Optical fiber humidity sensor and method of manufacturing the same

Abstract

The application discloses a kind of optical fiber humidity sensor and its preparation scheme, optical fiber humidity sensor includes micro-nano optical fiber and porous film;Wherein: the micro-nano optical fiber, at least one bending part is formed;The porous film is coated on the surface of the bending part;The porous film surface has a plurality of holes, which is configured to be able to swell by water absorption, so that the duty cycle of hole becomes smaller.The present application utilizes the evanescent field of U-shaped micro-nano optical fiber to couple with the porous film to realize humidity sensing.Under different relative humidity environment, the size of the hole of the porous film on the surface of the U-shaped micro-nano optical fiber is different, so that the size of the energy scattered by it is different, and the change of the optical power at the output end of the optical fiber is different, thereby realizing humidity sensing.The present application has the advantages of high sensitivity, fast response speed, high repeatability and stability.

Description

Technical Field

[0001] This invention relates to the field of sensor technology, and more specifically, to an optical fiber humidity sensor and its fabrication method. Background Technology

[0002] Environmental humidity detection is widely used in fields such as chip manufacturing, food processing, pharmaceuticals, storage, human health, and meteorological monitoring. Currently, various sensing technologies are available for humidity detection, such as resistive, capacitive, thermally conductive, gravity-based, and optical methods. Optical methods typically utilize micro / nano optical fibers as sensing units. When the fiber diameter is as small as micrometers or even nanometers, a strong evanescent field exists outside the fiber. This evanescent field interacts with matter, resulting in energy and momentum exchange and changes in output optical power. Due to the advantages of low cost, resistance to electromagnetic interference and corrosion, and the ability to perform detection in harsh environments, micro / nano optical fibers offer high detection sensitivity and fast response time, leading to their rapid development in recent years.

[0003] Currently, the sensing principles of fiber optic humidity sensors mainly include fiber optic grating sensing, evanescent field sensing, and interferometric sensing. Ordinary bare optical fibers have low humidity response, requiring the addition of active materials to increase sensor sensitivity. By attaching special sensitizing materials to the optical fiber, such as graphene, reduced graphene, reduced-oxidation graphene, and molybdenum disulfide, the large surface area or volume of the material increases the interaction between the material and water molecules, achieving high-sensitivity humidity sensing. However, these sensors often have long response and recovery times, making them unsuitable for applications requiring rapid humidity detection. Furthermore, they suffer from poor repeatability, short lifespan, and inability to achieve long-term stable detection. Summary of the Invention

[0004] The present invention aims to overcome at least one of the defects of the prior art and provide an optical fiber humidity sensor and its preparation method, which has higher sensitivity and faster response and recovery rate in the low humidity range compared with other optical fiber humidity sensors.

[0005] The technical solution adopted in this invention is:

[0006] A fiber optic humidity sensor includes micro / nano optical fibers and a porous thin film; wherein:

[0007] The micro / nano optical fiber has at least one bend.

[0008] The porous film is coated on the surface of the curved portion; the surface of the porous film has multiple pores, which are configured to swell by absorbing water, thereby reducing the pore occupancy ratio.

[0009] Preferably, the micro-nano fiber is a U-shaped micro-nano fiber, which comprises a first straight section and a second straight section in parallel, and the two ends of the curved section are connected with the first straight section and the second straight section respectively.

[0010] Preferably, the diameter of the curved section is smaller than the diameters of the first straight section and the second straight section, and at the connection of the curved section and the two straight sections, the diameter of the fiber gradually increases from the curved section to the straight sections.

[0011] Preferably, the diameter of the curved section is 1-10 microns, and the length is 10-50 mm.

[0012] Preferably, the bending diameter of the curved section is 2-3 mm.

[0013] Preferably, the porous film is a porous film with water absorption.

[0014] Preferably, the porous film is a porous methacryloyloxyethyl trimethyl ammonium chloride film.

[0015] The embodiment of the present application also provides a preparation method of the fiber humidity sensor as described above, which comprises:

[0016] providing a micro-nano fiber;

[0017] heating and fusion tapering the micro-nano fiber to form a waist section in the middle of the micro-nano fiber, the diameter of the waist section being smaller than the diameters of other sections, and being tapered at the connection;

[0018] bending the waist section to form a curved section;

[0019] placing the curved section in a porous solution and heating to coat the curved section with a porous film to obtain the fiber humidity sensor.

[0020] Preferably, the porous solution is a porous methacryloyloxyethyl trimethyl ammonium chloride solution.

[0021] Preferably, the heating is performed to 100 DEG C for 1.5 hours.

[0022] In summary, the optical fiber humidity sensor is a high-sensitivity humidity sensor, which utilizes the evanescent field of the U-shaped micro-nano fiber and the porous film to realize humidity sensing. Compared with the traditional humidity sensor, the sensor has a sensitivity of 2.995 dB / %RH in the humidity range of 47%RH to 57%RH, which is much larger than the sensitivity of the traditional humidity sensor. At the same time, the response time of the sensor is 0.692 s, and the recovery time is 0.023 s, which is much smaller than the humidity sensor prepared by wrapping the polymer on the outer surface of the micro-nano fiber. In addition, the sensor is based on the optical fiber structure, which is simple to manufacture, low in cost, and compatible with the fields of physical chemistry, biomedicine, etc. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 A structure diagram of the optical fiber humidity sensor provided by the embodiment of the present application is provided.

[0024] Figure 2 A schematic diagram of the optical fiber humidity sensor after fusion tapering provided by the embodiment of the present application is provided.

[0025] Figures 3(a)-3(c) A working principle diagram of the optical fiber humidity sensor provided by the embodiment of the present application is provided.

[0026] Figure 4 A humidity detection device diagram of the optical fiber humidity sensor is provided.

[0027] Figure 5 A diagram of the output power and relative humidity of the optical fiber humidity sensor changing with time is provided.

[0028] Figure 6 A diagram of the output power and relative humidity of the optical fiber humidity sensor is provided.

[0029] Figures 7(a)-7(b) A response speed and repeatability test diagram of the optical fiber humidity sensor is provided.

[0030] Figure 8 A diagram of the output power and relative humidity of the U-shaped micro-nano fiber without the porous DMC film attached to the surface changing with time is provided. DETAILED DESCRIPTION

[0031] The drawings of the present application are only used for illustrative explanation, and cannot be understood as a limitation of the present application. In order to better illustrate the following embodiments, some components of the drawings may be omitted, enlarged or reduced, and do not represent the actual product size; for those skilled in the art, it is understandable that some well-known structures and their descriptions in the drawings may be omitted.

[0032] Please refer to Figure 1 and Figure 2The optical fiber humidity sensor provided by the embodiment of the present application comprises:

[0033] The micro-nano optical fiber 10 is formed with a bending part 11.

[0034] In the embodiment, the reason for using the micro-nano optical fiber is that:

[0035] When the diameter of the optical fiber is as small as microns, the light propagating in the optical fiber will generate a strong evanescent field outside the optical fiber, which propagates along the surface of the optical fiber, and the transverse intensity thereof decreases exponentially with the distance from the optical fiber, and the transverse intensity decay range is usually hundreds of nanometers to several microns, which is related to the diameter of the optical fiber and the wavelength of the incident light.

[0036] In the embodiment, a stronger evanescent field will be generated outside the micro-nano optical fiber 10 at the bending part 11, so that the sensitivity of detection can be improved.

[0037] In the embodiment, in particular, the micro-nano optical fiber 10 is in a U shape, which comprises a first straight segment 12 and a second straight segment 13, and the two ends of the bending part 11 are connected to the first straight segment 12 and the second straight segment 13 respectively.

[0038] Of course, the overall shape of the micro-nano optical fiber 10 can also be set according to actual needs, as long as there is at least one bending part 11, and these schemes are within the protection scope of the present application.

[0039] Taking the U-shaped micro-nano optical fiber 10 as an example, the preparation process thereof is described as follows:

[0040] First, the prepared micro-nano optical fiber is heated and fused by a hydrogen-oxygen flame to make a biconical micro-nano optical fiber 1 as shown in Figure 2 .

[0041] As can be seen from Figure 2 , the diameter of the biconical micro-nano optical fiber 1 gradually decreases from both sides to the middle and tends to be uniform at the waist 2.

[0042] Then, the biconical micro-nano optical fiber 1 is bent at the position of the waist 2, and the U-shaped micro-nano optical fiber 10 of the embodiment is obtained.

[0043] In the embodiment, the diameter at the waist 2 is between 1 μm and 10 μm, and the diameter is controlled to be between 1 μm and 10 μm mainly to enable the evanescent field generated by the incident wavelength to effectively interact with the porous film 20.

[0044] The length of the waist 2 is about 10-50 mm, which can ensure that the diameter of the waist 2 is between 1 μm and 10 μm, and at the same time, the biconical micro-nano optical fiber 1 is easy to bend and not easy to break.

[0045] After bending, the bending diameter of the bending part is 2-3mm, which enables the light to generate a strong evanescent field when passing through the bending part 11, and meanwhile the U-shaped micro-nano fiber 10 will not be easily broken.

[0046] In particular, in this embodiment, when the incident light wavelength is 1310nm, the waist part 2 has a diameter of 7.59μm, and the bending diameter of the bending part 11 is 2.264mm. At this time, the light propagating in the U-shaped micro-nano fiber 10 will generate a strong evanescent field outside the bending part 11 and propagate along the surface of the U-shaped micro-nano fiber 10. Of course, it should be understood that for incident light of different wavelengths, the optimal waist diameter or bending diameter may be slightly different, but these schemes are within the protection scope of the present application.

[0047] A porous film 20 is coated on the surface of the bending part 11, and the surface of the porous film is formed with a plurality of holes 21 which are configured to be able to form swelling by absorbing water so that the duty cycle of the holes 21 is reduced.

[0048] In this embodiment, the porous film 20 is a porous water-absorbing film which is able to absorb water to generate swelling so that the duty cycle of the holes 21 on its surface is reduced.

[0049] For example, the porous film 20 can be a porous dimethylaminoethyl methacrylate chloride (DMC) film, and of course can also be other porous films with water-absorbing capability, which are not specifically limited by the present application.

[0050] In this embodiment, the porous film can be coated on the surface of the bending part 11 by immersing the bending part 11 in a corresponding porous solution and heating.

[0051] Taking the porous DMC film as an example, the bending part 11 can be immersed in a porous DMC solution and heated to 100℃ for 1.5 hours to coat the porous DMC film on the bending part 11. It can be understood that for different porous solutions, the coating method, heating temperature or heating duration can be different, but these are within the protection scope of the present application.

[0052] The specific working principle of this embodiment is described in detail as follows:

[0053] In this embodiment, as shown in Fig. 3(a), a small section of the bending section 11 of the micro-nano optical fiber 10 is analyzed, which can be regarded as a straight micro-nano optical fiber. When the incident light passes through the micro-nano optical fiber 10, there is a strong evanescent field 7 on the outer surface of the micro-nano optical fiber 10. As shown in Fig. 3(b), since the evanescent field is an electromagnetic wave, its propagation path is sensitive to the change of the refractive index of the external environment. When the porous film 20 is attached to the outer surface of the micro-nano optical fiber 10 and is within the lateral intensity decay range of the evanescent field 7, the evanescent field 7 will interact with the holes 21 in the porous film 20, and a part of the energy will be scattered out through the holes 21, so that the power at the output end of the optical fiber is reduced. As shown in Fig. 3(c), when the relative humidity of the environment increases, the water makes the porous film 20 swell, the duty cycle of the holes 21 rapidly decreases, the porous film 20 coated on the surface of the micro-nano optical fiber 10 becomes smooth, the scattered light 9 decreases, and the optical power at the output end of the micro-nano optical fiber 10 gradually increases. Under different relative humidity environments, the duty cycles of the holes 21 of the porous film 20 on the surface of the micro-nano optical fiber 10 are different, so that the sizes of the energy scattered through the holes 21 are different, the changes of the optical power at the output end of the optical fiber are different, and thus the humidity sensing is realized.

[0054] In addition, since the thickness of the porous film 20 is in the order of nanometers, when the environmental humidity decreases, the water in the porous film 20 evaporates easily, the duty cycle of the holes 21 rapidly increases, and when the environmental humidity increases, the porous film 20 rapidly absorbs water, the duty cycle of the holes 21 rapidly decreases, so that the sensor has a very fast humidity response capability, and at the same time, the small changes of the holes 21 can change the optical power output by the micro-nano optical fiber 10, so that the sensor has a very high sensitivity.

[0055] In order to increase the understanding of the present application, the application of the present application will be described below with some practical examples.

[0056] Figure 4 A schematic diagram for detecting humidity by using the optical fiber humidity sensor of this embodiment.

[0057] Figure 4 In this embodiment, the optical fiber humidity sensor is placed in a constant temperature and humidity box 30. When placed, the micro-nano optical fiber 10 is fixed on the same side of the glass slide by means of the purple glue, so that the bending section 11 of the micro-nano optical fiber 10 is in a suspended state; the refractive index of the purple glue is less than the core refractive index of the micro-nano optical fiber 10; so as to reduce the scattering of the evanescent field by the high refractive index, and effectively reduce the influence of the purple glue on the humidity detection result.

[0058] The constant temperature and humidity box 30 can control the relative humidity inside, in addition, commercial humidity sensor 31 is placed in the constant temperature and humidity box 30, for recording the relative humidity of surrounding environment and comparing and scaling with the optical fiber humidity sensor;Laser 40 is connected with the input end of the micro-nano optical fiber 10, the output end of the micro-nano optical fiber 10 is connected with optical power meter 50, optical power meter 50 is connected with computer 60, the relative humidity of constant temperature and humidity box 30 is changed, and the change of output optical power is monitored in real time through computer 60.

[0059] Figure 5 It is the change graph of the output power of the optical fiber humidity sensor and the relative humidity with time.The solid line is the change curve of the relative humidity in the constant temperature and humidity box 30, and the dotted line is the corresponding output power change curve;It can be seen that the output power changes with humidity in steps.

[0060] Figure 6 It is the scaling relationship between the output power of the optical fiber humidity sensor and the relative humidity.When the relative humidity in the constant temperature and humidity box 30 changes from 47% RH to 95% RH, the relative humidity and the output power are nonlinearly related, and the relative humidity and the output power are linearly related in the three ranges of 47% RH to 57% RH, 57% RH to 75.8% RH and 75.8% RH to 95% RH.In the humidity range of 47% RH to 57% RH, the sensitivity is 2.995 dB / % RH, and the linear correlation coefficient is 98.3%;In the humidity range of 57% RH to 75.8% RH, the sensitivity is 0.96 dB / % RH, and the linear correlation coefficient is 98.9%;In the humidity range of 75.8% RH to 95% RH, the sensitivity is 0.454 dB / % RH, and the linear correlation coefficient is 98.3%.It can be seen that in the humidity range of 47% RH to 57% RH, the optical fiber humidity sensor has ultra-high sensitivity.

[0061] Fig. 7 (a) and Fig. 7 (b) are the response speed and repeatability measurement of the optical fiber humidity sensor.The constant temperature and humidity box 30 is controlled to change between 10% RH and 90% RH for multiple cycles, and the periodical change curve of the sensor output power is obtained, which can be seen that after multiple periodical measurements, the output power of the sensor remains basically unchanged, indicating that the sensor has good repeatability and stability, and the response time of the sensor is 0.692 s and the recovery time is 0.023 s.

[0062] Figure 8 It is the control experiment of the optical fiber humidity sensor of the application.A micro-nano optical fiber 10 without porous DMC film attached on the surface is made in this embodiment, and the change of the output power with the relative humidity is measured. Figure 8The U-shaped micro-nano fiber 10 with no porous DMC film attached to the surface shows that the output power remains almost unchanged when the relative humidity changes, and the humidity response is extremely low. Therefore, the fiber humidity sensor proposed in the embodiment is based on the coupling of the evanescent field 7 of the U-shaped micro-nano fiber 10 and the porous film 20 to realize humidity sensing, and the scattering light 9 capability of the pores 21 of the porous film 20 plays an important role, and the U-shaped micro-nano fiber 10 with no porous DMC film attached has no humidity response performance.

[0063] In summary, the fiber humidity sensor proposed in the application is a high-sensitivity humidity sensor, which realizes humidity sensing by coupling the evanescent field 7 of the U-shaped micro-nano fiber 10 and the porous film 20. When the relative humidity of the environment increases, water causes the porous film 20 to swell, the duty cycle of the pores 21 rapidly becomes small, the porous film coated on the surface of the U-shaped micro-nano fiber 10 becomes smooth, the scattering light 9 decreases, and the output light power of the U-shaped micro-nano fiber 10 gradually increases. Under different relative humidity environments, the size of the pores 21 of the porous film 20 on the surface of the U-shaped micro-nano fiber 10 is different, so that the size of the energy scattered through the pores is different, and the change of the output light power of the fiber is different, thereby realizing humidity sensing. Compared with the traditional humidity sensor, the sensor proposed in the embodiment has a sensitivity of 2.995 dB / %RH in the humidity range of 47%RH to 57%RH, which is much larger than the sensitivity of the traditional humidity sensor. At the same time, the response time of the sensor proposed in the embodiment is 0.692s, and the recovery time is 0.023s, which is much smaller than the humidity sensor prepared by wrapping a polymer on the outer surface of a micro-nano fiber reported in the prior art. In addition, the sensor proposed in the embodiment is based on a fiber structure, which is simple to manufacture, low in cost, and compatible with the fields of physical chemistry, biomedicine, etc.

[0064] The second embodiment of the application also provides a preparation method of the fiber humidity sensor as described above, which comprises the following steps:

[0065] providing a micro-nano fiber;

[0066] heating and melting the micro-nano fiber to form a waist portion in the middle of the micro-nano fiber, the diameter of the waist portion being smaller than that of other portions, and the connecting portion being tapered;

[0067] bending the waist portion to form a bending portion;

[0068] placing the bending portion in a porous solution and heating to coat a porous film on the surface of the bending portion to obtain the fiber humidity sensor.

[0069] Preferably, the porous solution is a porous methacryloyloxyethyl trimethylammonium chloride solution.

[0070] Preferably, the heating is performed to 100℃ for 1.5 hours.

[0071] Obviously, the above embodiments of the present application are only examples for clearly illustrating the technical solutions of the present application, and are not intended to limit the specific embodiments of the present application. Any modification, equivalent replacement and improvement, etc. made within the spirit and principle of the present application should be included in the protection scope of the present application.

Claims

1. An optical fiber humidity sensor, characterized by, The micro-nano optical fiber and the porous film are included; wherein: The micro-nano optical fiber is formed with at least one bending part; The porous film is coated on the surface of the bending part; the surface of the porous film has a plurality of pores, which are configured to be able to swell by water absorption, so that the duty cycle of the pores becomes smaller; wherein the micro-nano optical fiber is a U-shaped micro-nano optical fiber, which includes a first straight segment and a second straight segment in parallel, and the two ends of the bending part are connected with the first straight segment and the second straight segment respectively; the porous film is a porous methacryloyloxyethyl trimethyl ammonium chloride film.

2. The optical fiber humidity sensor of claim 1, wherein, The diameter of the bending part is smaller than the diameters of the first straight segment and the second straight segment, and at the connection of the bending part and the two straight segments, the diameter of the optical fiber gradually increases from the bending part to the straight segments.

3. The optical fiber humidity sensor of claim 1, wherein, The diameter of the bending part is 1 μm-10 μm, and the length is 10 mm-50 mm.

4. A fiber-optic moisture sensor according to claim 1, wherein, The bending diameter of the bending part is 2 mm-3 mm.

5. A method of manufacturing an optical fiber humidity sensor according to any one of claims 1 to 4, characterized in that, The method comprises: providing a micro-nano optical fiber; heating and melting the micro-nano optical fiber to form a waist part in the middle of the micro-nano optical fiber by fusion tapering, the diameter of the waist part is smaller than that of other parts, and it is tapered at the connection; bending the waist part to form a bending part; putting the bending part into a porous solution and heating to coat a porous film on the surface of the bending part to obtain an optical fiber humidity sensor.

6. The production method according to claim 5, wherein The porous solution is a porous methacryloyloxyethyl trimethyl ammonium chloride solution.

7. The production method according to claim 6, wherein When heating, heat to 100℃ and maintain for 1.5 hours.

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