A pendulum type optical fiber flow rate sensor with temperature compensation function

Abstract

This invention discloses a pendulum-type fiber optic flow velocity sensor with temperature compensation and its fabrication method, belonging to the field of fiber optic sensing technology. The sensing unit of this flow velocity sensor is located on the end face of a single-mode fiber, including a rigid part with a high thermo-optic coefficient and a hollow structure. A flexible cantilever beam is housed within the hollow structure, with its end connected to a rubber ball. The flow velocity sensing unit on the fiber end face is a three-beam interference structure. When the fluid to be measured flows laterally through the hollow structure, the fluid impact force pushes the rubber ball, causing the cantilever beam to bend and deform. This deformation causes a change in the optical path difference of the interference cavity. Simultaneously, the rigid material is temperature-sensitive, and its thermal expansion or contraction causes an additional change in the optical path difference. Both of these factors modulate the phase of the three-beam interference spectrum. A phase demodulation algorithm is used to separate the contributions of strain caused by flow velocity and temperature to the phase, obtaining flow velocity and temperature data separately. This invention utilizes femtosecond laser two-photon polymerization technology for direct microfabrication and integration on the fiber end face, resulting in miniaturized structure, fast response, and simple fabrication. It can be widely applied to the synchronous monitoring of local flow velocity and temperature in microfluidics, microchannels, and biomedicine.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensing technology, and in particular to a pendulum-type fiber optic current sensing device with temperature compensation function and its fabrication method. Background Technology

[0002] In numerous sub-scenarios such as microfluidic chip operational monitoring, micro-pipeline fluid parameter verification, biomedical microscale fluid analysis, and microchemical reactor flow control, the synchronous real-time monitoring of fluid velocity and temperature within confined spaces is an indispensable key link for closed-loop process control, operational risk assessment, and experimental data calibration. Current mainstream detection solutions typically deploy independent velocity sensing elements (thermal film flow meters, differential pressure flow meters) and independent temperature sensing elements (thermocouples, thermistors) separately within the measured area. However, this separate deployment faces three major technical limitations: First, the two types of sensing elements occupy limited micro-area installation space, making it difficult to meet the stringent miniaturization requirements of microfluidic chips; second, the measurement points of the two types of sensing elements cannot be completely overlapped, resulting in inherent spatial deviations in velocity and temperature data; and third, the timestamps of the two independent acquisition systems are difficult to synchronize precisely, introducing significant dynamic measurement errors under transient flow conditions. The aforementioned three limitations significantly reduce the overall accuracy of multi-parameter synchronous detection in microscale scenarios, making it difficult to meet the stringent data requirements of high-precision microfluidic experiments and micro-volume biological fluid detection.

[0003] Single-mode fiber optic interferometric sensors, with their core advantages of small size, resistance to electromagnetic interference, and long-distance, low-loss optical signal transmission, have become the mainstream technology for sensing physical quantities in confined spaces. Among them, the three-beam interferometric structure utilizes the optical path difference of multiple reflected beams to form an interference spectrum. When an external physical quantity acts on the interference cavity, the optical path difference changes, and the phase of the interference spectrum shifts accordingly. High-precision inversion of the external physical quantity can be achieved through phase demodulation. However, for flow velocity measurement, fiber optic sensors also face the inherent challenge of temperature cross-sensitivity. Fluctuations in ambient temperature cause thermo-optical effects and thermal expansion and contraction in the sensing structure, leading to additional changes in the optical path difference. This change, combined with the strain optical path difference change caused by flow velocity, is superimposed on the same phase signal, resulting in temperature crosstalk errors in the flow velocity calculation. The current industry-standard solution is to add a second independent reference structure or an external dedicated temperature reference unit to cancel out cross-sensitivity through differential measurement. However, multi-structure arrangements increase the overall size of the sensing probe, adding microfabrication steps and increasing fabrication difficulty, making it difficult to adapt to applications with extremely strict device size constraints, such as microfluidic channels.

[0004] To address the aforementioned issues, this invention proposes a single-ended integrated temperature self-compensating sensing structure. Using a single optical fiber end as a substrate, it achieves both high-sensitivity flow velocity response and self-compensation for temperature crosstalk through integrated microstructure design. The structure comprises three functional layers along the optical axis: a partially reflective film on the fiber end face serves as a reference reflective surface (corresponding to component 1); a rigid material with a high thermo-optic coefficient serves as a temperature-sensitive and structural support layer (corresponding to component 2); and a pendulum-shaped cantilever beam integrally formed from a flexible column and the inner wall of the rigid material, along with a rubber ball fixed to its free end, serves as the flow velocity sensing and response unit (corresponding to component 3). These three components are arranged sequentially along the optical axis to form a three-beam interference cavity. Component 1 provides the reference light, component 2 converts temperature changes into an additional optical path difference to provide a compensation signal, and component 3 converts flow velocity information into interference cavity strain. At the fabrication level, this invention employs femtosecond laser two-photon polymerization technology, leveraging its true three-dimensional processing capabilities, sub-micron-level processing precision, and excellent compatibility with various polymer materials. This allows for the integrated molding of rigid materials, flexible columns, cantilever beams, and hollow structures within a 100-micron scale region at the fiber end, avoiding multi-step alignment and bonding processes and ensuring the structural integrity and dimensional accuracy. Through the joint calculation of thermo-optical effect models, elasticity models, and a three-beam interference phase demodulation algorithm, the temperature drift contribution can be accurately separated and eliminated using only the interference spectrum phase data output from a single fiber, thereby outputting a high-precision flow velocity value. This fundamentally avoids the inherent limitations of multi-structure compensation schemes in terms of volume and cost. Summary of the Invention

[0005] This invention proposes a method for fabricating a pendulum-type fiber optic flow velocity sensor with temperature compensation function, comprising the following steps:

[0006] S1. Take a single single-mode optical fiber (1), remove the coating layer at its end, cut the end face of the optical fiber flat, clean and pre-treat the end face, and deposit a partial reflective film on the end face of the optical fiber.

[0007] S2. Fix a rigid material (2) with a high thermo-optic coefficient on the end face of the optical fiber (1) so that the rigid material is fixedly connected to the end face of the optical fiber.

[0008] S3. A hollow structure (5) is formed in the rigid material (2). The hollow structure penetrates the rigid material and serves as a channel for the fluid to flow laterally.

[0009] S4. Inside the hollow structure (5), a cantilever beam (4) is integrally formed from the inner wall of the rigid material outward using femtosecond laser two-photon polymerization technology. The cantilever beam is connected to the inner wall of the rigid material through a flexible column, and the other end of the cantilever beam is a free end. The fiber end face, the interface between the rigid material and the cantilever beam, forms a fixed interference cavity, and the cavity length is basically unaffected by temperature and flow rate. The free end face of the cantilever beam, the interface between the rigid material and the cantilever beam, forms a sensing interference cavity, and the cavity length changes with the deformation of the cantilever beam.

[0010] S5. Apply glue balls (3) to the free end of the cantilever beam (4). The glue balls serve as receiving elements for fluid impact force.

[0011] S6. When the fluid flows laterally through the hollow structure (5), the fluid impact force pushes the rubber ball (3) to cause the cantilever beam (4) to bend and deform. The strain of the cantilever beam only changes the length of the sensing interference cavity, causing a change in the optical path difference of the three-beam interference cavity. At the same time, the rigid material (2) with a high thermo-optic coefficient senses the change in ambient temperature and brings about a significant change in refractive index through the thermo-optic effect. Thermal expansion only produces a slight cavity length shift, and the geometric length of the fixed interference cavity remains approximately unchanged, thereby generating an additional change in optical path difference. The optical fiber (1) outputs a three-beam interference spectrum, which contains the first phase signal corresponding to the fixed interference cavity between the first and second reflecting surfaces. And the second phase signal corresponding to the sensing interference cavity between the second and third reflecting surfaces. ;Demodulation via dual-cavity phase separation, utilizing Calibration of ambient temperature changes , and then from The phase drift caused by temperature is removed to obtain the cantilever beam strain ε caused only by flow velocity, and then the flow velocity value after temperature compensation is inverted.

[0012] Preferably, in S1, the optical fiber (1) is a single-mode optical fiber, and the end face of the optical fiber is coated with a partially reflective film by electron beam evaporation or sputtering coating process, with a reflectivity between 4% and 30%. The end face of the optical fiber serves as the first reflecting surface for three-beam interference.

[0013] Preferably, in S2, the rigid material (2) is selected from polymers or glass materials with high thermo-optic coefficients and moderate coefficients of thermal expansion, and its thermo-optic coefficient is... The absolute value is not less than 1×10⁻ 4 / °C, coefficient of thermal expansion Preferably 1×10⁻ 4 ~3×10⁻ 4 / °C. Rigid materials are fixed to the fiber end face by UV-curing adhesive or electroplating to convert changes in ambient temperature into measurable changes in optical path difference, thereby providing reference information for temperature compensation in subsequent signal processing.

[0014] Preferably, in S3, the hollow structure (5) is formed by femtosecond laser processing, photolithography or mechanical drilling, and its size is designed according to the flow range of the fluid to be measured. The ratio of the flow area to the cross-sectional area of ​​the pipe to be measured is between 0.3 and 0.8.

[0015] Preferably, in S4, the cantilever beam (4) and the flexible column are made of the same material as the rigid material, and are integrally formed by stretching from the inner wall of the rigid material outward using femtosecond laser two-photon polymerization technology. Femtosecond laser two-photon polymerization technology utilizes near-infrared femtosecond laser pulses to induce a two-photon absorption effect within the photosensitive material, achieving three-dimensional micro / nano structure processing beyond the diffraction limit. Its processing accuracy can reach the hundred-nanometer level, and it requires no mask or complex alignment process, making it particularly suitable for constructing hollow structures and complex three-dimensional microstructures such as cantilever beams in narrow spaces like fiber end faces. The thickness of the cantilever beam is less than the thickness of the rigid material, and the ratio of the cantilever beam length L to its thickness h, L / h, is between 10 and 30. The cross-sectional area of ​​the flexible column is smaller than that of the cantilever beam to reduce structural stiffness and improve the response sensitivity to fluid impact forces.

[0016] Preferably, in S5, the glue ball (3) is an elastic polymer microsphere, which is fixed to the free end of the cantilever beam by micro-doping or electrostatic adsorption. The diameter of the glue ball is adapted to the size of the hollow structure, and the ratio of the diameter of the glue ball to the hydraulic diameter of the hollow structure is between 0.4 and 0.8.

[0017] Preferably, in S6, during the dual-cavity phase separation and demodulation, the phase change of the interference cavity is fixed. With temperature change The relationship is shown in equation (1), which represents the phase change of the sensing interferometer cavity. With respect to the strain ε and temperature change of the cantilever beam The relationship is shown in equation (2); by solving equations (1) and (2) simultaneously, the flow velocity v after temperature compensation and the measured phase change are obtained. , The sensing equation (3) between them has the same meaning as in claim 1.

[0018] Preferably, changes in ambient temperature Causes changes in the refractive index of rigid materials and minute geometric length changes The two methods have different effects on the two types of interference cavities: the optical path change of the fixed interference cavity is mainly due to the thermo-optical effect of refractive index, and the change in geometric cavity length can be ignored; the sensing interference cavity has no thermo-optical refractive index modulation, and there is only a cavity length shift caused by the slight thermal expansion of the rigid material. The two types of additional optical path changes are used as temperature compensation terms in the demodulation algorithm to remove the phase drift caused by temperature from the total phase change.

[0019] In this invention, the proposed method for fabricating a pendulum-type fiber optic flow velocity sensor with temperature compensation directly uses the fiber optic end as the processing platform. It employs femtosecond laser two-photon polymerization technology to progressively construct a rigid material, a flexible column, a hollow structure, a cantilever beam, and a sphere. All sensing structures are integrated within a 100-micron-scale region at the fiber optic end, making the fabrication process clear and controllable. The bending strain of the cantilever beam corresponds to the fluid velocity, while the thermo-optic effect of the rigid material corresponds to changes in ambient temperature. These two physical effects jointly modulate the phase of the same three-beam interference spectrum. The phase change caused by temperature is not output as an independent measurement object but rather as a compensation signal. This compensation signal is removed from the total phase using an algorithm, thus obtaining the true phase change caused only by the flow velocity, achieving high-precision temperature-compensated flow velocity measurement.

[0020] The present invention also proposes a pendulum-type fiber optic flow velocity sensor with temperature compensation function, which is prepared by the above-described preparation method;

[0021] The sensing device includes a single-mode optical fiber (1), a rigid material (2), a glue ball (3), and a cantilever beam (4). The rigid material (2) is fixed to the end face of the optical fiber (1), and a hollow structure (5) is provided in the rigid material (2). The cantilever beam (4) is set inside the hollow structure, with one end connected to the rigid material through a flexible column and the other end being a free end. The glue ball (3) is fixed to the free end of the cantilever beam (4). The end face of the optical fiber (1) is coated with a partial reflective film as the first reflective surface, the connection interface between the rigid material and the cantilever beam is the second reflective surface, and the free end face of the cantilever beam is the third reflective surface.

[0022] Preferably, the hollow structure (5) penetrates the rigid material (2) to allow the fluid to be tested to flow laterally, and the fluid impacts the rubber ball (3) to cause the cantilever beam (4) to bend in a pendulum-like manner.

[0023] Preferably, the rigid material (2) has a high thermo-optic coefficient. and the coefficient of thermal expansion α, and changes in ambient temperature. Causes changes in the refractive index of rigid materials and minute length changes The change in refractive index and the change in geometric length together cause an additional optical path change and are transmitted to the phase of the interference spectrum. The additional optical path of the fixed interference cavity is shown in Equation (4), and the additional optical path of the sensing interference cavity is shown in Equation (5). This additional optical path change is used as a temperature compensation term in the demodulation algorithm to remove the phase drift caused by temperature from the total phase change, thereby eliminating the crosstalk of temperature to flow velocity measurement.

[0024] Preferably, the partial reflective film on the end face of the optical fiber (1) and the free end face of the cantilever beam (4) form a three-beam interference cavity. The three reflected beams are: the reflected beam from the first reflective surface of the optical fiber end face, the reflected beam from the second reflective surface at the interface between the rigid material and the cantilever beam, and the reflected beam from the third reflective surface at the free end of the cantilever beam. The three reflected beams generate an interference spectrum. The phase of the interference spectrum is simultaneously affected by the strain of the cantilever beam and the thermal expansion-thermal-optical coupling effect of the rigid material. The quantitative relationship is shown in equations (1) and (2). By using the three-beam interference phase demodulation algorithm and combining the temperature compensation model of equations (4) and (5), the flow velocity value after temperature compensation is obtained by solving the equations simultaneously. The result is consistent with equation (3), thereby realizing accurate flow velocity measurement with temperature compensation function.

[0025] This invention presents a pendulum-type fiber optic velocity sensor with temperature compensation. It utilizes femtosecond laser two-photon polymerization technology to directly construct a micro-cantilever beam-glue ball structure and a three-beam interference cavity at the fiber optic end. The temperature compensation signal is provided by leveraging the thermo-optic effect of a rigid material with a high thermo-optic coefficient, achieving high-precision, interference-resistant velocity measurement. This compact structure features fast response and strong compatibility with existing fiber optic systems, making it suitable for real-time, high-precision monitoring of fluid velocity in micro-areas. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the overall structure of a pendulum-type fiber optic flow velocity sensor with temperature compensation function proposed in this invention.

[0027] Figure 2 This is a schematic diagram of the optical field based on the three-beam interference principle of a pendulum-type fiber optic velocity sensor with temperature compensation function proposed in this invention. Detailed Implementation

[0028] like Figure 1 As shown, Figure 1 This is a schematic diagram showing the complete sensor device of the present invention with all reference numerals.

[0029] Reference Figure 1 The present invention proposes a method for fabricating a pendulum-type fiber optic flow velocity sensor with temperature compensation function, comprising the following steps:

[0030] S1. Take a single single-mode optical fiber (1), remove the coating layer at its end, cut the end face of the optical fiber flat, clean and pre-treat the end face, and deposit a partial reflective film on the end face of the optical fiber.

[0031] Specifically, the optical fiber (1) is a standard single-mode optical fiber (Corning SMF-28 or equivalent). The coating layer of approximately 1-2 cm at the end is removed using wire strippers, and a flat end face is cut using a fiber cleaver (cutting angle not exceeding 0.5°). The end face is then cleaned with anhydrous ethanol to ensure it is free of contaminants. An electron beam evaporation coating process is used to deposit a titanium / silicon dioxide composite film with a thickness of approximately 50-100 nm on the end face of the optical fiber. The reflectivity is controlled between 4% and 30%, serving as the first reflecting surface for three-beam interference. The function of this end-face reflective film is to provide a stable power reference reflected light, which, together with the reflected light from the subsequent two interfaces, forms three-beam interference. The choice of reflectivity must consider both the contrast of the interference fringes and the signal strength.

[0032] S2. Fix a rigid material (2) with a high thermo-optic coefficient on the end face of the optical fiber (1) so that the rigid material is fixedly connected to the end face of the optical fiber.

[0033] Specifically, the rigid material (2) is selected from polymer materials with high thermo-optic coefficients (such as SU-8 photoresist, whose thermo-optic coefficient is high). Approximately -1.5 × 10⁻ 4 / °C) or doped glass materials. A thin layer of approximately 50-200 μm thickness is formed on the fiber end face by drop coating or spin coating, and then cured with ultraviolet light or heat to firmly adhere the material to the fiber end face. The thermo-optic coefficient of rigid materials The absolute value should be no less than 1 × 10⁻ 4 / °C, to ensure a sufficiently significant optical path difference signal is generated within the target temperature range. This thermo-optical effect causes a change in refractive index. It is the physical basis of the temperature compensation function—it transforms changes in ambient temperature into quantifiable changes in optical path difference, thereby forming an identifiable temperature contribution component in the interference spectrum, which is easy for subsequent demodulation algorithms to identify and remove from the total phase.

[0034] S3. A hollow structure (5) is formed in the rigid material (2). The hollow structure penetrates the rigid material and serves as a channel for the fluid to flow laterally.

[0035] Specifically, rectangular or circular through-holes are cut into rigid materials using femtosecond laser 3D machining or focused ion beam. The width of the through-holes is designed according to the application flow rate, typically ranging from 50 to 300 μm. The perforated structure penetrates the rigid material, allowing fluid to flow laterally from the side. The ratio of the flow area to the cross-sectional area of ​​the pipe being measured is controlled between 0.3 and 0.8—the lower limit of this ratio ensures that the fluid exerts sufficient impact force on the ball to drive the cantilever beam to produce measurable deformation, while the upper limit avoids weakening the flow velocity sensitivity due to an excessively large flow area, while controlling the pressure loss within an acceptable range.

[0036] S4. Inside the hollow structure (5), a cantilever beam (4) is integrally formed from the inner wall of the rigid material to the outside using femtosecond laser two-photon polymerization technology. The cantilever beam is connected to the inner wall of the rigid material through a flexible column, and the other end of the cantilever beam is a free end.

[0037] Specifically, using the same material as the rigid material, flexible columns and cantilever beam structures are extended from one side of the hollowed-out inner wall through femtosecond laser two-photon polymerization technology within the hollowed-out area. The core principle of femtosecond laser two-photon polymerization technology is that a near-infrared femtosecond laser pulse is focused onto the interior of the photosensitive material through a high numerical aperture objective lens, inducing a two-photon absorption effect in the focal region. This causes the photosensitive material in a very small volume (approximately hundreds of nanometers to micrometers) near the focal point to undergo polymerization and cross-linking reactions, while the material outside the focal point remains unpolymerized. By controlling the spatial scanning path of the laser focal point through a three-dimensional translation stage, any designed three-dimensional microstructure can be written point by point. This technology requires no mask or complex alignment steps, and the processing resolution can reach the hundreds of nanometer scale. It is particularly suitable for the integrated molding of three-dimensional microstructures such as hollowed-out cavities, flexible columns, and cantilever beams in space-constrained and structurally complex scenarios such as fiber optic end faces. The cantilever beam has a length L of approximately 100-300 μm, a thickness h of approximately 10-30 μm, and a width b of approximately 20-50 μm, with its free end extending towards the hollowed-out central region. The cross-sectional area of ​​the flexible column is designed to be smaller than that of the cantilever beam (for example, the width of the flexible column is 1 / 3 to 1 / 2 of the width of the cantilever beam). This aims to reduce the equivalent stiffness of the overall structure, making the cantilever beam more prone to pendulum-like oscillation under fluid impact, thereby improving the sensitivity of flow velocity measurement. The integrally molded flexible column-cantilever beam structure has a dual function: firstly, it efficiently transfers the fluid impact force on the ball into the bending strain of the cantilever beam; secondly, the introduction of the flexible column makes the displacement-force relationship at the free end more sensitive, facilitating the acquisition of measurable interference spectral phase shifts under low flow velocity conditions.

[0038] S5. Apply glue balls (3) to the free end of the cantilever beam (4). The glue balls serve as receiving elements for fluid impact force.

[0039] Specifically, a micro-dispensing needle is used to apply a drop of UV-curable adhesive to the free end of the cantilever beam, forming microspheres with a diameter of approximately 30-50 μm. After curing, the microspheres adhere firmly to the end of the cantilever beam. The ratio of the microsphere diameter to the hydraulic diameter of the hollow structure is controlled between 0.4 and 0.8 to ensure sufficient fluid impact capture area and maintain unobstructed flow channels. The microspheres are made of a low-modulus polymer material (Shore A hardness 30-50) to increase compliance under fluid impact, allowing even small changes in flow velocity to generate sufficient impact force to drive the deformation of the cantilever beam.

[0040] S6. When the fluid flows laterally through the hollow structure (5), the fluid impact force pushes the rubber ball (3) to cause the cantilever beam (4) to undergo pendulum-like bending deformation. This deformation causes a change in the optical path difference of the three-beam interference cavity. At the same time, the rigid material (2) with a high thermo-optic coefficient senses the change in ambient temperature and causes an additional optical path difference change through the thermo-optic effect. This additional optical path difference is used to compensate for the interference of temperature on the flow rate measurement. The optical fiber (1) outputs the three-beam interference spectrum and realizes the flow rate measurement with temperature compensation function through phase demodulation. Specifically, during measurement, the fiber optic end is placed in the fluid channel or microfluidic channel, allowing the fluid to flow laterally through the perforation perpendicular to the fiber axis. The fluid flows at a velocity... Impact ball, fluid impact force The relationship with the flow velocity v satisfies:

[0041] in The drag coefficient (depending on the shape of the rubber ball and the Reynolds number, usually taken as 0.5-1.0) For fluid density, This represents the windward surface area of ​​the rubber ball.

[0042] The rubber ball causes the cantilever beam to bend. According to the deflection theory of the cantilever beam end under concentrated force in mechanics of materials, the strain ε generated in the cantilever beam under the action of force F satisfies: In the formula The length of the cantilever beam. For elastic modulus, The width of the cantilever beam. Let be the thickness of the cantilever beam. This formula establishes a quantitative relationship between the mechanical load and the internal strain of the cantilever beam.

[0043] The change in optical path difference of the three-beam interferometer cavity caused by strain of the cantilever beam satisfy: In the formula Let be the effective length of the interference cavity. Let the complex amplitudes of the three reflected beams be respectively... , , ,in , , The light intensities of the three reflected beams are respectively (corresponding to) Figure 2 In , , ), , , Let I be the initial phases of the three beams. Then the total light intensity I of the interference of the three beams satisfies:

[0044] in, The phase difference between the reflected light from the first reflecting surface (M1) and the second reflecting surface (M2) is... The phase difference between the reflected light from the second reflecting surface (M2) and the third reflecting surface (M3) is the phase difference between them. The phase difference between the reflected light from the first reflecting surface (M1) and the third reflecting surface (M3) is denoted as M1. This is the optical path length (fixed cavity length) between M1 and M2. is the optical path length between M2 and M3 (sensing cavity length, which varies with the strain of the cantilever beam); n is the effective refractive index of the medium inside the interference cavity; Let λ be the wavelength of the incident light in a vacuum. This equation fully describes the intensity distribution of the interference spectrum formed by the superposition of three reflected beams, and is the physical basis for subsequent extraction of external physical quantities through spectral phase demodulation.

[0045] The total phase change of the three-beam interference spectrum The phase change is modulated by both strain and temperature. Specifically, the phase change of the fixed interference cavity is only sensitive to temperature, and its magnitude... With temperature change The relationship is shown in equation (1); the phase change of the sensing interference cavity is affected by both flow velocity, strain, and temperature, and its phase change amount is... With respect to the strain ε and temperature change of the cantilever beam The relationship is shown in equation (2).

[0046] Meanwhile, changes in ambient temperature The optical path lengths of the two interference cavities are affected by different mechanisms. For the fixed interference cavity (M1-M2), the rigid material undergoes both thermal expansion and thermo-optical effects, and its additional optical path length change is shown in Equation (4). For the sensing interference cavity, the fluid to be measured is inside the hollow structure, and its thermo-optical coefficient is extremely small and negligible. The optical path length change caused by temperature only comes from the cavity length change caused by the thermal expansion of the rigid material, and its additional optical path length change is shown in Equation (5).

[0047] In the demodulation algorithm, the ambient temperature is first calibrated in real time using equation (1) to obtain the current change in ambient temperature. The calculation formula (6); the result Substituting into equation (2), the contribution of thermal expansion caused by temperature is eliminated, and the pure flow velocity strain calculation equation (7) is obtained; combining equations (8) and (9), the final flow velocity sensing equation after complete temperature compensation can be obtained, and the result is consistent with equation (3).

[0048] In this embodiment, the proposed fabrication method uses the fiber optic end as a processing platform and employs femtosecond laser two-photon polymerization technology to progressively construct rigid materials, flexible pillars, hollow structures, cantilever beams, and glue spheres. All sensing structures are integrated within a 100-micron scale region at the fiber optic end. The advantages of two-photon polymerization technology are: firstly, its true 3D processing capability allows for in-situ construction of cantilever beam structures within the hollow cavity, eliminating the need for multi-step bonding or alignment; secondly, its 100-nanometer-level processing precision ensures precise control over the dimensions of the cantilever beams and flexible pillars, thereby guaranteeing consistent mechanical response; and thirdly, the abundance of photosensitive polymer materials allows for the design of stepped moduli for both the rigid substrate and flexible pillars within the same material system. Due to the high thermo-optic coefficient and strong temperature sensitivity of the rigid material, significant temperature compensation signals can be generated. The cantilever beam-flexible pillar-glue sphere structure constitutes a fluid impact response structure with high flow velocity sensitivity. Both share a three-beam interference cavity within the same fiber as the signal channel, achieving high-precision, anti-interference measurement of flow velocity through phase demodulation and temperature compensation algorithms, eliminating the need for additional leads or multi-structure compensation, fundamentally ensuring device miniaturization and measurement accuracy.

[0049] Figure 1 The complete structure of the pendulum-type fiber optic flow sensor with temperature compensation function in this embodiment is fabricated using the above-described preparation method. Specifically, the sensor includes a single-mode fiber (1), a rigid material (2), a rubber ball (3), and a cantilever beam (4). The rigid material has a hollow structure (5). The fiber (1) is coated with a partial reflective film as the first reflective surface, the interface between the rigid material and the cantilever beam is the second reflective surface, and the free end face of the cantilever beam is the third reflective surface. The three components constitute a three-beam interference cavity. The end face of the fiber is connected to the rigid material, and the cantilever beam (4) extends from the hollow inner wall through a flexible column, with the rubber ball (3) fixed at its free end.

[0050] In this embodiment, when the sensing device is working, the fluid flows laterally through the hollow structure (5), impacting the rubber ball (3) and causing the cantilever beam (4) to bend in a pendulum-like manner, resulting in a change in the distance between M2 and M3. The optical path difference strain contribution is shown in equation (10). Simultaneously, changes in ambient temperature cause changes in the refractive index and geometric dimensions of the rigid material (2), with its optical path temperature contribution shown in equations (4) and (5). In the demodulation algorithm, based on equation (1), the fluid... Solution According to equation (2), After deducting the temperature contribution, the strain ε is separated and finally substituted into equation (3) to obtain the real-time flow rate value after temperature compensation.

[0051] The device in this embodiment has a compact structure, with the maximum outer diameter at the device end not exceeding twice the diameter of the optical fiber plus the thickness of the rigid material (typical value <500 μm). It is directly compatible with standard optical fiber connectors, facilitating plug-and-play integration with existing optical fiber sensing systems. The fabrication method employs femtosecond laser two-photon polymerization technology, requiring no masks or complex alignment, exhibiting good process repeatability, and is suitable for mass production. The phase demodulation resolution of the interference spectrum can reach 10⁻⁻⁴. 4 The flow rate is on the order of rads, corresponding to a flow velocity resolution better than 0.01 m / s; the temperature compensation function effectively eliminates the interference of ambient temperature fluctuations on the flow velocity measurement results, and the measurement error can be controlled within ±1% in the temperature range of 5~60°C; the response speed is fast (millisecond level), which is suitable for real-time high-precision monitoring of tiny flow rates in microfluidic chips.

Claims

1. A method for fabricating a pendulum-type fiber optic flow velocity sensor with temperature compensation function, characterized in that, Includes the following steps: S1. Select a single single-mode optical fiber (1), remove the coating layer covering the end, cut the fiber end face flat, perform a clean pretreatment on the end face, and deposit a partial reflective film on the fiber end face. S2. Fix a rigid material (2) with a high thermo-optic coefficient to the processing end face of the optical fiber (1) to complete the end face sealing and fixing of the two. S3. A through-hole structure (5) is processed inside the rigid material (2), and the hollow cavity serves as a flow channel for the transverse flow of the fluid to be tested; S4. Based on the inner wall of the hollow structure (5), a cantilever beam (4) is integrally formed using femtosecond laser two-photon polymerization technology: one end of the cantilever beam is firmly connected to the inner wall of the rigid material through a flexible column, and the other end is set with an unconstrained free end. The free end face of the cantilever beam serves as the third reflecting surface of the three-beam interference; the fiber end face is the first reflecting surface, and the interface between the rigid material and the cantilever beam is the second reflecting surface. The first and second reflecting surfaces constitute a fixed interference cavity with a basically constant cavity length; the second and third reflecting surfaces constitute a sensing interference cavity with a cavity length that changes with the deformation of the cantilever beam. S5. Rubber balls (3) are placed at the free end of the cantilever beam (4). The rubber balls are used to bear the fluid impact load. S6. When the fluid to be tested passes laterally through the hollow structure (5), the fluid impacts the rubber ball (3), causing the cantilever beam (4) to undergo pendulum-like deflection, changing the length of the sensing interference cavity. This deformation causes a change in the optical path difference of the three-beam interference cavity. At the same time, the rigid material (2) with a high thermo-optic coefficient senses the ambient temperature fluctuation. The temperature mainly changes the refractive index of the material through the thermo-optic effect, generating an additional optical path difference. The optical fiber (1) outputs a three-beam interference spectrum, which contains the first phase signal corresponding to the fixed interference cavity between the first and second reflecting surfaces. And the second phase signal corresponding to the sensing interference cavity between the second and third reflecting surfaces. ; Demodulation via dual-cavity phase separation, utilizing Calibration of ambient temperature changes , and then from The phase drift caused by temperature is removed to obtain the cantilever beam strain ε caused only by flow velocity, and then the flow velocity value after temperature compensation is inverted.

2. The method for fabricating a pendulum-type fiber optic flow velocity sensor with temperature compensation function according to claim 1, characterized in that, In S1, the reflectivity of the partially reflective film coated on the fiber end face is between 4% and 30%, and the fiber end face serves as the first reflecting surface of the three-beam interference; the interface between the rigid material and the cantilever beam serves as the second reflecting surface of the three-beam interference; the free end face of the cantilever beam serves as the third reflecting surface of the three-beam interference; the first and second reflecting surfaces constitute a fixed interference cavity with a basically constant cavity length, and the second and third reflecting surfaces constitute a sensing interference cavity with a cavity length that changes with the deformation of the cantilever beam.

3. The method for fabricating a pendulum-type fiber optic flow velocity sensor with temperature compensation function according to claim 1, characterized in that, In S2, the rigid material (2) is selected from polymers or glass materials with high thermo-optic coefficients. The absolute value is not less than 1×10⁻ 4 / °C; Rigid materials are fixed to the fiber end face by UV-curing adhesive or electroplating to convert temperature changes into significant optical path difference changes, thereby providing temperature compensation signals.

4. The method for fabricating a pendulum-type fiber optic flow velocity sensor with temperature compensation function according to claim 1, characterized in that, In S3, the hollow structure (5) is formed by femtosecond laser processing, photolithography or mechanical drilling. The cross-sectional dimensions of the hollow structure are designed according to the flow range of the fluid to be measured. The ratio of the flow area to the cross-sectional area of ​​the pipe to be measured is between 0.3 and 0.

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5. The method for fabricating a pendulum-type fiber optic flow velocity sensor with temperature compensation function according to claim 1, characterized in that, In S4, the cantilever beam (4) is made of the same material as the rigid material and is integrally formed by stretching from the inner wall of the rigid material to the outside through femtosecond laser two-photon polymerization technology; the thickness of the cantilever beam is less than the thickness of the rigid material, and the ratio of the length L of the cantilever beam to the thickness h, L / h, is between 10 and 30; the cross-sectional area of ​​the flexible column is less than the cross-sectional area of ​​the cantilever beam to reduce the structural stiffness and improve the response sensitivity to fluid impact force.

6. The method for fabricating a pendulum-type fiber optic flow velocity sensor with temperature compensation function according to claim 1, characterized in that, In S5, the glue ball (3) is an elastic polymer microsphere, which is fixed to the free end of the cantilever beam by micro-dispensing or electrostatic adsorption. The diameter of the glue ball is adapted to the size of the hollow structure. The ratio of the diameter of the glue ball to the hydraulic diameter of the hollow structure is between 0.4 and 0.

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7. The method for fabricating a pendulum-type fiber optic flow velocity sensor with temperature compensation function according to claim 1, characterized in that, In S6, the dual-cavity phase separation demodulation satisfies the following relationship: Fixed interference cavity phase change With temperature change The relationship is: Phase change of the sensing interferometer cavity With respect to the strain ε and temperature change of the cantilever beam The relationship is: In the formula, λ is the incident light wavelength, and α is the thermal expansion coefficient of the rigid material. denoted as the thermo-optic coefficient of the rigid material, n is the refractive index of the rigid material, l1 is the optical path length between the first and second reflecting surfaces, and l2 is the optical path length between the second and third reflecting surfaces. Solving the above two equations simultaneously yields the temperature-compensated flow velocity v and the measured phase change. , The sensing equation between them: In the formula, E is the elastic modulus of the cantilever beam material, b is the width of the cantilever beam, and h is the thickness of the cantilever beam. Let ρ be the drag coefficient of the rubber ball, ρ be the fluid density, A be the windward area of ​​the rubber ball, and L be the length of the cantilever beam.

8. A pendulum-type fiber optic flow velocity sensor with temperature compensation function, characterized in that: The device is manufactured using the process described in any one of claims 1–7; the sensing device includes a single-mode optical fiber (1), a rigid material (2), a force-bearing rubber ball (3), and a cantilever beam (4); the rigid material (2) is fastened to the end face of the optical fiber (1), and a through-hole structure (5) is opened inside the rigid material; the cantilever beam (4) is housed and arranged inside the hollow cavity, with one end rigidly connected to the inner wall of the rigid material cavity and the other end being a deformable free end, and the free end face of the cantilever beam forming the third reflecting surface of the three-beam interference; the rubber ball (3) is fixedly assembled at the free end position of the cantilever beam; the end face of the optical fiber (1) is coated with a partial reflective film as the first reflecting surface, the connection interface between the rigid material and the cantilever beam as the second reflecting surface, and the free end face of the cantilever beam as the third reflecting surface, and the three are arranged sequentially along the optical axis to form a three-beam interference cavity; the first and second reflecting surfaces form a fixed interference cavity with a basically constant cavity length, and the second and third reflecting surfaces form a sensing interference cavity with a cavity length that changes with the deformation of the cantilever beam.

9. A pendulum-type fiber optic flow velocity sensor with temperature compensation function according to claim 8, characterized in that, The hollow structure (5) penetrates the rigid material (2) to allow the fluid to be tested to flow laterally. The fluid impacts the rubber ball (3) to cause the cantilever beam (4) to bend in a pendulum-like manner.

10. A pendulum-type fiber optic flow velocity sensor with temperature compensation function according to claim 8, characterized in that, Rigid materials (2) have high thermo-optic coefficients and the coefficient of thermal expansion α, and changes in ambient temperature. Causes changes in the refractive index of rigid materials and minute geometric length changes This change in refractive index, together with the change in geometric length, causes an additional optical path change, which is then transmitted to the phase of the interference spectrum. Additional optical path length for fixed interference cavity: Additional optical path of the sensing interferometer cavity: In the formula, l1 and l2 are the effective lengths of the corresponding interference cavities; for a fixed interference cavity, the geometric change in cavity length is extremely small, and the change in refractive index is dominated by the temperature optical path contribution; this additional optical path change is used as a temperature compensation term in the demodulation algorithm to remove the phase drift caused by temperature from the total phase change; according to formula (1) Solution Then, based on equation (2), from After deducting the temperature contribution, the strain ε is separated and finally substituted into equation (3) to obtain the flow velocity v after temperature compensation.

11. A flow velocity demodulation method for a pendulum-type fiber optic flow velocity sensor with temperature compensation function obtained by the preparation method according to any one of claims 1-7, characterized in that, Includes the following steps: D1. Acquire the three-beam interference spectrum and extract the phase change of the fixed interference cavity. Phase change of the sensing interferometer cavity ; D2. Calculate the change in current ambient temperature according to equation (1). D3, will Substituting into equation (2), the strain of the cantilever beam is isolated from it. D4. Based on the bending model of a cantilever beam under concentrated force at its end, introduce the strain-force relationship. Formula for fluid impact force After combining the equations, the flow velocity value after temperature compensation is inverted, and the result is consistent with equation (3).

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