Fiber Bragg grating humidity sensor and preparation method thereof

Abstract

The present invention provides a fiber Bragg grating (FBG) humidity sensor and its preparation method, comprising the following steps: preparing a moisture-sensitive coating modification solution containing potassium acetate and polyacrylic acid; immersing a polyimide-coated fiber Bragg grating (FBG) into the moisture-sensitive coating modification solution for surface modification; annealing the surface-modified fiber Bragg grating to obtain a cured polyimide-coated grating; and stress-relieving the cured polyimide-coated grating to obtain a fiber Bragg grating humidity sensor. The preparation method of the fiber Bragg grating humidity sensor of the present invention can improve the stability of the adhesion between the polyimide coating and the optical fiber of the fiber Bragg grating humidity sensor, reduce moisture hysteresis, and increase the service life of the sensor.

Description

Technical Field

[0001] The present invention belongs to the field of sensing and monitoring technology, and more specifically, relates to a fiber grating humidity sensor and a preparation method thereof. Background Art

[0002] Fuel cell test benches have high gas flow rates and rapid temperature fluctuations. Hydrothermal conditions, such as internal test bench temperature and membrane moisture content, are closely related to the performance and lifespan of the fuel cell stack. Therefore, real-time monitoring of the temperature and relative humidity of gas entering the fuel cell stack from the test bench's thermal management system and humidification system is crucial for high-precision comprehensive testing and diagnosis of fuel cells.

[0003] Currently, comprehensive fuel cell test systems widely use MEMS capacitive humidity sensors for relative humidity detection. The sensor probe consists of a silicon substrate, a humidity-sensitive electrolyte layer, and a moisture-permeable electrode. When the relative humidity around the sensor probe changes, the capacitance of the humidity-sensitive dielectric changes accordingly. The MEMS capacitive sensor converts the capacitance measurement into the required output signal. However, most MEMS capacitive humidity sensors are sheet-like structures. In high-humidity environments, temperature fluctuations can easily cause condensation to form inside the sensor, which can adhere to the humidity-sensitive electrolyte and cause relative humidity measurement failure.

[0004] Optical fiber offers advantages such as immunity to electromagnetic interference, compact size, and corrosion resistance. Combining optical fiber with humidity-sensitive films to form fiber-optic humidity sensors may meet the need for relative humidity measurement in some extreme environments. Different humidity-sensitive films vary significantly in performance and mechanism of action. Light intensity modulation and interferometric fiber-optic sensors employ humidity-sensitive films coated on the end face of optical fibers for sensing. These sensors link changes in ambient humidity to the intensity and optical path difference of the transmitted light. However, in practical applications, light intensity and optical path difference are susceptible to interference from external factors such as pressure and temperature, leading to significant errors in humidity measurements.

[0005] Fiber Bragg grating humidity sensors utilize a moisture-sensitive film coated around a Bragg grating. The polymer film absorbs water and expands in a humid environment. The resulting stress causes a change in the grating period, which in turn shifts the central wavelength of the grating reflection peak. By monitoring this change in central wavelength, changes in relative humidity can be detected. This type of sensor exhibits a linear response to ambient humidity and is relatively resistant to environmental interference. While suitable for high-humidity detection to a certain extent, prolonged exposure to high humidity and high temperature environments can still lead to separation between the coating and the fiber cladding, high moisture hysteresis, and a short service life. Summary of the Invention

[0006] The object of the present invention is to provide a fiber Bragg grating humidity sensor and a preparation method thereof, so as to improve the stability of the adhesion between the coating layer and the optical fiber of the fiber Bragg grating humidity sensor, reduce moisture hysteresis and increase the service life of the sensor.

[0007] To achieve the above object, the first aspect of the present invention provides a method for preparing a fiber Bragg grating humidity sensor, comprising the following steps:

[0008] preparing a moisture-sensitive coating modification solution containing potassium acetate and polyacrylic acid;

[0009] Immersing the polyimide-coated fiber Bragg grating into the moisture-sensitive coating modification solution for surface modification;

[0010] annealing the surface-modified fiber Bragg grating to obtain a cured polyimide-coated grating;

[0011] The cured polyimide coated grating is subjected to stress release treatment to obtain a fiber Bragg grating humidity sensor.

[0012] Furthermore, in the moisture-sensitive coating layer modification solution, the concentration of the polyacrylic acid is 12 wt % to 20 wt %.

[0013] Furthermore, in the moisture-sensitive coating layer modification solution, the concentration of potassium acetate is 1 wt% to 2 wt%.

[0014] Furthermore, the surface modification treatment is performed at a temperature of 20 to 30° C., a humidity of 40% RH to 60% RH, and a treatment time of 4 to 6 hours.

[0015] Furthermore, the polyimide-coated fiber Bragg grating is prepared by the following method: immersing the pretreated fiber Bragg grating in a polyimide slurry, forming a polyimide coating layer on the surface of the fiber Bragg grating by pulling the coating and heating and curing; the pretreatment is to perform hydroxylation treatment and silane coupling agent modification treatment on the surface of the fiber Bragg grating.

[0016] Furthermore, the pretreatment is performed by the following method: firstly placing the fiber Bragg grating with the coating removed in a piranha solution for surface hydroxylation treatment, and then placing the fiber Bragg grating in a silane coupling agent solution for silane coupling agent modification treatment.

[0017] Furthermore, the silane coupling agent is aminopropyltriethoxysilane, such as WD-50 and KH-550.

[0018] Furthermore, the annealing treatment is performed at a temperature of 200 to 220° C. and for a time of 2 to 4 hours.

[0019] Furthermore, the stress release treatment is performed at a temperature of 20 to 30° C., a humidity of 40% RH to 60% RH, and a treatment time of more than 20 hours.

[0020] A second aspect of the present invention provides a fiber Bragg grating humidity sensor, which is obtained by any of the above-mentioned preparation methods.

[0021] Compared with the prior art, the present invention has the following technical effects:

[0022] The present invention provides a preparation method for a fiber Bragg grating humidity sensor. Before annealing, a polyimide-coated fiber Bragg grating is immersed in a moisture-sensitive coating modification solution for surface modification. The polyacrylic acid in the modification solution serves as a wire adhesive, thereby achieving a better bonding effect between the polyimide coating and the fiber Bragg grating cladding (Si material). In addition, the polyacrylic acid and polyimide in the modification solution can form an amphoteric copolymer network, which can increase the hydrophobic angle of the surface of the polyimide film (fiber coating material) and allow water droplets to quickly escape from the fiber coating surface. The potassium acetate in the modification solution can increase the internal pore space of the polyimide coating, thereby increasing the specific surface area of ​​the material, increasing the expansion volume of the polyimide coating after absorbing water molecules during subsequent humidity sensing, and the rate at which water vapor passes through the coating, thereby improving the response speed of the sensor and reducing the wet hysteresis, thereby improving the sensitivity of the sensor. In addition, the polyacrylic acid in the modified solution has a small volume thermal expansion coefficient, which can effectively inhibit the thermal volume expansion of the polyimide coating, prevent the polyimide coating from detaching from the fiber grating cladding layer due to long-term exposure to high temperature and high humidity, and improve the stability of the adhesion between the polyimide coating and the optical fiber of the fiber grating humidity sensor, thereby increasing the service life of the sensor.

[0023] The fiber grating humidity sensor of the present invention uses polyimide as the humidity-sensitive material. Polyimide has low hysteresis and high temperature resistance and can remain stable under high temperature conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0025] Figure 1 is a SEM image of the optical fiber end face of the fiber Bragg grating humidity sensor prepared in the comparative example of the present invention;

[0026] Figure 2 is a SEM image of the optical fiber end face of the fiber Bragg grating humidity sensor prepared in Example 1 of the present invention;

[0027] Figure 3This is a test result diagram of the humidity sensitivity coefficient of the fiber Bragg grating humidity sensor prepared in Example 1 of the present invention between 30% RH and 100% RH;

[0028] Figure 4 This is a graph showing the long-term stability test results of the fiber Bragg grating humidity sensor prepared in Example 1 of the present invention between 30% RH and 100% RH. DETAILED DESCRIPTION

[0029] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0030] The terms used in the embodiments of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The singular forms "a", "an", "the" and "the" used in the embodiments of the present invention and the appended claims are also intended to include plural forms unless the context clearly indicates otherwise.

[0031] The weights of the relevant components mentioned in the examples of the present invention may not only refer to the specific content of each component, but also represent the weight ratios between the components. Therefore, as long as the content of the relevant components is proportionally increased or decreased according to the examples of the present invention, it is within the scope disclosed in the examples of the present invention. Specifically, the mass described in the examples of the present invention may be μg, mg, g, kg, or other mass units commonly known in the biochemical field.

[0032] The present invention provides a fiber Bragg grating humidity sensor and a method for manufacturing the same, comprising the following steps:

[0033] (1) preparing a moisture-sensitive coating modification solution containing potassium acetate and polyacrylic acid;

[0034] (2) immersing the polyimide-coated fiber Bragg grating into a moisture-sensitive coating modification solution for surface modification;

[0035] (3) annealing the surface-modified fiber Bragg grating to obtain a cured polyimide-coated grating;

[0036] (4) The cured polyimide coated grating is subjected to stress release treatment to obtain a fiber Bragg grating humidity sensor.

[0037] In the above step (1), the concentration of polyacrylic acid in the moisture-sensitive coating modification solution is 12wt% to 20wt%. If the concentration of polyacrylic acid is greater than 20wt%, the viscosity of the modified solution will be too high, making it impossible for potassium acetate to be evenly dispersed, and its corrosion effect will be weakened, ultimately resulting in the modification failing to achieve the expected effect; if the concentration of polyacrylic acid is less than 12wt%, its bonding effect is poor, and the coating layer and the optical fiber grating cladding layer cannot be well combined.

[0038] Furthermore, the concentration of potassium acetate in the moisture-sensitive coating modification solution is between 1 and 2 wt%. If the potassium acetate concentration is greater than 2 wt%, the corrosion effect is too strong, causing irreversible damage to the polyimide polymer film. If the potassium acetate concentration is less than 1 wt%, the corrosion effect fails to effectively increase the porosity, thereby failing to improve the polymer film's ability to absorb and desorb moisture, and thus failing to enhance the sensor's sensitivity.

[0039] The polyacrylic acid in the moisture-sensitive coating modification solution contains a carboxyl group with strong polarity. Before the polyimide coating is annealed, it is immersed in the modification solution. The polyacrylic acid in the modification solution can be used as a wire adhesive, so that the polyimide coating has a better bonding effect with the optical fiber grating cladding (Si material). In addition, the polyacrylic acid and polyimide in the modification solution can form an amphoteric copolymer network, which can increase the hydrophobic angle of the polyimide film (optical fiber coating material) surface, so that water droplets are quickly separated from the optical fiber coating surface. In addition, the potassium acetate added to the moisture-sensitive coating modification solution can cause the polyacrylic acid to swell after mixing with the potassium acetate aqueous solution. The rate becomes larger, and the potassium acetate solution has a certain corrosiveness to the polymer polyimide film material, which can permanently increase the internal pore space of the polyimide polymer film (i.e., the polyimide coating layer), thereby increasing the specific surface area of ​​the material, and increasing the expansion volume of the polyimide polymer film after absorbing water molecules during subsequent humidity sensing and the rate at which water vapor passes through the coating layer, thereby improving the response speed of the sensor and reducing the hysteresis, so as to improve the sensitivity of the sensor; in addition, the fiber grating humidity sensor of the present invention uses polyimide as the humidity-sensitive material. Polyimide has low hysteresis and high temperature resistance and can remain stable under high temperature conditions.

[0040] In addition, the polyacrylic acid in the modified solution has a small volume thermal expansion coefficient, which can effectively inhibit the thermal volume expansion of the polyimide polymer film, prevent the polyimide coating from detaching from the fiber grating cladding layer due to long-term exposure to high temperature and high humidity, and improve the stability of the adhesion between the polyimide coating and the optical fiber of the fiber grating humidity sensor, thereby increasing the service life of the sensor.

[0041] In an embodiment of the present invention, the moisture-sensitive coating layer modification solution can be prepared specifically according to the following method:

[0042] (101) Dissolve 0.055–0.19 g of potassium acetate in 3.895 g of deionized water and stir magnetically for 5 min to completely dissolve it;

[0043] (102) 1.57–5.42 g of a 35 wt % aqueous solution of polyacrylic acid (PAA, 523925) was sonicated for 5 min to obtain a uniformly dispersed polymer solution;

[0044] (103) The solutions obtained in steps (101) and (102) were placed in the same beaker and mixed, and magnetically stirred at 600 rpm for 15 minutes to obtain a moisture-sensitive coating layer modified solution.

[0045] In the above step (2), the polyimide-coated fiber Bragg grating can be prepared by the following method: immersing the pretreated fiber Bragg grating in a polyimide slurry, forming a polyimide coating layer on the surface of the fiber Bragg grating by pulling the coating and heating and curing; the pretreatment is to perform hydroxylation treatment and silane coupling agent modification treatment on the surface of the fiber Bragg grating. The hydroxylation treatment can make the fiber surface more hydrophilic, enhance the surface reaction efficiency of the grating periodic structure and the subsequent stability of its combination with the humidity-sensitive film. The silane coupling agent modification treatment can make the silicon-based material subsequently couple with the humidity-sensitive film, further enhancing the bonding force between the humidity-sensitive film and the fiber grating area. The silane coupling agent can be aminopropyltriethoxysilane, such as WD-50, KH-550, etc.

[0046] In an embodiment of the present invention, the pretreatment is performed by placing the fiber Bragg grating (FBG) with the coating removed in a piranha solution for surface hydroxylation, and then placing the fiber Bragg grating in a silane coupling agent solution for silane coupling agent modification. Specifically, the following steps are included:

[0047] (201) Open the fume hood and use a pipette to slowly pipette 7 mL of 98% concentrated sulfuric acid along the wall of the beaker and add it into the 30 mL beaker;

[0048] (202) Use a pipette to transfer 3 mL of 30% hydrogen peroxide solution along a glass stirring rod and drip it into the beaker in step (201);

[0049] (203) The mixed solution obtained in step (202) was magnetically stirred at 180 rpm for 5 min to obtain 10 mL of a uniformly dispersed piranha solution;

[0050] (204) Fix the grating area of ​​the Bragg fiber grating after removing the coating layer in a 50 mm curved glass surface dish groove, and transfer 0.5 mL of the evenly dispersed piranha solution obtained in step (203) to the grating area and allow it to react for 20 minutes;

[0051] (205) Pipette the piranha solution from the glass watch glass into a beaker, and rinse the Bragg grating area alternately with deionized water and alcohol. After washing, dry it with a hot air gun. A fiber Bragg grating with surface hydroxylation treatment is obtained.

[0052] (206) 7.2 mL of anhydrous ethanol, 2 mL of KH-550 (RSiY3), and 1.6 mL of deionized water were added to an iron container and magnetically stirred for 5 min;

[0053] (207) immersing the surface hydroxylated fiber Bragg grating region obtained in step (205) into the silane coupling agent solution of step (206) for 10 minutes;

[0054] (208) The grating in step (207) is taken out and placed in a 120°C oven for 20 minutes to obtain a pre-treated bare coated fiber Bragg grating.

[0055] The piranha solution in the embodiment of the present invention can effectively remove organic impurities on the grating surface. After hydroxylation treatment with the piranha solution, the optical fiber surface has stronger hydrophilicity, which can enhance the surface reaction efficiency of the grating periodic structure and the subsequent stability of its combination with the humidity-sensitive film.

[0056] After step (208) is completed, the bare coated fiber Bragg grating is immersed in a polyimide slurry, and a polyimide coating layer is formed on the surface of the fiber Bragg grating by pulling the coating and heating and curing. The polyimide slurry can be prepared using existing technology. The specific preparation process of the polyimide slurry in the embodiment of the present invention refers to CN112409265A. The embodiment of the present invention uses 3,3',4,4'-biphenyltetracarboxylic dianhydride (BDPA) and 2-(4-aminophenyl)-5-aminobenzimidazole (PABZ) as raw materials for dianhydride and diamine, and the copolymer is 4,4'-diaminodiphenyl ether (ODA). ODA and BDPA are stirred with equal molar mass to prepare a polyamic acid solution, and then the target polyimide slurry is prepared by two-step thermal imidization. The embodiment of the present invention can control the water absorption rate of the polyimide slurry by selecting specific dianhydride and diamine, thereby improving the humidity sensitivity coefficient of the polyimide optical fiber coating layer and improving the sensitivity of the sensor.

[0057] In the embodiment of the present invention, the polyimide coating can be specifically prepared according to the following steps:

[0058] (209) The bottom of a 0.5 mL transparent centrifuge tube was pierced using a micro-grinder with a drill bit of 1 mm in diameter;

[0059] (210) A custom hollow glass tube (length L: 10 mm, inner diameter r: 0.5 mm, outer diameter R: 1 mm) was fixed vertically to the bottom of the centrifuge tube in step (209) using UV glue;

[0060] (211) Align the central axis of the centrifuge tube and the ceramic hot pressing ring, and fix them with a bracket just below the mechanical arm of the immersion coating machine;

[0061] (212) The optical fiber Bragg grating processed in step (208) is passed through a ceramic hot pressing ring, a centrifugal tube, and a hollow glass tube in sequence, and the optical fiber head is suspended on a mechanical arm of a dipping and pulling coating machine;

[0062] (213) Manually control the mechanical arm of the dip-pull coating machine to adjust the position of the optical fiber grating area, set the starting and ending points of the pull, the starting point is the middle of the centrifuge tube, and the end point is the middle of the ceramic hot pressing ring, record the pull height h, and place the grating area at the starting position;

[0063] (214) The polyimide slurry is injected into the centrifuge tube fixed in position in step (213), and the first dipping of the gate area begins, and the dipping time is 1 min;

[0064] (215) Adjust the immersion coating machine to manual operation mode, set the pulling height to the value h recorded in step (213), the pulling speed to 180 μm / s, and the descending speed to 180 μm / s. After the grating is pulled to the specified height h, stay for 15 seconds and start to descend to the origin position;

[0065] (216) Repeat step (215) twice to complete the first three coating layers. The first three coating layers are manually completed to ensure the uniformity of the moisture-sensitive film combined with the optical fiber cladding.

[0066] (217) The dip-pull coating machine was adjusted to the automatic operation mode, and the pulling speed was set to 400 μm / s, the descending speed was set to 400 μm / s, the dipping time was 30 s, the dwell time was 10 s, and the number of cycles was 6 to complete the coating.

[0067] The fiber Bragg grating with a polyimide coating prepared in the embodiment of the present invention has a coating thickness of 15 μm and an outer diameter of 155 μm at the grating, which is thinner than the original coating diameter.

[0068] After the polyimide coating is completed, the fiber Bragg grating needs to be immediately immersed in a moisture-sensitive coating modification solution for modification. In the practice of the present invention, the modification treatment temperature is 20-30°C, the humidity is 40% RH to 60% RH, and the treatment time is 4-6 hours. By controlling the modification temperature, modification humidity, and modification time, the properties of the modification solution can be maintained stable and the modification effect can be optimized.

[0069] In step (3), the annealing temperature is 200-220°C for 2-4 hours. The annealing temperature of the polyimide material can affect its thermal expansion coefficient and water absorption rate. Within this annealing temperature range, the effect of thermal expansion on humidity expansion can be effectively reduced, the impact of the hysteresis effect at high temperatures can be reduced, and internal stress can be eliminated to a certain extent.

[0070] In step (4), the stress release treatment is performed at a temperature of 20°C to 30°C, at a humidity of 40% RH to 60% RH, and for a duration of at least 20 hours. By controlling the temperature, humidity, and duration of the stress release treatment, the stress introduced during the processing can be fully released.

[0071] The following examples illustrate a fiber Bragg grating humidity sensor and its preparation method according to embodiments of the present invention. In these examples, the specific preparation process of the polyimide-coated fiber Bragg grating can be prepared according to the above method and will not be repeated here. The following examples only investigate the effects of humidity-sensitive coating modification solutions with different material contents on the performance of the prepared fiber Bragg grating humidity sensor.

[0072] Example 1

[0073] Embodiment 1 of the present invention provides a fiber Bragg grating humidity sensor and a method for preparing the same, comprising the following steps:

[0074] (a) Dissolve 0.105 g of potassium acetate in 3.895 g of deionized water and stir magnetically for 5 min to completely dissolve.

[0075] (b) 3 g of a 35 wt% polyacrylic acid (PAA, 523925) aqueous solution was sonicated for 5 min to obtain a uniformly dispersed polymer solution;

[0076] (c) The solutions obtained in steps (a) and (b) were placed in a beaker and mixed, and magnetically stirred at 600 rpm for 15 minutes to obtain a moisture-sensitive coating layer modified solution.

[0077] (d) immersing the polyimide-coated fiber Bragg grating into 10 mL of the moisture-sensitive coating modification solution of step (c) for modification; the modification treatment is: treating in a clean room experimental environment at 25° C. and 60% RH for 4 hours;

[0078] (e) annealing the modified fiber Bragg grating in an oven at 220° C. for 4 h to obtain a cured polyimide-coated grating;

[0079] (f) The cured polyimide-coated grating was placed in a clean room experimental environment at 25°C and 60% RH for 24 hours. After the stress was released, a fiber Bragg grating humidity sensor was obtained.

[0080] Example 2

[0081] The difference from Example 1 is that the amount of potassium acetate used in step (a) is 0.055 g, the amount of polyacrylic acid used in step (b) is 1.57 g, and other process conditions are the same.

[0082] Example 3

[0083] The difference from Example 1 is that the amount of potassium acetate used in step (a) is 0.19 g, the amount of polyacrylic acid used in step (b) is 5.42 g, and other process conditions are the same.

[0084] Comparative Example

[0085] The difference from Example 1 is that the polyimide-coated fiber Bragg grating is directly subjected to annealing and stress release treatments without being treated with a moisture-sensitive coating layer modification solution.

[0086] The fiber Bragg grating humidity sensors prepared in Example 1 of the present invention and the comparative example were subjected to high and low temperature cycle treatment for 6 times. The specific treatment process is as follows:

[0087] (1) The sensors prepared in Example 1 of the present invention and the comparative example were respectively placed in a temperature and humidity test chamber (including a humidity generator (L-LRH) and a chilled mirror precision dew point meter) and installed. The sensors were initially tested under normal laboratory atmospheric conditions.

[0088] (2) Adjust the test chamber temperature to A (the initial value of A is -30°C) and maintain it for 10 minutes until the test sample and the chamber reach a stable temperature state and maintain this temperature state for 30 minutes;

[0089] (3) Increase the temperature to 160°C and maintain it at this temperature for 30 minutes;

[0090] (4) Repeat steps (2) and (3) 6 times;

[0091] (5) Adjust the temperature in the test chamber back to room temperature and maintain it for 1 hour. Then open the door and allow the sensor to return to a stable state under normal laboratory atmospheric conditions.

[0092] After the high and low temperature cycle treatment, the end faces of the fiber Bragg grating humidity sensor prepared in Example 1 of the present invention and the comparative example were subjected to SEM analysis. The SEM results are as follows: Figure 1 、 Figure 2 shown. Figure 1This is the fiber end face diagram of the fiber Bragg grating humidity sensor prepared in comparative example. Figure 2 This is a fiber end face diagram of the fiber Bragg grating humidity sensor prepared in Example 1 of the present invention. Figure 1 There is obvious separation between the polyimide coating and the optical fiber cladding. Figure 2 The optical fiber cladding and the polyimide coating are stably bonded, indicating that the stability of the direct bonding between the polyimide coating and the optical fiber cladding can be significantly enhanced after the polyimide-coated fiber Bragg grating is treated with the moisture-sensitive coating modification solution.

[0093] The humidity sensitivity coefficient of the fiber Bragg grating humidity sensor prepared in Example 1 of the present invention was tested between 30% RH and 100% RH. The testing process was as follows:

[0094] (1) The sensors prepared in Example 1 of the present invention and the comparative example were respectively placed in a temperature and humidity test chamber and installed, and the sensors were initially tested under normal laboratory atmospheric conditions;

[0095] (2) Adjust the test chamber temperature to 25°C and humidity B (the initial value of B is 25% RH) for 15 minutes to allow the sensor and the chamber to reach a stable temperature and humidity state. Record the channel center wavelength value stored in the optical performance monitoring module during the stable state period.

[0096] (3) Keep the temperature constant, increase the humidity to 35% RH, repeat step (2) and record the results;

[0097] (4) Keep the temperature constant, increase the humidity to 45% RH, repeat step (2) and record the results;

[0098] (5) Keep the temperature constant until the humidity reaches 55% RH, repeat step (2) and record the results;

[0099] (6) Keep the temperature constant until the humidity reaches 55% RH, repeat step (2) and record the results;

[0100] (7) Keep the temperature constant until the humidity reaches 65% RH, repeat step (2) and record the results;

[0101] (8) Keep the temperature constant until the humidity reaches 75% RH, repeat step (2) and record the results;

[0102] (9) Keep the temperature constant until the humidity reaches 85% RH, repeat step (2) and record the results;

[0103] (10) Keep the temperature constant until the humidity reaches 95% RH, repeat step (2) and record the results;

[0104] (11) Adjust the temperature in the test chamber to normal temperature and humidity, maintain it for 1 hour, then open the door to allow the sensor to return to a stable state under normal laboratory atmospheric conditions.

[0105] The test results are as follows Figure 3 As shown, from Figure 3 As can be seen in the figure, the sensitivity of the fiber Bragg grating humidity sensor prepared in Example 1 of the present invention is 3.9 pm / % RH, with a goodness of fit of 0.9989. Testing of the humidity sensitivity coefficient of the fiber Bragg grating humidity sensor prepared in the comparative example of the present invention between 30% RH and 100% RH revealed that, with the same polyimide coating thickness, the sensitivity coefficient of the fiber Bragg grating humidity sensor prepared in Example 1 of the present invention is approximately four times that of a conventional polyimide-coated humidity sensor (i.e., the humidity sensor prepared in the comparative example).

[0106] The long-term stability of the fiber Bragg grating humidity sensor prepared in Example 1 of the present invention was tested between 30% RH and 100% RH. The testing process was as follows:

[0107] 1) The sensor prepared in Example 1 of the present invention was placed in a temperature and humidity test chamber and installed, and the test piece was initially tested under normal laboratory atmospheric conditions;

[0108] (2) Adjust the test chamber temperature to 25°C and humidity B (the initial value of B is 35% RH) for 15 minutes to allow the test sample and the chamber to reach a stable temperature and humidity state. Record the chamber temperature and humidity values ​​and the sensor demodulated humidity values ​​every 1 minute for 5 minutes. Save the central wavelength values ​​of all channels stored in the optical performance monitoring module during the stable state period.

[0109] (3) Keep the temperature constant, increase the humidity to 55% RH, repeat step (2) and record the results;

[0110] (4) Keep the temperature constant, increase the humidity to 75% RH, repeat step (2) and record the results;

[0111] (5) Keep the temperature constant until the humidity reaches 95% RH, repeat step (2) and record the results;

[0112] (6) Adjust the temperature in the test chamber to normal temperature and humidity, maintain it for 1 hour, then open the door to allow the sensor to return to a stable state under normal laboratory atmospheric conditions;

[0113] The test results are as follows Figure 4 As shown, from Figure 4 It can be seen that the response value deviations of the fiber Bragg grating humidity sensor prepared in Example 1 of the present invention are all within 1% RH, which shows that the fiber Bragg grating humidity sensor prepared in Example 1 of the present invention has good stability in high humidity measurement and a long service life.

[0114] In this embodiment of the present invention, a moisture-sensitive coating modification solution is used to modify the unannealed polyimide coating, enhancing the bonding stability between the optical fiber cladding and the polyimide coating. This prevents the polyimide coating from separating from the optical fiber cladding even when the sensor is exposed to high humidity and high temperature environments for extended periods of time, extending the sensor's service life and improving its stability in high-humidity measurement environments. Furthermore, the sensor's sensitivity can be enhanced.

[0115] The above embodiments merely illustrate several implementations of the present invention, and while their descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art would be able to make numerous modifications and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be determined by the appended claims.

Claims

1. A method for preparing a fiber Bragg grating humidity sensor, characterized in that: The following steps are involved: preparing a moisture-sensitive coating modification solution containing potassium acetate and polyacrylic acid; Immersing the polyimide-coated fiber Bragg grating into the moisture-sensitive coating modification solution for surface modification; annealing the surface-modified fiber Bragg grating to obtain a cured polyimide-coated grating; performing stress release treatment on the cured polyimide coated grating to obtain a fiber Bragg grating humidity sensor; In the moisture-sensitive coating layer modification solution, the concentration of the polyacrylic acid is 12 wt% to 20 wt%; The concentration of potassium acetate is 1 wt% to 2 wt%.

2. The method for preparing a fiber Bragg grating humidity sensor according to claim 1, wherein: The surface modification treatment is performed at a temperature of 20 to 30° C., a humidity of 40% RH to 60% RH, and a treatment time of 4 to 6 hours.

3. The method for preparing a fiber Bragg grating humidity sensor according to claim 1, wherein: The polyimide-coated fiber Bragg grating is prepared by the following method: immersing the pretreated fiber Bragg grating in a polyimide slurry, forming a polyimide coating layer on the surface of the fiber Bragg grating by pulling the coating and heating and curing; the pretreatment is to perform hydroxylation treatment and silane coupling agent modification treatment on the surface of the fiber Bragg grating.

4. The method for preparing a fiber Bragg grating humidity sensor according to claim 3, wherein: The pretreatment is performed by the following method: firstly placing the fiber Bragg grating with the coating removed in a piranha solution for surface hydroxylation treatment, and then placing the fiber Bragg grating in a silane coupling agent solution for silane coupling agent modification treatment.

5. The method for preparing a fiber Bragg grating humidity sensor according to claim 4, wherein: The silane coupling agent is aminopropyltriethoxysilane.

6. The method for preparing a fiber Bragg grating humidity sensor according to any one of claims 1 to 5, characterized in that: The annealing treatment is performed at a temperature of 200 to 220° C. and for a time of 2 to 4 hours.

7. The method for preparing a fiber Bragg grating humidity sensor according to any one of claims 1 to 5, characterized in that: The stress release treatment is performed at a temperature of 20 to 30° C., a humidity of 40% RH to 60% RH, and a treatment time of more than 20 hours.

8. A fiber Bragg grating humidity sensor, characterized in that: The method is obtained by the preparation method according to any one of claims 1 to 7.

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