Arc-shaped photonic crystal fiber refractive index sensor based on surface plasma resonance

Abstract

The invention relates to the technical field of optical fiber sensors, in particular to a surface plasma resonance arc-shaped photonic crystal fiber refractive index sensor which comprises an optical fiber substrate and a to-be-detected object. An air hole is formed in the optical fiber substrate, the wall surface of the optical fiber substrate is provided with an arc-shaped polishing surface which is inwards concave towards the circle center side, and the polishing surface is plated with a gold film, the polishing surface is of an arc-shaped structure, the surface of the arc-shaped structure is coated with a film, and the contact surface of the polishing surface is large, so that the polishing surface is easier to contact with an analyte to be detected; the stability of the sensor is improved while the sensor has higher sensitivity, the metal layer coated on the arc-shaped polished surface is easier than the metal layer coated in the air hole, the preparation process is more mature, the metal layer coating is more uniform, and by optimizing the structural parameters and selecting the optimal parameters, the stability of the sensor is improved. The measurement range of the sensor is 1.35 to 1.42, and the sensitivity of the sensor can reach 24900 nm / RIU.

Description

Technical Field

[0001] The invention relates to the technical field of optical fiber sensors, and in particular to an arc-shaped photonic crystal optical fiber refractive index sensor of surface plasma resonance. Background Art

[0002] As a new type of sensing technology, fiber optic sensors have developed rapidly in recent years. They use optical fibers as sensitive elements and transmission media, and have the advantages of anti-electromagnetic interference, corrosion resistance, small size, light weight, and long-distance transmission. They are widely used in industry, medical treatment, environmental protection and other fields.

[0003] As fiber optic sensor technology continues to develop, surface plasmon resonance technology has gradually attracted people's attention. Surface plasmon resonance is a physical phenomenon that occurs at the interface between metal and medium. When a beam of light of a specific wavelength is irradiated on the metal surface, it will excite the free electrons on the metal surface to oscillate collectively, forming a surface plasma wave. This surface plasma wave is very sensitive to the change of refractive index at the interface. A small change in refractive index will cause a significant change in the resonance angle or resonance wavelength. Combining surface plasmon resonance technology with fiber optic sensors can greatly improve the sensitivity and selectivity of fiber optic sensors. Photonic crystal fiber has attracted widespread attention due to its many unique optical properties, such as non-cutoff single-mode transmission, high nonlinearity, and controllable mode field area. Combining photonic crystal fiber with surface plasmon resonance technology can give full play to the advantages of both and realize high-sensitivity and high-selectivity sensing functions.

[0004] Existing surface plasmon resonance photonic crystal fibers mostly use multi-layer coatings, special and complex fiber structures and slotting methods to improve sensor performance. Therefore, our company has developed a surface plasmon resonance photonic crystal fiber refractive index sensor with a relatively simple structure, without multi-layer coatings, by optimizing fiber structural parameters and designing an arc structure to enhance mode coupling, to achieve high-sensitivity detection of the analyte to be measured.

[0005] Therefore, the present application develops a surface plasmon resonance arc photonic crystal fiber refractive index sensor to solve the problems existing in the prior art. Summary of the invention

[0006] The purpose of the present invention is to provide a surface plasmon resonance arc photonic crystal fiber refractive index sensor to solve the problem of difficulty in coating on the surface plasmon resonance arc photonic crystal fiber in the prior art.

[0007] The technical solution of the present invention is: a surface plasmon resonance arc photonic crystal fiber refractive index sensor, comprising: an optical fiber substrate, an object to be detected;

[0008] The optical fiber base has an air hole inside, and the wall surface of the optical fiber base has an arc-shaped polishing surface which is concave inwardly toward the center of the circle, and the polishing surface is plated with a gold film.

[0009] Preferably, the optical fiber base is provided with a first layer of air holes, a second layer of air holes, and a third layer of air holes in sequence radially from the center of the circle, and the first layer of air holes, the second layer of air holes, and the third layer of air holes each include a plurality of air holes with the same diameter, and the plurality of air holes in each layer are evenly distributed on the optical fiber base in an arc shape with the center of the optical fiber base as the center of the circle.

[0010] Preferably, the radius of the optical fiber substrate is r=6.5 μm, and the distance between the lowest point of the concave portion of the polishing surface and the center of the optical fiber substrate is 3.5 μm, that is, the polishing depth h=3.5 μm.

[0011] Preferably, the diameters of the first layer of air holes, the second layer of air holes and the third layer of air holes increase successively along the radial direction of the optical fiber substrate, and the difference between the radius of the arc where the third layer of air holes is located and the radius of the arc where the second layer of air holes is located is equal to the difference between the radius of the arc where the second layer of air holes is located and the radius of the arc where the first layer of air holes is located, and is greater than the radius of the arc where the first layer of air holes is located.

[0012] Preferably, the diameter of the first layer of air holes is d1 = 0.7 μm, and the diameter of the arc where the first layer of air holes is located is r1 = 1.7 μm.

[0013] Preferably, the diameter of the second layer of air holes is d2 = 1 μm, and the diameter of the arc where the second layer of air holes is located is r2 = 3.6 μm.

[0014] Preferably, the diameter of the air holes in the third layer is d3=1.4 μm, and the diameter of the arc where the air holes in the third layer are located is r3=5.5 μm.

[0015] Compared with the prior art, the advantages of the present invention are:

[0016] (1) The polishing surface is set to an arc-shaped structure, and a film is coated on the surface of the arc-shaped structure. Since the contact surface of the polishing surface is large and easier to contact with the analyte to be measured, the sensor has higher sensitivity and improves the stability of the sensor;

[0017] (2) Gold film can produce a strong electromagnetic field enhancement effect in the visible light band, thereby greatly improving the interaction between light and matter. In SPR sensors, it improves the sensitivity and resolution of sensing.

[0018] (3) It is easier to coat the metal layer on the curved polished surface than in the air holes, the preparation process is more mature, and the metal layer coating is more uniform;

[0019] (4) By optimizing the structural parameters and selecting the best parameters, the sensor can measure a range of 1.35-1.42 and a sensitivity of 24900nm / RIU;

[0020] (5) By reducing the air holes in the vertical direction, the mode coupling between the SPP mode and the core mode is enhanced. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The present invention will be further described below in conjunction with the accompanying drawings and embodiments:

[0022] Figure 1 The cross section of the curved photonic crystal fiber refractive index sensor of the surface plasmon resonance of the present invention;

[0023] Figure 2a This is a comparison diagram of the loss spectrum and sensitivity of the sensor under different gold film thicknesses of the present invention;

[0024] Figure 2b This is a comparison diagram of the sensitivity of the sensor under different gold film thicknesses of the present invention;

[0025] Figure 3a The loss spectrum of the sensor at different distances between the first layer of air holes and the center of the optical fiber substrate of the present invention;

[0026] Figure 3b This is a sensitivity comparison diagram of the sensor at different distances between the first layer of air holes and the center of the optical fiber substrate of the present invention;

[0027] Figure 4a The loss spectrum of the sensor at different distances between the second layer of air holes and the center of the optical fiber substrate of the present invention;

[0028] Figure 4b This is a sensitivity comparison diagram of the sensor at different distances between the second layer of air holes and the center of the optical fiber substrate of the present invention;

[0029] Figure 5a The loss spectrum of the sensor at different distances between the third layer air hole and the center of the optical fiber substrate of the present invention;

[0030] Figure 5b This is a sensitivity comparison diagram of the sensor at different distances between the third layer air hole and the center of the optical fiber substrate of the present invention;

[0031] Figure 6a The loss spectrum of the sensor with different first-layer air hole diameters according to the present invention;

[0032] Figure 6b This is a sensitivity comparison diagram of sensors with different first-layer air hole diameters according to the present invention;

[0033] Figure 7a The figure is a loss spectrum diagram of the sensor with different second-layer air hole diameters according to the present invention;

[0034] Figure 7b This is a sensitivity comparison diagram of sensors with different second-layer air hole diameters according to the present invention;

[0035] Figure 8a The figure is a loss spectrum of the sensor with different third-layer air hole diameters according to the present invention;

[0036] Figure 8b This is a sensitivity comparison diagram of sensors with different third-layer air hole diameters according to the present invention;

[0037] Figure 9a The loss spectra of the sensors with different polishing depths described in the present invention are as follows;

[0038] Figure 9b This is a sensitivity comparison diagram of different polishing depth sensors of the present invention;

[0039] Fig.10 This is a comparison diagram of the loss spectra of the sensor under different refractive indices according to the present invention;

[0040] Fig.11 It is a fitting diagram of a curved photonic crystal fiber refractive index sensor of a surface plasmon resonance according to the present invention;

[0041] Fig.12 This is a schematic diagram of simulation software for a surface plasmon resonance curved photonic crystal fiber refractive index sensor according to the present invention.

[0042] Wherein: 1. optical fiber substrate; 11. air holes; 12. polishing surface; 13. first layer of air holes; 14. second layer of air holes; 15. third layer of air holes; 2. object to be detected; 3. gold film. DETAILED DESCRIPTION

[0043] The present invention is further described in detail below in conjunction with specific embodiments:

[0044] like Figure 1As shown, a surface plasmon resonance arc photonic crystal fiber refractive index sensor includes an optical fiber substrate 1 and an object to be detected 2. The optical fiber substrate 1 has an air hole 11 inside. The wall of the optical fiber substrate 1 has an arc-shaped polished surface 12 which is concave inward toward the center of the circle. The polished surface 12 is plated with a gold film 3. In this application, the outer coating of the cladding is adopted instead of the inner coating of the air hole 11. When the inner coating of the air hole 11 is performed in the air hole 11, it is necessary to ensure that the metal film can evenly cover the inner wall of the air hole 11. The technical requirements for the coating are as follows: It is very high, and due to the irregular shape and size of the air hole 11, the uniformity of the coating is difficult to ensure, and special coating methods and equipment, such as atomic layer deposition, are required, which greatly increases the complexity and cost of the coating process; while in the outer cladding coating, the outer surface of the optical fiber cladding is relatively flat, and it is easier to achieve the uniformity of the film layer during the coating process, and only some common coating methods are used, such as magnetron sputtering coating, etc. These methods are relatively mature in process and simple to operate, and can quickly and efficiently form high-quality metal films on the outer surface of the optical fiber cladding.

[0045] In this embodiment, the wall surface of the optical fiber substrate 1 has an arc-shaped polishing surface 12 that is concave toward the center side, which can better match the field distribution of the SPP mode, making the coupling between the SPP mode and the core mode easier and more efficient, helping to reduce the reflection and scattering losses of light on the polishing surface 12, thereby improving the transmission efficiency of light, and the arc-shaped polishing surface 12 can respond more sensitively to changes in the external refractive index, because the arc-shaped structure makes the transmission path of light in the optical fiber longer, increasing the interaction time between light and the external environment, helping to enhance the SPR effect, so that the sensor has higher sensitivity within a specific refractive index range, and at the same time, the arc-shaped polishing surface 12 enables the sensor to maintain stable performance within a wider refractive index range, thereby improving the application range of the sensor.

[0046] The highest sensitivity and other performances based on different structures are shown in Table 1:

[0047] Table 1: Maximum sensitivity and resolution of different structures

[0048]

[0049] The structure adopted in the present application is an arc-shaped photonic crystal fiber. It can be seen from Table 1 that the polishing surface with an arc shape is superior to the polishing surfaces with other different curved surfaces in terms of sensitivity and resolution.

[0050] In this embodiment, the radius of the optical fiber base is 6.5 μm, and the distance between the lowest point of the concave part of the polishing surface 12 and the center of the optical fiber base 1 is 3.5 μm, that is, the polishing depth h=3.5 μm. The polishing depth directly affects the interaction between the optical fiber surface and the plasma material. An appropriate polishing depth can optimize this interaction, thereby improving the sensitivity of the sensor. A shallow polishing depth will result in insufficient contact area between the optical fiber surface and the plasma material, affecting the SPR effect, while excessively deep polishing may destroy the structural integrity of the optical fiber.

[0051] An object to be detected 2 is provided along the axial direction of the light beam, and the diameter of the object to be detected 2 is larger than the diameter of the optical fiber base. When the diameter of the object to be detected 2 is larger than the diameter of the optical fiber base, the surface plasmon resonance effect can be effectively enhanced, which helps to improve the detection sensitivity of the sensor, so that it can more accurately measure the refractive index of the object to be detected 2. At the same time, it can also better seal the optical signal in the optical fiber, reduce the possibility of light leakage, and help to improve the stability and accuracy of the sensor.

[0052] Among them, a gold film 3 is plated on the polished surface 12. Gold, as a metal, exhibits a significant surface plasmon resonance effect in the visible light band, so that the gold nanostructure can produce a strong electromagnetic field enhancement effect when excited by light, thereby greatly improving the interaction between light and matter. In the SPR sensor, this helps to improve the sensitivity and resolution of the sensor. Because the sensor needs to maintain stability and reliability when working, gold as a surface plasma material can ensure that the sensor maintains stable performance for a long time, thereby achieving accurate detection of the sensor. The surface plasmon resonance effect of the gold nanostructure is very sensitive to changes in the surrounding environment, including slight changes in parameters such as refractive index, temperature, and pressure. In the SPR sensor, it is more conducive to achieving high-sensitivity detection.

[0053] In this embodiment, the optical fiber substrate 1 is provided with a first layer of air holes 13, a second layer of air holes 14 and a third layer of air holes 15 in sequence from the center along the radial direction. The first layer of air holes 13, the second layer of air holes 14 and the third layer of air holes 15 respectively include air holes 11 of the same diameter but different diameters, wherein the diameter of the first layer of air holes 13 is 0.7 μm, the diameter of the second layer of air holes 14 is 1 μm, and the diameter of the third layer of air holes 15 is 1.4 μm, and the air holes of each layer are uniformly distributed on the optical fiber substrate 1 in an arc shape with the center of the optical fiber substrate 1 as the center of the circle. The air holes distributed in an arc shape can effectively adjust the coupling strength between the core mode and the plasma mode, thereby enhancing the SPR effect. When the refractive index of the medium to be measured changes, the change in the refractive index can be more sensitively captured, thereby improving the sensitivity of the sensor. In the SPR sensor, the medium to be measured usually needs to flow through the air holes 11 in the optical fiber, and the air holes 11 distributed in an arc shape will form a smoother fluid channel, reduce the resistance and eddy current when the fluid flows, thereby improving the response speed and measurement accuracy of the sensor.

[0054] The air holes with circular arc distribution can more effectively constrain the propagation of light waves inside the optical fiber, reduce light scattering and leakage, and thus improve the optical waveguide effect. The air holes with polygonal distribution or other irregular shapes will cause the propagation path of the light waves to be disturbed, reducing the optical waveguide efficiency. The optical waveguide effect can constrain the light waves to propagate in a specific mode, reduce scattering and leakage, and help improve the signal-to-noise ratio of the sensor, thereby enhancing the sensitivity. Therefore, the air holes with circular arc distribution have higher sensitivity. At the same time, the air holes with circular arc distribution can optimize the plasma resonance conditions on the surface of the optical fiber, making the sensor more sensitive and accurate in detecting specific substances, while the air holes with polygonal or other shapes are not conducive to the optimization of plasma resonance, thereby affecting the sensitivity of the sensor. Moreover, in the polygonal distribution, since the arrangement of the air holes is relatively regular, the distribution of the electromagnetic field in the optical fiber will change differently, so that the air holes may concentrate or disperse the electric field in a certain area, thereby affecting the propagation characteristics of the surface plasma wave. Especially when multiple air holes interact with each other, it may cause a more complex optical coupling effect, resulting in different sensing results.

[0055] Among them, the diameters of the first layer of air holes 13, the second layer of air holes 14 and the third layer of air holes 15 increase successively along the radial direction of the optical fiber substrate 1. The gradient air holes make the cladding of the microstructure optical fiber form a structure similar to that of a gradient refractive index optical fiber. The refractive index gradually changes from the core to the cladding. This gradient refractive index distribution can make the light continuously refracted and totally reflected during the transmission process, thereby enhancing the binding ability of light in the optical fiber, reducing light leakage and energy loss, improving the transmission efficiency of light, making the propagation path of light in the optical fiber longer, and increasing the interaction time and intensity between light and the biosensitive material or the biological substance to be detected in the core, which helps to improve the detection sensitivity of the sensor to biological molecules or substances.

[0056] Furthermore, the difference in radius between the arc where the third-layer air holes 15 are located and the arc where the second-layer air holes 14 are located is equal to the difference in radius between the arc where the second-layer air holes 14 are located and the arc where the first-layer air holes 13 are located, and is greater than the radius of the arc where the first-layer air holes 13 are located. By adjusting the radius difference of the air holes 11, the refractive index of the optical fiber surface can be more effectively changed, thereby enhancing the SPR effect, so that the sensor can more sensitively capture the change in the refractive index and improve the measurement accuracy.

[0057] In this embodiment, the base material of the optical fiber is silicon dioxide, and the calculation formula of the limiting loss of the loss spectrum is as follows:

[0058]

[0059] Im(n eff ): imaginary part of the effective refractive index of the core mode in the optical fiber;

[0060] α: Limitation loss of the core mode.

[0061] like Figure 2a and Figure 2b As shown, by calculating the loss of different wavelengths and drawing a loss spectrum, the loss of the core mode is observed, and the control variable method is used to change a specified parameter and keep other parameters unchanged. The sensor performance is compared and the best parameters are selected to optimize the sensor structure parameters. In the optimization process, the thickness of the gold film 3, the air hole spacing, the size of the air hole 11, and the polishing depth are four important parameters. Among them, the change in the thickness of the gold film 3 will affect the peak loss, half-wave full width (FWHM) and sensitivity of the sensor loss spectrum, and the peak loss cannot be too low. The lower peak loss will affect the surface plasma resonance effect, thereby affecting the final result analysis. Therefore, in order to obtain appropriate peak loss, narrower FWHM, and higher sensitivity, the selected gold film 3 thickness t = 28nm.

[0062] like Figure 3a , Figure 3b , Figure 4a , Figure 4b , Figure 5a , Figure 5b As shown, the change in the distance between the first layer of air holes 13 and the second layer of air holes 14 and the center of the optical fiber substrate 11 most directly affects the peak loss, FWHM and sensitivity of the loss spectrum, while the change in the diameter of the arc where the third layer of air holes 15 is located has little effect on the sensor performance. Therefore, in order to meet the parameter selection requirements of the sensor, the diameter r1 of the arc where the first layer of air holes 13 is located is selected to be 1.7μm, the diameter r2 of the arc where the second layer of air holes 14 is located is selected to be 3.6μm, and the diameter r3 of the arc where the third layer of air holes 15 is located is selected to be 5.5μm.

[0063] like Figure 6a , Figure 6b , Figure 7a , Figure 7b , Figure 8a , Figure 8b As shown, the change in the diameter of the first layer of air holes 13 and the second layer of air holes 14 will cause the change in the peak loss of the sensor loss spectrum and the sensitivity, while the change in the diameter of the third layer of air holes 15 in a small range has little effect on the sensor performance. In order to obtain higher performance, the diameter of the first layer of air holes 13 is d1 = 0.7 μm, the diameter of the second layer of air holes 14 is d2 = 1 μm, and the diameter of the third layer of air holes 15 is d3 = 1.4 μm.

[0064] like Figure 9a and Figure 9b As shown, the polishing depth has a direct impact on the sensor performance, because the polishing depth directly affects the distance of coupling between the SPP mode and the core mode. In order to obtain better sensing performance, the selected polishing depth h = 3.5 μm.

[0065] like Figure 10-11 As shown, after the corresponding parameters are obtained and set, the loss spectrum in the refractive index range of 1.35-1.42 is analyzed, and multiple fittings are performed on the resonance wavelength and the refractive index of the analyte to obtain a fitting curve with a fitting degree of 0.98909, which has higher sensitivity and higher nonlinear fitting degree than existing sensors.

[0066] The above embodiments are only for illustrating the technical concept and features of the present invention, and their purpose is to enable people familiar with this technology to understand the content of the present invention and implement it accordingly, and they cannot be used to limit the scope of protection of the present invention. For those skilled in the art, it is obvious that the present invention is not limited to the details of the above exemplary embodiments, and the present invention can be implemented in other specific forms without departing from the spirit or basic features of the present invention. Therefore, no matter from which point of view, the embodiments should be regarded as exemplary and non-restrictive. The scope of the present invention is limited by the attached claims rather than the above description, and it is intended to include all changes within the meaning and scope of the equivalent elements of the claims in the present invention.

Claims

1. A surface plasmon resonance arc photonic crystal fiber refractive index sensor, characterized in that: include: Optical fiber substrate (1), object to be detected (2); The optical fiber base (1) has an air hole (11) inside, and the wall surface of the optical fiber base (1) has a polishing surface (12) which is concave inwardly toward the center of the circle and is in an arc shape, and the polishing surface (12) is plated with a gold film (3); The optical fiber base (1) is provided with a first layer of air holes (13), a second layer of air holes (14), and a third layer of air holes (15) in sequence from the center of the circle along the radial direction; the first layer of air holes (13), the second layer of air holes (14), and the third layer of air holes (15) each include a plurality of air holes (11) with the same diameter, and the plurality of air holes (11) in each layer are uniformly distributed on the optical fiber base (1) in an arc shape with the center of the optical fiber base (1) as the center of the circle.

2. The surface plasmon resonance curved photonic crystal fiber refractive index sensor according to claim 1, characterized in that: The radius of the optical fiber substrate (1) is r=6.5 μm, and the distance between the lowest point of the inner concave of the polishing surface (12) and the center of the optical fiber substrate (1) is 3.5 μm, that is, the polishing depth h=3.5 μm.

3. The surface plasmon resonance curved photonic crystal fiber refractive index sensor according to claim 1, characterized in that: The diameters of the first layer of air holes (13), the second layer of air holes (14) and the third layer of air holes (15) increase successively along the radial direction of the optical fiber substrate (1); the difference between the radius of the arc where the third layer of air holes (15) is located and the radius of the arc where the second layer of air holes (14) is located is equal to the difference between the radius of the arc where the second layer of air holes (14) is located and the radius of the arc where the first layer of air holes (13) is located, and is greater than the radius of the arc where the first layer of air holes (13) is located.

4. The surface plasmon resonance arc photonic crystal fiber refractive index sensor according to claim 1, characterized in that: The diameter d1 of the first layer of air holes (13) is 0.7 μm, and the diameter r1 of the arc where the first layer of air holes (13) are located is 1.7 μm.

5. The surface plasmon resonance curved photonic crystal fiber refractive index sensor according to claim 1, characterized in that: The diameter of the second layer of air holes (14) is d2 = 1 μm, and the diameter of the arc where the second layer of air holes (14) are located is r2 = 3.6 μm.

6. The surface plasmon resonance curved photonic crystal fiber refractive index sensor according to claim 1, characterized in that: The diameter d3 of the third layer of air holes (15) is 1.4 μm, and the diameter r3 of the arc where the third layer of air holes (15) are located is 5.5 μm.

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