Preparation method of semicircular spiral plastic optical fiber sensor and liquid concentration measuring device

Abstract

The invention provides a preparation method of a semicircular spiral plastic optical fiber sensor and a liquid concentration measuring device, and relates to the technical field of optical fiber sensing, the method comprises the following steps: pouring on a sensing area with a set position and length by using an ethyl acetate solution, and removing a cladding; flattening the plastic optical fiber sensing area with the cladding removed to obtain a flat plastic optical fiber sensing area; twisting the flat plastic optical fiber sensing area to obtain a spiral plastic optical fiber probe of which the thread number N is equal to 4; the sensing area of the spiral plastic optical fiber probe is wound on a cylindrical metal rod with the set radius and the temperature of 100-110 DEG C, after the cylindrical metal rod is naturally cooled to the room temperature, the shaped semicircular spiral plastic optical fiber probe is taken down from the cylindrical metal rod, and the plastic optical fiber sensor of the semicircular spiral structure is obtained. The sensor designed by the invention has the advantages of high resolution, simple preparation process and low cost.

Description

Technical Field

[0001] The invention relates to the technical field of optical fiber sensing, and in particular to a method for preparing a semicircular spiral plastic optical fiber sensor and a liquid concentration measuring device. Background Art

[0002] With the rapid development of optical fiber technology and optical communication technology, optical fiber sensors, as an emerging sensing technology, have shown broad application prospects in many fields such as national defense, aerospace, energy and environmental protection, medicine and health, and household appliances due to their unique advantages. Optical fiber sensors use light as an information carrier and modulate the properties of light (such as light intensity, wavelength, frequency, phase, polarization state, etc.) with the help of the external environment to be measured. After subsequent processing and analysis of the optical signal, they can accurately obtain information about the physical quantity to be measured. This technology not only has many advantages such as lightness, sensitivity, anti-electromagnetic interference, high sensitivity, small size, and remote operation, but also realizes high-performance sensing of dozens of physical quantities such as temperature, pressure, flow, displacement, liquid level, concentration, etc., becoming a key indicator of a country's scientific and technological level.

[0003] In the field of liquid concentration measurement, with the increasing awareness of food safety and environmental protection among the public, the demand for concentration detection of liquids such as drinking water and pesticides has become more urgent. Therefore, the development of high-performance liquid concentration measurement fiber optic sensors has become a hot topic in the current academic community. At present, common liquid concentration fiber optic sensors include surface plasmon resonance type, long period grating type, corroded Bragg grating type and tilted Bragg grating type. Most of these sensors use single-mode quartz optical fiber, but the quartz material is hard and has poor toughness, making it difficult to modify and transform the structure, and requires expensive test system support. In contrast, plastic optical fiber has received widespread attention in the field of sensing technology due to its soft texture, strong plasticity, and easy fiber structure modification and transformation. In particular, the large core diameter of commercial plastic optical fiber makes it extremely easy to couple and align it with the light source and detector, making it an ideal material for realizing a simple and low-cost liquid concentration sensor.

[0004] However, existing optical fiber sensors for measuring liquid concentration still have many shortcomings. For example, the liquid concentration measurement method using a U-shaped optical fiber sensor [1], although the structure is simple, the optical fiber made of quartz material has high requirements during the manufacturing process, is easy to break, and has strict restrictions on the bending radius, which affects the sensitivity and practicality. Although the concentration sensor based on single-ended long-period fiber grating [2] has fast response, high accuracy, corrosion resistance, anti-electromagnetic interference, and is sensitive to changes in the refractive index and concentration of the surrounding medium, it is limited by the refractive index sensitivity characteristics and its measurement accuracy and convenience need to be improved. Although the liquid mass fraction sensor based on helical optical fiber [3] can achieve rapid measurement, the manufacturing conditions of the optical fiber probe are high and the sensitivity improvement is not obvious. Although the helical plastic optical fiber surface plasmon resonance refractive index sensor [4] can be used to measure liquid concentration, the magnetron sputtering coating process is complex and costly, and the sensor is directly placed in the liquid, which affects the experimental repeatability and system error.

[0005] In summary, the existing optical fiber sensors for measuring liquid concentration have limitations to varying degrees in terms of material selection, manufacturing process, measurement accuracy and convenience. Therefore, it is urgent to design a liquid concentration measurement device with low cost, high accuracy, simple manufacturing and easy promotion to meet the growing demand for liquid concentration detection.

[0006] References:

[0007] [1] Zhang Na, Fan Naimei, Chen Yan, et al. Research on liquid concentration measurement system of U-type optical fiber sensor [J]. Optical Technology, 2011, 37(4):4.

[0008] [2] Guan Shouhua, Yu Qingxu, Song Shide. Concentration sensor based on single-ended long period fiber grating [J]. Journal of Sensor Technology, 2005, 18(3):4.

[0009] [3] Xu Mingyuan, Huang Wanling, Qin Yan, et al. Liquid mass fraction sensor based on helical optical fiber [J]. Physical Experiment, 2023, 43(8): 56-60.

[0010] [4]Xu Yue, Xue Peng, Zhang Rui, et al. Helical plastic optical fiber surface plasmon resonance refractive index sensor[J]. Applied Optics, 2023, 44(1):8. Summary of the invention

[0011] To this end, the embodiments of the present invention provide a method for preparing a semicircular spiral plastic optical fiber sensor and a liquid concentration measuring device, which are used to solve the problem that the existing liquid concentration measurement optical fiber sensors in the prior art have limitations to varying degrees in terms of material selection, manufacturing process, measurement accuracy and convenience.

[0012] In order to solve the above problems, an embodiment of the present invention provides a method for preparing a semicircular spiral plastic optical fiber sensor, the method comprising:

[0013] Step S1: Select a certain length of plastic optical fiber, use ethyl acetate solution to cast it on the sensing area of ​​the set position and length, and remove its cladding;

[0014] Step S2: flattening the plastic optical fiber sensing area with the cladding removed to obtain a flat plastic optical fiber sensing area;

[0015] Step S3: performing a twisting operation on the flat plastic optical fiber sensing area to obtain a spiral plastic optical fiber probe with a thread number N=4;

[0016] Step S4: Wrap the sensing area of ​​the spiral plastic optical fiber probe around a cylindrical metal rod with a set radius and a temperature of 100-110°C. After the cylindrical metal rod is naturally cooled to room temperature, the semicircular spiral plastic optical fiber probe that has been formed is removed from the cylindrical metal rod to obtain a semicircular spiral structure plastic optical fiber sensor.

[0017] Preferably, in step S1, a certain length of plastic optical fiber is selected, and ethyl acetate solution is used to cast it on a sensing area of ​​a set position and length, and the method for removing its cladding is:

[0018] A certain length of plastic optical fiber is selected, and a sensing area with a length of 3.14 cm is selected at the middle position thereof. The sensing area is cast with an ethyl acetate solution to remove the cladding thereon.

[0019] Preferably, in step S2, the method of flattening the plastic optical fiber sensing area with the cladding removed to obtain a flat plastic optical fiber sensing area is:

[0020] First, fix a pair of smooth copper blocks in the jaws of a vise, place the plastic optical fiber sensing area with the cladding removed and a length of 3.14 cm between the two copper blocks, and tighten the vise handles to tightly clamp the plastic optical fiber sensing area.

[0021] Then, the vise with the plastic optical fiber sensing area is placed on a constant temperature heating platform, and the heating temperature of the platform is set to 120 degrees. This temperature is maintained to continuously heat the vise and the plastic optical fiber sensing area clamped therein;

[0022] After heating is completed, rotate the vise handle to apply a certain stress to flatten the plastic optical fiber sensing area between the copper blocks. By controlling the pressing force and time, ensure that the thickness of the plastic optical fiber sensing area is evenly and accurately compressed to the set thickness. After maintaining this pressing state for a few seconds, release the vise handle and take out the flat plastic optical fiber sensing area from between the copper blocks.

[0023] Preferably, the set thickness is 0.5 mm.

[0024] Preferably, in step S3, the method of twisting the flat plastic optical fiber sensing area to obtain a spiral plastic optical fiber probe with a thread number N=4 is:

[0025] Use a fiber optic clamp to clamp the two ends of the flat plastic optical fiber sensing area and place it 1mm to 2mm above the constant temperature heating platform for heating;

[0026] After the plastic optical fiber sensing area softens, use coaxial and unidirectionally rotating gears to twist the two ends of the plastic optical fiber sensing area in opposite directions at a certain angle for a few seconds, then move away from the heat source and keep this state for a while to allow the plastic optical fiber to cool naturally to room temperature, thereby obtaining a spiral plastic optical fiber probe with a thread number N=4.

[0027] Preferably, in step S4, the set radius is 1 cm.

[0028] The embodiment of the present invention further provides a liquid concentration measuring device, the device comprising a light source, an aperture, a convex lens, an optical fiber holder, a semicircular spiral plastic optical fiber sensor, and an optical power meter;

[0029] An aperture, a convex lens, and an optical fiber holder are sequentially arranged on the horizontal optical path of the light source;

[0030] The semicircular spiral plastic optical fiber sensor is prepared by the above-mentioned preparation method of the semicircular spiral plastic optical fiber sensor, and the semicircular spiral plastic optical fiber sensor is composed of a plastic optical fiber, a semicircular spiral plastic optical fiber probe, and an optical fiber fixing fixture;

[0031] One end of the semicircular spiral plastic optical fiber sensor is connected to the optical fiber holder through the plastic optical fiber, and the other end is connected to the optical power meter through the plastic optical fiber.

[0032] Preferably, the light source is a laser with a central wavelength of 638 nm.

[0033] Preferably, the plastic optical fiber is a multimode plastic optical fiber with an optical fiber diameter of 980 μm and a cladding thickness of 10 μm.

[0034] Preferably, the optical fiber fixing fixture is used to fix two arms of a semicircular spiral plastic optical fiber probe.

[0035] It can be seen from the above technical solutions that the present invention has the following beneficial effects:

[0036] (1) The preparation process is simple and the cost is low: The present invention prepares a semicircular spiral plastic optical fiber sensor through simple steps (including removing the cladding using an ethyl acetate solution, flattening treatment, twisting operation and shaping treatment, etc.) and at a low cost.

[0037] (2) High resolution: The resolution of the sensor in measuring the concentration of sodium chloride solution reaches 0.09%, achieving high measurement accuracy.

[0038] (3) Good measurement effect and convenient operation: By utilizing the characteristics of optical fiber for light transmission, transmissivity and reflection, the optical fiber sensor measurement device designed in the present invention is easy to operate, has good measurement effect, and is suitable for measuring liquids of different concentrations. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] In order to more clearly illustrate the implementation cases of the present invention or the technical solutions in the prior art, the following is a brief description of the drawings required for use in the embodiments. By referring to the drawings, the features and advantages of the present invention will be more clearly understood. The drawings are schematic and should not be understood as limiting the present invention in any way. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work. Among them:

[0040] Figure 1 A flow chart of a method for preparing a semicircular spiral plastic optical fiber sensor provided in an embodiment;

[0041] Figure 2 Schematic diagram of the optical fiber wave leakage principle in the embodiment;

[0042] Figure 3 Schematic diagram of the structure of a semicircular spiral plastic optical fiber probe in an embodiment;

[0043] Figure 4 is a schematic diagram of a liquid concentration measuring device provided in an embodiment;

[0044] Figure 5 It is a schematic diagram of the fitting curve between the concentration of sodium chloride solution and the normalized light intensity at the output end in the embodiment.

[0045] Figure numerals in the specification: 1. light source; 2. aperture; 3. convex lens; 4. optical fiber holder; 51. plastic optical fiber; 52. semicircular spiral plastic optical fiber probe; 53. optical fiber fixing fixture; 6. optical power meter. DETAILED DESCRIPTION

[0046] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0047] Embodiment 1

[0048] In order to solve the problem that the existing liquid concentration measurement optical fiber sensors in the prior art have limitations in material selection, manufacturing process, measurement accuracy and convenience. Figure 1 As shown, an embodiment of the present invention provides a method for preparing a semicircular spiral plastic optical fiber sensor, the method comprising:

[0049] Step S1: Select a certain length of plastic optical fiber, use ethyl acetate solution to cast it on the sensing area of ​​the set position and length, and remove its cladding;

[0050] Step S2: flattening the plastic optical fiber sensing area with the cladding removed to obtain a flat plastic optical fiber sensing area;

[0051] Step S3: performing a twisting operation on the flat plastic optical fiber sensing area to obtain a spiral plastic optical fiber probe with a thread number N=4;

[0052] Step S4: Wrap the sensing area of ​​the spiral plastic optical fiber probe around a cylindrical metal rod with a set radius and a temperature of 100-110°C. After the cylindrical metal rod is naturally cooled to room temperature, the semicircular spiral plastic optical fiber probe that has been formed is removed from the cylindrical metal rod to obtain a semicircular spiral structure plastic optical fiber sensor.

[0053] From the above technical scheme, it can be seen that the present invention provides a method for preparing a semicircular spiral plastic optical fiber sensor, by selecting a plastic optical fiber, removing the cladding of the sensing area with an ethyl acetate solution, flattening, twisting to form a spiral structure, and shaping it on a cylindrical metal rod and then cooling it down to obtain a plastic optical fiber sensor with a semicircular spiral structure. This scheme is simple in process and low in cost, can effectively improve the measurement resolution to 0.09%, has the advantages of compact structure, convenient operation and high resolution, and is suitable for accurate measurement of liquid concentration.

[0054] Fiber optic sensing technology shows unique advantages in liquid concentration measurement. Its principle is based on the transmission characteristics of light in optical fiber and the change of light intensity caused by the interaction between optical fiber and the external environment. This paper will deeply explore the basic principle of optical fiber measurement of liquid concentration, analyze the relationship between optical fiber microbending loss and liquid concentration. After understanding the basic principle of optical fiber measurement of liquid concentration, we further analyze how the bending state of optical fiber affects the measurement process, explore the influence of bending radius on light cone angle and measurement sensitivity, and finally introduce an innovative semicircular spiral plastic optical fiber sensor design. We not only explain the advantages of the semicircular spiral plastic optical fiber sensor proposed in this invention, but also show its great potential and application prospects in the field of liquid concentration measurement.

[0055] 1. Principle of optical fiber measurement of liquid concentration

[0056] like Figure 2 As shown in the figure, when light is transmitted in the optical fiber, total reflection will occur at the interface between the core and the cladding. Assuming the refractive index of the core is n1, the coating and cladding of the optical fiber are stripped off, and the bare optical fiber is placed in a solution with a refractive index of n2, a part of the transmitted light will penetrate into the liquid medium wrapping the optical fiber, which is a rapidly decaying evanescent wave.

[0057] Remove a section of the optical fiber's coating and cladding, and place the fiber core in the solution. The surface of the fiber core is regarded as a local plane, and the boundary between n1 and n2 is taken as the zero point of the x-axis. There is a field strength of

[0058] E1(x)=E0exp(jk0n2cosθ2)exp[j(βz-ωt)] (1)

[0059] Where E0 is the initial field intensity, E1 is the field intensity in the outer dielectric layer, k0 is the wave vector in vacuum, j represents the imaginary unit, β is the propagation constant of light waves in liquid solution, and if the incident angle is θ1, the refraction angle θ2 can be derived:

[0060]

[0061] get

[0062]

[0063] Formula (3) shows that the transmitted wave propagates along the z direction and decays exponentially along the x direction. The degree of attenuation has a certain relationship with the refractive index n2 of the external liquid, and the refractive index has a corresponding relationship with the solution concentration when the temperature is constant. Therefore, the liquid concentration can be measured by measuring the change in the light intensity after the light passes through the sensing area in the optical fiber.

[0064] 2. Relationship between optical fiber microbend loss and liquid concentration

[0065] Designing optical fiber into a curved shape will produce microbend loss. Microbend loss is the power loss caused by the coupling between the higher-order conduction mode and the radiation mode caused by the random distortion of the optical fiber. For step-index optical fiber, the loss rate α can be approximately calculated by the following formula:

[0066]

[0067] Where W1 and W2 are called auxiliary mode field radii, k0 is the wave number propagating in vacuum, and k2 is the wave number propagating in solution. P is the curved spectrum function, which can be approximately expressed as formula (5):

[0068]

[0069] In the formula, σ and L C are the root mean square value and autocorrelation length of the lateral deviation of the optical fiber axis, respectively. Equations (4) and (5) show that the microbend loss and size are proportional to the negative sixth power of n2. Immersing in the solution actually increases the n2 of the external medium of the optical fiber, and the increase in n2 reduces the microbend loss. Therefore, as the refractive index of the optical fiber immersed in the solution (which has a corresponding relationship with the liquid concentration) increases, the intensity of light lost during propagation decreases, which ultimately causes the intensity of light output at the end of the optical fiber to increase. Therefore, the relationship between light intensity loss and refractive index can be established by designing the bending shape.

[0070] 3. Relationship between bending radius and light cone angle

[0071] According to the principle of measuring liquid concentration with optical fiber, the concentration of the measured liquid can control the amount of leaked light wave mode, and then control the light cone angle. for:

[0072]

[0073] Where d is the diameter of the optical fiber core and R is the radius of curvature of the bent optical fiber.

[0074] From formula (6), we can see that the curvature radius R of the bent optical fiber can be adjusted Corresponding to n2, when the refractive index n2 of the solution to be measured increases, the light cone angle gradually decreases, the leakage light intensity changes, and the relationship between the residual light intensity and the refractive index can be established. Changing the bending radius R of the curved optical fiber can increase the change of the light cone angle, making the optical fiber more sensitive to concentration measurement.

[0075] 4. Design of semicircular spiral plastic optical fiber sensor

[0076] For curved optical fiber sensors, the smaller the bending radius, the better the sensitivity. However, the bending radius has a limit value. The greater the bending degree, the smaller the light detected by the light intensity meter, and the greater the system error of the light intensity meter. Therefore, when designing the sensor, it is necessary to ensure the performance balance between the bending radius and the light intensity detection. Based on the flat optical fiber, we rotate it along the axis to form a spiral optical fiber sensor with a certain number of threads, and then bend it to form a semicircle to make a semicircular spiral optical fiber sensor. Figure 3 As shown, the structure of the semicircular spiral fiber optic sensor probe is demonstrated.

[0077] The preparation process of the semicircular spiral plastic optical fiber sensor designed by the present invention is described in detail below.

[0078] In step S1, a certain length of plastic optical fiber is selected, and a sensing area with a length of 3.14 cm is selected at the middle position thereof, and the sensing area is cast with an ethyl acetate solution to remove the cladding thereon, in preparation for the subsequent deformation treatment, ensuring that the optical fiber in the sensing area can be deformed freely.

[0079] In step S2, the plastic optical fiber sensing area with the cladding removed is flattened to obtain a flat plastic optical fiber sensing area, which specifically includes:

[0080] First, fix a pair of smooth copper blocks in the jaws of a vise, place the plastic optical fiber sensing area with the cladding removed and a length of 3.14 cm between the two copper blocks, and tighten the vise handles to clamp the plastic optical fiber sensing area tightly.

[0081] Then, the vise with the plastic optical fiber sensing area is placed on a constant temperature heating platform, and the platform's heating temperature is set to 120 degrees. This temperature is maintained and the vise and the plastic optical fiber sensing area clamped are continuously heated for about 10 minutes. The purpose of this is to soften the optical fiber material, making it easier to perform subsequent deformation operations.

[0082] After heating is completed, the handle of the vise is rotated to apply a certain stress so that the plastic optical fiber sensing area is flattened between the copper blocks. By controlling the pressing force and time, the thickness of the plastic optical fiber sensing area is ensured to be evenly and accurately compressed to the set thickness (the set thickness in this embodiment is 0.5 mm). After maintaining this pressing state for a few seconds, the handle of the vise is released, and the flat plastic optical fiber sensing area is taken out from between the copper blocks.

[0083] In step S3, a twisting operation is performed on the flat plastic optical fiber sensing area to obtain a spiral plastic optical fiber probe with a thread number N=4, which specifically includes:

[0084] Use an optical fiber clamp to clamp the two ends of the flat plastic optical fiber sensing area, and place it 1mm to 2mm above the constant temperature heating platform for heating.

[0085] After the plastic optical fiber sensing area softens, use coaxial and unidirectionally rotating gears to twist the two ends of the plastic optical fiber sensing area in opposite directions at a certain angle for a few seconds (about 10 seconds), then keep away from the heat source and maintain this state for a period of time (about 1 minute) to allow the plastic optical fiber to cool naturally to room temperature, thereby obtaining a spiral plastic optical fiber probe with a thread number N=4.

[0086] In step S4, the sensing area of ​​the spiral plastic optical fiber probe is wound around a cylindrical metal rod with a set radius (the set radius in this embodiment is 1 cm) and a temperature of 100-110°C. After the cylindrical metal rod is naturally cooled to room temperature, the semicircular spiral plastic optical fiber probe that has been formed is removed from the cylindrical metal rod to obtain a semicircular spiral structure plastic optical fiber sensor.

[0087] The invention can obtain flat plastic optical fiber sensing areas of different thicknesses by controlling the stress of the knob vise; can obtain spiral optical fiber sensing areas of different pitches by changing the torsion angle; and can obtain semicircular spiral plastic optical fiber probes of different curvature radii by replacing cylindrical metal rods of different radii.

[0088] The length of the semicircular spiral plastic optical fiber probe prepared by the present invention is 3.14 cm (during the twisting process, because the distance between the two optical fiber rotating clamps remains unchanged, the length of the prepared spiral plastic optical fiber probe is the same as the length of the flat plastic optical fiber sensing area), the probe is semicircular, and the bending radius is 1 cm.

[0089] Embodiment 2

[0090] like Figure 4 As shown, the present invention provides a liquid concentration measuring device, which includes a light source 1, an aperture 2, a convex lens 3, an optical fiber holder 4, a semicircular spiral plastic optical fiber sensor, and an optical power meter 6;

[0091] A diaphragm 2, a convex lens 3, and a fiber holder 4 are sequentially arranged on the horizontal optical path of the light source 1;

[0092] The semicircular spiral plastic optical fiber sensor is prepared by the preparation method of the semicircular spiral plastic optical fiber sensor proposed in Example 1. The semicircular spiral plastic optical fiber sensor is composed of a plastic optical fiber 51, a semicircular spiral plastic optical fiber probe 52, and an optical fiber fixing fixture 53;

[0093] One end of the semicircular spiral plastic optical fiber sensor is connected to the optical fiber holder 4 through the plastic optical fiber 51 , and the other end is connected to the optical power meter 6 through the plastic optical fiber 51 .

[0094] In this embodiment, the light source 1 is a laser with a central wavelength of 638 nm; the plastic optical fiber 51 is a commercial multimode plastic optical fiber with a fiber diameter of 980 μm and a cladding thickness of 10 μm.

[0095] In this embodiment, the optical fiber fixing fixture 53 is used to fix the two arms of the semicircular spiral plastic optical fiber probe 52 .

[0096] The working principle of the liquid concentration measuring device proposed by the present invention is as follows: the light source 1 emits light with a wavelength of 638nm, which is modulated into a regular circular light spot by the aperture 2, incident on the convex lens 3 and converged on the optical fiber port on the optical fiber holder 4, and transmitted to the input end of the semicircular spiral plastic optical fiber probe 52 through the plastic optical fiber 51. Finally, the output end of the semicircular spiral plastic optical fiber probe 52 is connected to the optical power meter 6 through the plastic optical fiber 51, and the optical power meter 6 detects the light intensity value. When the semicircular spiral plastic optical fiber probe 52 is immersed in liquids of different concentrations, the refractive index of the medium in contact with the semicircular spiral plastic optical fiber probe 52 changes (the concentration changes), so that the output light intensity changes. By calibrating the output light intensity corresponding to solutions of different concentrations, drawing a calibration curve, and then measuring the light intensity at the output end when the semicircular spiral plastic optical fiber probe 52 is immersed in the solution to be measured, the concentration can be measured.

[0097] The semicircular spiral plastic optical fiber sensor of this embodiment is prepared by the preparation method of the semicircular spiral plastic optical fiber sensor proposed in Embodiment 1. Therefore, its specific implementation can refer to the description of the corresponding embodiments, and will not be repeated here to avoid redundancy.

[0098] In order to further illustrate the advantages of the present invention, it is described below in conjunction with specific experiments.

[0099] The prepared semicircular spiral plastic optical fiber sensor was used for experimental measurement. The semicircular spiral plastic optical fiber probe 52 was immersed in sodium chloride solutions of different concentrations. The fitting curve between the concentration of the sodium chloride solution and the normalized light intensity at the output end was obtained as shown in FIG. Figure 5 shown.

[0100] All the sodium chloride solutions tested were conducted at 25°C, and the solution concentration measurement range was 0-25%. The solution concentration is defined as the mass percentage concentration of the solution = solute mass / solution mass × 100%.

[0101] After obtaining the fitting curve, we measured with prepared sodium chloride solutions with concentrations of 2%, 7%, 12%, 18%, and 22%, and compared the measured concentrations with the actual concentrations. The results are shown in Table 1.

[0102] Table 1 Measurement results of sodium chloride solutions with concentrations of 2%, 7%, 12%, 18% and 22%

[0103]

[0104]

[0105] The absolute error of the measurement is calculated using the results in Table 1 and the standard concentration value, and the uncertainty formula is used The Class A uncertainty of each concentration measurement is calculated, as shown in Table 2. As can be seen from Tables 1 and 2, the minimum resolution of the present invention is 0.09%. The probe structure parameters can be optimized later to further improve the sensor measurement range, sensitivity and resolution. In addition, the sensor designed by the present invention has potential application value in other liquid concentration measurements.

[0106] Table 2 Uncertainty and absolute error table

[0107]

[0108] In summary, the resolution of the liquid concentration measuring device provided by the present invention in measuring the concentration of sodium chloride solution reaches 0.09%, achieving a high measurement accuracy. At the same time, by utilizing the characteristics of optical fiber transmission, transflection and other properties of light, the optical fiber sensor measuring device designed by the present invention is simple to operate, has good measurement effect, and is suitable for measuring liquids of different concentrations.

[0109] Obviously, the above embodiments are merely examples for clear explanation and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived from them are still within the protection scope of the invention.

Claims

1. A method for preparing a semicircular spiral plastic optical fiber sensor, characterized in that: include: Step S1: Select a certain length of plastic optical fiber, use ethyl acetate solution to cast it on the sensing area of ​​the set position and length, and remove its cladding; Step S2: flattening the plastic optical fiber sensing area with the cladding removed to obtain a flat plastic optical fiber sensing area; Step S3: performing a twisting operation on the flat plastic optical fiber sensing area to obtain a spiral plastic optical fiber probe with a thread number N=4; Step S4: Wrap the sensing area of ​​the spiral plastic optical fiber probe around a cylindrical metal rod with a set radius and a temperature of 100-110°C. After the cylindrical metal rod is naturally cooled to room temperature, the semicircular spiral plastic optical fiber probe that has been formed is removed from the cylindrical metal rod to obtain a semicircular spiral structure plastic optical fiber sensor.

2. The method for preparing a semicircular spiral plastic optical fiber sensor according to claim 1, characterized in that: In step S1, a certain length of plastic optical fiber is selected, and ethyl acetate solution is used to cast on a sensing area of ​​a set position and length, and the method for removing its cladding is as follows: A certain length of plastic optical fiber is selected, and a sensing area with a length of 3.14 cm is selected at the middle position thereof. The sensing area is cast with an ethyl acetate solution to remove the cladding thereon.

3. The method for preparing a semicircular spiral plastic optical fiber sensor according to claim 1 or 2, characterized in that: In step S2, the method of flattening the plastic optical fiber sensing area with the cladding removed to obtain a flat plastic optical fiber sensing area is: First, fix a pair of smooth copper blocks in the jaws of a vise, place the plastic optical fiber sensing area with the cladding removed and a length of 3.14 cm between the two copper blocks, and tighten the vise handles to tightly clamp the plastic optical fiber sensing area. Then, the vise with the plastic optical fiber sensing area is placed on a constant temperature heating platform, and the heating temperature of the platform is set to 120 degrees. This temperature is maintained to continuously heat the vise and the plastic optical fiber sensing area clamped therein; After heating is completed, rotate the vise handle to apply a certain stress to flatten the plastic optical fiber sensing area between the copper blocks. By controlling the pressing force and time, ensure that the thickness of the plastic optical fiber sensing area is evenly and accurately compressed to the set thickness. After maintaining this pressing state for a few seconds, release the vise handle and take out the flat plastic optical fiber sensing area from between the copper blocks.

4. The method for preparing a semicircular spiral plastic optical fiber sensor according to claim 3, characterized in that: The set thickness is 0.5 mm.

5. The method for preparing a semicircular spiral plastic optical fiber sensor according to claim 1, characterized in that: In step S3, the method of twisting the flat plastic optical fiber sensing area to obtain a spiral plastic optical fiber probe with a thread number N=4 is: Use a fiber optic clamp to clamp the two ends of the flat plastic optical fiber sensing area and place it 1mm to 2mm above the constant temperature heating platform for heating; After the plastic optical fiber sensing area softens, use coaxial and unidirectionally rotating gears to twist the two ends of the plastic optical fiber sensing area in opposite directions at a certain angle for a few seconds, then move away from the heat source and keep this state for a while to allow the plastic optical fiber to cool naturally to room temperature, thereby obtaining a spiral plastic optical fiber probe with a thread number N=4.

6. The method for preparing a semicircular spiral plastic optical fiber sensor according to claim 1, characterized in that: In step S4, the set radius is 1 cm.

7. A liquid concentration measuring device, characterized in that: The device comprises a light source, an aperture, a convex lens, an optical fiber holder, a semicircular spiral plastic optical fiber sensor, and an optical power meter; An aperture, a convex lens, and an optical fiber holder are sequentially arranged on the horizontal optical path of the light source; The semicircular spiral plastic optical fiber sensor is prepared by the preparation method of the semicircular spiral plastic optical fiber sensor according to any one of claims 1 to 6, and the semicircular spiral plastic optical fiber sensor consists of a plastic optical fiber, a semicircular spiral plastic optical fiber probe, and an optical fiber fixing fixture; One end of the semicircular spiral plastic optical fiber sensor is connected to the optical fiber holder through the plastic optical fiber, and the other end is connected to the optical power meter through the plastic optical fiber.

8. The liquid concentration measuring device according to claim 7, characterized in that: The light source is a laser with a central wavelength of 638 nm.

9. The liquid concentration measuring device according to claim 7, characterized in that: The plastic optical fiber is a multimode plastic optical fiber with an optical fiber diameter of 980 μm and a cladding thickness of 10 μm.

10. The liquid concentration measuring device according to claim 7, characterized in that: The optical fiber fixing fixture is used for fixing two arms of a semicircular spiral plastic optical fiber probe.

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