A fiber-optic-based ultrasensitive stress sensor structure and system

Abstract

This invention discloses a fiber-optic-based ultrasensitive stress sensor structure, comprising: a helical structure coaxially disposed on the end face of the optical fiber and a flat plate structure disposed at the other end of the helical structure and parallel to the end face of the optical fiber, wherein an FP resonant cavity is formed between the end face of the optical fiber and the flat plate structure. This invention also provides a fiber-optic-based ultrasensitive stress sensor system. The FP-type stress sensor of this invention can be directly fabricated on the end face of a single-mode optical fiber using two-photon polymerization 3D processing technology. This technology has a simple fabrication process and high processing precision (resolution better than 200nm). Combined with high-performance photoresist, the fabricated micro / nano structure has extremely low surface roughness (generally below 80nm). Furthermore, the resulting sensor achieves a sensing sensitivity of up to 0.43nm / nN, and the force sensing limit reaches the nanonewton (nN) level, representing an improvement of four orders of magnitude compared to existing fiber-optic stress sensors.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensor technology, and in particular to a structure and system for an ultrasensitive stress sensor based on optical fiber. Background Technology

[0002] Fiber optic stress sensors offer numerous advantages, maintaining high sensitivity while also possessing flexibility, biocompatibility, and resistance to electromagnetic interference. Therefore, fiber optic stress sensors have been a hot research topic in recent years, with FP-type fiber optic stress sensors being widely used due to their simple design and good sensing linearity.

[0003] Currently, FP-type fiber stress sensors are mainly fabricated using methods such as capillary fusion splicing, focused ion beam etching, and fiber splicing. Among these, FP cavities based on capillary fusion splicing typically suffer from drawbacks such as large surface roughness, complex operation, and low sensing accuracy (>0.6μN). (Reference 1: Donlagic D, Pevec SA miniature all-fiber force sensor[J]. Optics Letters,2020,45(18).). In particular, when the surface roughness of the FP cavity is large, reducing reflectivity, the quality factor (Q-factor) of the FP cavity will decrease significantly, reducing detection accuracy.

[0004] Focused ion beam etching (Reference 2: Gong Y, Yu CB, Wang TT, et al. Highly sensitive force sensor based on optical microfiber asymmetrical Fabry-Perot interferometer[J]. Optics Express, 2014.) is time-consuming and inefficient in fabricating relatively smooth FP cavity structure surfaces. Furthermore, to meet the fabrication requirements, the fiber diameter typically needs to be tapered to below 40 μm, and this extremely small fiber diameter significantly reduces the sensor's mechanical strength, making it prone to breakage.

[0005] Fiber splicing (Reference 3: Liu Y, Qu S, Qu W, et al. A Fabry–Perot cuboid cavity across the fiber for high-sensitivity strain force sensing[J]. Journal of Optics, 2014, 16(10): 105401.) refers to splicing different types of optical fibers directly or at different angles to form an FP cavity structure between the two splicing interfaces. However, this fabrication method usually requires multiple fiber cutting and splicing operations, which is complex and very time-consuming.

[0006] All three methods described above, employing traditional processing techniques, suffer from low stress sensing accuracy. They are ill-suited to accurately sensing minute forces in demanding scenarios.

[0007] Patent document (Publication No. 114659963A, Publication Date 2022-06-24) proposes a nanonewton-level force detection device, which utilizes a spring structure to significantly improve stress sensing accuracy. Under the same force, an elastic structure can produce relatively large deformation. However, in extreme cases, when using this device to measure "microforces," the minute deformation (<100nm) produced by the structure cannot be measured in real time by detectors (optical microscopes, industrial cameras, etc.), limiting its performance.

[0008] Therefore, there is an urgent need to develop a new design and fabrication method for fiber optic FP cavity stress sensors to solve the problem that existing sensors of this type cannot simultaneously achieve the advantages of flexibility, electromagnetic interference resistance, and high sensitivity. Summary of the Invention

[0009] This invention provides a fiber-optic-based FP cavity ultra-high precision stress sensor structure and its application.

[0010] To solve the above problems, the present invention adopts the following technical solution:

[0011] A fiber-optic-based ultrasensitive stress sensor structure includes: a spiral structure coaxially disposed on the end face of the fiber and a flat plate structure disposed at the other end of the spiral structure and parallel to the end face of the fiber, wherein an FP resonant cavity is formed between the end face of the fiber and the flat plate structure.

[0012] By employing the structure of this invention, a high-quality FP cavity structure can be formed using the fiber end face and the planar structure, thereby achieving high sensitivity.

[0013] Preferably, the optical fiber is a single-mode optical fiber. The spiral structure is a 3D spiral structure; the 3D spiral structure connects the flat structure to the optical fiber; the hollow portion between the end face of the single-mode optical fiber and the flat structure is the FP cavity structure required for sensing. Simultaneously, the excellent mechanical properties (extremely low k-value) of the 3D spiral structure enable the sensor to achieve an accuracy of 0.43 nm / nN, further ensuring high sensitivity.

[0014] Preferably, the flat plate structure is a disk structure, and the 3D spiral structure, the flat plate structure, and the single-mode fiber end face are coaxially arranged.

[0015] When the 3D spiral structure is subjected to a force (which can be pressure or tension) perpendicular to the fiber end face (or along the spiral structure axis), it will undergo compressive deformation, which will lead to a change in the height of the FP cavity structure and ultimately a change in the position of the interference spectrum valley.

[0016] Preferably, the spiral structure and the flat plate structure constitute a 3D micro-nano structure, and the 3D micro-nano structure is an integral structure.

[0017] Preferably, the 3D micro / nano structure is prepared using a two-photon polymerization 3D printing process.

[0018] Preferably, the flat plate structure is a disk structure, and the single-mode fiber end face of the flat plate is coaxially arranged.

[0019] Preferably, the diameter of the effective reflective area of ​​the flat plate structure is greater than or equal to the diameter of the fiber mode field.

[0020] The spiral structure and planar structure of this invention constitute a 3D micro / nano structure, which can be fabricated on the end face of an optical fiber using a two-photon polymerization method. As a preferred embodiment, an ultra-high precision stress sensor based on an optical fiber FP cavity is provided, comprising: a single-mode optical fiber and a 3D micro / nano structure. The FP cavity is formed between the end face of the single-mode optical fiber and the planar structure (disk structure).

[0021] Preferably, the structural dimensions of the spiral structure are less than 1000 micrometers.

[0022] Preferably, the spiral structure consists of one or more single spirals with the same spiral direction.

[0023] Preferably, the number of single spirals is 1 to 5. The number of turns of a single spiral is 1 to 3. More preferably, it is 1 to 2 turns.

[0024] As a further preferred embodiment, when there are multiple 3D spiral structures (composed of multiple single spirals), they should be arranged uniformly along the circumference.

[0025] Preferably, the cavity formed by the fiber end face, the spiral structure, and the flat plate structure has a height of 60–100 micrometers and a diameter of 70–100 micrometers; the cross-sectional area of ​​the spiral structure (single spiral) is 5–20 micrometers.

[0026] As a further preferred embodiment, the cross-section of the single helix is ​​a regular polygon such as a rectangle, circle, or triangle, or other irregular cross-sectional structure. The thickness of the flat plate structure is 2–8 μm.

[0027] As an experimental example, the 3D spiral structure (single spiral) has a rectangular cross-section with dimensions of 3μm * 3.5μm and a spiral height of 80μm. The disk structure has a diameter of 90μm and a thickness of 2μm.

[0028] The present invention also provides an ultrasensitive stress detection system, comprising:

[0029] The fiber-optic-based ultrasensitive stress sensor structure described in any of the above technical solutions;

[0030] Tunable lasers provide optical signals;

[0031] An optical power meter receives reflected light signals;

[0032] Fiber optic connectors are used to achieve corresponding connections between the tunable laser, the optical power meter, and the sensor structure.

[0033] Preferably, the fiber optic connector is a fiber optic circulator. During connection, the tunable laser is connected to the first end of the fiber optic circulator, the second end of the fiber optic circulator is connected to the optical power meter, and the third end of the fiber optic circulator is connected to the fiber-optic-based FP cavity stress sensor.

[0034] The FP-type fiber optic stress sensor described in this invention can measure minute forces. This invention can detect tensile or pushing forces as low as 0.5 nN; for example, it can detect the weight or tensile force of an object. The force that this invention can detect is generally no higher than 30 nN (calculated from the maximum accurate spectral drift).

[0035] This invention also provides an application demonstration of the sensor, which can be used to weigh ceramic micron-sized particles. This application can also be considered a calibration experiment for the sensor's accuracy.

[0036] The micron-sized particles are made of silicon oxide (SiO2) and are regular spherical particles. The example shows three specifications with diameters of 45μm, 60μm, and 75μm, and weights of 1.11nN, 2.64nN, and 5.14nN, respectively.

[0037] This invention employs two-photon polymerization 3D printing technology to fabricate the 3D micro / nano structure. The two-photon polymerization 3D printing technology specifically includes the following steps: photoresist dispensing (using negative photoresist in this method), two-photon polymerization, development, and cleaning; that is, firstly, photoresist is dispensing; then, based on the three-dimensional structural data of the 3D micro / nano structure, a femtosecond laser is used to perform two-photon polymerization on the photoresist; after polymerization, a developer is used to dissolve the untreated portions; finally, a cleaning solution is used to wash the printed target, ultimately obtaining the 3D micro / nano structure.

[0038] In this invention, the photoresist used is a conventional photoresist, including but not limited to IP-Dip, IP-S and IP-L.

[0039] As an experimental example, the photoresist used is IP-Dip.

[0040] In this invention, the two-photon laser parameters used are: wavelength of 700–900 nm; laser power of 10–40 mW; and laser scanning speed of 500–2000 μm / s.

[0041] As an experimental case, the two-photon laser parameters used were: wavelength 780nm; laser power 20mW; laser scanning speed 1000μm / s.

[0042] In this invention, the developing agent used is propylene glycol methyl ether acetate (PGMEA); the cleaning solution used is isopropanol (IPA).

[0043] The stress sensor described in this invention can be formed in one step on the end face of an optical fiber using a two-photon polymerization process, offering significant advantages such as simple fabrication, good surface roughness of the FP cavity, and high structural mechanical strength. Furthermore, a testing system adapted to this sensor is also disclosed in this invention, and mass sensing experiments were conducted on silica micron-sized particles, verifying the sensor's performance.

[0044] This invention provides an optical fiber-based FP cavity stress sensor and system, which has the following features and advantages compared with the prior art:

[0045] The stress sensor of this invention is an FP-type stress sensor. The FP-type stress sensor is directly fabricated on the end face of a single-mode optical fiber using two-photon polymerization 3D processing technology. This technology has a simple fabrication process and high processing precision (resolution better than 200nm). Combined with high-performance photoresist, the fabricated micro / nano structure has extremely low surface roughness (generally below 80nm). In particular, the stress sensor's sensing sensitivity can reach 0.43nm / nN, far superior to existing fiber optic stress sensors, and the force sensing limit can reach the nanonewton (nN) level, representing a four-order-of-magnitude improvement compared to existing fiber optic stress sensors. Attached Figure Description

[0046] Figure 1 This is a schematic diagram of the structure of an FP cavity stress sensor based on optical fiber provided by the present invention.

[0047] Figure 2 The image shows an electron microscope (EM) image of a fiber-optic-based FP cavity stress sensor provided for this invention.

[0048] Figure 3 A schematic diagram of the sensor manufacturing process is provided for this invention.

[0049] Figure 4 This is a diagram of the sensing system device provided by the present invention.

[0050] Figure 5 The interferometric spectrum of the fiber optic stress sensor provided by this invention, and the spectral drift diagram of the trough position before and after placing silicon oxide particles of different sizes.

[0051] Figure 6 The graph shows the linear relationship between the spectral drift of the stress sensor prepared according to an embodiment of the present invention and the applied weight. Detailed Implementation

[0052] To facilitate understanding, the present invention will be further described below with reference to the accompanying drawings. Note that the following description is merely a preferred embodiment of the present invention and is intended only to aid in understanding the invention; therefore, it should not be construed as limiting the scope of the invention.

[0053] A schematic diagram of a fiber-optic-based FP cavity stress sensor provided by this invention is shown below. Figure 1 As shown, the sensor includes a single-mode optical fiber (including an optical fiber core 101 and an optical fiber cladding 102) and a 3D micro / nano structure. The 3D micro / nano structure includes a 3D spiral structure 103 and a top plate structure 104. The plate structure is aligned with the center of the optical fiber end face, parallel to the optical fiber end face, and the diameter of the effective reflective area is greater than or equal to the diameter of the optical fiber mode field. The 3D spiral structure is used to connect the optical fiber end face and the plate structure. At the same time, the spiral structure has a very low k-value (force-displacement ratio), which can produce a large deformation under the same force to amplify the force effect. There can be one or more 3D spirals. In this embodiment, there are three 3D spirals, which are uniformly arranged circumferentially along the annular base (i.e., the optical fiber end face) and the plate structure. An FP resonant cavity is formed between the single-mode optical fiber end face, the 3D spiral structure, and the plate structure.

[0054] As an experimental case, the 3D spiral structure cross-section (i.e., the cross-section of a single spiral) is rectangular (though circular, elliptical, triangular, or other regular polygonal or irregular shapes can be chosen as needed), with cross-sectional dimensions (w*t) of 3μm*3.5μm and a spiral height of H = 90μm. The flat plate structure has a diameter of 90μm and a thickness of 2μm. The electron microscope image of the product prepared using two-photon polymerization 3D printing technology is shown below. Figure 2 As shown.

[0055] The 3D spiral structure is coaxially arranged with the fiber core 101, and the inner diameter of the enclosed FP resonant cavity is greater than or equal to the fiber mode field diameter. The number of turns in the 3D spiral structure is 1 to 3. In this embodiment, it is approximately 1 turn.

[0056] The fiber-optic-based FP cavity stress sensor provided by this invention is obtained through two-photon polymerization fabrication, and its fabrication process is as follows: Figure 3 As shown, the process consists of the following steps: photoresist drop application (this method uses negative photoresist), two-photon polymerization, development, and cleaning.

[0057] The processing parameters are as follows: the two-photon laser parameters are: wavelength 780nm; laser power 20mW; laser scanning speed 200μm / s; the developer used is propylene glycol methyl ether acetate (PGMEA); the cleaning solution used is isopropanol (IPA). The photoresist is IP DIP.

[0058] Figure 4 This is a schematic diagram of a fiber-optic FP-cavity stress sensing system provided by the present invention. The fiber-optic FP-cavity refractive index sensing system includes: a tunable laser, a fiber optic circulator, an optical power meter, and... Figure 1 The fiber-optic-based FP cavity stress sensor;

[0059] The tunable laser is connected to the first end of the fiber optic circulator to transmit the laser signal to the stress sensor. The second end of the fiber optic circulator is connected to the optical power meter to receive the reflected signal. The third end of the fiber optic circulator is connected to the fiber-optic FP cavity stress sensor for inputting the laser signal and outputting the reflected signal, etc.

[0060] Figure 5 The interferometric spectrum of the fiber optic stress sensor provided by this invention and the changes in the trough position before and after placing silicon oxide micron-sized particles of different weights are shown. The results show that before and after placing the particles (Figures a-c, with weights of 1.11 nN, 2.64 nN, and 5.14 nN respectively), the spectrum exhibits blue shifts of 0.48 nm, 1.115 nm, and 2.249 nm, respectively. Figure 6As shown, the spectral drift can be calculated from the spectrum obtained by the optical power meter. The spectral drift is linearly related to the applied weight, verifying the excellent performance of this micro-force sensor. The calculated sensing accuracy of this stress sensor is 0.43 nm / nN, which is far superior to existing fiber optic stress sensors. For 3D micro / nano structures of different sizes, the magnitude of the external force on the 3D micro / nano structure can be directly obtained through simple calculation based on the linear relationship between the pre-detected spectral drift and the applied weight (or tension). Working principle: The fiber-optic FP cavity stress sensor provided by this invention mainly adopts the FP cavity interference principle. The light intensities reflected from the fiber end face and the flat structure are denoted as I1 and I2, respectively. Therefore, the interference signal between the two can be expressed as:

[0061]

[0062] Where n represents the ambient refractive index, L represents the cavity length of the FP cavity, and λ represents the incident light wavelength. This indicates the initial phase. The trough positions of the interference spectrum must satisfy the following phase condition:

[0063]

[0064] Where m represents an integer, λ m To represent the position of the m-th wave trough, the above formula can be transformed into:

[0065]

[0066] As can be seen from the above equation, the wavelength of the interference trough decreases as the cavity length of the FP decreases (blue shift), and there is a correlation between the two.

Claims

1. A fiber-optic-based ultrasensitive stress sensor structure, characterized in that, include: A spiral structure coaxially disposed on the end face of an optical fiber and a flat plate structure disposed at the other end of the spiral structure and parallel to the end face of the optical fiber, wherein an FP resonant cavity is formed between the end face of the optical fiber and the flat plate structure. The spiral structure is a 3D spiral structure; when the 3D spiral structure is subjected to a force perpendicular to the end face of the optical fiber, it will undergo compression deformation, which will lead to a change in the height of the FP resonant cavity structure. The optical fiber is a single-mode optical fiber; The flat plate structure is a disk structure, and the 3D spiral structure, the flat plate structure and the center of the single-mode fiber end face are coaxially arranged. The effective reflective area of ​​the flat plate structure has a diameter greater than or equal to the diameter of the fiber mode field.

2. The fiber-optic-based ultrasensitive stress sensor structure according to claim 1, characterized in that, The spiral structure and the flat plate structure constitute a 3D micro-nano structure, which is an integral structure.

3. The fiber-optic-based ultrasensitive stress sensor structure according to claim 2, characterized in that, The 3D micro / nano structure was prepared using a two-photon polymerization method.

4. The fiber-optic-based ultrasensitive stress sensor structure according to claim 1, characterized in that, Its features are, The structural dimensions of the spiral structure are less than 1000 micrometers.

5. The fiber-optic-based ultrasensitive stress sensor structure according to claim 1, characterized in that, The spiral structure consists of one or more single spirals with the same spiral direction.

6. The fiber-optic-based ultrasensitive stress sensor structure according to claim 5, characterized in that, The number of single helices ranges from 1 to 5.

7. The fiber-optic-based ultrasensitive stress sensor structure according to claim 6, characterized in that, The cavity formed by the fiber end face, helical structure, and flat plate structure has a height of 60-100 micrometers and a diameter of 70-100 micrometers; the cross-sectional area of ​​a single helix is ​​5-20 micrometers.

8. A supersensitive stress detection system, characterized in that, include: The fiber-optic-based ultrasensitive stress sensor structure according to any one of claims 1 to 7; Tunable lasers provide optical signals; An optical power meter receives reflected light signals; Fiber optic connectors are used to achieve corresponding connections between the tunable laser, the optical power meter, and the sensor structure.

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