Optical fiber vector curvature sensor based on polydimethylsiloxane and preparation method thereof

Abstract

The invention belongs to the technical field of optical fiber sensing and curvature measurement, and particularly relates to an optical fiber vector curvature sensor based on polydimethylsiloxane and a preparation method thereof.The optical fiber vector curvature sensor based on polydimethylsiloxane comprises a coreless optical fiber, and the outer side of the coreless optical fiber is divided into an upper semicircular area and a lower semicircular area; wherein a layer of metal film is arranged on the outer side of one semicircular area, a layer of high-refractive-index film is deposited on the outer side of the metal film, and a layer of polydimethylsiloxane is arranged on the outer side of the high-refractive-index film; the invention discloses a preparation method of an optical fiber vector curvature sensor based on polydimethylsiloxane. The preparation method comprises the following steps: forming a surface plasma resonance area on the outer side of a coreless optical fiber; depositing a metal film on the surface of the surface plasma resonance area; depositing a high-refractive-index film on the outer side of the metal film; and coating polydimethylsiloxane on the surface of the high-refractive-index thin film. Vector detection is realized by using an asymmetric structure, curvature sensitivity is improved by compounding SPR angle modulation and a refractive index detection mechanism, and the bending direction can be continuously detected and identified in a wide curvature range.

Description

Technical Field

[0001] The present invention belongs to the technical fields of fiber optic sensing and curvature measurement, and particularly relates to a fiber optic vector curvature sensor based on polydimethylsiloxane and a preparation method thereof. Background Art

[0002] Fiber optic curvature sensors have shown significant advantages in various application scenarios. Firstly, they can achieve high-precision curvature measurement, which is crucial for structural health monitoring. For example, in the materials or structures of infrastructure such as bridges, buildings, tunnels, and dams, accurately detecting curvature changes can prevent potential failures and catastrophic damages. In addition, in engineering applications, curvature measurement not only optimizes the performance of components but also improves efficiency. In intelligent manufacturing and automation systems, precise curvature measurement enhances the flexibility and accuracy of robots in complex tasks, ensuring efficient operation in complex environments.

[0003] Currently, various fiber optic curvature sensors with different structures have been proposed in domestic and foreign patents, such as interferometric, fiber grating, and intensity types. These sensors have their own advantages and disadvantages, and their sensitivity and sensing range are limited by the mode and structural design.

[0004] In addition, surface plasmon resonance (SPR) fiber optic sensors have received extensive attention due to their high sensitivity in various applications. Traditional SPR fiber optic curvature sensors usually combine microfabrication techniques and require the design of complex fiber structures. Although these methods can improve sensitivity, they also increase the manufacturing complexity, reduce the overall robustness of the fiber, and impose angular limitations, restricting the measurement range.

[0005] In the prior art "Multi-core Bragg Grating Curvature Sensor Based on Matched Filter Demodulation", Zheng Di et al. used multi-core optical fiber to make an optical fiber curvature sensor. Bragg gratings were inscribed in two cores of the multi-core optical fiber that were axisymmetric about the center to form a matched filter mode. When the multi-core optical fiber was bent, the area of the overlapping region of the reflection spectra of the two fiber Bragg gratings changed, affecting the intensity of the output signal light. Therefore, curvature demodulation could be achieved by measuring the power of the matched filter signal. However, the fabrication of this sensor was cumbersome and the requirements for the demodulation system were relatively high. In the present invention, a spectral analyzer is used, and the sensing signals are collected and processed by a computer, and the measurement results can be processed simply and quickly. In the prior art "Fiber Optic Sensor for the Measurement of Respiratory Chest Circumference Changes", Anatoly Babchenko et al. made a multi-turn helical tube structure from graded-index plastic optical fiber and used the sensing principle that the macro-bending of the optical fiber caused a change in optical transmission loss to detect structural deformation. However, since the output parameters of the intensity modulation type optical fiber sensor are light intensity or optical power, any environmental factor fluctuations in the system will affect the output parameters. Therefore, the stability of the intensity modulation type optical fiber sensor is not good enough and it is difficult to improve the measurement accuracy. The diameter of the optical fiber used in the present invention allows for the propagation of multiple optical fiber modes, which makes this design more sensitive to bending. The Chinese patent with the patent application number CN201910760427.4 uses a single-mode optical fiber and a hollow-core optical fiber to form Mach-Zehnder interference fringes and constitutes a Fabry-Perot resonator; the built fiber curvature and temperature measurement system based on the fiber curvature sensor can obtain the transmission spectrum of the sensor, and by calculating the wavelength intervals of the Mach-Zehnder interference peaks and resonance absorption peaks in the transmission spectrum, the measurement of fiber curvature and temperature can be realized.

[0006] In contrast, the fiber optic vector curvature sensor based on polydimethylsiloxane proposed by the present invention has high sensitivity and a wide detection range, achieving significant improvements in design and function compared to the prior art. When the optical fiber is bent, the incident angle of light in the optical fiber changes with the curvature, thereby affecting the resonance condition of SPR, resulting in a change in the position of the SPR resonance valley. Polydimethylsiloxane is a high-quality material widely used in the optical field, not only having good optical properties but also excellent flexibility and ductility. When polydimethylsiloxane is subjected to pressure interference, the refractive index inside it changes, and this change is detected by the fiber optic SPR, causing a shift in the position of the SPR resonance valley. Both the angle modulation and refractive index detection mechanisms of SPR are sensitive to curvature changes, and the direction of change of the resonance valley is the same, which not only improves the sensitivity of the sensor but also enhances its reliability. The present invention further adopts an enhanced SPR sensing technology by depositing a high refractive index material on the metal layer to increase the dielectric constant of the metal layer, thereby significantly improving the sensitivity of the sensor. Specifically, the deposition of the high RI thin film causes a red shift in the resonance wavelength, further enhancing the sensor's ability to detect curvature changes. The present invention utilizes an asymmetric structure to achieve the vector detection function and combines the SPR angle modulation and refractive index detection mechanisms to further improve the sensitivity of the sensor. This sensor can continuously detect and identify the bending direction within a wide curvature range, providing a more accurate and reliable technical solution for curvature sensing. Summary of the Invention

[0007] The object of the present invention is to provide a fiber optic vector curvature sensor based on polydimethylsiloxane and its preparation method, which can have high sensitivity and can continuously detect and identify the bending direction within a wide curvature range.

[0008] The technical solutions adopted by the present invention are specifically as follows:

[0009] A fiber optic vector curvature sensor based on polydimethylsiloxane includes a coreless optical fiber. The outer side of the coreless optical fiber is divided into upper and lower semi-circular regions. A metal thin film is provided on the outer side of one of the semi-circular regions. A high refractive index thin film is deposited on the outer side of the metal thin film, and a layer of polydimethylsiloxane is provided on the outer side of the high refractive index thin film.

[0010] A preparation method of a fiber optic vector curvature sensor based on polydimethylsiloxane includes the following steps:

[0011] Step 1: Process the coreless optical fiber to form a surface plasmon resonance region on the outer side of the coreless optical fiber;

[0012] Step 2: Deposit a metal thin film on the surface of the surface plasmon resonance region;

[0013] Step 3: Deposit a high refractive index film on the outer side of the metal film;

[0014] Step 4: Coat a layer of polydimethylsiloxane on the surface of the high refractive index film.

[0015] Further, the polydimethylsiloxane in the above step is cured at a temperature of degrees Celsius.

[0016] The technical effects achieved by the present invention are as follows:

[0017] A fiber optic vector curvature sensor based on polydimethylsiloxane of the present invention removes the coating layer of a coreless fiber and deposits a metal film and a high refractive index film thereon. Through the changes in the SPR excitation angle and modal coupling caused by the bending of the coreless fiber, polydimethylsiloxane is coated on the outer surface of the high refractive index film to provide a stable RI environment. Compared with traditional intensity-type, fiber grating-type, and interferometer-type fiber optic curvature sensors, it has a rapid response, can continuously and accurately identify curvature and bending direction within a wide sensing range, has high sensitivity, can continuously detect and identify the bending direction within a wide curvature range, combines the PDMS material with a high light absorption material, improves the photosensitive properties of the PDMS material itself, and optimizes its mechanical properties so that it can better perform curvature measurement and direction recognition capabilities. Description of the Drawings

[0018] Figure 1 is a schematic diagram of depositing a gold film in the present invention;

[0019] Figure 2 is a schematic diagram of depositing a high refractive index film in the present invention;

[0020] Figure 3 is a schematic diagram of controlling the filling of the PDMS coating using a mold in the present invention;

[0021] Figure 4 is a schematic diagram of the structure of the present invention;

[0022] Figure 5 is the curvature sensing curve of the prepared vector curvature sensor in the present invention in the positive and negative directions.

[0023] In the drawings, the list of components represented by each reference numeral is as follows:

[0024] 1. Coreless fiber; 2. Metal film; 3. High refractive index film; 4. Polydimethylsiloxane; 5. Mold. Detailed Embodiments

[0025] In order to make the objectives and advantages of the present invention more clear and understandable, the present invention will be specifically described below in conjunction with embodiments. It should be understood that the following text is only used to describe one or several specific implementation manners of the present invention, and does not strictly limit the scope of protection specifically claimed by the present invention.

[0026] As Figures 1 - 5 shown, a fiber optic vector curvature sensor based on polydimethylsiloxane includes a coreless fiber 1 with a diameter of 10 - 300 μm. The coreless fiber 1 does not require splicing and processing of the fiber structure, and the light transmitted inside is relatively stable, being less affected by bending.

[0027] The outer side of the coreless fiber 1 is divided into upper and lower semi-circular regions, that is, the outer side of the coreless fiber 1 is divided into two parts with the axis of the coreless fiber 1 as the center. A metal thin film 2 is provided on the outer side of one of the semi-circular regions. The setting method can be depositing a metal thin film 2 on the surface of the coreless fiber 1 using a plasma sputtering device. The thickness of the metal thin film 2 is preferably 30 - 70 nm. The metal thin film 2 is the excitation condition for SPR, and a thickness of 30 - 70 nm results in a better SPR effect.

[0028] A high refractive index thin film 3 is deposited on the outer side of the metal thin film 2 using a magnetron sputtering system, and the thickness of the high refractive index thin film 3 is preferably 0 - 100 nm. The high refractive index thin film can improve the sensing sensitivity. The high refractive index thin film 3 includes zinc oxide ZnO, germanium antimony tellurium mixture GST, indium tin oxide ITO, and titanium dioxide TiO2.

[0029] A layer of polydimethylsiloxane 4 PDMS is provided on the outer side of the high refractive index thin film 3 as a sensing medium, and the thickness of the polydimethylsiloxane 4 is 10 - 300 μm.

[0030] The metal thin film 2 on the surface of the coreless fiber 1 excites SPR. The bending of the coreless fiber 1 causes changes in the SPR excitation angle and modal coupling, which enables the curvature to be detected. The polydimethylsiloxane 4 is formed by mixing PDMS material with a high light absorption rate material. The PDMS material is light and soft with a high light transmittance, and the high light absorption rate material can exhibit high absorption of light in a specific wavelength band. By controlling the thickness of the polydimethylsiloxane 4, the sensitivity can be adjusted.

[0031] The process of fiber optic vector curvature measurement is as follows: when the coreless fiber 1 bends, the polydimethylsiloxane 4 is subjected to stress from the external curvature, resulting in a change in the internal refractive index (RI). This RI change is detected by the SPR phenomenon, leading to a measurable shift in the SPR angle corresponding to the fiber curvature, thereby realizing the measurement of the fiber optic vector curvature.

[0032] A preparation method of a fiber optic vector curvature sensor based on polydimethylsiloxane includes the following steps:

[0033] Step 1: Process the coreless optical fiber 1, remove the fiber protective layer near the sensing area, and form a surface plasmon resonance (SPR) area on the outside of the coreless optical fiber 1;

[0034] First, take a 1-meter-long coreless optical fiber 1. To ensure the accuracy and consistency of subsequent steps, use precision tools to remove a 1-cm-long coating and cladding at the midpoint of the coreless optical fiber 1. This processed area will be designated as the surface plasmon resonance SPR area. This step is very crucial because the SPR area is the core part of the entire sensor and is used to detect changes in the external environment.

[0035] Step 2: Deposit a metal thin film 2 on the surface of the surface plasmon resonance SPR area;

[0036] Next, use a plasma sputtering device to uniformly deposit a pure metal thin film 2 on the surface of the SPR area. To ensure that the thickness of the metal thin film 2 is smooth and uniform, we have specifically designed a motor system and integrated it into the vacuum chamber of the plasma sputtering system. During the coating process, this motor rotates the coreless optical fiber 1 at a constant speed, enabling the metal thin film 2 to uniformly cover the entire surface of the SPR area. This uniform metal thin film 2 is crucial for improving the sensitivity and stability of the sensor.

[0037] Step 3: Deposit a high refractive index thin film 3 on the outside of the metal thin film 2;

[0038] Then, use a magnetron sputtering system to deposit a 30-nm-thick high refractive index thin film 3 on the already formed metal thin film 2, apply a sputtering voltage of 300 volts, and adjust the thickness of the high refractive index thin film 3 by precisely controlling the deposition time. This step further enhances the sensor's response ability to changes in the external environment.

[0039] Step 4: Coat a layer of polydimethylsiloxane 4 on the surface of the high refractive index thin film 3.

[0040] Finally, with the aid of a micro-operation system, perform a coating operation on the mixed PDMS (Dow Corning 184), and use a customized mold 5 to precisely control the thickness of the polydimethylsiloxane 4 filled therein. The thickness of the filled polydimethylsiloxane 4 is controlled by a capillary tube to ensure the stability of the experiment. To ensure the stability and repeatability of the experimental results. Degas the mixture of polydimethylsiloxane 4 to remove air bubbles, and then cure it at a temperature of 65 degrees Celsius for 1 hour to form the final fiber optic vector curvature sensor.

[0041] Light undergoing total internal reflection (TIR) in the fiber waveguide generates an evanescent wave on the metal thin film 2. When the propagation constant of the evanescent wave matches that of the surface plasmon wave (SPW), part of the incident light energy is transferred to the evanescent surface wave, resulting in a decrease in reflectivity at the resonance wavelength. Therefore, the transmission spectrum exhibits an SPR resonance dip, and the resonance dip redshifts as the RI increases. The p-polarized light oscillates the free electrons on the surface of the metal thin film 2, generating plasmons. The evanescent wave decays exponentially in the horizontal direction, and its wave vector can be expressed as Equation 1:

[0042]

[0043] where n1 is the RI of the coreless fiber 1, ω is the angular frequency of the incident light, θ is the incident angle, and λ is the resonance wavelength. At the junction of the metal thin film 2 and the coreless fiber 1, the free electrons on the surface of the metal thin film 2 are excited, generating oscillating charges. This phenomenon leads to the formation of surface plasmons, which can be expressed as:

[0044]

[0045] where n s represents the RI of the analyte, ε m and ε s represent the dielectric constants of the metal thin film 2 and the analyte, respectively. When k ev = Re(k sp ), SPR is excited. At this time, the incident light is absorbed, resulting in a decrease in the resonance wavelength in the output spectrum. According to Equation 2, the change in n s causes a shift in the resonance wavelength. However, n s will be affected by θ and ε m .

[0046] The bending of the coreless fiber 1 leads to mode coupling, which changes the distribution and range of the incident angle of the light wave. In this case, when bending occurs, the resonance tilt wavelength of θ changes, and bending in both the positive and negative x-directions will cause corresponding changes in the SPR incident angle, and the changes are continuous. Therefore, this method can achieve the detection of the bending direction.

[0047] In addition, we coated a layer of polydimethylsiloxane 4 with a thickness of 10 - 300 μm on the outside of the coreless fiber 1, which provides a sensing medium for SPR to sense curvature changes. PDMS provides a stable RI environment for SPR. When pressure is applied to the polydimethylsiloxane 4, n decreases as the pressure increases. This change in RI is related to the stretching and reorientation of the siloxane chains in PDMS. In the case of angle change and pressure change, the resonance tilt angle of SPR redshifts and blueshifts in the same direction, and the two together significantly enhance the bending sensitivity of SPR.

[0048] Increasing the dielectric constant ε of the metal thin film 2 m can improve the sensitivity of the SPR sensor. The dielectric constant ε of the metal thin film 2 m affects the propagation constant k of the surface plasmon wave sp , and thus affects the resonance condition of SPR. Therefore, the detection of RI changes becomes more accurate and sensitive. It is found that adding 3 layers of high refractive index thin films causes a significant red shift of the resonance wavelength. The addition of 3 layers of high refractive index thin films increases ε m , resulting in a red shift of the resonance wavelength and a significant improvement in sensitivity.

[0049] As Figure 5 shown by detection, this curvature sensing probe has high sensitivity and can identify bending in the range of 1.74 nm / m from 0 to 64 m in the positive x direction -1 and bending in the range of 6.21 nm / m from 0 to -21.3 m in the negative x direction -1 . -1 -1

[0050] In summary, after removing the coating layer of the coreless optical fiber 1, a metal thin film 2 and a high refractive index thin film 3 are plated. By the changes in the SPR excitation angle and mode coupling caused by the bending of the coreless optical fiber 1, polydimethylsiloxane 4 is coated on the outer surface of the high refractive index thin film 3 to provide a stable RI environment. Compared with traditional intensity-type, fiber grating-type, and interferometer-type fiber curvature sensors, it has a rapid response, can continuously and accurately identify curvature and bending direction within a wide sensing range, has high sensitivity, can continuously detect and identify the bending direction within a wide curvature range, combines the PDMS material with a high light absorption material, improves the photosensitive properties of the PDMS material itself, and optimizes its mechanical properties to enable better curvature measurement and direction recognition capabilities.

[0051] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made. For example, the specific thickness of the gold film or the high refractive index thin film can be adjusted, or the PDMS curing conditions can be changed, etc. These improvements and refinements should also be regarded as the protection scope of the present invention. These improvements and refinements should also be regarded as the protection scope of the present invention. The structures, devices, and operation methods not specifically described and explained in the present invention are implemented according to the conventional means in the art without special description and limitation.

Claims

1. A fiber optic vector curvature sensor based on polydimethylsiloxane, characterized in that: It includes a coreless optical fiber (1). The outer side of the coreless optical fiber (1) is divided into upper and lower semi-circular regions. A metal thin film (2) is provided on the outer side of one of the semi-circular regions. A high refractive index thin film (3) is deposited on the outer side of the metal thin film (2). A layer of polydimethylsiloxane (4) is provided on the outer side of the high refractive index thin film (3).

2. The fiber optic vector curvature sensor based on polydimethylsiloxane according to claim 1, characterized in that: The diameter of the coreless optical fiber (1) is 10 - 300 μm.

3. The fiber optic vector curvature sensor based on polydimethylsiloxane according to claim 1, characterized in that: The thickness of the metal thin film (2) is 30 - 70 nm.

4. The fiber optic vector curvature sensor based on polydimethylsiloxane according to claim 1, characterized in that: The thickness of the high refractive index thin film (3) is 0 - 100 nm.

5. The fiber optic vector curvature sensor based on polydimethylsiloxane according to claim 1, wherein: The high refractive index thin film (3) includes a mixture of zinc oxide, germanium antimony telluride, indium tin oxide, and titanium dioxide.

6. The fiber optic vector curvature sensor based on polydimethylsiloxane according to claim 1, wherein: The high refractive index thin film (3) is deposited on the outer side of the metal thin film (2) by using a magnetron sputtering system.

7. A fiber optic vector curvature sensor based on polydimethylsiloxane according to claim 1, characterized in that: A layer of polydimethylsiloxane (4) is coated on the outer side of the coreless optical fiber (1). The thickness of the polydimethylsiloxane (4) is 10 - 300 μm.

8. A preparation method of a fiber optic vector curvature sensor based on polydimethylsiloxane, using the fiber optic vector curvature sensor described in any one of claims 1-6, characterized in that: It includes the following steps: Step 1: Process the coreless optical fiber (1) to form a sensing region on the outer side of the coreless optical fiber (1). Step 2: Deposit the metal thin film (2) on the surface of the surface plasmon resonance region. Step 3: Deposit a layer of high refractive index thin film (3) on the outer side of the metal thin film (2). Step 4: Coat a layer of polydimethylsiloxane (4) on the surface of the high refractive index thin film (3).

9. The preparation method of a fiber optic vector curvature sensor based on polydimethylsiloxane according to claim 7, characterized in that: In Step 4, the polydimethylsiloxane (4) is cured at a temperature of 65 degrees Celsius.

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