A liquid level sensor and a method of constructing the same, a liquid level detection system and a detection method

Abstract

The application belongs to the technical field of liquid level detection, and discloses a liquid level sensor, a construction method thereof, a liquid level detection system and a detection method. The liquid level sensor comprises a first single-mode optical fiber, a capillary optical fiber and a second single-mode optical fiber which are sequentially fused. The tail end of the second single-mode optical fiber is plated with a reflective film. An F-P cavity string is arranged on the capillary optical fiber. The F-P cavity string comprises a plurality of identical F-P cavities. The application can be applied to complex environments such as high-risk, high-pollution and high-temperature environments, and improves the sensitivity and detection accuracy. Moreover, the application has a simple structure and low cost.

Description

Technical Field

[0001] This invention belongs to the field of liquid level detection technology, and more specifically, relates to a liquid level sensor and its construction method, a liquid level detection system and detection method. Background Technology

[0002] Liquid level sensors are widely used in various fields of life, especially in industrial production and environmental monitoring. There are many types of liquid level sensors available, which can be classified according to whether they come into contact with the liquid, including float-type liquid level sensors, capacitive liquid level sensors, magnetostrictive liquid level sensors, and ultrasonic liquid level sensors. Float-type liquid level sensors are designed based on Archimedes' principle and the principle of magnetic coupling. They consist of a float, spring, magnetic chamber, and indicator. The indicator uses a magnetic induction element and a transmission device to indicate the liquid level. Capacitive liquid level sensors are a type of variable dielectric capacitive liquid level sensor that utilizes the change in capacitance caused by changes in the surface of the measured medium, converting the measured non-electrical quantity into a change in capacitance to measure the liquid level. Magnetostrictive liquid level sensors calculate the accurate location of the intersection point by detecting the strain pulse signal generated when two different magnetic fields intersect, with a measurement range of 50–3000 mm. Ultrasonic liquid level sensors are sensors developed using the properties of ultrasound and are composed of piezoelectric crystals. These sensors contain sophisticated electronic components and are well-packaged. It exhibits superior performance under favorable operating conditions, but struggles to perform effectively in complex environments, such as those involving flammable compounds or contaminated biochemical solutions.

[0003] Fiber optic liquid level sensors offer advantages such as remote control, small size, corrosion resistance, and resistance to electromagnetic interference, making them suitable for application in complex environments such as high-risk and high-pollution environments. While numerous studies have been conducted on fiber optic liquid level sensors, further improving their sensitivity and detection accuracy remains a crucial research topic in this field. Summary of the Invention

[0004] This invention provides a liquid level sensor and its construction method, a liquid level detection system and detection method, thereby solving the problems of low sensitivity and low detection accuracy of existing fiber optic liquid level sensors.

[0005] In a first aspect, the present invention provides a liquid level sensor, comprising: a first single-mode optical fiber, a capillary optical fiber, and a second single-mode optical fiber; a first end of the capillary optical fiber is fused to the first single-mode optical fiber, a second end of the capillary optical fiber is fused to the second single-mode optical fiber, and a reflective film is coated on the tail end of the second single-mode optical fiber; a FP cavity string is disposed on the capillary optical fiber, the FP cavity string comprising a plurality of FP cavities, the plurality of FP cavities having the same structure and the same size.

[0006] Preferably, the coating layers are removed from the first single-mode fiber, the capillary fiber, and the second single-mode fiber, and the depth of the FP cavity is the difference between the cladding radius and the core radius of the capillary fiber.

[0007] Preferably, the capillary fiber, the first single-mode fiber, and the second single-mode fiber all have the same outer diameter, ranging from 124.3 to 125.7 μm; the first single-mode fiber and the second single-mode fiber have the same inner diameter, ranging from 8.5 to 9.7 μm; the capillary fiber has an inner diameter of 5 to 50 μm and a length of 9800 to 10200 μm; each FP cavity has a cavity length of 11 to 15 μm, the cavity spacing between two adjacent FP cavities is 520 to 560 μm, and the cavity width of each FP cavity is 28 to 32 μm.

[0008] Preferably, the liquid level sensor further includes: a fixing device; the fixing device is used to fix the straightened fusion splice structure, the fusion splice structure being the structure obtained by sequentially fusing the first single-mode optical fiber, the capillary optical fiber, and the second single-mode optical fiber.

[0009] Secondly, the present invention provides a method for constructing the above-mentioned liquid level sensor, comprising the following steps: simulating the structural parameters of a capillary optical fiber using optical simulation software to obtain simulation parameter information that meets preset conditions; the structural parameters include the inner diameter and length of the capillary optical fiber, the cavity spacing between two adjacent FP cavities, and the cavity length and cavity width of each FP cavity; selecting a capillary optical fiber with a suitable inner diameter and length based on the simulation parameter information, depositing a reflective film at the tail end of a second single-mode optical fiber, and sequentially fusing the first single-mode optical fiber, the capillary optical fiber, and the second single-mode optical fiber; and fabricating an FP cavity string on the capillary optical fiber based on the simulation parameter information to obtain the liquid level sensor.

[0010] Preferably, when simulating the structural parameters of the capillary optical fiber, the inner diameter and length of the capillary optical fiber are first determined based on the simulation results of the inner diameter, length, and normalized reflected optical power of the capillary optical fiber; secondly, the cavity length of each FP cavity and the cavity spacing between two adjacent FP cavities are determined based on the simulation results of the cavity length, cavity spacing, and normalized reflected optical power of the capillary optical fiber; then, the cavity width of each FP cavity is determined based on the simulation results of the cavity width and normalized reflected optical power of the capillary optical fiber; finally, the number of FP cavities contained in the FP cavity string is calculated based on the length of the capillary optical fiber, the cavity length of each FP cavity, and the cavity spacing between two adjacent FP cavities.

[0011] Before processing the capillary fiber, the structure obtained by sequentially fusing the first single-mode fiber, the capillary fiber, and the second single-mode fiber is straightened and fixed on a fixing device. The fixing device is placed on an optical platform, and the FP cavity string is processed on the capillary fiber using a femtosecond laser.

[0012] Thirdly, the present invention provides a liquid level detection system, comprising: a light source, a circulator, a spectrometer, a data processing device, and the aforementioned liquid level sensor; the light source is connected to the input end of the circulator, the first output end of the circulator is connected to the first single-mode optical fiber of the liquid level sensor, the liquid level sensor is inserted into the liquid to be tested in the liquid container, the second output end of the circulator is connected to the input end of the spectrometer, and the output end of the spectrometer is connected to the data processing device.

[0013] Fourthly, the present invention provides a liquid level detection method, comprising the following steps:

[0014] Step 1: Insert the liquid level sensor into the container of the liquid to be tested, connect the light source to the input end of the circulator, connect the first output end of the circulator to the first single-mode optical fiber of the liquid level sensor, connect the second output end of the circulator to the input end of the spectrometer, and connect the output end of the spectrometer to the data processing device.

[0015] Step 2: Turn on the light source. The signal light emitted by the light source enters the first single-mode fiber of the liquid level sensor through the circulator, is incident on the FP cavity string of the capillary fiber of the liquid level sensor, and returns after passing through the reflective film of the second single-mode fiber of the liquid level sensor. The reflected light is then output to the spectrometer after passing through the circulator.

[0016] The liquid to be tested is gradually and quantitatively added to the container to gradually increase the liquid level. During the process of gradually increasing the liquid level, the spectral data of the spectrometer is recorded. The data processing device extracts spectral feature information based on the spectral data. The data processing device obtains a database as a detection standard based on the information including the liquid level data and its corresponding spectral feature information.

[0017] Step 3: Insert the liquid level sensor into the liquid to be tested in the liquid container to obtain the detection spectral feature information, and compare the detection spectral feature information with the database to obtain the detection liquid level height.

[0018] Preferably, at different solubilities, the process of obtaining the database in step 2 is performed to obtain a two-dimensional database, which includes liquid level height data and spectral feature information of the solution to be tested at different concentrations; in step 3, the detection spectral feature information and the detection solubility are compared with the two-dimensional database to obtain the detection liquid level height at the detection concentration.

[0019] Preferably, the process of obtaining the database in step 2 is performed at different temperatures and different solubilities to obtain a three-dimensional database. The three-dimensional database includes liquid level height data and spectral feature information of the test solution at different temperatures and concentrations. In step 3, the detection spectral feature information, detection temperature, and detection solubility are compared with the three-dimensional database to obtain the detection liquid level height at the detection temperature and the detection concentration.

[0020] One or more technical solutions provided in this invention have at least the following technical effects or advantages:

[0021] The liquid level sensor provided by this invention includes a first single-mode optical fiber, a capillary optical fiber, and a second single-mode optical fiber fused together in sequence. The tail end of the second single-mode optical fiber is coated with a reflective film. A FP cavity string is arranged on the capillary optical fiber, and the FP cavity string includes several identical FP cavities. The liquid level sensor provided by this invention is a fiber optic sensor, and therefore can be applied to complex environments such as high-risk, high-pollution, and high-temperature environments. Compared with existing fiber optic liquid level sensors, this invention has higher sensitivity and higher liquid level detection resolution because it connects multiple identical FP cavities in series. In addition, this invention is made by fusing single-mode optical fiber and capillary optical fiber, which has a simple structure and low cost. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the structure of a liquid level sensor provided in Embodiment 1 of the present invention;

[0023] Figure 2 The simulation results of the inner diameter, length and normalized reflected light power of the capillary optical fiber in the construction method of the liquid level sensor provided in Embodiment 2 of the present invention are shown in the figure.

[0024] Figure 3 The simulation results of the cavity length, cavity spacing and normalized reflected light power of the capillary fiber FP cavity in the liquid level sensor construction method provided in Embodiment 2 of the present invention are shown in the figure.

[0025] Figure 4 The simulation results of the cavity width of the capillary fiber FP cavity and the normalized reflected light power in the liquid level sensor construction method provided in Embodiment 2 of the present invention are shown in the figure.

[0026] Figure 5This is a schematic diagram of a liquid level detection system provided in Embodiment 3 of the present invention. Detailed Implementation

[0027] To better understand the above technical solutions, the following will provide a detailed explanation of the technical solutions in conjunction with the accompanying drawings and specific implementation methods.

[0028] Example 1:

[0029] Example 1 provides a liquid level sensor, see [link to example]. Figure 1 The assembly includes: a first single-mode fiber 110, a capillary fiber 120, a second single-mode fiber 130, and a fixing device 140; the first end of the capillary fiber 120 is fused to the first single-mode fiber 110, the second end of the capillary fiber 120 is fused to the second single-mode fiber 130, and the tail end of the second single-mode fiber 130 (the end not fused to the capillary fiber) is coated with a reflective film; an FP cavity string 121 is provided on the capillary fiber 120, the FP cavity string 121 includes a plurality of FP cavities, and the plurality of FP cavities have the same structure and the same size; the fixing device 140 is used to fix the straightened fusion structure, the fusion structure being the structure obtained by sequentially fusion of the first single-mode fiber 110, the capillary fiber 120, and the second single-mode fiber 130.

[0030] In this configuration, the coating layers of the first single-mode fiber 110, the capillary fiber 120, and the second single-mode fiber 130 are all removed, and the depth of the FP cavity is the difference between the cladding radius of the capillary fiber 120 and the core radius of the capillary fiber.

[0031] The capillary fiber 120, the first single-mode fiber 110, and the second single-mode fiber 130 all have the same outer diameter, ranging from 124.3 to 125.7 μm. The first single-mode fiber 110 and the second single-mode fiber 130 have the same inner diameter, ranging from 8.5 to 9.7 μm. The capillary fiber 120 has an inner diameter of 5 to 50 μm and a length of 9800 to 10200 μm. Each FP cavity has a cavity length of 11 to 15 μm, a cavity spacing of 520 to 560 μm between two adjacent FP cavities, and a cavity width of 28 to 32 μm.

[0032] The liquid level sensor provided in Example 1 is composed of a first single-mode fiber, a capillary fiber, and a second single-mode fiber sequentially fused together. The second single-mode fiber is coated with a high-reflectivity film at its tail end, and the capillary fiber has several FP cavities. Therefore, when constructing a liquid level detection system using this sensor, the light source enters through the first single-mode fiber, is incident on the FP cavity string of the capillary fiber, then enters the second single-mode fiber and the high-reflectivity film before returning, forming a specific interference spectrum. Since multiple FP cavities exist within the capillary fiber, the spectral characteristics change as the solution sequentially submerges the FP cavity string. After the spectral characteristics are extracted by the data processing device and matched one-to-one with the liquid level gauge scale, the liquid level gauge function can be realized. The liquid level sensor provided in Example 1 is small in size, low in cost, highly sensitive, and has high detection accuracy.

[0033] Example 2:

[0034] Example 2 provides a method for constructing a liquid level sensor as described in Example 1, comprising the following steps: simulating the structural parameters of a capillary optical fiber using optical simulation software to obtain simulation parameter information that meets preset conditions; the structural parameters include the inner diameter and length of the capillary optical fiber, the cavity spacing between two adjacent FP cavities, and the cavity length and cavity width of each FP cavity; selecting a capillary optical fiber with a suitable inner diameter and length based on the simulation parameter information, depositing a reflective film at the tail end of a second single-mode optical fiber, and sequentially fusing the first single-mode optical fiber, the capillary optical fiber, and the second single-mode optical fiber; and fabricating an FP cavity string on the capillary optical fiber based on the simulation parameter information to obtain the liquid level sensor.

[0035] Specifically, the simulation of the structural parameters of the capillary optical fiber includes the following steps:

[0036] (1) First, based on the simulation results of the inner diameter, length and normalized reflected light power of the capillary fiber, the inner diameter and length of the capillary fiber are determined.

[0037] See Figure 2During simulation, the normalized power of capillary fibers with different inner diameters (d) at the same length was displayed using the length of the capillary fiber as the horizontal axis and the normalized power as the vertical axis. When d = 20 μm, the normalized power was relatively large and stable. While the normalized power was large at d = 30 μm and a length of 10000 μm, it was not adopted due to processing errors and other external environmental factors. The final selected capillary fiber had an outer diameter of 125 μm and an inner diameter of 20 μm. The designed length was 10000 μm, and considering the influence of actual fusion splicing errors, the actual fusion splicing length ranged from 9800 to 10200 μm. The first and second single-mode fibers were of the same type, using G.652 fiber with a low water peak, an outer diameter range of 124.3–125.7 μm, and an inner diameter range of 8.5–9.7 μm.

[0038] (2) Secondly, based on the simulation results of the cavity length, cavity spacing and normalized reflected light power of the capillary fiber, the cavity length of each FP cavity and the cavity spacing between two adjacent FP cavities are determined.

[0039] See Figure 3 When the cavity spacing between two adjacent FP cavities (i.e. Figure 3 When the distance between the FP cavities is 540 μm and the cavity length of each FP cavity is 13 μm, the normalized power is relatively high. Considering the influence of processing accuracy, the actual processed cavity spacing ranges from 520 to 560 μm, and the actual processed cavity length ranges from 11 to 15 μm. The etching depth of the FP cavity is the difference between the fiber cladding radius and the core radius, i.e., 52.5 μm. The actual processed FP cavity etching depth ranges from 51.85 to 53.85 μm.

[0040] (3) Then, based on the simulation results of the cavity width of the capillary fiber and the normalized reflected light power, the cavity width of each FP cavity is determined.

[0041] See Figure 4 When the cavity width of each FP cavity is 30 μm, the normalized power is relatively high. Considering the influence of machining accuracy, the actual machined cavity width ranges from 28 to 32 μm.

[0042] (4) Finally, the number of FP cavities contained in the FP cavity string is calculated based on the length of the capillary fiber, the cavity length of each FP cavity and the cavity spacing between two adjacent FP cavities.

[0043] To prevent sensor bending or fiber breakage, before processing the capillary fiber, the structure obtained by sequentially fusion splicing the first single-mode fiber (with the coating removed), the capillary fiber, and the second single-mode fiber can be straightened and fixed on a fixing device (e.g., the fused structure can be bonded to a glass slide). Then, the fixing device is placed on an optical platform, and the FP cavity string (i.e., several FP cavities with the same structural size and equal spacing) is processed on the capillary fiber using a femtosecond laser, making the interior of the capillary fiber connected to the external air medium. During processing, a stable light source output can be continuously provided using a light source, and spectral changes can be monitored using a spectrometer to achieve online FP cavity fabrication.

[0044] Example 3:

[0045] Example 3 provides a liquid level detection system, see [link to example]. Figure 5 The system includes: a light source 210, a circulator 220, a spectrometer 240, a data processing device 250, and a liquid level sensor 230 as described in Example 1; the light source 210 is connected to the input end of the circulator 220, the first output end of the circulator 220 is connected to the first single-mode optical fiber of the liquid level sensor 230, the liquid level sensor 230 is inserted into the liquid to be tested in the liquid container, the second output end of the circulator 220 is connected to the input end of the spectrometer 240, and the output end of the spectrometer 240 is connected to the data processing device 250.

[0046] The liquid level detection system provided in Example 3 can be used to realize liquid level detection, which will be described below through Example 4.

[0047] Example 4:

[0048] Example 4 provides a liquid level detection method, including the following steps:

[0049] Step 1: Insert the liquid level sensor into the container of the liquid to be tested, connect the light source to the input end of the circulator, connect the first output end of the circulator to the first single-mode optical fiber of the liquid level sensor, connect the second output end of the circulator to the input end of the spectrometer, and connect the output end of the spectrometer to the data processing device.

[0050] Step 2: Turn on the light source. The signal light emitted by the light source enters the first single-mode fiber of the liquid level sensor through the circulator, is incident on the FP cavity string of the capillary fiber of the liquid level sensor, and returns after passing through the reflective film of the second single-mode fiber of the liquid level sensor. The reflected light is then output to the spectrometer after passing through the circulator.

[0051] The liquid to be tested is gradually and quantitatively added to the container to gradually increase the liquid level. During the process of gradually increasing the liquid level, the spectral data of the gradually changing spectrometer is recorded. The data processing device extracts spectral feature information based on the spectral data. The data processing device obtains a database as a detection standard based on the information including the liquid level data and its corresponding spectral feature information.

[0052] For example, a glass slide containing the first single-mode fiber, the capillary fiber, and the second single-mode fiber is placed in a precision graduated cylinder, and a quantitative solution is injected into the precision graduated cylinder using a micro-syringe.

[0053] Step 3: Insert the liquid level sensor into the liquid to be tested in the liquid container to obtain the detection spectral feature information, and compare the detection spectral feature information with the database to obtain the detection liquid level height.

[0054] Step 3 involves conducting a liquid level sensing experiment.

[0055] This invention can achieve liquid level height measurement in at least the following three situations.

[0056] (1) Execute the process of obtaining the database in step 2 and obtain a database in which the liquid level height data and spectral feature information correspond one-to-one; in step 3, compare the detected spectral feature information with the database to obtain the detected liquid level height.

[0057] For example, a broadband light source provides a stable light source to a capillary optical fiber. Pure water immerses the FP cavity, causing a change in the refractive index within the capillary fiber, which in turn alters the spectrum displayed by the spectrometer. As multiple FP cavities are successively submerged, their corresponding spectra also change. By recording the data from each change in the spectrometer, and using algorithms to extract spectral features, a one-to-one correspondence between the liquid level height and the spectral feature data is established. This data is then processed by a data processing device (e.g., a computer) to form a database. In this scenario, a liquid level gauge function can be implemented.

[0058] (2) At different solubilities, the process of obtaining the database in step 2 is performed respectively to obtain a two-dimensional database. The two-dimensional database includes the liquid level height data and spectral feature information of the solution to be tested at different concentrations. In step 3, the detection spectral feature information and the detection solubility are compared with the two-dimensional database to obtain the detection liquid level height at the detection concentration.

[0059] For example, at room temperature, a stable light source is provided to the capillary optical fiber using a broadband light source. A 1% NaCl solution is used to immerse the FP cavity, causing a change in the refractive index within the capillary fiber, which in turn alters the spectrum displayed by the spectrometer. As multiple FP cavities are successively submerged, the corresponding spectra also change. The data from each change in the spectrometer is recorded, and the spectral characteristics are extracted using an algorithm. The liquid level height corresponds one-to-one with the spectral characteristic data. The container and the liquid level sensor itself are then rinsed thoroughly with pure water. Next, a 2% NaCl solution is used to immerse the FP cavity, causing a change in the refractive index within the capillary fiber, which in turn alters the spectrum displayed by the spectrometer. As multiple FP cavities are successively submerged, the corresponding spectra also change. The data from each change in the spectrometer is recorded, and the spectral characteristics are extracted using an algorithm. The liquid level height corresponds one-to-one with the spectral characteristic data. This process continues, and after processing by a data processing device, a two-dimensional database is formed. In this case, a liquid level gauge function for liquids of different concentrations can be established.

[0060] (3) At different temperatures and different solubilities, the process of obtaining the database in step 2 is performed to obtain a three-dimensional database. The three-dimensional database includes the liquid level height data and spectral feature information of the solution to be tested at different temperatures and different concentrations. In step 3, the detection spectral feature information, detection temperature and detection solubility are compared with the three-dimensional database to obtain the detection liquid level height at the detection temperature and the detection concentration.

[0061] For example, at 0°C, a stable light source is provided to the capillary fiber using a broadband light source. A 1% NaCl solution is used to immerse the FP cavity, causing a change in the refractive index within the capillary fiber, which in turn alters the spectrum displayed by the spectrometer. As multiple FP cavities are successively submerged, the corresponding spectra also change. The data from each change in the spectrometer is recorded, and the spectral characteristics are extracted using an algorithm. The liquid level height corresponds one-to-one with the spectral characteristic data. The container and the liquid level sensor itself are then rinsed thoroughly with pure water. Next, a 2% NaCl solution is used to immerse the FP cavity, causing a change in the refractive index within the capillary fiber, which in turn alters the spectrum displayed by the spectrometer. As multiple FP cavities are successively submerged, the corresponding spectra also change. The data from each change in the spectrometer module is recorded, and the spectral characteristics are extracted using an algorithm. The liquid level height corresponds one-to-one with the spectral characteristic data, and so on. A data table of liquid level height and spectral characteristics for different concentrations of NaCl solution at 0°C is obtained.

[0062] The temperature was adjusted to 1℃, and a stable light source was provided to the capillary optical fiber using a broadband light source. A 1% NaCl solution was used to immerse the FP cavity, causing a change in the refractive index within the capillary optical fiber, which in turn altered the spectrum displayed by the spectrometer. As multiple FP cavities were successively submerged, the corresponding spectra also changed. The data from each change in the spectrometer was recorded, and the spectral characteristics were extracted using an algorithm. A one-to-one correspondence was established between the liquid level height and the spectral characteristic data. The container and the liquid level sensor itself were then thoroughly rinsed with pure water. Next, a 2% NaCl solution was used to immerse the FP cavity, causing a change in the refractive index within the capillary optical fiber, which in turn altered the spectrum displayed by the spectrometer. As multiple FP cavities were successively submerged, the corresponding spectra also changed. The data from each change in the spectrometer module was recorded, and the spectral characteristics were extracted using an algorithm. A one-to-one correspondence was established between the liquid level height and the spectral characteristics of NaCl solutions of different concentrations at 1℃. By changing the temperature and liquid concentration, the data was processed by a data processing device to form a three-dimensional database. In this case, a liquid level gauge function for liquids of different temperatures and concentrations can be established.

[0063] In summary, this invention is based on the principle of spectral interference, analyzes the one-to-one correspondence between spectral characteristic values ​​and liquid level height, and can measure liquid level height. The sensor is small in size, has high detection accuracy, and is made by fusion splicing single-mode optical fiber and capillary optical fiber, which has a simple structure and low cost.

[0064] Finally, it should be noted that the above specific embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to examples, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A liquid level sensor, characterized in that, include: A first single-mode fiber, a capillary fiber, and a second single-mode fiber; the first end of the capillary fiber is fused to the first single-mode fiber, the second end of the capillary fiber is fused to the second single-mode fiber, and the tail end of the second single-mode fiber is coated with a reflective film; a FP cavity string is provided on the capillary fiber, the FP cavity string includes a plurality of FP cavities, and the plurality of FP cavities have the same structure and the same size. The inner diameter of the capillary fiber is 5–50 µm, and the length of the capillary fiber is 9800–10200 µm; the cavity length of each FP cavity is 11–15 µm, the cavity spacing between two adjacent FP cavities is 520–560 µm, and the cavity width of each FP cavity is 28–32 µm. The number of FP cavities contained in the FP cavity string is calculated based on the length of the capillary fiber, the cavity length of each FP cavity, and the cavity spacing between two adjacent FP cavities.

2. The liquid level sensor according to claim 1, characterized in that, The coating layers of the first single-mode fiber, the capillary fiber, and the second single-mode fiber are all removed, and the etching depth of the FP cavity is the difference between the cladding radius and the core radius of the capillary fiber.

3. The liquid level sensor according to claim 1, characterized in that, The capillary fiber, the first single-mode fiber, and the second single-mode fiber all have the same outer diameter, ranging from 124.3 to 125.7 µm; the first single-mode fiber and the second single-mode fiber have the same inner diameter, ranging from 8.5 to 9.7 µm.

4. The liquid level sensor according to claim 1, characterized in that, Also includes: A fixing device; the fixing device is used to fix the straightened fusion splice structure, which is the structure obtained by sequentially fusing the first single-mode fiber, the capillary fiber and the second single-mode fiber.

5. A method for constructing a liquid level sensor as described in any one of claims 1-4, characterized in that, Includes the following steps: The structural parameters of the capillary optical fiber were simulated using optical simulation software to obtain simulation parameter information that meets the preset conditions. The structural parameters include the inner diameter and length of the capillary fiber, the cavity spacing between two adjacent FP cavities, and the cavity length and cavity width of each FP cavity. Based on the simulation parameter information, a capillary fiber with a suitable inner diameter and length is selected, a reflective film is deposited at the tail end of the second single-mode fiber, and the first single-mode fiber, the capillary fiber, and the second single-mode fiber are sequentially fused together. Based on the simulation parameter information, an FP cavity string is fabricated on the capillary fiber to obtain the liquid level sensor.

6. The method for constructing a liquid level sensor according to claim 5, characterized in that, When simulating the structural parameters of the capillary fiber, firstly, based on the simulation results of the inner diameter, length, and normalized reflected optical power of the capillary fiber, the inner diameter and length of the capillary fiber are determined; secondly, based on the simulation results of the cavity length, cavity spacing, and normalized reflected optical power of the capillary fiber, the cavity length of each FP cavity and the cavity spacing between two adjacent FP cavities are determined; then, based on the simulation results of the cavity width and normalized reflected optical power of the capillary fiber, the cavity width of each FP cavity is determined; finally, based on the length of the capillary fiber, the cavity length of each FP cavity, and the cavity spacing between two adjacent FP cavities, the number of FP cavities contained in the FP cavity string is calculated. Before processing the capillary fiber, the structure obtained by sequentially fusing the first single-mode fiber, the capillary fiber, and the second single-mode fiber is straightened and fixed on a fixing device. The fixing device is placed on an optical platform, and the FP cavity string is processed on the capillary fiber using a femtosecond laser.

7. A liquid level detection system, characterized in that, include: The device comprises a light source, a circulator, a spectrometer, a data processing device, and a liquid level sensor as described in any one of claims 1-4; the light source is connected to the input end of the circulator, the first output end of the circulator is connected to the first single-mode optical fiber of the liquid level sensor, the liquid level sensor is inserted into the liquid to be tested in the container of the liquid to be tested, the second output end of the circulator is connected to the input end of the spectrometer, and the output end of the spectrometer is connected to the data processing device.

8. A liquid level detection method, characterized in that, Includes the following steps: Step 1: Insert the liquid level sensor as described in any one of claims 1-4 into the container of the liquid to be measured, connect the light source to the input end of the circulator, connect the first output end of the circulator to the first single-mode optical fiber of the liquid level sensor, connect the second output end of the circulator to the input end of the spectrometer, and connect the output end of the spectrometer to the data processing device. Step 2: Turn on the light source. The signal light emitted by the light source enters the first single-mode fiber of the liquid level sensor through the circulator, is incident on the FP cavity string of the capillary fiber of the liquid level sensor, and returns after passing through the reflective film of the second single-mode fiber of the liquid level sensor. The reflected light is then output to the spectrometer after passing through the circulator. The liquid to be tested is gradually and quantitatively added to the container to gradually increase the liquid level. During the process of gradually increasing the liquid level, the spectral data of the spectrometer is recorded. The data processing device extracts spectral feature information based on the spectral data. The data processing device obtains a database as a detection standard based on the information including the liquid level data and its corresponding spectral feature information. Step 3: Insert the liquid level sensor into the liquid to be tested in the liquid container to obtain the detection spectral feature information, and compare the detection spectral feature information with the database to obtain the detection liquid level height.

9. The liquid level detection method according to claim 8, characterized in that, At different solubilities, the process of obtaining the database in step 2 is performed to obtain a two-dimensional database, which includes liquid level height data and spectral feature information of the test solution at different concentrations; in step 3, the detection spectral feature information and detection solubility are compared with the two-dimensional database to obtain the detection liquid level height at the detection concentration.

10. The liquid level detection method according to claim 8, characterized in that, At different temperatures and concentrations, the process of obtaining the database in step 2 is performed to obtain a three-dimensional database, which includes liquid level height data and spectral feature information of the test solution at different temperatures and concentrations. In step 3, the detection spectral feature information, detection temperature, and detection concentration are compared with the three-dimensional database to obtain the detection liquid level height at the detection temperature and the detection concentration.

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