Fiber-optic tilted grating surface plasmon resonance sensing device and spectral processing method

Abstract

The application discloses a fiber tilt grating surface plasmon resonance sensing device and a spectrum processing method, and comprises the following steps: obtaining a polarization-independent SPR reflection spectrum of the sensing device in a refractive index matching liquid; obtaining a polarization-independent TFBG reflection spectrum in air based on the polarization-independent SPR reflection spectrum in the refractive index matching liquid; obtaining a polarization-independent TFBG transmission spectrum in air based on the polarization-independent TFBG reflection spectrum in air; and obtaining a polarization-independent SPR transmission spectrum in the refractive index matching liquid based on the polarization-independent TFBG transmission spectrum in air. The application uses the polarization-independent reflection spectrum in air as a reference spectrum without using a polarization controller to adjust the polarization, and effectively reduces the proportion of the S polarization component.

Description

Technical Field

[0001] This invention relates to the field of optical refractive index sensing and detection, specifically to a fiber optic tilted grating surface plasmon resonance sensing device and a spectral processing method. Background Technology

[0002] Currently, most fiber optic surface plasmon resonance sensors on the market adopt structures such as D-type fiber, wedge fiber, tapered fiber, heterogeneous core fusion splicing, long-period grating, and tilted grating. The measurement device of the tilted grating surface plasmon resonance sensor usually uses a polarizer and a polarization controller to control the polarization state to ensure the generation of surface plasmon resonance.

[0003] For fiber surface plasmon resonance sensors based on traditional D-type fiber, wedge fiber, tapered fiber, etc., it is necessary to destroy the fiber structure to generate a strong evanescent field and thus generate surface plasmon resonance. Moreover, the sensing wavelength range of this type of sensor is in the visible light band, and halogen tungsten lamp light source is often used. The light source has poor stability, and the destruction of the fiber structure will also affect the stability of the sensing to a certain extent.

[0004] Fiber tilted gratings (SPRs) utilize a method that couples the core mode into the cladding without damaging the fiber's external structure, generating a cladding mode and thus surface plasmon resonance (SPR). Due to the periodicity of the tilted grating, its sensing wavelength range extends into the communication band, and its spectrum contains multiple high-order cladding modes with narrow linewidths, enabling relatively accurate location of the resonant wavelength. Since only P-polarized light can excite SPR, traditional tilted grating measurement devices often employ polarizers and polarization controllers to control the light's polarization state. However, these devices are susceptible to external disturbances that can alter the polarization state, leading to spectral changes or even the disappearance of the SPR attenuation envelope. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention proposes a fiber optic tilted grating surface plasmon resonance sensing device and spectral processing method. Based on the principle of polarization multiplexing, the polarization state of the incident light is orthogonally converted, achieving constant transmission of the P-polarization component in the tilted grating surface plasmon resonance sensor. A polarization beamsplitter is used to obtain mutually orthogonal polarization components. By aligning the slow axes of the two outgoing polarization-maintaining fibers of the polarization beamsplitter with the grating surface of the tilted grating, the incident light at both ends of the tilted grating is P-polarized, effectively utilizing the polarization of the light source to achieve polarization independence. Based on the same principle, a Faraday rotator mirror is used instead of the polarization beamsplitter to convert the input polarization into a polarization state orthogonal to it, obtaining a stable ratio of P-polarization to S-polarization.

[0006] To achieve the above objectives, the present invention provides the following solution:

[0007] A fiber tilted grating surface plasmon resonance sensing device includes: a broadband light source, a circulator, a spectrometer, and an optical polarization unit;

[0008] The broadband light source, circulator, and optical polarization unit are connected in sequence.

[0009] The circulator is also connected to the spectrometer;

[0010] The optical polarization unit is used for reverse polarization of the broadband light source.

[0011] Preferably, the optical polarization unit includes a polarization beam splitter, a tilting grating sensing structure, and a rotating fixture;

[0012] One end of the polarization beam splitter is connected to the circulator;

[0013] The other end of the polarization beam splitter is connected to both ends of the tilted grating sensing structure via two polarization-maintaining optical fibers.

[0014] The tilted grating structure is equipped with rotating clamps at both ends.

[0015] Preferably, the prepared tilted grating sensing structure is fused to ports 1 and 2 of the polarization beam splitter, respectively. Refractive index matching liquids with different refractive indices are dropped onto the tilted grating sensing structure. As the refractive index increases, the reaction wavelength of the surface plasmon resonance reaction redshifts, thereby achieving sensitive sensing of the external refractive index.

[0016] Preferably, the specific process of the spectrometer performing spectral measurements is as follows:

[0017] The spectrum of surface plasmon resonance exhibits an attenuation envelope at a specific SPR resonance wavelength;

[0018] Based on the attenuation envelope, different refractive index sensing is achieved by detecting the change of SPR resonant wavelength with the external refractive index.

[0019] Preferably, the optical polarization unit includes a tilted grating sensing structure and a Faraday rotating mirror;

[0020] The circulator, the tilted grating sensing structure, and the Faraday rotator are connected in sequence.

[0021] Preferably, port 1 of the circulator is connected to the broadband light source to transmit unpolarized light, and port 2 of the circulator is connected to the Faraday rotator through the tilted grating sensing structure. The Faraday rotator converts the input light into mutually orthogonal P-polarized light and S-polarized light, and makes the proportions of P-polarized light and S-polarized light the same.

[0022] The present invention also provides a spectral processing method, which is implemented based on the aforementioned fiber tilted grating surface plasmon resonance sensing device, and includes the following steps:

[0023] Acquire the reflection spectrum of the polarization-independent SPR of the sensing device in a refractive index-matched liquid;

[0024] Based on the reflection spectrum of the polarization-independent SPR in the refractive index-matched liquid, the polarization-independent TFBG reflection spectrum in air is obtained;

[0025] Based on the polarization-independent TFBG reflectance spectrum in air, obtain the polarization-independent TFBG transmission spectrum in air;

[0026] Based on the polarization-independent TFBG transmission spectrum in air, the polarization-independent SPR transmission spectrum in the refractive index-matched liquid is obtained.

[0027] Preferably, the expression for the transmission spectrum of the polarization-independent SPR in the refractive index-matched liquid is:

[0028]

[0029] Among them, T n (λ) represents the reflection spectrum of the polarization-independent SPR in the refractive index-matched liquid, T S (λ) is the reflection spectrum of the polarization-independent SPR excited by S-polarized light, T P (λ) represents the reflection spectrum of a polarization-independent SPR excited by P-polarized light in a refractive index-matched liquid, and T air (λ) is the reflection spectrum of the polarization-independent SPR in air, T P,air (λ) is the reflection spectrum of a polarization-independent SPR excited by P-polarized light in air.

[0030] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0031] This invention utilizes a tilted grating to couple the core mode into the cladding, and uses the cladding mode to generate surface plasmon resonance. This avoids the instability of the sensing structure caused by changes in the original structure of the fiber, such as D-type fiber, tapered fiber, and wedge fiber. Furthermore, the narrow linewidth cladding mode of the tilted grating can more accurately locate the sensing wavelength.

[0032] This invention utilizes a circulator and a PBS ring structure to convert between S-polarization and P-polarization states, replacing the traditional polarizer and polarization controller to generate P-polarized light. This ensures that the vector sum of P-polarized light passing through the tilted grating sensing structure is a constant value, achieving the independence of the tilted grating sensing structure from the polarization of the light source and improving the stability of the sensing device.

[0033] This invention utilizes a circulator and Faraday rotating mirror reflection structure to replace the traditional polarizer and polarization controller, reducing system complexity, realizing a light source polarization-independent sensing structure, reducing the impact of external interference, and improving system stability.

[0034] This invention proposes a spectral optimization process that uses the polarization-independent reflection spectrum in air as a reference spectrum without using a polarization controller to adjust polarization, effectively reducing the proportion of the S-polarization component. Attached Figure Description

[0035] To more clearly illustrate the technical solution of the present invention, the drawings used in the embodiments are briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0036] Figure 1 This is a diagram of the tilted grating sensing structure of the present invention;

[0037] Figure 2 This is a diagram of the surface plasma resonance device structure of the light source polarization-free tilted grating in an embodiment of the present invention;

[0038] Figure 3 Two diagrams illustrate the structure of the light source polarization-free tilted grating surface plasma resonance device in this embodiment of the invention.

[0039] Figure 4 This invention enables the sensing of spectra with minute refractive index changes;

[0040] Figure 5 This is a flowchart of the spectral optimization process in an embodiment of the present invention;

[0041] Figure 6 This is a comparison image before and after spectral processing according to the present invention.

[0042] Figure descriptions: 1-Broadband light source; 2-Circulator; 3-Polarization beam splitter; 4-Tilted grating sensing structure; 5-Rotating fixture; 6-Spectrometer; 7-Faraday rotator. Detailed Implementation

[0043] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0044] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0045] The polarization-independent sensing and measurement device based on polarization multiplexing and surface plasmon resonance of a tilted fiber grating proposed in this invention uses a gold film with a tilt angle of 4°-12° and a surface coating of 30-70 nm thick on the fiber grating region. Figure 1 As shown.

[0046] The fiber tilted grating surface plasmon resonance sensing device of the present invention includes: a broadband light source 1, a circulator 2, a spectrometer 6, and an optical polarization unit;

[0047] Broadband light source 1, circulator 2, and optical polarization unit are connected in sequence;

[0048] Circulator 2 is also connected to spectrometer 6;

[0049] The optical polarization unit is used for reverse polarization of broadband light source 1.

[0050] Specifically, the optical polarization unit may include a polarization beam splitter 3, a tilted grating sensing structure 4, and a rotating fixture 5;

[0051] One end of the polarization beam splitter 3 is connected to the circulator 2;

[0052] The other end of the polarization beam splitter 3 is connected to both ends of the tilted grating sensing structure 4 via two single-mode optical fibers.

[0053] Both single-mode optical fibers are equipped with rotating clamps 5.

[0054] Specifically, the optical polarization unit may also include a tilted grating sensing structure 4 and a Faraday rotating mirror 7;

[0055] The circulator 2, the tilted grating sensing structure 4, and the Faraday rotator 7 are connected in sequence.

[0056] This invention provides two fiber tilted grating surface plasmon resonance sensing devices, as detailed below:

[0057] Example 1

[0058] The surface plasmon sensing device based on polarization multiplexing provided by the present invention includes: a broadband light source 1, a circulator 2, a polarization beam splitter (PBS) 3, a tilted grating sensing structure 4, a rotating fixture 5, and a spectrometer 6.

[0059] The circulator 2 has three ports. Port 1 is connected to the broadband light source 1 via an optical fiber. Port 2 of the circulator 2 is connected to Port 3 of the PBS 3. Port 2 of the PBS 3 passes through the rotating clamp 5. Port 2 and Port 1 of the PBS 3 are respectively connected to the two ends of the fiber tilt grating sensing structure 4. Port 3 of the circulator 2 is connected to the spectrometer 6.

[0060] Light emitted from the broadband light source enters port 1 of the circulator and exits from port 2. It then connects to port 3 of the PBS via a single-mode fiber. The PBS couples the input light into two polarization-maintaining fibers, outputting two orthogonally polarized beams. Due to the directionality of the tilted grating surface, a CO2 fusion splicer is used to manually spin the polarization-maintaining fiber to make its slow axis parallel to the tilted grating surface. The beam then passes through the tilted grating and is connected to a spectrometer for real-time monitoring and SPR spectrum determination. The tilted grating sensing structure 4 is connected to ports 1 and 2 of the PBS. Since the tilted grating sensing structure 4 still contains a single-mode fiber, rotating clamps are placed at both ends of the structure to fine-tune the fiber, ensuring that the light passing through the tilted grating sensing structure is P-polarized in both opposite directions. The light is then recombined and returned to the PBS, where it is connected to a spectrometer via port 3 of the circulator for spectral measurement. The spectrum of surface plasmon resonance exhibits an attenuated envelope at a specific SPR resonance wavelength. By detecting the change in SPR resonance wavelength with the external refractive index, different refractive index sensing is achieved. The overall structure of the device is shown in the figure below. Figure 2 As shown.

[0061] In this embodiment, the circulator connected to the PBS converts non-linearly polarized light into mutually orthogonal linearly polarized light. Then, the fixture rotates port 2 by 90° to convert S-polarized light into P-polarized light, so that all light coupled into the TFBG-SPR sensor from different directions is P-polarized light.

[0062] In this embodiment, the prepared tilted grating sensing structure is fused to port 1 (port 1) and port 2 (port 2) of PBS, respectively. Refractive index matching liquids with different refractive indices are dropped onto the tilted grating sensing structure. As the refractive index increases, the reaction wavelength of the surface plasmon resonance reaction redshifts, thereby achieving high-sensitivity sensing of the external refractive index.

[0063] Example 2

[0064] The present invention also provides a surface plasmon sensing device based on polarization multiplexing, comprising: a broadband light source 1, a circulator 2, a tilted grating sensing structure 4, a Faraday rotator 7, and a spectrometer 6.

[0065] The circulator 2 has three ports. Port 1 is connected to the broadband light source 1 via optical fiber. Port 2 of the circulator 2 is connected to the fiber tilt grating sensing structure 4. The other end of the fiber tilt grating sensing structure 4 is connected to the Faraday rotating mirror 7. Port 3 of the circulator 2 is connected to the spectrometer 6.

[0066] Port 1 of the circulator is connected to a broadband light source, and port 2 is connected to the tilted grating sensor sample. Since unpolarized light can be considered as a rapidly changing random combination of P-polarized and S-polarized light, the Faraday rotating mirror reflects the input light polarization state at a 90° orthogonal polarization direction, converting it into a polarization state orthogonal to it and returning it to the tilted grating sensor. Therefore, the P-polarization and S-polarization of the input light are mutually converted and then pass through the TFBG-SPR sensor again. The S-polarized and P-polarized light passing through the TFBG-SPR sensor maintain the same ratio, thus achieving the polarization independence of the tilted grating sensor. Port 3 of the circulator is connected to a spectrometer to monitor the sensing spectrum of different refractive indices in real time. The device diagram is shown below. Figure 3 As shown.

[0067] In this embodiment, port 1 (port 1) of the circulator is connected to a broadband light source to transmit unpolarized light, and port 2 (port 2) of the circulator is connected to a Faraday rotator. The Faraday rotator converts the input light into mutually orthogonal polarized light, so that the proportion of P-polarized light and S-polarized light passing through the TFBG-SPR sensor is the same, and the proportion remains unchanged under any polarization state of the input light.

[0068] In this embodiment, different refractive index matching liquids are dropped into the tilted grating region, and their sensing spectra are measured on a spectrometer. The sensor is sensitive to minute refractive indices in the external environment; as the refractive index changes, the amplitude of its cladding mode changes significantly, such as... Figure 4 As shown.

[0069] Example 3

[0070] The present invention also provides a polarization-multiplexed spectral processing method, which is implemented using a polarization-multiplexed surface plasmon sensing device and includes the following steps:

[0071] Acquire the reflection spectrum of the polarization-independent SPR of the sensing device in a refractive index-matched liquid;

[0072] Based on the reflection spectrum of polarization-independent SPR in refractive index-matched liquid, the polarization-independent TFBG reflection spectrum in air was obtained;

[0073] Based on the polarization-independent TFBG reflectance spectrum in air, the polarization-independent TFBG transmission spectrum in air is obtained;

[0074] Based on the polarization-independent TFBG transmission spectrum in air, the transmission spectrum of the polarization-independent SPR in the refractive index matched liquid was obtained.

[0075] In this embodiment, the spectral optimization process is as follows: Figure 5 As shown, T is defined i(λ), i=p,s are the transmission spectra of TFBG-SPR under P-polarized light and S-polarized light excitation, respectively, and the nonlinearly polarized light I in For P-polarized light I P and S-polarized light I s The superposition of polarized light and the interconversion of incident P-polarized and S-polarized light by the FRM result in an I-polarized light output. out =(I P +I S )T P ·T S Therefore, the normalized reflectance spectrum is At this point, the reflection spectrum of the TFBG-SPR sensor in the refractive index-matched liquid is obtained. Since only the cladding mode excited by P-polarized light can excite surface plasmon resonance, while the TFBG transmission spectrum coupled by S-polarized light does not undergo SPR resonance, the superposition of the SPR spectra of the two orthogonally polarized beams will reduce the degree of SPR attenuation. Therefore, the spectrum T excited by S-polarized light should be removed. s (λ). In the optimization process, T(λ) is proposed as the ratio of input to output. Theoretically, it should be logarithmically calculated to obtain a relative value. After the logarithmic operation, T(λ) becomes T. P (λ) and T S The sum of (λ) is only the surface plasmon resonance spectrum T excited by P-polarized light. P (λ) can be obtained by subtracting T from T(λ). S (λ) is obtained. Since the transmission spectrum measured by the TFBG-SPR sensor in air is simultaneously excited by both P-polarized and S-polarized light, T S (λ) is insensitive to changes in the external refractive index, while the excited SPR spectrum of P-polarized light in air is the surface plasmon resonance occurring in a medium with a refractive index of 1. Its resonance wavelength is far from the resonant wavelength and is insensitive to changes in the external refractive index. At this time, T S (λ) and T P Since the (λ) envelopes are the same, the reflection spectrum directly measured in air using a polarization-insensitive device can be used as the reference spectrum. Therefore, the final transmission spectrum can be written as: Among them, T n (λ) represents the reflection spectrum of the polarization-independent SPR in the refractive index-matched liquid, T S (λ) is the reflection spectrum of the polarization-independent SPR excited by S-polarized light, T P (λ) represents the reflection spectrum of a polarization-independent SPR excited by P-polarized light in a refractive index-matched liquid, and T air (λ) is the reflection spectrum of the polarization-independent SPR in air, T P，air (λ) represents the reflection spectrum of a polarization-independent SPR excited by P-polarized light in air. For example... Figure 6 As shown, although the excitation spectrum of S-polarized light cannot be completely eliminated, the proportion of S-polarized components is effectively reduced.

[0076] The key technical point of this invention lies in proposing two tilted grating surface plasmon resonance sensing structures independent of light source polarization: a circulator and a PBS circulator structure, and a circulator and a Faraday rotating mirror reflection structure. Experiments verify that both structures are insensitive to changes in light source polarization, thus enhancing system stability. A spectral optimization process is also proposed.

[0077] This invention proposes a tilted grating sensing device with a ring structure composed of a circulator and a polarization-maintaining fiber (PBS) to achieve polarization insensitivity of the light source. It includes a fine-tuning fiber that ensures the slow axis of the polarization-maintaining fiber is parallel to the grating surface of the tilted grating, and finally connects to the system to ensure that P-polarized light is transmitted in both directions of the tilted grating, effectively improving the utilization rate of the light source.

[0078] This invention proposes a reflection circuit composed of a circulator and a Faraday rotator. The Faraday rotator is used to achieve orthogonal polarization transformation of the reflected light, so that the proportions of the P polarization component and the S polarization component are the same after passing through the TFBG-SPR region, thereby achieving polarization-independent characteristics.

[0079] This invention proposes an SPR spectral optimization process that reduces the proportion of the S-polarization component and increases SPR loss without requiring a polarizer or polarization controller, making it easier to identify SPR modes and obtain stable and reliable sensing data.

[0080] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made to the technical solutions of the present invention by those skilled in the art without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A fiber optic tilted grating surface plasmon resonance sensor device, characterized by, include: Broadband light source (1), circulator (2), spectrometer (6), and optical polarization unit; The broadband light source (1), circulator (2), and optical polarization unit are connected in sequence; The circulator (2) is also connected to the spectrometer (6); The optical polarization unit is used for polarization multiplexing of the broadband light source (1); The optical polarization unit includes a polarization beam splitter (3), a tilted grating sensing structure (4), and a rotating fixture (5). One end of the polarization beam splitter (3) is connected to the circulator (2); The other end of the polarization beam splitter (3) is connected to both ends of the tilted grating sensing structure (4) via two polarization-maintaining optical fibers. Both ends of the tilted grating sensing structure (4) are provided with rotating clamps (5). The polarization beam splitter couples the input light into two polarization-maintaining fibers and outputs two beams of orthogonally polarized light. The polarization-maintaining fibers are manually fused using a CO2 fusion splicer to make the slow axis of the polarization-maintaining fibers parallel to the grating surface of the tilted grating. The fibers are finely adjusted by rotating clamps so that the light from the tilted grating sensing structure is P-polarized in both opposite directions. The beams are then recombined and returned to the polarization beam splitter, and spectral measurements are performed by a spectrometer connected to a circulator.

2. The fiber tilted grating surface plasmon resonance sensing device according to claim 1, characterized in that, The prepared tilted grating sensing structure (4) is fused to the two ports of the polarization beam splitter (3). Refractive index matching liquids with different refractive indices are dropped onto the tilted grating sensing structure (4). As the refractive index increases, the reaction wavelength of the surface plasma resonance reaction redshifts, thereby realizing sensitive sensing of the external refractive index.

3. The fiber tilted grating surface plasmon resonance sensing device according to claim 1, characterized in that, The specific process of spectral measurement performed by the spectrometer (6) is as follows: The spectrum of surface plasmon resonance exhibits an attenuation envelope at a specific SPR resonance wavelength; Based on the attenuation envelope, different refractive index sensing is achieved by detecting the change of SPR resonant wavelength with the external refractive index.

4. A spectral processing method, wherein the method is implemented using the fiber tilted grating surface plasmon resonance sensing device according to any one of claims 1-3, characterized in that, Includes the following steps: Acquire the reflection spectrum of the polarization-independent SPR of the sensing device in a refractive index-matched liquid; Based on the reflection spectrum of the polarization-independent SPR in the refractive index-matched liquid, the polarization-independent TFBG reflection spectrum in air is obtained; Based on the polarization-independent TFBG reflectance spectrum in air, obtain the polarization-independent TFBG transmission spectrum in air; Based on the polarization-independent TFBG transmission spectrum in air, the polarization-independent SPR transmission spectrum in the refractive index-matched liquid is obtained.

5. The spectral processing method according to claim 4, characterized in that, The expression for the transmission spectrum of the polarization-independent SPR in the refractive index-matched liquid is: ； in, The reflection spectrum of polarization-independent SPR in refractive index-matched liquid. The reflection spectrum of a polarization-independent SPR excited by S-polarized light. The reflection spectrum of a polarization-independent SPR excited by P-polarized light in a refractive index-matched liquid. The reflection spectrum of polarization-independent SPR in air. This is the reflection spectrum of a polarization-independent SPR excited by P-polarized light in air.

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