Optical fiber SPR sensor and preparation method thereof

Abstract

The invention belongs to the technical field of optical fiber sensors, and discloses an optical fiber SPR (Surface Plasmon Resonance) sensor and a preparation method thereof. The optical fiber SPR sensor provided by the invention comprises a first optical fiber, a capillary optical fiber and a multimode optical fiber, the first optical fiber is an optical fiber capable of realizing double-path optical transmission, the surface of the capillary optical fiber is plated with a metal film, and two ends of the capillary optical fiber are respectively welded with the first optical fiber and the multimode optical fiber; an optical signal transmitted along a first path of the first optical fiber enters the inner wall of the capillary optical fiber to be totally reflected, and interacts with the metal film to generate SPR (Surface Plasmon Resonance) which is received by the multimode optical fiber for external refractive index sensing; and the optical signal transmitted along the second path of the first optical fiber enters the air cavity of the capillary optical fiber and is received by the multimode optical fiber after passing through the air cavity, and the two paths of optical signals at the output end form Mach-Zehnder interference for external temperature sensing. According to the invention, external refractive index and temperature change can be measured at the same time, cross-talk can be avoided, and high sensitivity is achieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical fiber sensors, and more specifically, relates to an optical fiber SPR sensor and a preparation method thereof. Background Art

[0002] As a label-free optical sensor, SPR (Surface Plasmon Resonance) sensor has high sensitivity and selectivity and is widely used in biosensing, chemical detection, medical diagnosis, environmental monitoring and other fields. Early SPR sensors were mainly prism-based SPR sensors, including Otto type and Kretschmann type. Such sensors are large in size and expensive in equipment, and cannot be used for long-distance sensing measurements. Compared with prism-type SPR sensors, fiber-based SPR sensors have the advantages of small size, compact structure, low cost, and can be used for online remote control, and have been rapidly developed. Some typical structures include online transmission structure, reflection structure, heterogeneous core structure and D-type fiber structure. Traditional fiber-optic SPR sensors can only sense and measure changes in external refractive index, and cannot monitor external temperature changes in real time. However, the thermo-optical effect of the material causes the refractive index to be closely related to temperature. Therefore, a fiber-optic SPR sensor that can simultaneously measure external refractive index and temperature is of great significance for studying the parameter performance of the sample to be tested.

[0003] In recent years, some scholars have proposed fiber optic sensor solutions that can simultaneously measure the external refractive index and temperature. However, there are some problems with the existing solutions. For example, refractive index sensing is achieved by measuring the intensity of the reflected signal, but the refractive index measurement uses light intensity modulation, which is easily interfered by ambient light and has low sensitivity. Summary of the invention

[0004] The present invention solves the problem of low sensitivity of the optical fiber sensor for simultaneously measuring the external refractive index and temperature in the prior art by providing an optical fiber SPR sensor and a preparation method thereof.

[0005] The present invention provides an optical fiber SPR sensor, comprising: a first optical fiber, a capillary optical fiber and a multimode optical fiber; the first optical fiber is an optical fiber capable of realizing dual-path optical transmission, the surface of the capillary optical fiber is plated with a metal film, one end of the capillary optical fiber is welded to the first optical fiber, the other end of the capillary optical fiber is welded to the multimode optical fiber, the first optical fiber is used as an input optical fiber, and the multimode optical fiber is used as an output optical fiber;

[0006] The optical signal transmitted along the first path of the first optical fiber is incident on the inner wall of the capillary optical fiber, undergoes total reflection, and interacts with the metal film plated on the surface of the capillary optical fiber to generate surface plasmon resonance SPR, which is received by the multimode optical fiber and used for external refractive index sensing;

[0007] The optical signal transmitted along the second path of the first optical fiber is incident on the air cavity of the capillary optical fiber, and is received by the multimode optical fiber after passing through the air cavity of the capillary optical fiber. At the output end, the two optical signals form Mach-Zehnder interference for external temperature sensing.

[0008] Preferably, the first optical fiber is a coaxial dual-waveguide optical fiber, and the coaxial dual-waveguide optical fiber includes a cladding, a ring core and a single-mode core; the optical signal transmitted along the ring core is used as the optical signal transmitted along the first path of the first optical fiber, and the optical signal transmitted along the single-mode core is used as the optical signal transmitted along the second path of the first optical fiber.

[0009] Preferably, the annular fiber core is aligned with the inner wall of the capillary optical fiber.

[0010] Preferably, the operating band of the surface plasmon resonance (SPR) is the visible light band, and the operating band of the Mach-Zehnder interferometer is the near-infrared band.

[0011] Preferably, the multimode optical fiber is a step-index multimode optical fiber or a gradient-index multimode optical fiber.

[0012] Preferably, the air cavity of the capillary optical fiber is filled with a thermosensitive material, and the thermosensitive material is one of PDMS, agar, ethanol, and glucose solution.

[0013] Preferably, the length of the capillary optical fiber is 500 to 1000 μm, and the capillary optical fiber is a columnar capillary optical fiber.

[0014] Preferably, the metal film is one of a gold film, a silver film, a copper film, and a composite metal film; the composite metal film is one of a gold-chromium composite film and a gold-silver composite film; in the gold-chromium composite film, gold is used as the lower layer and chromium is used as the upper layer; in the gold-silver composite film, silver is used as the lower layer and gold is used as the upper layer.

[0015] On the other hand, the present invention provides a method for preparing the optical fiber SPR sensor as described above, comprising the following steps:

[0016] welding one end of the capillary optical fiber to the first optical fiber;

[0017] Cutting the capillary optical fiber so that the length of the capillary optical fiber reaches a preset length;

[0018] Welding the other end of the cut capillary optical fiber to the multimode optical fiber;

[0019] A metal film is plated on the surface of the capillary optical fiber to prepare an optical fiber SPR sensor.

[0020] Preferably, before welding one end of the capillary optical fiber to the first optical fiber, the method further comprises: performing corrosion treatment on the capillary optical fiber so that the inner wall thickness of the capillary optical fiber reaches a preset thickness.

[0021] One or more technical solutions provided in the present invention have at least the following technical effects or advantages:

[0022] (1) The present invention provides an optical fiber SPR sensor comprising a first optical fiber, a capillary optical fiber and a multimode optical fiber, wherein the first optical fiber is an optical fiber capable of realizing dual-path optical transmission, a metal film is coated on the surface of the capillary optical fiber, one end of the capillary optical fiber is welded to the first optical fiber, and the other end of the capillary optical fiber is welded to the multimode optical fiber, the first optical fiber serves as an input optical fiber, and the multimode optical fiber serves as an output optical fiber; an optical signal transmitted along a first path of the first optical fiber is incident on the inner wall of the capillary optical fiber, undergoes total reflection, and interacts with the metal film coated on the surface of the capillary optical fiber to generate surface plasmon resonance SPR, which is received by the multimode optical fiber for external refractive index sensing; an optical signal transmitted along a second path of the first optical fiber is incident on the air cavity of the capillary optical fiber, and is received by the multimode optical fiber after passing through the air cavity of the capillary optical fiber, and the two optical signals form Mach-Zehnder interference at the output end for external temperature sensing. That is, the present invention utilizes wavelength division multiplexing technology to generate surface plasmon resonance SPR and Mach-Zehnder interference MZI to respectively perform refractive index and temperature sensing. The two channels are independent of each other, avoiding the problem of crosstalk. It can not only measure the external refractive index and temperature simultaneously, but also has the advantage of high sensitivity.

[0023] (2) The present invention uses the inner wall of the capillary optical fiber as the refractive index sensing area. The inner wall of the capillary optical fiber is directly exposed to the air, causing a strong evanescent field, and the surface area can be made larger than that of the traditional fiber core, which can make the surface plasmons and the analyte overlap more, and can further improve the refractive index sensitivity and response speed. The present invention uses the air cavity of the capillary optical fiber as the temperature sensing area, introduces the cavity mode and the inner wall mode to produce Mach-Zehnder interference, and can achieve temperature response.

[0024] (3) When the first optical fiber in the present invention adopts a coaxial dual-waveguide optical fiber, a dual-path waveguide is introduced by using the coaxial dual-waveguide optical fiber to realize dual-parameter measurement, which can simplify the device preparation process and improve the device integration, thereby obtaining a compact optical fiber sensor.

[0025] (4) When the capillary optical fiber in the present invention adopts a cylindrical capillary optical fiber, compared with a square or other polygonal capillary optical fiber, the sensing area can be maximized, so that the surface plasmons and the object to be detected have the maximum overlap, further improving the detection accuracy.

[0026] (5) The present invention can further improve the temperature sensing performance by filling the air cavity with thermosensitive materials such as PDMS, agar, ethanol and glucose solution. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 A schematic diagram of the structure of an optical fiber SPR sensor provided in Example 1 of the present invention;

[0028] Figure 2 A schematic diagram of a coaxial dual-waveguide optical fiber in an optical fiber SPR sensor provided in Example 1 of the present invention; wherein, Figure 2 (a) is a schematic diagram of the end face structure of a coaxial dual-waveguide optical fiber. Figure 2 (b) is the refractive index distribution diagram at the dotted line of the coaxial dual-waveguide optical fiber end face;

[0029] Figure 3 A schematic diagram of the end face structure of a capillary optical fiber in an optical fiber SPR sensor provided in Example 1 of the present invention;

[0030] Figure 4 A schematic diagram of using optical fiber precision cutting technology to accurately control the length of a capillary optical fiber in a method for preparing an optical fiber SPR sensor provided in Example 2 of the present invention;

[0031] Figure 5 A diagram of an experimental setup for conducting experiments based on the optical fiber SPR sensor provided by the present invention.

[0032] Among them, 1-coaxial dual-waveguide optical fiber, 2-capillary optical fiber, 3-multimode optical fiber, 4-gold film, 5-microscope objective, 6-one-dimensional adjustment frame, 7-blade, 8-supercontinuum light source, 9-sensing cavity, 10-spectrometer;

[0033] 1-1-ring core, 1-2-single mode core, 1-3-cladding;

[0034] 2-1-inner wall, 2-2-air cavity. DETAILED DESCRIPTION

[0035] In order to better understand the above technical solution, the above technical solution will be described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0036] Embodiment 1:

[0037] Embodiment 1 provides an optical fiber SPR sensor, comprising: a first optical fiber, a capillary optical fiber and a multimode optical fiber; the first optical fiber is an optical fiber capable of realizing dual-path optical transmission, the surface of the capillary optical fiber is coated with a metal film, one end of the capillary optical fiber is welded to the first optical fiber, and the other end of the capillary optical fiber is welded to the multimode optical fiber, the first optical fiber serves as an input optical fiber, and the multimode optical fiber serves as an output optical fiber; an optical signal transmitted along a first path of the first optical fiber is incident on the inner wall of the capillary optical fiber, undergoes total reflection, and interacts with the metal film coated on the surface of the capillary optical fiber to generate surface plasmon resonance SPR, which is received by the multimode optical fiber for external refractive index sensing; an optical signal transmitted along a second path of the first optical fiber is incident on the air cavity of the capillary optical fiber, and is received by the multimode optical fiber after passing through the air cavity of the capillary optical fiber, and the two optical signals form Mach-Zehnder interference at the output end for external temperature sensing.

[0038] The surface plasmon resonance (SPR) operates in the visible light band, and the Mach-Zehnder interferometer operates in the near-infrared band.

[0039] The multimode optical fiber is a step-index multimode optical fiber or a gradient-index multimode optical fiber. If a step-index multimode optical fiber is used, more modes can be excited, which is conducive to mode matching to generate interference. If a gradient-index multimode optical fiber is used, it has a self-focusing effect, which can reduce the distance where interference occurs, has fewer modes, and can reduce spectral noise under the premise of satisfying interference.

[0040] That is, the output optical fiber in the present invention adopts a step-index / gradient-index multimode optical fiber with a large core diameter, which can simultaneously receive the optical signals transmitted by the inner wall of the capillary optical fiber and the air cavity.

[0041] In a preferred embodiment, the air cavity of the capillary optical fiber may be filled with a thermosensitive material, such as PDMS, agar, ethanol, glucose solution, etc.

[0042] The length of the capillary optical fiber is 500-1000 μm, and the capillary optical fiber is a columnar capillary optical fiber.

[0043] The metal film is one of a gold film, a silver film, a copper film, and a composite metal film; the composite metal film is one of a gold-chromium composite film and a gold-silver composite film; in the gold-chromium composite film, gold is used as the lower layer and chromium is used as the upper layer; in the gold-silver composite film, silver is used as the lower layer and gold is used as the upper layer.

[0044] For example, the first optical fiber is a coaxial dual-waveguide optical fiber, which includes a cladding, a ring core and a single-mode core; the optical signal transmitted along the ring core is used as the optical signal transmitted along the first path of the first optical fiber, and the optical signal transmitted along the single-mode core is used as the optical signal transmitted along the second path of the first optical fiber.

[0045] The annular core is aligned with the inner wall of the capillary optical fiber.

[0046] In addition, the first optical fiber may also be another optical fiber that is different from the coaxial dual-waveguide optical fiber, such as a dual-core optical fiber, a multi-clad optical fiber, etc., that can realize dual-path optical transmission.

[0047] The present invention is further described below by taking the case where the first optical fiber is a coaxial dual-waveguide optical fiber and the metal film is a gold film as an example and combining the parameters.

[0048] See also Figure 1 The present invention provides an optical fiber SPR sensor comprising a coaxial dual-waveguide optical fiber 1, a capillary optical fiber 2 and a multimode optical fiber 3, wherein the surface of the capillary optical fiber is plated with a gold film 4.

[0049] See also Figure 2 The coaxial dual-waveguide optical fiber 1 is composed of a cladding 1-3, a ring core 1-1 and a single-mode core 1-2, wherein: Figure 2 (a) is a schematic diagram of the end face structure of a coaxial dual-waveguide optical fiber. Figure 2 (b) is a refractive index distribution diagram of the coaxial dual-waveguide optical fiber end face at the dotted line. The diameter of the coaxial dual-waveguide optical fiber 1 is 125-200 μm, the diameter of the single-mode fiber core 1-2 is 6-10 μm, the inner diameter of the annular fiber core 1-1 is 50-70 μm, and the outer diameter of the annular fiber core 1-1 is 100-120 μm.

[0050] See also Figure 3 The capillary optical fiber 2 includes an inner wall 2-1 and an air cavity 2-2.

[0051] The coaxial dual-waveguide optical fiber 1 is welded to the capillary optical fiber 2 by an optical fiber welding machine. A part of the optical signal transmitted along the annular core 1-1 is incident on the inner wall 2-1 of the capillary optical fiber 2 to generate total reflection, and interacts with the gold film 4 on the surface of the capillary optical fiber 2. When the transmitted light meets the resonance condition, the SPR phenomenon is generated and is received by the multimode optical fiber 3 at the output end for external refractive index sensing; another part of the optical signal transmitted along the single-mode core 1-2 passes through the air cavity 2-2 in the capillary optical fiber 2, is incident on the core of the multimode optical fiber 3, and generates Mach-Zehnder interference with the inner wall mode of the capillary optical fiber 2 for external temperature sensing, thereby realizing simultaneous measurement of the external refractive index temperature.

[0052] The working principle of the above-mentioned optical fiber SPR sensor is as follows: light is incident from a light source into the coaxial dual-waveguide optical fiber 1, a part of which is transmitted along the annular core 1-1, and the other part is transmitted along the single-mode core 1-2. After the light passes through the welding point between the coaxial dual-waveguide optical fiber 1 and the capillary optical fiber 2, the optical signal transmitted along the annular core 1-1 is incident on the inner wall 2-1 of the capillary optical fiber 2, undergoes total reflection, and forms an inner wall mode. When the resonance condition is met, a part of the mode interacts with the gold film 4 on the surface of the capillary optical fiber 2 to produce an SPR phenomenon, and is received by the multimode optical fiber 3. Since the SPR is sensitive to the external refractive index, it can be used for external refractive index sensing; the optical signal transmitted along the single-mode core 1-2 is incident on the air cavity 2-2 of the capillary optical fiber 2, and is received by the multimode optical fiber 3 after passing through the air cavity 2-2. The introduction of the air cavity 2-2 causes the two optical signals to produce an optical path difference, and forms Mach-Zehnder interference with the inner wall mode at the output end. Since the effective refractive index of the air cavity 2-2 is easily affected by the external temperature, when the external temperature changes, the optical path difference changes, resulting in changes in the interference spectrum at the output end, which can be used for external temperature sensing measurement. The present invention combines Mach-Zehnder interference and SPR phenomenon, can achieve simultaneous measurement of external temperature and refractive index, and the dual-channel bands will not overlap, avoiding the problem of crosstalk, and has high sensitivity.

[0053] The length of the capillary optical fiber 2 can be controlled by using optical fiber precision cutting technology to ensure that the cavity length satisfies the output end optical fiber to generate stable Mach-Zehnder interference.

[0054] In addition, in a preferred solution, the thickness of the inner wall 2-1 of the capillary optical fiber 2 can be controlled by chemical etching to change the number of inner wall modes of the capillary optical fiber 2, optimize the output spectrum, and reduce spectral noise.

[0055] Specifically, increasing the thickness of the inner wall 2-1 will increase the number of transmission light modes and generate more high-order modes. The central core diameter of the coaxial dual-waveguide optical fiber 1 is relatively small, and after propagating through the air cavity 2-2 for a certain distance, a divergence angle will be generated, and more modes will be generated when reaching the multimode optical fiber 3. If the number of inner wall modes is also large, multiple interference spectra will be superimposed, increasing spectral noise. Therefore, the present invention can generate stable Mach-Zehnder interference by controlling the thickness of the inner wall 2-1 of the capillary optical fiber 2 and the length of the capillary optical fiber 2.

[0056] The thickness of the gold film 4 on the surface of the capillary optical fiber 2 should be sufficient to generate the SPR phenomenon.

[0057] Embodiment 2:

[0058] Embodiment 2 provides a method for preparing the optical fiber SPR sensor as described in Embodiment 1, which mainly comprises the following steps:

[0059] welding one end of the capillary optical fiber to the first optical fiber;

[0060] Cutting the capillary optical fiber so that the length of the capillary optical fiber reaches a preset length;

[0061] Welding the other end of the cut capillary optical fiber to the multimode optical fiber;

[0062] A metal film is plated on the surface of the capillary optical fiber to prepare an optical fiber SPR sensor.

[0063] In addition, before welding one end of the capillary optical fiber to the first optical fiber, the method may further include: performing corrosion treatment on the capillary optical fiber so that the inner wall thickness of the capillary optical fiber reaches a preset thickness.

[0064] The present invention is further described below by taking the case where the first optical fiber is a coaxial dual-waveguide optical fiber and the metal film is a gold film as an example and combining the parameters.

[0065] See also Figures 1 to 3 First, the capillary optical fiber 2 is corroded to optimize the thickness of the inner wall 2-1 so that the annular fiber core 1-1 is aligned with the inner wall 2-1 of the capillary optical fiber 2, including the following steps:

[0066] (1) Cut a section of the capillary optical fiber 2 with a length of 10 to 15 cm, and use Miller pliers to remove 1 to 2 cm of the coating layer at both ends of the capillary optical fiber 2.

[0067] (2) A 30-40% ammonium chloride solution and a 30-40% hydrofluoric acid solution are mixed in a volume ratio of 1:1.5 to prepare a corrosion mixture. A 2 mL syringe is used to draw 1.5-2 mL of the corrosion solution. One end of the capillary optical fiber 2 is connected to the syringe, and the interface is sealed with ultraviolet glue to prevent leakage.

[0068] For example, using a corrosion mixture of 40% ammonium chloride and 40% hydrofluoric acid (1:1.5) can improve the smoothness of the inner wall 2-1 of the capillary optical fiber 2 after corrosion.

[0069] (3) By injecting a corrosive liquid into the capillary optical fiber 2 by pressurizing the syringe, the inner wall of the capillary optical fiber 2 is corroded. The initial inner diameter of the capillary optical fiber 2 is 10 to 50 μm. After etching for 20 to 40 minutes at room temperature, a capillary optical fiber 2 with an inner diameter of 50 to 90 μm can be obtained.

[0070] See also Figure 4, using optical fiber precision cutting technology to accurately control the length of the capillary optical fiber 2, including the following steps:

[0071] (4) Using an optical fiber welding machine, weld one end of the coaxial dual-waveguide optical fiber 1 and the corroded capillary optical fiber 2 together, so that the annular fiber core 1-1 is aligned with the inner wall 2-1 of the capillary optical fiber 2 to generate an inner wall waveguide.

[0072] (5) Fix the capillary optical fiber 2 in the fixture of the optical fiber cutting knife, find the welding point under the microscope objective 5, and move the optical fiber through the one-dimensional adjustment frame 6 to align the blade 7 with the welding point. Move the optical fiber again through the screw micrometer of the one-dimensional adjustment frame 6 to make the welding point away from the blade 7 for a certain distance. Cutting the optical fiber can accurately control the length range of the capillary optical fiber 2. The whole process is carried out under a microscope, and the cutting accuracy can reach the order of 10μm. For example, the capillary optical fiber 2 is cut using an optical fiber precision cutting knife, and a length of 500 to 1000μm is retained as the sensing area.

[0073] (6) A multimode optical fiber 3 with a core diameter of 65 to 105 μm is used as the output optical fiber, and the other end of the sensing area is welded to the multimode optical fiber 3 by an optical fiber welding machine. The welded optical fiber is fixed on a glass slide, and then placed in a closed air chamber of an ion sputtering instrument. The position of the optical fiber is adjusted so that the gold target can be deposited on the surface of the sensing area to form the gold film 4. The air pump is turned on to evacuate the air, and the vacuum degree in the air chamber is adjusted to be stable at 2 to 2.5 mbar. The current is adjusted to 6 to 8 mA, and the deposition time is set to 2 to 3 minutes. After the gold film 4 is plated on the surface of the sensing area, the optical fiber is taken out. At this time, the thickness of the gold film 4 is 45 to 55 nm, which is the ideal thickness range for exciting SPR. The structure of the optical fiber SPR sensor obtained after the production is completed can be seen in Figure 1 .

[0074] After the fiber SPR sensor is prepared, an experimental device can be built to perform experimental measurements. Figure 5 , the supercontinuum light source 8 is connected to the sensing area (i.e., the capillary optical fiber 2) through the coaxial dual-waveguide optical fiber 1, and the sensing area is placed in a closed sensing cavity 9. The output optical fiber adopts the multimode optical fiber 3 with a large core diameter, and is connected to the spectrometer 10 for data acquisition. In the experiment, the sample solution is introduced into the sensing cavity 9, and the temperature of the sample solution in the sensing cavity 9 is controlled by water bath heating. As the temperature changes, the refractive index of the sample changes accordingly, causing the SPR resonance spectrum line to shift, and the refractive index change of the sample solution can be obtained by demodulation; at the same time, due to the change in external temperature, the effective refractive index of the air cavity 2-2 changes, causing the interference spectrum line to shift, which can be used to measure the external temperature and realize the measurement of the sample thermo-optical coefficient.

[0075] Finally, it should be noted that the above specific implementation methods are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to examples, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the spirit and scope of the technical solution of the present invention, which should be included in the scope of the claims of the present invention.

Claims

1. An optical fiber SPR sensor, characterized in that: include: A first optical fiber, a capillary optical fiber, and a multimode optical fiber; The first optical fiber is an optical fiber capable of realizing dual-path optical transmission, the surface of the capillary optical fiber is coated with a metal film, one end of the capillary optical fiber is welded to the first optical fiber, and the other end of the capillary optical fiber is welded to the multimode optical fiber, the first optical fiber is used as an input optical fiber, and the multimode optical fiber is used as an output optical fiber; The optical signal transmitted along the first path of the first optical fiber is incident on the inner wall of the capillary optical fiber, undergoes total reflection, and interacts with the metal film plated on the surface of the capillary optical fiber to generate surface plasmon resonance SPR, which is received by the multimode optical fiber and used for external refractive index sensing; The optical signal transmitted along the second path of the first optical fiber is incident on the air cavity of the capillary optical fiber, and is received by the multimode optical fiber after passing through the air cavity of the capillary optical fiber. At the output end, the two optical signals form Mach-Zehnder interference for external temperature sensing.

2. The optical fiber SPR sensor according to claim 1, characterized in that: The first optical fiber is a coaxial dual-waveguide optical fiber, which includes a cladding, a ring core and a single-mode core; the optical signal transmitted along the ring core is used as the optical signal transmitted along the first path of the first optical fiber, and the optical signal transmitted along the single-mode core is used as the optical signal transmitted along the second path of the first optical fiber.

3. The optical fiber SPR sensor according to claim 2, characterized in that: The annular core is aligned with the inner wall of the capillary optical fiber.

4. The optical fiber SPR sensor according to claim 1, characterized in that: The working band of the surface plasmon resonance (SPR) is the visible light band, and the working band of the Mach-Zehnder interference is the near infrared band.

5. The optical fiber SPR sensor according to claim 1, characterized in that: The multimode optical fiber is a step-index multimode optical fiber or a gradient-index multimode optical fiber.

6. The optical fiber SPR sensor according to claim 1, characterized in that: The air cavity of the capillary optical fiber is filled with a thermosensitive material, and the thermosensitive material is one of PDMS, agar, ethanol, and glucose solution.

7. The optical fiber SPR sensor according to claim 1, characterized in that: The length of the capillary optical fiber is 500 to 1000 μm, and the capillary optical fiber is a columnar capillary optical fiber.

8. The optical fiber SPR sensor according to claim 1, characterized in that: The metal film is one of a gold film, a silver film, a copper film, and a composite metal film; the composite metal film is one of a gold-chromium composite film and a gold-silver composite film; in the gold-chromium composite film, gold is used as the lower layer and chromium is used as the upper layer; in the gold-silver composite film, silver is used as the lower layer and gold is used as the upper layer.

9. A method for preparing a fiber SPR sensor according to any one of claims 1 to 8, characterized in that: The following steps are involved: welding one end of the capillary optical fiber to the first optical fiber; Cutting the capillary optical fiber so that the length of the capillary optical fiber reaches a preset length; Welding the other end of the cut capillary optical fiber to the multimode optical fiber; A metal film is plated on the surface of the capillary optical fiber to prepare an optical fiber SPR sensor.

10. The method for preparing the optical fiber SPR sensor according to claim 9, characterized in that: Before welding one end of the capillary optical fiber to the first optical fiber, the method further includes: performing corrosion treatment on the capillary optical fiber to make the inner wall thickness of the capillary optical fiber reach a preset thickness.

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