A micro-nano fiber evanescent field coupling based acoustic wave sensor and a preparation method thereof

Abstract

The present application relates to the technical field of optical fiber sensor, more particularly, to a kind of acoustic wave sensor based on micro-nano fiber evanescent field coupling and preparation method thereof, the purpose of the present application is to provide acoustic wave sensor with low cost, demodulation simple, photoelectric system compatibility good many aspects of advantages.The acoustic wave sensor of the present application includes substrate, transducer diaphragm, thin film modulation layer and bipyramidal micro-nano fiber;Substrate is provided with through hole, transducer diaphragm is pasted on the upper surface of substrate;Thin film modulation layer is pasted and fixed on the upper surface of transducer diaphragm, bipyramidal micro-nano fiber is fixed on thin film modulation layer, the waist region of bipyramidal micro-nano fiber is suspended above transducer diaphragm, and the position corresponds with through hole, coupling modulation space is formed between the waist region of bipyramidal micro-nano fiber and transducer diaphragm.The present application also provides a kind of method for preparing the acoustic wave sensor.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensor technology, and more specifically, to an acoustic wave sensor based on evanescent field coupling of micro-nano optical fibers and its fabrication method. Background Technology

[0002] Acoustic wave detection has important applications in medicine, industry, and military fields. Fiber optic acoustic sensors have attracted widespread attention due to their advantages such as resistance to electromagnetic interference, high sensitivity, and corrosion resistance. Currently, fiber optic acoustic sensors are mainly divided into intensity-modulated, wavelength-modulated, and phase-modulated types. Intensity-modulated sensors have a simple structure and low cost, but limited accuracy and weak anti-interference capabilities; wavelength-modulated sensors are easy to multiplex, but suffer from temperature crosstalk problems; phase-modulated sensors typically use transducers to make the acoustic wave act on a fiber optic interferometer, and then demodulate the acoustic wave signal by changes in light intensity or phase. Phase-modulated sensors have high sensitivity and a large frequency response range, but the demodulation speed is limited by the computing power of the demodulation system, and the demodulation system required for large-scale deployment of array sensors is very expensive.

[0003] Among phase-type sensors, the Fabry-Perot interferometer (FPI) type is highly valued due to its simple structure and ease of miniaturization. Its performance primarily depends on mechanical and optical sensitivity. Mechanical sensitivity is affected by the size of the sensing diaphragm and the Young's modulus of the material; a larger radius, thinner thickness, and lower modulus result in higher sensitivity. Optical sensitivity depends on the reflectivity of the sensing diaphragm. Polymer materials with moduli as low as 2 GPa have relatively low reflectivity, while materials with better reflectivity are mostly metallic materials such as aluminum, gold, and silver, with moduli reaching tens of GPa. Therefore, when using polymer sensing diaphragms to adapt to low-frequency sound wave sensing, FPI-type intensity demodulation sensors struggle to maintain adequate optical sensitivity.

[0004] It is evident that existing acoustic detection technologies still face challenges in achieving high sensitivity, compact structure, cost control, and wide detection range, and urgently require further improvement. Summary of the Invention

[0005] The present invention aims to overcome at least one of the defects of the prior art and provide an acoustic wave sensor based on micro-nano fiber evanescent field coupling and its fabrication method, which has the advantages of low cost, simple demodulation and good compatibility with optoelectronic systems.

[0006] The first aspect of this invention is to propose an acoustic wave sensor based on evanescent field coupling of micro-nano optical fibers, comprising a substrate, a transducer diaphragm, a thin film modulation layer, and a biconical micro-nano optical fiber; The substrate has through holes extending through its upper and lower surfaces. The transducer diaphragm is attached and fixed to the upper surface of the substrate, and the transducer diaphragm covers the through holes. The thin film modulation layer is adhered and fixed to the upper surface of the transducer diaphragm, wherein the orthographic projection of the thin film modulation layer on the substrate avoids the through hole; The biconical micro / nano fiber is fixed on the thin film modulation layer, and the thin film modulation layer is further configured such that the waist region of the biconical micro / nano fiber is suspended above the transducer diaphragm, and the waist region of the biconical micro / nano fiber corresponds to the position of the through hole. A coupling modulation space is formed between the waist region of the biconical micro / nano fiber and the transducer diaphragm. The refractive index of the transducer diaphragm is greater than that of the coupling modulation space. The signal light is input from the first end of the biconical micro / nano fiber, and after generating an evanescent field on the outer wall of its waist region, it is output from the second end of the biconical micro / nano fiber. The evanescent field is coupled into the transducer diaphragm through the coupling modulation space, thereby reducing the signal light power output by the biconical micro / nano fiber.

[0007] In this invention, the signal light propagating in the biconical micro / nano fiber generates a strong evanescent field on the outer wall of the waist region and propagates along its surface. The transverse intensity of the evanescent field decays exponentially in the range of hundreds of nanometers to several micrometers. When the acoustic signal is transmitted through the through-hole and causes the transducer diaphragm to vibrate, the micro-gap between the transducer diaphragm and the biconical micro / nano fiber changes, and the coupling range between the evanescent field and the transducer diaphragm changes accordingly, forming a coupling modulation space. The evanescent field is sensitive to changes in the refractive index of the external environment. When the refractive index of the transducer diaphragm is greater than the refractive index of the coupling modulation space, some of the energy of the evanescent field will couple to the transducer diaphragm, causing a decrease in optical transmission power. When the acoustic wave causes the transducer diaphragm to vibrate, the gap between the biconical micro / nano fiber and the transducer diaphragm decreases, further increasing the coupling range of the evanescent field, resulting in more optical energy coupling to the transducer diaphragm and reducing the output power of the signal light. Because the deformation of the transducer diaphragm varies under different acoustic pressures, the energy coupled to the transducer diaphragm varies. This process occurs periodically under the action of acoustic waves, thereby realizing acoustic wave sensing. The acoustic wave sensor of the present invention does not require an expensive demodulation system, the cost of the transducer diaphragm is also very low, and it is compatible with existing optoelectronic systems. It also has many advantages such as fast response speed and high sensitivity.

[0008] In some embodiments, the thin film modulation layer is a PI film modulation layer.

[0009] This invention can reduce the manufacturing difficulty of the modulation layer and provide stable and reliable modulation performance.

[0010] Furthermore, the biconical micro / nano fiber is bonded and fixed to the PI film modulation layer using a first violet adhesive, wherein the refractive index of the first violet adhesive is less than the refractive index of the core of the biconical micro / nano fiber.

[0011] This invention utilizes a low-refractive-index violet gel to reduce the scattering of evanescent fields by high-refractive-index materials, effectively reducing light loss.

[0012] In other embodiments, the thin film modulation layer is formed by curing a second violet adhesive, and the biconical micro / nano fiber is directly bonded to the second violet adhesive, wherein the refractive index of the second violet adhesive is less than the refractive index of the core of the biconical micro / nano fiber.

[0013] Furthermore, the thin film modulation layer includes a first modulation layer and a second modulation layer. The first modulation layer and the second modulation layer are disposed one after the other on the upper surface of the transducer diaphragm along the length direction of the biconical micro / nano fiber. The positions of the first modulation layer and the second modulation layer correspond to the two sides of the through hole, respectively. The first end of the biconical micro / nano fiber is fixedly disposed on the first modulation layer, and the second end of the biconical micro / nano fiber is fixedly disposed on the second modulation layer.

[0014] Furthermore, the thickness of the thin film modulation layer is 500nm-2000nm.

[0015] It is understandable that the thickness of the thin film modulation layer is used to change the micro gap between the biconical micro / nano fiber and the transducer diaphragm in order to optimize the required coupling modulation space. The thickness of the PI thin film modulation layer can be flexibly adjusted according to the monitored acoustic frequency and sound pressure to achieve the best coupling effect.

[0016] Furthermore, the substrate is a silicon dioxide glass substrate.

[0017] Furthermore, the through hole is a circular through hole with a radius of 1mm-10mm.

[0018] In this invention, the size of the opening determines the effective size of the transducer diaphragm. According to calculations, the larger the aperture, the larger the transducer diaphragm, and the more sensitive it is to low-frequency sound waves. The size of the opening can be flexibly adjusted according to the monitored sound wave frequency and sound pressure to achieve the best coupling effect.

[0019] Furthermore, the waist region of the biconical micro / nano fiber passes directly above the center of the circular through-hole.

[0020] In this invention, the effective working area of ​​the transducer diaphragm is the circular area of ​​the opening in the glass substrate. Under the action of sound waves, the maximum deformation area of ​​the transducer diaphragm corresponding to the circular through-hole area is at the center of the circle. Therefore, this invention can achieve the best coupling sensitivity.

[0021] Furthermore, the waist diameter of the biconical micro / nano fiber is 200 nm. 2000nm.

[0022] The diameter range of the present invention ensures that the range of the evanescent field generated by the signal light matches the micro-gap coupling modulation space of the transducer diaphragm, ensuring that the evanescent field and the transducer diaphragm can interact effectively.

[0023] Furthermore, the waist length of the biconical micro / nano fiber is 10 mm. 50mm.

[0024] The length range of this invention can be matched with the waist diameter of biconical micro / nano fiber ranging from 200nm to 2000nm.

[0025] Furthermore, the transducer diaphragm is a polydimethylsiloxane diaphragm.

[0026] Furthermore, the thickness of the transducer diaphragm is 20μm-200μm.

[0027] Furthermore, the transducer diaphragm exhibits high transmittance for the signal light.

[0028] Furthermore, the refractive index of the transducer diaphragm is greater than the refractive index of the coupling modulation space.

[0029] Furthermore, the wavelength of the signal light is 500nm-2000nm.

[0030] The intensity and penetration depth of the evanescent field are related to the diameter of the biconical micro / nano fiber and the incident wavelength of the signal light. The wavelength selected in this invention is matched with the diameter of the biconical micro / nano fiber to control the vibration matching between the evanescent field and the transducer diaphragm, so as to achieve the optimal coupling effect.

[0031] Furthermore, the biconical micro / nano optical fiber is fabricated by heating and melting the micro / nano optical fiber with an oxyhydrogen flame and then drawing it into a cone shape. The diameter of the cone region gradually decreases from both sides to the middle and tends to be uniform in the waist region.

[0032] The second aspect of the present invention is to provide a first method for preparing the aforementioned acoustic wave sensor, comprising: Single-mode optical fiber is heated, melted, and drawn into biconical micro / nano optical fibers; A transducer diaphragm is attached to the upper surface of a substrate with a through hole, so that the transducer diaphragm completely covers the through hole. A thin film modulation layer is attached to the upper surface of the transducer diaphragm, and the orthogonal projection of the thin film modulation layer on the substrate avoids the through hole. The biconical micro / nano fiber is suspended above the transducer diaphragm, with the waist region of the biconical micro / nano fiber corresponding to the through hole. The two ends of the biconical micro / nano optical fiber are fixed on the thin film modulation layer using a first violet light adhesive.

[0033] A third aspect of the present invention is to provide a second method for preparing the aforementioned acoustic wave sensor, comprising: Single-mode optical fiber is heated, melted, and drawn into biconical micro / nano optical fibers; The transducer diaphragm is attached to the upper surface of the substrate with through holes, so that the transducer diaphragm completely covers the through holes; The biconical micro / nano fiber is suspended above the transducer diaphragm, with the waist region of the biconical micro / nano fiber corresponding to the through hole. The two ends of the biconical micro / nano optical fiber are fixed to the transducer film on both sides of the corresponding through hole using a second violet adhesive.

[0034] Compared with the prior art, the beneficial effects of the present invention are as follows: The acoustic wave sensor based on micro-nano fiber evanescent field coupling provided by the present invention has a simple structure. It can realize the sensing of acoustic waves by simply using the evanescent field of micro-nano fiber and the transducer diaphragm to couple with each other. It does not require a complex demodulation system and has the advantages of high sensitivity, fast response, strong stability and compatibility with existing optoelectronic systems for easy deployment. Attached Figure Description

[0035] Figure 1 This is a structural diagram of the acoustic wave sensor based on evanescent field coupling of micro-nano optical fibers according to the present invention. In order to clearly show each structure, the transducer diaphragm is separated from the substrate.

[0036] Figure 2 This is a flowchart illustrating the fabrication method of the high-sensitivity acoustic wave sensor based on micro / nano fiber evanescent field coupling according to the present invention.

[0037] Figure 3 This is a schematic diagram illustrating the working principle of the acoustic wave sensor based on evanescent field coupling of micro / nano optical fibers according to the present invention. Figure 1 .

[0038] Figure 4 This is a schematic diagram illustrating the working principle of the acoustic wave sensor based on evanescent field coupling of micro / nano optical fibers according to the present invention. Figure 2 .

[0039] Figure 5 The diagram shows the relationship between sound pressure level and output signal in the acoustic wave sensing test for the example.

[0040] Reference numerals: 1. Substrate; 11. Through-hole; 2. Transducer diaphragm; 3. Biconical micro / nano fiber; 31. Waist region; 4. Thin film modulation layer; 41. First modulation layer; 42. Second modulation layer; 5. First ultraviolet adhesive; 6. Evanescent field; 7. Acoustic signal. Detailed Implementation

[0041] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0042] Unless otherwise specified, the experimental methods used in the following examples are conventional methods. Unless otherwise specified, the materials, reagents, methods, and instruments used are all conventional materials, reagents, methods, and instruments in the art, and can be obtained commercially by those skilled in the art.

[0043] Example 1 like Figure 1 , 3 As shown in Figure 4, the present invention first proposes a high-sensitivity acoustic wave sensor based on evanescent field coupling of micro-nano optical fiber, including a substrate 1, a transducer diaphragm 2, a thin film modulation layer 4, and a biconical micro-nano optical fiber 3. The substrate 1 has through holes 11 extending through its upper and lower surfaces. The transducer diaphragm 2 is attached and fixed to the upper surface of the substrate 1, and the transducer diaphragm 2 covers the through holes 11. The thin film modulation layer 4 is attached and fixed to the upper surface of the transducer diaphragm 2. The thin film modulation layer 4 is set to avoid the position on the upper surface of the transducer diaphragm 2 corresponding to the through hole 11. That is, it is ensured that the orthogonal projection of the thin film modulation layer 4 on the substrate 1 avoids the through hole 1. The biconical micro / nano fiber 3 is fixed on the thin film modulation layer 4, and the position of the thin film modulation layer 4 covering the upper surface of the transducer diaphragm 2 is such that the waist region 31 of the biconical micro / nano fiber 3 is suspended above the transducer diaphragm 2, and the waist region 31 of the biconical micro / nano fiber 3 corresponds to the position of the through hole 11, and a coupling modulation space is formed between the waist region 31 of the biconical micro / nano fiber 3 and the transducer diaphragm 2. In use, the signal light is input from the first end of the biconical micro / nano fiber 3. After generating an evanescent field 6 on the outer wall of its waist region 31, it is output from the second end of the biconical micro / nano fiber 3. The evanescent field 6 is coupled into the transducer diaphragm 2, reducing the signal light power output by the biconical micro / nano fiber 3. It can be understood that when different sound pressures act on the transducer diaphragm 2 through the through-hole 11, the deformation of the transducer diaphragm 2 is different, resulting in different energies coupled to the transducer diaphragm 2, thereby realizing sound wave sensing.

[0044] In practical applications, the signal light propagating in the biconical micro / nano fiber 3 generates a strong evanescent field 6 on the outer wall of the waist region 31 and propagates along its surface. The transverse intensity of the evanescent field 6 decays exponentially in the range of hundreds of nanometers to several micrometers. The acoustic signal 7 enters from bottom to top through the through-hole 11, causing the transducer diaphragm 2 to vibrate. This changes the micro-gap between the transducer diaphragm 2 and the biconical micro / nano fiber 3, thus altering the coupling range between the evanescent field 6 and the transducer diaphragm 2, forming a coupling modulation space. The evanescent field 6 is sensitive to changes in the refractive index of the external environment. When the refractive index of the transducer diaphragm 2 is greater than that of the coupling modulation space, some energy from the evanescent field 6 is coupled to the transducer diaphragm 2, causing a decrease in optical transmission power. When the acoustic signal 7 causes the transducer diaphragm 2 to vibrate, the gap between the biconical micro / nano fiber 3 and the transducer diaphragm 2 decreases, further increasing the coupling range of the evanescent field 6, resulting in more optical energy coupling to the transducer diaphragm 2 and reducing the output power of the signal light. Because the deformation of the transducer diaphragm 2 varies under different sound pressure levels, the energy coupled to the transducer diaphragm 2 varies. This process occurs periodically under the action of sound waves, thereby realizing sound wave sensing. The sound wave sensor of the present invention does not require an expensive demodulation system, the cost of the transducer diaphragm 2 is also very low, and it is compatible with existing optoelectronic systems. It also has advantages such as fast response speed and high sensitivity.

[0045] In some embodiments, the thin film modulation layer 4 is implemented using a PI film, and the biconical micro / nano fiber 3 is raised by the PI film. Specifically, the two ends of the biconical micro / nano fiber 3 are fixed to the thin film modulation layer 4 using a first violet adhesive 5. The refractive index of the first violet adhesive 5 is less than the refractive index of the core of the biconical micro / nano fiber 3. In this way, the scattering of the evanescent field 6 by the high refractive index can be reduced, and the optical loss can be effectively reduced.

[0046] In some other embodiments, the thin film modulation layer 4 can also be directly formed by curing the second ultraviolet adhesive. In this case, the biconical micro / nano fiber 3 is directly bonded to the second ultraviolet adhesive. Similarly, in order to reduce optical loss, the refractive index of the second ultraviolet adhesive is less than the refractive index of the core of the biconical micro / nano fiber 3.

[0047] Continue to refer to Figure 1 To reduce material usage, the thin-film modulation layer 4 includes a first modulation layer 41 and a second modulation layer 42. The first modulation layer 41 and the second modulation layer 42 are disposed along the length of the biconical micro / nano fiber 3 on the upper surface of the transducer diaphragm 2. The first modulation layer 41 and the second modulation layer 42 correspond to the two sides of the through-hole 11, respectively. The first end of the biconical micro / nano fiber 3 is fixed to the first modulation layer 41 using a first ultraviolet adhesive 5, and the second end of the biconical micro / nano fiber 3 is fixed to the second modulation layer 42 using the same first ultraviolet adhesive 5. It is understood that to ensure stable operation of the coupling modulation space, the first modulation layer 41 and the second modulation layer 42 have the same material and thickness.

[0048] In a preferred embodiment, the thickness of the thin film modulation layer 4 is 500nm-2000nm. By adjusting the thickness of the thin film modulation layer 4, the micro gap between the biconical micro / nano fiber 3 and the transducer diaphragm 2 can be changed to optimize the required coupling modulation space. In actual use, the thickness of the thin film modulation layer 4 can be flexibly adjusted according to the monitored acoustic frequency and acoustic pressure characteristics to achieve the best coupling effect.

[0049] In practice, substrate 1 is a silicon dioxide glass substrate.

[0050] In some embodiments, the through hole 11 is a circular through hole. In other embodiments, the through hole 11 may also be rectangular, elliptical, or other shapes. When a circular through hole is used, the analysis of the shape variables at various points of the transducer diaphragm 2 can be based on the plate vibration model in elasticity. The formula of this model utilizes circular symmetry, which is convenient for describing the shape variables at various points of the transducer diaphragm 2.

[0051] In a preferred embodiment, the radius of the circular through hole is 1mm-10mm. It can be understood that the size of the through hole 11 determines the effective size of the transducer diaphragm 2. According to calculations, the larger the aperture, the larger the transducer diaphragm 2, and the more sensitive it is to low-frequency sound waves. The size of the aperture can be flexibly adjusted according to the monitored sound wave frequency and sound pressure to achieve the best coupling effect.

[0052] Preferably, the waist region 31 of the biconical micro / nano fiber 3 passes directly above the center of the circular through-hole. It can be understood that the effective working area of ​​the transducer diaphragm 2 is the circular region corresponding to the through-hole 11 of the substrate 1. Under the action of acoustic waves, the maximum deformation of the circular region of the transducer diaphragm 2 is at the center. The arrangement of the waist region 31 in this invention can achieve optimal coupling sensitivity.

[0053] In some embodiments, the waist region 31 of the biconical micro / nano fiber 3 has a diameter of 200 nm. 2000nm. The diameter of the waist region 31 ensures that the range of the evanescent field 6 generated by the signal light matches the micro-gap coupling modulation space of the transducer diaphragm 2, ensuring that the evanescent field 6 and the transducer diaphragm 2 can interact effectively.

[0054] In some embodiments, the waist region 31 of the biconical micro / nano fiber 3 has a length of 10 mm. The diameter of 50mm is sufficient to match the waist region 31 of biconical micro / nano fiber with a diameter of 200nm-2000nm, which helps to construct a coupling modulation space.

[0055] In practical implementation, the transducer diaphragm 2 is required to have high transmittance to the signal light. Preferably, the material of the transducer diaphragm 2 is transparent to the wavelength of the signal light, and the refractive index of the transducer diaphragm 2 is greater than the refractive index of the coupling modulation space. In practice, the transducer diaphragm 2 is a polydimethylsiloxane diaphragm, and further, the thickness of the transducer diaphragm 2 is 20μm-200μm.

[0056] In some embodiments, the wavelength of the signal light is 500nm-2000nm. It can be understood that the intensity and penetration depth of the evanescent field 6 are related to the diameter of the biconical micro / nano fiber 3 and the incident wavelength of the signal light. The wavelength selected in this invention is matched with the diameter of the biconical micro / nano fiber 3 to control the vibration matching between the evanescent field 6 and the transducer diaphragm 2, so as to achieve the optimal coupling effect.

[0057] Specifically, let the penetration depth of the evanescent field 6 be d, the incident wavelength of the signal light be λ, and the diameter of the biconical micro / nano fiber 3 be D, where: d can be approximated as being related to... There is a positive correlation.

[0058] like Figure 2 As shown, the present invention also provides a method for fabricating the acoustic wave sensor based on micro / nano fiber evanescent field coupling, comprising the following steps: S1: Fabrication of biconical micro / nano fiber 3. In specific implementation, single-mode fiber is heated, melted, and drawn into biconical micro / nano fiber 3. Specifically, a standard single-mode fiber (SMF28) with a diameter of 125μm is first melted and drawn into biconical micro / nano fiber 3 using a hydrogen flame heating device. The biconical micro / nano fiber 3 is fabricated by heating and melting with an oxyhydrogen flame and drawing it into a tapered shape. Two tapered structures are formed at both ends of the biconical micro / nano fiber 3. The two tapered structures are connected by a waist region 31 with a uniform diameter. The diameter of the biconical micro / nano fiber 3 gradually decreases from both sides to the middle and tends to be uniform in the waist region 31.

[0059] S2: The transducer diaphragm 2 and the thin film modulation layer 4 are sequentially fixed on the substrate 1. In specific implementation, a through hole 11 is first made on the substrate 1, and the transducer diaphragm 2 is pasted and fixed on the substrate 1 so that the transducer diaphragm 2 completely covers the through hole 11. Then, the thin film modulation layer 4 made of PI film is attached to the upper surface of the transducer diaphragm 2, ensuring that the thin film modulation layer 4 is positioned away from the position on the upper surface of the transducer diaphragm 2 corresponding to the through hole 11. In a preferred embodiment, the transducer diaphragm 2 is directly attached to the substrate 1, and the thin film modulation layer 4 is directly attached to the transducer diaphragm 2, without the need for adhesive.

[0060] S3: Adjust the positions of the biconical micro / nano fiber 3 and the transducer diaphragm 2. Specifically, suspend the biconical micro / nano fiber 3 above the transducer diaphragm 2, so that the waist region 31 of the biconical micro / nano fiber 3 corresponds to the through-hole 1. In practice, a three-dimensional precision displacement stage is used to suspend the biconical micro / nano fiber 3 above the transducer diaphragm 2. Preferably, the through-hole 11 is circular, and the waist region 31 of the biconical micro / nano fiber is positioned above the center of the circular through-hole. It can be understood that during operation, the effective working area of ​​the transducer diaphragm 2 is the circular area corresponding to the circular through-hole. Under the action of acoustic waves, the maximum deformation area of ​​the transducer diaphragm 2 in the fundamental mode is located at the center. Therefore, the biconical micro / nano fiber can achieve the best coupling sensitivity by passing above the center.

[0061] S4: Fix the biconical micro / nano fiber 3. Specifically, fix both ends of the biconical micro / nano fiber 3 to the thin film modulation layer 4 on both sides of the corresponding through-hole 11. It can be understood that in step S2, the thin film modulation layer 4 also needs to ensure sufficient coverage of the upper surface of the transducer diaphragm 2 to ensure that the biconical micro / nano fiber 3 can be suspended above the transducer diaphragm 2 with the support of the thin film modulation layer 4. The waist region 31 of the biconical micro / nano fiber 3 corresponds to the through-hole 11. In some embodiments, ultraviolet adhesive is dropped onto the surface of the thin film modulation layer 4 corresponding to both sides of the through-hole 11, and the biconical micro / nano fiber 3 is moved upwards and tightly adhered to the thin film modulation layer 4 using a nano-displacement stage. Preferably, the biconical micro / nano fiber 3 is moved upwards by 0.4 μm-2 μm through the thin film modulation layer 4 to ensure that the distance between the biconical micro / nano fiber 3 and the transducer diaphragm 2 is slightly less than the lateral range of the evanescent field 6, in order to achieve effective coupling. In a preferred embodiment, the refractive index of the violet adhesive used is lower than that of the core of the biconical micro / nano fiber 3, so as to reduce the scattering of the evanescent field 6 by the high refractive index.

[0062] Test case In this test example, the opening of the substrate 1 is a circular hole with a radius of 5 mm. The purpose is to make the effective working size of the transducer diaphragm 2 a circular area with a radius of 5 mm, so as to adapt to the target acoustic signal 7 to be detected and generate effective interaction.

[0063] The transducer diaphragm 2 is made of polydimethylsiloxane material with a thickness of 50 μm. The effective working area corresponds to the circular hole of the substrate 1 and is a circle with a radius of 5 mm. Under the action of sound pressure, the deformation range of the transducer diaphragm 2 is several hundred nanometers to ensure full coupling with the evanescent field 6.

[0064] The thickness of the thin film modulation layer 4 made of PI film is 1 μm. This is because, under the conditions of the size of the biconical micro / nano fiber 3 and the wavelength of the signal light in this test example, the transverse intensity attenuation range of the evanescent field 6 is about 1 micrometer. The thickness of the thin film modulation layer 4 provides the transducer diaphragm 2 with a suitable deformation space to ensure full coupling with the evanescent field 6. In addition, the refractive index of the violet photoresist used in this test example is 1.37.

[0065] The waist region 31 of the biconical micro / nano fiber 3 has a diameter of 751 nm, and the incident signal light wavelength is 1550 nm. Under these conditions, the signal light propagating in the biconical micro / nano fiber 3 will generate a strong evanescent field 6 outside the waist region 31 of the biconical micro / nano fiber 3. The evanescent field 6 propagates along the surface of the biconical micro / nano fiber 3. The transverse intensity distribution of the evanescent field 6 decreases exponentially with the distance away from the surface of the biconical micro / nano fiber 3. The attenuation range of the transverse intensity is about 1 micrometer. This range is related to the diameter of the waist region 31 of the biconical micro / nano fiber 3 and the wavelength of the incident signal light.

[0066] For ease of understanding, the acoustic wave sensing principle of the acoustic wave sensor based on micro / nano fiber evanescent field coupling in this invention is as follows: Figure 3 and Figure 4 As shown. First refer to Figure 3 When the spacing between the biconical micro / nano fiber 3 and the transducer diaphragm 2 is within the attenuation range of the transverse intensity of the evanescent field 6, the evanescent field 6 will interact with the transducer diaphragm 2. Specifically, a portion of the signal light energy will be coupled to the transducer diaphragm 2, resulting in a reduction in the power at the output end of the biconical micro / nano fiber 3. (Continue to refer to...) Figure 4 Under the action of acoustic signal 7, the transducer diaphragm 2 deforms. During the deformation process, the gap between the transducer diaphragm 2 and the biconical micro / nano fiber 3 decreases, which leads to more signal light energy being coupled to the transducer diaphragm 2, further reducing the optical power at the output end of the biconical micro / nano fiber 3. It can be understood that under different sound pressures, the deformation of the transducer diaphragm 2 is different, resulting in different energy being coupled to the transducer diaphragm 2, thereby realizing acoustic wave sensing.

[0067] refer to Figure 5 The invention demonstrates the actual acoustic wave sensing test of the acoustic wave sensor based on evanescent field coupling of micro-nano optical fiber. The test conditions are a signal light with a wavelength of 1550nm and a double-conical micro-nano optical fiber 3 with a waist region diameter of 751nm, an input acoustic wave with a frequency of 200Hz, and the sound pressure is gradually increased while monitoring the output amplitude of the sensor. Figure 5 The results show that as the sound pressure increases, the distance between the waist region 31 of the biconical micro-nano light sensor and the transducer diaphragm 2 changes more significantly. This means that the energy variation of the signal light coupled to the transducer diaphragm 2 also increases, resulting in a larger output signal amplitude. The sensor measured a sensitivity of 168 mV / Pa with a goodness of fit of 0.983.

[0068] In summary, this invention proposes a high-sensitivity acoustic wave sensor based on evanescent field coupling of micro / nano fiber optics. Acoustic wave sensing is achieved by coupling the evanescent field 6 of a biconical micro / nano fiber 3 with a transducer diaphragm 2 substrate. When an acoustic wave acts on the transducer diaphragm 2, it causes the diaphragm 2 to vibrate, reducing the gap between the biconical micro / nano fiber 3 and the transducer diaphragm 2. This allows more energy to be coupled into the transducer diaphragm 2, reducing the optical power at the output end of the biconical micro / nano fiber 3. It can be understood that, under different sensing requirements, transducer diaphragms 2 with different parameters and thin-film modulation layers 4 with different thicknesses can be used to change the coupling modulation space to achieve optimal sensitivity. Compared with traditional fiber optic acoustic wave sensors, this invention does not require a complex demodulation system based on interferometer sensing, is easy to manufacture, low-cost, and compatible with existing optoelectronic systems. The high-sensitivity acoustic wave sensor based on evanescent field coupling of micro / nano fiber optics provided by this invention has broad application prospects in medical diagnosis, industrial monitoring, disaster early warning, geological research, and resource exploration.

[0069] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the technical solution of the present invention, and are not intended to limit the specific implementation of the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the claims of the present invention should be included within the protection scope of the claims of the present invention.

Claims

1. An acoustic wave sensor based on evanescent field coupling of micro / nano optical fibers, characterized in that, Includes a substrate, a transducer diaphragm, a thin-film modulation layer, and a biconical micro / nano optical fiber; The substrate has through holes extending through its upper and lower surfaces. The transducer diaphragm is attached and fixed to the upper surface of the substrate, and the transducer diaphragm covers the through holes. The thin film modulation layer is adhered and fixed to the upper surface of the transducer diaphragm, wherein the orthographic projection of the thin film modulation layer on the substrate avoids the through hole; The biconical micro / nano fiber is fixed on the thin film modulation layer, and the thin film modulation layer is further configured such that the waist region of the biconical micro / nano fiber is suspended above the transducer diaphragm, and the waist region of the biconical micro / nano fiber corresponds to the position of the through hole. A coupling modulation space is formed between the waist region of the biconical micro / nano fiber and the transducer diaphragm. The refractive index of the transducer diaphragm is greater than that of the coupling modulation space. The signal light is input from the first end of the biconical micro / nano fiber, and after generating an evanescent field on the outer wall of its waist region, it is output from the second end of the biconical micro / nano fiber. The evanescent field is coupled into the transducer diaphragm through the coupling modulation space, thereby reducing the signal light power output by the biconical micro / nano fiber.

2. The acoustic wave sensor according to claim 1, characterized in that, The thin film modulation layer is a PI film modulation layer.

3. The acoustic wave sensor according to claim 2, characterized in that, The biconical micro / nano optical fiber is bonded and fixed to the PI film modulation layer using a first violet adhesive, the refractive index of which is less than the refractive index of the core of the biconical micro / nano optical fiber.

4. The acoustic wave sensor according to claim 1, characterized in that, The thin-film modulation layer includes a first modulation layer and a second modulation layer; The first modulation layer and the second modulation layer are disposed one after the other along the length direction of the biconical micro / nano fiber on the upper surface of the transducer diaphragm, and the positions of the first modulation layer and the second modulation layer correspond to the two sides of the through hole, respectively. The first end of the biconical micro / nano fiber is fixed to the first modulation layer, and the second end of the biconical micro / nano fiber is fixed to the second modulation layer; and / or, The thickness of the thin-film modulation layer is 500nm-2000nm; and / or, The thin film modulation layer is formed by curing a second ultraviolet adhesive, and the biconical micro / nano fiber is bonded to the second ultraviolet adhesive. The refractive index of the second ultraviolet adhesive is less than the refractive index of the core of the biconical micro / nano fiber.

5. The acoustic wave sensor according to claim 1, characterized in that, The substrate is a silicon dioxide glass substrate; and / or, The through hole is a circular through hole with a radius of 1mm-10mm.

6. The acoustic wave sensor according to claim 5, characterized in that, The waist region of the biconical micro / nano fiber passes directly above the center of the circular through-hole.

7. The acoustic wave sensor according to any one of claims 1-6, characterized in that, The waist diameter of the biconical micro / nano fiber is 200 nm. 2000nm; and / or, The waist region of the biconical micro / nano fiber is 10 mm long. 50mm; and / or, The transducer diaphragm is a polydimethylsiloxane diaphragm; and / or, The thickness of the transducer diaphragm is 20μm-200μm; and / or, The transducer diaphragm exhibits high transmittance for the signal light; and / or The refractive index of the transducer diaphragm is greater than the refractive index of the coupling modulation space.

8. The acoustic wave sensor according to any one of claims 1-6, characterized in that, The wavelength of the signal light is 500nm-2000nm.

9. A method for preparing the acoustic wave sensor according to any one of claims 1-8, characterized in that, include: Single-mode optical fiber is heated, melted, and drawn into biconical micro / nano optical fibers; A transducer diaphragm is attached to the upper surface of a substrate with a through hole, so that the transducer diaphragm completely covers the through hole. A thin film modulation layer is attached to the upper surface of the transducer diaphragm, and the orthogonal projection of the thin film modulation layer on the substrate avoids the through hole. The biconical micro / nano fiber is suspended above the transducer diaphragm, with the waist region of the biconical micro / nano fiber corresponding to the through hole. The two ends of the biconical micro / nano optical fiber are fixed on the thin film modulation layer using a first violet light adhesive.

10. A method for preparing the acoustic wave sensor according to any one of claims 1-8, characterized in that, include: Single-mode optical fiber is heated, melted, and drawn into biconical micro / nano optical fibers; The transducer diaphragm is attached to the upper surface of the substrate with through holes, so that the transducer diaphragm completely covers the through holes; The biconical micro / nano fiber is suspended above the transducer diaphragm, with the waist region of the biconical micro / nano fiber corresponding to the through hole. The two ends of the biconical micro / nano optical fiber are fixed to the transducer film on both sides of the corresponding through hole using a second violet adhesive.

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