SPR (Surface Plasmon Resonance) refractive index sensor based on double-C-type microstructure optical fiber

Abstract

According to the SPR refractive index sensor based on the double-C-type microstructure optical fiber and preparation of the SPR refractive index sensor, the sensor is of a cylinder structure, the cross section of the cylinder is of a double-C-type quasi-circular structure, and the SPR refractive index sensor comprises a silicon dioxide substrate, air circular holes, a gold thin film layer and a titanium dioxide thin film layer; the silicon dioxide substrate is of a double-C-shaped quasi-circular structure, the double-C-shaped quasi-circular structure is formed by polishing and grinding a section of C-shaped notch on each of two opposite side surfaces of the silicon dioxide substrate, four air circular holes are formed in the silicon dioxide substrate and are uniformly distributed in the double-C-shaped quasi-circular structure in a square shape, the metal thin film layer is attached to the outer side of the C-shaped notch, and the titanium dioxide thin film layer is attached to the outer side of the metal thin film layer. During preparation, the sensor is formed by drawing, drilling, stacking and assembling, stretching and polishing deposition in sequence, and the prepared sensor has good sensing application performance. The preparation method is simple in process flow, provides an important theoretical basis for the design of the microstructure optical fiber sensor, and has a good application prospect in the fields of biomedical detection, chemistry and chemical engineering analysis and the like.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensors, specifically to an SPR refractive index sensor based on a double-C microstructure fiber and its fabrication. Background Technology

[0002] With the rapid development of fields such as medical testing, environmental monitoring, and chemical testing, the demand for high-sensitivity, low-noise sensors is becoming increasingly urgent. Fiber optic sensors based on surface plasmon resonance (SPR) technology have been widely used in many fields due to their significant advantages, including ultra-high sensitivity, real-time detection, label-free operation, strong birefringence, miniaturization, and multifunctional integration.

[0003] The development of traditional SPR fiber sensors has long been limited by the phase matching problem, the core of which lies in the effective refractive index matching requirement between the core mode and the plasma mode. The effective refractive index of the core of conventional single-mode fiber is usually greater than 1.45, while the effective refractive index of plasma resonance in biosensing is about 1.3. Matching between the two can only be achieved under high-frequency conditions, but this limits the penetration depth of the plasma, resulting in reduced sensitivity. Microstructured fiber (MOF) can customize the structure to form MOF-SPR and precisely control the optical properties to achieve optical refractive index sensing. However, existing MOF-SPR sensors mostly require chemical vapor deposition or electroplating techniques to coat the inner wall of micron-level air holes. For example, patent CN102353655A proposes a surface plasmon resonance sensor based on photonic crystal fiber, in which metal nanoparticles or nanofilms are deposited on the inner wall of the air holes near the core of the photonic crystal fiber, and the cladding air holes directly serve as microfluidic channels. Alternatively, the results may be complex and difficult to manufacture. For example, Chinese Patent Publication No. CN102590143A discloses an SPR sensor based on annular aperture microstructure optical fiber. This sensor requires the fabrication of support arms of different shapes, increasing fabrication time and cost. Therefore, designing MOFs with fewer air holes, simplified structure, and simpler manufacturing process is crucial for improving the performance of refractive index sensors. Summary of the Invention

[0004] The purpose of this invention is to overcome the above-mentioned shortcomings of the prior art and provide an SPR refractive index sensor based on a double C-shaped microstructure optical fiber and its fabrication.

[0005] To achieve the above objectives, the present invention provides the following technical solution: an SPR refractive index sensor based on a double-C-shaped microstructure optical fiber. The sensor has a cylindrical structure with a cross-sectional shape exhibiting a double-C-shaped quasi-circular structure. Specifically, it includes a silicon dioxide substrate, air holes, a gold thin film layer, and a titanium dioxide thin film layer. The double-C-shaped quasi-circular structure is formed by polishing a C-shaped notch on each of the two opposite sides of the silicon dioxide substrate. The distance between the arc-shaped top of the C-shaped notch and the center of the silicon dioxide substrate is the polishing depth. There are four air holes on the silicon dioxide substrate, which are evenly arranged in a square within the double-C-shaped quasi-circular structure. The metal thin film layer is attached to the outside of the C-shaped notch, and the titanium dioxide thin film layer is attached to the outside of the metal thin film layer. The cylinder is made of silicon dioxide or germanium, and the metal thin film layer is made of gold or silver.

[0006] A further technical solution of the present invention is as follows: the length of the cylinder is 10mm to 50mm, the polishing depth is 2.76 to 2.96μm, the radius of the silicon dioxide substrate is 4 to 6μm, the radius of the air hole is 1 to 3μm, the distance from the center of the air hole to the center of the silicon dioxide substrate is 2.5 to 3.5μm, the radius of the double C-shaped quasi-circular shape is 3 to 5μm, the thickness of the metal thin film layer is 38 to 42nm, and the thickness of the titanium dioxide thin film layer is 6 to 8nm.

[0007] Another technical solution provided by this invention is the fabrication of an SPR refractive index sensor based on a double-C microstructure optical fiber, comprising the following steps:

[0008] Step 1: Take multiple silica rods of suitable length and diameter as optical fiber substrates, remove the coating layer on the outer surface of the silica rods, and use optical fibers to cut the two ends of the silica rods flat to form prefabricated silica rods.

[0009] Step 2: Using standard drawing process, some pre-made silica rods are drawn into thin silica rods, with a diameter approximately half that of the original silica rods; using ultrasonic drilling technology, air holes penetrating both ends of the pre-made silica rods are precisely machined at the center of the cross-section.

[0010] Step 3: Stack and assemble the prefabricated silica rods, fine silica rods, and prefabricated silica rods with air holes in a specific order to form a stacked prefabricated rod, and fix the stacked prefabricated rod structure with adhesive to ensure accurate alignment between the three components.

[0011] Step 4: Perform precision stretching on the stacked preform structure. By uniformly applying tension, the preformed silica rod, fine silica rod, and preformed silica rod with air holes are fused into a seamless microstructured optical fiber semi-finished product.

[0012] Step 5: Perform fine polishing on the microstructured optical fiber semi-finished product, and use an optical polishing machine to polish the opposite sides of the microstructured optical fiber semi-finished product to create a C-shaped arc with the same structure, remove surface defects and adhesive, and obtain a smooth double C-shaped microstructured optical fiber semi-finished product.

[0013] Step 6: Deposit a metal thin film layer and a titanium dioxide thin film layer in the left and right arc-shaped regions of the double-C microstructured optical fiber semi-finished product using chemical vapor deposition technology to obtain the refractive index sensor of the double-C microstructured optical fiber.

[0014] A further technical solution of the present invention is: the specific sequence is that four fine silica rods are evenly distributed in a cross shape on the outer periphery of the preformed silica rod, and four preformed silica rods containing air holes are respectively arranged between two fine silica rods.

[0015] A further technical solution of the present invention is: in step four, an optical fiber drawing tower is selected for the stretching operation.

[0016] Another technical solution provided by this invention is the application of an SPR refractive index sensor based on a dual-C microstructure optical fiber. This sensor is applied to analytes with a refractive index (RI) ranging from 1.38 to 1.42. The analytes are distributed around the optical fiber refractive index sensor. The drift of the loss spectrum under different incident wavelengths is observed, and the sensor's sensitivity, full width at half maximum (FWHM), quality factor (FOM), and number of air holes are used to evaluate the sensing performance.

[0017] Compared with the prior art, the present invention has the following characteristics:

[0018] (1) By setting a plasma layer gold film and a sensitivity-enhancing layer titanium dioxide film and optimizing their thickness, this invention realizes a refractive index sensor with advantages such as high sensitivity, low FWHM, and high FOM in the near-infrared band of 0.7μm to 1.1μm. The polishing shortens the distance between the fiber core and the metal thin film layer, which can accelerate the detection response and increase the resonance intensity of the fiber core and the metal thin film layer, thereby improving the performance of the refractive index sensor.

[0019] (2) The fiber optic refractive index sensor manufacturing process used in this invention is simple and easy to operate, which is conducive to the large-scale production of refractive index sensors.

[0020] The detailed structure of the present invention will be further described below with reference to the accompanying drawings and specific embodiments. Attached Figure Description

[0021] Figure 1(a) , 1(b) The images show a 3D view and a cross-sectional view of the double-C microstructure fiber refractive index sensor, respectively.

[0022] Figure 2The diagram shows the refractive index distribution and loss distribution of the refractive index sensor.

[0023] Figure 3(a) , 3(b) The figures show the effects of the radius of the double C-type quasi-circular shape and the polishing depth on the SPR mode loss spectrum under different refractive indices.

[0024] Figure 4(a) , 4(b) The figures show the effect of the thickness of the gold thin film and the titanium dioxide thin film on the SPR mode loss spectrum under different refractive indices.

[0025] Figure 5(a) , 5(b) Figures 5 and 5c show the variations in the SPR mode loss spectrum, sensitivity S, and quality factor FOM under different refractive indices. Detailed Implementation

[0026] Example 1, as shown in Figures 1-5, describes an SPR refractive index sensor based on a double-C-shaped microstructure optical fiber. The sensor has a cylindrical structure, and the cylinder is made of silicon dioxide to achieve refractive sensing characteristics in the near-infrared band. Other materials such as germanium can also be selected according to actual needs. The length of the cylinder is 10mm to 50mm, and may even be longer depending on the application. The cross-sectional shape of the cylinder is a double-C-shaped circular structure, specifically including a silicon dioxide substrate 1, air holes 2, a metal thin film layer 3, and a titanium dioxide thin film layer 4. The double-C-shaped circular structure is specifically achieved by polishing a C-shaped notch on each of the two opposite sides of the silicon dioxide substrate 1. The distance between the arc-shaped top of the C-shaped notch and the center of the silicon dioxide substrate is the polishing depth H, which is 2.76μm. There are four air holes 2 on the silicon dioxide substrate 1, which are evenly arranged in a square within the double-C-shaped circular structure. The metal thin film layer 3 is attached to the outside of the C-shaped notch, and the titanium dioxide thin film layer 4 is attached to the outside of the metal thin film layer 3. The silicon dioxide substrate 1 has a radius r2 of 4 μm, the air hole 2 has a radius r1 of 1 μm, the distance h from the center of the air hole 2 to the center of the silicon dioxide substrate 1 is 2.5 μm, and the radius R of the double C-shaped quasi-circular structure is 3 μm. The metal thin film layer 3 has a thickness tg of 38 nm and is made of gold or silver, while the titanium dioxide thin film layer 4 has a thickness tt of 8 nm.

[0027] In this embodiment, the refractive index of air is set to 1, and the refractive index of the silicon dioxide substrate 1 is calculated using the widely used Sellmeier formula as follows:

[0028]

[0029] Where λ is the incident wavelength, in μm.

[0030] Gold and silver are commonly used in SPR refractive index sensor structures due to their significant absorption properties in the near-infrared band. Silver provides a sharper absorption peak compared to gold; however, silver is easily oxidized in air. Therefore, in this embodiment, the more stable gold is used as the metal thin film layer 3 to provide SPR resonance in the sensor design. The dielectric constant of this metal thin film layer 3 is calculated using the Drude model as follows:

[0031]

[0032] Where, ε Au ε represents the dielectric constant of gold. ∞ ω represents the high-frequency dielectric constant. p The plasma frequency of gold is represented by ω = 2π / λ, and the angular frequency of the incident light is represented by ω. τ This represents the electron scattering frequency. The dielectric constant ε of gold at high frequencies... ∞ The value is 9.75, which is the plasma frequency ω of gold. p The value is 1.36 × 10¹⁶ rad / s, and the electron scattering frequency ω τ The value is 1.45 × 10¹⁴ rad / s.

[0033] The titanium dioxide thin film layer 4 is attached to the outside of the metal thin film layer 3. It can serve as both a sensitizing film to enhance the sensor's sensitivity and a protective film to protect the metal thin film layer 3. Its refractive index is calculated using the following formula:

[0034]

[0035] In use, the fiber optic refractive index sensor is placed in the analyte, with a refractive index ranging from 1.38 to 1.42. The performance of the fiber optic refractive index sensor is evaluated by observing the drift of the loss spectrum at different incident wavelengths using the wavelength scanning method. The loss value is calculated using the following formula:

[0036]

[0037] Where Im(n) eff The value represents the imaginary part of the effective mode refractive index of the optical fiber. Sensor sensitivity (S), a crucial performance indicator, is measured by the magnitude of the resonant wavelength shift. The figure of merit (FOM), another important performance indicator, is the ratio of sensor sensitivity S to the full width at half maximum (FWHM) of the resonant loss spectrum.

[0038] Based on the finite element method, COMSOL Multiphysics was used to design and simulate the fiber optic refractive index sensor, and the results were obtained as follows: Figure 2The diagram shows the refractive index distribution and loss distribution of the refractive index sensor. From... Figure 2 As can be seen, the dotted and solid lines represent the variation of the real part of the effective refractive index of the Surface Plasmon Polariton (SPP) mode and the SPR mode with wavelength, respectively. Both show a decreasing trend with increasing wavelength and intersect at a wavelength of 0.906 μm, meaning that their real refractive indices are the same. At this point, the real part of the effective mode refractive index of the SPR mode undergoes a slight jump, which just satisfies the phase matching condition. The solid and dashed lines represent the loss spectra of the SPR mode and the core fundamental mode LP01, respectively. The loss spectrum of the SPR mode first increases and then decreases, reaching a peak at a wavelength of 0.906 μm. This clearly shows the energy coupling process between the core fundamental mode LP01 and the SPP mode, confirming the existence of a significant surface plasmon resonance phenomenon at this wavelength.

[0039] Example 2: Fabrication of a SPR-based double-C microstructure fiber optic refractive index sensor, comprising the following steps:

[0040] Step 1: Take multiple silica rods of suitable length and diameter as optical fiber substrate 1, remove the coating layer on the outer surface of the silica rods, and use optical fibers to cut the two ends of the silica rods flat to form prefabricated silica rods.

[0041] Step 2: Using standard drawing process, some pre-made silica rods are drawn into thin silica rods, with a diameter approximately half that of the original silica rods; using ultrasonic drilling technology, air holes 2 are precisely machined at the center of the cross-section of another pre-made silica rod, penetrating both ends of the pre-made silica rod.

[0042] Step 3: Assemble the prefabricated silica rods, fine silica rods, and prefabricated silica rods with air holes in a specific order to form a stacked prefabricated rod structure. Secure the stacked prefabricated rod structure with adhesive, ensuring accurate alignment between the three components. Specifically, the specific order involves four fine silica rods evenly distributed in a cross shape around the outer periphery of the prefabricated silica rods, and four prefabricated silica rods with air holes positioned between two fine silica rods. Choosing four fine silica rods and four prefabricated silica rods with air holes allows for better resonance with the outer gold film layer during use, resulting in a higher SPR (Surface Resonance Ratio).

[0043] Step 4: Perform precision stretching on the stacked preform structure. By uniformly applying tension, the preformed silica rod, fine silica rod, and preformed silica rod containing air holes are fused into a seamless microstructured optical fiber semi-finished product, while maintaining the integrity of the air hole structure. To maintain the accuracy of the stretching process, an optical fiber drawing tower is selected for operation.

[0044] Step 5: Perform fine polishing on the microstructured optical fiber semi-finished product, and use an optical polishing machine to polish the opposite sides of the microstructured optical fiber semi-finished product to create a C-shaped arc with the same structure, remove surface defects and adhesive, and obtain a smooth double C-shaped microstructured optical fiber semi-finished product.

[0045] Step 6: In the left and right arc-shaped regions of the double-C microstructure fiber semi-finished product, a metal thin film layer 3 and a titanium dioxide thin film layer 4 are deposited respectively using chemical vapor deposition technology to optimize the optical properties of the fiber and obtain the refractive index sensor of the double-C microstructure fiber.

[0046] In the above process, only drilling, stretching and polishing of the silica rod are required. Therefore, the process is simpler than some existing irregular shapes or a large number of hole arrangements, as well as the addition of other materials. Moreover, the circular shape has lower requirements for drilling, which significantly reduces the complexity of the process.

[0047] Example 3: The SPR refractive index sensor based on double C-shaped microstructure optical fiber in Example 3 is basically similar in structure to that in Example 1. The difference lies in the different radii R of the double C-shaped circular structure and the different polishing depth H. Figure 3(a) , 3(b) The effects of varying the radius of the double-C-shaped circular core (3 μm, 4 μm, and 5 μm) and the polishing depth (H) (2.56 μm, 2.76 μm, and 2.96 μm) on the wavelength of the resonance peak in the loss spectrum are shown in the figures. When the refractive index of the analyte is between 1.40 and 1.41, the change in the resonance wavelength is relatively small. However, when the refractive index is 1.42, a significant redshift occurs in the resonance peak as the radius R of the double-C-shaped circular core decreases. The wavelength of the resonance peak at R = 3 μm is 42 nm redshifted compared to that at R = 5 μm. This is mainly attributed to the change in the internal structure of the microstructure fiber caused by the change in the radius of the double-C-shaped circular core, which in turn affects the excitation conditions of surface plasmon resonance. The results indicate that the SPR sensor achieves the highest sensitivity and optimal resolution when the radius of the double-C-shaped circular core is 3 μm.

[0048] Example 4: The SPR refractive index sensor based on double C-shaped microstructure optical fiber in Example 4 is basically similar in structure to that in Example 1. The difference is that the thickness tg of the metal thin film layer 3 and the thickness tt of the titanium dioxide thin film layer 4 are different. Figure 4(a) , 4(b)The effects of varying thicknesses of the metal thin film layer 3 (tg) of 38 nm, 40 nm, and 42 nm, and the titanium dioxide thin film layer 4 (tt) of 6 nm, 8 nm, and 10 nm, on the SPR mode loss spectrum are shown when the refractive index of the analyte varies in 0.01 steps within the range of 1.40 to 1.42.

[0049] As shown in Figure 4(a), the change in the thickness tg of the metal thin film layer 3 significantly affects the peak intensity and position of the resonance wavelength. The peak intensity of the resonance wavelength decreases monotonically with increasing tg. This phenomenon is attributed to the increased spacing between the fiber surface and the plasma mode due to the increased gold film thickness, which weakens the energy transfer efficiency from the fiber core fundamental mode to the plasma mode, thereby reducing the loss peak. In the refractive index range of 1.40–1.41, the resonance wavelength exhibits a significant redshift; when the refractive index reaches 1.42, this phenomenon tends to weaken. Through comprehensive evaluation of various performance indicators, a tg of 38 nm was determined to be the optimal gold film thickness for the sensor. Under this parameter, the sensor exhibits the best working performance. As shown in Figure 4(b), the resonance wavelength exhibits a significant redshift with increasing thickness tt of the titanium dioxide thin film layer 4. This phenomenon is similar to the effect produced by increasing the gold film thickness, and its physical mechanism is the redshift of the phase matching point caused by the increase in the effective refractive index (neff) of the plasma. When the refractive index of the analyte is 1.40, the peak loss increases with increasing tt thickness. However, when the refractive index is between 1.41 and 1.42, the peak value of the SPR loss spectrum first increases and then decreases with increasing tt thickness. Therefore, the SPR sensor reaches its highest peak loss and lowest FWHM value when tt = 8 nm. Thus, the sensor achieves optimal performance when the titanium dioxide film thickness tt = 8 nm.

[0050] Example 5 illustrates the application of the SPR refractive index sensor based on dual-C microstructure fiber as described in Examples 1 to 4. After parameter optimization of the aforementioned microstructure fiber refractive index sensor, under the conditions of R = 3 μm, H = 2.76 μm, tg = 38 nm, and tt = 8 nm, the refractive index sensor achieves optimal performance parameters, making it more suitable for practical sensing applications. When the refractive index RI of the analyte varies in 0.01 steps within the range of 1.38 to 1.42, the SPR mode loss spectrum on the refractive index sensor surface is shown in Figure 5(a). It can be seen from the figure that as the refractive index of the analyte increases, the resonant wavelength exhibits a significant redshift, and the degree of redshift increases with increasing refractive index. This characteristic is consistent with the inference from the block curve in Figure 5(b). Furthermore, in the refractive index range of 1.41 to 1.42, the SPR mode loss spectrum curve shows a significant and sharp peak, indicating that it possesses an extremely low full width at half maximum (FWHM) and an extremely high quality factor (FOM). From Figure 5(b), the FWHM of the loss spectrum varies with refractive index. It can be seen that when the refractive index changes from 1.38 to 1.42, the average FWHM of the SPR loss spectrum is 25.4 nm. Particularly at a refractive index of 1.41, the FWHM reaches a minimum of 21.9 nm, significantly lower than most other types of SPR sensors. This low FWHM characteristic not only helps in identifying adjacent resonant loss spectra but also improves the FOM value. Figure 5(c) shows the sensor sensitivity and FOM as a function of refractive index. It can be seen that when the refractive index is in the range of 1.39 to 1.42, the sensor sensitivity continuously increases with increasing refractive index, reaching a peak of 13800 nm / RIU at a refractive index of 1.42, corresponding to a resolution of 7.246 × 10⁻⁶. -6 RIU-1. Meanwhile, the FOM value also continues to increase, reaching a maximum of 543.3 RIU-1 at a refractive index of 1.42, with an average value as high as 322 RIU-1 in the refractive index range of 1.39 to 1.42. In practical SPR sensing applications, high FOM is a key parameter for achieving accurate and noise-free sensing.

[0051] Furthermore, the SPR refractive index sensor based on double C-shaped microstructure optical fiber in this embodiment is compared with the performance of existing related sensors. The comparison focuses on key indicators such as sensitivity, full width at half maximum (FWHM), quality factor (FOM), and structural complexity (characterized by the number of air holes), resulting in the following comparison table 1.

[0052] Table 1 Comparison of performance indicators of the sensor proposed in this paper with existing sensors.

[0053]

[0054] As can be seen from Table 1, when the refractive index (RI) of the analyte is in the range of 1.38 to 1.42, the refractive index sensor proposed in this scheme exhibits superior overall performance, with a sensitivity value of 13800. It also achieves superior values ​​in both FWHM and FOM indicators. This characteristic helps to more clearly distinguish adjacent resonance loss peaks, thereby effectively reducing sensor noise.

[0055] The above implementation examples reflect the application levels and occasions of the present invention. Any technical solution that achieves the purpose of the present invention by essentially the same means falls within the protection scope of the present invention.

Claims

1. An SPR refractive index sensor based on a double-C microstructure optical fiber, characterized by: The sensor has a cylindrical structure with a double-C-shaped quasi-circular cross-section. Specifically, it includes a silicon dioxide substrate, air holes, a gold thin film layer, and a titanium dioxide thin film layer. The double-C-shaped quasi-circular structure consists of a C-shaped notch polished on each of the two opposite sides of the silicon dioxide substrate. The distance between the arc-shaped top of the C-shaped notch and the center of the silicon dioxide substrate is the polishing depth. There are four air holes on the silicon dioxide substrate, evenly arranged in a square within the double-C-shaped quasi-circular structure. The metal thin film layer is attached to the outside of the C-shaped notch, and the titanium dioxide thin film layer is attached to the outside of the metal thin film layer. The cylinder is made of silicon dioxide or germanium, and the metal thin film layer is made of gold or silver.

2. The SPR refractive index sensor based on a double-C microstructure optical fiber as described in claim 1, characterized in that: The cylinder has a length of 10mm to 50mm, a polishing depth of 2.76 to 2.96μm, a silicon dioxide substrate radius of 4 to 6μm, an air hole radius of 1 to 3μm, a distance from the center of the air hole to the center of the silicon dioxide substrate of 2.5 to 3.5μm, a double C-shaped near-circular shape radius of 3 to 5μm, a metal thin film layer thickness of 38 to 42nm, and a titanium dioxide thin film layer thickness of 6 to 8nm.

3. The fabrication of the SPR refractive index sensor based on a double-C-shaped microstructure optical fiber as described in claim 1 or 2, characterized in that: Includes the following steps, Step 1: Take multiple silica rods of suitable length and diameter as optical fiber substrates, remove the coating layer on the outer surface of the silica rods, and use optical fibers to cut the two ends of the silica rods flat to form prefabricated silica rods. Step 2: Using standard drawing process, some pre-made silica rods are drawn into thin silica rods, with a diameter approximately half that of the original silica rods; using ultrasonic drilling technology, air holes penetrating both ends of the pre-made silica rods are precisely machined at the center of the cross-section. Step 3: Stack and assemble the prefabricated silica rods, fine silica rods, and prefabricated silica rods with air holes in a specific order to form a stacked prefabricated rod, and fix the stacked prefabricated rod structure with adhesive to ensure accurate alignment between the three components; Step 4: Perform precision stretching on the stacked preform structure. By uniformly applying tension, the preformed silica rod, fine silica rod, and preformed silica rod with air holes are fused into a seamless microstructured optical fiber semi-finished product. Step 5: Perform fine polishing on the microstructured optical fiber semi-finished product, and use an optical polishing machine to polish the opposite sides of the microstructured optical fiber semi-finished product to create a C-shaped arc with the same structure, remove surface defects and adhesive, and obtain a smooth double C-shaped microstructured optical fiber semi-finished product. Step 6: Deposit a metal thin film layer and a titanium dioxide thin film layer in the left and right arc-shaped regions of the double-C microstructured optical fiber semi-finished product using chemical vapor deposition technology to obtain the refractive index sensor of the double-C microstructured optical fiber.

4. The fabrication of the SPR refractive index sensor based on a double-C-shaped microstructure optical fiber as described in claim 3, characterized in that: The specific sequence is that four fine silica rods are evenly distributed in a cross shape on the outer periphery of the pre-made silica rod, and four pre-made silica rods containing air holes are respectively arranged between two fine silica rods.

5. The fabrication of the SPR refractive index sensor based on a double-C-shaped microstructure optical fiber as described in claim 3 or 4, characterized in that: In step four, an optical fiber drawing tower is selected for the stretching operation.

6. The application of the SPR refractive index sensor based on a double-C microstructure optical fiber as described in claim 1 or 2, characterized in that: The analyte with a refractive index (RI) ranging from 1.38 to 1.42 is applied to the fiber optic refractive index sensor. The analyte is distributed around the fiber optic refractive index sensor. The drift of the loss spectrum at different incident wavelengths is observed, and the sensor's sensitivity, FWHM, FOM, and number of air holes are used to evaluate the sensor's performance.

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