A steel corrosion sensor based on photonic crystal fiber probe

Abstract

The present invention belongs to the field of sensor technology, and discloses a steel corrosion sensor based on a photonic crystal fiber probe. The steel corrosion sensor includes a single-mode optical fiber: a core, a cladding, and a coating layer; a photonic crystal optical fiber: a core, a cladding, cladding pores, and an iron film. The photonic crystal fiber excited state cladding mode of the sensor is highly sensitive to the refractive index of the external medium. The iron film, which is similar to the main component of steel, will change the refractive index of its surroundings after corrosion. The steel corrosion condition can be indirectly and effectively monitored by real-time monitoring of the external refractive index of the crystal optical fiber. The sensor has the advantages of simple manufacture, small size, high strength, and high sensitivity. It can effectively realize non-destructive, real-time, and accurate measurement of the steel corrosion condition in construction projects, and has broad application prospects.

Description

Technical Field

[0001] The invention relates to a steel corrosion sensor based on a photonic crystal optical fiber probe, belongs to the field of structural health monitoring, and is particularly used for corrosion monitoring of steel in reinforced concrete structures and steel structures. Background Art

[0002] Steel is one of the commonly used engineering materials in building structures. For example, steel bars in reinforced concrete structures, steel sections in rigid structures, and steel structures are all made of steel. Steel corrosion is one of the important factors affecting the durability of these building structures. Every year in my country, there are many accidents of building structure damage and even collapse caused by steel corrosion, which has caused huge economic losses and casualties. Therefore, it is necessary to effectively monitor the corrosion of steel in building structures, strengthen and repair damaged structures in time, and prevent disasters.

[0003] The current methods for monitoring steel corrosion can be roughly divided into two categories: electrochemical methods and non-electrochemical methods. Electrochemical methods directly monitor corrosion parameters to determine the corrosion state of steel, such as corrosion potential, corrosion resistance and corrosion rate. Non-electrochemical methods usually monitor the degree of 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, ultrasonic broadcasting and digital image correlation. Acoustic emission belongs to passive monitoring and can only monitor expanding cracks. In addition, the monitoring range of acoustic emission sensors is not easy to determine, and the acquisition signal is easily interfered by environmental factors. Digital image correlation has the advantages of convenient measurement, wide measurement range and high image sampling frequency, but it is difficult to identify the cracking of the pixel block position of the corresponding point or the uneven deformation caused by stress concentration. If it is applied to actual engineering, the layout of its image acquisition equipment also has certain requirements for the surrounding environment and space of the component to be measured.

[0004] Fiber optic sensors have been increasingly widely used in recent years due to their small size, light weight, anti-electromagnetic interference, strong corrosion resistance and other advantages. Some fiber optic sensors used to monitor steel corrosion have also been developed. However, traditional fiber optic sensors for monitoring steel corrosion have problems such as long monitoring cycle, low monitoring sensitivity and low monitoring accuracy. Therefore, it is very necessary to provide a fiber optic sensor that is simple to manufacture, highly sensitive to steel corrosion status, and has accurate and reliable monitoring results. Summary of the invention

[0005] In order to solve the above technical problems, the present invention provides a steel corrosion sensor based on a photonic crystal fiber probe. The biggest feature of this sensor is that it is very sensitive to changes in the refractive index of the surrounding environment, and can indirectly monitor the degree of steel corrosion by measuring changes in the external refractive index. This steel corrosion sensor based on a photonic crystal fiber probe is simple to make, can non-destructively and real-time monitor the corrosion of steel in components, and effectively judge the degree of damage to the structure, thereby ensuring the safety of important engineering structures.

[0006] The technical solution of the present invention:

[0007] A steel corrosion sensor based on a photonic crystal fiber probe comprises a single-mode optical fiber 1, a photonic crystal fiber (PCF) probe 2, a three-port circulator 3, an amplified spontaneous emission (ASE) laser 4 and a spectrum analyzer 5; wherein the single-mode optical fiber 1 comprises a core 6, a cladding 7 and a coating 8; the photonic crystal fiber 2 comprises a core 11, an air channel 10, a cladding 12 and an iron film 14;

[0008] One end of the single-mode optical fiber 1 is connected to one of the interfaces of the three-port circulator 3, and the other end is fused with the photonic crystal optical fiber 2. There is an air bubble 9 at the fusion point between the single-mode optical fiber 1 and the photonic crystal optical fiber 2, and the air bubble 9 serves as a first reflector 15;

[0009] The other two ports of the three-port circulator 3 are connected to an ASE laser 4 and a spectrum analyzer 5 respectively;

[0010] The other end of the photonic crystal fiber 2 is a sphere 13 formed by fusion, and the tip of the sphere is a second reflector 16 .

[0011] The thickness of the iron film 14 is adjusted according to the sensor sensitivity and service life requirements.

[0012] The length of the photonic crystal fiber probe 2 is adjusted according to the monitoring object and monitoring sensitivity requirements.

[0013] The iron film 14 is wrapped around the outer surface of the photonic crystal fiber probe 2 and covers the entire length of the photonic crystal fiber probe 2 .

[0014] The thickness of the iron film 14 is adjusted according to the sensor sensitivity and service life requirements.

[0015] The transmission principle of the steel corrosion sensor based on the photonic crystal fiber probe is:

[0016] The principle of the steel corrosion sensor based on the photonic crystal fiber probe is shown in Figure 5(a). The light source emitted by the ASE laser is transmitted to the core of the single-mode fiber through a three-port circulator. As can be seen from Figure 5(a), when the light propagating in the single-mode fiber reaches the air bubble in the splicing area with the photonic crystal fiber, part of the light is reflected back; the other part of the light passes through the bubble, and the air bubble acts as a divergent lens to diffract the light. Part of the light is excited to be transmitted into the cladding to form a cladding mode. The excited cladding mode is very sensitive to changes in the external refractive index. Changes in the external refractive index will cause changes in the effective refractive index of the cladding mode. The effective refractive index of the cladding mode is a function of the external refractive index of the wavelength, that is, Where λ is the wavelength of the transmitted light, n ext is the external refractive index. The core mode is wavelength-dependent, that is,

[0017] The mode propagating through the photonic crystal fiber is reflected back when it reaches the spherical end face at the end of the device. When it reaches the splicing point again, the core mode and cladding mode with phase difference interfere with each other. The working principle of the device is essentially a Michelson interferometer of multi-beam interference. Finally, the transmission light returned to the single-mode fiber is transmitted to the spectrum analyzer again through the three-port circulator.

[0018] The total reflected electric field of the reflected light is estimated by the sum of the total reflected electric fields of the two mirrors:

[0019]

[0020] Where E0 is the incident light field, α1 represents the transmission loss of the first reflector, R1 and R2 represent the reflectivity of the first reflector and the second reflector, respectively, which can be expressed as

[0021]

[0022] Where n i represent the refractive index of single-mode fiber, bubble fiber and photonic crystal fiber respectively. represents the phase, which can be expressed as

[0023]

[0024] L i represents the distance between the junction of the photonic crystal fiber and the single-mode fiber and the tip of the solid sphere. This value is a constant. The interference spectrum intensity of the reflected light is finally expressed as

[0025]

[0026] Since the main component of low-carbon steel is iron, an iron film deposited on the outside of the photonic crystal fiber is used to simulate the low-carbon steel widely used in engineering. When the iron film is corroded, the refractive index of the medium around the photonic crystal fiber changes from n to n', which eventually causes the wavelength drift and light intensity change of the overall response spectrum of the reflected light received by the spectrometer, such as Figure 2 As shown. By analyzing the reflection spectrum, the corrosion state of the iron film can be indirectly monitored.

[0027] Let the change in spectral intensity be ΔI(λ), that is, the reflected light spectrum ΔI R The difference between (λ) and the incident light spectrum ΔI0(λ)

[0028] ΔI(λ)=ΔI R (λ)-ΔI0(λ)(5)

[0029] The relationship between the percentage of mass loss of the iron film caused by corrosion, η, and the change in spectral intensity, ΔI, can be expressed by the following formula:

[0030] ΔI=γ(η) (6)

[0031] Where γ is the functional relationship between the mass loss percentage η of the iron film and the spectral intensity change ΔI, which is determined by experimental fitting.

[0032] Therefore, the steel corrosion sensor based on the photonic crystal fiber probe is arranged around the steel components in the building structure. By analyzing the change ΔI of the reflection spectrum of the steel corrosion sensor of the photonic crystal fiber probe, the mass loss percentage γ of the iron film can be obtained according to the relationship (6), and the quantitative monitoring of the corrosion state of the iron film can be realized. Because the composition of the iron film is similar to that of low-carbon steel, the quantitative monitoring of the corrosion state of the tested steel can also be realized.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] (1) The present invention indirectly monitors the amount of steel corrosion by changing the reflected light spectrum of the steel corrosion sensor based on the photonic crystal fiber probe.

[0035] (2) The present invention cleverly combines single-mode optical fiber and photonic crystal optical fiber, and controls the size of the collapsed area by reasonably selecting welding parameters to form a more excellent interference spectrum, which makes steel corrosion monitoring more accurate.

[0036] (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.

[0037] (4) The present invention has high sensitivity. The cladding mode excited by the air bubble is very sensitive to the change of the refractive index of the external medium, making the sensor highly sensitive to the corrosion condition of the surrounding steel.

[0038] (5) The present invention has a high resolution and can achieve high-precision monitoring of steel corrosion.

[0039] (6) The photonic crystal fiber probe of the present invention has high strength, which ensures the long-term stable operation of the sensor.

[0040] (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 building structures, is suitable for promotion, and has a high application prospect. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] FIG1(a) is a schematic diagram of a steel corrosion sensor based on a photonic crystal fiber probe according to the present invention; FIG1(b) is a schematic diagram of a single-mode optical fiber; FIG1(c) is a schematic diagram of a photonic crystal fiber probe;

[0042] Figure 2 It is a cross-sectional view of the AA section of the steel corrosion sensor based on the photonic crystal fiber probe of the present invention;

[0043] Figure 3 It is a cross-sectional view of the B1-B1 section of the steel corrosion sensor based on the photonic crystal fiber probe of the present invention;

[0044] Figure 4 It is a cross-sectional view of the B2-B2 section of the steel corrosion sensor based on the photonic crystal fiber probe of the present invention;

[0045] FIG5(a) is a schematic diagram of the sensing mechanism of the steel corrosion sensor based on the photonic crystal fiber probe of the present invention; FIG5(b) is a comparison diagram of the incident light spectrum and the reflected light spectrum;

[0046] Figure 6 A schematic diagram of the arrangement of the steel corrosion sensor based on the photonic crystal optical fiber probe of the present invention applied to the monitoring of steel bars in an actual concrete structure;

[0047] In the figure: 1 single-mode optical fiber; 2 photonic crystal fiber probe; 3 three-port circulator; 4 amplified spontaneous emission (ASE) laser; 5 spectrum analyzer; 6 single-mode optical fiber core; 7 single-mode optical fiber cladding; 8 single-mode optical fiber coating; 9 air bubbles; 10 air holes inside the photonic crystal fiber cladding; 11 photonic crystal fiber core; 12 photonic crystal fiber cladding; 13 sphere formed by fusion; 14 iron film; 15 first reflector; 16 second reflector; 17 incident light; 18 core mode; 19 cladding mode; 20 first reflected light; 21 second reflected light; 22 corrosion products. DETAILED DESCRIPTION

[0048] 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.

[0049] As shown in Fig. 1-6, the present invention provides a steel corrosion sensor based on a photonic crystal fiber probe and a manufacturing method thereof, characterized in that the steel corrosion sensor based on a photonic crystal fiber probe comprises a single-mode optical fiber 1, a photonic crystal optical fiber 2, a three-port circulator 3, an amplified spontaneous emission (ASE) laser 4, and a spectrum analyzer 5. The single-mode optical fiber 1 comprises a core 6, a cladding 7, and a coating 8; the photonic crystal optical fiber 2 comprises a core 11, a cladding 12, and an iron film 14;

[0050] The diameter of the single-mode optical fiber core 6 is 9 μm, the diameter of the cladding 7 is 125 μm, and the diameter of the coating layer 8 is 255 μm;

[0051] The photonic crystal fiber 2 is a photonic crystal fiber of a hexagonal silica lattice, and the crystal fiber has a core 11 and a cladding 12, and two layers of air channels 10 are distributed inside the cladding, as shown in FIG5 ;

[0052] One end of the single-mode optical fiber 1 is connected to one of the interfaces of the three-port circulator 3, and the other end is fused with the photonic crystal optical fiber 2. There is an air bubble 9 at the fusion point between the photonic crystal optical fiber and the single-mode optical fiber 1, and the air bubble acts as a first reflector 15;

[0053] The other two ports of the three-port circulator 3 are connected to an ASE laser 4 and a spectrum analyzer 5 respectively;

[0054] The photonic crystal cladding 12 has air holes 10 surrounding the fiber core 11. The photonic crystal fiber is coated with an iron film 14. The other end of the photonic crystal fiber is a sphere 13 formed by fusion. The end of the sphere is a second reflector 16.

[0055] Furthermore, the air bubble 9 acts as a first reflector 15, and part of the light transmitted to the air bubble is reflected, and part passes through the air bubble and excites the cladding mode 19;

[0056] The cladding mode 19 is highly sensitive to changes in the refractive index of the external medium;

[0057] The length of the photonic crystal fiber probe 2 can be adjusted according to the sensor sensitivity requirements and the size of the monitored object;

[0058] A layer of iron film 14 is deposited on the outer surface of the photonic crystal fiber probe 2 .

[0059] Furthermore, the thickness of the iron film 14 is adjusted according to the sensor sensitivity and service life requirements;

[0060] Furthermore, the iron film 14 is wrapped around the outer surface of the photonic crystal fiber probe 2 made of photonic crystal fiber, and covers the entire length of the photonic crystal fiber probe.

[0061] Furthermore, the cladding mode 19 in the photonic crystal fiber excited by the light source through the bubble 9 is sensitive to the change of the refractive index of the surrounding environment. The core mode 18 and the cladding mode 19 reflected to the splicing point by the second reflector 16 interfere with each other due to the phase difference;

[0062] In an embodiment of the present invention, a method for preparing a steel corrosion sensor based on a photonic crystal fiber probe includes the steel corrosion sensor based on a photonic crystal fiber probe described in any one of the above, and comprises the following steps:

[0063] Step 1: Take a section of single-mode optical fiber, remove the outer coating at both ends of the optical fiber with a fiber stripper, and wipe the surface with alcohol to clean it.

[0064] Step 2: Take a section of photonic crystal fiber and use a blade to peel off the coating on its surface, leaving a cladding part with a diameter of 125μm. Then wipe the surface of the crystal fiber with the coating removed with alcohol to clean the surface.

[0065] Step 3: Use the overmelt collapse method to fuse one end of the photonic crystal to the end of the single-mode optical fiber stripped of the coating layer, select reasonable welding parameters, create a photonic crystal fiber part with completely folded air holes, produce air collapse, and reasonably control the size of the air bubbles.

[0066] Step 4: Place the other end of the photonic crystal fiber between the connecting electrodes and retract it so that its end forms a sphere.

[0067] Step 5: Clean the spliced ​​single-mode fiber-photonic crystal fiber with 20% NaOH solution, rinse with distilled water, and then dry in an oven.

[0068] Step 6: Use a radio frequency magnetron sputtering system to perform sputtering deposition coating on the cleaned photonic crystal fiber probe using a Fe target with a purity of 99.99%;

[0069] Step 7: Fix the cleaned optical fiber on a stainless steel fixture to ensure that the optical fiber remains straight during the iron film deposition process. Flip the fixture over and plate again to ensure that the iron film is deposited evenly.

[0070] 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. A steel corrosion sensor based on a photonic crystal fiber probe, characterized in that: The steel corrosion sensor based on the photonic crystal fiber probe comprises a single-mode optical fiber (1), a photonic crystal fiber probe (2), a three-port circulator (3), an amplified spontaneous emission laser (4) and a spectrum analyzer (5); wherein the single-mode optical fiber (1) comprises a single-mode optical fiber core (6), a single-mode optical fiber cladding (7) and a coating layer (8); the photonic crystal fiber probe (2) comprises a photonic crystal optical fiber core (11), an air channel (10), a photonic crystal optical fiber cladding (12) and an iron film (14); One end of the single-mode optical fiber (1) is connected to one of the interfaces of the three-port circulator (3), and the other end is fused to the photonic crystal optical fiber probe (2). An air bubble (9) exists at the fusion point between the single-mode optical fiber (1) and the photonic crystal optical fiber probe (2), and the air bubble (9) serves as a first reflector (15); The other two ports of the three-port circulator (3) are connected to an ASE laser (4) and a spectrum analyzer (5) respectively; The other end of the photonic crystal fiber probe (2) is a sphere (13) formed by fusion, and the tip of the sphere is a second reflector (16); The iron film (14) is wrapped around the outer surface of the photonic crystal optical fiber probe (2) and covers the entire length of the photonic crystal optical fiber probe (2); When the light propagating in the single-mode fiber reaches the air bubble in the splicing area with the photonic crystal fiber, part of the light is reflected back; the other part of the light passes through the bubble, and the air bubble acts as a divergent lens to diffract the light. Some of the light is excited to the cladding for transmission, forming a cladding mode. The excited cladding mode is very sensitive to changes in the external refractive index. Changes in the external refractive index will cause changes in the effective refractive index of the cladding mode. The mode propagating through the photonic crystal fiber is reflected back when it reaches the spherical end face at the end of the device. When it reaches the splicing point again, the core mode and cladding mode with phase difference interfere with each other. The transmission light finally returns to the single-mode fiber and is transmitted to the spectrum analyzer again via the three-port circulator.

2. The steel corrosion sensor based on photonic crystal fiber probe according to claim 1 is characterized in that: The length of the photonic crystal optical fiber probe (2) is adjusted according to the monitoring object and monitoring sensitivity requirements.

3. The steel corrosion sensor based on photonic crystal fiber probe according to claim 1 or 2, characterized in that: The thickness of the iron film (14) is adjusted according to the sensitivity and service life requirements of the sensor.

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