A fiber optic corrosion sensor based on surface plasmon resonance technology

Abstract

The present invention belongs to the field of structural health monitoring, and discloses an optical fiber corrosion sensor based on surface plasmon resonance technology, including a multimode optical fiber: a core, a cladding, and a coating layer; a single-mode optical fiber: a core, a cladding, a silver film, and an iron film. The present invention realizes corrosion monitoring of steel bars based on plasma resonance technology. After the iron film, which is similar to the main component of the steel bar, corrodes, it will change the refractive index of the medium surrounding the single-mode optical fiber. The plasma between the silver film and the iron film interface of the single-mode optical fiber is highly sensitive to the change in the refractive index of the surrounding medium, and the steel bar corrosion condition is indirectly monitored by monitoring the external refractive index of the single-mode optical fiber. The sensor has a simple structure, low cost, high sensitivity, and high measurement accuracy. It can effectively realize non-destructive, real-time, and accurate measurement of the steel bar corrosion condition in reinforced concrete structures. It has broad application prospects in the field of steel corrosion monitoring and is suitable for promotion.

Description

Technical Field

[0001] The invention relates to an optical fiber corrosion sensor based on surface plasmon resonance technology, belongs to the field of structural health monitoring, and is particularly used for corrosion monitoring of steel bars in reinforced concrete structures. Background Art

[0002] Reinforced concrete structures are the most widely used structural form in civil engineering, combining the advantages of high tensile strength of steel bars and high compressive strength of concrete. However, corrosion of steel bars is the most difficult problem facing reinforced concrete structures. The corrosion of steel bars not only weakens their own cross-sectional area, reducing the bearing capacity of steel bars, but also reduces the bond strength of concrete, affecting the collaborative working performance of the two. In addition, the volume of steel bars expands after corrosion, causing concrete to crack or even fall off, ultimately leading to reduced stress performance and durability of concrete structures.

[0003] The current methods for monitoring steel corrosion can be roughly divided into two categories: electrochemical methods and non-electrochemical methods. The electrochemical method directly monitors the corrosion of steel bars. Common electrochemical methods include resistance method, potential method, electrochemical noise technology, etc. The electrochemical monitoring of steel bar corrosion methods generally have disadvantages such as long monitoring time, complex operation, high cost, and susceptibility to interference from external signals. Non-electrochemical methods usually monitor steel corrosion indirectly by indirectly monitoring the structural volume deformation, delamination and cracking caused by corrosion. Common non-electrochemical corrosion monitoring methods include acoustic emission method, ultrasonic guided broadcast method and digital image correlation method. These methods are difficult to achieve direct contact with the steel bars, and their measurement results have great uncertainty, and cannot accurately and real-time monitor the corrosion state of the steel bars.

[0004] Fiber optic sensors are small in size, light in weight, resistant to electromagnetic interference, and highly resistant to corrosion. They can be placed inside reinforced concrete to achieve non-destructive, real-time, and long-term monitoring of steel corrosion. Compared with traditional fiber optic sensors, fiber optic corrosion sensors based on surface plasmon resonance technology are simple to make, highly sensitive, and have reliable measurement results. Their monitoring results can provide an important basis for durability assessment and reinforcement and maintenance of reinforced concrete structures. Summary of the invention

[0005] In view of the problems of the prior art, the present invention provides an optical fiber corrosion sensor based on surface plasmon resonance technology, which is highly sensitive to changes in the refractive index of the surrounding environment and can indirectly monitor the corrosion degree of the steel bars by measuring changes in the external refractive index. This optical fiber corrosion sensor based on surface plasmon resonance technology can be buried near the steel bars in reinforced concrete structures to monitor the corrosion state of the steel bars in real time. The measurement results are reliable, thus providing an important basis for durability assessment and reinforcement and maintenance of reinforced concrete structures.

[0006] The technical solution of the present invention:

[0007] An optical fiber corrosion sensor based on surface plasmon resonance technology, characterized in that the optical fiber corrosion sensor based on surface plasmon resonance technology comprises a first multimode optical fiber 1, a single-mode optical fiber 2, and a second multimode optical fiber 3. The first multimode optical fiber 1 comprises a core 4, a cladding 5, and a coating 6; the single-mode optical fiber 2 comprises a core 7, a cladding 8, a silver film 9, and an iron film 10;

[0008] The two ends of the single-mode optical fiber 2 are fused with the first multi-mode optical fiber 1 and the second multi-mode optical fiber 3 respectively;

[0009] The radius of the cladding 5 of the first multimode optical fibers 1 and 3 is equal to the radius of the cladding 8 of the single-mode optical fiber 2;

[0010] The length of the single-mode optical fiber 2 is adjusted according to the size requirements of the coating instrument, the length of the material to be tested, and the sensitivity requirements of the monitored object;

[0011] The outer surface of the cladding 8 of the single-mode optical fiber 2 is uniformly coated with a layer of silver film 9, and there is no gap between the silver film 9 and the cladding 8;

[0012] A layer of iron film 10 is uniformly coated on the outer surface of the silver film 9 of the single-mode optical fiber 2, and a vacuum is formed between the iron film 10 and the silver film 9;

[0013] The silver film 9 may be plated by vacuum evaporation, vacuum push rod method, silver mirror reaction method or other coating methods.

[0014] The iron film 10 may be coated by vacuum evaporation, vacuum push rod method, plasma sputtering technology or other coating methods;

[0015] The thickness of the silver film 9 can be adjusted according to the sensor sensitivity requirements;

[0016] The silver film 9 covers the entire length of the single-mode optical fiber cladding 8;

[0017] The thickness of the iron film 10 can be adjusted according to the sensor sensitivity requirements, service life requirements and size limitations of the structure to be measured;

[0018] The iron film 10 covers the entire length of the silver film 9 .

[0019] The transmission principle of the optical fiber corrosion sensor based on plasma resonance technology is as follows:

[0020] The principle of the optical fiber corrosion sensor based on plasma resonance technology is shown in Figure 5. The laser enters the single-mode optical fiber 2 through the first multimode optical fiber 1. A part of the light 11 propagates in the core 7 of the single-mode optical fiber, and the other part of the light 12 enters the cladding 8 of the single-mode optical fiber and propagates in the cladding. As can be seen from Figure 5, when the light beam that is totally reflected in the cladding reaches the interface with the silver film 11, it does not directly generate reflected light, but first passes through the silver film 9 to a depth of about one wavelength, then flows along the interface for a distance of about half a wavelength and finally returns to the cladding 8. This part of the wave that passes through the silver film medium is the evanescent wave 14. The evanescent wave 14 passes through the silver film 9 and resonates with the surface plasma 13 at the interface between the iron film and the silver film, thereby forming a surface plasma resonance phenomenon.

[0021] The corrosion of the iron film 10 will cause the refractive index of the medium around the sensor to change, and the plasma between the interface of the iron film and the silver film is very sensitive to the change of the refractive index of the external medium. Finally, the spectrum of the transmitted light transmitted from the second multimode optical fiber 3 changes. The corrosion of the iron film can be indirectly monitored by analyzing the change in the transmission spectrum.

[0022] The above evanescent wave can be decomposed into an S polarized wave perpendicular to the interface and a P polarized wave parallel to the interface. Only P polarized light can cause surface plasmon resonance. The intensity reflection coefficient of P polarized light at the interface between the core and the cladding can be expressed by the transfer matrix as follows:

[0023]

[0024] In the formula, q i = cosθ i / n i , i represents the number of layers, and M represents the transfer matrix. Assume that the refractive index coefficient of the optical fiber is n cor The refractive index of the cladding is n cla , then the amount of reflected light occurring in the optical fiber 7 and the cladding 8 can be expressed by the following formula

[0025] N=L / Dtanθ(2)

[0026] Where L represents the transmission length of the single-mode optical fiber 2, D represents the radius of the optical fiber, and θ represents the incident angle. cr The light between 90 degrees and 100 degrees has different transmission modes. The power of each transmission mode light varies with the incident angle and is roughly Gaussian distributed. The expression is as follows:

[0027]

[0028] The transmittance of P polarized light in the optical fiber is

[0029]

[0030] Since only P-polarized light can cause surface plasmon resonance, the light transmittance can be expressed as

[0031]

[0032] The above-mentioned plasma resonance will cause different transmittances of light of different wavelengths passing through the sensor. There will be a trough in the wavelength-transmittance spectrum received by the spectrometer. The change of the external refractive index will cause the trough position to shift significantly. Therefore, the change of the external refractive index can be determined by monitoring the shifted position of the trough of the spectrum curve. The characteristic wavelength corresponding to the trough position is λ.

[0033] The main component of steel bars is iron. The iron film 10 wrapped around the outermost layer of the single-mode optical fiber 2 simulates the corrosion of steel bars in an actual reinforced concrete structure. When the iron film is corroded, the refractive index of the medium surrounding the single-mode optical fiber changes from n to n', and accordingly, the characteristic wavelength changes from λ to λ'. The change in the characteristic wavelength Δλ and the corrosion rate η of the iron film 10 can be expressed as follows:

[0034] Δλ=α(η) (6)

[0035] The fiber optic corrosion sensor based on surface plasmon resonance technology is arranged near the steel bars in the reinforced concrete structure. By analyzing the drift of the characteristic wavelength of the transmission spectrum of the fiber optic corrosion sensor based on surface plasmon resonance technology and substituting it into the relationship (6), the mass loss percentage η of the iron film can be obtained, and the corrosion degree of the iron film can be determined. The main components of the steel bar are the same as those of the iron film, and the environment of the two is exactly the same, so the corrosion degree of the steel bar under test can also be quantitatively determined.

[0036] Compared with the prior art, the present invention has the following beneficial effects:

[0037] (1) The present invention indirectly monitors the amount of steel bar corrosion by changing the transmitted light spectrum of the optical fiber corrosion sensor based on surface plasmon resonance technology.

[0038] (2) The present invention plates a silver film for surface plasmon resonance sensing on the outer surface of the cladding of a single-mode optical fiber. Compared with other metals, the silver film provides the clearest surface plasmon resonance spectrum and has a stronger sensitivity to measuring changes in the external refractive index.

[0039] (3) The present invention is very small in size and can be buried in the material to be tested to monitor steel corrosion without affecting the performance of the material to be tested, thereby achieving non-destructive monitoring of the building structure.

[0040] (4) The plasma at the interface between the iron film and the silver film of the present invention is very sensitive to the change of the refractive index of the external medium, so that the optical fiber sensor has a higher sensitivity.

[0041] (5) The present invention has a long service life, and the thickness of the iron film can be adjusted according to the service life requirements and sensitivity requirements;

[0042] (6) The present invention has a high resolution and can achieve high-precision monitoring of steel corrosion.

[0043] (7) The present invention is simple to manufacture, easy to deploy, and low in cost. It can realize real-time monitoring of steel corrosion in reinforced concrete structures, is suitable for promotion, and has a high application prospect. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 A schematic diagram of an optical fiber corrosion sensor based on plasma resonance technology of the present invention;

[0045] Figure 2 It is a cross-sectional view of the AA section of the optical fiber corrosion sensor based on the plasma resonance technology of the present invention;

[0046] Figure 3 It is a cross-sectional view of the B1-B1 section of the optical fiber corrosion sensor based on the plasma resonance technology of the present invention;

[0047] Figure 4 It is a cross-sectional view of the B2-B2 section of the optical fiber corrosion sensor based on the plasma resonance technology of the present invention;

[0048] FIG5 is a schematic diagram of the sensing mechanism of the optical fiber corrosion sensor based on the plasma resonance technology of the present invention, (a) before the iron film is corroded; (b) after the iron film is corroded; (c) schematic diagram of the incident spectrum and the transmission spectrum;

[0049] Figure 6 A schematic diagram of the arrangement of the optical fiber corrosion sensor based on the surface plasmon resonance technology of the present invention applied to the monitoring of steel bars in an actual concrete structure;

[0050] In the figure: 1 first multimode optical fiber; 2 single-mode optical fiber; 3 second multimode optical fiber; 4 multimode optical fiber core; 5 multimode optical fiber cladding; 6 multimode optical fiber coating; 7 single-mode optical fiber core; 8 single-mode optical fiber cladding; 9 silver film; 10 iron film; 11 single-mode optical fiber core transmission light; 12 single-mode optical fiber cladding transmission light; 13 surface plasma; 14 evanescent wave; 15 corrosion product. DETAILED DESCRIPTION

[0051] In order to make the purpose, features and advantages of the present invention more intuitive and easy to understand, the technical solutions 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 embodiments described below are only 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.

[0052] like Figure 1-6 As shown, the present invention provides an optical fiber corrosion sensor based on surface plasmon resonance technology, characterized in that the optical fiber corrosion sensor based on surface plasmon resonance technology comprises a first multimode optical fiber 1, a single-mode optical fiber 2, and a second multimode optical fiber 3. The first multimode optical fiber 1 comprises a core 4, a cladding 5, and a coating 6; the single-mode optical fiber 2 comprises a core 7, a cladding 8, a silver film 9, and an iron film 10;

[0053] The two ends of the single-mode optical fiber 2 are fused with the first multi-mode optical fiber 1 and the second multi-mode optical fiber 3 respectively;

[0054] The diameter of the core 4 of the first multimode optical fibers 1 and 3 is 62.5 μm, the diameter of the cladding 5 is 125 μm, and the diameter of the coating 5 is 255 μm;

[0055] The core 7 of the single-mode optical fiber 2 has a diameter of 9 μm, the cladding 5 has a diameter of 125 μm, the thickness of the silver film is 15 nm, and the thickness of the iron film is 20 μm;

[0056] The length of the single-mode optical fiber 2 is adjusted according to the size requirements of the coating instrument, the length of the material to be tested, and the sensitivity requirements of the monitored object;

[0057] The outer surface of the cladding 8 of the single-mode optical fiber 2 is uniformly coated with a layer of silver film 9, and there is no gap between the silver film 9 and the cladding 8;

[0058] A layer of iron film 10 is uniformly coated on the outer surface of the silver film 9 of the single-mode optical fiber 2, and a vacuum is formed between the iron film 10 and the silver film 9;

[0059] The silver film 9 may be plated by vacuum evaporation, vacuum push rod method, silver mirror reaction method or other coating methods.

[0060] The iron film 10 may be coated by vacuum evaporation, vacuum push rod method, plasma sputtering technology or other coating methods;

[0061] Furthermore, the thickness of the silver film 9 can be adjusted according to the sensor sensitivity requirements;

[0062] Furthermore, the silver film 9 covers the entire length of the single-mode optical fiber cladding 8;

[0063] Furthermore, the thickness of the iron film 10 can be adjusted according to the sensor sensitivity requirements, service life requirements and size restrictions of the structure to be measured;

[0064] Furthermore, the iron film 10 covers the entire length of the silver film 9 .

[0065] In an embodiment of the present invention, a method for preparing an optical fiber corrosion sensor based on surface plasmon resonance technology includes any one of the optical fiber corrosion sensors based on surface plasmon resonance technology described above, and comprises the following steps:

[0066] Step 1: Take two sections of multimode optical fiber, use a blade to peel off the coating at the end of the multimode optical fiber, and wipe the surface with alcohol to clean it.

[0067] Step 2: Take a 20mm single-mode optical fiber and use a fiber stripper to remove the coating on its surface, leaving a cladding part with a diameter of 125μm. Then wipe the surface of the crystal optical fiber with the coating removed with alcohol to clean the surface.

[0068] Step 3: Use a fiber optic cutter to cut the end of the single-mode optical fiber with the coating stripped off to ensure that the end face is flat. After cutting, wipe the surface with alcohol to clean it.

[0069] Step 4: - Use a ruby ​​knife to scratch the cross section of the multimode optical fiber and break the optical fiber at the scratch to ensure that the end face is flat. After cutting, wipe with alcohol to clean the surface;

[0070] Step 5: Use a fusion splicer to weld the two ends of the single-mode optical fiber to the two ends of the single-mode optical fiber. Since the core diameters of multi-mode optical fiber and single-mode optical fiber are different, manual welding mode is selected.

[0071] Step 6: Clean the spliced ​​multimode-singlemode-multimode optical fiber with 20% NaOH solution, rinse with distilled water, and then dry in an oven.

[0072] Step 7: Secure the cleaned fiber to a stainless steel fixture to ensure that the fiber remains straight during subsequent film deposition.

[0073] Step 8: Use the silver mirror reaction method to uniformly deposit a layer of silver film on the surface of the single-mode optical fiber cladding.

[0074] Step 9: Use a radio frequency magnetron sputtering system to sputter an iron film on the outer surface of the single-mode optical fiber that has been coated with a silver film using a Fe target with a purity of 99.99%; flip the fixture over and plate again to ensure that the iron film is evenly deposited.

[0075] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. An optical fiber corrosion sensor based on surface plasmon resonance technology, characterized in that: The optical fiber corrosion sensor based on surface plasmon resonance technology comprises a first multimode optical fiber (1), a single-mode optical fiber (2), and a second multimode optical fiber (3); wherein the first multimode optical fiber (1) and the second multimode optical fiber (3) comprise a first fiber core (4), a first cladding (5), and a coating layer (6); and the single-mode optical fiber (2) comprises a second fiber core (7), a second cladding (8), a silver film (9), and an iron film (10); The two ends of the single-mode optical fiber (2) are fused to the first multi-mode optical fiber (1) and the second multi-mode optical fiber (3) respectively; The radius of the first cladding (5) of the first multimode optical fiber (1) and the second multimode optical fiber (3) is equal to the radius of the second cladding (8) of the single-mode optical fiber (2); The outer surface of the second cladding (8) of the single-mode optical fiber (2) is uniformly coated with a layer of silver film (9), and there is no gap between the silver film (9) and the second cladding (8); the outer surface of the silver film (9) of the single-mode optical fiber (2) is uniformly coated with a layer of iron film (10), and there is a vacuum between the iron film (10) and the silver film (9); Laser light enters a single-mode optical fiber (2) through a first multi-mode optical fiber (1), a portion of the light (11) propagates in a second core (7) of the single-mode optical fiber, and another portion of the light (12) enters a second cladding (8) of the single-mode optical fiber and propagates in the second cladding (8); when the light beam that is totally reflected in the second cladding (8) reaches an interface with a silver film (9), it first passes through the silver film (9) to a depth of one wavelength, then flows along the interface for a distance of half a wavelength and finally returns to the second cladding (8); this portion of the wave that passes through the silver film medium is an evanescent wave (14); the evanescent wave (14) passes through the silver film (9) and resonates with a surface plasma (13) at an interface between the iron film and the silver film, thereby forming a surface plasma resonance phenomenon.

2. The optical fiber corrosion sensor based on surface plasmon resonance technology according to claim 1 is characterized in that: The length of the single-mode optical fiber (2) is adjusted according to the coating size requirements, the length of the material to be measured and the sensitivity requirements of the monitored object.

3. The optical fiber corrosion sensor based on surface plasmon resonance technology according to claim 1 or 2, characterized in that: The silver film (9) is plated by a vacuum evaporation method, a vacuum push rod method or a silver mirror reaction method; the iron film (10) is plated by a vacuum evaporation method, a vacuum push rod method or a plasma sputtering technique.

4. The optical fiber corrosion sensor based on surface plasmon resonance technology according to claim 1 or 2, characterized in that: The silver film (9) covers the entire length of the second cladding (8) of the single-mode optical fiber; the thickness of the silver film (9) is adjusted according to the sensitivity requirements of the sensor.

5. The optical fiber corrosion sensor based on surface plasmon resonance technology according to claim 1 or 2, characterized in that: The iron film (10) covers the entire length of the silver film (9); the thickness of the iron film (10) is adjusted according to the sensor sensitivity requirements, service life requirements and the size of the structure to be measured.

6. The optical fiber corrosion sensor based on surface plasmon resonance technology according to claim 3 is characterized in that: The iron film (10) covers the entire length of the silver film (9); the thickness of the iron film (10) is adjusted according to the sensor sensitivity requirements, service life requirements and the size of the structure to be measured.

7. The optical fiber corrosion sensor based on surface plasmon resonance technology according to claim 4 is characterized in that: The iron film (10) covers the entire length of the silver film (9); the thickness of the iron film (10) is adjusted according to the sensor sensitivity requirements, service life requirements and the size of the structure to be measured.

Images