Hydrogen sulfide concentration sensor based on asymmetric double-core fiber long-period fiber grating

Abstract

The invention provides a manufacturing method of a hydrogen sulfide gas concentration optical fiber sensor based on an asymmetric double-core optical fiber long-period optical fiber grating structure and a detection system of the hydrogen sulfide gas concentration optical fiber sensor. The sensor comprises an asymmetric double-core optical fiber (3), and the asymmetric double-core optical fiber (3) is provided with a middle fiber core (6) and an eccentric core (5) deviating from the central axis. And two ends of an eccentric core (5) of the asymmetric double-core optical fiber (3) are respectively connected with the first section of single-mode optical fiber (1) and the second section of single-mode optical fiber (4). A CO2 laser is used for writing a long-period fiber grating (7) on the asymmetric double-core optical fiber (3), and the surface of the asymmetric double-core optical fiber (3) is coated with a zinc oxide film (2). The sensor can realize high-sensitivity gas concentration detection, and has the characteristics of compact structure and high repeatability.

Description

Technical Field

[0001] The present invention relates to the technical field of fiber optic sensing, and particularly to a design of a hydrogen sulfide concentration sensor based on an asymmetric dual-core fiber long-period fiber grating. Background Art

[0002] Fiber optic gas sensors have the advantages of being structurally compact, anti-electromagnetic interference, and high sensitivity, and are widely used in various gas detection environments. A long-period fiber grating is a fiber with a periodically varying core refractive index, which can couple the fundamental mode to the high-order cladding mode and has high sensitivity to changes in the external environment. It is widely used in sensing such as refractive index, torsion, and gas concentration. Hydrogen sulfide is a common by-product in fertilizer manufacturing, petroleum, natural gas refining, and power equipment, and is a highly toxic flammable gas. Hydrogen sulfide not only has a pungent rotten egg odor. When the hydrogen sulfide concentration is higher than 20 ppm, it will irritate the human lungs and eyes; when the concentration is higher than 300 ppm, it will cause damage to the eyes; when the concentration is higher than 1000 ppm, it will lead to respiratory failure and even death. Therefore, monitoring the hydrogen sulfide concentration is of great significance for ensuring the safety of production and life.

[0003] In 2017, Deng et al. (see reference [1] D.S. Deng, W.L. Feng, et al., “Trace hydrogen sulfide gas sensor based on tungsten sulfide membrane-coated thin-core fiber modal interferometer,” Applied Surface Science, vol. 423, pp. 492-497, 2017.) used a thin-core fiber Mach-Zehnder interferometer combined with a 320-nm tungsten sulfide thin film to improve the hydrogen sulfide detection sensitivity to 18.37 pm / ppm in the range of 0-80 ppm; in 2018, Qin et al. (see reference [2] X. Qin, W. Feng, et al., “Molybdenum sulfide / citric acid composite membrane-coated long period fiber grating sensor for measuring trace hydrogen sulfide gas”, Sensors and Actuators B: Chemical, vol. 272, pp. 60-68, 2018.) achieved the detection of hydrogen sulfide gas with a sensitivity of 10.52 pm / ppm in the range of 0-70 ppm by preparing a molybdenum sulfide / citric acid composite membrane on the surface of a long period fiber grating. In 2020, Huang et al. fused a photonic crystal fiber between two multimode fibers (see reference [3] G.J. Huang, Y.J. Li, et al., “Hydrogen sulfide gas sensor based on titanium dioxide / amino-functionalized graphene quantum dots coated photonic crystal fiber,” Journal of Physics D: Applied Physics, vol. 53, pp. 325102, 2020), and coated a composite material of titanium dioxide / amino-functionalized graphene quantum dots as a sensitive film. This sensor achieved hydrogen sulfide sensing with a sensitivity of 26.62 pm / ppm in the range of 0-55 ppm. The existing detection technologies have low sensitivity and need to further improve the detection sensitivity. Summary of the Invention

[0004] Technical problem to be solved by the present invention: To further improve the sensitivity of hydrogen sulfide detection, the present invention designs a hydrogen sulfide concentration sensor based on an asymmetric dual-core fiber long-period fiber grating.

[0005] The technical solution of the present invention is as follows:

[0006] A hydrogen sulfide concentration sensor based on an asymmetric dual-core fiber long-period fiber grating, comprising an asymmetric dual-core fiber 3, the asymmetric dual-core fiber 3 having an intermediate core 6 and an eccentric core 5 offset from the central axis, the two ends of the eccentric core 5 being respectively connected to a first single-mode fiber 1 and a second single-mode fiber 4, long-period fiber gratings 7 being inscribed on both the intermediate core 6 and the eccentric core 5, and a zinc oxide thin film 2 being coated on the surface of the asymmetric dual-core fiber 3.

[0007] The asymmetric dual-core fiber 3 has a cladding diameter of 120 - 180 μm, a core diameter of 5 - 15 μm, a distance between the two cores of 30 - 50 μm, and a length of 2 - 4 cm; the long-period fiber grating 7 has a period of 300 - 500 μm and a number of periods of 30 - 40; the thickness of the zinc oxide thin film 2 on the surface is 100 nm - 120 nm.

[0008] When the light output from the first single-mode fiber 1 is incident on the eccentric core 5, the optical field is transmitted along the core in the fundamental mode; at the resonant wavelength, the fundamental mode is coupled to the cladding mode through the grating. Due to the unique asymmetric structure of the dual-core fiber, high-order asymmetric modes sensitive to the external environment will be excited, and at the same time, part of the energy in the cladding mode will be coupled to the intermediate core 6. When the reconstructed fundamental mode in the intermediate core 6 satisfies the phase matching condition, secondary coupling will occur, causing the energy to be recoupled back to the eccentric core 5.

[0009] A preparation method of a hydrogen sulfide concentration sensor based on an asymmetric dual-core fiber long-period fiber grating: After the end faces of the first single-mode fiber 1, the asymmetric dual-core fiber 3, and the second single-mode fiber 4 are cut flat, the core of the single-mode fiber is aligned and fusion-spliced with the eccentric core 5 of the asymmetric dual-core fiber 3; a CO2 laser is used to inscribe long-period fiber gratings 7 on the intermediate core 6 and the eccentric core 5 of the asymmetric dual-core fiber 3; an atomic layer deposition technique is used to deposit a zinc oxide thin film 2 on the surface of the asymmetric dual-core fiber 3, and the thickness of the zinc oxide film is controlled by controlling the deposition time during deposition.

[0010] A detection system of a hydrogen sulfide concentration sensor based on an asymmetric dual-core fiber long-period fiber grating, comprising the above-mentioned fiber optic sensor 9 for hydrogen sulfide detection, the fiber optic sensor 9 being placed in a gas chamber 10, and a broadband light source 8, the fiber optic sensor 9 in the gas chamber 10, and a spectrometer 11 being connected in sequence through a single-mode fiber.

[0011] The working wavelength of the broadband light source 8 is 600 nm - 1700 nm.

[0012] After the light emitted by the broadband light source 8 is transmitted to the fiber optic sensor 9, the transmitted light is transmitted to the spectrometer 11 for monitoring.

[0013] Hydrogen sulfide gas flows through the gas chamber 10. When the concentration of hydrogen sulfide changes, a redox reaction occurs between the zinc oxide thin film 2 on the surface of the asymmetric twin-core fiber 3 and the hydrogen sulfide gas, resulting in a change in the refractive index of the thin film, and further causing a shift in the transmission spectrum. Compared with the prior art, the present invention has the following remarkable advantages:

[0014] The asymmetric twin-core fiber has a unique asymmetric structure. Writing a long-period fiber grating on the asymmetric twin-core fiber can excite higher-order asymmetric cladding modes, improving the sensitivity to environmental changes. The sensor includes a single-mode fiber and an asymmetric twin-core fiber. After the first section of the single-mode fiber, the asymmetric twin-core fiber, and the second section of the single-mode fiber are cut flat at the end faces, they are sequentially fused using a special fiber fusion splicer. After writing a long-period fiber grating on the asymmetric twin-core fiber using a CO2 laser, an atomic layer deposition is used to coat a zinc oxide thin film on the surface of the asymmetric twin-core fiber, and the thickness of the thin film can be controlled by controlling the deposition time during deposition. The sensor prepared by this method has the characteristics of high sensitivity and compact structure.

[0015] This fiber optic sensor uses a CO2 laser to write a long-period grating in the asymmetric twin-core fiber, transmits the highly sensitive asymmetric mode to environmental changes through the side core, and combines the cooperative coupling mechanism of the two fiber cores to significantly improve the sensing sensitivity. An atomic layer deposition process is used to coat a zinc oxide sensitive thin film on the surface of the grating, realizing high-sensitivity detection of hydrogen sulfide gas. The sensor of the present invention has a compact structure, strong stability, and a mature preparation process, and has great application potential in the field of gas detection. Brief Description of the Drawings

[0016] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts. Among them:

[0017] Figure 1 It is a schematic diagram of the overall structure of the hydrogen sulfide concentration sensor of the present invention.

[0018] Figure 2 It is a detection system diagram of the hydrogen sulfide concentration sensor of the present invention.

[0019] Figure 3 It is the sensing spectrum of the hydrogen sulfide concentration sensor of the present invention.

[0020] Figure 4This is the sensing spectrum of the present invention when no grating is inscribed on the middle core of the asymmetric dual-core fiber long-period fiber grating sensor.

[0021] Figure 5 This is the experimental spectrum and sensing sensitivity of the hydrogen sulfide concentration sensor of the present invention.

[0022] Among them, the reference numerals are: 1. The first section of single-mode fiber; 2. Zinc oxide thin film; 3. Asymmetric dual-core fiber; 4. The second section of single-mode fiber; 5. Eccentricity of the asymmetric dual-core fiber; 6. Middle core of the asymmetric dual-core fiber; 7. Long-period grating on the asymmetric dual-core fiber; 8. Broadband light source; 9. Fiber optic sensor; 10. Gas chamber; 11. Spectrometer. Specific embodiments

[0023] In order to more clearly illustrate the technical content of the present invention, specific embodiments are provided below, and the specific embodiments of the present invention are further described in conjunction with the accompanying drawings.

[0024] The following description provides many specific details to fully understand the content of the present invention. However, in addition to the described manner, the present invention can also adopt other different implementation manners. Those skilled in the art can make similar changes and modifications without departing from the principle of the present invention. Therefore, the present invention is not limited by the following specific embodiments and can have a wider application and implementation.

[0025] Secondly, the "one embodiment" or "embodiment" mentioned refers to the specific features, structures or characteristics included in at least one implementation manner of the present invention. In this specification, the appearance of "in one embodiment" does not all refer to the same embodiment, nor is it a separate or selectively exclusive embodiment from other embodiments.

[0026] Thirdly, the purpose of the device structure schematic diagram is to assist in understanding and explaining the principle and implementation manner of the present invention, rather than an accurate scale drawing. Therefore, the dimensions and proportions in the schematic diagram may not be exactly the same as those of the actually manufactured device, and the schematic diagram is only for example and does not limit the protection scope of the present invention. In actual manufacturing, the accuracy of three-dimensional spatial dimensions such as length, width and depth should be considered.

[0027] Refer to Figure 1, The hydrogen sulfide concentration sensor based on an asymmetric twin-core fiber long-period fiber grating includes a first single-mode fiber 1, an asymmetric twin-core fiber 3, and a second single-mode fiber 4. After the end faces of each part are cut flat with a cutting knife, they are successively fused using a special optical fiber fusion splicer. Among them, the asymmetric twin-core fiber has an intermediate core 6 and an eccentric core 5 that deviates from the central axis. The two ends of the eccentric core of the asymmetric twin-core fiber are respectively connected to the first single-mode fiber 1 and the second single-mode fiber 4. A long-period fiber grating 7 is inscribed on the asymmetric twin-core fiber using a CO2 laser, and a zinc oxide thin film 2 is coated on the surface of the asymmetric twin-core fiber.

[0028] Specifically, the first single-mode fiber and the second single-mode fiber have the same parameters. Their core and cladding diameters are 8.3μm and 125μm respectively, and their refractive indices are 1.4681 and 1.4628 respectively; the cladding diameter of the asymmetric twin-core fiber is 150μm, the diameters of both cores are 9μm, the core pitch is 42μm, and the length is 3cm; the period of the long-period fiber grating is 400μm, and the number of periods is 30 - 40; the thickness of the zinc oxide is 100nm - 120nm. This data is obtained from simulation and experimental experience. Based on these parameters, the sensor can achieve high-sensitivity sensing. Conversely, the effectiveness of the sensor cannot be guaranteed.

[0029] Refer to Figure 2 , The hydrogen sulfide concentration sensing system based on an asymmetric twin-core fiber long-period fiber grating includes a broadband light source 8, a gas chamber 10, a spectrometer 11, and an optical fiber sensor 9. The optical fiber sensor 9 is placed in the gas chamber 10. The broadband light source 8 emits continuous light. After being transmitted to the optical fiber sensor 9, the transmitted light is transmitted to the spectrometer 11, enabling real-time monitoring of the transmission spectrum.

[0030] Refer to Figure 3 and Figure 4 , When hydrogen sulfide gas flows through the gas chamber, when the hydrogen sulfide concentration changes, the zinc oxide thin film 2 on the surface of the asymmetric twin-core fiber will undergo an oxidation-reduction reaction with the hydrogen sulfide gas, resulting in a change in the refractive index of the thin film and a shift in the transmission spectrum. When the refractive index of the thin film changes within the range of 1.899 - 1.929, the sensitivity of the intermediate core of the asymmetric twin-core fiber without an inscribed grating is -766.46nm / RIU, and the sensitivity of the proposed asymmetric twin-core fiber long-period fiber grating sensor can reach -876.38nm / RIU, which is greatly improved compared to when the intermediate core does not have an inscribed long-period grating.

[0031] Refer to Figure 5 , Hydrogen sulfide gas with different concentrations of 5 - 40ppm is introduced into the gas chamber 10, and the spectrometer is used to observe the change of the transmission spectrum of the sensor in real time. It can be observed that as the gas concentration increases, the resonant wavelength of the transmission spectrum of the sensor shifts towards the long-wavelength direction, and the sensitivity can reach 56.82nm / ppm, and the linear fitting degree R2 is 0.9914.

[0032] Among them, the output wavelength of the broadband light source 8 is 600 nm - 1700 nm.

[0033] The light output by the broadband light source 8 is transmitted through the first - stage single - mode optical fiber 1 and reaches the eccentric core 5 of the asymmetric dual - core optical fiber. Since the long - period fiber grating 7 is inscribed on the eccentric core, the fundamental mode transmitted by the first - stage single - mode optical fiber can be coupled to the high - order asymmetric cladding mode. After the cladding mode is transmitted through the second - stage single - mode optical fiber 4, it is output to the spectrometer 11 for monitoring. By analyzing the transmission spectrum, the change in the concentration of hydrogen sulfide in the gas chamber can be detected.

[0034] Sensing mechanism description: When hydrogen sulfide gas flows through the gas chamber, when the concentration of hydrogen sulfide changes, the zinc oxide thin film on the surface of the asymmetric dual - core optical fiber will undergo a redox reaction with the hydrogen sulfide gas, resulting in a change in the refractive index of the thin film, and then causing a shift in the transmission spectrum.

[0035] In the long - period fiber grating of the asymmetric dual - core optical fiber, the core is periodically modulated. When the effective refractive index of the fundamental mode and the effective refractive index of the cladding mode meet the phase - matching condition, the fundamental mode and the high - order cladding mode will undergo mode coupling. Among them, the phase - matching condition is obtained by the following formula:

[0036]

[0037] In the formula, λ is the resonant wavelength, and Λ is the period of the long - period fiber grating. When the surface of the long - period fiber grating of the asymmetric dual - core optical fiber is coated with a zinc oxide thin film, the effective refractive index of the cladding mode will be affected by the zinc oxide thin film. When the concentration of hydrogen sulfide changes, the zinc oxide thin film on the surface of the asymmetric dual - core optical fiber will undergo a redox reaction with the hydrogen sulfide gas, resulting in a change in the refractive index of the thin film and a change in the effective refractive index of the cladding mode. Since the redox reaction is reversible, by analyzing the shift of the resonant wavelength, the change in the concentration of hydrogen sulfide can be monitored in real time.

[0038] Since long - period gratings 7 are inscribed on both the middle core and the eccentric core of the asymmetric dual - core optical fiber, when the light output from the single - mode optical fiber is incident on the eccentric core, the light is transmitted in the core in the fundamental mode. At the resonant wavelength, the light is coupled from the fundamental mode to the cladding, and at the same time, the energy in the cladding mode will be coupled to the middle core. When the reconstructed fundamental mode in the middle core meets the phase - matching condition, a secondary coupling will occur and return to the eccentric core. Therefore, the change in the refractive index of the thin film will cause a change in the effective refractive index of the cladding mode. Due to the coupling between the two cores, inscribing long - period fiber gratings on both cores of the asymmetric dual - core optical fiber can improve the sensitivity of the sensor.

[0039] The manufacturing process of a hydrogen sulfide concentration sensor based on an asymmetric dual-core fiber long-period fiber grating includes cutting, fusion splicing, writing the long-period fiber grating with a CO2 laser, and coating a zinc oxide thin film on the surface of the long-period fiber grating.

[0040] During the preparation, first use a wire stripper to remove the coating layers of the first single-mode fiber 1, the asymmetric dual-core fiber 3, and the second single-mode fiber 4, and clean the bare fibers. Then, use a special fiber fusion splicer to adjust the fusion splicing parameters so that the cores of the single-mode fibers are aligned and fused with the eccentric part 5 of the asymmetric dual-core fiber. Next, use a CO2 laser to write a long-period fiber grating 7 on the asymmetric dual-core fiber. Finally, use atomic layer deposition technology to deposit a zinc oxide thin film 2 on the surface of the asymmetric dual-core fiber. During the deposition, control the thickness of the zinc oxide film by controlling the deposition time.

[0041] Through the above steps, the preparation and processing of the sensor structure are completed.

[0042] It is important to note that the construction and arrangement of the multiple different exemplary embodiments shown in the present invention are only illustrative. Although only a few embodiments are described in detail in this disclosure, those who read this disclosure should be able to understand that many modifications are possible without departing from the subject matter, novel teachings, and advantages described in this application. These modifications include, but are not limited to, changes in the dimensions, scales, structures, shapes, proportions of various elements, as well as parameter values (such as temperature, pressure, etc.), installation arrangements, use of materials, colors, orientations, etc. For example, an element that is exemplarily presented as being integrally formed may be composed of multiple parts or elements, the position of the element may be inverted or otherwise changed, and the nature, quantity, or position of discrete elements may be modified or changed. Therefore, all these modifications are intended to be included within the scope of the present invention. The order or sequence of any process or method steps may be changed or reordered according to alternative embodiments. In the claims, any "means-plus-function" clause is intended to cover the structure that implements the described function, and is not limited to structural equivalency. Without departing from the scope of the present invention, the design, operating state, and arrangement of the exemplary embodiments may be subject to other alternatives, modifications, changes, and omissions. Therefore, the present invention is not limited to specific embodiments, but extends to various modifications that still fall within the scope of the appended claims.

[0043] In addition, in order to provide a concise description of the exemplary embodiments, features that are irrelevant to the best mode currently considered for implementing the present invention or features that are irrelevant to the implementation of the present invention in the actual embodiments may be omitted.

[0044] It should be understood that, during the development of actual embodiments, as in any engineering or design project, many decisions regarding specific embodiments can be made. Such development work may be complex and time-consuming, but for those of ordinary skill in the art who benefit from the present disclosure, excessive experimentation is not required. The described development work will become routine work in design, manufacturing, and production.

[0045] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not restrictive. Although the present invention has been described in detail with reference to the best embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently substituted without departing from the core idea and scope of the technical solutions of the present invention. These modifications or equivalent substitutions should all be included within the scope of the claims of the present invention.

Claims

1. A hydrogen sulfide concentration sensor based on an asymmetric dual-core fiber long-period fiber grating, comprising an asymmetric dual-core fiber (3), characterized in that: The asymmetric dual-core optical fiber (3) has an intermediate fiber core (6) and an eccentric core (5) that deviates from the central axis. The two ends of the eccentric core (5) are respectively connected to the first section of single-mode optical fiber (1) and the second section of single-mode optical fiber (4). Long-period fiber gratings (7) are engraved on the intermediate fiber core (6) and the eccentric core (5). The surface of the asymmetric dual-core optical fiber (3) is coated with a zinc oxide film (2).

2. The hydrogen sulfide concentration sensor based on asymmetric dual-core fiber long-period fiber grating according to claim 1 is characterized in that: The cladding diameter of the asymmetric dual-core optical fiber (3) is 120-180 μm, the core diameter is 5-15 μm, the distance between the two cores is 30-50 μm, and the length is 2-4 cm; the period of the long-period fiber grating (7) is 300-500 μm, and the number of periods is 30-40; the thickness of the zinc oxide film (2) on the surface is 100 nm-120 nm.

3. The hydrogen sulfide concentration sensor based on asymmetric dual-core fiber long-period fiber grating according to claim 1 is characterized in that: When the output light of the first section of single-mode optical fiber (1) is incident on the eccentric core (5), the light field is transmitted along the core in the form of a fundamental mode; at the resonant wavelength, the fundamental mode is coupled to the cladding mode through the grating. Due to the unique asymmetric structure of the dual-core optical fiber, a high-order asymmetric mode that is sensitive to the external environment is excited. At the same time, part of the energy in the cladding mode is coupled to the intermediate core (6). When the fundamental mode reconstructed in the intermediate core (6) meets the phase matching condition, secondary coupling will occur, so that the energy is re-coupled back to the eccentric core (5).

4. A method for preparing a hydrogen sulfide concentration sensor based on an asymmetric dual-core fiber long-period fiber grating, characterized in that: After the end faces of the first section of single-mode optical fiber (1), the asymmetric dual-core optical fiber (3) and the second section of single-mode optical fiber (4) are cut flat, the core of the single-mode optical fiber and the eccentric core (5) of the asymmetric dual-core optical fiber (3) are aligned and fused; a long-period fiber grating (7) is written on the middle core (6) and the eccentric core (5) of the asymmetric dual-core optical fiber (3) using a CO2 laser; and a zinc oxide film (2) is plated on the surface of the asymmetric dual-core optical fiber (3) using atomic layer deposition technology, and the thickness of the zinc oxide film is controlled by controlling the deposition time during deposition.

5. A detection system of a hydrogen sulfide concentration sensor based on an asymmetric dual-core fiber long-period fiber grating, comprising the optical fiber sensor (9) for hydrogen sulfide detection according to claim 1, characterized in that: The optical fiber sensor (9) is placed in the air chamber (10), and the broadband light source (8), the optical fiber sensor (9) in the air chamber (10) and the spectrometer (11) are connected in sequence through single-mode optical fibers.

6. The detection system of the hydrogen sulfide concentration sensor based on the asymmetric dual-core fiber long-period fiber grating according to claim 5 is characterized in that: The working wavelength of the broadband light source (8) is 600nm-1700nm.

7. The detection system of the hydrogen sulfide concentration sensor based on the asymmetric dual-core fiber long period fiber grating according to claim 5 is characterized in that: After the light emitted by the broadband light source (8) is transmitted to the optical fiber sensor (9), the transmitted light is transmitted to the spectrometer (11) for monitoring.

8. The detection system of the hydrogen sulfide concentration sensor based on the asymmetric dual-core fiber long-period fiber grating according to claim 7 is characterized in that: Hydrogen sulfide gas flows in the gas chamber (10). When the concentration of hydrogen sulfide changes, the zinc oxide film (2) on the surface of the asymmetric dual-core optical fiber (3) undergoes a redox reaction with the hydrogen sulfide gas, causing the refractive index of the film to change, thereby causing the transmission spectrum to shift.

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