Hot-wire-type wind speed sensor based on double-layer film synergistic enhanced micro-nanofiber and sensing device of hot-wire-type wind speed sensor

Abstract

The invention discloses a hot-wire type wind speed sensor based on a double-layer film synergistic enhanced micro-nano optical fiber and a sensing device thereof. The optical fiber wind speed sensor is characterized in that a single-mode optical fiber-coreless optical fiber-single-mode optical fiber structure is tapered to form a conical optical fiber; and the surface of the conical region is sequentially sputtered with a gold film and coated with graphene oxide to form the hot-wire type wind speed sensor. According to the invention, 980 nm laser is used as pump light to heat the gold film and the graphene oxide layer, so that the temperature of the sensing unit is increased; and meanwhile, a broadband light source is used as a detection light source, and optical signals of the Mach-Zehnder interference effect generated by the tapered optical fiber sputtering the gold film and the tapered optical fiber coated with the graphene oxide film are received through a spectrograph. When airflow flows through the sensing structure, the cooling effect causes temperature change, the effective refractive index and the phase difference of the interferometer are changed through the thermo-optic effect, and finally resonant wavelength drift of a transmission spectrum is caused. The accurate measurement of the wind speed can be realized by detecting the wavelength drift distance. The micro-channel wind speed sensor has the advantages of no temperature crosstalk, compact structure, high sensitivity and the like, and can be suitable for wind speed detection of an actual micro-channel.

Description

TECHNICAL FIELD

[0001] The present application relates to a kind of hot-wire type wind speed sensor based on double-layer film synergistically enhanced micro-nano optical fiber and its sensing device, belong to optical fiber sensing and fluid measurement technical field. BACKGROUND

[0002] Wind speed measurement plays a crucial role in meteorological monitoring, environmental assessment, industrial process control, aerospace and other fields. Traditional anemometers such as mechanical anemometers have problems such as large inertia, high starting wind speed, and easy wear; while hot-wire anemometers have fast response and high precision, but they are based on electrical principles and are susceptible to electromagnetic interference, and are not suitable for use in flammable and explosive environments. Optical sensing technology provides a new solution for wind speed measurement due to its advantages such as electromagnetic interference resistance, intrinsic safety, corrosion resistance, and the ability to achieve remote distributed measurement. Existing optical anemometers are mostly based on Doppler effect and particle image velocimetry, but these methods are usually complex and costly, or require tracer particles, limiting their widespread application.

[0003] In recent years, optical fiber anemometers have shown great potential. A microfluidic flow rate sensing chip, detection system and detection method proposed by Zhejiang University is based on the preparation of a Bragg grating on a photothermal optical fiber and the etching of a microfluidic channel in the middle of the grating to form a microstructure phase shift grating. However, the use of fiber gratings as a sensitive unit has limited thermal sensitivity and requires complex packaging to enhance heat exchange efficiency. Another invention discloses a microfluidic chip flow rate sensor based on micro-nano optical fiber, which utilizes the bending of a flexible film under pressure caused by fluid flow, the increase in bending loss of the micro-nano optical fiber, and the subsequent decrease in output light intensity signal to achieve flow rate sensing. However, its mechanical strength is weak, and it lacks efficient photo-thermal conversion materials, resulting in low heating efficiency and narrow wind speed measurement range. Therefore, it is of great significance to develop an optical fiber anemometer that combines high sensitivity, fast response, good stability, and simple preparation.

[0004] The present application uses a tapered optical fiber as the base structure, which has a strong evanescent field that makes it more sensitive to environmental changes. The metal film and graphene oxide film provide a layer of efficient and stable photo-thermal conversion material, resulting in high photo-thermal conversion efficiency and further improving sensitivity and response speed. SUMMARY

[0005] The present application aims to overcome the shortcomings of the prior art and provide a hot-wire type wind speed sensor based on double-layer film synergistically enhanced micro-nano optical fiber. This sensor combines the micro-nano optical fiber Mach-Zehnder interferometer structure with the high-efficiency photo-thermal conversion characteristics of the metal film and graphene oxide double-layer film, achieving high sensitivity, fast response, and temperature cross-interference-free wind speed measurement.

[0006] The object of the present application can be achieved by the following technical solutions:

[0007] A hot-wire type wind speed sensor based on a double-layer film synergistically enhanced micro-nano optical fiber, comprising: a single-mode optical fiber, a hollow-core optical fiber;

[0008] The single-mode optical fiber, the hollow-core optical fiber and the single-mode optical fiber are sequentially fused, and after fusion, the middle hollow-core optical fiber segment is subjected to a tapering treatment by using a fiber tapering machine to obtain a tapered optical fiber, and a light-heat conversion element layer is coated on the tapered region surface of the tapered optical fiber.

[0009] Preferably, the waist diameter of the tapered optical fiber is 20-50 μm.

[0010] Preferably, the light-heat conversion element layer is a double-layer film, the first layer is a gold film, and the second layer is a graphene oxide film.

[0011] Preferably, the metal film / graphene oxide composite humidity-sensitive film is coated by a layer-by-layer self-assembly method, and the thickness is 100 nm-2 μm. The metal film is a gold film deposited on the tapered region surface by a magnetron sputtering technology to a thickness of 20-50 nm, which has good optical fiber adhesion, stable chemical properties, is not easy to oxidize, and a nanoscale thin film can achieve high absorption rate. The graphene oxide serves as a high-efficiency light-heat conversion layer and a stable layer, and its two-dimensional sheet structure provides a support skeleton for the gold film and improves the mechanical stability of the film, and on the other hand, has high light-heat conversion efficiency. The two-dimensional sheet structure has extremely high thermal conductivity in the plane, can quickly and uniformly transfer the heat generated by the gold film to the entire sensing unit and the surface, and reduces the thermal response time. The huge specific surface area and rich oxygen-containing functional groups greatly increase the collision probability with air molecules, greatly optimize the heat exchange efficiency with flowing air, and amplify the cooling effect.

[0012] The gold film has intrinsic light absorption characteristics in the near-infrared waveband, can synergistically act with the graphene oxide layer, efficiently converts the absorbed light energy into heat energy, increases the temperature of the tapered region, and ultimately significantly improves the response speed and sensitivity of the sensor.

[0013] The application also provides an optical fiber wind speed sensing device comprising the above-mentioned hot-wire type wind speed sensor.

[0014] Preferably, the optical fiber wind speed sensing device comprises a 980 nm pump light source, a broadband light source, a coupler and a spectrometer.

[0015] The light emitted by the pump laser light source and the broadband light source is coupled by the coupler, the output end of the coupler is connected to the hot-wire type wind speed sensor, and the other end of the sensor is connected to the spectrometer. The coupler has a splitting ratio of 90:10, wherein the end with a ratio of 90 is connected to the pump light source, and the end with a ratio of 10 is connected to the broadband light source. The fixed input power can obtain a larger heating light source and a larger heat.

[0016] The signal light emitted by the broadband light source will excite different transmission modes when it is transmitted in the tapered fiber. These modes interfere after propagation to form a Mach-Zehnder interference spectrum. The spectrometer obtains the environmental wind speed information by monitoring the wavelength shift amount of the interference peak.

[0017] The working principle of the present application is as follows: when there is no airflow, the sensing unit dissipates heat through heat conduction and natural convection to reach a stable equilibrium temperature, at which time the transmission power or resonance wavelength of the interference spectrum is fixed. When airflow blows over the tapered surface coated with gold film and graphene oxide, the forced convection heat transfer effect will accelerate heat dissipation, resulting in a decrease in the temperature of the sensing unit. Changes in temperature will cause changes in the refractive index of the fiber material and the two layers of film (thermo-optic effect), thereby changing the phase difference of light propagation in the interferometer. Changes in the phase difference directly cause the interference peak in the transmission spectrum of the signal light to shift. By detecting the wavelength shift amount of the interference peak with a high-precision spectrometer, and through a calibration experiment using a commercial digital anemometer and the wavelength value of the interference peak beforehand, the wind speed value can be accurately demodulated.

[0018] The technical solution provided by the present application has the following beneficial effects:

[0019] Compact structure and short response time: the physical size of the sensing unit is extremely small (taper waist diameter is microns, length is millimeters), and the excellent in-plane thermal conductivity of graphene oxide can quickly transfer heat from the gold film to the entire surface of the sensing unit, forming a uniform "heat zone" and maximizing the effective heat exchange area with the air.

[0020] High wind speed sensitivity: the strong evanescent field generated by the tapered fiber makes the signal light extremely sensitive to changes in the external environment; the double-layer composite structure composed of gold film and graphene oxide realizes efficient absorption and heat conversion of the pump light, ensuring significant wind speed response sensitivity.

[0021] Stability: gold film is chemically stable and not easily oxidized; the graphene oxide coating is tightly combined with the gold film through physical adsorption and van der Waals force to form a protective layer. This structure is more reliable than sensors that simply rely on surface adsorption of particles or unstable chemical coatings, and can work stably for a long time in various temperature and humidity environments. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 FIG. 1 is a structural schematic diagram of a hot-wire type wind speed sensor based on a double-layer film synergistically enhanced micro-nano fiber.

[0023] Figure 2 FIG. 2 is a schematic diagram of a test system for a hot-wire type wind speed sensor based on a double-layer film synergistically enhanced micro-nano fiber.

[0024] Figure 3A flow chart of a wind speed measurement method of a hot-wire type wind speed sensor based on a double-layer film synergistically enhanced micro-nano optical fiber is provided. DETAILED DESCRIPTION

[0025] In order to make the technical solutions and advantages of the present application clearer, the present application is further described in detail below in combination with the drawings and examples. It should be understood that the specific examples described herein are only used to explain the present application and do not limit the present application. The drawings are only used for illustrative description and cannot be understood as limiting the present application.

[0026] REFERENCE Figure 1 The hot-wire type wind speed sensor based on a double-layer film synergistically enhanced micro-nano optical fiber provided by the present application needs to first perform tapering treatment on a single-mode-no-core-single-mode optical fiber structure, then perform magnetron sputtering of a metal film and coating of graphene oxide material on the surface of the taper region of the tapered optical fiber to form a light-heat conversion layer, and finally fix the two sections of the structure to a glass slide. The following is a detailed description of the preparation of the hot-wire type wind speed sensor.

[0027] Preparation of the tapered optical fiber: fix the SNS structure on the optical fiber tapering machine, control the hydrogen-oxygen flame scanning and the motor stretching through the program, align the scanning center in the middle of the coreless optical fiber, and prepare a tapered optical fiber with a tapered waist diameter of about 40 μm and a taper region length of about 12 mm. The entire process is monitored by the interference transmission spectrum of the spectrometer to ensure the consistency of the tapering process.

[0028] Preparation of the tapered optical fiber: fix the SNS structure on the optical fiber tapering machine, control the hydrogen-oxygen flame scanning and the motor stretching through the program, align the scanning center in the middle of the coreless optical fiber, and prepare a tapered optical fiber with a tapered waist diameter of about 40 μm and a taper region length of about 12 mm. The entire process is monitored by the interference transmission spectrum of the spectrometer to ensure the consistency of the tapering process.

[0029] Sputtering of the metal film: fix the tapered optical fiber with the desired size on the sample table of the magnetron sputtering instrument, and the control parameters of the magnetron sputtering instrument are as follows: the vacuum cavity is pumped to 5.0×10 -6 Pa, high-purity argon gas is introduced as the sputtering gas, the working gas pressure is set to 1 Pa, and the gold target is pre-sputtered for 5 minutes at a sputtering current of 50 mA to clean the gold target. By controlling the sputtering rate and time, a uniform gold film with a thickness of about 40 nm is sputtered on the surface of the taper region.

[0030] Coating of graphene oxide film: The gold-coated tapered fiber was sequentially ultrasonically cleaned in propanol, ethanol and deionized water for 5 minutes each to remove surface organic contamination. Then the fiber was treated with oxygen plasma or strong acid to make the surface hydrophilic and carry hydroxyl groups (-OH), which is conducive to the adsorption of positively charged polyelectrolyte. The pre-processed tapered fiber was immersed in a polydiallyldimethylammonium chloride solution (PDDA) for 15 minutes, so that the positively charged PDDA molecules would be electrostatically adsorbed on the negatively charged fiber surface. After taking it out, it was immersed in deionized water for 3 times, each for 1 minute, to wash away the physically adsorbed molecules. The fiber was immersed in a nanocellulose suspension for 15 minutes, and the negatively charged graphene oxide was electrostatically adsorbed on the positively charged PDDA layer. After taking it out, it was immersed in deionized water for 3 times. After each coating, it was placed in a constant temperature environment at 40 °C for 1 hour to ensure that the material was fully adsorbed on the fiber surface. After assembly, it was placed in a 60 °C oven for 1 hour to completely remove the water between the layers, enhance the interaction between the layers, and form a uniform composite light-to-heat conversion layer. The final thermal-wire type wind speed sensor based on a double-layer film synergistically enhanced micro / nano fiber is shown in the structural schematic diagram of Figure 1 .

[0031] System building and testing Figure 2 may be shown. The 980 nm laser and the broadband light source are connected to the incident port of the 90 / 10 coupler, the outgoing port of the coupler is connected to one end of the prepared thermal-wire type wind speed sensor based on a double-layer film synergistically enhanced micro / nano fiber, and the other end of the sensor is connected to the optical spectrum analyzer. The sensor is placed in the built wind speed measurement platform, and a commercial digital anemometer is placed at the same time to record the wind speed information of the surrounding environment and compare and calibrate the thermal-wire type wind speed sensor.

[0032] Light-to-heat conversion efficiency calibration: First, in a constant temperature (25 °C) environment without airflow, turn on the 980 nm laser and set the input current to change its output power from 5-40 mW, with an increase interval of 5 mW. After the spectrometer receives stable spectral information, a "pump laser input power-transmission spectrum wavelength" fitting relationship is established, and the heat response provided by the heating light source at zero wind speed is recorded. Then the sensor is placed in a controllable temperature box, and the environmental temperature is increased from 25-50 °C with an increase interval of 5 °C. After the spectrometer receives stable spectral information, a "temperature-transmission spectrum wavelength" fitting relationship is established, and finally the light-to-heat conversion efficiency of the transmission structure is obtained.

[0033] Wind speed measurement: The wind speed sensor is placed horizontally in the center of the wind speed measurement platform to ensure that it is perpendicular to the airflow direction The double-layer film as a light-heat conversion element, the heat taken away by different air flow rates causes the change of the effective refractive index in the structure, so as to measure the wind speed by detecting the wavelength shift of the interference spectrum, and realize the wind speed sensor with high contrast, wide range and high sensitivity.

[0034] The test results show that when there is no air flow through the sensor, the 980 nm laser actively heats the sensor to increase the temperature of the double-layer film, and the initial wavelength shifts from λ1 to a long wave to λ2. For a given laser power, the temperature of the sensor reaches the maximum. With the increase of the wind speed in the air flow channel, the interference peak in the spectrum shifts to a short wave direction, and λ2 decreases to λ3. This is because when the wind speed increases, the heat taken away by the air flow increases, resulting in a decrease in the temperature when the heat balance is established, and the effective refractive index of the fiber micro-nano structure decreases. In the low wind speed range, the temperature difference between the anemometer probe and the air flow is large, and the rapid change rate increases rapidly with the increase of the air flow speed. When the wind speed is in a certain range, the temperature difference between the anemometer and the air flow is relatively small, the change rate becomes relatively stable, and gradually reaches the saturation effect.

[0035] The above is only the preferred specific embodiment of the present application, but the protection scope of the present application is not limited thereto. Any person skilled in the art can make equivalent replacement or change according to the technical method and the inventive concept of the present application within the technical range disclosed by the present application, which should be covered within the protection scope of the present application.

Claims

1. A hot-wire anemometer based on a dual-layer thin-film synergistic enhancement micro / nano optical fiber, characterized in that, include: Single-mode fiber (1) and coreless fiber (2); Single-mode fiber (1), coreless fiber (2) and single-mode fiber (1) are fused together in sequence. After the fusion is completed, the coreless fiber (2) section in the middle is tapered using a fiber tapering machine to obtain a tapered fiber. A metal thin film (3) and a graphene oxide thin film (4) are sequentially set on the tapered area surface of the tapered fiber.

2. The hot-wire anemometer according to claim 1, characterized in that, The metal thin film (3) is a gold film with a thickness of 20-60 nm, which is used to efficiently absorb the light energy of the pump light source and generate heat.

3. The hot-wire anemometer according to claim 1, characterized in that, The graphene oxide film (4) is coated on the metal photothermal conversion layer to synergistically enhance the photothermal conversion efficiency and serve as a sensitive medium for heat exchange with the airflow.

4. The hot-wire anemometer according to claim 1, characterized in that, The tapered optical fiber has a waist diameter of 20-50 μm and a length of 10-18 mm, thereby generating a strong evanescent field in the tapered region, which interacts strongly with the surface metal film and graphene oxide layer.

5. A fiber optic wind speed sensing device, characterized in that, Including the hot-wire anemometer according to any one of claims 1-4.

6. The fiber optic wind speed sensing device according to claim 5, characterized in that, It also includes a pump light source (5), a broadband light source (6), a coupler (7), and a spectrometer (8). The pump light source (5) uses a 980 nm pump laser with adjustable optical power and is used to provide a heating light source. The broadband light source (6) is used to provide a detection light source. The pump light source (5) and the broadband light source (6) are coupled through a coupler (7). The output port of the coupler (7) is connected to one end of the hot-wire anemometer, and the other end of the hot-wire anemometer is connected to the spectrometer (8). The wind speed value is obtained by demodulating the wavelength shift of the interference spectrum from the hot-wire anemometer (9) by detecting and processing it.

7. A wind speed measurement method, employing a hot-wire anemometer as described in any one of claims 1-6, characterized in that, The steps for implementing the method include: S11. First, by changing a single parameter, an experimental fitting relationship between "pump laser input power - transmission spectrum wavelength" and "temperature - transmission spectrum wavelength" is established to obtain the photothermal conversion efficiency of the sensing structure. S12, turn on the pump light source, and use the heating light to heat the hot-wire anemometer until it reaches its initial equilibrium temperature; S13, the signal light passes through the wind speed sensor and records the initial transmission spectrum of the interference light in the windless state; S14, when the airflow passes over the surface of the wind speed sensor, the cooling effect causes the sensor temperature to change, which in turn causes the wavelength of the signal light transmission spectrum to change; after the heat generation and heat dissipation reach a dynamic balance, the signal light transmission spectrum is recorded in real time by a spectrometer. S15 converts the detected resonant wavelength change value into a real-time wind speed value based on the pre-established "transmission spectrum wavelength-wind speed" calibration curve.

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