Displacement sensing system and measurement method based on large-taper conical optical fiber

Abstract

The invention relates to the technical field of optical sensing, in particular to a displacement sensing system based on a large-taper conical optical fiber and a measuring method, and the displacement sensing system based on the large-taper conical optical fiber comprises a tunable laser and a photoelectric detector. The large-taper conical optical fiber is positioned between the tunable laser and the photoelectric detector and is used for transmitting light waves; the system further comprises a polarization controller. The cavity at one end of the optical microcavity is close to the large-taper conical optical fiber, the displacement device is connected with the other end of the optical microcavity, and the optical microcavity and the large-taper conical optical fiber are always in a critical coupling state; and the computer is respectively connected with the tunable laser and the photoelectric detector. The problems that an existing small-taper conical optical fiber is poor in stability and anti-interference capacity, and a sensing system is low in measurement sensitivity are solved, the optical microcavity and the large-taper conical optical fiber are combined, high-precision displacement measurement can be achieved, and the large-taper conical optical fiber displacement sensor is suitable for measurement occasions with the high sensitivity requirement.

Description

Technical Field

[0001] The present invention relates to the field of optical sensing technology, and more specifically, to a displacement sensing system and a measuring method based on a large-taper tapered optical fiber. Background Art

[0002] Fiber optic sensing technology was initially used in the field of communications. With the development of fiber optic technology and the increase in application demand, fiber optic sensing technology has gradually been introduced into the research of various sensors. After development, fiber optic sensors have become an important supporting technology in cutting-edge fields such as military, aerospace, and medicine, with the characteristics of compact structure, high sensitivity, anti-electromagnetic interference, and good repeatability. Among them, displacement sensing technology is widely used in various precision measurement fields, such as micro-nano processing, precision machinery, medical diagnosis, sensor technology, etc. Traditional displacement sensors include laser interferometers, fiber optic displacement sensors, capacitive displacement sensors, etc. Although these technologies have certain measurement accuracy and wide application, they still have some technical bottlenecks in some high-sensitivity or small-size occasions. Fiber optic displacement sensors have become an important development direction in displacement sensing technology in recent years due to their advantages such as high precision, anti-electromagnetic interference, and strong adaptability; especially the application of tapered optical fibers, which have good performance in measuring small displacements or detecting tiny objects.

[0003] Tapered optical fiber has unique optical properties. The light field intensity distribution at its tip is relatively concentrated, which can effectively couple with the external medium, thereby improving the sensitivity and measurement accuracy of the sensor. However, the tapered optical fiber displacement sensors currently used all use small-taper optical fibers. Small-taper optical fibers often encounter some problems in practical applications: First, small-tapered tapered optical fibers usually have a longer tapered length and a smaller waist diameter, which makes them easy to deform under the same external force, resulting in creeping lag of relative displacement, affecting the accuracy and stability of displacement measurement. The waist diameter of small-tapered tapered optical fibers is usually about 2μm. Due to the small size of the optical fiber itself, its structure is relatively fragile and is easily broken due to changes in the external environment or mechanical stress during use, thereby limiting its reliability in high-intensity or long-term use; secondly, due to its small taper angle, the contact area between the surface of the small-tapered tapered optical fiber and the external medium is relatively small, resulting in low optical coupling efficiency with the external medium, which in turn affects the measurement sensitivity of the sensor, especially in measurement occasions with high sensitivity requirements. How to ensure the high sensitivity of the sensor while improving its stability and anti-interference ability is still a technical problem that needs to be solved urgently in this field of technology. Summary of the invention

[0004] The object of the present invention is to overcome the problems of poor stability and anti-interference ability of the existing small-tapered optical fiber and low measurement sensitivity of the sensing system, and to provide a displacement sensing system and a measurement method based on a large-tapered optical fiber. The present invention combines an optical microcavity with a large-tapered optical fiber to achieve high-precision displacement measurement and is applicable to measurement occasions with high sensitivity requirements.

[0005] To solve the above technical problems, the technical solution adopted by the present invention is:

[0006] A displacement sensing system based on a large-tapered optical fiber, comprising a tunable laser for generating light waves of a specific wavelength, a photodetector for receiving an optical signal and converting it into an electrical signal, and a large-tapered optical fiber located between the tunable laser and the photodetector and used for transmitting light waves;

[0007] It further includes a polarization controller located on the input side of the large-tapered optical fiber and used for adjusting the polarization state of the light wave;

[0008] It further includes an optical microcavity with one end of the cavity close to the large-tapered optical fiber, and a displacement device connected to the other end of the optical microcavity. The optical microcavity and the large-tapered optical fiber are always in a critically coupled state;

[0009] It further includes a computer respectively connected to the tunable laser and the photodetector.

[0010] It should be noted that the computer can set the tunable laser to generate light waves with tunable wavelengths and input them into the large-tapered optical fiber; the polarization controller is used for adjusting the polarization state of the light wave before the light wave enters the large-tapered optical fiber and sending the light wave into the large-tapered optical fiber;

[0011] The large-tapered optical fiber is generally placed parallel. During the working process, one end of the cavity of the optical microcavity is close to the large-tapered optical fiber and is always in a critically coupled state with the large-tapered optical fiber. When light propagates in the tapered part of the large-tapered optical fiber, part of the light field will leak to the outside to form an evanescent wave, and the intensity of these evanescent waves decays exponentially with the distance d, that is, E(d)∝e -e / ζ ;

[0012] In this way, a part of the light wave propagates in the large-tapered fiber, and another part diffuses outwards. The light wave distributed outside the large-tapered fiber exists in the form of an evanescent field. When the mode field of the optical microcavity coincides with the evanescent field of the large-tapered fiber, the light wave that satisfies the phase matching condition will generate a resonance phenomenon in the optical microcavity, forming a resonant light wave signal, while the light wave that does not satisfy the phase matching condition will return to the large-tapered fiber and continue to propagate; when the light wave is coupled into the optical microcavity, it will propagate around the axis in a spiral shape and reflect back to the original place at the turning point, and the light rays that satisfy the phase condition will form a stable optical whispering gallery mode;

[0013] When the displacement device fixedly connected to the other end of the optical microcavity generates a displacement, it will drive the optical microcavity to generate a displacement; the photodetector collects the resonant light wave signals propagated by the large-tapered fiber under different displacements, and converts the light wave signals into electrical signals and outputs them to the computer for processing. When the optical microcavity generates each displacement, the light wave signals collected at the output end of the large-tapered fiber will change to a certain extent; when the optical microcavity generates n displacements, n resonant spectra can be collected at the output end of the fiber waveguide; when the displacement device generates a displacement, the characteristic parameters of each resonant mode in the resonant spectrum will change regularly. By monitoring these changes, the displacement can be accurately measured. For example, the characteristic parameters of each resonant mode, such as the Q value or transmittance, etc. will change regularly. By monitoring these changes, the displacement can be accurately measured.

[0014] Therefore, by combining the optical microcavity and the large-tapered fiber, the present invention can achieve high-precision displacement measurement. The large-tapered fiber is manufactured by the cooperation of an optical fiber fusion splicer and a fiber tapering machine, has a larger optical coupling area, can effectively improve the contact strength between the light wave and the fiber surface, thereby enhancing the coupling effect and improving the sensitivity of the sensing system; and has a shorter overall length and a shorter coupling distance, can reduce the packaging volume and is convenient for packaging; the thicker fiber has higher strength and stiffness in structure, can reduce the deformation caused by external forces, improve the stability and durability of the displacement sensing system, can effectively avoid the creep hysteresis phenomenon under the same adsorption force, and improve the accuracy of displacement measurement and the ability to resist interference stably. By continuously changing the spatial position between the fiber waveguide and the optical microcavity, and collecting the changes of the output signal in real time, plotting the resonant spectrum and analyzing its characteristics, accurate displacement measurement can be achieved. This method has the advantages of simple structure and low manufacturing cost.

[0015] Further, the distance between the optical microcavity and the large-tapered fiber is 50 nm - 200 nm.

[0016] It should be noted that the evanescent wave of the small-tapered fiber attenuates slowly, so it can remain strongly coupled even when slightly away from the optical microcavity; while the evanescent wave of the large-tapered fiber attenuates rapidly, so it needs to be closer to the optical microcavity to maintain sufficient coupling strength. During coupling, since the taper region length of the small-tapered fiber is generally several millimeters to more than ten millimeters, the distance between the small-tapered fiber and the optical microcavity is generally 100 - 500 nm during coupling. In contrast, the distance between the large-tapered fiber and the optical microcavity only needs to be maintained at 50 nm - 200 nm. Therefore, the distance between the large-tapered fiber and the optical microcavity can be relatively smaller, which can reduce the volume during its packaging, facilitate packaging, and at the same time improve the stability and durability of the displacement sensing system.

[0017] Further, the large-tapered fiber includes a waist section and a straight section, and the minimum diameter of the waist section is d min , and 4μm ≤ d min ≤ 6μm.

[0018] For a tapered fiber, the shorter its length and the thicker its diameter (i.e., the smaller the aspect ratio), the greater its stiffness and the worse its deformation ability. The minimum diameter of the waist section of the large-tapered fiber is about 4μm ≤ d min ≤ 6μm, while the minimum diameter of the waist section of the small-tapered fiber is usually less than 2μm. In comparison, the large-tapered fiber has higher strength, which can improve the overall durability and stability.

[0019] Further, the taper angle of the large-tapered fiber is θ:

[0020] θ = tan -1 ((R1 - R2) / L),

[0021] wherein, R1 is the radius of the straight section, R2 is the radius at any point of the waist section, and L is the length of the unilateral tapered transition region of the waist section.

[0022] Further, the minimum taper angle θ of the large-tapered fiber min > 2°.

[0023] It should be noted that in the present invention, the waist section of the large-tapered tapered optical fiber exhibits a curved change and a linear change, which is different from the simple linear change of the small-tapered tapered optical fiber. The large-tapered tapered optical fiber is usually from several degrees to dozens of degrees, and the taper angle is greater than 2°; while the taper angle of the small-tapered tapered optical fiber is generally less than 2°. When the diameters of the thinnest parts of the large-tapered tapered optical fiber and the small-tapered tapered optical fiber are both 5 μm, it is observed that there is no obvious phenomenon in the small-tapered tapered optical fiber, while the phenomenon in the large-tapered tapered optical fiber is obvious. This is because when the diameters are the same, the local taper angle of the large-tapered tapered optical fiber is larger, which is easy to meet the mode conversion condition, resulting in an obvious mode conversion phenomenon; while the local taper angle of the small-tapered tapered optical fiber is smaller. If the local taper angle is less than the critical taper angle, then mode conversion is difficult to occur, resulting in no obvious phenomenon. The large-tapered tapered optical fiber has a large contraction angle at the tip, significant optical characteristics, and can couple more effectively with the external medium, and is suitable for measurement occasions with higher sensitivity requirements.

[0024] Further, the local tapering length z of the tapered transition region of the waist section t whose reciprocal is greater than or equal to the beat length z b , that is:

[0025] z t ≥z b ,

[0026] z t =Ω / d,

[0027] z b =(β1 - β2) / 2π,

[0028] wherein, Ω is the local taper angle of the waist section, d is the local diameter of the waist section, and β1 and β2 are the propagation constants of two local coupling modes respectively.

[0029] Further, the local taper angle Ω of the tapered transition region of the waist section is greater than the critical taper angle Ω1, that is:

[0030] Ω > Ω1,

[0031] Ω = d(β1 - β2) / 2π,

[0032] Ω1 = d(βm - βn) / 2π,

[0033] wherein, Ω1 is the critical taper angle of the waist section, βm is the propagation constant of a certain specific coupling mode (such as the fundamental mode), and βn is the propagation constant of another specific coupling mode (such as the high-order mode).

[0034] It should be noted that when the local cone angle Ω of the fiber optic tapered transition region is greater than the critical angle Ω1, the tapered transition region will cause the coupling and energy conversion between the fundamental mode and the higher-order mode, and then the mode conversion can be achieved. During the coupling process between the tapered fiber and the optical microcavity, the mode conversion directly affects the coupling efficiency and energy transmission.

[0035] Furthermore, the total length of the waist section of the taper is 0.8 mm to 1.2 mm. For a tapered optical fiber, the shorter its length and the thicker its diameter (i.e., the smaller the aspect ratio), the greater its stiffness and the worse its deformation ability. Therefore, under the action of the same adsorption force, a shorter and thicker tapered optical fiber is not prone to the creep lag phenomenon of relative displacement. The large-taper tapered optical fiber adopted in the present invention has a smaller aspect ratio, and avoids the fracture problem caused by too small a diameter during the manufacturing process.

[0036] Furthermore, the optical microcavity is one of a SNAP microcavity, a microcylindrical cavity or a sausage structure cavity. The large-taper tapered optical fiber and the optical microcavity are generally placed horizontally. During the working process, the large-taper tapered optical fiber always maintains a small distance from the optical microcavity, so that when the optical microcavity moves along the axial direction of the optical microcavity, it is always in a critical coupling state with the large-taper tapered optical fiber. When light waves are coupled into the optical microcavity, they will propagate in a spiral shape around the axis and reflect back to the original place at the turning point. Among them, the light rays that meet the phase condition will form a stable optical whispering gallery mode.

[0037] The present invention also provides a measurement method for a displacement sensing system based on a large-taper tapered optical fiber as described above, specifically including the following steps:

[0038] A computer controls a tunable laser to generate light waves with a specific wavelength. After the polarization state of the light waves is adjusted by a polarization controller, the light waves are input into the large-taper tapered optical fiber;

[0039] The light waves enter the large-taper tapered optical fiber, and the light waves distributed outside the large-taper tapered optical fiber exist in the form of an evanescent field. When the light waves pass through an optical microcavity arranged close to the large-taper tapered optical fiber, when the mode field of the optical microcavity coincides with the evanescent field of the large-taper tapered optical fiber, the light waves that meet the phase matching condition will generate a resonance phenomenon in the optical microcavity, forming a resonance light wave signal, and the light waves that do not meet the phase matching condition will return to the large-taper tapered optical fiber and continue to propagate;

[0040] When the light waves propagate to a photodetector, the photodetector converts the collected light wave signal into an electrical signal and sends it to a computer for processing to obtain an output resonance spectrum;

[0041] When the displacement device connected to the optical microcavity moves in a certain direction, the optical microcavity is displaced relative to the large-tapered fiber, thereby changing the output resonance spectrum. By collecting and processing the optical wave signals at different displacements, each time the optical microcavity generates a displacement, the optical wave signals collected by the photodetector will change to a certain extent;

[0042] The computer processes the electrical signals sent by the photodetector, and then n resonance spectra can be obtained; the characteristic parameters of each resonance mode in the n resonance spectra will change regularly. By monitoring these changes, the displacement can be accurately measured.

[0043] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0044] (1) The present invention combines an optical microcavity with a large-tapered fiber, which can achieve high-precision displacement measurement. The large-tapered fiber has a large contraction angle at the tip, significant optical characteristics, and can be more effectively coupled with the external medium. The large-tapered fiber has a larger optical coupling area, which can effectively enhance the contact intensity between the optical wave and the fiber surface, thereby enhancing the coupling effect and improving the sensitivity of the sensing system, making it suitable for measurement occasions with high sensitivity requirements.

[0045] (2) The large-tapered fiber of the present invention has a shorter overall length and a shorter coupling distance. Under the action of the same adsorption force, the shorter and thicker tapered fiber is not prone to the creep hysteresis phenomenon of relative displacement. The present invention can reduce the packaging volume and is convenient for packaging; the thicker fiber has higher strength and stiffness in structure, which can reduce the deformation caused by external forces, improve the stability and durability of the displacement sensing system, effectively avoid the creep hysteresis phenomenon under the same adsorption force, and improve the accuracy of displacement measurement and the ability to resist interference stably.

[0046] (3) The present invention can achieve precise displacement measurement by continuously changing the spatial position between the fiber waveguide and the optical microcavity, collecting the changes of the output signals in real time, plotting the resonance spectrum, and analyzing its characteristics. This method has the advantages of simple structure and low manufacturing cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 is a schematic structural connection diagram of the displacement sensing system based on the large-tapered fiber in the present invention;

[0048] Figure 2 is a schematic contour diagram of the large-tapered fiber in the present invention;

[0049] Figure 3 is a schematic contour diagram of a traditional small-tapered fiber;

[0050] Figure 4This is a physical comparison diagram of the large-tapered conical fiber and the small-tapered conical fiber in the present invention (where a is a schematic diagram of the thinnest part of the large-tapered conical fiber, and b is a schematic diagram of the thinnest part of the small-tapered conical fiber);

[0051] Figure 5 This is a schematic diagram of the coupling position between the large-tapered conical fiber and the sausage structure cavity in the present invention (where c is a schematic diagram of the coupling position between the large-tapered conical fiber and the sausage structure cavity under top view, and e is a schematic diagram of the coupling position between the large-tapered conical fiber and the sausage structure cavity under front view);

[0052] Figure 6 This is a schematic diagram of the resonance spectrum image when the large-tapered conical fiber is coupled with the sausage structure cavity;

[0053] Figure 7 This is a schematic diagram of the change in the transmittance of the nth resonance mode when the sausage structure cavity is displaced relative to the small-tapered conical fiber in the present invention;

[0054] Figure 8 This is a schematic diagram of the change in the transmittance of the resonance mode when the sausage structure cavity is displaced from 0 μm to 2 μm relative to the small-tapered conical fiber in the present invention;

[0055] Figure 9 This is a schematic diagram of the change in the transmittance of the resonance mode when the sausage structure cavity is displaced from 2 μm to 4 μm relative to the small-tapered conical fiber in the present invention;

[0056] Figure 10 This is a schematic diagram of the change in the transmittance of the resonance mode when the sausage structure cavity is displaced from 0 μm to 2 μm relative to the large-tapered conical fiber in the present invention;

[0057] Figure 11 This is a schematic diagram of the change in the transmittance of the resonance mode when the sausage structure cavity is displaced from 2 μm to 4 μm relative to the large-tapered conical fiber in the present invention.

[0058] The illustration markings are explained as follows:

[0059] 1 - Computer, 2 - Tunable laser, 3 - Polarization controller, 4 - Optical microcavity, 5 - Large-tapered conical fiber, 6 - Photoelectric detector, 7 - Displacement device. Specific embodiments

[0060] The present invention will be further described below in conjunction with specific embodiments. Among them, the drawings are only for illustrative purposes, showing only schematic diagrams, not physical diagrams, and should not be construed as a limitation of this patent; in order to better illustrate the embodiments of the present invention, some components in the drawings will be omitted, enlarged or reduced, and do not represent the dimensions of actual products; for those skilled in the art, it is understandable that some well-known structures and their descriptions in the drawings may be omitted.

[0061] In the drawings of the embodiments of the present invention, the same or similar reference numerals correspond to the same or similar components; in the description of the present invention, it should be understood that if there are terms such as "upper", "lower", "left", "right", etc. indicating the orientation or positional relationship, they are based on the orientation or positional relationship shown in the drawings. This is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. Therefore, the terms describing the positional relationship in the drawings are only for illustrative purposes and cannot be construed as a limitation of this patent. For those of ordinary skill in the art, the specific meanings of the above terms can be understood according to specific circumstances.

[0062] Embodiment 1

[0063] As Figure 1 shown, this is the first embodiment of the present invention. A displacement sensing system based on a large-tapered fiber optic includes a tunable laser 2 for generating light waves of a specific wavelength, a photodetector 6 for receiving optical signals and converting them into electrical signals, and a large-tapered fiber optic 5 located between the tunable laser 2 and the photodetector 6 and used for transmitting light waves.

[0064] As Figure 1 shown, it further includes a polarization controller 3 located on the input side of the large-tapered fiber optic 5 and used for adjusting the polarization state of the light wave.

[0065] As Figure 1 shown, it further includes an optical microcavity 4 with one end of the cavity close to the large-tapered fiber optic 5, and a displacement device 7 connected to the other end of the optical microcavity 4. The optical microcavity 4 and the large-tapered fiber optic 5 are always in a critically coupled state.

[0066] As Figure 1 shown, it further includes a computer 1 respectively connected to the tunable laser 2 and the photodetector 6.

[0067] It should be noted that the computer 1 can set the tunable laser 2 to generate light waves with tunable wavelengths and input them into the large-tapered fiber optic 5; the polarization controller 3 is used for adjusting the polarization state of the light wave before the light wave enters the large-tapered fiber optic 5 and sending the light wave into the large-tapered fiber optic 5.

[0068] The large-tapered fiber optic 5 is generally placed parallel. During the working process, one end of the cavity of the optical microcavity 4 is close to the large-tapered fiber optic 5 and is always in a critically coupled state with the large-tapered fiber optic 5. When light propagates in the tapered part of the large-tapered fiber optic 5, part of the light field will leak to the outside to form an evanescent wave. The intensity of these evanescent waves decays exponentially with the distance d, that is, E(d) ∝ e -e / ζ ;

[0069] In this way, a part of the light wave propagates in the large-tapered fiber 5, and another part diffuses outward. The light wave distributed outside the large-tapered fiber 5 exists in the form of an evanescent field. When the mode field of the optical microcavity 4 coincides with the evanescent field of the large-tapered fiber 5, the light wave that satisfies the phase matching condition will generate a resonance phenomenon in the optical microcavity 4, forming a resonant light wave signal, while the light wave that does not satisfy the phase matching condition will return to the large-tapered fiber 5 and continue to propagate; when the light wave is coupled into the optical microcavity 4, it will propagate around the axis in a spiral shape and reflect back to the original place at the turning point. Among them, the light rays that satisfy the phase condition will form a stable optical whispering gallery mode;

[0070] When the displacement device 7 fixedly connected to the other end of the optical microcavity 4 generates a displacement, it will drive the optical microcavity 4 to generate a displacement; the photodetector 6 collects the resonant light wave signals propagated by the large-tapered fiber 5 under different displacements, and converts the light wave signals into electrical signals and outputs them to the computer 1 for processing. When the optical microcavity 4 generates each displacement, the light wave signals collected at the output end of the large-tapered fiber 5 will change to a certain extent; when the optical microcavity 4 generates n displacements, n resonant spectra can be collected at the output end of the fiber waveguide; when the displacement device 7 generates a displacement, the characteristic parameters of each resonant mode in the resonant spectrum will change regularly. By monitoring these changes, the displacement amount can be accurately measured. For example, the characteristic parameters of each resonant mode, such as the Q value or transmittance, etc. will change regularly. By monitoring these changes, the displacement amount can be accurately measured.

[0071] Therefore, by combining the optical microcavity 4 and the large-tapered fiber 5, the sensing system of this embodiment can achieve high-precision displacement measurement. The large-tapered fiber 5 is manufactured by cooperating a fiber fusion splicer and a fiber tapering machine, has a larger optical coupling area, can effectively improve the contact strength between the light wave and the fiber surface, thereby enhancing the coupling effect and improving the sensitivity of the sensing system; and has a shorter overall length and a shorter coupling distance, can reduce the packaging volume and is convenient for packaging; the thicker fiber has higher strength and stiffness in structure, can reduce the deformation caused by external forces, improve the stability and durability of the displacement sensing system, can effectively avoid the creep hysteresis phenomenon under the same adsorption force, and improve the accuracy of displacement measurement and the ability of stable anti-interference. By continuously changing the spatial position between the fiber waveguide and the optical microcavity 4, and real-time collecting the changes of the output signal, drawing the resonant spectrum and analyzing its characteristics, accurate displacement measurement can be achieved. This method has the advantages of simple structure and low manufacturing cost.

[0072] Embodiment 2

[0073] This embodiment is similar to Embodiment 1, the difference is that:

[0074] In this embodiment, the operating wavelength of the tunable laser 2 is in the range of 1354.84 nm - 1357.84 nm.

[0075] Figure 2 Shown is a schematic contour diagram of a large-tapered tapered fiber.

[0076] Figure 3 Shown is a schematic contour diagram of a traditional small-tapered tapered fiber.

[0077] As Figure 2 shown, the large-tapered tapered fiber 5 includes a cone waist section and a straight section. The diameter of the straight section is 125 μm, and the minimum diameter of the cone waist section is d min , d min = 5 μm. The large-tapered tapered fiber 5 is obtained by drawing a single-mode fiber with a fusion splicer.

[0078] For a tapered fiber, the shorter its length and the thicker its diameter, that is, the smaller its aspect ratio, the greater its stiffness and the worse its deformation ability. The minimum diameter of the cone waist section of a small-tapered tapered fiber is usually less than 2 μm. In contrast, the large-tapered tapered fiber 5 has higher strength, which can improve the overall durability and stability.

[0079] Figure 4 Shown is a physical comparison diagram of a large-tapered tapered fiber and a small-tapered tapered fiber. Among them, Figure a is a schematic diagram of the thinnest part of the large-tapered tapered fiber. The total length of the cone area and the waist area is 0.9 nm, and the diameter of the thinnest part of the cone waist area is about 5 μm; Figure b is a schematic diagram of the thinnest part of the small-tapered tapered fiber, and the diameter of the thinnest part of the cone waist area is 2 μm.

[0080] In this embodiment, the total length of the cone waist section is 900 μm.

[0081] For a tapered fiber, the shorter its length and the thicker its diameter, that is, the smaller its aspect ratio, the greater its stiffness and the worse its deformation ability. Therefore, under the action of the same adsorption force, a shorter and thicker tapered fiber is not likely to have the creep hysteresis phenomenon of relative displacement. The large-tapered tapered fiber 5 adopted in the present invention has a smaller aspect ratio, and avoids the fracture problem caused by too small a diameter during the manufacturing process.

[0082] In this embodiment, the taper angle of the large-tapered tapered fiber 5 is θ:

[0083] θ = tan -1 ((R1 - R2) / L),

[0084] wherein, R1 is the radius of the straight section, R2 is the radius of any part of the cone waist section, and L is the length of the unilateral tapered transition area of the cone waist section.

[0085] Specifically, L is 450 μm and R1 is 62.5 μm.

[0086] In this embodiment, the minimum taper angle θ of the large-tapered tapered optical fiber 5 min > 2°.

[0087] It should be noted that in the present invention, the waist section of the large-tapered tapered optical fiber 5 shows a curve change. Figure 3 The figure shows a schematic contour diagram of a traditional small-tapered tapered optical fiber. Different from the linear change of the small-tapered tapered optical fiber, the taper angle of the large-tapered tapered optical fiber 5 of the present invention is usually several degrees to dozens of degrees, and the minimum taper angle is greater than 2°; while the maximum taper angle of the small-tapered tapered optical fiber is generally less than 2°.

[0088] In this embodiment, the local tapering length z of the tapered transition region of the waist section t The reciprocal of is greater than or equal to the beat length z b , that is:

[0089] z t ≥z b ,

[0090] z t = Ω / d,

[0091] z b = (β1 - β2) / 2π,

[0092] Wherein, Ω is the local taper angle of the waist section, d is the local diameter of the waist section, and β1 and β2 are the propagation constants of two local coupling modes respectively.

[0093] In this embodiment, the local taper angle Ω of the tapered transition region of the waist section is greater than the critical taper angle Ω1, that is:

[0094] Ω > Ω1,

[0095] Ω = d(β1 - β2) / 2π,

[0096] Ω1 = d(βm - βn) / 2π,

[0097] Wherein, Ω1 is the critical taper angle of the waist section, βm is the propagation constant of a certain specific coupling mode such as the fundamental mode, and βn is the propagation constant of another specific coupling mode such as the high-order mode.

[0098] It should be noted that when the local taper angle Ω of the optical fiber tapered transition region is greater than the critical angle Ω1, the tapered transition region will cause coupling and energy conversion between the fundamental mode and the high-order mode, and mode conversion can be achieved. During the coupling process of the tapered optical fiber and the optical microcavity 4, mode conversion directly affects the coupling efficiency and energy transmission.

[0099] In this embodiment, the optical microcavity 4 is a sausage-structured cavity. The first converging section of the sausage cavity is a conical section or a frustum section, and the second converging section is a conical section or a frustum section. The apex angle of the conical section or the frustum section is 10 - 60°; the length of the straight section is 0.2 - 10 mm, the straight section is a cylindrical structure, and the radius of the cylindrical structure is 62.5 um.

[0100] Figure 5 The following shows the schematic diagram of the coupling position between the large-tapered fiber and the sausage-structured cavity in this embodiment. Among them, Figure c is the schematic diagram of the coupling position between the large-tapered fiber and the sausage-structured cavity under top view, and Figure e is the schematic diagram of the coupling position between the large-tapered fiber and the sausage-structured cavity under front view.

[0101] The sausage-structured cavity is placed perpendicular to the large-tapered fiber and maintains a small distance, enabling the coupling to be in a critical coupling state. The sausage-structured cavity is used to confine the light wave input by the waveguide and enable it to form a stable whispering gallery mode in the cavity. The large-tapered fiber 5 and the optical microcavity 4 are generally placed horizontally. During the working process, the large-tapered fiber 5 always maintains a small distance from the optical microcavity 4, so that when the optical microcavity 4 moves along the axial direction of the optical microcavity 4, it is always in a critical coupling state with the large-tapered fiber 5. When the light wave is coupled into the optical microcavity 4, it will propagate in a spiral shape around the axis and reflect back to the original place at the turning point. Among them, the light rays that meet the phase condition will form a steady optical whispering gallery mode.

[0102] In this embodiment, the distance between the optical microcavity 4 and the large-tapered fiber 5 is 50 nm - 200 nm.

[0103] It should be noted that the evanescent wave of the small-tapered fiber attenuates relatively slowly, so it can be slightly away from the optical microcavity 4 and still maintain a strong coupling; while the evanescent wave of the large-tapered fiber 5 attenuates relatively quickly, so it needs to be closer to the optical microcavity 4 to maintain sufficient coupling strength; during coupling, the cone region length of the small-tapered fiber is generally several millimeters to more than ten millimeters, and the cone region length of the large-tapered fiber is several hundred micrometers to several millimeters. Moreover, during coupling, the distance between the small-tapered fiber and the optical microcavity 4 is generally 100 - 500 nm, while the distance between the large-tapered fiber 5 and the optical microcavity 4 only needs to be maintained at 50 nm - 200 nm. Therefore, it can reduce the volume during its packaging, facilitate packaging, and at the same time improve the stability and durability of the displacement sensing system.

[0104] Figure 6 It is the schematic diagram of the resonance spectrum image when the large-tapered fiber is coupled with the sausage-structured cavity.

[0105] Figure 7Schematic diagram of the change in the transmittance of a certain resonant mode when the sausage structure cavity is displaced relative to the small-tapered fiber waveguide; at this time, the minimum diameter of the waist region of the small-tapered fiber waveguide is about 5 μm, and no obvious phenomenon can be observed. Therefore, it can be seen that when the diameters of the large-tapered fiber 5 and the thinnest part of the small-tapered fiber are both 5 μm, the effect of the small-tapered fiber is not good, while the effect of the large-tapered fiber 5 is obvious. This is because when the diameters are the same, the local cone angle of the large-tapered fiber is larger, which is easy to meet the mode conversion condition, resulting in an obvious mode conversion phenomenon; while the local cone angle of the small-tapered fiber is smaller. If the local cone angle is less than the critical cone angle, then mode conversion is difficult to occur, resulting in no obvious phenomenon. The large-tapered fiber 5 has a large contraction angle at the tip, significant optical characteristics, and can couple more effectively with the external medium, and is suitable for measurement occasions with higher sensitivity requirements.

[0106] Figure 8 Schematic diagram of the change in the transmittance of the resonant mode when the sausage structure cavity in the present invention is displaced from 0 μm to 2 μm relative to the small-tapered fiber waveguide;

[0107] Figure 9 Schematic diagram of the change in the transmittance of the resonant mode when the sausage structure cavity in the present invention is displaced from 2 μm to 4 μm relative to the small-tapered fiber waveguide;

[0108] Figure 10 Schematic diagram of the change in the transmittance of the resonant mode when the sausage structure cavity in the present invention is displaced from 0 μm to 2 μm relative to the large-tapered fiber waveguide;

[0109] Figure 11 Schematic diagram of the change in the transmittance of the resonant mode when the sausage structure cavity in the present invention is displaced from 2 μm to 4 μm relative to the large-tapered fiber waveguide.

[0110] It can be seen from Figures 8 to 11 that when the initial position of the coupling is set to 0 μm, the displacement increment is 1 μm. Figure 8 and Figure 10 depict the change in the optical resonant mode transmittance of the small-tapered fiber waveguide and the large-tapered fiber 5 when the sausage structure cavity is displaced from 0 μm to 2 μm; Figure 9 and Figure 11 then show the change in the optical resonant mode transmittance of the small-tapered fiber waveguide and the large-tapered fiber 5 when further displaced from 2 μm to 4 μm. During the entire displacement process of the sausage structure cavity from 0 μm to 4 μm, 5 groups of resonant spectrum data can be collected.

[0111] After analysis and processing, it can be observed that as the displacement of the sausage structure cavity occurs, the transmittance of the resonance peak of the large-tapered fiber 5 changes significantly. Only partial displacement changes are shown in this example of the solution. Therefore, the present invention combines the optical microcavity 4 with the large-tapered fiber 5, which can achieve high-precision displacement measurement. The large-tapered fiber 5 has a large contraction angle at the tip, significant optical characteristics, and can be more effectively coupled with the external medium. The large-tapered fiber 5 has a larger optical coupling area, which can effectively enhance the contact intensity between the light wave and the fiber surface, thereby enhancing the coupling effect and improving the sensitivity of the sensing system, and is suitable for measurement occasions with high sensitivity requirements.

[0112] Embodiment 3

[0113] The present invention also provides a measurement method for the large-tapered fiber displacement sensing system as described in Embodiment 1 above, specifically including the following steps:

[0114] The computer 1 controls the tunable laser 2 to generate light waves of a specific wavelength. After the polarization state of the light waves is adjusted by the polarization controller 3, the light waves are input into the large-tapered fiber 5.

[0115] The light waves enter the large-tapered fiber 5, and the light waves distributed outside the large-tapered fiber 5 exist in the form of an evanescent field. When the light waves pass through the optical microcavity 4 arranged close to the large-tapered fiber 5, when the mode field of the optical microcavity 4 coincides with the evanescent field of the large-tapered fiber 5, the light waves that satisfy the phase matching condition will generate a resonance phenomenon in the optical microcavity 4, forming a resonance light wave signal, and the light waves that do not satisfy the phase matching condition will return to the large-tapered fiber 5 and continue to propagate.

[0116] After the light waves propagate to the photodetector 6, the photodetector 6 converts the collected light wave signal into an electrical signal and sends it to the computer 1 for processing to obtain an output resonance spectrum.

[0117] When the displacement device 7 connected to the optical microcavity 4 moves in a certain direction, the optical microcavity 4 is displaced relative to the large-tapered fiber 5, which will change the output resonance spectrum. By collecting and processing the light wave signals under different displacements, for each displacement of the optical microcavity 4, the light wave signal collected by the photodetector 6 will change to a certain extent.

[0118] The computer 1 processes the electrical signals sent by the photodetector 6, and n resonance spectra can be obtained; the characteristic parameters of each resonance mode in the n resonance spectra will change regularly. By monitoring these changes, the displacement amount can be accurately measured.

[0119] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, rather than limitations on the implementation manners of the present invention. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to enumerate all the implementation manners here. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the claims of the present invention.

Claims

1. A displacement sensing system based on a large taper tapered optical fiber, characterized in that, It includes a tunable laser (2) for generating light waves of a specific wavelength, a photodetector (6) for receiving an optical signal and converting it into an electrical signal, and a large-tapered tapered fiber (5) located between the tunable laser (2) and the photodetector (6) and used for transmitting light waves; It further includes a polarization controller (3) located on the input side of the large-tapered tapered fiber (5) and used for adjusting the polarization state of the light wave; It further includes an optical microcavity (4) with one end of the cavity close to the large-tapered tapered fiber (5), and a displacement device (7) connected to the other end of the optical microcavity (4). The optical microcavity (4) and the large-tapered tapered fiber (5) are always in a critically coupled state; It further includes a computer (1) respectively connected to the tunable laser (2) and the photodetector (6).

2. The displacement sensing system based on a large taper tapered optical fiber according to claim 1, characterized in that The distance between the optical microcavity (4) and the large-tapered tapered fiber (5) is 50 nm - 200 nm.

3. The displacement sensing system based on a large taper tapered optical fiber according to claim 1, characterized in that, The large-tapered tapered optical fiber (5) includes a tapered waist section and a straight section, and the minimum diameter of the tapered waist section is d min , and 4 μm ≤ d min ≤ 6 μm.

4. A displacement sensing system based on a large taper tapered optical fiber according to claim 3, characterized in that, The taper angle of the large-tapered tapered fiber (5) is θ, and: θ = tan -1 ((R1 - R2) / L), wherein, R1 is the radius of the straight section, R2 is the radius at any point of the tapered waist section, and L is the length of the single-sided tapered transition region of the tapered waist section.

5. A displacement sensing system based on a large taper tapered optical fiber according to claim 4, characterized in that, The minimum taper angle θ of the large-tapered tapered optical fiber (5) min > 2°.

6. The displacement sensing system based on a large taper tapered optical fiber according to claim 3, characterized in that, The reciprocal of the local tapered length of the tapered transition region of the tapered waist section is greater than or equal to the beat length.

7. A displacement sensing system based on a large taper tapered optical fiber according to claim 6, characterized in that, The local taper angle of the tapered transition region of the tapered waist section is greater than the critical taper angle.

8. A displacement sensing system based on a large taper tapered optical fiber according to claim 3, characterized in that The total length of the tapered waist section is 0.8 mm - 1.2 mm.

9. A displacement sensing system based on a large taper tapered optical fiber according to claim 1, characterized in that, The optical microcavity (4) is one of a SNAP microcavity, a microcylindrical cavity or a sausage structure cavity.

10. A measurement method of a displacement sensing system based on a large taper tapered optical fiber as described in any one of claims 1-9, characterized in that, Specifically, it includes the following steps: The computer (1) controls the tunable laser (2) to generate light waves of a specific wavelength. After the polarization state of the light wave is adjusted by the polarization controller (3), the light wave is input into the large-tapered tapered fiber (5); The light wave enters the large-tapered tapered fiber (5). The light wave distributed outside the large-tapered tapered fiber (5) exists in the form of an evanescent field. When the light wave passes through the optical microcavity (4) arranged close to the large-tapered tapered fiber (5), when the mode field of the optical microcavity (4) coincides with the evanescent field of the large-tapered tapered fiber (5), the light wave that satisfies the phase matching condition will generate a resonance phenomenon in the optical microcavity (4) to form a resonant light wave signal, and the light wave that does not satisfy the phase matching condition will return to the large-tapered tapered fiber (5) and continue to propagate; After the light wave propagates to the photodetector (6), the photodetector (6) converts the collected light wave signal into an electrical signal and sends it to the computer (1) for processing to obtain an output resonance spectrum; When the displacement device (7) connected to the optical microcavity (4) moves along a certain direction, the optical microcavity (4) is displaced relative to the large-tapered tapered fiber (5), thereby changing the output resonance spectrum. By collecting and processing the light wave signals under different displacements, for each displacement of the optical microcavity (4), the light wave signals collected by the photodetector (6) will change to a certain extent; The computer (1) processes the electrical signals transmitted by the photodetector (6), and then n resonance spectra can be obtained; the characteristic parameters of each resonance mode in the n resonance spectra will change regularly. By monitoring these changes, the displacement can be accurately measured.

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