CN111239076A - Surface plasma resonance optical fiber sensor - Google Patents
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- 239000013307 optical fiber Substances 0.000 title claims abstract description 43
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Abstract
The invention provides a surface plasma resonance optical fiber sensor, comprising: the sensing optical fiber is a few-mode optical fiber and comprises a fiber core and a cladding, wherein a sensing area of the cladding is processed to form two mutually perpendicular sections, and the distance between the intersection line of the two sections and the central axis of the fiber core is larger than that between the intersection line of the two sections and the central axis of the fiber core
R is the radius of the fiber core, and the lengths of the vertical lines from the central axis of the fiber core to the two tangent planes are the same; and each section is provided with a nano metal wire group, each nano metal wire group comprises at least two nano metal wires which are parallel to each other, the central axes of all the nano metal wires are parallel to the central axis of the fiber core, and the radii and the lengths of all the nano metal wires are equalThe cross-sectional shapes are the same. The invention can realize polarization insensitive sensing and effectively reduce the requirements of sensing on light sources and optical fiber devices.
Description
Technical Field
The invention relates to the field of optical fiber sensing, in particular to a surface plasma resonance optical fiber sensor.
Background
Surface plasmon resonance is a particular physical optical phenomenon. When light is totally reflected at the interface, evanescent waves can be generated near the interface, and the evanescent waves can excite free electrons at the metal medium interface to generate surface plasma waves. When the wave vector penetrating into the interface of the metal medium is equal to the wave vector of the surface plasma wave, the two waves are strongly coupled, and the surface plasma resonance phenomenon occurs. At this time, free electrons in the metal absorb energy of incident light, so that energy of reflected light is attenuated, and a resonance absorption peak appears in a reflection spectrum. The surface plasmon resonance phenomenon is very sensitive to the refractive index change of a metal surface medium, and is often used to measure the refractive index change of a liquid and a physical quantity related to the refractive index change.
When light is transmitted in the optical fiber, evanescent waves of the sensing area and plasma waves on the metal surface resonate under certain conditions, absorption peaks appear in a transmission spectrum, and the position of resonance peaks of an SPR spectral curve can be changed due to the change of an external environment medium, so that the detection of parameters of the external environment medium is realized. The side polished optical fiber is often used for manufacturing an optical fiber surface plasma resonance sensor due to the strong evanescent field effect of the side polished optical fiber. A common side polishing optical fiber surface plasmon resonance sensor is characterized in that a polishing area is plated with a metal film, the structure has high loss for a y polarization fiber core mode, and has low loss for an x polarization fiber core mode, and the tuning of the range of the resonance wavelength and the control of the coupling loss of a surface plasmon mode and a fiber core mode are difficult. Compared with a metal film, the structure of polishing and grinding the planar arrangement nano metal wires has higher coupling loss to fiber core molds in two polarization states, and meanwhile, through reasonable arrangement of the diameter, the number and the spacing of the nano metal wires, the range of tuning resonance wavelength can be effectively controlled, and the number of surface plasma body molds is reduced, so that more obvious loss peaks are formed, the coupling loss of the surface plasma body molds and the fiber core modes is enhanced, but the structure still has the problem that the resonance wavelengths corresponding to the two polarization states are different.
Disclosure of Invention
Aiming at the defects in the prior art, the invention provides the surface plasma resonance optical fiber sensor which can realize polarization insensitive sensing and effectively reduce the requirements of sensing on a light source and an optical fiber device.
The present invention achieves the above-described object by the following technical means.
A surface plasmon resonance optical fiber sensor comprising:
the sensing optical fiber is a few-mode optical fiber and at least supports LP within the working wavelength range11The sensing optical fiber comprises a fiber core and a cladding, a sensing area of the cladding is processed to form two mutually perpendicular sections, and the distance between the intersection line of the two sections and the central axis of the fiber core is larger than
R is the radius of the fiber core, and the lengths of the vertical lines from the central axis of the fiber core to the two tangent planes are the same; and
each section of the nano metal wire group is provided with one nano metal wire group, each nano metal wire group comprises at least two mutually parallel nano metal wires, the central axes of all the nano metal wires are parallel to the central axis of the fiber core, and the radii, the lengths and the section shapes of all the nano metal wires are equal.
Preferably, the length of a perpendicular line from the central axis of the fiber core to the two tangent planes is H, and H-R is more than or equal to 0 and less than or equal to 1 mu m.
Preferably, the material of the nano metal wire is gold or silver, and the radius r of the nano metal wire is 50-200 nm.
Preferably, the distance between the centers of two adjacent nano metal wires in the nano metal wire group is d, which satisfies the requirement
Preferably, the group of nano-metal wires includes an odd number of the nano-metal wires.
Preferably, the middle nanowire in each nanowire group is located at a position a, where the position a is a projection position of the central axis of the fiber core on the tangent plane, and the remaining nanowires are symmetrically distributed on two sides of the position a.
Preferably, the group of nano-metal wires includes an even number of the nano-metal wires.
Preferably, the nano metal wires in the nano metal wire group are symmetrically distributed at two sides of a position B, and the position B is a projection position of the central axis of the fiber core on the tangent plane.
Preferably, the number of the nano metal wires in the nano metal wire group is n, and 2r + (n-1) d < 2H, n is an integer greater than 1.
Preferably, the radius R of the fiber core is 4-10 mu m, and the refractive index n of the fiber corecRefractive index n of the claddingcladThe difference satisfies: 0.001<(nclad-nc)<0.05。
The invention has the beneficial effects that:
the cladding of the few-mode optical fiber sensing part is polished or cut to form two vertical sections which are used as leakage windows of the optical fiber sensing area, and compared with the common single-side polished optical fiber, the optical fiber sensing area has the advantages that the evanescent field effect of transmitted light is more obvious, and the resonance loss is stronger. By arranging a certain number of nano metal wires on two vertical sections, the defect that the conventional single-side polished fiber based on surface plasma sensing is sensitive to polarization is overcome, polarization-insensitive sensing is realized, the requirements of sensing on a light source and an optical fiber device can be effectively reduced, and an ideal technical scheme is provided for low-cost sensing; the coupling effect of the fiber core high-order mode and the surface plasma body mode is enhanced by coupling the fiber core high-order mode and the surface plasma body mode, and the defect that the fiber core high-order mode and the surface plasma body mode are difficult to couple is overcome; the defects that the number of multimode fiber modes is large, and resonance spectral line broadening caused by mode superposition is avoided, so that the sensing sensitivity is reduced. All the metal wires have the same parameters, are respectively coupled with the fiber core module, and realize the purposes of enhancing the coupling loss and the loss peak value through the coupling of a plurality of metal wires and the proper arrangement among the metal wires.
By reasonably setting the diameter, the number and the spacing of the nano metal wires, the invention can effectively control the range of the tuned resonance wavelength and reduce the number of the surface plasmon mode, thereby forming more obvious loss peak and enhancing the coupling loss of the surface plasmon mode and the fiber core mode. Compared with an optical fiber sensing device based on a metal film, the scheme of the invention can effectively inhibit the number of surface plasma modes, thereby reducing the number of resonance peaks between the surface plasma modes and a fiber core mode, narrowing the loss peak at the resonance wavelength and forming high-sensitivity sensing.
Drawings
FIG. 1 is a schematic structural diagram of a surface plasmon resonance fiber sensor according to the present invention;
FIG. 2 is a first cross-sectional view of a surface plasmon resonance fiber sensor according to the present invention;
FIG. 3 is a schematic cross-sectional view of a surface plasmon resonance fiber sensor according to the present invention;
fig. 4 is a dispersion curve of the fiber core model and the surface plasmon model of the optical fiber sensor according to the present invention.
FIG. 5 is a graph of different core mode losses for a fiber optic sensor.
FIG. 6 is a core mode loss curve for a fiber optic sensor, wherein: the graphs (a) and (c) are respectively the core fundamental mode and LP when the nano metal wires are arranged in a single section11The mode loss curves of the fiber core fundamental mode and LP are shown in the graphs (b) and (d) respectively when the two metal nanowires are arranged in the vertical section11The mode loss curve.
FIG. 7 shows the core LP at different external refractive indices11The mode loss curve.
Reference numerals:
1-fiber core, 2-cladding, 3-nano metal wire.
Detailed Description
Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like or similar reference numerals refer to the same or similar elements or elements having the same or similar function throughout. The embodiments described below with reference to the drawings are illustrative and intended to be illustrative of the invention and are not to be construed as limiting the invention.
In the description of the present invention, it is to be understood that the terms "central," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "axial," "radial," "vertical," "horizontal," "inner," "outer," and the like are used in the orientations and positional relationships indicated in the drawings for convenience in describing the present invention and for simplicity in description, and are not intended to indicate or imply that the referenced devices or elements must have a particular orientation, be constructed and operated in a particular orientation, and are not to be considered limiting. Furthermore, the terms "first", "second" and "first" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of the present invention, "a plurality" means two or more unless specifically defined otherwise.
First, a surface plasmon resonance optical fiber sensor according to an embodiment of the present invention will be described in detail with reference to the drawings.
Referring to fig. 1-3, a surface plasmon resonance fiber sensor according to an embodiment of the invention includes a sensing fiber and a set of nano-metal wires.
The sensing fiber is a few-mode fiber, i.e. it supports at least LP in the operating wavelength range11The sensing optical fiber comprises a fiber core 1 and a cladding 2, wherein the cladding 2 of a sensing part is polished or cut to form two mutually perpendicular sections, and the distance between the intersection line of the two sections and the central axis of the fiber core 1 is larger than
R is the radius of the core 1, and the lengths of the perpendicular lines from the central axis of the core 1 to the two cleaved surfaces are the same.
Each section is provided with a nano metal wire group which comprises at least two nano metal wires 3 which are parallel to each other, compared with a metal film, the nano metal wires 3 only excite a small amount of surface modes, and the defects that the loss peak is not obvious and the sensing sensitivity is influenced because the excessive metal surface plasma body modes excited on the surface of the metal film are coupled with the fiber core model are avoided. In order to ensure the relative independence and stable transmission of the surface plasmon mode excited by the nano metal wires 3, the central axes of all the nano metal wires 3 are parallel to the central axis of the fiber core 1, so that the surface plasmon mode is strongly coupled with the fiber core model at a specific wavelength position. Therefore, in the case where the refractive index of the external environment medium is the same, the resonance losses of the surface plasmon mode and the fiber core mode excited by the nano-metal lines in the horizontal direction and the vertical direction are the same. All the nano metal wires 3 have the same radius, length and section shape, each nano metal wire 3 is respectively coupled with the fiber core model, and the coupling loss and the loss peak value of the nano metal wires 3 are enhanced through the coupling of a plurality of nano metal wires 3 and the proper arrangement among the nano metal wires 3.
The invention adopts few-mode fiber instead of single-mode fiber or multi-mode fiber, because the mode field of the high-order mode is more in the cladding region and the extension region is larger, the high-order mode is easier to be strongly coupled with the surface plasma mode, thereby leading the loss peak to be more obvious. The multimode fiber can cause strong coupling between a plurality of fiber core modes and a surface plasma body mode, so that loss peaks among different modes are overlapped, and the sensing sensitivity of the multimode fiber is reduced. Fig. 4 shows dispersion curves of the core mode and the surface plasmon mode of the few-mode fiber, where the intersection point of the two is a phase matching point, and the real parts of the core mode and the surface plasmon mode are equal to each other, so as to achieve the maximum coupling loss. In the figure, the dotted line represents a second-order surface plasmon mode, the solid line represents a first-order surface plasmon mode, and the number of surface plasmon modes of the nano-metal wire 3 is determined by the number of nano-metal wires 3. FIG. 5 is a graph of different core mode loss curves, LP, for a few-mode fiber11Modulus ratio LP01The loss of the mode is stronger, and the phase matching with the surface plasma mode is easier to achieve.
According to the invention, a certain number of nano metal wires 3 are arranged on two vertical sections, so that the defect that the conventional single-side polished fiber based on surface plasma sensing is sensitive to polarization is overcome, and polarization-insensitive sensing is realized. Fig. 6 is a core mode loss curve of the nano-metal wires 3 arranged in two normal cuts and a single cut, wherein: in the case of the graphs (a) and (c) respectively, the nano metal wires 3 are arranged in a single section,core fundamental mode and LP11The mode loss curves, graph (b) and graph (d) are the fundamental mode and LP of the fiber core when the two nanowires 3 are arranged in a vertical section11The mode loss curve. As can be seen from fig. (a) and (c), the core mode of the single sectional-arrangement nano-wire 3 has different losses of x-polarization and y-polarization, where x-polarization corresponds to the resonance loss of the second-order surface plasmon mode and y-polarization corresponds to the resonance loss of the first-order surface plasmon mode. From the graphs (b) and (d), the core mode x-polarization and y-polarization losses of the two perpendicular-to-plane arranged nano-metal wires 3 are the same, and the loss in one polarization state is shown in the graph, which is equivalent to the sum of the losses of the single-section core mode x-polarization and y-polarization. Since the surface plasmon modes of the nano-metal wires 3 in the x and y polarization states are different on the same plane, the coupling with the fiber core mode is also different, so-called polarization dependence is formed, that is, the coupling characteristics in the two polarization states are different. And the difference between the surface plasma body model of the nano metal wire 3 on the horizontal section and the surface plasma body model of the nano metal wire 3 on the vertical section is just 90 degrees due to the arrangement direction, and the characteristics of the x polarization mode on the horizontal section are just the same as the characteristics of the y polarization mode on the vertical section. Thus, the properties of the two polarizations are complementary, so that the loss properties of the core 1 for x-and y-polarizations are exactly the same. Because the characteristics of the two polarizations are completely the same, the input field of the two polarizations does not need to realize the input of a single polarization by methods such as a polarization controller and the like, and the cost and the complexity of the system can be effectively reduced.
Due to LP11Mode field characteristics of the mode, such that LP11aThe mode corresponds to a second order surface plasmon mode, LP11bThe mode corresponds to the resonance loss of the first-order surface plasmon mode, so LP is selected11aMold and LP11bThe mode researches the resonance loss change and the resonance wavelength drift of the second-order surface plasma body and the first-order surface plasma body under different external environment medium refractive indexes. As shown in fig. 7, when the refractive index of the external environment medium changes from 1.33RIU to 1.35RIU, the resonance loss of the second-order surface plasmon mode and the first-order surface plasmon mode increases, and the resonance wavelength shifts in the long wavelength direction.
When the sensing optical fiber is manufactured, a wheel type side polishing method can be used, the sensing optical fiber is placed on a controllable rotating wheel, one side of the optical fiber is polished by the rotation of the rotating wheel, a part of the optical fiber is immediately turned over for 90 degrees, and the other side of the optical fiber is processed and polished. The method has the advantages of simple process, environmental protection, flat polishing area and controllable parameters of residual polishing depth, polishing length and the like. During processing, the smaller the residual polishing depth is, the stronger the coupling between the fiber core mold and the surface plasma mold is, but because the optical fiber is brittle, the smaller the residual polishing depth is, the optical fiber is easy to break, and meanwhile, a certain distance needs to be ensured between the fiber core mold and the surface plasma mold to realize the strong wavelength dependence of the loss curve, preferably, the length H of a perpendicular line from the central axis of the fiber core 1 to two tangent planes of the fiber core 1 meets the condition that H-R is not less than 0 and not more than 1 mu m.
When the arrangement of the nano metal wires 3 is too close, the surface plasmon modes are coupled with each other, which may cause a situation that a plurality of peaks appear in the transmission line, and thus the mode field diameter of the surface plasmon mode needs to be considered. The radius of the nano metal wire 3, the radius of the fiber core 1 and the residual polishing depth obtained by simulation by a finite element method have little influence on the mode field diameter of the surface plasma body model, and the mode order has great influence on the mode field diameter, so that the invention requires that the center-to-center distance d between two adjacent nano metal wires 3 in the nano metal wire group meets the requirement of the invention
Where r is the radius of the nanowire 3 in nm and m is the surface plasmon mode order coupled to the fiber core mode. In order to enhance the coupling between the surface plasmon mode and the fiber core mode and ensure that the coupling has strong wavelength dependence, the nano metal wires 3 are required to be arranged in an array form and have symmetry. The specific requirements for the arrangement of the nano metal wires are as follows: when the number of the nano metal wires 3 in the nano metal wire group is odd, the middle nano metal wire 3 in the nano metal wire group is positioned at a position A, the rest nano metal wires 3 are symmetrically distributed at two sides of the position A, and the position A is a projection position of the central axis of the fiber core 1 on a tangent plane; when the nano metal wire 3 is cut at one positionWhen the number of the surfaces is even, the nano metal wires 3 in the nano metal wire group are symmetrically distributed at two sides of a position B, and the position B is the projection position of the central axis of the fiber core 1 on the tangent plane.
Gold or silver commonly used in an SPR sensor is used as a material of the nano metal wire 3, the radius r of the nano metal wire 3 is 50-200 nm, the number of the nano metal wires 3 on a single section is n, n is an integer larger than 1, and 2r + (n-1) d is smaller than 2H. The variation of the diameter size and number of the nano-metal wires 3 can tune the range of resonant wavelength and control the coupling loss of the surface plasmon mode and the core mode.
As before, there can not be too many modes in the optical fiber, and in order to ensure that the sensing optical fiber is a few-mode optical fiber and is easy to be strongly coupled with the surface plasmon mode, the requirements on the radius and the refractive index difference of the fiber core 1 are as follows: the radius R of the fiber core is 4-10 μm, and the refractive index n of the fiber core 1cRefractive index n of the cladding 2cladThe difference satisfies: 0.001<(nclad-nc)<0.05。
The following is a preferred embodiment of the present invention:
the diameter of the core 1 is 10 μm, the diameter of the cladding 2 is 125 μm, the refractive index of the core 1 is 1.467, and the refractive index of the cladding 2 is 0.005 smaller than the refractive index of the core 1. The number of the nano metal wires 3 on one section is 3, the projection of the center axis of the fiber core 1 on the section is provided with one nano metal wire 3, and other nano metal wires 3 are distributed on two sides of the nano metal wire 3 and are symmetrically distributed. The radius of the nano metal wire 3 is 100nm, the material is silver, and the center distance between two adjacent nano metal wires 3 is 1.4 μm. The radius, the section shape and the length of all the nano metal wires 3 are respectively the same, and the nano metal wires 3 positioned on the same tangent plane are arranged in parallel. The perpendicular lengths from the central axis of the core 1 to both tangent planes are the same and 5.2 μm. As shown in FIG. 7, the refractive index of the external environment medium changes from 1.33RIU to 1.35RIU from LP11aMold and LP11bThe resonance loss curve of the mode shows that the resonance loss of the second-order surface plasmon mode and the first-order surface plasmon mode is increased, and the resonance wavelength moves to the long wavelength direction. The refractive index of the medium in the external environment is changed from 1.33RIU to 1.35RIU,the resonance wavelength corresponding to the second-order surface plasmon mode is shifted from 574nm to 606nm, the average sensitivity is 1600nm/RIU, the resonance wavelength corresponding to the first-order surface plasmon mode is shifted from 596nm to 630nm, and the average sensitivity is 1700 nm/RIU. LP11aThe half widths of the resonance lines of the modes are respectively 9.96nm, 8.02nm, 7.54nm and LP11bThe half-widths of the resonance lines of the modes are respectively 11.54nm, 12.57nm and 11.85nm, which is much smaller than that of the conventional side-polished fiber SPR sensor.
In the description herein, references to the description of the term "one embodiment," "some embodiments," "an example," "a specific example," or "some examples," etc., mean that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the invention. In this specification, the schematic representations of the terms used above do not necessarily refer to the same embodiment or example. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.
Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention, and that variations, modifications, substitutions and alterations can be made in the above embodiments by those of ordinary skill in the art without departing from the principle and spirit of the present invention.
Claims (10)
1. A surface plasmon resonance fiber optic sensor, comprising:
the sensing optical fiber is a few-mode optical fiber and at least supports LP within the working wavelength range11The sensing optical fiber comprises a fiber core and a cladding, a sensing area of the cladding is processed to form two mutually perpendicular sections, and the distance between the intersection line of the two sections and the central axis of the fiber core is larger than
R is the radius of the fiber core, and the lengths of the vertical lines from the central axis of the fiber core to the two tangent planes are the same; and
each section of the nano metal wire group is provided with one nano metal wire group, each nano metal wire group comprises at least two mutually parallel nano metal wires, the central axes of all the nano metal wires are parallel to the central axis of the fiber core, and the radii, the lengths and the section shapes of all the nano metal wires are equal.
2. The surface plasmon resonance optical fiber sensor of claim 1 wherein the length of the perpendicular from the central axis of the fiber core to both of the cleaved surfaces is H, and 0. ltoreq. H-R. ltoreq.1 μm.
3. The surface plasmon resonance optical fiber sensor according to claim 1, wherein the material of the nano-metal wire is gold or silver, and the radius r of the nano-metal wire is 50 to 200 nm.
4. The surface plasmon resonance fiber sensor of claim 1, wherein the distance between the centers of two adjacent nanometal wires in the nanometal wire group is d, which satisfies the condition that
r is the radius of the nano-metal wire.
5. A surface plasmon resonance fiber sensor according to claim 1, wherein the group of nanometal lines comprises an odd number of the nanometal lines.
6. A surface plasmon resonance fiber sensor according to claim 5, wherein the middle nanowire in each nanowire group is located at position A, which is a projection position of the central axis of the fiber core on the tangent plane, and the rest of the nanowires are symmetrically distributed on two sides of the position A.
7. A surface plasmon resonance fiber sensor according to claim 1, wherein said group of nanometal lines comprises an even number of said nanometal lines.
8. The surface plasmon resonance fiber sensor of claim 7, wherein the nano-metal wires in the nano-metal wire group are symmetrically distributed on two sides of a position B, and the position B is a projection position of the central axis of the fiber core on the tangent plane.
9. The surface plasmon resonance fiber sensor of claim 1, wherein the number of the nano-metal wires in the nano-metal wire group is n, and 2r + (n-1) d < 2H, n being an integer greater than 1.
10. A surface plasmon resonance fiber sensor according to claim 1, wherein the radius R of the fiber core is 4 to 10 μm, and the refractive index n of the fiber core iscRefractive index n of the claddingcladThe difference satisfies: 0.001<(nclad-nc)<0.05。
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| CN203479701U (en) * | 2013-07-30 | 2014-03-12 | 深圳大学 | Optical fiber sensor and measurement system |
| CN107121410A (en) * | 2017-05-04 | 2017-09-01 | 南昌航空大学 | Based on D-type optical fiber SPR refractive index sensing models |
| CN107389618A (en) * | 2017-06-14 | 2017-11-24 | 暨南大学 | Surface plasma resonance sensor of titanium dioxide enhanced sensitivity and preparation method thereof |
| WO2018018666A1 (en) * | 2016-07-28 | 2018-02-01 | 江苏大学 | Few-mode fibre device |
| CN209460387U (en) * | 2018-12-26 | 2019-10-01 | 暨南大学 | A kind of magnetic field vector sensor based on side throwing optical fiber surface plasmon resonance body |
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Patent Citations (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN203479701U (en) * | 2013-07-30 | 2014-03-12 | 深圳大学 | Optical fiber sensor and measurement system |
| WO2018018666A1 (en) * | 2016-07-28 | 2018-02-01 | 江苏大学 | Few-mode fibre device |
| CN107121410A (en) * | 2017-05-04 | 2017-09-01 | 南昌航空大学 | Based on D-type optical fiber SPR refractive index sensing models |
| CN107389618A (en) * | 2017-06-14 | 2017-11-24 | 暨南大学 | Surface plasma resonance sensor of titanium dioxide enhanced sensitivity and preparation method thereof |
| CN209460387U (en) * | 2018-12-26 | 2019-10-01 | 暨南大学 | A kind of magnetic field vector sensor based on side throwing optical fiber surface plasmon resonance body |
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