ultrasonic sensing structure is as follows: Prepare a section of single-mode fiber with a stripped coating, cut the end face of the single-mode fiber flat, and clean it with alcohol. Then, insert the end of the single-mode fiber into a flexible probe preparation mold, inject a mixed solution of polydimethylsiloxane and curing agent into the flexible probe preparation mold, and cure it by heating to form a polydimethylsiloxane probe integrated with the single-mode fiber. Remove the flexible probe preparation mold, fill the sound-transmitting body under a microscope, drop a solution of chloroprene rubber or butyl rubber onto a glass slide, and continuously move the flexible probe using a precision displacement stage to allow the chloroprene rubber or butyl rubber solution to fully penetrate into the axial through-hole of the polydimethylsiloxane. After removing excess chloroprene rubber or butyl rubber solution, cure it to finally form a fiber optic ultrasonic sensing structure with a Fabry-Perot interference structure of single-mode fiber-polydimethylsiloxane-chloroprene rubber or butyl rubber.

4. The flexible fiber optic Fabry-Perot interferometric ultrasonic sensor according to claim 3, characterized in that, The flexible probe fabrication mold includes a tube and a central column, wherein the length of the central column is less than the length and diameter of the tube, and the diameter of the central column is less than the inner diameter of the tube.

5. The flexible fiber optic Fabry-Perot interferometric ultrasonic sensor according to claim 3, characterized in that, The method for preparing the mixed solution of polydimethylsiloxane and curing agent is as follows: mix polydimethylsiloxane and curing agent in a ratio of 10:1, place them on a mixer and stir evenly for 10 minutes to ensure that the polydimethylsiloxane and curing agent are fully mixed and uniform.

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