CN117889983A - SPR optical fiber sensing device for measuring solution temperature and refractive index - Google Patents
SPR optical fiber sensing device for measuring solution temperature and refractive index Download PDFInfo
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- CN117889983A CN117889983A CN202311582227.7A CN202311582227A CN117889983A CN 117889983 A CN117889983 A CN 117889983A CN 202311582227 A CN202311582227 A CN 202311582227A CN 117889983 A CN117889983 A CN 117889983A
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Abstract
The invention provides an SPR optical fiber sensing device for measuring solution temperature and refractive index. The SPR optical fiber sensing device consists of a broadband continuous light source, a single-mode optical fiber, a circulator, a sensing probe, a spectrometer and a computer. The sensing probe is a wedge-shaped structure ground at the end of the optical fiber and an optical fiber Bragg grating is etched in the fiber core. The fiber Bragg grating is used for detecting the external temperature to eliminate the interference of the temperature on the SPR effect, the wedge-shaped structure plated with the metal film is used for exciting the SPR effect and reflecting light waves back to the fiber core, and the circulator couples reflected light into the spectrometer. The SPR sensing mechanism is utilized to convert small changes in the refractive index RI of the liquid analyte into measurable changes in the loss peak, thereby realizing refractive index sensing. The device has the advantages of high sensitivity, compact structure, strong stability and the like, and can be widely applied to the fields of biochemical analyte detection, water pollution monitoring and magnetic field measurement.
Description
Technical Field
The invention relates to an SPR optical fiber sensing device for measuring solution temperature and refractive index, belonging to the field of optical fiber sensing.
Background
In 1902 Wood first discovered the phenomenon of surface plasmon resonance in an optical experiment. Fano in 1941 explained this phenomenon based on excitation of electromagnetic waves at the sensing surface of the metal and air interface. SPR sensing devices have become a research hotspot in the field of International sensing devices since Lied-berg et al used SPR technology in the field of chemical sensing device research.
SPR sensing devices are largely classified into two types, prism type and optical fiber type. Prism-based SPR sensing devices, which are relatively bulky, not portable and relatively expensive, have been developed in which optical fibers are used as media, because total reflection must be generated at the prism-metal interface in order to generate SPR. The patent with the application number of CN201210067372.7 proposes a D-type optical fiber SPR sensing device based on graphene film sensitization, wherein the invention adopts a D-type optical fiber as a light transmission medium, and a silver plating film on the surface of the D-type optical fiber forms an SPR structure; then, each applicant respectively provides an SPR optical fiber refractive index sensing device, a preparation method and application (CN 114088664A); the utility model provides a spiral microstructure optical fiber refractive index sensing device (CN 109655430A) based on SPR effect and grid-shaped film double-core D-type photonic crystal optical fiber refractive index sensing device (CN 110441260A) based on SPR effect, which are all used for exciting SPR by different structures and realizing the measurement of refractive index, but can only detect refractive index and cannot eliminate the interference of temperature on SPR; the square hole photon crystal fiber refractive index sensing device and method (CN 112432924A) based on SPR also carries out temperature compensation while measuring refractive index, but is of a transmission structure, and has the problems of larger sensing area, easy breakage and the like; and these are all special optical fibers, which are relatively costly. In 2022, zijian Hao et al proposed a reflective SPR optical fiber sensing device based on a multimode optical fiber wedge structure [ Hao Zijian. Nanopho-to-nics, 2022,11 (15) ], which places the sensing region at the end of the optical fiber, so that the sample is easy to detect and clean, but the full width at half maximum of the absorption peak of the SPR is larger due to more optical wave modes transmitted in the multimode optical fiber, and the sensitivity is still to be improved.
The invention uses common single-mode fiber, which not only has low cost, but also has smaller sensing area, and the area of the sensing area is less than 100 mu m 2 Only transmitted light wavesIn one mode, the full width at half maximum of the absorption peak of SPR is smaller, so that a narrower absorption peak can be obtained, and the accuracy is higher. The invention also cascades the fiber Bragg gratings to eliminate the interference of temperature on the SPR effect so as to improve the detection accuracy.
Disclosure of Invention
The invention designs an SPR optical fiber sensing device for measuring the temperature and the refractive index of a solution, which is used for measuring the refractive index and the temperature of the solution.
The purpose of the invention is realized by the following technical proposal:
an SPR optical fiber sensing device for measuring solution temperature and refractive index, which is characterized in that: the device consists of a broadband continuous light source, a single-mode fiber, a circulator, a sensing probe, a spectrometer and a computer; the optical fiber is connected with the broadband continuous light source, the optical wave is transmitted through the optical fiber Bragg grating and the wedge-shaped reflection area, the optical fiber Bragg grating reflects the light with the Bragg wavelength, the other optical waves excite SPR (surface plasmon resonance) on the sensing surface of the wedge-shaped reflection structure and are reflected to the reflection surface, then the reflected light is vertically reflected back to the fiber core, and the reflected light is coupled into the spectrometer through the circulator.
The invention discloses a device sensing probe for realizing refractive index sensing, which mainly comprises two parts, wherein the first part is a wedge-shaped reflecting structure at the tail end of a single-mode fiber, and aims to resonate incident light with a gold film on a wedge-shaped sensing surface to form an SPR effect, and the incident light is reflected back to a fiber core by a reflecting surface, so that the purpose of realizing reflective real-time refractive index measurement is realized. The second part is a Fiber Bragg Grating (FBG) which is used for detecting the external temperature and eliminating the interference of the temperature on the SPR effect.
The main principle of the present invention is the SPR effect. Optical Surface Plasmon Resonance (SPR) is an optical physical phenomenon. When a P polarized light beam (parallel plane of polarization direction) is incident on the prism end face within a certain angle range, a surface plasmon wave is generated at the interface of the prism and the metal film (Au or Ag). When the propagation constant of the incident light wave matches that of the surface plasmon wave, free electrons in the metal film are caused to resonate, that is, surface plasmon resonance. The condition that electromagnetic waves resonate is that two waves have the same frequency and wave vector (i.e., wavelength) and the propagation directions are identical. If the evanescent wave and the surface plasmon have the same frequency and wave vector, they will resonate. The condition of total reflection at the interface at resonance will be destroyed and attenuated total reflection will occur, i.e. the reflectivity will be at a minimum. The reflectance value is a function of the incident wavelength or angle of incidence, so that the point at which its minimum occurs is the resonant wavelength or angle. The sample to be detected is contacted with the sensing surface, the sample can cause the refractive index change of the gold film surface, the SPR angle or the resonance wavelength is finally changed, and the information such as the concentration, the affinity, the kinetic constant, the specificity and the like of the analyte can be obtained by detecting the change of the SPR angle or the wavelength.
The core of the invention is a wedge-shaped structure at the end of the optical fiber, which can reflect SPR excitation back into the fiber core and couple the SPR excitation back into a spectrometer through a circulator. The principle of the wedge structure of the present invention for implementing reflective SPR sensing will now be described in an enlarged view of the sensing probe (4) of FIG. 1. The invention designs a wedge-shaped reflecting structure, wherein the angle of a sensing surface is alpha, the surface is required to grind a fiber core, so that the angle of the whole fiber core is the same as that of a cladding, the thickness of a metal film of the sensing surface is d1, d1 is less than or equal to 40nm and less than or equal to 60nm, the angle of a reflecting surface is beta, the surface is only required to grind the cladding, the surface is not required to grind to the fiber core, and the thickness of the metal film is d2, and d2 is 150nm and less than or equal to 200nm. The angle of the compound meets the formula
2α+β＝90° (1)
When light waves enter the wedge-shaped structure meeting the formula from the fiber core, the light waves are reflected to the reflecting surface after the sensing surface excites SPR, and the angle of the light waves meets the formula, so that the reflected light of the sensing surface and the reflecting surface are just perpendicular, and the reflected light returns along the original path.
The invention cascades fiber Bragg gratings. The FBG is perpendicular to the fiber axis, essentially equivalent to a mirror, and when the incident light enters the fiber bragg grating, the light of the wavelength satisfying the phase matching condition, i.e., the bragg wavelength, is reflected. When the incident light passes through the fiber Bragg grating, the fiber core mode of forward transmission and the fiber core mode of backward transmission in the fiber core are coupled, the incident light is reflected, and the reflected light meets the phase matching condition and is in a certain specific wavelength. The phase matching condition, i.e. the wave vector of the diffracted beam is equal to the sum of the wave vector of the fiber bragg grating and the wave vector of the incident beam. In FBG, the fiber bragg grating vector is defined as
β i ＝2π/Λ (2)
Wherein Λ is the fiber Bragg grating period measured along the fiber axis; the incident light wave vector is consistent with the reflected light wave vector, and the FBG temperature and stress response have good linear correlation. All reflected wave combinations satisfying phase matching have a Bragg wavelength lambda B . The following expression is an expression of the phase matching condition,
λ B ＝2ΛN eff (3)
in N eff Is the effective refractive index of the fiber. The principle that the fiber Bragg grating reflects specific wavelength is utilized to monitor the external temperature. After passing through the fiber Bragg grating, the incident light reflects light with a specific wavelength, and the light with other wavelengths continues to propagate along the fiber core. When the sensing device is placed in a temperature-changing environment, the central wavelength of the fiber Bragg grating is directly caused to change due to the thermo-optical effect of the optical fiber and the thermal expansion effect of the optical fiber material, so that the absorption peak is displaced. The displacement of the absorption peak is in linear relation with the temperature change, so that the temperature change of the solution to be measured can be obtained through the fiber Bragg grating. The interference to the SPR resonance peak due to temperature variation is eliminated by an algorithm.
Compared with the prior art, the invention has the following advantages:
the sensing area of the SPR optical fiber sensing device is extremely small and is only in the micron order, so that the high-precision and high-sensitivity sample refractive index detection analysis is realized, and the SPR optical fiber sensing device has good prospects in various detection fields.
The transmission optical fiber of the SPR optical fiber sensing device is a single-mode optical fiber, is not easy to consume and distort in long-distance transmission, and is suitable for being connected with an optical fiber network based on the single-mode optical fiber transmission at present.
Drawings
FIG. 1 is a schematic diagram of a SPR fiber optic sensing device system. The SPR optical fiber sensing device system consists of a broadband continuous light source 1, a circulator 2, a transmission single-mode optical fiber 3, a sensing probe 4, an optical fiber spectrometer 5 and a computer (6). The sensing probe 4 of fig. 1 is a schematic structural diagram of a coated SPR optical fiber sensing area in the embodiment; wherein 4-1 is a single-mode fiber cladding, 4-2 is a single-mode fiber core, and 4-3 is a fiber Bragg grating etched in the single-mode fiber core; 4-4 is the sensing surface grinding angle alpha of the wedge-shaped structure, and 4-5 is the reflecting surface grinding angle beta; 4-6 is SPR excitation metal film, 4-7 is reflection metal film.
FIG. 2 is a simulated spectrum of SPR formants at different refractive indices.
FIG. 3 shows SPR resonance wavelengths at different refractive indices, with slope indicating sensitivity, and good linearity at the 1.33-1.35 refractive index band, suitable for use as the refractive index sensing region.
Detailed Description
The patent of the invention is further described below with reference to the accompanying drawings and examples. It will be apparent that the described embodiments are only some, but not all, embodiments of the invention. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
In this embodiment, the end of the optical fiber is first obliquely polished to obtain angles α and β, where α is typically a small angle, α is 10 ° or more and less than or equal to 30 °, preferably 20 °, β is typically a large angle, β is 30 ° or more and less than or equal to 70 °, preferably 50 °, and 20 ° is a bevel to the core, the depth of which is typically about 75 μm, and the 50 ° bevel is typically about 45 μm. After finishing grinding, observing two angles by using a microscope, ensuring that the inclined plane of 20 degrees is ground through the fiber core, and the two inclined plane angles meet the angle formula in the claims.
When the external temperature changes, the spectrum data of the existing SPR optical fiber sensing device can change correspondingly, and the shift of formants is embodied, so that the accuracy of refractive index measurement is reduced. Therefore, certain measures are required to compensate for the temperature-induced spectral data variations.
And etching the fiber Bragg grating on the grinded fiber near the grinding end by using a phase mask method so as to distinguish the fiber Bragg grating from an absorption peak of SPR reflection by using a spectrometer.
And coating a film on the optical fiber of the etched optical fiber Bragg grating, wherein a 20-degree inclined plane only needs to be coated with a 50nm metal film for exciting SPR, and a 50-degree inclined plane needs to be coated with a 200nm total reflection film for ensuring that most of light waves can be reflected back to the fiber core and received by a spectrometer.
At room temperature of 25 ℃, a broadband light source is transmitted to the SPR sensing area through an optical fiber, a surface plasma resonance effect is excited in the sensing area, a sample to be tested is a solution, and when the refractive index of the solution changes, the position of an SPR resonance peak can be influenced. As can be seen from fig. 3, the SPR formants red shift with increasing refractive index. Similarly, by changing the temperature of the solution and observing the grating reflection spectrum demodulated by the spectrometer, the relation of resonance peak displacement corresponding to specific temperature change is obtained, and then the interference of the temperature on the SPR resonance peak is removed through an algorithm.
The above is a preferred embodiment of the present invention, and all changes made according to the technical solution of the present invention belong to the protection scope of the present invention when the generated functional effects do not exceed the scope of the technical solution of the present invention.
Claims (3)
1. An SPR optical fiber sensing device for measuring solution temperature and refractive index, which is characterized in that: the device consists of a broadband continuous light source, a single-mode fiber, a circulator, a sensing probe, a spectrometer and a computer; the single-mode fiber is connected with the broadband continuous light source, the light wave is transmitted through the single-mode fiber and is coupled into the fiber Bragg grating and the wedge-shaped reflection area of the sensing probe through the circulator, the fiber Bragg grating reflects the light with the Bragg wavelength, the rest light waves excite SPR (surface plasmon resonance) on the surface (4-6) of the wedge-shaped reflection structure and are reflected to the surface (4-7) and then vertically reflected back to the fiber core, and the reflected light is coupled into the spectrometer through the circulator.
2. The SPR fiber optic sensing apparatus for measuring solution temperature and refractive index according to claim 1, wherein: the wedge-shaped reflecting structure is characterized in that the tail end of a single-mode fiber is ground into a wedge shape according to a certain inclination angle, a sensing surface of the wedge-shaped reflecting structure needs to be ground from a fiber core to a lower cladding, the angle is alpha, a reflecting surface only needs to be ground from the cladding, the angle is beta, and the two angles meet the formula 2alpha+beta=90 degrees.
3. The SPR fiber optic sensing apparatus for measuring solution temperature and refractive index according to claim 1, wherein: the surface (4-6) is an SPR excitation surface, the thickness of the metal film is d1, the surface (4-7) is a reflecting surface, and the thickness of the metal film is d2.
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