Fiber optic gas sensor
The optical fiber gas sensor addresses the challenge of costly uniform film formation by using a tubular design with a sensing member and through holes, enabling easy manufacturing and high sensitivity gas detection through Bragg wavelength variation.
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Patents
- Current Assignee / Owner
- CMIWS
- Filing Date
- 2024-01-31
- Publication Date
- 2026-06-03
AI Technical Summary
Existing optical fiber gas sensors require uniform film formation with uniform thickness over the entire circumference of the optical fiber, which is time-consuming and costly, especially when multiple detection points are needed.
An optical fiber gas sensor design featuring a tubular case with an FBG portion fixed inside, a sensing member promoting catalytic combustion, and through holes covered by a hydrophobic sealing member, allowing easy manufacturing and high sensitivity detection by varying the Bragg wavelength through case distortion.
The sensor can be manufactured easily and detects target gases with high sensitivity by leveraging the Bragg wavelength shift due to case deformation from catalytic reactions, reducing false detections from external forces.
Smart Images
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Abstract
Description
Technical Field
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[0001] The present invention relates to an optical fiber gas sensor including a FBG (Fiber Bragg Grating).
Background Art
[0002] Conventionally, an electric gas sensor has been used to detect gas leakage and the like. However, when detecting a combustible gas such as hydrogen gas, the electric gas sensor can be an ignition source, and thus it cannot be used without appropriate explosion-proof treatment. To solve such problems, gas sensors using optical fibers have been proposed (for example, Patent Documents 1 and 2). Gas sensors using optical fibers do not become an ignition source because they do not require wiring for measurement and power supply like electric sensors. They also have the feature that a large number of sensors can be easily arranged on a measurement object such as a pipe.
[0003] Patent Document 1 discloses a hydrogen gas sensor in which a gas sensing layer is disposed on the entire circumference of the cladding of a long-period fiber grating structure in an optical fiber having a long-period fiber grating structure around the fiber core of the optical fiber. In this configuration, the concentration of hydrogen gas is identified based on the transmission power loss of the long-period fiber grating structure that varies as the gas sensing layer adsorbs hydrogen. Patent Document 2 discloses a hydrogen gas sensor in which a tungsten oxide thin film supporting a platinum catalyst, which is a hydrogen-sensitive substance, is disposed on the entire circumference of the cladding of the FGB portion in an optical fiber in which an FBG portion is formed in the fiber core. In this configuration, the concentration of hydrogen gas is identified based on the change in the wavelength characteristics of the light transmitted or reflected by the FBG portion due to heat generation or deformation in the reaction process between the tungsten oxide thin film supporting the platinum catalyst and hydrogen.
Prior Art Documents
Patent Documents
[0004] <00[Patent Document 2] Japanese Patent Publication No. 2005-351651 [Overview of the project] [Problems that the invention aims to solve]
[0005] In the technologies disclosed in Patent Documents 1 and 2, a film consisting of a gas-sensing layer that adsorbs the target gas and a sensitive material that reacts with hydrogen is formed with a uniform thickness over the entire circumference of the optical fiber, covering the long-period fiber lattice structure and FBG section. However, forming such a film with a uniform thickness over the entire circumference of an optical fiber is not easy and requires time and effort, making it expensive. Furthermore, if gas detection sections are formed at multiple locations on the same optical fiber, the cost may increase even further.
[0006] This invention has been made in view of the problems of the prior art, and aims to provide an optical fiber gas sensor that can be manufactured relatively easily and can detect the target gas with high sensitivity. [Means for solving the problem]
[0007] To achieve the above-mentioned objectives, the present invention employs the following technical means. The optical fiber gas sensor according to the present invention comprises a tubular case, an optical fiber having an FBG (Fiber Bragg Grating) portion, a fixing member, a sensing member, a through hole, and a sealing member. The fixing member fixes the optical fiber to the case in a state aligned with the axial direction of the case and with tension applied to the FBG portion. The sensing member covers the FBG portion inside the case. The sensing member is also composed of a catalyst that promotes the catalytic combustion reaction of the gas to be detected. The through hole is provided on the wall surface of the case. The sealing member covers the through hole. The sealing member is also composed of a hydrophobic material that allows the gas to be detected to pass through.
[0008] The optical fiber gas sensor of the present invention has a configuration in which the fiber optic gas generator (FBG) portion of an optical fiber fixed inside a case is covered with a sensing element. Therefore, it can be manufactured more easily than a configuration in which the sensing element is arranged as a film of a constant thickness around the entire circumference of the optical fiber. Furthermore, with this configuration, because the optical fiber is fixed to the case, the Bragg wavelength of the FBG portion can be greatly varied by the distortion of the case caused by the heat generated by the catalytic combustion reaction. As a result, the target gas can be detected with high sensitivity.
[0009] In this optical fiber gas sensor, the case is preferably made of a material having a larger coefficient of thermal expansion than the optical fiber. Alternatively, a configuration can be adopted in which the sensing element is filled into a portion of the axial region of the case. For example, a platinum-supported silica catalyst can be used as the sensing element. Furthermore, a configuration can be adopted in which multiple through-holes are provided at positions excluding the contact area between the FBG portion and the case. Additionally, a configuration can be adopted that further includes a cylindrical outer case housing the case in which the optical fiber is fixed. Furthermore, a configuration can be adopted in which the optical fiber further includes an FBG portion for temperature compensation. [Effects of the Invention]
[0010] According to the present invention, an optical fiber gas sensor capable of detecting a target gas with high sensitivity can be manufactured relatively easily. [Brief explanation of the drawing]
[0011] [Figure 1] Figure 1 is a schematic perspective view showing an example of the appearance of an optical fiber gas sensor in one embodiment of the present invention. [Figure 2] Figure 2 is a schematic longitudinal cross-sectional view showing an example of the internal structure of an optical fiber gas sensor in one embodiment of the present invention. [Figure 3] Figure 3 shows an example of the use of an optical fiber gas sensor in one embodiment of the present invention. [Figure 4]Figure 4 is a schematic perspective view showing an example of the appearance of another optical fiber gas sensor in one embodiment of the present invention. [Figure 5] Figure 5 is a schematic longitudinal cross-sectional view showing an example of the internal structure of another optical fiber gas sensor in one embodiment of the present invention. [Figure 6] Figures 6(a) to 6(d) show examples of measurement results of an optical fiber gas sensor in one embodiment of the present invention. [Modes for carrying out the invention]
[0012] Embodiments of the present invention will be described in more detail below with reference to the drawings. Figure 1 is a schematic perspective view showing the external appearance of the optical fiber gas sensor 1 in this embodiment. Figure 2 is a longitudinal cross-sectional view showing the internal structure of the optical fiber gas sensor 1 in this embodiment. Note that Figure 2 is a longitudinal cross-sectional view along the axial direction of the optical fiber gas sensor 1 shown in Figure 1.
[0013] As shown in Figures 1 and 2, the optical fiber gas sensor 1 has a structure in which an optical fiber 2 and a sensing member 4 are arranged inside a tubular double case 10. Although not particularly limited, a cylindrical double case 10 is exemplified here. The double case 10 is composed of an inner case 12 and an outer case 13, with the fiber 2 and sensing member 4 arranged inside the inner case 12. The materials of the inner case 12 and the outer case 13 are not particularly limited, but it is preferable that the material of the inner case 12 has a thermal expansion coefficient greater than that of the optical fiber 2. In this example, the material of the double case 10 is stainless steel. Although not particularly limited, the outer diameter of the double case 10 is approximately 5 mm.
[0014] The optical fiber 2 has a structure in which a core 2a that propagates light, a cladding 2b that surrounds the core and reflects light propagating through the core back towards the core, and an outer resin 2c that surrounds the cladding and protects the core and cladding are arranged in order from the center. The optical fiber 2 also includes an FBG (Fiber Bragg Grating) section 21. As is well known, the FBG section 21 reflects light of a wavelength defined by the Bragg wavelength. The FBG section 21 is composed of a plurality of diffraction gratings formed at predetermined intervals on the core of the optical fiber 2. The Bragg wavelength is proportional to the product of the refractive index of the optical fiber and the spacing of the diffraction gratings. Therefore, the wavelength of light reflected by the FBG section 21 increases due to an increase in the refractive index caused by a rise in temperature and an increase in the spacing of the diffraction gratings due to the elongation of the optical fiber 2. Conversely, the wavelength of light reflected by the FBG section 21 decreases due to a decrease in the refractive index caused by a drop in temperature and a decrease in the spacing of the diffraction gratings due to the contraction of the optical fiber 2. For convenience, in the diagram, the FBG section 21 of optical fiber 2 is represented by a black and white striped pattern.
[0015] As shown in Figure 2, the inner case 12 is housed in the outer case 13. Although not particularly limited, the outer diameter of the inner case 12 is slightly smaller than the inner diameter of the outer case 13. The optical fiber 2 is arranged along the axial direction of the inner case 12, and the optical fiber 2 on both sides of the FBG portion 21 is fixed to the inner wall of the case 10 by a fixing member 3 such as an adhesive, with the FBG portion 21 in contact with the inner wall surface of the inner case 12. When fixing the optical fiber 2, tension (pretension) is applied to the FBG portion 21. Although not particularly limited, in this case, an adhesive applied in a spot pattern is used as the fixing member 3.
[0016] The sensing member 4 covers the FBG section 21 inside the inner case 12. The sensing member 4 is composed of a catalyst that promotes the catalytic combustion reaction of the gas to be detected. The sensing member 4 can be appropriately selected according to the type of the gas to be detected. For example, when the gas to be detected is hydrogen, a platinum-supported silica catalyst can be used as the sensing member 4. Although not particularly limited, in this example, the sensing member 4 is arranged in a state of covering the FBG section 21 and filling a part of the inside (axial direction) of the inner case 12.
[0017] Note that after the opening end of the inner case 12 is filled with the sensing member 4, it is sealed with a resin sealing plug 15. Thereby, leakage of the sensing member 4 to the outside of the inner case 12 can be prevented. Also, the opening end of the outer case 13 is sealed with a one-touch joint 16. The one-touch joint 16 also has a function of fixing the optical fiber 2 to the double case 10 by sandwiching the optical fiber 2 (outer skin resin 2c).
[0018] The inner case 12 also has a through hole 11. Although not particularly limited, in this embodiment, the through hole 11 is circular and is provided at a position facing the FBG section 21 fixed to the inner case 12. The number, size, and shape of the through hole 11 are not particularly limited. For example, a plurality of through holes 11 can be provided at positions excluding the contact area between the FBG section 21 and the inner case 12. The outer case 13 has a corresponding through hole 14 at a position overlapping the through hole 11 in a state where the inner case 12 is accommodated in the outer case 13. Although not particularly limited, the through hole 14 is circular with the same diameter as the through hole 11. The through hole 11 of the inner case 12 is covered with a sealing member 5. The sealing member 5 is composed of a hydrophobic film that allows the gas to be detected to permeate. For example, when the gas to be detected is hydrogen, PTFE (polytetrafluoroethylene) can be used as the sealing member 5.
[0019] In an atmosphere where the gas to be detected exists, the optical fiber gas sensor 1 having the above configuration allows the gas to be detected to enter the inner case 12 through the through hole 14, the sealing member 5, and the through hole 11 and come into contact with the sensing member 4. At this time, the sensing member 4 generates heat by a catalytic combustion reaction. The inner case 12 is stretched and deformed by the heat being propagated to the inner case 12 or the sensing member 4 expanding due to the heat.
[0020] As described above, since both ends of the FBG section 21 are fixed to the inner case 12 in a state of contacting the inner case 12, when the inner case 12 is deformed, the Bragg wavelength of the FBG section 21 increases. Also, the amount of heat generated by the sensing member 4 due to the catalytic combustion reaction increases as the concentration of the gas to be detected increases. Therefore, the amount of deformation of the case 10 also increases as the concentration of the gas to be detected increases. Thus, by previously obtaining the correspondence relationship between the shift amount of the Bragg wavelength of the FBG section 21 and the concentration of the gas to be detected, it becomes possible to identify the concentration of the gas to be detected based on the shift amount of the Bragg wavelength of the FBG section 21.
[0021] In this embodiment, since the inner case 12 is housed without being fixed to the outer case 13, even if an external force acts on the double case 10 from the outside during the detection operation, it is possible to suppress the inner case 12 from being deformed by the external force. That is, it is possible to suppress false detection due to an external force.
[0022] As described above, the optical fiber gas sensor 1 has a configuration in which the FBG section 21 of the optical fiber 2 fixed to the inner wall surface of the inner case 12 is covered by the sensing member 4. Therefore, it can be easily manufactured as compared with a configuration in which the sensing member is arranged as a film with a constant thickness around the entire circumference of the optical fiber 2. Also, according to this configuration, since the FBG section 21 is in contact with the inner case 12, the Bragg wavelength of the FBG section 21 can be greatly varied due to distortion of the case or the like caused by the heat generated by the catalytic combustion reaction. As a result, the gas to be detected can be detected with high sensitivity.
[0023] Here, we will explain an example of the use of the optical fiber gas sensor 1. Figure 3 is a diagram illustrating an example of the use of the optical fiber gas sensor. Figure 3 shows an example of its use in a gas transport pipe through which gas is being transported.
[0024] As shown in Figure 3, the gas transport pipe 6 has a double-pipe structure consisting of an inner pipe 61 and an outer pipe 62 that houses the inner pipe 61. In this configuration, even if gas leaks from the inner pipe 61, the leaked gas is retained in the outer pipe 62, preventing it from leaking to the outside.
[0025] In such a gas transport pipe 6, the optical fiber gas sensor 1 can be installed on the outer wall surface of the inner pipe 61 or the inner wall surface of the outer pipe 62. In Figure 3, for illustrative purposes, an opening is provided in the outer pipe 62 to expose the optical fiber gas sensor 1. In actual use, such an opening does not exist in the outer pipe 62. Furthermore, as is well known, multi-point measurement can be performed by providing multiple FBG sections 21 on a single optical fiber 2. That is, by arranging optical fibers 2 equipped with multiple FBG sections 21 along the axial direction of the gas transport pipe 6, information about the approximate location of leakage can also be obtained.
[0026] As mentioned above, the Bragg wavelength is determined by the refractive index of the optical fiber 2 and the grating spacing of the diffraction grating. Therefore, the Bragg wavelength will also fluctuate due to changes in the refractive index caused by temperature changes, as well as expansion and contraction of the optical fiber. In other words, if a rapid temperature change occurs in a short period of time during detection operation, it may be impossible to distinguish whether the fluctuation in the Bragg wavelength is due to deformation of the inner case 12 or due to the temperature change.
[0027] As a countermeasure, an FBG section for temperature compensation can be provided. Figure 4 is a schematic perspective view showing the external appearance of another optical fiber gas sensor 7 in this embodiment. Figure 5 is a longitudinal cross-sectional view showing the internal structure of the other optical fiber gas sensor 7 in this embodiment. Note that Figure 5 is a longitudinal cross-sectional view along the axial direction of the optical fiber gas sensor 7 shown in Figure 4.
[0028] As shown in Figures 5 and 6(a) to 6(d), the optical fiber gas sensor 7 differs from the optical fiber gas sensor 1 in that it further includes an FBG section 22 for temperature compensation. The other components are the same as those of the optical fiber gas sensor 1, and the same reference numerals are used for components that have the same effects as those of the optical fiber gas sensor 1.
[0029] As shown in Figures 4 and 5, the optical fiber gas sensor 7 comprises a cylindrical double case 70 in which two inner cases 72 are housed in one outer case 73. Although not particularly limited, the outer diameter of the inner cases 72 is slightly smaller than the inner diameter of the outer case 73. An FBG unit 21 for gas detection is arranged inside one of the inner cases 72. An FBG unit 22 for temperature compensation is arranged inside the other inner case 72. Similar to the optical fiber gas sensor 1, the optical fibers 2 are arranged along the axial direction of each inner case 72, and the optical fibers 2 on both sides of the FBG unit 21 are fixed to the inner wall of the corresponding inner case 72 by a fixing member 3 such as an adhesive, with the FBG unit 21 in contact with the inner wall surface of the inner case 72. Similarly, the optical fibers 2 on both sides of the FBG unit 22 are fixed to the inner wall of the corresponding inner case 72 by a fixing member 3 such as an adhesive, with the FBG unit 22 in contact with the inner wall surface of the inner case 72.
[0030] In this example, FBG sections 21 and 22 are formed adjacent to each other on the same optical fiber 2. Here, the Bragg wavelength of FBG section 21 and the Bragg wavelength of FBG section 22 are different. Such a configuration can be achieved, for example, by forming adjacent FBG sections having the same Bragg wavelength on the same optical fiber 2, and when fixing each FBG section 21 and 22 to the corresponding inner case 72 with a fixing member 3, by making the pretension applied to FBG section 21 and the pretension applied to FBG section 22 different. In this embodiment, since FBG sections 21 and 22 are fixed to separate inner cases 72, such different pretensions can be easily applied.
[0031] Furthermore, each inner case 72 is provided with a through hole 71. Although not particularly limited, in this embodiment the through hole 71 is circular and is provided at positions opposite to the FBG portion 21 and FBG portion 22 fixed to the inner case 72. The number, size, and shape of the through holes 71 are not particularly limited. For example, multiple through holes can be provided at positions excluding the contact area between the FBG portions 21 and 22 and the inner case 72. The outer case 73 is provided with a corresponding through hole 74 at a position that overlaps with the through hole 71 when the inner case 72 is housed inside the outer case 73. Although not particularly limited, the through hole 74 is circular with the same diameter as the through hole 71. Similar to the optical fiber gas sensor 1, the through holes 71 of the inner case 72 are covered by a sealing member 5.
[0032] In the optical fiber gas sensor 7, the FBG portion 21 is covered by the sensing member 4, but the FBG portion 22 is not covered by the sensing member 4. Therefore, when the optical fiber gas sensor 7 is placed in an atmosphere where the target gas is present, the FBG portion 21 is affected by the heat generated by the catalytic combustion reaction of the sensing member 4, but the FBG portion 22 is not affected by this heat. In other words, the FBG portion 22 is only affected by the ambient temperature. Note that here, the example shows the FBG portion 22 exposed without being covered by other members, but it may be covered by other members. For example, it may be covered by a member that has a coefficient of thermal expansion similar to that of the sensing member 4 and does not undergo a catalytic combustion reaction with the target gas.
[0033] In the optical fiber gas sensor 7 having the above configuration, when the target gas is present, the target gas enters the inner case 72 through the through-hole 74, the sealing member 5, and the through-hole 71 and comes into contact with the sensing member 4. As a result, as described above, the Bragg wavelength of the FBG section 21 shifts. At this time, if the ambient temperature is fluctuating, the amount of wavelength shift due to the temperature change will be superimposed on the amount of Bragg wavelength shift of the FBG section 21. However, this amount of wavelength shift due to the temperature change can be obtained as the amount of wavelength shift of the FBG section 22. Therefore, by calculating the difference between the amount of wavelength shift of the FBG section 21 and the amount of wavelength shift of the FBG section 22, it is possible to obtain a wavelength shift amount that excludes the effect of ambient temperature.
[0034] Figures 6(a) to 6(d) show examples of measurement results from the optical fiber gas sensor 7. Figure 6(a) shows the time change in the concentration of the target gas (hydrogen) introduced into the ambient atmosphere surrounding the optical fiber gas sensor 7. Figure 6(b) shows the time dependence of the target gas concentration in the ambient atmosphere, identified based on the Bragg wavelength shift amount of the FBG unit 21. Figure 6(c) shows the time dependence of the value obtained by converting the Bragg wavelength shift amount of the FBG unit 22 to the target gas concentration in the ambient atmosphere. Figure 6(d) shows the time dependence of the target gas concentration in the ambient atmosphere, identified based on the Bragg wavelength shift amounts of the FBG unit 21 and the FBG unit 22. Note that in Figure 6(d), the data shown in Figures 6(b) and 6(c) is shown with a dashed line for reference. From Figures 6(a) and 6(d), it can be seen that the optical fiber gas sensor 7 is able to detect the target gas concentration well.
[0035] As described above, according to the present invention, an optical fiber gas sensor capable of detecting a target gas with high sensitivity can be manufactured relatively easily.
[0036] It should be noted that the embodiments described above do not limit the technical scope of the present invention, and various modifications and applications are possible within the scope of the present invention, even those not already described. For example, in the embodiments described above, a particularly preferred form was described in which the FBG sections 21 and 22 are fixed in contact with the inner wall surfaces of the inner cases 12 and 72, but it is sufficient for both ends of the FBG sections 21 and 22 to be fixed to the inner cases 12 and 72, and it is not essential that they be fixed in contact with the inner wall surfaces. Also, in the embodiments described above, a particularly preferred form was described in which the inner cases 12 and 72 are housed in the outer cases 13 and 73, but it is also possible to adopt a configuration in which there is no outer case. Furthermore, in the embodiments described above, a particularly preferred form was described in which through holes 11 and 71 are provided in positions other than the contact area between the FBG sections 21 and 22 and the inner cases 12 and 72, but it is also possible to provide through holes in the contact area between the FBG sections 21 and 22 and the inner cases 12 and 72. Moreover, the shape of the cases, the size, quantity, and arrangement of the fixing members are merely examples and can be changed as desired. Furthermore, the physical shape of each of the elements described above can also be arbitrarily changed within the range that achieves the effects of the present invention. [Industrial applicability]
[0037] According to the present invention, an optical fiber gas sensor capable of detecting a target gas with high sensitivity can be manufactured relatively easily and is useful as an optical fiber gas sensor. [Explanation of Symbols]
[0038] 1. Optical fiber gas sensor 2 Optical Fibers 3 Fixing member 4 Sensing member 5 Sealing member 10, 70 Double Case 11, 14, 71, 74 through holes 12, 72 Inner case (case) 13, 73 Outer case 21 FBG section 22 FBG section (for temperature compensation)
Claims
1. A tubular case, An optical fiber equipped with an FBG section, A fixing member for fixing the optical fiber to the case in a state aligned with the axial direction of the case and with tension applied to the FBG portion, A sensing member comprising a catalyst that promotes the catalytic combustion reaction of the gas to be detected, and which covers the FBG portion inside the case, A through hole provided in the wall surface of the case, A hydrophobic sealing member that covers the through hole and allows the gas to be detected to pass through, A fiber optic gas sensor equipped with [specific features / features].
2. The optical fiber gas sensor according to claim 1, wherein the case is made of a material having a larger coefficient of thermal expansion than the optical fiber.
3. The optical fiber gas sensor according to claim 2, wherein the sensing member is filled in a portion of the axial region of the case.
4. The optical fiber gas sensor according to claim 3, wherein the sensing member is a platinum-supported silica catalyst.
5. The optical fiber gas sensor according to claim 4, wherein a plurality of through holes are provided in positions excluding the contact area between the FBG portion and the case.
6. The optical fiber gas sensor according to any one of claims 1 to 5, further comprising a cylindrical outer case for housing the aforementioned case.
7. The optical fiber gas sensor according to claim 6, wherein the optical fiber further comprises an FBG section for temperature compensation.